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The swelling of membranes of polyelectrolyte complexes ... achieved after which they slowly shrink to an equilibrium value. ..... retention capacity for Cu 2+.
Polymer Bulletin 31,471-478 (1993)

Polymer Bulletin 9 Springer-Verlag 1993

Swelling of membranes from the polyelectrolyte complex between chitosan and carboxymethyl cellulose W. ArgOelles.Monal 1, O. L. Hechavarria 2, L. Rodriguez 1, and C. Peniche3'* IlMRE, University of Havana, Havana 10400, Cuba 2Faculty of Chemistry, University of Havana, Havana 10400, Cuba 3Centro de Biomateriales, University of Havana, Havana 10400, Cuba SUMMARY The s w e l l i n g of m e m b r a n e s of p o l y e l e c t r o l y t e complexes f o r m e d from c h i t o s a n and c a r b o x y m e t h y l c e l l u l o s e is reported. These m e m b r a n e s adsorb water until a maximum swelling is achieved after w h i c h they slowly shrink to an equilibrium value. The m a x i m u m s w e l l i n g v a l u e and the time at w h i c h it is attained i n c r e a s e s as the pH of f o r m a t i o n of the c o m p l e x inc r e a s e s from 4.0 to 5.8. S h r i n k a g e o b s e r v e d at longer times is the result of the s e g m e n t a l m o b i l i t y of the p o l y e l e c t r o l y t e c h a i n s in the s w o l l e n state w h i c h a l l o w s the c o m p l e t i o n of the interpolyelectrolyte reaction. The w a t e r u p t a k e up to m a x i m u m s w e l l i n g obeys s e c o n d order kinetics.

INTRODUCTION P o l y e l e c t r o l y t e c o m p l e x e s (PEC) are f o r m e d w h e n d i s s o l u tions of m a c r o m o l e c u l e s carrying opposite c h a r g e s are mixed. Essentially, this is the result of e l e c t r o s t a t i c i n t e r a c t i o n s between both polymers. D e p e n d i n g on a variety of conditions, it may provoke the c o m p l e x to s e p a r a t e into a c o n c e n t r a t e d coacervate phase, or in a more or less c o m p a c t h y d r o g e l or precipitate. The b e s t k n o w n a p p l i c a t i o n of p o l y e l e c t r o l y t e complexes, as an end product, is in the p r e p a r a t i o n of membranes. These membranes can find a p p l i c a t i o n in controlled-release devices for drugs and agricultural pesticides based on swellable p o l y m e r matrices. Due to its u n i q u e c a t i o n i c character, c h i t o s a n ((l-4)-2a m i n o - 2 - d e o x y - ~ - D - g l u c a n ) has found i n c r e a s i n g a t t e n t i o n as a p o l y m e r c o m p o n e n t in a v a r i e t y of such c o m p l e x e s (1-5). A m o n g them, the s t o i c h i o m e t r i c polyelectrolyte complex formed w i t h carboxymethyl cellulose was formerly studied by F u k u d a (6,7), and later in our l a b o r a t o r y (8,9). However, no i n f o r m a t i o n has b e e n p r o v i d e d as to the s w e l l i n g b e h a v i o u r of m e m b r a n e s prep a r e d f r o m this PEC. In the p r e s e n t p a p e r the p r e p a r a t i o n of m e m b r a n e s from the i n t e r p o l y m e r complex between c h i t o s a n and carboxymethyl cellulose, as w e l l as the. e v a l u a t i o n of their s w e l l i n g b e h a v lout, has b e e n considered. *Corresponding author

472

EXPERIMENTAL

Materials Crab c h i t o s a n from Protan (~v = 6.9"10 s) w i t h an B2% deacetylation degree was employed. Carboxymethyl cellulose (CMC; E v = 7 . 7 - 1 0 4 , s u b s t i t u t i o n d e g r e e 0.7) was purchased from BDH. The m o l e c u l a r w e i g h t of c h i t o s a n w a s d e t e r m i n e d viscometrically at 25 • O . O I ~ in the solvent proposed by Roberts (I0), and the d e g r e e of d e a c e t y l a t i o n w a s m e a s u r e d by the U V - f i r s t d e r i v a t i v e m e t h o d (ii). Deionized water was employed in all e x p e r i m e n t s . All o t h e r r e a g e n t s w e r e a n a l y t i c a l grade.

