Relaxations in Polyethylene: Orientation of the Lamellar Crystals

Abstract

A torsion pendulum has been used to make mechanical measurements on linear polyethylene between −175° and +110°C at about 1 Hz. Samples in which orientation had been induced by crystallization in a temperature gradient were measured together with unoriented but otherwise comparable samples. The data are analyzed to show that the a relaxation in polyethylene is sensitive to the orientation of the deformation with respect to the structural and morphological symmetry. For shearing in the a-c face of the unit cell and lamella boundaries the relaxation average over all directions is different from the average for shearing in all possible planes in the cell and boundaries. The γ relaxation does not exhibit this difference. Various viscoelastic parameters such as the real part of the modulus and the magnitude of the relaxations do not correlate with lamella thickness in the present samples.