Temperature dependent microstructure of MTES modified hydrophobic silica aerogels

摘要

Hydrophobic silica aerogels were prepared using a single step sol-gel process followed by ethanol supercritical drying. Using tetraethoxysilane (TEOS) as a precursor and ammonium hydroxide as a catalyst the aerogel surface was chemically modified with methyltriethoxysilane (MTES). A MTES/TEOS molar ratio of 0.5 (M5) was used. The microstructure of the surface modified aerogels was evaluated as a function of heat treatment temperature over a range of 200-500 ℃. The thermal stability was analyzed by simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC) and the microstructure was evaluated by physisorption analysis (BET) and scanning electron microscopy (SEM). The chemical composition and hydrophobicity/hydrophilicity of the aerogels were investigated by Fourier Transform-Infrared (FT-IR) spectroscopy. The M5 aerogels, which were initially hydrophobic, exhibited partial hydrophilicity at treatments above 244.5 ℃ and complete hydrophilicity above 429.9℃. The surface area of the aerogels ranged from 776.65-850.20 m~2/g. Pore size increased after heat treatment, ranging from 16.25 to 18.52 nm vs. an initial pore size of 14.71 nm. The maximum pore size of 18.52 nm was found at the lowest heat treatment temperature (～200 ℃). Heat treatment had a mixed effect on the pore volume, as pore volumes decreased at lower treatments (～200-400 ℃) and increased at higher heat treatments (～450-500 ℃) relative to the untreated aerogels. With initial heat treatment the Si-CH_3 group began to oxidize to Si-OH. Aerogels heated above 429.9 ℃ exhibited hydroxyl polymerization leading to aerogels with large particles and a dense microstructure.