at CMMPE — Materials
— Introduction to liquid crystals (page
2 of 4)
Liquid crystals are further characterised into different mesophases depending upon the degree of orientational and positional order that they possess. Some liquid crystals may only exhibit one particular type of liquid crystal mesophase, whilst others may exhibit many different phases at different temperatures. In the following section, some of the common phases of calamatic liquid crystals (ie: rod-shaped) are described, although some of these phases have anaolgies in discotic and sanidic (ie: planar-shaped) liquid crystals also.
The least ordered phase is the nematic which has only long-range orientational order (ie: no positional order). In this case, the long axes of the molecules point on the average in the same direction, which is defined by a unit vector commonly known as "the director" (n). The macroscopic optic axis lies along the same direction as the director. The nematic phase can also be considered as centrosymmetric since the physical properties are indistinguishable with an inversion of the director. This mesophase has complete rotational symmetry about the director. As the temperature is decreased the order of the phase is increased.
Due to the microstructure of the nematic phase, many of the macroscopic properties are anisotropic. For example, nematics exhibit a birefringence, because for polarised light, the refractive index in a direction along the long axis of the molecules (ordinary refractive index) is different to the refractive index in the orthogonal direction across the width of the molecules (extraordinary refractive index). Furthermore, the molecules can undergo a reorientation (electro-optic switching) due to an anisotropy in the distribution of electrical charge across the molecule (ie: the anisotropy in dielectric permittivities). Therefore, nematic liquid crystals possess a number of useful material properties such as a high birefringence, sensitivity to low frequency electric fields and structural flexibility. This combination forms the basis of operation of many of the liquid crystal devices in production today. Nematic phases can be identified using optical polarising microscopy by two and four point defects in the resulting "Schlieren" texture.
The nematic phase: (left) an illustration of the orientation of the
A variant of the nematic phase is the chiral nematic (also known as the cholesteric phase) which is a spontaneously forming macroscopic helical structure. This phase was first observed by Reinitzer in 1888 in derivatives of cholesterol. Consequently, the phase is often referred to as cholesteric. The presence of chiral molecules causes the director profile to assume a twisted configuration through the medium. In the figure below, a somewhat naïve picture illustrates the rotation of the director about a single axis, the helix axis. The revolution of the director results in a macroscopic helix. There is actually no layered structure and the local ordering is identical to that of the nematic phase. The defining length scale of this phase is the pitch, the distance for which the director rotates through an angle of 2pi. However, due to the fact that this phase is also non-polar, the invariance with director values of n and -n means that the periodicity is only half the pitch. Unlike the nematic phase, the optic axis is aligned along the helix axis. Macroscopically, they are therefore uniaxially negative.
When the chiral nematic phase occurs in naturally chiral materials with a preference for twisted molecular stacking, the convention is to call this phase "cholesteric". However, the equivalent structure is also achievable with nematics, which intrinsically do not have this chiral twist. In this case, a small concentration of chiral dopant molecules can be added to the nematic, which give give chiral properties to the liquid crystal structure. Furthermore, higher concentrations of dopant molecules can be used to increase twisting power, and to shorten the pitch as required.
Illustrations of the chiral nematic phase (top) and their corresponding
textures (below) when viewed with polarising microscopy.
The N*LC phase can also be identified using optical polarising microscopy. When contained between the walls of a cell the supramolecular helical structure will predominantly orient such that the helix axis is along the normal of the substrates (vertically aligned helix). If viewed along the helix using a polarising microscope a uniform colour is seen which does not change or disappear with a rotation of the sample between crossed polarisers due to the fact that the optic axis is parallel to the direction of propagation of the light. However, a different texture is observed if the helical axis of the chiral nematic is oriented within the plane of the cell (lying helix). In this state, the characteristic 'fingerprint texture' can be observed under polarising microscopy, as the director randomly wanders within the plane of the cell. Finally, if no surface alignment layer is present in the cell, then a random ordering of the helix occurs, resulting in the characteristic 'focal conic' texture.
Chiral nematic liquid crystals possess a number of useful optical properties that are not found with the nematic phase. For example, when the pitch is of the same order of magnitude as the wavelength of light, selective reflection can occur. Consequently, the phase behaves as a one-dimensional Bragg reflector for circularly polarised light that matches the rotation sense of the helix. Furthermore, as a result of the sensitivity of the pitch to thermal changes, different wavelengths are reflected for different temperatures. Such a property has already been exploited for the purpose of thermometry applications. The recent realisation that these are naturally forming photonic band gap materials has also opened up a multitude of potential applications. With the advent of high twisting power chiral additives, nematogens that contain no chiral centres can be made to form a chiral nematic phase with only a small concentration of additive. Therefore, properties such as the local birefringence and ordering are the same as that of the bulk nematic.
