Enhancement of diffusion in a high-permeability polymer by the addition of nanoparticles

A nanocomposite was formed by adding fumed silica to the high-permeability random copolymer of tetrafluoroethylene (TFE) and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (PDD). The self-diffusion constant of pentane in cast films of the pure polymer and nanocomposites containing 10, 20, 25, and 30 wt % fumed silica was measured. The addition of 30 wt % fumed silica increased the self-diffusion constant by an order of magnitude. For the pure polymer and all four nanocomposite compositions, the apparent self-diffusion constants are a function of the time over which diffusion is observed, Delta, in the pulse field gradient experiment. At a given level of fumed silica, diffusion appears to be faster when observed over shorter times and decreases toward a plateau value at long times. This result is qualitatively indicative of tortuous diffusion. The addition of fumed silica not only increases the apparent self-diffusion constants but also changes the dependence of the apparent self-diffusion constants on Delta. The self-diffusion constants decrease more slowly as a function of Delta, indicating better connectivity of the more permeable domains. The apparent diffusion constants also increase as a function of time after the introduction of pentane. This phenomenon is observed in the pure polymer as well and is described as conditioning of the membrane. Conditioning of the membrane increases diffusion constants initially; but after a few months, aging of the membrane leads to a decrease in translational mobility.