Effects of non-uniform solvation on thermal response in poly(N-isopropylacrylamide) gels

The quantitative analyses on the dehydration of poly(N-isopropylacrylamide) (NIPA) hydrogels were conducted by means of differential scanning calorimetry (DSC) as a function of polymer volume fraction, phi. It was found that the thermal properties of NIPA gels were strongly dependent on the history of the gel treatment before DSC measurements. In particular, a disagreement in the hydrophobic dissociation temperature at which an endothermic peak appears, T-hd, and in the enthalpy of dehydration, Delta H, was clearly detected between gels having different histories: one prepared by adding a given amount of water to a dried gel (non-equilibrated gels) and the other made by gradual shrinking along the isobar line by heating (equilibrated gels). The phase diagram, i.e. the plot of T-hd vs. phi, for the equilibrated gels has a convex function of phi, whereas that of the non-equilibrated gel shows an anomalous dip for phi > phi(st) approximate to 0.4 (i.e. above the stoichiometric concentration of NIPA-water hydration). The values of Delta H fbr the non-equilibrated gels are less than those for the equilibrated gels in the concentration regime of phi > phi(st). This implies that the non-equilibrated gels have a higher degree of non-uniform solvation than the equilibrated gels. (C) 1999 Elsevier Science Ltd. All rights reserved.