Studies on rheological properties of methyltriethoxysilane (MTES) based flexible superhydrophobic silica aerogels

The flexible and superhydrophobic properties of silica aerogels are extremely important requirements for their long-term applications. Therefore, the present paper deals with the synthesis and characterization of flexible and superhydrophobic silica aerogels utilizing a two-step acid–base catalyzed sol–gel process with a novel precursor, methyltriethoxysilane (MTES), followed by supercritical drying. Solvent and catalysts used for the synthesis were methanol (MeOH), oxalic acid (C2H2O4) and ammonium hydroxide (NH4OH), respectively. Silica alcosol was prepared by varying the MeOH/MTES molar ratio (S) from 6.45 to 19.35 by keeping the each of oxalic acid (0.001 M)/MTES and maintaining a constant ammonium hydroxide (10 M)/MTES molar ratio at 4. It was observed that the Young's modulus (Y) decreased from 15.03 × 104 to 3.95 × 104 N/m2 with an increase in S value from 6.45 to 19.35. Utilizing constant molar ratio of MeOH/MTES at 19.35, experiments produced monolithic, less shrinkage and flexible aerogels. The effect of various sol–gel parameters such as acid and base catalysts concentration and gel-aging on the flexibility and hydrophobicity of the aerogels were investigated. The aerogels have been characterized by bulk density, percentage of volume shrinkage, Young's modulus, elastic limit measurements, strain energy, Poisson's ratio and contact angle measurements. The microstructural studies were carried out using Transmission Electron Microscopy. The hydrophobicity was confirmed by Fourier Transform Infrared (FTIR) spectroscopy.