Lattice Model for Thermodiffusion in Polymer Solutions

When a temperature gradient is applied to a polymer solution, the polymer typically migrates to the colder regions of the fluid as a result of thermal diffusion (Soret effect). However, in recent thermodiffusion experiments on poly(ethylene-oxide) (PEO) in a mixed ethanol/water solvent it is observed that for some solvent compositions the polymer migrates to the cold side, while for other compositions it migrates to the warm side. In order to understand this behavior, a two-chamber lattice model for thermodiffusion in liquid mixtures and dilute polymer solutions has been developed. For mixtures of PEO, ethanol, and water, the compressibility and hydrogen bonding between PEO and water molecules are taken into account and Soret coefficients are calculated for a given temperature, pressure, and solvent composition. The sign of the Soret coefficient of PEO is found to change from negative (polymer enriched in warmer region) to positive (polymer enriched in cooler region) as the water content of the solution is increased, in agreement with experimental data. A close relationship between the solvent quality and the partitioning of the polymer between the two chambers is noted, which may explain why negative Soret coefficients for polymers are so rarely observed. The Soret effect in ethanol/water mixtures is also investigated and a change in sign of the Soret coefficient of water is found at high water concentrations, in qualitative agreement with experimental data.