SANS study of hybrid silica aerogels under "in situ" uniaxial compression

Abstract

We have modified the inorganic silica network of aerogels with polydimethylsiloxane (PDMS), a hydroxyl-terminated polymer, to obtain an organic modified silicate (ORMOSIL). Reactions were assisted by high-power ultrasounds. The resulting gels were dried under supercritical conditions of the solvent to obtain a monolithic sono-aerogel. The mechanical behaviour of these aerogels can be tuned from brittle to rubbery as a function of the organic polymer content. In order to determine the links between the mechanical behaviour and modifications made to the microstructure, SANS (small-angle neutron scattering) experiments were carried out. To measure the intensities under "in situ" uniaxial compression of the aerogel, a specific sample-holder was built. Under uniaxial compression the 2D-diagrams were significantly anisotropic (butterfly pattern), indicating the rearrangement of the polymer. The form factor of these aerogels is described well by two correlation lengths, small microporous silica clusters surrounded by entangled polymer chains of 6 nm average size (blobs), which form a larger secondary level of agglomerates governed by the "frozen-in" elastic constraints.