Calamitic Liquid Crystals—Nematic and Smectic Mesophases
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3 Calamitic liquid crystals—nematic and smectic mesophases 3.1 CALAMITIC MESOPHASE STRUCTURES A material is defined as a crystalline solid when the structure has long-range order of the molecular positions in three dimensions. A fully ordered crystal will also have long-range orientational ordering of its constituent molecules. When a fully ordered molecular crystal (C) is heated, the thermal motions of the molecules within the lattice increase and eventually the vibrations become so intense that the regular arrangement of molecules is broken down with the loss of long-range orientational and positional order to give the disorganised isotropic liquid (I) (T1, Figure 3.1). The temperature at which this process occurs is called the melting point. However, this process, which takes a compound from being very well ordered to being totally disordered in one step, is a very destructive one and is not universal for all compounds. For many compounds, this process occurs by way of one or more intermediate phases as the temperature is increased. These phases are called mesophases and some of these mesophases are liquid crystalline. Liquid crystalline phases have properties which are intermediate between those of the fully ordered crystalline solid (C) and the isotropic liquid (I); liquid crystalline mesophases are fluids which, due to partial orientational ordering of the constituent molecules, have material properties such as permittivity, refractive index, elasticity and viscosity which are anisotropic (i.e., their magnitude will differ from one direction to another). Mesogenic (i.e., mesophase-producing) compounds generally consist of long, narrow, lath-like and fairly rigid molecules (see Figure 3.1). In the crystal state (C), the molecules are held together by strong intermolecular forces of attraction which, due to the rod-like structure, are anisotropic. In simple terms, the smectic phase arises if the lateral intermolecular forces of attraction are stronger than the terminal forces and so, on heating, the terminal forces break down first, in-plane translational order is lost and this results in a lamellar arrangement of molecules in which the layers are not perfectly defined (T2). Due to possible correlations within the layers and between the layers, there are five true smectic modifications and a further six quasi-smectic disordered crystal mesophases (see Figure 3.2). T3 represents the loss of both in-plane and out-of-plane translational order to leave a statistically parallel arrangement of molecules (orientational order) in the nematic phase. When the smectic phase is heated, either out-of-plane translational ordering is lost (T4), which produces the nematic phase, or additionally orientational ordering is lost (T5), which gives the isotropic liquid (I). T6 depicts the loss of orientational ordering of the nematic phase to give the isotropic liquid. No single liquid crystalline material exhibits all liquid crystal phase types but many compounds do exhibit two or three different types of liquid crystalline phases. 44 Introduction to Liquid Crystals Figure 3.1. Possible melting sequences for a liquid crystalline material. Liquid crystal phases can be identified by their individual birefringent textures when viewed between crossed polarisers under a light microscope (see Chapter 9). Such studies by Friedel in 1922 led to the nematic phase (nematic is Greek for thread-like) being so named because its optical texture appeared as a series of optically extinct threads (defects) on a coloured (birefringent) background. A nematic liquid crystal phase can be generated by both calamitic molecules (long and lath-like structures) and discotic molecules (disc-like structures, see Chapter 4); however, although both variations exhibit the same optical texture, these two nematic phases are not miscible and hence are not the same. Friedel did not recognise the existence of more than one smectic phase but he noticed that the ‘smectic phase’ had a soap- like appearance and hence was named because smectic is Greek for soap-like. Additionally, chiral variations of the nematic and smectic phases exist and these will be discussed in Chapter 6. Figure 3.1 sufficiently illustrates the molecular orientational order of the nematic (N) phase. In an isotropic liquid (properties are identical regardless of the direction in which they are measured), the constituent molecules are completely disordered with respect to each other; however, they do not possess enough thermal energy to break into the gas phase. In the nematic phase the constituent molecules are also completely disordered with respect to each other but the long molecular axes statistically point in a preferred direction known as the director ( ). This one degree of ordering of the nematic phase makes it the least ordered liquid crystal phase with a high degree of fluidity. This fluidity combined with the anisotropic nature of the molecules is the basis for the operation of liquid crystal displays. Calamitic Liquid Crystals—Nematic and Smectic Mesophases 45 The fluid nematic phase has a low viscosity and nematic materials can be designed so that molecular orientation can be switched by an electric field; the different optical properties of the two orientations enables display applications (see Chapter 13). Figure 3.2. Plan views of smectic mesophase structures. The smectic mesophase is more ordered than the nematic phase and furthermore, whereas only one nematic phase exists, the ‘smectic phase’ exhibits polymorphism (see Figure 3.2); i.e., there are many different types of smectic phases. As for the nematic phase, the smectic phases can be identified by optical polarising microscopy (see Chapter 9). In 1917, Grandjean was studying (by microscopy) a sample of smectic liquid crystal (later classified as smectic A) which showed stepped edges, indicating that the smectic phase was lamellar in nature. The lamellar nature of smectic phase allows various combinations of molecular correlations both within the ‘layers’ and between the ‘layers’, each of which constitutes a different type of smectic mesophase. 46 Introduction to Liquid Crystals Initially smectic phases can be subdivided into the true liquid crystal smectics and the even more ordered crystal smectics. The crystal smectics have no liquidity and are crystals (the molecules possess long-range positional order in three dimensions); however, there is considerable disorder of molecular orientation and so they are mesophases but not liquid crystal phases. The true smectic liquid crystals are considerably less ordered and are liquids. A second crucial subdivision of smectic phases is made depending upon whether the constituent molecules are tilted, or not, with respect to the layer normal. Figure 3.2 shows idealised plan views of the molecular organisation within the different smectic mesophases. The constituent molecules of the smectic A (S ) A are not tilted and they have no positional ordering within the layers. The smectic C (S ) C phase is the tilted analogue of the S phase. The smectic B (S ) phase is more ordered A B than the SA phase with the constituent molecules adopting a hexagonal ordering (bond orientational ordering) but the hexagonal lattices only have a repeat positional order over ~150–600 Å. The hexagonal nature of the S phase generates two tilted analogues called B the smectic I (S ) phase and the smectic F (S ) phase where the molecules are tilted such I F that the hexagonal lattice tilts towards the apex and the side, respectively (the tilt direction is depicted by the direction of the ‘triangular’ molecules in Figure 3.2). In the crystal B (B) phase the molecules are also hexagonally ordered but additionally the positions of the hexagonal lattices are predictable over a long range in three dimensions. The crystal E (E) phase develops from a contraction of the hexagonal lattice which confers a herring- bone-like structure with restricted rotation. The crystal J (J) and the crystal G (G) phases are tilted analogues of the B phase and the crystal K (K) and the crystal H (H) phases are the respective tilted analogues of the E phase; the tilt direction is shown by the arrows. 3.2 STRUCTURE-PROPERTY RELATIONS This chapter considers the molecular structure of rod-like (calamitic) liquid crystal materials and how those structures can be tailored to generate specific liquid crystal phases at specific temperatures. An enormous number of liquid crystal materials has now been prepared and although many are very similar, each has its own specific combination of structural moieties which confer a certain phase morphology and particular values of melting point and transition temperatures. Additionally, the combination of structural moieties determine the physical properties of materials which are very important when materials are being considered for specific applications. Accordingly, much care is required in the design and synthesis of liquid crystal materials in order to generate the desired liquid crystal properties and the necessary general physical properties. When account is taken of the phenomenal number of organic compounds that have been prepared, and can possibly be prepared, then only a very small proportion of this total will exhibit any liquid crystal phases. The generation of liquid crystal phases is limited by both steric and polarity considerations, i.e., liquid crystal phases can only be exhibited by materials of specific molecular structures. To be suitable for an application, not only does a material require the