Cross-Linked Polymers - American Chemical Society

Chapter 29

Structure and Properties of Polydimethacrylates Dental Applications

Downloaded by UNIV OF CINCINNATI on February 18, 2015 | http://pubs.acs.org Publication Date: April 18, 1988 | doi: 10.1021/bk-1988-0367.ch029

D. T. Turner, Z. U . Haque, S. Kalachandra, and Thomas W. Wilson Department of Operative Dentistry and Dental Research Center, University of North Carolina, Chapel Hill, NC 27514

Dimethacrylate monomers were polymerized by free radical chain reactions to yield crosslinked networks which have dental applications. These networks may resemble ones formed by stepwise polymerization reactions, in having a microstructure in which crosslinked particles are embedded in a much more lightly crosslinked matrix. Consistently, polydimethacrylates were found to have very low values of Tg by reference to changes in modulus of elasticity determined by dynamic mechanical analysis. Also, mechanical data on the influence of low volume fractions (0.03-0.05) of rigid filler particles provide evidence of a localized plastic deformation which would not seem understandable by reference to a uniformly crosslinked network. A non-uniformly crosslinked matrix might also be invoked to account for insensitivity of the rate of diffusion of water on the apparent degree of crosslinking. However, an observed increase in the uptake of water with apparent degree of crosslinking remains unexplained. C r o s s l i n k e d polymers a r e w i d e l y used as d e n t a l m a t e r i a l s ( 1 - 3 ) . Perhaps the most c h a l l e n g i n g a p p l i c a t i o n i s i n the r e s t o r a t i o n o f t e e t h ( 4 ) . The monomers must be n o n - t o x i c and c a p a b l e o f r a p i d p o l y m e r i z a t i o n i n the p r e s e n c e o f oxygen and w a t e r . The p r o d u c t s s h o u l d have p r o p e r t i e s comparable t o t o o t h enamel and d e n t i n and a s e r v i c e l i f e o f more t h a n a few y e a r s . I ncurrent restorative m a t e r i a l s such p r o p e r t i e s are sought u s i n g s o - c a l l e d " d e n t a l c o m p o s i t e s " w h i c h c o n t a i n h i g h volume f r a c t i o n s o f p a r t i c u l a t e i n o r g a n i c f i l l e r s ( 5 - 7 ) . However i n the p r e s e n t a r t i c l e a t t e n t i o n i s c o n c e n t r a t e d on one commonly used c r o s s l i n k e d p o l y m e r i c component, and on the way i n w h i c h some o f i t s p r o p e r t i e s are i n f l u e n c e d b y low volume f r a c t i o n s o f f i l l e r s . Up t o the p r e s e n t t i m e , use has been made a l m o s t e n t i r e l y o f d i m e t h a c r y l a t e s w h i c h are p o l y m e r i z e d b y f r e e r a d i c a l mechanisms t o y i e l d c r o s s l i n k e d p r o d u c t s ( 8 ) . P o l y m e r i z a t i o n i s i n i t i a t e d by redox systems, such as b e n z o y l p e r o x i d e / a r o m a t i c amine, and b y 0097-6156/88/0367-0427$06.00/0 © 1988 American Chemical Society In Cross-Linked Polymers; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


