MOLECULAR DYNAMICS SIMULATION OF THE RELAXATION OF A FULLY EXTENDED POLYETHYLENE CHAIN

摘要

Molecular dynamics simulation of the relaxation at 300 K of a fully extended polyethylene chain of800 CH2 units has been carried out by following the changes in morphology, van der Waals energy, radius ofgyration in the sense of mechanics and gyration radius in the sense of Flory, population of trans-conformation and orientation factor. The relaxation went through three stages: （1） relaxation from themorphology of a straight rod of 100 nm length to the molphology close to a random coil of gyration radius5.9 nm in 110 ps; （2） collapse of the morphology of a coil to a highly compact globule close to a sphere ofgyration radius 1.3 nm after 178 ps as the result of intersegmental van der Waals attractive interactions; （3）lateral ordering of the folded chain segments in the globule without appreciable changes in the chaindimension up to 1600 ps, the time limit of present simulation. Nearly complete relaxation of local segmentalorientation was performed much faster than the relaxation of globule chain orientation even for a single chainof low degree of polymerization and at a temperature some 155℃above its Tg. The lateral ordering of thechain segments during the period 178 to 680 ps of the simulation time was found to obey the Avramiequation with an Avrami index of 1 .44.