Motion in polyethylene. II. Vibrations in crystalline polyethylene

Using the specific heats of completely crystalline polyethylene as a basis, the vibrational spectrum is discussed. The vibrational spectrum is found to consist of three completely separate parts. (A)The high?frequency CH2?stretching vibrations between 2850 and 2930 cm-1 which contribute little to the specific heat below 350°K. (B)The low?frequency optical vibrations between 720 and 1480 cm-1 which contribute above 150°K increasingly to the specific heat. (C)The acoustical vibrations which stretch from zero to approximately 500 cm-1 and are at 260°K to 90 percent excited. A and B are adequately known from infrared and Raman experiments. C can be approximated by a Tarasov?type treatment leading to ? temperatures of 123° and 540° for the three?dimensional and one?dimensional parts, respectively. More detailed spectra are discussed, and a semiempirical best fit is given. The one-dimensional Debye function necessary for this discussion has been computed to five places and is tabulated in steps of hv/kT=0.01.