The Interactions of Water with Epoxy Resins - ACS Symposium Series

30 The Interactions of Water with Epoxy Resins P. ΜΟY and F. E. KARASZ

Downloaded by UCSF LIB CKM RSCS MGMT on December 2, 2014 | http://pubs.acs.org Publication Date: August 19, 1980 | doi: 10.1021/bk-1980-0127.ch030

Polymer Science and Engineering, University of Massachusetts, Amherst, M A 01003

The s t r u c t u r a l p r o p e r t i e s o f a s y n t h e t i c p o l y m e r c a n o f t e n be m o d i f i e d by t h e s e r v i c e c o n d i t i o n s t o w h i c h i t becomes e x p o s e d , an example b e i n g i n t e r a c t i o n w i t h a t m o s p h e r i c m o i s t u r e . Water i n d u c e d p l a s t i c i z a t i o n i s a common o c c u r r e n c e , y e t d e p e n d i n g upon t h e c h e m i s t r y and m o r p h o l o g y o f t h e p o l y m e r e n c o u n t e r e d , t h e n a t u r e o f t h e i n t e r a c t i o n p r o c e s s may t a k e w i d e l y d i f f e r e n t routes. Some d e g r e e o f s p e c i f i c i t y must be assumed i n d i s c u s s i o n s r e l a t i n g the s t a t e o f sorbed water i n d i f f e r e n t polymeric systems. The r e s u l t s p r e s e n t e d i n t h i s p a p e r a r e p a r t o f a c o n t i n u i n g study d e a l i n g w i t h the w a t e r - i n d u c e d p l a s t i c i z a t i o n o f a high p e r formance epoxy r e s i n . The r e s i n , b a s e d on a t e t r a f u n c t i o n a l epoxy monomer c u r e d w i t h an a r o m a t i c d i a m i n e , i s a h i g h l y c r o s s - l i n k e d s y s t e m w i t h a h i g h a t t a i n a b l e Tg (>200°C). T h i s s y s t e m has u t i l i t y as a m a t r i x m a t e r i a l f o r r e i n f o r c e d c o m p o s i t e s w i d e l y used i n aerospace a p p l i c a t i o n s . Although the g l a s s t r a n s i t i o n temperature o f t h e d r y r e s i n i s f a r above a m b i e n t t e m p e r a t u r e s , t h e m o i s t u r e d e p r e s s e d Tg o f t h e m a t e r i a l c a n f a l l w i t h i n t h e use t e m p e r a t u r e of the r e s i n . I n s t a b i l i t i e s i n the mechanical p r o p e r t i e s brought a b o u t by t h e o n s e t o f t h e g l a s s t r a n s i t i o n i s sometimes s u f f i c i e n t to i n i t i a t e f a i l u r e processes i n these m a t e r i a l s . In an e a r l i e r p a p e r (1_), t h e a m b i e n t s o r p t i o n b e h a v i o r o f t h i s r e s i n was e x a m i n e d . These s t u d i e s have now been e x t e n d e d t o high temperatures. More r e l e v a n t i n s i g h t may be g a i n e d i n t h i s manner s i n c e t h e s e e x p e r i m e n t s w i l l more c l o s e l y s i m u l a t e t h e actual c o n d i t i o n s a t which the p l a s t i c i z a t i o n process o c c u r s . Knowledge o f t h e e q u i l i b r i u m w e i g h t g a i n o f t h e r e s i n a t v a r i o u s temperatures a l s o permits a q u a n t i t a t i v e e v a l u a t i o n of the p l a s t i c i z a t i o n process i n terms o f t h e o r e t i c a l e x p r e s s i o n s . The e f f e c t o f s o r b e d w a t e r on t h e s t a t e o f t h e h y d r o g e n bonds i n t h e e p o x y system and i t s p o s s i b l e c o r r e l a t i o n t o g l a s s t r a n s i t i o n d e p r e s s i o n phenomena has a l s o been e x a m i n e d i n t h i s s t u d y . F i n a l l y , i t was found i n the course o f these s t u d i e s t h a t the observed d i s c o n t i n u i t y i n the heat c a p a c i t y o f t h i s network system a t the g l a s s t r a n s i t i o n i s a very s t r o n g f u n c t i o n of the degree o f c r o s s - l i n k i n g . In f a c t , a t h i g h e x t e n t s o f c u r e , t h e t r a n s i t i o n becomes i m p e r c e p 0-8412-0559-0/ 80/47-127-505305.00/ 0 © 1980 A m e r i c a n Chemical Society

