A note on sector boundaries in polymer crystals

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Abstract
Monolayer and bilayer polymer crystals have been examined in the electron microscope using dark-field illumination. The change in spacing and direction of moiré fringes on crossing sector boundaries in polyethylene crystals has been interpreted in terms of a shear of the sub-cell in the (001)s plane. A difference of spacing between (110)s and (1 0)s of ∼ 0·007 Å is indicated. In both polyethylene and polyoxymethylene monolayers sector boundaries are revealed as light or dark lines alternately. The contrast is a consequence of the curvature of the Ewald sphere and involves rotations of fold stems in the boundaries through approximately the Bragg angle. The pattern of contrast is explained on the basis of packing asymmetric chain-folds. It is concluded that chain-folds have a well-defined shape and in a crystal sector have a common orientation at each fold surface, with the orientations at the two surfaces being the inverse of each other. It is suggested further that the asymmetry normal to a folded ribbon is that which affects fold packing the more. Chair configurations are absent in the polyethylene crystals examined.