High-temperature quasi-hexagonal phase in the simplest model of a polymer crystal (original) (raw)

2008, Doklady Physical Chemistry

Polymorphism and structural phase transitions in polymer crystals have been extensively studied for many years. However, even in polyethylene (PE), a model polymer, the molecular mechanisms of structural transitions are still unclear. Several types of crystallites, including orthorhombic and quasi-hexagonal (packing of round cylinders), have been found in PE. The transition of the orthorhombic phase to the quasihexagonal one occurs upon heating at high pressure or when geometric constraints are imposed on the sample . The structure of this phase has not been determined thus far. Most popular is the model of a conformationally disordered (condis) crystal in which chains are, on average, parallel but the fraction of gauche conformations in them is very large. A model was also proposed in which long sequences of trans conformations are separated by short sequences composed of a few gauche conformations and flat chain segments can rotate as a whole like chains in paraffins . Another model implies that the orthorhombic cell locally (within short segments of several neighboring chains) persists in the crystal, which is a mosaic of differently oriented pieces of the orthorhombic structure. All these models predict the existence of a hexagonal lattice in the crystal.

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