Moisture Transport Phenomena in Epoxies for ... - ACS Publications

Chapter 25

Moisture Transport Phenomena in Epoxies for Microelectronics Applications 1

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 24, 2016 | http://pubs.acs.org Publication Date: September 5, 1989 | doi: 10.1021/bk-1989-0407.ch025

D. J . Belton , E. A. Sullivan, and M . J . Molter Philips Research Laboratories, Sunnyvale, CA 94086-3409

In epoxy materials for microelectronic encapsulation, moisture is known to have a deleterious effect upon device r e l i a b i l i t y . In this paper we are concerned with moisture uptake as a function of relative humidity. The effects of temperature, sample thickness, and processing history were systematically examined for a single commercially important material via conjugate moisture sorption experiments. As temperature or penetrant activity was increased the transport behavior was observed to change in character. The change was in the direction from diffusion to Case II control. The overall character appeared dominated by the diffusion contribution. Post mold curing a sample led to an increase in both the diffusion coefficient and total moisture uptake. This result was explained in terms of both volume recovery during aging, and an increase in sample defect volume. The e f f e c t s o f t e m p e r a t u r e and r e l a t i v e h u m i d i t y on t h e k i n e t i c s o f m o i s t u r e s o r p t i o n i n epoxy m a t e r i a l s f o r microelectronics encapsulation a r e n o t g e n e r a l l y known. In a p r e v i o u s p a p e r Q J we examined m o i s t u r e s o r p t i o n as a f u n c t i o n o f t e m p e r a t u r e under c o n d i t i o n s o f 100 p e r cent r e l a t i v e humidity. C o n j u g a t e s o r p t i o n measurements were combined w i t h m e c h a n i c a l , d i e l e c t r i c and t h e r m a l methods o f a n a l y s i s t o examine m o i s t u r e r e l a t e d m i c r o structural alterations. E x p o s u r e o f a p o l y m e r t o a l i q u i d o r gaseous pene1

Current address: Signetics Korea Company, Ltd., Seoul, Korea 0097-6156/89/0407-0286$09.75A) © 1989 American Chemical Society

In Polymeric Materials for Electronics Packaging and Interconnection; Lupinski, John H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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Moisture Transport Phenomena in Epoxies

t r a n t r e s u l t s i n a s o r p t i o n of that penetrant w i t h i n the polymeric matrix. The generally accepted transport mechanism f o r s m a l l m o l e c u l e s i n p o l y m e r s above t h e i r g l a s s t r a n s i t i o n temperature involves simple solution followed by diffusion (2.3) . Here, solution is d e s c r i b e d by Henry's law, and d i f f u s i o n i s F i c k i a n i n nature. The t r a n s p o r t o f p e n e t r a n t s i n g l a s s y p o l y m e r s c a n n o t be d e s c r i b e d by such a s i m p l e model. Generally, anomalous b e h a v i o r i s o b s e r v e d , and has been a s c r i b e t o such phenomena as dual mode sorption (4-10), f l u c t u a t i o n s i n the s u r f a c e boundary c o n d i t i o n s (11.12), polymer relaxation controlled kinetics (13-15), p e n e t r a n t c l u s t e r i n g (16), a h i s t o r y o r t i m e dependent d i f f u s i o n c o e f f i c i e n t (2J, c r a z i n g (2.17), o r s t r e s s and o r i e n t a t i o n e f f e c t s Ci) . In the absence of structural defects and orientation e f f e c t s , anomalous p e n e t r a n t transport is largely the result of a concentration gradient c o n t r o l l e d d i f f u s i o n s u p e r i m p o s e d upon a r e l a x a t i o n c o n t r o l l e d swelling. The r e l a t i v e c o n t r i b u t i o n s o f t h e s e e f f e c t s v a r y w i t h t h e system, and w i t h i n a g i v e n s y s t e m as a f u n c t i o n o f t e m p e r a t u r e , p e n e t r a n t a c t i v i t y , and sample geometry t o yield a wide r a n g e o f behaviors (15.18.19). F i c k i a n d i f f u s i o n d e f i n e s t h e net transport o f a p e n e t r a n t under i d e a l c i r c u m s t a n c e s , and as s u c h represents a l i m i t i n g case f o r g l a s s y polymers. Fickian behavior has been designated, therefore, as Case I transport (13). I f one examines m o i s t u r e u p t a k e v e r s u s t h e s q u a r e r o o t o f t i m e , c e r t a i n c r i t e r i a must be fulf i l l e d t o c h a r a c t e r i z e t h e k i n e t i c s as F i c k i a n . These are : 1. A b s o r p t i o n and desorption curves linear. 2. Beyond t h e l i n e a r r e g i o n t h e c u r v e s the a b c i s s a . 3. Reduced curves do not display a sample t h i c k n e s s .

are are

initially concave

dependence

to on

In some c a s e s d e v i a t i o n s from F i c k i a n b e h a v i o r i n glassy epoxy polymers (19-25) have been adequately d e s c r i b e d u s i n g d u a l mode s o r p t i o n t h e o r y (22-24). This theory is based upon the premise that the sorbed penetrant exists in two thermodynamically distinct populations. These p o p u l a t i o n s consist of molecules a d s o r b e d i n " h o l e s " , and s p e c i e s s i m p l y d i s s o l v e d i n t h e polymer m a t r i x . A second limiting transport process finds the weight gain of penetrant a l i n e a r f u n c t i o n of time over the e n t i r e s o r p t i o n range. T h i s p r o c e s s has been t e r m e d Case I I T r a n s p o r t , and i s m e c h a n i s t i c a l l y q u i t e d i f f e r ent from Fickian diffusion. The rate controlling phenomena a r e p e n e t r a n t i n d u c e d p o l y m e r i c r e l a x a t i o n s . A c o m b i n a t i o n o f Case I and Case I I p r o c e s s e s has been



