Vapor transport in silica-based hybrid and mixed matrix glassy membranes

In this study we analyzed and compared the vapor transport behavior into two different composites based on PTMSP and obtained i) as mixed matrices by loading and hydrophobic fumed silica nano-particles in PTMSP and ii) as hybrid matrices formed by PTMSP and SiO2 generated in situ from tetraethoxysilane (TEOS) via a sol-gel reaction. The experimental data show that n-butane solubility and diffusivity in the mixed matrices increase monotonously with the silica fraction, while the transport rates in hybrid matrices are lower than those of the pure polymer and much lower than those observed in mixed matrices at equal content of silica. None of these trends can be described by the Maxwell's model, which assumes absence of interactions between polymer and filler: in particular, the diffusivity in the mixed matrices is higher than predicted, while that in hybrid matrices is lower than the estimated value. The data were thus modeled using a different approach that is able to represent the transport behavior of both types of composite membranes, based on a small set of physical and empirical parameters. The model adopted describes the gas diffusion as a function of one parameter, i.e. the fractional free volume of the polymeric matrix, which can be estimated based on sorption data of one penetrant in the polymer and in the composite matrices, and the NELF model for glassy polymers. The data relative to hybrid and mixed matrices lie on a same mastercurve when the diffusivity in the matrix is plotted against the fractional free volume estimated by means of the NELF model. It was concluded that, in mixed matrices additional free volume is created at the interface between polymer and filler, maybe due to poor interactions between the phases; for hybrid matrices the inorganic phase is deeply interconnected to polymeric chains and acts as an additional constraint on the open structure of PTMSP, lowering its fractional free volume.