Elasticity, one of the most important properties of a soft material, is difficult to quantify in nanoparticle/polymer networks because of the presence of topological molecular defects. Furthermore, the impact of these defects on bulk elasticity is unknown. The macroscopic characteristics of a material are based on its composition across several length scales, in particular on interactions occurring on different time scales which jointly determine its effective macroscopic properties. Therefore, the physics and chemistry of nanocontacts within a material a central issue for the design of the nanocomposites.
We have studied the elasticity of in a network of functionalized TiO2 nanoparticles interacting via through covalent or weak non-covalent intermolecular forces. The interactions were tuned by molecular defects introduced through a partial substitution of surface ligands. The substitution on the nanocrystal surface was tracked by 1H- and 13C-NMR spectroscopy. The functionalized nanocrystals were further characterized by transmission/scanning electron microscopy and dynamic light scattering to determine particle size and arrangement, hydrodynamic diameter, and zeta potential. Brillouin light scattering (BLS) was employed on samples prepared by spin coating to obtain the longitudinal modulus M, which is related to E via the Poisson ratio. BLS directly measures the frequency f of the thermally activated acoustic phonons due to the inelastic scattering. From the BLS spectra recorded at different phonon wave vectors q, the dispersion f(q) for homogenous samples can be used to determine the phase sound velocity c = 2πf/q. The method is very sensitive to probe the properties of the nanocontacts. In particular, we observe and evaluate how much the subnanometric molecules present at nanocontacts affect the coherent acoustic phonon propagation along the network of the interconnected silica nanoparticles. Finally, we show that the technique provides quantitative estimates of the rigidity/stiffness of the nanocontacts and the elasticity of the nanocomposite.