In Water in Polymers; Rowland, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

WATER IN P O L Y M E R S

506

t i b l e by s c a n n i n g c a l o r i m e t r y . F u r t h e r d i s c u s s i o n w i l l be g i v e n t o t h i s f i n d i n g and i t s r e l a t i o n t o t h e o b s e r v e d p l a s t i c i z a t i o n phenomenon.


Experimental The p r i n c i p a l r e s i n i n v e s t i g a t e d i n t h e s e s t u d i e s i s f o r m u l a t e d f r o m a 27 w e i g h t p e r c e n t m i x t u r e o f 4 , 4 ' - d i a m i n o d i p h e n y l s u l f o n e (DDS) and 73 w e i g h t p e r c e n t t e t r a g l y c i d y l - 4 , 4 ' - d i a m i n o d i p h e n y l methane (TGDDM). C u r i n g was e f f e c t e d t h e r m a l l y a t 177°C f o r 24 h o u r s . Samples u s e d i n s o r p t i o n measurements were m o l d e d f i l m s , 10 t o 15 m i l s t h i c k . The i s o t h e r m a l s o r p t i o n b e h a v i o r o f t h e TGDDM-DDS s y s t e m a t e l e v a t e d t e m p e r a t u r e s was s t u d i e d u s i n g a q u a r t z s p r i n g m i c r o b a l a n c e (2). Samples s u s p e n d e d f r o m c a l i b r a t e d s p r i n g s o f s e n s i t i v i t y 3 mm/mg were s e a l e d i n e v a c u a t e d g l a s s t u b e s t o g e t h e r w i t h bulbs c o n t a i n i n g water. Measurements were i n i t i a t e d f o l l o w i n g t h e r e l e a s e o f t h e w a t e r by s u c c e s s i v e f r e e z e - t h a w c y c l e s . The t e m p e r a t u r e o f t h e measurement was r e g u l a t e d by a f u r n a c e e n c l o s i n g the s p r i n g assembly. The p r e s s u r e w i t h i n t h e s y s t e m was v a r i e d by changing the temperature o f a heated bath i n t o which the lower p o r t i o n o f t h e s o r p t i o n c e l l was i m m e r s e d . Changes i n t h e w e i g h t o f t h e s a m p l e were d e t e r m i n e d by m o n i t o r i n g t h e e x t e n s i o n o f t h e s p r i n g assembly w i t h a c a t h e t o m e t e r . The d e p r e s s e d g l a s s t r a n s i t i o n t e m p e r a t u r e s o f t h e r e s i n a t v a r i o u s c o n c e n t r a t i o n s o f s o r b e d w a t e r were d e t e r m i n e d u s i n g s c a n n i n g c a l o r i m e t r y . Samples o f t h e r e s i n were f i r s t c o n d i t i o n e d w i t h an e x c e s s o f w a t e r i n l a r g e volume s t a i n l e s s s t e e l DSC pans a t p r e d e t e r m i n e d t e m p e r a t u r e s t h e n s c a n n e d a t 20 d e g / m i n . Heat c a p a c i t y measurements were a l s o made w i t h a P e r k i n - E l m e r DSC-2 scanning c a l o r i m e t e r . The i n f r a r e d a n a l y s i s was p e r f o r m e d u s i n g s a m p l e s c u r e d b e tween s i l v e r c h l o r i d e p l a t e s . C o n d i t i o n i n g o f t h e r e s i n was a c h i e v e d by i m m e r s i o n i n w a t e r . H i g h t e m p e r a t u r e s c a n s were made u s i n g a t e m p e r a t u r e r e g u l a t e d sample c e l l . S p e c t r a were o b t a i n e d w i t h a P e r k i n - E l m e r 283 s p e c t r o p h o t o m e t e r . Results

