Liquid crystals: rediscovered materials

Liquid crystals (LCs) have been a part of research for more than a century. The first LC was found as a cholesteric phase of cholesteryl acetate by Reinitzer in 1888 [1]; since then, a vast number of LC discoveries have been reported, in various LC phases, compounds, and applications. However, most recent research has focused on the display applications, i.e., liquid crystal displays (LCDs). However, there are still many diverse aspects of LCs to be explored and applied into modern materials, such as solar cells or artificial cell membranes.

LC materials can be divided into two categories, thermotropic and lyotropic LCs. The former is a pure compound that exhibits LC phases, while the latter is a mixture of one or more compounds in a solute–solvent system and is able to show LC phases at a certain concentration, both within a specific LC temperature range [1]. LC materials have the ability to self-assemble into various LC phases.

When a thermotropic LC material is heated from its solid state, it melts at its melting point forming an LC phase. With continuous heat its clearing point, the compound transforms from a LC phase into an isotropic liquid where all LC properties diminish. In each LC compound, there may be more than one LC phase within the LC temperature range. With distinct optical birefringence properties and the ability to twist the plane of polarized light (in chiral LCs), LC materials are widely used in display applications. An example of the thermotropic LC compounds introduced here is methyl 4-(4′-octylphenyl)benzoate (MC8PB). MC8PB exhibits two interesting LC phases, i.e. smectic B around 86 °C and crystal E at 62 °C. The cover image is the smectic B texture of the compound under a polarized light microscope on cooling from its isotropic state [2].

The smectic B phase possesses a 3D molecular orientation, with hexagonal molecular arrangement inside each perpendicular molecular layer. The dark area of the image shows where molecules are in their isotropic state, while the colorful area is where the molecules have self-assembled into the smectic B phase. Apart from display applications, MC8PB and other LC compounds may be used to produce numerous desirable materials, for example, a template in the production of nanofilms. By making use of the 3D structure of MC8PB, the interlayer spacing of the smectic B phase allows the substrate molecules to penetrate, thus forming a nanofilm. If the substrate molecules can penetrate into the LC layer, the substrate molecules form a multilayer nanoporous material.

The other type of LC is the lyotropic mesophase. Even though lyotropoic phases have been known since the 1970s, the applications of these phases are not as great as the thermotropic LCs. However, lyotropic LCs do come into play within the field of biomaterials: most lyotropic LC materials are of biological origin, e.g. lecithin (extracted from soya beans). Mixing with a water–ethanol solvent, lecithin forms lyotropic bilayer textures which can be observed under a polarized light microscope [1]. The bilayer structures of lecithin may be in the form of myelin textures, Maltese crosses, bilayer tubes, or other micellar phases. Most of the structures may be in the form of microtubes or colloidal particles, possessing lecithin bilayer walls, thus forming focal conic patterns [3]. If micro tubular structures are needed in the construction of a desired material, as in the formation of nanocellulose [4], the structures may be applied to aid the tubular formations. The bilayer structure can be further applied to packaging technology by coating the soya lecithin film onto paper for use as food packaging. The coated paper has been reported to have the ability to extend the shelf-life of packaged food [5].

One of the most powerful tools in liquid crystal investigations is the polarized light microscope. To achieve the highest contrast, as in the image featured on the cover of this issue of Materials Today, a simple modification of the microscope setup was employed (dark field and interference reflection). A second light source (a halogen lamp) equipped with a third polarizer was included at a slightly tilted angle. By adjusting the polarity of the polarizers, the angle of the second light source, and the intensities of the light sources, the highest contrast was obtained. The modified setup of the microscope may not only be applied to LC phase investigations, but also to the study of different types of materials. In studies of the microstructure of materials, either scanning or transmission electron microscopy (SEM or TEM) is usually employed. However, with the modified polarized light microscope, the microstructures can be clearly observed, together with the inner configuration. Moreover, if the second light source is replaced by a UV lamp, fluorescence properties of the sample will be observed. The limitation of this setup is the low magnification, which is usually no more than 1500×.

The rediscovery of LC in current material science will expand the possibility of producing nanostructures and microstructures in novel materials and biomaterials for various applications.

Further reading

[1] D. Demus et al.; Handbook of Liquid Crystals, vol. 1 Wiley-VCH, Weinhein, Germany (1998)

[2] S. Tantrawong, PhD Thesis, Magnetomesogens: Vanadium-containing Liquid Crystals. The University of Hull, UK, 1994.

[3] S. Tantrawong, unpublished results.
[4] A. Dufresne; Mater. Today, 16 (6) (2013), pp. 220–227
[5] Tantrawong; J. Pac. Sci. Tech., 23 (2) (2014), pp. 1–7

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DOI: 10.1016/j.mattod.2014.03.001