Free-Volume-Dependent Fluorescence Probes of Physical Aging in

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Free-Volume-Dependent Fluorescence Probes of Physical Aging in Polymers Scott D. Schwab and Ram L. Levy McDonnell Douglas Research Laboratories, St. Louis, MO 63166

Free-volume-dependent

fluorescence

(FVDF)

probes were used to

monitor physical aging in several types of polymers.

Changes in

fluorescence intensity (I ) of the FVDF probes correlate with changes f

in microscopic free volume. The glass transition temperature (T ) of g

probe-containing

specimens was indicated by a slope change in a plot

of I as a function of temperature. f

I increased monotonically f

with

safo-T anneal time. The ability of these probes to monitor the therg

moreversibility of the physical aging process was also demonstrated.

THE

U S E O F POLYMERS AS E N G I N E E R I N G MATERIALS for aerospace systems

r e q u i r e s a t h o r o u g h u n d e r s t a n d i n g o f the n a t u r e a n d extent o f the p r o p e r t y changes t a k i n g place u n d e r service c o n d i t i o n s . P h y s i c a l a g i n g o f a m o r p h o u s p o l y m e r s is a u n i v e r s a l , t h e r m o r e v e r s i b l e process that occurs b e l o w t h e material's glass transition t e m p e r a t u r e (T ). P h y s i c a l a g i n g i n d u c e s changes g

i n m a n y o f the p r o p e r t i e s o f p o l y m e r i c glasses, a n d a g r o w i n g n u m b e r o f e x p e r i m e n t a l t e c h n i q u e s is b e i n g u s e d to m o n i t o r the k i n e t i c s a n d m a g n i t u d e of these changes. S o m e t e c h n i q u e s measure changes i n macroscopic p r o p erties s u c h as v o l u m e , e n t h a l p y , a n d stress relaxations (1-3); s t r e s s - s t r a i n b e h a v i o r (2); a n d u l t i m a t e t e n s i l e s t r e n g t h (2); o t h e r t e c h n i q u e s r e s p o n d d i r e c t l y to changes i n m i c r o s c o p i c parameters (i.e., l o c a l free v o l u m e ) o r m o l e c u l a r m o b i l i t y s u c h as d i e l e c t r i c (4\ I R (5), N M R (6), a n d p o s i t r o n a n n i h i l a t i o n (7) spectroscopies as w e l l as U V - v i s i b l e spectroscopy for m o n itoring local

free-volume-dependent

photoisomerization of

photochromic

probes a n d labels (8-11). T h e s e m e t h o d s , h o w e v e r , are not p a r t i c u l a r l y s u i t e d to c o n t i n u o u s , i n - s e r v i c e m o n i t o r i n g o f p h y s i c a l aging. T h i s c h a p t e r

0065-2393/90/0227-0397$06.00/0 © 1990 American Chemical Society

In Polymer Characterization; Craver, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1990.

398

POLYMER CHARACTERIZATION

describes an application of

free-volume-dependent

probes as a novel method for monitoring the s u b - T

fluorescence g

(FVDF)

free-volume relaxation

that occurs during the physical aging of amorphous polymers.

Physical Aging of Polymeric Glasses

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The volume (and enthalpy) relaxation in amorphous, glassy polymers below T

g

is a fundamental property of the glassy state (I) and results from the

nature of the glass transition. This phenomenon has long been studied in thermoplastics (J, 12), and in the last several years has been observed in some of the high-performance thermosets used in the aerospace industry (2, 13). A general diagram for how the relaxation process arises is given in Figure 1. As the polymer is cooled through T , segmental mobility becomes severely g

restricted, and excess, nonequilibrium volume is trapped in the matrix. Physical aging at a specified temperature, T , is defined as the slow loss of a

this excess free volume as the material approaches equilibrium. Because of the decrease in free volume, segmental mobility is further reduced, leading to embrittlement of the material. Thus, the physical and mechanical properties of the matrix are time-dependent and can change substantially from their initial values. Matrix properties strongly influence the overall properties of composite materials, and Kong and co-workers (2, 13) showed that the ultimate tensile strength, strain to break, toughness, and water sorption of carbon-epoxy composites decrease after physical aging. Physical aging, as opposed to chemical aging (e.g., oxidative degradation), is thermoreversible. If the polymer is heated above T followed by a g

rapid quench to below T , the free volume lost during physical aging is g

restored. The temperature range over which physical aging occurs is generally limited to between T and the highest temperature secondary transition ( T g

p

in Figure 1). In most cases, the rate of physical aging increases as the temperature approaches T and then decreases at temperatures near T (1). g

g

Many polymeric materials used in aerospace systems operate in a service temperature envelope that includes at least part of their physical aging range. Therefore, the ability to monitor the aging process is important for understanding the long-term performance of these polymers.

