Knowledge Bank

The availability of highly oriented clear ``solid-state” extrudates of polyethylene have enabled us to measure a number of important but hitherto obscure or unobserved features in the Raman spectrum of the polymethylene chain, and in addition to determine the polarization of nearly all bands in the spectrum. About 20 bands due to two-phonon Raman scattering have been found in the region 4000-1600 $cm-1$ These bands are characteristically broad and asymmetric, and their polarization shows they belong to chain symmetry species $AgorB1g(\alpha xy\prime \ne 0$, where z is the chain axis). Many of the bands can be interpreted in terms of dispersion curves of the isolated chain. In a few cases their intensities are enhanced by Fermi resonance interaction with nearby fundamentals. The assignment of some of the fundamentals will be discussed in relation to their observed polarization. A broad low frequency longitudinal acoustic mode vas found near 12 $cm-1$. Its shape has been measured, and from it the distribution of lengths of extended chains has been determined. This analysis is based in part on Raman band shape measurements of the longitudinal acoustical nodes of crystalline n-paraffins.