Simultaneous enhancement of CO2 permeability and CO2/CH4 and CO2/N2 selectivity via incorporating dense, rubbery and CO2-philic vinyl acetate-based copolymers into poly(ethylene oxide-b-amide 6) membranes

In this work, blend membranes based on poly(ethylene oxide-b-amide 6), i.e. Pebax 1657, and 30 wt% of dense, rubbery and CO2-philic vinyl acetate/ dibutyl maleate (VAc/DBM) copolymers with different chemical compositions were prepared. Depending on the copolymer composition, glass transition temperature (T-g) of copolymers were measured to be in the range of 7-23 degrees C. Copolymer composition dependency of the copolymers' density was also evaluated. Interaction of the various components present in as well as morphology of the blend membranes were investigated by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimeter (DSC) and field emission scanning electron microscopy (FE-SEM) analyses. The permeability of CO2, CH4 and N-2 gases was determined at a pressure of 3-9 bar and a temperature higher than T-g of CO2-philic VAc/DBM copolymers, i.e. 30 degrees C. Although copolymers had densities higher than that of pristine Pebax 1657; however, CO2 diffusion increased by adding these copolymers. It can be attributed to the increased volume fraction of amorphous portion in the blend membranes, which is responsible for gas transport, as well as to enhanced mobility of polymer chains. CO2 permeability and CO2/CH4 and CO2/N-2 selectivities were found to increase simultaneously in all of the blend membranes in comparison with the pristine Pebax 1657. It may be attributed to enhanced CO2-philicity of the blend membranes, resulting in the enhanced CO2/CH4 (or N-2) solubility selectivity, as well as to the enhanced fractional free volume in the blend membranes with free volume sizes suitable for transport of smaller molecules of CO2, resulting in the enhanced CO2/CH4 (or N-2) diffusivity selectivity. The best results were found for a blend membrane containing 30 wt% VAc/DBM copolymer with 72 mol% VAc in the chains, where CO2 permeability and CO2/CH4 and CO2/N-2 selectivities increased about 198%, 157% and 156%, respectively, compared to the pristine Pebax. Both CO2 permeability and selectivity increased by increasing pressure in the range of 3-9 bar, while plasticization phenomenon was not observed, indicating that the blend membranes prepared in the present work can practically be used for efficient separation of CO2 from the mixed gases.