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Synthesis and Characterization ofPoly(acrylic acid)/modified BentoniteSuperabsorbent PolymerShiv Sankar Bhattacharya a , Kalyan Kumar Sen a , Suma OommenSen a , Subham Banerjee b , Santanu Kaity a , Ashoke Kumar Ghosh b
& Animesh Ghosh ca Department of Pharmaceutics, Gupta College of TechnologicalSciences, Asansol, West Bengal, Indiab College of Pharmacy, Institute of Foreign Trade and Management,Moradabad, Uttar Pradesh, Indiac Department of Pharmaceutical Sciences, Birla Institute ofTechnology, Ranchi, Jharkhand, IndiaPublished online: 29 Nov 2011.
To cite this article: Shiv Sankar Bhattacharya , Kalyan Kumar Sen , Suma Oommen Sen ,Subham Banerjee , Santanu Kaity , Ashoke Kumar Ghosh & Animesh Ghosh (2011) Synthesisand Characterization of Poly(acrylic acid)/modified Bentonite Superabsorbent Polymer,International Journal of Polymeric Materials and Polymeric Biomaterials, 60:13, 1015-1025, DOI:10.1080/00914037.2011.557807
To link to this article: http://dx.doi.org/10.1080/00914037.2011.557807
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Synthesis andCharacterization ofPoly(acrylic acid)/modified BentoniteSuperabsorbent Polymer
Shiv Sankar Bhattacharya,1 Kalyan Kumar Sen,1
Suma Oommen Sen,1 Subham Banerjee,2 Santanu Kaity,1
Ashoke Kumar Ghosh,2 and Animesh Ghosh3
1Department of Pharmaceutics, Gupta College of Technological Sciences,Asansol, West Bengal, India2College of Pharmacy, Institute of Foreign Trade and Management, Moradabad,Uttar Pradesh, India3Department of Pharmaceutical Sciences, Birla Institute of Technology,Ranchi, Jharkhand, India
A novel poly(acrylic acid)=modified bentonite superabsorbent polymer (SAP) wassynthesized through chemical crosslinking by a polymerization technique in a completeaqueous environment. This SAP was fabricated effectively by dispersing modifiedbentonite in a monomeric solution, using N,N0-methylenebisacrylamide as crosslinkerand ammonium persulfate as initiator. Fourier transform infrared (FTIR) spectralanalysis showed that the XG chains had intercalated into bentonite sheets. Theinfluence of crosslinking density and XG content were investigated. Results showedmodified bentonite not only effectively increases water absorbency, but also improveswater retention ability. This can be further used as a carrier matrix for the develop-ment of a drug delivery system.
Received 24 October 2010; accepted 22 January 2011.The authors are grateful to the authority of Gupta College of Technological Sciences,Asansol, West Bengal, India, for providing necessary facilities for this work.Declaration of Interest: The authors report no conflicts of interest. The authors aloneare responsible for the content and writing of the paper.Address correspondence to Mr. Shiv Sankar Bhattacharya, Gupta College of Techno-logical Sciences, Ashram More, G.T. Road, Asansol-713 301, West Bengal, India.E-mail: [email protected]
International Journal of Polymeric Materials, 60:1015–1025, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0091-4037 print=1563-535X online
DOI: 10.1080/00914037.2011.557807
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Keywords bentonite, superabsorbent polymer, water absorbency, xanthan gum
INTRODUCTION
Superabsorbents are three-dimensional polymeric networks that can absorb
and retain large volumes of water because of superior properties compared
to traditional absorbents (such as sponge, cotton and pulp, etc.). Superabsor-
bents are widely used in many fields, such as hygienic products, horticul-
ture, gel actuators, drug-delivery systems, as well as water-blocking tapes
and coal dewatering [1,2]. In recent years, a number of studies have focused
on the preparation and utilization of polysaccharidic superabsorbents
because of their biodegradability, biocompatibility, renewability, and non-
toxicity [3–6]. In particular, xanthan gum (XG) is a high molecular weight,
anionic extracellular polysaccharide that is produced by gram-negative
bacterium Xanthomonas campestris. It is widely used in food, cosmetics,
and pharmaceuticals because of its encouraging reports on safety [7,8]. On
the basis of short-term and long-term feeding studies, XG was cleared by
the U.S. Food and Drug Administration (FDA) in 1969, permitting its use
in food products without any specific quantity limitations. Bentonite is a
silicate of aluminium hydroxide. Due to its hydrophilic nature bentonite is
suitable for use in SAPs as an additive.
