Cholesteric textures observed by transmission electron microscopy in diffraction contrast

Abstract

We have studied the structure of a side chain cholesteric macromolecule by transmission electron microscopy. Up to now, the structure of cholesteric 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 of the texture without radiation damage.

In this paper we compare the morphologies obtained in specimens prepared from solution and in ultramicrotomed specimens. The small thickness of the sample prepared from solution and the influence of the carbon film oblige the cholesteric axis to lie in the substrate plane. The dark and bright lines therefore form various patterns, such as fingerprints and spirals, with edge dislocations and different types of π disclinations.

The existence of a contrast of dark and bright lines related to the cholesteric periodicity in sections obtained by ultramicrotomy is not well understood. In this paper, we also investigate the contribution of three types of contrast (orientation of a single molecule, thickness modification under radiation and diffraction contrast). From the evidence of electron diffraction patterns, dark field and bright field images in transmission electron microscopy, we demonstrate that diffraction contrast is a contributing mechanism in undamaged samples prepared from solution or by ultramicrotomy. In ultramicrotomed samples, this contrast quickly fades away during the observation, while another contrast due to the electron radiation appears. The orientation of the molecule is not relevant.

We also demonstrate that the diffraction contrast of the dark lines in bright field is due to an intermolecular distance of 4–7 Å oriented along the cholesteric axis. This is in agreement with the traditional image of a cholesteric, in which the mesogenic molecules lie perpendicular to the cholesteric axis.