Polyhedral oligomeric silsesquioxane-based fluoroimide-containing poly(urethane-imide) hybrid membranes: Synthesis, characterization and gas-transport properties

The purpose of this work is to study the gas permeation rates of O₂, N₂ and CO₂ gases and selectivity of O₂/N₂ and CO₂/N₂ using synthesized fluorinated poly(urethane-imide) polyhedral oligomeric silsesquioxane (FPUI-POSS). FPUI-POSS membranes having different amounts of fluorinated imide were synthesized via simple condensation reaction of isocyanate terminated prepolyurethane (PU) and anhydride terminated fluorinated prepolyimide (FPI). All the membranes were characterized for structural details [attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR)], thermal stability [thermogravimetric analysis (TGA)], surface morphology and porosity [scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM)], mechanical strength [dynamic mechanical analysis (DMA)], and polarity (contact angle). The density and the fractional free volume (FFV) were determined to study and to correlate the structure-gas transport properties of these membranes. From the surface morphology studies, root mean square (RMS) surface roughness value of higher percentage of fluorinated membrane (FPUI-30-POSS) showed 48nm compare to the other membranes. From the dynamic mechanical analysis (DMA), storage modulus decreases with increase in the imide content. Thus, DMA of the membrane with higher imide content (FPUI-30-POSS) shows lower storage modulus due to decrease in the urethane crosslink density. Higher imide content membrane has lowest density of 1.02g/cm³ and resulting in higher free volume due to the hindrance in the chain packing of rigid C(CF₃)₂ groups. There is a strong relationship between fractional free volume and the gas permeability. Both FFV and gas permeability can also be further correlated with density. It was concluded that the fluorinated imide content increased in the polymeric membranes simultaneously increases the surface roughness and thereby lowering the density because of the loss of uniformity and flexibility of the hybrid membranes.