ScienceWeek

3. LIQUID CRYSTALS

ON APPLICATIONS OF LIQUID CRYSTALS

The following points are made by C. Denniston and J.M. Yeomans (Phys. Rev. Lett. 2001 87:275505):

1) In most practical applications of liquid crystals, such as traditional display devices, defect structures in liquid crystals destroy the optical properties and are undesirable. However, novel display designs, such as bistable displays or multidomain nematics, exploit defect properties. Most attempts to control the defect motion have made use of bulk electric fields.

2) Liquid crystals are typically comprised of highly anisotropic rod-shaped molecules. The "nematic" phase occurs when the molecules align parallel, giving rise to long-range orientational order with the direction of alignment indicated by the so-called "director field". In a typical display device, the liquid crystal is confined between two plates a few microns apart, and the director configuration on the plates is fixed. When an electric field is switched on, the molecules align in the direction preferred by the field. After switching off the field, long-range elastic interactions ensure that the molecules reorient themselves in the direction preferred by the surfaces. These devices can be used as displays because different liquid-crystal orientations have different optical properties. The switching of such traditional liquid-crystal devices is well understood.

3) However, there is now considerable interest in developing bistable devices that can retain a memory of two distinct states with different liquid-crystal orientations and thus different optical properties, even when the field is switched off. It has been demonstrated by both theory and experiment that it is possible to produce two metastable states with different orientations of the director field in the bulk by having different configurations of defects near the surface. However, the process of switching between these two states is not understood. The authors describe the physics behind the switching dynamics of a simple bistable nematic device.

Related Material:

GIANT SUPRAMOLECULAR LIQUID CRYSTAL LATTICE

The following points are made by G. Ungar et al (Science 2003 299:1208):

1) Molecular self-assembly into a variety of bulk phases with two-dimensional (2D) and 3D nanoscale periodicity, such as cubic, cylindrical, or mesh phases, has been researched intensely in lyotropic (e.g., surfactant-water) liquid crystals (LCs) (1), block copolymers (2-4), and thermotropic (solvent-free) LCs (5). Lyotropics can provide templates for porous inorganic materials with well-defined structures, and LC complexes of DNA with cationic and neutral lipids are potential carriers for gene delivery. Complex organic nanostructures may serve as scaffolds for photonic materials and other nanoarrays and for surface nanopatterning.

2) In the case of bicontinuous cubic phases (1,2,5), the same structures have been observed in lyotropics, thermotropics, and block copolymers. The recent inclusion of self-assembling dendrons into the category of nanostructured soft matter has shown their tapered shape to be responsible for creating similar bulk phases and enabling "self-processing" of electronics components. It has also become apparent that manipulating the size and distribution of "micelles" aggregated from self-assembling dendrons can be used in controlling polymerization. Regarding creation of structural diversity, dendron shape can be fine-tuned in ways unavailable in lyotropic LCs or block copolymers, and hence have the potential of forming hitherto unobserved phases.

3) In summary: Self-organized supramolecular organic nanostructures have potential applications that include molecular electronics, photonics, and precursors for nanoporous catalysts. Accordingly, understanding how self-assembly is controlled by molecular architecture will enable the design of increasingly complex structures. The authors report a liquid crystal (LC) phase with a tetragonal three-dimensional unit cell containing 30 globular supramolecular dendrimers, each of which is self-assembled from 12 dendron (tree-like) molecules. The present structure is one of the most complex LC phases yet discovered. A model explaining how spatial arrangement of self-assembled dendritic aggregates depends on molecular architecture and temperature is proposed.

References (abridged):

1. J. M. Seddon, Ber. Bunsenges. Phys. Chem. 100, 380 (1996)

2. E. L. Thomas, D. M. Anderson, C. S. Henkee, D. Hoffman, Nature 334, 598 (1988)

3. I. W. Hamley, The Physics of Block Copolymers (Oxford Univ. Press, Oxford, 1998)

4. M. A. Hillmyer, et al., Science 271, 976 (1996)

5. A. M. Levelut and M. Clerc, Liq. Cryst. 24, 105 (1998)

Related Material:

SELF-ASSEMBLY OF PHASE-SEGREGATED LIQUID CRYSTAL STRUCTURES

The following points are made by Takashi Kato (Science 2002 295:2414):

1) Molecules in the liquid crystalline (LC) state, although still mobile, have an orientational ordering that depends on external conditions, such as temperature or electric fields (1,2). The best known example would be the thermotropic LC materials, which exhibit different phases as a function of temperature, that are used in displays (3). In contrast to these high-tech materials, biological self-assembled systems consist of a variety of discrete molecules forming heterogeneous and hierarchical structures (structures within structures) on various scales of length ranging from the nanometer to the micrometer scale. For example, lipids exhibit lyotropic LC states in which phase formation depends on solution concentration (4). Biological cell membranes are formed through self-assembly of lipids with other biomolecules such as proteins and steroids.

