Aggregation of Fillers Blended into Random Elastomeric Networks: Theory and Comparison with Experiments

Significant aggregation takes place when nanoparticles are blended into a random amorphous polymeric medium, contrary to in situ generation which results in uniform size distributions. In the present paper, we propose a theoretical model to study the size and distribution of aggregates that take place when the particles are blended into a random amorphous mobile elastomeric medium. We test the predictions of the model with experimental results on silica-filled end-linked poly(dimethylsiloxane) (PDMS) networks and explain the increase in the degree of aggregation with increase in filler content. Organic/inorganic nanocomposites with different levels of combinations have important applications in optics, electronics, and rubber elasticity. The efficiency of the nanocomposites depends on dispersion, size distribution of the guest inorganic particles in host organic matrix, and their specific interactions with polymer chains. The homogenous dispersion of uniform-sized nanoparticles is the most desired characteristic of these materials. However, clustering or aggregation of inorganic nanoparticles is often a problem, especially when the particles are incorporated into a matrix by blending rather than in situ generation (throughout the paper we use the terms cluster and aggregate interchangeably). The smaller (non-aggregated) particles provide larger contact area between the organic and inorganic components due to large surface-to-volume ratio Summary: A theoretical model describing aggregation of filler particles in amorphous elastomers is proposed. The model is based on a counting technique originally used in genome analysis to characterize the size and distribution of overlapping segments randomly placed on a DNA molecule. In the present model, the particles are first assumed to aggregate randomly uponmixing into the elastomer and their sizes are calculated. The sizes and distributions of aggregates are also studied in the presence of attractive interparticle forces. Results of the proposed model are compared with experimental data on silica-filled end-linked poly(dimethylsiloxane) networks. Comparison of the theory and experiment shows that the random aggregation assumption where no attractive forces exist between the particles is not valid and a significant attraction between the silica particles is needed in the theory to justify the experimental data obtained using atomic force microscopy. For filler content below 1.45 vol.%, the model agrees, qualitatively, with experiment and shows the increase in cluster size with increasing amount of filler. It also explains the increase in the dispersion of aggregate sizes with increasing amount of filler.