Steady-state and transient transport studies of gas permeation through dense membranes using on-line mass spectrometry

Pervaporation and Gas Separation Properties of High-Molecular Ladder-like Polyphenylsilsesquioxanes

This paper presents the results of studies on the pervaporation properties (for benzene/hexane mixtures) and gas permeability (for He, H2, N2, O2, CO2, CH4, C2H6, and C4H10) of ladder-like polyphenylsesquioxanes (L-PPSQ) with improved physical and chemical properties. These polymers were obtained by condensation of cis-tetraphenylcyclotetrasiloxanetetraol in ammonia medium. The structure of L-PPSQ was fully confirmed by a combination of physicochemical analysis methods: 1H, 29Si NMR, IR spectroscopy, HPLC, powder XRD, and viscometry in solution. For the first time, a high molecular weight of the polymer (Mn = 238 kDa, Mw = 540 kDa) was achieved, which determines its improved mechanical properties and high potential for use in membrane separation. Using TGA and mechanical analysis methods, it was found that this polymer has high thermal (Td5% = 537 °C) and thermal-oxidative stability (Td5% = 587 °C) and good mechanical properties (Young's module (E) = 1700 MPa, ultimate tensile stress (σ) = 44 MPa, elongation at break (ε) = 6%), which is important for making membranes workable under various conditions. The polymer showed a high separation factor for a mixture of 10% wt. benzene in n-hexane (126) at a benzene flow of 33 g/(m2h).

A Novel Time Lag Method for the Analysis of Mixed Gas Diffusion in Polymeric Membranes by On-Line Mass Spectrometry: Pressure Dependence of Transport Parameters

This paper presents a novel method for transient and steady state mixed gas permeation measurements, using a quadrupole residual gas analyser for the on-line determination of the permeate composition. The on-line analysis provides sufficiently quick response times to follow even fast transient phenomena, enabling the unique determination of the diffusion coefficient of the individual gases in a gas mixture. Following earlier work, the method is further optimised for higher gas pressures, using a thin film composite and a thick dense styrene-butadiene-styrene (SBS) block copolymer membrane. Finally, the method is used to calculate the CO₂/CH₄ mixed gas diffusion coefficients of the spirobisfluorene-based polymer of intrinsic microporosity, PIM-SBF-1. It is shown that the modest pressure dependence of the PIM-SBF-1 permeability can be ascribed to a much stronger pressure dependence of the diffusion coefficient, which partially compensates the decreasing solubility of CO₂ with increasing pressure, typical for the strong sorption behaviour in PIMs. The characteristics of the instrument are discussed and suggestions are given for even more versatile measurements under stepwise increasing pressure conditions. This is the first report on mixed gas diffusion coefficients at different pressures in a polymer of intrinsic microporosity.