Synthesis and swelling properties of silk sericin-gpoly(acrylic acid/attapulgite) composite superabsorbent

Transcription

1 DOI /s y ORIGINAL PAPER Synthesis and swelling properties of silk sericin-gpoly(acrylic acid/attapulgite) composite superabsorbent Xiaobin Hu Yuehua Deng Received: 21 June 2014 / Revised: 11 October 2014 / Accepted: 26 November 2014 Ó Springer-Verlag Berlin Heidelberg 2014 Abstract A composite superabsorbent silk sericin-g-poly(acrylic acid/attapulgite) was synthesized by the aqueous solution copolymerization method with potassium persulfate sodium sulfite as the redox initiation system and N,N 0 -methylenebisacrylamide as the crosslinker. The structure of the attapulgite, silk sericin and the product was characterized via Fourier transform infrared absorption spectroscopy, scanning electron microscopy, and transmission electron microscopy. The influences of reaction variables on the swelling properties of the composite superabsorbent were studied and optimized. The addition of the natural silk sericin improved the swelling properties of the acrylic acid/attapulgite bipolymer. The maximum swelling capacities of the composite superabsorbent silk sericin-gpoly(acrylic acid/attapulgite) in pure water and in 0.9 % NaCl aqueous solution were 1,236 and 108 g/g, respectively. The optimal conditions were initiator, 0.6 %; acrylic acid, 2.5 M; crosslinker, 0.1 %; silk sericin, 10 %; attapulgite, 12 %; reaction temperature, 65 C; and time, 4 h. Keywords Composite superabsorbent Acrylic acid Silk sericin Attapulgite Introduction Superabsorbent polymers (SAPs) have great advantages over traditional waterabsorbing materials and are widely used in many fields, such as agriculture [1], X. Hu (&) School of Life Science, Huzhou Teachers College, Huzhou , People s Republic of China xiaobinhu001@163.com Y. Deng State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing , People s Republic of China

2 horticulture [2], hygienic products [3], chemical engineering [4], pharmaceuticals [5, 6], food industry [7], water treatment [8], and biomaterials [9]. SAPs are slightly crosslinked hydrophilic polymers that can absorb and retain pure water at amounts that are a thousand times relative to their own mass [10]. Although, the hydrogels made from artificial monomers, such as acrylic acid, acrylamide and their analogs possess excellent water-absorbing properties; their toxicity and non-biodegradability might cause longtime environmental problems and limit their widespread use [11, 12]. On the other hand, the water absorptivity of SAPs is usually very low in salt solutions (i.e., 0.9 % NaCl) compared with their absorptivity in pure water [13]. However, the practical applied circumstances usually contain salt. Therefore, during the last decade, attention has been focused on synthesizing salttolerating, water-absorbent gels by changing the monomer constitution and structure of SAPs. One of the important attempts used is the preparation of organic inorganic superabsorbent composites [14]. At the same time, many attempts have been focused on grafting or blending of plastic materials with cheap and biodegradable natural biopolymers, such as starch [15], protein [16, 17], chitosan [18], cellulose [19], dextrin [20], and agar [21] to produce natural-based SAP hydrogels which are degradable partly or entirely in natural environment in a relatively short time. Attapulgite (ATP) is a magnesium aluminum phyllosilicate with a possible formula of (Mg, Al, and Fe) 5 Si 8 O 20 (OH) 2 (OH 2 ) 4 4(H 2 O) and forms gel structures in fresh and saltwater by establishing a lattice structure of particles that are connected via hydrogen bonds. ATP particles can be considered as charged particles with zones of positive and negative charges [22, 23]. The bonding of these alternating charges allows the particles to form gel suspensions in salt and fresh water. China has abundant reserves of high-quality ATP clay [24]. Hu et al. [25] reported the synthesis of poly(acrylic acid/attapulgite) superabsorbent via aqueous solution polymerization. Wang et al. [26] prepared superabsorbent polyacrylamide/attapulgite via free radical polymerization. Liu et al. [27] used attapulgite to prepare sodium carboxymethyl cellulose-g-poly(acrylic acid/ attapulgite) superabsorbent nanocomposites via solution polymerization. The incorporation of clay has become a preferred method for the preparation of superabsorbent composites because this method is low cost and significantly improves the comprehensive water-absorbing properties of superabsorbent materials. Silk sericin (SS) enveloped the fibroin is a second type of silk protein, which made up % of total cocoon weight, contains 18 amino acids. The total amount of hydroxy amino acids in sericin is more than 40 % [28]. SS is a biocompatible and biodegradable material owing to its proteinous nature and susceptibility to the action of proteolytic enzymes. So far, most of SS dissolved in wastewater from silk filature factory was still discharged to environment and resulted in pollution [29]. Few studies have been reported about SS-based hydrogels [30 32]. SS protein is environmentally friendly and easy-obtained natural material. It can be copolymerized or crosslinked, and blended with other macromolecular materials, especially artificial polymers, to produce materials with improved properties [33].