Complex preparation 1 g of c h i t o s a n w a s d i s p e r s e d in 25 ml of w a t e r and diss o l v e d by a d d i n g 5 ml glacial acetic acid. C M C (1.7 g) was dissolved in 25 ml of water. Both p o l y e l e c t r o l y t e solutions w e r e a l l o w e d to s t a n d overnight. The s o l u t i o n s w e r e mixed, and 1.5 ml h y d r o c h l o r i c acid w e r e added. T h e n the p H of the s o l u t i o n w a s a d j u s t e d to the desired v a l u e by dropwise addition of 10% (w/v) NaOH under vigorous agitation. The fine dispersed hydrogel formed was p o u r e d into a plane sintered glass filter (~ = 8.5 cm), and a l l o w e d to d e c a n t and dry for a few days. A transparent, rigid and b r i t t l e m e m b r a n e w a s obtained.

Swelling measurements D r i e d and previously weighted membranes w e r e i m m e r s e d in an e x c e s s of w a t e r at pH = 5.5 ( a p p r o x i m a t e l y 1 liter). The solvent was r e g u l a r l y c h a n g e d in o r d e r to e l i m i n a t e the low molecular weight electrolytes d i f f u s e d out of the membrane, which was monitored b y c o n d u c t i m e t r i c m e a s u r e m e n t s . D u r i n g the e x p e r i m e n t the t e m p e r a t u r e w a s kept at 25~ The w e i g h t of the membrane was measured p e r i o d i c a l l y after carefully removing the e x c e s s of s o l v e n t from its surface. The s w e l l i n g degree, M, w a s e s t i m a t e d as M-

P-P~

Po

It]

Po

where Po is the w e i g h t of the dry membrane, and P is the weight of the s w e l l e d membrane. Experiments w e r e c a r r i e d out u s i n g t h r e e r e p l i c a t e s , and the s w e l l i n g d e g r e e s r e p o r t e d are a v e r a g e values.

Cupric ion retention measurements A strip of a s w e l l e d m e m b r a n e w a s w e i g h t e d ( a p p r o x i m a t e l y 1 g) and s t i r r e d d u r i n g 3 h o u r s in 0.05 m o l / l CuCI2 aqueous s o l u t i o n at 25~ After this, the membrane w a s once washed w i t h w a t e r and left to dry. A portion of this membrane was weighted, digested with n i t r i c acid and the copper content w a s m e a s u r e d in a Philips SP-9 a t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t e r at A = 2 3 8 . 4 nm w i t h a i r / a c e t y l e n e flame. The c o p p e r uptake was then r e p o r t e d as g ram s of Cu m§ per g r a m of dry m e m b r a n e .

473

RESULTS

AND DISCUSSIQ~

The p r o p e r t i e s of P E C m e m b r a n e s are s t r o n g l y d e p e n d e n t on the p r e p a r a t i o n c o n d i t i o n s -pH, ionic strength, temperature, concentration and m o l a r ratio of reacting polyelectrolytes, among othersand t h e r e f o r e it w a s a p r e m i s e of this w o r k that the m e m b r a n e s prepared should exhibit a reproducible behavlout. This w a s accomplished by mixing equivalent amounts of c h i t o s a n and C M C s o l u t i o n s and l o w e r i n g the pH of the m i x t u r e b e l o w 2. At this pH v a l u e the solution remains homogeneous, since the c o m p l e x is not f o r m e d (8). The s o l u t i o n pH w a s then adjusted to the desired value by addition of d i l u t e NaOH solution and the p o l y e l e c t r o l y t e c o m p l e x p r e c i p i t a t e d as small hydrogel particles. The mixture was then poured into a sintered glass f i l t e r in o r d e r to e l i m i n a t e the supernatant s o l u t i o n and the s o l i d w a s a l l o w e d to dry as a m e m b r a n e w h e n left at room t e m p e r a t u r e in a dry p l a c e for a few days. Following c a r e f u l l y this p r o c e d u r e it w a s p o s s i b l e to r e p r o d u c ibly o b t a i n P E C m e m b r a n e s . M