Free-Volume-Dependent Fluorescence The F V D F probe approach is based on the observation that the fluorescence quantum yields () of certain compounds are strongly dependent on the f

viscosity and free volume of the medium (14, 15). This dependence is the result of nonradiative deactivation of the excited state by intramolecular twisting or torsional motions that lead to a low value for in low-viscosity, f

high-free-volume media. However, when the free volume of the medium


23.

SCHWAB A N D LEVY

Free-Volume-Dependent

Fluorescence

Probes

399

A


Specific volume

Equilibrium line

Figure 1. Specific volume as a function of temperature for an amorphous polymer in the region of the glass transition temperature (Tg)- T is the aging temperature and Tp is the highest temperature secondary transition. a

decreases, these torsional motions are more inhibited, leading to an increase in 4> and giving rise to F V D F behavior (14-16). f

The utility of F V D F probes in polymer studies was first demonstrated by Oster and Nishijima (14). Loutfy exploited the F V D F approach to monitor the polymerization of vinyl monomers (16) and to determine polymer chain tacticity (17). Subsequently, we used the same F V D F probe to monitor epoxy cure kinetics (18). Loutfy (19) suggested the general utility of such "molecular rotor" fluorescence probes for monitoring polymer free volume. H e showed that r = ^ e x

P

0 -

(1)

where K is the rate of radiative decay, KJ is the intrinsic rate of molecular relaxation of the probe molecule, V is the occupied (van der Waals) volume of the probe molecule, and 3 is a constant for the particular probe. r

0

Experimental Details Materials. The F V D F probes were [p-(N,N-dialkylamino)benzylidine malononitrile)] (DABM) and 4-(N,N-dimethylamino)-4'-nitrostilbene (DMANS, Aldrich Chemical Co.) (see structures). These probes were dissolved in diglycidyl ether of bisphenol A epoxy (DGEBA, Dow D E R 3 3 2 ) at concentrations rangingfrom5 to 40 ppm and then cured with the stoichiometric amount of phthalic anhydride (PA, Aldrich), 1,6-diaminohexane (DAH, Aldrich), N,N-dimethyl-l,6-diaminohexane (DDH, Aldrich), or a mixture of the amines. Epoxy-amine samples were cured at 50 °C in silicone rubber molds according to the method described by Fanter (20).


400



H

DMANS To ensure that all chemical reaction had been completed, each sample was then postcured at 150 °C for 3 h. Epoxy-anhydride samples were cured at 135 °C for 24 h followed by a postcure at 170 °C for 6 h. DABM-poly(methyl methacrylate) (PMMA, Aldrich) samples were prepared by dissolving a mixture of the probe and polymer in methylene chloride and solvent casting onto a glass plate. The solvent was removed by allowing the sample to stand at room temperature for 48 h, followed by 24 h in a vacuum oven at 150 °C. D A B M could not be used in amine-cured systems because it reacts with amines to form a nonfluorescent (at visible wavelengths) compound. DMANS was found to be stable in amine-cured D G E B A provided it was stored in the dark. Prolonged exposure to light at wavelengths less than 450 nm caused the formation of an unidentified fluorophore whose maximum emission occurs at 415 nm. Because of the possibility of photodegradation, all samples containing DMANS were stored in the dark, and fluorescence measurements were made with the minimum possible excitation power. Instrumentation. All fluorescence measurements were made with an S L M 4800fluorescencespectrometer. The fluorescence intensity collected at a right angle to the excitation beam was focused onto a photomultiplier tube. Appropriate optical filters were used to isolate the fluorescence from the scattered excitation light. Physical aging was carried out in the spectrometer sample chamber, where the temperature was controlled to 0.1 °C with a water circulator. Spectra were corrected for the wavelength variation of the instrument response. Differential scanning calorimetry (DSC) was used for determination of T and enthalpy relaxation and was carried out on a DuPont 9900 thermal analyzer. All scans were made at 10 °C/min in a flowing N atmosphere. The T s of the four samples were, in degrees Celsius, PMMA, 105; D G E B A - P A , 90; D G E B A - D D H - D A H , 47; and D G E B A - D D H , 27. g

2

g

Results and Discussion The s u b - T physical aging of epoxy specimens containing F V D F probes was g

monitored as a function of time by measurement of the fluorescence in-


23.

