Chitosan-g-Poly(Acrylic Acid)/Kaolin Superabsorbent Composite: Synthesis and Characterization

Transcription

1 Chitosan-g-Poly(Acrylic Acid)/Kaolin Superabsorbent Composite: Synthesis and Characterization Chitosan-g-Poly(Acrylic Acid)/Kaolin Superabsorbent Composite: Synthesis and Characterization A. Pourjavadi* and G.R. Mahdavinia Polymer Research Laboratory, Department of Chemistry, Sharif University of Technology, Azadi Ave., P.O. Box , Tehran, Iran Received: 9 May 2005 Accepted: 25 July 2005 SUMMARY A novel superabsorbent composite based on chitosan was prepared by graft copolymerization of acrylic acid (AA) in the presence of kaolin powder using methylenebisacrylamide (MBA) as a crosslinking agent and ammonium persulfate (APS) as an initiator. The synthetic variables (i.e. AA, MBA, and APS concentration, and the kaolin amount) affecting the water absorbency of the resulting superabsorbent composite were studied. Evidence of grafting and kaolin interaction was obtained by comparing the FTIR spectra of the initial substrates with that of the superabsorbent composite. A new absorption band at 1722 cm -1 in the composite spectrum confirmed the kaolin-organic polymer linkage. The effect of kaolin amount showed that increasing this parameter caused the water absorbency of the superabsorbent composite to decrease. The effect of ph on the water absorbency of the superabsorbent was also studied. INTRODUCTION In recent years, there has been considerable interest in water-swellable superabsorbent polymers capable of absorbing and holding large amounts of water that the absorbed water is hardly removable even under some pressure. Superabsorbents are threedimensional networks of hydrophilic polymers held together by crosslinks of covalent bonds or ionic and/or secondary forces in the form of hydrogen bonds or hydrophilic interactions 1-3. Their affinity for water makes them useful especially for agriculture, personal hygiene products, industrial absorbents, medicine, and cosmetics 4-6. Because of their exceptional properties, i.e. biocompatibility, biodegradability, renewability, and non-toxicity, polysaccharides form the main part of the natural-based superabsorbent hydrogels. The higher production cost and low gel strength of these superabsorbents, however, restrict their application severely. To overcome these limitations, low cost inorganic compounds can be used. The introduction of inorganic fillers to a polymer matrix increases its strength and stiffness. Among inorganic compounds, special attention has been paid to clay minerals for use in nanocomposites because of their small particle size and intercalation properties. Mineral powders are hydrated layered aluminosilicates with reactive hydroxyl groups on the surface. The interaction of mineral powders with the reactive sites of natural polymers and monomers results in a superabsorbent composite. Superabsorbent composites based on synthetic polymers 7,8 or natural polymers 9,10 have already been reported. Chitosan is a linear natural polysaccharide composed of a partially deacetylated material of chitin. It is a basic polymer, having amine side groups 11. Due to their excellent biocompatibility and biodegradability, chitosan and its derivatives have been widely used in the fabrication of biomedical materials, enzyme and cell immobilization, and especially for drug delivery. *Corresponding author: purjavad@sharif.edu Rapra Technology Limited, 2006 Graft copolymerization of hydrophilic vinyl monomers onto polysaccharides is an efficient Polymers & Polymer Composites, Vol. 14, No. 2,

