THERMODYNAMIC STUDY OF POLY(N-VINYL CAPROLACTAM) HYDRATION AT TEMPERATURES CLOSE TO LOWER CRITICAL SOLUTION TEMPERATURE

The lower critical solution temperature of aqueous solutions of poly(N -vinyl caprolactam) falls in the 305-307 K range and depends on the mo lecular weight of the polymer. The thermodynamic functions of mixing a t 298 K have been calculated from measurements of vapor pressures and heats of dissolution and dilution. Partial Gibbs energy, partial entha lpy, and partial entropy of mixing were negative over the entire range of composition. Increasing temperature resulted in a decrease in the exothermal character of mixing. Excessive heat capacity values, calcul ated from the dependencies of enthalpy of mixing on temperature, were positive over the entire composition range. Heat capacity of dilute so lutions was measured at 298 K and partial heat capacity of poly(N-viny l caprolactam) at infinite dilution was shown to be positive. The data obtained point out the hydrophilic and hydrophobic hydration of poly( N-vinyl caprolactam) in aqueous solutions. Hydrophobic hydration domin ates at temperatures close to binodal curve. As a result, the mutual m ixing of the polymer with water is decreased and phase separation take s place.