Reference

NBS

PUBLICATIONS AlllOb 034333

NBSIR 85-3197

Molecular and Microstructural

Factors Affecting Mechanical

Properties of Polymeric Cover

Plate Materials

Elizabeth J. Clark

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Gaithersburg, MD 20899

July 1 985

Prepared for: U.S. Department of Energy

Office of Solar Heat Technologies —QC—

ating and Cooling Division

j qq

)n, DC 20585

. U56

85-3197

1985

NATIONAL BUREAU OF STANDARDS

LIBRARY

NBSIR 85-3197

MOLECULAR AND MICROSTRUCTURAL

FACTORS AFFECTING MECHANICAL

PROPERTIES OF POLYMERIC COVER

PLATE MATERIALS

Elizabeth J. Clark

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Gaithersburg, MD 20899

July 1985

Prepared for: U.S. Department of Energy

Office of Solar Heat Technologies

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

Table of Contents

Page

1 . Introduction 2

2. Background 3

3. Aspects of Polymer Physics and Chemistry 4

3.1 Polymer Molecules 5

3.2 Bonding 7

3.3 Solidification ...... 7

3.4 Glass Transition 10

4. Polymeric Cover Plate Materials 12

5. Deformation and Mechanical Properties of Polymers 16

5.1 Polymer Deformation and Fracture 16

5.2 Mechanical Properties 23

6. Effect of Microstructural and Molecular Factors on Mechanical

Properties 27

6.1 Molecular Weight 28

6.1.1 General 28

6.1.2 Effect of Molecular Weight on Mechanical Properties . . 30

6.2 Crystallinity 35

6.3 Crosslinking and Branching 40

6.4 Copolymerization 41 6.5 Plasticizers 41 6.6 Orientation 42

6.7 Residual Stresses 45

7. Degradation of Polymeric Cover Plates 46

8. Discussion 52

8.1 General 52

8.2 Polymer Microstructure 52

8.3 Methods to Study Polymer Degradation and Deformation .... 55

9. Summary 57

10. References 58

Abstract

This paper reviews the dependence of mechanical properties of polymer

on various microstructural factors. The microstructural and molecular

factors considered are: molecular weight, crystallinity, crosslinking,

branching, copolymerization, plasticization, orientation, and residual

stresses. The types of mechanical properties considered are: direct

loading, fatigue, creep, wear and abrasion, and environmental stress

cracking. The effects of polymer deformation and fraction at the

molecular level are discussed. Cracking, crazing, and shear yielding are

described. Polymeric cover plate materials are discussed and their

degradation reviewed. Methods to measure microlevel changes in polymers

are identified.
1 . Introduction

In solar collectors the cover plate simultaneously serves as a window

for the sun and a structural component of the collector, thus placing

prime importance on the optical and mechanical properties of the material.

Polymers have long been used in applications requiring transparency,

strength, and durability, for example, as windows, in greenhouses, and as

jet plane canopies, so it is natural to consider them in collector glazing

applications. Polymeric cover plate materials may play an important role

in the future development of solar energy collectors if they can be shown

to have adequately long service life. Use as solar collector cover plates

imposes more severe demands on polymeric materials than most past

large-scale uses because degradation may occur when polymers are used as

collector glazings. Possible advantages of polymeric, rather than

inorganic glass cover plates are: higher transmittance, better impact

resistance (from some materials), lower cost, easier processing, lighter

weight, and greater design flexibility. On the other hand, polymers are

not as inert to their environment as the inorganic glasses they might

replace.

The most significant difference between solar collector cover plates

and other polymer applications requiring transparency, strength, and

durability, is that cover plates are exposed simultaneously to elevated

temperatures and natural outdoor weather. Optical and mechanical

properties of polymers can be reduced by exposure to elevated temperatures

or to outdoor weathering, and simultaneous exposure to both can lead to

loss of solar transmittance and embrittlement of the polymer. Service

life prediction of polymeric cover materials is difficult because

knowledge of the relationships between polymer use, composition,

morphology, and long term behavior are not well enough developed. As

2
knowledge in polymer structure-property relationships is gained, and as

correlations are established between weather factors and polymer

degradation, service life prediction will improve. Past research on cover

plate materials has emphasized transmittance changes as a measure of

polymer degradation. This paper reviews the dependence of polymer

mechanical properties on their composition and microstructure and shows

the relationship between cover plate polymer degradation and

microstructural changes.

2. Background

Mechanical properties are associated with resistance of a material to

deformation under load. The mechanical properties of polymers are complex

due to their viscoelastic nature which is demonstrated by a strong

dependence of the mechanical properties on time and temperature [1]. This

is a critical consideration for cover plate materials which encounter

extended use at temperatures above ambient. Failure, in the sense of

mechanical properties, occurs when the material breaks, tears, cracks,

flows, or crazes.

The mechanical properties of polymers are dependent on many factors

including microstructure, chemical composition, residual stresses, flaws,

processing methods and history. Mechanical properties are properties of

the bulk material, but they reflect phenomena occurring at the microscopic

scale down to the molecular level. Polymer degradation begins on a

molecular level with alteration of the polymer molecules and generally is

accompanied or followed by changes in morphological properties (e.g.,

crystallinity or orientation) at the microscopic level. As degradation

continues, the molecular and microscopic changes manifest themselves as

macroscopic changes, e.g., changes in strength, elongation, and hardness.

Measures of the microscopic level changes which occur earlier in the

3
degradative process than macroscopic types, may be Important as early

degradation indicators. For service life prediction, it is desirable to

have diagnostic tools able to detect and measure early changes which are

clearly precursors in degradation processes. Since mechanical properties

are so dependent on structural properties, monitoring microstructural

changes should give an indication of mechanical property deterioration.

Understanding the relationships between structure and properties of

polymers has received increased emphasis in recent years but is not as

well-developed as for metals and ceramics. There remains a major

technical challenge to develop a basic understanding of the relationships

between structure and end use performance and the scientific and

technological framework needed for improving the durability of polymeric

materials. As insight develops into the influence of microstructure on

mechanical properties, it permits better choice of materials, improved

processing, and an improved ability to predict if a material will meet the

requirements of the application. It has been recognized that additional

polymer research is needed to determine accurately the service life of

commercial polymers used as glazing materials and to develop new polymers

with long term durability under the exposure conditions of solar

collectors [2,3].

3. Aspects of Polymer Physics and Chemistry

To understand the mechanical properties of polymers, it is necessary

to appreciate the nature of polymers. The two important considerations

are the types of molecules and bonds which in turn influence the molecular

morphology of polymers. These factors, along with the chemical

composition control the solidification, flexibility, strength, and

stability of polymers.
3 . 1 Polymer Molecules

Polymer molecules consist of very long chains of covalently bonded

atoms containing multiple sequences of a chemical structure, this

repeating unit being unique for each polymer. Polymer properties are

dependent on chemical composition and physical arrangement of the long

molecular chains in space. The simplest polymer chain, polyethylene, is

made up primarily of covalently bonded carbon atoms where the side

structure on these carbon atoms is a hydrogen atom,

(i.e. -$-<?-).

H H

Polymers vary by having other atoms or molecules substitute for C in the

main chain backbone or H as the side attachment. The flexibility of

polymer chains depends on the atoms in the chain backbone, the size of the

side groups, bond angles, the ease of rotation about the bonds, and bond

energies. Bulky side groups make rotation and packing more difficult and

may reduce structural regularity and interchain bonding. Ring structures

in the main chain increase rigidity. Oxygen in the main chain provides

greater ease of rotation and increases flexibility [4]. The flexibility

of the bulk polymer is a direct result of the flexibility of the polymer

chain on a microscopic level.

The structural repeat unit of several common cover plate polymers is

shown in table 1. Poly(vinyl fluoride) (PVF) and poly( vinylidene

fluoride) (PVDF) have the most simple repeat unit with two C atoms in the

backbone and F substituted for some of the side H atoms. Fluorinated

ethylene propylene (FEP) is a random copolymer consisting of 80-90 mol# of

the -C

2

- Fij- units and 10-20 mol# of the -CF2 -CF-CF

3 -units. Poly( methyl

methacrylate) (PMMA) has two C atoms in the main chain but bulky side

5
Table 1

Common

Cover

Plate

Polymers

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in 6

Fiber

reinforced

polyester

crosslinked
groups. The repeat units of polycarbonate (PC) and poly(ethylene

terephthalate) (PET) are large and oontaln a number of atoms in the main

chain including both benzene rings and oxygen atoms.

3.2 Bonding

The strength and mechanical properties of polymers are influenced by

the bonding between the molecules and the three-dimensional nature of the

bulk material. Several types of bonding exist within polymers: chemical

bonds between atoms within an individual polymer chain; secondary bonding

( intermolecular forces) between adjacent molecules (e.g., London

dispersion forces, induced polar-dipolar forces, and dipolar forces

including hydrogen bonding); and physical entanglements of the long chains

with adjacent chains [1],

3.3 Solidification

Most polymeric materials used as cover plates are fabricated by

casting or extrusion of the molten material. The microstructure obtained

on solidification is dependent on the polymer composition, rate of

cooling, and stresses or orientation resulting from processing. The

solidification of a linear polymer from the liquid gives either an

amorphous solid or a partly crystalline one. In amorphous polymers the

polymer chains are disordered. Semicrystalline polymers contain regions

having both crystalline and amorphous material.

The crystallization behavior of polymers is unusual and results in a

complex morphology due to the chain-like structure of the polymer

molecules. Polymers crystallize by their constituent chains folding back

and packing parallel to one another into thin platelets, as shown in

figure 1 . As a crystal grows, the platelet thickness remains fixed while

7
arrrarp’nous

region

1 . Semicrystalline polymer illustrating chain folded crystals, tie

molecules, and the amorphous regions.

the other two dimensions grow very large in comparison. The thickness of

polymer crystals is on the order of 50-200 A [5]. This plate-like crystal

structure is a major difference between crystals from polymers and

nonpolymeric materials. Most polymers crystallize only in randomly

distributed crystallites of microscopic size.

