Elastic organic–inorganic hybrid aerogels and xerogels

Novel aerogels and xerogels with methylsilsesquioxane (MSQ, CH₃SiO_1.5) networks have been prepared by a modified sol–gel process using surfactant and urea as a phase-separation inhibitor and as an accelerator for the condensation reaction, respectively. Optimized aerogels dried under a supercritical condition not only showed the similar properties as conventional pure silica aerogels such as high transparency and porosity etc, but also demonstrated outstanding mechanical strength against compression; the aerogel drastically shrank upon loading and then recovered when unloaded, which is called a “spring-back” behavior. On ambient pressure drying, the wet gel also exhibited the similar response against compression stress originated from the capillary pressure, and thus xerogels with the comparative structure and properties to those of corresponding aerogels have also been obtained. This unusual mechanical behavior is attributed to the trifunctional flexible networks of MSQ, low silanol concentration which prevents the irreversible shrinkage, and high concentration of a hydrophobic methyl group directly attached to every silicon atom which helps re-expansion after the temporal shrinkage.