DSC was done in the DSC unit (Netzsch DSC-200 PC Phox, Germany). The samples were heated in a close aluminum pan at the rate of 40��C/min under nitrogen flow (50mL min?1). TGA was performed using a thermogravimetric analyzer (TGA 951, TA Instruments, selleck chemicals llc USA). Samples were heated at the rate of 10��C/min with temperature range of 30�C400��C.2.4.4. Drug Loading and Release Kinetics Dried hydrogel discs were immersed in 10mL of isosorbide mononitrate solution (1% w/v) prepared in ethanol-water mixture (50:50% v/v). These hydrogels were kept at ambient temperature without stirring for 7 days to attain equilibrium swelling. After reaching equilibrium swelling point, discs were removed from the loading solution, blotted with filter paper, and dried in an oven at 45��C for 7 days.

The amounts of drug loaded were calculated by recurrently extracting the drug from the hydrogels in ethanol-water mixture (50:50% v/v) and the concentration of drug in pooled extract was monitored spectrophotometrically at 210nm. The experiments were conducted in triplicate.Drug release studies were carried out in USP II dissolution apparatus (Pharma Test, Germany) using 0.05M USP phosphate buffer solutions at various physiological pHs (1.2, 6.5, and 7.5). The weighed hydrogel discs were immersed in 500mL dissolution medium stirred at a rate of 100rpm, and maintained at 50��C. With each sampling, 5mL release media was withdrawn at predetermined time and immediately replenished with the same volume of fresh medium to maintain sink conditions. The determination of isosorbide mononitrate release was carried out at 210nm for up to 12h at regular intervals.

Drug release data were fitted to various kinetic models including zero-order, first-order, Higuchi, GSK-3 and Korsmeyer-Peppas models [27, 28]. These models are generally used when more than one type of release phenomena is involved.3. Results and Discussion3.1. Swelling BehaviourThe poly(acrylic-co-vinylsulfonic) acid hydrogels were synthesized using AA as the monomer, PVSA as the polymer, and EGDMA as the cross-linker in the presence of BPO as an initiator of free radical polymerization. Schematic depiction of the synthesis of cross-linked hydrogel is shown in Figure 1. Since AA and VSA are negatively charged polyelectrolytes in the polymerization medium; therefore, strong electrostatic repulsive forces would operate between AA (COO?) and VSA (SO3?) groups. It is envisaged that a possible expanded network of poly(AA/VSA) hydrogel would be obtained that has a higher swelling capacity [29]. Figure 1Schematic representation of hydrogel synthesis.Swelling behaviour of hydrogels against external stimuli (temperature or pH) is a measure of their usefulness as biomaterials in the field of pharmaceuticals.