Interfacial constraints in ultrathin poly(1-trimethylsilyl-1-propyne) (FTMSP) membranes yielded gas permeabilities and CO2/helium selectivities that exceed bulk PTMSP membrane transport properties by up to three-fold for membranes of submicrometer thickness. Maximum permeability coefficients of 110 x 10(3) Barrer and 27 x 10(3) Barrer for carbon dioxide and helium, respectively, were found to occur in membranes of similar to 750 nm thickness. Indicative of a free volume increase, a molecular energetic mobility analysis (involving intrinsic friction analysis) revealed enhanced methyl side group mobility. This was evidenced by a minimum in the activation energies of similar to 4 kcal/mol in thin PTMSP membranes with maximum permeation, compared to similar to 5.5 kcal/mol in bulk films. Aging studies conducted over the timescales relevant to the conducted experiments signify that the free volume states in the thin film membranes are highly unstable in the presence of sorbing gases such as CO2. These results are discussed and contrasted to PTMSP bulk membrane systems, which were found to be unaffected by aging over the equivalent experimental time scale. (C) 2012 Elsevier Ltd. All rights reserved.