Atomistic molecular dynamics simulation study on the mechanical behavior and dispersion of surface functionalized graphene/polypropylene nanocomposites

The effect of methyl surface functionalization of single-layer graphene on the mechanical behavior and interfacial load transfer of polypropylene (PP) nanocomposites is studied by using molecular dynamics (MD) simulations. In the molecular model, both pristine and 20 methyl-group-functionalized graphenes are embedded into an amorphous PP matrix to constitute unit cell models of transversely isotropic periodic nanocomposites. The stress-strain curves of nanocomposites under uniaxial tension and shearing are determined based on non-equilibrium ensemble simulations with a constant true strain rate. Due to the degradation of graphene by the methyl groups, the longitudinal Young's modulus and in-plane shear modulus of the nanocomposites are degraded. On the other hand, the longitudinal shear moduli of the nanocomposites, which depend on the interfacial shear load transfer, are considerably improved upon methyl functionalization. To establish the relationship between the improved interfacial properties in nanocomposites and methyl functionalization, the intrinsic adhesion energy between graphene and PP matrix and the arithmetic mean surface roughness of graphenes are determined and compared. The dispersibility of graphenes are also studied in terms of the solubility parameters.