Organic-inorganic nanocomposite membranes for vapor separations

Barometric and gravimetric sorption in combination with mixed and pure gas permeation measurements have been employed to study transport in high-free-volume, vapor-selective poly(4-methyl-2-pentyne) [PMP] containing nanometer-sized, nonporous fumed silica. Contrary to behavior in traditional filled polymer systems, addition of fumed silica to glassy, amorphous PMP increases penetrant permeability coefficients by as much as 240%. Gas and vapor solubility in PMP are essentially unaffected by the presence of fumed silica at up to 40 wt%. Penetrant diffusion coefficients in PMP increase regularly with increasing fumed silica content, and it is this rise in diffusivity that is responsible for elevated permeability in the nanocomposites. Fumed silica addition to PMP augments the permeability of large penetrants more than that of small gases, consistent with a weakening of diffusivity selectivity. Consequently, PMP experiences a favorable increase in vapor selectivity with increasing finned silica concentration. These data are consistent with fumed silica increasing free volume in PMP presumably through disruption of polymer chain packing.; Positron annihilation lifetime spectroscopy [PALS] data confirm transport results revealing that fumed silica addition increases accessible free volume in PMP. Large free volume elements increase in size systematically with increasing fumed silica content in PMP, and there is an excellent correlation between relative PALS accessible free volume and penetrant permeability coefficients in the nanocomposites. Fumed silica aggregate sizing from transmission electron microscopy indicates that these particles are relatively well dispersed in PMP, consistent with weak polymer-filler interactions. Primary particle size appears to be more important than particle surface chemistry for inducing permeability enhancement, with smaller particles being favored.; Transport in filled PMP is compared and contrasted with that in two additional filled high-free-volume polymers, vapor-selective poly(1-trimethylsilyl-1-propyne) [PTMSP] and size-selective poly(2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole- co-tetrafluoroethylene) [AF2400]. Similar to PMP, fumed silica addition increases the permeability of both polymers. The size sieving ability of AF2400 weakens with increasing fumed silica content, and in fact, this polymer experiences a selectivity reversal, transforming from methane selective to n-butane selective at high fumed silica loadings. In contrast to PMP and AF2400, the vapor selectivity of PTMSP is reduced by fumed silica addition. Filling of PTMSP apparently increases free volume in this polymer to the extent that free phase transport mechanisms, such as Knudsen diffusion, become important. AF2400's glass transition temperature is unaffected by the addition of fumed silica particles, indicating the filler has little impact on chain stiffness or mobility. The enhancement in permeability at a given fumed silica loading decreases as the permeability of the base polymer being filled increases. This result suggests that the polymer with the greatest amount of free volume can accommodate fumed silica particles with the least disruption of polymer chain packing and, experience therefore, the smallest permeability enhancement and vice versa.