12

O( |

0

i

I

i

i

~

f

i

20

4.0

80

80

100

120

140

180

t (h) Figure

1 Swelling

ent

values:

pH

pH

curves o f PEC = 4 . 2 (O) ; D H

=

membranes Drepared 5 . 0 (*) ; p H = 5 . 7

at (o)

differ-

These membranes, though transparent and b r i t t l e in the dry state, b e c a m e opaque and f l e x i b l e w h e n left for some time in a h u m i d place, since b e c a u s e of their h i g h hydrophilicity they e a s i l y adsorb w a t e r from the s u r r o u n d i n g medium. This r e s u l t s in a l o w e r i n g of Tg of the P E C due to the p l a s t i c i z i n g e f f e c t of the a d s o r b e d water. The swelling behaviour of the membranes prepared at d i f f e r e n t pH v a l u e s w h e n i m m e r s e d in w a t e r at 25~ is s h o w n in F i g u r e i. The c u r v e s e x h i b i t a c h a r a c t e r i s t i c p a t t e r n invaria b l y found in t h e s e e x p e r i m e n t s : the m e m b r a n e s a d s o r b w a t e r at a c o n s i d e r a b l e rate u n t i l a m a x i m u m s w e l l i n g is achieved, but

474

for p r o l o n g e d i m m e r s i o n t i m e s they s t e a d i l y loose w e i g h t until a final equilibrium v a l u e is reached. The maximum swelling value, and the time at w h i c h it is attained, i n c r e a s e s as the pH of f o r m a t i o n of the P E C increases. T h i s kind of d e p e n d e n c e , of the s w e l l i n g d e g r e e of the P E C w i t h the pH v a l u e at w h i c h it w a s formed, h a s b e e n r e p o r t e d b e f o r e for P E C s p r e p a r e d from o t h e r w e a k p o l y e l e c t r o l y t e s (12). During the first part of the s w e l l i n g p r o c e s s - w h i c h is show n e n h a n c e d inside F i g u r e ithe m a x i m u m s w e l l i n g is attained as a r e s u l t of the equilibration between the o s m o t i c force c a u s i n g the p e n e t r a t i o n of w a t e r inside the m e m b r a n e and the o p p o s i n g e l a s t i c force of the s t r e s s e d ionic n e t w o r k of the PEC. It m u s t be r e c a l l e d that the PEC has been obtained by reacting equimolar q u a n t i t i e s of C M C and c h i t o s a n hydrochloride a c c o r d i n g to S c h e m e i. Scheme

%a

I:

...... ~ HSHe

+ nH % nOI-

M The degree of conver14 sion, e, - d e f i n e d as the ratio of c o n c e n t r a t i o n of in12 t e r c h a i n salt b o n d s f o r m e d to the initial c o n c e n t r a t i o n of functional g r o u p s of any of lo the polyelectrolytes. is strongly dependent on the pH of f o r m a t i o n of the P E C (8, 8 13). It is e v i d e n t that as e i n c r e a s e s the a m o u n t of free 8 ionic g r o u p s in the P E C decreases and the complex bec o m e s less h y d r o p h i l i c . 4 The higher swelling cao p a c i t y of m e m b r a n e s f o r m e d at 2 pH 5.7 indicates that this PEC has more free ionic i i i groups t h a n the ones formed o 4,5 5 5,5 at pH 5 and 4.2. In other words, the d e g r e e of converpH sion, 8, of the P E C s decreas- Figure 2 Dependence o f (*) m a x es as the pH of formation imum swelling, M .... , a n d (s) increases from 4 to 5.8. The e q u i l i b r i u m swelling, M , q , on high slope of the curve ob- the p H o f f o r m a t i o n o f the PEC t a i n e d w h e n p l o t t i n g the maximum s w e l l i n g value, Mm~• as a f u n c t i o n of the pH of f o r m a t i o n of the P E C (Figure 2, curve i) is a direct consequence of the s t r o n g d e p e n d e n c e of e on pH, t y p i c a l of c o o p e r a t i v e t r a n s i t i o n s . The steady shrinkage o b s e r v e d for these highly swelled

475

membranes when maintained immersed in w a t e r for a prolonged p e r i o d of time can be a c c o u n t e d for if one c o n s i d e r s that at the pH v a l u e at which swelling experiments were carried out (pH = 5.5), the free c a r b o x y l i c g r o u p s of C M C in the P E C are p r e s e n t as sodium carboxylate and the free amino groups of chitosan are protonated, so that new interpolyelectrolyte saline b o n d s can be f o r m e d t h r o u g h the reaction depicted in S c h e m e 2. Scheme 2:

~

1"-I~OI- "Ha-O00C~- § *Na-O0(~

H~ OI- *Ne-O00"

