The Study Of Low-temperature Pyrolytic Membrane And Carbon Membrane Prepared From Polyetherimide

Gas separation membranes have been developed fast due to their high separation efficency, low energy consumption and simple operation. However, the applications of polymeric membrane are greatly limited on large scale and some rigorous environment due to their low gas separation properties which can not exceed the "Robsen" upper bound. The carbon membrane as a novel inorganic membrane has greatly attracted people's attention since their nanopore structure and exellent gas separation performance. As a novel membrane material, the low-temperature pyrolytic membrane is prepared at 400-500℃in inert gas, air (only occurring part oxidizing reaction) or under vacuum. The low-temperature pyrolytic membranes are highly interested recently, which have the intermixed properties of polymer membranes and carbon membranes, such as toughness and good flexibility for polymer membranes, high gas permeability with reasonably high selectivity for carbon membranes.In this paper, the low temperature pyrolytic membranes are prepared from the novel precursors of the prepolymer of polyetherimides by pyrolyzed the polymer at 400-480℃in air or under vacuum. The thermal decomposition and cross-link behavior is characterized by TG, ATR-FTIR and EA, and the effects of pyrolytic atmosphere, pyrolytic temperature and duration time on the microstructure and gas separation performances are investigated. The results show that the free radicals are formed by the decompositions of imide groups and ether linkage, and react with the oxygen. At last, the cross-link structures are formed by oxygen bridges and the imide groups of inter-molecular. The cross-link structures are formed by the abstraction of hydrogen due to the absence of oxygen under vacuum. The varieties of decomposition and cross-link degree make the microstructure different. The gas separation properties of pyrolytic membranes prepared in air are better than under vacuum.The pyrolytic membranes with the best gas separation performances are prepared at 460℃a duration time 30 min.The carbon membranes derived from low temperature pyrolytic membranes of Polyetherimide are prepared at 550-850℃in Ar flow or under vacuum. The effects of pretreatment and conbonization conditions on the microstructure and gas separation are investigated. The results show that the gas separation properties are best when the pretreatment temperature is 460℃in air. The O₂ permeability is 108.9 Barrer, and the O₂/N₂ selectivity is 14.1. For the carbon membrane pretreated at different temperature under vacuum, the maximum permeability value appears at 460℃, and the maximum selectivities value appears at 480℃. With the increasing of carbonization temperature, the degree of graphitization increases gradually. The carbon membrane prepared at 850℃show the best O₂/N₂ separtion properties with the selectivity of 32.3.