Beam-Induced Contrast in the Electron-Microscope Images of Polymers as Observed in Ultrathin Sections

Abstract

Sections of spherulitic polyethylene used as control specimens in the development of cryo-ultramicrotomy were found to display a concentric ring structure under the electron microscope corresponding to the familiar extinction bands revealed by polarizing microscopy. This observation is unexpected as the source of contrast must be quite different in the two cases. The present note deals with the nature and possible origin of this contrast. It was observed that the bands develop during beam irradiation after all Bragg diffraction ceased and that the surface of the section becomes correspondingly undulating at the same time. Transmission and stereoscan micrographs are included which demonstrate these effects. The contrast of beam-induced band structure depends on the relation of the cut to the spherulite center. High dose rates induce additional larger scale movements of the material in the section. Selective etching and material rearrangement are thought to be responsible for the electron-beam-induced changes in the polymeric specimens. The significance of these findings for the understanding of the spherulitic texture and for the interpretation of transmission and scanning micrographs of polymers in general is pointed out.