Effects of heat treatment on various properties of organic–inorganic hybrid silica derived from phenyltriethoxysilane

• Residual silanol in hybrid silica from phenyltriethoxysilane is eliminated by heat treatment.
• The elimination is visibly indicated by insolubility in acetone.
• The elimination is also indicated by absence of FT-IR absorbance due to hydroxyl groups.
• Heat treatment at 300 °C for 192 h completely eliminates the residual silanol.
• Å-scale voids are eliminated during the process of the elimination of the residual silanol.

Microparticles of organic–inorganic hybrid silica derived from a tri-functional ethoxysilane (phenyltriethoxysilane (PTES)) still have non-negligible amount of silanol (Si–OH) moiety after it is solidified in its outside appearance. Those residual silanol moiety formed in the Stöber process resumes further polycondensation in a heat treatment at temperatures above 250 °C. The rate of the polycondensation reaction of the residual moiety is dominantly influenced by the temperature at which the heat treatment was carried out. Heating at 300 °C for 192 h completes the polycondensation and entirely eliminates the residual silanol groups. The completion of polycondensation of the residual silanol is detected by visually observable insolubility of the heat-treated powder in acetone. The elimination of the silanol moiety due to the completion of the polycondensation is shown by the complete absence of the infrared absorbance corresponding to hydroxyl groups at approximately 950 cm−1. The completion of the polycondensation leads to densification of the siliceous polymeric network as indicated by the prevention of amphiphilic organic solvent (e. g., N,N-dimethylformamide) from permeating into the bulk matrix of the organic–inorganic hybrid silica. The thermogravimetric measurement shows that phenyl groups directly bonded to Si remains intact at the temperature region (≈300 °C) wherein the polycondensation reaction of the residual silanol groups is strongly promoted.