Deformation-induced structure evolution of oriented β-polypropylene during uniaxial stretching

β phase isotactic polypropylene (β-iPP) with the β phase lamellae oriented parallel to the melt extrusion direction, was employed to investigate the deformation-induced structure evolution at various temperatures (25, 80, 110, 130 and 140 °C). The orientation change of β phase during deformation greatly influences the β to α phase transformation and its temperature dependence. At temperatures lower than 110 °C, the orientation of β phase is almost unchanged during deformation, and void or crack forms before fragmentation and reorientation of β phase. The crystal size change of β phase is small, and the defolding of the β lamellae triggers the β to α phase transformation. As the deformation temperature rises to 130 and 140 °C, the reorientation of β phase occurs gradually upon stretching, and the size of micro-voids decreases due to the fact that less β crystal fragmentation takes place at high draw temperature than that at low temperature. The β to α phase transformation is mainly induced by intra-lamella slip, and the trend of crystal size change of β phase is larger. The chains orientation changes from perpendicular to the stretching direction in β phase to parallel to the stretching direction in α phase is achieved by the chains defolding of β phase along the stretching direction at temperatures lower than 110 °C, and it is through the chains reorientation of β phase along the stretching direction at temperatures of 130 and 140 °C. The crystal size of α phase of the deformed β-iPP during deformation depends on the dynamic balance of the breakage of existing α crystal and the formation of new crystal through phase transformation. Specially, at draw temperature of 25 °C, the slippage of β phase relieves the breakage of α phase crystal, which indicates that the high content of β phase crystal really accounts for the toughening effect on iPP.