Crosslinked poly[1-(trimethylsilyl)-1-propyne] membranes: Characterization and pervaporation of aqueous tetrahydrofuran mixtures

Abstract

To enhance their applicability in a broader range of pervaporation feed streams, poly[1-(trimethylsilyl)-1-propyne] (PTMSP) membranes have been successfully crosslinked, using a 3,3′-diazido-diphenylsulfone crosslinker. Both photochemical and thermal processes were used to activate the bis(azide) and thus initiate the crosslink reaction. The presented photochemical crosslink process, has an insufficient efficiency, due to the unreacted bis(azide) and the formation of by-products such as carboxylic acids. On the other hand, thermal annealing at temperatures of at least 160 °C allows successful crosslinking of PTMSP. In contrast to photochemical crosslinking, the bis(azide) completely decomposes after thermal treatment, rendering the membranes insoluble in solvents that dissolve the uncrosslinked polymer, such as tetrahydrofuran (THF), n-heptane and methyl-tert-butyl ether. All membranes were extensively characterized by means of infrared analysis, solid-state 1H-wideline NMR, positron annihilation lifetime spectroscopy, swelling capacity measurements and pervaporation measurements. These techniques, allowed to gain insight in the crosslink reaction mechanism, crosslinking density of the crosslinked polymer network, changes in the free volume cavity sizes, solvent resistance and pervaporation performance, respectively. The potential of the thermally crosslinked PTMSP membranes in the removal of demanding solvents from aqueous mixtures was illustrated by pervaporation tests on dilute THF/water mixtures. The membrane containing 15 wt.% of crosslinker and treated at 180 °C during 1.5 h showed specific permeation rates that are approximately 4 times higher than those of the commercially available polydimethylsiloxane-based membranes, combined with competitive THF/water separation factors. Feed streams containing 10 wt.% THF could be enriched up to 84 wt.% THF in the permeate.