Novel flexible, hybrid aerogels with vinyl- and methyltrimethoxysilane in the underlying silica structure

Abstract

Flexible, monolithic and superhydrophobic silica aerogels were obtained by combining methyltrimethoxysilane (MTMS), vinyltrimethoxysilane (VTMS) and tetramethylorthosilicate (TMOS) (50:30:20 mol%) in a one-step base-catalyzed co-precursor sol–gel procedure. Polybutylacrylate (PBA) and polystyrene (PS) were grafted and cross-linked in the gel aiming to enhance the mechanical performance. Fourier transform infrared spectroscopy, thermogravimetry analysis and scanning electron microscopy confirmed the presence of the polymers as a binding coating on the 3D silica network, primarily formed by firmly connected 3–5 μm secondary particles. When compared to the MTMS-based aerogels, the VTMS–MTMS–TMOS-derived aerogels, either reinforced or not, show a threefold increase of the bulk density (to ~150–160 kg m−3) and a consequent decrease in the surface area and average pore size; the thermal conductivity also increases to 60–70 mW m−1 K−1, a 50 % increase over the values of MTMS-derived aerogels. Although these tendencies are more marked in the polymer-reinforced materials, the change of the silica skeleton from MTMS to VTMS–MTMS–TMOS is responsible for the main differences. The VTMS–MTMS–TMOS underlying structure gives a fourfold increase in compressive strength relatively to the MTMS-derived aerogels, even when not reinforced. In addition, it retains a high elongation at break (40–50 %) and flexibility—modulus of 25 kPa for the PBA-reinforced aerogel, the more flexible aerogel, and modulus of 91 kPa for PS-reinforced aerogel, the stiffer and stronger material. The obtained aerogels have touch feeling that resembles that of expanded polystyrene foams, and also show negligible particle shedding, which is a valued characteristic for aerospace applications.