Morphology, crystal structure and melting of syndiotactic polypropylene lamellar crystals

The morphology, crystallization, melting, and crystal structure of syndiotactic polypropylene (sPP) has been studied extensively within recent years. Regular, faceted, and lath-shape lamellar crystals grown from sPP thin films show sectors with substantial thickness differences. Generally, in polymer lamellar crystals, the sectorization can only be observed after decoration or due to the presence of a sector boundary. Our main objectives were to study the difference between sectors in both morphology and crystal structure of sPP, the temperature and molecular weight dependence of sectorization, and the thermodynamic stabilities of sectors.; Elongated rectangular lamellar crystals can be isothermally grown from the melt in a wide temperature range (70°C to 140°C). From transmission electron microscopy (TEM) and atomic force microscopy observation, sectorization can be found in lamellar crystals grown at different crystallization temperatures, except the highest crystallization temperature (140°C). All single crystals from samples of different molecular weights display sectorization, and the thickness difference between sectors ranges from 1/7 to 1/3 of crystal thickness. With the selective area electron diffraction and tilting experiments, the crystal structures in sectors were determined to be the same. However, the melting experiments indicate two melting temperatures in one sPP lamellar crystal, and it was observed that sector (100) has higher melting temperature. The melting temperature difference is small, in the range of one or two degrees. The sectors are formed from the very beginning of crystallization, as shown in the in-situ observation of the growth and melting of sPP lamellar crystals. The thickness difference changes with crystallization time.; Sector (100) has a wave-like morphology and sector (010) has a radial morphology, as seen from the dark field images and the multi-dark field images. These morphologies are probably related to the different chain folding directions in different sectors.