The prototypical solution-processable polymer of intrinsic microporosity, PIM-1, and derivatives thereof offer combinations of permeability and selectivity that make them potential candidate materials for membrane-based gas separations. Paramount to the design and evaluation of PIMs for economical natural gas sweetening is a high and stable CO2/CH4 selectivity under realistic, mixed-gas conditions. Here, amidoxime-functionalized PIM-1 (AO-PIM-1) was prepared and examined for fundamental structure/property relationships. Qualitative NLDFT pore-size distribution analyses of physisorption isotherms (N-2 at -196 degrees C; CO2 at 0 degrees C) reveal a tightened microstructure indicating size-sieving ultra-microporosity ( < 7 angstrom). AO-PIM-1 demonstrated a three-fold increase in alpha(D)(CO2/CH4) over PIM-1, surpassing the 2008 upper bound with P(CO2)= 1153 Barter and ideal alpha(CO2/CH4)=34. Under a 50:50 CO2:CH4 mixed-gas feed, AO-PIM-1 showed less selectivity loss than PIM-1, maintaining a mixed-gas alpha (CO2/CH4) similar to 21 across a 20 bar pressure range. Conversely, PIM-1 endured up to 60% increases in mixed-gas CH4 permeability over pure-gas values concurrent with a selectivity of only similar to 8 at 20 bar. A pervasive intermolecular hydrogen bonding network in AO-PIM-1 predominantly yields a rigidified microstructure that mitigates CO2-induced matrix dilations, reducing detrimental mixed-gas CH4 copermeation. (C) 2014 Elsevier B.V. All rights reserved.