Crystallinity effects in the electron microscopy of polyethylene

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Abstract
Single crystals of polyethylene were examined electron-microscopically with special precautions to avoid damage due to the electron beam. A variety of unexpected beam-sensitive features were observed. They can be classified as (1) moirés. (2) morphological features. In connection with (1) the general principles of moirés are discussed. It is shown that moirés can be due either to double diffraction, the conditions of the formation of which are defined, or to dislocations arising through contact deformation of superimposed crystals. Our observations on polyethylene crystals are analysed in the light of this general discussion. It is found that the moirés& in polyethylene are to a large extent due to rotational superposition of layers. Crystal growth with such a rotation is exemplified by a unique preparation. In places the moiré makes a pseudo-image of the crystal lattice itself, in others only of particular lattice planes. There is evidence for strong local lattice distortions, and also for possible local departures from orthorhombic symmetry. Various other features are also discussed and fundamental difficulties in the interpretation of image contrast are pointed out. (2) We found a surface corrugation strongly heightened by extinction effects which disappeared within a short time after the preparation of the specimens. We tentatively interpret this as resulting from a change in the molecular fold length, when the crystals are dried down. The corresponding striations did not follow low index crystallographic directions. There is evidence for the existence of distinguishable quadrants within the crystals, and also of internal buckling, both of which are consistent with other considerations. Repeated asymmetric twinning along (010) and to a lesser extent along (100) is observed which can account for a variety of observed crystal shapes.