Crystal Vibrations of Polyethylene

Abstract

Several problems involving the internal and external vibrations of the polyethylene crystal have been studied. The splittings of some of the internal vibration bands arising from transition dipole coupling have been evaluated and found to have small but nonnegligible values as compared with the splittings calculated from the intermolecular H···H interaction potential. On the other hand, interactions between permanent CH2 dipole moments in different chains have been shown to make quite insignificant contributions to the translational lattice frequencies. The effects on the vibrational frequencies of cell contraction with decreasing temperature have been calculated, and the experimentally observed upward shift of a lattice frequency is found to be explainable primarily on this basis. The effect caused by the change of the setting angle of each chain in the unit cell has also been examined. The short-range H···H interaction force constants and the dispersion curves of normal and deuterated polyethylenes have been obtained.