and

Discussion

I s o t h e r m a l s o r p t i o n p l o t s o f t h e TGDDM-DDS s y s t e m a r e shown i n F i g u r e 1. From t h e s e r e s u l t s two d i s t i n c t r e g i m e s o f s o r p t i o n b e h a v i o r may be d i s c e r n e d . While the lower temperature p l o t s are s i g m o i d a l c u r v e s o f t h e t y p e II i s o t h e r m s i n t h e BET c l a s s i f i c a t i o n (3»), t h e h i g h t e m p e r a t u r e i s o t h e r m s a p p r o a c h a l i n e a r d e p e n dence a t t h e h i g h e r p a r t i a l p r e s s u r e s . The f o r m e r i s o f t e n i d e n t i f i e d w i t h a m u l t i l a y e r s o r p t i o n process i n which i n i t i a l sorbed w a t e r m o l e c u l e s a r e a b l e t o i n t e r a c t w i t h b i n d i n g s i t e s on t h e polymer w h i l e subsequent molecules a s s o c i a t e w i t h the primary layer in l i q u i d water-like structures. The l a t t e r r e g i m e h o w e v e r , approaches simple s o l u t i o n s o r p t i o n . At elevated temperatures,


A N D KARASZ

Epoxy

Resins

507


ΜΟΥ

Figure 2.

Glass temperature depression data: (—) Equation i

calculated line as predicted by



508

t h e a v e r a g e t h e r m a l e n e r g y o f t h e s y s t e m e x c e e d s t h a t due t o any l o c a l i z e d p o l a r i z a t i o n e f f e c t s . The s o r p t i o n p r o c e s s may t h e n be a s y m p t o t i c a l l y a p p r o x i m a t e d by a H e n r y ' s Law dependence on t h e a c t i v i t y o f the p e n e t r a n t . The e q u i l i b r i u m w e i g h t g a i n s a t t h e s e t e m p e r a t u r e s a r e com p i l e d i n T a b l e I.


T a b l e I. Glass T r a n s i t i o n Temperature Depression Conditioning Temp. ( ° K ) 348 373 398 423 448

Τ

E q u i l i b . Wt. G a i n o f W a t e r (wt %)

9

Experimental (°K)

Data Τ 9

Calculated (°K) 443 455 460 461 462

438 454 457 461 467

1.27 0.85 0.77 0.68 0.67

From t h e s e v a l u e s , one may c a l c u l a t e and compare t h e e f f e c t i v e g l a s s t r a n s i t i o n t e m p e r a t u r e o f t h e r e s i n as a f u n c t i o n o f d i l u e n t c o n c e n t r a t i o n . Gordon e t a l . have r e c e n t l y d e r i v e d an e x p r e s s i o n r e l a t i n g the g l a s s temperature of a p o l y m e r - p l a s t i c i z e r m i x t u r e t o t h e g l a s s t e m p e r a t u r e s o f t h e components on t h e b a s i s o f the configurât!onal entropy theory o f glass formation (4). T h e i r d e r i v e d e x p r e s s i o n was a l s o o b t a i n e d f r o m a c l a s s i c a l t h e r modynamic argument by Couchman and K a r a s z ( 5 j . The e x p r e s s i o n was given as:

Τ (

φΤ _aî

α 1

φ )

=

+ Φ +

9

(Ι-Φ)ΤΚ

(i)

