Analysis of the Intensities of the Longitudinal Acoustic Vibtrations in n-Alkanes and Polyethylene

Abstract

Simple theoretical considerations indicate, that the integral intensity of the "longitudinal acoustic modes" (LAM) in the Raman spectrum of n-alkanes should be independent of the chain length. The experiment in fact confirms this expectation. Using this property it becomes possible for polyethylene samples, to derive from the measured LAM-intensity the mass fraction of all-turns sequences contributing to this band. For solution crystallized samples this fraction coincides with the crystallinity, whereas in melt crystallized samples it generally turns out as smaller. The observed difference is likely to be due to the occurrence of local conformational defects (kinks, chain torsions) which interrupt the all-turns sequences within the crystallites. Temperature dependent experiments speak in favour of a defect creation for temperatures above 70°C.

Dedicated to Prof. Dr. G. Rehage on the occassion of his 60th birthday.