Reference

NBS

PUBLICATIONS

Alllot, 0 3 4 0 7 7

Institute for Materials Science and Engineering

POUTMERS

NBSIR 85-3190

U.S. Department of Commerce

National Bureau of Standards

-QC

100

- U56

85-3190

1985

Technical Activities

1985
Idealized representation of a crosslinked epoxy network. The dark

circles show the portions of the molecule labeled with deuterium

atoms to provide points of contrast for small angle neutron

scattering. Neutron scattering experiments can provide quantitative

characterization of the degree of disorder in crosslinked networks, an important factor in the mechanical performance of these

polymers used as adhesives and matrix materials for advanced

composites.
ms

K^EARCHKrcrr r cnrsz?

institute for Materials Science and Engineering

POUTMERS

L. E. Smith, Chief

B. M. Fanconi, Deputy

NBSIR 85-3190

U.S. Department of Commerce

National Bureau of Standards

U.S. Department of Commerce

National Bureau of Standards
POLYMERS DIVISION

CHIEF

Leslie E. Smith

Phone: (301) 921-3734

DEPUTY CHIEF

Bruno M. Fanconi

Phone: (301) 921-3734

ASSISTANT CHIEF

Martin G. Broadhurst

Phone: (301) 921-3734

GROUP LEADERS

Electrical Properties

G. Thomas Davis

Phone: (301) 921*3851

Chemical Performance & Standards

Francis W. Wang

Phone: (301) 921*3321

Mechanical Performance

Bruno M. Fanconi

Phone: (301) 921-3734

Polymer Composites

Donald L. Hunston

Phone: (301) 921*3318

Polymer Blends

Charles C. Han

Phone: (301) 921-3251

Dental & Medical Materials

John A. Tesk

Phone: (301) 921*3336
Polymers

Division

LESLIE E. SMITH, Chief. Dr. Smith was born in New York. He

earned degrees in chemistry from

Case Institute of Technology and

Catholic University. Except for a

three year period of graduate study,

he has been with NBS since 1964.

BRUNO M. FANCONI, Deputy

Chief. Dr. Fanconi was born in Merced, California. He earned

degrees in physical chemistry from

the University of California at Los

Angeles and the University of

Washington. He joined the staff of

NBS in 1971.

MARTIN G. BROADHURST,

Assistant Chief. Dr. Broadhurst was

born in Washington, D.C., and

earned degrees in physics at Western Maryland College and Pennsylvania

State University. After teaching at Pennsylvania State, he joined NBS in 1960.

G. THOMAS DAVIS, Group

Leader, Electrical Properties. Dr. Davis was born in Montour Falls, New York. He earned degrees in chemical engineering at Cornell

University and physical chemistry at Princeton University. Prior to joining NBS in 1964, he worked for Esso

Research and Engineering Co. and

taught at the University of Virginia.

FRANCIS W. WANG, Group

Leader, Chemical Performance and

Standards. Dr. Wang was born in Peikang, Taiwan. He earned the B.S. and M.S. in chemical engineering at California Institute of Technology

and the Ph.D. in physical chemistry

at the University of California, in San

Diego. He joined the staff of NBS in 1972 after a postdoctoral period at the Polytechnic Institute of New

York.

DONALD L. HUNSTON, Group

Leader, Polymer Composites. Dr. Hunston was born in Springfield, MI. He earned degrees in mathematics and chemistry at Kent State University. Prior to joining NBS in 1980, he was a Postdoctoral Fellow at Northwestern University and worked

in the Chemistry Division at the Naval Research Laboratory.

JOHN A. TESK, Leader, Dental and

Medical Materials. Dr. Tesk was

born in Chicago, Illinois. He holds

the degrees of B.S., M.S. in Metallurgy and Ph.D. in Materials Science from Northwestern University. Prior to joining NBS in 1978, lie was at the University of Illinois, Chicago, on the staff at Argonnc Na

tional Laboratory, Director of R&D

with the Dental Division of

Howmedica, Inc. and Director of Education Services at IGT.

CHARLES C. HAN, Group Leader,

Polymer Blends. Dr. Han was born

in Szuchuan, China. He earned his B.S. in chemical engineering at National Taiwan University, M.S. in physical chemistry at the University

of Houston and his Ph.D at the University of Wisconsin. He joined

the staff of NBS in 1974.

BRUNO M. FANCONI, Group

Leader, Mechanical Performance.

TABLE OF CONTENTS PAGE

INTRODUCTION iii

RESEARCH STAFF v

TECHNICAL ACTIVITIES 1

TASK 14332 - POLYMER STANDARDS AND CHEMICAL PERFORMANCE. ... 1

FY 85 Accomplishments 1 Characterization and Standards. . . . . . . ....... 2

Chemical Durability ...... 9

TASK 14333 * BLENDS PROCESSING 12

FY 85 Accomplishments 12

Experimental 13

Theory. ................... 14

SAXS and Pole Figure Facilities ............. 16

TASK 14335 * BIOMATERIALS SCIENCE, METROLOGY AND STANDARDS

FOR IMPROVED DURABILITY, APPLICATIONS AND

MANUFACTURING OF DENTAL AND MEDICAL MATERIALS . . 19

FY 85 Accomplishments 19

Dental Composite and Cement Sealant and Adhesion

Chemistry 20

Wear, Durability, Clinical Studies and Related

Properties of Dental Composites, Amalgams,

Cements, and Glass Ionomer Cements 22

Dental Alloys, Ceramics, Metrology and Analysis ..... 24

Calcium Phosphate Chemistry, Biochemistry, Structure

and Clinical Implications 25

Improved Surgical Bone Cement .............. 27

TASK 14336 “ MECHANICAL PERFORMANCE OF POLYMERS 28

FY 85 Accomplishments 29

Characterization of Polymer Structure and

Deformation Processes 29

Mechanical Properties Measurement and Modeling. ..... 39

Mechanical Durability of Polymeric Materials. ...... 45
TASK 14337 “ PROCESSING AND RELIABILITY OF POLYMER

COMPOSITES. 49

FY 85 Accomplishments 50

Cure Monitoring and Process Control for Polymers

and Their Composites 50

Analysis of Structure in Polymers and Their

Composites 53

Performance of Polymers and Their Composites. ...... 56

TASK 14339 “ MICROSTRUCTURE AND PERFORMANCE OF

DIELECTRIC PLASTICS 61

FY 85 Accomplishments 62

Automated Dielectric Measurements and Models for

Dielectric Relaxation 62

Piezoelectric Polymers and Pressure Transducers ..... 64

Space Charge and Ion Transport in Polymers. ....... 66

OUTPUTS/INTERACTIONS 69

Publications 69

Technical and Professional Committeet Leadership

Activities 77

Industrial and Academic Interactions. .......... 81

li
INTRODUCTION

POLYMERS DIVISION

Leslie E. Smith, Chief

Bruno M. Fanconi , Deputy Chief

The Polymers Division is responsible for providing standards,

measurements, and fundamental concepts of polymer science to assist U.S.

industries that produce, process, or use polymers as an essential part of

their business. Plastics, elastomers, and synthetic fibers now form the

basis of industries that add over $100 billion of value by manufacture to

the U.S. economy and these industries are growing rapidly relative to

other basic materials industries. This growth rate is due primarily to

the extreme versatility of synthetic polymers, whose wide range of

possible properties offer opportunities for scientists to create

innovative materials for nearly any new product. The polymer industry has

vigorously pursued these opportunities and continues to introduce new polymers, process methods, and applications at a high rate. Examples of

these new applications include trends for the increased use of polymers in

packaging, construction, transportation, national security, electronics,

information handling, clothing, electrical appliances, dentistry, and

medicine. The Division’s program are structured to deliver outputs that

meet current needs as well as prepare for the changing needs of the

industry over the next decade.

The organization of the Division consists of three property-oriented

Groups and three material-oriented ones. The Groups concerned with

electrical, chemical, and mechanical performance of polymers concentrate

on the relationship of polymer structure to properties and especially on

the measurement of the response of polymers to electrical, chemical, or mechanical stress. The programs of these Groups focus on the use of

fundamental science to provide standards, measurement methods, and

solutions to technological problems.

The Groups concerned with polymer blends and composites are responsible

for all aspects of the processing and performance of these advanced

materials. The formation of these Groups reflects the growing importance

of blends and composites to the polymer-related industries. Polymer

blends are one effective way for domestic polymer producers to respond to

competitive pressure from foreign producers with access to very

inexpensive feedstocks. New products can be tailored to meet specific

market requirements by blending two or more polymers, without abandoning

present production facilities and without the large capital expenditures

necessary to produce a totally new polymer. The principal industrial need

in this field is for a fundamental science base to underlie the design and

processing of new blends. This includes needs for measurement methods to

produce data for polymer phase diagrams, data on polymer diffusion and the

kinetics of phase separation, and theoretical models that accurately

describe the thermodynamics and kinetics of blends.

Although only about "\ 0 % of polymer production is now used In polymer

matrix composites, the current and potential impact of these materials on other industries is dramatic. Military and aerospace uses of advanced

composites are already large and diverse; major components of automobile

iii
are beginning to be constructed of fiber-reinforced composites and many

opportunities exist in the construction, machinery, and electronics

markets. Two major factors have inhibited the growth of composites in

mass markets: inability to process the materials inexpensively and

reliably; and uncertainty about the performance of these anisotropic

materials under complex loads, particularly at failure.

The Composites Group has programs aimed at solving some of the measurement

and basic science aspects of these two problems. More efficient

processing will come from automated manufacture aided by appropriate

feedback control sensors. The identification of materials properties that

can be monitored and development of measurement methods suitable for

process control are major objectives of the Group with additional support

from the NBS Non-Destructive Evaluation program.

Failure behavior of composites is very complex and the fundamental science

of heterogeneous materials is not advanced enough to be confident of

predictions based on much less than prototype testing. Much work needs to

be done in this area but progress is being made in understanding the

resistance to composites to interlaminar shear failure, at least under

simple types of loads. Our current emphasis is on developing reliable

test methods for interlaminar shear strength and trying to understand

failure behavior in terms of the properties of the constituent materials.

The Dental and Medical Materials Group is a model of effective private

sector-government cooperation. For over sixty years the American Dental

Association (ADA) and NBS have worked together to improve dental materials

and devices. Virtually all the major materials used by dentists in the

United States today have been developed or influenced by this joint

program. This year the NBS program achieved another dimension when

Research Associates from the National Institutes of Dental Research (NIDR)

joined with those from the ADA, the dental industry, and domestic and

foreign guest workers to become part of what is now the most comprehensive

program of its kind anywhere in the world.

Each of these programs is described in more detail in this report. For

management purposes, the technical program is divided into six task areas,

as follows:

POLYMER STANDARDS AND CHEMICAL PERFORMANCE

BLENDS PROCESSING

BIOMATERIALS SCIENCE, METROLOGY AND STANDARDS FOR IMPROVED

DURABILITY, APPLICATIONS AND MANUFACTURING OF DENTAL AND

MEDICAL MATERIALS

MECHANICAL PERFORMANCE OF POLYMERS

PROCESSING AND RELIABILITY OF POLYMER COMPOSITES

MICROSTRUCTURE AND PERFORMANCE OF DIELECTRIC PLASTICS

iv
RESEARCH STAFF

Antonucci, Joseph M.

Barnes, John D.

Bauer, Barry J.

Blosser, Richard L.*

Bowen, Rafael L.*

Brauer, Gerhard M.

Broadhurst, Martin G.

Brown, Daniel W.

Brown, Walter E.*

Bur, Anthony J.

Carey, Clifton M.#

Chang, Shu Sing

^Research Associate

• Synthetic and polymer chemistry

• Dental composites and cements

• Antioxidants

• Gas and vapor transport in polymers

• X-ray scattering

• Computer applications in polymer

measurements

• Polymer synthesis

• Neutron, x-ray and light scattering

• Adhesion measurements

• Adhesion

• Polymer composites

• Biocompatibility

• Synthetic and polymer chemistry

• Dental composites and cements

• Adhesion

• Orthopedic materials

• Dielectric measurement

• Piezoelectric and pyroelectric

modeling and theory

• Equation of state of polymers

• Ionic conduction

• Pyrolysis of polymers

• Lifetime of polyurethanes and

polyesters

• Solution chemistry of calcium

phosphate (CAPH) compounds,

• Biological CAPH compounds,

• Atherosclerotic plaque

• Dielectric properties of polymers • Piezoelectric, pyroelectric polymers

• Polymeric transducers

• Dental plaque

• Microanalytical analysis techniques

• Thermal Properties of Polymeric

Materials

• Electronic Packaging

• Polymer Phase Transitions

• Precision Electrical and Temperature

Measurements

v
Chiang, C. K. • Electrical properties of polymeric,

ceramic and organic materials

• AC electrical impedance

spectroscopy

Chow, Laurence C.* • Calcium phosphate compounds

• Dental and bioimedical cements

• Solution chemistry

• Topical dental fluorides

Crissman, John M. • Mechanical Behavior

• Environmental Stress-Cracking

0 Failure and Fracture of Polymers

Davis, G. Thomas 0 Piezoelectricity in polymers

0 Polymer crystallization

0 X-ray diffraction of polymers

Debelius, Elizabeth E. 0 Mechanical Properties of Paper

0 Mechanical Testing of Composites

0 Measurement of Dental Materials

Dehl, Ronald E. 0 Surface Analysis

0 Porosity Measurements

0 Ellipsometry

0 Bonding of Polymers and Composites

DeReggi, Aime S. 0 Polarization-depth profiles in

polymers

0 Space charge in dielectrics

0 Ferroelectric polymers

0 Polymeric piezo- and pyroelectric

devices

de Rijk, Waldemar G.* 0 Clinical dentistry

0 Ceramic materials

0 Expansion characteristics of solids

and semi-solids

0 Statistical analysis

Dickens, Brian 0 Kinetics of thermal degradation

" of polymers

0 Photodegradation of poly

(methyl methacrylate)

0 Curing studies of thermosetting

printing inks

Di Marzio, Edmund A. 0 Statistical mechanics of polymers

0 Phase transitions

0 Glasses

0 Polymers at interfaces

^Research Associate

vi
Eanes, Edward D.# • Structure of bones and teeth

• Calcium phosphate compounds

• Effects of biological molecules on precipitation of calcium phosphates

• Liposome studies

Eidelman, Naomi N.* • Atherosclerotic plaque

Fanconi , Bruno M. • Infrared and Raman Spectroscopy

of polymers

• Structure of polymers

• Polymer fracture

• Process monitoring of

polymer composites

Fowler, Bruce 0.* • Laser Raman structural analysis

of calcium phosphates

Giuseppetti, Anthony A .* • Casting of dental alloys

• Titanium alloys

Grant, Warren H. • Adsorption of polymers

• Fluorescence measurements of

polymeric systems

Gregory, Thomas M.* • Calcium phosphate compounds

• Surface chemistry

Guttman, Charles M. • Solution properties of polymers

• Diffusion of small molecules

into polymeric systems

• Semicrystalline polymer

chain configurations

• Kinetics of polymer crystal growth

Han, Charles C. • Polymer blends

• Polymer characterization

• Small angle neutron scattering

• Static and quasi-elastic light scattering

Hailer, Arthur W.* • Chemical reactions, chemical analysis

Howell, Barbara F. • Electron microscopy

• Spectrophotometry

• Light microscopy

Hunston, Donald L. • Adhesion Science and Technology

• Fracture Behavior of Polymers

• Processing and Failure Behavior

of Composites

• Flow Behavior of Dilute High

Polymer Solutions

• Macromolecule-Small Molecule Binding

^Research Associate
Johnson, Allen D.*

Khoury, Freddy A.

Kryder, Samuel J.

Lowry, Robert E.

Mathew, Mathai*

Maurey, John R.

Mazur, Jacob

McCrackin, Frank L.

McKenna, Gregory B.

McKinney, John E.

Misra, Dwarika N.*

^Research Associate

• Nuclear magnetic resonnance

• Mossbauer studies

• Chemical adhesion

• Crystallization, Structure and

morphology of polymers

(including polymer blends)

• Analytical Electron Microscopy

of polymers

• Wide angle and small angle

x-ray diffraction

• Structure and Mechanical Property

Relationships

« Electronic circuit design and

construction

• Electronic troubleshooting and

repair

• Applications of fluorescence

spectroscopy to polymeric systems

• Synthesis of chromophore-labeled

polymers

• Crystallography

• Calcium phosphate compounds

• Ultracentrifugation

• Rayleigh light scattering

• Osmometry

• Densimetry • Refractometry

• Crystallographic defects in polymers

• Molecular dynamics

• Computer modeling of crystalline

polymers

• Vibrational spectroscopy of polymers

• Monte Carlo computer simulations

• Ellipsometry analysis

• Size exclusion chromatography

• Nonlinear Viscoelasticity

• Molecular Rheology

• Physics of Polymer Glasses

• Rubber Elasticity

• Tribology of dental composites,

cements and alloys

• Surface chemistry

• Adhesion

• Chemisorption

viii
Mopsik, Frederick I.

Paffenbarger , George C.*

Passaglia, Elio

Penn, Robert W.

Phillips, Joseph C.

Pummer, Walter J.

Reneker, Darrell H.

Roth, Steven C.

Rubin, Robert J.

Rupp, Nelson W.*

^Research Associate

• Dielectric measurements and behavior

• Automated measurement design

• Computerized data analysis and

programming

• Electrical properties of polymers

• Dental materials

• Dental standards

• Amalgams

• Fracture and crazing

• Linear Viscoelasticity

• Crystallization theory

• Rheology

• Properties of polymers and composites

• Statistical analysis

• Viscosity & Normal Stress

Measurements

• Environmental Effects on Transport

and Mechanical Properties

• Curing Behavior of Thermoset Inks

• Synthesis and characterization

of aromatic polyimides

• Synthesis of specialty compounds

• Blending of radio-tracers into polymers

• Crystallographic defects in polymers

• Polymer Structure

• Electron microscopy of polymers

• Vibrational Spectrosocopy of polymers

• Piezoelectric polymer transducersfabrication and applications

• Vacuum deposition of metals

• Calibration of polymer transducers

• Statistics of polymer chain

configurations

• Theory of Brownian motion

• Random walk models with traps

• Polymers near surfaces

• Clinical dentistry

• Amalgams

• Gold foils

• Dental standards

• Composites

• Titanium castings

ix
Sanchez, Isaac C. • Statistical thermodynamics of polymers

• Polymer blends

• Interfacial phenomena

Schreiber, Carol J.* • Topical dental fluorides

Setz, Louise E.* $ Adhesion

• Chemical analysis

Sieck, Barbara* 9 Calcium phosphate compounds

9 Chemical analysis

Smith, Leslie E. 9 Adsorption of polymers

9 Polymer degradation reactions

Stansbury, Jeffrey W. 9 Synthetic chemistry

9 Polymers and polymer composites

9 Polymerization expanding monomers

Takagi, Shozo* 9 Crystallography

9 X-ray diffraction

9 Calcium phosphate compounds

Tesk, John A. 9 Casting of alloys

9 Strength of dental systems

9 Thermal expansion and properties of

dental materials

9 Finite element studies

9 Porcelain-metal system

Tomazic, Branko* 9 Atherosclerotic plaque

9 Biological calcium phosphate compounds

Tung, Ming S.* 9 Biological solution chemistry of

calcium phosphate compounds

9 Equilibrium studies

VanderHart, David L. 9 Measurement of orientation in

polymer fibers and films

9 Solid State NMR of polymers

9 Measurement of polymer morphology

on the 1-1 Onm scale

Venz, Sabine* 9 Clinical dentistry

9 Dental composites

9 Dental polymers

Verdier, Peter H. 9 Polymer solution properties

9 Polymer chain dynamics in solution

9 Statistical analysis of data

9 Error estimation

9 Computer simulation of polymer

chain dynamics

*Research Associate

X
Vogel, Gerald L.*

Wagner, Herman L.

Wahlstrand, K. J.

Wang, Francis W.

Wu, Wen-li

Zapas, Louis J.

^Research Associate

• Dental plaque, biological solution

chemistry of calcium phosphates

• Micro analytical techniques

• Dilute solution properties

of polymers

• Molecular parameters

• Limiting viscosity number

• Size exclusion chromatography

• Osmotic pressure

• Mathematical analysis

• Computer simulation

• X-ray pole figures

• Photophysics and photochemistry

of polymers

• Fluorescence spectroscopy

• Thermodynamic and frictional

properties of macromolecules

• Neutron and X-ray Scattering

• Electron Microscopy

• Mechanical Behavior of Polymers

and Composites

• Continuum Mechanics & Rheology

• Non-linear mechanical behavior of

polymers

• Static fatigue of plastics

• Finite Elasticity (Rubbers)

xi
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POLYMER STANDARDS AND CHEMICAL PERFORMANCE

Task 1 4332

The objective of this task is to provide to the polymer industry standard

reference materials (SRM’s) as well as measurement methods and data for

predicting the lifetimes of polymers used in high volume or critical applications

.

Polymeric SRM's certified for molecular weight are needed by the polymer

industry to calibrate gel permeation chromatographs (GPC’s) which are used

routinely to measure polymer molecular weights and molecular weight dis*“

tributions. The latter are the most important physical properties in determining a polymer’s processibility and its end-use properties.

Improved techniques for polymer characterization and better understanding

of polymer solution properties are essential to the efficient production

of polymeric SRM’s. Consequently, experimental and theoretical studies of

polymer-solution properties are actively pursued in this task.

The most promising areas of growth in polymer markets are in construction,

transportation, and electrical systems where long-term durability is es- sential to competitive performance. To compete successfully in these markets, our country needs scientific research on the mechanisms by which

polymeric materials are degraded because of environmental and other imposed stresses. In fact, a recent National Research Council report

states, ’’Mechanistic data on degradation are essential (1) in identifying

possible stabilization processes to slow degradation, and (2) in developing short-term methodologies for predicting service life.”

Protective additives are needed for fabrication and long-term use of most

polymers. In use, these additives inevitably diffuse through the polymer

to some extent and may migrate out of the polymer entirely. The polymer

is then left with poorer properties or made more vulnerable to degradation

and failure. This is a major failure mode of polymers, and lifetime prediction methods explicitly include the rates of additive diffusion along

with the rates of the inhibited and uninhibited degradation reactions.

The number of commercially usable polymer-additive combinations is too

large to be considered efficiently on a case-by-case basis. The emphasis

in this task is, therefore, on the development of general principles or models of diffusion that are applicable to the widest range of generically

important systems. In addition, knowledge of the general principles which

control diffusion through solid polymers is important to the design and

evaluation of improved additive systems.

FY 85 Significant Accomplishments

• A series of poly(methyl methacrylate) is being produced with support

from the Navy and the Office of Standard Reference Materials. These

SRM’s will be used for quality control of antifouling paints and as calibrating materials for the analysis of acrylic polymers. The

limiting viscosity number (intrinsic viscosity) and the number-av^r.i,'

molecular weight for the highest molecular weight polymer in the series

have been determined.
• Two "linear low density" polyethylenes used in gas piping (one with,

and the other without pigment) are being characterized for certification as Standard Reference Materials, with support from the Gas Research Institute and the Office of Standard Reference Materials. Such

a standard would be useful not only to the gas distribution and associated industries but also to the rest of the scientific community. Preliminary experiments have established the conditions necessary for size

exclusion chromatography, such as the type of columns, concentration,

and temperature.

• No significant difference has been found between the diffusion

coefficients of the cyclic polystyrene polymer and that of the linear

polystyrene polymer when they have the same molecular weight. The

diffusion coefficients of fluorophone-labelled cyclic polystyrene in

dilute and semi-dilute linear polystyrene solutions were measured by a novel method called Fluorescence Recovery After Photobleaching (FRAP)

.

Characterization and Standards

Subtask 1 of Task 1 4332

F. W. Wang, C. M. Guttman, P. H. Verdier, H. L. Wagner, J. R. Maurey,

R. E.' Lowry, W. H. Grant, and E. S. Wu^

University of Maryland, Baltimore County

Work in this Subtask proceeds primarily along four lines: Production of

polymer standards; critical evaluation of experimental data on polymers;

development of improved methods for characterizing polymeric materials and

development of fundamental concepts related to the solution properties of

polymers. The linear low density polyethylene standards currently under

development, which are partly supported by the Gas Research Institute,

will aid in the development of polyethylene gas pipes with improved

properties. The poly(methyl methacrylate) standards currently in

production will aid the Navy, which is partially supporting their

production, in the development of long-lived antifouling paints for ships'

hulls. In addition, both kinds of standards will be generally useful in

the plastics and related industries, both for equipment calibration and as materials for research. The critical evaluation of literature data on the

relationships between limiting viscosity number and molecular weight for

atactic polystyrenes will aid in materials characterization in this

important segment of the plastics industry. Development of a self-calibrating gel permeating chromatograph should eventually result in

substantial reductions in the cost of producing polymeric Standard

Reference Materials, and should allow much more detailed characterization

of polymeric materials than has thus far been practical. Fluorescence

measurements of polymer diffusion and polymer-polymer interactions will

provide data that are needed for welding of polymers, for designing

polymerization reactors, and for producing molecularly reinforced

composites. Finally, the theoretical studies of translational diffusion

constants and chain-molecule dynamics will yield fundamental concepts that

will not only guide the experimental work on polymer-solution properties

but also give insight on the processibility and the end-use properties of

polymeric materials.

2
Characterization of Gas Research Institute Polyethylene

Polyethylene is widely used for piping in gas distribution systems. It

has been recognized lately that better coordination of research and development activities and facilitated comparison of techniques and test data

would result if a single well characterized resin were available to all

research workers in the gas distribution and associated industries. Such

a resin should be available from a single supplier over a number of years.

As a result the Gas Research Institute and OSRM are supporting work on the

characterization of two "linear low density" polyethylenes used in gas

piping, one with, and the other without pigment. This new type of polyethylene is considerably higher in molecular weight than any of the polyethylene SRM’s now available. Such a standard would be useful to the rest

of the scientific community because there is a growing need for higher molecular weight polyethylene standards.

The natural or unpigmented resin, designated SRM 1496, will be certified

for limiting viscosity number, and a supplementary (non-certif ied ) distribution in molecular weight. For the pigmented resin, designated SRM

1497, a certified melt flow rate and a supplementary (non-certif ied) molecular weight distribution will be determined. Later on the molecular

weight of SRM 1496 will be measured for certification.

A thousand pound shipment of the unpigmented resin has been received and

preliminary experiments have established the conditions necessary for size

exclusion chromatography, such as the type of columns, concentration, and

temperature. It was also found that a temperature of 160 °C for one hour

is necessary for complete solution. This lot of resin has also been

tested for uniformity by determining the viscosity number in

tri chlorobenzene at 130 °C for samples from 10 different locations in

the container.

Poly(methyl methacrylate) Standard Reference Materials

A series of three poly (methyl methacrylate) Standard Reference Materials,

certified for molecular weights and limiting viscosity numbers, will be

produced. These standards will be used by the Navy, which is providing

partial support for their production, in quality control procedures for

Navy specifications for commercial antifouling paints used on ships'

hulls. They will also be widely used in the plastics and related industries for the calibration of size-exclusion chromatographs used for quality control, and for research and development.

