A 1 1 1 D 2 t> ^ b 7 0 0

NBSIR 87-3581

In-Situ Characterization of the

Interface of Glass Reinforced

Composites

PUBLICATIONS

F. W. Wang

B. M. Fanconi

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Institute for Materials Science and Engineering

Polymers Division

Gaithersburg, MD 20899

June 1987

Prepared for: —QC Research Office

100 12211

Triangle Park, NC 27709

#87-3581

1987

C.2

NBSIR 87-3581

^search Information Center

Aational Bureau of Slanuai. ds

IN-SITU CHARACTERIZATION OF THE

INTERFACE OF GLASS REINFORCED

COMPOSITES

F. W. Wang

B. M. Fanconi

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Institute for Materials Science and Engineering

Polymers Division

Gaithersburg, MD 20899

June 1987

Prepared for: U.S. Army Research Office

P. 0. Box 12211

Research Triangle Park, NC 27709

/

/

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

In- situ Characterization of the Interface of Glass

Reinforced Composites

Final Report

F.W. Wang and B.M. Fanconi

June, 1987

U.S. ARMY RESEARCH OFFICE

MIPR ARO 111-84

Polymers Division

Institute for Materials Science and Engineering

National Bureau of Standards

Gaithersburg, MD 20899

APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED
The view, opinions, and/or findings contained in this report are those of the

author (s) and should not be construed as an official Department of the Army

position, policy, or decision, unless so designated by other documentation.
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1. REPORT NUMBER

Final Report

2. GOVT ACCESSION NO. 3. RECIPIENT'S CATALOG NUMBER

4. TITLE (and Subtitle) IN-SITU CHARACTERIZATION OF THE INTERFACE OF

GLASS REINFORCED COMPOSITES

5. TYPE OF REPORT & PERIOD COVERED

Final Report 11/83 - 11/8 t

6. PERFORMING ORG. REPORT NUMBER

7. AUTHORf*;

F.W. Wang

B.M. Fanconi

8. CONTRACT OR GRANT NUMBER^)

MIPR ARO 111-84

9. PERFORMING ORGANIZATION NAME AND ADDRESS

Polymers Division, National Bureau of Standards

Institute for Materials Science & Engineering

Gaithersburg, MD 20899

10. PROGRAM ELEMENT, PROJECT, TASK

AREA 4 WORK UNIT NUMBERS

address

Post Office Box 12211

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11. CO 12. REPORT DATE

June, 1987

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58

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SUPPLEMENTARY NOTES

s»

19. KEY WORDS NOSnQ f(Continue C*.r\r* Hnn« <vi on reveree rmxrmr mm miaiderim Ifif neceeeary nmrmmjwtv mnri and identify irlmnti fv hvby hlnrle block ntrmhmr number)

cure monitoring, viscosity, epoxies, fluorescence, optic fibers, remote sensing

20. ABSTRACT (Continue an rarerwe aim ft neneeeary 'and. identify by block number)

A technique has been developed to monitor the viscosity of curing epoxies.

Fluorescence spectroscopy of viscosity-sensitive organic dyes is used together

with optic fibers as a means of delivering the probing light to the measurement

site and retrieving the modulated light for detection. The technique is adaptable

to remote sensing through the use of optic fibers. The evanescent wave of the

guided radiation is used to excite probe moTecules lying at the interface between

the fiber and surrounding matrix. Therefore, the technique is adaptable to

in-situ characterization of the interface of glass reinforced composites.

w> ,;srw 1473 EDITION OF 1 NOV 65 IS OBSOLETE Unclassified

SECURITY CLASSIFICATION OF THIS PAGE fWran Dafa Entered)
LIST OF ILLUSTRATIONS

Page

1. Diagram of Cure Monitoring System. 4

2. Dependence of the Fluorescence Spectrum of 1 , 3 -bis ( 1 -pyrene) 9

propane on Solvent Viscosity.

3. Cure Curve of an Epoxy Monitored with Excimer Dye. 12

4. Fluorescence and Excitation Spectra of ( 1

- (4-dimethylamino) 14

-6 -phenyl- 1 , 3 , 5 hexatriene)

5. Fluorescence Intensities of Viscosity Sensitive Dye and 15 . 9,10-diphenyl anthracence in a Curing Epoxy.

6. Cure Curve of an Epoxy Monitored with Dye Pair of Fig. 5. 16

7. Comparisons among Cure Monitoring Data for an Epoxy System.

a. Fluorescence and Viscometry 18

b. Fluorescence and Ultrasonics 19

c. Viscometry and DSC 21

8. Excitation and Emission Spectra of Poly (amide acid). 24

9. Optical System for Measurement of Fluorescence using 29

Optical Fiber Probes.

10. Fluorescence Spectrum of Rhodamine-B. 30

11. Dependence of Fluorescent Light Intensity Collected by 3.1 Waveguide -Type Optic Fiber on Fiber Length.

12. Dependence of Fluorescent Intensity Collected by Optic 33

Fiber on Solvent Refractive Index.

13. Evanescent Wave Amplitude. 42

14. Collection Efficiency of Optic Fiber Sensor. 44

15. Angular Distribution of Radiation from Dipole Source. 50

16. Comparison of Calculated and Observed Collection 52

Efficiency of Optic Fiber Sensor.

LIST OF TABLES

1. Input Parameters for Modeling Optic Fiber Collection 38

Efficiency.

ii
INTRODUCTION

Polymer matrix composite materials are increasingly used in Army

applications owing to the advantages of light weight, corrosion resistance and

high specific strength and stiffness 1. It is apparent that wider use of these

materials to benefit the Army could be made if more rapid processing of higher

quality products could be achieved. Current production methods are slow and

labor intensive, and there is a demonstrable need to develop on-line methods

for process monitoring, in particular with respect to the interfacial regions

coupling the strong and brittle reinforcement fibers to the polymer matrix2

. In-service monitoring of polymer composites is also important, and as in the

case of processing, the interfacial region is primarily of interest. In this

report, we present results of a research program to develop sensors capable of

process monitoring with emphasis on interfacial properties. The sensors are based on optical fibers which become a permanent part of the polymer composite

component after processing and which have the added advantage that they may

function as performance monitors when the component is placed in service.

Processing of polymer composites differs from that of the more common

thermoplastics in two important aspects. First, polymerization of

thermosetting resins occurs in the final stages of product manufacture, and

second, the presence of reinforcements, frequently at volume ratios greater

than 50%, not only introduces inhomogeneities in thermal expansion

coefficients, but also leads to large interfacial regions which can have a pronounced effect on resultant properties 3

. The development of the interface

in terms of consolidation of reinforcement plies and fiber wetting becomes an important aspect of processing, and consequently should be probed by the

process monitoring sensors.
2 The interfacial region is also important in the performance of polymer

composites. The accumulation of chemical by-products during manufacture at the

interface and of environmental contaminants, such as moisture, may be

detrimental to the adhesion between the reinforcements and the matrix resin.

Moisture accumulation is known to have an adverse effect on the performance of

advanced composites of the sort used in aircraft4

. One aspect of our program

addresses the identification of probes that are sensitive to polarity, and

hence may be used to monitor moisture uptake in the interfacial region.

Current trends in polymer composite manufacture to more rapid and

automated processing are also driving the demand for on-line process sensors

and control technologies. In addition, on-line process monitoring techniques

would facilitate the introduction of modified or new resin systems. Currently,

extensive laboratory studies must be conducted to optimize the cure cycles

before modified or new resin systems can be commercialized. With proper online monitoring, the amount of laboratory work can be greatly reduced. Thus,

process monitoring should lead to more rapid introduction of improved composite

materials and optimization of cure cycles as well as facilitate more rapid and

automated processing technologies.

Complex chemical and physical processes are associated with the cure of

thermosets, such as epoxy resin materials. In the usual cure cycle, the resin

initially exists as a viscous liquid that thins as it is warmed to the

temperature of cure. The exothermic cross-linking reactions also contribute to

the temperature rise. The growing molecular weight of the thermoset matrix

eventually controls the viscosity which increases to a value typical of a gel,

and in the latter stages of cure to that of a brittle solid5

. Chemical

reactions may also produce by-products of the cross-linking reactions that may
3

affect processing and use properties.

Of the various changes that occur, the viscosity is perhaps the most

important from a processing standpoint as it influences fiber wetting,

devolatilization, uniformity of resin distribution, and consolidation of the

reinforcement plies. In the processing environment, the resin viscosity

initially decreases as the resin temperature rises to that of the autoclave,

and then increases as the molecular weight of the cross-link network dominates

the viscosity. If the viscosity decreases too much during the autoclave

process, an excess of resin will flow out of the product leading to poor

adhesion between the reinforcement plies. If, on the other hand, the viscosity

remains too high, or increases too rapidly, the flow may be insufficient to

achieve good consolidation of the plies. Thus, knowledge of the time

dependence of the viscosity during the cure cycle is necessary for proper

adjustment of the autoclave temperature and pressure to achieve high quality

products. The time intervals during which the polymer composite is held at

specific temperature and pressure may. be varied to produce high quality

products6

. The key aspect is to have an on-line monitor of viscosity so that

the time- temperature -pressure can be adjusted to optimize processing.

