Study of the distribution of the molecular orientation in thick polyethylene samples by X-ray diffraction, infra-red dichroism and Raman spectroscopy

Abstract

The molecular orientation in thick polyethylene samples has been studied by wide-angle X-ray diffraction, i.r. dichroism and Raman spectroscopy. The original specimens, with dimensions of the order of a centimetre, were cut to obtain 1 mm thick platelets on which the measurements were made. The mean coefficient of the second-order Legendre polynomial, 〈 P 2〉, was calculated from X-ray diffraction and from the 1894 cm −1 i.r. band for the crystalline phase, from the 909 cm −1 i.r. band for the vinyl end groups and from the 1130 and 1060 cm −1 Raman bands for the all- trans C-C conformers. The fourth-order coefficient, 〈 P 4〉, was also determined from X-ray diffraction and Raman spectroscopy for a series of cylindrical rods of draw ratios (λ) ranging from 6 to 20. An excellent correlation is observed between the 〈 P 2〉 coefficients measured from different X-ray reflections and from the 1894 cm −1 i.r. band. The Raman spectroscopy results show that the all- trans bonds located in the amorphous phase are aligned perpendicular to the extrusion direction for the λ = 6 rod, and gradually reorient towards the fibre axis for λ values up to 20, while the 〈 P 2〉 and 〈 P 4〉 coefficients calculated for the crystalline phase remain constant at λ ≥ 12. The variation of the orientation through the thickness of the samples was investigated for the cylindrical rods and for an H-shaped moulding produced by extrusion and rolling. Minor differences in the degree of molecular orientation were detected between the centre and the surface of the rods, whereas important variations were measured for the H-shaped sample.