Higher order liquid crystal phases are the so-called smectic phases which combine orientational order with positional order. Unlike the nematic and chiral nematic phases the density is not uniform as there is some correlation between the molecules’ centre of mass. This results in a diffuse layered structure. However, the interlayer forces are weak in comparison to the intralayer forces between the molecules and therefore the layers have some degree of freedom to move. Due to the layered structure smectics are usually more viscous than conventional nematics and tend to appear at lower temperatures.
There are a number of different smectic phases and these are characterised by the packing formation and the tilt angle with respect to the layer normal. Each phase being classified by a different letter of the alphabet in the order they were discovered (eg: SmA, SmB, SmC, etc, ranging from A to K). The most commonly used smectics used in this research group are SmA and SmC:
Smectic phases, showing layered structure: (left) Smectic A, and (right) Smectic C (tilted).
Whilst the smectic A phase does possess a layered structure, the molecules within the layer are oriented similar to that of a nematic. It is thus the least ordered of the smectic phases as it has only one-dimensional positional order. This was identified by the fan-shaped focal conic texture, which unlike the nematic and chiral nematic phases do not flash under shearing forces.
Discotic and sanidic liquid crystals also can form phases similar to that of the smectic phase in calamatics. For example, the stacking of discotics into regular columns, packed together in regular patterns, is described by the 'columnar phase'. This is analogous to the smectic B phase in calamatics, where molecues form regular rows in one direction, and are hexagonally packed in the perpendicular direction. Bent-core molecules have also recently been developed that form smectic phases. Their unique shape has led then to be known as 'banana phases'. More information on how molecular shape determines liquid crystal phases can be found here.
Helical smectic phases are also known to exist such as the chiral smectic C phase (SmC*). This is similar to the chiral nematic structure except that the molecules are tilted at an angle to the layer normal and it is the precession of the tilt that gives rise to the macroscopic helical structure. Smectic C* liquid crystals exhibit ferroelectric qualities, including bistability and fast switching speeds.
The chiral smectic C phase, where the director tilt of each layer precesses around a circle upwards through the structure.
In the case of liquid crystal molecules that possess an electric dipole, each molecular layer within the smectic structure will have a net electrical polarisation. The polarisation vector will rotate from one layer to the next, resulting in a material with overall zero electrical polarisation. We call such liquid crystals 'ferroelectric' (FLC). Antiferroelectric structures are also possible, in which the director is orientated to either one side of the cone or the other in every alternate layer, resulting in the electric polarisation reversing itself from one layer to the next. Meanwhile, as one observes every other layer, the directors continue to precess around the cone. A third form is also possible, whereby only occasional layers have the reverse electric polarisation. More layers are therefore located on one side of the cone than the other, and the material has a net electrical polarisation. We call this a 'ferrielectric" liquid crystal.
The ferro-, ferri- and antiferro-electric phases. (Structures are drawn in their unwound state for simplification purposes).
Also, there exists a twist grain boundary phase (TGB). Twist grain boundary phases tend to appear between chiral nematic and smectic phases or alternatively isotropic and smectic phases. This frustrated state usually occurs as a result of the competition between layer ordering and twist/bend deformations.
Blue phases can exist between the chrial nematic phase and the isotropic liquid phase of liquid crystalline materials with high chirality. They usually exist over only a very narrow temperature range (approximately 0.5 to 2 degrees C), although recent work at CMMPE has demonstrated blue phase materials with much wider temperature stabilities. There are three types of blue phase: BP I*, BP II* and BP III*, in order of appearance when heating towards the isotropic state. The three blue phases differ in the amount of order they possess and the structures that the chiral molecules form.
Blue phases occur becuase the helicoidal structure of the chiral neamtic phase is not the lowest energy configuration for chiral molecules. In the chiral nematic, molecules lie in quasi-nematic layers and rotate when going from one layer to the next. However, the free energy is actually lower if the molecules twist in two dimensions simultaneously. This leads to the formation of the double twist cylinder, where the molecules all rotate about a central axis.
double twist cylinder structure from above (left) and from a perspective
The double twist cylinder is not stable over large distances (this is known as 'frustration', and hence blue phases are often termed 'frustrated phases'). The twist of a cylinder is limited to a maximum angle of 45 degrees between the directors of the axial molecules and those at the outer surface of the cylinder. The cylinders therefore pack together to form cubic structures, interlaced with disclinations. For example, BP II* has been shown to have a simple cubic structure, whilst BP I* packs into a face-centred cubic (fcc) structure.
phase liquid crystals are structured by intersecting double twist cylinders.
In BP I*, the cylinders pack into a
When blue phases are viewed under a microscope using reflected light, domains are observed which reflect different colours. This is an example of Bragg reflection associated with the photonic band gap of chiral liquid crystals. However, different colours are reflected in different regions because the reflected light is dependent upon both the spacing of reflective planes and also the angle between the light and the reflective planes.
A collection of three example blue phase textures viewed under a microscope.