Knowledge Bank

A number of the prominent two phonon absorption bands observed in the Infrared spectrum $(2800-1500cm-1)$of crystalline polyethylene have been interpreted in terms of density of state curves derived from the dispersion of the fundamental vibrations of the polymer. Combination bands observed in the infrared spectra of the crystalline n-paraffins play a major role in the analysis. The appearance of these latter are found to be determined primarily by approximate conservation of wave vector. In consequence, the number of such modes allowed is small enough to permit detailed assignment. In certain cases it is possible to determine from the combination series the shape of a dispersion curve of a fundamental mode in a region not previously accessible; such is the case for $\nu 5$ and $\nu 7$.