Indian Journal of Chemistry

Vol. 48A, August 2009, pp. 1085-1090

Agar/sodium alginate-graft-polyacrylonitrile, a stable hydrogel system

Mahesh U Chhatbar, Ramavatar Meena, Kamalesh Prasad & A K Siddhanta*

Marine Biotechnology & Ecology Discipline, Central Salt & Marine Chemicals Research Institute,

Council of Scientific & Industrial Research, GB Marg, Bhavnagar 364 002, Gujarat, India

Email: aks@csmcri.org

Received 16 April 2009; revised and accepted 13 July 2009

Polyacrylonitrile grafted agar/sodium alginate (Agar/Na-Alg-graft-PAN) has been synthesized in aqueous medium under

reflux conditions in the presence of potassium persulphate as a free radical initiator. By varying the reaction parameters,

e.g., concentrations of acrylonitrile monomer and K2S2O8, reaction time and temperature, the optimum grafting conditions

have been identified as that having the highest grafting ratio (Gr 1.87), total conversion (Ct 1.05) and grafting efficiency

(Ge 0.89). The blend and grafted products have been characterized by FT-IR, X-ray diffraction, differential scanning

calorimeter and scanning electron microscopy. The swelling capacity of Agar/Na-Alg-graft-PAN is found to be 8.5g/g at

pH 1.2 and the swelled material is stable for over 24 h. This copolymer hydrogel system may be exploited in various

applications utilizing its swelling properties and stability.

Keywords: Polymers, Graft polymers, Copolymers, Biopolymers, Alginates, Polyacrylonitriles, Hydrogels

IPC Code: Int. Cl.9 C08B37/04; C08F251/00

Agar is a seaweed polysaccharide and chemically

consists of alternating 3-O-linked D-galactopyranose

and 4-O-linked 3,6-anhydro-L-galactopyranose1.

Alginates are linear anionic polysaccharides of

(1,4)-linked α-guluronic acid and β-D-mannuronic

acid residues2. Both biopolymers are biodegradable

and are used in the adhesive, food, cosmetics and

pharmaceuticals industries. Sodium alginates

obtainable from Indian seaweeds are of low viscosity,

which limits their applications3, 4

.

Modification of natural polymers by grafting

technique is a promising method for the preparation

of new materials. This method enables one to

introduce special properties and widen the field of the

potential applications. Acrylonitrile has been grafted

onto various natural and modified polysaccharides

(e.g., gum arabic, gum tragacanth, xanthan gum,

sodium alginate, chitosan, sodium carboxymethyl

cellulose, hydroxy-ethyl cellulose, methyl cellulose)

by using the ceric-carbohydrate redox initiating

system5. Under the ongoing program of our laboratory

on value addition of seaweed and seaweed

polysaccharides, we have reported different stable

hydrogel systems with improved properties6-8

.

We report herein a one-pot synthesis by graft

copolymerization of acrylonitrile (AN) onto the blend

of agar and sodium alginate (Agar/Na-Alg) in

presence of the initiator, potassium persulphate. The

grafted product has better swelling properties and

stability in aqueous media of pH 1.2, 7.0 and 12.5. To

our knowledge, this is the first report of grafting of

PAN onto the agar-sodium alginate blend.

Materials and Methods Agar and sodium alginate used herein were

extracted from seaweeds, viz., Gelidiella acerosa and

Sargassum tenerrimum, growing in Indian waters. The

agar and alginate were extracted following the methods

reported in the literature9-11

. The manuronic acid to

guluronic acid ratio (M/G ratio) of sodium alginate

used in this study was 0.37 and the weight average

molecular weight of agar was 2.65 × 105 Dalton.

Potassium persulfate (KPS; AR grade), a water-soluble

initiator, and acrylonitrile (AN) AR grade were

purchased from SD Fine Chemicals, Mumbai.

Agar and Na-Alg blend were prepared by mixing

agar and Na-Alg in 1:3 (w/w) ratios, as described in

our previous work6.

