Crystallization, melting and morphology of syndiotactic polypropylene fractions. 4. In situ lamellar single crystal growth and melting in different sectors

Over a wide crystallization temperature (T-c) range, elongated rectangular lamellar single crystals of syndiotactic polypropylene (s-PP) grown from th e melt of thin films exhibit sectorization along both diagonal directions t o form the (010) [with the (010) free edge] and the (100) [with the (100) f ree edge] sectors. They can be observed by transmission electron and atomic force microscopy (TEM and AFM). Electron diffraction experiments show that the crystals in both sectors are form III orthorhombic structure, as recen tly proposed.(1-4) The observation of this sectorization is due to the thic kness difference between these two sectors with different fold planes. In s itu observations of the single-crystal growth are monitored using an AFM co upled with a hot stage. The T-c-depedent crystal growth rates along both th e (100) and(010) normal can be measured, and they are anisotropic. This lea ds to a large aspect ratio of the rectangular single crystal. AFM results a lso show that the thickness difference between the two sectors is dependent upon crystallization time (t(c)). In the initial stage of single-crystal d evelopment, the ratio between the thickness difference and the (100) sector thickness is large [similar to 35% of the thickness of the (100) thick sec tors]. With increasing t(c), this ratio of the thickness difference gradual ly decreases to similar to 15%. Multiple-step cooling and isothermal experi ments in a successive T-c-decreasing sequence reveal that each T-c produces a unique lamellar thickness. With reducing T-c the thickness decreases. Ro om-temperature TEM observations show that the (010) thin sectors are molten while the (100) thick sectors remain after the single crystals are anneale d on a temperature-gradient bar for a prolonged time. In situ single crysta l melting monitored by AFM reveals different melting pathways of these two sectors. When the isothermally grown single crystals are heated to 5 degree s C above T-c (120 degrees C), the melting first appears in the (100) thick sectors in the form of molten domains. These domains can be repaired via r ecrystallization if the samples are annealed at this temperature for a long period of time. As further increasing the temperature, the melting of the (100) thick sectors again starts in molten domains along the (100) free edg es of the single crystals. In the (010) thin sectors the melting takes plac e on the free edges of the single crystals, and progresses toward the inner area of the sectors.