Until recently, only dielectric measurements have been exploited for insitu monitoring of polymer composites cure 7-9

. Measurements on curing resins

have shown that the dielectric response is dominated by ionic conductivity,

even though the concentration of ions is very low in these materials 10 . The

variability in type and concentration of adventitious ions poses problems for

quantification of dielectric cure data. Preliminary work has shown that UV-vis

absorption and fluorescence spectroscopy can be used to determine the

concentration of some reactive species in chemically labeled epoxies 11

, and
4

Pig.

1

Monitoring

the

cure

of

epoxy

resins

using

viscosity

sensitive

fluorescence

of

organic

dyes

and

optic

fibers.
5 that the fluorescence spectra of probe molecules 12 and of specific epoxysystems 13 ' 14 are sensitive to local viscosity. The question of the sensitivity

of these methods to the interface or of how to make measurements in a remote

sensing environment have not been addressed.

On the basis of the above-mentioned considerations, the project reported

herein was designed to address measurement of viscosity during processing,

albeit in a relative sense, with specificity to the fiber-reinforcement

interface. The measurement system was also designed to include the flexibility

to monitor chemical changes, at least in selected resin formulations. The

major components of the process monitoring approach developed in this project

are fluorescent probes in the form of low molecular weight organic dyes that

exhibit sensitivity to microviscosity and optic fibers as remote sensors. The

application of the method is illustrated in figure 1. Laser light is

propagated through an optic fiber imbedded in the composite; probe molecules

lying near the optic fiber interface are excited by the evanescent wave of the

propagated light; fluorescence from the probe molecules is trapped by the fiber

and observed at the end of the fiber.

The fluorescence spectra of organic molecules may be sensitive to the

chemical environment at the molecular site as well as to microviscosity 15

. In

addition, the high quantum yields obtainable with many dyes, laser sources

capable of delivering high power in narrow frequency intervals, and

sophisticated spectroscopic equipment capable of detection and differentiation

on exceedingly low concentrations of dye molecules make the approach viable.

In the course of this project, several dye systems have been identified for

monitoring viscosity during processing; and detailed investigations have been

conducted to demonstrate their application to epoxy resin systems 16-18 . In
6

addition, fluorescence spectra of polyimides have been shown to be sensitive to

the chemical environment 18 .

For neat resins, conventional fluorimetry has been used to measure the

intensities of viscosity probes. This method is generally limited to the

surface of a reinforced specimen, particularly for opaque samples. To probe

the interior of a specimen, optic fiber sensors have been investigated as a means of transmitting the excitation light to the measurement site and for

collecting and transmitting a portion of the fluorescence light to the

detector. Optic fibers have the advantage that they can be selectively placed

in the specimen to develop an image of the viscosity profile across the

specimen. This is particularly important for thick specimens in which a significant temperature gradient between exterior and interior regions may

result in non-uniform cross-linking.

The combination of fluorescence probes and optic fiber sensors shows

promise as a useful method to monitor the viscosity of a curing matrix resin

on-line. Both experimental and theoretical studies have been conducted to

determine the optimal conditions for use of optic fibers in viscosity

monitoring and for interrogation of the interface region. In the latter use,

the interface is between the optical waveguide and the surrounding medium.

Since the waveguides are constructed of glass the immediate application is to

glass reinforced composites where the reinforcements themselves may serve as

the optic fibers.

APPROACH

The approach adopted in this work combines fluorescence probes that

exhibit sensitivity to medium viscosity, or the chemical environment, with

optic fibers as a means of transmitting the excitation light to the interior of
7

a composite part as well as for conducting a portion of the modulated light to

the detector for analysis.

Previous Work

Sung et al 11,19 used diaminoazobenzene (DAA) as a probe molecule to

mimic the reactivity of the cure agent diamino diphenylsulfone . The probe

molecules used in our work do not participate in the cross-linking chemistry

whereas DAA is a tetrafunctional amine whose UV-VIS absorption and fluorescence

spectra are sensitive to the replacement of amine hydrogens by functional

groups as occurs during cure. The spectroscopic measurements using DAA track

the chemistry of the probe which may or may not be indicative of the cure

agent. Also, this probe is restricted to very specific epoxy systems. The

absorption technique is limited also by the necessity of relatively high probe

concentrations, >.1%, and lack of sensitivity after vitrification. Sung and

co-workers have used the fluorescence of DAA as a probe of the cross-linking

chemistry 11 . Comparisons with the fluorescence yields of model compounds have

shown that the fluorescence sensitivity of DAA probes reflects the amine

functionality rather than viscosity 11

. The fluorescence of the DAA probe does

exhibit changes beyond vitrification, but is limited by the lack of an internal

standard.

Other investigators have attempted to use fluorescence associated with

certain types of epoxy resins as a means of process monitoring 13 . This

approach was discarded early in our work because the approach could not be

generalized to other epoxy systems. It has been observed in the course of our work that the band shape and frequency maximum of epoxy fluorescence changes

with excitation frequency, source of the epoxy, and state of cure. It is

likely that most of this fluorescence signal does not arise from the diglycydil
8

ether of bisphenol-A, or from epoxy- type intermediaries in the polymerization.

If a significant portion of the intensity arose from an impurity, as appears

likely, then the spectral response should vary from batch- to -batch as we have

observed. For these reasons cure monitoring of epoxies by fluorescence

spectroscopy is best accomplished by the intentional addition of a fluorescence

probe with a well-defined spectral response. Cure monitoring of polyimides by

naturally occurring fluorescence has been demonstrated in our work.

Viscosity-Sensitive Fluorescence

Fluorescence probes that were identified for specificity to the

microviscosity fall into two classes, one being the excimer type that has the

added advantage of self -calibration, and the second type combining a viscosity

sensitive dye with an insensitive dye, the latter of which served as an

internal standard 17

. The sensitivity of the fluorescence spectra of some organic dye molecules

to viscosity stems from the competition between radiative and non-radiative

processes for de-excitation of the electronically excited states 20 . As the

solvent viscosity increases, the degrees of motion of the solute dye molecules

are affected to the extent that the probability of large scale movement is

diminished. For example, translational modes and rotational modes of a probe

molecule are converted into internal modes when the molecule is trapped in a highly viscous medium. Inhibiting molecular mobility reduces the probability

of non-radiative decay of the electronic excitation, and fluorescence is

enhanced. There are molecules, however, for which the reverse is true, that is

the fluorescence intensity of some bands diminishes with decreasing mobility

rather than increasing. The latter situation occurs in excimer- forming dyes in
9

Fig.

2

Dependence

of

the

fluorescence

spectrum

of1

,3-bis

(1

-pyrene)

propane

on

solvent

viscosity.

All

spectra

are

normalized

to

constant

monomer

intensity.

The

solvent

viscosity

progressively

increases

from A

to

E:.

A,

0.44

cp;

B,

0.91

cp;

C,

1.36

cp;

D,

2.81

cp;

E,

4.0

cp.
10

which mobility enhances the formation of excited electronic state complexes

which fluoresce at different wavelengths than the normal emission21 . Excimer Probes

One of the two types of probe molecules investigated was excimer- forming

An example is the molecule 1 , 3 -bis- (1 -pyrene) propane the structure of which is

shown in figure 2. This molecule contains two pyrene rings, the chromophores

involved in the electronic excitation, attached to either end of a propane

linkage. When the molecule is excited into the excited electronic states of

the pyrene group the excitation can follow one of three pathways to deexcitation. One involves non-radiative decay through coupling of the

electronic motion to nuclear motions (molecular vibrations, rotations, and

translations), another process results in the return to the ground state with

emission of light (fluorescence) after a period of time termed the lifetime of

the pyrene excited electronic state, and the third pathway involves creation of

a new electronic state (excimer state) in which the excitation is shared by

both pyrene groups. The relative probabilities of these three processes

determine the course of de-excitation. Fluorescence from the excited state of

the single pyrene chromophore takes place approximately 100ns after excitation,

the lifetime of its excited state, provided that de-excitation has not occurred

through the other mechanisms. The probability of non-radiative decay is

affected by the strength of the coupling between the electronic and nuclear

motions 22,23 . The nuclear motions can be affected by the local viscosity of

the medium which surrounds the molecule as discussed previously. The

probability of the third process, the formation of an excimer state, depends on

the relative arrangement of the two pyrene groups. Configurations which

maximize overlap of the II- electrons of the two pyrene groups have a high
11

probability of excimer formation, whereas other conformations have low

probability. Maximum overlap occurs when the two pyrene groups adopt a sandwich configuration, that is, the planes of the pyrene rings are face-toface. Both the inter -chromophore distance and the relative orientation of the

two pyrene rings affect the probability21

. Internal rotations about the propane segment of 1 , 3 -bis (1 -pyrene) propane

allow conformations such as the sandwich conformation. An electronically

excited molecule undergoes transitions among the various conformations with

transition probabilities proportional to the conformational energy barriers.

