Annealing studies of solution and bulk crystallized polyethylene using the Raman-active longitudinal acoustical vibrational mode

The longitudinal acoustical (LA) mode has been studied in solution and bulk-crystallized polyethylene by Raman spectroscopy. In bulk-crystallized polyethylene the crystalline repeat, estimated from the LA frequency, corresponds to the larger of the two small angle repeat distances. Observations of the LA linewidth indicate that upon annealing the distribution of lamellar thicknesses broadens for single crystal mats and narrows for quenched bulk material. In successive annealing experiments on preannealed polyethylene where the spectra were recorded at the annealing temperature, the crystalline core maintains a constant thickness and does not melt-inward from the fold surfaces. The intensity of the LA mode decreases during such annealing. These results are interpreted in terms of the α-relaxation process which increases the number of defect chains in the crystalline core upon annealing.