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Chapter 18
Semiinterpenetrating Networks Based on Triazine Thermoset and N-Alkylamide Thermoplastics J. A. Feldman and S. J . Huang Institute of Materials Science, University of Connecticut, Storrs, CT 06268
In this study a semi-interpenetrating network (SIPN), is developed using bisphenol-A dicyanate with polyethyloxazoline and polyvinylpyrrolidone. The resulting SIPN is characterized with thermal analysis, infrared spectroscopy and electron microscopy. A water etching process shows that the polyethyloxazoline system is more compatible than the polyvinylpyrrolidone system. Phase domains of the thermoplastics range in size from 0.5 µm to 5.0 µm, respectively. This is the effective size for thermoset toughening. The results also indicate that the material may have useful biomedical applications. The porous surface may enhance tissue-polymer interactions, thus increasing the compatibility of implant prostheses. The end use o f a m a t e r i a l i s o f t e n d e c i d e d from an e n g i n e e r i n g v i e w p o i n t . The m o l e c u l a r l e v e l , a l t h o u g h i m p o r t a n t , i s n o t t h e o n l y concern when t h e a p p l i c a t i o n r e q u i r e s b u l k p r o p e r t i e s t o be i n a s p e c i f i c range. Many s y n t h e t i c polymers a r e t a i l o r made such t h a t t h e p h y s i c a l p r o p e r t i e s w i l l be optimum f o r a p a r t i c u l a r end use. I n t h e p a s t t h i s meant d e v e l o p i n g c o m p l e t e l y new p o l y m e r s , and a l t h o u g h t h e r e i s p r a c t i c a l l y no l i m i t t o t h e number o f p o l y mers t h a t can be made, o t h e r s have found t h a t b y combining two known p o l y m e r s , one can a l s o o b t a i n a s p e c i a l i z e d system. Combini n g v a r i o u s known systems t o g e t h e r produces p h y s i c a l p r o p e r t i e s averaged a c c o r d i n g t o t h e volume o f each component. C o n v e r s e l y , polymers c a n be p h y s i c a l l y mixed w i t h t h e r e t e n t i o n o f t h e i r p r o p e r t i e s . ( l ) I n t h i s way t h e d e s i r e d m e c h a n i c a l p r o p e r t i e s and p r o c e s s i n g c a p a b i l i t i e s can be o b t a i n e d . However, some problems e x i s t from a thermodynamic s t a n d p o i n t . The degree o f m i x i n g i s d i r e c t l y r e l a t e d t o t h e c o m p a t i b i l i t y between t h e two polymers. I f t h e c o h e s i v e energy d e n s i t i e s a r e q u i t e d i f f e r e n t , t h e n phase s e p a r a t i o n i s l i k e l y t o o c c u r and t h e f i n a l p r o d u c t w i l l not be homogenous i n b u l k c o m p o s i t i o n , ( l ) A b l e n d o f t h i s t y p e may show a n i s o t r o p y as a f u n c t i o n o f domain s i z e and p o s i t i o n . Whether t h i s i s b e n e f i c i a l o r d e t r i m e n t a l w i l l depend on i t s a p p l i c a t i o n . I f 0097-6156/88/0367-0244$07.00/0 © 1988 American Chemical Society Dickie et al.; Cross-Linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
18.