428

CROSS-LINKED POLYMERS

p h o t o p o l y m e r i z a t i o n w i t h e i t h e r u l t r a - v i o l e t or l o n g e r wavelength l i g h t . A c o n s i d e r a b l e f r a c t i o n o f the double bonds remains u n r e a c t e d because o f i m m o b i l i z a t i o n , due t o v i t r i f i c a t i o n o r s t e r i c i s o l a t i o n ( 9 - 1 4 ) . Even so, the s h r i n k a g e w h i c h accompanies the c o n v e r s i o n o f double t o s i n g l e bonds i s b e l i e v e d t o be the cause o f f a i l u r e s i n s e r v i c e (15.16). One approach t o t h i s problem i s t o reduce the c o n c e n t r a t i o n o f double bonds i n r e a c t a n t s by u s i n g d i m e t h a c r y l a t e s o f h i g h m o l e c u l a r w e i g h t such as u r e t h a n e d i m e t h a c r y l a t e s (17.18) and BIS-GMA i . e . the adduct o f b i s p h e n o l - A and g l y c i d y l m e t h a c r y l a t e ( 1 9 ) . A more a m b i t i o u s approach i s t o a v o i d s h r i n k a g e by r e c o u r s e t o r i n g o p e n i n g p o l y m e r i z a t i o n r e a c t i o n s , b u t t h i s i n e v i t a b l y r e q u i r e s a l o n g term e f f o r t because o f the d i f f i c u l t i e s o f d e v e l o p i n g any new polymer to t h e s t a g e o f a p p l i c a t i o n (20.21). The main d e f i c i e n c y o f p o l y d i m e t h a c r y l a t e s f o r the r e s t o r a t i o n of t e e t h i s t h e i r poor wear r e s i s t a n c e ( 4 - 7 ) . T h i s can be improved by i n c l u s i o n o f p a r t i c u l a t e f i l l e r s w h i c h a r e h a r d e r t h a n the p o l y m e r i c m a t r i x . An a m b i t i o u s g o a l would be t o match the r e m a r k a b l e p r o p e r t i e s o f d e n t a l enamel, w h i c h c o n t a i n s more t h a n 95 v o l - % o f h y d r o x y a p a t i t e c r y s t a l l i t e s t i g h t l y packed i n t o an i n t r i c a t e m i c r o s t r u c t u r e . I n comparison, t h e c u r r e n t composite r e s t o r a t i v e m a t e r i a l s have a crude m i c r o s t r u c t u r e w i t h no more t h a n 65 v o l - % i n o r g a n i c f i l l e r . T h i s i s n o t f o r want o f a p p l i c a t i o n o f c u r r e n t knowledge o f the t e c h n o l o g y o f p a r t i c u l a t e polymer c o m p o s i t e s . A wide range o f f i l l e r s o f v a r y i n g shapes and s i z e s , r a n g i n g from c o l l o i d a l dimensions t o t e n s o f m i c r o n s , i s b e i n g u s e d i n v a r y i n g c o m b i n a t i o n s . S i l a n e c o u p l i n g agents a r e b e i n g used, i n e x t e n s i o n o f Bowen's p i o n e e r i n g work ( 1 9 ) , t o bond t h e s e p a r t i c l e s t o the p o l y m e r i c m a t r i x . Y e t , the c u r r e n t composite m a t e r i a l s have much l o w e r wear r e s i s t a n c e t h a n the s i l v e r amalgams w h i c h t h e y a r e d e s i g n e d t o r e p l a c e . D e s p i t e t h i s d e f i c i e n c y t h e r e a r e d i v e r s e cogent r e a s o n s , such as e s t h e t i c s and a v o i d a n c e o f mercury p o l l u t i o n o f the environment, w h i c h spur on t h e i r f u r t h e r development. C r o s s l i n k e d polymers a r e a l s o used t o bond composite r e s t o r a t i o n s t o d e n t i n (22) and t h e r e b y m i n i m i z e the o c c u r r e n c e o f m a r g i n a l gaps w h i c h c o u l d r e s u l t i n b a c t e r i a l i n v a s i o n ( 2 3 ) . I n t h i s a p p l i c a t i o n , t h e r e i s a c a r r y - o v e r o f the e x p e r i e n c e g a i n e d w i t h the use o f d i m e t h a c r y l a t e monomers i n composite r e s t o r a t i v e m a t e r i a l s . As a f u r t h e r development adducts o f POCI3 w i t h h y d r o x y groups i n m e t h a c r y l a t e monomers a r e used i n the hope t h a t phosphonate groups, formed by h y d r o l y s i s , would f a v o r i o n i c b o n d i n g t o the h y d r o x y a p a t i t e component o f d e n t i n ( 2 4 ) . A q u i t e d i f f e r e n t approach i s t o use a h y d r o p h i l i c c o m b i n a t i o n o f g l u t a r a l d e h y d e and h y d r o x y e t h y l m e t h a c r y l a t e ( 2 5 ) . P o s s i b l y the r e s u l t i s an i n t e r p e n e t r a t i n g network between a s y n t h e t i c polymer and a more d e n s e l y c r o s s l i n k e d c o l l a g e n . These and o t h e r d e n t i n b o n d i n g agents (26-28) have been r e p o r t e d t o p r e v e n t m a r g i n a l l e a k a g e i n e x t r a c t e d t e e t h and t o g i v e a t e n s i l e a d h e s i v e s t r e n g t h as h i g h as 10 MPa. F o r comparison, the two m a t e r i a l s w h i c h a r e bonded t o g e t h e r have t e n s i l e s t r e n g t h s as f o l l o w s : d e n t i n , 40-50 MPa; d e n t a l c o m p o s i t e , 30-60 MPa. I n d e n t a l a p p l i c a t i o n s where c r o s s l i n k e d polymers do n o t need to be p r e p a r e d i n the mouth, h i g h e r degrees o f c r o s s l i n k i n g can be

