Spiro-biindane containing fluorinated poly(ether imide)s: Synthesis, characterization and gas separation properties

A new series of fluorinated poly(ether imide)s (PEIs) was synthesized from the bis(ether amine), 6,6′-bis-[2″-trifluoromethyl 4″-(4‴-aminophenyl)phenoxy]-3,3,3′,3′-tetramethyl-1,1′-spirobiindane containing rigid spiro-biindane unit with different aromatic dianhydrides, e.g., BPADA, 6-FDA, BTDA, ODPA and PMDA. All the PEIs were well characterized by elemental analysis, NMR, FTIR spectroscopy and gel permeation chromatography (GPC). Significant characterizations of their physical properties were done to assess their applicability as membranes for gas separation application. The PEIs exhibited excellent thermo-oxidative stability (409–491 °C for 5% weight loss), high glass transition temperatures (232–288 °C) and high tensile strengths (up to 60 MPa). Gas transport properties of the PEI membranes were systematically investigated at three different temperatures (35, 45 and 55 °C) under an applied upstream pressure of 3.5 bar. PEI ‘8b’ containing hexafluoroisopropylidene (6-FDA) unit showed the highest permeability coefficient in barrer for all the gases (PCO2 = 52.98, PO2 = 36.08) whereas PEI ‘8a’ exhibited the highest permselectivity towards CO2 relative to CH4 (68.2) and PEI ‘8d’ showed the highest permselectivity towards O2 over N2 (11.67) among the series. All the membranes exhibited excellent separation performance for O2/N2 gas pair surpassing the latest upper boundary limit drawn by L.M. Robeson. The effect of the presence of the cardo spiro-biindane unit and structural variations of the aromatic dianhydrides in these poly(ether imide) membranes towards the glass transition temperatures and fractional free volume (FFV) of the polymers and their consequences on gas permeation of four gases (e.g., CO2, O2, N2 and CH4) and permselectivity for pair of gases were correlated. Temperature dependencies of the permeation and diffusion processes were utilized to calculate the activation energies of the permeation and diffusion processes for these four different gases through these PEI membranes.