F E L D M A N AND H U A N G
Semiinterpenetrating
Networks
245
t h e c o h e s i v e energy d e n s i t i e s a r e s i m i l a r , t h e n t h e f r e e energy o f m i x i n g w i l l he o p t i m i z e d f o r a homogenous b l e n d , b e t t e r a p p r o x i mated as a polymer s o l u t i o n ( 2 ) . The p h y s i c a l p r o p e r t i e s o f a polymer s o l u t i o n a r e c l o s e r t o a t r u e average t h a n a b l e n d . There t h e two polymers a r e h i g h l y i n t e r t w i n e d . The degree o f m i x i n g w i l l be h i g h but t h e r e i s a g r e a t e r dependency on t h e e n t r o p y o f m i x i n g . The change i n e n t h a l p y may enhance m i x i n g , but t h e change i n e n t r o p y needed t o b r i n g one polymer i n t o c o n t a c t w i t h t h e o t h e r may be t o o l a r g e . T h i s would l i m i t t h e homogeneity o f t h e polymer solution. One way t o maximize t h e m i x i n g o f two polymers i s t o a c t u a l l y p o l y m e r i z e t h e monomers a f t e r t h e y a r e m i x e d , f o r m i n g an i n t e r p e n e t r a t i n g network, ( I P N ) . The f r e e energy o f m i x i n g must be m i n i m i z e d a c c o r d i n g t o t h e e n t h a l p y and e n t r o p y o f t h e system. The e n t h a l p y o f m i x i n g between two c h e m i c a l s can be m i n i m i z e d by matching t h e s o l u b i l i t y paramet e r s o r c o h e s i v e energy d e n s i t i e s . Monomers w i t h t h e same g e n e r a l p r i m a r y and secondary f o r c e s s h o u l d e x h i b i t s i m i l a r thermodynamic p r o p e r t i e s . The e n t h a l p y o f m i x i n g w i l l be f a v o r a b l e i n t h i s c a s e . One r e c o g n i z e s t o o , t h a t t h e e n t r o p y o f m i x i n g s h o u l d be m a x i m i z e d ^ 3) T h i s may not be e a s i l y a c c o m p l i s h e d u s i n g h i g h m o l e c u l a r weight p o l y m e r s , but by m i x i n g t h e monomers t o g e t h e r f i r s t , one f i n d s t h e t o t a l i n c r e a s e i n e n t r o p y t o be much h i g h e r . The r e s u l t i s a l o w e r f r e e energy o f m i x i n g f o r t h e monomers r e l a t i v e t o t h e polymers. The same argument w i l l h o l d , but t o a l e s s e r degree, f o r a two p a r t system u s i n g a monomer and a polymer i n s t e a d o f two monomers. A f t e r t h e compounds a r e m i x e d , t h e f i n a l parameter t o be concerned w i t h i s t h e d i f f u s i o n r a t e . Assuming F i c k i a n b e h a v i o r , one e x p e c t s t h e d i f f u s i o n r a t e t o be a f u n c t i o n o f t h e square r o o t o f t i m e . Ample t i m e s h o u l d be a l l o w e d t o o b t a i n a homogenous system. The f i r s t s t i p u l a t i o n t o be r e c o g n i z e d i n o b t a i n i n g a t r u e IPN i s t h a t each monomer s h o u l d o n l y p o l y m e r i z e w i t h i t s e l f . Separate i n i t i a t o r s may be needed t o accommodate each monomer. As l o n g as t h e p o l y m e r i z a t i o n mechanism i s s u f f i c i e n t l y d i f f e r e n t one s h o u l d o b t a i n an i n t e r p e n e t r a t i n g network, IPN. On a m o l e c u l a r l e v e l t h e polymers s h o u l d be i n t e r t w i n e d . There s h o u l d be a h i g h degree o f entanglement but no c o v a l e n t bonding between s p e c i e s . T h i s t y p e o f i n t e r l o c k i n g has been c o i n e d " c a t e n a n e s " . ( l ) T h e o b s e r v e d m e c h a n i c a l p r o p e r t i e s s h o u l d be c l o s e l y approximated by r u l e o f m i x t u r e theories. There i s s t i l l c o n t r o v e r s y t o t h e boundary between polymer b l e n d s , polymer s o l u t i o n s and i n t e r p e n e t r a t i n g networks. The work p r e s e n t e d i n t h i s paper seems t o have an i n t e r p e n e t r a t i n g morphology even though s e p a r a t e domains a r e observed. The o r i g i n a l d e f i n i t i o n o f IPN r e q u i r e s b o t h c h e m i c a l s p e c i e s t o be c r o s s l i n k e d , f o r m i n g "catenanes". However i f one component i s l i n e a r w h i l e t h e o t h e r i s c r o s s l i n k e d t h e n t h e f i n a l system w i l l o n l y be p a r t i a l l y , but s e l e c t i v e l y c r o s s l i n k e d . The s e m i - i n t e r p e n e t r a t i n g n e t w o r k s , SIPNs, a r e a c t u a l l y s y n t h e s i z e d by s e q u e n t i a l a d d i t i o n o f a monomer t o a polymer.(3) There a r e two t y p e s o f SIPNs. The d i s t i n c t i o n a r i s e s from t h e o r d e r o f a d d i t i o n . The o r i g i n a l polymer i s d e s i g n a t e d I and t h e a d d i t i o n a l monomer i s d e s i g n a t e d II. The SIPN o f t h e f i r s t k i n d i s o b t a i n e d when I i s c r o s s l i n k e d and I I i s s w o l l e n i n t o i t , p o l y m e r i z e d , but not c r o s s l i n k e d . The l i n e a r polymer i s not f o r m a l l y c r o s s l i n k e d . However i t w i l l be h i g h l y e n t a n g l e d . The f i n a l m a t e r i a l s h o u l d be l e s s r i g i d t h a n a