SCHWAB A N D L E V Y

Free-Volume-Dependent Fluorescence Probes

401

tensities a n d was v e r i f i e d b y D S C . D S C m o n i t o r i n g of the e n t h a l p y changes that a c c o m p a n y p h y s i c a l aging (21, 22) verifies that the o b s e r v e d changes i n the

fluorescence

intensities are d u e to p h y s i c a l aging. A t y p i c a l e x a m p l e o f

the changes that o c c u r i n the t h e r m o g r a m w i t h s u b - T a n n e a l i n g t i m e i n an g

a m i n e - c u r e d D G E B A p o l y m e r is s h o w n i n F i g u r e 2. A n e n d o t h e r m i c peak that grows w i t h a n n e a l i n g t i m e c o r r o b o r a t e d the

fluorescence

data a n d i n -


d i c a t e d that p h y s i c a l aging, not a d d i t i o n a l c h e m i c a l reaction, was responsible for the o b s e r v e d results. W e found a l i n e a r relationship b e t w e e n the excess e n t h a l p y (as c o m p a r e d to the q u e n c h e d sample) a n d the l o g a r i t h m of t h e a n n e a l i n g t i m e . T h i s relationship has b e e n r e p o r t e d for m a n y types o f a m o r p h o u s p o l y m e r s (2, 2 1 , 22). The

fluorescence

intensities (I ) of P M M A a n d D G E B A - P A c o n t a i n i n g f

J

0.1 W/g

ft min

_

60 min

^

250 min

900 min ^ Endotherm

0

1

20

1

U

1

1

40 60 80 Temperature (°Q

1

100

Figure 2. DSC scans of DGEBA cured with a mixture of DDE and DAE after the indicated physical aging times at 35 ° C following a quench from 100 ° C .


402


D A B M increase with aging time at T

g

- 25 °C after a quench from T

g

+

25 °C, as shown in Figure 3. The difference in slope between samples indicates a slower apparent aging rate for D G E B A - P A . The presence of crosslinks in the epoxy network decreases the mobility of the chain segments above T and reduces the difference in free volume between the glassy and g

rubbery states. In a cross-linked system, as opposed to a linear thermoplastic such as P M M A , a sudden quench from above to below T produces a struc-


g

ture that has less nonequilibrium volume. Because the cross-linked structure starts the physical aging cycle closer to equilibrium, it approaches equilibrium at a slower rate. The results depicted in Figure 3 are consistent with the findings of Lee and McKenna (23), who determined from stress relaxation experiments that the physical aging rates of epoxy networks decreased with increasing cross-link density. The study of physical aging in amine-cured epoxies was conducted with D M A N S as the F V D F probe because D A B M reacts with amines. The fluorescence spectrum of D M A N S is highly sensitive to changes in the mobility and polarity of the local environment in liquids (24, 25) and thermoplastic polymers (26, 27). Substantial changes in both the intensity and wavelength of maximum emission (XSSd of DMANS-containing D G E B A were observed as a function of cure time (Figure 4). The intensity increases in proportion to the free-volume decrease as bonds are formed in the curing reaction. The change in X ^ i is the result of the probe's large dipole moment in the excited

10

100

1000

Physical aging time (min) Figure 3. Fluorescence intensity of DABM in PMMA (D) and DGEBA-PA (•) after a quench from T + 25 °C to T - 25 ° C . Percent change is relative to intensity immediately after quench (time t = 0). G

G


In Polymer Characterization; Craver, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1990. x

Figure 4. Fluorescence intensity (Π) and k™ (·) of DMAN S during DGEBA-DDH

cure at 30 °C.


to

^ Ο CO

CO

3

Co

ο

8"

