agar/sodium alginate-graft -polyacrylonitrile, a...
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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: [email protected]
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
1086
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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