Polymer crystals are surrounded by amorphous material but are

connected by polymer chains that loop out of the crystal through the

amorphous regions and participate in other crystallites. These tie chains

give strength to the material by providing physical connections between

the crystallites which hold the structure together.

The melting point, T

m> of a semicrystalline polymer is one of the most

important characteristics governing its usefulness. Tm

is influenced by a

number of characteristics of the repeating unit, the most important of

which are: (1) structural regularity, (2) bonding flexibility,

(3) close-packing ability, and (A) interchain attraction. High melting

points are generally associated with either highly regular structures,

rigid molecular structures, or a combination of these [4], For example,

8
PC has a high melting temperature, which is related to the molecular

stiffness of the backbone which has two benzene rings.

A feature in the structure of amorphous polymers which affects

mechanical properties is entanglements. The coiling motion of the polymer

chains in the melt causes them to become entangled. However, a critical

molecular length is necessary before the entanglements affect flow. Once

chains are long enough to become entangled, flow becomes much more

difficult because forces applied to one polymer chain become transmitted

to and distributed among many other chains. Below the critical molecular

weight, Mw> polymer viscosity is roughly proportionately to the weight

average molecular weight, Mw . Above the critical molecular weight,

polymer viscosity is related to Mw raised to about the 3.4th power [6].

The relationship between flow viscosity and 1^ is illustrated in figure 2.

For most polymers, the critical molecular weight is between - 10,000 and

40,000 [6]. Since the viscosity increases as the molecular weight

increases, more energy is required to process high molecular weight

polymers. Thus an optimum range based on processability is usually

selected for commercial polymers.

2. Flow viscosity as a function of molecular weight.

9
3.4 Glass Transition

Most polymers have a characteristic glass transition temperature, Tgf

above which the thermal motion of the polymer molecules is increased. This

is important in cover plate applications since mechanical properties of

polymers show profound changes in the region of the glass transition

temperature. For this reason, knowledge of the T_ can be vital in ©

selecting a polymer for applications in which mechanical properties are

important [7]. Above T

g| the continuous vibrational motion of segments of

amorphous chains results in a myriad of different shapes of the polymer

chain and, if the chain is long enough, the motion enables it to form

physical entanglements with itself and its neighbors. Below T gt the

thermal energy is insufficient to activate movement of chain segments [8].

The T

g

is related to chain stiffness and the geometry of the polymer

chain. The practical uses of various polymers depend greatly on their T

g

value. The main requirement for a polymer to serve as a useful plastic at

room temperature is that T

g

for amorphous polymers and Tra , in the case of

crystalline polymers, occur well above room temperature. The T

g

is always

less than T ra

. An approximate empirical relationship of 1/2 < T

g

/Tm < 2/3

(with temperature in K) has been developed [9].

The segmental motion above T

g

is temperature dependent and it is the

basis for polymer viscoelasticity [8] which implies behavior similar to

both viscous liquids and to purely elastic solids. In viscous systems all

work done on the system is dissipated as heat whereas in elastic systems

all work is stored as potential energy as in a stretched spring [7],

The relative importance of the various deformation modes changes with

the temperature, and it is convenient to distinguish four physical states

of polymers, although in practice the divisions are not always clear cut.

Starting at the low temperature end, the states are: (1) glassy,

10
(2) retarded highly elastic (leathery), (3) instantaneous highly elastic

(rubbery), and (4) viscous [10]. In the glassy state ideally there is

only normal elasticity, although in reality some creep will be found.

Above Tg the glassy state gives way to the leathery state where thermal

agitation is sufficient to promote retarded high elasticity and viscous

flow. About 30 degrees above Tg the leathery state merges into the

rubbery state where the motion of molecules as a whole becomes dominant

and where viscous flow rate occurs at an increased rate. Finally, at

still higher temperatures the viscous flow rate increases to a point at

which it dominates the other deformation modes. Below T the material is ©

*

a hard, rigid glass. As the temperature is increased for high molecular

weight polymers, the material becomes leathery, then rubbery, and finally

liquid. Only the Tg is well marked, the other divisions being somewhat

intermediate due to overlapping of the deformation modes [10]. Polymers

are normally used at temperatures which put them into the glassy or

leathery state. The load versus deformation curves for glassy,

LOAD

Semicrystalline

polymer

Rubbery polymer

ELONGATION

3. Load-elongation curve for glassy, semicrystalline,

and rubbery polymers.
semicrystalline, and rubbery polymers are illustrated in figure 3* One

can observe the change in elongation as the temperature is increased from

below to above T

g#

4. Polymeric Cover Plate Materials

The polymers used most frequently in cover plate applications are

listed in table 1, along with their chemical repeat unit, glass transition

temperature, and melting temperature. A brief description of each

material follows.

Poly( vinylidene fluoride) is a semicrystalline polymer with a degree

of crystallinity ranging from 35~65 percent, depending on thermal history

[11]. The symmetric arrangement of the H and F atoms attached to the C

backbone contributes to the ease of crystallization. Tm

of commercial

films is about 170 °C [11,12], although it depends on the method of

polymerization and/or crystallization history. The T

g

is -45 °C. PVDF is

extruded into films which can be oriented to increase mechanical strength.

Cover plate film is about 0.13 mm thick. PVDF has high levels of

mechanical and impact strength, abrasion resistance, resistance to most

chemicals and solvents, and reportedly has good resistance to sunlight.

Poly(vinyl fluoride) is a tough, flexible semicrystalline polymer

having one fluorine atom on every other carbon atom in the backbone. Since

F has a Van der Waals radius near that of H, the regularity of the main

chain is retained and PVF readily crystallizes. The degree of

crystallinity ranges from 20-60 percent, depending on the polymerization

method [13]. The melting temperature is about 200 °C [13] and the maximum

continuous use temperature is 107 °C [14]. PVF polymer is converted to

film by melt extrusion. As a cover plate it is used in thicknesses of

about 0.1 mm. The extruded film can be biaxially oriented to varying

degrees to improve tensile strength, tear resistance, and impact

12
resistance. Because of the high temperatures needed for processing, heat

stabilizers are added to PVF; however no plasticizers are used [14]. PVF

has exceptional resistance to water adsorption, chemical attack and

unusual solvent resistance. PVF has low solubility in all solvents at

temperatures below 100 °C and is reported to have excellent resistance to

damage from sunlight. However, the early PVF cover plate films discolored

and embrittled [15]. Apparently the combination of elevated temperature

and sunlight caused degradation. Inhibitor systems in PVF have since been

altered.

Fluorinated ethylene propylene (FEP) is a semicrystalline copolymer

produced by copolymerization of tetrafluoroethylene and

hexaf luoropropylene. FEP has a crystalline melting temperature of 290 °C

and a T

g

between -5° and 29 °C [12,16]. The continuous use temperature of

FEP is 204 °C [16]. FEP cover plates are made from extruded film having a

thickness of about 0.025 mm. The crystallinity of FEP after processing

depends on the cooling rate of the molten polymer but can reach 70 percent

[16]. FEP is a relatively soft material with lower tensile strength, wear

resistance, and creep resistance than many polymers. Its strength

decreases with increasing temperature. FEP is chemically inert to

essentially all industrial solvents and chemicals and is virtually

unaffected by sunlight.

Poly(methyl methacrylate) used in cover plates is an amorphous polymer

with a Tg -105 °C. The bulky side groups off the main chain raise the T

g

and decrease the regularity, preventing the polymer from crystallizing.

The maximum recommended use temperature is about 80-85 °C [17] which can

present a problem for certain cover plate applications. PMMA cover plate

materials are made by a casting process. Since it is a glassy material

which is fairly brittle at ambient temperature, the thickness of PMMA

13
cover plates is generally 1-1.5 mm or greater to provide sufficient

strength to span across the collector. Prolonged subjection to even quite

low stresses can produce surface crazing [17]. Differences in temperature

and/or humidity on the inner and outer surfaces in glazing may bow the

sheet in the direction of the higher value. Temperature differences act

rapidly, within a few hours, but recovery is also fast when the gradient

is removed. Differences in humidity act slowly over weeks [17]. Although

PMMA has a history of good resistance to outdoor weathering, UV

stabilizers are added to improve resistance to sunlight [18]. When

cleaning a cover material the chemical resistance must be considered. PMMA

is chemically resistant to many liquids, however aromatic and chlorinated

hydrocarbons, ethers, and ketones attack it.

Poly( ethylene terephthalate) is a thermoplastic polyester which has

varying degrees of crystallinity, depending on its thermal treatment [19].

The T

g

is reported as 69 °C and Tm

as high as 280°, although the Tm

of

most commercial products ranges from 255~265° due to reduced crystallinity

resulting from chemical impurities [20]. In semicrystalline PET both

crystalline and amorphous regions contribute to the overall properties and

a range of degrees of order exist. In the absence of nucleating agents

and plasticizers, PET crystallizes slowly and performs best in

applications where strength and crystallinity can be increased through

orientation which is accomplished by stretching the film and inducing a

high level of molecular chain alignment and extension. Orientation

significantly improves creep resistance, tensile strength, and modulus. If

an oriented film is heated above its T

g

, additional crystallization may

occur; thus PET cover plates may increase crystallinity if the

temperatures exceed 70 °C. Oriented PET has good chemical resistance being

resistant to most organic solvents and mineral acids [21]. Unaged PET is
a tough, strong film, however its properties degrade substantially on

weathering. PET used as a cover plate has a thickness of 0.13 mm and is

made by an extrusion process. PET is particularly susceptible to

hydrolysis which can result in decreases in molecular weight and

associated reductions in mechanical properties [20]. Upon exposure to

sunlight, PET loses elasticity, breaks easily and develops a crazed

surface. It is common for both UV and heat stabilizers to be added to PET

or for UV absorber to be applied on the surface.