In Cross-Linked Polymers; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


29.

TURNER ET AL.

Polydimethacrylates:

Dental

Applications

429

a c h i e v e d m e r e l y by h e a t i n g t o h i g h e r t e m p e r a t u r e s . A h i g h degree o f c r o s s l i n k i n g i s d e s i r a b l e i n crown and b r i d g e p r o s t h e s e s where a h i g h modulus o f e l a s t i c i t y i s sought i n o r d e r t o r e p l a c e m e t a l s , such as g o l d a l l o y s . The major p r o b l e m i s t o i n c r e a s e t h e r i g i d i t y o f p o l y m e r i c m a t e r i a l s y e t w i t h o u t i n c u r r i n g the e s t h e t i c a l l y u n a c c e p t a b l e appearance a t t e n d i n g the use o f most rigid f i l l e r particles. The b e s t approach has been t o use a v a r i e t y o f h y d r o p h o b i c d i m e t h a c r y l a t e copolymers w h i c h y i e l d s t i f f polymer c h a i n s and h i g h degrees o f c r o s s l i n k i n g (29.30). However, i n p u s h i n g i n t h i s d i r e c t i o n account must be t a k e n o f the o n s e t o f e m b r i t t l e m e n t ( 3 1 ) . An i n t e r e s t i n g p o s s i b i l i t y f o r a m e l i o r a t i o n i s the development o f polymer b l e n d s ( 3 2 ) . C u r r e n t ways i n w h i c h many s c i e n t i s t s a r e a d d r e s s i n g problems r e l a t e d t o the above a p p l i c a t i o n s can be j u d g e d by r e f e r e n c e t o a b s t r a c t s o f the I n t e r n a t i o n a l A s s o c i a t i o n o f D e n t a l R e s e a r c h (33). For the remainder o f t h i s a r t i c l e , a few l i n e s o f i n v e s t i g a t i o n i n j u s t one l a b o r a t o r y w i l l be o u t l i n e d i n the b e l i e f t h a t t h e y i n c l u d e p o i n t s o f g e n e r a l i n t e r e s t t o workers on o t h e r a s p e c t s o f h i g h l y c r o s s l i n k e d polymers. Dynamic M e c h a n i c a l A n a l y s i s

(DMA)