Previous researchers concluded that blending of polymers with another
polymer or inorganic fillers appears to be an easier alternative [9], because
simple blends have been found to have shown poor mechanical properties along
with unstable morphological characteristics. The compatibilization of such
blends is well accepted [10]. Hence we have used xanthan gum as a bentonite
modifier in this study. One reason is that XG can intercalate into bentonite
sheets [11] and it has good compatibility with PAA due to the presence of a
hydrophilic group in its moiety.
Polymer intercalation has been proven to be an excellent technique
because of its versatility and environmentally benign character due to the
absence of solvent [12]. Here, it has been found that xanthan gum can also
intercalate into bentonite sheets and it has good compatibility with poly(acrylic
acid). The intercalation of xanthan gum into bentonite makes the bentonite
nanolayers more hydrophilic, enabling them to be exfoliated by acrylate mole-
cules more easily. Another important reason for XG as the modifier is that XG
has high water absorbency and it may be used to prepare SAP with enhanced
water absorbing capacity [13].
In the literature of material science we are not aware of the synthesis and
characterization of SAP with poly(acrylic acid)=xanthan gum modified benton-
ite where the entire polymerization process was carried out in a complete
aqueous environment.
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Therefore, we paid attention to the synthesis and characterization of poly-
meric superabsorbents by introducing modified bentonite in poly (acrylic acid)
solution through chemical crosslinking by a polymerization technique using
N,N0-methylenebisacrylamide as a crosslinker and ammonium persulfate as
an initiator in a complete aqueous environment. The structure and morphology
of the superabsorbent base were characterized by Fourier transform infrared
spectroscopy (FTIR) and scanning electron microscopy (SEM), respectively.
The swelling kinetics and water absorbency of the superabsorbent base were
also investigated systematically. The objectives of this study were: (i) to
synthesis PAA=XG-bentonite SAP; and (ii) to study the influence of modified
bentonite along with MBA on water absorbency and water retention ability
of SAP.
MATERIALS AND METHODS
MaterialsXanthan gum (XG, food grade) was obtained from Loba Chemie Private
Limited, Mumbai, India. N,N0-methylenebisacrylamide (MBA, chemically
pure) was recrystallized from methanol purchased from Loba Chemie
Private Limited, Mumbai, India. Acrylic acid (AA, analytical grade) was
distilled under reduced pressure before use and was commercially pur-
chased from Merck Specialties Private Limited, Mumbai, India. Bentonite
was milled and passed through a 120-mesh screen and supplied by Central
Drug House Private Limited, New Delhi, India. Ammonium persulfate
(APS) was recrystallized twice by distilled water and supplied by Qualigens
Fine Chemicals Private Limited, Mumbai, India. Sodium hydroxide (ana-
lytical grade) was supplied from BDH Chemicals, India. Other agents used
were all of analytical grade and all solutions were prepared with double
distilled water.
Preparation of Xanthan Modified Bentonite0.5 gm bentonite was added to 50 ml distilled water and mechanically
stirred (Remi Equipments Private Limited, Mumbai, India), followed by
ultrasonic treatment (FS-600, Frontline Electronics and Machinery Pvt.
Ltd., Gujarat, India) for 15 min, which resulted in good bentonite dispersion.
The XG solution obtained by dissolving 1 gm XG into 50 ml distilled water
under vigorous stirring was added to the bentonite suspension and the mix-
ture was stirred vigorously for 2 hour at 70�C temperature. The modified
bentonite suspension was dried in an oven (Lunar amalgamated suppliers,
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Kolkata, India) at 70�C temperature to constant weight and then milled
through a 120-mesh screen.
