Chain diffusion in polyethylene andn-alkane crystals observed by carbon-13 NMR

Abstract

Molecular chain diffusion in polyethylene crystals is studied using carbon-13 NMR longitudinal relaxation. Conventional slow-cooled polyethylene samples have been treated by means of electron-beam irradiation in acetylene, which forms crosslinks mainly at the fold surfaces. Following saturation, these samples show a systematic decrease in the rate of recovery of the crystalline signal with gel fraction, which cannot be interpreted by the usual dipolar spin–lattice relaxation mechanism. The results are attributed to and modelled by diffusion of chains from the crystal to the interfacial region, where they experience an efficient relaxation process, and back into the crystal during the experimental recycle delay. The diffusion coefficient appears to be independent of molecular weight in the 105–106 range, but increases with lamellar thickness. As the lamellar thickness has been increased by the process of high pressure annealing, the increase in the diffusion coefficient is attributed to a decrease in the amorphous-phase entanglement density, so that there are fewer molecular constraints to the longitudinal motion of the crystalline chain stems. From the temperature dependence of the diffusion coefficient, an activation energy of 27 ± 0·7 kJmol-1 has been calculated. Preliminary results are reported on a monodisperse n-alkane, C294H590, which has been quenched so that the lamellae comprise chains in the once-folded conformation. Here, the diffusion process is faster than for slow-cooled and pressure-annealed polyethylene, which is consistent with the progressive removal of constraints. © 1998 Society of Chemical Industry