Ind. Eng. Chem. Res. 2007, 46, 2497-2502
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PROCESS DESIGN AND CONTROL Preparation and Properties of Chitosan-g-poly(acrylic acid)/Montmorillonite Superabsorbent Nanocomposite via in Situ Intercalative Polymerization Junping Zhang,†,‡ Li Wang,†,‡ and Aiqin Wang*,† Center of Eco-material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
A novel chitosan-g-poly(acrylic acid)/montmorillonite superabsorbent nanocomposite with water absorbency of 160.1 g g-1 in distilled water and 46.6 g g-1 in 0.9 wt % NaCl solution was prepared by in situ intercalative polymerization among chitosan, acrylic acid, and montmorillonite in aqueous solution, using N,N′methylenebisacrylamide as a cross-linker and ammonium persulfate as an initiator. Chitosan could intercalate into layers of montmorillonite and form superabsorbent nanocomposites through in situ graft-polymerization with acrylic acid according to XRD, IR spectra, TEM, and SEM analysis. The one-step method is more convenient, and the corresponding nanocomposites have higher swelling ability and pH-responsivity as compared to the two-step method. Factors influencing water absorbency of the superabsorbent nanocomposite were investigated, such as weight ratio of acrylic acid to chitosan and montmorillonite content. The introduced montmorillonite could form a loose and porous surface and improve water absorbency of the chitosan-gpoly(acrylic acid) superabsorbent. 1. Introduction Superabsorbents are a cross-linked network of hydrophilic polymers that can absorb and retain aqueous fluids up to thousands of times their own weight, and the absorbed water is hardly removable even under some pressure. Because of the excellent properties of traditional water-absorbing materials (such as sponge, cotton, and pulp, etc.), superabsorbents have raised considerable interest and are widely used in many fields, such as hygienic products, horticulture, gel actuators, drugdelivery systems, as well as water-blocking tapes and coal dewatering.1-5 Material’s biodegradability has been widely focused on because of the renewed attention to environmental protection issues.6 However, about 90% of superabsorbents are used in disposable articles, and most of them are synthetic polymers, which are poor in degradability, and then there remains an environmental problem.7 Incorporation of biodegradable and renewable natural high polyols, such as starch,7 cellulose,8 and chitosan,9 is a convenient way to improve biodegradability of corresponding superabsorbent materials and has drawn much attention. Chitosan, a polysaccharide from chitin, is one of the most abundant biomasses in the world. Reactive -NH2 and -OH of chitosan are convenient for graft-polymerization of hydrophilic vinyl monomers onto it, and this is an efficient way to acquire hydrogels with novel properties. It has been reported previously that superabsorbent from chitosan has antibacterial activities and could be used in infant diapers, feminine hygiene products, and other special fields.10,11
Recently, clay became the focus for the preparation of inorganic-organic superabsorbent composite to improve swelling properties, enhance hydrogel strength, and reduce production cost of corresponding superabsorbents. Clays, including montmorillonite,12 kaolin,13 mica,14,15 attapulgite,16 and sericite,17 have already been incorporated into poly(acrylic acid) or polyacrylamide polymeric networks. Chitosan could intercalate into layers of montmorillonite under acidic conditions.18,19 Qiu et al.20 reported the preparation of the poly(acrylic acid)/ chitosan-intercalated montmorillonite (PAA/CTS-intercalated MMT) nanocomposite; however, the preparation process is somewhat redundant, for that montmorillonite must be modified first with chitosan, and then polymerized with acrylic acid to form the nanocomposites (designated as two-step method). On the basis of our previous work about superabsorbent composite21-26 and chitosan,27,28 a novel chitosan-g-poly(acrylic acid)/montmorillonite (CTS-g-PAA/MMT) superabsorbent nanocomposite was prepared via in situ intercalative polymerization in this study (designated as one-step method). In fact, in situ intercalative polymerization for the preparation of clay-polymer nanocomposites has drawn much attention because their preparation procedure is convenient and requires no pretreatment of the clay mineral.29 Properties, for example, water absorbency and pH sensitivity, of the superabsorbent nanocomposites were compared to those prepared according to the two-step method. Reaction mechanism, surface morphology, and factors influencing water absorbency of the nanocomposite were also investigated. 2. Experimental Section
* To whom correspondence should be addressed. Tel.: 86 931 4968118. Fax: 86 931 8277088. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Graduate University of the Chinese Academy of Sciences.
