Physical properties of liquid crystals

The order parameter S = 0.5(3(cos ) — I) characterizes the long-range order of molecules in a mesophase, where 0 is the momentary angle between the long axis of the molecule and the director. In an ideal crystal the order parameter S equals 1, and it equals 0 in an isotropic liquid. In a nematic phase the order parameter hes in the range 0.5-0.7. 

One of the most useful properties for the apphcation of liquid crystals is the anisotropy of their reflactive index An = e — where rig is the extraordinary and rio 

The orientation of molecules in an electric field is determined by the sign of the dielectric anisotropy (As = — ex) e and ex are the dielectric constants measured parallel and perpendicular to the director. Some nematic LCs can change their sign of Ae depending upon the frequency of the applied field. 

The majority of mesogens are diamagnetic. The diamagnetic anisotropy (A/) characterizes the behavior of a LC under the influence of a magnetic field A/ = X — XX, where x and xx are the diamagnetic susceptibilities parallel and perpendicular to the director. 

The basic methods for the determination of phase transition temperatures (7 ) are DSC (differential scanning calorimetry), DTA (differential thermal analysis), and polarization microscopy. Every method has its advantages and restrictions. DSC allows one to determine the enthalpies of phase transitions (A// ). Microscopy allows one both to determine the phase transition temperatures and to identify the t)fpe of mesophase. DTA gives reliable results for melting temperatures of LCs that show solid-state polymorphism, and of LC mixtures. The differences in that can be found in publications by different authors arise from different measurement techniques and the presence of impurities. We have selected the data with the higher 7 values in such cases. 

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General reviews of the structure and properties of liquid crystals can be found in the following G. H. Brown, J. W. Doane, and V. D. Neff. "A Review of the Structure and Physical Properties of Liquid Crystals." CRC Press, Cleveland, Ohio, 1971 P. J. Collings and M. Hind, Introduction to Liquid Crystals. Nature s Delicate Phase of Matter," Taylor and Francis, Inc., Bristol. Pennsylvania, 1997 P. J. Collins, "Liquid Crystals. Nature s Delicate Phase of Matter," Princeton University Press. Princeton. New Jersey, 1990. A thermodynamic description of the phase properties of liquid crystals can be found in S. Kumar, editor, "Liquid Crystals in the Nineties and Beyond, World Scientific, Riven Edge, New Jersey, 1995. 

D. Demus, J.W. Goodby, G.W. Gray, H.-W. Speiss and V. Vill (Eds.), Physical Properties of Liquid Crystals, Wiley-VCH, Weinheim, 1999. 

Physical properties of liquid crystals are generally anisotropic (see, for example, du Jeu, 1980). The anisotropic physical properties that are relevant to display devices are refractive index, dielectric permittivity and orientational elasticity (Raynes, 1983). A nematic LC has two principal refractive indices, Un and measured parallel and perpendicular to the nematic director respectively. The birefringence An = ny — rij is positive, typically around 0.25. The anisotropy in the dielectric permittivity which is given by As = II — Sj is the driving force for most electrooptic effects in LCs. The electric contribution to the free energy contains a term that depends on the angle between the director n and the electric field E and is given by... 

Synthesis and Physical Properties of Liquid Crystals An Interdisciplinary Experiment 118... 

Brown G (1971) A review of the structure and physical properties of liquid crystals. Butterworths, London... 

Gray, G. (1962) Molecular structure and the properties of liquid crystals. NY Academic Press 23Brown, G. Doane, J. Neff, V. (1971) A Review of the Structure and Physical Properties of Liquid Crystals. Boca Raton, FL CRC Press. 

Comparisons of values quoted in the literature for the physical properties of liquid crystals are often of dubious validity due to differences in the methods of assessment often carried out at different absolute temperatures (e.g. 22°C or 25°C) or reduced temperatures (e.g. T -j— 10°C or 0.95 x 7V /). The use of extrapolated data from a wide variety of nematic mixtures of different composition and properties at various concentrations is also common. Unfortunately non-ideal behaviour is common for such mixtures and non-linear behaviour is not unusual, i.e. the values extrapolated to 100% are more often than not dependent on the matrix used and the concentration of the compound to be evaluated. However, although the absolute values of the data collated in Table 3.13, measured in the same way at the same reduced temperature (0.96 X r r-/), are lower than those reported for the same compounds in the literature, usually measured at 22°C the trends and relative values are very similar. 

D. Dunmur, A. Fukuda, and G. Luckhurst (eds.). Physical Properties of Liquid Crystals Nematics, esp. p. 268, INSPEC Institution of Electr. Engs., London (2001). 

H. March and M. P. Tosi Coulomb Liquids, Academic Press, New York, London, 1984. [l09] D. Demus, J. Goodby, G. W. Gray, H.-W. Spiess, and B. Vill (eds.) Handbook of Liquid Crystals, Vols. 1-4, Wiley-VCH, Weinheim/Germany, 1998. [110] D. Demus, J. Goodby, G. W. Gray, H.-W. Spiess, and B. Vill (eds.) Physical Properties of Liquid Crystals, Wiley-VCH, Weinheim/ Germany, 1999. 

Physical Properties of Liquid Crystals Nematics, editors D.A. Dunmur, A. Fukuda and... 

Wiiiflinger, A., and Sandmann, M., Equation of state for nematics, in Physical Properties of Liquid Crystals Nematics, Dunmur, D., Fukuda, A., and Luckhurst, G., Eds. INSPEC, London, 2001. 

A.G. Petrov, Measurements and interpretation of flexoelectricity. In eds. D.A. Dunmur, A. Pukuda and G.R. Luckhurst, Physical Properties of Liquid Crystals, Vol 1 Nematics, Inspec, London, 2001. pp. 251-264. 

W. H. de Jeu, Physical Properties of Liquid Crystal Materials, Gordon and Breach, New York... 

Thoen, J. Thermal methods. In Demus, D., Goodby, J., Gray, G.W., Spiess, H.-W., Vill, V. (eds.) Physical Properties of Liquid Crystals, pp. 208-232. Wiley-VCH, Weinheim (1999) Pikin, S.A. Structural Transformations in Liquid Crystals. Gordon Breach, New York (1981)... 

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