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POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION

invoked in a number of instances to provide a description of the observed k i n e t i c s (1,14,18-21,25). S e p a r a t i o n o f d i f f u s i o n and r e l a x a t i o n p a r a m e t e r s has been p r e s e n t e d f o r a c a s e where d i f f u s i o n i s c o n s i d e r e d as F i c k i a n ( 1 5 ) . Such an a n a l y s i s has a l s o been a p p l i e d t o m o i s t u r e u p t a k e by an epoxy s y s t e m ( 2 5 ) . In t h i s p a p e r we examine m o i s t u r e s o r p t i o n i n an epoxy m o l d i n g compound f o r m u l a t i o n u s e d f o r s e m i c o n d u c tor encapsulation. In p a r t i c u l a r , we w i l l be c o n c e r n e d w i t h m o i s t u r e u p t a k e as a f u n c t i o n o f r e l a t i v e h u m i d i t y . The e f f e c t s o f t e m p e r a t u r e , sample t h i c k n e s s , and p r o c e s s i n g h i s t o r y w i l l be s y s t e m a t i c a l l y examined f o r a s i n g l e commercially important m a t e r i a l . Experimental A l l samples were p r e p a r e d from a c o m m e r c i a l l y a v a i l a b l e epoxy c r e s o l n o v o l a c - p h e n o l f o r m a l d e h y d e novolac-tertiar y amine b a s e d m o l d i n g compound. Pelletized preforms were h e a t e d t o 85°C i n a RF p r e h e a t e r p r i o r t o b e i n g t r a n s f e r molded a t 180°C/68 atm. f o r 90 s e c . Molded samples were c o o l e d i n a i r t o room t e m p e r a t u r e and s t o r e d i n a d e s i c c a t e d e n v i r o n m e n t u n t i l t e s t i n g o r subsequent thermal t r e a t m e n t . P o s t mold c u r i n g , PMC, was a c c o m p l i s h e d i n a g r a v i t y oven a t 175°C f o r a p e r i o d o f 4 hours. Samples without post mold curing are d e s i g n a t e d by NPMC. Gravimetric sorption measurements were conducted u s i n g a M e t t l e r a n a l y t i c a l b a l a n c e a c c u r a t e t o 0.02 mg. M o i s t u r e u p t a k e was m o n i t o r e d as a f u n c t i o n o f p o s t mold curing schedule, sample t h i c k n e s s , r e l a t i v e humidity, and t e m p e r a t u r e . For each e x p e r i m e n t a l c o n d i t i o n the a v e r a g e o f f i v e samples was u s e d i n t h e w e i g h t g a i n / l o s s determination. The samples were e x p o s e d t o a s e r i e s o f conditions i n order to construct conjugate sorption isotherms. A c o n j u g a t e s o r p t i o n i s o t h e r m i s d e f i n e d as a s e t o f d a t a encompassing a s o r p t i o n t e s t f o l l o w e d i n sequence by a d e s o r p t i o n t e s t and a r e s o r p t i o n test. Samples f o r the sorption and resorption tests were exposed t o w a t e r immersion (100% R.H.) or the vapor above e q u i l i b r a t e d s a l t s o l u t i o n s i n s e a l e d v e s s e l s a t the a p p r o p r i a t e temperature. Temperatures ranged from 25°C t o 100°C, and r e l a t i v e h u m i d i t y from 31% t o 100%. E a c h sample s e t was removed from i t s v e s s e l , t h e s u r f a c e w a t e r b l o t t e d away, and t h e n weighed. D e s o r p t i o n measurements were t a k e n a t a p p r o p r i a t e t i m e i n t e r v a l s on samples s t o r e d i n c o n v e c t i o n ovens a t t h e t e m p e r a t u r e o f interest. Results A F i c k i a n d e s c r i p t i o n f o r t h e amount o f p e n e t r a n t t a k e n up by a p l a n e s h e e t o f t h i c k n e s s 1 i n a t i m e t , Mj., i s g i v e n by Q ) :


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289

(1)


i s t h e m o i s t u r e u p t a k e a t e q u i l i b r i u m , and constant diffusion coefficient. For short E q u a t i o n 1 can be a p p r o x i m a t e d by CD : u t - 4 M

~

D

is a times,

[ÛT

Ύπΐ

2

(2)

Reduced c o n j u g a t e s o r p t i o n d a t a can t h e r e f o r e be o b t a i n e d by e x a m i n i n g m o i s t u r e u p t a k e as a f u n c t i o n o f t°* /l. M o i s t u r e u p t a k e i s p l o t t e d i n t h e form o f l\./W , where W i s t h e o r i g i n a l d r y w e i g h t o f t h e p o l y m e r (.Li) . T h i s a p p r o a c h w i l l a l l o w a c o m p a r i s o n o f s o r p t i o n and r e s o r p t i o n data f o r experiments where d e s o r p t i o n does not r e s u l t i n a complete l o s s o f sorbed m o i s t u r e QJ . C l a s s i c Case I I t r a n s p o r t b e h a v i o r finds weight g a i n a l i n e a r f u n c t i o n o f t i m e (18) . A c o n s t a n t r a t e o f a b s o r p t i o n w i l l be t h e r e s u l t o f a c o n s t a n t r a t e r e l a x a t i o n process i f d i f f u s i o n of penetrant to the r e l a x i n g b o u n d a r y i s r a p i d when compared t o p e n e t r a n t induced relaxations. A r e l a t i o n d e s c r i b i n g p e n e t r a n t u p t a k e as a f u n c t i o n o f t i m e has been g i v e n (26): 5

0

Q

Mt

k

0

t

Moo

C

0

a_

(3)

where k i s a r e l a x a t i o n c o n s t a n t , C i s the e q u i l i b r i u m s o l u b i l i t y p a r a m e t e r , and a i s t h e h a l f t h i c k n e s s . For a s l a b t h e exponent η i s u n i t y , and a l i n e a r dependence on t i m e i s p r e d i c t e d . Q

0

Moisture S o r p t i o n Under Conditions of 1QQ% Relative Humidity. Conjugate s o r p t i o n data are g i v e n i n F i g u r e 1 f o r NPMC samples c o n d i t i o n e d a t 100% r e l a t i v e h u m i d i t y , (R.H.), 25°C. The PMC c o u n t e r p a r t s a r e g i v e n i n F i g u r e 2. The conjugate data sets deviate from Fickian c r i t e r i a i n t h a t there i s a crossover i n the s o r p t i o n desorption curves. Otherwise the curves f o r s o r p t i o n and desorption appear initially linear, and become concave toward the a b c i s s a w i t h time. I t i s apparent from t h e s e d a t a t h a t n o t a l l o f t h e m o i s t u r e gained during a sorption cycle i s lost during desorption. The moisture uptake d u r i n g the r e s o r p t i o n c y c l e i s observed to increase beyond that experienced during the absorption cycle. In F i g u r e 3 we present only the first sorption



290


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·

·

SORPTION

•

•

•

DESORPTION

Λ

*

A

RESORPTION

THK « 0.07"

Q Ζ

ύζ/ι Figure

1:

C o n j u g a t e s o r p t i o n d a t a f o r samples w i t h o u t p o s t mold c u r e and e x p o s e d t o w a t e r i m m e r s i o n a t 25°C. (Abscissa u n i t s are h o u r s / i n c h . ) 1 / 2



25.

BELTON ET AL.

