Sol–Gel Reaction of Tetraethoxysilane, Hexaethoxydisiloxane, and Octaethoxytrisiloxane: Differences in Siloxane Precursor Oligomers Depending on Raw Materials

Abstract

Silica materials were prepared by the hydrolysis and condensation of alkoxysilane. Recently, we found that the hydrolysis behavior is dependent on the structure of ethoxyoligosiloxanes, such as tetraethoxysilane, hexaethoxydisiloxane, and octaethoxytrisiloxane. In this work, we report the preparation and properties of polysiloxanes using ethoxyoligosiloxanes and their transformation into silica thin films and xerogels with controlled porosity. Polysiloxane precursor solutions were prepared by the hydrolysis−condensation of ethoxyoligosilixanes. To characterize the macromolecular precursors, condensates (Cs) were obtained by trimethylsilylation of precursor solutions with a molecular weight of approximately 3500. Cs were separated into low-and highmolecular-weight components (Cs−LS and Cs−HS) as a distillate and a residue in vacuum distillation, respectively. Cs−LS are mainly composed of 6-membered ladder oligosiloxanes, showing that Cs are composed of broken ladder-and cage-like oligosiloxanes. Silica thin films (SFs) and xerogels (XGs) were obtained by spin-coating or aging of precursor solutions, followed by heating at 100−900 °C. SFs and XGs exhibit porous structures at 100−700 °C and nonporous structures at 700−900 °C. The micropore diameters of XGs increased from approximately 1.0 to 1.3 nm by increasing the number of silicon atoms in ethoxyoligosiloxanes, respectively.