3 So far, there are few reports about the composite hydrogel whose constitution contains both natural SS and ATP. In this paper, a composite superabsorbent SS-gpoly(AA/ATP) was synthesized in the presence of a crosslinking agent and an initiator via aqueous solution copolymerization. The effect of reaction variables on the water absorbency and swelling behavior of the hydrogel was investigated. The novel hydrogel SS-g-poly(AA/ATP) displayed better water adsorptivity than poly(aa/atp). Materials and methods Chemicals and reagents Acrylic acid (AA) and the crosslinker N,N 0 -methylenebisacrylamide (MBA) were purchased from Shanghai RichJoint Chemical Reagent Corporation Limited. SS powder was purchased from Huzhou Xintiansi bio-tech Corporation Limited and used as received. AA was distilled before use. Potassium persulfate (KPS), sodium bisulfite (NaHSO 3 ), and sodium hydroxide (NaOH; all from Nanjing Chemical Reagent Factory and analytical grade) were used as received. Attapulgite was purchased from Xuyi County, Jiangsu Province. The ATP powder was milled through a 320-mesh screen followed by treatment with 37 % HCl for 48 h and washed with distilled water until a ph of 7 was reached. The powder was then dried at 105 C for 8 h before use. All solutions were prepared using distilled water. Synthesis of superabsorbent hydrogel The hydrogel in this study was prepared as shown in Scheme 1. A typical procedure for synthesizing the superabsorbent composite in water is described as follows: AA (4.08 g, 56.6 mmol; titrated by 10 % NaOH to a neutralization percentage of 75 %), SS (0.41 g), ATP (0.33 g), and crosslinker MBA (0.004 g) solution were poured into a 100-mL three-neck flask equipped with a mechanical stirrer and placed in a water bath with digital display and at a stable temperature. A certain amount of deionized water was then added into the flask. The mixture was stirred until a completely uniform suspension was obtained. Before the polymerization reaction, the solution was bubbled with pure nitrogen for 15 min to remove the dissolved oxygen. KPS (0.025 g) and the NaHSO 3 (0.01 g) were added to the mixture to start the polymeric reaction while keep stirring and stable N 2 flow introducing during the whole reaction period. The temperature of the mixture was increased rapidly and controlled to a setting value (65 C) within a few minutes. After 4 h of reaction, the product was cut into small pieces and then immersed in 100 ml dried ethanol for 2 h for dewatering. The ethanol was subsequently decanted, and another 100 ml of fresh ethanol was added. The particles were allowed to stand for 4 h in ethanol to be dewatered. After that, the product was dried in a vacuum oven at 60 C for 10 h, and the small particles were then ground into powder using an agate mortar. The powdered superabsorbent hydrogel was stored in a desiccator. The acrylic acid/

