Nematic materials possess long range orientational order but little if any positional order. This results in their properties being in between those of liquids and crystalline solids: For example they can have liquid-like flow properties and solid-like tensile strengths. Of particular practical significance are polymeric liquid crystals or liquid-crystalline polymers (LCP's). These materials offer the advantages of being stable, inexpensive, and (relatively) easy to fabricate in thin films.

Polymeric liquid crystals are currently used primarily as high strength fibers. An example is Kevlar (developed by duPont), which is a polyamide with very high tensile strength and is found in (among other things) tires, bullet proof vests, cables, and sports equipment. Liquid crystalline polymers have biological importance which arises from the fact that they can form rigid structures that contain some fluid properties; phospholipid bilayers, which are responsible for most of the compartmentalization within cells, are liquid crystalline in nature. Liquid crystalline behaviour may be of medical importance in blood clotting, atherosclerosis, and sickle cell anemia.

The phase behaviour of LCP's is not well understood from a theoretical perspective. The purpose of our research is to obtain an understanding of LCP's, at the molecular level, using the methods of statistical mechanics and computer simulation. Our long term goal is to relate the chemical structure of liquid crystalline polymers to the phase behaviour and mechanical properties, thus aiding in the design of new materials. In the process, we hope to gain considerable insight into the behaviour of polymers and liquid crystals, and obtain a more thorough understanding of the phase behaviour of macromolecules in general.

We have been investigating the phase behaviour of simple hard chain polymer models as a function of the stiffness of the chain backbone. If the chains are very flexible, then the only stable phase is isotropic. A snapshot from a simulation of freely-jointed chains is shown below:

In this case the fluid is isotropic. If the chains are sufficiently stiff and the density is sufficiently high, then a nematic phase is possible. A snapshot from a simulation of rod-like molecules is shown below:

Notice how the rods are aligned preferentially in one direction. Sometimes we encounter unexpected (in this case non-equilbrium) ``phases" that are stable over the length of our simulation but are not the thermodynamically stable state of the system. An example is:

If you would like to know more about liquid crystals, here are some excellent books:

``Liquid Crystals", by S. Chandrasekhar
``The Physics of Liquid Crystals", by P. -G. deGennes
``Liquid Crystals: Nature's Delicate Phase of Matter", by Peter J. Collings