High-Sensitivity Isothermal and Scanning Microcalorimetry in PNIPA Hydrogels around the Volume Phase Transition

Abstract

Mechanical and dynamic light scattering measurements show that at 20 °C poly(N-isopropylacrylamide) (PNIPA) hydrogels are in good solvent conditions. At this temperature, the average value of the collective diffusion coefficient is D = 4 × 10-7 cm2/s. The value of the enthalpy found by isothermal microcalorimetry is roughly twice that reported in the literature. Even at extremely low scanning rates (0.02 °C/min), full thermal equilibrium was not achieved in DSC. Small-angle X-ray scattering measurements confirmed that the gel collapse involves two stages. The first is prompt microphase separation in which the polymer chains form clusters of size approximately 10 nm, separated by distances of several hundred nanometers. The second stage involves slow relaxation and expulsion of solvent, with an estimated diffusion coefficient at 40 °C equal to about 10-17 cm2/s. The slow relaxation of the frozen network is responsible for the hysteresis in the DSC measurements and can explain the discrepancies in the reported values of the enthalpy.