Defect modes for (200), GGTGG,tight fold re-entry in polyethylene single crystals

Lattice dynamical calculations on trans-planar polyethylene chains containing conformational defects of the type GGTGG have been carried out. The purpose of the work is to predict the points in the vibrational infra-red and/or Raman spectrum at which a tight, (200) type, fold should show absorptions and/or scattering. Calculations are verified for the cyclic hydrocarbon C₃₄H₆₈ which contains two GGTGG defects. The usefulness and limitations of the vibrational spectrum as evidence for the existence of (200) tight folds are discussed. Two infra-red bands near 1342 cm^?1 and a third near 700 cm^?1 may indicate GGTGG defects in single crystals. Raman spectra seem insensitive to such defects.