Control of porosity in SiO₂:PDMS Polycerams through variations in sol-gel processing and polymer content

A series of optically transparent SiO₂: Polydimethylsiloxane (PDMS) Polyceram monoliths have been synthesized by two-step acid/base sol-gel processes. Two different processing routes are discussed and compared; one synthetic route (Route 1) utilizes lower water content, shorter reflux times, and faster drying conditions compared to the other synthetic route (Route 2). The Route 1 Polycerams were all essentially non-porous at all PDMS contents examined (20-80 volume % PDMS). In contrast, the porosity of the Route 2 Polycerams varied dramatically as a function of PDMS content. The surface area and pore volume for a 0% PDMS Route 2 Polyceram were 573 m²/gm and 0.59 cm³/gm, respectively; the surface area and pore volume decreased with increasing PDMS content. The amount of porosity within the Polycerams is proposed to be controlled by the relative rates of condensation and evaporation during processing and by the amount of PDMS trapped in the pores. This ideas supported by the differences in the drying behavior with processing and by the structural information obtained by magic angle spinning solid-state ²⁹Si NMR of the Polyceram monoliths. Quantitative evaluation of the ²⁹Si NMR and porosity data are utilized to formulate structural models of these Polycerams. The structural models are then specifically used to describe the effect of porosity on the photostabilization of a laser dye doped within these Polyceram monoliths.