Three poly(methyl methacrylate) polymers with molecular weights of

approximately 8,000, 30,000 and 120,000, which are to become SRM* s 1487,

1488, and 1489, respectively, have been blended, bottled, and examined by

nuclear magnetic resonance (NMR) for chemical identify and tacticity. The

limiting viscosity number of SRM 1489 has been determined by capillary

viscosity measurements for certification. The number average molecular

weight, which is also to be certified, has been determined by membrane

osmometry for this same SRM. Both types of measurements were made at 25°C

in toluene.

3
Critical Evaluation of Mark-Houwink Relations for Atactic Polystyrene

One of the most important physical properties of a polymer is its average

molecular weight, which largely determines processibility and performance

properties. Since absolute determinations of molecular weight are time

consuming and are not made routinely, relative methods, such as solution

viscosity and size exclusion chromatography (also called gel permeation

chromatography) are used instead.

The limiting viscosity number (intrinsic viscosity), [ n ] , and the

molecular weight, M, are related, empirically, by a simple power law:

Uni = KM a

where K and a are the empirically determined Mark-Houwink parameters,

which depend on the specific solvent and temperature. An incomplete

compilation of these data can be found in the handbooks, but a critical

assessment is not yet available. Yet these data are used extensively to

obtain molecular weights as well as to calibrate size exclusion

chromatography columns.

Detailed examination of the literature revealed that many of the determinations of K and a were made by indirect rather than by absolute measurements of molecular weight, or by the use of whole polymers rather than

fractions, leading to large uncertainties. Determinations such as these

were rejected in favor of direct determinations of molecular weight by absolute methods such as light scattering or osmotic pressure.

The literature for atactic polystyrene has been critically reviewed and

the K and a values for a variety of solvents often used for viscosity

measurements have been recommended. The solvents are benzene, toluene,

1,2,4 trichlorobenzene, tetrahydrof uran (THF) , o-dichlorobenzene,

2-butonone, and two theta solvents, cyclohexane and decalin. In addition,

the Mark-Houwink parameters for several other solvents, not used as frequently., are provided. A review paper has been written and submitted

to the Journal of Physical and Chemical Reference Data.

A similar critical evaluation has been undertaken for poly (methyl

methacrylate). The data for this polymer in 11 solvents are being

reviewed. These are acetone, benzene, chloroform, butanone, THF, toluene,

nitroethane, ethyl acetate, acetonitrile, dichloroethane , and methyl

isobutyrate, as well as a number of theta solvents.

Self-Calibrating Gel Permeation Chromatography

Gel permeation chromatography (GPC) is a widely accepted technique for es- timating the molecular-weight distribution (MWD) of high polymers.

However, the usefulness of the conventional GPC apparatus is limited by

the need to provide calibrants for each polymer measured of known molecular

weight over the entire molecular weight range in which the MWD is

significantly different from zero. The calibration depends, among other

things, upon the chemical nature, degree of branching, etc., of the

polymeric material so that each new material requires a fresh calibration.

The so-called "universal calibration" hypothesis, while useful, is limited

to comparisons of polymers of similar shape and, in any event, is

inadequate for quantitative determinations. Some instruments, one of

which is commercially available, are intended to circumvent the need for
callbrants by adding a single-angle light scattering detector to the usual

concentration-sensitive detector. However, this does not allow the

extrapolation to zero scattering angle which is required, in principle, to

relate scattering intensity to molecular weight. In addition, qualitative

information on the variation of scattering with angle, normally required

to give assurance that meaningful results are being obtained, is not

available.

A light-scattering detector for the GPC has been designed and builtat NBS

which measures, in real time, scattered intensity as a function of

scattering angle. The apparatus is controlled by a dedicated minicomputer

in a way that allows scattering to be measured as a function of both

scattering angle and concentration. Continuous monitoring of the

variation of scattering with angle while the chromatogram is being

obtained allows Immediate identification of difficulties such as association, microgel formation, etc., which would affect the validity of

the molecular weights obtained, an important consideration for work on new

and unstudied materials. In addition to molecular weight, the mean-square

radius (radius of gyration) can be obtained as a function of molecular

weight, at least in the higher ranges of molecular weight, providing

useful information for the characterization of branched polymers. The

second virial coefficient, a measure of polymer-polymer interactions of

both current and continuing interest to solution-properties theorists, can also be obtained as a function of molecular weight.

A new sample injection system has been designed by us and installed which

permits quantitative injection of samples into the GPC columns and

injection of callbrants etc. directly into the light-scattering detector,

bypassing the columns.

Diffusion of Linear and Cyclic Polymers in Semi-dilute Solutions by

Photobleaching Method

Although the reptation model of de Gennes successfully accounts for the

diffusion of linear polymers, there remains a controversy whether it adequately describes the diffusion of cyclic or star-shaped ones. Resolution

of this controversy has been hampered by the lack of a rapid method for

determining the diffusion coefficients of polymers and by the difficulty

in preparing and purifying cyclic polymers. We have recently applied a novel method, called Fluorescence Recovery After Photobleaching (FRAP), to

rapidly determine the diffusion coefficients of linear and cyclic polymers

in semi-dilute solutions. The FRAP method can be used to measure diffusion coefficients in the range 10* 11 to 10“^ cm

2 /s.

Linear and cyclic polystyrene polymers of narrow molecular weight

distribution have been labelled with nitrobenzoxadiazole (NBD). The

diffusion of NBD-labelled cyclic polystyrene in dilute and semi-dilute

linear polystyrene solutions has been measured by the FRAP method over a concentration range from 0.02 to 0.3 g/ml. No significant difference has

been found between the diffusion of the cyclic polystyrene polymer and the

diffusion of the linear polystyrene polymer when they have the same

molecular weight.

5
Fluorescence Spectroscopy of Rod-like Polymers

Recently, heterocyclic rod-like polymers, exhibiting thermal and oxidative

resistance as well as superior mechanical properties, have attracted

considerable interest. The high strength of these polymers arises from

their ability to achieve a high degree of molecular order during film or fiber processing. Such order arises from the high degree of rigidity that

these polymers show as well as the degree of interaction they have with

themselves and the surroundings. Even in the solution, the polymers show

anisotropic liquid crystalline states as well as isotropic states. In all

states the polymers seem to exhibit a fully extended configuration.

In cooperation with the Materials Laboratory of the Air Force Wright Aeronautical Laboratories (AFWAL) we are developing fluorescence methods to

study single chain and multiple chain properties of these systems. By

looking at the time-resolved fluorescence intensities from dilute solutions of the polymers, we hope to provide an estimate of the torsional

rigidities of these polymers which we expect will relate directly to their

performance properties. From studies of the fluorescence of the solutions

of these polymers as a function of concentration, we hope to characterize

the polymer-polymer interactions as well as the degree of order in the

various phases.

We have measured the excitation and emission spectra of a dilute solution

of poly (p-phenylene benzbisthiazole) (PBT) in methanesulfcnic acid. We

have also measured these spectra for a dilute methanes ulfonic acid solution of 2, 6-diphenylbenzo( 1

, 2-d: 4,5“d’ )bisthiazole (hereafter referred to

as t-bisthiazole), a model compound of PBT. We have found that the emission spectrum of the PBT solution has its most intense band at 466 nm when

the excitation wavelength is any of the following wavelengths: 270 nm,

320 nm, and 440nm. The emission band for the PBT solution occurs at

longer wavelength than the one for the t-bisthiazole solution at 41 6 nm,

indicating that the phenyl and the heterocyclic moieties in PBT are highly

conj ugated.

Because PBT films, and blends containing PBT are often cast from

methanesulfonic acid, they inevitably contain some residual acid. It is

therefore important to study the effect of protonation on the emission

spectrum of t-bisthiazole, the model compound of PBT. We have measured

the emission spectrum of a dilute solution of t-bisthiazole in chloroform

as a function of the concentration of trifluoroacetic acid (TFA) added to

the solution. We have found that as the concentration of TFA increases, a broad structureless band at 460 nm appears and increases in intensity at

the expense of the structured band at 416 nm. However, as the concentration of TFA is further increased, the intensity of the band at 416 nm in- creases while that at 460 nm decreases and nearly disappears when the

solvent is a strong acid, for example, methanesulfonic acid.

We have also measured the lifetimes of electronically excited PBT and

t-bisthiazole in methanesulfonic acid by nanosecond fluorescence spectroscopy. The lifetimes are estimated to be 0.2 to 0.3 ns for t-bisthiazole

and 0.1 ns or less for PBT. We plan to get better estimates of the

lifetimes by picosecond fluorescence spectroscopy.

6
Fluorescence Studies of Polymer Solutions

Current interest in solution properties of polymers focuses on the shape

of the polymeric chain in dilute solutions and the chain’s interactions

with itself, with other chains, and with solvents. The fluorescence

quenching study provides information on many of these properties. The

chemistry of attaching fluorescent and quenching groups to a polystyrene

chain has been worked out, and chains with varying concentrations of

fluorophores and quenchers have been made. Preliminary experiments on the

properties of single chains in solution have been done. Theoretical development establishing the relationship between the fluorescence properties of isolated chains, or two chains in solution and other chain

properties is being carried out in connection with the experimental studies. Our long range plan is to study the two-chain properties of the

polymeric chain in more concentrated systems so we will be able to obtain

a better understanding of the interactions of polymers in blends.

Translational Diffusion Constants of Linear and Branched Chains

With the advent of experimental equipment in which both static and

quasi-elastic light scattering may be done on the same sample, the ability

to obtain both a radius of gyration and a hydrodynamic radius on the same

system is possible. For branched chains this offers the hope of obtaining

structural information about branching here-to-fore unavailable. Since

the character and details of the branching of a polymer molecule determine

its processibility , as well as its ultimate physical properties and

mechanical strength, such information on branching is invaluable.

One theory currently available to relate these two quantities to the polymer structure is the Kirkwood-Riseman theory of the translational

diffusion constant in solution. However, there has been found to be a discrepancy between the experimental value of the translational diffusion

constant of a polymer molecule, both branched and unbranched, in dilute

solution and that obtained from Kirkwood-Riseman theory using an analytical Gaussian model of the chain. The discrepancy is 1556 for linear

chains and nearly 5056 for the ideal star branched chains studied.

Current work at this laboratory approaches this problem in various ways.

In one, the hydrodynamic radius is computed using the Kirkwood-Riseman

formulation for both linear and branched chains created by a computer

simulation in which both excluded volume and interaction energies are

included. These calculations have shown that about half of the 1556 deviation found between theory and experiment can be explained by this

modeling. Previously, dynamic corrections to Kirkwood-Riseman theory were

invoked to explain these discrepancies. Work on branched chains is in

progress.

In a second approach, analytical modeling was done of chains in which

local chain structure, but not excluded volume, is taken into account.

Earlier work showed that the analytical model currently available to

compare theory and experiment shows a very slow approach to its large

molecular weight, M, limit. It is only in this limit that the Gaussian

theory and experiment can be meaningfully compared. Thus, correction:, t the large M values used to compare with experiments were not possible even

for polymers of high molecular weight. In this effort analytical modeling

of chains with local structure and stiffness was done, and the first two

corrections to the large M limit of the hydrodynamic radius of the chain:-.

7
were computed exactly. It was shown the effect of chain structure on each

of these three terms was different. Thus, the simple ideas of statistical

length used earlier was found to need improving. This work has been

published.

In another approach Monte Carlo chains have been created in which

corrections only up to the second cluster integral or the second virial

coefficient are included in the modeling. Current work shows that such

chains at the theta point are akin to the so called ’'true” self avoiding

walks. In the future, we plan to look into the importance of the third and

higher cluster terms on size related properties, like hydrodynamic radius,

for both linear and branched chains.

Dynamics of Polymer Chains

The dynamic behavior of polymer chains is important to improving both

characterization and processibility of polymers. The principal methods

used to estimate molecular weight, and, in particular, gel permeation

chromatography, the only method available at present for estimating

distribution in molecular weight, are dynamical, nonequilibrium

experiments which depend in part upon the relaxation of polymer chains in

dilute solution. In its present state, the theory of these systems does

not allow the calculation of molecular weight directly from measured

quantities; a series of calibrants of known molecular weight is required.

Improvements in the theory could reduce, or even remove, the need for

calibrants, allowing the determination of molecular weight distribution of

polymers for which calibrants are not available.

The processibility and the final characteristics of both plastics and

elastomers depend in large part upon the relaxation behavior of the

high-polymer chains of which these materials are primarily composed. More

realistic theoretical treatments of polymer melts and elastomers, which

must start with, more realistic treatment of the dynamical behavior of the

individual chains, will lead to more efficient processing and better fabricated end products. A major shortcoming of the present theory is its

inability to treat chain entanglements in a realistic way. Present work

is aimed primarily at the study of entanglement effects, using a combination of analytical and computer simulation techniques. The simulation results provide data on simple model systems and serve to guide the

development of better theoretical treatments.

We have examined the concentration dependence of the translational

diffusion constant of polymer chains in non-dilute solutions by direct

computer simulation for simple lattice-model chains. In agreement with

several recent experimental studies, the results show no sign of regions

of constant power-law dependence of diffusion constant upon concentration

predicted by some theoretical models. These results also appear to

suggest that the major part of the concentration dependence may be

accounted for by simple free-volume considerations.

8
Chemical Durability

Subtask 2 of Task 1 4332

D. W. Brown, B. Dickens, F. W. Wang, W. H. Grant, R. E. Lowry,

W. J. Pummer, and B. F. Howell

Work in this Subtask proceeds primarily along two lines: kinetics and

mechanisms of polymer degradation and diffusion of additives in polymer

matrices. The experimental study of the durability and legibility of

polyester based recording media will lead to a lifetime prediction method

that will help the National Archives determine when the archival

microfilms and magnetic tapes should be copied. The experimental study of

the degradation of poly(methyl methacrylate) will result in a predictive

method for the lifetime of this material in outdoor applications. Since

almost all polymers in use contain additives to maintain and enhance their

useful properties, the experimental study of the diffusion-controlled formation of photodimers in polymer matrices will provide the data needed for

predicting the lifetimes of polymers containing additives. Finally, the

newly developed computer programs for gel permeation chromatography will

facilitate the molecular weight determinations frequently needed for polymer-degradation studies.

Predicting the Lifetime of Polyester-Based Recording Media

Recording media have information-bearing coatings on a base of

poly(ethylene terephthalate) . Our earlier work has shown that the base

polymer will last for 1000 to 2000 years at 20-25 °C and 5056 relative humidity. The primarily polyester polyurethane tape binder which holds the

oxide on the tape degrades severely under mild conditions by hydrolysis of

the ester bond.

Magnetic tape binder is cross-linked, and is not completely soluble in any

solvent. Other workers have reported that the soluble portion of binder

increases at high relative humidities due to hydrolytic scission of the

ester linkage, but decreases at low relative humidities due to the reverse

of this reaction. They concluded that magnetic tape should be stable

indefinitely at 20 °C and 40% relative humidity. We found at low relative

humidity that magnetic tape binder cross-links further while polyester

polyurethanes and undegraded model tape binder do not. Thus, the

prediction of infinite stability, based on the concept of simple reversal

of hydrolysis in model compounds, may not apply to magnetic tapes. We are currently aging tapes on which data have been written and are testing the

readability of the tapes with a recently acquired commercial tape

transport. The tapes soon became unreadable at 85 °C, the actual time

depending on the level of relative humidity. None of the pre-written

tapes has become unreadable when kept at 35 °C or lower temperatures, but

tapes aged at 35 °C and 1 00% relative humidity for over 2 years and

started before the tape transport became available can either not be

written on successfully or require an abnormally large spacing between

data.

The force required to peel the binder layer from the base and the elongation at which the binder layer breaks loose from the base when the tape in

stretched decrease greatly with aging. The water uptake of the tap'

different relative humidities increases somewhat with aging. These quantities can be measured on relatively small pieces of tape and will b< un- to predict the tape lifetime.

9
Degradation of Poly(methyl methacrylate)

The thermal degradation of commercial PMMA at temperatures up to 125 °C

has been followed using gel permeation chromatographic determinations of

molecular weight from which the number of chain scissions in the PMMA

molecules is calculated. Gas chromatography has been used extensively to

monitor the levels of small molecules in PMMA during degradation, and the

results have been correlated with UV spectral measurements. Comparison of

results from commercial and purified specimens showed that the degradation

is initiated by some small molecular weight material which can be

extracted from the PMMA using methanol. The low molecular weight

material, which initiates degradation, is definitely not monomer nor methyl pyruvate. We have found no evidence for peroxide linkages in

PMMA, which would decompose thermally to give chain scissions and methyl

pyruvate. Our results suggest that the agent which initiates thermal

degradation in PMMA at these temperatures is an oxidation product of

molecular weight of 116 or 116x2, which is closely related to monomeric

methyl methacrylate. We have shown that once scissions occur in PMMA, the

monomer concentration approaches the level of 0.3$ required by the

equilibrium between propagation radicals and monomeric material. In the

presence of oxygen and when the concentration of monomer is above 0.3$»

the oxidation product is formed faster than it is destroyed, and the

degradation rate of PMMA increases with time. As a steady state in the

concentrations of monomer and oxidation product is approached, the

degradation rate also becomes independent of time. In the absence of

oxygen, additional amounts of oxidation product are not produced and the

degradation rate falls with time. Publications dealing with the

techniques of analysis, the combined effects of photo and thermal

degradation, and lifetime prediction are planned.

Photodimers in Polymer Matrices

The photodimers formed by polycyclic aromatic hydrocarbons and their

derivatives can be photodissociated to the original monomers. When the

photodimer is held in a sufficiently rigid matrix, the dimer can be re- formed efficiently because the two monomers are held in the correct relationship for photodimerization. Since both the monomer and dimer states

are stable at room temperature, a system with such a photodimer embedded

in a matrix is attractive for reversible phase holographic recording with

nondestructive readout.

Glassy polymers, such as poly(methyl methacrylate), are good prospective

candidates for the matrix material. To obtain a system suitable for

holographic recording, however, dimers with suitably high solubility and

with appropriate reorientation and diffusion characteristics in the matrix

must be found, and the matrix must be shown to be sufficiently immutable

under the conditions of use.

We have measured the photophysical properties of the photodimers of

anthracene and octadecyl anthroate. We have found that when a poly(methyl methacrylate) film containing dianthracene is exposed to UV

light at 280nm, dianthracene dissociates to form anthracene. A readily

measurable fraction of dianthracene is recovered when the film is

subsequently irradiated with UV light at 365 nm.

10
Computer Programs for Gel Permeation Chromatography

Many special features have been incorporated into our GPC computer

programs. We anticipate that these computer programs will be useful to the

coatings industry, which routinely tries to compare chromatograms.

Calibration of the GPC columns is a problematic step. The chromatograph

can be calibrated from a single standard sample of wide molecular weight.

The peak positions of narrow molecular weight standards can be picked out

of their chromatograms automatically. A powerful least squares program

allows easy and rapid comparison of observed and calculated quantities in

the estimation of the 5-degree polynomial used to represent the

calibration function of the columns. The user interacts with the

calibration calculations to throw out bad points, examine the fit in

detail, and quickly get a good calibration while having some feeling for

what he is doing. The data collection program also invokes the data

analysis program so that analyses can be made immediately after the data

have been collected without operator intervention. To make this possible,

the data analysis program contains a procedure which determines the limits

of the peak in the chromatogram and estimates the best place at which the

baseline may be determined.

Specimens may be characterized in terms of elution volume, which is

dependent on the calibration of the columns, or log hydrodynamic volume,

which is independent of the calibration of the columns. This latter

method allows the meaningful comparison of chromatograms taken on

different sets of columns, or at different times on columns, which are

changing in calibration. Averages analogous to the various molecular

weight averages are calculated from the distribution of hydrodynamic

volumes. This allows quantitative comparisons to be made. Specimens for

which Mark-Houwink parameters are available may also be characterized in

terms of their molecular weight distributions and averages.

A program has been written to allow quantitative comparison and

manipulation of chromatograms. This procedure was designed for specimens

for which molecular weight distributions can not be calculated, either

because the Mark-Houwink parameters relating hydrodynamic volume to

molecular weight are not known, or, more often, because the chromatograms

are from materials which are mixtures or branched or curing crosslinked

polymers. A similar program has also been written to allow quantitative

comparisons of molecular weight distributions, to produce compound

molecular weight distributions from those of single components, and to

decompose multicomponent molecular weight distributions into components.

This program examines the shape of the molecular weight distribution and

allows one to examine the calibration of the columns in this way.

11

BLENDS PROCESSING

Task 14333

Approximately 15 to 20$ of the estimated 47 billion pounds of plastics to

be produced in the United States during 1985 will be polymer blends. In

the same way that stainless steels are alloys of iron, chromium, and

nickel to make metal resistant to corrosion, polymer blends are a mixture

of two or more polymers to create plastic alloys with new properties.

Mechanical properties, flame retar dance , processability, as well as many

other polymer properties can be improved by blending.

While the blending of low cost polymers into new alloys is a relatively

inexpensive way to produce materials with new properties, most polymers

fail to form a homogeneous phase during the blending process. In a majority of instances, the heterogeneous alloy has poorer mechanical

properties, but in a small number of cases, large and favorable

synergistic effects are obtained. For example, the impact resistance

(toughness) of a polymer can sometimes be increased by an order of

magnitude whereas such synergism is never observed in a homogeneous alloy.

The properties of a heterogeneous alloy are dependent on the details of

the microstructure and interfacial tension between phases. The key to

understanding how to produce desirable heterogeneous alloys is a betterknowledge of the fundamental factors that control interfacial adhesion and

the relationship between the kinetics (rates of separation) and the

thermodynamics (static properties) of polymer blends.

To obtain basic information for predicting whether or not polymers are miscible and will mix together to form a homogeneous alloy and to control

the rate of demixing and the size of phase separated microdomains if the

polymers are immiscible, two research techniques are being used: small

angle neutron scattering (SANS) and temperature-j ump light scattering

(TJLS). These methods allow the phase diagram to be determined and the

phase separation kinetics to be studied. In addition to these two

methods, Forced Rayleigh Scattering (FRS), small-angle X-ray scattering

(SAXS) , and electron microscopy are also used in polymer-blend studies.

FY 85 Significant Accomplishments

• Temperature, composition, and molecular weight dependences of the binary

interaction parameter, x> have been measured by small-angle neutron

scattering for the first time for a binary polymer blend

(polystyrene/ poly(vinylmethyl ether ) (PS/PVME). Using a new experimental

technique developed at NBS, TJLS spinodal decomposition has been clearly

observed in PS/PVME blends over a period of hours. At later stages, a hydrodynamic mechanism dominates the phase separation process. In this

mechanism surface tension forces drive the phase separation and the

domains grow linearly with time.

• The phase diagram, crystallization kinetics, and morphology of the

polyvinylidene fluoride (PVDF) and polyethylacrylate (PEA) blend system

have been determined. This system exhibits overlapping phase separation

and crystallization “behavior with the intersection of the cloud point

and melting point curves occurring at about 1 80 °C and at a composition

of 50/50 PVF2 /PEA (by weight). The binary interaction parameter, x» measured from the PVF2 melting point depression is -0.1 6 (at 170°C). In- crystallization kinetics and morphology studies performed on blend')

12

containing less than 50# PVF2 yield information on the concentration

dependence of the crystal growth rate, crystal habit, and lamellar

structure in PVF

2

.

• Scattering functions (monomer-monomer correlation functions), stability

limits (spinodals) , and the periodicity of mesophases have been

calculated for block copolymer/homopolymer blends, graft copolymers, and

n-armed diblock copolymers. A graft copolymer is more difficult to

phase separate than a diblock copolymer of the same composition. A

n-arm diblock copolymer (n = 1,2,..) with each arm having equal amounts

of A and B monomers, phase separates at the same temperature for all n.

• A new facility for obtaining x-ray pole figures on specimens of

semicrystalline polymers was implemented. This facility has been

extensively used to characterize the orientation present in sheets of

piezoelectric polymers and in thin films of polyethylene. Major

components of the 10-meter small-angle x-ray scattering camera have

arrived and are being installed. This camera is expected to be placed

in service early in FY 1986.

Experimental

Subtask 1 of Task 1 4333

C. C. Han, B. J. Bauer, F. L. McCrackin, R. M. Briber, F. A. Khoury,

M. Okada 1

, Y. Muroga2

, and Q. Tran-Cong^

1 Tokyo Institute of Technology; 2Nagoya University; ^Kyoto University

Polystyrene/Poly(vinylmethylether) Blends

Twenty years ago, John Cahn of IMSE developed his now classic theory of

spinodal decomposition (SD). In the intervening years experimentalists

have searched for examples of this phase separation mechanism in both

ceramic and metallic systems. Although the theory is widely accepted,

experimental evidence to support it is sparse. Unequivocal experimental

evidence of SD has now been found in PS/PVME blends.

PS/PVME blends exhibit lower critical solution temperatures at about

150°C. Small angle neutron scattering was used to determine for the first

time the equilibrium phase diagram and the spinodal lines for this system.

SANS was also used to determine the x interaction parameter as a function

of temperature and composition. The phase separation kinetics were

measured using a temperature jump light scattering technique. Spinodal

decomposition has been observed in the PS/PVME blend over a period of

hours and the characteristic spacing of the SD network is of the order of

1 micrometer. Cahn’s theory has been stringently tested and is able to

describe the experimental observations in detail.

At a well-defined time, which is determined by the size of the temperature

jump into the two-phase region, another mechanism of phase separation

supercedes SD. This transition is marked by the feature that domain sizes

now increase linearly with time. This is quite unlike the classical

Lifshitz-Slyozov mechanism of phase ripening where domain size increase

with the cube root of time. This new mechanism is dominated by

hydrodynamics in which surface tension drives the phase separation. This

13
hydrodynamic mechanism was first suggested by Eric Siggia at Cornell.

Again the PS/PVME system appears to provide the best experimental evidence

for the existence of this particular mechanism of phase separation.

Poly ( vinyl i dene fluoride) and Poly(ethylacrylate) Blends

A partially miscible blend system consisting of a crystallizable component

and a noncrystallizable one (PEA) has been studied with the goal of

determining the phase diagram. In order to establish the phase

boundaries, the cloud point curve and the equilibrium melting point curve were determined. The melt exhibits phase separation upon heating

(indicative of a lower critical solution temperature) to 10“50°C above the

melting point of the PVF2, depending on the composition. The cloud point

and equilibrium melting point curve intersect at about 180°C and a composition of 50/50 (by weight) PVF

2 /PEA.

The polymer-polymer interaction parameter, x> was calculated from the

equilibrium melting point data from samples containing 50$ PVF

2 and less.

The value obtained was “0.16 at (170°C), which is consistent with the

miscibility of the blend in this composition range.

In addition to the phase diagram, the morphology of PVF2 crystallized from

the homogeneous melt (<50$ PVF2 ) has been studied. Spherulite growth

rates for the alpha crystal form of PVF

2 have been measured as a function

of composition and temperature. Analysis of the data yields a value of

the product of the surface free energies oo

e

= 4 x 10“^ J

2 /m 4 (assuming

regime II kinetics) which appears to be independent of PEA content. While

the spherulite growth rates depend only moderately on composition the

spherulite morphology changes substantially with the texture becoming more

open and the spherulite ring spacing becoming larger with increasing PEA

content at a given crystallization temperature. At very low PVF2 contents

(0.5$) and relatively high crystallization temperatures (T

c

= 150°C)

single crystals of alpha“PVF2 are formed. The basic crystal habit is

diamond shaped bounded by four (110) faces and with the long axis of the

crystal corresponding to the b axis of the unit cell. At lower

crystallization temperatures enhanced dendritic growth occurs along the

axis, which also corresponds to the radial growth direction observed in

alpha spherulites.

