Polymeric films based on blends of 6FDA–6FpDA polyimide plus several copolyfluorenes for CO2 separation

Abstract

Three emitting copolyfluorenes, based on 2,7-(9,9-dihexyl)fluorene and different aryl groups (1,4-bencene, PFH-B; 1,4-bencen-1,2,5-thiadiazole PFH-BT; 1,4-naphthalen-1,2,5-thiadiazole, PFH-NT), showing diverse acceptor character, in different proportions were blended with a polyimide 6FDA–6FpDA to make a series of films. These copolyfluorene–polyimide blends were prepared and characterized in the solid state, using several techniques. The fluorescence of conjugated polymers can be used as a tool to understand the formation of the membrane and also to increase permeability and selectivity in comparison to films which do not fluoresce. The relationship between the intrinsic fluorescence of conjugated polyfluorenes and their gas separation properties has been explored in order to establish the influence of the composition and the nature of the aryl group in the conjugated polymer, on the gas separation performance. In all cases, a low proportion of copolyfluorene (<1% weight) gives better CO₂/CH₄ permselectivity properties than the original pure polyimide matrix. The best results were found for the samples that contain PFH-NT. These samples gave over 23% increase in the CO₂ permeability with a 15% increase in CO₂/CH₄ selectivity. Finally, the loss of efficiency in conjugation mechanisms of absorption and emission of the samples could be explained on the basis of the π-stacking of the polymer chains, produced when a certain low percentage of conjugated polymers in the blend is surpassed. When this π-stacking starts, the gas permeation properties start to decline too.