Oriented crystallization of long chain branched polypropylene induced by step-shear deformation in pre-crystallization regime

The oriented crystallization of long chain hyper-branched polypropylene (LCB-PP) induced by shear deformation in the pre-crystallization regime was investigated through simultaneous time-resolved synchrotron wide-angle and small-angle X-ray scattering (WAXS and SAXS) measurements and compared with that of linear PP (L-PP). Highly oriented crystallization was induced in LCB-PP by the application of a step-shear in the pre-crystalline regime; in contrast, the crystalline lamellae of L-PP were slightly oriented. In addition, LCB-PP formed daughter lamellae whose a-axis was oriented along the shear direction. The oriented crystallization of LCB-PP induced by the step-shear was related to its primary structure in the following manner. Since the existence of branched chains increased the relaxation time of the LCB-PP chains, the elongated LCB-PP chains were remained for longer after the step-shear deformation. In addition, the branching points of LCB-PP aided the formation of point-like precursors for crystallization. Consequently, these point-like precursors were arrayed along the shear direction and transformed into thread-like precursors. This caused the rapid formation of highly oriented shish-kebab structures. Furthermore, since short chains that caused the formation of daughter lamellae were covalently tethered to the kebab parts in LCB-PP, the growth of the daughter lamellae were spatially restricted owing to which the a-axis of the daughter lamellae in LCB-PP was oriented along the shear direction. [Display omitted] •The oriented crystallization was intensively induced in branched PP by step-shear.•The a-axis oriented daughter lamellae was formed in branched PP.•The crystallization induced by step-shear was dominated by relaxation behavior.