6533b7d3fe1ef96bd12608de

RESEARCH PRODUCT

Enthalpy and Entropy Contributions to Solvent Quality and Inversions of Heat Effects with Polymer Concentration

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Vapor pressures above the solutions of polystyrene in either cyclohexane (32-60 °C) or toluene (10-70 °C) have been measured by a combination of head space sampling and gas chromatography as a function of the volume fraction φ of the polymer. The thus obtained Flory-Huggins interaction parameters Χ (φ; 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 Χ H and Χ s . Furthermore these experimental findings were modeled by means of an approach splitting the mixing process conceptually into two parts and leading to the expression Χ = Χ fc + Χ cr . The first step takes place at fixed conformation of the components and yields Χ fc ; equilibrium is reached in the second step, during the conformational relaxation, contributing Χ 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 Χ fc and Χ cr .