Prediction of Polymer Thermal Diffusion Coefficients from Polymer-Solvent Interaction Parameters: Comparison with Thermal Field Flow Fractionation and Thermal Diffusion Forced Rayleigh Scattering Experiments

Abstract

In this study a model that allows the qualitative prediction of polymer thermal diffusion coefficients from polymer-solvent interaction parameters is presented. The Flory-Huggins lattice theory served as a starting point for the thermal diffusion model. From this model it follows that D T is determined by the temperature dependence of the polymer-solvent interaction parameter ( h ), the segmental mobility of the polymer chain, and the polymer concentration. In agreement with literature data, the model predicts that D T is independent of the molecular mass of the polymer, D T increases with temperature, and D T is strongly dependent on the interaction between the polymer and the solvent. Furthermore, it follows from the model that D T should decrease with concentration. A good qualitative agreement with experimental data has been found. In order to study the potentials of the model, the D T values of polystyrene (PS) in several solvents were predicted. The required temperature-dependent polymer-solvent interaction parameters were calculated from Hildebrand's solubility parameters of the polymer and of the solvent. The solvent's solubility parameters were estimated using Lee and Kesler's generalized thermodynamic equations of state. The polymer solubility parameters were taken from the literature. Lack of data complicated the comparison. However, a good agreement for PS in THF, MEK, dioxane, and cyclohexane was found. Thermal diffusion values predicted for PS in several aromatic solvents were, however, significantly low. Still, the overall result is satisfactory considering the fact that the Flory-Huggins theory is a highly simplified polymer-solvent theory that was never intended to be used for quantitative purposes. Polymer thermal diffusion model Thermal field-flow fractionation Polymer-solvent interaction parameter Thermal diffusion forced Rayleigh scattering