CHOLESTERIC LIQUID-CRYSTALS - IMAGE-CONTRAST IN THE TEM

Bright-field image contrast of thermotropic cholesteric liquid crystal line materials in the transmission electron microscope (TEM) is invest igated. Possible sources of contrast for these systems are discussed i n terms of their molecular anisotropy. A cholesteric side-chain liquid crystalline compound was investigated with TEM, low voltage high reso lution scanning electron microscopy (LVHRSEM), and atomic force micros copy (AFM) to determine the origin of the strong contrast observed in these systems using bright-field transmission electron microscopy. Ini tial contrast of thin microtomed sections, as viewed with TEM low dose techniques and an image intensifier, was much weaker than observed un der normal viewing conditions. The periodic steady state contrast typi cally observed for these materials is the result of beam damage. Furth ermore, the surface of microtomed samples (parallel to the cholesteric helical axis) is corrugated with a periodicity of 1/2 the pitch due t o a preferred fracture path in the glassy cholesteric state. AFM profi le analysis shows an average peak to valley height of approximately 20 -25 nm. AFM of free surfaces from aligned films also indicates a corru gation with a periodicity equal to 1/2 the pitch with substantially sm aller average corrugation depths. TEM indicates a series of + 1/2 and - 1/2 disclination lines at the surface due to a rotation of the prefe rred helix direction parallel to the surface, consistent with previous ly reported models.