Crystalline−Noncrystalline Structure and Chain Diffusion Associated with the 180° Flip Motion for Polyethylene Single Crystals As Revealed by Solid-State 13C NMR Analyses

Abstract

The structure of the noncrystalline overlayer and molecular motion in the crystalline region for polyethylene single crystals have been investigated by solid-state 13C NMR spectroscopy and transmission electron microscopy. Our previous finding that the noncrystalline overlayer is mainly composed of loose loops is confirmed for the single crystals with a typical lozenge shape by the line shape analysis of the fully relaxed DD/MAS 13C NMR spectrum with the aids of 13C spin−lattice and spin−spin relaxation time measurements. The occurrence of the 180° flip motion in the crystalline region is clarified by using similar methods performed in the previous work. Moreover, the chain diffusion associated with the flip motion in the crystalline region is also observed at 78 °C in the 2D 13C exchange NMR spectrum. A one-dimensional random walk simulation is carried out to clarify the correlation between the 180° flip motion and the chain diffusion. The loose loop structure of the noncrystalline overlayer is still kept even after the annealing for 7 days at the temperature where the chain diffusion occurs. It is suggested that loose loops are probably the quasi-equilibrium structure for the noncrystalline overlayer of the polyethylene single crystals.