Molecular-weight dependent thermal diffusion in dilute polymer solutions

Thermal diffusion of long polymers is independent of the molecular weight M_{w}. Accounting for the molecular structure of the solvent, we derive an additional contribution of opposite sign which is significant for short chains but vanishes as M_{w}→∞. Our findings explain the dependence on M_{w} observed recently for polystyrene, and its inverse Soret effect at very small M_{w}. Moreover, they bridge the gap between the macroscopic hydrodynamics description for large solutes and the heat-of-transport picture for small molecules