Nematocrystalline''chain packing: Three-dimensional structure of low molecular weight linear polyethylene

The three-dimensional orthorhombic crystal structure of low molecular weight linear polyethylene PE600 (polydispersity: 1.11) has been determined in space group Pnam from experimental electron diffraction intensities. Although a bridged lamellar structure was proposed in a preliminary investigation based on projected diffraction data from annealed samples, it is now clear that a true lamellar separation, with attendant lateral shifts of sequential chain layers, can never occur. In the most ordered form of this polymer, a superlattice exists for the extended chain packing with periodic clustering of nonunitary methylene group occupancies to account for the observed layer repeat. The apparent longitudinal spacing of chain layers is 57.77 +/- 0.12 Angstrom, comparable to the true lamellar repeat of a C40-chain paraffin. Accordingly, because of the small amount of high molecular weight component, there is only a nematocrystalline, ordering of the polymethylene chains.