EUROPEANPOLYMER

European Polymer Journal 40 (2004) 1399–1407

www.elsevier.com/locate/europolj

JOURNAL

Modified chitosan4. Superabsorbent hydrogels

from poly(acrylic acid-co-acrylamide) grafted chitosanwith salt- and pH-responsiveness properties

G.R. Mahdavinia a, A. Pourjavadi a,*, H. Hosseinzadeh a, M.J. Zohuriaan b

a Department of Chemistry, Polymer Research Laboratory, Sharif University of Technology, Azadi Avenue,

P.O. Box 11365-9516, Tehran, Iranb Superabsorbent Hydrogel Division, Iran Polymer and Petrochemical Institute (IPPI), P.O. Box 14965-115, Tehran, Iran

Received 24 November 2003; received in revised form 25 January 2004; accepted 27 January 2004

Available online 18 March 2004

Abstract

Graft copolymerization of mixtures of acrylic acid (AA) and acrylamide (AAm) onto chitosan was carried out by

using potassium persulfate (KPS) as a free radical initiator in the presence of methylenebisacrylamide (MBA) as a

crosslinker. The effect of reaction variables, such as MBA concentration and AA/AAm ratio on the water absorbency

capacity have been investigated. The polymer structures were confirmed by FTIR spectroscopy. Water absorbencies

were compared for the hydrogels before and after alkaline hydrolysis. In the non-hydrolyzed hydrogel, enhanced water

absorbency was obtained with increasing AA in monomer feed. However, after saponification, the sample with high

AAm ratio exhibited more water absorbency. These behaviors were discussed according to structural parameters. The

swelling kinetics of the superabsorbing hydrogels was studied as well. The hydrogels exhibited ampholytic and

reversible pH-responsiveness characteristics. The swelling variations were explained according to swelling theory based

on the hydrogel chemical structure. The hydrogels exhibited salt-sensitivity and cation exchange properties. The pH-

reversibility and on–off switching properties of the hydrogels make the intelligent polymers as good candidates for

considering as potential carriers for bioactive agents, e.g. drugs.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Chitosan; Hydrogel; Superabsorbent; Acrylic acid; Acrylamide; Salt; pH

1. Introduction

Superabsorbent polymers are lightly crosslinked net-

works of hydrophilic polymer chains. The most efficient

water absorbers are polymer networks that carry disso-

ciated, ionic functional groups. The network can swell in

water and hold a large amount of water while maintain-

ing the physical dimension structure [1]. Hydrogels which

swell and contract in response to external condition have

* Corresponding author. Tel./fax: +98-218-719-585.

E-mail address: purjavad@sharif.edu (A. Pourjavadi).

0014-3057/$ - see front matter � 2004 Elsevier Ltd. All rights reserv

doi:10.1016/j.eurpolymj.2004.01.039

been explored [2,3]. These smart hydrogels have potential

use in site-specific delivery of drugs to specific regions of

the gastrointestinal tract and have been prepared for

delivery of low molecular weight protein drugs [4]. Many

structural factors (e.g. charge, concentration and pKa ofthe ionizable group, degree of ionization, crosslink den-

sity and hydrophilicity) influence the degree of swelling of

ionic polymers [5–7]. In addition, properties of the

swelling medium (e.g. pH, ionic strength and the counter

ion and its valency) affects the swelling characteristics [8–

10]. These responsive or smart hydrogels have become an

important area of research and development in the field

of medicine, pharmacy and biotechnology.

ed.

1400 G.R. Mahdavinia et al. / European Polymer Journal 40 (2004) 1399–1407

Chitosan, is an aminopolysaccharide produced from

chitin, the most abundant biomass in the world [11]. It

has potential applications ranged from biomedicine and

pharmacy to water treatment [12,13]. Chitosan has both

reactive amino and hydroxyl groups that can be used to

chemically alter its properties under mild reaction con-

ditions. Chitosan is a weak base and easy bioadsorber,

with gel forming ability at low pH [14]. An efficient

approach to modify swelling behavior of chitosan hy-

drogels in various pHs, is graft polymerization of vinylic

monomers such as acrylic acid [15], acrylamide [16],

acrylonitrile [17], vinyl pyrrolidone [18] onto chitosan.