2 A. Pourjavadi and G.R. Mahdavinia route to achieving new superabsorbent polymers. Superabsorbents based on chitosan and its graft copolymerization with acrylamide, acrylic acid and acrylonitrile have been reported In this work, we attempt to synthesize and characterize new superabsorbent composites based on chitosan in the presence of kaolin particles. The presence of amine groups, with their protonation ability, was our main idea for the synthesis of chitosan-based superabsorbent composites. The preparation of the biopolymer-based superabsorbent composites can also give improved mechanical properties and can lower the cost of the finished product compared with all-synthetic counterparts as well as providing biodegradable characteristics. The reaction variables affecting the water absorbency of the chitosan-g-poly(acrylic acid)(chitosan-g- PAA)/kaolin and the ph-sensitivity of the hydrogels were investigated. EXPERIMENTAL Materials Chitosan (DD, degree of deacetylation, 0.76) was prepared from chitin (extracted from shrimp shell) in our laboratory 15. Acrylic acid (Merck) was used after vacuum distillation. Ammonium persulfate (Merck) was used without purification. Methylenebisacrylamide (Fluka) and kaolin (from Khorassan Co., Iran, KPS grade, particle size <5 µm) were used as received. All other chemicals were of analytical grade. Superabsorbent Composite Synthesis Chitosan solution was prepared in a 1-litre reactor equipped with mechanical stirrer and gas inlet. Chitosan ( g) was dissolved in 30.0 ml degassed distilled water containing 1 %wt of acetic acid. After complete dissolution of chitosan, various amounts of kaolin powder ( g) were added to the chitosan solution and allowed to stirred (200 rpm) for 15 min. The reactor was placed in a water bath at 60 C. Then 0.10 g of APS initiator was added and stirred for 10 in at 60 C. After APS, variable amounts of AA (1 4 g) were added to the chitosan solution. MBA solution ( g in 5 ml H 2 O) was added to the reaction mixture after the addition of the monomer and the mixture was continuously stirred (200 rpm) for 1 h under argon. After 60 min, the reaction product was allowed to cool to ambient temperature and neutralized to ph 8 by addition of 1 M NaOH solution. Methanol (500 ml) was added to the gelled product while stirring. After complete dewatering for 24 h, the hardened superabsorbent composite particles were filtered, washed with fresh methanol (2 50 ml) and dried at 50 C. Swelling Measurements A small bag (i.e. a 100 mesh nylon screen) containing an accurately weighed powdered sample (0.50 g) with average particle sizes between mesh was immersed entirely in distilled water (250 ml) or desired solutions with various ph values (100 ml) and allowed to soak for 3 h at room temperature. The bag was hung up for 15 min in order to remove the excess liquid. The maximum absorbency or equilibrated swelling (ES) was measured twice and calculated using the following equation: ES (g/g) =( W s W d )/W d (1) where W s and W d are the weights of the swollen superabsorbent composite and the dry sample, respectively. Swelling at Various ph Values Individual solutions with acidic and basic ph values were prepared by dilution of NaOH (ph 13.0) and HCl (ph 1.0) aqueous solutions to achieve ph 6 and ph<6, respectively. The ph values were precisely checked by a ph-meter (Metrohm/620). Then, 0.50 g of the dried composite was used for the swelling measurement according to Equation 1. FT-IR Analysis FT-IR spectra of samples in the form of KBr pellets were recorded using an ABB Bomem MB-100 FT-IR spectrophotometer. RESULTS AND DISCUSSION Synthesis and Characterization The superabsorbent composites were prepared by graft copolymerization of acrylic acid onto chitosan in the presence of a crosslinking agent and powdery kaolin. Ammonium persulfate was used as an initiator. The persulfate is decomposed under heating and produced sulfate anion-radicals that abstract hydrogen from the hydroxyl groups of chitosan backbones. So, this persulfate-saccharide redox system results in active centres capable of 204 Polymers & Polymer Composites, Vol. 14, No. 2, 2006

3 Chitosan-g-Poly(Acrylic Acid)/Kaolin Superabsorbent Composite: Synthesis and Characterization initiating free radical polymerization of acrylic acid to form a graft copolymer. Since a crosslinking agent, e.g. MBA, is present in the system, the copolymer has a crosslinked structure. The mechanism of the reaction is shown in Scheme 1. FT-IR spectroscopy was used for identify the product. The FT-IR spectra of the initial substrates and the composite graft copolymer are depicted in Figure 1. Figure 1b represents the spectrum of kaolin. In the layer silicate structure, -OH groups show absorption bands at cm -1. In Figure 1a, the bands cm -1 (O-H and N-H stretching), 2911 and 2866 cm -1 (C-H stretching), cm -1 (C-O stretching of secondary alcohol) and cm -1 (C-O stretching of the primary alcohol) were common to both spectra because of the chitosan backbone. Two new absorption peaks at 1574 and 1722 cm -1 appeared in the spectrum of the composite (Figure 1c). The characteristic band at 1574 cm -1 was attributed to C=O asymmetric stretching in the carboxylate anions, reconfirmed by another peak at 1401 cm -1 which is related to the symmetric stretching mode of the carboxylate groups 16. Figure 1. FTIR spectra of (a) chitosan, (b) kaolin, (c) chitosan-g-paa/kaolin composite, and (d) mixture of chitosan/ kaolin/poly(acrylic acid) Polymers & Polymer Composites, Vol. 14, No. 2,

