(231d) Vapor Transport in Silica-Based Hybrid and Mixed Matrix Glassy Membranes

Authors: 
Galizia, M., University of Bologna
Puccini, I., Università di Modena e Reggio Emilia,
Messori, M., Università di Modena e Reggio Emilia,
Sarti, G. C., University of Bologna


Nanocomposite materials obtained combining inorganic fillers with high free volume glassy polymers have been investigated in the last decade with the aim to improve the permselectivity of polymeric membranes for gas and vapor separation. The glassy matrix chosen for the present study is poly[1-(trimethylsilyl)-1-propyne] (PTMSP), for which relatively large information about the gas and vapour transport and the microscopic morphology is available in the literature. The object of this study is an experimental analysis of n-pentane sorption and diffusion into: i) mixed matrices formed by PTMSP and hydrophobic nanometric silica added in solution (mass fractions ranging from 10 to 50wt %); ii) hybrid matrices formed by PTMSP and Si-SiO2 obtained from tetraethoxysilane (TEOS) via sol-gel reaction, with silica content ranging from 10 to 30%. The first aim of the study is to compare the transport behaviour of the two types of matrices at similar silica content and to understand the role of the interactions between polymer and silica domains in driving the transport behavior. In the second place, we looked for a comprehensive model that can represent the transport in mixed and/or hybrid matrices based on a small set of physical and empirical parameters. The experimental data collected show that n-pentane solubility and diffusivity in the mixed matrices increases monotonously with increasing the silica content, while the transport rates in hybrid matrices are lower than in 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 that the polymer phase is not altered by interactions with the filler: the permeability in mixed matrix is higher than predicted by this model, while the permeability in hybrid matrices is lower than the estimated value. Clearly, therefore, the interactions between polymer and silica play an important role in both cases. The studies dealing with the transport properties of mixed matrices of PTMSP and silica, as well as of hybrid matrices obtained by combining PTMSP with a precursor of the silica phase such as an alkoxysilane, did not provide a definite and unique structure-property correlation to link the permeability and diffusivity to a single physical parameter of the composite matrix. In this work we apply a model which describes the gas diffusion as a function of one single parameter, i.e. the fractional free volume of the polymeric matrix. The approach has been previously tested on mixed matrices, and it is proven here that this approach can describe also the transport properties in the hybrid matrices obtained from PTMSP and TEOS. Moreover, 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 (FFV). The estimation of the fractional free volume in the glassy polymer phase of mixed (or hybrid) matrix state is a non trivial problem, which can be afforded, however through reasonable assumptions. To this aim we considered that silica maintains in the composites the same density and adsorption capacity for vapours as in the state of pure solid, while the polymer density and sorption capacity do vary with the silica content. The vapour sorption is uniquely related to the polymer density through the NELF model for gas solubility in glassy polymers. In this work, we used experimental sorption capacity as input to evaluate, from the NELF model, the polymer density and, from it, the fractional free volume. From this analysis it has been seen that, for mixed matrices, the FFV is higher than that of the pure polymer while hybrid matrices show lower FFV values with respect to the unloaded PTMSP. This result actually explains the different transport trend in the composite matrices, and is consistent with an empirical correlation based on the free-volume theory which is shown in the Figure, that is able to depict in a single curve the data of hybrid and mixed matrices. We believe that in mixed matrices additional free volume is created at the interface between polymer and filler 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.

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