Post-combustion carbon dioxide capture has the greatest near-term potential compared to other technological pathways to mitigate CO2 emissions. The hybrid membrane and cryogenic distillation process developed at American AirLiquide allows sub-ambient temperature (-50 degrees C to -20 degrees C) CO2 capture to be cost effective. In previous work, Matrimid (R) hollow fiber membranes with so-called "nodular" selective layers were successfully formed and showed a combination of high selectivity and high permeance at temperatures below -20 degrees C. Despite good performance, considering the low CO2 concentration and large volume of flue gas to be treated, the permeance of the "nodular" Matrimid (R) fibers still required improvement. In this work, a new membrane material with higher fractional free volume than Matrimid, 6FDA/BPDA-DAM (1:1), was investigated for sub-ambient temperature (SAT) CO2 capture. Defect free 6FDA/BPDA-DAM hollow fiber membranes were successfully formed; however, the bare fibers lost selectivity at temperatures below 0 degrees C, possibly due to the development of minor defects in the thin selective layer. This problem was corrected by post-treatment with polydimethyl siloxane (PDMS). The PDMS post-treated 6FDA/BPDA-DAM hollow fiber membranes displayed much higher permeance than the Matrimid (R) fibers with a high selectivity. The high selectivity is presumably due to additional polymer backbone rigidity at SAT conditions combined with possible local chain orientation; and the high permeance is believed to be a result of hindered packing of the polymer backbone. (C) 2016 Elsevier B.V. All rights reserved.