Preparation of Poly (Acrylic Acid)/modified BentoniteSuperabsorbent BaseA series of samples with different amounts of MBA and XG were prepared
using chemical crosslinking by a polymerization method [11]. In brief, AA 10 ml
was dissolved in 10 ml distilled water and then neutralized in an ice bath with a
predetermined amount of aqueous sodium hydroxide solution until the neutra-
lization was completed in a 250 ml four-necked flask equipped with a stirrer, an
efficient reflux condenser, a drip funnel, and a thermometer. Then the required
amount of modified bentonite was added to the above neutralized solution and
stirred vigorously until the modified bentonite was well dispersed. Thereafter
20 mg of APS was charged as initiator and varying amounts of MBA as cross-
linker into the reaction mixture dropwise. The mixture was stirred and heated
to 70�C in a heating mantle (Lunar amalgamated suppliers, Kolkata, India) for
4 hrs. After complete polymerization, the resulting product was washed with
double distilled water and dried in an oven at 70�C temperature for 24 hrs.
The dried material was pulverized into particles and screened through a
120-mesh screen. The feed composition of all samples used to prepare a PAA=
modified bentonite superabsorbent polymer are shown in Table 1.
Preparation of PAA/XG-Bentonite Superabsorbent BaseThe preparation process of PAA=XG-Bentonite SAP was obtained by
adding separately XG and bentonite to the acrylic acid solution and then
the mixture was stirred. The polymerization process was carried out under
the aforementioned conditions.
Table 1: Feed compositions of poly(acrylic acid)=modified bentonitesuperabsorbent
Formulationcode
Samplescode
Acrylicacid (ml)
InitiatorAPS (mg)
CrosslinkerMBA (mg)
Xanthangum (gm)
F1 MBA1 10 20 20 1.00F2 MBA2 10 20 15 1.00F3 MBA3 10 20 10 1.00F4 MBA4 10 20 05 1.00F5 m-B1 10 20 15 0.30F6 m-B2 10 20 15 0.60F7 m-B3 10 20 15 0.90F8 m-B4 10 20 15 1.20
(MBA¼N,N0-methylenebisacrylamide and m-B¼modified bentonite).
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Scanning Electron Microscope (SEM) AnalysisThe purpose of the SEM study was to obtain topographical characteristics
of PAA=modified bentonite and PAA-XG-Bentonite superabsorbent polymer. A
small amount of the above polymer samples were deposited on one side of a
double adhesive NEM Tape (Nisshin Em. Co. Ltd., Tokyo, Japan). These
samples were mounted on the scanning electron microscope (Jeol Datum Ltd,
JSM-6360, Tokyo, Japan). SEM photographs were taken at the required
magnification at room temperature. The working distance of 39 mm was
maintained and acceleration voltage used was 20 kV with the secondary
electron image as a detector.
Fourier Transform Infrared (FTIR) Spectral AnalysisFTIR spectra of the pristine XG, pristine bentonite, modified bentonite,
pristine PAA, PAA= modified bentonite and polymer prepared by separate
addition of PAA-XG and bentonite were recorded in a Perkin Elmer FTIR
Spectrometer (Spectrux BX, Perkin-Elmer, UK) from 4,000 to 400 cm�1 at a
resolution of 4 cm�1 using a KBr pellet press technique. The pellets were made
by applying a pressure of 10 tons for 15 min in a hydraulic pellet press
(Type-KP, Kimaya Engineers, India).
Measurement of Water AbsorbencyThe water uptake swellings of different formulations were measured
by immersing 100 mg PAA=modified bentonite superabsorbent polymer into
800 mL distilled water. To ensure complete equilibration, superabsorbent
polymer samples were allowed to swell completely for about 24 hours to attain
equilibrium at 37�C temperature. The excess surface-adhered liquid droplets of
the swollen samples were removed by blotting with soft tissue papers and the
swollen polymer was weighed using a Dhona single pan balance (Dhona Instru-
ments Private Limited, Kolkata, India). In order to maintain the accuracy,
experiments were carried out in triplicate for all formulations to obtain repro-
ducible results [14]. The % equilibrium water uptake was calculated using the
equation:
% equilibrium water uptake ðQÞ ¼Mass of swollen polymer ðWaÞ � Mass of dry polymer ðW0Þ
Mass of dry polymer ðW0Þ
� �� 100
Measurement of Swelling KineticsThe swelling kinetics of the superabsorbent was measured according to
the following process [14]. 100 mg sample was tied in a muslin cloth, poured
Poly(acrylic acid)=modified Bentonite Superabsorbent 1019
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into 250 mL 5 mmol=L NaCl solution, and was weighed at 10-minute intervals
for 1 hour, then followed up by 6 hours at 1-hour intervals. The measurement
condition was the same as the equilibrium water absorbency measured in
distilled water according to the previous equation.
