Molecular modeling techniques are applied to study the cavity size distributions and transport properties of two very permeable polymers, poly (1-trirnethylsilyl-l-propyne) (PTMSP) and a random copolymer of tetrafluoroethylene and 2,2-bis(trifluoromethyl)-4,5-difluoro-l,3-dioxole (TFE/BDD), which have very similar and large fractional free volumes, but very different permeabilities. Using atomistic models, cavity size (free volume) distributions determined by a combination of molecular dynamics and Monte Carlo methods are consistent with the observation that PTMSP is more permeable than TFE/BDD. The average spherical cavity size in PTMSP is 11.2 A, whereas it is only 8.2 A in TFE/BDD. These cavity size distributions determined by simulation are also consistent with free volume distributions determined by positron annihilation lifetime spectroscopy (PALS). The diffusivity, solubility and permeability of CO2 in these polymers were also obtained through molecular simulation. The diffusivity and permeability of CO2 in PTMSP are higher than in TFE/BDD. Good agreement is observed between the simulation and experimental data.
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