Dynamical Modeling and Experimental Analysis on the Swelling Behavior of the sIPN Hydrogels Saman Sotoudeh, † Ghazaleh Pourfallah,* ,† Abolfazl Barati, † Reza Davarnejad, † Mohammad Aliabadi Farahani, ‡ and Amir Memar § Department of Chemical Engineering, Faculty of Engineering, Arak UniVersity, Arak 38156-8-8349, Iran, Department of Chemical Engineering, Isfahan UniVersity of Technology, Isfahan, Iran, and Fuel Cell Institute; UniVersity Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia In this article, polyethylene glycol-acrylamide (PEG-AAm) hydrogels were experimentally prepared with a matrix structure and their swelling behaviors were tested. Different amounts of polyethylene glycol, acrylic acid, and acrylamide were added, and an optimum case was discovered. Effect of some parameters on the swelling was then investigated. A dynamic model based on the Maxwell-Stefan equation, Nernst-Planck equation, and Donnan theory was developed to consider the swelling behavior of hydrogels. These equations were applied to model the buffer diffusion inside the hydrogels in the swelling process. COMSOL software was also applied to simulate the swelling behavior in the hydrogels. The results showed that the data obtained from the modeling were in good agreement with the experimental data (the deviation value was around 16%). Further, COMSOL was able to simulate water diffusion process in the hydrogels, properly. Introduction Hydrogels are chemically or physically cross-linked by macromolecules with segments of hydrophilic homopolymers or copolymers. 1-7 In the swollen state, they are soft and rubbery and exhibit excellent behaviors, such as high water affinity, high thermal, mechanical stability, and biocompatibility. These properties make a swollen hydrogel very applicable in various areas, such as chemical engineering, medicine (controlled drug delivery), biomedical and biotechnological fields, 2 tissue engi- neering, pharmaceuticals, and food and agriculture. 3,4 Hydrogels are often sensitive to some conditions, such as temperature, pH, ionic concentration, electric field, and light. Its reason is the presence of certain functional groups along the polymer chains. 5,6 Temperature and pH are very important parameters in biochemical area. 7 When a hydrogel network is contacted water or another aqueous solution, its volume is increased by absorbing water or solution. This phenomenon is called swell- ing. 8 Synthetic hydrogels are currently used in various applications, such as superabsorbents in diapers, separation media in ion exchange, and size exclusion chromatography. In the pharma- ceutical industries, hydrogels control humidity and drug release (drug delivery system). They let compressed powder tablets expand in the stomach. 9 A gel degree of swelling is an essential feature, which is controlled during a polymer network charac- terization. The degree of swelling is determined by the intermolecular interactions and elastic forces in a cross-linked polymer. 10 These materials still have some problems regarding mechanical strength and biocompatibility. 11 The ampholytic hydrogels show semi-interpenetration during polymerization process. 12 According to this process, two polymers are blended as one of them is cross-linked in another one to produce a mixture with desirable morphology. Further, no covalent interaction between two polymers would affect on the morphology, mechanical properties, and thermal properties of a sIPN gel. 13-16 The present research is aimed to prepare the sIPN hydrogels and model the data based on the Maxwell-Stefan equation. For this purpose, the experimental data were used to investigate the equation of deformation (using Matlab software (version 7.1)) and obtain the modeled data. The experimental data were compared with the modeled data as well. Then, by applying the recent equation obtained from Matlab software in COMSOL- Multiphysics software (version 3.4), the deformation of hydro- gels was simulated and sIPN hydrogels swelling behavior was studied. Experimental Section Materials. Polyethylene glycol [(PEG) with M W ) 1000 g · mol -1 ], acrylamide, acrylic acid, ammonium persulphate [(APS) as the initiator], N,N,N,N-tetra-methylethylendiamin (TEMED) as the accelerator were purchased from Merck com- pany. N,N′-methylene bisacryamide as a cross-linker was purchased from Sigma Company. Phosphate buffer saline [(PBS) with pH ) 7.8 containing NaCl 0.138 M, KCl 0.0027 M, and phosphate buffer saline 0.01 M)] was purchased from Merck company and used in vitro swelling study. PEG-AAm Hydrogel Preparation. Two grams of PEG, 1.42 g of acrylamide, and 0.023 g of N,N′-methylene bisacrya- mide were dissolved in 20 mL of twice distilled water and mixed for 1 h at ambient temperature. Six milliliters of acrylic acid was then added to the solution and mixed again for 1 h. The slurry solution was put in a 250 mL flask equipped with a stirrer and a nitrogen line. APS as the initiator and N,N,N,N-tetra- methylethylendiamin (TEMED) as the accelerator were added. Since APS is oxidized with atmospheric oxygen so, nitrogen was injected into the flask to vent oxygen (because oxygen is lighter than nitrogen). The obtained hydrogel should be remained around 24 h 17 to complete the polymerization reaction. Then the products were removed from flask and cut to small pieces and dried to reach a constant weight. Figures 1 and 2 show the vertical and horizontal cross sec- tional area of an obtained hydrogel, respectively. A unique * To whom correspondence should be addressed. Tel.: + 98 (311) 4432813. Fax: + 98 (861) 2225946. E-mail: ghh_pf82@yahoo.com. † Arak University. ‡ Isfahan University of Technology. § University Kebangsaan Malaysia. Ind. Eng. Chem. Res. 2010, 49, 10111–10115 10111 10.1021/ie101062d  2010 American Chemical Society Published on Web 09/21/2010