A criterion for the normal properties of graphene/polymer interface

Is the interfacial strength of graphene/polymer composite on the normal direction higher or lower than that of the polymer matrix? This issue has attracted fewer attention than the interfacial shear properties, but plays a key role in understanding the complex interfacial properties of the graphene/polymer composite as well as the CNT/polymer composite. This work studies the mechanical properties of the graphene/polymer interface on the normal direction by using molecular dynamic simulations. A representative structure of graphene and polymer matrix was constructed to assess the force-separation response of the interface between the graphenes and the polymer matrix including polyethylene, polyurethane and polystyrene. Simulations of this composite systems under uniaxial tension were conducted with pristine or modified graphenes at different stretching velocities. The comparisons of obtained results show that the ratio of the interfacial strength vs the polymer strength plays a key role in determining whether the damage takes place in the polymer matrix or at the interface, hence determining whether the interface is strong or weak. Thus a criterion is proposed to determine the failure behavior of the interface on the normal direction. Furthermore, a theoretical model is established to calculate the interfacial strength in place of the MD results to extend the application of the criterion.