If during the 100ns lifetime of the pyrene excited state, the molecule adopts

the sandwich conformation, there is a high probability for excimer formation

and fluorescence occurs at longer wavelengths. The conformational energy

barriers are dependent on the solvent viscosity which gives the dependence of

excimer fluorescence intensity on medium viscosity. An example of the

viscosity dependence of excimer fluorescence is given in figure 2 in which

fluorescence spectra of 1 , 3-bis(l-pyrene)propane in solvents having viscosities

in the range 0.44 to 4cp are shown. Two fluorescence bands are evident, one from a singly- excited pyrene group (monomer) , and the longer wavelength band

from the excimer state (dimer) . As the solvent viscosity increases, the

relative intensity of the excimer emission to the monomer fluorescence

decreases or alternately, the ratio of the monomer to excimer fluorescence

increases linearly with viscosity.

An advantage of excimer forming dyes for viscosity monitoring is that the

monomer fluorescence intensity may be used as an internal standard. The

monomer and excimer fluorescence wavelengths are sufficiently well -separated to

permit independent intensity measurements. A number of factors, characteristic
25

20

15

10

5

12

10 20 30 40

Cure Time, minutes

50

'he cure curve of an epoxy system [EPON 828 cured with 4,4'-

lethylene-bis (cyclohexamine) ] monitored by the fluorescence ratio

>f the excimer dye, 1, 3-bis (1-pyrene) propane.
13

of both the sample and measurement system, may change during the course of

cure. For example, the refractive index of the resin may change, or inhomogeneities may affect the emission intensity. For these reasons, it is

highly desirable to have an independent measure of the efficiency of the

fluorescence excitation and detection system. The monomer emission of an

excimer forming molecule serves this purpose, as long as the active probe

concentration does not change as cure proceeds

We have applied the excimer fluorescence technique to monitor the

viscosity change during the cure of an epoxy resin. A trace amount of 1,3-bis-

( 1 -pyrene) propane was dissolved in a stoichiometric mixture of Epon 828 1

(diglycidil ether of bisphenol-A and 4,4' -methylene -bis- (cyclohexylamine)

The intensity ratio, Fm/Fd, where Fm and Fd are the fluorescence intensities of

the pyrene monomer and pyrene -excimer (dimer) fluorescence, respectively, was

measured as a function of time after the mixture was heated to 60 C. The

measured values of the intensity ratio, figure 3, successfully indicated the

initial decrease of the resin viscosity due to heating and exothermic

reactions, and the subsequent increase of the resin viscosity due to

polymerization. The cure curve, figure 3, also shows a drop in the

fluorescence ratio at a cure time of 40 minutes. We believe this drop

indicated that photochemical reactions were occurring and became appreciable

1 Certain commercial materials and equipment are identified in this paper

in order to specify adequately the experimental procedure. In no case does

such identification imply recommendation or endorsement by the National Bureau

of Standards, nor does it imply necessarily the best available for the purpose.
14

(sj. j u n Ajejuqjv)

A)jsua)U| 0ou0osejon|-j

llie

fluorescence

(dashed

curve)

and

excitation

(solid

curve)

spectra

of

the

viscosity

sensitive

dye

(1-

(4

-dimethylamino)

-6-phenyl

-1

,3,5

hexatriene)

,

DMA-DPH,

in

solution.
15

:

GO

CO

CO

0)

a>E

a>3

o

CM

(sjjUfi AjBJijqjv)

A)jsu3)u| eouaosejonij U-5

bO •H

P-,

The

fluorescence

intensity

of

DMA-DPH,

filled

circles,

and

9,10

diphenyl

anthracene,

DPA,

filled

triangles,

in

an

epoxy

system,

DER

332

cured

with

4,4'

methylene-bis-

(cyclohexyl

amine)

,

BPACM,

as

a

function

of

cure

time.
16

Hd

,dj

Fig.

6

The

cure

curve

of

the

epoxy

system

DER

332

cured

with

BPACM

monitored

with

the

dye

pair

DMA-DPH/DPA.
17

when the resin viscosity was such that probe molecules no longer diffused in

and out of the sampling volume. For this reason we have examined other dye

molecules for use as viscosity probes.

Use of Dve as Internal Standard

The selection of suitable probe molecules is governed by the following

considerations: (1), sensitivity of the fluorescence intensity over a wide

range of microviscosity, (2)

chemical stability in the cure environment

(chemical reactions and photochemistry) , (3)

excitation and fluorescence

frequencies, (4), solubility in the epoxy resin, and (5), internal intensity

standard. As stated above, the excimer-type dye satisfies 3-5, but is

photosensitive which affects its suitability to monitor the latter stages of

cure. Investigations to date have indicated that the optimal probe system is a combination of two dyes. One dye of the pair, eg DMA-DPH, displays

fluorescence intensity sensitivity to microviscosity while the other, 9,10-

diphenylanthracene , has a fluorescence intensity that is essentially

independent of viscosity. The two dyes emit at different frequencies, so that

in the course of cure monitoring the luminescence from both can be

independently determined. The excitation and emission spectra of the viscosity

sensitive molecule we have selected are shown in figure 4. The dependence of

their emission intensity on viscosity as measured during the cure of an epoxy

resin is shown in figure 5, and the ratioed intensity versus cure time is given

in figure 6. In the case of the cure curve, figure 6, the initial time, t=0

corresponded to when the resin temperature equaled the temperature of cure and

for this reason, the dip in the ratio at early times, corresponding to the rise

in resin temperature to that of cure, is missing.
EPOXY

332/BPACM

CURED

AT

45

C

18

00 OJ

Oiq.Dy 0OU3OS0JOn{J

L_

GO CD ID 00

i OJ

co

tZ (Q DrJUJ') XaiSOOSIA 6ot

Comparisons

among

cure

monitoring

data

for

the

epoxy

resin

DER

332/BPACM

at

4S°C.

a.

viscosity

and

fluorescence

data
EPOXY

332/BPACM

CURED

AT

45

C

19

CO OJ

(i 0J/8P) uoiiDnusiiv ss30x 3

o

lD

z?

•M

cc <Crsj

tV'

’J) 0

5-

>>X

c

C.0

0

C£C

U

cmc

o

E

CCcmc

o

X)

5-

ccC.s

o • CJ Z2

-0

0- co OJ cm

• rH U. oiq.Dy 0oueos0jon| -j

ultrasonics

and

fluorescence

data
20

Comparisons with Other Techniques

The cure curve of figure 6 has been compared to those generated by other

experimental techniques and these comparisons are shown in figure 7. In figure

7a, the fluorescence cure curve is compared to that obtained from viscometry.

The viscometric data were determined with an automated concentric cylinder

viscometer utilizing shear rates between 50s -1 and .02s-l. The departure of

the two curves at longer cure times may not be significant because the

viscometric data may not be accurate over the entire cure for the following

reasons. The measured viscosity is a function of the shear rate, particularly

at times corresponding to the rapid rise in viscosity. As the sample

approaches gelation, the high stresses involved and the rapid changes in the

sample limit the measurement to low shear rates, and thus the shear rate

dependence cannot be measured. Moreover, it is difficult to assure that steady

state flow is achieved, and thus the data may be only a qualitative measure of

viscosity at the highest values. For this reason, the departure between the

fluorescence measurement and viscometry at the latter stages of cure

does not necessarily indicate a lack of sensitivity on the part of the

fluorescence probe.

Figure 7b compares the fluorescence cure curve to that obtained in

an ultrasonics measurement of the shear mechanical properties 24 . The

ultrasonic wave propagation technique uses either a quartz plate or rod

that is coated or placed in contact with the sample. 25 The data presented

in figure 7b were obtained with the rod geometry. In this system, a shear

wave is generated at one end of the rod and is monitored by a transducer

at the end opposite to that placed in contact with the sample. By

measuring the reflection coefficient at the coated end (the attenuation
EPOXY

332/BP

ACM

CURED

AT

45

C

L_

o

a

CT)

(S DJW) X^TSOOST/\ Qo\

—L_

Q

a

OJ

a

o

(6/0 H

a

j

Cure

Time

(min)

Fig.

7c

Comparisons

among

cure

monitoring

data

for

the

epoxy

resin

DOR

332/BPACM

at

4

c.

DSC

and

viscosity

data
22

and phase shift of the reflected wave) , the mechanical properties of the

sample can be determined. Since the attenuation of shear motion is very

high until the sample becomes rigid, the measurement is independent of

sample thickness. The ultrasonics shear wave method provides a direct

measure of the dynamic shear storage (elastic) and loss (viscous) moduli.

If desired, the absolute values of these materials properties can be

measured and followed during cure. Often, however, it is the relative

changes that are of prime interest, and in this case the measurement of

the amplitude attenuation coefficient (loss in wave amplitude) per

reflection can be used. As the sample cures the attenuation increases

markedly but eventually levels off as the cure approaches completion.

The shape of the ultrasonics and fluorescence cure curves are very

similar and can be superimposed if shifted along the cure time axis. It

has not been determined if there is some inherent difference in the data

of the two techniques as relates to viscosity, or if the offset merely

reflects a difference in the initial measurement time. Both sets of data

were obtained on epoxy resin from the same batch.

