Investigation on double yielding behavior under tensile loading in isotactic polypropylene

•We first discovered a peculiar phenomenon of double yielding in α-PP.•The inter-spherulitic deformation is predominant in the first yielding.•The intra-spherulitic deformation is preferential in the second yielding.•The two plastic processes contribute cooperatively in the yield process of α-PP.

In this article, a peculiar phenomenon of double yielding was first discovered in isotactic polypropylene (iPP) under tensile loading. The results of differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarized light microscopy (PLM) show that all the three samples, which were subjected to different crystallization procedures, only form α-crystals that are composed of radial lamellae and tangential lamellae. These α-PP samples display different double yield behaviors under tensile loading. PP-quenched sample exhibits double yield points when stretched at low cross-head speed (CHS), while one single yield point appears accompanied with a marked shear band when stretched at high CHS. However, in the case of PP-annealed, only one yield point appears at low CHS accompanied with the formation of a large number of crazes in the necked region, meanwhile, a second yield point gradually develops with increasing CHS. Furthermore, as for PP-isotherm, only one yield point is observed with homogenous deformation and concomitant whitening along the whole sample at any CHS. Based on the characterization of crystalline structure changes after yielding, we propose two plastic processes that contribute cooperatively in the yield process of α-PP, namely the inter-spherulitic deformation and intra-spherulitic deformation. The inter-spherulitic deformation which is prone to be initiated in the sample of strong spherulites is predominant in the first yield process, while the intra-spherulitic deformation enters into action after the appearance of the second yield point in the case of weak spherulites. Moreover, due to the polydispersity of lamellae thickness, the two deformation processes are co-existed and operate competitively in double yielding of α-PP.