Volumetric Studies of Aqueous Polymer Solutions Using Pressure Perturbation Calorimetry:A New Look at the Temperature-Induced Phase Transition of Poly(N-isopropylacrylamide) in Water and D_2O

摘要

We report the first application of pressure perturbation calorimetry (PPC) to determine the hydration properties of poly(N-isopropylacrylamide) (PNIPAM) in H_20 and in D_2O as the solutions . undergo a temperature-induced phase transition. The technique, which measures the heat change resulting from a pressure change above a solution of PNIPAM placeil in a microcalorimeter cell, yields the temperature dependence of the coefficient of thermal expansion, a.p, of the polymer in solution and the change in volume of the solvation layer around the polymer chain. In the temperature ranges below and above the phase transition, a.p ofPNIPAM in H_2O increased linearly with temperature. It underwent a sharp increase at the transition temperature, T_(max), then rapidly decreased. The phase transition was accompanied by an increase in the partial specific volume of the hydrated polymer. This increase was significantly higher for solutions of PNIPAM in D_2O, compared to H_2O. A study by PPC of the phase transition ofhydrophobicallymodified PNIPAMsamples that undergo micellization in water demonstrated that the hydration of the polymeric micelles varies significantly as a function of the degree of hydrophobic substitution and length of the alkyl group linked to the polymer.