Liquid crystal displays (LCD) are used in watches, calculators, and flat-panel displays found on televisions or mobile phones. In an LCD, liquid crystals are aligned between two polarizers and two transparent electrodes.
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However, the question remains: how can a crystalline material also be a liquid? Going back to 1888, Austrian botanist Friedrich Reinitzer was conducting research where he observed cholesteryl benzoate under a microscope and saw that the compound did not go directly from a solid to a transparent liquid at 145 ºC, but instead to a cloudy fluid.
A more transparent liquid was obtained when the temperature increased to 178 ºC. Cholesteryl benzoate had two melting points.
In 1889, German physicist Otto Lehman discovered that the molecules displayed a crystalline structure in that cloudy state. He named them ‘liquid crystals’ as they are now known.
A liquid crystal is defined by the compound's molecular structure when it is between the ordered crystalline solid phase and the disordered liquid phase. The key features of these phases are a combination of the anisotropy of the solid state and the fluidity and mobility of the liquid state. These molecular orders are called ‘mesomorphs’. When seen through a polarized-light microscope, they have striking textures, as shown in the image.
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How the mesophase is generated delineates the two main types of liquid crystals. Thermotropic liquid crystals are generated by temperature, whereas the effect of a solvent generates lyotropic liquid crystals. ‘Amphoteric’ varieties demonstrate both types of performances.
As previously mentioned, liquid crystals are anisotropic materials because their physical properties vary according to their orientation. Liquid crystals can be formed of rod-shaped molecules (calamitic liquid crystals) or disc-shaped molecules (discotic or columnar liquid crystals). These are the main molecule types that affect the appearance of the liquid crystal state due to temperature.
Can Polymers be Liquid Crystals?
Some polyamides, such as Kevlar, are liquid-crystal polymers (LCP). These materials rely on obtaining polyamide fibers from sulfuric acid when a lyotropic liquid crystal phase has occurred. Due to the orientation and organization of the fibers, the result is a textile with excellent properties.
A very common example of thermotropic Liquid Crystal Polymers is Vectran, an aromatic copolyester created by the polycondensation of 4-hydroxybenzoic acid and 6-hydroxynaphthalene 2-carboxylic acid. Vectran is often utilized in sports equipment like racquets, fishing lines, mountain climbing gear, etc.
The orientation of liquid crystal monomers allows the plastic parts gained from these monomers to be stable, with precise dimensions, high rigidity, and excellent chemical resistance. LCPs have unique advantages, but also some disadvantages. The anisotropic nature of the material causes a lack of strength in the joining and welding lines where the material is in different molecular orientations.
Are they purely synthetic? Liquid crystal-type organizations exist in nature, particularly in biological systems. Phospholipids are the most popular example, as they are the main element of cell membranes. These biological systems have inspired cosmetic products. Based on liquid crystals, hollow spheres can be obtained where the surface is made of phospholipids or other derivatives. Active principles can be stored inside and gradually released.
Liquid crystals have also been used in windows to make them transparent or opaque at the flip of a switch. The switch creates an electric field that enables the molecules to move from an orderly state to parallel alignment to the field which creates light distortion, causing the device to lose transparency. Filters can create different colors, such as red, green, or blue.
Organic Light-Emitting Diodes (OLED)
Among many possible applications, the most well-known is in screens. However, a new competitor has recently emerged: the OLED device. This offers brighter colors and greater lightness, but it also degrades faster and is more complex to manufacture.
Research continues in light of the vast array of uses. The University of Wisconsin-Madison recently published an article in Nature that describes liquid crystals as being able to self-regulate drug releases accurately and in repeated doses. A film is made of liquid crystals, and microscopic drops of active agents can be stored inside. Following placement inside, these agents come into contact with a stimulus and are expelled.
Professor Nick Abbot explained how bacteria can be placed on the film surface and, thanks to the movement of flagella, they create a strain on the surface that produces the release.
Liquid crystal is a unique state of aggregation of matter with many applications. Some are already in use, while others still have a long way to go.
This information has been sourced, reviewed and adapted from materials provided by AIMPLAS.
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