Future work will center around understanding the lamellar morphology of

the spherulites grown in more concentrated blends (>15$ PVF2

) and in

following the evolution of the spherulites from simple lamellar crystals

to more complex aggregates (and eventually spherulites) as the PVF2

concentration is increased.

Theory

Subtask 2 of Task 14333

I. C. Sanchez, E. A. Di Marzio, R. J. Rubin, B. J. Bauer, M. Olvera de la

Cruz 4

, and A. C. Balazs 4

^University of Massachusetts

Effect of Molecular Weight Distribution on Equilibrium Compositions

of Polymer Blends

Flory-Huggins thermodynamics have been applied to the problem of

14
equilibrium compositions of polymer blends with any molecular weight

distribution. A numerical solution can be obtained giving the volumes,

average compositions, and molecular weight distributions of the phases in

equilibrium

.

The calculations are greatly simplified by taking the limit of one of the

phases going to zero volume. This corresponds to the experimentally

accessible cloud point. The cloud point at the critical point is

dependent on the weight average molecular weights. Away from the critical

point, the cloud point diverges from the values calculated by assuming

that the weight average acts as a monodisperse polymer of that weight. In

the case where the amount of one of the polymers in the blend becomes

small, the cloud point is affected greatly by the highest molecular weight

portion of the distribution.

Effect of Sequence Distribution on the Miscibility of Polymer/Copolymer

Blends

Previous theories describing the phase behavior of copolymer blends have

ignored the sequence distribution of monomer units in the copolymer. We

introduce a parameter, 6, which describes the binary sequence distribution

of the monomers in a copolymer chain; 0=1 for an alternating copolymer, =0

for a block copolymer, and =1/2 for a random copolymer. It is assumed

that the interaction energy between a monomer of homopolymer C and the

monomer A (or B) in the copolymer AB is mediated by the nearest neighbors

chemically bonded to the A (or B) structural unit. It is found that the

sequence distribution significantly affects the degree of compatibility

between the polymers AB and C. For a fixed composition, there is an optimal range of 0 values (or sequence distributions) for which the C/AB

system is miscible.

Block Copolymer/Homopolymer Blends

The general phase diagram of a blend of a AB block copolymer and a homopolymer has been calculated. The mesophase transition temperature of

the block copolymer may either increase or decrease when the copolymer is

blended with a small amount of the homopolymer. When larger amounts of

the homopolymer are added, a liquid-liquid phase transition may occur. All

of the appropriate scattering functions (monomer-monomer correlation

functions) and mesophase periodicities have also been calculated.

Phase Behavior of Graft and Star Diblock Copolymers

Scattering functions, mesophase periodicities, and limits of phase

stability (spinodals) have been calculated for graft copolymers and n arm star diblock copolymers. The fraction of A monomers in the graft

copolymer of the AB type is f and 0 is the fraction along the A chain

backbone at which the B chain is chemically linked (when 0 = 0 or 1 the

graft copolymer becomes a diblock copolymer). Leibler has previously

shown that when f = 0.5 that a di block copolymer containing N monomer

units undergoes a mesophase transition when yN = 10.5. According to the

present theory, a graft copolymer with f = 0.5 and 0 = 0.5 is more

difficult to phase separate and its critical yN value is 13.5. The

critical yN values monotonically decrease to a value of 10.5 as 9 approaches 0 or 1 . In view of this result it is indeed surprising to find

15
that for a star polymer in which each of the arms are block copolymers of

composition f = 0.5, the critical x N value is always 10.5 independent of

the number of arms, n.

Long Time Relaxation in Polymers Undergoing Phase Transitions

The kinetics of phase transitions is an important subject that is now just

beginning to receive the attention that it deserves. Normally, the rate

of forming new phase is faster the larger the supercooling (or more

generally the thermodynamic force), however there are situations in which

the phase may be locked in kinetically. As an example consider a chain

that reptates in a potential field. The rate of reptation is less the

larger the field gradient because the chain tends to get stuck in the

deeper wells. Thus the rate of formation of the new phase will be

inhibited. A good model is a periodic ramp potential. The slope of the

ramp is proportional to the thermodynamic driving force and tends to empty

out the wells. However, the depth of the wells can also increase with the

thermodynamic driving force so that trapping results. This general idea

has been used to explain very slow melting in DNA. It probably can explain slow relaxations in glasses and the formation of gels.

It is now widely recognized that relaxation in polymers rarely displays

exponential decay, and that many times, the Kohlrausch-Williams-Watts

(KWW) form exp(-t/i)8 is an accurate representation of the data where t is the relaxation time and 8 a parameter between 0 and 1. Models of

polymers that lead to the KWW form are being formulated. When the

representative phase point must diffuse out of a deep well' to effect a motion, and when there is a distribution in well depths, then we obtain

KWW behavior.

Random Walk in a Random Array of Traps

We have obtained a generating function solution for a random walk on a one-dimensional lattice where the probability of stepping from lattice

site i to site j is proportional to exp{-a/ ( i-j ) } and where M lattice

sites, R-j <R 2

< . . .<R M , are designated as trapping sites. Among the many

quantities of interest which we have evaluated are: (1) the probability

of starting at an arbitrary point r Q in the presence of two traps and

ultimately being trapped at R

1

; and (2) the probability of starting to the

right of M traps and ultimately ending up at a site to the left of the

traps

.

SAXS and Pole Figure Facilities

Subtask 3 of Task 14333

J. D. Barnes and D. F. Krintz

Digital Camera for Small-Angle X-ray Scattering

The small angle X-ray scattering laboratory is essentially ready to

receive the 10-meter, small angle X-ray camera. The X-ray generator is in

place on its rotatable mount, and the masonry piers for supporting the

beam path Lave been installed. Most of the major components of the beam

path have been built, and the entire camera will be installed as soon as the support for the entrance window and the specimen chamber come out of

the shops. Electronic alignment of the detector system is underway.

Further work on computer interfacing is required.

16
The software for the color graphics display system is essentially complete

and supporting tools for statistical analysis and modelling have been

installed on the VAX computer. Prototype versions of the software for the

dedicated data-acquisi tion computer have been written. We will be writing

software for integrated data collection, experiment control, and on-line

data display shortly. The techniques used in our previous laboratory

automation efforts will be used to provide simple operation of the system.

Further work is underway to implement additional software for contour

plotting, 3-dimensional plotting, non-linear least squares fitting, and

resolution modelling.

X-ray Pole Figure

The NBS Polymers Division has installed a new facility for obtaining X-ray

pole figures on polymer specimens. Dr. Edward Clark, a consultant to the

Division, provided material assistance in designing software to control

data acquisition on this machine. Additional software for plotting the

data has been implemented on the small angle x-ray data analysis facility.

Users of the facility have collected more than 300 data sets since

September of 1984.

The quality and quantity of the numerical data obtained with this facility

have created a need for improved tools for describing the results. Dr.

Clark has prepared a report on the use of orientation functions of various

kinds to characterize the materials being studied. Dr. Clifford

Speigelmann of the NBS Statistical Engineering Laboratory has been working

closely with Polymers Division staff to develop suitable descriptive

measures based on probability density functions defined on a spherical

domain.

The NBS work in developing data descriptions based on explicit probability

density functions is pioneering work that should provide data that can characterize the products of materials forming processes in a more

morphologically descriptive way. This approach contrasts with the

standard technique of calculating orientation functions from data with

lots of noise. The new approach produces functions that have all of the

correct characteristics of pole figure data in that they are smooth

non-negative functions whose integral over the surface of the sphere is

finite. Because these probability density functions are defined on a spherical domain, they also possess all of the correct periodicities.

Suitable statistical estimation techniques can be used to determine the

values of width parameters, location parameters, and other characteristics

of the distributions.

Initial applications of these modelling techniques to the study of

piezoelectric activity in polymers and to the study of orientation in

blow-molded films are encouraging. Future efforts will be devoted to

finding functions that fit the data better and to applying these

techniques to inverse pole figures.

The pole figure facility has created a need for software to calculate

structure factors, absorption corrections, and plots of molecular

structures. We are implementing these tools on the VAX as they are

needed. In some instances we must adapt software used for work with

17
single crystals to deal with the kinds of specimens that we use in our

work. Absorption corrections are a good example because the usual

software does not deal with the finite size of the incident beam.

Before the data from the new pole figure system can be regarded as truly

quantitative, we must develop procedures to correct for background and

absorption. Techniques for resolving contributions from overlapping

diffraction maxima are also needed. We expect to pursue design studies

aimed at adapting a one-dimensional position-sensitive detector to the

apparatus. We expect to improve our background estimates by collecting

data over a range of angles around the diffration maxima of interest. We

should also be able to resolve overlapping peaks and to simultaneously

collect data from multiple reflections. The diffraction geometry for such

a system is complex because the data from different parts of the detector

arises from different chi and phi values.

18
BIOMATERIALS SCIENCE, METROLOGY AND STANDARDS FOR IMPROVED DURABILITY,

APPLICATIONS AND MANUFACTURING OF DENTAL AND MEDICAL MATERIALS

Task 14335

The objective of this research is to provide basic materials science,

engineering, and test methods which may be used by sectors of the

health-care industry for the development of new or improved materials,

delivery systems, and standards. Although no current efforts on diagnostic systems or therapeutic equipment exists, there are plans to

include these activities in the future.

Materials-research efforts are aimed at development of materials and

techniques for promoting adhesion of restorative materials to tooth dentin

and enamel. Novel approaches are pursued for reducing marginal leakage of

restorations via utilization of adhesives and durable space occupying

materials for use in cavity preparations. These include new composite

resin formulations with reduced polymerization shrinkage and more

resistance to attack by oral fluids as well as macroscopic bulky space

fillers bonded to cavity walls. Strong, more durable, versatile dental

cements are also explored as are methods for improving properties of

surgical bone cement such as durability, radiopacity, and coupling between

the bone-metallic implant system. Studies of calcium phosphate compounds

for endodontic sealants, bone augmentation, or caries-reducing agents are being investigated. The basic chemistry of precipitation of calcium

phosphates is explored as it relates to formation of mineral deposits such

as bone, teeth, and atherosclerotic plaque is under investigation.

Theoretical and experimental studies of stresses within bonded dental

systems are explored with the objectives of defining the most compatible

material-property combinations and geometric designs for

porcelain-to-metal , tooth-to-resin, resin-to-filler bonding and so forth.

Techniques for characterizing casting processes and effects of variables

including alloy compositions are being studied with a view toward improved

alloys and processing methods.

FY 85 Significant Accomplishments

• Cooperative work with an American company lead to the development of a new casting investment suitable for titanium metals and the clinical

placement of the first-known dental titanium casting in the United

States. General effects of variables such as casting investment on precision casting of dental alloys were also identfied.

• Tough, deformable dental-cement formulations which expand slightly on setting have been developed. Prepolymer resins for dental composites

which reduce polymerization shrinkage and show promise for higher in

vivo durability were also developed. These include monomers which

expand during free-radical polymerization. A dental composite using a glass filler developed at NBS was made commercially available by

American Industury.

• A hardening calcium phosphate cement (using only calcium phosphate

compounds) suitable as an endodontic sealant and with other potential

biomaterial applications have been developed. Monomers which yield

surgical bone-cement radiopaque were also synthesized. A new dental

cement with improved adhesion and durability was developed. The iron

19
cation species responsible for bonding between dentin, a surface active

compound and a coupling agent of a dentin bonding system was also

identified. This coupling agent was developed at the American Dental

Association Health Foundation, Paffenbarger Research Center (ADAHF/PRC)

at NBS.

• Some service conditions under which dental glass-ionomer cements are prone to catastrophic failure have been identified. Along similiar

lines it was demonstrated that dental composites evaluated by diametral

tensile testing undergo surface induced failure rather than

internal-flaw induced fracture.

Dental Composite and Cement Sealant and Adhesion Chemistry

Subtask 1 of Task 1 4335

J. M. Antonucci, R. L. Blosser 1

, R. L. Bowen 1, G. M. Brauer, E. E. Cobb 2

, A. D. Johnston 1

, R. W. Penn, N. W. Rupp 1 , L. E. Setz 1

, J. W. Stansbury,

and S. Venz3

American Dental Association Health

Center (ADAHF/PRC) at NBS

2 Dental School, Georgetown University,

^Dentsply International, Milford, DE

Foundation, Paffenbarger Research

Washington, DC

Vinyl-based thermosetting composites, the primary esthetic anterior dental

restorative material, also are increasingly being used in stress-bearing

applications traditionally reserved for amalgam fillings. However,

despite recent improvements in many properties of dental composites, they

still are deficient in wear resistance and margin adaptability. Recent

research at NBS has indicated that the oral environmental resistance (OER)

of polymer-based dental materials is a significant factor in determining

their _in vivo performance and ultimate service life both in relatively

stress-free as well as stress-bearing situations. To enhance the OER of

these materials two complementary approaches are being pursued in our laboratories. One approach is designed to increase the degree of

polymerization and crosslink density of dental vinyl-resin systems by the

synthesis and formulation of flexible, multifunctional methacrylates and

chain transfer agents (e.g. polythiols). The other approach involves

designing resin systems that have solubility parameters significantly

lower than that found in the oral environment. To this end we have

synthesized and formulated resin systems that yield composites containing

significant amounts of fluorocarbon or siloxane segments in the matrix

phase and still have acceptable mechanical properties. Composites

prepared using a siloxane-containing resin showed improved OER and

acceptable strength. Emphasis also is directed to monomer systems that

have minimal (preferably zero or slightly positive) changes in volume on polymerization. The difficulty of increasing the degree of polymerization

while simultaneously minimizing polymerization shrinkage is recognized.

Two methods are being pursued, (1) the synthesis of prepolymer monomers

and (2) monomers that expand on polymerization. A polyfluorinated

oligoether with pendant urethane ethyl methacrylate groups (PFUMA) was synthesized which on formulation yield composites with excellent esthetics

and mechanical strength comparable to commercial composites. These low

surf ace- energy resin-based composites also exhibit low water sorption,

residual unsaturation, and polymerization shrinkage. A new synthetic

procedure has been developed which permits the formation of free-radical

polymerizable, asymmetric spiro orthocarbonate monomers which expand upon

20
polymerization. A series of these monomers containing various substituent

groups has been prepared. Formulations comprised of these spiro monomers

in conjunction with commercial monomers common to dental composites and

adhesives are under evaluation.

In another study the chemical stability of dental resin is being

investigated regarding (1) the color changes of a wide variety of dental

resins resulting from the exposure to different radiation sources or to

thermal exposure, under diverse environmental conditions for various

periods of time, (2) correlation of the experimental aging test results

with the composition of the materials as well as with its clinical

performance. It was found that dental composities are more susceptable to

undergo color changes on aging than denture-base resins or plastic teeth.

Therefore, a series of composites were exposed to the Xenon or RS lamp for

time periods ranging from six hours to seven days. Composites were also

aged at 60 °C in the absence of light. The ingredients of the materials

studied are currently being analyzed by liquid chromatography and

infra-red analysis. The results of this investigation should establish

which constituents cause the color instability of dental composites.

A main disadvantage of most dental restorative materials is the lack of

adhesive bonding to tooth structures in the oral environment.

Investigation continues on the materials and methods of the ADAHF bonding

system which has been shown to chemically unite composite resins to dentin

as well as to enamel. The bonding system consists of treating the tooth

surface with an aqueous solution of ferric oxalate, rinsing with water,

and then applying solvent solutions of a surface-active compound and a coupling agent, one after the other. This treatment prepares the surface

for very strong adhesive bonding with composites. Recent studies have

centered on determining the best level of acidity of the ferric oxalate

solution. The same degree of acidity gave maximum bond strengths to both

enamel and dentin, a fortunate coincidence. Mossbauer spectroscopy has

been used to study the role of the iron (ferric ion) component in this new adhesion technology. This spectroscopy has given information regarding

the nature of the immediate chemical environment of the iron cation as it

reacts with the tooth surfaces and the other components. Also, other

methods have been used to determine the nature of complexes which form

between the iron, the surface-active compound and the coupling agent. For

the ultimate clinical success of this new capability, it is essential to

clearly characterize the structural modifications of the tooth surfaces

and the chemical reactions which the compounds undergo to bring about the

initiation of polymerization of the active molecular species bound to the

surfaces. Gratifying 'progress has been made during this year toward these

important ends.

Experimental work on improving posterior-composite durability is exploring

a glass reinforcing filler with a refractive index which renders composite

restorations esthetically translucent, X-ray opacity, which improves

diagnostic properties of the restorations, and phase separability, which

makes it possible to acid etch the filler particles for improved bonding

between glass filler and resin binder. The first commercial product based

on this glass composition was introduced to the dental profession thi

year. The ADAHF/PRC at NBS obtained a patent on this product.

Another objective of this subtask has been to synthesize specific monomer.", or oligomers such as allyl 2-cyanoacrylate or oligomers

2-isocyanatoethyl methacrylate and to prepare suitable formulat

i

on:! using

21
these ingredients which may bond to dentin in an aqueous environment. A

neat monomer of allyl cyanoacrylate adhered to biosurfaces. However, the

storage stability of the inhibited monomer even when refrigerated was only

a few months. A copolymer of n-butyl methacrylate (35 wt %) ethyl

acrylate (35 wt %) and 2-isocyanatoethyl methacrylate (30 wt %) was also

synthesized. The vitreous colorless solid containing pendant isocyanate

groups as established from infra-red spectra will be reacted with

mineralized tissue such as bone or dentin to determine if grafting of this

polymer and collagen-containing surfaces takes place.

Non-eugenol containing dental cements based on vanillate esters,

o-ethoxybenzoic acid (EBA) and zinc oxide, have also been developed in

this laboratory. These cements have many advantages compared to the

presently used zinc oxide-eugenol (ZOE) or EBA cements: (1) excellent

strength, (2) lower solubility, (3) cure in conjunction with composite

filling materials to which they adhere, (4) are compatible with acrylic

monomers, (5) adhere strongly, in water, to non-precious metals, procelain

and composites. The present objectives of this study are to modify these

cements by additions of small concentrations of additives such as acids,

metals, or fluorides to improve their properties. Hexyl vanillate or ethylhexyl syringate cements that are prepared with fluoride salts could

possess caries reducing properties. The biological safety evalution of

the hexyl vanillate (HV)-EBA-ZnO cements have been completed and the

cements are now considered safe for clinical studies.

The feasibility of utilizing halatopolymers derived from dimer acid (DA)

and trimer acid (TA) as the organic phases of dental cements is also being

explored. A translucent cement which matches tooth color and resists

brittle fracture under compression was made from DA and calcium hydroxide.

This cement is hydrophobic, compatible with resin-based dental materials

and, because of the alkaline constituents, it may promote secondary dentin

formation. Reinforced DA/Ca(0H)2 exhibited improved dimensional

stability, and strength, low solubility and, in some cases, adequate

radiopacity. Remarkably, some of these DA cement formulations expand

slightly on setting.

Wear, Durability, Clinical Studies and Related Properties of Dental

Composites, Amalgams, Cements, and Glass-Ionomer Cements

Subtask 2 of Task 14335

J. M. Antonucci, R. L.

Hinman^, j. e. McKinney,

and R. M. Waterstrat 1

Bowen 1

, W. de Rijk 2, A. A. Giuseppetti 1

, R. G. C. Paff'enbarger 1

, N. E. Rupp 1

, R. W. Penn,

W.

1 ADAHF /PRC

p Dental School, University of Maryland, Baltimore, MD

^Naval Dental Clinic, San Diego, CA

In our laboratories wear measurements have been extended to include

ionomer cements. Prior to wear measurements the specimens were

preconditioned in air, water, and 0.02 N lactic acid for one week. With

the air-preconditioned specimens, wear studies were essentially impossible

due to catastrophic failure resulting from brittle fracture of the

dehydrated specimens. With the lactic acid preconditioning, the hardness

fell markedly with a corresponding decrease in wear resistance. Since

lactic acid is generated during plaque formation, it is intraorally

22
omnipresent in many people. This kind of chemical instability is a serious limitation to the in vivo application of ionomer cements as presently formulated.

Experimental composites with flexible-chain polymers were evaluated.

Specimens were preconditioned in organic solvents with solubility parameters

encompassing those applicable to the intraoral environment. The

results indicated that these composites softened by amounts similar to

those observed on present commercial composite restorative materials;

however, the maximum swelling corresponded to lower values of solubility

parameters as intended.

Wear measurements were initiated on an experimental composite containing a 70/30 PFUMA/bis (methacryl oxypropyl) tetramethyl-disiloxane (BIS-MPTMS)

matrix polymer. The specimens were preconditioned in 1 00% ethanol and in

pure water for which the former produced a large amount of swelling and

the latter, none. Although the wear rate for this material was larger

than that usually observed for the commercial materials, it was essentially unaffected by preconditioning. This distinction between

hardness and wear behavior may result from the high degree of crosslinking

in this polymer and its more rubberlike character which allow softening,

but limit decomposition or breakdown during wear. Studies of glass

transition temperatures (Tg) are also considered important for

characterization of wear test materials. T measurements are carried out

using differential scanning calorimetric (DSC) and torsional rheometric

(TR) methods. Specimens of 70% 2, 2-bis[p-( 2* -hydroxyl-3’ -methacryl

oxypropoxy )-phenyl]propane (BIS-GMA) - 30 % triethylene glycol

dimethacrylate (TEGDMA) were cured with benzoyl peroxide, over a temperature range of 37 °C to 1 83 °C. The TR results of the specimens

cured below 100 °C indicated the existence of two glass transition peaks;

the position of both peaks are somewhat sensitive to the curing

temperature, T

c

. Only one prominent glass transition peak could be

observed for specimens cured above 100 °C. The value of the lower glass

transition temperature for specimens cured at 37 °C is 55 °C, which is

dangerously close to the service temperature.

With respect to recent public concern over potential health hazards caused

by dental amalgams, considerable time and effort has been devoted during

the past year to reviewing the literature and preparing lectures and

papers related to the potential hazards of mercury or glavanism from these

restorations. To date no evidence has been discovered that indicates

hazard to the vast majority of patients.

On April 11, 1985, the first clinical cementation of a dental titanium

casting in the United States was performed. This was the result of

American Dental Association supported research. The casting has excellent

margin adaptation and retention. The titanium is in approximal contact

with a gold crown, and the occluding bicuspid has an amalgam restoration;

yet, there is no galvanic response, unusual taste, or other problem

related to the restoration. Additional castings are being made for oth'-r

patients. A new investment material and casting technique unique to

titanium that made this casting possible. (See Subtask 3)

23
Dental Alloys, Ceramics, Metrology, and Analysis

Subtask 3 of Task 1 4335

A. A. Giuseppetti 1

, W. de Rijk2

, 0. 0kuno3, r. w. Penn, J. A. Tesk, and

R. W. Waterstrat^

1 ADAHF/PRC

p Dental School, University of Maryland, Baltimore, MD

^Tokyo Medical & Dental University, Tokyo

The ability to cast new dental alloys into precise prosthetic forms is an important consideration. Using a technique developed here, a castability

value C

y is defined as the number (or fraction) of completely cast

segments from a polyester grid pattern. Recent work has focussed on determining the temperature dependent effects of various elements on C ytf

(a previously defined, statistically relevant transform of C

y

). Boron was found to greatly improve C

y^, but as the alloy temperature was raised, its

effect became less important. Silicon was also found to have a positive

effect on C yt> but its effect became more pronounced as the alloy

temperature was raised. The effects of other elements as function of

alloy and mold temperatures are being assessed. Preliminary studies also

indicate that various batches of investment produce the same temperature

dependent effects, but that there are shifts in the magnitude of C

yt with

each batch for a given set of temperature conditions. To ensure control

of the variable of casting force, the automatic centrifugal casting

machine was instrumented to measure that force during the casting process.

Work continues to progress on development of technique and materials for

making titanium dental castings. A new zirconium oxide investment

material has been developed containing a proprietary additive for

producing a predetermined expansion of the mold at its firing temperature.

This permits casting of any metal or alloy into room-temperature molds

without loss of dimensional accuracy. A dental crown of pure titanium has

already been cast using this mold material and is presently undergoing a clinical study. This is the first clinical titanium casting ever used in

the U.S. and it marks the beginning of more extensive studies in our ongoing research program on titanium dental castings.

Efforts on characterizing the strength of porcelain-metal system

continues. Weibull statistical analysis shows three dental systems are

not normally distributed. Progress is being made on implementing maximum

likelihood algorithm for defining characteristic strength parameters.

Porcelain-metal thermomechanical-stress compatibility also is being

investigated. Finite-element programs are underdeveloped to account for

thermal gradients and viscous stress relaxation during cooling from

porcelain fixing temperature. This effort is in cooperation with the

University of Illinois, Chicago.

The application of Weibull statistics for evaluation of the strength of

dental composites is being explored. The high strength of an experimenal

industrial composite material was shown by Weibull Statistics analyis to

be the result of the non-standard (small) specimen size used by the

manufacturer. The analysis suggests that the fracture initiating flaws

are distributed on the surface of the specimen rather than through its

bulk.

24
In a cooperative program with the University of Michigan the Diametral

Tensile Test (DTT) was evaluated for applicability to the newer dental

composite restoratives (DCR ) . Samples from four OCR’s were tested at

crosshead speeds from0.5 to 0.005 cm/min. All of the DCR's were more

brittle than the brittle cement. It was concluded that the DTT remains a viable method for determining the tensile strength of the more recent

DCRs

.

Finally, a mercury dilatometer was constructed for measuring the

polymerization shrinkage of commercial dental composites and newly

formulated NBS dental resins, cements, and composites. This instrument

will be a key measurement device for the program tasks aimed at improving

such materials.

Calcium Phosphate Chemistry, Biochemistry, Structure and Clinical

Implications

Subtask 4 of Task 1 4335

W. E. Brown 1

, C. M. Carey 1, L. C. Chow 1

, N. Eidelman 1

, E. D. Eanes 2

, B. 0. Fowler 2

, T. M. Gregory 1

, A. W. Hailer 2

, M. Mathew 1

, D. N. Misra 1, B. A. Sieck 1

, S. Takagi 1

, B. Tomazic 1

, M. S. Tung 1

, and G. L. Vogel 1

1 ADAHF/PRC

^National Institutes of Dental Research (NIDR) at NBS

An improved topical dental fluoridation procedure is being studied. In

recent studies a treatment which forms dicalcium phosphate dihydrate in

enamel was found to greatly increase enamel reactivity towards low levels

(0.2 to 2 parts per million) of fluoride. Repeated treatment induces

additional fluoride uptake. This treatment regimen would significantly

enrich the tooth enamel with permanently bound fluoride and should enhance

the tooth resistance to decay. This would constitute a fluoride-enhancing

regimen without the use of additional fluoride applications.

Remineralization of white spot lesions also is being investigated.

Slurries containing tetracalcium phosphate and dicalcium phosphate in a carboxymethyl cellulose gel were found to effectively remineralize

artificial caries-like lesions in enamel. Because of the ease of

handling, these slurries have good potential for remineralizing incipient

lesions jji vivo .

A hardening calcium phosphate cement consisting of only calcium phosphate

compounds was developed during the previous report period. This cement

has now been tested as an endodontic root-canal filling material in a pilot animal study. The results were often superior to those obtained

with gutta-percha. Since the cement is highly biocompatible, it may prove

to be permissible for the cement to protrude beyond the apical foramen

without deleterious effects.