0-Φ)κ

i n w h i c h φ i s t h e volume f r a c t i o n o f p l a s t i c i z e r p r e s e n t , T g i ^ i s t h e g l a s s t r a n s i t i o n t e m p e r a t u r e o f t h e d i l u e n t and polyffief r e s p e c t i v e l y and Κ i s a p a r a m e t e r i n i t i a l l y a d j u s t a b l e t o y i e l d a b e s t f i t o f t h e d a t a . However, Κ may a l s o be f o r m a l l y i d e n t i f i e d w i t h the r a t i o Δ0ρ2/Δ0 where C p i , 2 r e p r e s e n t s t h e d i s c o n t i n u i t y i n heat c a p a c i t y a t T respectively. Using values of Τ = 134°K, T q = 483°K, p ( p o l y m e r d e n s i t y ) » 1.26 gm/ml and Κ = 0 . 1 2 7 , a c o m p a r i s o n o f t h e c a l c u l a t e d and e x p e r i m e n t a l l y o b t a i n e d glass temperatures of the mixture according to equation ( i ) i s shown i n F i g u r e 2 . I t i s seen t h a t t h e a g r e e m e n t i s q u i t e good a t s m a l l volume f r a c t i o n s o f p l a s t i c i z e r . S i n c e e q u a t i o n ( i ) was d e r i v e d f o r b i n a r y m i x t u r e s w h i c h obey t h e l a w s o f r e g u l a r s o l u t i o n s , i t may be assumed t h a t under t h e s e e x p e r i m e n t a l c o n d i t i o n s , t h e e p o x y - w a t e r s y s t e m may a l s o be t r e a t e d as s u c h . The i m p l i c a t i o n s o f the c o m p a r a t i v e l y low value o f Κ are d i s c u s s e d below. A

ρ ι

q l

2

g

2

2


i

30.

