'Stimuli' responsive copolymers of N-acryloyl-N'-ethyl piperazine (AcrNEP) and methyl methacrylate (MMA) were synthesized by free radical solution polymerization. The copolymers were analyzed as thin films by FTIR spectroscopy. The monomer reactivity ratios were determined by linearization methods of Fineman-Ross (F-R) and Kelen-Tudos (K-T) giving the results r(1) (AcrNEP) = 0.58 and (r)2 (MMA) = 0.91 by the F-R method and r(1) = 0.72 and r(2) = 1.08 by the K-T method. The latter r values in turn yielded Q = 0.59 and e = -0.12 for AcrNEP. Crosslinked copolymer hydrogels of AcrNEP and MMA with various compositions were prepared in bulk and solution by photo-initiated free-radical polymerization. The gels were dual responsive to pH and temperature. The response to pH was reversible with a response time of 100 min with good reversibility and with no loss in swelling capacity. Water sorption of the gels was investigated gravimetrically and the collective diffusion coefficients were determined at 10, 25, and 50 degrees C. The water sorption of the gels in water was Fickian. The temperature dependence of the equilibrium water content was studied by the Gibbs-Helmholtz equation. The enthalpy of mixing decreased with an increase in the hydrophilic content (AcrNEP) of the gel. Other parameters such as type and amount of crosslinker, preparative conditions, nature of buffers, and salts were found to influence the swelling behavior. (c) 2006 Elsevier Ltd. All rights reserved.