2.1. Materials. Acrylic acid (AA, distilled under reduced pressure before use), ammonium persulfate (APS, recrystallized from distilled water before use), and N,N′-methylenebisacryla-
10.1021/ie061385i CCC: $37.00 © 2007 American Chemical Society Published on Web 03/09/2007
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mide (MBA, used as received) were supplied by Shanghai Reagent Corp. (Shanghai, China). The degree of deacetylation and viscosity-average molecular weight of chitosan (CTS, Zhejiang Yuhuan Ocean Biology Co., China) are 85% and 9.0 × 105, respectively. Montmorillonite (MMT) with a cationexchange capacity of 102.8 mmol/100 g was purchased from the Longfeng Montmorillonite Co. (Shandong, China) and was milled through a 320-mesh screen before use. All other reagents used were of analytical grade, and all solutions were prepared with distilled water. 2.2. Preparation of CTS-g-PAA/MMT Superabsorbent Nanocomposites (One-Step Method). A series of CTS-g-PAA/ MMT nanocomposites from CTS, AA, and MMT were prepared according to the following procedure. An appropriate amount of CTS was dissolved in 30 mL of acetic acid solution (1%) in a 250 mL four-neck flask, equipped with a mechanical stirrer, a reflux condenser, a funnel, and a nitrogen line. After being purged with nitrogen for 30 min to remove the oxygen dissolved from the system, the solution was heated to 60 °C, and then 0.10 g of APS was introduced to initiate CTS to generate radicals. Ten minutes later, the mixed solution of 3.60 g of AA, 0.15 g of MBA, and a certain amount of MMT was added. The water bath was kept at 60 °C for 3 h. The resulting granular product was transferred into a sodium hydroxide aqueous solution (1 M) to be neutralized to pH ) 7, and then dehydrated with methanol. After excessive methanol was wiped off of the surface using filter paper, the samples were dried in an oven at 70 °C to a constant weight. The product was milled, and all of the samples used for the test had a particle size in the range of 40-80 mesh. The preparation procedure of CTS-g-PAA was similar to that of the superabsorbent nanocomposites except without the addition of MMT. 2.3. Preparation of PAA/CTS-Intercalated MMT Superabsorbent Nanocomposites (Two-Step Method). (1) Modification of MMT with CTS. 4.0 g of MMT was dispersed in 100 mL of distilled water. CTS solution was prepared by dissolving 0.132, 0.660, or 3.30 g of CTS in a 1% (v/v) acetic acid aqueous solution, and then the pH was adjusted to 4.90 using a 20 wt % NaOH aqueous solution. The CTS solution was slowly added to the MMT suspension followed by stirring at 60 °C for 6 h to obtain the CTS-intercalated MMT. The products were washed with distilled water until pH ) 7, and then dried at 60 °C for 12 h. CTS content in the CTS-intercalated MMT was determined by thermogravimetric analysis. (2) Polymerization with AA. The procedure of preparation of PAA/CTS-intercalated MMT was similar to that of the preparation of the CTS-g-PAA/MMT superabsorbent composite except that CTS had been intercalated into the layers of MMT and no dissolution in acetic acid solution is necessary. 2.4. Measurement of Water Absorbency. 0.05 g of sample was immersed in an excess of distilled water (500 mL) at room temperature for 8 h to reach the swelling equilibrium. Swollen samples were then separated from unabsorbed water by filtering through a 100-mesh screen under gravity for 30 min and no blotting of samples. Water absorbency in distilled water of the superabsorbent composite, Qeq, was calculated using the following equation:
Qeq )
m2 - m1 m1
Figure 1. XRD patterns of (a) MMT and CTS-intercalated MMT with weight ratios of CTS to MMT of (b) 3.02 wt %, (c) 11.04 wt %, and (d) 17.16 wt %, respectively.