In previous works, we reported pH-sensitivity of a

chitosan-based hydrogel named H-chitoPAN [19,20].

The present report reveals the synthesis and swelling

properties of chitosan-g-poly(AA-co-AAm) as a novel

chitosan-based hydrogel.

2. Experimental

2.1. Materials

Chitosan sample (DD 0.76) was prepared from chitin

(extracted from shrimp shell) in our laboratory [17].

Acrylic acid (AA, Merck), was used after vacuum dis-

tillation. Acrylamide (AAm, Fluka), was used after

crystallization in acetone. Potassium persulfate (KPS,

Merck) was used without purification. Methylenebis-

acrylamide (MBA, Fluka), was used as received. All

other chemicals were of analytical grade.

2.2. Preparation of hydrogel

Chitosan solution was prepared in a 1-l reactor

equipped with mechanical stirrer and gas inlet. Chitosan

was dissolved in degassed distillated water containing

1 wt.% of acetic acid. In general, 0.50 g of chitosan was

dissolved in 30.0 ml of distillated degassed 1 wt.% acetic

acid solution. The reactor was placed in a water bath

preset at 60 �C. Then 0.10 g of KPS as an initiator wasadded to chitosan solution and was allowed to stir for

10 min at 60 �C. After adding KPS, variable amounts ofAA and AAm (AA 0.40–1.60 g, AAm 0.40–1.60 g) were

added simultaneously to the chitosan solution. There-

fore, the chitosan/synthetic monomers weight ratio was

0.25. MBA solution (0.050–0.15 g in 5 ml H2O) was

added to the reaction mixture after the addition of

monomers and the mixture was continuously stirred

(600 rpm) for 1 h under argon. After 60 min, the reac-

tion product was allowed to cool to ambient tempera-

ture and neutralized to pH 8 by addition of 1 N NaOH

solution. Methanol (500 ml) was added to the gelled

product while stirring. After complete dewatering for

24 h, the hardened gel particles product were filtered,

washedwith freshmethanol (2 · 50ml) and dried at 50 �C.

The chitosan-g-poly(AA-co-AAm) (0.50 g), was then

saponified using 20 ml aqueous 1 N NaOH solution in a

loosely stopper 100-ml flask at 100 �C for 60 min. Thereaction product was then allowed to cool to ambient

temperature and neutralized to pH 8 by addition of 10

wt.% acetic acid solution. Methanol (200 ml) was added

to the gelled product while stirring. After complete

dewatering (4 h), the yellow product was filtered, washed

with fresh methanol (2 · 50 ml) and dried at 50 �C.

2.3. Infrared analysis

The samples were crushed with KBr to make pellets.

Spectra were taken on an ABB Bomem MB-100 FTIR

spectrophotometer.

2.4. Swelling measurements

A chitosan-g-poly(AA-co-AAm) sample (0.10 g) was

put into a weighed tea bag and immersed in 100 ml

distilled water and allowed to soak for 2 h at room

temperature. The equilibrated swollen gel was allowed

to drain by removing the tea bag from water and

hanging until no drop drained (�20 min). The bag wasthen weighed to determine the weight of the swollen gel.

The absorbency (equilibrium swelling) was calculated

using the following equation:

Absorbency ¼ ðWs � WdÞ=Wd ð1Þ

where Ws and Wd are the weights of the swollen gel andthe dry sample, respectively. So, absorbency was calcu-

lated as grams of water per gram of resin (g/g). The

accuracy of the measurements was ±3%.

2.5. Swelling in buffer solutions

Two buffer solutions with pH 3 (citric acid/hydro-

chloric acid) and pH 10 (boric acid/potassium chloride–

sodium hydroxide) were used to study of pH-sensitivity

of the hydrogel. The pH values were precisely checked

by a pH-meter (Metrohm/820, accuracy ±0.1). Then

0.10 g of dried sample was used for the swelling mea-

surements in both buffers according to the above-

mentioned method.

