Effects of acrylic acid incorporation on the pressure-temperature behaviorand the calorimetric properties of poly(N-isopropylacrylamide) in aqueous solutions

We studied the effects of pH on the pressure-temperature dependence of coil -collapse transition for aqueous solutions of copolymers of N-isopropylacry lamide and acrylic acid (Ac). At low pressures. the transition temperature (T-tr) increased with pressure, but T-tr decrease with increasing pressure at pressures higher than 50-100 MPa. By increasing the pH, the transition c ontour shifted to a higher temperature. When the Ac content was increased, the effects of pH became more evident. From a calorimetric study at atmosph eric pressure, Delta H-tr was found to become smaller by increasing the por tion of the ionized residues in the copolymer, The ratio to the van't Hoff enthalpy changes became larger with an increase in pH, which indicated that the production of charge decreased the cooperative domain size.