Free Volume and Mesopores in Polymer Nanocomposites

Abstract: The free volume in polymers is coupled to several important physical properties, such as density, viscosity and gas diffusivity. The physical properties of polymers close to a solid surface have been shown to be significantly perturbed as compared to their bulk state. The behavior of polymers close to surfaces comes into play within the field of polymer composites, where the properties of the interface between the filler and the polymer matrix plays a key role in determining the final performance of the composite. This thesis is aimed at studying the effect of surfaces, introduced as nano-sized fillers, on the free volume and physical properties of polymers. Positron annihilation lifetime spectroscopy (PALS), which involves measurement of ortho-positronium (o-Ps) lifetime was used to determine free volume cavity sizes. Nano-sized fillers, fumed silica and layered silicate, were observed to have a relatively small effect on the mean free volume cavity size in poly(dimethylsiloxane) and poly(amide 6), respectively. The polymer free volume sizes were generally increased upon addition of filler. In poly(1-trimethylsilyl-1-propyne) (PTMSP) nano-sized fumed silica filler was discovered to have a pronounced effect on free volume sizes. The free volume cavity sizes in PTMSP increased with increasing filler content. Large cavities, in the mesopore range, were observed in PTMSP fumed silica nanocomposites. The mesopores were not present in unfilled PTMSP. Increasing gas permeability and decreasing selectivity with increasing filler content, suggested the presence of mesopores to be of importance for the gas transport properties.