Bridged lamellae: Crystal structure(s) of low molecular weight linear polyethylene

The crystal structure of a low molecular weight polyethylene (M-w = 595) with low polydispersity (1.11) was determined from electron diffraction intensity data collected from samples epitaxially oriented on benzoic acid and subsequently annealed. A distribution of local microcrystalline structures was observed, somewhat resembling orthorhombic n-parafiins: n-C37H76 to n-C44H90. However, a quantitative structure determination based on the 0kl single-crystal intensities reveals that the lamellae are not totally separated in the microcrystalline array, apparently due to residual bridging molecules that span adjacent chain layers. An atomic model for a frequently observed structure, comparable to n-C41H84, packs in space group A2(1)am with a = 7.42, b = 4.96, and c = 109.80 Angstrom and corresponds well to the observed diffraction amplitudes (R = 0.21).