The third comparison, figure 7c, is between the fluorescence data

and heat evolution measurements. The latter were made by differential

scanning calorimetry which measures the heat evolved during the cross-linking

reaction. Excellent agreement is found between these two measurements.

It is concluded from comparisons between the fluorescence cure curve and

those of more traditional techniques of cure and viscosity monitoring that

the fluorescence method does provide a reasonable measure of the degree

of cure and of the viscosity during epoxy cure. We conclude from the foregoing that we have demonstrated that
23

fluorescence probes are effective polymer matrix cure monitors. The

fluorescece probes have the added advantage, as will be discussed below,

of being adaptable to remote sensing.

CURE MONITORING OF POLYIMIDES

Preliminary work on polyimides has shown that the natural fluorescence

from species generated by the cure chemistry can be used as a measure of the

degree of cure. Synthesis of Polvimide

The polyimide was prepared from 2 , 2-bis(3 ,4-dicarboxyphenyl)

hexafluoropropane dianhydride (American Hoechst 6F) 1 and 2,2-bis[4(4-

aminophenoxy) phenyl] hexafluoropropane (Morton Thiokol 4BDAF)

Both 6F and 4BDAF are soluble in dry glyme at 25 °C. 4BDAF (0.5g,

9 . 6x10' 4 mol) was dissolved in 3 ml of dry glyme at 25°C with stirring in a 25

ml glass -stoppered flask. When dissolution was completed (usually within 3

minutes), 0.42g (9. 6x10” 4 mol) of solid 6F in small portions (O.lg each) was

added to the solution of 4BDAF at 25 °C. Within 5 minutes after the 6F was

added, stirring was impeded by the increased solution viscosity. After 15

minutes of manually swirling the contents, the solution viscosity decreased

sufficiently to allow normal stirring to proceed. The reaction was stopped

after 44 hours at 25°C. This solution (26% solids) of the poly(amide acid) in

glyme was used to prepare films for fluorescence spectroscopy as described

below.

A few drops of the poly(amide acid) solution were spread on a clear

quartz slide by drawing a wedge of the solution beneath another clean slide.

Room temperature solvent evaporation and all heat treatments were carried out

in air. The film was cured for 0.5 hour at each of seven temperatures ranging
24

(simp AjBJJiqjy)

A);su9)U| eouaosejonij

oo

ci> •rt tu

Excitation

and

emission

spectra

of

poly(amide

acid)

after

heat

treatments.
25

from 80° to 350° C, with oven warmup and cooiing down times of up to 0.5 hour

each. Front surface fluorescence from the same film region was measured at

room temperature after each heat treatment. Film thickness averaged 13^m.

Results

Figure 8 shows the uncorrected excitation and emission spectra of a film

of the poly(amide acid) after thermal treatment in an air filled oven for 30

minutes at each of the temperatures ranging from 80 °C to 350°C. The excitation

and the emission wavelengths were 480 nm and 550 run, respectively. The

fluorescence intensity at 550 nm increased steadily when the cure temperature

was raised from 80°C to 350°C. The uncorrected excitation spectra were more

complex. A band at 460 nm appeared at the expense of the one at 330 nm after

the second heating. However, the intensity of the band at 330 nm relative to

the one at 460 nm increased steadily with additional thermal treatments. In

any event, the formation of this polyimide polymer can be readily monitored insitu by fluorescence spectroscopy.

OPTIC FIBER SENSORS

Conventional fluorimetry can be used to monitor the fluorescence of probe

molecules in the processing environment, particularly if the manufactured part

is transparent or if surface measurements suffice. Indeed, there have been

reports of its use in the manufacture of circuit boards 26 - For general

applications in the autoclave environment, or for measurements of cure in the

interior of a composite part, it is necessary to use optic fibers as a means of

bringing the excitation light to the sampling volume and for transmitting a portion of the modulated radiation to the detector.

Several optic fiber probes specifically designed for fluorescence

detection are available commercially, including devices that may be suitable
26

for use in an autoclave under certain conditions. A common aspect of

commercial devices is distal sensor, that is, sample illumination, and in some

designs the collection of modulated light, occur at the end of the optic

fiber27 . The distal end of the optic fiber may have an optic element, such as

a lens, incorporated into it. With this sensor design, the material under

inspection is situated in a small volume located at the end of the optic fiber.

The dimensions of the volume of material interrogated in such sensor designs

depend on the numerical aperture and diameter of the fiber, as well as the

optic element incorporated into it, if any.

Another type of optic fiber sensor uses the evanescent wave to excite the

fluorescence 24,28-30 . The volume element, termed the evanescent volume, is

rather different from that sensed by the distal- type of optic fiber probe. In

the evanescent- type design, the material surrounding the optic fiber to a depth

ranging from 30-100nm is measured by the fluorescence technique. Both optic

fiber probe designs make use of the guided waves to transmit the radiation to

the point of measurement as well as to guide modulated radiation to the

detection system. No commercial sensors based on the evanescent wave

excitation are currently available, although the design offers advantages in

characterization of interfaces and through -specimen sampling. By using an

array of waveguide- type sensors, an image of the state-of -cure as a function of

cure time of a thick and complex composite part can be made. There are

technical difficulties with evanescent- type sensors that must be resolved

before this design becomes commercially viable. Our work on optic fiber

sensors has explored the potential of the evanescent- type to monitor the

fluorescence of probe molecules.
27

EVANESCENT WAVE SENSORS

In the evanescent- type sensor, the fluorescence is excited by evanescent

waves. The evanescent wave arises from total internal reflection at the

waveguide -medium interface 31 . As long as the refractive index of the

surrounding medium is less than that of the fiber, there will be an angle of

incidence at the waveguide -medium interface beyond which light is totally

reflected. That angle, termed the critical angle, is given by

9 = sin" 1 (rijjj/ng ) [1]

where ng is the refractive index of the fiber, is that of the medium and 9

is the angle that the ray incident at the fiber -medium interface makes with the

surface normal. The salient feature of optic waveguides is that the electric

field amplitude of a totally reflected ray does not vanish at the waveguide

interface but penetrates a short distance into the surrounding medium of lower

refractive index. The electric field in the lower refractive index medium is

termed the evanescent wave and its magnitude at a distance z into the medium is

proportional to exp(-z/d), where d is given by

d = A/2II n8

[ s in ( 6 )

- (n,,./^) 2 ]* [2]

In this equation, A is the wavelength of light in vacuum, 9 is the angle of

incidence and , ng are as defined previously. The distance d is referred to

as the penetration depth and equals the distance at which the evanescent wave

magnitude decreases to 1/e its value at the interface. The magnitude of the

electric field at the interface also depends on the angle of incidence, 9 .
28

As a result of the evanescent wave, light propagating in a waveguide can

excite probe molecules that are located within a distance from the interface

approximately equal to the penetration depth. This distance is of the order of

lOOnm and according to equation 2 depends on the wavelength of the exciting

radiation, the refractive indices of the waveguide and the medium, and the

angle of incidence that the propagating light makes at the wave guide/medium

interface. The probability of excitation depends on the electric field

amplitude and therefore falls off exponentially away from the surface. For

this reason, evanescent wave excited fluorescence samples the interfacial

region, in contrast to the distal -type sensor design.

EXPERIMENTAL STUDIES

Experiments were conducted to investigate factors that may affect or limit the usefulness of optic fibers in cure monitoring systems. One potential

advantage of the waveguide- type sensor is that the fluorescence signal is

generated along the entire length of the fiber, and therefore descriptive of

the entire system. We have examined the dependence between the fluorescence

signal and the length of fiber exposed to the medium. Another aspect

investigated was the effect of the difference in the refractive indices of the

optic fiber and surrounding medium on the fluorescence signal. In order for a fiber to operate as a waveguide, the refractive index of the fiber must be

higher than that of the surrounding material. The penetration depth of the

evanescent wave and the efficiency of the waveguide to collect the fluorescent

light depend on the refractive index difference. In addition, the refractive

index of the resin system is expected to change during cure as a result of

changes in density and chemistry6

. To explore the degree to which the observed

fluorescence is affected by the refractive index difference, experiments were
29

cn

cij

Optical

setup

for

measurement

of

fluorescence

intensity

using

waveguide

type

optic

fiber

probes.
30

a

LD

r^

oo

o

LO

CO

o

LD

O

CO

CO

X

I

LDX

LU

_J

LU

>

<

a

LD

LD

A1ISN31NI Fig.