2.6. Swelling in salt solutions

Absorbency of the chitosan-g-poly(AA-co-AAm)

hydrogel sample was evaluated in 0.15 M solutions of

NaCl and CaCl2 according to the above method de-

scribed for swelling measurement in distilled water.

2.7. Swelling kinetics

Hydrogel samples (40–60 mesh, 0.10 g) were poured

into numbers of weighed tea bags and immersed in

G.R. Mahdavinia et al. / European Polymer Journal 40 (2004) 1399–1407 1401

100 ml distilled water. At consecutive time intervals, the

water absorbency of the samples was measured accord-

ing to Eq. (1).

3. Results and discussion

3.1. Synthesis and spectral characterization

PAA and PAAm were simultaneously grafted onto

chitosan in a homogeneous medium using KPS as a

radical initiator and MBA as a crosslinking agent under

an inert atmosphere.

The crosslinker concentration and the monomer

ratios, two important variables affected on swelling

capacity of hydrogel, were investigated before and after

hydrolysis of hydrogel. The mechanism of copolymeri-

zation of AA and AAm onto chitosan in the presence of

MBA is shown in Scheme 1. The persulfate initiator is

decomposed under heating to generate sulfate anion-

radical. The radical abstracts hydrogen from the hy-

droxyl group of the polysaccharide substrate to form

alkoxy radicals on the substrate. So, this persulfate–

saccharide redox system is resulted in active centers on

the substrate to radically initiate polymerization of AA

Scheme 1. General mechanism for radical graft polymerization of acr

crosslinker.

and AAm led to a graft copolymer. Since a crosslinking

agent, e.g. MBA, is presented in the system, the co-

polymer comprises a crosslinked structure.

The superabsorbency, pH- and salt-sensitivity of this

hydrogel were investigated. To obtain a hydrogel with

high swelling capacity, the chitosan-g-poly(AA-co-

AAm) hydrogel copolymer was hydrolyzed with NaOH

solution. During the saponification, ammonia evolves

and amide groups are converted to carboxylate salt. This

reaction can be shown as below:

O

R NH2

OHR

O

HH2N

O-

--NH3

O

R O-

For identification of the hydrogel, infrared spectro-

scopy was used. Fig. 1 shows the IR spectroscopy of

chitosan-g-poly(AA-co-AAm) hydrogel. The superab-

sorbent hydrogel product comprises a chitosan back-

bone with side chains that carry sodium carboxylate and

carboxamide functional groups that are evidenced by

peaks at 1563 and 1670 cm�1 respectively. The very in-

tense characteristic band at 1563 cm�1 is due to C@O

ylamide and acrylic acid onto chitosan in the presence of MBA

Fig. 1. FTIR spectra of (a) chitosan-g-poly(AA-co-AAm) and (b) hydrolyzed chitosan-g-poly(AA-co-AAm), AAm–AA ratio is 1.

y = 1.6654 x-0.975

y = 2.8008 x-0.6705 0

50

100

150

200

250

300

0 0.01 0.02 0.03 0.04MBA, mol/L

Swel

ling,

g/g

after hydrolysis

before hydrolysis

(b)(a)

1402 G.R. Mahdavinia et al. / European Polymer Journal 40 (2004) 1399–1407

asymmetric stretching in carboxylate anion that is

reconfirmed by another sharp peak at 1401 cm�1 which

is related to the symmetric stretching mode of the car-

boxylate anion. Combination of absorption of the car-

boxylate and alcoholic O–H stretching bands are

appeared in the wide range of 3500–2550 cm�1 [21]. As

shown in this Fig. 1(b), the intensity of carboxylate

groups (1563 cm�1) is increased after hydrolyzing the

hydrogel. This is attributed to conversion of amide

groups to carboxylate salt.

To obtain an additional evidence of grafting, a sim-

ilar polymerization was conducted in absence of the

crosslinker. After extracting the homopoly(AA-co-

AAm) (5%), appreciable amount of grafted chitosan was

concluded. The graft copolymer spectrum was very

similar to Fig. 1(a).

