Biphenylene units possessing flammable and nonflammable characteristics in fluoroalkyl end-capped vinyltrimethoxysilane oligomeric silica gel matrices after calcination at 800 °C

Abstract

Two kinds of fluoroalkyl end-capped vinyltrimethoxysilane oligomer [RF-(VM)n-RF] silica nanocomposites containing biphenylene units were prepared by the sol-gel reactions of the corresponding oligomer with biphenylene-bridged ethoxysilanes or 4,4′-biphenol under alkaline conditions, respectively. One is the fluorinated oligomer/silica nanocomposites containing biphenylene units [RF-(VM-SiO2)n–RF/Ar-SiO2], of whose biphenylene units were incorporated into nanocomposite cores through the siloxane bondings, and the other is the fluorinated oligomer/silica nanocomposites containing biphenylene units [RF-(VM-SiO2)n–RF/Biphenol], of whose biphenylene units were directly encapsulated into nanocomposite cores through the sol–gel process. Interestingly, the shape of RF-(VM-SiO2)n–RF/Ar-SiO2 nanocomposites is morphologically controlled cubic particles; although the shape of RF-(VM-SiO2)n–RF/Biphenol nanocomposites is spherically fine particles. Thermogravimetric analyses 2H magic-angle spinning nuclear magnetic resonance, Ultraviolet visible, and fluorescent spectra showed that biphenylene units in RF-(VM-SiO2)n–RF/Ar-SiO2 nanocomposites have a flammable characteristic after calcinations at 800 °C; in contrast, biphenylene units in RF-(VM-SiO2)n–RF/Biphenol nanocomposites have a nonflammable characteristic even after calcination at 800 °C. X-ray photoelectron spectroscopy analyses of these two kinds of fluorinated nanocomposites showed that nonflammable characteristic toward biphenylene units in the silica gel matrices is due to the formation of ammonium hexafluorosilicate during the sol–gel process.