Novel polyimide ionomers: CO2 plasticization, morphology, and ion distribution

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 °C. 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 ∼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.