Fig. 2. Effect of crosslinker concentration on swelling capacity

of chitosan-g-poly(AA-co-AAm) before (a) and after (b)

hydrolysis. The regression for curves (a) and (b) are 0.9418 and

0.9055, respectively. AA–AAm weight ratio is 1.0.

3.2. Effect of crosslinker concentration on swelling of

hydrogel

Crosslinks have to be present in a hydrogel in order

to prevent dissolution of the hydrophilic polymer chains

in an aqueous environment. Efficiency of crosslinker

incorporation controls the overall crosslink density in

the final hydrogel. The effect of the extent of crosslinking

on water absorbency of chitosan-g-poly(AA-co-AAm)

hydrogel is shown in Fig. 2. In this reaction series, the

AA/AAm ratio in monomer feed was chosen to be 1.

The results indicate power law behaviors of swelling–

[MBA], so that the higher the crosslinking agent, the

lower the water absorbency will be. Higher crosslinker

concentration produce more crosslinked points in

polymeric chains and increases the extent of crosslinking

of the polymer network, which results in less swelling

when it is brought into contact with the solvent. Similar

observations have been reported in the literature [6,22].

In addition, the swelling capacity of hydrolyzed hydro-

gel is higher than that of non-hydrolyzed hydrogel. This

comparison shows that, since the hydrolyzed hydrogel

comprises more carboxylate groups, its swelling capacity

is enhanced due to electrostatic repulsion from these

carboxylate groups as the main driving forces. However,

the non-hydrolyzed hydrogel is less sensitive to the

amount of the crosslinker (lower sloped curve). The

200

G.R. Mahdavinia et al. / European Polymer Journal 40 (2004) 1399–1407 1403

swelling capacity for hydrogel before and after hydro-

lysis is 90 and 220 g/g, respectively, at a fixed MBA

concentration of 0.0053 mol/l.

3.3. Effect of monomer ratio on swelling capacity

The swelling capacity of the hydrogels prepared with

various ratios of monomers, is shown in Fig. 3. Before

hydrolysis, a high swelling capacity is obtained with high

ratio of AA. Since pH of the polymerization mixture was

adjusted at 8.0 after the reaction, the superabsorbency of

chitosan-g-poly(AA-co-AAm) hydrogel is due to both

functional groups of ionic carboxylate (from neutralized

AA) and non-ionic carboxamide (from AAm). The

presence of the ionic groups in polymer chains results in

increasing of swelling because the ions are more strongly

solvated rather than non-ionic groups in the aqueous

medium. When the hydrogels are hydrolyzed with

NaOH solution, higher swelling capacities are obtained

from employing higher initial ratios of AAm/AA. Since

the sol (soluble) content of the hydrogel network was

measured to be as little as 2%, we assumed the AAm

has totally been involved in the network. Therefore,

the swelling enhancement versus higher AAm/AA ratio

can be attributed to the formation of high carboxylate

groups in the ‘‘saponified’’ samples. In these hydrolyzed

hydrogels, the carboxylate groups in network are

appreciably increased and higher swelling capacity is

achieved.

3.4. Swelling in salt solutions

Chitosan-g-poly(AA-co-AAm) superabsorbents are

ionized hydrogels that their swelling behavior depends

0

50

100

150

200

250

0 1 2 3 4 5

AAm/AA

Swel

ling,

g/g

before hydrolysisafter hydrolysis

Fig. 3. Effect of monomer ratio on swelling capacity of the

chitosan-based hydrogels. The pH of both hydrogels has been

adjusted at 8.0.

on both the characteristics of the chemical structure and

the medium. Generally, the extent to which the hydrogel

swells at equilibrium increases with (a) increase in the

concentration of functional ionizable groups on the

network, and (b) decrease in the extent of crosslinking

occurred during the synthetic step(s). The swelling of the

absorbents in saline solutions was appreciably decreased

comparing to the values measured in deionized water.

