Ingo Dierking

Liquid crystal-carbon nanotube dispersions

Parallel alignment of nanotubes can be obtained by dispersion in a self-organizing anisotropic fluid such as a nematic liquid crystal. Exploiting the cooperative reorientation of liquid crystals, the overall direction of the nanotube alignment can be controlled both statically and dynamically by the application of external fields. These can be electric, magnetic, mechanic, or even optic in nature. Employing multiwall as well as single-wall carbon nanotubes, we show their parallel alignment along a uniform liquid crystal director field and electrically verify their reorientation behavior for two complementary geometries. These demonstrate electrically controlled carbon nanotube OFF–ON and ON–OFF switches. Further applicational potential will be outlined.

Magnetically steered liquid crystal-nanotube switch

Liquid crystals LCsRef. 1 are spontaneously selforganized anisotropic fluids. They combine fluidity with the anisotropy of physical parameters and the elasticity observed for solids. The simplest of the LC phases is the nematic, which exhibits solely orientational order of the long molecular axes along a preferred direction called the director. LC displays LCDs based on nematics are by now a mature technology utilized in a wide range of applications from simple black and white pocket calculators to high-end full color television screens. All of these applications are based on the electric Freedericksz transition, a collective reorientation of the LC director along the direction of an applied electric field for positive dielectric anisotropy. Carbon nanotubes 2 are also highly anisotropic materials. They exhibit a large elastic modulus and electric conductivity along the tube axis, while being flexible and nonconductive in the perpendicular direction. Proposed applications range from nanoelectronics 3 to biochemical sensors. 4 It lies at hand to combine these two modern materials to design novel nondisplay devices based on LC-nanotube dispersions. It has recently been shown that the LC order through

INVESTIGATIONS OF THE STRUCTURE OF A CHOLESTERIC PHASE WITH A TEMPERATURE INDUCED HELIX INVERSION AND OF THE SUCCEEDING SC-ASTERISK PHASE IN THIN LIQUID-CRYSTAL CELLS

Abstract Investigations of 4-[(S, S)-2, 3 epoxyhexyloxy]-phenyl-4-(decyloxy)-benzoate by polarizing microscopy, the Cano-Grandjean method, optical rotation dispersion and UV-VIS spectroscopy reveal a cholesteric phase with temperature induced reversal of the helical twist. Switching time experiments in the Sc* phase show that the intrinsic helix can be unwound reversibly and irreveribly by application of electric fields of different strengths.

Chiral Liquid Crystals: Structures, Phases, Effects

The introduction of chirality, i.e., the lack of mirror symmetry, has a profound effect on liquid crystals, not only on the molecular scale but also on the supermolecular scale and phase. I review these effects, which are related to the formation of supermolecular helicity, the occurrence of novel thermodynamic phases, as well as electro-optic effects which can only be observed in chiral liquid crystalline materials. In particular, I will discuss the formation of helical superstructures in cholesteric, Twist Grain Boundary and ferroelectric phases. As examples for the occurrence of novel phases the Blue Phases and Twist Grain Boundary phases are introduced. Chirality related effects are demonstrated through the occurrence of ferroelectricity in both thermotropic as well as lyotropic liquid crystals. Lack of mirror symmetry is also discussed briefly for some biopolymers such as cellulose and DNA, together with its influence on liquid crystalline behavior.

Recent developments in polymer stabilised liquid crystals

Polymer stabilised liquid crystals describe systems in which a polymer network is formed within an anisotropic liquid crystal matrix. During the polymerisation process a bi-continuous system is formed, where a continuous polymer network permeates a continuous liquid crystal phase. The order and structure of the liquid crystal phase are transferred onto the polymer network, which thus mechanically stabilises the phase it was formed in. Such systems have attracted increasing interest over the last 15 years, as they are serious candidates in the development of electronic paper, but also for the mechanical stabilisation of smectic ferroelectric liquid crystals, which exhibit some very superior electro-optic properties over their nematic counterparts found in most common LCDs today. In the last few years some remarkable developments in the field of polymer stabilised liquid crystals took place, some of more academic interest, but others certainly also of interest for future display and optical component applications. Some examples of both will be discussed.

Reorientation Dynamics of Liquid Crystal–Nanotube Dispersions

Single wall carbon nanotubes (SWNT), dispersed in a nematic liquid crystal, exhibit orientational order due to the influence of the liquid crystal molecules. The direction of this order, and the associated material properties, can be altered by the application of an electric field, allowing for liquid crystal controlled switches. We have experimentally investigated the reorientation dynamics as a function of temperature and find two characteristic timescales. The short timescale is associated with the reorientation of the liquid crystal texture and the so induced disruption of the conducting nanotube percolation network. The longer timescale is associated with the orientational reorientation of the carbon nanotubes.

HORIZONTAL CHEVRON CONFIGURATIONS IN FERROELECTRIC LIQUID-CRYSTAL CELLS INDUCED BY HIGH ELECTRIC-FIELDS

Abstract Application of a high electric field to a S∗c ferroelectric liquid crystal cell may cause the formation of horizontal chevron configurations with the smectic layers tilted by the amount of the chevron angle (in the case of the present investigation equal to the director tilt angle) with respect to the normal to the rubbing direction of the cell substrates. The layer structure resembles that of the well-known vertical chevron configuration, but in the plane of the substrate instead of perpendicular to it, and is similar to that recently reported for the stripe-shaped texture. Between crossed polarizers, the two domain types appear to switch in opposite directions when an a.c. electric field is applied. The temperature dependence of the observation of horizontal chevron structures was investigated and an explanation is proposed analogous to that for the stripe texture model.

One- and two-dimensional fluids

To be frank from the beginning, I really like this book. It avoids the sometimes artificial boundaries between thermotropic and lyotropic phases, as well as those of different types of mesogens, may they be calamitic, bent-core, discotic or bowlic. General concepts are introduced by means of liquid crystal phases, and topics are discussed which often do not find their way into the standard texts about our subject, one example being soap films, which are often treated separately from liquid crystals.

Electro-optic properties of polymer-stabilized ferroelectric liquid crystals before, during and after photo-polymerization

The electro-optic behaviour of polymer-stabilized polar smectic liquid crystals is studied in detail before and after photo-polymerization of a bifunctional monomer in the fluid SmA* and SmC* phases. We investigate the effect of a stabilizing, phase-separated polymer network on the electro-optic performance of smectic liquid crystals in relation to the electroclinic as well as the ferroelectric electro-optic effects. For the first time we also report time-dependent data obtained during the polymerization process, which allow the study of the dynamics of photo-polymerization in a liquid crystal host and provide evidence for how this polymerization process is accomplished and why the favourable electro-optic properties of ferroelectric liquid crystals are retained in polymer-stabilized FLCs.

Horizontal chevron domain formation and smectic layer reorientation in SmC* liquid crystals stabilized by polymer networks

The influence of a polymer network, stabilizing an initial texture of horizontal chevron geometry, on the in-plane smectic C* layer reorientation process is studied for different applied electric field conditions. As expected, the reorientation of smectic layers is strongly slowed down and eventually suppressed by the network, even at rather low monomer concentrations. Polymer network formation in a uniformly reoriented smectic layer state reveals that the network acts in two ways: first it gives a biased elastic torque counteracting a field of such symmetry as to cause a change from the templated layer direction; second it introduces an increased effective viscosity counteracting the reorientation in both directions. The behaviour of samples stabilized by two different kinds of polymer networks, created in between the smectic layers (intra-layer) and across them (inter-layer), is then investigated and discussed.