Influence of the matrix on the fractal properties of precipitated silica in composites

The influence of different interactions between polymer chains and the particle surface on the morphology of hierarchically structured silica were studied by means of small-angle X-ray scattering experiments and a thorough analysis. To realize different interaction strengths, the particles were dispersed in natural and nitrile rubber, i.e. matrices having different polarities. Changing the silica fraction renders the respective influences of particle-particle, cluster-cluster and polymer-particle interactions accessible. Thus, the interplay between external mechanical forces from the mixer, internal forces, e.g. caused by silica-silica collisions, and forces mediated by the polymers are addressed in detail. Mixing of particles and polymers affects all parameters related to the clusters, but not the primary particle structure. It is demonstrated that the external forces cause a change in cluster size as well as the internal forces arising from silica-silica collisions. There is no evidence that the different interaction strengths between polymer chains and particle surfaces influence the morphology. Hence, the mixing process dominates the final structure at the macroscopic scale but not the different interaction strength on the molecular scale.