Figure 2. XRD patterns of PAA/CTS-intercalated MMT nanocomposites (two-step method). Weight ratios of CTS to MMT for CTS-intercalated MMT are (a) 3.02 wt %, (b) 11.04 wt %, and (c) 17.16 wt %, respectively. MMT content in the feed is 10 wt %.
Water absorbencies in 0.9 wt % NaCl solution and in solutions of various pH values, Qeq′, were tested according to the same procedure. Buffer solutions with various pH values were made by combining the NaH2PO4, Na2HPO4, HCl solution, and NaOH solution properly. Ionic strengths of all of the buffer solutions were adjusted to 0.2 M by addition of NaCl solution.30 The above pH values were determined on a pH meter (DELTA320). 2.5. Characterization. IR spectra of samples were recorded on a Thermo Nicolet NEXUS TM spectrophotometer in KBr pellets. The micrographs of samples were obtained on a SEM (JSM-5600LV, JEOL, Ltd.). Before the SEM observation, all samples were fixed on aluminum stubs and coated with gold. Powder XRD analyses of the specimens were performed using an X-ray power diffractometer with Cu anode (PAN alytical Co. X’pert PRO), running at 40 kV and 30 mA, scanning from 4° to 18° at 3°/min. TEM observation was operated on a transmission electron microscope (JEOL 1200) at 80 kV. The sample preparation for the TEM is as follows. A small amount of the composite powder was dispersed in ethanol, and a drop of the dispersion was taken and put on a TEM grid. The drop on the grid was allowed to dry in the open atmosphere. 3. Results and Discussion
(1)
where m1 and m2 are the weights of the dry sample and the swollen sample, respectively. Qeq is calculated as grams of water per gram of sample.
3.1. XRD Analysis. Wang et al. prepared CTS-intercalated MMT nanocomposites and discovered that the flocculatedintercalated nanostructure was formed at high MMT content and the intercalate-exfoliated nanostructure was formed at low MMT content.18 Figures 1 and 2 show XRD patterns of CTS-
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Figure 5. SEM micrographs of (a) CTS-g-PAA and (b) CTS-g-PAA/MMT superabsorbent nanocomposite prepared using the one-step method. Weight ratio of AA to CTS is 7.2; MMT content in the feed is 10 wt %. Figure 3. XRD patterns of (a) MMT and CTS-g-PAA/MMT nanocomposites (one-step method) with weight ratios of CTS to MMT of (b) 3.02 wt %, (c) 11.04 wt %, and (d) 17.16 wt %, respectively. MMT content in the feed is 10 wt %.
Figure 6. TEM image of the CTS-g-PAA/MMT superabsorbent nanocomposite prepared using the one-step method.
Figure 4. IR spectra of MMT, CTS, CTS-g-PAA, and CTS-g-PAA/MMT prepared using the one-step method. Weight ratio of AA to CTS is 7.2; MMT content in the feed is 10 wt %.
intercalated MMT and corresponding PAA/CTS-intercalated MMT nanocomposites, respectively. As can be seen from Figure 1, the typical diffraction peak of MMT shifted from 6.94° (basal spacing 12.74 Å) to 5.84° (basal spacing 15.13 Å) with increasing CTS content to 11.04 wt % of MMT. This typical diffraction peak disappeared with further increasing CTS content to 17.16 wt % of MMT. Information in Figure 1 indicates that CTS intercalated into layers of MMT and formed an intercalateexfoliated nanostructure or a flocculated-intercalated nanostructure. However, after polymerization of these CTS-intercalated MMTs with AA, this typical diffraction peak of MMT cannot be detected no matter how much CTS-intercalated MMT was introduced as shown in Figure 2, which indicates the exfoliation of MMT. For the CTS-g-PAA/MMT nanocomposites prepared according to the one-step method, the typical diffraction peak of MMT (6.94°) disappeared when MMT was incorporated as shown in Figure 3, which indicates the exfoliation of MMT in the superabsorbent nanocomposite. In acidic solutions, CTS shows an extended structure that may facilitate its intercalation into layers of MMT.31 In addition, -NH2 of CTS was protonated in acidic solution, and then the extended and protonated CTS could intercalate into layers of MMT through a cation-exchange process or be adsorbed in the acetate salt form as has been reported by Darder.19 Consequently, CTS could first intercalate into layers of MMT, and then formed the CTS-g-PAA/MMT nanocomposite through in situ graft-polymerization with AA.
