Scattering and order in thermotropic liquid crystals

Hamley, Ian William (1990) Scattering and order in thermotropic liquid crystals. University of Southampton, Doctoral Thesis.

Abstract

This thesis is largely concerned with the information on ordering in nematic and smectic liquid crystals provided by neutron and X-ray diffraction. Both orientational and positional molecular ordering is considered, the latter to a lesser extent. Chapter 1 introduces theoretical treatments used to quantify molecular orientational order for rigid nematogens and describes molecular field theories for nematics. The theory for neutron, X-ray or electron scattering from fluid phases is outlined. In particular, intramolecular neutron diffraction from liquid crystals is considered as it provides information on single molecule ordering. Mesogenic molecules are invariably of low symmetry and non-rigid. Chapter 2 includes calculations of the intramolecular neutron diffraction from biaxial molecules and it is shown that the deviations from cylindrical symmetry have a minor effect on this scattering. An analogy with NMR is developed where order parameters for internuclear vectors are specified. Chapter 3 shows that molecular flexibility can influence the anisotropic intramolecular neutron scattering components extracted at low Q from scattering by mixtures of protonated and deuterated nematogens containing a single terminal alkyl chain. The magnitudes of these anisotropic terms depend on the ordering and weightings of individual conformers. The sensitivity of X-ray or neutron diffraction experiments to molecular conformation may be enhanced by bromination of molecules. This is illustrated by calculations of electron diffraction curves for n-butane and 1,4-dibromobutane. An analysis of experimental intramolecular neutron diffraction data extracted at low Q to determine orientational order parameters is presented in chapter 4. The use of the technique, which can in principle give more complete information on molecular orientational ordering than NMR, is critically evaluated. Singlet distribution functions calculated from the order parameters via the maximum entropy method often show maxima away from the direction of preferred orientation. The splitting of the wide angle peak observed in the neutron diffraction pattern from 4-methoxy-4'-cyanobiphenyl may be due to local molecular biaxial packing. Molecular mechanics calculations are used to model possible dimers. Results of a computer simulation are used in chapter 5 in calculations of the diffraction pattern from ensembles of linear molecules in isotropic, nematic, smectic A and smectic B phases. The intra- and inter- molecular scattering is readily separated so that the separate influence of molecular orientational order and positional organisation on the diffraction pattern may be studied. Thus, information on molecular packing is readily obtained which may not be obvious from the simulation. The features of the calculated diffraction patterns resemble those for real mesogens.

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