Enhanced Gas Transport Properties and Molecular Mobilities In Constrained Poly[1-(trimethylsilyl)-1-Propyne] Membranes

The impact of interfacial confinements on the gas transport properties of poly(l-trimethylsilyl-1-propyne) (PTMSP), as found in nanocomposites, were studied by means of ultrathin film analysis. Bulk deviating gas permeabilities and selectivities were observed in aluminum oxide sieve supported thin film membranes of PTMSP, as a function of film thickness, revealing permeation maxima for carbon dioxide and helium in ~700 nm films. The reverse selectivity of CO₂/helium from bulk to the interfacial region was found to monotonically increase by a factor of ~ 1.5 to 2.0. An energetic analysis of the polymer mobility conducted by intrinsic friction analysis (IFA)) revealed an enhancement in the backbone methyl-group rotations in thin films between 400-800 nm with highest mobility at  ~600 nm thickness, identified by the lowest enthalpic energy of ~4 kcal/mol, compared to ~5.5 kcal/mol and ~7 kcal/mol above 800 nm and below 300 nm, respectively.  While the enhancement in mobility between 400-800 nm is supporting evidence for increased free-volume, the high enthalpic energy in the ultrathin film region below 400 nm is indicative of a decreased mobility/free volume configuration of PTMSP compared to the bulk. A three layer model is employed to interpret the observed flux and mobility behavior of confined PTMSP membranes.