Temperature Effects on Interlamellar Chain Entanglement and Structural Changes in Isotactic Polypropylene during Uniaxial Tensile Deformation

\textit{In-situ} small-angle x-ray scattering, wide-angle x-ray diffraction and \textit{ex-situ} atomic force microscopy techniques were carried out to investigate the structural and morphological changes of isotactic polypropylene (iPP) films during uniaxial tensile deformation at varying temperatures (i.e., room temperature, 60$^{\circ}$C and 160$^{\circ}$C). The mass fractions of amorphous, mesomorphic, and crystal phases were determined. Results indicate that at room temperature, the dominant deformation-induced transition occurs between the crystal and mesomorphic phases, while at high temperature ($>$ 60$^{\circ}$C), the dominant transition occurs between amorphous and crystal (i.e., monoclinic $\alpha $-form) phases. This behavior can be explained by the concept of chain mobility, and the relative strength between the interlamellar entangled amorphous network and the surrounding crystal lamellae.~ It appears that at low temperatures, the interlamellar entanglement network is stronger than the adjacent lamellae, resulting in lamellar fragmentation and formation of oriented mesomorphic phase.~ In contrast, an opposite scenario occurs at high temperatures, resulting in disentanglement of amorphous chains and the growth of crystalline lamellae.