10

Fluorescence

spectrum

of

rhodamine-B

ina

solvent

mixture

of

propanol-1

and

dichlorobenzene

obtained

with

waveguide

type

optic

fiber

(+)

,

and

fluorescence

spectrum

of

optic

fiber

(*).
31

ID in in in in • • • • ID xfr CO OJ > < EO

oc

LU

CDw

LL

CD

LlI

COO

CLX

LU

X1ISN31NI 33N33S3aomd O

5-1 X

5-4 Omp0

in40) XI

•pH

4-iU

•HXa00X&so 1 • r—1 0 X T3 2 X X

§3 ° 0> 0

033 -0

03 0X>S X X o

T3 X 0x -o O 0 0) m x O x p. O X 0 0 ^ 0 X 0

•H XV) XP0X Xc o

•PI X X X X 00 ao p X 0 X 0 C X0 X X X3 0 x X O x m X X O I 0 0 P O x P E 0 o3 P O 0 Xa 5- 0Q 03

ti

•rH PL
32

conducted using dyes dissolved in mixtures of two solvents having different

refractive indices

All experiments were carried out using one of two setups, both of which

simulated arrangements envisaged for real time cure monitoring. The setup

diagrammed in figure 9 collects the fluorescence from the proximal end of the

fiber, whereas in the other setup the fluorescence is collected at the distal

end. In either arrangement, light from an argon ion laser is focussed onto one end (proximal) of the optic fiber. The fiber is held in a commercial fiber

mount which can be adjusted to position the fiber at the focal point of the

objective. The fiber, which is stripped of cladding, if any, passes through

the sample. The fluorescence light is collected at the distal end of the fiber

in one arrangement and in the other, as shown in figure 9, collected through a beamsplitter from the proximal end of the fiber. In either arrangement, the

fluorescence light is focussed onto the entrance slit of a double 0.25M

monochromator, and the intensity measured as a function of wavelength by photon

counting electronics.

The dependence of the fluorescence signal on exposed fiber length was

obtained for a 100 micrometer diameter waveguide made of ordinary silica,

refractive index 1.46, and the dye rhodamine-B at a concentration of 10-5M in a 30/70 by volume mixture of dichlorobenzene and propanol-1. The fluorescence

spectrum of rhodamine-B obtained with excitation at 488nm is shown in figure

10, and the peak intensity as a function of the length of fiber exposed to the

solution is depicted in figure 11. The findings given in figure 11 show a linear relationship between fiber length and fluorescence intensity. In these

experiments, no attempt was made to optimize the fluorescence signal so that

the minimum length of fiber needed to obtain good quality data has not been
FLUORESCENCE

MONITORING

USING

OPTIC

FIBERS

REFRACTIVE

INDEX

EFFECT

33

u t in

in o

o

cn

LU>-Q

in CD

. X — LU

CD

LU>

CJ

c

or

LL. LU

or

CO in

in

CO

CO*”4

P

ao

-O

'"G0P

O

0

o

u

m

c

o

I—l 3

0

cfl PQ

1 0

c

•rHI

T30

4=P

1

£“• * <P X

0 P T3

,C C

CU}th

•rH rH 0

> P *P G P

0 U

U CO Vj p 0 Ph P 0

O P

PH

C 6oi) AJ.ISN31NI 33N33S3a0nid

Ph >

O r—

O

>s V/ •H 0

C/5 rC C P

0

p c c o 0 VO rC ^

PHO X0

0 T3 o c

G -p 0

T3C P

0 0 GG0 -ri Q P

bO

•rH Uh
34

addressed. Other factors such as the total change in fluorexcence intensity

during a cure cycle, fiber characteristics (diameter, refractive index

composition) , laser intensity, and dye concentration will influence the minimum

length of exposed fiber required for measurement of state of cure.

The straight line of figure 11 extrapolates to the origin, zero intensity

at zero length, which provides an internal check on the data. This observation

differs from that made by Andrade et al 28 who used a somewhat similar design to

determine fluorescence from solutions. In their optic fiber system, the fiber

terminates in the solution; the distal end is blackened, and the signal is

observed from the proximal end of the fiber. With this design, their plot of

signal intensity versus immersion depth does not extrapolate to zero intensity,

but to a finite value. A possible explanation is that guided light may escape

the fiber at the air- solution interface and excite molecules situated outside

the fiber-solution interfacial region. Light will escape the fiber at the airsolution boundary because the critical angle for fiber/air is less than that

for fiber/solution owing to the difference in refractive index between air and

the solution. With proper selection of the optics coupling the laser beam into

the fiber, the effect can be minimized as demonstrated by the data shown in

figure 11.

Experiments also verified that a linear relationship existed between the

dye concentration and the fluorescence intensity.

As the refractive indices of epoxies are in the range 1.5 -1.6 one

potential problem is the availability of fibers of high refractive index. It

is likely that the fiber index would have to be close to that of the resin.

For this reason, we have examined the dependence of the fluorescence intensity

observed with a fiber on the difference between refractive indices of the
35

waveguide and surrounding medium. In figure 12 is shown the fluorescence

intensities observed with a glass fiber (index 1.46) immersed in rhodamine-B

solutions of different refractive indices. Refractive indices in the range

1.34 to 1.56 were obtained with mixtures of propanol -1 and dichlorobenzene.

Care was taken to ensure that the same length of fiber was exposed to the

solution for each measurement. The observed fluorescence increases by three

orders of magnitude when the solution refractive index is less than that of the

glass fiber. The residual intensity observed with the solution index higher

than that of the fiber is attributed to scattering of the propagating light at

the air- liquid interface. The scattering results from a change in the minimum

angle of incidence for total internal reflection when the surrounding medium

refractive index increases at the air- liquid interface. The observed

fluorescence intensities are also sensitive to the effects of the refractive

index mismatch on the efficiency of the fiber to collect some of the

fluorescence light for transmission through the fiber to the detector. The

effects of the refractive index and fiber diameter on the collection efficiency

has been analyzed theoretically and will be discussed in the next section.

The dye systems described in the preceding section require excitation in

the near UV and UV regions of the electromagnetic spectrum. Unfortunately, most

glasses, particularly those with refractive indices higher than that of epoxy

resins, fluoresce strongly upon excitation by radiation in this wavelength

range. Optic fibers made of high purity quartz have been used to excite and

collect fluorescence of probe molecules which have been excited at 363. 8nm by

an argon laser with UV reflectors. However, the refractive index of quartz is

too low for use as a waveguide with epoxy resins. Owing to the limitations of

glass fibers we have explored the potential of plastic fibers made of
36

polystyrene, PS, and poly(methyl methracrylate) , PMMA. The refractive index of

PS is 1.6 and PMMA has an index of 1.499. The latter material, PMMA, may be

marginally acceptable for some epoxy systems, whereas the former has general

application. Both plastic fibers are chemically inert in an epoxy system.

Another possibility, which may be worthy of consideration, is the fabrication

of optic fibers from epoxy resins. This is attractive for the following

reasons. The refractive index of the epoxy fiber would be higher than that of

the epoxy prepolymer resin because of densification, and subsequent to cure the

fiber would have a chemical composition very similar to that of the cured

resin. The latter effect could be important if the optic fibers made of glass

serve as failure initiation sites when the composite parts in which they are imbedded are placed in service.

THEORETICAL CONSIDERATIONS

It is desirable to improve the efficiency of fiber optics

fluorescence excitation and collection systems in order to reduce fiber

diameters and probe concentrations, as well as to permit the use of less

expensive light sources and detection systems.' Efficiency, as used here, is

defined as the product of the probabilities of fluorescence excitation with

evanescent wave light and of observing a photon of fluorescent light at the

detector end of the optic waveguide. Calculations were conducted to determine

the effect of various factors, listed in Table 1, on the excitation and

collection probabilities. In addition to the factors listed in Table 1, we

considered the effect of the probe molecule absorption on the refractive index

of the resin-probe medium and we will discuss this aspect first.

As evident from figure 12, small changes in the refractive index of the

epoxy-probe system when the waveguide index is only slightly larger can have a
37

demonstable effect on the waveguide excitation and collection efficiency. The

high refractive index of epoxies relative to most waveguide materials is likely

to limit the available range of refractive index differences between the

waveguide and medium. It is possible, therefore, that the medium index would

be only slightly less than that of the fiber waveguide and the effect of the

dye on the refractive index should be considered. Although the probe

concentration is low its effect on the medium refractive index at the

excitation and emission wavelengths may be non-negligible owing to anomalous

dispersion.

Dispersion in the refractive index, , in the frequency range of the

probe absorption was modeled by the Lorenz -Lorentz relation32

,

2 2 222 22 22

n^ = n„ + 4n Ne f(w' )/me [(o>' -c/) + y w ] [3]

In this equation, nQ is the refractive index of the epoxy without the probe, N

is the probe concentration in number of molecules per cc, e and me are the

charge and mass of the electron, u> is the frequency of the light, 7 and f are

the halfwidth and oscillator strength of the electronic transition,

respectively, and w' is the electronic transition frequency shifted by

-4 II e

2 f/3 me

. The frequency dependence of the refractive index given by

equation 3 has implications for both the excitation and collection of

fluorescent light by an optic fiber. The presence of the probe molecule has no effect on the refractive index for u> = w' , increases the refractive index above

nQ for u<w' , and lowers the refractive index below nQ at frequencies greater

than u>' . Based on equation 3 it may be possible to select an excitation frequency

greater than w' thereby decreasing the refractive index of the medium.