Dynamic t e s t i n g o f p r o p r i e t a r y d e n t a l composites has been u s e d t o determine Young's modulus, damping ( 5 ) , and t o a s s i g n a v a l u e o f Tg from a maximal v a l u e o f t a n 8 ( 3 4 ) . However f o r p r e s e n t p u r p o s e s , a t t e n t i o n w i l l be c o n c e n t r a t e d on u n f i l l e d networks made from m i x t u r e s o f known components ( 3 5 ) . The f o l l o w i n g m i x t u r e was p o l y m e r i z e d by exposure t o l i g h t o f wavelength > 400 run: BIS-GMA (75 wt%)+ t r i e t h y l e n e g l y c o l d i m e t h a c r y l a t e (25%) c o n t a i n i n g d l camphoroquinone (0.2%) and Ν,Ν-dimethylaminoethyl m e t h a c r y l a t e ( 0 . 1 % ) . Only about 50% o f the double bonds r e a c t e d , as d e t e r m i n e d by c a l o r i m e t r y ( 3 6 ) . The BIS-GMA copolymer was t e s t e d a t 11 Hz on an A u t o v i b r o n apparatus. The t r a n s i t i o n from the g l a s s y t o the r u b b e r y s t a t e o c c u r s over a wide temperature range o f more t h a n 100°C ( F i g . 1 ) . F o l l o w i n g a c o n v e n t i o n u s e d i n DMA, a v a l u e o f Tg - 48°C may be a s s i g n e d by r e f e r e n c e t o the maximal v a l u e o f m e c h a n i c a l l o s s , i . e . o f t a n 8. However, an a l t e r n a t i v e assignment may be made a t the temperature where the modulus o f e l a s t i c i t y (Ε') f i r s t b e g i n s to d e c r e a s e , i . e . a t Tg - -25°C. The h i g h e r v a l u e , from t a n 5, i s i n c o n s i s t e n t w i t h the o b s e r v a t i o n t h a t the p o l y m e r i c p r o d u c t i s f l e x i b l e a t room temperature. A l s o the lower v a l u e from E' i s c l o s e r t o the v a l u e o f Tg a s s i g n e d by r e f e r e n c e t o changes i n the c o e f f i c i e n t o f t h e r m a l e x p a n s i o n . As t h i s q u e s t i o n o f assignment i s o f g e n e r a l importance i t w i l l be i l l u s t r a t e d f o r the case o f a more h i g h l y c r o s s l i n k e d polymer ( c . f . r e f . 37) made by i r r a d i a t i o n o f BIS-GMA ( F i g . 2 ) . I t w i l l be seen t h a t s i m u l t a n e o u s measurements o f specimen l e n g t h v e r s u s temperature ( F i g . 3) gave a v a l u e o f Tg - 145°C w h i c h i s c l o s e r t o the v a l u e from E' (138°C) t h a n t o t h a t from t a n 8 (195°C). I n summary o f t h i s and o t h e r r e l a t e d work on epoxy systems ( 3 8 ) , i t appears t h a t though, g e n e r a l l y , i t i s more d i f f i c u l t t o a s s i g n a p r e c i s e v a l u e o f Tg by r e f e r e n c e t o Ε', n e v e r t h e l e s s t h i s v a l u e i s i n c l o s e r agreement w i t h the c l a s s i c a l method o f assignment by r e f e r e n c e t o the c o e f f i c i e n t o f thermal expansion.




430

Fig. 1

Dynamic m e c h a n i c a l p r o p e r t i e s o f a BIS-GMA/TGDM copolymer p r e p a r e d by p h o t o p o l y m e r i z a t i o n .

Tq=l38°C 1

195 °C J

ο ο

Ταηδ

3

-2 7

1

-100

Fig. 2

\

-50

1

1

1

1

0 50 100 150 TEMPERATURE (°C)

1

200

250

Dynamic m e c h a n i c a l p r o p e r t i e s o f a polymer p r e p a r e d b y Y - i r r a d i a t i o n o f BIS-GMA:Dose=75 Mrad.

TEMPERATURE (°C) Fig. 3

P e r c e n t change i n l e n g t h o f t h e polymer p r e p a r e d b y Y i r r a d i a t i o n o f BIS-GMA.


29.

TURNER ET AL.


