Pressure effects on the aggregation of poly(N-isopropylacrylamide) and poly(N-isopropylacrylamide-co-acrylic acid) in aqueous solutions

Pressure effects on the phase separation of aqueous solutions of poly(N-isopropylacrylamide) (pNIPA) and poly(N-isopropylacrylamide-co-acrylic acid) (pNIPA/AAc) were examined at pressures up to 250 MPa. The cloud point temperature first increased and then decreased as the pressure was raised. The copolymer solutions had higher maximum temperature and pressure than the pNIPA solutions. Though the cloud point curve of pNIPA solutions had no concentration dependence, the cloud point curve of pNIPA/AAc solutions shifted toward higher temperature and pressure with decreasing concentration. The behavior of the specific volume of a 3.0 wt % pNIPA solution during the phase separation suggested that the hydrophobic interaction is not the main factor in the pressure dependence of the cloud point of pNIPA solution at high pressure.