Polycarbonate used in cover plates is an extruded polyester based on

bisphenol A. The T

g

is about 150 °C, however, due to the size and

flexibility of the repeat unit and vibrational motion of parts of the

chain, PC does not become brittle until -100 °C. Polycarbonate

crystallizes only after lengthy annealing at 190 °C in the absence of

oxygen and moisture [22]. Thus it is unlikely that cover plates contain

crystalline PC. However, PC can crystallize from amorphous material by

treatment with swelling agents or their vapors. Polycarbonate is soluble

in or swelled by a number of aromatic and chlorinated hydrocarbons and

other chemicals. With solar collectors it may be necessary to assure that

heat transfer fluids and cleaners are compatible with the polymer. PC

also exhibits some hydrolytic sensitivity [23], The heat distortion

temperature is 135-1 *10 °C [22]. Annealing below T

g

leads to an increase

in order and in stiffness [22]. Stresses can be removed by annealing

above 100 °C. PC is a tough, stiff, impact resistant material, however it

is not scratch resistant. Scratch resistant coatings have been developed

[2*1] . Strong sunshine combined with humidity causes superficial yellowing

and embrittlement to fracture. Thus thermal and UV stabilizers are added

to PC [2*1].

15
Fiber reinforced plastics ( FRP) are generally thermosetting polyester

resins with glass fiber added for strength. Due to the highly crosslinked

nature of these thermosets, they have no T

g or Tm< FRP i mpac t resistance

and strength are aided by the presence of chopped glass fibers. As with

most polyesters, FRP materials are generally susceptible to moisture

damage and they are discolored by exposure to heat.

5. Deformation and Mechanical Properties of Polymers

Mechanical properties of a polymer are those properties involving

loading or deformation of the material under the influence of applied

forces. Polymer mechanical properties are complex because of their

viscoelastic nature. Viscoelastic behavior is demonstrated by a strong

dependence of the mechanical properties on time and temperature [1]. A

brief overview of the deformation of polymers and their mechanical

properties is provided here. The reader is directed elsewhere for more

detailed discussions of polymer deformation and mechanical properties

[7,10,24-40].

5. 1 Polymer Deformation and Fracture

When a polymer system is subjected to an applied stress, a number of

chemical and rheological events (often having opposing effects) may occur

at a given temperature. The net effect of the several competing processes

depends on many factors, including the temperature, the time scale of the

experiment with reference to the time scale of the process, the

environment, the basic molecular properties of the polymer, and the

composition [35].

At sufficiently low stresses and strains, the mechanical response is

reversible while at higher stresses and strains an irreversible component

of deformation becomes evident. Thus, during deformation and fracture, a

variety of events takes place, including homogeneous shearing,

16
heterogeneous shearing (shear banding), dilatational processes, and bond

rupture [35]. Polymers exhibit a wide range of elastic properties

depending upon their structure and the testing conditions. Figure 4 shows

the variation of Young's modulus, E, with temperature for an amorphous

polymer. At low temperatures the polymer is glassy with a relatively high

modulus. The modulus falls rapidly through the region of the T where the O

polymer is viscoelastic and the modulus is very rate- and

temperature-dependent . At a sufficiently high temperature the polymer

becomes rubbery. If the polymer is crosslinked the modulus remains

approximately constant with increasing temperature, otherwise the polymer

flows like a viscous liquid [33].

Semicrystalline polymers behave like amorphous polymers below the T

O

of the non-crystalline phase. However, they have interesting and useful

properties between T

g

and Tm where the structure consists of crystals in a

rubbery matrix. The modulus of the material depends upon the degree of

crystallinity and degree of orientation. The modulus values for different

types of polymer are summarized in table 2 [33].

Typical variation of Young's modulus, E, with temperture for a polymer

showing the effect of crosslinking upon E in the rubbery state [33].

17
Table 2

Typical Values of Young's Modulus, E, and Fracture Strength, o it for Different Types of Polymers Measured at Room Temperature and

Moderate Testing Rates [33]

Type of Polymer Modulus

E(MPa)

Fracture Strength

Oi(MPa)

Rubber 1 - 10

Semicrystalline

polymer (above T ) - 100 ~ 30

Semicrystalline

polymer (below Tg) -1000 - 50

Glassy polymer -1000 - 70

Two major deformation micromechanisms of importance in polymers are

shear yielding and crazing. They represent the main source of energy

absorption in the material. Both involve localized, or inhomogeneous,

plastic deformation of the material. The difference between the

mechanisms is that shear yielding occurs essentially at constant volume

whereas crazing occurs with an increase in volume. Thus, unlike shear

yielding, crazing is a cavitation process.

Ductile plastics tend to have a fairly well-defined yield point with

the value of yield strain typically of the order of 5~1 0 percent—much

higher than the value for metals or ceramics. Some semicrystalline

polymers can be cold-drawn to extension ratios in excess of 20 during

which a stable neck extends along the specimen. Most amorphous glassy

polymers are brittle in tension but yield and flow to high strains in

compression or pure shear [ 331 . Shear deformation consists of a change of

shape without significant change in volume. Molecules translate past each

other during shear yielding. The shear yielding of polymers is highly

18
relevant to the mechanics and mechanisms of polymer fracture. Shear

yielding limits the strength of the polymer if brittle fracture can be

suppressed. A polymer must have a high yield stress in order to be strong

and if bulk, homogeneous yielding does occur the polymer is likely to be

tough. Recent evidence suggests that shear yielding, in the form of

microshear bands, plays a key role in the initiation of cracks. Brittle

crack propagation in polymers usually involves localized viscoelastic and

plastic energy dissipative processes occurring in the vicinity of the

crack tip [ 33 H -

A craze is initiated when an applied tensile stress causes microvoids

to nucleate at points of high stress concentrations in the polymer created

by scratches, flaws, cracks, dust particles, molecular heterogeneities,

etc. The microvoids do not coalesce to form a true crack since they

become stabilized by fibrils of plastically deformed, oriented polymeric

material spanning the craze. The resulting localized yielded region

therefore consists of an interpenetrating system of voids and polymer

fibrils.

Molecular entanglements play an important role in controlling craze

geometry. Unlike a crack, a craze is capable of transmitting loads

between its faces. However, when cracks do initiate and grow, they do so

by means of the breakdown of the fibrillar structure in a craze. This

typically results in slow cavity expansion followed by crack extension

through the craze, and finally crack propagation during which a craze or

crazes precedes the crack tip through the remainder of the material [33].

Crazes may initiate, grow and break down to give cracks at stresses below

those necessary to cause bulk shear yielding and so lead to brittle

fracture requiring only a relatively low value of fracture energy, Gj

c> 0r stress-intensity factor, Kj

c [33]. Crazing therefore plays an important

19
role in determining the strength of a polymer. Temperature is important

in determining the balance between crazing and shear response. The

relationship between yield stress, crazing stress, and microvoid

nucleation stress for polystyrene is shown as a function of temperature in

figure 5. Both the crazing stress and yield stress decrease as

temperature rises. More detailed discussion of the mechanisms of crazing

and shear yielding is given in references [ 33 ] and [35].

Whatever the detailed yielding response may be, attainment of the

critical energy for crack propagation depends on the balance between

energy stored and energy dissipated and, hence, on the relative importance

of the various modes of deformation and their consequences in terms of

strength of the molecular aggregations that oppose the crack's progress

[35]. Many possible combinations of deformations and defects may exist

thus making mechanistic correlations difficult. While fracture mechanics

may serve to characterize a material, a given level of behavior may arise

from a multiplicity of different competing molecular events [35]. The

5. Relationship between yield stress, crazing stress, and

microvoid nucleation stress in polystyrene [33].

20
X

///

6. General deformation for a polymer: stress-strain curve as observed in one material as a function of temperature of rate of strain, or for a wide variety of materials. (I) Hookean elasticity; also linear

viscoelasticity; (II) nonlinear viscoelasticity; (III) yield point;

(Y) yield stress; (H) hysteresis of return curves; (N) necking region;

(IV) beginning of strain hardening; (S) set, permanent deformation,

incipient failure; (R) rupture; (T) tearing; (V) plastic flow; (ER)

partial elastic recovery, elasticoviscous, or plasticoelastic ; (VI)

strain hardening and rupture; (DR) ductile rupture (VII) ductile

failure [35].

general deformation of a polymer in simple tension is illustrated in

figure 6, and as shown, the response may be quite complex. The events

shown have been described by Hertzberg and Manson [35] as follows.

"At first (Region I) bonds are stretched and energy is stored in a

quite reversible manner (Hookean elasticity, linear viscoelasticity). As

the stress rises further, the response (Region II) becomes nonlinear;

energy is still recoverable on release of the stress, but slowly

(anelastic response). Eventually, yielding occurs (Region III),

accompanied by the onset of components of irreversible deformation. (This

is not to say that no reversible component exists in this region; up to

some point, even crazes may exhibit a high degree of reversibil ity .

)

During such deformation, extensive cooperative sequential motions take

21
place, and energy is, of course, dissipated in a viscous manner. The

nominal stress, then, usually drops somewhat (Region IV) and extension

frequently proceeds easily, often with the formation of a neck that

continuously undergoes drawing. Fracture (Region V) may occur shortly

after necking begins or, especially with crystalline polymers such as

polyethylene, fracture may be delayed due to an orientation of the

molecules (analogous to strain hardening in metals) and the stress may

actually increase, as shown in Region VI.”