For grafting, the blend (1 g) was dissolved in 85 mL

distilled water by heating for 2 min. To this was added

KPS (0.01–0.08 g in 5 mL water) with stirring

followed by addition of a solution of AN (1.0–2.5 g in

10 mL water). The reaction mixture was heated under

reflux conditions at 70 oC for 5h with constant stirring,

INDIAN J CHEM, SEC A, AUGUST 2009

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cooled to room temperature to make the gel, cut into

small pieces and dehydrated with IPA to isolate the

solid graft copolymer (Agar/Na-Alg graft-PAN). The

solid product was filtered through a nylon cloth under

reduced pressure and unreacted acrylonitrile

homopolymer (PAN) was removed from the product by

washing with dimethyl formamide (DMF) and aqueous

methanol. Finally it was washed with pure methanol.

The unreacted homopolymer (PAN) was recovered from

the washings by evaporating to dryness. The grafted

copolymer was dried at 50 oC for 2 h, and ground

(30–40 mesh) using a mortar and pestle.

The grafted product was characterized by FT-IR

analysis on a Perkin- Elmer Spectrum GX FT-IR

system (USA) in KBr pellets (10.0 mg sample per

600 mg KBr). All spectra were an average of two

counts with 10 scans each and a resolution of 5 cm-1

.

Powder X-ray diffractions were recorded on a Philips

X’pert MPD X-ray powder diffractometer with

2θ ranging from 5o

to 60o. Differential scanning

calorimetry (DSC) was carried out on Mettler Toledo

DSC822 equipment (Switzerland). Experiments were

run at a heating rate of 10 oC/min. Crystallinity was

calculated using the methods reported earlier12,13

. The

apparent viscosity was measured on a Brookfield

viscometer (Synchrolectric Viscometer, Stoughton,

MA 02072, USA) using Spindle No. 1 at 60 rpm. The

gel strength (g cm-2

at 20 oC) was measured using a

Nikkansui-type gel tester (Kiya Seisakusho Ltd,

Tokyo, Japan). The measurements were performed on

1.5% w/w gel samples (cured overnight at 10 oC)

14,

using a solid cylindrical plunger of 1 cm in diameter.

Scanning electron microscopy (SEM) was measured

on (Carl-Zeiss Leo VP 1430 instrument) at an

accelerating voltage of 20 kV and ×202

magnification, using powdered samples having

particle size between 20-30 µm under identical

conditions. In this study, gelling and melting

temperatures of copolymer gels were measured as

described by Craigie and Leigh15

. The manuronic acid

to guluronic acid ratio (M/G) of sodium alginate was

measured using the method described by

Chhatbar et al.16

The grafting parameters i.e., total conversion (Ct),

Grafting ratio (Gr), grafting efficiency (Ge), add-on

(Ad) and homopolymer content (Hp) were determined

according to the known weight-basis expressions17

,

Ct = W2/W1 … (1)

Gr = W3/Wo … (2)

Ge = W3/W2 … (3)

Ad = (W3-W0)/W3 … (4)

Hp = (W2-W3)/W2 or Hp = 1-Ge … (5)

where Wo, W1, W2 and W3 are the weights of initial

substrate, monomer used, the product mixture

(i.e., grafted copolymer and unreacted PAN) and pure

graft copolymer (after DMF washed) respectively.

A weighed sample of dried parent agar

polysaccharide, blend polysaccharides and Agar/Na-

Alg-graft-PAN copolymers with particle size between

30 and 40 mesh were immersed in aqueous medium

of selected, pH i.e., 1.2, 7.0 and 12.5, in separate

experiments. Equilibrium swelling (ES) capacity was

calculated as described in our previous reports6,18

using Eq. (6),

E S = (Ws – Wd)/Wd … (6)

where Ws and Wd are the weights of the swollen and

dry samples respectively.

