Nanoscale elastic–plastic deformation in clay-reinforced nanostructured materials: The response of phase and structural morphology

Abstract

The study described here combines materials science and engineering and mathematical analysis to fundamentally obtain insights underlying nanoscale deformation in polymer nanocomposites. The objective was achieved by examining the surface deformation response of two polymer nanocomposite systems with significant differences in ductility during scratching with a nanoindenter. The model host systems are a ductile polyethylene and a less ductile polypropylene. The model reinforcement material is nanoclay. The two systems are unique nanostructured materials because the distance between the individual clay layers is comparable to their thickness and also to the polymer segments. Thus, it is a morphology that is truly dominated by nanoscale features. Furthermore, we have used pressure-induced crystallization approach to alter the structure of the investigated two systems and study nanoscale surface deformation response. Our hypothesis is that the susceptibility to scratch deformation of nanostructured materials compared to their respective unreinforced counterpart depends on the shift of von Mises stress from the surface to the sub-surface region, leading to reduction in the maximum tensile stress induced by the scratch. To test the hypothesis, we have addressed two important aspects influencing the mechanics of surface deformation response of nanostructured materials. They include: (a) the determining role of nanoparticles on physical and mechanical properties and (b) the primary effect of nanoparticles on the structure of the host matrix. In the test of hypothesis, we have elucidated the relationship between structure, physical, and mechanical properties on the mechanism of nanoscale deformation via careful electron microscopy of the deformed surface. The nanoscale deformation response of polypropylene with large spherulites and α-phase was characterized by ripple-type deformation tracks that formed by a stick–slip mechanism. However, with decrease in spherulite size and evolution of toughness enhancing γ-phase, induced by high crystallization pressure and nanoclay, ironing represented the primary mode of deformation. In contrast, the highly ductile polyethylene system experienced multiple or single ironing process. In summary, the surface deformation topography implied that the nanoscale deformation response was material specific. Furthermore, electron microscopy topography of scratch tracks confirmed that reinforcement of polymers with nanoclay is a viable route to decrease the susceptibility of polymeric materials to nanoscale deformation and can be discussed in terms of physical and mechanical properties of materials, notably refinement of structure, percentage crystallinity, and elastic recovery. It was also intriguing to note that during nanoscratching, elastic recovery was high, in the range 77–85%, implying that a large fraction of the scratch-deformed volume was recovered. Finally, the electron microscopy findings and mechanical properties are related to mathematical analysis.