Kinetic Theory of Thermal Diffusion in Dilute Polymer Solutions

Abstract

A simple Brownian motion model is used to discuss thermal diffusion in dilute polymer solutions. The thermal diffusion coefficient is found to be independent of polymer at sufficiently large molecular weights and to be given by D_T= − (D_sE_s) / RT² where D_s is the self‐diffusion of the solvent and E_s is the activation energy for this diffusion. This suggests that the thermal diffusion ratio α= TD_T / D₀ should show a marked temperature dependence, a molecular weight dependence proportional to M^½, and a concentration dependence governed for the most part by that of D₀. The results are shown to agree with published data in most respects and are contrasted to the results of other theoretical models. The results indicate that thermal diffusion is not a particularly promising method of fractionating various molecular weight molecules.