Structure of polyethylene from X-ray powder diffraction: Influence of the amorphous fraction on data analysis

A sample of commercial semicrystalline polyethylene (PE), characterized by a M-w of 300,000 and an estimated crystallinity of 73%, was structurally characterized through constant wavelength (CW) X-ray powder diffraction and the Rietveld method. The space group is Pnam; the cell parameters are a = 7.4241(7) Angstrom; b = 4.9491(5) Angstrom; c = 2.5534(1) Angstrom. The structure of crystalline PE was refined to a respectable level for X-ray powder diffraction experiments, including isotropic displacement parameters and hydrogen atom coordinates. The refinement indicates a C-C bond distance (ca. 1.53 Angstrom) and a C-C-C (ca. 113 degrees) intrachain bond angle, comparable to those reported for other polymers and PE. The inclusion of the amorphous fraction, through a Debye-type function, and some 1% by weight of monoclinic PE allows the proper fitting of the broad band in the 10 degrees-30 degrees 2 theta. The derived correlation distances r of the amorphous PE are in substantial agreement with those reported in reference data from especially suited experiments. The correlation limit has been estimated to be of the order of 23 Angstrom.