ΜΟΥ

A N D KARASZ

Epoxy

Resins

509

I t has been shown t h a t t h e m a j o r i t y o f t h e p o l a r m o i t i é s i n e p o x y - d i a m i n e n e t w o r k s , s u c h as h y d r o x y ! g r o u p s g e n e r a t e d by t h e c r o s s - l i n k i n g r e a c t i o n and r e s i d u a l amino h y d r o g e n s , t a k e p a r t i n i n t e r - o r i n t r a m o l e c u l a r h y d r o g e n bonds (6.,7). Indeed, a t ambient temperatures, the i n f r a r e d hydroxy! s t r e t c h frequency o f epoxydiamine systems are c h a r a c t e r i s t i c a l l y s h i f t e d from i t s f r e e value o f 3600 cm-1 t o a hydrogen bonded v a l u e o f 3440 c m " (8). W i l l i a m s and D e l a t y c k i have shown t h a t f o r a homologue o f t h e e p o x y - d i a m i n e s y s t e m s s t u d i e d , t h e i n t e r n a l h y d r o g e n bonds a r e o n l y d i s r u p t e d when t h e p o l y m e r s a r e h e a t e d t o above t h e i r g l a s s t e m p e r a t u r e s (7_). C o n v e r s e l y , one may e x p e c t t h a t t h e s t a t e o f t h e h y d r o g e n bonds w i l l have an e f f e c t on t h e r e l a x a t i o n a l b e h a v i o r of the network. In F i g u r e 3 , t h e wave number o f t h e h y d r o x y ! s t r e t c h f r e q u e n c y o f t h e TGDDM-DDS s y s t e m i s p l o t t e d as a f u n c t i o n of temperature. I t can be s e e n t h a t w i t h i n c r e a s i n g t e m p e r a t u r e , t h e band d i s p l a y s a t r a n s i t i o n - l i k e p r o c e s s i n a p p r o a c h i n g i t s unassociated or free value. Noteworthy a l s o i s the f a c t t h a t the t r a n s i t i o n process i s i n i t i a t e d at correspondingly lower tempera t u r e s f o r t h e wet r e s i n t h a n t h e d r y . The i m p l i c a t i o n s o f t h i s f i n d i n g may be more f u l l y e x p l o r e d on a m o l e c u l a r b a s i s . As w a t e r molecules are introduced i n t o the network, i t i s not unreasonable t o assume t h a t some d e g r e e o f exchange w i l l t a k e p l a c e between t h e h y d r o g e n bonded g r o u p s and t h e w a t e r m o l e c u l e s . S i n c e t h e s e i n t e r a c t i o n s a r e l a r g e l y d i p o l a r i n n a t u r e , t h e h y d r o g e n bonds f o r m e d between p o l y m e r segments and t h o s e between p o l y m e r segments and w a t e r m o l e c u l e s w i l l be e n e r g e t i c a l l y s i m i l a r . I t i s then e x p e c t ed t h a t t h e d e g r e e o f i n t e r c h a n g e w i l l be d e p e n d e n t s o l e l y on t h e a c t i v i t y o f the water. However, as t h e t e m p e r a t u r e o f t h e s y s t e m i s r a i s e d , t h e r e l a t i v e d i f f e r e n c e i n m o b i l i t y between p o l y m e r segments and t h e s o r b e d w a t e r m o l e c u l e s become s i g n i f i c a n t . The h y d r o g e n bonds c o n t a i n i n g w a t e r b r i d g e s w i l l be w e a k e r i n t h i s r e g a r d and w i l l be e x p e c t e d t o d i s s o c i a t e a t l o w e r t e m p e r a t u r e s , due t o t h e e n h a n c e d t h e r m a l m o b i l i t y o f t h e w a t e r , t h a n t h o s e f o r m e d between p h y s i c a l l y c o n s t r a i n e d p o l y m e r s e g m e n t s . The o n s e t o f segmental f l e x i n g w i l l i n t r o d u c e f l u c t u a t i o n s i n the f r e e v o l ume o f t h e s y s t e m . Such s e g m e n t a l m o b i l i t y and c h a n g e s i n f r e e volume w i l l a l l have t h e e f f e c t o f i n i t i a t i n g t h e g l a s s t r a n s i t i o n in the system. T h i s i s one mechanism by w h i c h p l a s t i c i z a t i o n i n e p o x y r e s i n s may p r o c e e d . I t was f o u n d t h a t t h e h e a t c a p a c i t y d i s c o n t i n u i t y a t t h e g l a s s t r a n s i t i o n o f t h e TGDDM-DDS s y s t e m i s s t r o n g l y d e p e n d e n t on the e x t e n t of cure. A t high degrees o f c r o s s - l i n k i n g , the g l a s s t r a n s i t i o n (which c o n c o m i t a n t l y i n c r e a s e s w i t h i n c r e a s e d c r o s s l i n k i n g ) becomes u n d e t e c t a b l e by s c a n n i n g c a l o r i m e t r y as A C approaches z e r o . To i n v e s t i g a t e t h i s p o i n t f u r t h e r a s l i g h t l y m o d i f i e d r e s i n s y s t e m was e m p l o y e d . T h i s overcame t h e p r o b l e m s a s s o c i a t e d w i t h c h a r a c t e r i z i n g an i n c o m p l e t e l y c u r e d s y s t e m and t h e p o s s i b i l i t i e s o f f u r t h e r r e a c t i o n when t h e r e s i n i s h e a t e d above i t s g l a s s t e m p e r a t u r e . By r e a c t i n g TGDDM w i t h a v a r y i n g s t o i c h i o m e t r i c m i x t u r e o f a n i l i n e and p - p h e n y l e n e d i a m i n e , a


1

n



510


Ο

X Ο

Δ Ο

Ô

Δ Ο

Δ

Ù

Ο ο tfry Δ wet

*

34001

§0

100 Temperature

Figure 3.

Spectroscopic transition of the TGDDM-DDS

1.0

system

o.s Equivalence

Figure 4.

110 *C

ο

Fraction of Aniline to ρ - P t w n y t · * · Diamine

Variation in A C at the glass transition for resins of TGDDM cured with different equivalence fractions of aniline and p-phenylenediamine P


30.

