The Thermodynamics of Water Sorption in Radiation-Grafted

22 The

Thermodynamics of Water Sorption in Radiation-

Grafted Hydrogels

Downloaded by UNIV OF NORTH CAROLINA on June 7, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0031.ch022

BERNARD KHAW,* BUDDY D. RATNER, and ALLAN S. HOFFMAN** Department of Chemical Engineering and Center for Bioengineering, University of Washington, Seattle, Wash. 98195 Hydrogels are lightly crosslinked, water swollen polymer networks and they have been suggested for or applied to a number of biomedical applications (e.g., 1-3). Since bulk hydrogels display relatively poor mechanical properties when water swollen, it is of interest to coat them onto stronger support materials. Radiation graft copolymerization is a useful technique for preparing a covalently bonded coating of one polymer on or within the surface region of another polymer and i t has been used to graft hydrogels to stronger polymer supports (3,4,5). It has been hypothesized that both the content and "structural character" of the water which is sorbed in hydrogels can be important determinants of the biological response at the interface with a hydrogel (6,7,8). It has become important, therefore, to study the manner in which water molecules are taken up by these hydrogel-grafted films, in order to find a correlation between the structure and character of the water in these gels and their biocompatibility. From this knowledge i t may be possible to design improved biomaterials by adjusting the water sorption behavior to some predetermined ideal. One way of studying the mode in which water molecules are taken up by hydrogels is by means of the thermodynamic parameters for the sorption of water vapor into the hydrogels. This paper describes the thermodynamics of water sorption in three different hydrogels, which were radiation grafted to silicone rubber fi1ms. Materials and Methods Three monomers were used in these studies: Hydroxyethyl methacrylate (HEMA), generously supplied by Hydro-Med Sciences of New Brunswick, N.J. and used as received; N-vinyl pyrrolidone (N-VP) purchased from Matheson, Coleman and Bell and purified by double distillation; and ethylene glycol dimethacry1 ate (EGDMA) purchased from Monomer-Polymer Laboratories, Philadelphia, PA. and used as received. Films of Medical Grade Silastic were * Present Address: ITT Rayonier, Shelton, Wash. ** To whom reprint requests should be directed. 295

In Hydrogels for Medical and Related Applications; Andrade, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

296

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p u r c h a s e d f r o m Dow-Corning C o r p , T h e i r ( n o m i n a l ) t h i c k n e s s was lOmil. The method used f o r r a d i a t i o n g r a f t i n g has been d e s c r i b e d i n p r e v i o u s p u b l i c a t i o n s (3,*0 . The r a d i a t i o n dose used h e r e was 0.25 Mrad. A q u a r t z s p r i n g v a p o r s o r p t i o n a p p a r a t u s was used t o o b t a i n s o r p t i o n isotherms at d i f f e r e n t temperatures f o r the three different films. From t h e s e d a t a , t h e s t a n d a r d d i f f e r e n t i a l h e a t s , (ΔΗ°) e n t r o p i e s , (AS°) and f r e e e n e r g i e s (AG ) o f s o r p t i o n were c a l c u l a t e d b a s e d on t h e method o f Othmer, e t , a l , (9). Downloaded by UNIV OF NORTH CAROLINA on June 7, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0031.ch022

0

R e s u l t s and

Discussion

The t h r e e f i l m s c h o s e n f o r s t u d y a r e d e s c r i b e d i n T a b l e I. They w e r e c h o s e n b e c a u s e o f t h e i r w i d e range o f w a t e r c o n t e n t s and a l s o b e c a u s e t h e y w e r e s u f f i c i e n t l y g r a f t e d t o y i e l d r e a d i l y measured w a t e r s o r p t i o n i s o t h e r m s . T a b l e I. Hydrogel Grafted

Monomer S o l u t i o n Composition ( v o l . % ) HEMA

NVP

EGDMA

20 10 0

0 10 20

1 1 1

HEMA HEMA/NVP NVP

R e m a i n i n g 73%

a)

HYPROGEL/S1LASTIC FILMS STUDIED a

( v o l . ) i s a 1/1

Extent of G r a f t i n g (mg/cm ) 2

%Η 0 i n Hydroge1 2

4.40 3.18 2.64

b

26 50 61

(vol.) mixture of Methanol/H 0. 2

b)

