ELECTRON-DIFFRACTION STRUCTURE-ANALYSIS OF POLYETHYLENE - A DIRECT PHASE DETERMINATION

Direct determination of crystallographic phases based on estimates of triplet and quartet structure invariants is used to solve the crystal structure of polyethylene using three-dimensional electron-diffraction intensity data from solution-crystallized and epitaxially crystallized specimens. In this analysis, the phase values of 40 out of 50 reflections are assigned ab initio. Valence parameters for the carbon zigzag chain, computed from the carbon atomic positions in the electrostatic potential map, are consistent with X-ray determination of alkane crystal structures. The chain setting angle calculated from the carbon atom position in the potential map is close to the value found for orthorhombic even-chain n-paraffins. A somewhat larger value is found when a zigzag chain model is rotated to find a minimum of the crystallographic residual, consistent with an earlier X-ray analysis.