No obvious difference can be seen when comparing Figure 3 with Figure 2. This means that MMT could be exfoliated and the nanocomposites could be acquired by both the method reported by Qiu et al.20 and that in this study. According to the two-step method, MMT must be modified with CTS first, and then polymerized with AA to form the PAA/CTS-intercalated MMT nanocomposite. However, the method reported in this study is more convenient in that MMT need not be modified first by CTS. 3.2. IR Spectra Analysis. IR spectra of MMT, CTS, CTSg-PAA, and CTS-g-PAA/MMT were shown in Figure 4. In the spectrum of CTS, the absorption bands at 1648, 1587, 1383, 1091, and 1030 cm-1 are ascribed to CdO of amide I, N-H, amide III, C3-OH, and C6-OH of CTS, respectively. However, the absorption bands at 1587, 1383, and 1091 cm-1 disappeared after the reaction with AA as shown in spectra of CTS-g-PAA and CTS-g-PAA/MMT, which reveals that both -NH2 and -OH groups of CTS took part in graftpolymerization with AA. The new absorption bands at 1456 cm-1 (C-H), 1405 cm-1 (symmetric -COO- stretching), and 1074 cm-1 indicate the existence of PAA chains. As can be seen from the spectrum of CTS-g-PAA, the absorption band at 1648 cm-1 (CdO of amide I) was overlapped by CdO of -COOH and asymmetric -COO- stretching and resulted in a broad absorption band in the range of 1557-1696 cm-1. A similar result has been reported by Mahdavinia et al. in the chitosan-g-poly(acrylic acid-co-acrylamide) system.9 Absorption bands at 3696, 3622, and 3422 cm-1 ascribed to -OH of MMT disappeared in the spectrum of CTS-g-PAA/ MMT as compared to that of MMT. Also, the absorption band at 1039 cm-1 ascribed to Si-OH of MMT shifted to 1026 cm-1, and the intensity decreased. These results indicate the participation of -OH group of MMT in the formation of the
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Figure 7. Schematic representation of the CTS-g-PAA/MMT superabsorbent nanocomposite via in situ polymerization. Table 1. Variation of Water Absorbency for the Nanocomposites Prepared via the Two Different Methods with Weight Ratio of CTS to MMTa CTS/MMT (%) method one-step two-step a
Qeq (g g-1) Qeq (g g-1)
3.02
11.04
17.16
332.3 283.7
340.5 224.3
223.1 193.1
MMT content in the feed is 10 wt %.