4 Scheme 1 Proposed pathway for the synthesis of SS-g-poly(AA/ATP) attapulgite (AA/ATP) bipolymer was prepared by the similar process except the addition of SS. SEM/EDS and TEM analysis The surface morphologies of the gel, SS, and the treated ATP were examined via scanning electron microscopy (SEM). The dried superabsorbent powder was coated with a thin layer of palladium gold alloy and imaged using an SEM instrument (HITACHI S-3400 N, 30 kv). Information about the elemental composition of the

5 ATP was obtained via energy dispersive spectroscopy (EDS). Transmission electron microscope images were obtained using JEM-2100 TEM. Fourier transform infrared spectroscopy analysis The infrared spectroscopy (IR) spectra of ATP, SS, and the composite superabsorbent SS/ATP/AA were obtained using a Nexus 870 Fourier transform infrared spectroscopy (FTIR) spectrophotometer (KBr disk). The infrared spectra were obtained in the region of 4, cm -1. Swelling measurements A weighted quantity of dry sample (50 mg) was introduced into a 500 ml beaker. About 400 ml of distilled water or saline solution was added to the beaker. The fully swollen hydrogel was not separated from the unabsorbed water until absorption equilibrium was reached and excess water was drained using a filter paper for 5 min. The hydrogel was then weighted. The relative water absorbency was calculated using q ¼ðm 2 m 1 Þ=m 1 ; ð1þ where m 1 and m 2 are the weights of the dry sample and the swollen sample, respectively. q was calculated as grams of water per gram of sample. The absorbency of the hydrogel was evaluated in tap water, 0.9 % NaCl, and 0.15 M MgCl 2 solutions based on the method for swelling measurement in distilled water. Swelling kinetics For studying the rate of absorption of the hydrogel, 50 mg of the sample with average particle sizes between 40 and 60 mesh was poured into a weighed tea bag and immersed in 400 ml of distilled water. At consecutive time intervals, the water absorbency of the hydrogel was measured based on the abovementioned method. Results and discussion The mechanism for graft copolymerization is shown in Scheme 1. First, the redox reaction of KPS and NaHSO 3 produced sulfate anion-radicals. The anion-radicals then attacked the double bond of the monomers AA to form the corresponding radicals. When the chain growth initiated by free radicals was increased, the microradicals formed from the monomers attacked the ATP chain to produce the ATP-co- AA chain. On the other hand, sulfate anion-radicals abstracted hydrogen from the OH groups of the ATP chains or from one of the functional groups (i.e., COOH, SH, OH, and NH 2 ) inside the chains of the SS to form the corresponding radicals, respectively. These radicals initiated partially neutralized AA grafting onto

6 inorganic ATP chains or SS polypeptide chains and resulted in the formation of a composite polymer. Similarly, a crosslinking reaction occurred in the presence of a crosslinker (MBA), and a three-dimensional network contained inorganic ATP chains, polypeptide chains and poly(acrylic acid) was obtained. Surface morphology of the hydrogel The information about the elemental composition of ATP provided by EDS (Fig. 1a) showed that ATP is a silicate mineral containing relatively large amounts of aluminum, iron, and magnesium. The TEM image (Fig. 1b, c) showed that the HCl-treated ATP was a claviform microstructure crystal. The ATP particles are composed of randomly packed short fibers with diameters of about nm and lengths of 200 1,000 nm. The SEM photograph of the hydrogel showed a porous structure (Fig. 1e). A porous structure usually favors swelling performance of the superabsorbent. Fig. 1 a SEM image and EDS of ATP; b and c TEM images of ATP; d SEM image of SS; and e SEM image of the composite superabsorbent SS-g-poly(AA/ATP)

