Microhardness of α- and β-modified isotactic polypropylene at the initial stages of plastic deformation: analysis of micromechanical processes

The microindentation hardness,H, of uniaxially deformed isotactic polypropylene samples was determined near the neck region, as a function of the draw ratio. The microhardness technique appears to be a valuable tool to describe mechanical properties in localized regions within a material and is capable of following changes in the semicrystalline structure during deformation. Differences in the microhardness behaviour of the two types of polymorphic forms, α and β, of isotactic polypropylene are discussed in terms of the two specific types of morphology, i.e. the cross-hatched arrangement of the crystalline lamellae for the α form and the parallel lamellar stacking for the β form. The changes of H as a function of λ are shown to be in accordance with the transformation in the neck from the spherulitic into the fibre structure. The steep H-decrease observed in the neck region is discussed in the light of the nanomechanical processes as revealed by scanning electron microscopy. These include lamellar separation, micro-void formation and fibrillation. Finally, microindentation experiments carried out in the neck allow an estimation of the local draw ratio at which the maximum pore content in the polypropylene samples occurs.