2019/01/18
What’s new in liquid crystals?
We have probably all heard about LCD displays. We can find them in a watch or a calculator, as well as in flat-panel displays of televisions or mobile phones. But, what does this acronym stand for and what materials do they use? These materials are liquid crystals and the acronym LDC refers to the electronic devices containing them: ‘Liquid Crystal Display’.
Inside this display, liquid crystals are placed aligned between two transparent electrodes and two polarizers.
But how can something crystalline be liquid at the same time? This is how a new state is defined. It all began in 1888, when the Austrian botanist Friedrich Reinitzer, during research in which he observed cholesteryl benzoate under a microscope, saw that at 145ºC the compound did not go directly from a solid to a more transparent liquid, but rather to a cloudy fluid. However, when the temperature increased to 178ºC, a more transparent liquid was obtained. Cholesteryl benzoate did not have one melting point, but two. The following year, the German physicist Otto Lehman discovered that, in that cloudy state, molecules had a certain crystalline structure, and so he called them ‘liquid crystals’, as these types of molecules are known nowadays.
Therefore, the definition of a liquid crystal describes the molecular structure that a compound has, between the ordered crystalline solid and a disordered liquid. The main characteristics of these phases combine the anisotropy of the solid state and the mobility and fluidity of the liquid state. These molecular orders are called ‘mesomorph’ and under a polarized-light microscope have textures as striking as those shown in the picture.
Two main types of liquid crystals can be found depending on how the mesophase is generated. In thermotropic liquid crystals, it is generated as a result of temperature, and in lyotropic liquid crystals, the mesophase is generated due to the effect of a solvent. Those with both types of performances are called ‘amphoteric’.
As mentioned above, liquid crystals are anisotropic materials, since the physical properties of the system vary depending on the orientation. There are liquid crystals formed of rod-shaped molecules (calamitic liquid crystals) and disc-shaped molecules (discotic or columnar liquid crystals). These two are the main types of molecules that give rise to the appearance of the liquid crystal state by the effect of temperature.
Can polymers be liquid crystals too?
An example of liquid-crystal polymers (LCP) are some polyamides, as known as Kevlar. This material is based on obtaining polyamide fibres from sulfuric acid when a lyotropic liquid crystal phase has been formed. Thanks to the orientation and organisation these fibres have, we obtain the excellent properties it has as a textile.
With regard to thermotropic Liquid Crystal Polymers, a very common example is Vectran, which is an aromatic copolyester produced by the polycondensation of 4-hydroxybenzoic acid and 6-hydroxynaphthalene 2-carboxylic acid. This polymer is used in sports articles such as racquets, fishing lines, mountain climbing, etc.
The orientation of liquid crystal monomers makes the plastic parts obtained from these monomers to have stable and precise dimensions, high rigidity and excellent chemical resistance. Although LCPs have unique advantages, they also have some disadvantages. Thus, the anisotropic nature of the material causes weakness in the joining and welding lines where the material is in different molecular orientations.
Are they just synthetic? In nature, specifically in biological systems, we find examples of liquid crystal-type organisations. One of the most popular examples are phospholipids, which are the main component of cell membranes. This similarity with biological systems has been transferred to the field of cosmetic products. Thanks to liquid crystals, hollow spheres are obtained where the surface consists of phospholipids or other derivatives and active principles, which will be gradually released, can be stored in its interior.
Other applications. Another well-known application is the use of liquid crystals in windows to make them transparent or opaque by actuating a switch. The pressure of the switch creates an electric field that allows molecules to pass from an orderly state to align parallel to the field and thus creates a light distortion, which causes the device to lose transparency. If filters are placed, they can produce different colours such as red, green or blue.
But if they have been around for a long time, what’s new?
While it is true that, among the possible applications, the best known is undoubtedly that of screens, in recent years, a new competitor has emerged, the OLED device (organic light-emitting diode). It allows obtaining brighter colours and has greater lightness; however, they also degrade faster and are more complex to manufacture.
In view of the wide range of applications, research on these materials continues. Last year, the University of Wisconsin-Madison published an article in Nature describing the ability of liquid crystals to self-regulate the release of drugs accurately and in repeated doses. Inside a film made of liquid crystals, microscopic drops containing active agents may be stored. These agents, after being placed inside and come into contact with a stimulus, can be expelled. Specifically, professor Nick Abbot explained how bacteria are able to be placed on the surface of the film and thanks to the movement of flagella, they transmit a strain on the surface that finally produces the release.
Liquid crystal is, therefore, a special type of state of aggregation of matter whose materials have several applications, many of them already exploited, while others still have a long way to go.
María José Clemente Oteo – Researcher at the Synthesis department of AIMPLAS
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