RESULTS AND DISCUSSION
Scanning Electron Microscope (SEM) AnalysisSEM micrographs of a PAA=modified bentonite and PAA=XG-bentonite
were observed and are shown in Figures 1 and 2, respectively. The surface mor-
phology of PAA=XG-bentonite superabsorbent is different from that of PAA=
modified bentonite. PAA=XG-bentonite exhibits a smooth and dense surface
(Fig. 2), while the superabsorbent polymer composites containing PAA=modi-
fied bentonite showed a relatively coarse and undulant surface (Fig. 1). This
surface (Fig. 1) is convenient for penetration of water into the polymeric net-
work, which may be of benefit to the water absorbency of the corresponding
superabsorbent. Another interesting observation was that the surface of a
PAA=modified bentonite matrix structure was held by a loose polymeric net-
work and reveals multiple porous structures, whereas the surface of the
PAA=XG-bentonite matrix structure was held by a rigid polymeric network
with less porous structure compared to the previous one. It may be due to
the better intercalation of bentonite sheets by the XG network by the exfoliated
and intercalated mode, which facilitates the superabsorbent nature of the
polymer with multiple porous structures to form a homogeneous composition.
Another interesting observation was that the surface of the PAA=modified
bentonite matrix structure of F4 was held by a loose polymeric network and
Figure 1: SEM micrographs of PAA=modified bentonite showing coarse and undulantsurface (72� 72 DPI).
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revealed a porous matrix structure (Figure 3), whereas the surface of F1 was
held by a rigid network, without showing any porous structure (Figure 4). This
may be due to the higher extent of the crosslinking influences rigid matrix
structure [15].
Fourier Transform Infrared (FTIR) Spectral AnalysisThe FTIR spectra are shown in Figure 5(a–f). In the case of pure XG, a sin-
gle peak was observed in the wave region of 1656.00 cm�1 due to the presence of
the carbonyl group, whereas for bentonite a single band was seen in the region
of 1647.68 cm�1 due to the presence of a similar group. However, it has been
observed that modified bentonite exhibits a single band at 1626.22 cm�1 and
Figure 2: SEM micrographs of PAA=XG-bentonite superabsorbent containing smoothand dense surface (72� 72 DPI).
Figure 3: SEM micrographs of PAA=modified bentonite containing porous matrixstructure of F4 (72�72 DPI).
Poly(acrylic acid)=modified Bentonite Superabsorbent 1021
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PAA=modified bentonite absorption band at 1560.41 cm�1. This could be
explained because a weak chemical interaction occurred between the two moi-
eties, which ultimately results in a shifting of carbonyl group stretching,
suggesting the chemical reaction of AA on modified bentonite. Finally, for
PAA=modified bentonite no sharp peak was observed in the wave region of
Figure 4: SEM micrographs of PAA=modified bentonite containing rigid matrix structureof F1 (72� 72 DPI).
Figure 5: FTIR-spectra of (a) pure XG; (b) modified bentonite; (c) pure bentonite; (d)PAA=modified bentonite; (e) pure PAA; (f) PAA=XG-bentonite (72�72 DPI). (Figure isprovided in color online.)
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1636.82 cm�1, which was previously observed in the case of pristine PAA. This
showed some interaction has occurred between PAA and modified bentonite.
The results obtained from FTIR analysis showed that the reaction of
both XG and bentonite with AA monomer took place during the polymerization
process. The free radicals on APS initiated the polymerization of AA
and modified bentonite, and then formed the superabsorbent polymeric
network.