Dental

Applications

431


From a p r a c t i c a l p o i n t o f v i e w i t i s i m p o r t a n t t o know whether s i m i l a r r e s u l t s a r e o b t a i n e d f o r the p r o p r i e t a r y composites c u r r e n t l y used t o r e s t o r e t e e t h . I n the examples s t u d i e d so f a r t h i s has been found t o be the case ( 3 5 ) , i n agreement w i t h e a r l i e r r e s u l t s ( 3 4 ) . Thus, t h e r e i s agreement about r e s u l t s b u t a d i f f e r e n c e i n how they s h o u l d be i n t e r p r e t e d . The v i e w t h a t t h e r e i s a low v a l u e o f Tg, w i t h i n the range o f temperatures (0-50°C) encountered i n s e r v i c e a t the s u r f a c e o f a r e s t o r a t i o n , w o u l d n o t seem t o bode w e l l f o r wear r e s i s t a n c e . I t would seem t o be i m p o r t a n t t o be a b l e t o c o n t r o l the p o l y m e r i z a t i o n r e a c t i o n , f o r example by means o f l i g h t i n t e n s i t y ( 3 9 ) , i n o r d e r t o impose any d e s i r e d v a l u e o f Tg and then t o e v a l u a t e i t s i n f l u e n c e on wear. Fillers The i n f l u e n c e o f f i l l e r s has been s t u d i e d m o s t l y a t h i g h volume f r a c t i o n s (40-42). However, i n a d d i t i o n , i t i s i n s t r u c t i v e t o s t u d y low volume f r a c t i o n s i n o r d e r t o t e s t c o n f o r m i t y w i t h t h e o r e t i c a l p r e d i c t i o n s t h a t c e r t a i n mechanical p r o p e r t i e s should i n c r e a s e m o n o t o n i c a l l y as the volume f r a c t i o n o f f i l l e r i s i n c r e a s e d ( 4 3 ) . For example, E i n s t e i n ' s t r e a t m e n t o f f l u i d s p r e d i c t s a l i n e a r i n c r e a s e i n v i s c o s i t y w i t h an i n c r e a s i n g volume f r a c t i o n o f r i g i d spheres. For g l a s s y m a t e r i a l s r e l a t e d comparisons can be made by r e f e r e n c e t o p r o p e r t i e s w h i c h depend m a i n l y on p l a s t i c d e f o r m a t i o n , such as y i e l d s t r e s s o r , more c o n v e n i e n t l y , i n d e n t a t i o n hardness. Measurements o f V i c k e r s h a r d n e s s number were made a f t e r p h o t o p o l y m e r i z a t i o n o f the BIS-GMA r e c i p e , d e t a i l e d above, c o n t a i n i n g v a r y i n g amounts o f a s i l a n t e d s i l i c a t e f i l l e r w i t h p a r t i c l e s o f tens o f m i c r o n s . Contrary to e x p e c t a t i o n , a minimum v a l u e was o b t a i n e d (44.45). f o r a volume f r a c t i o n o f 0.03-0.05 ( F i g . 4 ) . Subsequently, s i m i l a r r e s u l t s (46) were o b t a i n e d w i t h a l l 5 o t h e r f i l l e r s t e s t e d (Table 1 ) . T a b l e 1.

I n f l u e n c e o f f i l l e r s on V i c k e r s Hardness Number

Filler S i l i c a t e (45) (surface treated) Silicate (surface treated) S i l i c a t e (ashed)

Volume f r a c t i o n at minimum 0.03-0.05

% Decrease i n VHN* 10

0.05

34

0.05

7

Silica (surface treated) S i l i c a (ashed)

0.03

16

0.03

18

Tribasic calcium phosphate Hydroxyapatite

0.03

13

0.03

5

*100

[VHN

(no f i l l e r )

- VHN

( a t minimum)]/VHN (no

filler)