Dickie et al.; Cross-Linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
246
C R O S S - L I N K E D
P O L Y M E R S
f u l l IPN when u s e d above 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 l i n e a r polymer. I t s h o u l d be t o u g h e r s i n c e t h e l i n e a r m o l e c u l e s may be f r e e t o move and d i s s i p a t e shock energy. A SIPN o f t h e second k i n d r e s u l t s i n a system o f t h e same c h e m i c a l n a t u r e as t h e f i r s t k i n d b u t w i t h a d i f f e r e n t topography and morphology. This t y p e o f SIPN s t a r t s w i t h a l i n e a r polymer t o w h i c h a m u l t i f u n c t i o n a l monomer i s added. The monomer i s p o l y m e r i z e d and c r o s s l i n k e d i n situ. I n t h e l a s t decade much i n t e r e s t has e v o l v e d i n t h e p r o p e r t i e s and a p p l i c a t i o n s o f polymer b l e n d s . I t i s o f t e n f o u n d t h a t m i x i n g two systems t o g e t h e r produces a s y n e r g i s t i c e f f e c t . An example i s the a d d i t i o n o f r u b b e r p a r t i c l e s t o e p o x i e s o r p o l y s t y r e n e s ( l + , 5 , 6 ) . A toughened m a t e r i a l i s o b t a i n e d i f t h e r u b b e r domain s i z e i s chosen c o r r e c t l y (7)· As a r e s u l t o f polymer b l e n d t e c h n o l o g y , m a t e r i a l s c o n v e n t i o n a l l y r e g a r d e d as b e i n g t o o b r i t t l e f o r c e r t a i n a p p l i c a t i o n s can now be used. Two u n d e r l y i n g p r i n c i p l e s a r e emphasized i n t h i s work. One i s analogous t o t h e t o u g h e n i n g mechanism d e s c r i b e d above, w h i l e t h e second i n c l u d e s i n t e g r a t i o n o f the m a t e r i a l f o r b i o m e d i c a l a p p l i c a t i o n s . The thermoset i n c l u d e d here i s d e r i v e d from b i s p h e n o l - A d i c y a n a t e . I t can be t h e r m a l l y t r i m e r i z e d y i e l d i n g a t r i a z i n e o r c y a n u r a t e network (8,9,10) as seen i n the r e a c t i o n scheme (Table 1 ) . The c r i t i c a l m o l e c u l a r weight between c r o s s l i n k s i s r e l a t i v e l y l o w , r e s u l t i n g i n an e x t r e m e l y t i g h t , b r i t t l e network. The m a t e r i a l i s u s u a l l y used as a prepeg because a t o t a l c u r e produces a h a r d , i n f u s i b l e , and i n s o l u b l e m a t r i x . I t p o s s e s s e s e x c e l l e n t a d h e s i v e p r o p e r t i e s and i s c u r r e n t l y used as a m e t a l c o u p l i n g agent. I t o f f e r s many s u p e r i o r p r o p e r t i e s r e l a t i v e t o c o n v e n t i o n a l e p o x i e s d e r i v e d from b i s p h e n o l - A . B i s p h e n o l - A t y p e e p o x i e s a r e w i d e l y e x p l o i t e d as a d h e s i v e s , s e a l a n t s and c o a t i n g s f o r a m u l t i t u d e o f a p p l i c a t i o n s . I t i s also used as a m a t r i x f o r f i b e r c o m p o s i t e s . One r e c o g n i z e s , though, t h a t when an epoxy r e a c t s t h e r e a r e a l c o h o l m o i e t i e s produced w h i c h s i g n i f i c a n t l y c o m p l i c a t e f u t u r e b e h a v i o r . Improving p r o p e r t i e s o f e p o x i e s i n d i f f e r e n t environments remains a major c o n c e r n , range from l a r g e t e m p e r a t u r e f l u c t u a t i o n s t o l a r g e h u m i d i t y f l u c t u a t i o n s (5,11,12). The epoxy m a t r i x , c o n t a i n i n g m u l t i p l e h y d r o x y l and amino g r o u p s , has a h i g h tendency t o adsorb w a t e r , a major cause o f m a t e r i a l f a i l u r e i n many a p p l i c a t i o n s . F i n a l l y , when one uses t h e epoxy as a f i b e r m a t r i x , t h e r e i s a d i s t i n c t p r o b a b i l i t y t h a t v o i d s w i l l appear due t o r e a c t i o n b y - p r o d u c t s w h i c h do n o t escape d u r i n g c u r e . Once t h e m a t e r i a l v i t r i f i e s , t h e m o b i l i t y o f t h e m o l e c u l e s i s r e d u c e d , making i t d i f f i c u l t f o r t h e s m a l l m o l e c u l a r weight p r o d u c t s t o be e l i m i n a t e d . The t r i a z i n e thermoset has been d e v e l o p e d t o combat t h e s e s i t u a t i o n s . R e a c t i o n c o n v e r s i o n s approach 100$ when t h e c u r i n g i s done s t e p w i s e f o l l o w i n g a t e m p e r a t u r e c y c l e . There a r e few u n r e a c t ed groups w h i c h can c o m p l i c a t e m a t e r i a l p r o p e r t i e s o v e r l o n g periods o f time. C o m p l i c a t i o n s may a r i s e as a r e s u l t o f s i d e r e a c t i o n s i f r e s i d u a l u n r e a c t e d groups a r e p r e s e n t t h e r e f o r e complete r e a c t i o n s a r e d e s i r e d . Another advantage i s t h a t d u r i n g the c o u r s e o f r e a c t i o n t h e r e a r e no secondary r e a c t i v e groups p r o d u c e d , o n l y t r i a z i n e r i n g s a r e formed, whereas e p o x i e s produce a l c o h o l groups. No r e a c t i v e c e n t e r i s produced. Finally, i t i s seen t h a t no v o l a t i l e m o l e c u l e s a r e produced i n t h e r e a c t i o n .
