Bercea, M., Wolf, B.A.

"Enthalpy and entropy contributions to solvent quality and inversions of heat effects with polymer concentration"

Macromolecules 2006, submitted.

Abstract: Vapor pressures above the solutions of polystyrene in either cyclohexane (32-60 degrees C) or toluene (10-70 degrees C) have been measured by a combination of head space sampling and gas chromatography as a function of the volume fraction phi of the polymer. The thus obtained Flory-Huggins interaction parameters chi (phi, T) were complemented by data for three other systems (tert-butyl acetate/polystyrene, cyclohexane/poly(vinyl methyl ether), 2-propanol/poly(butyl methacrylate)) reported in the literature and analyzed in terms of their enthalpy and entropy parts chi(H) and chi(S). Furthermore these experimental findings were modeled by means of an approach splitting the mixing process conceptually into two parts and lading to the expression chi = chi(fc) + chi(cr). The first step takes place at fixed conformation of the components and yields chi(fc); equilibrium is reached in the second step, during the conformational relaxation, contributing chi(cr). The results demonstrate that mixing can either be driven enthalpically or entropically. By contrast the conformational response acts in all cases towards homogeneous mixing. The inversion of heat effects observed for two of the five systems can be well modeled in terms of the composition dependence of the enthalpy parts of chi(fc) and chi(cr).

preprint number: 278