432

The p a r t i c l e s v a r i e d i n s i z e from c o l l o i d a l dimensions up t o tens o f m i c r o n s and d i f f e r e d i n s u r f a c e c h a r a c t e r i s t i c s , i n c l u d i n g t r e a t m e n t w i t h s i l a n e c o u p l i n g agents. A g e n e r a l e x p l a n a t i o n was b a s e d on t h e knowledge t h a t r i g i d i n c l u s i o n s r e s u l t i n a h i g h l y l o c a l i z e d s t r e s s c o n c e n t r a t i o n on a p p l i c a t i o n o f an e x t e r n a l f o r c e (47). I t was s u g g e s t e d t h a t t h i s may r e s u l t i n a l o c a l i z e d p l a s t i c d e f o r m a t i o n and hence i n a r e d u c t i o n o f m a c r o s c o p i c p r o p e r t i e s w h i c h depend on y i e l d i n g , such as y i e l d s t r e s s and i n d e n t a t i o n h a r d n e s s . W i t h h i g h e r volume f r a c t i o n s o f f i l l e r , e f f e c t s due t o i s o l a t e d p a r t i c l e s become u n i m p o r t a n t and t h e r e i s an e v e n t u a l i n c r e a s e i n p r o p e r t y v a l u e s , as p r e d i c t e d theoretically. Two l i n e s o f e v i d e n c e a r e c o n s i s t e n t w i t h t h e above suggestion. F i r s t , f r a c t u r e surfaces obtained w i t h the s i l a n t e d s i l i c a t e have l i n e a r f e a t u r e s w h i c h o r i g i n a t e from p a r t i c l e s and w h i c h e x t e n d away from t h e d i r e c t i o n i n w h i c h t h e c r a c k t i p advanced ( F i g . 5 ) . I n f a c t such f e a t u r e s h a d been p r e d i c t e d on the grounds t h a t t h e p a r t i c l e s would s e r v e t o r o t a t e t h e t e n s i l e s t r e s s f i e l d ahead o f t h e c r a c k t i p . R o t a t i o n o f t h e s t r e s s f i e l d was e x p e c t e d t o generate i n t e r s e c t i n g m i c r o c r a c k s , f o r reasons d i s c u s s e d p r e v i o u s l y ( 4 8 ) . From t h e p r e s e n t p o i n t o f v i e w t h e most p e r t i n e n t p o i n t i s t h a t , a t h i g h e r m a g n i f i c a t i o n , t h e l i n e a r f e a t u r e s c a n be seen t o have a rounded c r o s s - s e c t i o n . I n contrast, microcrack i n t e r s e c t i o n s i n very b r i t t l e materials give f e a t u r e s w i t h sharp edges. T h e r e f o r e , t h e rounded c r o s s - s e c t i o n i s an i m p o r t a n t o b s e r v a t i o n w h i c h has been i n t e r p r e t e d as e v i d e n c e of l o c a l i z e d p l a s t i c deformation. A second l i n e o f e v i d e n c e i s p r o v i d e d by an experiment i n w h i c h t h e degree o f c r o s s l i n k i n g o f p h o t o p o l y m e r i z e d specimens was p r o g r e s s i v e l y i n c r e a s e d by exposure t o γ-rays. The e x p e c t a t i o n was t h a t c r o s s l i n k i n g s h o u l d reduce p l a s t i c d e f o r m a t i o n by p r e v e n t i n g macromolecules from s l i p p i n g p a s t one another. T h e r e f o r e i f t h e s o f t e n i n g i s , indeed, due t o l o c a l i z e d p l a s t i c d e f o r m a t i o n t h e n t h e d i f f e r e n c e between u n f i l l e d specimens and ones w i t h a l o w volume f r a c t i o n o f f i l l e r s h o u l d be reduced by r a d i a t i o n c r o s s l i n k i n g . T h i s was found t o be t h e case ( F i g . 6 ) . I n f a c t a f t e r t h e h i g h e s t dose, t h e specimen c o n t a i n i n g f i l l e r was as h a r d as t h e u n f i l l e d specimen. A p r a c t i c a l consequence o f t h i s work i s t h a t an i s o l a t e d f i l l e r p a r t i c l e c a n a c t as a s i t e o f m e c h a n i c a l weakness. I n l i g h t l y c r o s s l i n k e d m a t e r i a l s t h i s can r e s u l t i n p l a s t i c deformation. Presumably i n h i g h l y c r o s s - l i n k e d m a t e r i a l s t h i s might r e s u l t i n b r i t t l e f r a c t u r e , e s p e c i a l l y i n f a t i g u e . I s o l a t e d f i l l e r p a r t i c l e s i t u a t i o n s c a n be e n v i s a g e d i n a v a r i e t y o f s e r v i c e a p p l i c a t i o n s . I n the f i e l d o f dental m a t e r i a l s , t h i s might o c c u r a t an i n t e r f a c e between a d e n t i n b o n d i n g agent and a composite f i l l i n g and t h e r e b y c o n s t i t u t e a zone o f m e c h a n i c a l weakness. Water S o r p t i o n Water s o r p t i o n o f d e n t a l m a t e r i a l s c a n r e s u l t i n u n d e s i r a b l e changes i n dimensions and t o a d e t e r i o r a t i o n i n m e c h a n i c a l p r o p e r t i e s . S t u d i e s have been made o f BIS-GMA copolymers o f t h e k i n d mentioned above (49.50) and a l s o o f polymers o f p o t e n t i a l use



29.

TURNER ET AL.


VOLUME

Fig. 4

Fig. 5

Dental

FRACTION OF

Applications

FILLER

I n f l u e n c e o f volume f r a c t i o n o f r i g i d f i l l e r p a r t i c l e s on V i c k e r s hardness number o f a BIS-GMA copolymer.

Generation o f l i n e a r features by p a r t i c l e s o f f i l l e r (X1200). Volume f u n c t i o n o f f i l l e r = 0.05. Crack p r o p a g a t e d from t o p t o bottom.


433


434


Ο

1

5

1

10

Τ

15

RADIATION DOSE (M rad)

Fig.

