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 ch aracterized through constant wavelength (CW) X-ray powder diffraction and t he 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 struc ture of crystalline PE was refined to a respectable level for X-ray powder diffraction experiments, including isotropic displacement parameters and hy drogen 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, compara ble to those reported for other polymers and PE. The inclusion of the amorp hous fraction, through a Debye-type function, and some 1% by weight of mono clinic 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 ar e in substantial agreement with those reported in reference data from espec ially suited experiments. The correlation limit has been estimated to be of the order of 23 Angstrom.