Soret coefficients for aqueous polyethylene glycol solutions and some tests of the segmental model of polymer thermal diffusion

A thermogravitational cell is used to measure Soret coefficients (s) for dilute binary aqueous solutions of ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, and polyethylene glycol (PEG) fractions with average molecular weights from 200 to 20,000 g-mol(-1). The cell design allows the top and bottom halves of the solution column to be withdrawn and injected into a high-precision HPLC differential refractometer detector for analysis. Previously reported mutual diffusion coefficients D and the measured Soret coefficients are used to calculate thermal diffusion coefficients D-T. s and D vary with the PEG molecular weight M as M+0.53 and M-0.52, respectively; hence, D-T = sD is essentially independent of M. The segmental model of polymer thermal diffusion predicts D-T = DsegUS/RT2, where D-seg is the segment diffusion coefficient, US the solvent activation energy for viscous flow, R the gas constant, and T the temperature. The predicted D-T values, although independent of M, are too large by a factor of five. Additional tests of the segmental model are provided using literature data for polystyrene + toluene, n-alkane + CCl4, and n-alkane + CHCl3 solutions. Agreement with experiment is not obtained. In particular, the measured D-T values for the alkane solutions are negative.