Core nematics

Sathyanarayana P, Mathew M, Sastry VSS, Kundu B, Le KV, Takezoe H, Dhara S (2010) Splay bend elasticity of a bent-core nematic liquid crystal. Phys Rev E (Rapid) 81 010702(R)-... 

Taushanoff S, Le KV, Williams J, Twieg RJ, Sadashiva BK, Takezoe H, Jakli A (2010) Stable amorphous blue phase of bent-core nematic liquid crystals doped with a chiral material. J Mater Chem 20 5893-5898... 

J. Harden, B. Mbanga, N. Eber, K. Eodor-Csorba, S. Sprunt, J.T. Gleeson and A. Jakli, Giant flexoelectricity of bent-core nematic liquid crystals, Phys. Rev. Lett. 97(15), 157802/1-4, (2006). doi 10.1103/PhysRevLett.97.157802... 

Giant flexoelectricity of bent-core nematics studied by the flexing... 

Instead of discussing these we have only noted those layered structures which are relevant for understanding the properties of bent-core nematics and especially the flexoelectricity of these materials. 

As described above, the kink in the molecular shape and the requirement to fill the space as effectively as possible are not compatible with a three-dimensional fluid order. In other words, when translating a bent-core molecule in the melt of the neighbouring bent-core molecules, it experiences a periodic potential with its periodicity determined by the length I of the molecules. To allow for fluidity at the macroscopic level, one needs to frustrate the bent-core structure so that they do not lock into smectic layers easily. Such a frustration can be introduced by some steric or electrostatic disturbance of the bare bent-core (or peeled banana ) shape, which has been seen in some modulated smectic phases.As discussed by Bailey and Jdkli,a steric or electrostatic inclusion in the core of the molecules leads to layer modulation, an SmCc structure and broken smectic layers that effectively correspond to a columnar phase, as the inclusions increase. Following this picture, here we postulate that bent-core nematics are probably more frustrated than the electrically unswitchable B7 (columnar) phases, in which the broken smectic ribbons are separated by melted fluid nematic regions. Such over-frustrated B7 materials are characterized... 

In the following sections of this chapter we will summarize the direct, as well as the converse, flexoelectric measurements in fluid and elastomeric (dry or swollen) bent-core nematic liquid crystals, and try to explain these observations using the structural model outlined above. 

The direct method introduced in the previous section was first employed for studying the flexoelectric response of a bent-core nematic liquid crystal. 

ClPbislOBB is not the only bent-core nematic that has a giant 63 flexo-coefficient for a few other BC compounds (whose chemical structures and phase sequences are shown in Fig. 3.11) and mixtures a similar magnitude of jesj has been obtained (see Table 3.1). 

The presence of clusters in BC nematics is now well established from various measmements. Recent studies " have in fact indicated a ferroelectric or an antiferroelectric response to an applied electric field, and an unusual low-frequency (presumably collective) mode has been detected in the dielectric spectra of bent-core nematics, which might also be related to clusters. In spite of the intense studies, however, the exact structure and the physical properties of the clusters are still unknown. Therefore, not surprisingly, a precise physical model for the role of polar clusters in the flexoelectric response of BC nematics and a quantitative estimation of the resulting increment of the flexocoefiicients has not yet been worked out. 

Summarizing, experimental observations suggest that the giant (direct or converse) flexoelectricity of bent-core nematics is related to the polar smectic clusters occurring in them. In order to explore the exact mechanism for how clusters contribute to the flexoelectric response, further experimental and theoretical studies are needed. 

Finally, it is worth mentioning that a phenomenon analogous to the difference between the normal and giant flexoelectricity of calamitic and bent-core nematics, respectively, exists in crystals, ceramics and polymers too. The flexoelectric response (defined in Eq. (3.1)) of perovskite-type ferroelectrics, " of relaxor ferroelectric ceramics and polyvinylidene fluoride (PVDF) films are four orders of magnitude larger than the flexoelectricity of dielectric crystals. In those sohd ferroelectric materials the polarization induced by flexing is evidently of piezoelectric origin. 

The frequency dependences of the bend fiexoelectric coefficients were also measured for the same BC nematic fluid monomer, BC nematic swollen in a calamitic liquid crystal elastomer (BCN-LCE) and for the bent-core nematic elastomer (BCLCE) as shown in Fig. 3.14. One can see that for each material the fiexoelectric effect was found to be zero below 1 Hz, then the response increases abruptly up to 2 Hz and then decreases slightly. The apparent absence of the response below 1 Hz is probably due to screening by free ions. The slow decrease of the fiexoelectric coefficient at higher / is not yet clear. We assume, however, that it is not a measurement error, because 5CB showed a constant value in this frequency range. 

S.H. Hong, R. Verduzco, J.C. Williams, R.J. Twieg, E. DiMasi, R. Pindak, A. Jakli, J.T. Gleeson and S. Sprunt, Short-range smectic order in bent-core nematic liquid crystals. Soft Matter 6(19), 4819-4827, (2010). 

J. Harden, R. Teeling, J.T. Gleeson, S. Sprunt and A. Jakli, Converse flexo-electric effect in a bent-core nematic Uquid crystal, Phys. Rev. E 78(3), 031702/1-5, (2008). doi 10.1103/PhysRevE.78.031702... 

J. Harden, M. Chambers, R. Verduzco, P. Luchette, J. Gleeson and S. Sprunt, Giant flexoelectricity in bent-core nematic liquid crystal elastomers, Appl. Phys. Lett. 96(10), 102907/1-3, (2010). doi 10.1063/1.3358391... 

Non-standard EC has also been observed in bent-core nematics, where < 0, Ca < 0 is also realized and where flexoelectricity is strong. There is still no detailed theoretical description, which would have to include the strong frequency dependence of as well as the unusual viscosity and elastic properties these might indicate smectic cluster formations not only in the nematic but even in the isotropic phase. 

Finally, we mention that the longitudinal rolls observed in a bent-core nematic have been associated by Wiant et with ns-EC patterns. In a recent paper, however, similar patterns were interpreted as fiexodomains and in this way the value of [ei — es 6 pC/m was estimated. This value is of the same order of magnitude as in calamitic nematics. Similarly, for the flexocoefficients of another bent-core nematic a value of 0(10 pC/m) was obtained by stud3ung the instability due to the surface polarization mechanism. ... 

P. Tadapatri, U.S. Hiremath, C.V. Yelamaggad and K.S. Krishnamurthy, Patterned electroconvective states in a bent-core nematic liquid crystal, J. Phys. Chem. B 114(1), 10-21, (2010). doi 10.1021/jp9058802... 

Bent-core liquid crystalline elastomer Bent-core nematic swollen in a liquid crystal elastomer Black lipid membranes Direct current Electroconvection Electrohydrodynamic Ferroelectric liquid crystal Hybrid-aligned nematic Human embryonic kidney Isotropic... 

Harden, J. Mbanga, B. Eber, N. Fodor-Csorba, K. Sprunt, S. Gleeson, J. T Jakli, A. Giant flexoelectricity of bent-core nematic hquid crystals. Phys. Rev. Lett. 2006, 97, 157802. 

Fig. 14 Ferroelectricity in LCs. (a) In the smectic-C phase an electric field (E) can reverse the tilt and thereby the direction of the polar axis perpendicular to the tilt direction, (b) Application of an electric field to a chiral smectic-A phase induces a tilt of the molecules and thereby a polar structure, (c) In an aligned bent-core nematic phase a dipole is present in the direction of the kink. [Ill, 112]...

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