This well-known phenomenon, commonly observed in

the swelling of ionic hydrogels [23], is often attributed to

a charge screening effect of the additional cations caus-

ing a non-perfect anion–anion electrostatic repulsion,

led to a decreased osmotic pressure (ionic pressure)

difference between the hydrogel network and the exter-

nal solution. Fig. 4 illustrates a power law relationship

between swelling and saline concentration. It indicates

that changing the NaCl concentration higher than �0.05mol/l has no appreciable influence on superabsorbency

of the chitosan-g-poly(AA-co-AAm) hydrogel.

The equilibrium swelling data obtained from the

chloride salt solutions of sodium and calcium with same

concentration are given in Table 1. The swelling capacity

is decreased with an increase in charge of the metal

cation (Ca2þ <Naþ). It may be explained by complexing

ability arising from the coordination of the multivalent

cations with chitosan-g-poly(AA-co-AAm) groups. This

ionic crosslinking mainly occurs at surface of particles

and makes them rubbery and very hard when they swell

in Ca2þ solution. In contrast, the chitosan-g-poly(AAm-

co-AA) hydrogel particles swollen in NaCl solution

0

40

80

120

160

0 0.25 0.5 0.75 1 1.25

[NaCl], mol/Lit

Swel

ling,

g/g

before hydrolysis

after hydrolysis

(a)

(b)

Fig. 4. Swelling capacity variation of chitosan-g-poly(AA-co-

AAm) hydrogel in various concentration of saline solutions.

The regression for curves (a) and (b) are 0.9418 and 0.9055,

respectively. AA–AAm weight ratio is 1.0.

Table 1

The swelling capacity of chitosan-g-poly(AA-co-AAm) hydro-

gel in 0.15 M solutions of NaCl and CaCl2 (AH: after hydro-

lysis, BH: before hydrolysis)

AAm/

AA

weight

ratio

Swelling values (g/g)

Before hydrolysis After hydrolysis

NaCl CaCl2 NaCl CaCl2

4.0 28.1 17.0 46.0 9.0

1.5 33.5 15.0 40.0 8.5

0.67 36.0 12.5 36.5 7.8

0.25 40.2 10.0 35.0 8.5

0.0 42.0 8.5 42.0 8.5

1404 G.R. Mahdavinia et al. / European Polymer Journal 40 (2004) 1399–1407

exhibit lower strength. In addition, in CaCl2 solution,

the swelling capacity of the non-hydrolyzed hydrogel

with high ratio of AAm, the swelling capacity is higher

in comparison with the counterpart hydrolyzed hydro-

gel. In fact, after saponification, –COO� groups are in-

creased and interaction of ionic groups with Ca2þ ions is

more than that of non-ionic –CONH2 groups. There-

fore, the capacity of swelling in Ca2þ solution is higher

for the non-hydrolyzed gel. Fig. 5 shows [MBA]–swell-

ing relationship for the non-hydrolyzed and hydrolyzed

gels swollen in saline. The power law behaviors are

different comparing to those observed in the case of

swelling in distilled water (Fig. 2). Here, lower slopes is a

main feature of the swelling-loss versus the crosslinker

concentration.

y = 8.6214x -0.3293

R2 = 0.9132

y = 6.2624x-0.3205

R2 = 0.949

0

10

20

30

40

50

60

0 0.01 0.02 0.03 0.04

MBA, mol/L

Swel

ling,

g/g

after hydrolysis

before hydrolysis

Fig. 5. Swelling capacity of chitosan-g-poly(AA-co-AAm) with

variable MBA concentration in saline solutions (0.15 M). AA–

AAm weight ratio is 1.0.

3.5. Swelling kinetics

A preliminary study was conducted on the hydrogel

swelling kinetics. Fig. 6 represents the dynamic swelling

behavior of a chitosan-g-poly(AA-co-AAm) superab-

sorbent sample with certain particle size (40–60 mesh) in

water. Initially, the rate of water uptake sharply in-

creases and then begins to level off. The equilibrium

swelling was achieved after 20 min. Power law behaviors

are obvious from Fig. 6. The data may be well fitted with

a Voigt-based equation (Eq. (2)) [24].

