A study of gas transport properties of semifluorinated poly (ether imide) membranes containing cardo diphenylfluorene moieties

This manuscript presents the gas transport properties of a series of semifluorinated poly(ether imide) (PEI) membranes containing diphenylfluorene moiety in the main chain. The effect of diphenylfluorene moiety towards four gases (e.g., CO2, O2, N2 and CH4) at three different temperatures (35, 45 and 55 °C) under an applied upstream pressure of 3.5 bar has been examined. The membranes were prepared by thermal imidization of the poly(amic acid)s, reaction products of a diphenylfluorene containing semifluorinated diamine monomer, 4,4′-bis-((2′-trifluoromethyl-4′-(4″-aminophenyl) phenoxy)-9-fluorenylidene (FBP) with five different commercial dianhydrides namely, PMDA, BTDA, 6FDA, ODPA, and BPADA in DMF. All these polymeric membranes showed higher gas permeability with comparable permselectivity. The polymer FBP-6FDA and FBP-BTDA showed better separation efficiency in comparison to many other FBP containing polymeric membrane of different classes (e.g., polysulfones, polycarbonates, polyacrylates, poly(ether ketone)s, poly(arylene ether)s, poly(arylene ether ketone)s etc.) reported earlier. Moreover highest ideal CO2 permeability of 53.09 Barrer with highest permselectivity (39.62 for CO2/CH4) of the FBP-6FDA membrane containing hexafluoro-isopropylidene group made this polymer interesting for gas separation purpose among the entire diphenylfluorene based polymeric membrane investigated so far. This FBP-6FDA membrane also showed the highest permeability for O2 (13.46 Barrer) with ideal permselectivity of 6.53 whereas FBP-BTDA also showed similar ideal permselectivity of 6.57 for O2/N2 gas pair but with lower permeability value (6.24 Barrer). The effect of temperature on the gas transport properties of these membranes was also studied. The activation energies for gas permeation and diffusion processes were calculated from the corresponding Arrhenius plots. An attempt has been taken to draw a structure property correlationship between the polymer structures and their gas transport properties.

Research highlights▶ Effect of diphenylfluorene unit on gas transprort properties. ▶ Carbon dioxide permeability of 53 barrer. ▶ Carbon dioxide/methane permselectivity of 39.62.