nanocomposite. After MMT was incorporated into the polymeric network, absorption bands of -COOH and -COO- shifted from 1696 and 1557 cm-1 to 1717 and 1565 cm-1, respectively, which indicates that the chemical environment of -COOH and -COO- has changed, and this may have some influence on the absorbing ability of the corresponding superabsorbent nanocomposite. It can be concluded from Figure 4 that graft-polymerization has taken place among CTS, AA, and MMT. 3.3. Morphology Analysis. SEM micrographs of CTS-g-PAA and CTS-g-PAA/MMT superabsorbent nanocomposite were observed and shown in Figure 5. Obviously, surface morphology of the CTS-g-PAA/MMT nanocomposite is different from that of CTS-g-PAA. CTS-g-PAA has a tight surface; however, the introduced MMT seems to have destroyed the tight surface and generated many pleats. The surface of the CTS-g-PAA/MMT nanocomposite seems to be more loose and porous as compared to that of CTS-g-PAA. This surface is convenient for the penetration of water into the polymeric network, and then may be of benefit to water absorbency of corresponding superabsorbent nanocomposites. Figure 6 shows a TEM image of the CTS-g-PAA/MMT nanocomposite prepared by the one-step method. As can be seen, MMT dispersed homogeneously in the nanocomposite, and no aggregation of MMT can be seen, although the MMT content in the nanocomposite is as much as 10 wt %. According to the results from Figures 3-6, Figure 7 showed formation of the CTS-g-PAA/MMT superabsorbent nanocomposite schematically. A part of protonated CTS intercalated into layers of MMT, and then free radicals on CTS were generated under the existence of initiator. These radicals would initiate
Figure 8. pH-responsive behaviors of the nanocomposites prepared with the two methods in a series of phosphate buffer solutions. MMT content in the feed is 10 wt %; weight ratio of CTS to MMT is 0.17.
the polymerization of AA and MBA, and then formed the superabsorbent nanocomposite polymeric network. 3.4. Water Absorbency of CTS-g-PAA/MMT Superabsorbent Nanocomposites. (1) Comparison between the OneStep Method and the Two-Step Method. Absorbing ability and pH-responsivity of the nanocomposites prepared with the two different methods were compared in this section. Table 1 shows the water absorbency in distilled water for the nanocomposites prepared using the two different methods. The amount of CTS in the nanocomposites prepared using the two-step method is small (weight ratio of CTS to MMT is 17.16 wt % at most), for only a small amount of CTS could be intercalated into layers of MMT; however, the weight ratio of CTS to MMT could be as much as 235 wt % for the one-step method, which could enhance biodegradability of corresponding nanocomposites. To compare the absorbing ability of the nanocomposites prepared using the two methods, the same amount of CTS was introduced. As can be seen from Table 1, water absorbency for the nanocomposites prepared using the one-step method is always higher than that of the two-step method. For nanocomposites prepared using the two-step method, CTS intercalated into the layer of MMT, which may restrict graft-polymerization of CTS with AA, and then decrease the water absorbency. In
Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007 2501 Table 2. Variation of Water Absorbency in Distilled Water and in 0.9 wt % NaCl Solution for the CTS-g-PAA/MMT Superabsorbent Nanocomposite Prepared Using the One-Step Method with MMT Contenta MMT content/wt % 0
2
5
10
20
30
g-1)
Qeq/(g 150.3 160.1 153.2 148.7 129.5 116.1 Qeq′(0.9 wt % NaCl)/(g g-1) 43.4 46.6 42.4 39.8 34.8 29.2 a
Figure 9. Variation of water absorbency in distilled water and in 0.9 wt % NaCl solution for the CTS-g-PAA/MMT superabsorbent nanocomposites prepared using the one-step method with weight ratio of AA to CTS. MMT content in the feed is 10 wt %.
