Thermal diffusion parameters such as the thermal diffusion factor and the thermal diffusion coefficient for polystyrene in toluene are measured by using a thermal field flow fractionation（TFFF） method. The dependence of these parameters upon temperature, concentration, and polymer molecular weight is examined.
The thermal diffusion coefficient is found to be independent of the molecular weight for sufficiently large polymer molecules, but on the other hand the ordinary diffusion coefficient is known to be pronouncedly dependent on the molecular weight.
This result indicates that the fractionation effect is primarily governed by the differences in ordinary diffusion coefficients.