AbstractThe infrared absorption spectra of selected crystalline and noncrystalline bands were studied in bulk‐crystallized specimens of linear polyethylene which encompassed the extremely wide density range of 0.92–0.99 g./cm.3. The analysis of the data obtained at room temperature yield degrees of crystallinity by infrared methods which are in very good accord with the values deduced from the density measurements. Studies of the infrared spectra as a function of temperature give fusion curves which are in agreement with those obtained by thermodynamic methods. However, in order to obtain these latter results cognizance must be taken of the large negative temperature coefficient of the specific extinction coefficients of the crystalline bands from room temperature to the melting point. The necessary data to account for this phenomena were obtained from studies of the spectra of then‐paraffin, C94H190, where molecular crystals are formed. Analysis of the twogauchebands, at 1352 and 1303 cm.−1, which are assigned to the noncrystalline regions demonstrate that for bulk‐crystallized samples of lowest densities the intensity ratio at room temperature is identical to that expected from the pure melt at this temperature. The conclusion is thus reached that the noncrystalline regions in these cases and the pure melt are structurally very similar. For samples of higher density, where the crystallite size is comparable to the extended chain length, the intensity ratio of the twogauchebands is altered. This change could reflect a change in the sequential distribution ofgauchebonds. This intensity ratio for crystals formed from dilute solution is very similar to that for the high‐density bulk‐crystallized material and indicates a similarity in structure of the noncrystalline regions in