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 & alpha; 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 degrees C) is in close correlation with the & alpha;2 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.

Automated summary

This work systematically investigates the deformation mechanisms of isotactic polypropylene (iPP) precursor films prepared under different supercooling conditions during stretching at elevated temperatures. Three deformation mechanisms, namely inter-spherulitic, intra-spherulitic, and mix deformation, were observed as the yielding stress changed with stretching temperature. The transition temperature (60 degrees C) correlates closely with the α2 relaxation of iPP, which activates chain diffusion within the crystal blocks. The competition between inter-spherulitic deformation and intra-spherulitic deformation is determined not only by the spherulite structures but also by the stretching temperatures. Further studies reveal that larger spherulite size, more daughter lamellae, and lower stretching temperature dominate the induction of inter-spherulitic deformation. This work is expected to guide the preparation of iPP films with desired structures and functionalities.