Gas-permeable fluorine-substituted poly(diphenylacetylene)s: Effect of number and position of fluorine atoms on gas permeability

Poly(diphenylacetylene)s are ideal for fabricating gas-permeable membranes in gas separation systems. We previously reported the superior gas permeability of fluorine-substituted poly(diphenylacetylene)s compared to their fluorine-free counterparts. In this study, novel poly(diphenylacetylene)s were synthesized by varying the number of fluorine atoms from three to five in the repeating unit (2a, 2b, and 2c). Comparing the gas permeability for 2a-c, the highest value was achieved for poly(diphenylacetylene) with three fluorine atoms in a repeating unit (2a). The oxygen permeability coefficient was 8,100 barrers for 2a, remarkably higher than that of the previously reported poly(diphenylacetylene) with two fluorine atoms in the repeating unit. The density and X-ray diffraction analyses revealed a decrease in the free volume of the membranes with an increase in the number of fluorine atoms. Additionally, 2a-c did not show desilylation because the trimethyl silyl groups were substituted on the benzene ring possessing an electron-withdrawing fluorine atom.