Gas and hydrocarbon vapor transport properties of novel disubstituted polyacetylene membranes

Poly[1-(trimethylsilyl)-1-propyne] (PTMSP) is a high-free-volume, disubstituted polyacetylene with the potential to be an optimum membrane material for vapor separations. PTMSP desirably combines high vapor permeability with high organic-vapor/permanent-gas selectivity, especially under mixture conditions. Recently, other members in the disubstituted polyacetylene family have been found to exhibit gas/vapor transport properties similar to those of PTMSP. To better understand the gas transport behavior of other disubstituted polyacetylenes and PTMSP, their permeation and sorption properties to gases and hydrocarbon vapors have been investigated.; Gas permeation properties of blends of PTMSP with less permeable poly(1-phenyl-1-propyne) [PPP] are very sensitive to composition. Pure-gas permeabilities decrease and selectivities increase dramatically with increasing PPP concentration. In the phase-separated blends, the less permeable PPP phase exists as highly extended, ellipsoidal dispersions in a PTMSP matrix. Correlation of gas permeability and blend morphology suggests that the PPP ellipsoids are probably oriented with their long axis perpendicular to the gas transport direction.; Poly[1-phenyl-2-[p-(trimethylsilyl)phenyl]acetylene], poly[1-(trimethylgermyl)-1-propyne], and poly(4-methyl-2-pentyne), like PTMSP, are characterized by high fractional free volumes and high gas permeabilities. Unlike conventional glassy polymers, they preferentially permeate large hydrocarbons (e.g., n-butane) over smaller, permanent gases (e.g., hydrogen). Permeation and sorption results indicate that these polyacetylenes are weakly size-sieving and that relative penetrant solubility differences play a key role in determining their transport properties.; Physisorption and combined kinetic sorption/permeation results demonstrate that PTMSP and perhaps other polyacetylenes possess microporous characteristics. Apparent internal surface areas are high for the polyacetylenes and are comparable to those measured for microporous materials. Conventional polymers, though, are essentially nonporous and have apparent internal surface areas near zero. Permeation behavior in PTMSP also does not conform to the solution-diffusion mechanism used to describe gas transport in nonporous polymers. This inconsistency, combined with its high apparent internal surface area, provides evidence that PTMSP is a microporous polymer with interconnecting free volume elements that span the material and provide efficient, continuous pathways for gas transport. Thus, high-free-volume, disubstituted polyacetylenes, particularly PTMSP, should be viewed as microporous polymers.