Theoretical birefringence of ultraoriented polyethylene from model structure calculations

Abstract The birefringence of model structures of ultraoriented polyethylene has been calculated by using the point‐dipole concept and the polarizability tensor of the methylene group derived recently by Pietralla. For well‐oriented chains the birefringence is determined by the combination of a negative anisotropy of the polarizability tensor of the methylene group and a positive anisotropy of the Lorentz tensor, both with respect to the chain direction. As a consequence it decreases with increasing density. The results are in agreement with experimental values of Mead, Desper, and Porter. In the model the amorphous regions are regarded as a mixture of planar and helical segments arranged in nematiclike bundles which additionally may have a certain distribution of orientation. The birefringence is not much affected by defects which, on the other hand, strongly influence the ultimate mechanical properties.