Transmission Electron Diffraction Intensities from Real Organic Crystals: Thin Plate Microcrystals of Paraffinic Compounds

Abstract

Solvent-grown microcrystals of long polymethylene compounds packing in the O⊥ methylene subcell are found to have some of the defect structures seen for crystalline polyethylene. Crystals composed either of rectangular layers or of oblique layers both exhibit the growth around a screw dislocation characterized earlier by other workers. In addition, rectangular layer crystals of orthorhombic n-hexatriacontane give evidence of intralamellar edge dislocations. When rectangular layer crystals of cetyl palmitate are grown from hot ethanol, the same surface corrugations are found as seen earlier for polyethylene and orthorhombic paraffins. Oblique layer crystals have not demonstrated these latter two defect structures. All crystals show evidence of appreciable bending. As was found to be true for rectangular layers of n-paraffins, bending has some influence on the electron diffraction intensities from an oblique layer. Another important factor is a perturbation due to defects along long chain axes which disrupts the ordered packing of polar regions, and causes incoherent scatter from these regions. In addition, observed screw dislocations give a nonuniform end plane, causing isolation of layers from one another. Diffraction models accounting for bending or isolation of chain layers in an oblique layer both explain the observed near invariance of certain intensities which are due to the coherent scattering from the line grating formed by rows of carbon atoms in the tilted chain aggregate. Forbidden reflections predicted by a bending model are less important than those anticipated by the multiple scattering from a laminated layer model. As these forbidden reflections are mostly identified for multilayer crystals and since crystal structure analysis indicates most of the long chain in a oblique monolayer to be diffracting coherently, the concentration of Reneker-type defects in the centers of chains is probably not great enough to isolate small chain length laminae within a monolayer.