Flexible acrylate-grafted silica aerogels for insulation purposes: comparison of reinforcement strategies

Vinyltrimethoxysilane (VTMS), methyltrimethoxysilane (MTMS) and tetramethylorthosilicate (TMOS) were mixed in a one-step basic catalyzed sol–gel chemistry, to produce flexible and good thermal insulator silica-based aerogels. Moreover, mechanical reinforcement of the aerogels with a polymer phase was accomplished via free radical polymerization using either the macromer poly(ethylene glycol) diacrylate (PEG-DA) or the monomer 2-(dimethylamino)ethyl methacrylate (DMAEMA), the latter case resulting in the grafting of PDMAEMA. Both the influence of using a monomer or a macromer and of applying two different polymer grafting approaches—one-pot synthesis or gel soaking—on the aerogels’ final properties were analyzed. It was concluded that gel soaking strongly limits the diffusion of the organic moieties, creating heterogeneous materials. This approach led to denser and less hydrophobic aerogels with worse mechanical properties. The incorporation of low amounts of polymer with one-pot synthesis led to an improvement in the aerogels properties, making them less dense, more flexible and better insulators. The one-pot method also allowed to obtain aerogels with a very well-defined microstructure, due to the porogen role of the polymer, hence generating homogenous materials. It was found that the major differences in the aerogels properties occurred with different grafting approaches and not with different polymer phases, and the molecular weight of the organic moieties was not determinant for the materials homogeneity, in the studied cases. The obtained bulk densities ranged from 122 to 181 kg m−3, the thermal conductivity from 0.050 to 0.072 W m−1 K−1 and the modulus achieved 327 kPa. The addition of DMAEMA generated the best results when using the one-pot approach, providing the lightest and uncommonly flexible thermal insulator aerogels (modulus of ~70 kPa).