Changes in orientation of crystallites during stretching and relaxation of polyethylene films

Abstract

Abstract A microbeam camera has been used in x‐ray diffraction studies of the changes which occur in orientation of crystallites in polyethylene film when the film is stretched and later relaxed. The collimator of the microbeam camera, 100 μ in diameter, allows one to follow the orientation changes very closely in the necked‐down region of a stretched sample. The stretching of polyethylene proceeds by alignment of the a ‐axes of the crystallites perpendicular to the stretching direction with the planes containing b ‐ and c ‐axes perpendicular to the a ‐axis but randomly oriented, followed by a gradual change to a state in which the c ‐axes become more nearly aligned in the stretching direction although perfect alignment is not achieved. On relaxation, either the orientation of the c ‐axis parallel to the original stretching direction becomes less perfect, or folding of the oriented crystallites about the b ‐axis takes place. In the latter case the preferred crystallographic directions parallel to the stretching direction have indices such as [ u 0 w ]. These orientation changes during stretching and relaxation of the stretched samples suggest that crystallites in polyethylene films consist of parallel arrangements of platelets containing b ‐ and c ‐axes. The distinction between directions in the reciprocal lattice and the crystal lattice is clarified. A number of x‐ray diffraction patterns and charts of dimensional changes illustrate the paper.