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Thermodynamic study of a pressure-temperature phase diagram for poly(N-isopropylacrylamide) gels



The volume change, ΔVh,, accompanying the hydrophobic hydration associated with the volume phase transition in Poly(N-isopropylacrylamide) gels was measured by a simple method. The hydration accompanies a negative ΔVh−2.5 cm3/mol. The P-T phase diagram, the coexistence curve, for the gels was determined from the swelling ratio-pressure curves up to 350 MPa for various constant temperatures. The contour of the coexistence curve is shaped like an ellipsoid on the P-T plain, which is a feature peculiar to the reversible pressure-temperature denaturation of a protein. The thermodynamic analysis of the Clausius–Clapeyron relation for the measured ΔVh elucidates that the obtained coexistence curve represents the phase boundary between thermodynamic different phases like the two phases, native and denatured, of a protein and gives the transition enthalpy, ΔH, 5.2kJ/mol by estimate, which well coincides with the transition heat measured by a calorimetric method. Considering the volume-dependent free energy, Δvmi · P, for the mixing free energy of the gel, we can fit the calculated curve to the measured swelling ratio-pressure curve of PNIPA gels. The value of Δvmi changes the sign from negative to positive above around 100MPa. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 405–412, 2005