Evidence for a partially ordered component in polyethylene from wide-angle X-ray diffraction

This paper reports a discrepancy found during the analysis of wide-angle X-ray diffraction patterns from a broad range of branched polyethylenes and describes its interpretation. The paper is the second in a sequence of three investigating the structure of polyethylene using new methods in X-ray diffraction and molecular modelling. The X-ray diffraction patterns were recorded from both unoriented and fibre sample forms, using reflection and transmission geometries, respectively. A common set of crystallisation conditions were used to prepare the unoriented samples and the fibre samples were drawn from these unoriented samples using a consistent set of drawing conditions. The X-ray diffraction patterns were fitted to two contributions, namely crystalline and amorphous components, according to standard practice for polymers. However, a discrepancy in crystalline peak positions between low and high angle regions of each diffraction pattern was found for all samples, in both unoriented and fibre forms. The discrepancy is interpreted in terms of an additional distinct contribution to the X-ray diffraction pattern of polyethylene, from a third structural component of intermediate order. The scattering is consistent with partially ordered material and is found to be correlated with not only the branch content but also the branch distribution of the polyethylene. This opens up the possibility of tailoring the influence of the partially ordered component on the polymer's microstructure, and hence properties, by varying the molecular architecture and thermal history.