Spatially Resolved Phase Transformation upon Uniaxial Stretching of Polybutene-1 Spherulites in Form II

The stress-accelerated form II to form I phase transformation in Polybutene-1 (PB-1) is critical to its mechanical properties, yet the phase transition kinetics at different microdomains within its spherulitic superstructure upon tensile elongation remains elusive. This study elucidates the in situ structural evolution and polymorphic transition within individual PB-1 spherulites in form II as a function of strain by means of the synchrotron microbeam wide-angle X-ray diffraction (WAXD) technique. The spatially resolved maps of crystallinity, phase content, and crystallite size across the spherulite at various strains confirm that localized inter- and intralamellar block slips were at first activated at the spherulite center at small deformations followed by the spread to the diagonal domains with the lamellar stacks oriented at 45° to the stretching direction and meanwhile the equatorial and polar ones started to develop a greater stress. The finding that the local stress in the diagonal domains is stronger than that in the equatorial and polar ones at moderate deformations is directly evidenced by the markedly faster growth rate of the form I fraction and decrease in the crystallite size L(200)II for the diagonal regions. On the other hand, the interspherulitic domains exhibit a significantly more fraction of the form I crystallites induced by stretching than those within the spherulite due to the sparse arrangement of crystalline lamellae and thus the higher local strain accommodated by the soft matrix. These direct, spatially resolved observations provide a comprehensive micromechanical picture linking the initial microstructure, the local strain fields, and the complex, heterogeneous pathways of stress-induced phase transformation in PB-1 spherulites.