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 3 04-313.5 K and critical concentrations in the range of 0.02-0.08 polym er 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 mix ing with water of PVPA and CP(VCL-VP) was negative and the curve was c oncave over the entire range of composition at 298 and 308 K. The exce ssive 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-V P), revealing hydrophilic hydration. Hydrophilic hydration was predomi nant in solutions which were homogeneous over a wide temperature range , whereas hydrophobic hydration predominated in solution of polymers w ith LCST.