As measured by b l o t t i n g and w e i g h i n g t h e g r a f t e d f i l m a f t e r e q u i l i b r a t i o n in H 0 ( s e e Ref. 4 ) . T y p i c a l i s o t h e r m s f o r S i l a s t i c and two o f t h e g r a f t e d f i l m s a r e shown i n F i g u r e 1 and 2. I t can be seen t h a t w a t e r s o r p t i o n in the S i l a s t i c i s n e g l i g i b l e i n comparison to t h a t i n the g r a f t e d hydrogels. I t i s a l s o c l e a r t h a t water s o r p t i o n i s always exo t h e r m i c , as e x p e c t e d . The d e r i v e d d a t a f o r s t a n d a r d d i f f e r e n t i a l h e a t s , e n t r o p i e s and f r e e e n e r g i e s o f w a t e r s o r p t i o n as a f u n c t i o n o f e x t e n t o f s o r p t i o n a r e shown i n F i g u r e s 3~5 f o r t h e t h r e e g r a f t e d f i l m s . I t s h o u l d be n o t e d t h a t a l l t h e v a l u e s f o r ΔΗ°, A S , and AG o b t a i n e d f o r t h e t h r e e h y d r o g e l f i l m s under s t u d y a r e more neg~ a t i v e than those f o r the condensation o f water vapor t o l i q u i d water; the values f o r the condensation of water vapor to l i q u i d w a t e r a t 26°C a r e : -AG° = 1.97 k c a l / m o l e , «AS° = 2 8 , 2 9 e.u, ( c a l deg" m o l e " ) and -ΔΗ° = 10.48 kcal/mole. This indicates that e n t h a l p y , e n t r o p y and f r e e e n e r g y changes more n e g a t i v e t h a n t h o s e f o r c o n d e n s a t i o n a r e a t work and t h a t , as one w o u l d e x p e c t , t h e s o r p t i o n p r o c e s s i s not m e r e l y w a t e r v a p o r c o n d e n s i n g on t h e f i l m s ; t h a t i s , h y d r a t i o n o r p o l a r i n t e r a c t i o n s between t h e h y d r o s p h i l i e g r o u p s i n t h e h y d r o g e l s and t h e w a t e r m o l e c u l e s a r e no doubt o c c u r r i n g . The same can be s a i d f o r t h e c u r v e s o b t a i n e d a t 2

0

1

1


0


KHAW

ET

AL.

Water

Sorption in Radiation-Grafted

P°

Hydrogels

(mmHg)

Figure 1. Sorption isotherms for water vapor in ungrafted Silastic films

mgH20

SORBEΡ

Figure 2. Sorption isotherms for water vapor in two radia tion-grafted hydrogel/Silastic films


HYDROGELS FOR MEDICAL AND RELATED APPLICATIONS


210

,1.0

-I Ο

,

,

,

,

20

40

60

80

, 100

,120

mg. H 0 SORBED g. DRY HYDROGEL 2

Figure 3. Standard differential heat of sorption of water vapor as a function of the extent of water sorption in three radiation-grafted hydro gel/Silastic films at 26° C

0

20

40

60

80

100

120


Figure 4. Standard differential entropy of sorption of water vapor as a function of the extent of water sorption in three radia tion-grafted hydrogel/Silastic films at26°C


22.

KHAW

Water

ET AL.

