Morphology of oriented linear polyethylene: A study by Raman spectroscopy

Measurements have been made of the low‐frequency Raman longitudinal acoustic mode (LAM) vibration in oriented linear polyethylenes. The oriented samples were prepared by tensile drawing of both slow‐cooled and quenched sheets, and included ultrahigh‐modulus materials from draw ratios up to 30. A LAM line is observed clearly in samples of low and intermediate molecular weight up to draw ratios of ca. 15. In all these cases the Raman spectrum has been used to calculate the whole distribution of crystal sizes. This procedure leads to values of the number‐average and weight‐average crystal size which are in good agreement with crystal size determinations by x‐ray diffractometry and gel permeation chromatography on etched samples. At higher draw ratios the peak intensity of the LAM line is diminished. This can be attributed to a change in the distribution of crystal thicknesses, consistent with data from x‐ray diffraction and nitric acid etching. Effects due to initial morphology, sample molecular weight, and draw temperature have also been examined.