Polymer ductility decreases as the stress field becomes increasingly

severe. Ductile/brittle transitions are due to competition between a

brittle fracture mechanism such as crazing and a ductile mechanism such as

bulk shear yielding. Fracture originates from local concentrations of

flaws, scratches or notches. All materials contain such inhomogeneities

which can propagate to cause failure. Crack propagation occurs when the

material can deform or flow no longer, i.e. when the limit of non-fracture

deformation mechanisms is reached. The fracture stress, o f> can be

related to the crack length, a, by using a modified Griffith fracture

equation [33] where

where G

c is the fracture energy and E is Young’s modulus. The fracture

energy is the energy required to form a unit area of crack. A material's

fracture strength may be characterized as the energy per unit area

required to produce new surfaces. Polymers tend to have lower fracture

strengths than metals and ceramics; typical values of fracture strength,

for different plastics are given in table 2. As with Young's modulus,

there is a wide variation. The concepts of fracture mechanics provide a

sound basis on which to describe polymer fracture. In some areas, such as

1 /2

for plane stress

22
alow crack growth in glassy polymers, there is quite a bit of information,

while in other areas, such as environmental crack growth and fatigue there

is little. Ductile failure is the most difficult to characterize [40].

The experimental methods used to characterize crack growth generally

employ quite large flaws or cracks and it is easy to use the data to

predict what will happen when similarly large defects are present.

However, when the flaws present are small, it is more difficult.

Information is scarce and this area will require more attention in the

future. The theories of molecular fracture were originally developed

without any direct evidence that molecular fracture actually occurs.

Techniques are now available to study molecular fracture directly. The

polymer fracture process can be broken down into three distinct steps:

1. The excitation of bonds under the action of an applied stress;

2. The scission of the excited over-stressed bonds by thermal

fluctuations; and

3. The formation of small submicrocracks or crazes which coalesce to

form larger cracks [33].

Experimental techniques are now available to study all phases of fracture.

The most widely employed technique has been electron spin resonance

spectroscopy (ESR) which can be used to follow scission. It allows the

direct observation of free radicals produced when covalent bonds are

broken. It is an extremely powerful technique which allows the number and

the chemical nature of the radicals to be determined. Other experimental

techniques have included infra-red and Raman spectroscopy, small-angle

x-ray scattering, and the measurement of molar mass variations [33].

5.2 Mechanical Properties

The mode of fracture is related to the manner in which the load or

force is applied to the polymer. There are at least five conditions of

stress and environment that cause fracture. The five conditions, which

23
correspond to various types of mechanical tests, are: (1) direct loading,

(2) fatigue, (3) creep, (4) wear and abrasion, and (5) environmental

stress cracking [31]. Direct loading involves applications of a stress

until fracture is produced. Tensile, compression, and Impact tests are

examples of direct loading. Tensile and compression tests determine the

capacity of a material to withstand static load [42]; they are performed

as stress-strain tests which provide information on the modulus

(stiffness), strength, and elongation of the polymer. A sample tensile

stress strain curve is shown in figure 6.

Toughness is an indication of the energy a material can absorb before

breaking. Ductile materials that cold draw are very tough due to the

large elongation to break, whereas brittle materials have low toughness.

Tensile properties of brittle materials are greatly influenced by flaws

and submicroscopic cracks. Tensile properties of several cover, plate

materials are listed in table 3 in the order of their tensile strength

[41].

Impact resistance is a measure of toughness under shock loading [42].

Impact tests measure the energy necessary to fracture a material by an

impulse load and provide an indication of notch toughness or notch

sensitivity resulting from internal stress raisers. Two important factors

which enter into impact behavior are the energy needed to initiate a crack

and the energy required to propagate a crack.

Fatigue is a measure of lifetime under cyclic loading [42]. It is a

function of stress, consequently as one increases the maximum stress per

cycle, the number of cycles to failure decreases. Fatigue fracture occurs

after many cycles even though the maximum stress applied during a cycle is

less than would be required to produce fracture under direct loading.
Table 3

Tensile Properties of Several Cover Plate Polymers [ 41

]

Polymer Tensile Strength

MPa

Elongation

at Break

Tensile Modulus

GPa

Poly(vinyl fluoride) 48-103 100-190 2.

Poly( vinylidene fluoride) 48 300 1.4

Poly( methyl methacrylate) 55-76 2. 0-7.0 2. 4-3.

4

Polycarbonate 55-66 60-100 2.4

Below some value of maximum stress, known as the fatigue or endurance

limit, the fatigue curve levels off, and the material can be subjected to

many cycles without failure [25]. Fatigue life generally decreases as the

temperature increases and the frequency of oscillations increases. Fatigue

failure is usually due to the gradual growth of cracks. Materials

generally contain flaws which develop into submicroscopic cracks as the

stress exceeds some critical value. The cracks grow a small amount during

the peak load of a cycle until one or more of them exceeds microscopic

dimensions. Fatigue life of polymers is also related to their mechanical

damping and the resulting heat build-up in the material, if the heat

produced is greater than the rate of heat dissipation. The strength of

polymers generally decreases with temperature and crack growth becomes

easier.

The dimensional stability of polymers is measured by creep and stress

relaxation tests [7]. Creep tests measure the continuing deformation of a

material with time as the load is held constant. Creep is a function of

stress and temperature. Stress relaxation is the time dependent decrease

in the stress of a material which is constrained to a constant

25
deformation. Where viscous flow occurs the stress can decay to zero at

sufficiently long times, however where there is no viscous flow the stress

decays to a finite value.

Wear is the unwanted progressive loss of substance from the surface of

a material brought about by mechanical action from the rubbing of one

surface against another [M3]. Abrasion, wear, and scratch resistance are

all similar and are closely related to friction. The ploughing component

of frictional force is especially important in abrasion and in the process

of scratching a surface. It is similar on a microscopic scale to tearing

[25]. Small particles of the material are torn or broken off the wearing

or abraded surface.

Environmental stress cracking (ESC) is induced by the interaction

between a polymer and a nonsolvent in the presence of mechanical stress.

ESC occurs when crack or craze growth is promoted by the presence of an

environment and a stress. The major part of the energy required for

fracture is provided by chemical or physical-chemical processes, and the

applied stress plays a secondary, though essential role [31]. Internal

residual stresses may be sufficient to induce stress cracking in the

absence of an external stress. The exact mechanism of ESC is not known,

but the most likely mechanism appears to be that the presence of stress

and a crack or flaw results in a local microvoided zone or craze at the

crack tip and it is the interaction within this zone which gives enhanced

crack growth. Fatigue and deterioration of polymers are frequently

attributable to ESC.

26
6.

Effect of Microstructural and Molecular Factors on Mechanical

Properties

The deformation of a polymer is dependent on internal material

characteristics as well as external environmental factors. The following

microstructural and molecular factors influence the mechanical properties

C73-

1 . Molecular weight

2. Crystallinity and crystal morphology

3. Crosslinking and branching

4. Copolymerization

5. Plasticization

6. Molecular orientation

7. Residual stress.

Their effects on mechanical properties are discussed below. In addition

to these, the following environmental or external variables are important

in determining mechanical behavior [7]:

1 . Temperature

2. Type of deformation (shear, tensile, biaxial, etc.)

3. Time, frequency, rate of stressing

4. Pressure

5. Stress and strain amplitude

6. Heat treatment or thermal history

7. Nature of surrounding atmosphere

8. Weathering.

The effect of temperature was discussed briefly in Section 3.4. The type

of deformation will be considered below. The other factors will not be

addressed.

27
6. 1 Molecular Weight

6.1.1 General

The molecular weight and molecular weight distribution of a

polymer have a large effect on mechanical properties. Commercial polymers

contain molecules having a broad distribution of chain lengths. The

molecular weight distribution (MWD) of polycarbonate is shown in figure 7.

The curve labeled o month illustrates the MWD for an unweathered

material. The width and shape of the distributions are established during

processing. The molecular weight is typically reported as an average

value which obscures any knowledge of the distribution. The width and

shape of the distribution are important because some mechanical properties

are governed by the number of short molecules while others are dependent

on the long chains. In general, as the molecular weight increases, the

span of the molecular weight distribution increases. The narrower the

molecular weight distribution curve, the more rapid is the increase in

7. Molecular weight distribution of the irradiated surface layer of

polycarbonate at various periods of exposure (0 to 42 months) to

outdoor weathering [44].

28
strength with molecular weight. Polymers of interest have molecular

weights that range from 10^ to 10^ and contain lengths (i.e., of the fully

extended molecule) that range from 0.1 to 10 jjm [8].

It has been known for some time that the mechanical properties of

linear polymers are dependent on molecular weight in the sense that the

polymer does not develop any significant strength until a certain critical

value of molecular weight is reached where entanglements occur. Molecular

entanglement must take place before a polymer can carry a load. Above

this critical molecular weight, mechanical properties increase until a

second critical value of molecular weight is reached beyond which the

dependence of mechanical properties on average molecular weights is

greatly reduced or nonexistent [10]. This point is illustrated in

figure 8. The exact shape and slope of the curve varies for different

properties, as does the critical minimum molecular weight. The limiting

value at high molecular weights will vary, depending on the specific

polymer. Polar polymers and polymers with hydrogen bonding chains reach

their maximum property values at lower molecular weights than do nonpolar

MOLECULAR WEIGHT

8. Relationship between molecular weight and polymer properties.

29
polymers, For most thermoplastic materials, moderate changes in molecular

weight do not appreciably influence mechanical properties such as yield

stress or modulus. Thus, they must be toward the upper region of

figure 8. On the other hand, properties relating to rupture such as

ultimate elongation, impact strength, and ultimate strength are affected

by moderate molecular weight increases. These must be toward the lower

region of figure 8.

6.1.2 Effect of Molecular Weight on Mechanical Properties

An overview of the relationships between molecular weight and

mechanical properties is provided here. A more comprehensive review is

contained in references 45 and 46.