Sorption

in Radiation-Grafted

Hydrogels

h i g h e r t e m p e r a t u r e s o f 31°, 37°, ^ 3 ° , 50° and 56°C; t h e c u r v e s d e r i v e d a t t h e s e t e m p e r a t u r e s showed e s s e n t i a l l y t h e same behav i o r a s a t 26°C. R e f o j o and Yasuda (1Q) a n d l a t e r Warren and P r i n s (JLL) showed t h a t when b u l k HEMA h y d r o g e l s were immersed i n w a t e r , t h e y d e s w e l l e d as t e m p e r a t u r e was r a i s e d up t o a b o u t 55 t o 60°C. These o b s e r v a t i o n s a l s o s u p p o r t an e x o t h e r m i c p r o c e s s o f w a t e r s o r p t i o n below 55 60°C. When t h e i n i t i a l w a t e r m o l e c u l e s a r e s o r b e d i n t h e g r a f t e d f i l m s they partake i n h i g h l y exothermic i n t e r a c t i o n s i n a l l t h r e e hydrogel f i l m s . P r o b a b l y t h e s e i n i t i a l i n t e r a c t i o n s i n HEMA o r HEMA/NVP f i l m s a r e l o c a l i z e d a t t h e most a c c e s s i b l e -OH g r o u p s ; the s t r o n g i n t e r a c t i o n and l o c a l i z a t i o n o f a w a t e r m o l e c u l e a t such a s i t e n o t o n l y e v o l v e s heat (high -ΔΗ ) but i t a l s o r e duces t h e e n t r o p y o f t h e s y s t e m s i g n i f i c a n t l y ( h i g h - A S ° ) a s the w a t e r m o l e c u l e l o s e s randomness. I t i salso possible that the h i g h -ΔΗ° and - A S ° c o n t a i n some c o n t r i b u t i o n f r o m i n c r e a s e d h y d r o p h o b i c bond f o r m a t i o n upon s o r p t i o n o f w a t e r m o l e c u l e s , which could " p l a s t i c i z e " l o c a l c h a i n motions. That i s , t h e i n t e r a c t i o n o f H 0 m o l e c u l e s w i t h a v a i l a b l e -OH s i t e s c o u l d p e r m i t a c e r t a i n amount o f i n c r e a s e d h y d r o p h o b i c g r o u p i n t e r a c t i o n s , w h i c h w o u l d o c c u r w i t h -ΔΗ° and - A S ° c h a n g e s . Then, i f any w a t e r m o l e c u l e s were i n v o l v e d , t h e s e c o u l d become more s t r u c t u r e d a r o u n d s u c h h y d r o p h o b i c d o m a i n s , c o n t r i b u t i n g t o -ΔΗ° a n d -Δ$° changes. I t i s most l i k e l y t h a t t h e p o l a r i n t e r a c t i o n s d o m i n a t e t h e i n i t i a l s o r p t i o n p r o c e s s i n t h e HEMA-contaiηing f i l m s and c a l c u l a t e d c l u s t e r i n g f u n c t i o n s r e f l e c t t h i s ( s e e below). In t h e NVP f i l m t h e r e i s p r o b a b l y a somewhat w e a k e r i n t e r a c t i o n and l o c a l i z a t i o n o f t h e H 0 m o l e c u l e a r o u n d t h e p y r r o l i d o n e c a r b o n y l o r n i t r o g e n g r o u p s , and t h e changes h e r e w i t h i n c r e a s i n g s o r p t i o n a r e l e s s m a r k e d , o v e r t h e range o f water contents s t u d i e d . As more w a t e r i s s o r b e d , t h e e v o l u t i o n o f h e a t and d e c r e a s e i n e n t r o p y b o t h d e c r e a s e i n m a g n i t u d e u n t i l most o f t h e r e a d i l y a c c e s s i b l e s o r p t i o n s i t e s a r e s a t u r a t e d , and t h e c u r v e s b e g i n t o l e v e l o f f . E v e n t u a l l y t h e w a t e r w i l l b e g i n t o p e n e t r a t e and s w e l l t h e b u l k o f t h e HEMA h y d r o g e l s , o p e n i n g up some new -OH s o r p t i o n s i t e s t o t h e H 0 m o l e c u l e s ; t h i s l e a d s t o an i n c r e a s e i n t h e ( d i f f e r e n t i a l ) exothermîcity and d e c r e a s e i n ( d i f f e r e n t i a l ) entropy o f sorption. T h i s c a n e x p l a i n t h e minima n o t e d i n b o t h t h e -ΔΗ° and - A S ° c u r v e s , f o r t h e HEMA c o n t a i n i n g h y d r o g e l s ; t h e s e minima o c c u r a t a p p r o x i m a t e l y 1 H 0 / 3 HEMA-OH f o r the HEMA s y s t e m and 1 H 0/1 HEMA-OH f o r t h e HEMA/NVP c o p o l y m e r system. A model has been p r o p o s e d f o r t h e s t r u c t u r e o f p o l y (HEMA) g e l s i n w h i c h many o f t h e OH-groups a r e t i e d up i n s t r o n g i n t r a m o l e c u l a r Η-bonded c r o s s l i n k s w h i c h f o r m o r g a n i z e d r e g i o n s i n t o w h i c h w a t e r c a n n o t e a s i l y p e n e t r a t e . (12) The r a t i o o f 1 H 0 m o l e c u l e p e r 3 HEMA-OH g r o u p s may i n d i c a t e t h a t t w o - t h i r d s o r more o f t h e h y d r o x y l g r o u p s i n poly(HEMA) a r e i n v o l v e d i n t h e s e r e g i o n s i n t h e d e h y d r a t e d g e l . The u p t u r n i n t h e -ΔΗ° a n d _