Ultramicro-analytical techniques previously developed for studies of

caries mechanisms were applied to the analysis of plaque samples collected

from individual sites in the human mouth. These analytical procedure:,

permit determinations of the variation in parameters between adjacent

sites in a single individual and facilitates studies of the mechanism.’, of

caries and other plaque-induced oral diseases.

25
Investigation of octacalcium phosphate (OCP), a precursor in the formation

of tooth mineral, and its role in biomineralization has continued. OCP

forms crystalline adducts with a number of aliphatic saturated

dicarboxylates. Based on the structures of OCP and calcium succinate a structural model for an OCP-succinate complex was proposed. Attempts are being made to prepare large crystals suitable for single-crystal studies

of this complex as well as complex crystals containing other dicarboxylic

acids. The unravelling of the mode of incorporation of these

dicarboxylate ions into OCP could have a significant impact in the

understanding of the chemistry of biomineralization and may have

implications of value in adhesive-bonding technology.

Pathological calcification in the cardiovascular system is being studied.

Extensive characterization of the mineral phase in cardiovascular plaque

and solubility measurements of a number of calcium phosphates in blood and

serum were made. During the present reporting period, we contributed

substantially to the physical and structural information on plaque

mineral. This information has major implications regarding pathological

calcium phosphate deposits: (1) These deposits should not form unless the serum becomes supersaturated with respect to octacalcium phosphate. (2)

According to our findings, serum is approximately saturated with respect

to octacalcium phosphate and slightly supersaturated with respect to

plaque mineral. Because of the close proximity of these values, it should

be possible to manipulate the calcium and phosphate levels in blood

relatively slightly so as to make it either undersaturated or supersaturated with respect to octacalcium phosphate and/or plaque

mineral, as might be required by the disease conditions; with this

knowledge, it may be possible to induce dissolution of mineral deposits or prevent their formation without the use of invasive techniques. (3) The

roles of surface-active agents that block crystal growth of octacalcium

phosphate or its conversion can be understood with greater precision. And

(4) the chemical properties of these materials (e.g., reactions with F“)

and their manipulation in treatment regimens can be anticipated.

One principal objective of the research done under this subtask is to

better understand biomineralization mechanisms. Current areas of interest

include the nucleation/ growth processes which occur during the initial

stages of mineral deposition in vertebrate hard tissue, the physiochemical

and structural properties of biological mineral and related synthetic

salts, and the interaction of these salts with compounds of biological

interest. There is growing evidence that the first mineral to form in

cartilage, dentin, and some bone tissues occurs in association with

submicron size, membrane- bound, extracellular structures known as matrix

vesicles. A method developed by NIDR members of this group uses synthetic

lipid vesicles (liposomes) to artifically duplicate many of the

precipitation steps observed in matrix vesicle calcification. In previous

studies using this method, it was shown that precipitation inside the

liposomes involved the sequential appearance of three distinct calcium

phosphate salts: amorphous calcium phosphate, octacalcium phosphate, and

hydroxyapatite. Electron microscopic techniques showed precipitation of

calcium phosphate salts was also found to disrupt the normally stable

membrane envelope. This disruption enables the internal precipitate to

induce additional precipitation outside the liposome as well. Since these

events parallel in many ways those which occur in and around matrix

vesicles, the liposome system provides a useful vehicle for investigating

the role membrane constituents native to matrix vesicles may have in

26
regulating these events. Results of studies involving phospholipids and

liposomes may prove useful in assessing the role lipids have in coronary

and vascular calcification.

A new study has been initiated on factors controlling apatite crystal

growth. X-ray diffraction line broadening analysis is being employed to

determine the size apatite crystals can attain under a variety of

physiologically-relevant solution conditions. Particular effort is being

directed toward elucidating the effect of fluoride ions in delimiting

crystal growth. Results from this study should also provide new insight

into the physiochemical processes controlling apatite crystal growth in

biological tissues.

Hydroxyapatite is considered the prototype structure for the apatitic

phases investing hard tissues. An important, ongoing area of research

under this subtask is the preparation and detailed characterization of

pure and compositionally modified (e.g. F, CO^ substituted)

hydroxyapatites suitable for use as standards and controls in studies on

the structure and chemistry of tooth enamel apatite. In a recently

completed study, data from synthetic hydroxyapatites were utilized to

elucidate factors contributing to the reduced solubility of

laser-irradiated enamel mineral. The understanding of such reductions in

solubility is an important consideration in developing rational, effective

clinical laser treatments for preventing tooth decay.

Improved Surgical Bone Cement

Subtask 5 of Task 1 4335

G. M. Brauer, J. W. Stansbury, and J. A. Tesk

The ever increasing use of plastics for medical and dental applications

makes it desirable (often mandatory) that such materials have adequate

radiopacity to detect their presence in various environments. Some

commercial materials (e.g. bone cements) incorporate fillers of heavy

metal salts such as barium sulfate or bismuth subnitrate which weakens the

material. More compatible monomers with a high percentage of atoms of

high moleular weight such as triodophenyl or pentabromophenyl methacrylate

have been synthesized. These monomers are compatible with monomers used

in bone cements and denture-base resins. The rate and kinetics of

polymerization of monomer-polymer doughs is not altered appreciably.

Addition of 10 to 15 percent of the brominated or iodinated monomer to

methyl methacrylate results in radiopaque polymers. Such compositions are more homogeneous than filler-containing compositions and have higher

strength than commercial bone cements with barium sulfate fillers. Other

properties of these copolymeric compositions are being investigated.

27

MECHANICAL PERFORMANCE OF POLYMERS

Task 1 4336

The approximate 8% annual growth of polymeric materials results from

substitution for other materials owing to enhanced properties at reduced

costs, the expanded need for all materials due to growth in population and

economic activity, as well as new products that rely on unique properties

of polymers. Important to wider use and acceptance of plastics is a firmer understanding of performance limitations and processes that affect

durability. Full exploitation of plastics has not been realized due to

the lack of adequate data and models upon which to design for long-term

use. Evaluation and control during processing of materials parameters

that affect performance should lead to more durable materials and reliable

performance for the intended service life.

The objectives of this task are to provide data, test methods, and

standards for evaluation of long-term mechanical performance of polymeric

materials and determination of those factors that limit performance. The

approach is to identify and analyze principal failure mechanisms of

commercially important thermoplastics under mechanical stress, to

determine the effects of chemical structure and fine structure morphology

on mechanical properties and failure mechanisms, and to develop

constitutive relationships that serve as the basis for accelerated test

methodology and lifetime prediction.

The approach to development and verification of predictive models of

mechanical performance are: (1) identify the principal molecular

mechanisms and processes associated with deformation and failure and

establish their relationships with molecular structure and fine texture;

(2) evaluate theories of bond rupture as models for degradation of

mechanical properties and for extrapolating short-time measurements to

predict long-term performance; (3) model mechanical properties behavior

and failure using time dependent constitutive equations; and (4) expand

fracture mechanics approach to viscoelastic materials to model craze and

crack-growth phenomena.

Joint programs on performance of plastics have been established with other

government agencies which are fostering the use of plastics in areas of

national concern. For example, a joint program with the Air Force is

applying advanced characterization techniques to elucidate the structure

of molecular composites formed by combining rigid-rod polymers with

thermoplastic matrix materials. This project is concerned with

establishing the relationships among the molecular structure, fine

texture, mechanical properties, and performance of these materials.

Elucidation of the molecular and microstructural features that limit the

performance of polyethylene film is supported by NASA to improve the

success rate of high altitude, heavy lift balloons used in stratospheric

research. Test methods for evaluating the performance of elastomers used

in tank track pads are under development in a joint project with the U.S.

Army tank & Automotive Command.

28

FY 85 Significant Accomplishments

• Solid state nuclear magnetic resonance measurements have been used to

elucidate the dispersion of the rigid-rod molecule,

poly(p^phenylbisbenzthiazole) , in the molecular composite using

polyamides as matrix materials. The degree of dispersion affects the

mechanical properties of molecular composites.

• The creep and recovery behavior of polyethylenes under small uniaxialy

deformations has been successfully described with a one-dimensional

equation consisting of a single integral hereditary term and a plasticity term.

• The molecular structure and fine texture of polyethylene film used in

heavy lift balloons has been identified. The uniaxial and biaxial

behaviors of the same films have been determined to correlate structure

with mechanical performance.

Characterization of Polymer Structure and Deformation Processes

Subtask 1 of Task 1 A 336

L. H. Bolz, E. A. DiMarzio, B. M. Fanconi , C. M. Guttman, B. F. Howell,

F. A. Khoury, J. Mazur, G. B. McKenna, E. Passaglia, D. L. VanderHart,

L. J. Fetters 1 , B. Hostetter^, K. L. Ngai^, E. Perez 11 , D. J. Plazek^

1 Exxon Research and Engineering Company

2Hercules Research, Inc.

^Naval Research Laboratory

^Fellow, Instituto de Plasticos y Caucho, Madrid, Spain

^University of Pittsburgh

The identification of molecular mechanisms and changes in physical

structure associated with deformation during and subsequent to processing

not only provides the basis for development of test methods for prediction

of long-term performance and for quality control, but also provides

insights into the selection, processing, and design of materials with

improved mechanical properties and performance. A key aspect is the

development of advanced techniques to elucidate the molecular structure

and fine texture of polymers. The nature of the organization of

individual polymer molecules in the semicrystalline, lamellar textured

morphology of the solid state is being addressed by model calculations of

crystal growth and small angle neutron scattering profiles, as well as morphological studies of the crystal habits of polymer crystals.

Vibrational spectroscopy and solid state nuclear magnetic resonance are used to elucidate the physical structures of semi crystalline and amorphous

polymers, as well as molecular composites. Calculational approaches are used to predict the spectral signatures of those crystal defects having

the lowest conformational energies. Defects of this sort are thought to

be involved in mechanisms associated with mechanical deformation, as well

as other physical processes.

Techniques are under development to produce single extended polym.r

molecules on conducting substrates which are suitable for investigation by

tunneling electron microscopy. By such investigations details of the ty; •• and distribution of branching in polymers as well as the architecture of

copolymers may be elucidated. Better methods are needed to measure

29
branching in polymers, as this aspect of molecular structure, such as in

linear low density polyethylenes, can lead to enhanced mechanical

properties

.

Recently, large-scale polymer motion, as in self-diffusion, has been

treated using reptation models which although successful at describing

some aspects have failed to adequately predict others. Work is directed

towards testing recent reptation models of polymer diffusion in the solid

state and melt, as well as the discrepancy between the observed dependence

of the melt viscosity on molecular weight and that predicted from

reptation theories. The linear viscoelastic behavior of cyclic polymer

molecules and their linear analogues has been determined as a test of

reptation models since cyclic polymer molecules should exhibit quite

different behavior than the linear analogues.

Chain Scission and Mechanical Performance of Polyethylene

The underlying chemical processes associated with mechanical degradation

of polymers involve scission or rupture of valence bonds and/or van der

Waals type bonds. The valence bonds comprise the skeletal backbone of the

polymer while the latter type involve interactions between different

polymer segments either on the same polymer or belonging to different

polymer chains. The initial rupture of valence carbon bonds produces

highly reactive primary free radicals which quickly undergo reactions to

yield secondary free radicals. The nature of the free radical reactions

is such that for most polymeric materials only the secondary radicals are observable by electron spin resonance (ESR). The secondary radicals

undergo propagation and termination reactions to produce stable chemical

species whose concentrations can be measured with infrared spectroscopy.

Infrared spectroscopy and ESR have been used to measure the levels of

chain scission associated with mechanical stress and degradation. Studies

have been conducted on polyethylene (PE) and polystyrene degraded by

grinding under liquid nitrogen, as well as on PE ruptured under sinusoidal

and constant loadings. Chain scission concentrations have been measured

as functions of the number of cycles in fatigue experiments and of strain

carried out using a stretching frame.

In the present work, we investigated the dependence of chain scission on strain produced under sinusoidal loadings (fatigue). This work was motivated by the following observation. Whereas the buildup in chain

concentration increased with the number of stress cycles in a fatigue

experiment, no detectible chain scissions were observed in specimens

deformed in a stretching frame up to the point of macroscopic rupture.

After macroscopic rupture the chain scission concentrations were similar

to those observed after rupture in fatigue experiments.

The fatigue experiments were carried out at constant peak stress and the

specimens underwent plastic deformation during the experiment. The

specimen elongation and chain scission concentration were measured as a function of the number of fatigue cycles in separate experiments. In this

manner, the chain rupture concentrations could be plotted as a function of

the elongation in a fatigue experiment and the results compared to those

obtained by deforming the polymer in a stretching frame. Data were

obtained at both 0.01 and 0.002 Hz.

30
Somewhat different results were observed at the two frequencies. At 0.002

Hz no increase in chain scission concentration was observed up to a strain

of 2.5, followed by an abrupt rise, and a plateau region until the strain

reached a value of 4. The chain rupture concentration then increased

linearly up to the rupture strain. At the higher frequency, 0.01 Hz, the

chain scission concentration increased linearly with strain up to a strain

of 2.5, followed by a steep rise, and another region of linear behavior up

to the strain at rupture. The steep rise is a common feature of both

sinusoidal loading frequencies. Current work is directed towards

developing a model of polymer deformation that adequately describes these

observations, as well as those obtained under quasi-static conditions.

Studies of the Intimacy of Molecular Mixing in High Strength Molecular

Composites by Solid State NMR

This project is a joint effort with the Air Force Materials Laboratory to

facilitate development of high strength polymeric materials for use in

structural elements of airplanes as well as other military goods. The Air

Force is seeking chemically stable, processable, high temperature, high

strength materials. One class of promising polymers consists of highly

conjugated aromatic moieties incorporated into the backbone of the polymer

structure. Some of these moieties are exceedingly stiff and tend to form

rod-like macromolecules. These polymers are difficult to process and

generally must be solution cast from strong acids. In order to increase

the ease of fabrication of these materials while taking advantage of the

inherent reinforcing capabilities of stiff, rod-like molecules, there has

been considerable effort to develop high strength composites by blending

these rod-like molecules with more flexible macromolecules. The

reinforcing efficiency of the rod-like component depends rather crucially

on whether the rods aggregate (low strength) or are well dispersed (high

strength). We are using advanced proton and C— 1 3 solid-state NMR

techniques to measure the intimacy of mixing of the two components in

molecular composites.

While problems relating to the variable levels of proton-containing

impurities render certain NMR approaches to this question impossible, a significant measure of success was realized in the characterization of a composite of nylon 6,6 and poly (p-phenylbenzbisthiazole) ( PBT) . In a 40$

nylon/60$ PBT sample coagulated out of methane sulfonic acid, washed in

water, and compression molded at 255°C, mixing was shown not to occur at

the molecular scale. Minimum domain dimensions were determined to be in

the 4-5nm range for both components with some possibly larger nylon

crystalline regions. Rates of proton spin diffusion between the nylon and

PBT domains are the NMR observables which yield minimum domain dimensions.

The observation of significant concentrations of mobile-proton containing

impurities in the coagulated and washed samples of both rigid-rod

homopolymers and molecular composites requires further Investigation. The

entrapment of impurities prior to heat treatment may lead to unwanted

chemical reactions during heat treatment. Also, impurities could

aggregate during heat treatment and prevent longei—range structur r. ordering, thereby limiting the attainable mechanical properties.

31
Partitioning of Chain Ends and Side Groups Between the Crystalline and

Non-Crystalline Regions of Melt-Crystallized polyethylenes

The concept of proton spin diffusion in solid polymers may be used to

argue for the nearly uniform spin temperature of adjacent chains. This

assumption, along with the ability to distinguish vinyl and ‘ methyl

end-group resonances and ethyl and methyl branch resonances from backbone

resonances in magic-angle sample spinning (MAS) C-1 3 spectra enable the

spectroscopist to locate chain ends or side branches with respect to the

crystalline or non-crystalline regions.

By suitably changing the preparation of proton magnetization prior to

cross-polarization (by which C-1 3 signals are generated), different

mixtures of crystalline and non-crystalline signals may be obtained and

through appropriate linear combinations of these spectra, "pure

crystalline" and "pure non-crystalline" signals may be determined. With

an independent knowledge of end group and side-group concentrations, the

partitioning of end groups and si degroups between the crystalline and

non-crystalline regions is obtained.

In a linear polymer, more than half of the methyl ends reside in the

crystal. For terminal vinyl groups on the order of 40$ are to be found in

the crystal. This is a substantially higher fraction than is found by

other workers in solution- grown crystals. In those studies only 2-15$ of

vinyl ends appear to be unavailable for chemical reaction with bromine or ozone

.

With respect to side branch incorporation in the crystal, preliminary

results indicate that methyl side branches are incorporated to a much

greater degree than ethyl side branches. The ethyl branches seem to be

rejected by the crystal rather completely, i.e. 90-95$ of the ethyl

branches are in the non-crystalline region.

We have also examined some radiation effects using these techniques since

sol and gel fractions can be simultaneously examined. Aside from the

rather quick disappearance of terminal vinyls upon gamma irradiation and

the buildup of non-terminal vinyl groups and methyl end groups, little can

be observed. The radiation-induced interior vinyls seem to be evenly

partitioned between the crystalline and non-crystalline phases.

Second-Order Dipolar Shifts for C-1 3 Nuclei in Solids

A small, but real, contribution to the chemical shift for protonated

carbons has been shown to be attributable to a second-order perturbation

contribution from a non-secular term of the carbon-proton dipolar

interaction. The importance of this contribution diminishes as one moves

to higher static magnetic fields. The recognition of the shift provides

the basis for a more accurate comparison of shift data obtained at

different magnetic fields. It also enables one to separate those

influences on chemical shifts due to structure and conformation (effects

considered interesting) from those which depend merely on the operating

magnetic field.

32
013 Spin Diffusion at Natural Abundance

The fact that 013 nuclei are only 1.1% abundant makes the most probable

distance between 013 nuclei in a polymer of the order of 0.7~0.8nm. At

such distances, 013 spin exchange due to the 013 dipolar interaction is

a slow process and its inefficiency is also attributable to the existence

of possibly strong Ol 3-proton dipolar interactions. However, given a crystal polymer with intrinsically long longitudinal relaxation times

(>100s) 013 spin diffusion becomes an important tool for probing the

environment around a given carbon over distance of 0.5 to 0.8 nm (one or two intermolecular distances).

C-13 spin diffusion studies of polyethylene have demonstrated the slow,

temperature-dependent relative motion of the two chains within the unit

cell. A similar study on cellulose has demonstrated crystalline

polymorphy in certain native celluloses, particularly the algal

celluloses. The latter finding may have some commercial implications

because the rates of hydrolysis in strong acid also seem to be different

for the two crystalline forms.

Vibrational Spectra of Crystallographic Defects

Short chains of polyethylene in a crystal tend to move as rigid rods,

particularly by rotating around or translating along their long axes. However, as the chains become longer, the misfit energy associated with

rotation or translation as rigid rods increases with the chain length.

Above a certain length, it becomes energetically favorable for the polymer

to create a localized defect, consisting of a short sequence of

dramatically changed conformations which permits the undistorted parts of

the chain to fit into the crystal lattice. For polyethylene, this

transition occurs when the chain reaches a length of around 30 carbon

atoms

.

Minimum energy conformations of these defects were calculated using

interatomic potential functions. Although defects can be modeled in short

chains, end effects are troublesome. The new supercomputer at NBS enables

calculations to be made in reasonably short times on longer chain segments

so that end effects are minimized. The minimum energy conformation and

the vibrational spectra of defects in chains containing up to 44 carbon

atoms can now be calculated in a matter of minutes. Periodic boundary

conditions were used to eliminate singularities at chain ends. These

boundary conditions create a long chain with a repeat unit as long as the

original chain. This repeat unit consists of a defect and a zigzag

segment. The zigzag segment tends to vibrate like an n-alkane having the

length of the segment. The limitations of a short segment are important

for some modes, particularly the low frequency acoustic modes for which

the frequencies depend strongly on the length of the vibrating segment.

Calculations of conformation and vibrational spectra were performed for

dispiration, dislocation, and disclination loops surrounding a single

chain, and for the partial disclination array known as a twist boundary.

These crystallographic defects or boundaries play important roles in

determining the physical properties of polyethylene and other polymer . In addition, similar calculations were performed for adjacent reentry

folds in (110) and (200) planes.

33
Infrared and Raman intensities were calculated for all modes using the

eigenvectors, a bond moment approach. The variations of the bond dipole

moments were expressed in terms of the set of electro-optical parameters.

For Raman intensities the electro-optical parameters are collected into

separate matrices of bond mean polarizability derivatives and bond

anisotropy derivatives. The elements of the polarizability tensor for

each methylene group in the defect-containing molecular chain were

calculated. The mean polarizability and the anisotropy of each CH 2 chain

were also calculated. The resulting computer program can be applied to

compare the observed 'and calculated Raman intensities of stretch oriented,

defect-containing polyethylene, and also for Raman spectra of polyethylene

powders

.

The interpretation of the vibrational spectra is greatly facilitated not

only by the increased chain length that can be handled in the new NBS

computer, but also by the refinements in the graphical methods for

displaying the displacements and infrared intensities of each CH

2 group.

The molecular graphics made features of the following sort visible and

clear. Some of the modes introduced by the presence of a defect derive

entirely from the motions of atoms in or very close to the defect. The

frequencies of these modes lie at values not encompassed by any part of

the dispersion curves for vibrations of the polyethylene lattice. Little

coupling of these defect vibrations with the lattice vibrations is

observed. The observed 680 cm” 1 band is of this nature, as this frequency

is outside the range of the dispersion curve for the rocking mode of the

polyethylene lattice. It was found to be associated with a gentle twist

of the polyethylene chains which occurs at twist boundaries. This band is

relatively intense in rapidly quenched samples. It was suggested that

twist is the basic morphological feature that results in the long period

observed in polyethylene fibers. Reports in the literature of an absorption band at 680 cm” 1 indicates that twist is present and provides

support for this suggestion. Other modes found near 1421 cm” 1 are also

identified with defect vibrations not coupled to lattice vibrations.

Vibrational modes associated with defects at frequencies that lie within

the ranges encompassed by the dispersion curves of the crystal generally

are strongly coupled to the lattice vibrations. In the absence of the

defect these lattice modes are not infrared active, but the presence of

the defect provides coupling between the light waves and the molecular

vibrations. There are many such modes in polyethylene containing defects.

Defect and crystal parts of the chain may vibrate in a way that both

regions contribute to the absorption intensity or, if the vector sum of

their motions tends to cancel, the absorption may be weaker than it would

be for an isolated oligomer fixed in the defect conformation. These

defect coupled modes are so closely spaced and so weak that it is unlikely

that many of them can be resolved experimentally. The observed effect of

such defect vibrations is to contribute to the broad background absorption

commonly observed in polyethylene.

The modes which are clearly associated with the defects are governed

primarily by the dihedral angles associated with about three successive

bonds. A band near 750 cm” 1 is associated with gauche-trans-gauche

sequences. A band near 1420 cm” 1 is associated with a different sequence

of trans and gauche dihedral angles. The band at 680 cm” 1 is an exception

to this rule, since it is associated with a longer sequence of dihedral

angles (about 9) which all differ from trans by less than 25°.

34
Techniques for Handling Single Molecules and Small Fibers

To examine single molecules at very high resolution in an electron

microscope or in the new scanning tunneling microscopes, it is desirable

that the molecules be extended and attached to a conducting substrate.

Techniques were developed to disperse single polymer molecules or thin

fibers containing from a single molecule to a few tens of molecules in a reasonably dense pattern that extended over a large area. Examination of

such samples is expected to reveal molecular features such as chain

branching and the distribution of comonomer units along a copolymer chain.

Since it is now possible to make computer models of structural features

such as twist boundaries in fibers containing a few tens of polymer

molecules, it may also be possible to compare observed and calculated

morphological features.

A variety of methods for preparing molecular scale polymer samples on cleaved mica surfaces from which the molecules can be transferred to the

electron microscope or to a scanning tunneling microscope were

investigated. These include spreading a dilute polymer solution on cleaved mica or trapping the solution between freshly cleaved mica

surfaces. Wiping solid polymer particles or partly dissolved polymer

across a mica surface was found to leave fibers which tapered to molecular

dimensions. Thin layers of very lightly crosslinked gel trapped between

mica layers offer another class of routes to polymer networks with the

properties sought.

These procedures are being developed using observations of platinum carbon

shadowed samples in the transmission microscope as a guide. Single

molecule particles, particles or crystals containing many molecules,

fibers, fibers with crystal overgrowths (shish-kebabs ) and folded chain

crystals with fibers drawn out of them have been prepared. the major

effort is on polyethylene since its morphological features are familiar.

PBT (poly-p-phenylene-benzbisthi azole) and poly-gamma-benzyl-glutamata

were examined because they are expected to be rodlike, but no rodlike

single molecules have yet been observed, probably because of the limits on resolution imposed by the platinum-carbon shadowing technique. Gelatin

and polyacrylamide crosslinked networks were investigated, the resolution

limits of the electron microscope methods set a lower limit on the size of

the finest fibers observed.

The smallest fibers seen in the electron microscope contain 30 or more

polymer molecules. This is already a small enough number to be

commensurate with the ensembles of molecules used in the computer modeling

of twist boundaries in polyethylene and other crystalline defects. The

fibers that we have already produced and observed in the transmission

microscope are of a suitable size for the observation of some of the

features modelled in the computer, particularly the twist boundary.

Development of the electron microscope and electron diffraction procedures

for observing the long period in small polyethylene fibers is underway.

Techniques for observing polymer molecules in the scanning tunneling

microscope are being developed in cooperation with groups at NBS and I San Jose, who are building scanning tunneling microscopes. Practical

scanning tunneling techniques for observing the structure of single

molecules will make it possible to observe features such as branching,

block copolymerization, and network architecture. These aspects of th<- molecular structure of polymers are not directly observable at present and

must be deduced, more or less reliably from the chemical history of th<‘

35
sample, NNffl measurements of short chemical sequences, end group analysis,

swelling measurements, or the like. The availability of a direct way of

observing the structure and connectivity of single polymer molecules will

be extremely valuable in the characterization of copolymers used in

polymer blends, biological molecules such as DNA or proteins, networks in

rubbers, or in the matrices of reinforced composites.

Looking further to the future, the ability to combine inelastic tunneling

spectroscopy with scanning tunneling microscopy would permit an elegantly

incisive analysis of the structure of very complex molecules. Also, the

ability of scanning tunneling microscopy to observe the spatial variation

of the work function and related electronic parameters will make possible

the observation of parameters needed to incorporate molecules into useful

electron devices.

Differences in the Molecular Weight and the Temperature Dependences of

Self Diffusion and Zero Shear Viscosity in Linear Polyethylene and

Hydrogenated Polybutadiene

Within the context of a general coupling model we can support the

hypothesis that while the modes of relaxation for self-diffusion (D) and

shear flow (n) are the same, the entanglement interactions are different.

We assume that there are two distinct coupling parameters n

D and n

n for D and n respectively. The model predicts the molecular weight, M, and

temperature (T) dependences to be scaled by the relevant coupling

parameters as: n«[M 2exp(E

a/RT) 3

1 /(

1

“n

n

) and Doc[M 2exp(Ea/RT)“ 1 /( for

melts with Arrhenius temperature-dependence . In these relationships, E a is the activation energy. We have found that n

D =0.43 and 0.42 for

polyethylene, PE, and hydrogenated polybutadene, HPB, which scale as m 3* 5 an(j m

3* 1*. Also, the respective apparent flow activation energy E

n of

6.35 and 7.2 kcal/mole scale to primitive activation energies of 3.6 and

4.2 kcal/mole. On the other hand, the M“2 dependence of D results in n

D

= 1/3. The reported activation energies for self-diffusion in PE and HPB of

5.49 and 6.2 kcal/mole scale to primitive activation energy E

a

' s of 3.7

and 4.1 kcal/mole, respectively.

