A new family of partially pyrolyzed membranes (PPMs) based on copolyimides containing poly(ethylene oxide) side chains (PI-PEOs) as precursors has been prepared. The synthesis of PI-PEOs was carried out by an esterification reaction of a copolyimide containing carboxylic acid groups and poly(ethylene glycol) monomethyl ethers of differing molecular weights. The degree of modification and the PEO content were determined by H-1-NMR and some properties such as crystallinity, water uptake and thermal behavior were investigated. The PI-PEOs membranes were pyrolyzed at a relatively low temperature (385 degrees C) to prepare the corresponding PPMs. During the pyrolysis the PEO and carboxylic acid groups were removed, increasing the free volume in the PPMs while the copolyimide framework was maintained. The study of the gas separation properties of both families of polymers revealed the positive effect of removing the PEO side chains because the gas permeability of the pyrolyzed membranes increased by two to six times compared to the corresponding PI-PEOs, depending on the gas tested, without significant loss of ideal selectivity. The membranes were characterized also by positron annihilation lifetime spectroscopy (PALS) to better understand the gas transport properties. Pyrolysis of the PEO groups was shown to increase the free volume element size and concentration whilst broadening the free volume size distribution. The increase in free volume element size and concentration is deemed responsible for the increase in permeability and diffusivity. (C) 2012 Elsevier B.V. All rights reserved.