Dr Verena Görtz
01904 324530
Email: verena.gortz@york.ac.uk
Self–organising Soft Materials
Soft materials possess unique responsiveness to various external stimuli, which is the reason behind their profound impact on modern technology. Soft materials can be polymers, colloids, or liquid crystals and the research interests in our group cover aspects of all three types. In particular, we are exploring the exciting prospects of nano- and micron-sized polymer particles with liquid-crystalline properties, ie a new kind of soft material that combines the properties of polymers, colloids, and liquid crystals.
Liquid Crystals
Liquid crystals are a class of materials that is widely known for their application in liquid crystal displays (LCDs). They uniquely combine the fluidity typical of liquids with some of the long range order and anisotropic physical properties, for example optical birefringence, typical of many crystals. As a result, liquid crystals have the ability to respond optically to small external stimuli, such as the presence of different surfaces or the application of electric fields. The unique fluid collective behaviour of LC molecules, responsible for their use in displays, is also central to many aspects of biosciences. It is the liquid-crystallinity of many biological materials that enables the amplification of changes on a molecular level, such as, for example, at cell surfaces, and therefore the sensing and transmission of information over larger length scales. Synthetic liquid crystals possess the same sensing capability, but lack the inherent confinement geometry of cells, which provides alignment over larger distances. Therefore, in applications of LCs, external alignment surfaces are required for their function.
Microscopy image of a liquid crystal texture in a droplet (crossed polars)
Polymer Particles
Polymer particles are beads of cross-linked polymer which are produced with optimised characteristics by heterogeneous polymerisation techniques. All applications require polymer particles with very specific properties such as size, bead morphology, and surface properties. The control of size, surface functionality, and the uniformity of the spheres in the production process is therefore of major interest. With typical sizes ranging from tens of nanometres to hundreds of micrometres, they find many applications which exploit the surface the particles provide and are extensively used in medical and biological applications, as polymer supports in catalysis, for chromatographic separation and solid-phase extraction (SPE), and as colloidal photonic crystals. For their function, the polymer beads are often decorated on the outside, i.e. on the outer surface, with functional chemical groups of various natures, chosen according to the desired scientific or commercial application. For other applications, polymer beads are produced with functional groups on the inside. As they are elastic and therefore able to swell in a solvent, they can present their functionality to small reagents diffusing into the bead.
Microscopy image of polystyrene particles in a liquid-crystalline host
Smart Liquid-crystalline Microparticles
Our research targets the synthetic development of nano- and micron-sized polymer particles with liquid-crystalline properties. By incorporating and confining liquid crystal order into small elastic polymer beads, particles with exceptional optical response properties are created. We investigate these properties by a range of specific material studies using techniques such as polarised light microscopy, differential scanning calorimetry, electron microscopy, electro-optic measurements, and dynamic mechanical analysis. In the design of the materials both the elasticity and surface properties of polymer beads and the optically responsive properties of liquid crystals are exploited to develop sensor materials that provide an optical read-out in response to changes in the external environment. This can be, for example, mechanical or electrical stimuli, changes in temperature, or the uptake and release of molecules.
Microscopy image of three liquid-crystalline polymer particles (crossed polars, particle diameters ~ 100 – 150 µm)
Bent-core Nematic Liquid Crystals
The order in liquid crystal phases is established by interactions that are extremely sensitive to changes in molecular shape and polarity. In recent years, liquid crystal phases formed by molecules with bent-core molecular structures have received considerable attention in the liquid crystal community. This is mainly due to the increasing recognition that the properties of liquid crystal phases formed by these compounds are very often rather different to those observed for classical rod-like or disc-like liquid-crystalline materials. Strong research efforts have recently succeeded in producing bent-core materials that exhibit nematic phases, the least ordered of all liquid crystal phases; thereby showing that it is possible to overcome the strong tendency of bent-shaped molecules to pack into higher ordered structures. The effort is primarily driven not only by fundamental scientific interest, but, due to the so-called biaxiality of some bent-core nematic phases, could lead also to an entirely new, potentially much faster, class of liquid crystal devices. The first biaxial nematic materials were only discovered a few years ago. Intriguingly, in our studies of the nematic phase of bent-core oxadiazole compounds we observed that this fluid phase can segregate spontaneously into chiral domains of opposite handedness, even though the constituent molecules are achiral, and unique filament structures can form at the phase transition. The observation of spontaneous chiral resolution in a fluid, solely orientationally organised phase is remarkable and current investigations highlight the unique character of the nematic phase in bent-core materials, which is a challenge to our understanding of nematic phase behaviour. The design of novel bent-core nematic materials and the development of concepts describing the specific supramolecular organisation in these unconventional systems is therefore an ongoing process.
Microscopy image of filament formation at the phase transition for a nematic bent-core oxadiazole material (crossed polars)
Selected Publications
- 'Liquid Crystal Elastomer Beads.'
V Görtz and E Bevis, WO2010/112831, 7 October, 2010. - Chiral Resolution in Bent-core Nematic Liquid Crystals.
V Görtz, Liquid Crystals Today, 2010, 19, 37. - Unusual Properties of a Bent-core Liquid-crystalline Fluid.
V Görtz, C Southern, N W Roberts, H F Gleeson and J W Goodby, Soft Matter, 2009, 5, 463. - Enantioselective Segregation in Achiral Nematic Liquid Crystals.
V Görtz and J W Goodby, Chem Commun, 2005, 3262. - New Photosensitive Polymers: Synthesis and Free Radical Polymerization of Oxypyridinium and Oxyisoquinolinium Functionalized Methacrylate and Styrene Derivatives.
V Görtz and H Ritter, Macromolecules, 2002, 35, 4258.