Dickie et al.; Cross-Linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
18.
FELDMAN AND H U A N G
TABLE MATERIAL
Semiinterpenetrating
1 MATERIAL
ABBREV PEOX
polyethyloxazoline
INFORMATION STRUCTURE
PRODUCER MW (g/m) Dow
247
Networks
-(CH CH N) 2
250,000
2
n
0=CCH CH 2
-(CH CH) 2
polyvinylpyrrolidone
PVP
Aldrich
n
360,000
° ώ bisphenol-A
dicyanate
BP ADC
UCONN
278
CH
3
CH
3
NEC-
REACTION SCHEME
CH
3
5 NEC-0Ç^Ô>0-CEN
^
O^V\
V N^N Ο
Χ I χ (_) -(_)- (_)
PREPOLYMER y/y
m CURED V THERMOSET
American Chemical Society Library 1155 16th St., N.W. Washington, D.C.Polymers 20036 Dickie et al.; Cross-Linked ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
3
248
CROSS-LINKED
P O L Y M E R S
Three t r i p l e bonds r e a c t t o g i v e t h e a r o m a t i c c y a n u r a t e r i n g w i t h o u t any b y - p r o d u c t s . This i s especially enticing f o r fiber m a t r i c e s where v o i d f o r m a t i o n w i l l be r e d u c e d . The m a t e r i a l s d e v e l o p e d s h o u l d a l s o be u s e f u l f o r b i o m e d i c a l applications. P l a s t i c s a r e c u r r e n t l y used i n t h e body w i t h a l a r g e range o f a p p l i c a t i o n s , b u t many problems e x i s t ( 1 3 ) . I t i s imperat i v e t o e v a l u a t e t h e b i o c o m p a t i b i l i t y o f any m a t e r i a l brought i n t o c o n t a c t w i t h t h e body whether one d i s c u s s e s p o l y m e r i c drug d e l i v e r y systems, i n t r a c o r p o r e a l s y n t h e t i c p r o s t h e s e s o r e x t r a c o r p o r e a l p l a s t i c pumps o r t u b i n g ( l H , 1 5 ) . A s s o r t e d polymer f o r m u l a t i o n s have been t e s t e d t o a s s e s s t h e degree o f c o m p a t i b i l i t y b u t no a b s o l u t e a n a l y s i s method has been e s t a b l i s h e d . D i s c u s s i o n p e r s i s t s i n t h e l i t e r a t u r e on whether o r n o t t h e s u r f a c e o f an a r t i f i c i a l component must be smooth t o maximize b i o c o m p a t i b i l i t y o r rough t o enhance polymer-tissue i n t e r a c t i o n s ( i k ) . A smooth s u r f a c e i s b e l i e v e d t o h e l p b l o o d components pass by w i t h o u t a g r e a t d e a l o f f r i c t i o n which can i n d u c e t h r o m b o s i s and h e m o s t a s i s . T h i s i s somewhat o f an i d e a l p i c t u r e s i n c e a p r o t e i n l a y e r i s d e p o s i t e d over t h e p l a s t i c soon a f t e r c o n t a c t w i t h t h e t i s s u e . The h e a l i n g p r o c e s s s t a b i l i z e s t h e p r o s t h e s i s i n i t s p o s i t i o n and s e g r e g a t e s i t from b o d i l y fluids. However, t h e r e i s no sound e v i d e n c e t h a t t h e p r o t e i n l a y e r i s smooth. One thought i s t h a t t h e p r o t e i n l a y e r f o l l o w s t h e c o n t o u r s o f t h e s u b s t r a t e , r e s u l t i n g i n a smooth s u r f a c e ( l 6 ) . T i s s u e growth i s q u i t e e x t e n s i v e a f t e r t h e f i b r i n l a y e r i s d e p o s i t ed on a s y n t h e t i c component. Others b e l i e v e t h a t a rough s u r f a c e i s needed which can intera.ct w i t h t h e t i s s u e a g r e a t d e a l m o r e ( l U ) . A b e t t e r bond i s r e a l i z e d i f t h e pore s i z e and p o r o s i t y a r e chosen correctly. The rough s u r f a c e w i l l , however, i n d u c e some v a r i a t i o n s i n b l o o d f l o w l