TEMPERATURE-DEPENDENCE OF STRUCTURE AND MORPHOLOGY OF SYNDIOTACTIC POLYPROPYLENE AND EPITAXIAL RELATIONSHIPS WITH ISOTACTIC POLYPROPYLENE

Structural investigation of syndiotactic polypropylene as a function of crystallization temperature revealed that at growth temperatures greater-than-or-equal-to 105-degrees-C this polymer crystallizes in the form of large, rectangular, single-crystal laths whose unit cell and interchain packing are in full agreement with our recently proposed alternative structural model. At lower temperatures of crystallization, an intermolecular packing disorder is progressively introduced along the b axis, which however does not affect packing in the a direction. The origin of this defect lies in kinetically driven incorporation of molecules of either chirality, whereas slow growth at the highest temperatures favors antichiral packing. At the same time, the morphology becomes axialitic and eventually spherulitic, although the tendency toward a rectangular lamellar habit is preserved. Within the syndiotactic polypropylene specimens, discrete populations of isotactic crystals were identified. This has important implications on the mechanism of syndiospecific polymerization and on the prevailing interpretation of stereoblock formation. The isotactic crystals grow as edge-on lamellae at the lateral sides of the syndiotactic laths through a specific epitaxial relationship discussed here. Because the heteroepitaxial match between isotactic and syndiotactic crystals is not as close as the homoepitaxial one within isotactic polypropylene, its crystals grow as stacks aligned parallel to the growth direction of the underlying syndiotactic laths.