High resolution electron microscopy of liquid crystalline polymers

Abstract

Special techniques of high resolution electron microscopy applied to liquid crystalline polymers in the smectic and discotic phases revealed characteristic distortions from the perfect lattice periodicity as well as defects. In order to verify the direct relationship between electron microscope images and structure, computer simulations were performed. Agreement was found between simulated image and structure proposed on the basis of evidence from electron diffraction imaging. A theory was developed to explain this unexpectedly good agreement. Image restoration by frequency filtering in reciprocal space was found to give good results for the discotic material

Electron crystallography and non-linear optics

“…We have shown previously (Voigt-Martin et al, 1990) that dynamical scattering already begins to affect the diffracted amplitudes of organic materials at 10 nm. CERIUS uses the well-known multi-slice methods in which the propagation and transmission functions are calculated in reciprocal space (Cowley, 1986;Saxton and Koch, 1982;Saxton et al, 1983;Self et al, 1983).…”

“…The method selected to deal with this difficulty depends on the problem that has to be solved. In the case of a regularly repeated structure, averaging methods, both frequency filtering in reciprocal space (Predere and Thomas, 1990;Voigt-Martin et al, 1990) and image averaging in real space (Henderson et al, 1990), have been successfully used for many years. Appropriate software such as MRC in Cambridge (Amos et al, 1982;Henderson and Unwin, 1975) and the EMS system in Martinsried (Hawkes, 1980) were developed.…”

“…We use SHELX93 (G. Shedrick: ''A program for crystal structure refinement,'' University of Göttingen) to calculate the structure factors, incorporating electron scattering factors instead of X-Ray scattering factors. If the organic sample is more than 10 nm thick, dynamical scattering must be taken into account (Voigt-Martin et al, 1990). The HRTEM module in CERIUS gives the Pendellösung plots for all reflections related to the model for different thicknesses and the appropriate intensities for different thicknesses can be extracted from them.…”

Structure and defects in discotic crystals and liquid crystals as revealed by electron diffraction and high-resolution electron microscopy

“…For thin organic samples, it is generally appropriate to use the weak phase approximation. 26 Subsequently the diffraction pattern of the in the literature over 20 years ago. "J8 Detailed measurements of mass and diffraction loss in organic samples by electron energy loss spectroscopy and diffraction have been published by many authors.lg For this reason, lowdose techniques and cry0 methods were developed, which involved (a) the use of a cryoholder for the sample, (b) the employment of a double condensor with a strongly defocused first condensor, and (c) focus of the objective lens in one area of the specimen and photographing an adjacent, previously unexposed area with the first electrons interacting with the sample.…”

“…26 The image is a very complex interference pattern, and it cannot be assumed a priori that it represents a projection of the potential distribution in the sample. For this reason, it is essential to perform image simulation taking account of dyrurmical scattering effects and of all instrumental parameters effecting the image.…”