7 FTIR analysis Infrared spectroscopy was conducted to confirm the chemical structure of the hydrogel. Figure 2 shows the FTIR spectra of poly (AA/ATP), SS-g-poly(AA/ATP), PAA, SS and ATP. The strong absorption bands observed about 1,571 cm -1 can be attributed to the carboxylate ( COO - ) groups stretching and the N H bending vibration in the polymers. The strong absorption bands observed between 1,656 and 1,540 cm -1 can be attributed to carboxylate ( COO - ) groups stretching in the hydrogels. The characteristic absorption bands at 1,656 and 1,540 cm -1 can be attributed to the overlapping of carboxamide absorption bands with symmetric and asymmetric stretching modes of carboxylate anions ( COO - ) in the SS. The broad bands between about 3,200 and 3,621 cm -1 are due to the stretching of O H and N H bonds. Absorption bands between 2,933 and 2,975 cm -1 correspond to the C H stretching vibration. The bands observed at 1,041, 779, and 469 cm -1 can be attributed to Si O Si asymmetric stretching, symmetric stretching, and bending vibration, respectively. The bands at 1,640 cm -1 in Fig. 2e correspond to the H O H bending vibration in ATP. The shape and position of the signal of Si O Si in Fig. 2b underwent an obvious change compared with that in Fig. 2e. This change may be explained by the chemical reaction between part ATP and the monomers. Although the stretching band of carboxylate ( COO - ) functional groups overlapped with the C=O stretching band and the N H bending band of the SS portion of the copolymer, the change of the signal shape of carboxylate ( COO - ) groups around 1,571 cm -1 (in Fig. 2b) is probably due to the presence of SS-g-poly(AA/ATP) polymer. Fig. 2 FTIR spectra of a poly (AA/ATP); b SS-g-poly(AA/ATP); c PAA; d SS; and e ATP

8 Fig. 3 Effect of initiator concentration on the swelling capacity of SS-g-poly(AA/ ATP). Reaction conditions: AA (75 % neutralization), 2.5 M; ATP, 12 %; SS, 10 %; and MBA, 0.1 % at 65 C, 4 h Effect of initiator concentration The effect of initiator concentration (0.4 to 0.9 %) on the swelling capacity of the hydrogel was investigated in this series of experiments (Fig. 3). A relatively low concentration of the initiator cannot initiate part of the monomers to take part in polyreaction, which results in an incomplete polymerization. Thus, the insufficient crosslinking density and strength of monomers resulted in a relatively low swelling capacity. The swelling capacity increased with increasing initiator concentration before reaching the optimum condition. The optimized initiator amount with the highest water absorbency is about 0.6 %. The reduction in the swelling capacity of hydrogel when a higher concentration of initiator was used may be attributed to excess short-chain polymers, which resulted in an undeveloped network in the polymer because of excess free radicals and an increase in chain termination reactions caused by the collision among free radicals. Effect of crosslinker concentration The effect of crosslinker MBA concentration on the water absorbency of the synthesized superabsorbent composite is shown in Fig. 4. The maximum swelling capacity of hydrogels was achieved when the amount of MBA was 0.1 %. At a lower concentration of MBA, lesser crosslinking resulted in an insufficient threedimensional network in the polymer. Thus, the loose network connected via hydrogen bond cannot hold water effectively. Moreover, a low crosslinker concentration is brought about by a relatively large amount of water-soluble polymers. By contrast, a high crosslinker concentration can lead to a high crosslinking degree. To a certain extent, the swelling capacity increases with a high crosslinking degree. But the polymer particles cannot expand adequately as water moves into the network, and the free space of the network unit in the polymer is significantly decreased if the crosslinking was excessively tight. Consequently, after

