The annealing of solution-crystallized polyethylene mats: an X-ray diffraction study

Abstract

Both low- and wide-angle X-ray diffraction measurements have been made on solution-crystallized mats of high density polyethylene which have been annealed for different periods at 125° C. The low-angle data, in addition to showing the rapid increase in both long period and density defect at each fold surface for the first stages of annealing, also indicate that after annealing for times in excess of 10⁵ sec the density defect is reduced and approaches the value for the unannealed crystal. The wide-angle X-ray studies have centred around the Fourier analysis of the 0 0 2 diffraction peak. The results of this type of measurement on unannealed mats have already been reported [20]. In extending the work to annealed material evidence has been obtained for preferred chain stem lengths which are multiples of the unannealed length. The combination of these observations with the low-angle results has led to the formulation of a model for crystal thickening which invokes the unlooping mechanism proposed by Dreyfuss and Keller [15], but also envisages thickening occurring preferentially from one fold surface to give an asymmetric crystal profile in the [0 0 1] direction.