291


0.010 η •

·

SORPTION

0.009

-β

•

DESORPTION

-Α-

A

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0.008

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R e s o r p t i o n > A b s o r p t i o n . Finally, D for t h e p o s t c u r e d sample a p p e a r s g r e a t e r than D f o r t h e sample w i t h o u t p o s t c u r e . I n F i g u r e 7 we summarize moisture u p t a k e f o r s o r p t i o n , r e s o r p t i o n c y c l e s as a f u n c t i o n o f thermal treatment. The d a t a i n F i g u r e 7 and in Table I coupled with the fact that t h e d e s o r p t i o n c u r v e always c r o s s e s o v e r t h e a b s o r p t i o n c u r v e summarize the s a l i e n t f e a t u r e s f o r each conjugate data s e t a t each temperature. That i s , conjugate s o r p t i o n curves a r e initially l i n e a r with respect t o t /l, then become concave t o t h e a b s c i s s a . D e s o r p t i o n i s i n i t i a l l y faster t h a n a b s o r p t i o n b u t becomes s l o w e r as t h e m o i s t u r e l o s s proceeds. The d e s o r p t i o n c y c l e does n o t r e s u l t i n a complete loss o f moisture. Resorption proceeds to h i g h e r l e v e l s than a b s o r p t i o n . The c r i t e r i a t h a t s o r p t i o n d a t a must s c a l e a c c o r d 0 , 5



294


u

• 0

1

1

100

200

1

300

1

400

1

1

1

500

600

700

1

800

A/l Figure

4:

C o n j u g a t e s o r p t i o n d a t a f o r samples w i t h o u t p o s t mold c u r e and e x p o s e d t o w a t e r i m m e r s i o n a t 100°C. (Abscissa u n i t s are h o u r s

1 / 2

/inch.)



25.

BELTON ET AL.

0

295


100

200

300

400

500

600

300

400

tV,// Figure

5:

C o n j u g a t e s o r p t i o n d a t a f o r samples w i t h a p o s t mold c u r e c y c l e and e x p o s e d t o w a t e r imm e r s i o n a t 100°C. (Abscissa u n i t s are h o u r s / i n c h . ) 1 / 2



296


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PMC NPMC THK=0.07"

0.001 H T

200

300

400

500

T

600

T

700

800

\>z/l Figure

6:

Comparison o f a b s o r p t i o n c y c l e f o r samples w i t h and w i t h o u t p o s t mold c u r i n g and e x p o s e d t o w a t e r i m m e r s i o n a t 100°C. (Abscissa u n i t s are h o u r s / i n c h . ) 1 / 2



Figure

7:

(Abscissa u n i t s are

hours

1 / 2

/inch.)

M o i s t u r e u p t a k e as a f u n c t i o n o f temperature f o r samples w i t h and w i t h o u t a p o s t m o l d c u r e during absorption and resorption cycles.

SAMPLE A


3

S

298



ing t o t h i c k n e s s a r e examined i n F i g u r e s 8 and 9. It can be seen t h a t the data f o r samples o f differing t h i c k n e s s do n o t l i e on t h e same c u r v e . The c o r r e s p o n dence, however, becomes b e t t e r as t e m p e r a t u r e i n c r e a s e s . T h i s t r e n d was o b s e r v e d a t a l l i n t e r m e d i a t e t e m p e r a t u r e s f o r a l l samples. M o i s t u r e S o r p t i o n as a F u n c t i o n o f R e l a t i v e H u m i d i t y . E x p o s u r e c o n d i t i o n s o t h e r t h a n t o t a l sample immersion lead to moisture t r a n s p o r t k i n e t i c s that are devoid of c e r t a i n a n o m a l i e s p r e s e n t e d above. F i g u r e s 10-13 d e p i c t conjugate s o r p t i o n d a t a f o r samples w i t h and without p o s t mold c u r e e x p o s e d t o two d i f f e r e n t R.H. environments a t 25°C. The d e s o r p t i o n curve l i e s below the absorption data i n a l l cases. In a d d i t i o n , t h e m o i s t u r e u p t a k e d u r i n g r e s o r p t i o n does n o t s u r p a s s t h a t o b s e r v e d during the f i r s t s o r p t i o n c y c l e . Samples e x p o s e d t o 31% and 75% r e l a t i v e h u m i d i t y , but d i f f e r e n t temperatures a r e r e p r e s e n t e d by t h e d a t a shown i n F i g u r e s 14 and 15. D u r i n g d e s o r p t i o n c o m p l e t e m o i s t u r e r e m o v a l was ensured t h r o u g h t h e use o f a vacuum c y c l e . A l l c y c l e s of the c o n j u g a t e p l o t now a p p e a r t o o v e r l a p . The p e r c e n t m o i s t u r e u p t a k e as w e l l as v a l u e s o f D f o r a b s o r p t i o n c y c l e s as f u n c t i o n s o f r e l a t i v e h u m i d i t y and t e m p e r a t u r e are given i n Table I I . I t can be seen t h a t b o t h m o i s t u r e u p t a k e and D i n c r e a s e as R.H. i n c r e a s e s f o r e i t h e r temperature. Both parameters are observed t o i n c r e a s e with temperature. F i n a l l y , both parameters are i n c r e a s e d f o r t h e p o s t mold c u r e d sample. The appearance of the conjugate sorption data p r e s e n t e d so f a r q u a l i t a t i v e l y i n d i c a t e s a s h i f t from a more t o a l e s s F i c k i a n c h a r a c t e r as t h e i n i t i a l s u r f a c e moisture content i n c r e a s e s . That i s , t h e a p p e a r a n c e o f the conjugate s o r p t i o n isotherms o b t a i n e d by totally i m m e r s i n g t h e samples v i o l a t e two of the c r i t e r i a by which F i c k i a n behavior i s d e f i n e d . The same c a n n o t be s a i d f o r t h o s e samples e x p o s e d t o l e s s t h a n 100% R.H., p a r t i c u l a r l y a t 25°C. T h i s q u a l i t a t i v e t r e n d f o r PMC i s f u r t h e r d e m o n s t r a t e d by F i g u r e s 16 and 17. Here N ^ / W Q i s p r e s e n t e d as a f u n c t i o n o f t i m e . F o r b o t h t h e t h i c k and the thin sample, as either temperature or relative humidity i s increased, the character of the curves p r o g r e s s e s towards p u r e Case I I d e s c r i p t i o n . That i s , t h e m o i s t u r e u p t a k e becomes l i n e a r w i t h t i m e up t o t h e p o i n t where a p l a t e a u i s a c h i e v e d i n t h e b e h a v i o r . Discussion Moisture Humidity.

