CHOLESTERIC TEXTURES OBSERVED BY TRANSMISSION ELECTRON-MICROSCOPY IN DIFFRACTION CONTRAST

We have studied the structure of a side chain cholesteric macromolecul e by transmission electron microscopy. Up to now, the structure of cho lesteric polymers has usually been studied by optical microscopy, and sometimes by scanning electron microscopy, but rarely by transmission electron microscopy, because of the difficulty of obtaining an image o f the texture without radiation damage. In this paper we compare the m orphologies obtained in specimens prepared from solution and in ultram icrotomed specimens. The small thickness of the sample prepared from s olution and the influence of the carbon film oblige the cholesteric ax is to lie in the substrate plane. The dark and bright lines therefore form various patterns, such as fingerprints and spirals, with edge dis locations and different types of pi disclinations. The existence of a contrast of dark and bright lines related to the cholesteric periodici ty in sections obtained by ultramicrotomy is not well understood. In t his paper, we also investigate the contribution of three types of cont rast (orientation of a single molecule, thickness modification under r adiation and diffraction contrast). From the evidence of electron diff raction patterns, dark field and bright field images in transmission e lectron microscopy, we demonstrate that diffraction contrast is a cont ributing mechanism in undamaged samples prepared from solution or by u ltramicrotomy. In ultramicrotomed samples, this contrast quickly fades away during the observation, while another contrast due to the electr on radiation appears. The orientation of the molecule is not relevant. We also demonstrate that the diffraction contrast of the dark lines i n bright field is due to an intermolecular distance of 4 . 7 Angstrom oriented along the cholesteric axis. This is in agreement with the tra ditional image of a cholesteric, in which the mesogenic molecules lie perpendicular to the cholesteric axis.