Deformation induced void formation and growth in β nucleated isotactic polypropylene

In-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) were carried out to investigate the deformation and the void formation behavior of β nucleated isotactic polypropylene during uniaxial stretching at 100 °C. WAXD results indicated that β crystal started to deform after yielding, followed by recrystallization to α-phase. A quasi-quantitative SAXS analysis was attempted to evaluate the length, width, and relative number of ellipsoidal nano-voids. The length of initial nano-void corresponded well to that of amorphous thickness of iPP. It strongly suggests that the nano-void was generated by the fragmentation of β lamellae. The nano-void length kept increasing exponentially up to the strain of 3, while the number of nano-voids decreased exponentially with the same rate constant as that of the length. In this strain region, the width kept constant. Above results clearly demonstrated that the nano-void grows only by one-dimensional coalescence along PP chain direction. The excellent ductility, i.e. the toughness, of β-PP was attributed to the plasticizing effect of nano-voids, which inhibits the stress concentration at the edge of the void. [Display omitted] •Nano-scaled void structure in deformed polypropylene is quantitatively investigated.•Voids grow by one-dimensional coalescence during stretching.•A novel mechanism for deformation-induced toughening in beta-phase polypropylene is proposed.