the case of nanocomposites prepared using the one-step method, only a part of CTS intercalated into layers of MMT, which did not influence graft-polymerization between CTS and AA, and then has higher water absorbency. Figure 8 shows the pH swelling curves of the nanocomposites prepared with the two methods. As can be seen, both of the nanocomposites swelled only a little when the pH was lower than 2.72. With increasing pH to 5.02, the water absorbency increased abruptly to 46.4 and 43.8 g g-1 for nanocomposites prepared according to the one-step method and two-step method, respectively. The water absorbency increased slowly with continuously increasing pH to 12.06. It is the repulsion among hydrophilic groups, for example, -COO-, that expanded the superabsorbent polymeric network; however, most -COOgroups on the nanocomposites polymeric network were protonated in acidic media, and then led to very small water absorbency. With increasing pH of external solution, -COOgroups are ionized gradually, which enhance electrostatic repulsion, and then enhance swelling capacity. A similar result has been reported by Zhao30 for hydrogels prepared with acrylic acid, 2-hydroxyethyl methacrylate, and ethylene glycol dimethacrylate. A characteristic of these curves indicates pH-responsive behavior of the nanocomposites. In addition, nanocomposite prepared by the one-step method showed higher pH-responsivity as compared to the nanocomposite prepared by the two-step method. Data in Figure 8 were fitted to sigmoidal to reveal transition points of water absorbency for the nanocomposites. The transition points for the nanocomposites prepared with the one-step method (pH ) 3.16) and two-step method (pH ) 3.22) are very similar, which indicates that the preparation method has no obvious influence on the phase transition point of the nanocomposites. (2) Effect of Weight Ratio of AA to CTS on Water Absorbency. The effect of weight ratio of AA to CTS on water absorbency of the CTS-g-PAA/MMT superabsorbent nanocomposite was shown in Figure 9. Apparently, both water absorbency in distilled water and that in 0.9 wt % NaCl solution increase continuously with increasing weight ratio of AA to CTS. The increasing water absorbency with increasing weight ratio is attributed to the following facts. As the weight ratio of AA to CTS increases, more AA molecules could be available in the vicinity of the chain propagating sites of CTS macroradicals, which enhances the hydrophilicity of the corresponding superabsorbent nanocomposite, and then the water absorbency is improved. A similar result has been reported by Pourjavadi et al. in the κ-carrageenan-g-(2-acrylamido-2-methylpropanesulfonic) hydrogel.32 In addition, more Na+ ions are generated
Weight ratio of AA to CTS is 7.2.
in the polymeric network because of the neutralization of grafted PAA in the nanocomposite. Consequently, the osmotic pressure difference between polymeric network and external solution increased. As is known, higher osmotic pressure is of benefit to water absorbency. Moreover, molecular weight of grafted PAA chains increases, which also contributes to the improvement of water absorbency. (3) Effect of MMT Content on Water Absorbency. The influence of MMT content on water absorbency of the CTSg-PAA/MMT superabsorbent nanocomposite was shown in Table 2. It is evident that MMT content is an important factor influencing water absorbency of the superabsorbent nanocomposite. Water absorbency of the superabsorbent nanocomposite in distilled water and in 0.9 wt % NaCl solution was increased from 150.3 and 43.4 g g-1 to 160.1 and 46.6 g g-1, respectively, as 2 wt % MMT was introduced. According to the IR analysis, the -OH of MMT participated in the formation of the nanocomposite, which may improve the polymeric network, and then enhance the water absorbency. In addition, MMT contains a lot of cations (such as Na+), and they are easily ionized and dispersed into the superabsorbent nanocomposite polymeric network, which enhances the hydrophilicity of the nanocomposite and makes it swell more. Further increasing MMT content to 30 wt % resulted in a sharp decrease of water absorbency to 116.1 and 29.2 g g-1 in distilled water and in 0.9 wt % NaCl solution, respectively. The decreasing tendency of water absorbency with increasing MMT content may be attributed to the following facts. The interaction among MMT, CTS, and AA became intensive gradually with increasing MMT content. Consequently, more chemical and physical cross-linkages were formed in the polymeric network, and then the elasticity of the polymer chains decreases, which decreased the water absorbency of the superabsorbent nanocomposite. Additionally, the hydrophilicity of MMT was lower than that of AA. The hydrophilicity of the nanocomposite is lower with a higher MMT content, which also restricts its swelling. 