PERVAPORATION SEPARATION FOR TOLUENE/n-HEPTANE MIXTURE BY POLYIMIDE MEMBRANES CONTAINING FLUORINE

Five kinds of polyimides were synthesized using five dianhydrides (including 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl] propane dianhydride (BPADA), 3,3',4,4'-diphenylsulfone-tetracarboxylic dianhydride (DSDA), 4,4'-(hexafluoroisopropylidene)-diphthalic anhydride (6FDA), 1,4-bis(3,4-dicarboxyphenoxy) benzene dianhydride (HQDPA), and 4,4'-oxydiphthlic dianhydride (ODPA)) and 2,2-bis[4-(4- aminophenoxy) phenyl] hexafluoropropane (BDAF) via the two-step method that included polyaddition to form the polyamic acid and subsequent chemical imidization at ambient temperature. The structures of polyimides were characterized by FTIR and NMR. The thermal properties were characterized by DSC and TGA. All five kinds of polyimides showed good thermal properties and solubility in organic solvents such as DMF, DMAc, NMP and THF at room temperature. The pervaporation (PV) experiments of polyimides for toluene/n-heptane mixture were carried out, and all the polyimides showed selective permeation towards toluene. The fluxes of 6FDA-BDAF, DSDA-BDAF, HQDPA-BDAF and ODPA-BDAF at 80 degrees C were 1.08, 0.96, 1.77 and 0.10 kg.mu m/(m(2).h), and the separation factors were 5.44, 1.64, 1.28 and 11.44, respectively. The increasing feed temperature resulted in higher flux and lower separation factor of the 6FDA-BDAF membrane.