A Comparison of the Linear Viscoelastic Behavior of Cyclic and Linear

Polystyrenes in the Melt

This study is part of an ongoing collaboration between several

laboratories to study the molecular rheology of cyclic polymers. In the

past year we have carried out studies of the recoverable compliance and zero shear viscosities of a series of cyclic polystyrenes having a range

of molecular weights from 19,400 to 390,000 g-mole.

This study used samples made at the Centre de Recherches sur les

Macromolecules in Strasborg and at the University of Akron (by Hostetter

and Fetters). As found previously [1], the zero shear viscosity of the

cyclic polystyrenes is virtually the same as that of the linear materials

above some critical molecular weight for entanglement. M . Our recent

results further substantiate the finding that n0

a m

3

,I{ for the cyclic

molecules as is the case for the linear molecules. Furthermore, the

critical molecular weight for entanglement coupling, M

c , from viscosity

measurements is found to be the same for both the cyclic and linear

molecules, i.e., M

c~32000.

36
On the other hand, the recoverable compliance of the cyclic and linear

molecules is found to differ significantly. The plateau compliance, J^,

for the cyclic molecules is found to be twice that for the linear

molecules, indicating that the molecular weight between entanglement

points for the cyclic polystyrene is twice that of the linear polystyrene.

Furthermore, it is found that the steady-state recoverable compliance, J°,

for the cycles is twice for the linear chains. Therefore, the breadth

of the relaxation spectrum JO/J|J = 2.5 is the same for both the cyclic and

linear systems.

1. G. B. McKenna, G. Hadziioannou, P. Lutz, G. Hild, C. Strazielle, P. Rempp and A. J. Kovacs, "The Molecular Weight Dependence of the Zero

Shear Viscosity of Cyclic Polystyrene Molecules in the Melt", to be

published.

A Model for the Calculation of the Degree of Non-Adjacent Re-Entry in

Crystal Growth

A simplified model for the calculation of the degree of non-adjacent

re-entry may be developed and solved using previously developed

theoretical methods [1], At the end of a strip growing on the lateral

edge of a lamella, three processes are viewed as occurring: (1) the

polymer chain at the terminal end of the growing strip may fold to make an adjacent re-entry fold; (2) a dangling cilium further back in the strip

may fold onto the growing end to form a non-adjacent re-entry fold; (3) a new chain from the supernatant may be incorporated at the end of the

strip.' While in principle this model may be solved for the folding of a chain to form a non-adjacent re-entry from any location behind the

terminal one, mathematical tractability has so far limited solution only

to the case where non-adjacent re-entry occurs from the penultimate

location in the growing strip. In that case, the configuration at the

terminal two positions may be considered "species" in the sense of the

previous theory [1]. For the case of n-paraffin crystals with a lamallar

thickness that is just half the length of the extended chain (i.e., a single fold in the chain), this leads to a 6x6 transition matrix with only

eleven non-zero elements, while for a polymer chain, the corresponding

matrix is 8x8 with 21 non-zero elements.

The "composition" of the chain may determined using previous methods.

Preliminary results for different under coolings, AT, are as follows for

n-paraffins and polyethylene at a concentration of 0.0156:

n-Paraffin Polymer

AT=20 AT=1 6 AT=20 AT=0

% Adjacent 99.6 96.2 89 46.45

% Non-adjacent 0.31 3.72 7 46. 45

% Cilia 0.03 0.06 4 7.1

*at equilibrium

The undercooling is from the melting point of the extended chain crystal.

37
The amount of non-adjacent re-entry increases with solution concentration

and with decreasing undercooling. These preliminary results will be

refined and extended to other cases.

[1] J. I. Lauritzen, Jr., E. A. DiMarzio, and E. Passaglia, J. Chem.

Phys. 45, 4444 (1966).

Monte Carlo Modeling of Chain Conformation in the Semicrystalline State of

Polystyrene

The character of the chain trajectory in the semicrystalline state of

polymer has been a matter of discussion for many years. The recent

application of Small Angle Neutron Scattering (SANS) to such polymer

systems allows a more direct measure of such chain trajectories. In

recent years, a number of polymers in the semi crystal line state have been

studied this way experimentally. However, most of the systems which have

been studied have the problem that the deuterated chains often segregate

from the protonated chains except under very rapid crystallization

conditions. Thus, only rapidly crystallized polymers can be studied.

Polystyrene is one of the few polymers in which the deuterated chains and

protonated chains show no signs of segregation under a variety of thermal

treatments in the semi crystalline state. Taking advantage of this fact,

J. M. Guenet of Centre de Recherches sur les Macromolecules, Strasburg,

France, has made a comprehensive experimental study of the SANS of

isotactic polystyrene in a semicrystalline matrix.

In this work, we are developing Monte Carlo models of chain trajectories

of isotactic polystyrene in the semicrystalline state with the purpose of

fitting Guenet’ s SANS data. The modeling is similar to that which we previously performed on polyethylene. Studies of models of folding on a variety of available crystal planes will be developed. The effects of

thermal treatment on chain trajectory will also be studied.

The Lateral Growth Habits and Sectored Character of Polyethylene Crystals

Previous studies [1] of the habits of lamellar (chain-folded) crystals of

polyethylene fractions (M.W, 10^-10^) grown from relatively poor solvents

(heptyl acetate and dodecanol) in the range 95°C-120°C revealed, inter

alia, the following features concerning their aspect ratio R (length

parallel to b-axis/length parallel to a-axis). For the same solvent, R was larger the higher the growth temperature, it was also larger at a given growth temperature the lower the molecular weight. For the same

molecular weight and growth temperature the aspect ratio was larger for

crystals grown from heptyl acetate than from dodecanol (which is the

poorer solvent). Overall R ranged from 1.8-6. The crystals are bounded

laterally by {100} and (200) faces along which growth occurs through the

accretion of chains in a folded conformation. The usefulness of these

crystals as models for the constituent lamellae in polyethylene

spherulites grown from the molten state has been pointed out [1], A

feature of interest and puzzlement concerning these crystals is that the

longer lateral sides [(200) faces] tend to exhibit more pronouncedly

curved profiles, the higher the growth temperature (and correspondingly the

larger the aspect ratio) [1]). This trend raises some questions

concerning the details of the mechanism of lateral growth of the crystals

and, correspondingly, the nature of the sectored character of the

crystals. This latter aspect is currently being investigated. Earlier

38
observations derived from an examination of the diffraction contrast

characteristics of the various crystals are being complemented by the

application of the recently developed surface decoration technique of

Wittman and Lotz [2], Examination of decorated crystals with aspect ratios

R=2. 5-4.5 in the transmission electron microscope revealed the following:

(1) The crystals are six-sectored, (2 ) the boundary between adjacent

{110} sectors is sharp in all the crystals, (3) the boundary between a {110} and an adjoining {200} sector, although sharp in crystals with

aspect ratio 2.5, becomes progressively more diffuse the larger the aspect

ratio. This trend may be associated with distortions in the conformations

of the crystals resulting from the accommodation (in the sector boundary

region) of differences in the tilt of the stems relative to the fold

surface in the {110} as compared to that in adjacent the {200} sector.

This speculation and its possible relevance in connection with the

increasingly curved character of the longer faces of the lamellae the

higher their aspect ratio are being delved into.

1. F. Khoury and L. H. Bolz, 38th Annual Proc. Electron Microscopy Soc.

Amer. San Francisco; G. W. Bailey Ed. p. 242 (1980).

2. J. C. Wittman and B. Lotz, J. Polymer Sci. 23_, 205 (1985).

Mechanical Properties Measurement and Modeling

Subtask 2 of Task 1 4336

J. M. Crissman, G. B. McKenna, L. J. Zapas, J. A. Hinkley^ , J.-M. Guenet 2

^ NASA Langley Research Center

2Centre de Recherches sur les Macromolecules, Strasburg, France

The mechanical performance and behavior of polymeric systems covers a modulus, G, range including the gel-like state (G-IOKPa), rubbery state

(G-IMPa), semicrystalline polymers (G~.3GPa) and the glassy state

(G-IGPa). The members of the Polymer Division staff are carrying out

research to characterize relationships between the molecular and continuum

physics of appropriate polymer systems and their consequent mechanical

performance.

The effects of thermal history on the volume of polymer glasses is well

known, and the physics of volume changes are reasonably well understood.

However, the impact of this so-called "physical aging" on the properties

of the polymers in the nonlinear viscoelastic range and near failure are

not well understood. Furthermore, the coupling between large mechanical

excitations and the volume recovery of the glasses is not at all

understood. The ongoing efforts to elucidate the physical and mechanical

aging behavior of polymers are described.

The characterization of the response of polyethylene, a semicrystall Inc

polymer, to small deformations has lead to an advancement of our understandiang of visco-plastic performance. The surprising necessity of

using a nonlinear constitutive law, even at small deformations, to

accurately describe the long-term creep behavior of this ublqul tl ou: polymer has been described.

39
The mechanical behavior of elastomers is a subject of both theoretical and

practical interest. Research is present which deals with the use of

continuum physics to relate to molecular theory of the behavior of

polymers in the rubbery state and to understand an anamoly discovered in

the behavior of natural rubber at extremely small deformations.

The mechanical behavior of physical gels has heretofore been treated

within the context of rubbery like behavior. Evidence suggesting that

this is not the case has been obtained from careful measurements of the

solvent and concentration dependences of the compressive properties of

isotactic polystyrene gels. Such results impact our understanding of gel

formation and the mechanical properties which result.

On the basis of investigations of the mechanical behavior of polymeric

systems which exhibit a variation of over 5 orders of magnitude in

modulus, major contributions are being made to the understanding of the

molecular and continuum physics required to predict and/or describe the

mechanical performance of macromolecular materials.

Effects of Physical Aging on the Creep Rupture Behavior of Poly(methyl

methacrylate)

Upon isobaric cooling through their glass transition range, under-cooled

liquids, including polymers, are known to depart from their thermodynamic

equilibrium. The resulting non- equilibrium glasses are intrinsically

unstable and evolve spontaneously towards their equilibrium structure.

The approach towards structural equilibrium is reflected by the time

dependence of the volume and enthalpy of the glass. The changes in

viscoelastic properties associated with the evolving structure of the

glass has come to be known as physical aging [1],

The extensive work of Struik [1], suggested that aging can be described by

merely shifting the retardation spectrum of the glass towards longer

times. Furthermore, in the linear viscoelastic range, the value of the

double logarithmic shift rate:

y

= d log a/d log t e (1)

is close to unity in a rather wide temperature interval below the glass

transition temperature Tg. Log a denotes here the shift along the

logarithmic creep time, t, axis to superpose the creep compliance,

<J(t,t e

), curves determined at increasing values of the t 0

, the aging time

or time after quenching from above T to the aging temperature.

o The effect of large stresses or deformations on the aging behavior of the

glass have been also investigated [1,2] with the finding that the shift

rate, p (see equation 1) decreases with increasing stress or deformation.

Little work has been carried out, however, to examine the effects of

physical aging on the time-dependent failure behavior of polymer glasses.

This study addresses specifically this issue.

Samples of a commercial grade poly (methyl methacrylate) (PMMA) were

machined into the shape of ASTM type dumbbell specimens, placed between

glass plates and heated to 120°C (where T for PMMA=110°C) for 1 hour.

The samples were subsequently removed from tne 120°C oven and placed in an oven at 80°C where they were aged for times ranging from 10 min. to 250

days. The samples were removed from the 80°C oven, left at 23°C for one

40
week, and then tested In creep at large stresses. Creep deformation

measurements were taken using a cathetometer to read the distance between

gage marks placed at the center of the sample. Time to rupture was recorded for each sample.

There are two salient features which result from the preliminary analysis

of the data obtained to date:

1) Aging of the glass as measured by the increased creep compliance with

aging times continues for the full duration of the testing, i.e., 250

days of aging.

2) At aging times of less than 10 days (at 80°C) the lifetime of the

glass increases with increasing aging times, while after 10 days of

aging the time-to-failure is independent of aging time.

Further work is underway to understand why the creep and creep rupture

behavior are not affected in the same way by the changes in glassy

structure associated with the physical aging of the glass.

1. L. C. E. Struik, Physical Aging in Amorphous Polymers and Other

Materials , Elsevier, Amsterdam (1978).

2. G. B. McKenna and A. J. Kovacs, Polymer Engr. and Science, 24^ (10),

1138 (1984).

Mechanically Induced Physical Aging and Nonlinear ViscoelasticityExamples for Polymer Solutions and Glasses

There are several recent [1,2] reports of using small deformations

superimposed on large deformations as a probe of molecular relaxation in

polymer glasses. Although the experiments themselves are quite

interesting, we find the use of linear viscoelasticity theory to interpret

the incremental response to be ambiguous, as shown below. There are two

important findings which describe the effects of mechanical deformations

on the incremental modulus (either AG(t) or AG (&)). First, AG*(oo) is

found to decrease with increasing deformation and to increase with

increasing time, t 0 , after the imposition of the large deformation [1].

Second, the relaxation spectrum associated with AG(t) is found to

increasingly shift to shorter times and to narrow as the deformation

increases or t e decreases. These results have been interpreted as a form

of mechanically induced physical ageing or change in glassy structure

[ 1 , 2 ].

The purpose of this work was to show that, although there may be

structural changes associated with mechanical deformations of polymer

glasses, the superposition of small strains on large as a probe of such

changes is ambiguous. We show this by carrying out experiments on a material which undergoes no structural change (a concentrated solution of

polyisobutylene (PIB) in a hydrocarbon oil) and rationalize the results in

the context of a single integral non-linear constitutive law.

The salient results obtained for the PIB solution are that AG(t) in simple

shearing histories, decreases with increasing deformation and Increases

with increasing ageing times. Representation of the incremental modules

as a Williams-Watts function AG( t ) = G

0exp[-(t/x) 6

] shows a shift of th"

relaxation spectrum to shorter times and a narrowing of the spectrum as

41
deformation increases or t Q decreases. The results are consistent with the

concept of additivity of responses in the constitutive equation and are within 1 0% of the predicted responses at shear deformations as large as 9.5. This result demonstrates the ambiguity of interpretations of these

experiments if one imagines the small strain as simply a linear

viscoelastic probe. It strongly suggests that other more direot methods

of measurements are called for.

In a PMMA glass the incremental response is found to behave in a qualitatively similar way as it does in a polymer solution, i.e. AG(t) is

smaller than G(t) and increases with increasing time, t 0l after imposition

of the large deformation. However, the BKZ theory predicts the effects to

be much greater than what is actually observed. This indicates that the

glass, if measured relative to the BKZ prediction, is hardening rather

than softening, as found using the conventional interpretation relative to

the linear response.

1. T. L. Smith and T. Ricco, "Physical Ageing of a Stretched

Polycarbonate Film", paper presented at the 1984 March meeting

of the American Physical Society, Detroit, MI

2. A. F. Yee, R. J. Bankert, K. L. Ngai, R. W. Rendell and

T. K. Lee, "Non-linear Viscoelastic Behavior of Glassy Polymers:

Exponential Results and Theoretical Model", paper presented at the

1984 March meeting of the American Physical Society, Detroit, MI

Creep and Recovery Behavior of Polyethylene Under Small Uniaxial

Deformations

It has been shown in previous work [1-2] that the creep and recovery

behavior of an ultra high molecular weight polyethylene (UHMWPE) , in the

region of small uniaxial deformations, can be described quite well with a one-dimensional equation consisting of two terms, one a single integral

hereditary term, and the other a plasticity term. This equation describes

plasto-viscoelasticity. It was found that, for creep times of up to about

4 x lCr seconds, the contribution of the plasticity term at 23 °C was proportional to t-j

1 ^ where t

1 is the time under creep. For creep times

greater than 4 x lCr seconds the plasticity term became constant and

independent of the time under creep. The plateau value was further found

to be independent of temperature, at least up to a temperature of 57 °C.

This work has now been extended to two other polyethylenes , one a linear

high density polyethylene, and the other an ethylene-hexene copolymer.

Data were obtained at temperatures in the range from 23°C to 57°C and

creep times of from 10 seconds to 4.3 x 10^ seconds. In order to

approximate constant true stress conditions all of the creep experiments

were carried out at the same value of applied stress (4 MPa) and the

change in strain during creep was in all cases less than 2 percent.

Comparison of the present results with those from the earlier work on the

UHMWPE shows that there is a great deal of similarity in the behavior of

all three materials. The occurrence of nonrecoverable creep, or "plasticity", is a phenomenon general to polyethylene, and its dependence

on temperature or duration of creep is very similar for all three

materials. The behavior of the "plasticity" term has several features in

common to all three materials: (1) The initial slope decreases with

increased temperature. (2) The time at which the plasticity term reaches

a constant value shifts to earlier times with increased temperature and

42
the shift occurs in the same manner for all three polymers. (3) For a given material, the plateau value is essentially independent of the

temperature. (4) The magnitude of the plateau value increases with

decreasing density.

1. L. J. Zapas and J. M. Crissman, Polymer, 25, 57*62 (1984).

2. J. M. Crissman and L. J, Zapas, J. Poly. Sci. (in press).

The Mechanical and Swelling Behavior of Well-Characterized Polybutadiene

Networks

There has been considerable renewed interest in rubber elasticity theory

recently due to the success of the junction constraint model of Flory

[1,2]. In this work, we have carried out mechanical and swelling

measurements on well characterized polybutadiene networks and compared the

results with the junction constraint model in a new way. To do this we

use the concept from continuum mechanics of the Valanls-Landel (VL) [3]

strain energy density function, i.e„, a function in the principal

stretches which can be represented as a separable sum of one function

evaluated at each of the three principal stretches. We have further

derived the V-L function for the junction constraint model of Flory, and

compare it with the experimental data.

There are two salient results from our work: (1) while the Flory model

fits the data extremely well, the parameters related to the network

topology are not in particularly good agreement with those calculated from

the chemistry involved in making the networks and (2) the assumption that

the elastic free energy of deformation is equal to the free energy of

mixing at swelling equilibrium can only be supported if one includes a logarithmic term in the strain energy function. The sign of this term is

found to be opposite that postulated by Flory [1].

1. P. J. Flory, J. Chem. Phys., 66, 5720-5729 (1977).

2. B. Erman and P. J. Flory, Macromolecules, 1_6, 1607*161 3 ( 1983).

3. K. C. Valanis and R. F. Landel, J. Appl. Phys., 38, 2997*3002 (1967).

Measurements of the Strain-Energy Density Function of Peroxide Crosslinked

Natural Rubber at Small Strains

In two recent articles [1,2], we described experiments in which the

torsion behavior and the tension-compression behavior of peroxide

crosslinked natural rubber was examined at smaller deformations than had

been previously reported. In this paper we present new results in which a sample of peroxide cured natural rubber was tested in torsion where both

torque and normal thrust were measured. In this study the strains, Y, at

which the normal force was measured were lower by a factor of 4 than those

which we reported previously (Y=.0046).

As before we determined the derivatives 8W/9I

1 and 9W/9I 2 of the strain

energy density function W with respect to the invariants I

1 and I 2

, and

have further confirmed the result from the prior study that for the

peroxide cured rubber systems which are freshly prepared in our laboratory, 9W/9I 2 does not become negative, even at extremely small

deformations. These results are contrary to results reported In th<-

literature by other measurement techniques [3,4] or with samples

undergoing a different crosslinking (curing) history [5,6].

43
We further calculate the extension and compression behavior from the

torsion data by determining the Valanis-Landel [7] function for the rubber

using the method of Kearsley and Zapas [6] and compare them with the

measured response. We find that the torsional data at small deformations

do not predict the anomaly [2] of a cusp in the tension-compression moduli

beginning at strains of e«2x10“3, Furthermore, the character of the

reduced stress (*E/3) measured at small deformations is different from

that calculated from the V-L function and torque/thrust data. This

suggests that the V-L function is not a correct representation of the

rubber's tension-compression behavior at low deformations. Kearsley [8]

has suggested that this is due to compressibility effects in the rubber at

extremely small extensions and compressions.

1. G. B. McKenna and L. J. Zapas, Polymer, 24, 1495-1501 (1983).

2. G. B. McKenna and L. J. Zapas, Polymer, 24., 1502-1506 (1983).

3. S. Kawabata and H. Kawai , Advances in Polymer Science , Vol. 24,

"Molecular Properties", Springer Verlag, New York, (1977), pp. 89-124.

4. G. W. Becker, J. Poly. Scl. C, } 6_, 2893-2903 ( 1967).

5. S. Kawabata, M. Matsuda, K.'Tei, and H. Kawai, Macromolecules, 1_4^

154-162 (1981).

6. E. A. Kearsley and L. J. Zapas, J. Rheol., 24(4), 483*500 (1980).

7. K. C. Valanis and R. F. Landel , J. Appl. Phys . , 38, 2997-3002 (1967).

8. E. A. Kearsley, Rheology Research, Inc., Private Communication.

Mechanical Properties of Isotactic Polystyrene Physical Gels

Isotactic polystyrene (IPS), solutions in cis-decalln, trans-decalin and

chlorodecane, when quenched to low temperatures form physical gels. The

mechanism of gel formation and the resulting structure of the gel is still

controversial. In this study we examined the effects of solvent and

polymer concentration on the room temperature compressive behavior of IPS

gels formed at -20°C. There are three important results which appear from

a preliminary analysis of the data which indicate that the gels do not

exhibit classical rubbery behavior as would chemically crosslinked gels.

1. The gels show a definite viscoelastic response over the range 10

seconds to 3600 seconds. The relaxation rate tends to accelerate with

time and the double logarithmic rate of stress relaxation, d log a/d

log t, is of the order 0.1 (It varies with time and solvent) which is

much greater than observed in crosslinked rubber (~0.02).

2. The isochronal modulus at 1 20S does not follow a scaling type

behavior. Rather the modulus-concentration dependence shows sharp

features reminiscent of a phase diagram over narrow concentration

ranges. The position of the sharp breaks in the modulus-concentration

plots are dependent upon the solvent system.

3. In the cis- and trans-decalin gels, the amount of the solvent loss is

independent of deformation and can be attributed to evaporation (~1.8$

for cis- and 3.6$ for trans-decalin). The chlorodecane gels show a solvent loss which increases with increasing deformation.

Work is ongoing to understand and extend the results obtained so far.

44
Mechanical Durability of Polymeric Materials

Subtask 3 of Task 1 4336

J. D. Barnes, G. W. Bullman, J. M» Crlssraan, B. M. Fanconl, K. M. Flynn,

F. A. Khoury, G. B. McKenna

The objectives of Subtask 3 are to develop measurement methods and models

of long term mechanical performance and to conduct joint projects with

other government agencies on mechanical durability issues of concern to

those agencies. Joint activities with other agencies provide

opportunities to evaluate various approaches to mechanical durability on

materials with known performance histories. The premature failure of

polyethylene balloons used to lift scientific payloads for stratospheri

c

research has been a concern to NASA. A joint project has been established

to identify molecular and structural aspects of the PE film that impact

performance.

The durability of rubber tank track pads has been of concern to the Army,

and a joint project has been established to develop predictive methods for

performance. The failure behavior of filled rubbers under static and

dynamic loads is being investigated.

A Comparative Study of the Structure and Mechanical Properties of

Polyethylene Films Used in Heavy-Lift Balloons

This project is sponsored by the Balloon Projects Branch of the National

Aeronautics and Space Administration (NASA). It is part of a broader

ongoing investigation undertaken by NASA to determine the causes of

catastrophic failures of helium filled heavy-lift balloons. These

balloons whose capacity at float altitude can be as large as 50x10^ cu.ft.

are used to raise instrument payloads of up to 11,500 lbs to elevations of

100,000-150,000 ft. for various research projects. The helium is

contained by proprietary low density polyethylene film (crystallinity

46.2-48.2$, [1]) which is 0.5-1 mil. thick.

Since 1980, NASA has experienced an increased incidence of catastrophic

balloon failures during ascent through the troposphere at elevations

between 40,000-60,000 ft where the ambient temperature is -70°C. The

causes of the increased failures of this type from 1980 onwards have been

elusive. Among the possibilities which are being pursued by NASA is that

there have been substantial changes in the proprietary polyethylene films

used prior to and since 1980. The purpose of this project is to help

establish a basis for determining whether poorer balloon performances are associated with differences in the fine structure and the mechanical

characteristics of the proprietary films. Accordingly, various aspects of

the structure and mechanical properties of films manufactured at different

periods, including one produced prior to 1980, were probed in an exploratory study [1]. The work described below on four films (I, III,

B1,F1) Is a sequel to that earlier study. Films I and B1 are currently

used in heavy lift balloons, whereas film III is representative of films

used prior to 1980. Film FI has as yet not been used In heavy-lift

balloons.

Wide angle and small angle X-ray diffraction have been used to com pare th*- orientation characteristics of the crystalline regions in the film:; and

probe the nature of their fine textures. The (200), (020) and {110} pole

figures of the four films were determined, and small angle X-ray

45
dlffraotion patterns were recorded with the X-ray beam parallel to each of

the following directions: machine (M) , transverse (T), and normal (N).

While the results confirmed the following similarities between films I,

III and B1 [1], film B1 differed in some respects from films I and III as will be indicated later. In these three films: (1) the a-axis is

preferentially oriented in the MT plane, and preferentially parallel to M

in that plane; (2) the b-axis is preferentially oriented in the NT plane

and preferentially oriented parallel to N in that plane.

The features outlined above coupled with the small angle X-ray diffraction

patterns exhibited by the three films can be interpreted in terms of an idealized working model in which it is envisaged that the films consist

mainly of two populations of twisted lamellar microcrystals: a "transcrystalline" population and a "row-structured" population. The

lamellae in the transcrystalline population are oriented with their b-axis

parallel to N, with the a and c axes randomly oriented parallel to the MT

plane. The occurrence of "row-structures" in polymer films has been

discussed in detail by Keller and Machin [2], The overall orientation

characteristics associated with this type of lamellar texture in

polyethylene are (1) the b-axis is oriented parallel to the NT plane and

randomized in that plane, and (2) averaged over all the lamellae, there is

a preferential orientation of both the a and c axes parallel to M.

Comparison of the pole figures and small angle diffraction patterns

exhibited by the three films indicate that the B1 film contains a smaller

proportion of the "row-structure" population than films I and III.

The FI film was found to differ distinctly from films I, III and B1 . Both

the pole figure data and the small angle X-ray data indicated that the

texture was predominantly of the "row-structure" type. There was no evidence of a transcrystalline type of population.