299

0

2

2

2

2

2

2



300

HYDROGELS FOR

MEDICAL AND

RELATED APPLICATIONS

- A S ° c u r v e s ( F i g u r e s 3 and k) f o r HEMA c o u l d i n d i c a t e t h a t a l t h o u g h t h e g e l s a r e s w e l l i n g and r e a r r a n g i n g , a l a r g e number o f f r e e OH-groups a r e not b e i n g opened up f o r e x o t h e r m i c H^bond~ ing w i t h t h e w a t e r . In t h e HEMA/NVP c o p o l y m e r f i l m s t h i s HEMA o r g a n i z e d s t r u c t u r e may be p a r t i a l l y b r o k e n up by t h e NVP rings and t h e r e f o r e not as many h y d r o x y l g r o u p s a r e o c c u p i e d i n t h e Hbond s t r u c t u r e . The NVP s y s t e m does not show any minimum (and may i n f a c t e x h i b i t a s m a l l maximum) i n t h e -ΔΗ° and -AS° c u r v e s . This may be due t o t h e l a c k o f a p o r o u s s t r u c t u r e i n t h i s g r a f t e d h y d r o gel system (discussed below), coupled w i t h s o r p t i o n of r e l a t i v e l y l a r g e amounts o f H 0. F i n a l l y a l l c u r v e s s h o u l d l e v e l o f f a t some e q u i l i b r i u m v a l u e o f -ΔΗ° and -AS° r e p r e s e n t i n g t h e combined p r o c e s s o f c o n d e n s a t i o n , and m i x i n g ( o r d i l u t i o n ) o f w a t e r i n t h e s w o l l e n hydrogel/ S i l a s t i c e l a s t i c n e t w o r k , where t h e r e i s a b a l a n c e between de c r e a s e d f r e e e n e r g y due t o s o r p t i o n v s . i n c r e a s e d f r e e e n e r g y due t o n e t w o r k s w e l l i n g . The g r a d u a l d e c r e a s e i n -AG° w i t h i n c r e a s ing s o r p t i o n ( F i g u r e 5) i s a r e f l e c t i o n o f t h e a p p r o a c h o f t h e o v e r a l l s o r p t i o n process to s a t u r a t i o n e q u i l i b r i u m . The r e s u l t s i n t h i s s t u d y f o r t h e t h r e e h y d r o g e l f i l m s a r e c o m p a r a b l e t o t h o s e o b t a i n e d by Masuzawa and S t e r l i n g (13) for t h e s o r p t i o n o f w a t e r v a p o r by a g a r , carboxymethy1 c e l l u l o s e , g e l a t i n and m a i z e s t a r c h and a l s o t o t h o s e r e p o r t e d by Bettelheim and E h r l i c h (1k) f o r t h e s o r p t i o n o f w a t e r v a p o r by m u c o p o l y s a c charides. In a d d i t i o n , t h e p r e s e n c e o f maxima and minima and t h e c l o s e p a r a l l e l s between t h e shapes o f t h e e n t r o p y and e n t h a l py c u r v e s s e e n h e r e were a l s o o b s e r v e d by t h e s e a u t h o r s . Both g r o u p s a l s o r e p o r t a g r a d u a l d e c r e a s e i n -AG° w i t h i n c r e a s e i n mg. H 0 s o r b e d / g s o r b a n t , as s e e n h e r e i n o u r d a t a , and e x p e c t e d as s a t u r a t i o n w a t e r s o r p t i o n i s a p p r o a c h e d . The t e n d e n c y f o r w a t e r m o l e c u l e s t o c l u s t e r ( o r not t o c l u s t e r ) t o g e t h e r w i t h i n t h e g r a f t e d h y d r o g e l m a t r i c e s may be e s t i mated by c a l c u l a t i n g t h e " c l u s t e r i n g f u n c t i o n " f o r w a t e r , a f t e r t h e t e c h n i q u e o f Zimm and L u n d b e r g (15)» These c a l c u l a t i o n s have been made, a s s u m i n g no change i n volume on m i x i n g , and r e s u l t s a r e shown i n F i g . 6. I t can be seen t h a t a l l v a l u e s o f t h e c l u s t e r i n g f u n c t i o n are s i g n i f i c a n t l y negative, suggesting that t h e w a t e r m o l e c u l e s a r e s t r o n g l y s i t e bound a t w i d e l y s e p a r a t e d s i t e s w i t h i n the hydrogel m a t r i x . This is e s p e c i a l l y true f o r the HEMA-containing polymers. Data o f Zimm and L u n d b e r g (15) for c o l l a g e n a r e r e p r o d u c e d i n F i g . 6 and show a s i m i l a r b e h a v i o r t o t h e h y d r o g e l s s t u d i e d h e r e . To q u o t e t h e i r d i s c u s s i o n o f t h e s e data: "The i n i t i a l w a t e r i s t i g h t l y bound" and as more w a t e r i s a b s o r b e d " t h e s o r p t i o n p r o c e s s changes . . . f r o m one o f s o r p t i o n on a few h i g h l y s p e c i f i c s i t e s t o a d i f f u s e s w e l l i n g phenomenon". 05) 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 i n t h e h y d r o g e l g r a f t e d f i l m s may be c a l c u l a t e d f r o m t h e i n i t i a l s l o p e o f t h e s o r p t i o n o r d e s o r p t i o n c u r v e when i t i s p l o t t e d as ( e . g . , see Réf. 16): 2

2


Water

KHAW ET AL.

Sorption

in Radiation-Grafted

Hydrogels

-Δο< (kcal ^mole)


\ *

NVP

HEMA/NVP

HEMA


Figure 5. Standard differential free energy of sorption of water vapor as a function of the extent of water sorption in three radia tion-grafted hydrogel/Silastic films at 26°C

CLUSTERING FUNCTION

G

„ „ , — -

The Thermodynamics of Water Sorption in Radiation-Grafted

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