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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: shivsankar.bhattacharya@gmail.com

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

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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)

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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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