Visualization of nanostructure of soft matter by 3D-TEM: Nanoparticles in a natural rubber matrix

Abstract

In order to visualize the three-dimensional (3D) state of silica or carbon black dispersion in nanocomposites, 3D-TEM, a technique that combines transmission electron microscopy (TEM) with computerized tomography, is used and the results obtained are discussed. This technique is sometimes called electron tomography. The examples presented here elucidate the dispersion of these nanoparticles in a natural rubber (NR) matrix, which is a technologically very important biomass. It is made clear that such nanoparticles coalesce to form aggregates and they further coalesce to form agglomerates in the rubbery matrix. Once 3D-TEM images are established, one can evaluate all structural parameters, depending on the availability of software for each specific purpose. Here, the volume of each component in the composite, the size, the size distribution, the distance between neighboring aggregates, and the aspect ratio of the aggregates, for example, are evaluated as structural parameters. In the case of silica, 3D images of conventional silica and in situ formed silica are compared. The further coalescing of aggregates of both silica and carbon black particles results in the formation of agglomerates, the visualized images of which show that they form a kind of network structure in the rubber. In the case of carbon black, measurement of volume resistivity or electron conductivity reveals that percolation occurs near a volume fraction of 0.17 as a threshold value. Combining the resistivity results with a structural parameter, i.e., the nearest neighbor distance from 3D-TEM images, it is estimated that electrons hop over the rubber phase (which is an insulator), about 3 nm thick, between carbon black aggregates, thereby exhibiting electron conductivity. It is suggested from the results of this study that establishing the correlation between the 3D nanostructures as revealed by 3D-TEM and physical properties can play an important role in advancing the discipline of nanotechnology as an exact science, especially in the area of soft nanocomposites.