Yielding behavior of isotactic polypropylene at elevated temperature understood at the spherulite level

It is of great significance to study the deformation mechanism of isotactic polypropylene (iPP) which differs considerably from that of traditional polymers in consideration of its unique cross-hatched structure of α crystal. In this work, the yielding mechanism of three iPP precursor films prepared via crystallizing in different supercooling during stretching under elevating temperature had been investigated systematically. The results show that three mechanisms of deformation had been aroused in the change of yielding stress with stretching temperature named as inter-spherulitic, intra-spherulitic and mix deformation, respectively. Moreover, the transition temperature (60 °C) is in close correlation with the α2 relaxation of iPP, where the chain diffusion within the crystal blocks is activated. The competition between inter-spherulitic deformation and intra-spherulitic deformation is not only determined by the spherulite structures, but also the stretching temperatures. Further studies indicate that, larger spherulitic size, more daughter lamellae and lower stretching temperature are dominant issues to induce inter-spherulitic deformation. This work is expected to provide a guideline for the preparation of iPP films with desired structures and functionalities.