Analysis of swelling behavior of poly(methacrylamide-co-methacrylic acid) hydrogels and effect of synthesis conditions on water uptake Sunil Kumar Bajpai * , Surinderpal Singh Polymer Research Laboratory, Department of Chemistry, Government Model Science College, Jabalpur, MP 482 001, India Received 4 April 2005; received in revised form 15 September 2005; accepted 19 September 2005 Available online 14 November 2005 Abstract An investigation of water uptake behavior of a novel pH-sensitive poly(methacrylamide-co-methacrylic acid) hydrogel system was carried out with two major objectives. First was to study the effect of various synthesis conditions like reaction temperature, initiator content and degree of dilution of feed mixture on the water absorbency of resulting hydrogels and the second was to interpret the water uptake data by various kinetic models. The 2.25 times dilution of the reaction mixture resulted in the formation of hydrogels with highest water uptake of 1423 ± 21% in the simulating intestinal fluid of pH 7.4 at 37 °C. Likewise, the optimum synthesis temperature and initiator concentration were found to be 80 °C and 11.04 · 10 2 mM, respectively. The kinetic models proposed by Peppas and Hopfenberg were applied on the water uptake data to obtain the best fit. The enthalpy of mixing DH mix was also evaluated for hydrogels with varying concentrations of monomer acid in the feed mixture. Finally, the gels demonstrated sharp change in their water absorbency and mesh size of networks with pH of the swelling media, thus suggesting their strong candidature for being used as oral drug-delivery sys- tems along the gastrointestinal tract. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogels; Diffusion; Swelling; Methacrylamide 1. Introduction Hydrogel are three-dimensionally crosslinked hydrophilic polymer networks that have capability to retain a large volume of water in the swollen state. Depending on the type of monomers (i.e., ionic or neutral) incorporated in the gels, they can be made to respond to a variety of external environ- mental changes such as pH, ionic strength, temper- ature or specific chemical compounds [1–3]. These materials are of great importance due to their prom- ising applications such as controlled drug-delivery systems [4–6], artificial muscles [7], sensor systems [8] and in tissue engineering [9], etc. These applica- tions mainly depend upon the swelling behavior of these polymeric hydrogels, which in turn, are con- trolled mainly by the crosslinking density. The degree of crosslinking in the polymer network struc- ture is critical as it dictates the mechanical strength, 1381-5148/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2005.09.003 * Corresponding author. E-mail address: mnlbpi@rediffmail.com (S.K. Bajpai). Reactive & Functional Polymers 66 (2006) 431–440 www.elsevier.com/locate/react REACTIVE & FUNCTIONAL POLYMERS