Crystal structures, polymers entanglement

Somani RH, Yang L, Zhu L, Hsiao BS (2005) Flow-induced shish-kebab precursor structures in entangled polymer melts. Polymer 46 8587-8623 Sorrentino A, De Santis F, Titomanlio G (2007) Polymer crystallization under high cooling rate and pressure a step towards polymer processing conditions. Lect Notes Phys 714 329-344 Stocker W, Magonov SN, Cantow HI, Wittmann JC, Lotz B (1993) Contact faces of epitaxially crystallized a- and y-phase isotactic polypropylene observed by atomic force microscopy. Macromolecules 26 5915-5923... 

Amorphous stereotactic polymers can crystallise, in which condition neighbouring chains are parallel. Because of the unavoidable chain entanglement in the amorphous state, only modest alignment of amorphous polymer chains is usually feasible, and moreover complete crystallisation is impossible under most circumstances, and thus many polymers are semi-crystalline. It is this feature, semicrystallinity, which distinguished polymers most sharply from other kinds of materials. Crystallisation can be from solution or from the melt, to form spherulites, or alternatively (as in a rubber or in high-strength fibres) it can be induced by mechanical means. This last is another crucial difference between polymers and other materials. Unit cells in crystals are much smaller than polymer chain lengths, which leads to a unique structural feature which is further discussed below. 

Very high molar mass polyethylene (M>2,000,000 g mol-1) crystallises without the, formation of a clear superstructure, sometimes referred to as the random lamellar structure [116]. The great many chain entanglements present in high molar mass polymers obstruct crystallisation and the crystals become small and their orientation less correlated with surrounding crystal lamellae. 

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