Unfortunately, the few laser lines available and the low probability of
38

electronic excitation off -resonance militate against achieving a lower

refractive index through the presence of the absorbing probe. In the case of

the emission, the refractive index of the epoxy and probe system would be

greater than that of the epoxy alone since the fluorescence frequencies are

less than u>' . We calculated the maximum change in refractive index to be 0.01 using a probe concentration of 10-4 mol/1, a frequency w' corresponding to 500nm, an

oscillator strength, f, of 1.0, and a bandwidth, 7, of 2000cm- 1. Reference to

figure 12 shows that a change of this magnitude would only affect the

efficiency of the optic fiber sensor if the difference in the refractive

indices of the fiber and epoxy resin was less than 0.04.

TABLE 1 Input Parameters for Modeling Fluorescent Light Excitation

and Collection Efficiency of Optic Fibers

ELEMENT PARAMETERS

laser source

optic system

optical fiber

frequency, beam profile and diameter

numerical aperture and power of

objective lens used to focus laser

light onto fiber

diameter and refractive index

medium refractive index, quantum efficiency

and concentration of probe

The calculation of the efficiency of the optic fiber excitation and

collection can be divided into five parts:

(1) The intensity profile of the focussed laser beam as a function of the

angle of incidence at the front (proximal) surface of the optic fiber; (2) the

evanescent wave amplitude as a function of the angle of incidence at the
39

waveguide -medium interface and distance from the waveguide surface; (3)

probability of absorption by probe molecule with photon emission; (4) portion

of the fluorescent light entering the waveguide as a function of the angle of

incidence at the medium/waveguide interface; and (5) transmission efficiency of

the collected light. We will outline the computational approach to each of

these 5 parts below.

The excitation and collection efficiency is given by the probability

of observing a photon of fluorescent light at the end of an optic fiber

collection system. The probability is given by

L rn/2 2 r r P(A0 )=nPo CQ P(A0 ,A')

|

I

rH/2 rsin-l (po/(po+z)

)

r

J o f(0 r ) I(0 r ,z,A0 ,n )d9 r

.g

) [Fr (z ,0,^,1^ ,n

g ] dzd<f>dipd£

[ 4 ] The integration variables of equation 4 are defined as follows: 0

r is

i

n/ £. p£>J-ll- 1 ^U/ )

- s in" 1

( p /

(

Po +z ) )

the angle of incidence of the excitation laser at the waveguide -medium

interface; 2 is the axial fiber distance measured from where the optic

fiber enters the sample and L is total length of fiber exposed to the

medium; z is the radial distance measured from the fiber surface; <f> and

ip are the angles of a spherical coordinate system located on the

fluorescing molecule and their integration limits are set to include all

emitted light rays that intersect the waveguide. Since the fiber optic

illumination and detection system has radial symmetry, the integration over

this angle variable is included in the factor multiplying the integral

term. The remaining quantities of equation 4 which determine the

excitation probability are as follows: CQ is the probe concentration;

is t*ie probability of absorption of a photon of energy
40

hc/A0 with the emission of light of wavelength A'; f(0 r ) is the portion of the

laser light beam intersecting the waveguide -medium interface at the angle

9 r ; and I (0 r , z , A0 , 1^ , ng ) is the intensity of the evanescent

wave at the radial distance z from the waveguide. The collection

probability is given by the product

Fi (z, ,n

g )Fr (z, ,n

g ^

which gives the probability that a photon emitted at z,i in the direction

arrives at the detector end of the optic fiber. Fi (z ,n

g

) gives the

fraction of light refracted into the fiber and Fr (z , <f> f iJ> , n,,, ,n

g ) N ( * • ^ » L '^) is the

portion of that light which reaches the detector. The exact form of Fi and Fr depends on whether the emitted light is evanescent as will be discussed below.

The computational approaches used in each of the previously mentioned

five parts are defined as follows:

1. In our experiments, we used the 363. 8nm UV line of an argon ion laser and a

20X, 0.4 NA microscope objective lens to focus the laser light onto the

proximal face of the fiber. From the characteristics of the lens, the laser

beam profile (Gaussian), and the beam diameter (1/e2 intensity values) we

determine the distribution of the incident light intensity, g (<f>) , as a function

of the angle of incidence on the proximal fiber face. The distribution of the

source intensity as a function of the angle of incidence, f(0 r ), at the

waveguide -medium interface is found using Snell's law. The intensity

distributions -are related

f(* r )=g(?) [5]

where

J=sin_1 (n

g cos(0 r ))

and the index of air is taken to be unity. Since only guided waves are of
41

interest, we limit consideration to angles of incidence at the waveguide

proximal face that satisfy total internal reflection at the waveguide -medium

interface, or7< sin" 1 ((ng 2 -t^ 2

) %) [6]

The maximum value of <f> is limited by the numerical aperture of the focussing

lens which can be selected to satisfy equation 6 and thereby to minimize light

loss at the air- fiber-medium interface.

2. The amplitude of the evanescent wave at the distance z from the

waveguide/medium interface is given by

exp [ —b- (sin (0 r ) - (r^/ng) ) ] [7]

where A0 is the wavelength of the excitation light in vacuum. The intensity is

proportional to the square of the product of this quantity and 2 cos (& r )/(l -

(r^/ng

)

2

) 1 1; the latter expression gives the functional dependence of the

electric field at the interface on the angle of incidence and the refractive

indices. In equation 1, the intensity form I (0 r

, z ,

0 , ,n

g

)

is

2

4 cos d

T -4nzng I(0 r ,z,A0 .r^.ng ) =

( 1

. (nm/n&) 2

) exP

[ (sin (0 r )-( ) ]

[ 8 ] This expression is plotted in figure 13 as a function of the angle of

incidence, d

T and distance, z.

3. The probability of a molecule emitting a photon at wavelength A' is given

by the sum of the probabilities of spontaneous and stimulated emission, with
EVANESCENT

WAVE

SPECTROSCOPY

PENETRATION

DEPTH

o

in

CO c\j o

A1ISN31NI 3AVM 1N33S3NVA3

X

O

0

C

re T3C

re

0

U -M c c

3 -H

M-i

x, 3

re -h 0 0

E

i 4-i 4

•fH 0

rH Xa•n-* E 4-

re 0 0 X

> +-> re3 +-< re

4-> c 0

0 u

u e

(A 0

0 -D C • i—i re U

> C X • iH

• pH ix

DISTANCE

INTO

MEDIUM

(anq)
43

the latter proportional to the energy density of radiation of the wavelength

A' .

P(A')

e

2

f A' E

2

8n

2 e2

f

4h me C c

2 A#

[9]

where e, me are the charge and mass of the electron, respectively; C is the

velocity of light; h is Planck's constant; f is the oscillator strength of the

electronic transition; A' is the wavelength of emission; and E is the electric

field amplitude at the molecule. Alternately, the probability of fluorescence

may be expressed as the product of the quantum efficiency, q and the

probability of absorption. The quantum efficiency is defined as the

probability that an excited molecule will emit light to return to the ground

state rather than take non- radiative pathways to electronic de- excitation. The

variation in the quantity q with medium viscosity gives the sensitivity of the

fluorescence method for viscosity monitoring.

4. A probe molecule situated at the distance z from the interface will

fluoresce with the probability p(A0 ,A') I(0 r ,z,

0 , 1^ ,n

g

)

Not all of the

fluorescent light will impinge on the detector. To determine the solid angle

intercepted by the fiber we treat the fluorescing molecule as a point source at

z and determine the angles of incidence that the wavefront normals make with

the fiber surface normal. This angle is given by

• - 1 = sm /{ —sin(<£) - r 2 , lx 2 . 2,1/2. — [p cos (V>) - p + z ] } [ 10 ]

where p—pQ + z, and <f>

V are the angles of a spherical coordinate system

situated on the fluorescing molecule at z. The *z axis of the spherical
A1ISN31NI “IVNOIlDYdd

RATIO

FIBER

RADIUS

TO

EMITTER-FIBER

CENTER

DISTANCE

Fig.

14

Fraction

of

fluorescent

light

incident

on

optic

fiber

as

function

of

the

ratio

of

fiber

radius

to

distance

between

emitter

and

fiber

center.
45

coordinate system is parallel to the optic fiber direction, 2 (of equation 4)

and <j) is the angle between *z and the wavefront normal. To include all light

refracted into the fiber which would propagate in the positive 2 direction, <f> varies between 0 and n/2 and \j> between -sin_1

(p 0 /p) and sin-1(p Q /p).

Figure 14 shows the fraction of light incident on the fiber as a function

of the fiber radius and distance from the fiber surface. Departures from the

optimal value exist only for exceedingly small fiber diameters and/or large

distances. For distances over which the amplitude of the evanescent wave is

significant (0<z<Ao ) z /p « 1 since p Q is typically in the range 25-100^m and

A0 ~ 500nm. This implies that approximately 1/2 of the light emanating from

the point z impinges on the optic fiber.

5 . Some of the fluorescent light incident on the optic fiber at the angle of

incidence 9

S , given by equation 10, will be refracted into the fiber and

propagate down the fiber to the detector. The efficiency of the collection and

transmission depends on whether the angle 9

S is greater or less than the

critical angle,

*sc = sin’^TAn/ng) [11]

All the refracted light for 9 S >9 SC and propagating towards the detector will

reach the detector end of the fiber unless losses occur through scattering from

voids, impurities, etc. Light refracted into the fiber at angles of incidence

less than the critical angle propagates towards the detector by successive

reflections at the waveguide -medium interface, but with intensity loss at each

reflection owing to refraction into the medium.