4. Conclusions A convenient method for the preparation of CTS-g-PAA/ MMT superabsorbent nanocomposite was reported in this study. CTS could intercalate into layers of MMT and resulted in exfoliation of MMT during the in situ intercalative polymerization process, and then formed the nanocomposites. -OH and -NH2 of CTS and -OH of MMT participated in graftpolymerization with AA. Both methods could be used to prepare the nanocomposite. As compared to the two-step method, the one-step method is more convenient and time-saving, and the corresponding nanocomposites have higher swelling ability and pH-responsivity. In addition, more CTS could be incorporated into the nanocomposite according to the one-step method. The introduced MMT could generate a loose and porous surface that is of benefit to swelling ability of the nanocomposite. The MMT content and weight ratio of AA to CTS have great influence on water absorbency of the nanocomposite. Introducing a small
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amount of MMT could improve the swelling ability of the CTSg-PAA superabsorbent. Acknowledgment This work was financially supported by the Western Action Project of CAS (No. KGCX2-YW-501) and by the “863” Project of the Ministry of Science and Technology, People’s Republic of China (No. 2006AA03Z0454). Literature Cited (1) Buchholz, F. L.; Graham, T. Modern Superabsorbent Polymer Technology; John Wiley and Sons: New York, 1998. (2) Dorkoosh, F. A.; Brussee, J.; Verhoef, J. C.; Borchard, G.; Tehrani, M. R.; Junginger, H. E. Preparation and NMR characterization of superporous hydrogels (SPH) and SPH composites. Polymer 2000, 41, 8213. (3) Ende, M.; Hariharan, D.; Peppas, N. A. Factors influencing drug and protein transport and release from ionic hydrogels. React. Polym. 1995, 25, 127. (4) Raju, K. M.; Raju, M. P.; Mohan, Y. M. Synthesis of superabsorbent copolymers as water manageable materials. Polym. Int. 2003, 52, 768. (5) Shiga, T.; Hirose, Y.; Okada, A.; Kurauchi, T. Bending of poly(vinyl alcohol)-poly(sodium acrylate) composite hydrogel in electric fields. J. Appl. Polym. Sci. 1992, 44, 249. (6) Lenzi, F.; Sannino, A.; Borriello, A.; Porro, F.; Capitani, D.; Mensitieri, G. Probing the degree of crosslinking of a cellulose based superabsorbing hydrogel through traditional and NMR techniques. Polymer 2003, 44, 1577. (7) Kiatkamjornwong, S.; Mongkolsawat, K.; Sonsuk, M. Synthesis and property characterization of cassava starch grafted poly[acrylamide-co(maleic acid)] superabsorbent via γ-irradiation. Polymer 2002, 43, 3915. (8) Farag, S.; Al-Afaleq, E. I. Preparation and characterization of saponified delignified cellulose polyacrylonitrile-graft copolymer. Carbohydr. Polym. 2002, 48, 1. (9) Mahdavinia, G. R.; Pourjavadi, A.; Hosseinzadeh, H.; Zohuriaan, M. J. Modified chitosan 4. Superabsorbent hydrogels from poly(acrylic acidco-acrylamide) grafted chitosan with salt- and pH-responsiveness properties. Eur. Polym. J. 2004, 40, 1399. (10) No, H. K.; Park, N. Y.; Lee, S. H.; Hwang, H. J.; Meyers, S. P. Antibacterial activities of chitosan and chitosan oligomers with different molecular weights on spoilage bacteria isolated from tofu. J. Food Sci. 2002, 67, 1511. (11) Dutkiewicz, J. K. Superabsorbent materials from shellfish waste-a review. J. Biomed. Mater. Res. 2002, 63, 373. (12) Kabiri, K.; Zohuriaan-Mehr, M. J. Porous superabsorbent hydrogel composites:. synthesis, morphology and swelling rate. Macromol. Mater. Eng. 2004, 289, 653. (13) Wu, J. H.; Wei, Y. L.; Lin, S. B. Study on starch-graft-acrylamide/ mineral powder superabsorbent composite. Polymer 2003, 44, 6513. (14) Lin, J. M.; Wu, J. H.; Yang, Z.; Pu, M. Synthesis and properties of poly(acrylic acid)/mica. superabsorbent nanocomposite. Macromol. Rapid Commun. 2001, 22, 422. (15) Lee, W. F.; Chen, Y. C. Effect of intercalated reactive mica on water absorbency for poly(sodium acrylate) composite superabsorbents. Eur. Polym. J. 2005, 41, 1605.
