Lower critical solution temperature and hydrophobic hydration in aqueous polymer solutions

Phase diagrams of aqueous solutions of poly(N-vinyl caprolactam) (PVCL), N-vinyl caprolactam copolymer with vinylamine (3.8 mol%) (CP(VCL-VA)), and poly(N-vinyl propylacetamide) (PVPA) were shown to be binodal curves with lower critical solution temperatures (LCST) in the range 304–313.5 K and critical concentrations in the range of 0.02–0.08 polymer weight fraction. Aqueous solutions of N-vinyl caprolactam copolymer with N-vinyl pyrrolidone (80 mol%) (CP(VCL-VP)) remained homogeneous in the entire region of the liquid state of water. The enthalpy of mixing with water of PVPA and CP(VCL-VP) was negative and the curve was concave over the entire range of composition at 298 and 308 K. The excessive heat capacity and partial heat capacity at infinite dilution of PVPA were positive, proving the hydrophobic character of hydration of this polymer. In contrast, these parameters were negative for CP(VCL-VP), revealing hydrophilic hydration. Hydrophilic hydration was predominant in solutions which were homogeneous over a wide temperature range, whereas hydrophobic hydration predominated in solution of polymers with LCST.