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The main objective of this study is to acquire a better understanding of the relationships between the chemical structure of polymers and their permeability to different gases. This information is required for the development of new membrane processes for the separation of industrial gases. Based on our previous investigation of structure/permeability relationships, this study provides additional information on gas diffusivity and solubility data in different polyimides. The control factors of this relationship are identified as the structures of diamine and dianhydride, the degree of curing, and the chain morphology and mobilities.
This study confirms that both the gas selectivity and permeability of polyimides can be enhanced simultaneously. The desired membrane materials can be achieved by synthesizing polyimides with over 60% aromatic rings and containing bulky functional groups, such as(CF3)2, which impede chain rotation and act as "spacers" to increase intersegmental distances. In addition, the diamine moieties must be short and rigid to produce a high gas selectivity. Polymide membranes, then, behave as polymeric "molecular sieves." This is demonstrated by the fact that the systematic changes in the structure of the polyimides affect mainly the diffusivity rather than the solubility of the penetrant gases.
