Dependence of the reinforcement of polymer-based nanocomposites upon the nanofiller geometry

The role of the geometry of nanofillers on the mechanical properties of polymer nanocomposites, such as Young's modulus and toughness, has been reviewed based on published data in the literature. The fillers considered include carbon black (CB), carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs), reinforcing either elastomers or epoxy resins that act respectively as examples of flexible and rigid polymers. The Young's modulus of nanocomposites reinforced by CB is revealed to follow the classical theories on particulate reinforcement. The Young's modulus of nanocomposites reinforced by GNPs and CNTs are analysed using composite micromechanics theories, i.e. a combination of the shear lag theory and rule of mixtures. A surprising finding is that if the Young's modulus of the nanofillers is considerably higher than that of the matrix, the Young's modulus of the nanocomposite is only dependent on the aspect ratio, volume fraction and orientation of the nanofiller, and is independent of its mechanical properties. The failure of the nanocomposites by crack propagation is analysed regarding the fracture toughness of the epoxy nanocomposites and the tear strength of elastomer nanocomposites. The addition of nanoparticles is found to improve the crack resistance through crack pinning whereby the crack front length increases by pinning at individual particles. It is revealed that the tear strength of the elastomer nanocomposites and the fracture toughness of the epoxy nanocomposites filled with either GNPs or CNTs is dominated by the pull-out of nanoparticles, while for elastomers other toughening mechanisms may also occur, such as cavitation.