Thermal contraction and isometric heating of oriented polyethylene obtained by shear deformation

Abstract

A study was made of morphological and structural changes taking place during thermal contraction and isometric heating of oriented, low density polyethylene samples. It was shown that in the initial stages of thermal contraction first slight interfibrillar and later intralamellar shear takes place. Subsequent increase in temperature and particularly, transition to the region of partial melting markedly increase contraction and result in a change in the shape of the sample. During the entire process of heating the polymer up to melting point no mutual disorientation of crystalline lamellae was observed in spite of the fact that before melting the order of these centres deteriorated considerably resulting in the disappearance of individual small angle X-ray scattering. Structural regions were formed which begin to melt mainly on increasing temperature. It was shown that three processes taking place in oriented low density polyethylene (extension of the long period, marked contraction and inhibition, followed by the cessation of internal stress increase in the sample) are observed in the same temperature interval, which may be due to one cause—diffusion mobility of macromolecules in polymer crystallites.