6

I n f l u e n c e o f Ύ-irradiation on V i c k e r s h a r d n e s s number o f BIS-GMA c o p o l y m e r s , w i t h and w i t h o u t f i l l e r s . Key: I-mixed w i t h p e s t l e ; I I - m i x e d w i t h s t i r r i n g r o d . Curve Α., w i t h o u t f i l l e r ( 0 ) ; curve B, w i t h 3 v o l % s i l a n a t e d s i l i c a t e f i l l e r (·).

i n crown and b r i d g e work (29.30.51). Here a t t e n t i o n w i l l be c o n f i n e d t o more s y s t e m a t i c r e s u l t s o b t a i n e d w i t h networks made by c o p o l y m e r i z a t i o n o f m e t h y l m e t h a c r y l a t e (MMA) and t r i e t h y l e n e g l y c o l d i m e t h a c r y l a t e (TGDM), u s i n g a redox i n i t i a t o r ( 5 2 ) . The d i f f u s i o n c o e f f i c i e n t f o r w a t e r was f o u n d t o be r a t h e r i n s e n s i t i v e t o the p r o p o r t i o n o f t h e d i m e t h a c r y l a t e c r o s s l i n k e r , e s p e c i a l l y i n s o r p t i o n ( F i g . 7 ) . T h i s i n s e n s i t i v i t y might be r a t i o n a l i z e d i n a number o f ways. One o f these i s c o n s i s t e n t w i t h the v i e w t h a t these systems have a l i g h t l y c r o s s l i n k e d m a t r i x w h i c h does l i t t l e t o impede t h e d i f f u s i o n o f w a t e r m o l e c u l e s . The c o n s i d e r a b l e i n c r e a s e i n w a t e r u p t a k e , as much as t w o - f o l d ( F i g . 8 ) , c a u s e d b y r e p l a c i n g MMA w i t h TGDM i s s u r p r i s i n g . I t does n o t seem t o be due t o i n c o m p l e t e p o l y m e r i z a t i o n because s i m i l a r r e s u l t s were c a l c u l a t e d from d a t a p r e v i o u s l y r e p o r t e d (29) f o r specimens made a t the much h i g h e r temperature o f 120°C. N e i t h e r c a n i t be a t t r i b u t e d t o the more h y d r o p h i l i c n a t u r e o f TGDM, because s i m i l a r r e s u l t s were o b t a i n e d f o r copolymers o f MMA and e t h y l e n e g l y c o l d i m e t h a c r y l a t e . An a l t e r n a t i v e h y p o t h e s i s was explored that c r o s s l i n k i n g results i n less e f f i c i e n t m a c r o m o l e c u l a r p a c k i n g , and hence t o i n c r e a s e d accommodation o f water i n microvoids. However, t h i s h y p o t h e s i s was t e s t e d b y



29.

T U R N E R E T A L .


Dental Applications

435

Feed of TGDM (wt-%)

Fig. 7

Dependence o f d i f f u s i o n c o e f f i c i e n t . D. on p r o p o r t i o n o f TGDM i n a monomer m i x t u r e p o l y m e r i z e d b y a redox initiator. Key: S o r p t i o n ( t ) ; d e s o r p t i o n ( 0 ) . Other monomer i s MMA.

50

100

Feed of TGDM (wt-%)

Fig. 8

Dependence o f uptake o f w a t e r on p r o p o r t i o n o f TGDM i n a monomer m i x t u r e w i t h MMA. Key: p r e s e n t work (0) ; r e s u l t s c a l c u l a t e d from r e f . 29.

(a).



436

m o n i t o r i n g changes i n d e n s i t y (53) and j u d g e d t o be inadequate ( 5 4 ) . Thus t h e i n f l u e n c e o f c r o s s l i n k i n g i n i n c r e a s i n g w a t e r uptake remains u n e x p l a i n e d .


