Structure and morphology development in syndiotactic polypropylene during isothermal crystallization and subsequent melting

Structure and morphology development during the isothermal crystallization and subsequent melting of syndiotactic polypropylene (sPP) was studied with differential scanning calorimetry (DSC), time-resolved simultaneous small-angle X-ray scattering (SAXS), and wide-angle X-ray diffraction (WAXD) methods with synchrotron radiation. The morphology of sPP isothermally crystallized at 100°C for 3 h was also characterized with transmission electron microscopy (TEM). Time- and temperature-dependent parameters such as the long period (L), crystal lamellar thickness (l_c), amorphous layer thickness (l_a), scattering invariant (Q), crystallinity (X_c), lateral crystal sizes (L₂₀₀ and L₀₁₀), and unit cell dimensions (a and b) were extracted from the SAXS and WAXD data. Results indicate that the decreases in L and l_c with time are probably due to the formation of thinner crystal lamellae, and the decreases in a and b are due to crystal perfection. The changes in the morphological parameters (Q, X_c, L, and l_c) during subsequent melting exhibited a two-stage process that was consistent with the multiple melting peaks observed in DSC. The two high-temperature peaks can be attributed to the melting of primary lamellae (at lower temperatures) and recrystallized lamellae (at higher temperatures). An additional minor peak, located at the lowest temperature, was also visible and was related to the melting of thin and defective secondary lamellae. TEM results are consistent with the SAXS data, which support the assignment of the larger value (l₁) from the correlation function analysis as l_c. WAXD showed that the thermal expansion was greater along the b axis than the a axis during melting.