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Liquid Crystal Cells
Published in Russell A. Chipman, Wai-Sze Tiffany Lam, Garam Young, Polarized Light and Optical Systems, 2018
Russell A. Chipman, Wai-Sze Tiffany Lam, Garam Young
A botanist in Austria, Friedrich Reinitzer, first described liquid crystals in 1888. He encountered a material, cholesteryl benzoate, that exhibited a mesophase between the solid and liquid states. When heating through 145°C, it melted into a viscous white fluid. At 179°C, it transitioned into a clear isotropic liquid. He shared his discovery with a physics professor at the Technical University Karlsruhe, Otto Lehmann, describing the two melting points. Lehmann observed that in the mesophase, the liquid demonstrated the double refraction effect, a characteristic of a crystal. Since cholesteryl benzoate showed both liquid and crystal characteristics, he named it a “fliessende crystal.” This translated into the English name “liquid crystal.” Hence, the molecules themselves are not the liquid crystals. Liquid crystal refers to the phase, the state between solid and liquid.
Supermolecular Structure of Polyamic Acids and Polyimides
Published in Michael I. Bessonov, Vladimir A. Zubkov, Polyamic Acids and Polyimides, 2020
Yu. G. Baklagina, I. S. Milevskaya
Hence, polyamic acids in the condensed state should not consist of randomly entangled coils. In contrast, considerable parts of their macromolecules are straight and form aggregates which, however, do not exhibit true crystalline order because meta-and para-isomers are randomly included in the chain. The longitudinal size of mesophase aggregates in polyamic acids is 80 to 100 Å, i.e., it is only 1.5 to 2 times smaller than that of crystallites in polyimides.45 This suggests that the nuclei of the crystalline order in polyimides are formed in the polyamic acid stage. Thermal treatment at moderate temperature increases the order of the mesophase, and subsequent treatment at higher temperatures leads to imidization and crystallization.
Polymers in Special Uses
Published in Manas Chanda, Plastics Technology Handbook, 2017
Ordinarily a crystalline solid melts sharply at a single, well-defined temperature to produce a liquid phase that is amorphous and isotropic. A different behavior is exhibited by a class of organic compounds known as liquid crystals. The oldest examples are cholesterol derivatives, e.g., cholesteryl benzoate. This substance, for instance, does not have a sharp transition to amorphous liquid at 145.5°C, but changes to a cloudy liquid, which becomes clear and isotropic only at 178.5°C. This cloudy intermediate state that possesses an ordered structure with some resemblance to a crystalline solid, while still in the liquid state, is called a mesophase or mesomorphic phase from the Greek mesos, meaning in between or intermediate.
Fluorinated twist-bend nematogens: the role of intermolecular interaction
Published in Liquid Crystals, 2021
Anamarija Knežević, Irena Dokli, Jurica Novak, Darko Kontrec, Andreja Lesac
As expected, the introduction of the fluorine substituent reduces the isotropization temperature compared to their non-fluorinated analogue [26]. However, its position substantially influences melting/crystallisation temperature and the mesophase formation. The dimer BBFE, which contains fluorine-substituent in the inner aromatic ring, shows a significant depression of all transition temperatures. The most affected are melting (reduced by 43°C) and isotropization (reduced by 49°C) temperatures, yielding a broad temperature range of the NTB phase. By moving the fluorine-substituent from inner to the outer ring, all relative transition temperatures increase. The thermal behaviour of o-FBBE deviates the least from the parent non-fluorinated dimer. It shows rather small depression of all transition temperatures. Analogous behaviour was observed for o-fluorine-substituted 4-phenyl 4′-cyanobenzoate [36] and methylene linked phenyl (4-alkoxy-2-fluorobenzoate) dimers [34]. Changing the location of the fluorine-substituent to meta position resulted in a dramatic change in thermal behaviour – a substantial increase of the melting temperature, narrowing the temperature range of the monotropic nematic behaviour and the absence of the NTB phase. The melting temperature is 10°C higher than observed for BBE. In contrast, the melting temperature of m-fluorine-substituted 4-phenyl 4′-cyanobenzoate is 50°C lower than the non-fluorinated analogue [36]. The fact that the same fluorine pattern may affect mesomorphic properties in such a different way, indicates the need for a more detailed consideration of the available interactions of fluorine with its environment.