Highly porous SiH containing hybrids prepared by a novel process: rapid gelation of hydrogensilsesquioxane under ambient pressure

Abstract

A novel fast gelation route for the preparation of highly porous materials is described. Commercially available hydrogensilsesquioxane was used as the precursor in this approach, leading through a simple, single step process to gels, within minutes. This novel approach relied on increase of wet gel pore radius ( r p) to reduce the capillary pressure upon drying. The increment of r p was achieved by the evolution of hydrogen gas during the gelation step, which was triggered by the use of activating agents. NH 4OH, formamide, and polyethylene glycol were used as activating agents for this purpose. The resulting porosity in fast gelation derived porous materials was much broader than those in the conventional aerogels, with macropores up to several hundred nm, along with mesopores. Systematic studies of this novel fast gelation process were carried out through careful variation of the experimental conditions to gain control in the mesopore region. NH 4OH was unable to control the amount of SiH conversion into SiOH during the fast gelation process, trace amount of NH 4OH (0.036%) was sufficient to convert all of the SiH functions. The resulting porous materials derived by this activating agent showed a broad distribution covering the entire mesopore region. Polyethylene glycol and formamide showed better control over SiH to SiOH conversion, with a higher amount of activating agents giving higher conversions. Similar trends of both activating agents level onto total pore volume in the resulting porous materials were observed through the porosity evaluations, with the mid range concentration leading to the highest total pore volume. Mesopores with a narrow size distribution between 30 and 50 Å were observed in porous materials generated by the lower concentration in both series.