The glass transition temperature (T_g) is routinely used to characterize polymers as it yields useful information about processability and physical properties. A lesser-known property of polymers is the thermal diffusion coefficient (D_T), which is a measure of flux induced by a temperature gradient. This transport process has been leveraged in thermal field-flow fractionation (ThFFF) to drive separations and characterize polymers. It has been shown that D_T is unique for polymers (molar mass >1 kDa) of different compositions and tacticities. This allows for the separation of polymers by size and composition. Recently it was demonstrated that ThFFF can also yield information about polymer branching.
Various models have been introduced to explain thermophoresis and its correlation with polymer and solvent attributes. Due to the dependence of D_T on molar mass (<1-10 kDa), Kohler predicted a dependence on chain stiffness. This is the basis for our hypothesis that there may be a correlation between the glass transition temperature T_g and D_T. Experimental T_g and D_T were obtained from literature sources and compared. A linear relationship with only a few exceptions was observed for polybutadiene (1-4 and 1-2), polyisoprene, poly(butyl acrylate), poly(methacrylate), poly(methyl methacrylate), polystyrene, and poly(a-methylstyrene). Polymer T_g, as described by the locally correlated lattice model, strongly correlates with the entropy per segment of a polymer chain. These relationships imply that segmental entropy may be used to further probe thermal diffusion phenomenon. Current models focus only on the enthalpic interactions between polymer and solvent. This work presents a step towards a more complete description of thermal diffusion of polymers in dilute solutions.

Yongfu Li

Dow Chemical Company

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