In the area of mechanical deformation, the biaxial (bubble geometry)

deformation behavior of the films under inflation was examined at both

23°C and -73°C. In the region of the pole of the bubble it was found that

films I and B1 both deformed by almost equal amounts in both the M and T

directions with no evidence of necking even up to the point of failure

indicating that the films are relatively "well balanced". On the other

hand, films III and FI exhibited necking in the T direction and as a result deformed significantly more in that direction than in the M

direction. Film III exhibited multiple necking, whereas only one necked

region occurred in the FI film. The results of the elongation to break

experiments in uniaxial extension at 23°C obtained earlier [1] for films

I, III and B1 , and more recently for film FI are consistent with the

behavior of the films under biaxial deformation. The imbalance in the

deformation in the T relative to the M direction exhibited by the FI film

is consistent with the distinct difference indicated above between its

fine structure and those of the other three films. Correspondingly, it

has also been found that the shrinkage behavior in the M relative to the T

direction exhibited by the FI film differed distinctly from the behavior

of the other three films. Thus, FI shrank by a factor of about twelve in

the M direction and expanded by a factor of 1.35 in the T direction. In

contras^, films I, III, and B1 shrank in both the M and the T directions.

The shrinkage factors for these three films were as follow: Film 1:4.29

(M), 2. 84 ( T) , ratio 1.51; film 111:4.69 (M), 2. 51 ( T) , ratio 1.87; film B1

4.22(M), 3. 1 1 (T) , ratio 1.36. The corresponding ratio of the

strain-to-failure in the machine to the transverse directions for the

three films were: Film 1:0.98; Film 111:1.40; film B1 :0.96. No clear-cut
correlations between differences in the balance of mechanical properties

in the M relative to the T direction, the shrinkage behavior, or the

orientation characteristics of films I, III and B1 (whose crystallinities

are very nearly equal, 46 . 4 $-48 . 2 % , see Ref.[1]) have emerged from the

experiments described above. It may be noted in this connection that the

shrinkage ratio in the M relative to the T direction for the B1 film

(which apparently contains less of the "row-structure" type of texture) is

1.35 as compared to 1.51 for film I, whereas the ratio of the strain to

break in the M relative to the T direction at 23°C is essentially the same

for both films.

It was established in our earlier study that the degree of branching in

the constituent polymer chains in films I, III and B1 was the same (2

methyl groups/100 carbon atoms [1]). Analysis of gel permeation

chromatograms showed however that on a relative basis the polymer in film

III had the highest average molecular weight (Mw ) followed in decreasing

order by films I and B1 . In the case of film FI, the chains have been

found to be less branched (0.5 methyl groups/100 carbon atoms. The

molecular weight characteristics and crystallinity of this film are currently being determined. The influence of the differences in molecular

weight on the observed mechanical properties of films I, III and B1

remains an open question.

In summary, although differences in either molecular weight, fine texture,

or mechanical properties have been identified between films I, III and B1

(which have been used in heavy-lift balloons) it remains unclear why these

differences should lead to the failure of one type of film and not another

in the "balloon environment". The question of ductility arises. All of

the tests employed in this study indicate that all three films retain

considerable ductility at -73°C. One aspect not addressed in this study

however is that of high-velocity impact. All of the measurement methods

used involved rates of deformation which are relatively low. For example,

the rate of strain in uniaxial tensile measurements was 991/6/min [1], As

the balloon ascends through the region of lowest temperature in the

troposphere, it is observed that folds in the balloon pop out at a high

velocity. If this velocity is sufficiently high the behavior of the

polymer will become more glass-like and the ductile to brittle

characteristics of the films may change. This is an area where

differences in molecular weight or molecular weight distribution may

become critical factors. This possibility bears examining more closely.

References

1. F. Khoury, J. M. Crissman, J. D. Barnes and M. Fanconi , NBSIR 84-2989.

2. A. Keller and M. J. Machin, J. Macromol. Sci . (Phys) B1_, 41 ( 1967).

Mechanical Durability of Rubber Compounds for Tank Track Pad Applications

Failure behavior of a carbon-black filled rubber (15TP-14AX, MIL —T— 1 1891

)

is being studied under uniaxial tensile testing conditions. Static and

dynamic (fatigue) testing have been performed to study the material

behavior within the context of a cycle shifted failure envelope model of

rubber rupture. Results from testing of a standard filled butyl rubber

formulation are presented for comparison. Our results show that the

behavior of the 15TP-14AX rubber, while showing a greater lifetime than

47
that of the butyl rubber, Is qualitatively similar. Failure times for

both rubbers decrease with increasing test frequency. Importantly, the

number of cycles to failure increases with increasing test frequency.

The failure lifetime of the rubbers can be related to the stress softening

behavior under cyclic loading conditions. If it is assumed that, in some

sense, a failure envelope exists, then it can be shown that under cyclic

loading the filled rubbers (which stress soften) should fail at a time

earlier than under constant loading conditions. We have been able to

predict failure lifetimes by using creep data obtained from short-time

cyclic loading extrapolated to the shifted failure envelope. This

approach relates the observed frequency dependence of lifetimes to the

creep behavior under cyclic loading conditions.
PROCESSING AND RELIABILITY OF POLYMER COMPOSITES

Task 1 4337

One of the most rapidly growing areas In the materials field Is polymer

based composites. Their high specific strength and stiffness combined with

their versatility in the manufacture of complex parts give them tremendous

potential for applications in many diverse areas ranging from

transportation to construction. For advanced composites, growth rates of

16 percent per year and higher are frequently cited. To maintain this

growth rate, however, it will be necessary to overcome major problems in

two areas. First, there is a need to make processing more rapid and

reliable. ' Second, improvements must be made in our ability to predict

performance, particularly long term durability.

To achieve these goals, it is essential that the underlying science base,

which is currently rather weak, be significantly strengthened. The

Composites Task is responding to this need by conducting long range, basic

research to provide industry and other Government agencies with test

methods, materials property data, and predictive models in the area of

composite processing and performance.

The overall objective in this Task is to establish the scientific basis

for the relationships between the generic parameters associated

formulation and processing and the properties important in the performance

of the final material. The programs in this Task are divided into three

highly interrelated areas: processing, structure, and performance.

Projects in the first Subtask deal with the development of new process

monitoring techniques and the application of these techniques to the study

of model systems. This provides both new measurement methods for process

monitoring and a better understanding of the events that occur during

processing. The second Subtask area seeks to bridge the gap between

processing and performance by developing techniques to determine the

structure of the fabricated material. Both macro-structure (i.e. flaws,

defects, phase separated domains, etc.) and micro-structure (the molecular

network) are of interest, but most of the work during the past year has

focused on the use of neutron-scattering techniques to study the molecular

network in epoxies. As these techniques are developed, they will be

applied to the analysis of materials whose processing and performance

properties are also being examined. The third Subtask focuses on the

determination of performance properties for polymers and their composites.

The major effort in this area during the past year has been the study of

toughening mechanisms and delamination behavior as a function of polymer

type.

To assist in conducting this broadly based effort, the Task is utilizing a number of cooperative programs as well as the the assistance of experts

who, in some cases, have spent time working in the Division during the

last year. A major new aspect of this cooperation was the initiation of a Composites Colloquium Series which brought many world renown scientists to

NBS for discussions. The cooperative efforts involve programs with NASA

Langley Research Center, the Department of Treasury, and the Industrial

research laboratories at Hercules, Hexcel, and Texaco. In addition, Joint

programs were conducted with Drs . S. S. Wang at the Univ. of Illinolr>, A. J. Kinloch at Imperial College (London), and R. S. Stein at the Unlv. of

Massachusetts. Guest workers and visiting scientists during the last year

have included Dr. H. Benoit from Centre de Recherches les Macromoleeul

in France and Mr. A. Gilet from the Ministry of Defense In Israel.

49
FY 85 Significant Accomplishments

• The previously developed theoretical basts for the characterization

of the molecular network structure in thermosetting polymers was refined and applied to the analysis of neutron-scattering

experiments. This analysis has successfully determined both the

chemical and topological heterogeneities in a model epoxy. This

represents the first time such important information could be

obtained.

® The formation of a superlattice-type molecular network was discovered

in certain epoxy systems. For these systems, cured with mixtures of

crosslinking agents of different molecular weights, the chemical

arrangement of the crosslinking molecules was found to be highly

regular; the high and low molecular weight species incorporating

themselves into the network in an alternating pattern. This was completely unexpected and experiments to determine why this happens

and how if affects the properties are now underway.

® The high fracture energy of toughened polymers is of great importance

for improved adhesives and composites. This high toughness has been

shown to result from the development Of a large crack-tip deformation

zone. Scanning electron microscope studies have established yielding

and plastic flow as the primary deformation mechanisms in this zone,

A non-linear finite element analysis was successfully used to model

the development and growth of this zone during fracture experiments.

® Delamination behavior was studied for six different thermoplastic

composites, and where comparisons could be made, the interlaminar

fracture energies relative to the toughness of the polymers

themselves were lower than in comparable toughened thermoset

composites. This less than optimal performance in such composites

was attributed to the poor f iber-polymer bonding that was found in

all but one case. This result has led to the initiation of new

research programs at NBS, NASA, and numerous companies aimed at

improving this bonding in thermoplastic composites

Cure Monitoring and Process Control for Polymers and Their Composites

Subtask 1 of Task 14337

S. S. Chang, R. E. Dehl, B. Dickens, J. C. Phillips, B. M. Fanconi

,

F. W. Wang, K. M. Flynn, F. I. Mopsik, G. W. Bullman, and D. L. Hunston

The changes that take place during the processing of a composite laminate

are very complex, involving both chemistry and flow. As a result, no single technique can provide enough information to monitor all of the

events that occur. Probably the most important cure feature is the change

mechanical properties that takes place sinceit controls the consolidation

of the composite and strongly influences the final properties of the

laminate. Unfortunately, mechanical property measurements do not explain

why the changes occur and are difficult to apply in actual production

situations. Consequently, it is highly desirable not only to develop

mechanical property testing techniques for laboratory specimens, but also

to compare the results with those from other cure-monitoring techniques,

particularly techniques that have on-line measurement capabilities.

50
In this subtask we are actively developing a variety of techniques based

on the measurement of mechanical, electrical, optical, and thermal

properties. Neutron-scattering techniques which are also being applied to

the study of cure will be covered in a separate Subtask. A major strength

in the program is the availability of a wide variety of new and

established techniques. The goal is not only to develop new methods but

also to apply a wide variety of techniques to the study of model systems

so that a basic understanding can be developed and processing models can

be formulated.

Mechanical Property Methods

Two measurement methods for mechanical properties are currently under

development: an ultrasonic technique which has been described extensively

in previous Annual Reports, and a torsional oscillation method similar to

that under study in other laboratories. This second technique employs a ribbon of fabric that has been impregnated with the prepolymer. The

mechanical properties of this coated fabric are monitored by

small- amplitude torsional oscillation and followed as the polymerization

cures. The effective elastic stiffness and the damping behavior are then

determined as a function of cure. Frequencies between 30 Hz and 0.00003 Hz

can be used.

To test this measurement method, initial experiments have been conducted

using a model system consisting of a standard epoxy {(diglycidyl ether of

bisphenol A, DGEBA) } and 2-ethyl-4-methyl imidazole (EMI) as catalyst.

Figure 1 shows the results for the elastic stiffness measured at a curing

temperature of 60 C. As shown in this Figure, the elastic stiffness shows

little change in the initial stages of cure, and then the stiffness

increases rapidly before leveling off at a final value. The successful

development of this test method means that comparison experiments can now

be conducted with other cure monitoring techniques using this model system

cured under identical conditions. Such experiments are now underway with

viscosity and ultrasonic measurements as well as tests based on the

electrical, optical, and thermal techniques described below.

Electrical Methods

The technique of dielectric spectroscopy has been shown to be an easily

implemented method for monitoring the cure of composite resins, and it can easily be automated and incorporated into process control. In the initial

stages of cure, however, the conductance often dominates the dielectric

loss and is a more useful measurement. Consequently, the experiments

performed here have monitored cure both with an automated, real-time AC

conductivity measurement system for the early stages of cure and with the

NBS dielectric spectrometer for the later stages of cure.

For the samples measured to date, the data have shown that AC conductivity

at 50 Hz (which reflects to some extent the changes in viscosity) can be

used to monitor the cure since conductance is the main feature for most of

the cure cycle. Typical results are shown in Figure 2, which represent.-!

data for the model system (DGEBA-EM1).

An important problem in composite processing is the limited stor.ig- tlm<*

for the starting materials, i.e., the fiber cloth or tape impregnated wire,

the uncured resin. One of the storage effects is the very v. w polymerization of the resin. If the cure state could be determine

51
processing might be adjusted to compensate. To examine this idea, a partially cured resin sample was reheated and the polymerization taken to

completion. A conductance curve was obtained for this sample and compared

with the curve for a normally cured sample. It was found that the data

for the partially cured resin was identical to the last part of the cure curve for the reference sample. This suggests that a conductance

measurement could be used to monitor the initial cure state of the resin,

both to estimate shelf-life and to determine the curing time needed for

that batch of resin.

Optical Properties Methods

Two optical methods are under development for cure monitoring: Fourier

transform infrared spectroscopy, which has been described in previous

Annual Reports, and a fluorescence spectroscopy technique. The second

method uses the sensitivity of the fluorescence intensity of some organic

molecules to the local (molecular level) viscosity. Optical fibers are used to bring the excitation light into the interior of a curing composite

component. Future work will seek to use the optical fiber to collect and

transmit the emitted light out of the composite for detection as well. The

light in the fiber excites probe molecules that are located within the

penetration depth of the evanescent wave around the optical fiber. The

penetration distance depends on the refractive indices of the waveguide

and surrounding matrix, as well as the angle of incidence that the

propagating light makes with the optical fiber-matrix interface. Thus,

this approach is particularly sensitive to the interfacial region, and the

proper control of this region is of critical importance for achieving

desirable mechanical properties and performance in the composite.

Polymerization studies on amine-cured epoxy resins had shown that the

excimer-type of fluorescence probe is sensitive to viscosity changes in

the early stage of curing. Other probe molecules with fluorescence

sensitivities over a wider viscosity range are now under investigation.

Experiments are also being used to study the user of a second probe

molecule, which has a viscosity insensitive fluorescence and can therefore

be used as an internal reference. At the low concentrations, around 5 X

10“^M or less, the probe molecules are not expected to have an effect on the resultant mechanical properties of the composite part.

A significant advantage of waveguide spectroscopy for composites is that

the sampled material lies near the fiber-matrix interface, and by varying

the penetration depth of the evanescent wave it may be possible to profile

the viscosity distribution during chemical change in the interfacial

region.

Thermal Property Methods

Differential scanning calorimetry (DSC) can be used to determine the

amount of heat released during the cure reaction and thereby to measure

the degree of cure or crosslinking. The reaction kinetics may be followed

either isothermally or in scanning mode. The DSC can also be used to

study the glass transition regions as a function of the degree of cure or

as function of curing history.

Experiments using DSC to monitor cure have been successfully conducted

with the model DGEBA-EMI system. Comparison studies are now underway

using DSC in combination with other methods to monitor the same cure

52
reactions and compare the results. Figures 1 and 2 show preliminary

results for such tests with dynamic mechanical experiments and conductance

measurements. The peak of the exothermic reaction in DSC occurs well

before the drastic increase in the stiffness or the several decade

reduction in the conductivity, even if some lag time is allowed for the

samples to reach the oven temperature in the mechanical and the

conductance measurements. Hence the mechanical and the dielectric

measurements seem to be more sensitive to the later stages of cure. This

is consistent with the idea that thermal measurements are sensitive to

pregelation chemistry while the increase in elasticity and viscosity are associated with events near and after gelation.

Investigation of the Curing of Intaglio Inks

As an example of the practical application of cure monitoring, one project

in this Task is employing various measurement and process monitoring

techniques to follow the mixing and cure of intaglio inks. The objective

in this work is to analyze the variability in these inks and identify the

sources of this variability. To study the uniformity of the inks, a thermograv.imetric ashing method was used to determine the percentage of

non-combustible material in samples taken from various points within

different ink batches. The degree to which the inks had cured during the

mixing and milling associated with fabrication was followed by examining

the tong oil component usingultraviolet (UV) absorption measurements

andgel-permeation chromatography (GPC) determinations, which estimate

relative molecular weight. Cure behavior was monitored with relative

viscosity measurements on samples dried for various times in air on a rubber mill to simulate some of the conditions on a printing press.

Just as in the work previously described, the results here show that

viscosity measurements are not sensitive to events that occur during the

early stages of cure while GPC, for example, shows significant changes are taking place. This explains why viscosity measurements on uncured inks

were not an effective quality control tool. The results also suggest a correlation sometimes exists between uniformity and cure behavior, and

this means that inadequate mixing is a problem. A large data base on ink

behavior has been generated and is now being analyzed to provide

recommendations for reducing the variability and improving the quality on

inks

.

Analysis of Structure in Polymers and Their Composites

Subtask 2 of Task 14337

W. L. Wu, B. J. Bauer, E. A. Debelius, J. T. Fossett, J. Prahinski,

H. Benoit, 1 R. S. Stein, 2

A. J. Kinloch,^ and D. L. Hunston

1 Centre de Recherches les Macromolecules, Strasburg, France.

^University of Massachusetts, Amherst.

^Imperial College, London.

The determination of structure in thermoset resins and their composites is a vital step in relating processing and formulation parameters to

performance. During the past year, two new techniques have been develop' d to address this problem. The first involves the determination of

morphology in phase separated systems and the second, which comprised th<- majority of effort during the last year, focused on network structure 1 r. epoxies

.

53
Morphology in Phase Separated Resins

Phase separated systems such as rubber-modified epoxies are of great

commercial importance because they are very tough. This toughness,

however, Is highly dependent on the morphology that is generated during

processing and the relationships among formulation, processingand

morphology are not known, Duringthepast year, a technique has been

developed in cooperation with Dr, A. J. Kinloch at Imperial college to

characterize the volume fraction and particle size distribution of the

phase-separated component using transmission electron microscope pictures

and an appropriate analysis. This method was applied to evaluate four

formulations of rubber-modified epoxies having very different

morphologies. The fracture behavior of these materials was also determined

and evaluated using procedures developed previously to handle highly

viscoelastic failure data. This represents the first time that detailed

data on both morphology and properties have been obtained for such

systems. Although the number of systems that have been analyzed so far is

limited, this work is the beginning of a data base that will eventually

establish clear structure-property relationships for these commercially

important materials.

Molecular-Network Structures of Single-Phase Resins

The network structure of thermosetting resins used as composite matrix

materials largely determines some of the key properties for composite

materials. This structure refers to the arrangement of chemical

crosslinks which converts the liquid monomers into a rigid solid. The

network structure in these materials is not amenable to characterization

by standard techniques such as small angle X-ray scattering, electron

microscopy, nuclear magnetic resonance, etc. One of the major aspects

characterizing the network structure within epoxies is the spatial

distribution of the crosslinks. Studies in the Composites Task, however,

have demonstrated that elastic neutron scattering has the unique potency

for addressing this problem. Neutron measurements have been carried out

successfully to characterize the molecular networks in one of the commonly

used types of resins - i.e., the epoxies. A partially deuterated

diglycidyl ether of Bisphenol A (DGEBA) and a curing agent based on polypropylene oxide chain were chosen as the model system for the study.

The neutron scattering results can be analyzed by expressing the scattered

intensities in terms of a form factor of an individual network of infinite

molecular weight. Based on this formulation, the average size of the mesh

and the rigidity of the network could be deduced from the scattering data.

In addition, a decades-old controversy concerning the homogeneity of the

networks within thermosets can now be treated analytically using this

neutron technique. The heterogeneity in epoxies can be divided into two

categories; the compositional fluctuation and the topological fluctuation.

The compositional fluctuation of a network is defined as the variation in

the chain length and the chemical composition among the linkages between

crosslinks. This fluctuation can be related to the zero-angle (q=0)

scattering intensity of a partially deuterated epoxy specimen through

equation 1

.

I(q=0) iii

< n

1 > + <n2

> <n-|> + <n2

> (1)

54
where 5

1 and 62 are the breadth of the molecular weight distribution in

the epoxide and the amine linkages between the crosslinks, <n^> and <n2>

are the average molecular weights, and <j>^ and <f>2 are the volume fractions

occupied by these two species.

As to the topological variation within a molecular network, even for

epoxies derived from an exclusive epoxide-amine reaction with monomers of a monodispersi ve molecular weight, contain topological variations such as tight loops, loose loops, and dangling ends as the inevitable consequence

of the network formation. This type fluctuation will have an obvious

effect on the swelling process of a bulk specimen and will result in a spacial variation of concentration for the swelling solvent. A neutron

scattering experiment on solvent-swollen fully cured epoxies has been

carried out, and the effect of the topological variation on the scattered

intensities has been observed experimentally.

An analytic expression for scattering intensity of a swollen and

topologically regular network with a compositional fluctuation has been

derived. The zero-angle scattering intensity of such a swollen network

follows the relation

i( q=0) , (1 -l.) ^2 t i_ 6Fo2 (2)

SW denotes the bulk swelling ratio and 6F C

^ denotes the right-hand term of

equation 1. The difference between the observed scattering intensities

and the one expressed by equation 2 can be taken as a measure of the

topological heterogeneity.

For all the epoxies studied, the extent of chemical heterogeneity is quite

low. The polydispersity between crosslinks , defined as the ratio of

weight average to number average molecular weights, is less than 1.50.

However, the epoxy networks are found to be highly irregular in terms of

their topological connections. A program has been launched to elucidate

the effect of these network heterogeni ties on the epoxy physical

properties. The independent variables included in this ongoing program are

the chemical composition of the monomers and the processing condition. In

addition to the network structure parameters mentioned above, the fracture

toughness (K ic ), glass transition temperature (T ) , and degree of cure

have been monitored in all the epoxy specimens included in this

composition - processing study. In the following, an example of some

preliminary findings in this program is described briefly.

Formation of Superlattice In Epoxy Network

In an effort to broaden the molecular weight distribution between

crosslinks, Jeffamines of different molecular weight were blended together

and cured with a stoichiometric amount of epoxy monomers. The molar ratio

of the long chain amine (D-2000) to the short chain amines (D-400 to

D-230) were kept at either 1.0 or 0.5. Each one of these cured mixtures

gave rise to a pronounced scattering maximum at a q value smaller than

those observed in the epoxy containing only one of the constituent amino-;

.

Theoretical consideration based on this experimental observation strongly

suggested that the long chain and the short chain amines Incorporat*-:

themselves within the epoxy network in a highly regular manner. Tr- connecting linkage between a pair of short chain amines Is a long chitn

amine and vice versa . Such a regularly alternating block structure- w 1 *

1 •.

55
a amorphous material is rather striking. Work is now in progress to

identify the mechanism of this regular network formation which can not be

understood in terms of our current knowledge about epoxies.

Theglass transition temperature, Tg’s, of the cured mixtures are always

between those for samples made with either of the constituent amines

alone. An example of results for one set of experiments is given in Table

1. The mixture containing one mole of D-400, one mole of D-230 and 4 mole

of epoxy is of particular interest; the toughness of this material is far

greater than that for either of the single-amine-cured samples.

Experiments to study these systems in more detail are underway.

TABLE 1

Effect of Curing Process and Composition on Epoxy Properties

Con position Curing Condition

Fracture

Toughness

(10“6 N/m3/2)

T g(°C)

Degree

Cure

DER332 (2)

D-400 (1)

95°C/24 hrs

.

1.14 ± 0.124 52 >95$

DER332 (2)

D-230

100°C/24 hrs. 0.991 ± 0.026 91.4 >95$

DER332 (4)

D-400 (1)

D-230 (1)

100°C/24 hrs. 9.177 ± 3.037 70 >95$

Performance of Polymers and their Composites

Subtask 3 of Task 1 4337

R. E. Dehl, W. L. Wu, G. W. Bullman, S. S. Chang, A. Gilat,"1 E. Lin, A. J. Kinloch, 2

S. S. Wang, 3 W. D. Bascom^'N. J. Johnston,

R. J. Moulton, and D. L. Hunston

12

3

45

6

Ministry of Defense, Isreal

Imperial College, London, England

University of Illinois, Urbana, Illinois

Hercules, Inc., Magna, Utah

NASA Langley Research Center, Hampton, Virginia

Hexcel Inc., Dublin, California

In studies of composite materials the measurement of performance

properties is almost always a critical element. Consequently, the

development of test methods and reference data is of great importance.

During the last year, work in this area has focused in four areas:

delamination, toughening mechanisms, adhesion, and environmental attack.

In most of the projects the objective was to understand the performance

properties of the polymeric system in terms of the constituent properties

and the processing/fabrication involved.

56
Delamination

In recent years there has been a great deal of interest in composite

delamination, i.e., the growth of cracks between layers of fibers either

in the polymer that holds the fibers together or at the fiber-polymer

interface. These cracks, which are thought to grow from impact damage

substantially reduce themechanical performance of a composite,

particularly in compression. The susceptibility of composites to this type

of damage is attributable in part to the brittleness of the polymers

normally used., To establish this connection at least for a simple type of

fracture, a series of 23 different composites representing a wide range of

resin types and fracture energies was examined. The mode-I fracture

energies for bulk polymer samples were determined and compared with

interlaminar fracture data on their composites. The results are shown in

Figure 3, and the trends are in agreement with those reported last year

based on preliminary results. The correlation can be qualitatively

understood by noting that fiber breakage and pull-out increases the

composite fracture energy while the constraint imposed by the fibers on

the size of the crack-tip deformation zone lowers the composite fracture

energy. The former effects are more important in brittle systems while

the latter predominates with tough resins.

Thermoplastic composites are a particularly interesting case since their

composites generally seem to exhibit a lower interlaminar fracture

energies relative to their resin toughnesses than do comparable thermoset

systems. Last year two thermoplastic composites were examined, and it was concluded that the lower interlaminar fracture energies resulted from poor

fiber-matrix bonding. Four additional systems have now been studied, and

similar results were found in all but one case. A new program to study

interfacial bonding in thermoplastic composites is now underway.

Toughening Mechanisms

The generation of high fracture-energy polymer systems for adhesives and

composites is a major industrial goal. To assist this effort, basic

studies of toughening mechanisms are being conducted in this Task. A

critical part of this work is the analysis of deformation mechanisms for

various thermoplastic and toughened thermoset resins. Scanning electron

microscope (SEM) pictures were taken of fracture surfaces in bulk resin

samples and composites. Evidence of extensive yielding and plastic flow

were found. This was confirmed by heating the samples above their T

g

's for

10 minutes and then reexamining the same regions of the fracture surfaces.

Substantial relaxation of the local deformations in the resin were found

in both the bulk resin samples and their composites. For many of the

tough systems, both the resin specimens and their composites exhibited

stress whitening of the fracture in the initiation and slow crack growth

regions. After the heat treatment, the original color was regained.

In an effort to model the toughening in such systems, high speed movies

were made of the growth of the crack-tip deformation zone during loading.

In cooperation with Prof. S. S. Wang at the Univ. of Illinois, a non-linear finite element analysis was conducted, and the results compar< : with data taken from the movies. The agreement was excellent. Future

work will expand this analysis to’ include time-dependent effects sine- the

failure behavior of these materials is known to be highly viscoelastic.

57
Adhesion

Adhesion plays an important role in the composites field. Numerous aspects

of adhesion, both fundamental and applied, have been addressed in this

Task. During the last year two particular topics were studied as part of an applied project. The project concerned the bonding of a highly filled

composite to glass, and the durability of this bond when subjected to

moisture and thermal cycling. The mismatch in thermal expansion

coefficient is a primary stress inducer during thermal fatigue. The

mismatch can be adjusted, however, by varying the filler content.

Consequently, to develop an optimum design, a basic study was conducted to

determine the relationship among particle size and size distribution,

packing density, and thermal expansion coefficient. Based on this study a particular system was selected for bonding of a highly filled composite to

glass

.