S o r p t i o n Under C o n d i t i o n s of 1QQ% fiejative S o r p t i o n o f m o i s t u r e i n g l a s s y p o l y m e r s and



25. BELTON ET AL.

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ι— 100 8:

200

300

400

—ι 500

1— 600

700

800

T h i c k n e s s s c a l i n g f o r samples w i t h o u t a p o s t m o l d c u r e and e x p o s e d t o w a t e r immersion a t 25°C. (Abscissa u n i t s are hours /inch.) 1 / 2



300


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ο Sorption, 0.07 inches • Sorption, 0.128 inches

0.009H

~ ο

oo

0. 0 0 8 -

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006-

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0.004-

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Figure

9:

ι— 200

300

— ι — 400

500

600

700

800

T h i c k n e s s s c a l i n g f o r samples w i t h o u t a p o s t mold c u r e and e x p o s e d t o w a t e r immersion a t 100°C. (Abscissa units are hours /inch.) 1 / 2



25.

BELTONETAL


-·

301

SORPTION DESORPTION

Τ

100

Γ

200

τ 300

1

400

Γ 500

600

ύζ/1 Figure

10:

C o n j u g a t e s o r p t i o n d a t a f o r samples w i t h a p o s t mold c u r e and e x p o s e d t o 31% R.H. at 25°C. (Abscissa u n i t s are hours /inch.) 1 / 2



302


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0.008

ο

ζ Ρ

0.007

800

Figure

11: C o n j u g a t e s o r p t i o n d a t a f o r samples w i t h a p o s t m o l d c u r e and e x p o s e d t o 75% R.H. a t 25°C. (Abscissa units are hours /inch.) 1 / 2



25.

BELTON ET

303


0.010 0.009

• -•

· SORPTION • DESORPTION

0.008 0.007 0.006 H 0.005 0.004 H 0.003 0.002 1

800

Figure

12: C o n j u g a t e s o r p t i o n d a t a f o r samples w i t h o u t a p o s t mold c u r e and e x p o s e d t o 31% R.H. a t 25°C. (Abscissa u n i t s are hours /inch.) 1 / 2



304


0.010 -ι -·

0.009 H • Q

0.008

-

SORPTION

•m DESORPTION •

•

RESORPTION

THK=0.07"

Z> I

0.007 -

ο ο ε

0.006 -

Figure

13:

C o n j u g a t e s o r p t i o n d a t a f o r samples w i t h o u t a p o s t mold c u r e and e x p o s e d t o 75% R.H. a t 25°C. (Abscissa u n i t s are hours /inch.) 1 / 2



25.

BELTON ET AU

305


•

SORPTION DESORPTION

A

RESORPTION

1— — ι —

I

300

400

500

τ 600

Τ 700

Ι 800

ύζ/Ι Figure

14:

C o n j u g a t e s o r p t i o n d a t a f o r samples w i t h o u t a p o s t mold c u r e and e x p o s e d t o 31% R.H. at 66°C. Desorption conducted under vacuum. (Abscissa u n i t s are

hours

1 / 2

/inch.)



306


0.010 π 0.009 §

0.008

Ο 5 Ο

0.007

- ·

SORPTION

-β

DESORPTION

-A

RESORPTION

ΤΗΚ«0.07

Μ

υ

Ε

0.006 Η

3M

0.005

σ»

0.004 Η

^

0.003 Η 0.002 Η o.ooi Η

300

400

500

τ 600

700

800

ύζ/ι F i g u r e 15:

Conjugate sorption data f o r samples w i t h a p o s t mold cure and e x p o s e d t o 75% R.H. a t 66°C. Desorption conducted under vacuum. (Abscissa u n i t s are hours /inch.) 1 / 2



25.

BELTONETAL.

307


0.010 η 0.009 Η

TIME ( H R S )

Figure

16:

Percent moisture uptake v e r s u s t h i c k n e s s i s 0.07 i n c h .

time.

Sample



308


0.010 η 0.009 -

0

100

200

300

400

500

600

700

800

TIME (HRS)

Figure

17:

Percent moisture uptake versus time. t h i c k n e s s i s 0.128 i n c h .

Sample


25.

Table


SAMPLE

309


BELTON ET AL.

I I . Percent Moisture Uptake and D i f f u s i o n Coe f f i c i e n t f o r Samples w i t h a n d w i t h o u t P o s t M o l d C u r i n g as F u n c t i o n s o f R e l a t i v e H u m i d i t y and Temperature

TEMPERATURE (°C)

R.H. (%)

% MOISTURE UPTAKE

2

8

D (cm /sec x l O " )

PMC NPMC

25 25

31 31

0.44 0.40

2.20 1.96

PMC NPMC

25 25

75 75

0.45 0.40

2.27 1.96

PMC NPMC

25 25

100 100

0.56 0.50

2.56 2.26

PMC NPMC

66 66

31 31

0.48 0.46

6.49 5.56

PMC NPMC

66 66

75 75

0.50

5.92

PMC NPMC

66 66

100 100

0.75 0.70

7.91 7.14

their composites i s a complex p r o c e s s which c a n be further complicated by t h e p r e s e n c e of structural, s t r e s s , and o r i e n t a t i o n a l e f f e c t s . I n i t i a l l y we w i l l consider conjugate moisture sorption kinetics t o be d e s c r i b e d by c o n t r i b u t i o n s from a c o n c e n t r a t i o n g r a d i e n t c o n t r o l l e d d i f f u s i o n , and r e l a x a t i o n c o n t r o l l e d swelling. The d a t a p r e s e n t e d h e r e i n a r e c o n c e r n e d w i t h i n t e g r a l s o r p t i o n as w e l l as i n c r e m e n t a l s o r p t i o n e x p e r i ments. Integral sorption kinetics describe transport i n a sample o r i g i n a l l y p e n e t r a n t f r e e , w h i l e i n i n c r e m e n t a l s o r p t i o n t h e r e i s an i n i t i a l penetrant concentration (15) . These two c a s e s c o r r e s p o n d t o t h e a b s o r p t i o n a n d r e s o r p t i o n c y c l e s o f the conjugate data s e t s . During the initial sorption cycle transport will always commence by a d i f f u s i v e process into the previously unoccupied polymer m a t r i x . D i f f u s i v e motion w i l l occur when a p e n e t r a n t m o l e c u l e jumps from i t s own p o s i t i o n t o an adjacent location, and i t s p r e v i o u s position i s



