Liquid permeation through PTMSP: One polymer for two different membrane applications

Abstract

The extra high free volume glassy polymer poly[1-(trimethylsilyl)-1-propyne] (PTMSP) is a perspective membrane material for two inherently different applications: (i) organic solvent nanofiltration (OSN), when a high solvent flux through the membrane is expected, and (ii) high pressure membrane contactors for CO2 capture that requires the absence of liquid permeation (no liquid leakage) through the membrane. The transport of wetting/non-wetting liquids through PTMSP was studied. Filtration experiments of binary water–ethanol mixtures revealed that no liquid permeation through PTMSP took place at trans-membrane pressures up to 30bar if the alcohol content is 40wt% or lower. The further increase of the ethanol concentration (50wt% and higher) provides the hydrodynamic flux through PTMSP; its value increases with increasing ethanol content. No single transport model available in the literature can satisfactorily describe the water–ethanol permeation through PTMSP. In this work, a new semi-empirical model was developed and it was successfully applied for describing the experimental data on liquids permeation through PTMSP dense membranes at pressure differences up to 150bar. The model takes into account the fractional free volume of polymer, swelling degree of the polymer in solvent environment, membrane compaction under elevated pressure difference across the membrane and physical–chemical macroscopic parameters of the liquid and polymer such as surface tension, viscosity and molar volume. The sorption and swelling degree of PTMSP were measured for the water–ethanol mixtures under study. The sorption and swelling data and liquid permeation results for binary ethanol–water mixtures revealed that, in spite of remarkable content of the liquid in the free volume elements of PTMSP (i.e. sorption is up to 0.42g/g and swelling degree is up to 0.12), there is no liquid flux through PTMSP at pressure differences of 10–30bar. Thus, the threshold values of the PTMSP swelling degree was estimated on the level of 0.12–0.19. It was shown that PTMSP assures zero liquid flux (no liquid leakage) for the PTMSP/liquid system the corresponding parameters of which are below the threshold region (important for high pressure membrane contactor applications). When sorption and swelling for a PTMSP/liquid system are above the threshold levels, the PTMSP membrane exhibits a certain hydrodynamic flux for the corresponding liquid (important for OSN applications).