Synthesis and characterization of soluble polyimide membrane materials for pervaporation of n-octane/thiophene mixtures

Three kinds of soluble polyimides containing flexible groups which can be applied to pervaporation of n-octane/thiophene mixtures are synthesized in our lab. Soluble polyimides were synthesized from three dianhydrides, 2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA), 1,4-bis(3,4-dicarboxyphenoxy)benzene dianhydride (HQDPA) and 2,2-bis[4-(3,4-dicarboxyphenoxy)-phenyl]propane dianhydride (BPADA), and one flexible diamine 4,4′-diamino-3,3′-dimethyldiphenylmethane (DMMDA)) via a two-step method. The obtained polyimides were readily soluble in polar solvent at room temperature. The chemical structure of the polyimides was characterized by FT-IR and NMR. The glass transition temperatures of the polyimides were 256,243 and 249° C, respectively. The polyimides remained fairly stable up to a temperature below 525° C.Asymmetric membranes of three polyimides could be prepared by the typical phase-inversion method. The total flux and the separation factor of the pervaporation membranes are in the range of 5.42-2.63 kg/m2h and 2.65-4.60, respectively for n-octanee/thiophene mixtures at the temperature of 350 K.