Polypropylene 1-form crystals

Name the three forms of tacticity. For polypropylene, which form crystallizes fastest and why ... 

Okada, K.N. et al. (2010). Elongational crystallization of isotactic polypropylene forms nano-oriented crystals with ultra-high performance. Polymer Journal Vol. 42 (2010), pp.464-473... 

During drawing at 120°C chains pulled out from polypropylene crystals posses much higher mobility and may form assemblies ready to crystallize into well ordered a form crystals. 

Samuels, R. J. Yee, R.Y. (1972). Characterization of structure and organization of beta-form crystals in type-iii and type-iv beta-isotactic polypropylene spherulites. Polym. Sci. PartA-2 Polymer Phys., Vol.lO, Issue 3, pp. 385-432. 

Isotactic polypropylene can be blended with its random copolymers to enhance toughness. This can be accomplished without loss of stiffness and impact strength by nucleating beta form crystallization for applications such as in cables, pipes, and fittings (Machl et al. 2012). 

Neat isotactic polypropylene (iPP) crystallized from melt exhibits spherulitic morphology of the crystalline phase (72,73). In some cases and under very specific conditions, cylindrites, axialites, quadrites, hedrites, and dendrites may be formed of iPP (74). In general, crystallization from quiescent melts results in spherulitic morphology, whereas crystallization fi-om melts subjected to mechanical loads results in cylindrites (75). Crystalline supermolecular structure caused by oriented crystal growth from heterogeneous surfaces is commonly termed transcrystallinity (76). 

Three examples provided here are molecular packing, S5unmetiy, and imit cells of orthorhombic form of PE (Fig. 6) (2,10), monoclinic a form of iPP (Fig. 7), (11), and orthorhombic form of syndiotactic polypropylene (sPP) crystals having antichiral packing in handedness (Fig. 8) (12). 

Isotactic polypropylene can crystallize in different forms (modifications), such as a, P, y, and smectic, which differ by their unit cell type and thus by their packing density." The most common are the a-form and the y-form. The a-modification is the preferred crystalline form of polypropylenes synthesized by conventional Ziegler-Natta catalysts." High molecular weight isotactic polypropylenes prepared by metallocene catalysts preferentially crystallize in the y-form. The different polymorphic behaviors of metallocene and Ziegler-Natta samples can be related to the... 

Polypropylene usually crystallizes in the q form, however a /3 form also occurs. The 0 forms are easily identified by X-ray diffraction, or polarized... 

Lovinger AJ, Chua JO, Gryte CC. Studies on the a and p forms of isotactic polypropylene by crystallization in a temperature gradient J Polym Sci B 1977 15 641-656. 

Isotactic polypropylene (iPP) exhibits a stable crystalline a form consisting of chains in the 3i helical conformation (tgtgtg), packed in a monoclinic unit cell. Left- and right-handed helices face each other in the a form. The metastable -form crystals contain hexagonally packed 3i helical chains arranged in groups of the same helical handedness (left or right). There is also a smectic, or partially ordered, form... 

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