4 A. Pourjavadi and G.R. Mahdavinia The absorption peak at 1722 cm -1 can be attributed to the ester groups formed during graft copolymerization. The carboxylic groups of the grafted poly(acrylic acid) can react with the OH groups on the kaolin surface. The replacement of the OH groups on the surface of kaolin by carboxylic ones results in ester formation. The reaction can be shown as follows: Figure 1d shows the spectrum of the mixture of chitosan, MBA-crosslinked poly(sodium acrylate) and the kaolin powder. As shown in this figure, the absorption bands at cm -1 appeared reconfirming the involvement of the OH groups on the kaolin surface in the polymerization reaction. Effect of MBA Concentration on Swelling Crosslinks are necessary to form a superabsorbent hydrogel in order to prevent dissolution of the hydrophilic polymer chains in an aqueous environment. The effect of MBA concentration on the water absorbency of the chitosan-g-paa/ kaolin composites was studied by varying the concentration of MBA from to 0.06 mol/l. All the other parameters in these series of reactions were held constant. As the concentration of the MBA was increased (from to 0.003), the water absorbency of the superabsorbent composite increased (Figure 2). The water absorbency of the superabsorbent composite was similar to that found for starch-g-poly(acrylamide)/clay 9 and poly(acrylic acid)/mica 7 superabsorbents. Clearly, a lower concentration of crosslinker does not produce enough crosslink points to construct a polymeric network. Especially, when Scheme 1. The proposed mechanism of the graft copolymerization of acrylic acid onto chitosan in the presence of MBA crosslinking agent and kaolin powder 206 Polymers & Polymer Composites, Vol. 14, No. 2, 2006

5 Chitosan-g-Poly(Acrylic Acid)/Kaolin Superabsorbent Composite: Synthesis and Characterization the crosslinker concentration is less than , the absorption network does not form effectively. As a consequence, the water absorbency of the sample cannot be measured. Beyond mol/l of MBA concentration, the water absorbency of the composite decreases. This is due to a decrease in the space between the polymer chains as the crosslinker concentration is increased. The maximum water absorbency (421 g/g) was obtained at mol/l of MBA. Effect of Monomer Concentration on Swelling The swelling capacity of superabsorbent composite as a function of acrylic acid concentration is illustrated in Figure 3. With increasing acrylic acid concentration, the swelling capacity of the composite increased, reached the maximum value of the swelling capacity. This increase in swelling capacity could be derived from the greater availability of monomer molecules in the Figure 2. The effect of MBA crosslinker concentration on the water absorbency of the superabsorbent composite Figure 3. The influence of acrylic acid concentration on the water absorbency of the superabsorbent composite Polymers & Polymer Composites, Vol. 14, No. 2,