Results and Discussion In the present investigation the optimum values of

the various parameters of the reaction were: Wo = 1 g;

W1= 2 g; W2 = 2.11 g and W3 = (2.11-0.24) g =1.87 g

(unreacted homopolymer (PAN) was 0.24 g). The

grafting parameters calculated for the product were as

follows: Ct = 1.05 (± 0.012); Gr = 1.87 (± 0.041); Ge

= 0.89 (± 0.055); Ad = 0.46 (± 0.033); Hp = 0.11

(± 0.0008). These data indicate that PAN was grafted

on to the polysaccharide blend. Optimization of the

reaction conditions, e.g., effects of monomer

concentration, reaction temperature, reaction time and

initiator concentration was also carried out after

detailed study19

.

A plausible mechanism for the formation of the

graft copolymer is proposed in Scheme 1. The

physical properties of agar, sodium alginate, Agar/Na-

Alg blend and Agar/Na-Alg-graft-PAN copolymer are

given in (Table 1). The viscosity of agar, sodium

alginate, Agar/Na-Alg blend and Agar/Na-Alg-graft-

PAN were 55.0 ± 0.55 cP, 26.0 ± 0.45 cP, 38.0 ± 0.50 cP

and 63.0 ± 0.51 cP respectively at 80 oC temperature,

indicating that the grafted copolymer has the greatest

viscosity (Table 1).

Gel strength of the gel sample prepared from

grafted copolymer was significantly lower than that of

the parent blend. The gel strength of copolymer gel

sample and parent blend gel sample were 110 g cm-2

and 180 g cm-2

, respectively (Table 1). Similarly, the

gelling and melting temperatures of the copolymer gel

CHHATBAR et al.: SYNTHESIS OF AGAR/SODIUM ALGINATE-GRAFT-POLYACRYLONITRILE

1087

Table 1— Physicochemical properties of the blend polysaccharide and the graft copolymer hydrogels

Equil. swelling a (g/g) a at pH= Samples

1.2 7.0 12.5

Apparent viscosity

at 80ºC (cP)a,b pH

(at 60ºC)

Gel strength

(g/cm2)a,b

Gelling temp.

(ºC) a,b

Melting temp.

(ºC) a,b

Agar 6.0 ± 0.54 5.0 ± 0.46 5.0 ± 0.43 55 ± 0.55 6.9 800 ± 7.2 41 ± 0.52 84 ± 0.48

Na-Alg NAc NAc NAc 26 ± 0.45 7.0 NAc NAc NAc

Agar/Na-Alg blend

(1:3 w/w)d 14 ± 0.55 10 ± 0.54 8.5 ± 0.45 38 ± 0.50 6.2 180 ± 4.5 28 ± 0.55 69 ± 0.52

Agar/Na-Alg-graft-

PAN (1:2 w/w)d 8.5 ± 0.45 7.2 ± 0.45 6.5 ± 0.5 63 ± 0.51 6.7 110 ± 6.3 25 ± 0.45 61 ± 0.55

a Mean of triplicate measurements ± SD; b Measurements in 1.5 wt% sol/gel; c NA = not applicable; d w/w = weight by weight.

INDIAN J CHEM, SEC A, AUGUST 2009

1088

sample were also less than that of the parent blend gel

sample (Table 1). The lowering of gel strength of the

copolymer hydrogel was presumably due to the fact

that the -OH groups, which take part in network

formation through hydrogen bonding in the hydrogel,

become part of new ether linkages that are formed

with PAN (Scheme 1). This results in weak network

formation in the hydrogel having still weaker gel

strength, which is also associated with lower gel

melting temperature.

The swelling capacity of the Agar/Na-Alg-graft-

PAN was lower in all pH media as compared to that

of parent polysaccharide blend at pH 1.2, 7.0 and

12.5. The swelling capacities of the copolymer

hydrogel were 8.5 ± 0.45, 7.2 ± 0.45 and 6.5 ± 0.5 g/g

at these pHs respectively, while those of parent

polysaccharide blend were 14.0 ± 0.55, 10.0 ± 0.54

and 8.5 ± 0.45 in acidic (pH 1.2), neutral and alkaline

media (pH 12.5), respectively (Table 1). It may be

noted that one of the blend components is agar which

is acid labile, while the grafted copolymer is stable

beyond 24 h in acidic as well as the other pHs studied.