St ¼ Seð1� e�t=sÞ ð2Þ

where St is swelling at time t (g/g), Se is equilibriumswelling (‘‘power parameter’’, g/g), t is time (s) forswelling St, and s stands for the ‘‘rate parameter’’ (s).The rate parameters for the non-hydrolyzed and

hydrolyzed gels are found to be 1.72 and 1.06 min,

respectively. According to the smaller s value of thehydrolyzed hydrogel, it swells faster than its hydrolyzed

counterpart.

The rate of water uptake in hydrolyzed hydrogel is

higher than that of non-hydrolyzed hydrogel. This dif-

ference can be originated from more hydrophilic groups

(COO�) in the hydrolyzed hydrogel. In addition, it may

also be attributed to porosity of particles obtained from

the alkaline hydrolysis. In the course of the hydrolysis,

the reaction mixture becomes gelly and pasty state that

prevents removing the evolved NH3 and water vapor

from the pasty medium (see Section 2). So, the removed

vapors creates pores in the gel. The porosity favors

faster water diffusion through the hydrogel network and

higher swelling rate.

0

40

80

120

160

200

0 10 20 30 40Time, min.

Swel

ling,

g/g

before hydrolysisafter hydrolysis

Fig. 6. Swelling ratio as a function of time for chitosan-g-

poly(AA-co-AAm) copolymeric hydrogel before and after

hydrolysis. AA–AAm weight ratio is 1.0.

60

G.R. Mahdavinia et al. / European Polymer Journal 40 (2004) 1399–1407 1405

3.6. pH-dependent swelling of the hydrolyzed hydrogel

To investigate the sensitivity of the hydrolyzed

hydrogel to pH, firstly the equilibrium swelling (ultimate

absorbency) of the hydrogel was studied at various pHs

ranged from 1.0 to 13.0 (Fig. 7). No additional ions

(through buffer solution) were added to medium for

setting pH because absorbency of a superabsorbent is

strongly affected by ionic strength [23]. Therefore, stock

NaOH (pH 13.0) and HCl (pH 2.0) solutions were di-

luted with distilled water to reach desired basic and

acidic pHs, respectively. According to Fig. 7, the two

sharp swelling capacity changes can be attributed to

high repulsion of –NH3þ groups in acidic media and

–COO� groups in basic media. However, at very acidic

conditions (pH 6 2), a screening effect of the counter

ions, i.e. Cl�, shields the charge of the ammonium ca-

tions and prevents an efficient repulsion. As a result, a

remarkable decreasing in equilibrium swelling is ob-

served (gel collapsing). Around pH 5, the carboxylic acid

component comes into action as well. Since the pKa ofthe weak polyacid is about 6.4 [21], its ionization

occurring above this value, may favor enhanced absor-

bency. But under pH 6.4, at a certain pH range 4–6, the

majority of the base and acid groups are as non-ionized

forms, so hydrogen bonding between amine and car-

boxylic acid (and probable carboxamide groups) may

lead to a kind of crosslinking followed by a decreased

swelling. A similar observation is recently reported in

the case of an interpenetrating network composed of

poly(N-isopropylacrylamide)–poly(acrylic acid) [25]. At

higher pHs, the carboxylic acid groups become ionized

0

50

100

150

200

250

300

350

0 2 4 6 8 10 12 14

pH

Swel

ling,

g/g

Fig. 7. Swelling dependency of partially hydrolyzed chitosan-g-

poly(AA-co-AAm) superabsorbent on pH. AA–AAm weight

ratio is 1.0.

and the electrostatic repulsive force between the charged

sites (COO�) causes increasing in swelling. Again, a

screening effect of the counter ions (Naþ) limits the

swelling at pH 8–11 and opposed the swelling at

pH>12, so that the hydrogel totally collapses at pH 13.

Such behavior has been reported for copolymeric gels

from acrylic acid (the anionic constituent) and methac-

rylamidopropyl trimethyl ammonium chloride (the cat-

ionic constituent) [26]. In this system, a combination of

attractive or repulsive electrostatic interactions and

hydrogen bonding are the main reasons for existence of

several phases observed in various environmental con-

ditions.