ΜΟΥ

A N D KARASZ

Epoxy

511

Resins

s e r i e s o f c o m p l e t e l y c u r e d c o m p o s i t i o n s was o b t a i n e d w i t h v a l u e s o f ACn d e p e n d e n t on t h e a v e r a g e c r o s s - l i n k d e n s i t y o f t h e r e s i n . A C as a f u n c t i o n o f t h e r e s i n c o m p o s i t i o n i s t h u s shown i n F i g u r e 4 and r e s u l t s a r e a l s o t a b u l a t e d i n T a b l e I I . p


Table II. H e a t C a p a c i t y Data and T r a n s i t i o n T e m p e r a t u r e s f o r D i f f e r e n t R e s i n Compounds Equivalence F r a c t i o n of A n i l i n e to p-Phenylene Diamine i n S t o i chiometric Cure o f TGDDM 1.0 0.9 0.8 0.7 0.6 0.5 0.4

Γ

Tq

y

(°K) 389 376 408 438 445 450 483

P

L

(

, Joules qm-deg 0.472 0.485 0.414 0.326 0.284 0.201 0.109

I t i s , o f c o u r s e , commonly u n d e r s t o o d t h a t t h e m a n i f e s t a t i o n o f t h e g l a s s t r a n s i t i o n d e c r e a s e s i n i n t e n s i t y as t h e c r o s s - l i n k d e n s i t y i n c r e a s e s , b u t i t s e f f e c t does n o t seem t o have been p r e v i o u s l y s t u d i e d q u a n t i t a t i v e l y . I f the formalism o f Wunderlich's r u l e o f c o n s t a n t heat c a p a c i t y i n c r e m e n t p e r " b e a d " i s used ( 9 ) , t h e a v e r a g e " b e a d " s i z e must i n c r e a s e w i t h t h e a v e r a g e c r o s s - T i n k d e n s i t y i n t h e r e s i n so t h a t a t v e r y h i g h d e g r e e s o f c r o s s - l i n k i n g t h e " b e a d " c o n t r i b u t i o n s o f 1 1 . 3 j o u l e s p e r mole beads p e r d e g r e e becomes v a n i s h i n g l y s m a l l on a u n i t mass b a s i s . However, t h e g e n e r a l i t y of this semi-empirical relationship is in part derived f r o m i t s a r b i t r a r i n e s s i n a s s i g n m e n t o f t h e " b e a d " u n i t . A more r i g o r o u s treatment o f the excess s p e c i f i c heat a t the q l a s s t r a n s i t i o n has r e c e n t l y been g i v e n by D i M a r z i o and D o w e l ! ( 1 0 ) . By i n c o r p o r a t i n g l a t t i c e v i b r a t i o n arguments i n t o t h e G i b b s - D i M a r z i o c o n f i g u r â t ! o n a l e n t r o p y t h e o r y o f g l a s s e s , an e x p r e s s i o n was d e r i v e d f o r t h e s p e c i f i c h e a t d i s c o n t i n u i t y . The t o t a l ACp i s c o n s i d e r e d t o a r i s e from t h r e e c o n t r i b u t i o n s . There i s a config u r â t ! o n a l t e r m a r i s i n g f r o m shape changes o f t h e m o l e c u l e s a t t h e g l a s s t r a n s i t i o n , a c o n f i g u r â t ! o n a l t e r m f r o m t h e volume e x p a n s i o n and a v i b r a t i o n a l c o n t r i b u t i o n f r o m t h e change o f f o r c e c o n s t a n t s w i t h t e m p e r a t u r e . The v a l u e o f Κ i n e q u a t i o n ( i ) f o u n d above i s thus understandable i n these terms. The p a r t i c u l a r r e s i n used was h i g h l y ( t h o u g h n o t c o m p l e t e l y ) c r o s s - l i n k e d and w o u l d c o r r e s p o n d i n g l y be e x p e c t e d t o have s m a l l ACp v a l u e s . The s i z e o f t h e h e a t c a p a c i t y d i s c o n t i n u i t y f o r w a t e r i s n o t known e x a c t l y , b u t i t s e s t i m a t e d v a l u e o f 35 t o 37 j o u l e s p e r mole d e g r e e l i e s i n t h e " n o r m a l " range o f low m o l e c u l a r weight m a t e r i a l s (Tl_,12). The i m p l i c a t i o n s o f K « l are i m p o r t a n t , f o r from e q u a t i o n " T i ) i t i s e a s i l y shown t h a t t h e m a g n i t u d e o f t h e d e p r e s s i o n i n T by p l a s q


