Structural Characteristics of Fibrillar Crystals in Uniaxially Stretched Isotactic Polypropylene Dominated by Temperature and Strain

The spherulitic morphology of isotactic polypropylene can be transformed into the oriented fibrillar morphology through hot stretching processes with varying temperature (T_s) or altering strain (ε_t). The effects of T_s and ε_t on the structural characteristics of fibrillar crystals are comprehensively investigated with respect to crystal orientation, long periodic spacing, lamellar thickness (L_c), crystallinity (X_c), melting point, and chain relaxation behavior. Small-angle X-ray scattering patterns illustrate that the fibrillar crystals consist of alternated stacks of crystalline lamellae and amorphous layers. High T_s leads to a low orientation degree of lamellae, whereas large ε_t facilitates a high orientation level. The X_c and mean L_c are improved continuously with the increasing of T_s or ε_t, indicating a stretching-enhanced crystallization behavior driven by the two factors. The endothermic profiles reveal that new chain-folded lamellae with relatively thinner thickness form during the hot stretching process. The formation of thinner lamellae is dominated by the melting–recrystallization mechanism. This work would provide guidance for optimizing process conditions to manipulate the microstructure of hot-stretched semicrystalline polymers.