The stretching and relaxing of polyethylene

By using an X-ray beam of very small cross-sectional area, a series of X-ray diffraction patterns has been obtained over the region of the neck that is produced when randomly oriented polythene is stretched. These indicate that the orientation over the region of the neck produced by stretching at room and lower temperatures is abnormal in that it does not consist of a gradual alinement of the polymer chains with the direction of stretch, but that at intermediate stretches the preferred orientation is such that the (011) axis is parallel to the stretch direction. At higher stretches, however, there is a gradual tilting of the crystallites until they are finally alined with their chain axis parallel to the direction of stretch. At temperatures in the region of 100° C, this intermediate step does not take place and the crystallites gradually aline themselves with their chain direction parallel to the axis of the monofilament. The relaxing of a polythene neck at 110° C has also been shown to be non-random. The direction of preferred orientation changes as relaxation proceeds from that in which the a axis of the crystallites is perpendicular to the axis of the monofilament, to that in which the a axis is parallel to this direction. A tentative explanation has been put forward to explain this behaviour.