The molecular weight and molecular weight distribution of the

polymer are probably the most significant microstructural features

influencing deformation. The relationship between molecular weight and

tensile or impact properties has been investigated for many polymers. The

general conclusions reached are that the tensile and impact strengths and

elongation at break increase to limiting values at high molecular weight.

9. Elongation at break versus molecular weight for polycarbonate [46].

30
10. Tensile stress at break versus peak molecular weight of polycarbonate

subjected to outdoor and artificial weathering conditions [44].

Yamasaki and Blaga [44] studied polycarbonate glazings and found that

ultimate tensile stress and elongation at break decrease as molecular

weight declines. Gardner and Martin [47] also obtained similar results.

Figure 9 illustrates the relationship between molecular weight of

polycarbonate and elongation at break while figure 10 shows the dependence

of tensile stress on molecular weight.

Lee and Turner [48] found the density of entanglements for PMMA

becomes significant only above a MW of 1 x 10 2

* but eventually levels off

above 1 x 10^. Stress-strain properties of semicrystalline polymers such

as polyethylene and polypropylene also depend on MW; however, this

dependence seems to be less apparent because the crystallites help hold

the material together, in much the same way as chain entanglements.

Margolies [49] and Perkins et al. [50] examined high density polyethylene

( HDPE) and found that the tensile strength increased with MW to 1.5 x 10^,

then leveled off. However Margolies found that the ultimate tensile

elongation increased to a maximum at 5 x 1 0^ to 7.5 x 1 0^ but then

31
decreased when the MW increased further. He concluded that the lower

elongation values resulted when the ultra high molecular weight chain

entanglement increased to a point where straightening was substantially

inhibited prior to break. He also proposed that an increase in

intermolecular forces between very high MW molecules became significant.

Margolies also studied impact resistance of HDPE and found that

Izod impact strength increased to a plateau then decreased gradually above

MW = 3 x 1C>6. This is illustrated in figure 11. Apparently, the forces

mentioned above that affect ultimate tensile elongation also affect impact

strength.

Matsuoka and Wang [51] found that the MW of a polymer affects how

much stress the material will be able to sustain beyond the yield point.

Experimentally they found that the critical value for MW above which a

polymer begins to sustain stress beyond the yield point is about equal to

the critical MW for entanglement in melt rheology. In recent years the

importance of the fatigue behavior of polymers has been acknowledged and

work in this area is expanding. Little work has been done to explore the

dependence of fatigue life and fatigue crack propagation (FCP) on MW [34].

From what has been done, it seems fatigue life increases as molecular

11. Izod impact of HDPE versus molecular weight [49].

32
weight increases. Pitman and Ward [52] found a substantial decrease in

resistance of PC to FCP as the MW of the polymer decreased. Kim et al.

[53] found that FCP rates of PMMA were dependent on MW even when fracture

toughness changed little. Typical FCP curves showing the effect of MW of

PMMA are shown in figure 12.

Michel et al. [54] have recently proposed a simple theory to

explain the strong influence of MW on rates of FCP in amorphous polymers.

They propose that FCP rates be expressed as the product of two functions,

one involving the stress intensity factor (AK) and the other

characterizing the relaxation process occurring in the plastic zone. To

provide a physical network in the plastic zone that will sustain fatigue

loading, they propose that one needs a sufficient fraction of molecular

fibrils per unit area (W*) whose lengths are greater than M

c> the critical

value of MW required for entanglement. They have correlated FCP data for

PMMA using this approach. The effect of MW on fracture toughness is also

of interest. From figure 13 it can be seen that values (converted to

12. Typical FCP curves showing the effect of MW on fatigue crack

propagation in bulk polymerized PMMA [53].

33
13. Effect of molecular weight on fracture energy of PMMA. Band indicates

the range of values [53].

fracture energy) for all types of polymers are reasonably consistent. This

also shows that fracture energy is fairly insensitive to MW above

M

v

- 10 5 .

The FCP response of some crystalline polymers such as PVDF is

superior to that of glassy amorphous polymers such as PMMA. The

substantial molecular reorientation, as evidenced by stress whitening at

the fracture surface, may be important to the improved FCP response of

these crystalline materials [55]. For example, both PMMA and PC are

amorphous and have similar elastic moduli, however, PMMA has a FCP growth

rate 1300 times faster than found in PC [54].

Creep and stress relaxation are almost unaffected by changes in

molecular weight below T

g

because the motions of the large segments in the

polymer chain are frozen in place. Above T

g

the behavior is strongly

dependent on MW because the mechanical response of such materials is

determined by their viscosity and elasticity resulting from chain

entanglements. Bueche [56] found the creep compliance of PMMA is

insensitive to MW in the glassy region, however at higher temperatures

and/or longer times, creep and creep rate Increase as the MW decreases.

34
Apparently little has been done to study directly the

relationship between wear or abrasion resistance and MW. However, since

abrasion involves tearing away of small pieces of material, the tensile

strength and fatigue life are probably important factors. Both of them

are improved by higher MW.

Environmental stress cracking (ESC) is not well understood and

little has been done relating it to MW. ESC behavior is dependent upon

the molar mass of the polymer, with lower molar mass material having the

worst ESC resistance. Herman and Biesenberger [57] studied the

relationship between molecular weight distribution (MWD) and ESC in

annealed PE and found that a narrow MWD shows superior stress crack

resistance to a polymer of similar average molecular weight but with a

broader MWD. They conclude that the greater amount of low molecular

weight material and a high molecular weight tail give rise to voids and

stresses causing low ESC resistance in broad distribution resins. Devries

et al. [58] found an 18 percent decrease in the MW of PET exposed to 0^

for four days.

6.2 Crystallinity

The mechanical response of a semicrystalline polymers is related to

the crystalline regions, the amorphous regions, and the tie chains which

join the lamellar regions into a mechanically integrated structure. Tie

molecules carry a high level of stress and are broken during deformation

and fracture. The behavior of tie molecules may be compared with

entanglements in amorphous thermoplastics—both are responsible for the

strength of the polymers and without them both types of polymer are

extremely weak. Figure 1 illustrates the crystalline/ amorphous matrix of

a semicrystalline material. A given polymer's tendency toward being

crystalline or amorphous is determined by the chemical structure. The

morphology of semicrystalline polymers and their degree of crystallinity

35
have profound effects on their mechanical behavior. Since these factors

can be varied over a wide range, the mechanical properties of

semicrystalline polymers take on a perplexing array of possibilities [7].

Polymers do not attain complete crystalline order and the crystallization

rate varies widely among polymers.

Crystallinity is generally favored by symmetrical chain structures

that allow close packing of the polymer units to take advantage of

secondary forces which are distance related. High interchain (secondary

forces) interactions also favor crystallinity. The extent of the

crystallization is limited by irregularities in the molecular structure of

the polymer. Examination of the repeat unit of cover plate materials in

table 1 illustrates these points. PVF and PVDF, which have fairly

symmetric chain structures, are fairly crystalline, whereas in PMMA and PC

the molecular structure is far less regular.

The degree of crystallinity is also affected by the rate at which the

polymer melt is cooled during fabrication. If it is cooled rapidly, the

polymer may solidify without reaching maximum crystallization.

Consequently, it may continue to crystallize at a very slow rate if the

temperature is increased. Such a situation could occur on a solar

collector where the cover plate is exposed to temperatures elevated above

ambient. Crystallinity can also be increased in polymers by mechanically

stretching to align and crystallize the polymer chains.

Unlike amorphous polymers the application of fracture mechanics to the

fracture of crystalline polymers is not very straightforward even for

isotropic materials. This is principally because crystalline polymers are

not normally brittle and their deformation can be non-linear but it is

also because there is often large-scale yielding in the vicinity of the

36
crack tip. Hence it is generally impossible to apply linear elastic

fracture mechanics unless the polymer is at low temperature or has been

embrittled through environmental action [333*

Isotropic semicrystalline polymers deformed at temperatures above the

T

g of the amorphous phase are usually ductile, and they yield and flow

forming a neck with the specimen often drawing up to 30 times its original

length. Figure 6 illustrates the stress-strain curve of a typical polymer.

The relative ease of plastic deformation means that semicrystalline

polymers are generally tough above T

g

of the amorphous phase. Below the

Tg the amorphous phase is much less compliant and the semicrystalline

sample has a higher yield stress but flow is unable to take place and so

failure occurs at a lower strain [333-

Semicrystalline polymers are often so ductile that they can be

deformed to very high draw ratios before the specimens fail. In these

polymers, the noncrystalline component is normally rubbery; this allows

the crystals to deform plastically and change shape without cleavage.

During the early stages of deformation the original spherulitic structure

becomes elongated through slip and other deformation mechanisms. The chain

axes of the crystals which are originally oriented randomly become aligned

parallel to the drawing direction. Eventually the original lamellar and

spherulitic structures break up and a new fibrous structure is formed with

the molecules aligned approximately parallel to the extension direction

[ 333 . Semicrystalline polymers can be induced to behave in a brittle

manner by reducing the temperature or using certain hostile environments.

Fracture surfaces of HDPE deformed at liquid-nitrogen temperature appear

relatively smooth when examined at low magnification but detailed studies

[593 have shown that plastic deformation occurs on a local level even when

37
the fracture appears to be completely brittle. Spherulite size has a

pronounced effect on the deformation of semicrystalline polymers. The

larger the spherulite size, the more brittle the polymer.

In semicrystalline polymers, the high molecular weight molecules

crystallize first resulting in a molecular fractionation. Low molecular

weight polymer accumulates at crystallite boundaries. Gedde et al. [60]

recently showed that for polyethylene, fracture preferentially develops

along the weakest link path defined by these regions of segregated low

molecular weight material. It is likely that other semicrystalline

polymers behave similarly.

