Crystal structure polypropylene

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

Film manufacture also requires special considerations in the case of polypropylene in order to ensure small crystal structures and hence high clarity. Chill-roll casting processes may be used and these give films of high clarity and minimal thickness variations at high rates of production. Blown film processes can, however, give superior mechanical properties and in addition equipment costs are lower, and in consequence the process is more economic for lower tonnage production. 

Syndiotactic polypropylene became commercially available about ten years ago with the advent of single-site catalysts. Unlike its atactic and isotactic counterparts, its manufacture presented serious challenges to polymer scientists and engineers. Even under the best conditions, its syndiotacticity rarely exceeds 75%, based on pentad sequences. It typically has both a lower melting point (approximately 138 °C relative to approximately 155 to 160 °C) and density (0.89 g/cm3 relative to 0.93 g/cm3) than isotactic polypropylene. Syndiotactic polypropylene crystallites have a much more complex structure than the isotactic form, which impedes its crystallization. Therefore, in general, the syndiotactic form of polypropylene crystallizes very slowly. 

Kuo SW, Chan SC, Chang FC. Correlation of material and processing time scales with structure development in isotactic polypropylene crystallization. Macromolecules 2003 36 6653-6661. 

Most polymers fall in the class of translucent resins. These include acetal, polyamide, polybutylene terephthalate (PBT), polyethylene, and polypropylene as examples. There are very few neat polymers that are truly opaque (this depends on thickness as well). Liquid crystal polymer (LCP) is an example of a typically opaque polymer. It is theorized that these semicrystalline and crystalline resins will scatter some portion of incident light due to spherulitic crystal structure and the amorphous-crystalline region interfaces themselves. 

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... 

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