We have studied the crossover from small-molecule to polymer behavior in the Soret effect of dilute solutions of polystyrene in seven different solvents. The molar masses range from the monomer to M ≈ 106g/mol. The thermal diffusion coefficient DT is molar mass independent in the high polymer regime, and the quantity ηDT is approximately constant and independent of the solvent. For shorter chains below M ≈ 10 kg/mol, DT decreases monotonously with M and ηDT does no longer follow a common master curve. For the two “monomers” ethylbenzene and 3,3′-dimethylbutylbenzene there is even a sign change in several solvents. We conclude that the thermal diffusion coefficient, albeit being molar mass independent in the high polymer limit, is not a property of the monomer but rather of correlated segments of the order of the Kuhn segment. Hydrodynamic interactions dominate the behavior of the Soret coefficient for sufficiently long chains, where swelling due to excluded volume interactions becomes important. For a given molar mass the Soret coefficient depends only on the effective hydrodynamic radius of the polymer coil and, hence, on the solvent quality.
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