512


t i c i z a t i o n i s approximately proportional to K " l . This f a c t prob a b l y a c c o u n t s f o r t h e u n u s u a l e f f e c t i v e n e s s o f w a t e r as a p l a s t i c i z e r f o r h i g h l y c r o s s - l i n k e d epoxy s y s t e m s o f t e n o b s e r v e d . Fur t h e r q u a n t i t a t i v e e v a l u a t i o n o f t h i s heat c a p a c i t y data i s i n p r o g r e s s and w i l l be r e p o r t e d i n a s u b s e q u e n t p a p e r .


Conclusion In a p r e v i o u s p a p e r , i t was shown t h a t t h e d i f f u s i o n p r o c e s s f o r w a t e r i n t h e TGDDM-DDS e p o x y - d i a m i n e s y s t e m was n o n - F i c k i a n a t n e a r a m b i e n t t e m p e r a t u r e (χ). Furthermore, evidence from b r o a d ! i n e NMR r e s u l t s was p r e s e n t e d w h i c h showed t h a t w a t e r i s probably sorbed i n the system i n a m u l t i - l a y e r process. I t was found t h a t a t low c o n c e n t r a t i o n s , the sorbed water e x h i b i t e d g r e a t l y r e s t r i c t e d m o b i l i t y so t h a t a d i s t i n c t l i q u i d phase was undetectable. T h i s was i n t u r n a t t r i b u t e d t o t h e l o c a l i z e d b i n d i n g o f w a t e r m o l e c u l e s by a d j a c e n t p o l a r g r o u p s i n t h e e p o x y n e t work. R e s u l t s p r e s e n t e d i n t h i s p a p e r may be u s e d t o f u r t h e r develop our understanding o f the epoxy-water i n t e r a c t i o n process. W h i l e t h e s o r p t i o n b e h a v i o r a t low temperatures d e v i a t e from Fickian d i f f u s i o n , i n d i c a t i o n i s given that ideal behavior i s approached w i t h i n c r e a s i n g temperature. This t r e n d i s o f course c o n s i s t e n t w i t h t h e e x p e c t a t i o n s t h a t t h e p r o b a b i l i t y o f any s i t e i n t e r a c t i o n w i l l be d i m i n i s h e d by i n c r e a s i n g t h e a v e r a g e e n e r g y o f the system. I n f r a r e d a n a l y s i s has i n d i c a t e d t h a t a f a c t o r i n t h e p l a s t i c i z a t i o n p r o c e s s may be t h e d i s r u p t i o n o f e x i s t i n g h y d r o g e n bonds i n t h e n e t w o r k . Furthermore, the f a c i l i t y w i t h which water s u b s t i t u t e d h y d r o g e n bonds a r e b r o k e n r e l a t i v e t o t h o s e between p o l y m e r segments i s i m p o r t a n t t o t h e d i s c u s s i o n o f t h e s t a t e o f sorbed water i n macromolecular systems. I t has been shown t h a t s t r o n g p e r t u r b a t i o n s are o f t e n observed f o r the thermodynamic p r o p e r t i e s o f the sorbed water i n p o l y m e r i c systems c o n t a i n i n g p o l a r g r o u p s , i . e . , t h o s e f r o m h e a t o f f u s i o n measurements ( 1 3 , ]±). The n o t i o n o f bound w a t e r i s one t h a t i s commonly i n v o k e d t o account f o r these f i n d i n g s . However, t h e c a l o r i m e t r i c d a t a o f Hoeve and K a k i v a y a a s w e l l as o t h e r s show t h a t t h e t h e r m a l s t a b i l i t y o f w a t e r bound t o a p o l y m e r i s n o t l a r g e r t h a n t h a t o f b u l k l i q u i d w a t e r (15,16.). T h i s r e i n f o r c e s t h e c o n c e p t t h a t d i p o l a r i n t e r a c t i o n s a r e n o t s i g n i f i c a n t l y d i f f e r e n t f o r monomeric o r polymeric species. I n s t e a d , one must c o n s i d e r t h e l i m i t i n g s p a t i a l f l u c t u a t i o n s t h a t p h y s i c a l l y c o n s t r a i n e d p o l y m e r segments a r e a b l e t o undergo r e l a t i v e t o t h o s e between two s m a l l m o l e c u l e s . At low t e m p e r a t u r e s , a l l p o l y m e r segments a r e e f f e c t i v e l y f r o z e n and i n t e r a c t i n g s m a l l m o l e c u l e s w i l l be e x p e c t e d t o d i s p l a y a h i g h d e g r e e o f i m m o b i l i t y . As t h e t e m p e r a t u r e o f t h e s y s t e m i s r a i s e d , t h e g r e a t e r t h e r m a l m o b i l i t y o f t h e s o r b e d w a t e r m o l e c u l e s w i l l be r e f l e c t e d by an e a r l i e r o n s e t o f the d i s s o c i a t i o n o f w a t e r c o n t a i n i n g h y d r o g e n bonded s e g m e n t s . These e x p e c t a t i o n s a r e i n a c c o r d a n c e w i t h t h e HWR and i n f r a r e d r e s u l t s .


