Polyimide-silica sol–gel membranes from a novel alkoxysilane functionalized polyimide: preparation, characterization and gas separation properties

Novel polyimide-organosilicate hybrid films were prepared by sol–gel process from a novel functionalized polyimide with alkoxysilanes as pendant groups that increase the affinity between inorganic and organic phases. The synthesis of this functionalized polyimide was carried out by an esterification reaction of a copolyimide containing carboxylic acid groups with allyl alcohol and subsequent hydrosililation. Tetraethoxysilane was used as precursor of silica in different amounts to obtain hybrid membranes with a silica content of 5, 10 and 20 %. The polymers and hybrid membranes were structural, mechanical and thermally characterized. The 29Si-NMR solid state spectroscopy confirmed that silica was covalently bonded to the polyimide. SEM pictures showed a good dispersion of the silica particles and an amorphous morphology was observed by WAXS. DSC analyses revealed an increase in rigidity with the increase in silica content. The mechanical strength of the hybrid membranes decreased with the silica amount, exhibiting a brittle behavior. The evaluation of the gas permeation properties revealed that the film with the lowest silica content showed the highest permeability coefficients for O2, N2, CH4, and CO2 with 34, 8, 6, and 128 barrers respectively, while all hybrid membranes showed similar permselectivities around 4 and 22 for O2/N2 and CO2/CH4 respectively. The fractional free volume of hybrid membranes determined by positron annihilation lifetime spectroscopy followed the same trend that permeability coefficients, confirming that the gas transport properties are mainly governed by the free volume elements.