Pyrolysis of polyimide membranes from the same dianhydride monomer and different diamines to form carbon membranes

Polyimide membranes were synthesized by the polymerization of five diamines, p-phenylenediamine (PPD), 2,3,5,6-tetramethyl-1,4-phenylenediamine (TMPPD), 4,4'-oxydianiline (ODA), 4,4'-(4,4'-isopropylidenediphenyl-1,1'-diyldioxy)dianiline (BAPP) and 4,4'-(hexafluoroisopropylidene)bis(p-phenyleneoxy) dianiline (BDAF), and the same dianhydride monomer, 1,2,4,5-benzenetetracarboxylic anhydride (PMDA). The chemical structures of the membranes pyrolyzed at different temperatures were investigated by TGA, TG-MS, FT-IR and XPS. Results indicate that there are five stages during pyrolysis: (a) the removal of the solvents, adsorbed oxygen and water, (b) imidization, (c) cross-linking and carbonization, which are accompanied by main chain breaking, (d) deoxygenation and dehydrogenation, (e) aromatization and turbostratic carbon formation. The chemical structure of the precursor is the main factor that causes the great differences in chemical structures during pyrolysis. Imidization in the pyrolysis of the PPD-PMDA and TMPPD-PMDA occurs at higher temperatures than those for ODA-PMDA, BDAF-PMDA and BAPP-PMDA. The order of temperatures for the release of CH₄, CO₂ and C₆H₆ during the pyrolysis is ODA-PMDA >BDAF-PMDA >BAPP-PMDA. Elemental F in the BDAF-PMDA disappears after pyrolysis at 700℃. The chemical structures of the polyimides have a great influence on the microstructures of the carbon membranes obtained.