Copolyimides were synthesized from dianhydride of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with various diamine contents of 4,4'-oxydianiline (ODA) and 2,3,5,6-tetramethyl-1,4-phenylenediamine (TeMPD) by chemical imidization in a two-step procedure. Polyimides (PIs) were characterized using thermogravimetric analysis, Fourier transform infrared spectroscopy, differential scanning calorimetry, as well as specific volume and free volume. The gas transport properties for pure gas and blends of CO2 and CH4 for the homopolymers and 6FDA-ODA/TeMPD copolymers were investigated at 35 degrees C and 150 psi pressure. In pure gas permeation, permeability of CO2 and CH4 increased with increasing TeMPD content in the diamine moiety, whereas the ideal selectivity decreased with increasing TeMPD content. In the mixed gas permeation, permeabilities and separation factor were measured as a function of CO2 feed molar fraction for five PI membranes. The behavior of pure gas and mixed gas permeabilities and separation factor of CO2/CH4 mixtures as the chemical nature of the diamine and the CO2 molar fraction in the feed gas were varied and are discussed in detail. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 128: 380-389, 2013