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Simulations of side-chain liquid crystal polymers and dendrimers

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.