Polypropylene, isotactic, crystal structur

These crystal modifications differ in their molecular and crystal structures as well as in their physical properties. Many types of crystalline modifications are reported, including a stable orthorhombic phase and metastable monoclinic phase for PE a, and y forms for isotactic polypropylene (/-PP) trigonal and orthorhombic phases for polyoxymethylene a and y forms for Nylon 6 and others. Poly(vinylidene fluoride) (PVF), for example, appears in at least four types of crystalline modification (Lovinger, 1985 Dunn Carr, 1989). 

Ewen, J.A. Jones, R.L. Elder, M.J. Rheingold, A.L. Liable-Sands, L.M. Sommer, R.D. Chiral flns fl-metallocenes with Cp ring-fused to thiophenes and pyrroles syntheses, crystal structures, and isotactic polypropylene catalysts. J. Am. Chem. Soc. 2001, 123, 4763. 

In 1995 Coates and Waymouth ° reported a catalyst that produced polypropylene-containing blocks of atactic polypropylene and isotactic polypropylene. This novel elastomeric material was referred to as elastomeric homopolypropylene (EHPP). The unit cell of the crystal structure of the EHPP catalyst precursor bis(2-phenylindenyl)zirconium dichloride was observed to contain two distinct conformers see Figure 1 for the structures. In one conformer. 8M. the indenyl ligands were syn to one another (a meso stereochemistry), while in the other conformer. 8R. the indenyl ligands were anti to one another (a rac stereochemistry). Production of isotactic blocks of polypropylene could be explained by polymerization from the rac active site and production of atactic blocks by polymerization from the meso active site. In 1996 Pietsch and Rappe published a molecular... 

The isotactic polypropylene is found in almost all of the polypropylene market. It is a crystalline thermoplastic material, so mechanical properties mainly depend on their molecular structure, their crystal structure and the macro-structure induced by the transformation process (Monasse Haudin, 1995 Rodriguez et al, 2004 Varga, 1992 Fujiyama et al, 2002). 

Leugering, H. J. Kirsch, G.(1973). Effect of crystallization from oriented melts on crystal-structure of isotactic polypropylene. Angew. Makromol. Chem., Vol.33, Issue OCT,... 

Whereas most of the early work on crystallization, etc., were concerned with predominantly isotactic chains, the recent developments in synthetic methodologies have enabled the preparation of highly syndiotactic polymers [13,14]. Since the high stereoregularity of these syndiotactic polymers facilitates their crystallization, several papers have been published on the x-ray crystal structure and polymorphism of syndiotactic polystyrene [15-18]. The chain conformation in the crystalline state has also been analyzed using NMR [19]. Similarly, the crystal structure of syndiotactic polypropylene has also been studied by a number of authors [20-22]. 

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). 

Except for the PP-C, all the films exhibited diffraction patterns consistent with the a-monoclinic crystal structure of isotactic polypropylene. There were, however, distinct differences in crystalline orientation among the latter films, as indicated in Fig. 1. 

Isotactic polypropylene (iPP) was the first stereo regular polymer to be synthesized. The determination of its crystal structure was instrumental in establishing that the synthesis of the polymer was indeed stereo specific. The TG helical conformation, the 3/1 helix, is the only conformation free of serious steric repulsions in the isotactic chain. Finding this as the observed helix in the crystal... 

Keywords epitaxy, isotactic polypropylene, syndiotactic polypropylene, crystal structure, crystal polymorphism, helical hand, nucleating agents, AFM, a-phase, 3-phase, lamellae. 

Finally, a few comments about the uniqueness of polymer crystal structures and phase space localization are warranted. Almost all crystallizable polymers exhibit polymorphism, the ability to form different crystal structures as a result of changes in thermodynamic conditions (e.g., temperature or pressure) or process history (e.g., crystallization conditions) [12]. Two or more polymorphs of a given polymer result when their crystal structures are nearly iso-energetic, such that small changes in thermodynamic conditions or kinetic factors cause one or another, or both, to form. Polymorphism may arise as a result of competitive conformations of the chain, as in the case of syndiotactic polystyrene, or as a result of competitive packing modes of molecules with similar conformations, as in the case of isotactic polypropylene. In some instances, the conformational change may be quite subtle isotactic polybutene, for example, exhibits... 

In this review the crystal structure and the super-molecular structure of the most used polyolefins is discussed. In particular the latest papers on the morphology of polyethylene, isotactic and syndiotactic polypropylene, isotactic poly(l-butene), and finally isotactic poly(4-methylpentene-l) are summarized and integrated with the fundamental work on the topic. After a short general introduction, the first part of the chapter is dedicated to the analysis of the order at the molecular level (the crystal structure), and the second part deals with the supermolecular structures. 

Jiang, X. L., Liu, T., Xu, Z. M., Zhao, L., Hu, G. H., and Yuan, W. K. 2009. Effects of crystal structure on the foaming of isotactic polypropylene using supercritical carbon dioxide as a foaming agent. Journal of Supercritical Fluids 48 167-175. 

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