Thermodiffusion of Polymer Solutions in Convectionless Cells

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This contribution reviews the study of polymer thermodiffusion in convectionless cells. First, the development of thermodiffusion theory is summarized. Next, the apparent independence of thermodiffusion and degree of polymerization in polymers is discussed. Thermodiffusion coefficients (D T) measured by thermal field-flow fractionation on several polymer-solvent mixtures are then summarized. Polymers include a variety of different molecular weight samples of polystyrene, poly(á-methyl styrene), polyisoprene, and polymethylmethacrylate, as well as copolymers of styrene and isoprene. Solvents include benzene, toluene, ethylbenzene, tetrahydrofuran, 2-butanone, ethyl acetate, and cyclohexane. Measured values of D T do not correlate with predictive models of thermodiffusion, but are found to correlate with polymer density and solvent viscosity. Finally, using a hydrodynamic model of polymer thermodiffusion in nonpolar solvents, an expression is derived that expresses D T in terms of physicochemical parameters that can be estimated or measured independently. These parameters include the thermal expansion coefficient, molar volume and viscosity of the solvent, the radius of a polymer repeat unit, and the Hamaker constants. The model predicts general trends in the thermodiffusion of polymethylmethacrylate and polystyrene in solvents where such parameters are available, but overestimates the values of D T by a factor of 2–4.