Liquid crystals

1.1.1 Background

The observation of an intermediate state of matter between the solid crystalline phase and the isotropic liquid phase was first recorded in the late 1800s when “double melting” behaviour was observed in cholesteryl acetate and cholesteryl benzoate.

Subsequent work revealed such phases to flow like a liquid, whilst maintaining characteristics of a solid, and the term “liquid crystal” was first used in 1889 to describe these systems. Further research revealed a wider range of compounds that exhibited this phase, which became associated primarily with rod-like molecular structures, and it was observed for the first time in the early 1900s that some compounds exhibited multiple liquid crystalline phases. Subsequently, the field of research expanded, particularly after the discovery of technological applications in the 1960s.

40 1.1.2 Phases

Liquid crystalline phases of matter lie between fully isotropic liquids that exhibit little or no short or long range order, and crystalline solids that exhibit both short range and long range positional and orientational order. Hence, it follows that the subdivision of phases within liquid crystals is primarily determined by the degree of ordering of the system. Thermotropic liquid crystals, which are defined by phases largely dictated by the temperature of the system, are discussed in this thesis. Lyotropic liquid crystals, in which the phase behaviour is largely dictated by the concentration of the species within a solvent, do not fall within the scope of this work.

The simplest of the liquid crystal phases is the nematic phase, in which the molecules exhibit a degree of short range orientational order, i.e. neighbouring molecules have a tendency to orient in the same direction, as shown in Figure 1.1. Typically, this phase is formed by calamitic (rod-like) molecules. In a nematic phase, domains of aligned molecules form, although the domains themselves may not be aligned with each other, giving the bulk material no net alignment. However, alignment of the bulk material may be induced by an external influence such as the presence of an aligned surface or the application of an electric field, and the resulting average molecular orientation is termed the director, n. The alignment in a nematic phase may generally be considered to be uniaxial, meaning that the alignment in the phase is cylindrically symmetric around the director.

Figure 1.1 Schematic diagram of a nematic liquid crystal phase and the director, n.

Smectic phases are also formed by rod-like molecules and exhibit orientational order, similar to that observed in nematic phases. In addition, they exhibit a degree of positional order in which the molecules form layers, as shown in Figure 1.2. The

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relationship between the orientational order and the positional order gives rise to several subdivisions of smectic phases, such as the smectic A phase in which the layers are perpendicular to the director, and the smectic C phase in which the layers lie at an angle other than 90˚ to the director, as also shown in Figure 1.2. Additional order such as positional ordering within the layers gives rise to further subdivisions of smectic phases.

Figure 1.2 Schematic diagram of a smectic A (left) and smectic C (right) liquid crystal phases and their director orientations.

Specific phases formed by chiral molecules are also observed, and have similarities to those formed by achiral species. The chiral nematic phase, for example, exhibits short range orientational order, but the local director orientation rotates around an axis perpendicular to this director, as shown in Figure 1.3. Chiral smectic phases are also observed in which the orientation of the tilt angle described above rotates around an axis normal to the layers.

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Figure 1.3 Schematic diagram of a chiral nematic liquid crystal phase and the local director orientations.

In addition to the phases described above, many other phases have been characterised, such as discotic phases formed by disc-like molecules, defect-driven blue phases that lie between the isotropic and nematic phases, ferroelectric phases, and phases formed by bent-core molecules. All of these phases demonstrate subtly different properties due to the variations in their degrees of ordering and symmetry, and, although they may be divided into many different categories, they share an underlying basis for the formation of the phases: molecular anisotropy.

To exhibit one or more of the liquid crystalline phases shown in Figures 1.1 - 1.3 above, a molecule must not only have a rod-like structure, but it must also be relatively rigid in order to maintain this shape. Typically, such structures comprise a core of two or more connected aromatic or cyclohexane rings. Not only does this molecular design provide the overall rod-like shape and a degree of rigidity, but the presence of extended conjugation through aromatic ring systems can also result in strong intermolecular interactions that can aid alignment. The ring systems may be bonded directly or via linking groups to alter the overall molecular shape and conjugation within the molecule.

