Studies of the thermal volume transition of poly(N-isopropylacrylamide) hydrogels by high-sensitivity differential scanning microcalorimetry. : 1. : Dynamic effects

We present first results of studies of the volume phase transition in poly(N-isopropylacrylamide) (NIPA) hydrogels by high-sensitive differential scanning calorimetry (HS-DSC). We examine the behavior of the low-concentrated gels at different scanning rates and discuss the dependencies of the transition parameters upon heating rate. It is shown that HS-DSC measurements at heating rate of 0.125 K/min provide results closely approximating the equilibrium ones in every respect except for the value of transition temperature. At higher heating rates, the transition temperature appears to be practically rate-independent, and network exhibits signs of spatially nonuniform structures developing in the course of collapse, with some parts of the system collapsing under the restrictions of others being almost unperturbed. The apparent enthalpy of this constrained transition is higher than that observed at equilibrium. We discuss the hypothesis that the transition occurs through the spinodal decomposition.