Measurement of Water AbsorbencyEffect of MBA Content on Water Absorbency
According to Flory’s network theory [16], the crosslinking density is a key
factor influencing water absorbency of superabsorbents and water absorbency
is in inverse proportion to crosslinking density. The effect of crosslinker con-
tent on water absorbency of the PAA=modified bentonite superabsorbent is
shown in Table 2. As can be seen, the equilibrium water uptake decreased
significantly from 334.89% to 163.56% with the increase of crosslinker content
in the matrices from 5 to 20 mg. This tendency was attributed to the fact that
crosslinking density of the superabsorbent increased with increasing MBA
content, and the elasticity of the polymeric network of the superabsorbent
decreased, which resulted in the decrease of porous nature of the SAP water
absorbency by making the SAP network more compact. Similar phenomena
have been previous reported previously [6].
Table 2: Effect of MBA concentration on waterabsorbency (n¼ 3)
Formulationcode
Samplecode
MBAcontent(mg)
% Waterabsorbency
(Q)
F1 MBA1 20 163.56 (�08.81)F2 MBA2 15 208.61 (�18.32)F3 MBA3 10 318.23 (�14.22)F4 MBA4 05 334.89 (�11.93)
Table 3: Effect of XG concentration on water absorbency (n¼ 3)
Formulationcode
Samplecode
XG content(gm)
% Waterabsorbency (Q)
F5 m-B1 0.30 174.18 (�10.52)F6 m-B2 0.60 203.58 (�12.34)F7 m-B3 0.90 249.47 (�11.98)F8 m-B4 1.20 361.29 (�09.78)
Poly(acrylic acid)=modified Bentonite Superabsorbent 1023
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Effect of XG Content on Water Absorbency
The effect of XG content on water absorbency of the superabsorbents is
shown in Table 3. It can be seen that the water absorbency increased with
increasing the amount of XG concentration from 0.30 to 1.20 gm. The
maximum water absorbency was obtained with a XG content of 1.20 gm. When
the amount of XG was low, the monomer was superfluous in the reaction
system. The superfluous AA turned out to be a homopolymer, which cannot
contribute to the water absorbency. The homopolymer content decreased with
the increase of XG content at fixed crosslinking density [17]. Compared with
the chitiosan-g-poly(acrylic acid)=sodium humate superabsorbent reported by
Liu et al. [18], the introduction of natural polysaccharide XG can obtain
much higher water absorbency, which may be attributed to the presence of
hydrophilic hydroxyl and carboxyl moiety of the XG ring [13].
Swelling Kinetics Measurement
The swelling kinetics curves of the PAA=modified bentonite superabsor-
bent in 5 mmol=L NaCl solution are shown in Figure 6. In NaCl solution, the
water absorbency increased with the prolongation of immersing time and the
superabsorbent reaches swelling equilibrium within 4 hours. Further increase
of immersing time had no evident influence on water absorbency of the super-
absorbent. This phenomenon, namely the superabsorbent first swelled to a
maximum value followed by a gradual deswelling until the equilibrium, is
actually known as the over-shooting effect, which can be interpreted as the
consequence of a swelling-deswelling process [19].
CONCLUSION
The superabsorbent polymer composed of AA monomer was prepared by the
polymerization method. The effects of factors such as MBA concentration
Figure 6: Dynamic swelling curve of PAA=m-B in NaCl solution (72� 72 DPI).
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and XG concentration were studied. The rate of water absorption was also
measured in distilled water and swelling kinetics behavior of PAA=modified
bentonite in NaCl solution. This superabsorbent polymer showed high water
absorbency and fast swelling capacity. Utilization of this superabsorbent poly-
mer could be carried out in the future in the polymer and pharmaceutical
industries because of the complete use of aqueous environment during the time
of synthesis. For future studies, water-soluble and water insoluble drugs can
be included in this novel polymer and its in-vitro drug release profile can be
further studied. High water absorbing capacity also enables this polymer to
be used in a gastro-retentive drug delivery system or oral osmotic pump
controlled drug delivery system.
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