Hs + H8

9 nNa +§ n~-

~H~

This reaction is f a v o u r e d by the m i g r a t i o n of Na § and CIions o u t s i d e the m e m b r a n e . The s e g m e n t a l m o b i l i t y of the p o l y electrolyte chains in the s w o l l e n state s h o u l d be sufficient to a l l o w the adequate spatial arrangements of the reacting g r o u p s for this r e a c t i o n to proceed. As a result of this, 6 increases, thus p r o d u c i n g the observed steady shrinkage until an equilibrium value, M~q, is attained. It can be s e e n in Figure 2 (curve 2) that M ~ q tends, in all cases, to a similar value, v e r y c l o s e to M m ~ x at pH 4. In order M to v e r i f y this 12 e x p l a n a t i o n the s w e l l i n g of P E C 10 membranes was studied in me! dia with dif8I ferent ionic strength. Swelling experiments were carried out as 4 before, but NaCI solutions 8 were used as well as water. To this end, CI p o r t i o n s of the 0 50 100 ~0 200 250 800 same membrane t(h) were subjected Figure 3 Swelling curves of PEC membranes to different f o r m e d at p H = 5 . 7 in s o l u t i o n s with differsaline concenent ionic strength: water (*); 0.001 mol/l trations and N a C I (o); i m o l / l N a C I (0) the r e s u l t s are s h o w n in Figure 3. As in all other c a s e s the p o i n t s r e p r e s e n t the a v e r a g e of three e x p e r i m e n t s . Two facts are m a n i f e s t e d in this Figure: the lower v a l u e of M ~ a x in lO-S m o l / l NaCI s o l u t i o n as c o m p a r e d to M ~ a x in water, and the s m a l l e r fall in the d e g r e e of s w e l l i n g w i t h time

476

in the s a l i n e solution after the maximum swelling had b e e n reached. U n d e r the above considerations this is to be expected, since in the p r e s e n c e of a 10 -3 m o l / l NaCI solution, the osmotic pressure i n d u c i n g the p e n e t r a t i o n of w a t e r inside the membrane decreases, consequently, decreasing Mm~x. On the other side, the m i g r a t i o n of Na § and CI- ions out of the swelled membrane o c c u r s in a minor extent when s w e l l i n g is c a r r i e d out in I0 -s m o l / l NaCI s o l u t i o n due to the smaller concentration gradient of the s o l u t i o n inside and o u t s i d e the m e m b r a n e , thus p r o d u c i n g a lower shrinkage. In the s o l u t i o n of much higher ionic s t r e n g t h (i m o l / l NaCI) there is a drastic d e c r e a s e in M ~ x , and no f u r t h e r c h a n g e in the s w e l l i n g degree is observed. The adM sorption capac8 ity for Cu 2§ ions of the P E C / ~ membrane at d i f f e r e n t stag6 @@ es of the swelling process gives a 4 further confir61 53 mechanism proposed. Since under the conditions selected for the experiments (pH = 5.5) the free amino g r o u p s are protonated, and

the

carboxylic

2

0 0

50

'

J

tOO

t50

' 8~

850

t(h) Figure

4 Cupric ion retention at different stages of the swelling curve. The value at the top of each bar is the uptake in g r Cu per gr of membrane (PEC p r e p a r e d at p H = 5 . 5 )

groups are as carboxylate, the i n t e r a C t i o n of Cu 2+ is e x p e c t e d to o c c u r w i t h the latter (14). As it can be seen in F i g u r e 4, the h i g h e r r e t e n t i o n c a p a c i t y for Cu 2+ ions is a c h i e v e d w h e n the m e m b r a n e is at the maximum swelling state. It is s m a l l e r before reaching M~x b e c a u s e the -COOg r o u p s are less a c c e s s i b l e at lower swelling, and d i m i n i s h e s as time i n c r e a s e s after reaching Mm~x b e c a u s e of the p r o g r e s sive d e c r e a s e in free -CO0- g r o u p s as the r e s u l t of the reaction r e p r e s e n t e d in S c h e m e 2. The d i f f u s i o n - c o n t r o l l e d u p t a k e of p o l y m e r films and memb r a n e s can be e x p r e s s e d by (15):

__MM_I_8_8_~ 24.

~2a. ~

1 (2n+l) 2

exp[-~2D(2n+l)2t] 4H2

[2]

477

w h e r e M~ is the maximum film, M is the u p t a k e at cient of the p e n e t r a t i n g f i l m and n is an integer. can be a p p r o x i m a t e d to

equilibrium u p t a k e by the polymer time t, D is the d i f f u s i o n coeffisolvent, H is the t h i c k n e s s of the For long s w e l l i n g t i m e s e q u a t i o n [2]

in

M.