9 Fig. 4 Effect of crosslinker concentration on the swelling capacity of SS-g-poly(AA/ ATP). Reaction conditions: AA (75 % neutralization), 2.5 M; ATP, 12 %; SS, 10 %; and initiator, 0.6 % at 65 C, 4 h Fig. 5 Effect of the concentration of AA on the swelling capacity of SS-gpoly(AA/ATP). Reaction conditions: ATP, 12 %; SS, 10 %; MBA, 0.1 %; and initiator, 0.6 % at 65 C, 4 h reaching a maximum value, the swelling capacity decreased. Similar phenomena have been reported by other researchers [34]. Effect of monomer concentration The effect of monomer concentration on the water absorptivity of the superabsorbent composite was investigated by changing the total concentration of the monomer from 1.0 to 4.0 M while keeping the ATP amount 12 % and the SS 10 % unchanged. Figure 5 shows that the water absorbent capacity increases first and then decreases with increasing monomer concentration. The initial increase in water absorbency can be attributed to a higher monomer concentration, which improved the effective crosslinking density of the hydrogel. A higher number of effective three-dimensional structural units and hydrophilic groups ( COO -, OH, and NH 2 ) results in a higher water holding capacity, which indicates that the swelling capability of hydrogel increases. The subsequent decrease after the peak point may

10 be attributed to an excessively high concentration of monomer, which leads to rapid polymerization and excessively high crosslinking density. Thus, the free space of the network unit in the polymer was significantly decreased. Moreover, a rapid increase in the viscosity of the reaction medium may hinder the movement of macroradicals and ATP dispersion, which may not be favorable for copolymerization and also lower the swelling capacity of the superabsorbent composite [35]. Effect of ATP amount The effect of ATP amount in the composite hydrogel on water absorbency was studied. As shown in Fig. 6, when the weight percentage was increased up to 12 %, the water absorbent capacity increased and decreased with further increase in ATP amount. The initial increase in water absorbency can be attributed to a higher osmotic pressure difference between the concentration inside and outside of the hydrogel, which is caused by the presence of Mg 2?,Na?, and K? in the ATP of the hydrogel. The subsequent decrease after maximum absorbency may be attributed to a relatively high percentage of ATP, which resulted in an obvious decrease in AA that contain COO - hydrophilic groups in the composite hydrogel. Moreover, ATP is insoluble in water and is dispersed by nano-aggregation. Thus, too much ATP hinders effective polyreaction and increases the useless weight of the superabsorbent composite, thereby decreasing the swelling capacity of the superabsorbent. Influence of reaction temperature The reaction temperature is an important factor that affects the swelling capacity of the synthesized hydrogels. The water absorbency initially increased when the temperature was increased to 65 C but then subsequently decreased, as shown in Fig. 7. A low temperature slowed down the polyreaction rate and prolonged the reaction process, whereas a high temperature accelerated the dissociation of KPS. Thus, a higher temperature favors the increasing rate of formation of radical centers, Fig. 6 Effect of ATP amount on swelling capacity of SS-gpoly(AA/ATP). Reaction conditions: AA (75 % neutralization), 2.5 M; SS, 10 %; MBA, 0.1 %; and initiator, 0.6 %; 65 C; 4 h

11 Fig. 7 Effect of the reaction temperature on the water absorbency of SS-g-poly(AA/ ATP). Reaction conditions: AA (75 % neutralization), 2.5 M; ATP, 12 %; SS, 10 %; MBA, 0.1 %; and initiator, 0.6 %; 4 h Fig. 8 Effect of the reaction time on the water absorbency of SS-g-poly(AA/ATP). Reaction conditions: AA (75 % neutralization), 2.5 M; ATP, 12 %; SS, 10 %; MBA, 0.1 %; and initiator, 0.6 %; 65 C which results in a high crosslinking degree. In addition, the reaction temperature changes the internal structure and further changes the swelling capacity of the hydrogel by changing the reaction process. Influence of reaction time The data in Fig. 8 show that the optimal reaction time in the specified conditions is 6 h. The water absorbency of the hydrogel reached a maximum value and then decreased with increasing reaction time. Given that the reaction is a free radical polymerization, the main reaction at the beginning of the reaction is chain propagation. Crosslinking degree increased with time, but a long reaction time can cause a high crosslinking degree in the hydrogel, which decreases the swelling capacity.