e a d i n g t o h e m o s t a s i s ( 1 7 ) . There i s s u f f i c i e n t d a t a t o promote each i d e o l o g y b u t t h e r e a r e many i n h e r e n t c o m p l i c a t i o n s i n d e f i n i n g rough VS. smooth s u r f a c e s ( 1 5 ) . The t e c h n i q u e u s e d t o determine t h e s u r f a c e c h a r a c t e r i s t i c s i s c r i t i c a l b u t a g a i n no a b s o l u t e method o r s t a n d a r d e x i s t s . Much a l s o depends on t h e i n t e r a c t i o n s c a l e which i s a f u n c t i o n o f t h e biological particle of interest. The b l o o d components range i n s i z e from s m a l l m o l e c u l e s t o p r o t e i n m o l e c u l e s and l a r g e r b l o o d cells. These can be s e l e c t i v e l y s e p a r a t e d by c h o o s i n g t h e c o r r e c t pore s i z e . Some s p e c i e s w i l l p r e f e r e n t i a l l y i n t e r a c t w i t h t h e p r o s t h e s i s due t o i t s d i m e n s i o n s . The e x t e n t o f i n t e r a c t i o n between v a r i o u s components i s measured u s i n g s e v e r a l methods. X-ray p h o t o e l e c t r o n s p e c t r o s c o p y and s c a n n i n g e l e c t r o n m i c r o s c o p y a r e u s e d t o a s s e s s t h e degree o f a d h e s i o n between t h e b i o l o g i c a l s p e c i e s and t h e p l a s t i c component g e n e r a l l y p r o v i d i n g q u a l i t a t i v e results(13,15,18,19). These methods a r e o f t e n u s e d t o d e c i d e whether o r n o t a m a t e r i a l i s b i o c o m p a t i b l e , which remains t h e p r i m a r y c o n c e r n when i n t r o d u c i n g a polymer t o t h e body. Once a polymer i s r e c o g n i z e d as b e i n g b i o c o m p a t i b l e i t s t i l l does n o t a s s u r e t h e system w i l l be a p p l i c a b l e f o r a p a r t i c u l a r a p p l i c a t i o n . A d h e s i o n between t h e two components i s v e r y i m p o r t a n t . B i o c o m p a t i b l e a c r y l i c cements have been w i d e l y employed t o combat s l i p p a g e o r complete f a i l u r e between t h e t i s s u e and t h e polymer(l8). These g l u e s a r e s a i d t o be s a f e i n s p e c i f i e d amounts but t h e monomers a r e t o x i c and i f e x c e s s i v e q u a n t i t i e s a r e u s e d over a p e r i o d o f time many u n d e s i r a b l e s i d e e f f e c t s may a r i s e . The
Dickie et al.; Cross-Linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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system developed h e r e may h e l p t o m i n i m i z e many o f t h e c u r r e n t problems. The composite d i s c u s s e d h e r e may improve t i s s u e - p o l y m e r stability. T h i s i s t h e main f a c t o r f o r c h o o s i n g t h e t h e r m o p l a s tics. P o l y e t h y l o x a z o l i n e and p o l y v i n y l p y r r o l i d o n e a r e n o n - t o x i c up t o r e l a t i v e l y h i g h doses. The former i s an e x p e r i m e n t a l polymer made by Dow C o r p o r a t i o n . The g l a s s t r a n s i t i o n temperature ( T g ) , i s 68°C, but i t remains v e r y tough below t h i s t e m p e r a t u r e . It is i m p o s s i b l e t o f r a c t u r e even when c o o l e d by l i q u i d n i t r o g e n . P o l y v i n y l p y r r o l i d o n e i s a polymer a c c e p t e d by t h e FDA t o be used i n t h e body a t moderate m o l e c u l a r w e i g h t s (
