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 D2O

We report the first application of pressure perturbation calorimetry (PPC) to determine the hydration properties of poly(N-isopropylacrylamide) (PNIPA M) in H2O and in D2O as the solutions undergo a temperature-induced phase t ransition. The technique, which measures the heat change resulting from a p ressure change above a solution of PNIPAM placed in a microcalorimeter cell , yields the temperature dependence of the coefficient of thermal expansion , alpha (p), of the polymer in solution and the change in volume of the sol vation layer around the polymer chain. In the temperature ranges below and above the phase transition, alpha (p) of PNIPAM in H2O increased linearly w ith temperature. It underwent a sharp increase at the transition temperatur e, T-max then rapidly decreased. The phase transition was accompanied by an increase in the partial specific volume of the hydrated polymer. This incr ease was significantly higher for solutions of PNIPAM in D2O, compared to H 2O. A study by PPC of the phase transition of hydrophobically modified PNIP AM samples that undergo micellization in water demonstrated that the hydrat ion of the polymeric micelles varies significantly as a function of the deg ree of hydrophobic substitution and length of the alkyl group linked to the polymer.