Concluding

Remarks

I t appears t h a t networks formed from d i m e t h a c r y l a t e s a r e n o t u n i f o r m l y c r o s s l i n k e d , as was o f t e n assumed i n p i o n e e r i n g s t u d i e s (9.10). I n s t e a d t h e y have some resemblance t o t h e " p o r r i d g e " m i c r o s t r u c t u r e f i r s t a t t r i b u t e d by Houwink t o Bakélite (55) and s u b s e q u e n t l y adopted t o account f o r m i c r o s t r u c t u r a l o b s e r v a t i o n s on o t h e r networks p r e p a r e d b y s t e p w i s e p o l y m e r i z a t i o n r e a c t i o n s ( 5 6 ) . As f a r as i s known, m i c r o g e l p a r t i c l e s have n o t been o b s e r v e d i n networks formed b y c h a i n p o l y m e r i z a t i o n r e a c t i o n s . However, i t seems n e c e s s a r y t o invoke t h e i r f o r m a t i o n i n o r d e r t o account f o r t u r b i d i m e t r i c observations ( 5 7 ) , the onset o f g e l a t i o n , and g e l p a r t i t i o n ( 3 9 ) . I n t h e p r e s e n t work a case has been made f o r i n v o k i n g something l i k e a " p o r r i d g e " m i c r o s t r u c t u r e i n o r d e r t o a c c o u n t f o r some m e c h a n i c a l p r o p e r t i e s . Acknowledgments T h i s work was s u p p o r t e d b y NIH g r a n t s DE-02668, DE 06201 and RR 0533.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Phillips, R.W., "Skinner's science of dental materials", 9th ed., Saunders, Philadelphia, 1982. Craig, R.G. ed. "Restorative dental materials", 6th ed., C.V.Mosby Company, St. Louis, 1980. Greener, E.H., Harcourt, J.K., Lautenschlager, E.P., "Materials Science in Dentistry", Williams and Wilkins Company, Baltimore, 1972. Taylor, D.F., Leinfelder, K.F., eds. "Posterior Composites", Intern. Symp., Chapel Hill, NC 1982. Braem, M. "An In-Vitro Investigations into the Physical Durability of Dental Composites", Doctoral thesis, Leuven (Belgium), 1985. American Dental Society Council on Dental Materials, "Posterior Composite Resins", JADA 1986, 112 707. Roulet, J.F. "Degradation of Dental Polymers", Karger, Basel, 1987. Ruyter, I.E., Oysaed, H. J. Biomed. Mater. Res. 1987, 21, 11. Loshaek, S., Fox, T.G. J. Amer. Chem. Soc. 1953, 75, 3544. Hwa, J.C.H. J. Polym. Sci., 1962, 58, 715. Horie, K., Otawaga, Α., Muraoka, M., Mita, I., J. Polym, Sci. (Chem. Ed), 1975, 13, 445. Ruyter, I.E., Svendsen, S.A. Acta Odont. Scand. 1978, 36, 75. Asmussen, E. Acta Odont. Scand. 1975, 33, 337. Ferracane, J.L., Greener, E.H. J. Dental Res. 1984, 63, 1093. Going, R.E. JADA 1972, 84, 1349. Bausch, J.R., deLange, Κ., Davidson, C.L., Peters, Α., deGee, A.J. J. Prosthetic Dent. 1982, 48, 59. Asmussen, E. Acta Odont. Scand. 1975, 33, 129.



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48.

Turner, D.T. in ACS Symp. on "Characterization of highly crosslinked polymers" (Eds. S.S. Labana and R.A. Dickie), 1984, Symp. No. 243, 185 49. Soderholm, K.J., J. Biomed. Mater. Res. 1984, 18, 271. 50. Kalachandra, S., Turner, D.T. J. Biomed. Mater. Res. 1987, 21, 329. 51. Cowperthwaite, G.F., Foy, J.J., Malloy, M.A. in "Biomedical and dental applications of polymers" (Eds C.G. Gebglein and F. K. Koblitz), Plenum Press, New York, 1981, p. 379. 52. Turner, D.T., Abell, A.K. Polymer 1987, 28, 297. 53. Turner, D.T. Polymer 1982, 23, 197. 54. Haque, Z.U., Turner, D.T. unpublished work. 55. Houwink, R. "Elasticity, plasticity and structure of matter", 2nd ed., Dover, New York, 1958. 56. Morgan, R.J., O'Neal, J.E. J. Mater. Sci., 1977, 12, 1966. 57. Roschupkin, V.P., Ozerkovskii, B.V., Kalmykov, Y.B., Korolev, G. V., Vysokomol soed. 1977, A19, 699. RECEIVED October 7, 1987


Cross-Linked Polymers - American Chemical Society

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