MORPHOLOGY AND KINETICS OF CRYSTALLIZATION OF POLYETHYLENE FROM CHAINDISENTANGLED MELT

Short description of crystallization processes in linear polyethylene with applied reptation concept and emphasized role of chain entangleme nts is presented. The influence of chain entanglements in Regimes of c rystallization is discussed. The estimation of time of self-diffusion and time required for complete pull-out of a macromolecular chain from melt by crystallization forces on the basis of reptation friction lea ds to the conclusion that a crystallizing macromolecule is reeled out of the melt very quickly (order of microseconds) while other macromole cules are relatively motionless. Fast crystallization of polyethylene does not change end-to-end distance of macromolecules while slow isoth ermal crystallization causes partial disentanglement of chains as it c ould be concluded from an increased end-to-end distance. The route to complete disentanglement is a slow crystallization at elevated tempera ture (similar to 230 degrees C) and under high pressure (similar to 6 kbars). High pressure crystallization delivers polyethylene samples wi th a high melting point (similar to 145 degrees C) and completely exte nded chains (end-to-end distance similar to 1.0). Macromolecular chain s in such samples after melting exhibit for a period of several minute s complete or partial disentanglement and can be used for studies of c hain entanglements dynamics and also for studies of crystallization fr om chain disentangled melt. The studies of crystallization of linear p olyethylene in disentangled melt showed almost 4 fold increase in the growth rate of spherulite at 126 degrees C (end of Regime II of crysta llization) and only 1.1 fold increase at 123 degrees C (beginning of R egime II of crystallization). The results concerning the kinetics of c rystallization and nucleation in chain disentagled PE melt as well as resulting morphology are discussed further in terms of Regimes and rep tation concepts.