Gas permeability and permselectivity of polyimides with large aromatic rings

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Abstract
Polyimides with large aromatic rings were prepared from 3,6‐diaminocarbazole (CDA), N‐ethyl‐3,6‐diaminocarbazole (ECDA), 2,7‐diaminofluorene (DAF), 2,7‐diaminofluorenon (DAFO), and dimethyl‐3,7‐diaminodibenzothiophene‐5,5‐dioxide (DDBT) with 2‐bis(2,4‐dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (BPDA). Their physical properties, including gas permeability and permselectivity, were investigated in comparison with those of the related polyimides from 1,3‐phenylenediamine (mPD). Glass transition temperatures of the polyimides with large aromatic rings were much higher than those of the mPD‐based polyimides as a result of increased rigidity of the former polymer chains. With changing diamine from mPD to the large aromatic diamines, charge transfer (CT) interaction between the moieties of acid anhydride and diamine seems to be enhanced, judging from the red shift of absorption edge of the polyimide films and the red shift of CT excitation band of the 6FDA‐based polyimides in solution. Fraction of free space (VF) was a little smaller for the polyimides with large aromatic rings except DDBT than for the mPD‐based polyimides, probably because of enhancement in polymer chain‐chain interactions as a result of the increased CT interaction. The DDBT‐based polyimides had large VF than the mPD‐based polyimides because of the nonplanar structure of neighboring dibenzothiophene‐5,5‐dioxide and imide rings. For the 6FDA‐based polyimides, permeability coefficients to H2, O2, N2, CO2, and CH4 were in the order, DAFO < mPD ∼ DAF < CDA < ECDA < DDBT. As for the membrane performance for H2/CH4, CO2/CH4, and O2/N2 systems, it is significant to change diamine from mPD to DDBT or CDA, but not to DAF or DAFO. The DDBT‐based polyimides were excellent for H2/CH4 and CO2/CH4 separations. © 1995 John Wiley & Sons, Inc.