Insight into reverse selectivity and relaxation behavior of poly[1-(trimethylsilyl)-1-propyne] by flux-lateral force and intrinsic friction microscopy

Novel scanning force microscopy techniques (SFM) were employed to investigate poly[l-(trimethylsilyl)-1-propyne] (PTMSP), a high-free volume, highly permeable reverse-selective membrane material. This study reports, for the first time, reverse selectivity in relation to the interfacial gas adsorption capacity of the PTMSP membrane with the gas permeants CO2 and helium. With flux-lateral force microscopy (F-LFM), mechanical property changes caused by permeant gas infiltration were recorded within the polymer interfacial downstream region. In conjunction with bulk permeation measurements and varying sequential exposure to the two permeants, CO2 is found to saturate the membrane faster, i.e. at a lower differential pressure by about 0.3 bar, in comparison to helium. It is also identified as modifying agent for PTSMP causing a significant change in the mechanical properties of the polymer matrix, which consequentially leads to a considerable helium transport reduction, and thus, an increase in reverse selectivity from 1.2 to 4.7. Also in this study, thermally available activation modes of 6–8 kcal/mol were revealed by intrinsic friction analysis (IFA) that were attributed to backbone methyl-group rotations in accordance with conformational calculations. Bulk thermally activated modes were found to be modestly affected by interfacial constraints on the sub-100 nanometer scale, which is an important finding for interpreting interfacial constraints in PTMSP nanocomposites involving silicon-oxides.