Effect of methyltrimethoxysilane as a co-precursor on the optical properties of silica aerogels

To produce monolithic durable hydrophobic silica aerogels with high direct optical transmittance and low diffusion of light, systematic and detailed experimental investigations were carried out by adding methyltrimethoxysilane (MTMS) in the sol–gel processing of silica alcogels. The alcogels were dried supercritically using the high temperature alcohol method. A series of aerogel samples of different molar ratio combinations (0–1.85) of MTMS/tetramethoxysilane (TMOS) was optically examined in the UV–Visible–NIR range by a spectrophotometer equipped with an integrating sphere. The overall transmittance of the aerogels in the visible range decreased from 93% to 10% with increased MTMS/TMOS molar ratio (A) from 0 to 1.65, respectively. The most relevant parameter being studied was the direct/hemispherical transmittance ratio (τ). The hydrophobicity of the aerogels was measured from the contact angle and it varied from 50° to 140° depending upon the molar ratio combination, the higher being for the higher A value. A good compromise of high direct optical transmittance (τ≈80% at 750 nm for 1 cm thick sample) for durable hydrophobic (θ≈110°) aerogels with low volume shrinkage (<5%) was obtained for an A value of around 0.7. With a view towards understanding the variation in optical properties, the aerogels have been characterized by scanning electron microscopy (SEM) and nitrogen adsorption BET analysis. The chemical bonds, causing the hydrophobic properties of the aerogels, have been identified by infrared spectroscopy (IR). The experimental results are discussed by taking into account the hydrolysis and condensation reactions and pore size distributions (PSDs).