310


f i l l e d b e f o r e i t can r e t u r n . When t h e p e n e t r a n t has a m o l e c u l a r s i z e much s m a l l e r t h a n t h e monomer u n i t o f a given polymer, and the thermodynamic interaction is weak, a l i m i t e d movement o f o n l y one o r two monomer u n i t s would be s u f f i c i e n t t o p r o v i d e t h e c r o s s s e c t i o n r e q u i r e d f o r a d i f f u s i v e jump. In t h i s l i m i t i n g case d i f f u s i v e m o t i o n can be d e s c r i b e d by F i c k ' s law, with H e n r y ' s law d e s c r i b i n g t h e p e n e t r a n t - p o l y m e r equilibrium. F i c k i a n b e h a v i o r may p e r s i s t throughout the time s c a l e of s o r p t i o n to e q u i l i b r i u m w i t h i n the proper r e gimes o f t h e t e m p e r a t u r e - p e n e t r a n t a c t i v i t y plane. A generalized diagram of the temperature-penetrant a c t i v i t y plane i s given f o r organic penetrants i n glassy p o l y m e r s i n F i g u r e 18 (14). Areas of concentration independent, and c o n c e n t r a t i o n dependent d i f f u s i o n are considered regions of F i c k i a n behavior. A t h i g h e r penet r a n t a c t i v i t i e s ( f o r a range o f t e m p e r a t u r e s below t h e e f f e c t i v e T ) s t r u c t u r a l r e a r r a n g e m e n t s become n e c e s s a r y g

i n o r d e r t o accommodate t h e e q u i l i b r i u m m o i s t u r e content. T h i s i s t h e regime o f Case I I t r a n s p o r t d e p i c t e d i n F i g u r e 18. As m o i s t u r e sorption progresses under Case I I c o n d i t i o n s a s h a r p b o u n d a r y w i l l d e v e l o p between an inner g l a s s y core of essentially zero penetrant concentration, and a swollen outer shell of uniform concentration. The structural changes accompanying s w e l l i n g a r e d e t e r m i n e d by t i m e dependent r e l a x a t i o n s , characterized by a spectrum of relaxation times. S o r p t i o n o f i n c r e a s i n g q u a n t i t i e s o f m o i s t u r e can t h e r e f o r e l e a d t o e x t e n s i v e s t r u c t u r a l r e a r r a n g e m e n t s and can alter the mass transport process from a diffusion controlled to a relaxation controlled process. During Case I I s o r p t i o n , t h e f i n a l p i c t u r e t h a t emerges i s r a p i d d i f f u s i o n o f p e n e t r a n t t o a boundary s e p a r a t i n g t h e s w o l l e n s h e l l from t h e g l a s s y c o r e . T h i s boundary p r o g r e s s e s t h r o u g h t h e sample a t a r a t e p r o p o r t i o n a l t o time. I f the sample thickness i s increased to a critical v a l u e , a t r a n s i t i o n from Case I I t o another transport mode may occur. As Case II transport p r o g r e s s e s , t h e boundary s e p a r a t i n g t h e s w o l l e n shell from t h e g l a s s y c o r e may move t o a d i s t a n c e where t h e time required for a diffusing species to reach that b o u n d a r y i s no l o n g e r n e g l i g i b l e w i t h r e s p e c t t o t h e r e l a x a t i o n times. Beyond t h i s p o i n t s i g n i f i c a n t d i f f u s i o n a l r e s i s t a n c e s may begin t o develop, impeding the otherwise rate determining Case II relaxations. F i n a l l y , t h e r e i s a r e g i o n l a b e l e d anomalous d i f f u s i o n where b o t h F i c k i a n d i f f u s i o n and Case I I b e h a v i o r combine to determine the transport kinetics. Samples t h i c k n e s s w i l l serve t o induce t r a n s p o r t t r a n s i t i o n s i n this region also. As d e s c r i b e d e a r l i e r , samples immersed i n water, r e g a r d l e s s of c u r i n g h i s t o r y or temperature, exhibited two deviations from the criteria defining Fickian b e h a v i o r . These a r e : 1) t h e i n i t i a l h i g h e r v a l u e o f D



25.

BELTON ET AL.

F i g u r e 18:


311

Generalized temperature - penetrant a c t i v i t y plane. (Reproduced w i t h p e r m i s s i o n f r o m r e f . 14. Copyright 1969 Wiley.)



312


d u r i n g d e s o r p t i o n when compared t o a b s o r p t i o n , and t h e c r o s s o v e r o f t h e c u r v e s , and 2) t h e i n a b i l i t y o f t h e data t o s c a l e with thickness. D e v i a t i o n s from F i c k i a n b e h a v i o r i n g l a s s y epoxy p o l y m e r s have been d e s c r i b e d , i n some c a s e s , u s i n g d u a l mode s o r p t i o n t h e o r y . This theory i s based upon t h e premise that the sorbed p e n e t r a n t e x i s t s i n two t h e r m o d y n a m i c a l l y d i s t i n c t popu lations. These p o p u l a t i o n s a r e : m o l e c u l e s adsorbed i n " h o l e " , C H ' and s p e c i e s s i m p l y d i s s o l v e d i n t h e p o l y m e r matrix, C D . The t o t a l c o n c e n t r a t i o n o f s o r b e d p e n e t r a n t i s g i v e n by: C

=

C

+ H

Simple

solution

C D

(4)

i s d e s c r i b e d by H e n r y ' s law, t h e r e f o r e :

C = kp (5) where k i s t h e Henry's law c o n s t a n t , and ρ i s t h e equilibrium pressure. The c o n c e n t r a t i o n a d s o r b e d i s d e s c r i b e d by a Langmuir i s o t h e r m : D

C

H

C ' H bp -—;—Γ

=

Ι + bp

(6)

where C H ' i s t h e h o l e s a t u r a t i o n c o n s t a n t , b i s t h e h o l e a f f i n i t y c o n s t a n t , and ρ i s t h e e q u i l i b r i u m p r e s s u r e . S u b s t i t u t i o n o f E q u a t i o n s 5 and 6 i n t o E q u a t i o n 4 y i e l d s t h e d u a l mode s o r p t i o n model: C'H c

=

k

*

+

bp

Γ Τ Ί ^

(7)

V a r i a t i o n o f the hole a f f i n i t y constant o f Equation 7 permits p r e d i c t i o n o f s o r p t i o n - d e s o r p t i o n curve shapes t h a t match t h o s e o f F i g u r e s 1,2,4 and 5 ( 2 2 ) . M o i s t u r e diffusion i n this s y s t e m c a n be g i v e n t h e f o l l o w i n g interpretation: During a s o r p t i o n c y c l e moisture i s undergoing o r d i n a r y d i s s o l u t i o n i n t h e polymer matrix, with a concurrent adsorption at s p e c i f i c s i t e s . (One does n o t e x p e c t a c o n t i n u o u s v o i d phase.) Those mole cules i n t e r a c t i n g at s p e c i f i c s i t e s w i l l d i f f u s e f u r t h e r into e i t h e r p o p u l a t i o n d e p e n d i n g upon t h e a d s o r p t i o n reversibility. A t e q u i l i b r i u m t h e v o i d s and t h e m a t r i x w i l l be s a t u r a t e d . V o i d s a t u r a t i o n w i l l have an immedi a t e e f f e c t upon t h e d e s o r p t i o n c h a r a c t e r i s t i c s . A high r e v e r s i b i l i t y w i l l s p e e d d e s o r p t i o n , an e f f e c t o p p o s i t e to that f o r sorption. A f i n i t e r e v e r s i b i l i t y w i l l give rise to a finite quantity of irreversibly sorbed moisture at reasonable experimental times, hence t h e crossover i n the curves. Dual-mode s o r p t i o n t h e o r y p e r -



