Pyrolytic Mechanism And Kinetics Of The Polyimide Membrane At Low Temperature

As a novel carbon based porous membrane material, carbon molecular sieve membrane (CMS membrane) has the developed nanoporous structure and excellent gas separation performance, which exceeds the "Robeson upper bound". CMS membrane has greatly attracted people’s attention by the potential application. However, high cost, brittleness, low gas permeability and other problems still limit the large-scale application of CMS membrane. The low-temperature pyrolytic membrane is a new membrane material, which fabricated by pyrolyzing the precursors partially under inert atmosphere or vacuum. The pyrolytic membrane contains the excellent flexibility and high gas permeability and selectivity. Therefore, it is important to research the pyrolytic mechanism and the change of micro-structure of polymer precursor during the low temperature pyrolysis. In addition, it has great significance on commercial application of the CMS membrane.In this paper, PMDA-ODA type polyamic acid (PAA) was used as the precursor. TG, TG-MS, ATR-FTIR, XPS were used to investigate the behavior and characteristics, the change of chemical structure and the crosslinking reaction of PAA in pyrolysis at low temperature. The pyrolytic kinetics of PAA membrane was investigated by the calculation of the apparent activation energies E and pre-exponential factor A using the Flylnn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunrose (KAS) method, iterative iso-conversional procedures, Coats-Redfern and Achar-Bridly-Sharp methods.The results show that solvent evaporation, imidization and the formation of cross-linked structure of polyimide reaction occur during the low temperature pyrolysis (100-500℃) Cyclization dehydration reaction plays a dominant role from150℃to300"C. Functional groups on macromolecular chains begin to decompose and cross-link from300℃to500℃. However, the structure remained the skeleton structure of polyimide under the500℃. Also, the results indicate that the pyrolysis of PAA membrane at low temperature can be divided into three stages, in which the adsorbed oxygen and water in PAA are removed in the first stage from r. t to150℃and in the second stage from150℃to300℃, the imidization of PAA occurs and the residual solvent and water formed by the cyclodehy-dration reaction was removed. From300℃-500℃, the ring opening reaction of imide ring and cleavage reaction of ether bond occur to form the cross-linked structure of polyimide. During the low temperature pyrolysis, the pyrolytic temperature plays an important role to the pyrolytic behavior and chemical structure of PAA membrane. During the imidization reaction from150℃to300℃, the structure of PAA is converted into that of polyimide. The imidization of PAA membrane are the three order chemical reactions. Their average apparent activation energy is108.61kJ/mol and the mean pre-exponential factor A is1.19×1014S-1. The compensation effect is clearly observed from the straight line of the plot of InA versus E.