Crystal-nematic

Research Focus Preparation of materials having a helical rigid-rod shape that exhibit cholesteric or nematic crystal-phase behavior. 

Gibbons, W. M., Shannon, P. J., Sun, S-T., and Swetlin, B. J. Surface-mediated alignment of nematic crystals with polarized light. Nature (London) 353, 49-50 (1991). 

FIGURE 5.2 Liquid crystals, (a) Smectic crystals the ends of the molecules are on a plane, (b) Nematic crystals the ends of the molecules do not match, (c) Cholesteric crystals the molecules in each layer are arranged in a manner similar to nematic crystals, but the angle changes from plane to plane of the molecules, forming a helix of pitch length p. 

D. Fekete, J. Au Yeung and A. Yariv, "Phase Conjugate Reflection by Degenerate Four Wave Mixing in a Nematic Crystal in the Isotropic Phase," Opt. Lett., Vol. 5, p. 51 [1980]. 

In a nematic liquid crystal, the molecules are aligned so that their long axes tend to point in the same direction but the ends are not aligned with one another. In smectic A and smectic C liquid crystals, the molecules maintain the long-axis alignment seen in nematic crystals, but in addition they pack into layers. 

Ilquld crystal A substance that flows like a liquid but has some order in its arrangement of molecules. Nematic crystals have long molecules all aligned in the same direction, but otherwise randomly arranged. Cholesteric and smectic liquid crystals also have... 

CARBONACEOUS MESOPHASE is a liquid crystaUine state of PITCH which shows the optical birefringence of disc-like (discotic) nematic crystals. It can be formed as an intermediate phase during thermolysis (p5Tolysis) of an isotropic molten PITCH, or by precipitation from PITCH fractions prepared by selective extraction. With continuous heat treatment, the CARBONACEOUS MESOPHASE coalesces to a state of BULK MESOPHASE before solidification to GREEN COKE, with further loss of hydrogen or other low molecular weight compounds. 

In many cases unique optical textures are observed for the various orientations and structures of the three classes of liquid crystals. Thin films of nematic crystals, for example, can be identified by the pattern of dark tlueads (isogyres) which can appear in the optical microscope in transmission with crossed polarizers. Hot stage polarizing optical nucroscopy is often used to identify the phases and the transition temperatures. In some cases, the optical texture is not uniquely identifiable and x-ray diffraction and thermal analysis by DSC are used to complement the microscopy. 

M. Dennin, G. Ahlers, D. Cannell Measurement of material parameters of the nematic crystal 152, in P. Cladis, P. Palffy-Muhoray (eds) Spatio-temporal patterns in nonequilibrium complex systems, Santa Fe Institute Studies in the Sciences of Complexity XXI, Addison-Wesley, New York, (1994)... 

The mean diamagnetic sucseptibility x is usually of the order of 10 . The local magnetic field locai in n nematic crystal is prac-... 

Tni is the clearing temperature, y< 1 is a fitting parameter. The absolute anisotropy Axq is found by extrapolation to 7 =0. In the vicinity of Tf, this approximation fails and a correction factor p has to be introduced. The fit of experimental data on low molecular weight nematic crystals to the generalized equation [5]... 

Experimental methods can be subdivided into two classes. The first class determined the magnetic dipolar field induced in the nematic crystal, or the force on a nematic sample in an inhomogeneous magnetic field these methods give absolute susceptibilities X- A second class of experiments measures the torque on nematic samples in magnetic fields these experiments are sensitive to t %. The anisotropic properties are by far more relevant to the characterization and application of nematic samples. Therefore the latter methods can be considered more direct, although they give no information on absolute susceptibilities. 

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