X-ray scattering study on the changes in the morphology of low-modulus polypropylene under cyclic uniaxial elongation

Abstract

Low-modulus polypropylene (LMPP) with controlled stereoregularity was prepared based on the original catalyst technology [Minami et al. (Polym J 47:227–34, 2015)]. The LMPP showed elastic recovery, while plastic deformation was dominant in isotactic polypropylene. To understand the underlying mechanism, changes in the morphology of LMPP under cyclic uniaxial elongation were investigated using in situ small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). During the first cycle, yielding appeared in the stress–strain (S–S) curve, and the residual strain was large. The SAXS pattern is a ring pattern at strain zero and then changes to a four-point pattern and a two-point pattern with increasing strain. This four-point pattern is derived from the undulating structure of the crystal lamellae. During the stretching process, this undulation is large, and fragmentation of the lamellae occurs. Therefore, the residual strain increases. During the second cycle, no yield appeared in the S–S curve, and the residual strain was small. The SAXS pattern changed quickly from a four-point pattern to a two-point pattern. This result suggests that little fragmentation of the lamellae occurs and that only the lamellae were rotating under elongation. Thus, plastic deformation hardly occurs, and the residual strain decreases. Based on these results, lamella fragmentation has a significant effect on the elastic-recovery rate.