EPSRC Grant GR/M21676/01 - Summary of Research

EPSRC Grant: GR/M21676/01 01/10/1999-30/09/2002
Computer simulation of liquid crystal phases using atomistic potentials.

Summary of the research

What are liquid crystals?
The work on the grant concerns computer simulation studies of liquid crystals. Liquid crystals are “in between phases” (mesophases). They occur between conventional solid and liquid phases. So when a crystal made up of elongated molecules is melted, it often goes through one (or more) liquid crystal phases before it forms a normal (isotropic) liquid.

Common liquid crystal phases are the nematic phase, where the molecules show orientational order but no long range positional order; and the smectic-A phase, where the molecules have orientational order but are also (on average) ordered in layers (positional order) . These are the two phases that this project has focussed on.

Why are liquid crystals important?
Thermotropic liquid crystals (phase behaviour depends on temperature)
These materials can be used in liquid crystal displays for phones, laptops, and TVs. The displays are low powered, portable and thin. In the next few years, large flat-panel LC displays will become preferred over CRTs. They also have many further uses, e.g. in adaptive optics for telescopes and in optical wave guides.

Lyotropic liquid crystal (phase behaviour depends on temperature and also on the concentration of a solvent)
Lyotropic phases form when a liquid crystal molecule is dissolved in a solvent. Nature makes use of lyotropic materials everywhere (e.g. in cell membranes where phospholipids form a lyotropic liquid crystalline bilayer).

What are atomistic simulations?
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What are the main results of this research?

1) Atomistic Simulations of Liquid Crystals
We have used atomistic simulation studies to look at liquid crystal molecules in the nematic liquid crystal phase and also in the pretransitional region (the liquid phase close to the phase transition to a nematic). In the liquid crystal phase we have used the simulations to make predictions for the rotational viscosity. This is one of the key material properties that determine how a nematic liquid crystal repsonds in a display. For future fast displays, low viscosity materials will be required.

We see the future of this work as helping researchers to do “molecular engineering”. Here, the aim is to help design better materials for the future by using computer simulations to make predictions about the properties of the materials in the bulk.

2) Simulations of side-chain liquid crystal polymers and dendrimers

In a side-chain liquid crystal polymer, liquid crystal groups are hung from a polymer backbone by means of connecting flexible spacer groups (side-chains).

We have studied an atomistic model of a siloxane side-chain liquid crystal polymer. The model uses united atom potentials (hydrogens combined with the atoms they are attached to) and Gay-Berne (non-spherical) potentials to describe the polymer. We find that the polymers undergo microphase separation, separating into “polymer-rich” and “mesogen-rich” regions. It is microphase separation that induces the formation of a smectic liquid crystalline phase.

Liquid crystalline dendrimers are macromolecules which radiate out from a central core, with a series of branch points. The number of branch points defines the generation of the dendrimer. Usually the molecules are functionalised with liquid crystal groups at the end of each branch.

For liquid crystalline dendrimers, a big challenge exists in understanding how chemical structure can be related to the phases that form. For example, we might expect a large dendrimer molecule to be spherical in shape, but the smectic and columnar phases that form suggest that molecules change shape to form rods or discs in bulk phases. To study this we have looked at semi-atomistic models of a generation three carbosilane liquid crystalline dendrimer.

In one model we studied a model dendrimer in a liquid crystalline solvent and studied the change in structure of the dendrimer as it was equilibrated in a nematic liquid crystal phase. When the solvent is further cooled into a smectic-A phase, the dendrimer changes structure again, and the liquid crystalline parts (mesogenic groups) become distributed over 5 separate smectic layers.

For this generation of dendrimer, a central core exists, into which solvent molecules can not easily penetrate. This means it is possible to design a more coarse-grained model for the dendrimer, in which the central core is represented by a single site. The advantage of the coarse-grained model is that the reduced number of interactions sites, means that it is suitable for simulation of bulk phases. We have done some intial simulation work for 100 dendrimer molecules using this model, and have studied the phase diagram. So far we have not see a liquid crystal phase, but this is on-going work.

3) Calculations of helical twisting power
When a nematic liquid crystal phase is doped with a small amount of a chiral molecule, the whole phase becomes chiral, and the liquid crystal director (which describes the preferred direction of alignment for the molecules) is twisted through space to form a helix. Some molecules are better than others at inducing this twist, and the effectiveness of an individual molecule is measured by its Helical Twisting Power (HTP). There is a lot of industrial interest in making new molecules with very large HTPs, to use as chiral dopants for chiral polymer films. However, it is difficult to relate chemical structure to HTP.

In this part of the project, we developed new methods for predicting HTP prior to synthesis.

a) A Monte Carlo program was developed that allowed a chiral molecule, represented by atomistic potentials, to be simulated in a Gay-Berne (GB) solvent. The GB solvent formed a twisted nematic phase, in which the director was forced to twist through 90 degrees over the simulation box through employment of twisted periodic boundary conditions. In a series of simulations the chiral dopant was gradually mutated into its enantiomer, and the free energy change for this process was measured. The measured free energy change leads directly to a quantitative measure of the helical twisting power.

b) In a second project several single molecule techniques, to see if any of these can yield accurate HTP values. This would allow very rapid screening of molecules for industrials prior to them embarking on an expensive and time-consuming synthetic programme. The most successful approach to date has been pursued in collaboration with Maureen Neal’s group in Coventry, using a scaled chirality tensor. This has predicted good HTP values for a large number of molecules with a rigid chiral framework. Further work into this and other methods is currently underway to extend these to the study of flexible chiral dopants, using the Monte Carlo program developed in the first part of this project.

Finally, our work suggests that each separate molecular conformation has a different HTP, and constant NVT Monte Carlo simulations show how the overall HTP value changes with increases in temperature (as higher energy conformations with different HTPs start to become populated). This mechanism explains the temperature-induced reversal in HTP that occurs with some materials. It also explains the fact that some molecules can have different HTPs in different solvents. The latter is caused by some conformations being preferentially selected in certain solvents.