Synthesis and characterization of partially fluorinated aerogels and xerogels from environmentally-compatible precursors

Abstract

A new fluorocyclic silane monomer was prepared from environmentally benign and commercially available starting reagents in two, high yielding steps. The monomer was characterized by multi-nuclear NMR (¹H, ¹³C, ¹⁹F, and ²⁹Si). The monomer was used in the preparation of the first known fluorocyclic aerogels and xerogels. Due to the hydrophobic nature of the fluorocyclic monomer, a modified sol–gel synthesis was utilized in which a minimum amount of water was used. The mol% of the fluorocyclic monomer was systematically varied in both types of silica gels and the effects on the properties of the aerogels and xerogels was analyzed by scanning electron microscopy, BET surface area measurements, differential scanning calorimetry, and thermogravimetric analysis. It was determined that in both aerogels and xerogels, increasing the mol% of the fluorocyclic monomer resulted in condensed materials (i.e., pore collapse). Increased monomer concentration affected the gels thermal stability as the monomer is more susceptible to thermal decomposition than the silica gel. Pore collapse accelerates this effect as the condensed medium dissipates heat less efficiently.