Morphology of isotropic and oriented linear polyethylene. Study by small-angle X-ray scattering, Raman spectroscopy and nitric acid etching

Abstract

In the first instance, measurements of the Raman longitudinal acoustic mode (l.a.m.) were combined with small-angle X-ray scattering (SAXS) and nitric acid etching, followed by gel-permeation chromatography (g.p.c.) to examine the morphology of bulk-crystallized linear polyethylene (LPE). It was concluded that there were unambiguous differences between the SAXS and the Raman data, provided that the Raman length is compared with the layer thickness deduced from the SAXS long spacing taking into account chain tilt. The nitric acid/g.p.c. results strongly support the conclusion that the l.a.m. relates to the all-trans molecular length, i.e., to the direct determination of the crystal single traverse. For isotropic samples there is a direct correspondence between the Raman length and the crystal thickness deduced from the SAXS measurements. The combined results from the Raman, SAXS and g.p.c. experiments therefore suggest a uniform morphology with regular lamellar stacks separated by amorphous material. Further measurements were then carried out on drawn samples, including ultra high modulus materials, which were prepared by drawing both slow cooled and quenched sheets to draw ratios up to 30. It was found that the Raman line due to the l.a.m. which is present in the isotropic polymer gradually reduces in intensity on drawing so that by a draw ration of ≈ 5 it is not detectable. By draw ratio ≈ 10 an l.a.m. line can again be identified at a frequency which depends on the draw temperature. There is an apparent decrease in the intensity of the l.a.m. line at high draw ratio. This can be attributed to an increase in the average crystal thickness, consistent with X-ray diffraction and g.p.c. data. A remarkable finding is that the morphology of the isotropic sheet can have a considerable effect on the Raman spectrum of the drawn material. For the same drawing conditions significant differences were observed between drawn samples prepared from slow-cooled and quenched sheets.