| Research > Novel smectic liquid crystal phases and Materials | ||
| Liquid crystal phases are ordered fluid phases which occur between the solid and liquid phases of matter. They are formed from anisotropic molecules (i.e. not globular or spherical) which pack together to form an anisotropic phase. The phases formed are typically birefringent (i.e. they have two different refractive indices) and are now used everywhere in the form of LCDs (although they also have many other applications) | ||
A molecule which forms liquid crystal phases in bulk by packing in an ordered fashion may change it's packing arrangement as a function of concentration in a solvent (lyotropic liquid crystals) or as a function of temperature (Thermotropic liquid crystals). Our group works on both forms of liquid crystal material. Thermotropics will be discussed on this page, see the section on lipids and membranes for an example of lyotropic liquid crystals. Thermotropic Phases Rod-like molecules will form either a nematic phase (molecules orient themselves in a preferred direction) or a smectic-type phase (molecules have a preferred direction and also arrange themselves into fluid-like layers). Some material have a rich phase diagram, exhibiting several different smectic phases at different temperatures. T If the molecules are chiral
they may exhibit ferroelectric switching. In this case a bulk molecular motion is induced by an applied electric field across the material. By moving the molecules in bulk the transmission of polarised light through the materials can be moduated - resulting in an optical switch. |
A good introduction to the physics of liquid crystals and their synthesis can be found in the book “An introduction to Liquid Crystals” by Peter Collins and Mike Hird. |
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Bent-Core Liquid Crystalline Phases (or Banana phases) We are currently investigating a relatively new class of materials "Bent-Core" liquid crystals, in which the central rigid core of the molecule has a significant bend angle. This bend can be symmetric or asymmetric and molecular structure plays an important role in bulk phase morphology, as with all liquid crystal materials. There are 8 known B phases to date, although on-going research in this area is sure to reveal more. We currently focus on the B1 and B2 phases which have demonstrated electrical switching properties and using electro-optical measurements and x-ray diffraction we are looking at structural changes in the B phases as a function of applied electric fields.This work may eventually lead to new, low energy materials for display devices or other optical modulators. Recent projects:
An electric field induced phase transition from the columnar B1 phase to a new switching phase. |
Smectic layer spacing as a function of temperature (deg C from the SmA transition) for a ferroelectric material. measured using x-ray diffraction. |
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Chiral Smectic Phases We are interested in a class of phases called the chiral tilted smectic phases (SmC*). In these phases, molecules are tilted with respect to the layer normal. There are several variants to the tilted smectic phases, characterized by the progression of molecular orientation from layer to layer. For example the antiferroelectric phase exhibits a two layer orientational super-lattice. Other interesting and more complicated phases include the the ferrielectric (intermediate) phases (SmC*FI1 and SmC*FI2) and the alpha phase (SmC*a). The detailed structures of the ferrielectric and alpha phases were only recently described and we are interested in understanding their stability. These phases may also have important device applicatons as many different stable orientational states can be accessed by varying an applied electric field. One of the key methods used to elucidate the molecular arrangement in a phase is resonant x-ray scattering. This is a synchrotron technique which used labeled molecules to determine teh superlattice structure of a phase. For more details on this take a look at our recent review article "Resonant x-ray Scattering: a tool for structure elucidation in liquid crystals"
A free-standing antiferroelectric liquid crystal
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Combining smectic layer spacing measurements with synchrotron studies of molecular orientation we hope to learn more about the stability of these interesting layered phases and their electrical properties. The layer orientation in liquid crystal devices (i.e. between glass plates) can also be investigated via conventional small angle x-ray scattering. This measurement provides important information on the behavior of materials in commercial geometries (e.g. in LCDs). Recent projects:
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