30.

ΜΟΥ

A N D KARASZ

Epoxy

Resins

513


Abstract The high temperature sorption behavior of an epoxy-diamine system has been investigated. Correlation of the resulting data is made with a theoretical expression relating the depressed glass transition temperature of the system to the concentration of sorbed water. The effect of moisture on the hydrogen bonding in the network has also been examined. Discussion is given to the mechanisms through which plasticization may occur. It was found that the heat capacity discontinuity at the glass transition for this resin is a very strong function of the cross-link density. At high extents of cure, the ∆Cp per unit mass may become vanishingly small and undetectable by scanning calorimetry. This find ing largely accounts for the unusually effective plasticization of cross-linked epoxies by water. Acknowledgement We a r e g r a t e f u l AFOSR 7 6 - 2 1 9 8 .

Literature

f o r t h e s u p p o r t o f t h i s r e s e a r c h by G r a n t

Cited

1. Moy, P.; Karasz, F.E., in press. 2. McBain, J.W.; Baker, A.M. J. Amer. Chem. Soc., 1926, 48, 690. 3. Brunauer, S. "The Absorption of Gases and Vapors"; Oxford University Press: London, 1945, p. 150. 4. Gordon, J.M.; Rouse, G.B.; Gibbs, J.H.; Risen, W.M. J. Chem. Phys., 1977, 66, 4971. 5. Couchman, P.R.; Karasz, F.E. Macromol., 1978, 11, 117. 6. Vladimirov, L.V.; Zelenetskii, A.M.; Oleinik, E.F. Vysokomol. Soed., 1977, A19.(9), 2104. 7. Williams, J.G.; Delatycki, O. J. Polym. Sci., 1970, A-2(8), 295. 8. Silverstein, R.M.; Bassler, G.C.; Morrill, T.C. "Spectrometric Identification of Organic Compounds", John Wiley and Sons, Inc.: New York, 1974, p. 91. 9. Wunderlich, B. J. Phys. Chem., 1960, 64, 1052. 10. DiMarzio, E.A.; Dowell, F.,in press. 11. Sugisaki, M.; Suga, H.; Seki, S. Bull. Chem. Soc. Jpn., 1968, 41, 2591. 12. Johari, G.P. Philosophical Mag., 1977, 35, 1077. 13. Lee, H.B.; Jhon, M.S.; Andrade, J.D. J."Colloid, and Interface Sci., 1975, 51(2), 225. 14. Illinger, J.; Karasz, F.E.,in press. 15. Hoeve, C.A.J.; Kakivaya, S.R. J. Phys. Chem., 1976, 80, 745. 16. Pouchly, J.; Biroŝ, J.; Beneŝ, S. Makromol. Chem., 1979, 180, 745. RECEIVED January 4, 1980.


The Interactions of Water with Epoxy Resins - ACS Symposium Series

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