The blend swelled and got dispersed after 12 h at all

pHs studied. The relatively lower swelling in the

copolymer hydrogel may be due to the grafting of

blend with hydrophobic PAN moieties.

FT-IR spectra of grafted blend, PAN, blend and

parent polysaccharides are presented in Fig. 1. The

existence of a sharp intense peak at 2245 cm-1

for

Fig. 1 FT-IR spectra of (a) agar, (b) sodium alginate,

(c) agar/Na-Alg blend, (d) PAN, and, (e) Agar/Na-Alg-graft-PAN.

Fig. 2 XRD patterns of (a) PAN, (b) Agar/Na-Alg blend, and,

(c) Agar/Na-Alg-graft-PAN.

CHHATBAR et al.: SYNTHESIS OF AGAR/SODIUM ALGINATE-GRAFT-POLYACRYLONITRILE

1089

Fig. 3DSC curve of (a) Agar/Na-Alg blend, (b) polyacrilonitryl (PAN), and, (c) Agar/Na-Alg- graft-PAN.

Fig. 4 SEM images of (a) agar, (b) Na-Alg, (c) PAN, (d) Agar/Na-Alg blend, and, (e) graft copolymer.

INDIAN J CHEM, SEC A, AUGUST 2009

1090

(C=N-) stretching vibration in the IR spectra of the

graft copolymers is definite evidence of grafting. This

absorption band arises from the stretching vibration

mode of the nitrile groups and at 1454 cm–1

for C-N

stretching (Fig. 1).Characteristic IR bands at 777, 890,

and 931 due to 3,6-anydro-β-galactose skeletal

bending in agar20,21

, which were also observed for the

blend and grafted product, indicate that the backbone

configuration remained unaltered during the grafting

reaction.

The X-ray diffraction pattern of PAN, Agar/Na-

Alg blend and Agar/Na-Alg-graft-PAN are shown in

Fig. 2. The XRD patterns showed that the parent

blend and PAN were amorphous and crystalline in

nature, respectively (Fig. 2), while one intense peak

appears in copolymer indicates enhanced crystallinity

of the modified blend (Fig. 2).

The results of DSC study confirm the grafting of

the blend with PAN (Fig. 3). The enthalpy of melting

(∆H) was calculated by integrating the areas of the

exothermic and endothermic peaks. The blend

polysaccharide shows two endothermic peaks

(Fig. 3a) indicating that it is amorphous in nature.

Polyacrylonitrile exhibits 94% crystallinity while

grafted copolymer blend shows 38% crystallinity due

to the insertion of crystalline PAN on to the blend12,13

.

The SEM images of the graft copolymer confirm

grafting when compared to those of parent

polysaccharides, PAN, blend and grafted product

(Fig 4). The SEM image of the grafted product shows

a surface morphology having an integrated and

convoluted floral structure. This is distinctly different

from the images of the parent ungrafted blend and

also of the individual base component polymers, each

of which shows surface morphology of a segregated

system with discrete form.

Conclusions

In the present study, readily water-soluble sodium

alginate, a non-gelling seaweed polysaccharide, has

been converted into a stable copolymer hydrogel

through blending with agar and grafting the blend

with acrylonitrile. This hydrogel is stable in aqueous

media over a wide range of pH (pH 1.2, 7.0 and 12.5).

The unmodified blend swells and is dispersed after

12 h in all pH studied while the grafted copolymer is

not dispersed even after 24 h. The copolymer

hydrogel will be useful in newer applications

especially due to its resilience to acidic pH medium.

Acknowledgement The authors thank the Ministry of Earth Sciences,

New Delhi for an award of a fellowship to MUC

(MoES/9- DS/6/2007-PC-IV).

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