Structural and transport properties of BTDA-TDI/MDI co-polyimide (P84)–silica nanocomposite membranes for gas separation

Abstract

Fumed nano-SiO2 particles were modified with a silane coupling agent (3-aminopropyl)triethoxysilane and they were subsequently incorporated into P84 polymer matrix to form nanocomposite membranes. Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) were used to determine the effectiveness of binding the (3-aminopropyl)triethoxysilane onto the surfaces of the SiO2 particles. Physical properties including the membrane density, the stress modulus, the tensile strength and the glass transition temperature of the resulting nanocomposite membranes were obtained. The morphology of the membranes was observed using scanning electron microscopy and transmission electron microscopy. The composite membranes based on the modified SiO2 showed better bonding morphology at the polymer/particle interfaces, higher stress modulus and tensile strength, and significantly higher glass transition temperature as compared with those of the composite membranes based on the pristine SiO2 particles. The effects of the SiO2 surface modification and the SiO2 loadings on the gas separation capability of the resulting composite membranes were investigated. The permeability of all gases (CO2, O2, N2 and He) increased with increasing volume fraction of the inorganic filler. The composite membranes with less than 0.14 volume fraction of the modified SiO2 had comparable selectivity as that of the pure P84 membrane and significantly higher tensile modulus and stress than the latter. A decrease in selectivity was observed with increasing modified SiO2 volume fraction. With the same weight percentage of the inorganic filler, the composite membranes based on the modified SiO2 had higher selectivity than that of the composite membranes based on the pristine SiO2. The permeability of the composite membranes was compared with the theoretical predictions based on the Maxwell equation and the free volume theory. The Maxwell equation predicted lower values than the experimental data whilst the free volume theory produced acceptable predictions except for a large discrepancy in the permeability value for the P84/25%SiO2 composite membrane.