The Role of Molecular Flexibility in Stabilizing Layered Liquid Crystal Phases


Principal Investigator

Peter Harrowell

School of Chemistry University of Sydney


Liquid crystals present us with a rich range of phases with order and properties which span the space between rigid crystals and liquids and, as a result, open doors to a range of important applications (displays, sensors, etc.) and a number of fundamental problems. One central problem is the relationship between the macroscopic anisotropy of the liquid crystal phase and the microscopic anisotropy of the molecules responsible for it. We are interested in the microscopic origins of the stability of smectic liquid crystals characterised by regular layering along one direction only. It is well known experimentally that flexible chains at either end of a rigid rodlike core will stabilize the smectic phase. Earlier work had focused on the role of the long range van der Waals interactions in understanding this correlation. We have been interested in establishing whether the short range steric interactions alone are able to stabilize the layered phase.




Alison Casey

School of Chemistry University of Sydney



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What are the results to date and the future of the work?

We have carried out Monte Carlo simulations of a liquid of molecules consisting of a rigid rodlike core with flexible chains attached to either end. Working only with aligned cores so far, we have established that this 'aligned' liquid undergoes transitions with increasing density into one of two smectic liquid crystals. One, the smectic A, has the core axes normal to the layers and is found over a range of densities and chain lengths in reasonable qualitative agreement with experiment. Imposing right angle bends in the chain, we find that a second liquid crystal, the smectic C, is stabilized. In this phase the core axes lies at 45 degrees to the layer normal. The richness of the resulting phase diagram has demonstrated the importance of the purely entropic effects.

We have also carried out preliminary Monte Carlo calculations on the size dependence of nematic and smectic transitions of hard spherocylinders in small clusters. We have shown that the phases appear with increasing cluster size in the same sequence as they appear in the bulk with increasing density, i.e a nematic appears in clusters of 55 particles while smectic order did not appear until the cluser consisted of 200 particles. The long terms goals of this work are to use cluster size to explore the role of many particle effects like entropy as compared to additive two particle interactions in stabilizing order and to examine the effect of the interface.




What computational techniques are used?

The simulation used a modified Monte Carlo algorithm in which, at each attempted move of a molecular core, an exact count was made of the change in the number of chain configurations which would result. This latter step would be prohibitive on a scalar machine. The chain count algorithm was 100% vectorized. The chain count algorithm neglected chain-chain interactions but could easily deal with restrictions on chain bond angles, a range of end terminations and free polymer chains.

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