Polymer nanocomposites continue to receive tremendous attention for application in areas such as microelectronics, organic batteries, optics, and catalysis. We have discovered that physical dispersion of nonporous, nanoscale, fumed silica particles in glassy amorphous poly(4-methyl-2-pentyne) simultaneously and surprisingly enhances both membrane permeability and selectivity for large organic molecules over small permanent gases. These highly unusual property enhancements, in contrast to results obtained in conventional filled polymer systems, reflect fumed silica-induced disruption of polymer chain packing and an accompanying subtle increase in the size of free volume elements through which molecular transport occurs, as discerned by positron annihilation lifetime spectroscopy. Such nanoscale hybridization represents an innovative means to tune the separation properties of glassy polymeric media through systematic manipulation of molecular packing.
In contrast to the performance of traditional filled polymer systems, penetrant permeability
coefficients in high-free-volume, glassy poly(4-methyl-2-pentyne) (PMP) increase systematically and substantially with increasing concentration of nonporous, nanoscale fumed silica
(FS). For instance, the permeability of PMP containing 40 wt % FS to methane is 2.3 times
higher than that of the unfilled polymer. Gas and vapor uptake in the PMP/FS nanocomposites is essentially unaffected by the presence of up to 40 wt % FS, while penetrant diffusion
coefficients increase regularly with increasing filler content. This increase in diffusivity is
responsible for elevated permeability in the PMP/FS nanocomposites. The addition of FS to
PMP augments the permeability of large penetrants more than that of small gases, consistent
with a reduction in diffusivity selectivity. Consequently, vapor selectivity in the nanocomposites increases with increasing FS concentration. Activation energies of permeation in
PMP decrease with increasing FS content, suggesting that penetrant diffusive jumps require
less energy at higher filler concentrations. Positron annihilation lifetime spectroscopy (PALS)
reveals that FS subtly increases the free volume in PMP available for molecular transport.
The accessible free volume measured by PALS correlates favorably with relative penetrant
permeability in the nanocomposites. Transmission electron microscopy confirms that the
FS nanoparticles are relatively well dispersed in PMP.
Mixed-matrix membranes (MMMs) are based on polymeric membranes filled with inorganic particles as a means to improve their gas separation performance. In this study, MMMs were prepared from polysulfone (PSf) containing embedded nonporous fumed silica nanoparticles and the gas permeation properties of the resulting membranes were investigated. Physical properties such as film density, thermal degradation and glass transition temperature of PSf/silica MMMs were characterized. The distribution of the silica nanoparticles in PSf was observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Furthermore, the interface between the polymer and silica agglomerates was studied in relation with the gas transport properties. The gas permeabilities of hydrogen, helium, oxygen, nitrogen, methane, and carbon dioxide were measured as a function of silica volume fraction and diffusion and solubility coefficients were determined by the time-lag method. The effect of silica nanoparticles in PSf membranes on gas permeability is in contrast with predictions based on the Maxwell model. The O 2 permeability is approximately four times higher and CH 4 permeability is over five times greater than in a pure PSf membrane. However, the performance comprising permeability versus selectivity of PSf/silica MMMs for O 2 /N 2 and CO 2 /CH 4 follows a similar slope to that of the trade-off upper bound with increasing silica content. Crown
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