MOLECULAR-DYNAMICS SIMULATION OF POLYMER ORDERING - 1 - CRYSTALLIZATION FROM VAPOR-PHASE

We investigate the molecular mechanism of secondary nucleation on a gr owth surface of the polymer crystal. We adopt a simplified model of po lyethylene molecule made of 500 beads connected by springs, and consid er the crystallization from a vapor phase neglecting surrounding molec ules. A strongly collapsed chain in a vacuum is placed near the infini tely wide lateral surface of the polymer crystal, and the molecular pr ocesses of adsorption and ordering are investigated by molecular-dynam ics simulation. It is found that the polymer chain is quickly adsorbed , in a stepwise manner, to form a layer structure: A multilayered at l ower temperature and a monolayered at higher temperature. The chain se gments adsorbed to the surface align parallel with each other and grad ually, within several ns, grow into a neat chain folded lamella with p redominantly adjacent-reentry folds. The thickness of the lamella sens ibly depends on the crystallization temperature and shows a rapid incr ease around the melting point. It is found that the polymer ordering o n the surface is not a sequential process from the chain-end but a rat her cooperative process of many segments. Furthermore, we investigate the effect of finite thickness of the substrate crystal, and find that the Limited thickness of the substrate gives rise to the frequent gen eration of chain loops at and the segregation of chain entanglements t oward the lamella surfaces. (C) 1998 American Institute of Physics.