A SIMPLE METHOD FOR INDEXING OF X-RAY WIDE-ANGLE REFLECTIONS OF SUBSTANCES WITH ORTHORHOMBIC ELEMENTARY CELLS . MEASUREMENTS IN POLYETHYLENE AND ETHYLENE-PROPYLENE COPOLYMERS

An explicit procedure has been developed for indexing X-ray wide angle reflexes on the basis of the theoretical derivations of P. M. de Wolff from substances exhibiting orthorhombic unit cells. The method proposed has been applied for polyethylene and an ethylene propylene copolymer. Evaluating all measurable peaks up to 2 θ=158° it has been confirmed that besides the homopolymer the copolymer too exhibits an exactly orthorhombic unit cell. Crystallizing linear polyethylene very slowly it was possible to get a diagram with the spectrometric powder technique where reflexes with (h k 3) are separated corresponding to CuK α1 and CuK α2 radiation. The specific advantage of the proposed treatment is that all measurable reflexes are used and that all possible ones may be predicted easily. Consequently the lattice parameters are determined more accurately, although the calculation of the reflexe is less time consuming than by the conventional procedure. The lattice parameters of linear polyethylene are a= 7,4184±0,001, b=4,9456±0,001, c=2,5455±0,001 resp. at 25 °C. The general treatment of P. M. de Wolff is not limited to orthorhombic lattices as applied here. The type of symmetry of the unit cell follows directly from this general consideration. © 1969 Dr. Dietrich Steinkopff Verlag.