Hybrid Silica/Polymer Aerogels Dried at Ambient Pressure

Abstract

A novel sol−gel route was developed to prepare monolithic hybrid silica/polymer aerogels, stable under atmospheric conditions and suitable for machining. The synthesis of the hybrid wet gels followed a two-step hydrolysis/polycondensation of tetraethoxysilane with excess water, in 2-propanol. Cohydrolysis with trimethoxysilyl-modified poly(butyl metacrylate-co-butyl acrylate) cross-linked nanoparticles (average diameter of 94 nm) was carried out. Different alcogels were prepared varying the hydrolysis and condensation catalysis conditions and the polymer content (0−50% in weight). The aged alcogels were subcritically dried in a quasi-saturated solvent atmosphere. The resulting xerogels were characterized by dry-flow pycnometry, nitrogen adsorption−desorption, scanning electron microscopy (SEM), and diffuse reflectance infrared spectroscopy (DRIFT), and their mechanical properties were evaluated by unidirectional compression tests. The hybrid aerogels show improved mechanical properties with respect to the corresponding inorganic aerogel (much higher energy is stored until fracture), without loss of structure or porosity. The hybrid aerogels' properties are not monotonously dependent on the polymer content: the lowest density (357 kg·m-3) is achieved for 3 wt %, the corresponding porosity being 83%, the specific surface area 766 m2·g-1, and the average mesopore diameter 11.5 nm. This is also the most stable aerogel and the one with the best mechanical properties.