12 Swelling kinetics A Voigt-based Eq. (2) was applied to describe the swelling behavior of SAP in water [36]. S t ¼ S e ð1 e t=s Þ; where S t (in grams per gram) is the swelling at time t, S e is equilibrium swelling [power parameter (in grams per gram)], t is the time (in minutes) for swelling S t, and s (in minutes) is the rate parameter. To calculate the rate parameter using the above formula (after a natural logarithm transfer), one can plot Ln (1 S t /S e ) versus time (t). The slope of the fitted straight line (slope =-1/s) shows the rate parameter. The water absorptivity of the superabsorbent composite in distilled water was tested at consecutive time intervals. From Fig. 9, the rate of water absorbency sharply increased at the beginning and then leveled off. The equilibrium swelling was established after approximately 25 min. The curve suggested an obvious power-law swelling behavior of the hydrogel in water. The data can be fitted with Eq. 2. The rate parameter for the swelling of the hydrogel (40 60 mesh) in water was calculated at about 3.3 min. The s value is a measure of swelling rate; thus, this value can be used for the comparative evaluation of the rate of water absorption of hydrogels in similar condition, including similar particle size. Effect of salt solution on the swelling The swelling ratio of SAP depends on the combination of internal and external factors. The characteristics of SAP are the internal factors, which include elasticity of the network, constitution of the monomers, density of hydrophilic functional groups, and crosslinking degree. The external factors are the characteristics of the swelling solution, such as charge number, ionic strength, and ionic radius. The swelling capacities of ionic hydrogels in salt solutions are much lower compared with that in distilled water. The osmotic pressure theory can explain this ð2þ Fig. 9 Swelling kinetics of SSg-poly(AA/ATP) (40 60 mesh)

13 phenomenon. The ions that exist in the polymer network are immobile and are considered to be separated from the external solution by a semi-permeable membrane. When the hydrogel is placed in pure water, the observed osmotic pressure is much higher than that in salt solution. The Flory and Huggins theory is widely used to express the swelling behavior of polymer gels [37]. The swelling equilibrium relationship is expressed by Eq. 3 h Q 5=3 i=2v u S 0:5 2þ i ð 0:5 x1 Þ=V 1 = ðv E =V 0 Þ; ð3þ where Q is the water absorbency; V E /V 0 is the crosslinking density; (0.5 - x 1 )/V 1 is the affinity for water; i/v u is the charge density fixed on the hydrogel; S is the ionic strength of external solution; and V u is the molar volume of the structural unit. The first term in the numerator reflects the effect of external solution on water absorbency. A large ionic strength (S) causes a decrease in water absorbency (Q). A more fixed charge (i) causes an increase in Q value. The second term reflects the affinity of functional groups in the hydrogel for water. Strong polar groups help increase Q compared with weak polar groups. V E /V 0, as the denominator, suggests that increasing crosslinking density decreases the Q value. However, the hydrogel in the state of water absorbing shows elastic behavior similar to rubber. The rigidity of the hydrogel is proportional to V E /V 0. A low crosslinking density increases the Q value. A relatively low crosslinking density causes the hydrogel to lose rigidity and even dissolve in water. In this series of experiments, the swelling capacity was measured in distilled water, tap water, NaCl (0.9 %), and MgCl 2 solutions (0.15 M). Figure 10 shows that the decrease in swelling was strongly dependent on the type of salt added to the swelling medium, and the water absorbency of the hydrogel was very susceptible to the presence of electrolyte in the solution. At a similar concentration of salt solution, the absorbency decreased with increasing valence charge of the metal cations from Na? to Mg 2?. A charge screening effect and ionic crosslinking are the main explanations for the significant decrease in swelling [38]. The ionic strength of the medium and the ionic crosslinking at the surface of the particles increased and the swelling capacity consequently decreased with increasing cation charge. Fig. 10 Swelling capacity of the superabsorbent composite in different water solutions

