Analysis of some two-phonon infrared absorption bands of polyethylene and the analogous binary combination bands of the triclinic n-paraffins

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Abstract
The interpretation of several two‐phonon (binary combination) bands in the infrared spectrum of polyethylene is reported. The analysis is based in large part on the infrared spectra of the triclinic n‐paraffins, n‐CnH2n+2 (n=20, 22, 24, and 26) whose corresponding binary combinations appear with appreciable intensity under a phase difference condition that is analogous to the K=0 condition for the polymer. Several observed two‐phonon bands of polyethylene that are uninterpretable in terms of factor‐group fundamentals, are found to be associated with critical points that are not at the zone center. A detailed assignment of a band progression of binary states involving the ν3 and ν5 bands leads to new data on the ν5 phonon dispersion curve. It is found that the critical point maximum occurs near 527±5 cm−1, in good agreement with neutron inelastic scattering results, but lower than the value derived from most reported normal vibration calculations.