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New Supramolecular Liquid Crystal Structures in Wedge-Shaped Molecules

Chaiprapa, Jitrin (2012) New Supramolecular Liquid Crystal Structures in Wedge-Shaped Molecules. PhD thesis, University of Sheffield.

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Abstract

The main subject of this thesis is structure determination of self-assembled wedge-shaped liquid crystal-forming molecules. The main part of the work involves 3,4,5-trialkoxygallate alkali metal salts. These salts, with two short and one long alkyl chains, are studied in search for close packed micellar phases, which have never been observed before in thermotropic liquid crystals. The molecular structures of these compounds were designed on the basis of the theoretical prediction of volume filling of Voronoi polyhedra of close-packed structures. The idea is that the longer alkyl chain in the molecule would help fill the octahedral interstices, furthest points from the centers of surrounding micelles, in a structure of close packed spheres. To fill the octahedral interstices even better, various proportions of n-paraffin CnH2n+2, where n= 15, 17, 19, are also added to these dendrons. For comparison, 3,4,5-tridodecyloxygallates are also studied with varying proportions of added C19H40. Small-angle X-ray diffraction (SAXD) experiments have been carried out to identify the phase structures at different temperatures. It is found that neither of the pure compounds yields a close-packed structure, and it is only in the mixtures of Na or Rb salts with added C19H40 which the hexagonal close-packed (HCP) phase is obtained. The first order phase transition from the hexagonal columnar phase to another hexagonal columnar phase is also observed in pure compounds of Na salts and their mixtures with added C19H40 whereas the hexagonal columnar superlattice is also obtained from the mixtures of Li salt and n-alkane CnH2n+2 (n=15, 17). The phase diagrams of the binary systems are constructed, to understand the principles of close-packed structure formation in self-assembled tapered mesogens and dendrimers. The second subject of this thesis is structure determination of wedge-shaped liquid crystal molecules of hybrid dendrons of methyl 3,4- -alkoxy biphenylmethyl-4-yloxy) benzoate, with different lengths of alkyl chains. The molecular structure of these dendrons is a combination of molecular structures of previously studied benzyl ether based dendrons and biphenyl methyl ether based dendrons. Using SAXD, it is found that these dendrons yield wiggly lamellar phases, as well as hexagonal columnar phases consisting of core-shell supramolecular columns. The core-shell structure is confirmed by the good results of the intensity fitting between experimental intensities and intensities calculated from geometric models, as well as by reconstructing electron density maps and performing molecular dynamics simulations.

Item Type: Thesis (PhD)
Keywords: Liquid crystals, supramolecular dendrimers, dendrons
Academic Units: The University of Sheffield > Faculty of Engineering (Sheffield) > Materials Science and Engineering (Sheffield)
Depositing User: Mr Jitrin Chaiprapa
Date Deposited: 19 Mar 2012 14:30
Last Modified: 08 Aug 2013 08:48
URI: http://etheses.whiterose.ac.uk/id/eprint/2076

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