6 A. Pourjavadi and G.R. Mahdavinia vicinity of the chain propagating sites of chitosan macroradicals. In addition, a higher acrylic acid content enhances the hydrophilicity of the hydrogel in the chitosan-g-paa/kaolin composite that, in turn, causes a stronger affinity for water. A further increase in monomer concentration, however, results in decreased absorbency. This is probably due to: (a) a preference homopolymerization over graft copolymerization, (b) an increase in the viscosity of the medium that hinders the movement of free radicals and monomer molecules, (c) the enhanced chance of chain transfer to monomer molecules. Effect of APS Concentration on Swelling The effect of the initiator (APS) concentration on the water absorbency of the superabsorbent composite was also studied (Figure 4). The APS concentration was changed from to mol/l. The water absorbency of the composite was increased initially with increasing the initiator concentration up to mol/l, but it decreased later, as shown in Figure 4. The increase in water absorbency with increasing initiator concentration may be ascribed to the increase in the active sites on the backbone of the chitosan, arising from the attack of sulfate anion-radical. An additional reason, according to Flory, is the imperfection of polymer networks obtained from high-initiator polymerization systems. The increase in the number of active sites on the chitosan led to an increase in poly(acrylic acid) grafting onto chitosan backbones. The decrease in water absorbency with initiator concentration beyond mol/l may be due to: (a) an increased number of radicals led to termination by bimolecular collision, (b) a predominance of homopolymerization over grafting, (c) molecular weight loss of the synthetic part of the polymer network, and (d) free radical degradation of chitosan substrate. Hsu et al. have recently reported a similar observation in the case of degradation of chitosan with potassium persulfate 17. Effect of Kaolin Amount on Swelling The effect of the kaolin content on the water absorbency of the composite was studied (Figure 5). The kaolin/chitosan weight ratio was varied from 0.33 to 3, while other reaction variables were constant. Figure 5 indicates that the water absorbency of the composite decreased with increasing amount of kaolin incorporated in the composite structure. The clay in the polymerization reaction may acts in two ways: (a) the kaolin particles act as a crosslinking agent. This means that the carboxylate groups of the poly (acrylic acid) chains react with kaolin, as obviously proved by FTIR spectra (Figure 1), and (b) kaolin prevents the growth of the polymer chains by a chain transfer mechanism 7. Figure 4. Influence of APS initiator concentration on the water absorbency of the superabsorbent composite 208 Polymers & Polymer Composites, Vol. 14, No. 2, 2006

7 Chitosan-g-Poly(Acrylic Acid)/Kaolin Superabsorbent Composite: Synthesis and Characterization Effect of ph on Swelling Figure 6 shows the ph dependence of the equilibrium swelling of chitosan-g-paa/kaolin superabsorbent composite at ambient temperature. The effective pka for chitosan is 6.5 and the effective pka for carboxylic acid groups is ~4.7. The dependence of the equilibrium swelling of the hydrogel is characterized by a curve with two maximum values at ph=3 and 8. The remarkable swelling changes are due to the presence of different interacting species in the swelling medium, depending on the ph. It can be assumed that the composite includes both chitosan and poly(acrylic acid) structures. Chitosan and PAA are ionizable. Therefore, assuming that the pk a of PAA is ~4.7 and the pk a of chitosan is 6.5, the species involved are NH 3 + and COOH (at ph=1-3), NH 2 and COO - (at ph=7-13) Figure 5. The effect of kaolin/chitosan weight ratio on the water absorbency of the superabsorbent composite Figure 6. The ph-dependent swelling behavior of chitosan-g-poly(acrylic acid)/kaolin superabsorbent composite at 25 C Polymers & Polymer Composites, Vol. 14, No. 2,