»·

•β

ι

Γ

ο

%

M

r

Ο

>

03

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Ω S3

CO

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404


state (28, 29). Before curing, when the matrix dipoles have sufficient mobility to stabilize the excited state during the fluorescence lifetime, the probe can relax to a lower energy level before emission. As the epoxy cures, however, the matrix dipole mobility decreases and the D M A N S molecules are forced to emit fluorescence from progressively less relaxed states. Consequently, fluorescence emission occurs from higher energy levels and X ^ shifts toward


the blue. Given that X ^ was still changing appreciably during the latter stages of cure, it may seem reasonable that XJSx should be sensitive to physical aging. However, after postcuring, Xj^x of D M A N S remained constant (within experimental error) with physical aging time. Apparently, postcuring induces chemical and physical changes that reduce mobility to the point where the dipole relaxation time of the matrix is much longer than the fluorescence lifetime, and X ^ i reaches a limiting value. T h e If as a f u n c t i o n of t e m p e r a t u r e b e h a v i o r of D M A N S i n D G E B A - D D H - D A H is given in Figure 5 and can be explained in terms of free-volume changes. The increase in free volume with temperature can be represented by the differences in the macroscopic volume expansivities (30). Therefore, the increase in the expansivity that occurs at T reflects an g

increase in the free-volume expansion as well. The slope is more negative above T

g

because, as equation 1 indicates, an inverse relationship exists

between If (proportional to 4>) and the free volume. The data presented in f

Figure 5, then, are analogous to a thermomechanical analysis where T

g

is

indicated by a slope change in the thermal expansivity. In contrast to chemical aging, the effects induced by physical aging are thermoreversible and can be "erased" (I, 2) by heating above T and then g

rapid quenching to s u b - T temperatures. Quenching from above T g

g

reini-

tiates the physical aging process. The thermoreversible behavior of D G E B A cured with a mixture of D D H and D A H was monitored with D M A N S , and the results are shown in Figure 6. Fluorescence measurements were made at 35 °C (T g

12 °C). During the physical aging cycle, If increases in ac-

cordance with the collapse of free volume. After each cycle, the sample was heated to 80 °C and subsequently quenched to 35 °C. Because of the restoration of the free volume in the sample after quenching, If decreases. Monitoring physical aging with F V D F probes provides information similar to that obtained from the other techniques mentioned. However, fluorescence-based techniques, when combined with fiber-optic fluorometry, are inherently more amenable to in-service monitoring. We are currently exploiting the F V D F phenomenon in conjunction with fiber-optic spectrofluorometry as a novel method for monitoring the composite curing process (31, 32). O n the the basis of the observations reported here, a logical extension of this work is to use embedded fiber-optic sensors not only to follow the curing process, but also to monitor the extent of physical aging during service.



Figure 5. Fluorescence intensity of DMANS in DGEBA-DDH-DAH as determined by DSC.

as a function of temperature. T


g

= 47 °C

-

οι

è

3

S

1

ο

I

&"

S>

ι

S

03

Π

In Polymer Characterization; Craver, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1990. g

Figure 6. Fluorescence intensity of DMANS containing DGEBA-DDH-DAH ( T = 47 °C) through three quench-anneal cycles. At times indicated by vertical dotted lines, sample was heated to 80 °C for 5 min, then rapidly cooled to 35 °C.


23.

SCHWAB A N D L E V Y

Free-Volume-Dependent

407

Fluorescence Probes

Summary W e s h o w e d that F V D F probes can b e u s e d to m o n i t o r the

free-volume

relaxation associated w i t h p h y s i c a l aging i n b o t h t h e r m o p l a s t i c a n d t h e r m o s e t p o l y m e r s . F l u o r e s c e n c e i n t e n s i t y increased m o n o t o n i c a l l y w i t h p h y s i c a l a g i n g t i m e c o r r e s p o n d i n g to the collapse of free v o l u m e i n the probe's local


e n v i r o n m e n t . T h e restoration o f free v o l u m e after a q u e n c h from above to b e l o w T c o u l d also b e f o l l o w e d b y changes i n I . T h e d e m o n s t r a t e d feasibility g

f

of u s i n g the F V D F p r o b e s to follow p h y s i c a l aging is i m p o r t a n t because this approach is p a r t i c u l a r l y suitable for i n - s e r v i c e a p p l i c a t i o n to composites t h r o u g h

fiber-optic

carbon-epoxy

waveguides e m b e d d e d i n t h e structure.

