CHARACTERIZATION OF THERMAL-DIFFUSION OF POLYSTYRENE IN BINARY-MIXTURES OF THF DIOXANE AND THF CYCLOHEXANE

The thermal diffusion coefficient (Dr) was determined for three polyst yrene standards of different molecular masses in binary mixtures of te trahydrofuran/dioxane and tetrahydrofuran/cyclohexane of various compo sitions. The DT values were obtained by combining retention data from thermal field-flow fractionation measurements with diffusion data from dynamic light scattering experiments. In agreement with earlier work of Schimpf and Giddings, the thermal diffusion coefficient was found t o be virtually independent of the molecular mass of the polymers. In t he binary mixtures of tetrahydrofuran and dioxane, both good solvents for polystyrene, the DT value was approximately equal to the average o f the DT values in the pure solvents, weighted according to the mole f ractions of the solvents in the mixture. However, for polystyrene in b inary mixtures of tetrahydrofuran and cyclohexane this linear behavior of the thermal diffusion phenomenon was not observed. The addition of cyclohexane to tetrahydrofuran has initially only a minor effect on t he molecular and thermal diffusion coefficients of the polystyrene sta ndards. Because cyclohexane is a theta solvent for polystyrene, the pr eferential solvation of polystyrene by tetrahydrofuran could be an exp lanation for these results. (C) 1996 John Wiley & Sons, Inc.