On a higher level of morphology, there is still not yet any clear idea

how factors such as crystal structure or spherulite size affect the

behavior. A few studies have shown that, in general, large spherulites

cause the polymer to be more brittle, but the micromechanisms are not yet

fully understood. This may be contrasted with the studies of the behavior

of glassy polymers where a great deal is known about the micromechanisms

of deformation and fracture. This is clearly an area where more work

needs to be done. Similarly, the application of fracture mechanics to

ductile semicrystalline polymers is in its infancy, again in contrast to

amorphous polymers and metals [33].

Impact resistance of semicrystalline polymers is generally better than

that of glassy polymers. Semicrystalline materials have better fatigue

resistance than amorphous polymers, probably because crystalline polymers

not only dissipate energy when crystallites are deformed, but they also

apparently reform a crystalline structure that is exceedingly strong [35].

In static fatigue, HDPE, a semicrystalline material, fails in a ductile

manner after short loading periods whereas after prolonged loading it

fails in a brittle manner [59]. For short loading periods the yield

38
stress is less than the fracture stress and the polymer is ductile whereas

for longer periods of loading the fracture stress is less than the yield

stress and so the material is brittle.

The premature failure of semicrystalline polymers in certain

aggressive environments is a long standing problem. ESC is characterized

by brittle fracture. Williams and co-workers [61,62] have found a unique

dependence of crack velocity upon K

Ic f0r crack propagation in PE in

environments such as methanol, ethanol, and a detergent. This is

illustrated in figure 14 for HDPE in hot (60 °C) detergent. Williams and

Marshall [62] have interpreted the change in slope in the plot between

10"7 and 10~6 ms

“1 as being due to a change in the fracture mechanism.

They suggested that in the slow growth region the crack growth rate is

controlled by viscoelastic "relaxation processes" in the material.

However, at higher velocities it is thought that the different slope

corresponds to crack propagation in this region being controlled by

transport of the environment to the crack tip. They were able to

14. Crack growth data for high density polyethylene in a detergent

environment at +60 °C. (Different points represent

individual tests) [ 62 ].

39
correlate the velocity at which the transition between the two types of

behavior occurred with the viscosity of the fluid, with the transition

taking place at the lowest crack velocities for fluids with the highest

viscosity.

6.3 Crosslinking and Branching

Crosslinking is a chemical linking established between adjacent

polymer chains which results in a three-dimensional network. Crosslinking

provides stability to the physical shape of a polymeric object because

when a load is applied, the crosslinks resist deformation. Rigidity of a

polymer is increased by crosslinking, depending on the degree of

crosslinking. The length of the crosslink and the distance between

crosslinks varies. The shorter, the crosslinks, or the smaller the

separation between them, the stronger and more rigid the polymer becomes.

Lightly crosslinked polymers are rubbery whereas heavily crosslinked

materials are rigid and may be brittle.

Branching occurs when the backbone of the main polymer chain has

chains of identical chemical composition branching from it. The length

and frequency of branching affects the properties of polymers. The

branches provide increased physical entanglement between adjacent

molecules and consequently increase strength and rigidity. However

branching results in decreased regularity of the chain and of the polymer

crystallinity. Branching is established at the time of processing and

does not change during a polymer's life. Chain branching affects

viscosity above T

g

. The effect is difficult to quantify because the

length and number of branches vary. Above T

g

, crosslinking and branching

in moderate amounts improve modulus, yield strength, flexural properties,

fatigue life, shear, and impact properties while decreasing elongation to

yield, elongation to break, and creep resistance. Environmental stress

cracking and wear abrasion also benefit.

40
6. 4 Copolymerization

Copolymerization results when two or more monomers are polymerized

together so that the polymer backbone contains alternating sequences of

one repeating unit then the other. Copolymerization is performed to

modify polymer properties. Generally, copolymerization decreases

crystallinity by introducing more irregularity. In addition, the T

g

i a

altered, with the T

g

of the copolymer falling between the parent

homopolymers. Copolymerization occurs during polymer processing and does

not change during the life of the polymer.

FEP is the only cover plate material that is a copolymer. Its

crystallinity is lower than that of poly( tetrafluorethylene) (PTFE) (i.e.

70 percent vs 98 percent). This is a result in the loss in chain

regularity from replacement of a fluorine atom by a perfluoromethyl group

The effect of copolymerization on mechanical properties depends on the

polymers involved. By judicious choice of the type and structure of the

monomers, a copolymer with specific desired properties can be designed.

6.5 Plasticizers

The addition of a plasticizer to a polymer is made to lower the glass

transition temperature, thus increasing the flexibility and toughness of

the material at ambient temperature. Plasticizers are low vapor pressure

liquids, with molecular weights in the range of 100 to 1000 which form a

highly concentrated solution with the polymer. The polymer chains in the

amorphous regions are forced apart by the plasticizer decreasing the

intermolecular forces between chains and allowing them to slip more

readily over one another. The absorption of small molecules such as water

or the entrapment of monomers has a similar effect to the addition of a

plasticizer. In general, the addition of plasticizers decreases

properties requiring strong interchain bonding such as tensile strength,

flexural modulus, and yield strength. Properties assisted by chain
flexibility and slickness such as elongation at break and elongation at

yield are increased. Conversely, loss of plasticizer is indicated by

increase in yield strength and a drop in ultimate elongation reflecting

embrittlement of the polymer. It is believed that plasticization of the

polymer by active chemical species plays a role in failure due to

environmental stress cracking.

6.6 Orientation

Orientation can be produced by either hot stretching of a molten

polymer followed by rapidly cooling of the melt, or by cold drawing, or by

cold-rolling. Most commonly, orientation is accomplished by stretching

the polymer at or above T

g> Fixing the orientation occurs when the

stretched polymer is cooled below T

g

before the molecules have had a

chance to return to their random orientation. Biaxial orientation occurs

when a polymer is stretched in two directions at right angles to each

other. The polymer chain segments tend to line up parallel to the plane

of the sheet. Compared with unoriented materials, biaxially oriented

sheets and films have higher tensile, tear, and impact strengths [62].

Typical tensile behavior of oriented brittle polymers is shown in

figure 15. Parallel to the orientation direction, the polymer may become

ductile and have a yield point and high elongation, but in the

perpendicular direction the polymer becomes more brittle with low strength

and elongation [25]. The tensile stress-strain behavior of ductile

polymers is illustrated in figure 16. When an oriented polymer is tested

in the parallel direction, it has a higher yield strength, but its

elongation to break may be less than that of an oriented polymer tested in

the perpendicular or transverse direction. The reason for this unusually

high transverse elongation is that on stretching the previously oriented

material in the perpendicular direction, the molecules first deorient and

42
BRITTLE POLYMER

15. The stress-strain behavior of typical polymers which are brittle in

the unoriented state. | | = tensile stress parallel to direction

of orientation. J_ = stress perpendicular to the direction of the

uniaxial orientation [25].

16. The tensile stress-strain behavior of ductile polymers: unoriented,

measured parallel to the direction of uniaxial orientation, and

measured perpendicular to the direction of the orientation [25].

43
then reorient in the direction of the applied force. Brittle materials

break before the reorientation process gets started [25]. Polymers have

high strengths and moduli in the direction parallel to the orientation

because the applied loads are carried largely by the strong covalent bonds

of the polymer chains. The properties are poor (at least for brittle

polymers) In the direction perpendicular to the orientation because the

loads are carried primarily by the weak Van der Waals’ bonds. Also, if

there are small cracks or other imperfections in the polymer, they become

oriented parallel to the orientation direction. These oriented cracks are

strong stress concentrators for loads applied perpendicular to the

orientation direction.

Broutman and McGarry [6M] looked at biaxially oriented PMMA and

uniaxially stretched PMMA. In the case of the biaxially oriented material

cracks could be propagated in different directions perpendicular to the

plane of the sheet with the interfacial fracture energy Gj

c

~ 1 300 J m~^,

whereas cracks growing in the plane of the sheet propagated with G

Ic

- 33 J m

-2

, showing clearly the relative ease of cleaving between

molecules rather than across the molecules. GIc for isotropic PMMA was

found to be - 250 J m

-2

[ 6H ]

.

In the uniaxially stretched PMMA, GIc for crack propagation parallel

to the orientation direction was found to drop as the degree of

orientation was increased, falling by over an order of magnitude for

polymer stretched by over 100 percent [64], In similar studies by Curtis

[65] on PMMA, it was found that Gj

c for crack propagation parallel to the

orientation direction could be directly correlated to the birefringence of

the oriented polymer, as shown for PMMA in figure 17. Again the value of

Gj

c drops by over an order of magnitude for the most highly oriented

samples [65].
17. Variation of Gj

c pmma with orientation (given by birefringence,

An) for crack propagation parallel to direction of orientation [65].

PVF is an oriented semicrystalline film and PET and PVDF may also be

oriented. When PVF film is heated above 100 °C, it shrinks three to five

percent [66]. Oriented PVF film is sometimes placed on a frame and heat

shrunk to cause the cover plate to appear taut. This practice places the

entire film under tension and undoubtedly induces localized stresses.

Since stressed areas are more prone to degradation, heat shrinking cover

plate materials onto frames may hasten the polymer degradation.

6. 7 Residual Stresses

During fabrication the polymer is subjected to both a definite thermal

cycle and a definite shear cycle which vary considerably between the

different methods of fabrication. The level of residual stresses in a

fabricated material depends upon the response of the material to this

combination of shear and thermal histories. They depend upon both the

level of orientation induced at a specific point throughout the

fabrication cycle and the opportunity for its relaxation during the

remaining part of the cycle. The rate of relaxation at a particular

temperature will also tend to decrease as the molecular weight increases,

and, in a given fabrication process, both these factors will contribute

towards a higher level of residual stresses. The improvement in

45
mechanical properties usually observed with increasing molecular weight on

isotropic specimens may not, therefore, be observed in a fabricated

material, owing to this higher overall level of residual stresses [63].