Acknowledgments T h i s w o r k was p e r f o r m e d u n d e r the M c D o n n e l l D o u g l a s I n d e p e n d e n t R e search a n d D e v e l o p m e n t p r o g r a m . W e thank R . O . L o u t f y for s u p p l y i n g the D A B M fluorescence p r o b e a n d T. C . S a n d r e c z k i a n d D . P. A m e s for h e l p f u l discussions.

References 1.

Struik, L. C. E . Physical Aging in Amorphous Polymers and Other

Materials;

8. 9. 10. 11.

Elsevier: Amsterdam, 1978. Kong, E . S. W. Adv. Polym. Sci. 1986, 80, 120. Cizmecioglu, M . ; Fedors, R. F.; Hong, S. D.; Moacanin, J. Polym. Eng. Sci. 1981, 21, 940. Gomez Ribelles, J. L . ; Dias Calleja, R. Polym. Bull. (Berlin) 1985, 14, 45. Joss, B. L . ; Wool, R. P. Polym. Mater. Sci. Eng. 1985, 53, 307. Kong, E . S. W.; Adamson, M . ; Mueller, L. Compos. Technol. Rev. 1984, 6, 170. Jean, Y. C.; Sandreczki, T. C.; Ames, D. P. Polym. Mater. Sci. Eng. 1985, 53, 185. Sung, C. S. P.; Lamarre, L . ; Chung, K. H . Macromolecules 1981, 14, 1839. Victor, J. G . ; Torkelson, J. M . Macromolecules 1987, 20, 2241. Lamarre, L . ; Sung, C. S. P. Macromolecules 1983, 16, 1729. Yu, W. C.; Sung, C. S. P.; Robertson, R. E . Macromolecules 1988, 21, 355.

12.

Kovacs, A. J. Fortschr. Hochpolym. Forsch. 1964,

2. 3. 4. 5. 6. 7.

3,

394.

13. Kong, E . S. W.; Lee, S. M . ; Nelson, H . G. Polym. Compos. 1982, 3, 29. 14. Oster, G.; Nishijima, Y. J. Am. Chem. Soc. 1956, 78, 1581. 15. Forster, T.; Hoffman, G. Z. Physik. Chem. 1971, 75, 63. 16.

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17. Loutfy, R. O.; Teegarden, D. M . Macromolecules 1983, 16, 452. 18.

Levy, R. L . ; Ames, D. P. In Adhesive Chemistry-Development

and Trends;

Lee,

L. E d . ; Plenum: New York, 1984; p 245. 19.

Loutfy, R. O. Pure Appl. Chem. 1986,

58,

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20. Fanter, D. L. Rev. Sci. Instrum. 1978, 49, 1005. 21. 22.

Petrie, S. E . B. J. Polym. Sci. Part A-2 1977, 10, 461. Petrie, S. E . B. In Physical Structure and the Amorphous

State; Allen,

Petrie, S. E . B., Eds.; Marcel Dekker: New York, 1977.


G.;

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Lee, A.; McKenna, G. B. Polymer 1988, 29, 1812. Lippert, E . Angew. Chem. 1961, 73, 605. "25. " Lippert, E.; Luder, W.; Moll, F. Spectrochim. Acta 1959, 10, 858. Eisenbaeh, C. D.; Sah, R. E.; Baur, G. J. Appl. Polym. Sci. 1983, 28, 1819. Eisenbaeh, C. D.; Fischer, K. Polym. Prepr. 1988, 29, 501. Czekalla, J.; Wick, G. Z. Elektrochem. 1961, 65, 727. Liptay, W.; Czekalla, J. Z. Elektrochem. 1961, 65, 721. Roberts, G. E.; White, E. F. T . In The Physics of Glassy Polymers; Haward, R. N . , E d . ; Wiley: New York, 1973; p 153. 31. Levy, R. L . ; Schwab, S. D. Polym. Mater. Sci. Eng. 1987, 56, 169. 32. Schwab, S. D.; Levy, R. L. Polym. Mater. Sci. Eng. 1988, 59, 591.


23. 24. 25. 26. 27. 28. 29. 30.

SUBMITTED

November 22, 1988. RECEIVED for review February 14, 1989. A C C E P T E D revised manuscript January 10, 1990.


Free-Volume-Dependent Fluorescence Probes of Physical Aging in

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