The quantification of anisotropy in graphene/natural rubber nanocomposites: Evaluation of the aspect ratio, concentration, and crosslinking

In the processing of nanocomposites, high shear stresses at elevated temperatures orient two-dimensional nanoparticles like graphene. This orientation leads to anisotropic mechanical, thermal or barrier properties of the nanocomposite. This anisotropy is addressed in this study by comparing graphene (few-layer graphene, FLG) with a nanoscaled carbon black (nCB) at a filler content of 3 phr, by varying the vulcanization, and by comparing different FLG contents. Transmission electron microscopy gives insight into the qualitative orientation in the nanocomposite with FLG or nCB. The storage moduli parallel and normal to the orientation reveal the direction dependency of reinforcement through dynamic mechanical analysis (DMA). Dimensional swelling measurements show a restriction of the expansion parallel to the FLG orientation, and an increased expansion normal to the orientation. The vulcanization system and crosslinking determine the respective level of property values, and higher crosslinking densities increase the anisotropy in DMA resulting in values of up to 2.9 for the quantified anisotropy factor. With increasing FLG content, the anisotropy increases. A comparison of the results reveals swelling measurements as the most suitable method for the determination of anisotropy. Compared to recent literature, the presented processing induces higher anisotropy, leading to higher reinforcing effects in the direction of orientation.