The pH-dependent swelling reversibility of the hy-

drogels was examined in buffered solutions (Fig. 8). The

figure demonstrates the hydrogel reversibility to absorb

water upon changing the pH in acidic and basic region

(pH 3.0 10). However, in the buffered solutions, the

overall swelling values are much lower than those in the

non-buffered media (see the swelling scales of Figs. 7 and

8). This is due to higher ionic strength of the buffer

solutions described earlier. Chitosan-g-poly(AA-co-

AAm) hydrogel has both amine (chitosan backbone)

and carboxylate (PAA chains) as functional groups.

Chitosan is a weak base with an intrinsic pKa of 6.5.PAA contains carboxylic groups that become ionized at

pH values above its pKa of 4.7. Since hydrogel swellsdifferently in media with different pHs, we have inves-

tigated its pH-dependent swelling reversibility. The

0

10

20

30

40

50

0 50 100 150 200Time, min.

Swel

ling,

g/g

BH

AH

pH=3 (On)

pH=10 (Off)

Fig. 8. On–off switching behavior as reversible swelling (pH

3.0) and deswelling (pH 10) of the pH-responsive superabsor-

bent hydrogel, chitosan-g-poly(AA-co-AAm). AH: after hydro-

lysis, BH: before hydrolysis. AA–AAm weight ratio is 1.0.

1406 G.R. Mahdavinia et al. / European Polymer Journal 40 (2004) 1399–1407

protonation of amine and carboxylic groups takes place

at pH<4.7 and NH2 groups are converted to NHþ3. But

at pH<7, the carboxylic functional groups are con-

verted to carboxylate ions. Either protonated (NHþ3) or

deprotonated (COO�) groups increase charge density on

the polymer causing an enhancement of the osmotic

pressure inside the gel particles because of the NHþ3–

NHþ3 or COO

�–COO� electrostatic repulsion. This

osmotic pressure difference between the internal and

external solution of the network is balanced by the

swelling of the gel.

3.7. Salt-sensitivity of chitosan-g-poly(AA-co-AAm)

hydrogel

Since the chitosan-based hydrogels are comprised

carboxylate groups, they exhibit various swelling

capacity in different salt solutions with same concen-

trations. These swelling changes are due to valency dif-

ference of salts. The networks contain PAA chains with

carboxylate groups that can interact with cations. As

given in Table 1, the swelling capacity of the hydrogels

in Ca2þ solution is lower than that of in Naþ solutions.

In the presence of the bivalent calcium ions, the cross-

linking density increases because of a double interaction

of Ca2þ with carboxylate groups leading to ‘‘ionic

crosslinking’’. The swelling–deswelling cycle of the non-

hydrolyzed hydrogel in sodium and calcium salts are

shown in Fig. 9. In sodium solution, swelling of the

hydrogel is increased with time. When this hydrogel is

immersed in calcium chloride solution, it deswells to a

0

5

10

15

20

25

30

35

0 20 40 60 80 100

Time (min)

swel

ling,

g/g

CaCl2 (Off)

NaCl (On)

Fig. 9. Reversible cation exchange ability of the chitosan-g-

poly(AA-co-AAm) hydrogel. AA–AAm weight ratio is 1.0.

collapsed form. When the shrinked hydrogel is im-

mersed in sodium chloride solution again, the calcium

ions are replaced by sodium ions. This ion exchange

disrupts the ionic crosslinks leading to swelling

enhancement. As a result, when hydrogel is treated

alternatively with NaCl and CaCl2 solutions with equal

molarity, the swelling reversibility of hydrogel is ob-

served. This chemical behavior of hydrogel is resulted

from the ion exchange ability of the carboxylate groups.

