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Recent Advances in the Characterization of Free Volume in Model Fluids and Polymers: Shape and Connectivity

F.T. Willmore and I.C. Sanchez
University of Texas, US

free volume, cavity size, nanopore

The solubility and diffusivity of a penetrant species in an amorphous material are determined by the nanoscale properties of its free volume. Positron Annihilation Lifetime Spectroscopy, mechanical measurements of equation of state behavior, and Voronoi tessellations have been applied to study free volume properties. A recent theoretical technique for sizing cavities in model fluids and polymers has been extended to free volume in terms of shape and connectivity. A set of shape parameters is introduced, characterizing nanopores in terms of surface area, volume, radius of gyration, and span. Results are presented for a Lennard-Jones fluid, a hard sphere fluid, water, and for two high free volume polymers of interest to membrane scientists: A random copolymer of tetrafluoroethylene with 2, 2-bis(trifluoromethyl)-4, 5-difluoro-1, 3-dioxole (TFE/BDD) and (poly-trimethyl-silyl-propane) poly(1-trimethylsilyl-1-propyne) (PTMSP). The free volume of the latter polymer exhibits a greater deal of connectivity, as observed using this new technique, in agreement with the higher permeability observed experimentally. Some computer renderings are produced which show the shapes explicitly.

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