An increase in molecular weight also results in an earlier onset of

melt flow defects such as melt fracture. The magnitude of these residual

stresses and flow defects can be minimized by increasing the temperature

at which the deformation is carried out, but this inevitably results in a

longer overall cycle which is economically undesirable. There are often

additional restrictions on the upper temperature that can be used owing to

side effects such as crosslinking or chain scission, development of odor,

color changes etc., resulting from volatilization of additives and

oxidation, which may be detrimental to the final performance and/or the

appearance of the component. Some improvements can often be achieved by

modifying the width of the molecular weight distribution, as the rate of

decrease in viscosity with increasing shear rate is higher for wide

distributions [ 63 ].

7. Degradation of Polymeric Cover Plates

The weathering which polymeric cover plates receive on solar

collectors frequently causes degradation and a resultant loss of

mechanical properties. The weathering and environmental parameters that

supply the energy necessary for cover plate deterioration are shown in

table 4. The first four parameters, i.e., UV radiation, oxygen, water,

and temperature, will be considered here.

Polymer degradation may be considered any type of chemical change of a

polymer involving the main-chain backbone and/or side groups. These

changes require the breaking of primary valence bonds, leading to lower

molecular weights, crosslinking, and cyclization. Although degradation is
Table 4

Weather and Other Environmental Parameters

Affecting Polymer Deterioration

Parameter Description

UV Radiation 0.29 to 0.^0 pm wavelength range of sunlight

Oxygen or Ozone O

2 or O

3

Water humidity, rain, snow, dew, frost, fog

Temperature absolute value, thermal gradient, and thermal

cycling

Active Chemical Environment heat transfer fluid contact

Chemical Pollutants S02 , NO2 , and NaCl

Particle Bombardment dust, sand, hail, bird droppings, and insects

Cyclic Loading wind, snow loads

a chemical process, not only is the chemical composition of the polymer

modified, but also physical parameters such as MW, MWD, crystallinity,

chain flexibility, crosslinking, and branching which are governed by the

chemical composition.

The main processes taking place as dominant events during degradation

are random chain scission, depolymerization, crosslinking, bond changes,

and side group changes. The type of degradation occurring under a given

set of circumstances is dependent upon the reaction rates. When multiple

weathering factors are present, several degradative processes can be

happening simultaneously.

Chain scission involves bond breakage in the main backbone of the

polymer. Each chain scission creates another molecule and lowers the MW.

Hydrolysis of a polyester (e.g., PET or PC) is an example of a random

chain scission process since the susceptibility of the bond does not

depend greatly on molecular size. Depolymerization is a chain scission

occurring in the polymer backbone, generally from an end. During

*»7
depolymerization molecules are repeatedly split off from the active end of

the polymer chain reducing its length until the polymer molecule no longer

exists. ....

Crosslinking occurs when the reactive end of a broken polymer chain

reacts with the backbone of another polymer chain forming a chemical link

to the second chain. Crosslinking entails a localized increase in density

which creates internal stresses and the development of microcracks that

embrittle a material and increase the surface area open to chemical

attack.

Changes in the polymer backbone can be induced without scission or

crosslinking. For example, when PVC is heated, a chlorine atom and an

adjacent hydrogen atom react to form HC1. This changes the backbone

structure to form a double bond which is more susceptible to free radical

attack than the fully saturated material. A zippering occurs leaving

behind successive conjugated double bonds. The remaining structure is

highly reactive with metal salts and with oxygen. Since the chain cannot

rotate around double bonds, it is stiffened even in the absence of

crosslinking [67]. The atoms within side groups can react with each other

to eliminate a molecule similar to the above reaction with PVC. Side

group changes such as these are least important in contributing to

degradation which affects the mechanical properties of polymers.

The three most dominant processes occurring during degradation in a

solar collector (i.e., chain scissions, bond changes in the main chain,

and depolymerization) result in shortened polymer chains meaning fewer

entanglements per chain. This translates to less deformation resistance.

On the other hand, crosslinking, which occurs less frequently, increases

the connections between chains and provides greater resistance to

deformation. During processing the material is subjected to very high

thermal and mechanical stresses. These high stresses may initiate a
• vriety of polymer degradation processes leading to a deterioration of

properties even during processing. On the other hand, the damaging of the

material may result in the introduction of various defects in the polymer

which will work as degradation sources during its subsequent service life.

For example, in the presence of oxygen traces during processing, carbonyl

groups can be formed. These will function as built in sensitizers for

photodegradation processes [68]. Because of the dual nature of

degradation during processing, i.e., deterioration and the introduction of

defects as potential sources of deterioration, it is very important to add

stabilizers to the polymer before processing.

Polymer degradation can occur either at the surface or in the bulk.

Degradation from photooxidation, thermal oxidation, and hydrolysis are

more severe at the surfaces. Damage at the surfaces occurs more readily

through the combination of degradation mechanisms. Thus, in considering

cover plate degradation, the surface exposed to the sun will undoubtedly

suffer the most degradation because it is exposed to the greatest

combination of factors.

0

II Cover plate materials having an ester linkage (-C-0-) (i.e., PC, PET,

and FRP) are subject to hydrolysis at the exposed surface which results in

lowering of the MW. In semicrystalline materials, such as PET, hydrolysis

occurs preferentially in amorphous regions. This is undoubtedly one of

the reasons PET is reported to become brittle so rapidly [66]. Surface

cracking has been reported in PC [44,69,70], PET [66,71], and FRP

[66,70-74]. The thickness of PC and FRP as cover plates is 1 mm or

greater, whereas PET is about 0.13 mm. Thus, if surface degradation took

place to an equal depth in all three, the PET would show loss of

mechanical properties more rapidly because of the higher ratio of the

volume of the damaged material to the total volume of material. The
number of chains available to resist deformation is drastically reduced

and fracture undoubtedly occurs through the amorphous regions where the

tie chains linking the crystallites have been severed.

The major cause of degradation in PMMA is chain scission caused by

sunlight. This occurs at the outer surface and results in a reduction in

MW. PC degrades through hydrolysis and photooxidation. Weathering studies

of PC have shown a loss of MW and tensile properties [J|4]. Figure 7 shows

the downward shift in molecular weight distribution of PC with longer

periods of weathering. Changes in elongation at break and elongation at

yield of PC exposed to outdoor and natural weathering are illustrated in

figure 18. The decline in elongations parallels the reduction in MW seen

in figure 7.

PET degrades through both photooxidation and hydrolysis, and loses

strength rapidly when exposed outdoors [66]. PVF and PVDF are fairly

resistant to degradation from sunlight. Thermal degradation is probably

the most important degradation mechanism. A likely mechanism of thermal

degradation is abstraction of HF molecules in a manner similar to that

18. Changes in net elongation at break and elongation at yield of PC

on exposure to outdoor and artificial weathering conditions [H4].

50
described for PVC earlier, the result is a reduction in MW. Loss in

tensile strength and elongation at break for PVF have been reported [66].

The most common mechanisms of degradation affecting cover plate

materials result in a reduction of molecular weight. Most frequently this

degradation occurs at the surface. On a molecular scale, this means that

the polymer molecules at the surface are less entangled because they are

shorter than those in the bulk polymer, or not entangled at all if their

length is short enough. (Short molecules may simply volatilize).

Consequently the surface region is less capable of resisting deformation.

On a macro scale, cover plate polymers can be subjected to small

deformations on a regular schedule, such as daily thermal expansion and

contraction, swelling through absorption of moisture followed by

contraction at its loss, or motions due to wind. While these seem small

on a macroscopic level, at the microscopic level these forces are applying

a cyclic stress to the polymer. When the MW is reduced so that it

contains few entanglements and offers little resistance to deformation,

small local regions readily deform and can initiate a microcrack at the

surface. With additional degradation, the deforming stress is then

concentrated at the crack tip and spread over a smaller number of chains

than previously, resulting in continued crack growth.

Fukushima [75] has shown that the deterioration of polymer materials

by the simultaneous action of UV-light and diffusive oxygen is a function

of depth from the surface and increases in proportion to the square root

of time. Thus, monitoring the MWD (i.e., loss of long molecules and gain

of short molecules) as a function from the surface should show a

degradation profile which can be related to a decline in mechanical

properties.

51
8. Discussion

8.1 General

Probably the single greatest consideration with respect to mechanical

properties of polymeric cover plates is the fact that they decline during

use due to degradation caused by weathering. Degradation is a chemical

process affecting the composition and other physical parameters. The

mechanical properties of polymers are dependent on a number of factors,

some of which are inherent in the specific polymer while others are

induced at the time of processing. The key factors relating to the

polymer are: glass transition temperature, molecular weight and molecular

weight distribution, crystallinity, crosslinking, copolymerization, and

plasticization. These factors are not independent variables, rather their

effects are frequently interactive and it is difficult to isolate the

effects of each individually. Fabrication of the polymer into a product

may introduce orientation, anisotropic (unwanted) residual stresses,

flaws, melt flaw defects, or weak reactive sites in the chain structure.

These factors are difficult to measure and will vary, depending upon the

specific process involved, processing temperatures, and the polymer

characteristics.

8.2 Polymer Microstructure

The glass transition temperature is the most important material

characteristic of a polymer as far as mechanical properties are concerned

because it determines whether a cover material reacts to an applied force

in a brittle or ductile manner. The T

g

is related to the flexibility of

the polymer chain structure and the presence of plasticizers. Plasticizer

added during processing may be lost during use by volatilization due to

heat or by leaching out by liquids. In some cases moisture or a

chemically active environment may plasticize a cover plate material and

lower the T

g

, The glass transition temperature can be measured in a

52
variety of ways including dilatometry, refractometry, differential thermal

analysis (DTA) , differential scanning calorimetry (DSC), infrared

spectroscopy, nuclear magnetic resonance (NMR), stress birefringence,

dielectric constant and loss, creep and stress relaxation, and dynamic

mechanical properties [ 76—78 ]

.

