Dielectric Properties of Nematic Liquid Crystals

Molecules consisting of atoms with different values of the electronegativity are polar. The dipole moment of a chemical bonding is a vector and therefore, a compensation or increase of the bond moments in a molecule can be observed [1, 2], Furthermore, the overall dipole moment of a molecule depends on the life time of different conformations. By the dielectric method, only a very small orientation of the molecular dipoles and the time for its reorientation can be measured as static dielectric constant and relaxation time T, respectively. Thereby the static dielectric constant can be produced by switching off an external electrical field in different steps as demonstrated in Fig. 1. 

It can be expected that the reorientation of the O-CH3 group (b) is faster than that of the whole molecule about the short mo- 

A second effect of the same sample shown in Fig. 3 is also interesting. By cooling the isotropic liquid deviations of the static dielectric constant q from the straight line by approaching the is/N transition were measured. Such behavior was at first detected by Bradshaw and Raynes [24], confirmed by Thoen and Menu [25] and explained as an antiparallel correlation of the strong dipoles in the nematic-like clusters formed in the isotropic phase. For less polar molecules the driving force for the antiparallel orientation can also be produced by a combination of the steric and dipolar forces as shown by Kresse and Kremer [26] and demonstrated in Fig. 3. 

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Deshmukh RR, Jain AK (2014) The complete morphological, electro-optical and dielectric study of dichroic dye doped polymer dispersed liquid crystal. Liq Cryst 41 960-975 Deshmukh RR, Malik MK (2014) A method for estimating mterfacial tension of liquid crystal embedded in polymer matrix forming PDLC. J Appl Polynn Sci. doi 10.1002/app.41137 Dixit S, Misra A, Manohar R, Arora AK (2013) Enhancement in dielectric properties of nematic liquid crystal by gamma irradiation. Mol Cryst Liq Cryst 571 77-85 Doane JW, Vaz NA, Wu BG, Zumer S (1986) Field controlled light scattering fi-om nematic micro-droplets. Appl Phys Lett 48 269-271... 

Dielectric properties of nematic liquid crystals (NLCs) depend on the frequency of the applied field through the dielectric relaxation. There are two important general consequences of the dielectric relaxation phenomena (1) the switching memory effect, i.e., the non-local time relationship between the electric displacement and the electric field (2) dielectric heating effect. 

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