Optimization study of polyethylene glycol and solvent system for gas permeation membranes

Cellulose acetate (CA) membranes blended with Polyethylene glycol (PEG) in acetone-water solvent system were synthesized by using solution-casting method that resulted in the formation of flexible, white membranes. Different molecular weight (MW) grades of PEG (including MW 1000, 10,000 and 20,000g/mol) were used. Cast membranes were tested for tensile strength and permeability at different loading of PEG MW 10,000 and 20,000g/mol. Excellent flexible membranes were produced in acetone-water solvent system in the presence of PEG, which were otherwise brittle. Surface structure and morphology were analysed using scanning electron microscopy. The presence of different functional groups was confirmed using Fourier transform infra-red spectroscopy and the mechanical characteristics were studied by tensile testing. The introduction of PEG caused an increase in permeability of the membranes. The increase in permeability is due to the opening up of pores as the membrane becomes more flexible, when the plasticizer is added. The permeability continues to increase with the addition of PEG. Moreover, the resulting membranes are not only more flexible, but also have largely improved tensile strength as compared to the CA membranes without PEG. This improved tensile strength can also be attributed to the improved flexibility of the membrane. A trade-off is reached between tensile strength and permeability as increasing amount of PEG improves tensile strength but the resulting membrane becomes too permeable to be used for gas separation. Moreover, using PEG of higher MW resulted in porous membranes, even at low amounts of PEG. Therefore, we concluded that CA membrane with less amount of low-MW PEG (i.e. 5% PEG of MW 1000g/mol) must be used to optimize both permeability and tensile strength of the membrane.