The other areas addressed in this study was the use of coupling agents to

promote adhesion. A variety of coupling agents and application methods

were examined to optimize the bonding in this system. An important

conclusion of the work is that both the filler type and the coupling agent

play significant roles in retaining bonding during thermal cycling in a moist environment.

Environmental Attack

In most composite applications there is a need for resistance to attack by

the various environments that can be present. Since polymer composites

and polymer fibers are now being used in seals, gaskets, and similar

applications, there are cases where this environment can be very severe.

In several applied projects conducted during the past year, such was the

case. As a result, tests were conducted to examine the resistance of

various polymers to oxidation in hot acid or base. A data base was established indicating the resistance to degradation and in some cases information regarding the degradation mechanisms and products for a variety of materials. These materials included polyesters, polyamides,

polyethers (such as cured epoxies), saturated hydrocarbons , fluorinated

hydrocarbons, and polythioethers

.

58
REL.

STIFFNESS

Figure 1. Isothermal Cure Monitoring of Epoxy Resin

by Relative Stiffness and DSC at 60°C

Figure 2. Isothermal Cure Monitoring of Epoxy Resin

by Conductance and DSC at 80°

C

59
Composite

Interlaminar

Fracture

Energy

(k

J/m

2)

2 3 4 5

Resin Fracture Energy (k J/m 2

)

Figure 3. Relationship Between Resin and Composite Fracture

Energies for a Wide Variety of Systems. Dashed

Lines Indicate the General Trends in the Data.

60
MICROSTRUCTURE AND PERFORMANCE OF DIELECTRIC PLASTICS

Task 14339

The goal of the task on electrical properties is to carry out long-range

research to provide the basis for new or improved measurements and provide

concepts and data which will lead to higher performance in the electrical

applications of polymers. The research encompasses a broad range from insulating materials with low loss to materials that conduct either

electrons or ions. An important part of the program includes research on

polymers that can be made to exhibit piezoelectric and pyroelectric

properties which has led to new types of transducers and new measurement

capabilities. Results of the research in the form of data, measurement

techniques, and scientific concepts go directly to funding organizations

through written reports, or in some cases, by the delivery of a measurement system. The information is made available to interested

parties in the general public by publication in archival journals, by

presentation at the meetings of technical societies, through research

associates who come to work with us, and by visits by interested to our laboratories.

The measurement of the dielectric constant and dielectric loss as a function of frequency and temperature has long been used to probe the

molecular motions in polymers and to determine the regions of frequency

and temperature where they can be expected to perform best as insulators.

Development of a time-domain dielectric spectrometer at NBS has extended

the utility of these measurements to follow dielectric properties which

evolve as a function of time since data for a wide range of frequency can

be obtained In a time interval comparable to the reciprocal of the minimum

frequency of interest. Dielectric measurements are being explored as a means of following the cure of epoxy resins and the change in polymer

properties under use conditions. Our competence in dielectrics is also

being applied to the development of a stable, reproducible system to

simulate the dielectric properties of human tissue to be used ininduced

hyperthermia studies in cancer research.

Polymers have long performed well as insulators but higher demands are now being made on their electric breakdown strength as the electric power com- panies increase voltage of power transmission. Longer lifetimes in more

hostile environments are required as the trend to underground cables con- tinues. A more fundamental understanding of electrical breakdown is re- quired to address these problems. A thermal pulse measurement technique

which has proven useful for determining distribution of polarization in

polymer films offers an opportunity to measure the distribution of charge

in insulation film and thus study the role of non-uniform electric fields

in determining the breakdown strength in polymers.

NBS has made a significant contribution to the understanding of the mechanism responsible for the piezoelectric and pyroelectric properties of

polymers. There is a need for predicting their long-term performance

through an understanding of the mechanism whereby polarization is lost at

elevated temperatures. Investigations are continuing on the factors which

influence the stability such as poling conditions, crystal structure,

chemical structure, and molecular motions. Concurrent with these investigations, we are working with research associates to disseminate the know - edge gained. We are also fabricating, calibrating, and employing polymer

transducers in new measurement systems for other government agencies.

61
Although polymers have long been used for their insulating properties,

electronic conduction and ionic conduction in polymers is an exciting new

area of solid-state physics. Ionic transport in polymers provides the basis for a large and growing class of important applications including the

production of chlorine and sodium hydroxide from the electrolysis of

brine, hydrogen production from low-head hydroelectric sources, solid

electrolytes for fuel cells and batteries, materials separation membranes

and electrochemical sensors. In most such applications, it is required

that the current be carried entirely by ions participating in an electrochemical reaction rather than by electrons which essentially

short-circuit the cell. NBS work involves the preparation of ion-conducting specimens, measurement of transport properties, measurement of electrical properties, characterization of structure, and relating the

structure and properties to provide a basis for transport mechanisms at

the molecular level. These concepts will then guide the development of

ion-transporting films with improved conductivity.

FY 85 Significant Accomplishments

• Pressure gauges using poly(vinylidene) , (PVDF) fluoride have been

designed, fabricated and used successfully in other agency projects.

Pressure transducers developed for imbedding in solid media for the

measurement of shock-wave propagation were successful in tests performed

at the Naval Surface Weapons Center. In work supported by the Bureau of

Engraving and Printing, a second series of tests using PVDF pressure

gauges on the Intaglio printing press at the Bank of England provided

new data on pressure distribution with multiple layers of rigging.

• An automated, real-time AC conductivity measurement system has been used

to monitor the curing of epoxy resins during early stages of cure. The

value of conductivity and its evolution with time at a constant

temperature shows promise of being a method of estimating the remaining

shelf-life in a pre-mixed resin. Dipolar loss as measured with the

dielectric spectrometer does not seem to be a useful technique on the

system studied so far.

• The thermal pulse measurement of polarization distribution in polymer

thin films has been tried using a variety of laser light sources. Based

on the results, procurement specifications have been written for a light

source thought to be suitable for examining the distribution of trapped

charges near the surface of insulating polymers.

Automated Dielectric Measurements and Models for Dielectric Relaxation

Subtask 1 of Task 1 4339

F. I. Mopsik and K. J. Wahlstrand

During the past few years we have developed a time-domain dielectric

spectrometer which records the transient charge response to an applied

step voltage for times from 10 ys to 3000 s. The response is then

numerically transformed into an equivalent frequency range of 10'* 11 to 101* Hz. A commercial version of the instrument is being developed by IMASS

Inc., Hingham, MA, through the Research Associate program at NBS. The

speed of the measurement now allows us to examine the dielectric spectrum

of materials which change as a function of time. To facilitate such

measurements, a real-time clock has been Incorporated in the

instrumentation.

62
The dielectric spectrometer has been used to monitor the curing of epoxy

resins but has been useful only in the later stages of cure when the

initially high conductivity has been reduced by gelation. An automated

real-time A-C conductivity measurement system was set up to monitor the

early stages of cure. Such measurements have shown that AC conductivity

at 50 Hz can monitor the cure. For the systems studied to date,

conductance is the main feature that can be followed and there does not seem to be a major dipolar contribution to the loss that would be useful.

There still remains the possibility that dipolar loss would be important

in other systems but this is a point that needs clarification.

Conductance measurements have also been made on an epoxy that was partially cured prior to measurement. The results, when compared to resin

that was not previously cured, show that once curing temperature is

reached, the curing process resumes, for the partially cured resin, with a curve identical to the last part of that for the initially uncured resin.

This suggests that a conductance measurement could be used to monitor the

initial state of the resin, both to estimate shelf-life and to estimate

the curing time needed for that batch of resin. A program is now being

set up to establish the validity of this finding.

In a recent theory we proposed a soliton model of dielectric relaxation

for the crystalline a-peak in polyethylene, accounting reasonably well for

the observed dielectric data at constant temperature. However,

experiments by Sayre et al. [1] show that the shape of the Cole-Cole plot

depends on temperature. In particular, it is found that the Cole-Cole

width parameter 3 increases with increasing temperature. We are exploring

the temperature dependence of the soliton model in an attempt to explain

this behavior and more generally characterize the model.

An analytical extension of the continuum limit theory to include

temperature-dependent conformational defect populations was carried out.

The defects were modelled as fixed infinite reflecting barriers to soliton

motion. The resulting dielectric decay function was found to be not

significantly different from the previously obtained Williams-Watts

result. In addition, the function scaled with temperature, predicting a temperature-independent Cole-Cole plot, in contrast to the polyethylene

experiments

.

We are currently performing more stochastic dynamics simulations of the

soliton model at varying coupling constants and temperatures to explore

the temperature behavior in the continuum (large coupling) and

noncontinuum limits. It has been found that the continuum limit

autocorrelation function scales with temperature, with only a weakly

temperature-dependent friction coefficient. However, simulation using

"one-soliton" autocorrelation function data in the noncontinuum limit

shows a power law behavior which changes strongly with temperature. The

power increases with the ratio of coupling constant to temperature from

negative values up to the continuum limit result of 0.5. This is believed

to be a multiple soliton effect. It is hoped to relate this behavior to

an analytical model as well as use it to explain the Sayre et al

.

experimental results.

[1] J. A. Sayre, S. R. Swanson and R. H. Boyd, J. Polym. Scl., Polym.

Phys. Ed. 16: 1739 (1978).

63
Piezoelectric Polymers and Pressure Transducers

Subtask 2 of Task 14339

J. D. Barnes, A. J. Bur, M. G. Broadhurst, S. C. Roth, and

K. J. Wahlstrand

Poly(vinylldene fluoride), (PVDF), can be made piezoelectric by the

temporary application of high electric fields. During the past several

years NBS has played an important role in explaining the origin of

piezoelectricity and pyroelectricity in this and other polymers. This

fundamental understanding of the properties of the polymer is now being

used in the development of pressure transducers for the U. S. Air Force,

the Defense Nuclear Agency, and the Bureau of Engraving and Printing. In

addition, we are examining the changes which occur at the molecular level

as the polymer loses its activity when subjected to successively higher

temperatures above 80 °C.

PVDF transducers offer the advantages of being tough, thin, flexible films

with a broad-band frequency response and an acoustic impedance close to

that of water (and therefore close to that of wet soil and human tissue).

We are currently constructing pressure transducers to be characterized

under conditions of high dynamic pressures (shock waves), varying thermal

environment and dynamic shear stress.

Pressure gages, which will be used in soil and concrete environments,

consist of two 12 pm thick films of PVDF which are laminated together

using epoxy. The films are made piezoelectrically active over a region 1 cm in diameter by poling this area with an electric field of 2 MV/cm at room temperature. In order to use the gage in soil containing sharp

particles, the PVDF is laminated between two 125 pm thick layers of

protective material such as polycarbonate. We are investigating the

response of such gages to shear stress and to shock generated pressure

pulses. Gage output from an applied shear stress is of interest because

in the' field use of the gage we wish the piezoelectric shear coefficient

to be as small as possible, i.e. a gage which responds to compression

stress only is the most desirable. According to the model, the

piezoelectric shear coefficient is zero, but, in practice, we measure a shear-stress response which is 10-20$ of the response to an equivalent

compression stress. Some modification of the model is needed. We

hypothesize that the shear response is caused by a tilting of the polar

crystallites in the shear field.

In collaboration with the Naval Surface Weapons Center, we are carrying

out shock-wave experiments using the PVDF transducer as a detector. For

these experiments, the bilaminate PVDF gage without protective layers was positioned between two 1.27 cm thick plates of PMMA using epoxy as an adhesive. The PMMA-PVDF assembly formed the target of a light gas gun.

The shock wave was faithfully reproduced by the charge vs. time profile of

the gage, an initial rapid rise due to the shock front followed by a plateau with a duration of 3“4 ps and then release to zero. The tests

demonstrated the reliability of the gage at high pressures and short time.

A calibration, carried out over the pressure range 0.4 to 1.6 GPa, showed

that the response deviated from a linear extrapolation of low pressure

(4-10 MPa) measurements. The nonlinear behavior is an expected

observation resulting from the decrease in compressibility of PVDF as pressure increases.

64
Printing as an art has been with us for centuries. Printing as an empirical science is new. The United States Bureau of Engraving and

Printing, as a participatory member of the Intaglio Research Group, is

attempting to define some of the parameters of that science. Central to

this attempt is the ability to make dynamic measurements of the pressure

in the nip, or contact region between the two cylinders, of an intaglio

printing press. The thinness, flexibility, and toughness of PVDF

transducers provides a means of obtaining these data. During the past

year, development experience gained in previous years was used to

construct gages, calibrate them at low-frequency loading, and use them on

a sheet fed intaglio press (similar to the type used to print all U.S.

currency). On the rotating press, repetitive pressure measurements with

pulse heights as high as 40,000 psi (2.8 x 10® Pa) and pulse widths on the

order of 10 ms were successfully recorded. Placement of gages at

different interfaces within the multiple layers of rigging material

verified the validity of the assumption that the multiple layers act as a single homogeneous medium.

An increasing fraction of the piezoelectric activity of PVDF is lost as temperature is increased between 75 °C and its melting point of 175 °C. We

have investigated changes in dipole orientation within the crystalline

phase of the polymer by following changes in X-ray pole figures which

accompany this thermal aging. The specimen of PVDF which we chose for

these measurements was taken from a 50% crystalline, 1mm thick sheet.

Processing of this sample, which was done at Thomson CSF, France, involved

simultaneous mechanical rolling and electrical poling. Rolling is done in

order to orient the c-axis of the crystals In the direction of roll (which

also orients the ab plane of the crystals) so that the dipoles, which are

normal to the c-axis, are placed in an orientation to receive the maximum

torque from an applied field. X-ray pole figure observations of the

as-received material revealed that the dipoles occupy sites which are ±30°

with respect to the direction normal to the electrode.

Thermal aging consisted of cycling the temperature of the specimens

between room temperature and successively higher maximum temperatures,

Tmax» with each cycle. At the end of each cycle, room temperature

measurements of piezo- and pyroelectric activity, density and X-ray pole

figures were made. Pyroelectric and piezoelectric coefficients decrease

linearly with increasing temperature between 75 °C and 164 °C. Values of

these coefficients at room temperature after heating to 164 °C were only

35% of their initial values (nearly independent of time at the elevated

temperature). Extrapolation of these data predicts complete loss of

activity at 207 °C, in close agreement with other predictions of a Curie

temperature for PVDF.

The crystalline phase of our specimens was the pseudo-hexagonal B phase

for which the positions of the dipoles in the ab plane have a near hexagonal configuration. A proposed six site model for dipolar

orientation in the crystalline field is supported by our direct

observation of the six sites in the spatial distribution of X-r.ty

intensity from the (200) and (110) diffraction planes. Changes In

diffracted X-ray intensity from these six sites as a function of thenr.

aging were interpreted as the rotation of dipoles by 60° steps away fr<

the direction of primary polarization. Using this model of therm r. depolarization, we calculated the fraction of dipoles which contri bu v

. M ‘

65
the piezoelectric activity as a function of the state of depolarization.

The loss of polarization is attributed to a Curie transformation which

extends over a very broad temperature range.

We have also continued our effort to understand these piezoelectric

polymers from a theoretical standpoint. We view crystalline polymer

molecules as having a rotational persistence length, several monomers

long, and treat these segments as individual dipole units of moment, m. In

the classical site model for dielectric relaxation in polymers, these

dipoles move independently in a crystal potential having 2 or more minima

separated by barriers. The model becomes quite interesting when (1) the

dipole moment-electric field interaction energy, mE, is comparable to both

the thermal energy, kT, and the crystal interaction energies, and (2) some cooperativity is included in the model. The rich and varied electrical

behavior of ferroelectric polymers makes it possible to collect a large

variety of both equilibrium and kinetic data with which to evaluate the

parameters that make up such a cooperative site model.

In the past year we sought a mathematical description that mimics, as simply as possible, the hysteresis effects observed in ferroelectric

polymers. We focused our attention on the predicted spacial averages,

<cos0> and <cos2e>, where 8 is the angle between a dipole moment and the

applied field, (E), for comparison with quasi-equilibrium displacement

charge, (D) vs. E and dielectric constant or infrared intensity vs E data

for polyvinylidene fluoride. Generally we found quite good agreement

between experimental data for PVDF and the behavior of the variable site

model

.

Space Charge and Ion Transport in Polymers

Subtask 3 of Task 14339

M. G. Broadhurst. C. K. Chiang, G. T. Davis, A. S. DeReggi, C. A. Harding

and T. Takahashi^

^Foreign guest scientist from Ube Industries, Japan (Returned February

1985) .

Polymers which can dissolve salts and conduct ions are an interesting and

important class of materials. Since they can be inert to lithium and can

be fabricated in thin films they have potential applications as electrolytes in high energy density batteries. Although the number of

polymers which exhibit these properties is gradually increasing, most of

them are based upon a high proportion of ether groups or amine groups in

the polymer backbone or on side chains. Ionic transport occurs primarily

within amorphous regions of semicrystalline polymers and the temperature

dependence of ionic conductivity can be qualitatively described by the WLF

relation which is based upon free volume concepts. A general phenomenon

is that the dissolution of salts involves strong interactions between

polymer molecules and the ions. Even monovalent cations act like

cross-links which cause a large increase in the glass transition

temperature and a corresponding decrease in mobility with increasing salt

concentration.

During the past year, we have continued to investigate the properties of

linear polyethylene imine (-CH

2 CH 2 NH-) n , (PEI), to which alkali metal

salts have been added. Crystallinity in this semicrystalline polymer is

destroyed upon dissolution of the salts but in the case of Nal a new high

66
melting crystalline phase involving the salt and the polymer is formed. As

with all other polymers which have been studied so far, the DC

conductivity passes through a maximum with increasing salt concentration.

We have succeeded in improving the mechanical properties of the polymer

and prevented crystallization of both PEI and PEI-Nal complex while

retaining the ability to dissolve salts by light cross-linking with

1,2, 7, 8 di epoxyoctane. At 1^0 °C, the conductivity is comparable to that

of polyethylene oxide (PEO) - salt systems but the conductivity of the

cross-linked PEI decreases with decreasing temperature much more than the

PEO system.

We have attempted to reduce the strong interaction between polymer chains

induced by the presence of ions by quaternizing the nitrogen in the

backbone of the polyethylene imine with butylbromide and ethylbromide . The

quaternized polymers seem to be able to dissolve Nal but the T

&

of the

starting material is around 50 °C and DC conductivity is disappointingly

low.

Most of our effort in the coming year will be directed towards obtaining a more basic understanding of the state of the ions in the solid polymer. In

collaboration with personnel from the Center for Analytical Chemistry, we have made preliminary measurements using neutron-depth profiling which

indicate that we should be able to measure the diffusion coefficient of

^Li + in polymeric electrolytes under a variety of conditions. From

measured DC conductivity and ionic diffusion coefficients, we hope to

deduce information about the fraction of the added ions that contribute to

conductivity. Mobility of ions will also be compared with the mobility of

uncharged species (e.g. cyclohexane) in the same systems to yield

information about the general validity of models for ion transport in

polymeric electrolytes.

Electromagnetically(EM) induced diathermy and hyperthermia for cancer treatment, and growth and healing of bones and tissue are of increasing

importance. Dielectric tissue phantoms (dielectric materials that will

behave electrically much like living biological tissue are used to

evaluate equipment intended for the EM generation of heat in animal

tissue. Presently-used phantoms are based on aqueous biopolymer gels

which exhibit undesirable long-term changes of their mechanical and

electrical properties. The Food and Drug Administration is seeking

alternative non-aqueous non-biopolymer-based phantoms with improved

stability. We are helping to design a synt he tic- polymer based phantom as

a measurement and calibration standard. This past year we have carefully

measured and interpreted the dielectric data for biological tissue over 12

decades of frequency in order to fully understand the mechanisms whereby

these materials are heated by EM fields. The heating is due to the

conduction of ions dissolved in the aqueous tissue fluid and is modified

by the partial blocking of the ion flow by the cell walls. Based on this,

knowledge we have designed a class of synthetic materials using propylene

carbonate and ethylene carbonate as the liquid phase, one of sever r. possible salts to supply mobile ions, and one of several possible g

1

forming polymeric materials to provide the interfaces to partially bloc,

ionic conduction and provide the concommitant increase in dl olentrt

constant. We are currently obtaining systematic data on several potenti

phantom systems to establish the range of electrical and other physi

properties that are obtainable with these synthetic phantom.! and t confirm our understanding of the behavior of ionic conductors In •

67
The Electric Power Research Institute is funding research at NBS directed

toward improved polymeric electrically insulating materials for use as dielectrics in high voltage cables. The specific task of NBS is to gain

an understanding of charging phenomena in nominally insulating polymers

under high fields and moderately elevated temperatures. The gradual

accumulation of charge and the kinetics controlling its distribution as a function of time are hypothesized to create potentially undesirable

conditions in the insulation such as redistribution of the internal fields

accompanied by local concentrations of the field strength, which

conditions can result in poor ageing characteristics and ultimately in

premature breakdown. An understanding of the charging behavior is

expected to facilitate the development of modified polymers not prone to

charging. The experimental phase of the work involves the measurement of

charge-depth profiles in polyethylene and related materials using the

thermal pulse method. In order to make measurements in the anticipated

situations where the volume densities of space charge are small and

sometimes concentrated near the sample surfaces, the experiment was redesigned to employ a high power, pulsed laser as the thermal pulse

generator

.

Preliminary laser thermal pulse experiments were performed using a variety

of laser types (solid state, CO2, dye) available outside the Polymers

Division (Optical Physics, Fire Research) and outside NBS (U. of Maryland,

Nav. Res. Lab.) with the aim of evaulating their suitability prior to

procuring one for the project. On the basis of these measurements, it was concluded that a nonstandard laser was needed to generate pulses in the

100 ns to 10 ys range. One problem with standard Q-switched lasers is

that the pulses generated are too short (10 ns) so that at useful energies

per pulse, the peak powers are so high that the electrodes are vaporized

in a single shot. Several design approaches were discussed with laser

vendors, including designs which were previously used in other experiments

with similar requirements. However, the procurement process, which was

begun in late February 1985, failed to turn up concrete bids from vendors

reluctant to provide NBS with a custom pulsed laser heating system until

the Fourth Quarter when a freeze in previously committed funds has delayed

the procurement until the next fiscal year.

The preliminary experiments showed that polyethylene films previously

subjected to an applied electric field of 0.1 MV/cm at ca. 100 °C for 1 hour gave thermal pulse-induced electrical response signals. These

signals indicated that charge carriers were present in the bulk. The

signal strength, mean duration and shape were observed to depend on charging field, temperature and time and furthermore to evolve with the

passage of time after charging. By contrast, samples subjected to

electric fields at room temperature showed negligible charging when the

measurements were made, usually one day after charging.

68
OUTPUTS/INTERACTIONS

Publications

Barnes, J. D. The role of interlaboratory test programs in quality

assurance, Proc. of ASTM Symp. on Quality Assurance in Plastics, 31;

1985.

Barnes, J. D.; Clark, E. S. An automated apparatus for x-ray pole

figure studies of polymers. ACS Polymer Preprints 52: 291; 1985.

Barnes, J. D.; Dickens, B. ; McCrackin, F. L. Software for liquid size

exclusion chromatography data collection and analysis. ACS Symp.

Proceedings 52: 392; 1985.

Briber, B. M. ; Thomas, E. L. The crystallization behavior of random

block copolymers of polybutylene terephthalate and polytetramethylene

ether glycol. Polymer 26: 8; 1985.

Briber, R. M. ; Adams, W. W. ; Sherman, E. S. ; Porter, R. S. ; Thomas,

E. L. Microstructure of high modulus solid state extruded polyethylene:

x-ray scattering studies. Polymer 26: 17; 1985.

Broadhurst, M. G. Introduction to polymer physics for physics news in

1984. Physics Today 38: 5; 1985.

Bur, A. J. Dielectric properties of polymers at microwave frequencies:

a review. Polymer 26: 963; 1985.

Bur, A. J.; Roth, S. C. Measurement of a piezoelectric d constant for

polyvinylidene fluoride transducers using pressure pulses, J. Appl.

Phys. 57: 113; 1985.

Bur, A. J.; Roth, S. C. A polymer pressure gauge for dynamic pressure

measurements. Proc. of 2nd Symp. on the Interaction of Non-nuclear

Munitions with Structures, April 1985.

Bur, A. J.; Roth, S. C. Characterization of polyvinylidene fluoride

pressure transducers. Proc. 5th Inti. Symp. on Electrets, Sept. 1985.

Chung, R. M.; Bur, A. J . Laboratory evaluation of an NBS polymer soil

stress gage. Proc. of 2nd Symp. on the Interaction of Non-nuclear

Munitions with Structures, April 1985.

Holder, R. J.; Chung, R. M.; Bur, A. J . Field evaluation of the

polymer soil stress gage. Proc. of 2nd Symp. on the Interaction of

Non-nuclear munitions with structures, April 1985.

Lemar, E. R.; Forbes, J. W.; Tasker, D. G.; Bur, A. J . Polyvinylidene

fluoride transducers for dynamic pressure measurements. Proc. 1985 To;-i

Conf. on Shock Waves in Condensed Matter, July 1985.

Chiang, C. K.; Davis, G. T. ; Takahashi, T. ; Harding, C. A. Poly(ethylene imide)-sodium iodide complexes. Macromolecules 18(M:

825; 1985.

69
Chow, L. C.; Beaudreau, G. M.; Brown, W. E. Enamel fluoride profile

construction from biopsy data, Caries Res. 19: 103; 1985.

DeReggi, A. S. Primary piezoelectricity in ferroelectric polymers.

Bull. Am. Phys. 30(8): 340; 1985.

Dickens, B. Intaglio ink considerations, NBSIR 85“3216, 1985.

Di Marzio, E. A. Motion of polymers near surfaces. Proc. Workshop on Polymer-Flow Interaction, La Jolla, Ca, July 1985.

Gaylord, R. J.; Joss, B. ; Bendler, J. T.; Di Marzio, E. A . Continuous-time randow walk description of the non-equilibrium

mechanical response of crosslinked elastomers. Brit. Poly. J. 17: 126;

1985.

Fanconi , B. M.; Sarazin, D.; McCrackin, F. L. Infrared bandshapes of

methylene-d

2 bending vibrations in N-C 35H74 , N-C35 D714 mixtures.

Polymer 26: 219; 1985.

Fanconi, B. M.; Rabolt, J. F. The determination of longitudinal

crystal moduli in polymers by spectroscopic methods. J. Poly. Sci.

Poly. Phys. Ed. 23: 1201; 1985.

Akcasu, A. Z.; Guttman, C. M . Remarks on the translational diffusion

coefficient of relatively short chains. Macromolecules 18(5): 938;

1985.

Han, C. C.; Okada, M.; Muroga, Y.; Bauer, B. J.; Tran-Cong, Q. Phase

decomposition phenomena of polystyrene/ (poly vinylmethylether ) . ANTEC

85: 306 ; 1985.

Han, C. C.; Schaefer, D. W. Quasielastic light scattering from dilute

and semidilute polymer solutions. Chapter 5, "Dynamic Light

Scattering" 1 81 ; 1985.

Hasegawa, H. ; Hashimoto, T.; Han, C. C . SANS and SAXS studies on molecular conformation of a block polymer in microdomain space.

Macromolecules 18: 67; 1985.

Leung, P. S.; Goddard, E. D.; Han, C. C .; Glinka, C. J. A study of

polycation anionic surfactant complexes, Colloid & Surfaces 13: 47;

1985.