25. BELTON ET AL.


313

mits i n t e r p r e t a t i o n o f our conjugate s o r p t i o n data, but does n o t r e s o l v e t h e i n a b i l i t y of the data t o scale according to thickness. R e l a x a t i o n c o n t r o l l e d m o i s t u r e s o r p t i o n can h e l p t o e x p l a i n both o f the anomalies observed i n the data. At sorption equilibrium polymeric relaxations will have d e f i n e d a matrix s t r u c t u r e through which d i f f u s i o n i s g r e a t l y enhanced. T h e r e f o r e , d u r i n g an e n s u i n g d e s o r p tion cycle D i n i t i a l l y w i l l be enhanced. Later the curves will cross since some of the moisture is i r r e v e r s i b l y sorbed. At intermediate times, a moisture concentration p r o f i l e should i n d i c a t e a trend toward reduced d i f f u s i v i t i e s . A proposed s e r i e s o f p r o f i l e s i s g i v e n i n F i g u r e 19. As t h e m o i s t u r e d e s o r b s , a number o f phenomena must o c c u r . During the i n i t i a l stages of moisture loss the s t i l l p l a s t i c i z e d matrix w i l l have s h o r t e r r e l a x a t i o n times than those i n t h e u n p l a s t i c i z e d state. Volume c o l l a p s e can o c c u r more r a p i d l y under these conditions. After a certain moisture loss, contraction o f the previously swollen matrix w i l l be h i n d e r e d because o f i n c r e a s i n g r e l a x a t i o n times, t h e r e b y p r e v e n t i n g a complete c o l l a p s e . During t h i s s e r i e s of events D should decrease. Continued desorption w i l l further decrease the concentration gradient, again r e t a r d i n g moisture removal and d r i v i n g the sorptiondesorption profiles i n the experimentally observed directions. F i n a l l y , as m e n t i o n e d above, m o i s t u r e l o s s will n o t be t o t a l , causing the d e s o r p t i o n curve t o u l t i m a t e l y r e s i d e below t h e a b s o r p t i o n c u r v e . The i n a b i l i t y of the data t o scale according t o t h i c k n e s s c a n be r e s o l v e d by c o n s i d e r i n g t h e e f f e c t o f matrix relaxations. F o r specimens o f d i f f e r i n g t h i c k ness, at equivalent fractional d i s t a n c e s through a sample, t h e c o n c e n t r a t i o n o f m o i s t u r e w i l l change more s l o w l y f o r a t h i c k e r sample. T h i s w i l l p r o v i d e more time f o r those molecular relaxations necessary to accommodate e q u i l i b r i u m amounts o f m o i s t u r e . The m o i s t u r e i n d u c e d c o n f i r m a t i o n a l t i m e dependence w i l l t h e r e fore differ for equivalent locations within such samples. S i n c e more t i m e i s a v a i l a b l e f o r m o l e c u l a r r e l a x a t i o n s a t e q u i v a l e n t l o c a t i o n s i n a t h i c k e r sample, one will observe an i n c r e a s e i n m o i s t u r e uptake as d e m o n s t r a t e d i n F i g u r e s 8 and 9. The t r a n s p o r t b e h a v i o r o b s e r v e d i n t h i s s y s t e m has been v a r i e d o v e r a wide r a n g e by t e m p e r a t u r e changes. The a n o m a l i e s observed indicate that the behavior cannot be d e s c r i b e d as Fickian. We c a n s p e c u l a t e , however, on t h e d i f f u s i o n o r Case II contributions t o the character of the observed p r o f i l e s as a f u n c t i o n o f t e m p e r a t u r e . At the lowest temperature-penetrant a c t i v i t y p l a n e , one would e x p e c t Fickian transport. A t low t e m p e r a t u r e s t h e r e l a x a t i o n t i m e i s much g r e a t e r t h a n t h e t i m e f o r d i f f u s i o n . In t h i s s i t u a t i o n t h e r e i s no p o s s i b i l i t y t h a t t h e s t r u c -




314



25.

BELTONETAL.

315


t u r a l r e a r r a n g e m e n t s n e c e s s a r y t o i n c r e a s e t h e volume o f sorbed moisture w i l l occur. Therefore, moisture enters i n t o t h e a v a i l a b l e f r e e volume by a d i f f u s i v e m o t i o n . As t h e t e m p e r a t u r e i n c r e a s e s , t h e r e l a x a t i o n t i m e s w i l l become c o m p a r a b l e t o t h e d i f f u s i o n t i m e . The t r a n s p o r t behavior will shift from a d i f f u s i o n t o a r e l a x a t i o n control. The mode does not change a b r u p t l y , b u t i s a gradual change t h a t w i l l d e f i n e a r e g i o n o f behavior where both modes are operative. Finally, as the temperature increases to values greater than the effective T o f t h e m a t e r i a l , t h e mode w i l l r e t u r n t o d i f f u s i o n c o n t r o l . T h i s i s because the r e l a x a t i o n times a r e now l e s s t h a n t h e t i m e s r e q u i r e d f o r d i f f u s i o n . g

From t h i s d i s c u s s i o n and an e x a m i n a t i o n o f F i g u r e s 16 and 17, we c o n c l u d e t h a t t h e c h a r a c t e r o f t h e t r a n s p o r t mode d e m o n s t r a t e s an i n c r e a s e d Case I I c o n t r i b u t i o n as t e m p e r a t u r e i s i n c r e a s e d . Even t h o u g h t h e c h a r a c t e r i s s h i f t i n g , the o v e r a l l transport behavior i s b e l i e v e d to be dominated by diffusion. This statement is s u p p o r t e d by e x a m i n i n g t h e magnitude o f t h e a c t i v a t i o n e n e r g y c a l c u l a t e d from t h e average d i f f u s i o n coefficients. S i n c e t h e d i f f u s i o n c o e f f i c i e n t i s g i v e n by:

(8) t h e a c t i v a t i o n e n e r g y i s e a s i l y c a l c u l a t e d from a p l o t o f l o g D v s . 1/T. The v a l u e c a l c u l a t e d was 5.61 kcal/gm mole. This i s t y p i c a l of values c h a r a c t e r i z i n g F i c k i a n d i f f u s i o n o f s m a l l m o l e c u l e s i n o t h e r systems ( 2 J . The m o i s t u r e u p t a k e f o l l o w i n g a d e s o r p t i o n c y c l e i s c o n s i d e r e d t o be an example o f an i n c r e m e n t a l sorption experiment. T h i s i s not s t r i c t l y t r u e however, s i n c e the initial penetrant concentration i s present in a m a t r i x s t r u c t u r e t h a t may be c o n s i d e r a b l y a l t e r e d from that present during the i n i t i a l sorption cycle. For samples t h a t had been t o t a l l y immersed, t h e desorption c y c l e s do n o t l e a d t o a c o m p l e t e l o s s o f m o i s t u r e . A subsequent resorption cycle has moisture uptake i n c r e a s i n g with temperature. The m o i s t u r e u p t a k e , when compared t o t h e a b s o r p t i o n c y c l e , a l s o i n c r e a s e s with i n c r e a s i n g temperature. This behavior i s demonstrated i n F i g u r e 7. R e l a x a t i o n s d u r i n g d e s o r p t i o n may n o t l e a d to a t o t a l c o l l a p s e of the p r e v i o u s l y swollen matrix. T h e r e f o r e , t h e volume a v a i l a b l e f o r m o i s t u r e u p t a k e may be d i f f e r e n t d u r i n g r e s o r p t i o n t h a n d u r i n g absorption. One c o u l d expect, however, t h a t t h e f r o z e n - i n volume w o u l d r e l a x more r a p i d l y a t h i g h e r t e m p e r a t u r e s l e a d i n g to a t r e n d the opposite of that observed. The f r o z e n i n volume i s considered t o be a minor e f f e c t , and we c o n s i d e r t h e i n c r e a s e d u p t a k e t o be t h e r e s u l t o f an i n c r e a s e i n t h e t o t a l d e f e c t volume. The n a t u r e o f t h e d e f e c t volume w i l l be e l a b o r a t e d upon more b e l o w .



316


M o i s t u r e S o r p t i o n as a F u n c t i o n o f R e l a t i v e H u m i d i t y . D i s t i n c t v a r i a t i o n s were seen i n t h e c o n j u g a t e s o r p t i o n d a t a a t 25°C as t h e p e n e t r a n t a c t i v i t y was d e c r e a s e d below 100% r e l a t i v e humidity (Figures 10-13). In p a r t i c u l a r , the crossover i n the absorption-desorption c u r v e s was no l o n g e r e x h i b i t e d , and t h e m o i s t u r e u p t a k e during r e s o r p t i o n no longer surpassed that during absorption. Certain criteria describing Fickian diffusion a r e f o l l o w e d , t h e r e f o r e , f o r PMC and NPMC samples a t r e l a t i v e h u m i d i t i e s o f 31% and 75% a t 25°C. The t h i c k n e s s s c a l i n g f a c t o r has n o t been i n v e s t i g a t e d under these c o n d i t i o n s . A t lower p e n e t r a n t activities more d i f f u s i o n control i s expected i n the transport process. The p o l y m e r m a t r i x does n o t r e q u i r e s t r u c t u r a l r e a r r a n g e m e n t s i n o r d e r t o accommodate t h e e q u i l i b r i u m moisture content. Consider Figure 20; t h i s is a s c h e m a t i c r e p r e s e n t a t i o n o f s o r b e d p e n e t r a n t p l o t t e d as a f u n c t i o n o f penetrant p a r t i a l pressure. Henry's law b e h a v i o r i s g i v e n by t h e d a s h e d l i n e . Predictions of d u a l mode s o r p t i o n t h e o r y w i l l d e v i a t e , as shown, from t h o s e o f H e n r y ' s law. The d a t a o f T a b l e I I d e m o n s t r a t e a d e v i a t i o n from l i n e a r b e h a v i o r . In f a c t , t h e data a p p e a r t o be b e s t d e s c r i b e d by t h e r e g i o n d e f i n e d by p o i n t s A and Β o f t h e d u a l mode s o r p t i o n t h e o r y . The t r a n s p o r t b e h a v i o r a t R.H. v a l u e s o f 31% and 75% c a n be d e s c r i b e d a s d o m i n a t e d by d i f f u s i o n phenomena, w h i c h c a n i n t u r n be b e s t d e s c r i b e d by d u a l mode s o r p t i o n t h e o r y . F u r t h e r s u p p o r t f o r t h e s e i d e a s i s g i v e n by c o n s i d e r i n g the r e s o r p t i o n behavior displayed i n Figures 10-13. Structural rearrangements will be reflected in a resorption cycle. That moisture uptake following resorption does not surpass moisture uptake during absorption suggests no s t r u c t u r a l changes. Complete removal o f sorbed moisture d u r i n g vacuum desorption f i n d s an o v e r l a p o f a l l c o n j u g a t e d a t a a t 66°C a s shown i n F i g u r e s 14 and 15. A g a i n t h i s c a n be i n t e r p r e t e d as r e s u l t i n g from a l a c k o f s t r u c t u r a l a l t e r a t i o n s d u r i n g t h e i n i t i a l s o r p t i o n c y c l e , and a dominance o f d i f f u s i v e t r a n s p o r t a t p e n e t r a n t a c t i v i t i e s l e s s t h a n 100% R.H. Finally, i f t h e moisture uptake i s examined as a f u n c t i o n o f time ( F i g u r e 17) i t c a n be s e e n t h a t t h e b e h a v i o r a t a c t i v i t i e s l e s s t h a n 100% R.H. i s d i s t i n c t l y non-linear, as s h o u l d be t h e c a s e during diffusion controlled transport. The r e s o r p t i o n b e h a v i o r a s a f u n c t i o n o f p e n e t r a n t a c t i v i t y up t o and i n c l u d i n g t o t a l immersion c a n now be discussed i n more detail. As penetrant activity increases at a sufficient temperature, t h e form o f t r a n s p o r t k i n e t i c s c a n v a r y from F i c k i a n t o Case I I t o Case I I accompanied by c r a z i n g (2J . This progression a p p e a r s t o be a p p l i c a b l e under t h e c o n d i t i o n s c i t e d i n this paper f o r moisture s o r p t i o n i n epoxy molding compounds. For penetrant activities o f 31% and 75% R.H., t r a n s p o r t a p p e a r s t o be d o m i n a t e d by d i f f u s i o n .



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Ρ Figure

20:

S c h e m a t i c r e p r e s e n t a t i o n o f t h e dependence o f c o n c e n t r a t i o n on p a r t i a l p r e s s u r e illustrat i n g p r e d i c a t i o n s o f Henry's law and d u a l mode sorption theory.


