Synthesis, swelling properties, and network structure of new stimuli-responsive poly(N-acryloyl-N′-ethyl piperazine-co-N-isopropylacrylamide) hydrogels

Poly(N-acryloyl-N'-ethyl piperazine-co-N-isopropylacrylamide) hydrogels were prepared by thermal free-radical copolymerization of N-acryloyl-N'-ethyl piperazine (AcrNEP) and N-isopropylacrylamide (NIPAM) in solution using N, N'-methylene bisacrylamide as the crosslinking agent. The gels were responsive to changes in external stimuli such as pH and temperature. The pH and temperature responsive character of the gels was greatly dependent on the monomer content, namely AcrNEP and NIPAM, respectively. The gels swelled in acidic (pH 2) and de-swelled in basic (pH 10) solutions with a response time of 60 min. With increase in temperature from 23 to 80 degrees C the swelling of the gels decreased continuously and this effect was different in acidic and basic solutions. The temperature dependence of equilibrium water content of the gels was evaluated by the GibbsHelmholtz equation. Detailed analysis of the swelling properties of these new gels in relation to molecular heterogeneity in acidic (pH 2) and basic (pH 10) solutions were performed. Water transport property of the gels was studied gravimetrically. In acidic solution, the diffusion process was non-Fickian (anomalous) while in basic solution, the diffusion was quasi-Fickian. The effect was more evident in solution of pH 2 than in pH 10. Various structural parameters of the gels such as number-average molar mass between crosslink (Mc), the crosslink density (?c), and the mesh size (?) were evaluated. The mesh sizes of the hydrogels were between 64 and 783 angstrom in the swollen state in acidic solution and 20 and 195 angstrom in the collapsed state in basic solution. The mesh size increased between three to four times during the pH-dependent swelling process. The amount of unbound water (free water) and bound water of the gels was also evaluated using differential scanning calorimetry. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012