Thermal and mass diffusion in a semidilute good solvent-polymer solution

The Soret coefficient S-T and collective (mass) diffusion coefficient D-c of polystyrene dissolved in the good-solvent toluene has been measured over a range of concentrations and molecular masses with an optical beam-deflection method. Our measurements indicate that S-T scales inversely with the polymer translational diffusion coefficient in dilute solutions, exhibits a power-law scaling with polymer concentration, and an independence of polymer molecular mass in semidilute solutions. These findings are consistent with the known scaling of 1/D-c in dilute and semidilute polymer solutions, the relative insensitivity of the thermal-diffusion coefficient D-th of polystyrene in toluene to polymer concentration, and the relation S-T=D-th/D-c from irreversible thermodynamics. We are able to represent our S-T and D-c data by theoretically motivated reduced-concentration master curves, but the concentration-molecular mass scaling variables are found to be different for each transport property, a result contrary to theoretical expectations. However, the asymptotic concentration scaling exponents deduced from these data fits are compatible with de Gennes' scaling arguments for D-c and with modern estimates of the chain-size exponent nu for swollen polymers in good solvents. (C) 1999 American Institute of Physics. [S0021-9606(99)51128-8].