The three-dimensional (3D) morphology of particulate fillers embedded in a rubbery matrix was examined by transmission electron microtomography (TEMT). Two types of nanofillers, i.e., carbon black (CB) and silica (Si) nanoparticles, were used as the nanofillers. Although the CB and Si nanoparticles were difficult to distinguish by conventional transmission electron microscopy (TEM), they appeared different by TEMT; the CB and Si nanoparticles appeared to be hollow and solid particles in the cross-sectional images of the TEMT 3D reconstruction, respectively, demonstrating that TEMT itself provided a unique particle-discriminative function. The nanoparticles were found to form aggregates in the matrix. It is intriguing that each aggregate was made of only one species; not a single aggregate contained both the CB and Si nanoparticles. A particle-packing algorithm was developed to estimate the positions of each primary nanoparticle inside the aggregates.
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