In this article, three isotactic polypropylene (iPP) samples which were subjected to different crystallization procedures were prepared. These iPP samples which were mainly composed
of a-crystals displayed different double yield behaviors under tensile loading. PP-quenched sample
exhibits double yield points when stretched at low cross-head speed (CHS), while in the case of
PP-annealed, a second yield point gradually develops with increasing CHS. Furthermore, for
PP-isotherm, only one yield point is observed with homogenous deformation along the whole sample
at any CHS. Based on the characterization of crystalline structure changes after yielding, we
proposed two plastic processes that contribute cooperatively in the yield process of a-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 ofweak spherulites. Moreover, due to the polydispersity oflamellae thickness,the two deformation processes are co-existed and operate competitively in double yielding ofa-PP.
