Reorientation of LCs

K shear torque coefficient, 77 viscosity, flat display RCA Co. (1968) Guesl-host effect the reorientation of LC molecules... 

Maximum ON-state transmittance occurs when the refractive index of polymer ( p) matches with the ordinary refractive index ( ) of LC. During the film formation, it is possible that some fraction of LC dissolved in polymer matrix can have a profound effect mi the PDLC film properties. Therefore, determination of partition of LC between the polymer and LC phases is an important factor in evaluating the performance of PDLC films. The reorientation of LC portion in a PDLC composite film is responsible for the optical non-linearity and electro-optical properties of the device. The absorption of LC into an isotropic polymer results in the LC becoming a part of the polymer phase. In this state, reorientation of LC does not happen with an applied electric field, leaving less amount of LC behind for scattering of light. Therefore, selection of suitable concentration of LC in PDLC films is crucial in optimizing film properties. LC dissolved in polymer matrix alters refractive index, dielectric constant, viscosity etc. of the host polymer. As explained earlier, for best electro-optical responses, a polymer and LC material are chosen on the basis of... 

Among spatial reorientation techniques, one can find MAS, OMAS, variable angle spinning (VAS) and switched angle spinning (SAS) techniques. Mechanical spinning of LC materials, which has dated back to the early seventies,63,64 can control the averaging of anisotropic spin interactions... 

Figure 5 Switching in Strategy 2. An H-PDLC, whose droplets of LC form primarily in the dark regions during the writing step, can be reversibly switched by application of an electric field. The field reorients the LC such that the refractive index matches that of the surrounding polymer and the diffraction is therefore switched off.

Fig. 36 Illustration of the effect of temperature on the original state 1 of PS-fc-PMAA(LC). Ila isotropization Ilb electric field reorientation of the LC layers [171]...

Due to their large optical anisotropies, liquid crystals (LCs) have a large optical nonlinearity which is the result of molecular reorientation (Freedericksz transition) in an external field which exceeds the critical field [1], The high external field inhibits the application of LCs, and decreasing the threshold as low as possible is a difficult task [2], LCs doped with a small amount of absorbing dyes that could decrease the needed optical field intensity have been reported [3]. The basic assumption is that the anomalous reorientation of the director results from the interaction between the excited dye molecules and the host. However, this sample would easily degrade under the influence of laser radiation. 

Molecular motions in LCs may occur as isolated or collective modes (see Fig. 4). For the latter mechanism, known as order director fluctuations, a continuous distribution of correlation times is expected [36, 37, 170-173]. Recent protean Tjz dispersion measurements of the LCPs 4 and corresponding LMLCs 7 and 8, carried out over a frequency range of five orders of magnitude (10 Hz < t0o/27t < 3 x 10 Hz), clearly show that collective order fluctuations contribute to the relaxation process only at extremely low frequencies in the kHz regime, whereas the conventional MHz range is dominated reorientations of individual molecules [174]. 

Figure 25 clearly shows the coexistence of LC and crystalline components, differing drastically in tl ir molecular dynamics. In that respect thermotropic main chain polymers [183,184] resemble ordinary polymers, which exhibit amorphous and crystalline phases [40-42,176,185-187]. However, a broad distribution of correlation times is generally evaluated in all but the lowest molecular weight systems. In contrast, molecular reorientation in the LCPs 4 above Tg appears to occur essentially by single processes, where any distribution of correlation times must be restricted to less than one decade. This is particularly obvious from Fig. 21, which depicts a sharp Tf minimum, characteristic of a single proce. It should be noted, however, that a distribution of correlation times of 2.5 decades is evaluated for the ring flip motions below Tg [182]. 

In addition to its hquid crystalline phase behavior, graphene has been demonstrated as a transparent electrode in LCD devices and a potential substitute for metal oxide electrodes [132]. It has been found that the electrooptical characteristics of such devices are superior in nature. In a similar fashion GO has been utilized in its reduced form in LC cells to study the field-induced reorientation of a nematic LC [133, 134]. These preliminary results are very encouraging for future LC devices. The advantages of graphene compared to conventionally used metal oxide electrodes in terms of low resistivity, high transparency and chemical stability hold great promise for large scale exploitation as transparent conductive electrodes. 

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