Gas permeation through a synthesized composite PDMS/PES membrane

In this paper, a defect-free polydimethylsiloxane (PDMS)-based composite membrane was synthesized and characterized. The membrane consisted of a thin PDMS film (4 μm) and polyethersulfone (PES) as a support material. At first, a macroporous PES support was prepared by phase inversion method and its thickness, porosity, pore size distribution and surface porosity were determined. PES characterization results confirmed suitability of the support for the composite membrane preparation. Then, PDMS solution, containing crosslinker and catalyst, was cast over the support. Sorption and permeation of C3H8, CO2, CH4 and H2 through the synthesized composite membrane were measured at various upstream pressures and based on solution-diffusion mechanism, their diffusion coefficients were calculated. Flory-Huggins (FH) interaction parameters, χ, of the gases with the polymer matrix were calculated based on Flory-Rehner (FR) expression for the crosslinked PDMS membrane and discussed. The concentration-averaged FH interaction parameters of H2, CH4, CO2 and C3H8 in the synthesized PDMS membrane were estimated to be 2.526, 0.102, 0.435 and 0.248, respectively. Higher χ values exhibited less favorable interactions between the gas and the membrane and vice versa. Chemical similarity of CH4 with backbone structure of PDMS, led to the lowest concentration-averaged χ values for CH4. Increasing feed pressure increased permeability, solubility and diffusion coefficients of bigger gases while decreased those of smaller ones. Hence, C3H8/gas ideal selectivity also increased with increasing feed pressure. Only, local effective diffusion coefficient of C3H8 increased with increasing penetrant concentration which indicated plasticization effect of C3H8 over the range of penetrant concentration studied.