Permeability, permselectivity, and penetrant-induced plasticization in fluorinated polyimides studied by positron lifetime measurements

The ortho-positronium (o-Ps) lifetime tau(3) and its intensity I-3 in various fluorinated polyimides were determined by the positron annihilation technique and were studied with the spin-lattice relaxation time T-1 and the propylene permeability, solubility, diffusivity, and permselectivity for propylene/propane in them. tau(3), I-3, and the distribution of tau(3) changed when the bulky moieties in the polyimides were changed. The polyimides, having both large tau(3) and I-3 values, exhibited a short T-1 and a high permeability with a low permselectivity. The propylene permeability and diffusivity were exponentially correlated with the product of I-3 and the average free-volume hole size estimated from tau(3). In highly plasticized states induced by the sorption of propylene, the permeability increased with the propylene pressure in excellent agreement with the change in the free-volume hole properties probed by o-Ps. The large and broad distribution of the free-volume holes and increased local chain mobility for the 2,2-bis(3,4-decarboxyphenyl) hexafluoropropane dianhydride-based polyimides are thought to be important physical properties for promoting penetrant-induced plasticization. These results suggest that o-Ps is a powerful probe of not only the free-volume holes but also the corresponding permeation mechanism and penetrant-induced plasticization phenomenon. (C) 2002 Wiley Periodicals, Inc.