Transition between cholesteric states

1 Transition between planar state and focal conic state 

In bistable Ch reflective display applications, it is desirable that the threshold of the transition from the planar state to the focal conic state be high, so that the cholesteric liquid crystal can remain in the planar state and the display does not exhibit flicker under colimin voltage in addressing. 

2 Transition between the fingerprint state and homeotropic state 

We first consider the unwinding of the hehcal structure. As the applied field is increased, the pi-waUs (the narrow regions in which the hquid crystal director rotates by n) are propelled apart horizontally and annihilated at the boundaries far away. In the fingerprint state, the liquid crystal director is given by = sin 6(y), riy = 0,n = cos 9(y). The free energy is given by 

Using the Euler-Lagrange equation to minimize the free energy, we obtain 

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Price and Wendorff31 > and Jabarin and Stein 32) analyzed the solidification of cholesteryl myristate. Under equilibrium conditions it changes at 357.2 K from the isotropic to the cholesteric mesophase and at 352.9 K to the smectic mesophase (see Sect. 5.1.1). At 346.8 K the smectic liquid crystal crystallized to the fully ordered crystal. Dilatometry resulted in Avrami exponents of 2, 2, and 4 for the respective transitions. The cholesteric liquid crystal has a second transition right after the relatively quick formation of a turbid homeotropic state from the isotropic melt. It aggregates without volume change to a spherulitic texture. This process was studied by microscopy32) between 343 and 355.2 K and revealed another nucleation controlled process with an Avrami exponent of 3. 

Also the N-N transition was observed in a compensated system, in which another non-photoresponsive chiral dopant 10 with opposite helicity induction was used to compensate the HTP of the photoresponsive dopant 11 to form a compensated N phase. Reversible phase transition between the compensated N phase and cholesteric phase can be achieved by photo switching the chiral dopant between the two states [49] (Fig. 5.11). 

The bistable states observed in nemato-cholesteric mixtures (Ae > 0) could be divided into two parts [57, 58] transitions between two ordered states of minimum energy separated by an energy barrier [59] and order-disorder transitions with the switched-off state in the form of an almost transparent spiral structure or a scattering structure [60-64]. 

N-N transition can also be achieved by the HTP variation of the photoresponsive chiral dopants in induced CLCs. In these systems, the HTPs of the chiral dopants have to exhibit dramatic variation between its two states. At a certain doping concentration, the isomer with higher HTP can efficiently induce a cholesteric phase, while the isomer with lower HTP only generates an apparent nematic phase. The light-induced isomerization between these two forms can reversibly switch the LC phase between the cholesteric and the nematic. 

Electric fields have been used to switch a cholesteric texture between the planar, the focal conic, and the homeotropic state. The planar and focal conic states persist after the field is turned off, the choice determined by the amplitude and frequency of the applied field and the rate at which it is turned off. These textural transitions were discovered by Heilmeier and Goldmacher [32] and are used today for color display panels that exhibit gray-scale memory [33,34]. 

Under certain conditions, cellulose derivatives possessing the characteristics of cholesteric liquid crystals present cholesteric helical structures dissolution and transition from the cholesteric to the nematic phase [98]. When shear is over, the system is relaxed over a determined time and intense, shifting to a transition state, where the energy of deformation is minimal and the orientation ordering is maintained, causing the appearance of band structures. When the external field is removed, the shear-induced anisotropy is affected by the inevitable relaxation of the macromolecular chains. Structural relaxation after removal of the external field depends on the shear history and relaxation mechanism [99,100]. Moreover, literature suggests a possible competition between the order induced by shear and thermodynamically, and also a correlation between the viscosity peak and the appearance of the anisotropic phase at low shear rates [ 101,102]. 

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