(16) Li, A.; Wang, A. Q. Synthesis and properties of clay-based superabsorbent composite. Eur. Polym. J. 2005, 41, 1630. (17) Wu, J. H.; Lin, J. M.; Zhou, M.; Wei, C. Synthesis and properties of starch-graft-polyacrylamide/clay superabsorbent composite. Macromol. Rapid Commun. 2000, 21, 1032. (18) Wang, S. F.; Shen, L.; Tong, Y. J.; Chen, L.; Phang, I. Y.; Lim, P. Q.; Liu, T. X. Biopolymer chitosan/montmorillonite nanocomposites: Preparation and characterization. Polym. Degrad. Stab. 2005, 90, 123. (19) Margarita, D. Biopolymer-clay nanocomposites based on chitosan intercalated in montmorrilonite. Chem. Mater. 2003, 15, 3774. (20) Qiu, H. X.; Yu, J. G.; Zhu, J. L. Polyacrylate/(chitosan modified montmorillonite) nanocomposite: water absorption and photostability. Polym. Polym. Compos. 2005, 13, 167. (21) Li, A.; Liu, R. F.; Wang, A. Q. Preparation of starch-graft-poly(acrylamide)/attapulgite superabsorbent composite. J. Appl. Polym. Sci. 2005, 98, 1351. (22) Li, A.; Wang, A. Q.; Chen, J. M. Studies on poly(acrylic acid)/ attapulgite superabsorbent composite. I. Synthesis and characterization. J. Appl. Polym. Sci. 2004, 92, 1596. (23) Zhang, J. P.; Chen, H.; Wang, A. Q. Study on superabsorbent composite. III. Swelling behaviors of polyacrylamide/attapulgite composite based on acidified attapulgite and organo-attapulgite. Eur. Polym. J. 2005, 41, 2434. (24) Zhang, J. P.; Chen, H.; Wang, A. Q. Study on superabsorbent composite. IV. Effects of organification degree of attapulgite on swelling behaviors of polyacrylamide/organo-attapulgite composites. Eur. Polym. J. 2006, 42, 101. (25) Zhang, J. P.; Li, A.; Wang, A. Q. Study on superabsorbent composite. VI. Preparation, characterization and swelling behaviors of starch phosphate-graft-acrylamide/attapulgite superabsorbent composite. Carbohydr. Polym. 2006, 65, 150. (26) Zhang, J. P.; Li, A.; Wang, A. Q. Synthesis and characterization of multifunctional poly(acrylic acid-co-acrylamide)/sodium humate superabsorbent composite. React. Funct. Polym. 2006, 66, 747. (27) Sun, S. L.; Wang, A. Q. Adsorption properties of carboxymethylchitosan and cross-linked carboxymethyl-chitosan resin with Cu(II) as template. Sep. Purif. Technol. 2006, 49, 197. (28) Sun, S. L.; Wang, A. Q. Adsorption kinetics of Cu(II) ions using N,O-carboxymethyl-chitosan. J. Hazard. Mater. 2006, 131, 103. (29) Stackhouse, S.; Coveney, P. V.; Sandre´, E. Plane-wave density functional theoretic study of formation of clay-polymer nanocomposite materials by self-catalyzed in situ intercalative polymerization. J. Am. Chem. Soc. 2001, 123, 11764. (30) Zhao, B.; Moore, J. S. Fast pH- and ionic strength-responsive hydrogels in microchannels. Langmuir 2001, 17, 4758. (31) Pan, J. R.; Huang, C.; Chen, S.; Chung, Y. C. Evaluation of a modified chitosan biopolymer for coagulation of colloidal particles. Colloids Surf., A 1999, 147, 359. (32) Pourjavadi, A.; Hosseinzadeh, H.; Mazidi, R. Modified carrageenan. 4. Synthesis and swelling behavior of crosslinked κC-g-AMPS superabsorbent hydrogel with antisalt and pH-responsiveness properties. J. Appl. Polym. Sci. 2005, 98, 255.
ReceiVed for reView October 27, 2006 ReVised manuscript receiVed December 8, 2006 Accepted January 15, 2007 IE061385I
