Polymer Deformation. VI. Twinning and Phase Transformation of Polyethylene Single Crystals as a Function of Stretching Direction

Abstract

The deformation occurring during the drawing of polyethylene single crystals consisting only of {110} fold domains was studied by means of electron diffraction and bright-field and dark-field electron microscopy. Samples of several types of polyethylene crystals were used; all crystals studied being drawn 25% on a Mylar substrate. This paper is particularly concerned with the relationship between the draw direction and the crystal axes as it affects the type of deformation. Four different types of deformation, involving different combinations of twinning and phase changes, can be distinguished as follows: (1) When the draw direction is near the b axis, a phase transformation from orthorhombic to monoclinic occurs such that (21̄0)Mo corresponds to (200)Or. (2) When the draw direction lies at an angle between b and 〈110〉, i.e., is nearly perpendicular to a growth face, the phase transformations described here under 1 and 3 both occur. (3) When the draw direction is nearly parallel to 〈110〉, a phase transformation to a monoclinic cell occurs such that (200)Mo corresponds to (200)Or. (4) When the draw direction is near the a axis {110}Or twinning is the primary result. It was concluded that the shear direction for the (110)Or twinning is [11̄0] and that the phase transformation occurs in bands parallel to a, these bands corresponding to striations previously seen in bright- and dark-field micrographs of similarly deformed crystals. The possibility of twinning in the monoclinic phase and the necessity to consider slip parallel to c and molecular tilting is also pointed out.