Mass transport in hybrid PTMSP/Silica membranes

Hybrid silica/poly(trimethylsilyl propyne) (PTMSP) membranes, in which the silica particles are generated in the polymer solution with a sol-gel reaction from a tetraethoxysilane (TEOS) precursor, were fabricated with silica loadings up to 30 wt%. The membranes are characterized with microscopic, thermogravimetric and pycnometric analysis; their sorption and diffusion properties with respect to n-C4 and n-C5 vapors are also measured at 25°C. A significant reduction of the polymer specific volume, organic vapour solubility and diffusivity is observed after incorporation of silica, according to a combination of free volume filling and compressive constraints. The behaviour of the hybrid membranes (HM) is opposite to that of mixed matrix membranes (MMM) obtained by physical mixing of preformed silica nanoparticles into PTMSP solutions, which show, on the contrary, free volume, vapour solubility and diffusivity values higher than pure PTMSP. The hybrid membranes are characterized by larger silica domains (around 1 m) than mixed matrix ones. Remarkably, however, the transport behaviour of both type of membranes can be interpreted, by the same theoretical background, on the basis of a single structural parameter, namely the density of the polymer phase. This parameter allows a direct estimation of the variation of free volume and vapour solubility and diffusivity in both type of membranes, through a NELF/free volume approach.