8 A. Pourjavadi and G.R. Mahdavinia and NH 3 + and COO - or NH 2 and COOH (at ph=4-7). Under acidic conditions, the swelling is controlled mainly by the amino group (NH 2 ) on the chitosan component. It is a weak base with an intrinsic pk a of about 6.5, so it gets protonated and the increased charge density on the polymer should enhance the osmotic pressure inside the gel particles because of the NH 3+ -NH 3 + electrostatic repulsion. This osmotic pressure difference between the internal and external solution of the network is balanced by the swelling of the gel. However, under very acidic conditions (ph<3), a screening effect of the counter ions, i.e. Cl, shields the charge of the ammonium cations and prevents efficient repulsion. As a result, a remarkable decrease in equilibrium swelling is observed (gel collapsing). Around ph>4.7, the carboxylic acid component comes into action as well. Since the pk a of the weak polyacid is about 4.7, its ionization occurs above this value, which may favour enhanced absorbency. But if the ph is under 6.4, at a certain ph range 4-6.4, the majority of the base and acid groups are NH 3 + and COO - or NH 2 and COOH, so ionic interaction between NH 3 + and COO - species (ionic crosslinking) or hydrogen bonding between the amine and carboxylic acid may lead to crosslinking followed by decreased swelling. At ph=8, the carboxylic acid groups become ionized and the electrostatic repulsive force between the charged sites (COO ) causes an increase in swelling. Again, a screening effect of the counter ions (Na + ) limits the swelling at ph ,14. CONCLUSIONS A novel superabsorbent composite containing natural polymer (i.e. chitosan) and inorganic filler (i.e. kaolin) was prepared by graft copolymerization of poly(acrylic acid) onto chitosan in the presence of a crosslinking agent. The resultant superabsorbent composite had a large degree of water absorbency. The FT-IR spectra show that a new absorption band at 1722 cm -1 in the composite spectrum appeared; it was attributed to ester formation from the replacement of the hydroxyl groups of kaolin with carboxylic groups grafted onto the polysaccharide backbone. The effect of the kaolin content and MBA concentration showed that on increasing either of these parameters, the water absorbency of the superabsorbent composite was decreased. The swelling of hydrogels in solutions of various ph values showed their high sensitivity to ph. We reported a crosslinking polymerization to achieve superabsorbent composite materials with lower cost. The hydrogel composite will most probably possess higher biodegradability (due to the chitosan part) and higher swollen gel strength (due to the inorganic parts). The latter properties are under consideration in our laboratory. REFERENCES 1. Buchhulz F.L. and Peppas N.A., Superabsorbent Polymer Science and Technology, ACS Symposium Series 573, American Chemical Society, Washington DC, (1994). 2. Buchholz F.L. and Graham A.T., Modern Superabsorbent Polymer Technology, Wiley, New York, (1997). 3. Hennink W.E. and Van Nostrum C.F., Adv. Drug. Deliv. Rev. 54, (2002), Peppas L.B. and Harland R.S., Absorbent Polymer Technology, Elsevier, (1990). 5. Po R., J. Macromol. Sci-Rev. Macromol. Chem. Phys. 34C, (1994), Hoffman A.S., Polymeric Materials Encyclopedia, Vol. 5, Ed., Salamone J.C., CRC Press: Boca Raton, FL, (1996). 7. Lin J., Wu J., Yang Z. and Pu M., Macromol. Rapid Commun., 22, (2001), Lin J., Wu J., Yang Z. and Pu M., Polymers and Polymer Composites, 9, (2001), Wu J., Lin J., Zhou M. and Wei C., Macromol. Rapid Commun. 21, (2000), Wu J., Wei Y., Lin J. and Lin S., Polymer, 44, (2003), Roberts G.A.F., Chitin Chemistry; the Macmilan Press Ltd: Houdmills, Basingstoke, Hampshire, London, (1992). 12. Pourjavadi A., Mahdavinia G.R. and Zohuriaan-Mehr M.J., J. Appl. Polym. Sci., 90, (2003), Pourjavadi A., Mahdavinia G.R. and Zohuriaan-Mehr M.J., Polym. Adv. Technol., 15, (2004), Pourjavadi A., Mahdavinia G.R., Hosseinzadeh H. and Zohuriaan-Mehr M.J., Eur. Polym. J., 40, (2004), Pourjavadi A., Mahdavinia G.R., Zohuriaan- Mehr M.J. and Omidian H., J. Appl. Polym. Sci., 88, (2003), Polymers & Polymer Composites, Vol. 14, No. 2, 2006

9 Chitosan-g-Poly(Acrylic Acid)/Kaolin Superabsorbent Composite: Synthesis and Characterization 16. Silverstein R.M. and Webster F.X., Spectrometric Identification of Organic Compounds, 6 th Edn., Wiley, New York, (1998). 17. Hsu S.C., Don T.M. and Chiu W.Y., Polym. Degrad. Stab., 75, (2002), 73. Polymers & Polymer Composites, Vol. 14, No. 2,

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