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 e xperimental investigations were carried out by adding methyltritmethoxysila ne (MTMS) in the sol-gel processing of silica alcogels. The alcogels were d ried supercritically using the high temperature alcohol method. A series of aerogel samples of different molar ratio combinations (0-1.85) of MTMS/tet ramethoxysilane (TMOS) was optically examined in the UV-Visible-NIR range b y a spectrophotometer equipped with an integrating sphere. The overall tran smittance of the aerogels in the visible range decreased from 93% to 10% wi th increased MTMS/TMOS molar ratio (A) from 0 to 1.65, respectively. The mu st relevant parameter being studied was the direct/hemispherical transmitta nce ratio (tau). The hydrophobicity of the aerogels was measured from the c ontact angle and it varied from 50 degrees to 140 degrees depending upon th e molar ratio combination, the higher being for the higher A value. A good compromise of high direct optical transmittance (tau approximate to 80% at 750 nm for 1 cm thick sample) for durable hydrophobic (theta approximate to 110 degrees) aerogels with low volume shrinkage (<5%) was obtained for an A value of around 0.7. With a view towards understanding the variation in o ptical properties, the aerogels have been characterized by scanning electro n 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 ta king into account the hydrolysis and condensation reactions and pore size d istributions (PSDs). (C) 2001 Elsevier Science B.V. All rights reserved.