Water sorption studies of new pH-responsive N-acryloyl-N′-methyl piperazine and methyl methacrylate hydrogels

New crosslinked copolymer hydrogels of N-acryloyl-N′-methyl piperazine and methyl methacrylate in various compositions were prepared in bulk and solution by photo-initiated free-radical polymerization. The gels exhibited both pH and temperature sensitivities. The response to pH was reversible with a response time of 150 min. The water sorption of the gels was investigated gravimetrically and the collective diffusion coefficients were determined according to the Fickian kinetics at 10°C, 25°C, and 50°C. The apparent activation energy was independent of the composition of the gel networks with a value of 6.26 kJ mol−1 corresponding to a typical diffusion process. The temperature dependence of the equilibrium water content was analyzed by the Gibbs–Helmholtz equation. The enthalpy of mixing decreased with an increase in the hydrophilic content of the gel.