Clearing process, nematics

Among the instabilities shown there are two isotropic modes. One of these is well known from experiments on isotropic liquids and occurs at very low frequencies due to some injection processes at electrodes (injection mode). The other is also observable in ordinary liquids (such as silicon oil), but is seen particularly clearly in nematics, due to their optical anisotropy. The latter mode occurs over a wide frequency range due to ion drift to the electrodes as in electrolysis (electrolytic mode). Both isotropic... 

Only after heating for 15 min does the crystallinity disappear after quenching (see Fig. 27c). This experiment suggests that diffusion processes in the melt are very slow and that there is a strong memory which persists well into the nematic state. In fact it is not clear that the primary mechanism acting to eliminate the memory is diffusion or interchain transesterification or possibly both. At this point it is safe to argue that both diffusion and interchain transesterification reactions are operative over the 15 min time span at 35 °C above Tcn. 

The nature of the two endothermal processes has been revealed by study with POM that at 434 °K is the melting of the crystals and the formation of a nematic phase, while at 467 °K is the isotropization of the mesophase. If the polymer is cooled down from its isotropic liquid state the curve (B) is obtained. In the cooling process the first exothermal peak occurs at 463 °K which is the formation of the nematic phase as revealed by POM. The second exothermal peak is centered at 387 °K corresponding to the crystallization of nematic polymer. The jump in (B) of the glass transition is not as clear as in the heating curve. This is understandable because the glass transition is not a genuine thermodynamic transition. 

The precursor 6 exhibits the enantiotropic nature of chiral nematic (N ), chiral smectic C (SmC ) and chiral smectic I (SmI ) phases. The shell-printed texture of the SmC phase and the rose-like texture of the SmI phase can be clearly seen in Figure 12.6. The thiophene monomers, M2 and M3, show enantiotropic N, SmA and SmC phases. The SmC phase is characteristic of ferroelectricity. The polymers show various mesophases. The phase transition temperatures are summarized in Table 12.4. PI shows an enantiotropic SmA phase. P2 shows enantiotropic SmA, SmC and SmB phases. The fan-shaped texture of the SmA phase and the striated fan-shaped texture of the SmC phase are shown in Figure 12.7. P3 shows an SmA phase in the heating process and SmA and SmX phases in the cooling process. XRD analysis suggests that the SmX phase of P3 might be a higher order smectic phase. 

We have presented a discussion of the theories and experiments on laser-induced optica nonlinearities and some recently observed wave-mixing processes in nematic liquid crystals based on the phase grating induced by two laser pulses. These studies have demonstrated again the unique and interesting physical characteristics of liquid crystals that have attracted the attention of fundamental and applied researchers alike. It is also clear that some practically useful nonlinear optical devices could be constructed. The nematic phase is but one of the several mesophases of liquid crystal that possess these interesting nonlinearities. Cholesterics and smectics [4] and other hybrid forms of nematics [6] also possess large nonlinearities. We anticipate that many more effects will be observed in the near future. 

In 1990, Janossy showed that a small amount of dye added to a nematic liquid crystal dramatically reduces the threshold intensity of the optical Freedericksz transition [68]. Subsequently, it was demonstrated that the underlying process is an optically driven Brownian ratchet mechanism [69-71]. Here, energy, but not momentum, from the radiation field causes unidirectional continuous rotation of dye molecules in the nematic, exerting a torque on the director that exceeds the direct optical torque by orders of magnitude. Similar mechanisms could, in principle, be realized in LCEs. Whether such processes are viable in overcoming the orienting effect of the network is not clear the viability of such Brownian motor processes in LCEs is an intriguing open problem. 

The most straightforward way to produce partially oriented solute molecules is to orient them in anisotropic solvents. Good solute orientation can be achieved in stretched polymer sheets [94, 95, 96]. Homogeneously oriented nematic liquid crystals are perfectly clear and are thus excellently suited as anisotropic solvents for optical polarization experiments. Moreover, the liquid crystal method allows the performance of polarization experiments in fluid media. This unique feature of the liquid crystal method can be exploited for polarization studies of metastable molecular species (e.g. excited complexes) formed by a diffusion-controlled process. The ordered glasses produced by rapid cooling of a uniformly aligned nematic phase can be used for phosphorescence polarization experiments. 

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