Understanding the dependence of the transitional properties of liquid crystal dimers on their molecular geometry
- 1 September 1996
- journal article
- research article
- Published by Informa UK Limited in Liquid Crystals
- Vol. 21 (3), 373-382
- https://doi.org/10.1080/02678299608032846
Abstract
The characteristic feature of liquid crystal dimers, in which two mesogenic groups are linked by a flexible spacer, is often thought to be the strong odd-even effect exhibited by their transitional properties. That is, the nematic-isotropic transition temperature and the entropy of transition are large for dimers with an even number of groups in the spacer in comparison with those for neighbouring dimers with an odd number of groups. However, the magnitude of the odd-even effect along a homologous series of dimers is found to depend strongly on the nature of the link between the mesogenic group and the spacer. This dependence is thought to originate in the variation of the molecular geometry with the linking group, a view which is supported by detailed molecular field theory calculations involving all of the conformational states. Here we are concerned with developing a more transparent understanding of this geometrical effect using a simple model of the dimers in which all of the conformational states are replaced by just two, a linear and a bent conformer. The model has been found to exhibit a strong odd-even effect as well as a nematic-nematic transition when the bond angle is tetrahedral. We have used this model to explore the dependence of the transitional properties of liquid crystal dimers on their geometry by varying the bond angle of the bent conformer. The behaviour predicted by the model for the nematic-isotropic transition is found to be in qualitative agreement with experiment. In addition, the nematic-nematic transition is observed to exhibit a critical behaviour as the bond angle is increased. At the other extreme, when the bond angle is reduced to its limiting value of 90° there is a very strong first order transition between a discotic and a calamitic nematic.Keywords
This publication has 14 references indexed in Scilit:
- Liquid crystal dimers and oligomers: Experiment and theoryMacromolecular Symposia, 1995
- Prediction of the transitional properties of liquid crystal dimers. A molecular field calculation based on the surface tensor parametrizationThe Journal of Chemical Physics, 1994
- Understanding the unusual transitional behaviour of liquid crystal dimersChemical Physics Letters, 1993
- An enhanced odd-even effect of liquid crystal dimers Orientational order in the α,ω-bis(4′-cyanobiphenyl-4-yl)alkanesLiquid Crystals, 1993
- A shape model for molecular ordering in nematicsMolecular Physics, 1992
- Liquid crystalline polyethers based on conformational isomerism. 20. Nematic-nematic transition in polyethers and copolyethers based on 1-(4-hydroxyphenyl)2-(2-R-4-hydroxyphenyl)ethane with R = fluoro, chloro and methyl and flexible spacers containing an odd number of methylene unitsMacromolecules, 1992
- On the relative propensities of ether and methylene linkages for liquid crystal formation in calamiticsLiquid Crystals, 1991
- A theory of orientational ordering in uniaxial liquid crystals composed of molecules with alkyl chainsProceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences, 1982
- The effect of internal motion on the orientational order parameters for liquid crystalline systemsMolecular Physics, 1980
- Chain ordering in liquid crystals. I. Even-odd effectThe Journal of Chemical Physics, 1974