Crystallization, Melting and Morphology of Syndiotactic Polypropylene Fractions. 4. In Situ Lamellar Single Crystal Growth and Melting in Different Sectors

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Abstract

Over a wide crystallization temperature (Tc) range, elongated rectangular lamellar single crystals of syndiotactic polypropylene (s-PP) grown from the melt of thin films exhibit sectorization along both diagonal directions to form the (010) [with the (010) free edge] and the (100) [with the (100) free 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 recently proposed.1-4 The observation of this sectorization is due to the thickness difference between these two sectors with different fold planes. In situ observations of the single-crystal growth are monitored using an AFM coupled with a hot stage. The Tc-dependent crystal growth rates along both the (100) and (010) normal can be measured, and they are anisotropic. This leads to a large aspect ratio of the rectangular single crystal. AFM results also show that the thickness difference between the two sectors is dependent upon crystallization time (tc). In the initial stage of single-crystal development, the ratio between the thickness difference and the (100) sector thickness is large [∼35% of the thickness of the (100) thick sectors]. With increasing tc, this ratio of the thickness difference gradually decreases to ∼15%. Multiple-step cooling and isothermal experiments in a successive Tc-decreasing sequence reveal that each Tc produces a unique lamellar thickness. With reducing Tc, the thickness decreases. Room-temperature TEM observations show that the (010) thin sectors are molten while the (100) thick sectors remain after the single crystals are annealed on a temperature-gradient bar for a prolonged time. In situ single crystal melting monitored by AFM reveals different melting pathways of these two sectors. When the isothermally grown single crystals are heated to 5 °C above Tc (120 °C), the melting first appears in the (100) thick sectors in the form of molten domains. These domains can be repaired via recrystallization 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 edges of the single crystals. In the (010) thin sectors the melting takes place on the free edges of the single crystals, and progresses toward the inner area of the sectors.

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