Stein, R. S. ; Han, C. C. Neutron scattering from polymers. Physics

Today 1 : 74; 1985.

Hong, Y. C.; Chow, L. C.; Brown, W. E. Enhanced fluoride uptake from

mouth rinses. J. Dent. Res. 64(2): 82; 1985.

Howell, B. F.; McCrackin, F. L. ; Wang, F. W. Diffusion coefficient

determination of butylated hydroxyanisole in low density polyethylene

by fluorescence measurements. Polymeric Materials Science and

Engineering Proceedings 51: 778; 1985.

70
Howell, B. F.; McCrackin, F. L. ; Wang, F. W. Fluorescence measurement

of the diffusion coefficient for butylated hydroxyanisole in low

density polyethylene. Polymer 26: 433 ; 1985.

Hunston, D. L. ; Bullman, G. W. Relationships between mechanical

properties and performance on inks as the basis for quality control

techniques: Part III. NBSIR 85~31 80 , 1985.

Hunston, D. L. ; Bullman, G. W. Viscoelastic fracture behavior for

different rubber-modified epoxy adhesive formulations. Int. J.

Adhesion and Adhesives 5(2): 69; 1985.

Hunston, D. L. Characterization of interlaminar crack growth in

composites with the double cantilever beam specimen. Proc. Nasa Workshop

on Tough Composites, 1985.

Hunston, D. L. ; Koutsky, J. A.; Weisshaus, H. Measuring rheological

changes in adhesives during bond formation and cure. Proc. Adhesion

Society, 1985.

Kinloch, A. J.; Hunston, D. L. The mechanics and mechanisms of

fracture in toughened epoxies, pp. 4. 1-4.7 in Proc. of the International

Conf. on Toughening of Plastics, II (The Plastics and Rubber Institute,

London, 1985).

Kase, H. R. ; Case, E. D.; Tesk, J. A. Elastic constants of two dental

porcelains, J. Matls. Sci. 20: 524; 1985.

Kase, H. R.; Tesk, J. A.; Case, E. D. Thermal expansion of U.S. and

Australian synroc B. Nuclear Tech. 68: 423; 1985.

Khoury, F. A.; Crissman, J. M. ; Fanconi, B. M.; Wagner, H. L. ; Bolz,

L. H.; Harding, C. A.; The structure and properties of polyethylene

films used in heavy lift balloons. NBSIR 84-2989, January 1985.

McKenna, G. B.; Zapas, L. J. Superposition of small strains on large

as a probe of nonlinear response in polymers. ANTEC 85, p. 583, 1985.

McKenna, G. B.; Zapas, L. J. The time dependent strain energy function

for a polymeric glass. Polymer: 26: 543; 1985.

McKenna, G. B.; Zapas, L. J. The superposition of small deformations

on large deformations: measurements of the incremental modulus for a polyisobutylene solution. J. Poly. Sci., Poly. Phys. Ed. 23: 1647;

1985.

McKenna, G. B. Measurement of the torque and normal force in torsion in

the study of the thermoviscoelastic properties of polymer glasses, in

Relaxations in Complex Systems, K. L. Ngai and G. B. Wright, eds.,

USGPO, 1985.

McKinney, J. E. Environmental damage and wear of dental composite

restorations. Proc. Inti. Symp. on Posterior Composites, January 1 98 : .

71
Paffenbarger , J. A.; Tesk, J. A.; Brown, W. E. Dental research at the

National Bureau of Standards: How it changed the practice of dental

health service. JADA 111: 83; 1985.

Passaglia, E. Allen Astin: A commemorative career biography, NBS

Special Publication 690, 1985.

Peterlin, A. The diffusion and permeability in a homogeneous non-ideal

membrane. Coll. Polymer Sci. 263: 35; 1985.

Peterlin, A.; deCandia, F. D.; Vittoria, V. The time dependence of the

mechanical and transport properties of drawn and annealed linear

polyethylene. J. Poly. Sci., Poly. Phys. Ed. 23: 1217; 1985.

Reneker, D. H.; Mazur, J. Modeling of chain twist boundaries in

polyvinylidenefluoride as a mechanism for ferroelectric polarization.

Polymer 26: 821; 1985.

Zupancic, I.; Lahajnar, G. ; Blinc, R.; Reneker, D. H. ; VanderHart, D. L.

NMR self diffusion study of polyethylene and paraffin melts. J. Poly.

Sci. Poly. Phys. Ed. 23: 387; 1985.

Sanchez, I. C. Universality, critical exponents and polymer solutions.

Physics Today 38: 522; 1985.

Sanchez, I. C. Critical properties, potential force constants, and

structure of organic molecules. AiChE Journal 31(9): 1563; 1985.

Sanchez, I. C. A universal coexistence curve for polymer solutions. J.

Appl . Phys. 58(8): 2871; 1985.

Smith, L. E.; Fanconi, B. M.; Davis, G. T. ; Wang, F. W.; Hunston, D.

L.; Sanchez, I. C.; Tesk, J. A. Polymers division annual report 1984.

NBSIR 84-2946, January 1985.

Stansbury, J. W.; Brauer, G. M. Divanillates and polymerizable

vanillates as ingredients of dental cements. J. Biomed. Mater. Res.

19: 715; 1985.

Bailey, W. J.; No, K.; Pan, C. Y.; Saigo, K.; Stansbury, J. W . Recent

advances in ionic polymerization with expansion in volume. Polymer

Preprints 26: 50; 1985.

Takagi, S.; Chow, L, C.; Brown, W. E. ; Dobbyn, R. ; Kuriyama, M.

Application of the x-ray image magnifier to the microradiography of

dental specimens. J. Dent. Res. 64(6): 866; 1985.

Tesk, J. A.; Brauer , G. M. ; Antonucci, J. M.; McKinney, J. E. ; Penn,

R. W.; Stansbury, J. W.; Wu, W. L. Properties and interactions of oral

structures and restorative materials, NBSIR 85— 3 1 19, 1985.

Tesk, J. A.; Okuno, 0.; Penn, R. W.; Hirano, S. ; Kase, H. R. The mesh

monitor for casting characterization in nobel metals fabrication of

technology seminar, Proc. Int. Precious Metals Institute, April 1985.

72
Hinman, R. W.; Teak, J. A .; Parry, E. E.; Eden, G. T. Improving the

casting accuracy of fixed partial dentures J. Prosthetic Dent. 5 3 ( ^ ) : 466;

1985.

Hinman, R. W.; Tesk, J. A .; Whitlock, R. P.; Parry, E. E.; Durkowski,

J. S. A technique for characterizing casting behavior of dental alloys. J.

Dent. Res. 64(2): 134; 1985.

Meiser, E. T., Jr.; deRijk, W. G.; Tesk, J. A .; Hinman, R. W.; Hesby,

R. A.; and Pelleu, G. B., Jr. Internal setting expansion of a dental

casting investment measured with strain gauges. J. Prost. Dent. 53(6):

870; 1985.

Tung, M. S.; Brown, W. E. Effects of ionic organic material on enamel

demineralization, Caries Res. 19: 72; 1985.

Tung, M. S.; Brown, W. E. The role of octacalcium phosphate in

subcutaneous heterotonic calcification. Cal. Tiss. Inti. 37: 329; 1985.

Tung, M. S.; Chow, L. C.; Brown, W. E. Hydrolysis of dicalcium

phosphate dihydrate in the presence or absence of calcium fluoride. J.

Dent. Res. 64(1): 2; 1985.

Havens, J. R.; VanderHart, D. L. Multiple pulse proton NMR of

pressure-crystallized linear polyethylene. J. Magn. Res. 61: 389; 1985.

Havens, J. R.; VanderHart, D. L . The morphology of poly (ethylene

terephthalate) fibers as studied by multiple-pulse ^ H NMR,

Macromolecules 18(9): 1663; 1985.

Verdier, P. H.; Kranbuehl, D. E. Concentration dependence of the

diffusion coefficient and the longest relaxation time of polymer

chains in solution. Macromolecules 18(8): 1638; 1985.

Wagner, H. L., The Mark-Houwink-Sakurada equation for linear

polyethylene. J. Phys. Chem. Ref. Data 14(2): 611; 1985.

Wang, F. W.; Lowry, R. E. Excimer fluorescence technique for study of

polymer-segment mobility: applications to pyrene-labeled poly

(methyl methacrylate) and poly (methyl acrylate) in solution. Polymer

26: 1046; 1985.

Wang, F. W.; Lowry, R. E.; Wu, E.-S. Synthesis and self-diffusion of

nitrobenzoxadiazole-labeled polystyrene. Poly. Mat. Sci. Eng. 52: 355;

1985.

Wang, F. W.; Lowry, R. E.; Fanconi, B. F. Cure monitoring of epoxy

resins by fluorescence spectroscopy. Poly. Mat. Sci. Eng. 53: 180;

1985.

Choi, I

. —S . ; Wu, E.-S.; Wang, F. W . ; McKenna, G. B. ; Lowry, R. E.

;

Hild, G.; Lutz, P.; Rempp, R. Diffusion of linear and cyclic polymer;

in semi-dilute solutions of linear polymers measured by fluorescence

recovery after photobleaching. Poly. Matl. Sci. Eng. 53: 330; 198 .

73
Wu, W. L.; Bauer, B. J. Network structure of epoxies: 1 neutron

scattering study (I). Poly. Comm. 26(2): 39; 1985.

Wu, W.; Wiswe, D.; Zachmann, H. G. ; Huhn, K. A SANS study of partially

segregated polyethylene terephthalate blends. Polymer 26(5): 655; 1985.

Wu, W. L. ; Wignall, G. D. Small-angle neutron scattering of partially

segregated blends of polyethylene and deuteroupolyethylene. Polymer

26(5): 661 ; 1985.

Benoit, H. ; Wu, W . ; Benmouna, M.; Mozer, B. ; Bauer, B. ; Lapp, A.;

Elastic coherent scattering from multi-component systems. Macromolecules

18: 986; 1985.

Zapas, L. J.; Wineman, A. Superposition of small shear deformations on large uniaxial extensions for viscoelastic materials. Polymer 26:

1105; 1985.

Antonucci, J. M. Resin based dental composites-an overview. Proc. of

the 2nd Int. Conf. on Polymers in Medicine, Capri, Italy, in press.

Brauer, G. M.; Antonucci, J. M. Dental applications (of polymers) in

Encyclopedia of Polymer Science and Engineering, Wiley Interscience,

NY. Vol. V., in press.

Brauer, G. M.; Stansbury, J. W.; Flowers, D. Modification of cements

containing vanillate or syringate esters. Dent. Mat., in press.

Chang, S. S. ; Maurey, J. M. Solubilities of BHT. J. Chem. Eng. Data;

(Oct. 1985).

Chang, S. S. ; Maurey, J. M. Migration of low molecular weight components

from polymers. IV. migration of styrene monomer from polystyrene.

Polymer, in press.

Crissman, J. M.; Zapas, L. J. On the mechanical preconditioning of

ultra high molecular weight polyethylene at small uniaxial

deformations. J. Poly. Sci., in press.

Di Marzio, E. A. The nature of the glass transition. Proc. of Workshop

on Glasses, in press.

Gaylord, R. J.; Di Marzio, E. A . ; Lee, A.; Weiss, G. H. Relaxlation of an unattached chain in a cross-linked network: a span analysis of

reptation. Polymer, in press.

Weiss, G. H. ; Di Marzio, E. A .; Gaylord, R. J. First passage time

densities for randow walk spans. J. Stat. Phys., in press.

Fanconi, B. F. ; Wang, F. W.; Hunston, D. L; Mopsik, F. I. Cure

monitoring for polymer matrix composites. Proc. 31st Sagamore Army

Research Conf., in press.

7M
Fanconi, B. F. Trends in polymer development and analytical

techniques. Proc. of the Inti. Conf. on Polymer Characterization, in

press

.

Guttman, C. M. Relationship of the unweighted Rosenbluth and

Rosenbluth walk to a polymer chain at the theta point, Macromolecules,

in press.

McKenna, G. B. Constraint release and self diffusion in polymer melts.

Poly. Comm., in press.

McKenna, G. B. ; Bullman, G. W.; Flynn, K. M.; Patt, J. Fatigue of a rubber tank track compound under tensile loading. Proc. 32nd Sagamore

Army Materials Research Conf, in press.

McKenna, G. B. Hadziiaonnou, G. ; Hild, G. ; Strazielle, C.; Lutz, P.;

Rempp, P.; Kovacs, A. J. The molecular weight dependence of the zero

shear viscosity of cyclic polystyrene molecules in the melt.

Macromolecules, in press.

McKenna, G. B. ; Ngai, K. L; Plazek, D. J. Differences in the molecular

weight and the temperature dependences of self-diffusion and zero

shear viscosity in linear polyethylene and hydrogenated polybutadiene.

Polymer, in press.

McKenna, G. B. ; Zapas, L. J. On the small strain behavior of peroxide

crosslinked natural rubber. Rubber Chem. Tech, in press.

McKinney, J. E.; Wu, W. L. Chemical softening and wear. J. Dent. Res.,

in press.

Mopsik, F. I. The transformation of time-domain relaxation data into

the frequency domain. IEEE Trans. Diel. Phenomena, in press.

Mopsik, F. I.; Hoffman, J. D. In honor of Robert H. Cole's 70th

birthday, IEEE Trans. Diel. Phenomena, in press.

Passaglia, E.; Di Marzio, E. A. Growth regimes and the time

development of lateral habits in polyethylene crystals. Polymer, in

press.

Passaglia, E. The economic effects of degradative process on materials, ACS Symp. on Degradation of Materials Due to Acid Rain, in

press

.

Reneker, D. H. ; Mazur, J. Defect diffusion models for internal

friction processes in polyethylene, Editions de Physique, in press.

Reneker,

680 cm”'

in press

D. H. ; Mazur, J.; Fanconi, B. M. Assignment of IR band at

in polyethylene to molecular twist boundaries. Poly. Comm.,

Sanchez, I. C. Effect of sequence distribution on the miscibility of

polymer-copolymer blends. Macromolecules, in press.

75
Tesk, J. A.; Waterstrat, R. M. Physical metallurgy of dental casting

alloys-Co-Cr-Mo system, Ni-Cr system, ferritic and austenitic steels,

titanium alloys. Article #343/00314, Encyclopedia of Materials Science

and Engineering, Pergamon Press, in press.

Tesk, J. A. The role of structure and composition on the physical and

biological properties of dental alloys. An Invited paper presented at

the Int. Workshop on Biocompatibility, Toxicity and Hypersensitivity

to Alloy Systems Used in Dentistry, Univ. of Michigan, June 1985, in

press

.

VanderHart, D. L. ; Wang, F. W.; R. K. Eby; ; Fanconi, B. F. ; DeVries,

K. L. Exploration of advanced characterization techniques for

molecular composites. Air Force Wright Aeronautical Laboratory

Technical Report, in press.

Wagner, H. L. The Mark-Houwink equation for atactic polystyrene. J.

Phys. Chem. Ref. Data, in press.

Wang, F. W.; Lowry, R. E. ; Wu, E.-S.. Self-diffusion in concentrated

polystyrene solutions measured by fluorescence recovery after

photobleaching. Polymer, in press.

Wang, F. W. ; Lowry, R. E.; Cavanagh, R. R. Picosecond excimer

fluorescence spectroscopy: to polymer-segment mobility and

polymerization monitoring. Polymer, in press.

Wu, W. L. ; Bauer, B. J. Epoxy network structure (II). Polymer, in

press.

Zapas, L. J.; Crissman, J. M. Necking phenomena and cold drawing.

Proceedings of the Symp. on Viscoelasticity and Rheology, Mathematics

Research Center, University of Wisconsin, in press.

76
Technical and Professional Committee: Leadership Activities

Academy of Operative Dentistry

N. W. Rupp, Past President

Alpha Chi Sigma

G. M. Brauer, Member, Awards Committee

American Academy of Gold Foil Operators

N. W. Rupp, President Elect

American Association for Dental Research/Inter national Association

for Dental Research

R. L. Bowen

Teller

W. E. Brown

Chairman, Biological Mineralization Research Award Committee

American Association of Dental Research - Washington Section

J. A. Tesk, President

American Chemical Society

G. M. Brauer

Member, Meetings and Expositions Committee

Councilor

American Dental Association

R. L. Bowen

Member, Biomedical Research Support Grant Committee

W. E. Brown

Member, Biomedical Research Support Grant Committee

American National Standards Committee, MD 156, Dental Materials,

Instruments, and Equipment

R. L. Bowen

Secretary, Subcommittee on Biological Evaluation of Dental

Materials

Member, Subcommittee on Direct Filling Resins

G. M, Brauer

Chairman, Subcommittee on Denture Resins

J. A, Tesk

Chairman, Dental Materials, Instruments and Equipment

Secretary, Dental Implants

Chairman, USA TAG for ISO TC 106

American National Standards Institute (ANSI)

J. A. Tesk

Member, Medical Devices Standards Management Board

77
American Physical Society

I. C. Sanchez

Chairman and Organizer: Introduction to Theoretical Polymers

Physics" Short Courses to be presented prior to the 1986 March

Meeting of the APS

G. B. McKenna

Division of High Polymer Physics

Member: Publications Committee

American Society of Mechanical Engineers

R. W. Penn

Member, Reinforced Thermoset Plastic Corrosion Resistant

Equipment

American Society for Testing and Materials (ASTM)

G. M. Brauer

Member, Executive Committee, F-4 Medical and Surgical Materials,

and Devices

Chairman, Polymeric Materials Section

Member various task forces

Member, TAG/TCI 50

C. M. Guttman

ASTM E-37 * Subcommittee on Thermal Analysis

Chairman, Task Group on Purity Standards and Methods

Committee on Research & Technical Planning

E. Passaglia

Member: Subcommittee on Research Chairman

D-11 Rubber and Rubber-Like Materials

G. B. McKenna, Member

H. L. Wagner

Member: D20:70.05: Molecular Weight Parameters,

Section Chairman

D-2 Petroleum Products and Lubricants

L. J. Zapas

Member: D2.07 Flow Properties

D2.07A Newtonian Viscometry

E-9 Fatigue

G. B. McKenna, Member

Chemical Society of Washington

J. M. Antonucci

Member, Awards Committee

Member, Board of Managers

78
G. M. Brauer

Chairman, Councilors Committee

Chairman, Publications Committee

Member, Executive Committee

Member, Public Relations Committee

Member, Long Range Planning Committee

Conference on Transport and Relaxation Processes in Random Materials

R. J. Rubin, Co-Chairman

Experimental NMR Conference

D. L. VanderHart, Member, Executive Board

International Association for Dental Research, Dental Materials Group

G. M. Brauer

Member, Sauder Award Committee

International Standards Organization

TCI 06, Dental Materials and Devices

N. W. Rupp

Member, Restorative Materials

Chairman, WG.01, U.S. Delegation

Chairman, TG7, Alloys for Dental Amalgams

J. A. Tesk

Chairman, TAG

Macromolecules

E. A. Di Marzio

Member: Editorial Advisory Board

National Bureau of Standards Committee on University of Maryland

Instructional Television System

E. A. Di Marzio, Member

National Bureau of Standards Research Advisory Panel

E. A. Di Marzio, Member

National Center for Small Angle Scattering

C. C. Han

Member: Users Committee

National Health and Medical Research Council of Australia

G. C. Paffenbarger

Consultant

National Institutes of Health

L. C. Chow

Member, Oral Biology and Medicine Study Section

National Science Foundation Advisory Board

Regional NMR Facility at Colorado State University

D. L. VanderHart

79
Office of Science and Technlogy Policy

D. H. Reneker

Executive Secretary, Committee on Materials (COMAT)

Plastics Institute of America

D. H. Reneker

Member: Board of Directors

Polymer Physics Gordon Conference

E. A. Di Marzio, Chairman 1985

Polymers Division Editorial Committee

R. J. Rubin, Chairman

Polymers Gordon Conference (West)

I. C. Sanchez, Vice-Chairman & Chairman

Elect, 1985-86

Society of Plastics Engineers

I. C. Sanchez

Member: Executive Board of the Engineering Properties

and Structure Division

Society of Rheology

G. B. McKenna

Member: Committee on Education

D. L. Hunston

Member: Committee on Education

Member: Site Selection Committee

VAMAS

C. C. Han

Member: Technical Working Party on Polymer Blends

Visiting Committee for Lehigh University

D. H. Reneker

Member, Department of Metallurgy & Materials Engineering

80
Industrial and Academic Interactions

Industrial

A joint program is underway with Hercules, Hexcel, and NASA Langley

Research Center to study delamination in composite materials. Hercules

and Hexcel are providing materials and some fracture surface analysis,

NASA is characterizing the samples, and NBS is determining the

interlaminar fracture behavior. The objective is to establish the

relationship between the properties of the composite and the behavior of

its constituent materials.

The natural gas utilities, materials suppliers, and Gas Research Institute

are cooperating with the Polymers Division to produce reference

polyethylene materials of the sort used in gas distribution systems. The

reference resin, pipes, and joints will be distributed to laboratories

conducting research and tests on polyethylene parts used in gas

distribution, and will provide for more meaniful inter comparisons of

results from different laboratories.

The American dental industries continued to display strong interest in and

support for the dental materials program. The research associate

agreement with Dentsply International was extended for one year, and

another dental manufacturer explored use of NBS user facilities and a research associate arrangement. Several dental manufacturers supplied

special alloys for casting studies, and others provided specimens of new

composites for NBS scientific research. A glass developed at NBS has been

incorporated into a commercial composite formulation, and NBS developed

vanillated cements have been used as the basis for a new line of products.

Plans were made for cooperative work with IBM San Jose Laboratories on vibrational spectroscopy of polyethylene and on scanning tunneling

microscopy of polymer molecules.

Industrial/Academic

A cooperative program has been initiated to investigate the effects of

processing on the network structure of epoxy resins. Texaco and Prof. R.

Stein of the University of Massachusetts are participating in this effort.

Texaco is providing the raw materials and using calorimetry to monitor the cure reactions that occur during processing and NBS is determining the

cure kinetics by IR measurements and characterizing the network structure

from neutron scattering experiments. A postdoctoral fellow at the

University of Massachusetts is working part time at NBS to assist In data

collection and analysis. The objective of the project is to understand

how processing influences the network structure in thermosetting resins.

In cooperation with the Polymer- Science and Engineering Department at the

University of Massachusetts and the Research Department of the Celanese

Corporation, NBS is participating in research on polymer blends under the

sponsorship of the Defense Advanced Research Projects Agency. The primary

goal of this academic/industrial/government research effort is to develop

new polymer alloys which can be used in engineering applications at high

81
temperatures ( 300“iJ50°C ) . The effort at NBS is focussed on relating the

thermodynamics and kinetics of phase separating blend systems to physical

properties

.

Joint research among scientists at the University of Pittsburgh, Exxon,

and NBS is aimed at characterizing the viscoelastic behavior of cyclic

macromolecules in the melt. Exxon scientists are providing the samples,

and researchers at NBS and the University are conducting the mechanical

properties measurements. The focus of the work is to compare the

viscoelastic properties of cyclic macromolecules with those of the linear

analogs as a test of reptation theories of macromolecules dynamics.

Academic

C. M. Guttman carries out theoretical studies of chain conformation in the

semi crystalline state of polystyrene, in collaboration with Prof. J. D.

Hoffman, University of Maryland and Dr. Z. Kifle, who is now at the

University of Cincinnati but was a Research Associate of the Polymers

Division until February 1985.

F. W. Wang carries out measurements of diffusion in polymeric systems by a photobleaching method in collaboration with Prof. E.-S. Wu, University of

Maryland, Baltimore County.

The failure behavior of toughened polymers is being studied in cooperation

with Dr. S. S. Wang at the University of Illinois and Dr. A. J. Kinloch at

Imperial College. NBS is determining the fracture behavior, the

University of Illinois is conducting finite element analysis studies and

Imperial College is characterizing the morphology, all on the same

multiphase polymer systems. The ultimate objective is the development of

structure - property relationships for such materials.

A joint research program to study the conformation of block copolymers in

phase-separation systems has been established with Prof. J. Koberstein of

Princeton University. As part of this program, a graduate student is

conducting neutron scattering measurements on these polymers at NBS.

A joint program to obtain an analytic solution for the correlation

function associated with neutron scattering by molecular networks such as epoxies is underway with Prof. H. Benoit of CRM Strasbourg)!, France.

Success in this effort would directly benefit other projects using neutron

scattering to characterize network structure.

Five guest workers from domestic (2) and foreign (3) universities are associated with the Dental Materials Group. A cooperative seminar series

was established with the University of Maryland Dental School. Several

finite element modeling studies on dental material-tooth systems are in

progress with the University of Illinois, Chicago. One guest worker from

Georgetown University is a key investigator on bonding of dental resins to

dentin. Guest workers (residents) from the Naval Dental School work with

several investigators within the group.

82
Associated Activities

Invited Talks

Division personnel presented a total of 141 invited talks on research

activities at the following types of organizations and symposia:

industrial laboratories, 17; international symposia, 17; universities, 45;

topical symposia, 26; national and government laboratories, 16;

professional society meetings, 11 and Gordon Conferences, 9.

Conferences Sponsored

The Society of Plastics Engineers (SPE) and the National Bureau of

Standards jointly hosted a symposium on October 15-17, 1984 to discuss

industrial applications of polymer phase diagrams. Phase diagrams are graphic displays that contain data and information about the physical and

chemical states of materials and the thermodynamic transformations between

these various states, or phases. The purpose of this Symposium was to

focus the attention of the polymer scientific community on the utility of

polymer phase diagrams in solving technical problems. The keynote speaker

at the Symposium was Professor Paul Flory, 1974 recipient of the Nobel

Prize in chemistry for his contributions to polymer science. The

Symposium concluded with a workshop discussion, led by a panel of

industrial, government, and academic scientists, that identified the types

of phase diagram information most needed in industrial applications and

suggested directions for future research in polymer phase equilibria.

SRM’s

Three rubber and rubber compounding standard reference materials have been

certified for physical properties. These materials are used by the rubber

industry for process control and to conduct standard ASTM tests.

The certificate for a poly (methyl methacrylate) SRM is being drafted.

This SRM will be used by the polymers industry to calibrate instruments

used to measure the molecular weight and molecular weight distributions of

acrylic type polymers.

83
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U.S.

DEPARTMENT

OF

COMMERCE

National

Bureau

of

Standards
U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

REPORT NO.

November 1985

SHEET (See instructions) NBSIR-85/3190

4. TITLE AND SUBTITLE

Polymers: Technical Activities 1985

5. AUTHOR(S)

L. E. Smith and B. M. 'Fanconi

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s) NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

S. Type of Report & Period Covered

Annual Report

10/1/84 - 9/30/85

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. Stote. ZIP)

National Bureau of Standards

Institute for Materials Science 6 Engineering

Polymers Division, Gaithersburg, MD 20899

10. SUPPLEMENTARY NOTES

~| Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual Summary of most significant Information. If document Includes a significant

bibliography or literature survey, mention it here)

Technical Activities of the Polymers Division for FY 85 are reviewed. Included

are descriptions of the 6 Tasks of the Division, project reports, publications,

and other technical activities

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; a nd separate key words t, i

r

National Bureau of Standards; Polymers Division; Reivew; Technical Activities

13. AVAILABILITY

1 x I Unlimited

1 1 For Official Distribution. Do Not Release to NTIS

I | Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

|X | Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

102

IS. Price

$ 1 6 . 9

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