The identification of the initial lamellar thickness of polyethylene crystals grown from the melt using synchrotron X-radiation

Abstract

The lamellar thickness of a sharp fraction of linear polyethylene has been monitored during crystallization at 128.5° C at very short crystallization times (30sec to 30min) using low angle X-ray diffraction (LAXD) by using the powerful synchrotron source at Daresbury. The observed thickness is much lower than expected and does not change with time. The same sample showed two Raman longitudinal accordion modes (LAM), one corresponding to the lamellar thickness observed by LAXD and the other to a much larger thickness. From these, and further parallel experiments using Raman spectroscopy, differential scanning calorimetry and electron microscopy we deduce that the lamellae as initially formed are much thinner than previously expected, that they first thicken by a single, large step-to the usually reported values for melt crystallized polyethylene-and only then do they exhibit a continuous increase in thickness with time corresponding to the isothermal thickening as reported in previous work.