Mechanical properties of hexylene- and phenylene-bridged polysilsesquioxane aerogels and xerogels

Bridged polysilsesquioxanes are increasingly used to prepare protective coatings, particulate chromatographic materials, and adsorbents. However, little is known about the mechanical properties of the materials and how they are influenced by the nature of the bridging group. In this paper, we have prepared monolithic xerogels and aerogels of hexylene- and phenylene-bridged polysilsesquioxanes and have measured their flexural strength and modulus. Consistent with their compact structure, the porous, glassy phenylene- and hexylene-bridged xerogels were hundreds of stronger than the analogous aerogels. The nature of the bridging group did not appear to affect the mechanical properties of the xerogels, in contrast, it presented a profound effect on the mechanical properties of the aerogels. Phenylene-bridged aerogels were brittle and 30% stronger than silica aerogels of the same density. However, the opaque hexylene-bridged aerogels were found to be elastic and appreciably weaker than the phenylene-bridged or silica aerogels.