On the Cooccurrence of Demixing and Thermoreversible Gelation of Polymer Solutions. 2. Thermodynamic Background

By the combination of different experimental methods (light scattering, vapor pressure measurements, and inverse gas chromatography), the Flory-Huggins interaction parameters x were determined for poly(n-butyl methacrylate) (PBMA) plus the 0-solvents 2-propanol and ethanol as a function of composition and temperature. the subdivision of x into its enthalpy and entropy parts xh and xs reveals fundamental differences for the present systems as compared with normal 0-solutions. For example, in the case of the system cyclohexane/poly(styrene), both of these contributions are positive and increase steadily with rising polymer content of the mixture. With the solutions of PBMA under investigation, however, one observes minima in xh and maxima in xs, where the former quantity remains positive at all compositions and temperatures, whereas the latter starts from slightly positive or negative values (depending on T) at low concentrations but becomes considerably negative with increasing polymer content.