Synthesis and characterization of poly(ethylene oxide) based copolymer membranes for efficient gas/vapor separation: Effect of PEO content and chain length

Due to their high CO2 selectivity, poly(ethylene oxide) (PEO)-based membranes have raised attention to CO2-related gas separations. Nevertheless, the fabrication of such membranes is quite challenging as PEO suffers from poor film forming ability and high crystallization tendency, which restricts gas transport. In this work, a new series of CO2 and water selective aromatic polyethers bearing flexible PEO side chains prepared via a polycondensation reaction is reported. The synthesized copolymers are mechanically and thermally robust, completely amorphous even at 49 wt% PEO and form flexible, soft membranes. As observed, membrane permeation properties are significantly influenced by both PEO chain length and PEO loading. CO2 permeability increases with PEO loading, which is assigned mainly to the enhanced CO2 solubility contribution of the PEO segments. In particular, when PEO content is higher than 39 wt%, copolymers go from the glassy to the rubbery state, thus resulting in increased CO2 permeability due to the significant contribution of favorable CO2-PEO interactions along with a minimal diffusivity increase. CO2 selectivities over H2 and CH4 also increase as a function of PEO content mainly due to the strong solubility contribution on CO2 permeability enhancement. The copolymer with the highest PEO loading (49 wt%) and the longer PEO chain (750) shows both a maximum CO2 permeability of 11.2 Barrer and maximum ideal CO2/H2 and CO2/CH4 selectivities of 9.6 and 68, respectively, at 30 °C. The CO2/H2 ideal selectivity of 9.6 surpasses the upper bound. Moreover, these copolymers show high water permeability that is also strongly dependent on the PEO weight content and PEO chain length. Likewise, the highest water permeability (30,100 Barrer) is delivered by the copolymer with the highest PEO loading (49 wt%) and the longer PEO chain (750) at 30 °C. The highest H2O selectivity over CO2 (2688) observed for this copolymer is comparable with other PEO-based copolymers having excellent separation performance. These results support the possible application of these copolymers in hydrogen purification and dehydration of CO2 gas streams.