14 Conclusion A superabsorbent composite, SS-g-poly(AA/ATP), was synthesized via the aqueous solution copolymerization method. The SS, ATP and the hydrogel were characterized by SEM, TEM, and FTIR. The swelling characteristics of the superabsorbent hydrogel were investigated. The effects of reactive conditions, such as the concentration of the initiator, crosslinker, and monomers, ATP percentage, time, and temperature, on swelling capacity were studied. The optimum conditions were AA, 2.5 M; ATP, 12 %; SS, 10 %; initiator, 0.6 %; MBA, 0.1 %; reaction temperature, 65 C; and time, 4 h. The maximum water absorbency was more than 1,200 and 100 g/g in pure water and in 0.9 % NaCl solution, respectively. The superabsorbent composite SS-g-poly(AA/ATP) had a higher swelling capability than poly(aa/atp). References 1. Chu M, Zhu S, Li H, Huang Z, Li S (2006) Synthesis of poly(acrylic acid)/sodium humate superabsorbent composite for agricultural use. J Appl Polym Sci 102: Chu M, Zhu S, Huang Z, Li H (2008) Influence of potassium humate on the swelling properties of a poly(acrylic acid-co-acrylamide)/potassium humate superabsorbent composite. J Appl Polym Sci 107: Abd El-Rehim HA (2005) Swelling of radiation crosslinked acrylamide-based microgels and their potential applications. Radiat Phys Chem 74: Kabiri K, Mirzadeh H, Zohuriaan-Mehra MJ, Daliri M (2009) Chitosan-modified nanoclay poly (AMPS) nanocomposite hydrogels with improved gel strength. Polym Int 58: Chang C, Duan B, Cai J, Zhang L (2010) Superabsorbent hydrogels based on cellulose for smart swelling and controllable delivery. Eur Polym J 46: Hoare TR, Kohane DS (2008) Hydrogels in drug delivery: progress and challenges. Polymer 49: Meena R, Prasad K, Siddhanta AK (2009) Development of a stable hydrogel network based on agar kappa-carrageenan blend cross-linked with genipin. Food Hydrocoll 23: Zhang J, Wang A (2010) Adsorption of Pb(II) from aqueous solution by chitosan-g-poly(acrylic acid)/attapulgite/sodium humate composite hydrogels. J Chem Eng Data 55: Yin L, Ding J, Zhang J, He C, Cui T, Yin C (2010) Polymer integrity related absorption mechanism of superporous hydrogel containing interpenetrating polymer networks for oral delivery of insulin. Biomaterials 31: Zou X (2002) Superabsorbent Materials, 2nd edn. Chemical Industry Press, China 11. Haselbach J, Berner T, Wright H, Dunlap E (2000) Single-dose oral toxicity study of a cross-linked sodium polyacrylate/polyvinyl alcohol copolymer in chickens (gallus domesticus). Regul Toxicol Pharm 32: Buchholz FL, Graham AT (1998) Modern superabsorbent polymer technology. Wiley-Vch, New York 13. Li A, Wang A, Chen J (2004) Studies on poly(acrylic acid)/attapulgite superabsorbent composite. I. Synthesis and characterization. J Appl Polym Sci 94: Chen J, Ding S, Jin Y, Wu J (2013) Semidry synthesis of the poly(acrylic acid)/palygorskite superabsorbent with high-percentage clay via a freeze thaw extrusion process. J Appl Polym Sci 128: Shogren RL, Willett JL, Biswas A (2009) HRP-mediated synthesis of starch polyacrylamide graft copolymers. Carbohydr Polym 75: Tang Q, Wu J, Lin J, Fan S, Hu D (2009) A multifunctional poly(acrylic acid)/gelatin hydrogel. J Mater Res 24:

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