Separation of CO2/CH4 is one of the most important separations in oil and gas industries. Polyimide membranes are remarkable material for this purpose. Robson’s curve considers our claim about polyimide. One of the famous commercial polyimide membranes is Kapton. The structure of Kapton comprised from pyromelliticdianhydride (PMDA) and 4,4 -Oxydianyline (ODA). The permeability of Kapton is low so it’s necessary to reclaim structure to promote their properties.In this study we tried to enhance the gas permeability in this polymer by adding the aliphatic diamine to the polymer structure. The copolyimide synthesized base on PMDA as anhydride and also ODA and 4,9-dioxa-1,12-dodecacanediamine as diamines.Polymer synthesis was performed via thermal imidization based on two-step procedure.The physical properties of the copolyimide were characterized by FT-IR and XRD analyses. The gas separation studies showed higher permeation of gases at the aliphatic based polyimides in comparison to the fully aromatic ones. Also the permselectivity of gases has not changed significantly.The order of the increment in gas permeation of gases was in the reverse order of the size of the gas molecules.

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