Synthesis of Polyimides Containing Fluorine for Pervaporation Separation of Toluene/N-Heptane Mixtures

Four kinds of polyimides containing fluorine were successfully synthesized based on various dianhydrides including 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 4,4’-oxydiphthlic dianhydride (ODPA) and 1,4-bis(3,4-dicarboxyphenoxy) benzene dianhydride (HQDPA), and various diamines including 2,2-bis[4-(4-aminophenoxy)phenyl]-hexafluoropropane (BDAF) and 2,2-bis[4-(4-aminophenoxy)phenyl] propypane (BAPP). The polyimides (6FDA-BAPP, 6FDA-BDAF, ODPA-BDAF, HQDPA-BDAF) were prepared according to a two-step method, low temperature polycondensation with total monomers content 15wt% in DMAc and next chemical imidization of viscous poly(amic acid)s. The polyimides thus obtained were characterized by FT-IR, NMR, DSC and so on. The results showed that the polyimides had expected structures, excellent thermal properties and good solubilities in strong polar solvents such as DMF, DMAc and NMP. Furthermore, the pervaporation (PV) performances of these polyimide membranes prepared by solution casting were investigated for separating aromatic from non-aromatic hydrocarbon. All the polyimide membranes showed permeabilities and selectivities towards toluene from n-heptane. Especially 6FDA-BDAF and 6FDABAPP membranes had better PV performances than others. And 6FDA-BDAF membranes had a specific permeation flux of 1.18kg·μm/m·h and separation factor of 5.28, while 6FDA-BAPP membranes had 1.15kg·μm/m·h and 3.43 for 20wt% toluene in the toluene/n-heptane mixtures at 85°C. With the feed temperatures increasing, the flux of the 6FDABDAF membranes was enhanced from 0.66 kg·μm/m·h to 1.18 kg·μm/m·h, but the separation factor was reduced from 6.49 to 5.28.