Effects of molecular structure on the permeability and permselectivity of aromatic polyimides

Permeability coefficients of H₂, O₂, and N₂ were measured under 10 atm at the temperature from ambient temperature up to 150°C in a series of structurally different aromatic homo-and copolyimides, which were prepared from 4,4′-oxydianiline (ODA) or 4,4′-methylene dianiline (MDA) with various aromatic dianhydrides. The study shows that the molecular structure of the polyimides strongly influences gas permeability and permselectivity. As a result, the permeability coefficients of the polyimide membranes for each gas vary by over two orders of magnitude. In general, among the polyimide membranes studied, the increase in permeability of polymers is accompanied by the decrease in permselectivity, and the MDA-based polyimide membranes have higher permeability than ODA-based ones. Among the polyimides prepared from bridged dianhydrides, the permeability coefficients to H₂, O₂, and N₂ are progressively increased in the order BPDA <BTDA <ODPA ～ TDPA <DSDA <SiDA <6FDA, while H₂/N₂ and O₂/N₂ permselectivity coefficients are progressively decreased in the same order. The copolyimide membranes, which were prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), bis(3,4-dicarboxyphenyl)dimethylsilane dianhydride (SiDA), and ODA, have favorable gas separation properties and are useful for H₂/N₂ separation applications. © 1996 John Wiley & Sons, Inc.