Surfactant-free, flexible polymethylsilsesquioxane foams

Abstract

Elastic, hydrophobic polymethylsilsesquioxane foams were prepared through a surfactant-free route. Three silane precursors, tetraethyl orthosilicate, methyltriethoxysilane and dimethyldiethoxysilane were used in a two-step, sol-gel process in a solution with variable water:ethanol ratios (from 0.36 to 0.71) to control the phase separation and particle and pore size. The foams were dried at 65 °C under ambient pressure. All samples display a spherical particle network structure, but with reduced particle size (from 4.5 to 3.5 µm) and thinner inter-particle necks for higher water contents. The morphological changes resulted in a higher surface area for higher water contents (SBET between 0.655 m2/g and 0.904 m2/g). The inter-particle neck thickness affected the mechanical properties, with less strain recovery when the water content was high (permanent densification ∼50%). The foams prepared here display a similar performance to those from the literature, but without the need for surfactants or urea in large quantities.