The principle of reciprocity in electromagnetic theory leads to the

prediction that emission from an evanescent wave excited molecule, located near
46

the interface, should give rise to a totally internally reflected wave in the

waveguide 33 . The results given in reference 19 and in figure 11 are

experimental verification of this prediction for the following reasons. If the

fluorescing molecule is treated as a point source, e.g., the molecule is

situated far from the interface, it is easily shown that no light would be

refracted into the fiber such that it has an angle of incidence with the

wave guide -medium interface greater than the critical angle, sin" 1 (r^/ng )•. A

consequence is that light entering the waveguide would not undergo total

internal reflection and a portion of the fluorescent light would escape the

waveguide because of refraction. Except near the critical angle, the loss

through refraction at each reflection would be high and owing to the large

number of successive reflections needed to propagate the light to the detector,

virtually no fluorescence would reach the detector. If no fluorescent light

propagates in the waveguide at angles of incidence greater than the critical

angle then only light that enters the fiber near the detector would have a high

probability of reaching the detector. As a consequence, the detector signal

would not depend linearly on the length of the fiber exposed to the medium but

would increase rapidly at lengths of the order of the fiber diameter and

plateau at longer lengths. The observed linear dependence of the fluorescence

signal with exposed fiber length as well as the observation of light entering

the waveguide at angles beyond the critical angle support the notion of the

emission of evanescent waves by excited molecules.

The theory of the emission of evanescent waves by excited molecules has

been treated by Carniglia, et al 33 and Lukost and Kunz 34 using a classical

treatment of the transition dipoles. We will outline the treatment of

Carniglia, et al 33 and expand on it as necessary for the system under
47

consideration here. We seek the electric field amplitude in the waveguide

originating from a transition dipole, P situated a distance z from the

waveguide -medium interface. Note that treatment of the excited molecule as a point source leads to the result that fluorescent light refracted into the

waveguide cannot propagate at angles of incidence (with the normal to the

waveguide -medium interface) greater than the critical angle. The presence of a wave propagating in the waveguide in a direction beyond the critical angle

would imply the emission of an evanescent photon. Whereas Carniglia et al 33

treated a transition dipole oriented in a plane parallel to the interface we

will use a transition dipole with random orientation, and then average over all

orientations. The treatment of a dipole oriented perpendicular to the

interface is presented in reference 34 . Let p x ,p2 >P 3 be the projections of the

transition dipole on three mutually perpendicular directions corresponding to

the radial (z) and axial (x) fiber directions with the third direction mutually

perpendicular to these two. The electric field on the surface of the waveguide

(z=0) is given in the plane wave spectrum representation as

where K is the wavevector in the medium with components, Ki.K2.K3 and K is the

angular frequency in units such that the velocity of light in vacuum is C=1

The plane wave expression for the electric field across a dielectric

boundary and originating from an oscillating dipole has been solved

numerically3

for various values of rK where r is the distance from the

observation point to the dipole. The range of rK in the calculation3

included

both the near and far field limits. The treatment in reference 34 is in the

near field limit. Both limits lead to identical expressions for the angular
48

distribution of intensity and dependence on the distance from the interface.

An analytical solution to equation 12 has been obtained in the far field

limit33 which has the advantage that the functional dependence of the intensity

on the observation angle, the refractive index ratio, and the distance between

the dipole and the interface can be shown. We will use a similar approach in

our treatment of radiation from a randomly oriented dipole in the far field

limit.

The term [(P*K)K-K2 P] is decomposed into the TE and TM components

A (P-K)K-K2 P = PQ [ae + /3 (K x e)] [13]

A

where e is the unit vector normal to K and the plane of incidence, (TE) , and K

A

is the unit wave vector, K/ K, so that Kxe is the TM polarization. PQ is the

dipole moment magnitude,

P0 =(Pi 2 + P2

2 + Ps2

) %

Expansion of equation 13 yields

a = K

2 (p2 K, - Pl K2 )/(K, 2 + K2

2 )* [14]

and

P = Oq 2 + K2

2 )* K( Pl iq + P2 k2 + P3 K3 )-K3 (KlPl + K2 P2 )/K3 (K1 2 + K2 2 )*

[15]

The TM term differs from those of reference 33 in that p3 components are present whereas the TE component is identical since TE is perpendicular to the

interface, and hence to z. Using the Fresnel formulas 35 we have for the field

in the waveguide

,

E(r ’ c) - rj dkidkz exp(iK3Z)

[gficr +

2nff(Kxe )1

k3 +n2 K3 J where n is the refractive index ratio, ng/n^.

far-field limit to yield

Equation 16 is solved in the
49

[17]

The k values now correspond to directions along r, the other components being

negligible in the far field approximation.

The intensity of light for the TE mode at the interface but inside the

waveguide and propagating in the direction 9

S measured from the interface

normal becomes

[18]

for 9 S <9 SC and

P

2 I(*S.R) =

[19]

Let 9 S

' be the angle of incidence in the lower index medium so that

k3 = ng K cos (0 S ) [20a]

and

K3 = K cos (0'

s ) [20b]

The angles 9

S and 9S

' are related by 0

S

' = sin" 1 (ng/r^ sin 9

S ), or |K3 | = rv K (n

g W sin2

(tf s )-l)* h'>hc [21]

Substituting equations 20 and 21 into equation 18 we have

I(0 s ,R) = [ 22 ]

and equation 19 becomes

(sin2 (^)-nm

2 /ns 2

)

J=) [23]

Integration of a

2

, where a is given by equation 12, over all orientations of

the transition dipole moment yields
ANGULAR

DISTRIBUTION

DIPOLE

EMISSION

A1ISN31NI

ANGLE

OF

REFRACTION
51

The expression 23 differs from equation 22 by the exponential term

exp(-2z|K3 |) which is identical in form to the evanescent wave decay. Thus,

the signal at the detector depends exponentially on the distance between the

fluorescing molecule and the fiber surface in the probabilities of both the

excitation and collection of fluorescent light. For this reason, the

evanescent wave type of sensor is particularly sensitive to the interface

region.

When 9 S '>9 SC equation 23 replaces the term Ft *Fr N

^ ^

L of equation 4

for the TE polarization. The TM mode contribution is determined in a similar

manner using the (3 expression. In figure 15 we show the angular distribution

of the fluorescent intensity observed in the waveguide and originating from a dipole situated in the lower index medium. The maximum intensity occurs at the

critical angle determined by the refractive index ratio. The intensity at

angles greater than the critical angle derives from evanescent wave emission

and propagates without loss to the detector. Light which enters the waveguide

at angles less than the critical angle decays in intensity rapidly as it

propagates down the waveguide owing to losses by refraction. The different

curves of figure 15 give the dependence of the emitted intensity observed in

the waveguide on the distance between the dipole and the waveguide interface.

Inserting equation 23 for the terms FL (z,^,^,^ ,n

g ) [Fr (z ,<f> , 1^ ,ng ) ] N( * * * •

in equation 4 we calculate the intensity distribution as a function of the

refractive indices and ng

. The calculated values for the refractive indices

of the data shown in figure 12 are compared to the experimental values in

figure 16. The calculated intensity was scaled to fit the experimental value

in the plateau region, refractive index ratio >1.04. The agreement over the
EVANESCENT

WAVE

SENSOR

REFRACTIVE

INDEX

EFFECT

52

+

*

+

*

+

' *

+

*

*

*

*

*

I inr^incDininin^inro • • cd in oo

( 6°d aiisn3j.ni aoNaasaaoma

.95

.97

.99

1.01

1.03

1.05

1.07

REFRACTIVE

INDEX

RATIO

Fig.

16

Collection

efficiency

of

optic

fiber

for

evanescent

wave

emission

0

0

as

function

of

s

sc

refractive

index

ratio;

experimental

points,

*,

calculated

noints,

+.
53

range of refractive index ratio in which evanescent wave excitation occurs

>1.0) is satisfactory.

We next consider the non- evanescent contribution to the fluorescent

intensity collected and transmitted by the waveguide. The portion of light

entering the fiber at the angle 9 S at the distance L-£ from the detector which

reaches the detector is given by Fr (9 S , 1^ ,n

g ) N ( e

s

'

• where Fr (^, 1^,^) is

the reflection coefficient at the fiber/medium interface averaged over the two

polarizations

tan2 (9 S

' - 9

S ) F; = [24a]

2 tan (0 S

' + 9 s ) and

-1- sin2 (0 S' - 9

S ) Fr = ; [24b]

sin2 (0 S

' + 9

S )

and N (9 S

'

, L-i) is the number of reflections necessary to propagate light

which enters the fiber at the angle 0

S

' with the interface normal the distance

L-i,

N(0 '

, L-i) =

(L' i)tan

[25]

2 Po

The functional dependence of the reflection and refraction coefficients

with the angle of incidence 9

S is such that where the refracted light intensity

is high (near unity) , the reflection coefficient (at the opposite side of fiber

interface) is low, and vice-versa. The refraction loss dominates the

transmission efficiency, however, owing to the large number of reflection
54

required to propagate the fluorescence light down the fiber (see equation 25)

The dominant contribution to the collected light intensity derives from the

evanescent wave, for which 9

S > 9 SC .
55

REFERENCES

1. National Materials Advisory Board, "Materials for Lightweight Military

Combat Vehicles", NMAB-396, National Research Council, Washington,

D . C . (1982)

2. National Materials Advisory Board, "Organic Matrix Structural

Composites: Quality Assurance and Reproducibility", NMAB-365, National

Research Council, Washington, D. C.(1981).