The molecular weight and molecular weight distribution are the most

significant microstructural features influencing deformation. Higher

molecular weight provides more strength through more entanglements in the

amorphous material and through tie chains between crystallites in

semicrystalline materials. The molecular weight of cover materials is

reduced by polymer degradation, i.e., bond breaking, due to UV, heat,

moisture, or mechanical stress during processing. The average molecular

weight can be determined by many methods, the more common of which are

intrinsic viscosity, light scattering, osmotic pressure, and gel

permeation chromatography [77,78]. These methods require the polymer to

be dissolved in a solvent which is difficult for some polymers. An

alternative method for determining molecular weight, which does not

require solvent, is a melt viscosity measurement [78]. This method is

actually more sensitive in identifying small changes in average molecular

weight. The most common method for determining molecular weight

distribution is gel permeation chromatography [77,78] which separates the

polymer molecules by size. In some cases it may be very difficult to

determine the molecular weight of cover plate materials if the material is

not readily soluble or if the melt viscosity is too high to be measured in

the temperature range of the melt rheometer. For example, the MW of

poly(vinyl fluoride) is estimated by its producers at MW - 1,000,000

because of its high MW and the difficulty in dissolving the polymer. The

tendency for a polymer to crystallize is influenced by the regularity and

flexibility of the polymer chain. The size of the crystallites and the

53
extent of crystallization Is generally determined by the rate of cooling,

although orientation can also produce crystallization. The film cover

plate materials are semicrystalline i.e,, PVF, PVDF, FEP, and PET.

Crystallinity provides the film with strength and toughness. Crystallites

contribute to deformation resistance by dissipating energy during

deformation, but they probably also reform into a strong crystalline

structure. Incomplete crystallization at the time of fabrication can lead

to problems for cover plate materials later if the temperature is raised

or the Tg lowered to a level where polymer crystallization can occur. Such

crystallization would result in a small volume change and induce stress

into the material. Crystallinity is detected and its amount estimated by

a number of methods: x-ray diffraction, infrared-absorption spectroscopy,

polarized light microscopy, density, calorimetry, differential thermal

analysis, and nuclear magnetic resonance spectroscopy [ 76 - 78 ].

Crosslinking occurs either at the time of processing or through

polymer degradation. With the exception of the fiber reinforced

polyester, the polymeric cover plates contain little, if any crosslinking

at the time of processing, although degradation may induce some

crosslinking at the surface. FRP is a thermoset material which is highly

crosslinked. The degree of crosslinking dictates the rigidity of a

polymer. Heavily crosslinked materials such as FRP are rigid and may be

brittle, whereas lightly crosslinked materials are rubbery. Degradation

of most polymers is likely to induce only light crosslinking. Because

crosslinking essentially forms a large polymer network, crosslinked

materials are insoluble, however they will swell when exposed to an

appropriate solvent. Crosslinking is measured through swelling

experiments or by analysis of the stress-strain curve of a polymer network

[78,79]. This latter technique is not appropriate to characterize surface

crosslinking as may occur in cover plate degradation.

5*J
Fluorinated ethylene propylene is the only cover plate material which

is a copolymer. Copolymerization is generally accomplished to selectively

control some property of the end product. Once a copolymer is formed, it

can be characterized in the same manner as other polymers.

Copolymerization offers one possibility of obtaining different materials

for cover plates. By judicious choice of the monomers involved, it may be

possible to design a copolymer with specific desired properties.

Plasticizers are added to polymers to lower Tg an(j to increase the

flexibility and toughness of the material. Cover plate materials probably

have very little plasticizer added since plasticizers can be driven out by

elevated temperatures. Water, in the form of humidity or moisture which

collects on cover materials may be absorbed into the surface layer and act

as a plasticizer that enhances the yielding processes which occur during

degradation. Blaga [70] has postulated a theory for the mechanism of

surface degradation of plastic based sheets, whereby surface microcracks

are produced by differential dimensional strains induced by humidity and

temperature, in conjunction with resin degradation.

Stresses in the cover plate, such as would result from orientation or

from anisotropy introduced during processing, can be measured with

birefringence, infrared dichroism, and polarization of fluorescence [78].

8.3 Methods to Study Polymer Degradation and Deformation

On a molecular level, cover plate degradation results in chain

scission in the polymer backbone, chemical reactions, e.g., oxidation of

the polymer, or crosslinking. In general, the main degradation mechanisms

for each cover plate material are known, although much more specific

knowledge is needed regarding initiation mechanisms, rates of degradation,

and synergistic effects of multiple causative factors. Many methods are

available to study various aspects of polymer degradation.

55
Changes in the polymer stability as a function of temperature can be

monitored using thermogravimetry (TG), DTA, DSC, and thermal

volatilization analysis (TVA) including evolved gas detection and analysis

[68]. Volatile products from polymer degradation can be studied using gas

chromatography (GC), gas liquid chromatography, mass spectrometry (MS),

and a combination of GC and MS [68].

Chemical techniques capable of detecting chemical changes related to

cover plate degradation include Fourier Transform Infrared Analysis

( FTIR) , Electron Spectroscopy for Chemical Analysis (ESCA), X-ray

Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES),

Chemiluminescence (Cl), and GC/MS [80]. The morphological features of

surface degradation can also be studied using both optical and electron

microscopy.

In cover plate materials, degradation has been exhibited as either

cracking and crazing of the surface toward the sun or embrittlement of the

bulk material eventually resulting in fracture. The surface degradation

relates to a reduction in the molecular weight of the surface layers

accompanied by yielding on a molecular level to permit cracking and

crazing to occur. The cracks and crazes gradually grow from a molecular

to a microscopic level and finally to a visible level. Their growth may

well be enhanced by plasticization from moisture or other chemically

active environment, or by fatigue, for example from thermal expansion and

contraction or wind. The deformation mechanisms of polymers in general

have been described in detail in Section 5.1. Polymer deformation

involves molecular fracture and experimental techniques are available to

study all phases of fracture. Chain scission can be followed using ESR

which allows direct measurement of the number and nature of free radicals

formed when bonds are broken. The reduction of molar mass due to bond

breakage and the increase in concentration of end groups as measured by. IR

56
can also be used to follow molecular fracture. The formation and growth

of microvoids from the accumulation of chain ends and the coalescence of

these voids into crazes and microcracks can be followed using small-angle

x-ray scattering [33]*

The concepts of fracture mechanics provide a sound basis on which to

describe polymer fracture, however, little has been done in this area on

polymers used in cover plates. In some areas, such as slow crack growth

in glassy polymers, e.g., PMMA, there is quite a bit of information, while

in other areas, such as environmental crack growth and fatigue life, there

is little. Ductile failure is the most difficult to characterize, and

most cover plate materials, other than PMMA, are ductile. Information is

scarce in the study of the growth of flaws or cracks in the polymers used

as cover materials. Failure of these semicrystalline materials is complex

due to the multiple mechanisms of failure. It is complicated by yielding

and reformation of crystals after an initial deformation. Study of the

growth of cracks and flaws in cover plate materials needs more attention.

9 . Summary

The mechanical properties of cover plate polymeric materials do not

remain constant during exposure on a solar collector, rather they decline

as the polymer ages. The mechanical properties are a measure of the

deformation resistance of the polymer under an applied force. Polymer

deformation is a complex phenomenon which is influenced by many factors

including type of applied stress, temperature, characteristics of the

polymer, and uniformity of properties within the polymer. The decrease in

cover plate mechanical properties is a result of polymer degradation which

occurs at a molecular level. Mechanical property tests are a measure of

the bulk material, and appreciable degradation must occur prior to

measurable change in test results. Since the decline in mechanical

properties results from polymer degradation, more direct measurement of

57
polymer material properties at the microstructural level would provide an

indication of whether the material is changing and whether subsequent

changes in mechanical properties might be expected. Monitoring

microstructural properties of cover plate materials from the new material

through various stages of degradation would provide degradation rate data

which would aid in the prediction of service life of polymer cover plates.

Unfortunately, the state of the art of structure-property knowledge for

polymers in general is not sufficiently well developed to permit this at

this time. However, a number of techniques are available to study cover

plate materials at the molecular level to begin to build the body of

knowledge necessary for accurate service life prediction. Application of

modern instrumentation to study microstructure, chemical degradation, and

polymer deformation and failure mechanisms will aid in developing more

exact relationships between the microstructure of cover plate materials

and their mechanical properties.

This study was supported by the Office of Solar Heat Technologies of

the U.S. Department of Energy.

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62
NBS-1 MA (REV. 2 «80 ) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 85-3197

2. Performing Organ. Report NoJ 3. Publication Date

July 1985

4. TITLE AND SUBTITLE

Molecular and Microstructural Factors Affecting Mechanical Properties of Polymeric

Cover Plate Materials

5. AUTHOR(S)

Elizabeth J. Clark

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8 . Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City . State. ZIP)

Department of Energy

Washington, DC 20585

10. SUPPLEMENTARY NOTES

j 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)

This paper reviews the dependence of mechanical properties of polymers

on various microstructural factors. The microstructural and molecular

factors considered are: molecular weight, crystallinity, crosslinking,

branching, copolymerization, plasticization, orientation, and residual

stresses. The types of mechanical properties considered are: direct

loading, fatigue, creep, wear and abrasion, and environmental stress

cracking. The effects of polymer deformation and fraction at the

molecular level are discussed. Cracking, crazing, and shear yielding are described. Polymeric cover plate materials are discussed and their

degradation reviewed. Methods to measure microlevel changes in polymers

are identified.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon . degradation; polymers; mechanical properties; microstructure; service life

13. AVAILABILITY 14. NO. OF

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