318



As t h e a c t i v i t y i n c r e a s e s t o 100% R.H. t h e c h a r a c t e r o f the t r a n s p o r t begins to illustrate some relaxational contribution. T h i s c o n t r i b u t i o n becomes s t r o n g e r w i t h i n c r e a s i n g temperature. The p o s s i b i l i t y o f c r a z i n g a l s o i n c r e a s e s with temperature. The n a t u r e o f t h e d e f e c t volume a l l u d e d t o e a r l i e r c o u l d , t h e r e f o r e , i n p a r t be due to crazing. Certainly, contributions from m a t r i x - f i l l e r breakdown cannot be n e g l e c t e d . The E f f e c t o f P o s t M o l d C u r i n g . In t h e system s t u d i e d here, the percent moisture u p t a k e and t h e d i f f u s i o n coefficient have been o b s e r v e d t o increase f o r both a b s o r p t i o n and r e s o r p t i o n c y c l e s f o l l o w i n g a p o s t mold curing cycle. D u r i n g t h e b u l k o f t h e p o s t mold c u r i n g c y c l e t h e aging temperature i s l e s s t h a n t h e sample's Tg, a n d t h e e f f e c t s o f b o t h p h y s i c a l and c h e m i c a l a g i n g must be c o n s i d e r e d (27,28) . T h i s s u b j e c t o f sub-Tg aging i n thermosets and i t s e f f e c t s upon moisture s o r p t i o n i s i n t e r e s t i n g and h a s n o t been examined t o date. Both the diffusion coefficient and t h e e q u i l i b r i u m m o i s t u r e u p t a k e a r e b e l i e v e d t o depend upon t h e f r e e volume i n g l a s s y p o l y m e r s (2, 3,20,22,23) . An i n c r e a s e i n f r e e volume i s n e c e s s a r y f o r an i n c r e a s e i n b o t h m o i s t u r e u p t a k e and D. An a l t e r n a t i v e e x p l a n a t i o n c o u l d r e s i d e i n an i n c r e a s e i n t h e d e f e c t volume ( m i c r o v o i d s o r p o l y m e r - f i l l e r i n t e r f a c i a l breakdown) p r o v i d i n g a l e s s t o r t u o u s p a t h and an i n c r e a s e d volume f o r m o i s t u r e s o r p t i o n . Arguments b a s e d upon e a c h o f t h e s e i d e a s a r e examined n e x t . M o i s t u r e has been o b s e r v e d t o a c c e l e r a t e p h y s i c a l aging a t temperatures w e l l below T o f t h e d r y sample CL, 29) . F o r t h o s e samples w i t h o u t p o s t m o l d c u r i n g and e x p o s e d t o 100% R.H. a t 100°C, i t h a s been o b s e r v e d t h a t T i s i n i t i a l l y d e p r e s s e d q u i t e s i g n i f i c a n t l y Q J . The d e p r e s s i o n f o r t h e p o s t mold c u r e d samples i s f a r l e s s . Following depression of Τ f o r t h e NPMC samples, c o n t i n u e d m o i s t u r e e x p o s u r e l e a d s t o an i n c r e a s e i n T i n d i c a t i n g r e s i d u a l cure. The i n i t i a l d e p r e s s i o n b r i n g s the e f f e c t i v e T o f the moisture e x p o s e d NPMC sample close t o t h e experimental temperature. Under such c o n d i t i o n s p h y s i c a l a g i n g i n t h e p r e s e n c e o f low l e v e l c u r e i s enhanced, t h e r e b y c a u s i n g b o t h D a n d m o i s t u r e u p t a k e t o be a d e c r e a s i n g f u n c t i o n o f t i m e . In t h e post mold c u r e d sample T i s c o n t i n u a l l y d e p r e s s e d ; however i t remains q u i t e h i g h ( q u i t e f a r above t h e e x p e r i m e n t a l temperature) suggesting low levels of volume contraction. T h i s e x p l a n a t i o n assumes more importance at higher temperatures. A t t h e low temperatures, however, t h e NPMC sample s t i l l e x h i b i t s l o w e r v a l u e s o f D and m o i s t u r e u p t a k e . I t seems l i k e l y , t h e n t h a t an a d d i t i o n a l mechanism i s o p e r a t i v e . The volume r e c o v e r y a s s o c i a t e d w i t h t h e p o s t mold c u r i n g c y c l e i s synonymous with a contraction o f the polymeric matrix. I t c a n be g

g

g

g

g


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envisioned that such a contraction could lead to a pulling away o f t h e m a t r i x m a t e r i a l from the rigid f i l l e r particles. The g e n e r a t i o n o f d e f e c t s by s u c h a p r o c e s s w o u l d d r i v e b o t h D and m o i s t u r e u p t a k e i n t h e observed d i r e c t i o n s . Conclusions


1.

2.

3.

4.

5.

Anomalous t r a n s p o r t was o b s e r v e d i n t h i s system a t 100% R.H. As t e m p e r a t u r e was i n c r e a s e d u n d e r s u c h conditions the nature of the t r a n s p o r t mode was o b s e r v e d t o e x h i b i t an i n c r e a s i n g Case I I c h a r a c t e r . Even though the c h a r a c t e r i s s h i f t i n g i n such a manner, t h e overall transport i s believed to be d o m i n a t e d by d i f f u s i o n . I n c r e a s e d m o i s t u r e u p t a k e under c o n d i t i o n s o f 100% R.H. d u r i n g a r e s o r p t i o n c y c l e was a s c r i b e d t o an i n c r e a s e i n d e f e c t volume. I t appears t h a t moisture t r a n s p o r t i n c o n d i t i o n s of less than 100% relative humidity is largely d o m i n a t e d by d i f f u s i o n . The d i f f u s i o n phenomenon i s b e s t d e s c r i b e d u s i n g d u a l mode s o r p t i o n t h e o r y . Resorption with penetrant a c t i v i t i e s l e s s than 100% R.H. results i n b e h a v i o r which superimposes with that of the i n i t i a l sorption c y c l e . This indicates l i t t l e s t r u c t u r a l rearrangement or d e g r a d a t i o n . P o s t mold c u r e d samples e x h i b i t a g r e a t e r m o i s t u r e uptake, and a l a r g e r d i f f u s i o n c o e f f i c i e n t . This has been a s c r i b e d t o t h e g e n e r a t i o n o f a s i g n i f i c a n t d e f e c t volume, as w e l l as t o p h y s i c a l a g i n g e f f e c t s .

Acknowledgments The authors would like to acknowledge the useful c r i t i q u e s o f P r o f . D. Soane, U.C. B e r k e l e y , d u r i n g t h e preparation of t h i s manuscript. Literature 1.

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3. 4. 5. 6. 7.

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RECEIVED May 30, 1989


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