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

Abstract

AbstractStructure 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 (lc), amorphous layer thickness (la), scattering invariant (Q), crystallinity (Xc), 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 lc 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, Xc, L, and lc) 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 supports the assignment of the larger value (l1) from the correlation function analysis as lc. WAXD showed that the thermal expansion was greater along the b axis than the a axis during melting. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2982–2995, 2001