Gas separation and pervaporation of chemically-modified poly(1-trimethylsilyl-1-propyne); membranes

Poly (1-trimethylsilyl-1-propyne) (PTMSP) has the highest gas permeability coefficient of any known polymer. It is a potential high productivity membrane material when used for gas separation and it is also one of the few polymers that permeates ethanol preferentially against water. Thus it is potentially useful for separating ethanol from dilute aqueous solution. However, its selectivity for gas separation and liquid separation is relatively low, and the high gas permeability has been reported to be unstable with time, with the permeability coefficient of oxygen decreasing to 1/10 of the original value after 100 days when stored under vacuum.; We believe the permeability decline is caused by the slow interdiffusion of polymer coils in the solid state. Several methods were attempted in this study to cross-link the PTMSP membranes in order to stabilize the permeability. PTMSP membranes were successfully cross-linked via addition of bis(aryl azides) and by preparing copolymers that contain azide groups poly (1-trimethylsilyl-1-propyne-co-1-(4-azidobutyldimethylsilyl)-1-propyne). Both photo irradiation and thermal treatment initiated the cross-linking. Photo-crosslinked membranes (either by bis(aryl azides) or copolymerization) showed higher selectivity for oxygen/nitrogen separation with lower permeability than PTMSP membranes. Photo-crosslinked membranes with certain cross-linking degrees showed higher selectivities than the commercial membranes with comparable permeabilities. Thermal-crosslinked membranes showed little changes in permeability and selectivity. All cross-linked membranes resulted in a stabilized permeability compared to PTMSP membranes when stored in vacuum. This is believed due to the restriction of inter-chain diffusion caused by the cross-linking, which helped to maintain the high free volume and therefore stabilized the permeabiility.; In studies of ethanol/water separation through PTMSP membranes, we attempted to use solubility parameters, {dollar}delta{dollar}, as a guide in the synthesis and modification of ethanol-selective membranes. A variety of modified PTMSP membranes were used as pervaporation membranes for ethanol/water separation. The solubility parameters of these membranes were obtained by inverse gas chromatography, swelling measurements, and the group contribution method. There is difficulty in achieving a quantitative relationship between the solubility parameter of polymer membranes and pervaporation results of ethanol/water separation, and both the free volume and solubility parameters of the membranes must be considered in the design of membranes. Some modified membranes showed higher separation factors compared to PTMSP membranes when used for ethanol/water separation.