Isomeric polyimides based on fluorinated dianhydrides and diamines for gas separation applications

The gas sorption and transport properties of two isomeric polyimides with hexafluoroisopropylidene moieties in the diamine and dianhydride monomers were characterized for a variety of gases at 35°C at pressures up to 60 atm. These materials have structural properties which inhibit intrasegmental rotational mobility and intersegmental chain packing. The effect of isomerism on the physical and gas separation properties of these rigid materials was investigated. The effect of isomerism on the hindrance to packing is reflected in the wide angle X-ray diffraction (WAXD) measurements of the average spacing between adjacent polymer chains. The para connected polyimide showed significant increases in permeability relative to a series of polyimides studied earlier with less packing-disruptive substituents on the polymer backbone. The permeability of the higher flux material was 64 barrers for CO₂ and 16 barters for O₂. The meta connected polyimide showed large decreases in permeability with corresponding increases in permselectivity when compared to its para counterpart. For example, the permselectivity of the meta material for O₂ relative to N₂ is 6.9 which is 50% greater than that of the para connected material. The differences in permeability and permselectivity are due to both penetrant solubility and diffusivity effects.