3. Weatherhead, R. G., "FRP Technology Fibre Reinforced Resin Systems",

Applied Science, London (1980).

4. Vinson, J. R. , ed. "Advanced Composite Materials -Environmental

Effects", STP 658, American Society for Testing and Materials, (1978).

5. Gillham, J. K. , and Enns , J. B. , J. Appl. Polym. Sci., 28., 2567(1983).

6. Tsai, S. W. , and Hahn, H. T. , "Introduction to Composite Materials",

Technonic Publishing Co., Westport, CT(1980), Chap. 8.

7. Sanjana, Z.N., Polym. Eng. Sci., 26., 373(1986).

8. Bidstrup, W.W. , Sheppard, N.F. , Senturia, S.D., Polym. Eng. Sci. 26, 358

(1986).

9. Kranbuehl, D. , Delos, S., Yi, E. , Mayer, J. and Jarvie, T. , Polym. Eng. Sci.

26, 338(1986).

10. Zukas , W. X., "Polymers and Polymer Matrix Composites Workshop", University

of Connecticut, Storrs, CT. , May, 1986.

11. Sung, C.S.P., Pynn, E. and Sun, H.-L., Macromolecules, 19_, 2922(1986).

12. Levy, R. L. , and Ames, D.P., in "Adhesive Chemistry Developments and

Trends", L.-H. Lee, ed. , Plenum Press, New York (1983).

13. Levy, R. L. , Polym. Mater. Sci. Eng., 50, 124(1984).

14. Levy, R. L. and Ames, D.P., Polym Mater. Sci. Eng. 53., 176(1985).

15. Lakowicz , J., "Principles of Fluorescence Spectroscopy", Plenum Press,

New York (1983), Chap. 7.

16. Wang, F. W. , Lowry, R. E. , and Fanconi, B., Polymer, 27, 1529(1986).
56

17. Fanconi, B. , Wang, F. W. , and Lowry, R. E., in "Review of Progress in

Quantitative Nondestructive Evaluation", Plenum Press, New York, 6B,

1287(1987).

18. Wang, F. W. , Lowry, R. E. , Pummer, W. J., Fanconi, B., Wu, E.-S., ACS

Symposium Series, in press.

19. Sung, C.S.P., Chin, I. J. and Wu, W.-C., Macromolecules, 13, , 1570(1985).

20. Oster, G. and Nishijima, Y. , J. Am. Chem. Soc., 78., 1581(1956).

21. Birks , J. B. , "Photophysics of Aromatic Molecules", Wiley

Interscience, New York(1970).

22. Kauzmann, W. , "Quantum Chemistry", Academic Press, New York(1957)

23. Law, K.Y. , Chem. Phys . Letrs. 75, 545 (1980).

24. Fanconi, B. , Wang, F. , Hunston, D. , Mopsik, F. , "Cure Monitoring for

Polymer Matrix Composites", in "Materials Characterization for Systems

Performance and Relibility" , J. W. McCauley and V. Weiss, eds .

,

Plenum Press, New York, (1986).

25. Harrison, G. and Barlow, A. J., "Dynamic Viscosity Measurements",

Vol. 19 pp 137-178, in "Methods of Experimental Physics", Academic

Press, New York(1981)

26. Private communication, AT&T Laboratories.

27. Hirschfeld, T. , Deaton, T. , Milanovich, F. , Klainer, S., Opt. Eng.

22, 527(1983).

28. Newby, K. , Reichert, M. L. , Van Wagenen, R. , and Andrade, J. D.

,

Appl . Opt. 23, 1812(1984).

29. Andrade, J. D. , VanWagenen, R. A., Gregonis , D. E. , Newby, K.

,

Lin, J.-N., IEEE Transactions on Electron Devices Vol ED-32, 1175

(1985)

30. Newby, K. , Andrade, J. D. , Benner, R. E. , and Reichert, W. M. , J.

Coll. Interface Sci., 111 . 280(1986).

31. Ditchburn, R. W. , "Light", Interscience, New York(1963), Chap. 14.

32. Ibid., Chap. 15.

33. Carniglia, C. K. , Mandel, L. , Drexhage , K. H. , J. Opt. Soc. Am., 62

479(1972).
57

34. Lukosz, W. and Kunz , R.E., J. Opt. Soc.Am. 67, 1607 (1977) and ibid, pg

1615.

35. Burghart, T. P. , and Thompson, N. L. , Biophys. J., 46, 729(1984).

36. Born, W. , and Wolf, E. , "Principles of Optics", Pergamon Press LTD.,

London (1965) , Chap . 1
58

LIST OF PUBLICATIONS UNDER ARO SPONSORSHIP

"Picosecond Excimer Fluorescence Spectroscopy: Applications to Polymer- Segment

Mobility and Polymerization Monitoring", F.W. Wang, R.E. Lowry and R.R.

Cavanagh, Polymer, 26., 1657 (1985).

"Cure Monitoring of Epoxy Resins by Fluorescence Spectroscopy", F.W. Wang, R.E.

Lowry, and B.M. Fanconi, Polymeric Materials Science and Engineering, 53., 180

(1985).

"Fluorescence Monitoring of Viscosity and Chemical Changes During

Polymerization", F.W. Wang, R.E. Lowry, and B.M. Fanconi, Polymer Preprints 27. (2), 306 (1986).

"Cure Monitoring for Polymer Matrix Composites", B.M. Fanconi, F.W. Wang, D.L.

Hunston, and F.I. Mopsik, in Materials Characterization for Systems Performance

and Reliability, eds . J.W. McCauley and V. Weiss, Plenum Publishing, p. 275

(1986).

"Fluorescence Methods for Cure Monitoring of Epoxy Resins", F.W. Wang, R.E.

Lowry, E.-S. Wu and B.M. Fanconi, Proceedings of the 1985 Scientific Conference

on Chemical Defense Research, 573 (1986).

"Novel Fluorescence Method for Cure Monitoring of Epoxy Resins", F.W. Wang,

R.E. Lowry and B.M. Fanconi, Polymer, 27, 1529 (1986).

"Process Monitoring of Polymer Matrix Composites Using Fluorescence Probes",

B.M. Fanconi, F.W. Wang, and R.E. Lowry in Review of Progress in Quantitative

Nondestructive Evaluation, eds. D.O. Thompson and D.E. Chimenti, Plenum

Publishing, Vol. 6B, p. 1287 (1987).

"Comparisons Among Process Monitoring Techniques", B.M. Fanconi and F.W. Wang,

ANTEC '87, 1100 (1987).

"In-Situ Characterization of the Interface of Glass Reinforced Composites",

F.W. Wang and B.M. Fanconi, Final Report of ARO Contract MIPR ARO 111-84, NBSIR

870302 (1987).

"Cure Monitoring of Epoxy Resins by Fluorescence Recovery After

Photobleaching" , F.W. Wang and E.-S. Wu, Polymer Communications, 28, 73 (1987).

"Fluorescence Monitoring of Viscosity and Chemical Changes During

Polymerization", F.W. Wang, R.E. Lowry, W. Pummer and B.M. Fanconi in

"Photophysics of Polymer Systems" (J.M. Torkelson and C.E. Hoyle, eds.), ACS

Symposium Series, in press.

"In- situ Fluorescence Monitoring of Viscosity and Chemical Changes During

Processing of Polymer Composites", F.W. Wang, R.E. Lowry, and B.M. Fanconi,

Proceedings of the 1986 Scientific Conference on Chemical Defense Research, in

press
NBS-114A (REV. 2 -ec) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 87-3581

2. Performing Organ. Report No. 3. Publication Date JULY 1987

4. TITLE AND SUBTITLE

FINAL REPORT - In-Situ Characterization of the Interface of Glass Reinforced

Composites

5. AUTHOR(S)

F. W. Wang & B. M. Fanconi

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) 10.

SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

A technique has been developed to monitor the viscosity of curing epoxies.

Fluorescence spectroscopy of viscosity-sensitive organic dyes is used together with

optic fibers as a means of delivering the probing light to the measurement site

and retrieving the modulated light for detection. The technique is adaptable to

remote sensing through the use of optic fibers. The evanescent wave of the guided

radiation is used to excite probe molecules lying at the interface between the

fiber and surrounding matrix • Therefore, the technique is adaptable to in-situ

characterization of the interface of glass reinforced composites.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) cure monitoring, viscosity, epoxies, fluorescence, optic fibers, remote sensing

13. AVAILABILITY

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