2) Although the intrinsic ordering within a LC material can be controlled by external parameters, the ordering can also be controlled internally through weak interactions to create more complex structures that add functionality to the material. One approach is to obtain self-assembled structures from a variety of different molecules (2,5). Another is to form phase-segregated structures of "block" molecules of longer polymers and smaller molecules on the nanometer and micrometer scale (2). The control of intermolecular interactions (noncovalent interactions) within these structures is critical because several competing effects are at work.

3) In the formation of the self-assembled LC structures, specific intermolecular interactions such as hydrogen bonding and ionic interactions play key roles. A notable example is the formation of supramolecular liquid crystals (5). In the case of these materials, well-defined structures such as rods and disks are made through self-assembly of two or more complementary molecular components by hydrogen bonding. For example, benzoic acids and pyridines form supramolecular rod-like complexes exhibiting thermally stable nematic and smectic (layered) LC phases (5). Moreover, identical molecules self-assemble into unconventional LC complexes. Studies of these materials have initiated and stimulated the development of supramolecular polymers.

4) In summary: Additional functionality can be incorporated into liquid crystalline materials by using phase segregation and self-assembly. Intermolecular interactions such as hydrogen bonding and ionic interactions play key roles in the formation of these complex structures. One-, two-, and three-dimensional phase-segregated structures on various scales of length are formed by self-assembly of a variety of partially incompatible molecules. Such structures can enhance anisotropic properties such as ionic conductivity.

References (abridged):

1. D. Demus, J. W. Goodby, G. W. Gray, H.-W. Spiess, V. Vill, Eds., Handbook of Liquid Crystals (Wiley-VCH, Weinheim, Germany, 1998)

2. J. W. Goodby, Curr. Opin. Solid State Mater. Sci. 4, 361 (1999)

3. J. H. Haaren and D. Broer, Chem. Ind. 1998, 1017 (1998)

4. C. E. Fairhurst, S. Fuller, J. Gray, M. C. Holmes, G. J. T. Tiddy, in (1), vol. 3, chap. 7.

5. T. Kato, Struct. Bonding (Berlin) 96, 95 (2000)

Related Material:

OBSERVATION OF SURFACE AND BULK PHASE TRANSITIONS IN NEMATIC LIQUID CRYSTALS

The following points are made by M.I. Boamfa et al (Nature 2003 421:129):

1) The behavior of liquid crystal (LC) molecules near a surface is of both fundamental and technological interest: it gives rise to various surface phase-transition and wetting phenomena(1-5), and surface-induced ordering of the LC molecules is integral to the operation of LC displays.

2) In a confined geometry, interactions between LC molecules and the surfaces of the confining walls give rise to different LC behavior at the surface and in the bulk. Upon undergoing the IN transition the delicate balance of intermolecular interactions tends to favor the LC–surface interaction in the nematic phase, whereas the LC–LC interaction tends to dominate in the isotropic phase. The new nematic phase therefore occurs first at the surface, a phenomenon known as "wetting"(1,2). The IN transition is relatively weak thermodynamically, so large pre-transitional effects take place before the transition. The interaction of these pre-ordered LC molecules with the surface is known as "pre-wetting"(2). In a magnetic field the diamagnetic LC molecules undergo a torque that forces them to orient relative to the field direction. Although at the level of individual molecules the magnetic field effect is negligibly small in the isotropic phase, the collective behavior of the mesophase causes the molecular contribution to add up constructively, resulting in a macroscopic orientation effect.

3) The authors report the observation of a pure isotropic–nematic (IN) surface phase transition -- clearly separated from the bulk IN transition -- in a nematic LC on a substrate. Differences in phase behaviour between surface and bulk are expected(1-4), but have hitherto proved difficult to distinguish, owing in part to the close proximity of their transition temperatures. The authors have overcome these difficulties by using a mixture of nematic LCs: small, surface-induced composition variations lead to complete separation of the surface and bulk transitions, which the authors then study independently as a function of substrate and applied magnetic field.

4) The authors find the surface IN transition to be of first order on surfaces with a weak anchoring energy and continuous on surfaces with a strong anchoring. The authors demonstrate that the presence of high magnetic fields does not change the surface IN transition temperature, whereas the bulk IN transition temperature increases with field. The authors attribute this to the interaction energy between the surface and bulk phases, which is tuned by magnetic-field-induced order in the surface-wetting layer.

References (abridged):

1. Cahn, W. J. Critical point wetting. J. Chem. Phys. 66, 3667-3670 (1977)

2. Nakanishi, H. & Fisher, M. E. Multicriticality of wetting, prewetting, and surface transitions. Phys. Rev. Lett. 49, 1565-1568 (1982)

3. Sheng, P. Phase transition in surface-aligned nematic films. Phys. Rev. Lett. 37, 1059-1062 (1976)

4. Sluckin, T. J. & Poniewierski, A. Novel surface phase transition in nematic liquid crystals. Phys. Rev. Lett. 55, 2907-2910 (1985)

5. Yokohama, H. et al. Boundary dependence of the formation of new phase at the isotropic-nematic transition. Mol. Cryst. Liq. Cryst. 99, 39-52 (1983)

ScienceWeek http://www.scienceweek.com