(605d) Nanoflux Analysis in Poly(1-trimethylsilyl-1-propyne) Membranes by Flux Lateral Force Microscopy

Poly(trimethyl silyl propyne) (PTMSP) membrane systems have garnered significant attention for their high reverse selective separation properties. PTMSP, a member of the polyacytelene family, possesses the highest gas permeability of all known synthetic polymeric systems. This has been attributed to the large free volume within the condensed phase (about 20%), which in turn is caused by the inefficient packing of the rigid backbone polymeric molecules with their and bulky pendant side groups. PTMSP exhibits high organic vapor/permanent-gas selectivity. Further enhancements in the permeabilities have also been observed in PTMSP membranes with the addition of silica nanoparticles.

Much research has focused on understanding the capabilities of reverse-selective nanocomposite systems on the bulk scale. Several attempts have been made to further enhance the transport properties, for instance, by the addition of nanoscale filler materials. As transport property characterizations like permeability measurements are bulk scale methods, access to local transport properties in multiphase systems, such as nanocomposites, are very limited. In this context our group has developed a flux-lateral force microscopy (F-LFM) technique, with which in-situ gas fluxes can be monitored on a local scale. This technique, which is based on atomic force microscopy (AFM) utilizes the lubricating effect and the mechanical property changes in the membrane to determine fluxes.

Original work focused on local gas permeability in inorganic stiff materials, such as thin MFI zeolite membranes, followed by water permeation studies in soft polyelectrolyte systems around the glass transition temperature. Thereby, lateral force vs. pressure curves offered a direct measure of local permeabilities. This method is highly versatile, as the nanoscale morphology can be recorded and forces, such as adhesive force measurements between nanoparticles and polymer matrix can be determined with the same probe.

In this paper, we will present F-LFM measurements on thin PTMSP membranes which are (a) free standing, and (b) supported by nano-sieve membranes. The local fluxes are compared to global fluxes obtained from conventional integrated methods. First resolved local reverse-selective flux data will be presented involving helium and carbon dioxide. The paper will also address the impact of aging, and flux dependence on film thickness.