4. Conclusion

Chitosan-g-poly(AA-co-AAm) hydrogel was synthe-

sized through simultaneous crosslinking and graft

polymerization of acrylic acid/acrylamide mixtures onto

chitosan. Swelling capacity of the hydrogels is affected

by the crosslinker (MBA) concentration and monomer

ratio, so that the swelling is decreased by increasing the

MBA concentration and AAm/AA ratio. However, for

the hydrolyzed hydrogel, swelling capacity is decreased

by increasing the AAm/AA ratio. It can be attributed to

the high carboxylate groups in the hydrolyzed sample

with the high AAm/AA ratio. The swelling of hydrogel

exhibited high sensitivity to pH. Study of effect of Hþ/

OH� concentration carried out at various pHs shows

that the swelling of hydrogel causes several large volume

changes. So we investigated the pH-sensitivity of the

hydrogel, chitosan-g-poly(AA-co-AAm). Ionic repulsion

between charge groups incorporated in the gel matrix by

an external pH modulation could be assumed as the

main driving force responsible for such abrupt swelling

changes. This superabsorbent polyampholytic network

intelligently responding to pH may be considered as an

excellent candidate to design novel drug delivery sys-

tems. The swelling capacity in CaCl2 is much lower than

that in NaCl solution and distillated water. The swell-

ing–deswelling process of the non-hydrolyzed hydrogel

alternatively carried out in CaCl2 and NaCl solutions

results in a high capability of ion exchanging of the

chitosan-based hydrogel.

References

[1] Buchholz FL, Graham AT. In: Modern superabsorbent

polymer technology. New York: Wiley; 1997.

[2] Hennink WE, Nostrum CF. Adv Drug Delivery Rev

2002;54:13.

[3] Qui Y, Park K. Adv Drug Delivery Rev 2001;53:321.

[4] Bronsted J, Kopecek J. Biomaterials 1991;12:584.

[5] Lee JW, Kim SY, Kim SS, Lee YM, Lee KH. J Appl

Polym Sci 1999;73:113.

[6] Wu J, Lin J, Wei C, Li G. Polym Int 2001;50:1050.

[7] Askadskii AA. Polym Sci USSR 1990;32:2061.

[8] Gupta P, Vermani K, Garg S. Drug Discov Today 2002;

7:569.

G.R. Mahdavinia et al. / European Polymer Journal 40 (2004) 1399–1407 1407

[9] Castel D, Richard A, Audebert R. J Appl Polym Sci

1990;39:11.

[10] Lee WF, Lin GH. J Appl Polym Sci 2001;79:1665.

[11] Roberts GAF. Chitin chemistry. London:Macmillan; 1992.

[12] Dinesh K, Alok R. J Macromol Sci, Rev Macromol Chem

Phys 2000;40:69.

[13] Huang C, Chen Y. J Chem Technol Biotechnol 1996;

66:227.

[14] Muzzarelli R, Baldassarre V, Conti F, Ferrara G, Biagini

G, Gazzaneli V. Biomaterials 1988;9:247.

[15] Yazdani-Pedram M, Retuert J, Quijada R. Macromol

Chem Phys 2000;201:923.

[16] Yazdani-Pedram M, Lagos A, Retuert J. Polym Bull

2002;48:93.

[17] Pourjavadi A, Mahdavinia GR, Zohuriaan-Mehr MJ,

Omidian H. J Appl Polym Sci 2003;88:2048.

[18] Yazdani-Pedram M, Retuert J. J Appl Polym Sci 1997;

63:1321.

[19] Pourjavadi A, Mahdavinia GR, Zohuriaan-Mehr MJ.

J Appl Polym Sci 2003;90:3115.

[20] Pourjavadi A, Mahdavinia GR, Zohuriaan-Mehr MJ.

Polym Adv Technol, in press.

[21] Silverstein RM, Webster FX. Spectrometric identifica-

tion of organic compounds. 6th ed. New York: Wiley;

1998.

[22] Wu J, Lin J, Zhou M, Wei C. Macromol Rapid Commun

2000;21:1032.

[23] Flory PJ, editor. Principles of polymer chemistry. Ithaca,

NY: Cornell University Press; 1953.

[24] Omidian H, Hashemi SA, Sammes PG, Meldrum I.

Polymer 1998;39:6697.

[25] Dhara D, Nisha CK, Chtterji PR. Macromol Chem Phys

2001;202:3617.

[26] Kost J. In: Mathiowitz E, editor. Encyclopedia of con-

trolled drug delivery, vol. 1. New York: Wiley; 1995.

p. 445.