Electric field effects, polymer liquid crystals

In conclusion, electric field effects in liquid crystals is a well-developed branch of condensed matter physics. The field behavior of nematic liquid crystals in the bulk is well understood. To a certain extent the same is true for the cholesteric mesophase, although the discovery of bistability phenomena and field effects in blue phases opened up new fundamental problems to be solved. Ferroelectric and antiferroelectric mesophases in chiral compounds are a subject of current study. The other ferroelectric substances, such as discotic and lyotropic chiral systems and some achiral (like polyphilic) meso-genes, should attract more attention in the near future. The same is true for a variety of polymer ferroelectric substances, including elastomers. 

All electrooptical effects known to the present time for polymeric liquid crystals may be divided into two groups. First of all there are so called orientational effects, which are due solely to the effect of the electric field (field effect) on LC polymers, but are not a result of a current flowing. The second group of electrooptical effects is attributed to the phenomena ascribed to the anisotropy of electrical conductivity (Act) of liquid crystals. These are called electrohydrodynamic effects. 

The change in light scattering of conventional polymers with electric and magnetic fields is small. However, much larger effects may be obtained with liquid crystalline systems that have cooperative orientation. As with Kerr effect devices (see Section 4.14), more rapid orientation times desirable for display devices are obtained using low molecular weight or side chain polymer liquid crystals. 

As the previous sections have shown, nematic polymer liquid crystals may be oriented by surface forces and in electric fields. It has been shown recently that such field-induced changes in orientation may also be used to orient pleochroic dyes through the guest-host effect. In such an effect either guest dyes dissolved in a nematic polymer " host or side-chain dye moieties in a nematic copolymer system (where A is a nematic moiety and B is a dye in Fig. 2b) undergo a cooperative realignment as the nematic director responds to the applied field. Since the pleochroic dye has its absorption transition... 

The storage effects could also be realized in polymer liquid crystals. On cooling, ferroelectric liquid crystal polymers with the electric field applied, the macroscopic polarization is frozen in the glassy state [74]. Thus, the polymer film becomes a pyroelectric and a piezoelectric. Unfortunately, the glassy state is too viscous to allow the field-induced reorientation of the polarization and the film cannot be considered to be a ferroelectric. 

H.-S. Kitzerow and P.P. Crooker, Electric field effects on the droplet structure in polymer dispersed cholesteric liquid crystals, Liq. Cryst 13, 31 (1993). 

In this section we wish to consider all the possible contributions to the electric permittivity of liquid crystals, regardless of the time-scale of the observation. Conventionally this permittivity is the static dielectric constant (i.e. it measures the response of a system to a d.c. electric field) in practice experiments are usually conducted with low frequency a.c. fields to avoid conduction and space charge effects. For isotropic dipolar fluids of small molecules, the permittivity is effectively independent of frequency below 100 MHz, but for liquid crystals it may be necessary to go below 1 kHz to measure the static permittivity polymer liquid crystals can have relaxation processes at very low frequencies. 

Liquid crystalline solutions as such have not yet found any commercial uses, but highly orientated liquid crystal polymer films are used to store information. The liquid crystal melt is held between two conductive glass plates and the side chains are oriented by an electric field to produce a transparent film. The electric field is turned off and the information inscribed on to the film using a laser. The laser has the effect of heating selected areas of the film above the nematic-isotropic transition temperature. These areas thus become isotropic and scatter light when the film is viewed. Such images remain stable below the glass transition temperature of the polymer. 

The use of an electric field is not the only effective way to influence the LC polymer structure, magnetic fields displays a closely similar effect167 168). It is interesting as a method allowing to orient LC polymers, as well as from the viewpoint of determining some parameters, such as the order parameter, values of magnetic susceptibility, rotational viscosity and others. Some relationships established for LC polymer 1 (Table 15), its blends with low-molecular liquid crystals and partially deuterated polyacrylate (polymer 4, Table 15) specially synthesized for NMR studies can be summarized as follows ... 

We shall mention here another property of liquid crystalline polymeric systems. As in the case of low-molar mass liquid crystals, when electric and magnetic fields are applied, liquid crstalline domains get oriented along the direction of the field. Rearrangement of a polymer structure under the effect of a magnetic field was demonstrated in for a PBA-dimethylacetamide system. However, the processes of... 

Apply the concept of liquid crystal networks crosslink the backbone of NLO side chain liquid crystalline polymers when they are in the liquid crystal phase. In the presence of a mechanical force the resultant sample may be well aligned because of the interaction between the network strands and the side groups. The mechanical effect is equivalent to the electric or magnetic field. The re-orientation response of the liquid crystal is a quadratic function of the applied electric field, but it is linearly proportional to the mechanical stress. Thus, the mechanical stress is more effective in aligning the liquid crystals and is expected to produce less defects and hence to promote the transparency of the sample. 

Chilaya, G., Hank, G., Koswig, H.D., Sikhamlidze, D. Electric-field controlled color effect in cholesteric liquid crystals and polymer-dispersed cholesteric liquid crystals. J. Appl. Phys. 80, 1907-1909 (1996)... 

Note that the effective aligning field Ep of the polymer network depends only on the density of the polymer fibers, but not on the dielectric anisotropy. Equation (11.70) shows that if the dielectric anisotropy of the liquid crystal is Ae, an electric field higher than Ep must be apphed in order to overcome the aligning effect of the polymer network such that the hquid crystal can... 

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