~2Dt

M.-M

Z42

[3]

E q u a t i o n [3] r e p r o d u c e s a first order rate e q u a t i o n as far as D and H are constant. H o w e v e r , Schott (16) has p o i n t e d out that for e x t e n s i v e s w e l l i n g n e i t h e r H nor D r e m a i n constant, and u n d e r these c i r c u m s t a n c e s a first o r d e r kinetics does not apply. In his study of the swelling behaviour of g e l a t i n and c e l l u l o s e films he w a s able to fit the d a t a to the empirical equation [4]

_~t-A+Bt

M which

he

demonstrated

s w e l l i n g kinetics.

In

that corresponds 1 this e q u a t i o n B-~-~,

to

a

s e c o n d order

the r e c i p r o c a l

of

maximum s w e l l i n g and A is the r e c i p r o c a l of the initial rate of swelling, since for short s w e l l i n g times A >> Bt, and in the limit e q u a t i o n [4] b e c o m e s llm ( - ~ )

i

[5]

Figure 5 shows o 2oo t (mln) aoo 6oo + j 200 the straight lines 7~ obtained by applyt/M t/M ing e q u a t i o n [4] to the initial part 150 -up to the maximum ~e0 swollen stateof the s w e l l i n g curves of PEC membranes 80 100 r e p r e s e n t e d in Figure i. The excellent fits obtained 50 at the three pH 40 values studied indicate that this PEC membranes obey I I 0 O a second order 0 ~0 m~ m~ s w e l l i n g kinetics, t(min) The values of F i g u r e 5 P l o t s a c c o r d i n g to eq. [4] of M= calculated with the first period o f PEC s w e l l i n g curves equation [4] w e r e s h o w n in F i g . I: DH = 4.2 (o)~ pH = 5.0 in good a g r e e m e n t (,); p H = 5 . 7 ( D w i t h those obtained experimentally. F u r t h e r m o r e , the s p e c i f i c rate constants e v a l u a t e d from the

478

aforementioned kinetic model increase as the pH of formation decreases, in concordance with the longer times required to attain the maximum swelling state at higher ~H values.

i. 2. 3. 4.

5. 6. 7. 8. 9. i0. ii. 12. 13.

14. 15.

16.

Y.Kikuchi, H.Fukuda (1974) Makromol. Chemie 175: 3593 H.Fukuda, Y.Kikuchi (1977) Makromol. Chemie 178: 2895 H.Fukuda, Y.Kikuchi (1978) Bull. Chem. Soc. Jpn. 51: 1142 Ye.Ye. Skorikova, G.A.Vikhoreva, R.I.Kalyuzhnaya, A.B.Zezin, L.S.Gal'braikh, V.A.Kabanov (1988) Vysokomol. Soedin. A 3 0 : 4 4 V.Chavasit, C.Kienzle-Sterzer, J.Torres (1988) Pol~n. Bull. 1 9 : 2 2 3 H.Fukuda, Y.Kikuchi (1979) Makromol. Chemie 1 8 0 : 1 6 3 1 H.Fukuda (1980) Bull. Chem. Soc. Jpn. 5 3 : 8 3 7 W.Arg~elles-Monal, C.Peniche-Covas (1988) Makromol. Chemie, R a p i d Con~n. 9 : 6 9 3 W.Arg~elles-Monal, M.G~rciga, C.Peniche-Covas (1990) Polym. Bull. 2 3 : 3 0 7 G.A.F.Roberts, J.G.Domzsy (1982) Int. J. Biol. Macromol. 4:374 R.A.A.Muzzarelli, R.Rocchetti (1985) Carbohydr. Polym. 5: 461 B.Philipp, L.T.Hong, K.J.Linow, W.Dawydoff, K.Arnold (1980) Acta Polym. 3 1 : 6 5 4 V.A.Kabanov, A.B.Zezin (1982) Water soluble non stoichiometric p o l y e l e c t r o l y t e complexes: a n e w class o f synthetic polyelectrolytes. In Volpin M.E. (ed.) Soviet Sci. Rev. Harwood Academic Publishers GmbH, New York (Vol 4, pp 207-282) N.M.Kabanov, A.I.Kokorin, V.B.Rogacheva, A.B.Zezin (1979) Vysokomol. Soedin. A 2 1 : 2 0 9 J.Crank (1957) Diffusion in Plane Sheet. In M a t h e m a t i c s o f Diffusion~ 2rid edn., Oxford University Press, Oxford (pp 45-40) H.Schott (1992) J. Macromol. Sci.-Phys. B31: 1

Accepted July 28, 1993

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