Role of the silica nanoparticle anisotropy on morphological and mechanical properties of Styrene Butadiene Rubber nanocomposites.

Rubber compounds enhance their mechanical properties when a rigid solid particulate (filler) is finely dispersed in the rubber structure, particularly in case of nanostructured silica. In fact, most of nanosilica properties that impact on rubber compounds have been investigated, in particular the size of the nanoparticle, its state of aggregation and surface chemistry. An extensive effort was paid to understand how nanostructured silica exert a reinforcing action on the resulting silica/rubber nanocomposite material. The intrinsic difficulty in obtaining silica nanoparticles with well defined shape has limited the discussion to nanoparticles different from silica, for example carbon nanotubes, nanoclays, thermoplastic polymeric fibres, cellulose nanowhiskers, ecc. It is therefore necessary to create a model system in which silica nanoparticle shape can be tuned adequately and its influence on the reinforcing mechanism can be worked out. In this context, the aim of the thesis is to investigate the impact of the anisotropic silica particles on the dynamic-mechanical behavior of model SBR nanocomposites, considering first the networking of differently shaped nanoparticles, and second the formation of the nanoscale rigid rubber at the filler interface. Silica nanoparticles featuring different shapes were obtained by sol-gel method, using a structuring agent based on an acqueous micellar solution of surfactant CTAB, that allowed to control the growth of the particles giving them different anisotropies, including spherical and rod-like particles with aspect ratio ranging between 2 and 7. Nanocomposites of silica nanoparticles and styrene butadiene rubber (SBR) were prepared by compounding methods, in order to evaluate the effect of shape on the mechanical properties of the material. The nanocomposites morphology and its effect on the dynamic-mechanical behavior were studied by a multi-technique approach. The combined use of TEM and AFM tapping mode analyses revealed that in all the nanocomposites spherical and anisotropic NPs are surrounded by a layer of about 15 nm of immobilized rubber. Spherical or nearly spherical particles show small contact zones sharing thin rubber layers. Anisotropic particles show instead oriented domains of rods preferentially aligned along the main axis separated by several layers of immobilized rubber. The formation of these self assembled domains causes an increase of rubber fraction trapped between the aligned particles. Low field 1H NMR measurements evidences that polymer chain dynamics of the rubber layers tightly bound to the silica particles are restricted by the interaction with the filler, increasing the stiffness with respect to that of polymer far from the particles. Dynamic- Mechanical measurements confirm that anisotropic particles with higher aspect ratio give a high reinforcement, as a result of spontaneous alignment of anisotropic particles that implies higher amounts of immobilized rubber, when compared to the system containing spherical particles. A similar approach was applied to anisotropic nanoparticles based on Sepiolite, a naturally occurring nanostructured silicate. Nanorods were prepared by a surface modification methodology and successfully compared with the model system proposed before showing improved reinforcement, particularly related to energy dissipation, that makes them potentially interesting for an industrial application. To conclude, the reinforcement mechanism passes through the immobilization of rubber on the surface of the nanoparticles, and it was found that the self-assembly of particles in anisotropic domains is crucial in order to further immobilize rubber and ensure a high reinforcement. The study confirmed that the control of filler morphology is mandatory to adequately modulate the nanocomposite mechanical properties.