Alkyl and alkoxy chains are commonly appended as terminal groups on the ring systems to provide the molecule with a degree of flexibility, favouring the formation of liquid crystalline phases in comparison with less flexible structures lacking such groups.

Polar terminal groups can be used in order to increase intermolecular interactions,

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favouring alignment between neighbouring molecules. Lateral substituents may also be added to the ring systems in order to tune the phase behaviour and dielectric properties.

A schematic diagram showing a generalised structure of a rod-like molecule that may exhibit liquid crystal behaviour is shown in Figure 1.4.

Figure 1.4 A general structure of a rod-like molecule including the ring groups (blue), linking group (yellow), terminal chain (orange), terminal polar group (red), and lateral substituents (green).

1.1.3 Properties

The order exhibited by liquid crystalline materials, along with the molecular anisotropy of their constituents, means that many of their bulk physical properties are also characterised by anisotropy. Properties including elasticity, magnetic susceptibility, electric permittivity, viscosity, and optical properties can vary significantly depending on the orientation in which they are measured relative to the director. It is the combination of the optical properties and the electric properties that has given rise to most of the current applications of liquid crystals.

1.1.4 Uses

Currently, the primary application for liquid crystals is in display technology. Such displays vary in their construction and mode of operation, but all rely on the optical anisotropy of aligned liquid crystal systems. Typically, a cell is constructed from two glass plates coated with a transparent electrode layer to enable an electric field to be applied across the cell. The inner surfaces of the glass plates are usually coated with an alignment layer, such as polyimide or nylon, to induce bulk alignment of the mixture, and polarisers may be present on one or both of the plates depending on the mode of operation of the device. A cell typically has a path length of the order of ca 10 μm and is filled with the liquid crystal.

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The first practical liquid crystal display (LCD) that was commercially successful was produced in 1973, and utilised an ion-doped liquid crystal mixture. In the “off” state of an ion-doped system, the liquid crystal is uniformly aligned and transparent, but on the application of an electric field, movement of the ions disrupts the liquid crystal alignment, causing light to scatter and the transparency to be lost. In spite of its initial success, this technology was limited by its power consumption and operating temperatures, and was soon superseded by a device utilising an alternative mode of operation: the twisted nematic (TN) display.

TN displays rely on the behaviour of plane polarised light passing through a liquid crystalline material that exhibits a helical twist much larger than the wavelength of the incident light. This mesophase configuration is typically achieved by sandwiching a nematic liquid crystal between two orthogonally aligned surfaces of a cell, as depicted in Figure 1.5. Due to the differences in the refractive indices of the liquid crystal measured parallel and perpendicular to the director, when the incident light is polarised with the electric component parallel to the director, the effect of the twist in the director orientation is to rotate the polarisation of the light as it passes through the cell. By placing polarisers parallel with the direction of molecular alignment on each surface of the device, a situation is achieved in which linearly polarised light may pass through the perpendicular polarisers and be transmitted through the device. On the application of an electric field between the surfaces, the dielectric anisotropy of the molecules causes them to rotate to lie perpendicular to the plane of the surfaces. In this configuration, the polarisation of the incident light is not rotated and is blocked by the second polariser, producing a dark state, as also shown in Figure 1.5.

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Figure 1.5 Schematic diagram of the bright “off” state (left) and dark “on” state (right) of a twisted nematic device.

This TN mode of operation overcame some shortcomings of the earlier scattering device, providing significantly lower power consumption and improved contrast between the off and on states. Further improvements have been made to the materials used, and TN displays comprise the majority of commercial LCDs produced.¹

Aside from their use in displays, liquid crystals have also been used in more diverse commercial applications, often utilising the temperature dependence of chiral nematic liquid crystals, with examples including thermometers, medical screening devices, battery testers, and jewellery.² Research is ongoing into further applications for liquid crystals including, but certainly not limited to, electrically switchable lenses,³ lasers,⁴ optoelectronic devices⁵ and biological applications.⁶