Crystallization and Melting of Polyethylene under Pressure

摘要

The crystallization conditions which lead to extended-chain polyethylene under elevated pressure were elucidated further. The dependence of polyethylene density on crystallization pressure on slow cooling was extended up to 6980 bars. No further improvement of crystal perfection or extension of polymer chains were observed. At 5000 bars the temperature of crystallization was varied between 170 to 270C. Temperatures between 195 and 225C were found suitable for crystallization of extended chain polyethylene. A time sequence at 200C and 5000 bar revealed some insight in the formation of extended chain polyethylene. It is thought to involve a two step process of nucleation of folded chain lamellae, followed by solid state chain extension. High pressure differential thermal analysis apparatus has been developed for measurement up to 3700 bar. Folded as well as extended chain crystals of polyethylene were analyzed. Only the extended chain crystals show an initial increase in melting point in accord with the Clausius Clapeyron equation. (Author)