Liquid crystal monomers (LCMs) are chemicals in the physical state of liquid crystal. Liquid crystal is an anisotropic liquid or partially ordered fluid, combining order and mobility. And the liquid crystalline phases are separated from the isotropic liquid and the crystalline solid through thermodynamic phase transitions, while their phase diagram can schematically be summarized as in Figure 1.
Fig. 1 Thermodynamic phase diagram with the inclusion of the liquid crystalline state, beside the three well-known states of matter, gases, liquids and solids.
Generally, there are two ways to classify liquid crystal materials:
According to the way liquid crystals formed, they can be classified into thermotropic and the lyotropic liquid crystals.
Most of the thermotropic liquid crystals are composed of rod-like molecules and they are formed on heating the crystalline solid or cooling the isotropic liquid. When these crystals are placed under a spectrometer magnet, molecules of the crystals tend to orient to a common direction. The direction is either along the external magnetic field or in a perpendicular direction to the field. The transition of thermotropic liquid crystalline is showed in figure 2.
Fig. 2 Thermotropic liquid crystalline transition
Lyotropic liquid crystals are formed in mixtures of amphiphiles (e.g., surfactants) and solvents, for example, detergents and water. Consequently, these phases are thermodynamically stable at defined temperatures, pressures, and concentrations. Like thermotropic, a variety of structurally distinct modifications exist, which are collectively known as lyotropic liquid crystals. In lyotropic liquid crystals, phase transition between mesophases may also be affected by the addition of appropriate solvents, such as surfactants, and their concentration, see figure 3.
Fig. 3 Lyotropic phases. Some lyotropic LC phases as a function of temperature and concentration of the amphiphilic molecules a) dissolved surfactant being in a disordered state, b) micellar, c) cubic, d) cylindrical and e) lamellar phases
According to the phase states, they can be classified nematic, smectic and cholesteric liquid crystals.
Nematic phase (N), the simplest liquid crystal phase, is characterized by a high degree of long range orientational order but no translational order. Hence nematic liquid crystals (NLCs) can be described as fluids with the orientational order of their anisotropic building blocks.
Smectic liquid crystals consist of flat layers of cigar-shaped molecules. The layers are able to slide over one another and their long axes oriented perpendicularly to the plane of the layer. Generally, there are two kinds of smectic phases (Figure 4). The smectic A phase has the molecules oriented along with the normal layer, whereas smectic C has its molecules tilted away from it.
Fig. 4 Schematic representation of smectic A phase and smectic C phase
Cholesteric liquid crystals, also known as chiral nematic liquid crystals, are a type of liquid crystals made only of chiral molecules and with a helical structure (Figure 5).
Fig. 5 Helical structure of the cholesteric liquid crystal phase
Liquid crystal monomers are widely used in liquid crystal displays (LCDs), which are dramatically changing the world due to the provision of convenient communication. While LCMs in LCDs may be released into the environment, especially in electronic waste recycling industrial parks with a high pollution risk.
Su[1] reported that 87 out of 362 LCMs showed potential as persistent and bioaccumulative chemicals, indicating that these LCMs would exhibit resistance to degradation and high mobility after entering the environment. Beyond that, it was observed that exposure to LCMs resulted in adverse effects during in vitro screening, by use of cell culture and toxicogenomic evaluation.
Based on environmental and economic considerations, the liquid crystal in the LCD panel should be recycled or disposed in a suitable way.
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