Aluminum crosslinked 6FDA-6FpDA:DABA 2:1 and 1:2 copolyimide membranes were synthesized and their CO2-induced plasticization was investigated. The membranes are unstable against CO2 plasticization in long-term permeation tests at 40 atm CO2 feed pressure at 35 degreesC. To correlate the plasticization with the polymer morphology, scanning transmission electron microscopy (STEM) was used to image the ionomers. STEM shows Al-rich aggregates in both ionomers, but the aggregate size and shape distribution in the 1:2 film is much more heterogeneous than in the 2:1. The 1:2 ionomer contains spherical aggregates with diameters from similar to 5 to 25 nm, chainlike assemblies of small spherical aggregates, vesicular aggregates, large distorted vesicular aggregates, and long band-like structures that extend up to 1000 nm in one dimension. The 2:1 ionomer only contains a few approximately spherical aggregates. Large fractions of both ionomers do not exhibit visible aggregates on the STEM length scale. IR spectroscopy reveals a neutralization level of the acid groups above 60% and transmission electron microscopy detects unreacted neutralizing agent in the polymer. Small angle and wide angle X-ray scattering patterns show scattering patterns that can be linked to the solid-state structure of the polymer backbone rather than the ionic aggregates. (C) 2003 Elsevier Science Ltd. All rights reserved.