By removing anhydride groups from crosslinked copolyimide membranes at 430 °C, partially pyrolysed membranes were obtained and gas separation properties were evaluated and related to the content of labile groups. The spaces occupied by the linking anhydride groups prevailed as micropores, which greatly improved the gas transport properties.
The release of the labile groups was confirmed by TGA and FT-IR spectroscopy which permitted us to establish that polymer main chains did not suffer any substantial change during the pyrolytic treatment. During the heating step new cross-linking points are created, no Tg below 450 °C could be observed and the process prevents the collapse of micropores.
The pyrolysed membrane obtained from the precursor with the lowest content of anhydride groups showed the highest permeability values: 101, 25, 17 and 446 barrers to O2, N2, CH4 and CO2 respectively, and a selectivity to CO2/CH4 of 26, good enough to place this membrane close to Robeson's new upper bound (2008), while the crosslinked precursor membrane fell below Robeson's old upper bound (1991).
