Nanoindentation mapping of multiscale composites of graphene-reinforced polypropylene and carbon fibres

The local mechanical properties of hierarchically reinforced laminates of graphene-isotactic polypropylene (iPP) and carbon fibres are investigated by nanoindentation. Carbon fibre fabrics are alternated with films of iPP reinforced with different graphene contents and types (pristine and modified with iPP brushes). The distribution of the mechanical data obtained around one isolated carbon fibre at the boundary with the polymer layer is compared with a similar analysis far away from any fibre. In the case of neat iPP, a high E′ tail can be identified with a micrometre-sized interphase showing distinct properties from those of the polymer matrix. With increasing amount of graphene, the E′ distribution including the high E′ tail progressively shifts to higher values. It is suggested that graphene platelets accumulate at the front of the ply due to the filtering effect of the carbon fibres giving rise to a transition region, superimposed on the interphase, with enhanced properties. Nanoindentation studies reveal that the chemical modification of graphene by short-chain iPP lowers the reinforcement of the nanofiller, both near and far away from the fibres front. On the other hand, indentation push-in tests suggest that modified graphene increases the resistance of the laminate under shear stress conditions. However, from the point of view of overall performance, it lowers the electrical conductivity across the laminate. The work offers a discussion of the role of graphene type and content on the properties of the multiscale composite.