Interlayer interactions and the dependence of biaxiality of the chiral smectic-C*phase on electric field in the helical unwinding process

Abstract

We investigate the unwinding process of pure and racemized mixtures of ͑1-methylheptyloxycarbonyl͒ phenyl-4Ј-carboctyloxy-biphenyl-4-carboxylate ͑MHPOOCBC͒ in homeotropic cells by studying ͑i͒ the optical texture under crossed polarizers and ͑ii͒ the tilted conoscopy as a function of the electric field for different temperatures. The tilted conoscopy yields biaxiality and the tilt angle for values of the latter up to 45°. We find that the unwinding process depends on the optical purity of the sample and the temperature. A cell with pure MHPOOCBC surprisingly exhibits a large positive biaxiality in the initial stages of the unwinding process. The magnitude of the positive biaxiality decreases with a decrease in the enantiomeric excess ͑EE͒ of the compound and eventually it becomes negative for a material having a low EE value. The negative biaxiality observed in a mixture with a low EE value is normal and can be explained by a double sine-Gordon equation proposed by Meyer. Microscopic observations show that the special unwinding behavior ͑positive biaxiality͒ is related to the helical fractures, that is, to the existence of discretely coiled helical structures. We show that these helical fractures arise from the existence of a narrow energy well in the synclinic position. This implies that the elastic constant near the synclinic position is significantly larger than in other positions. By adopting a semiempirical equation for the interlayer interactions which replaces the conventional equation, we successfully simulate the various unwinding behaviors. We reasonably conclude that the special interlayer interaction is due to the steric interactions between the two adjacent surfaces formed by somewhat tilted terminal parts of the molecules.

Mechanism of field-induced unwinding of Sm C * helix and bias field dependencies of dielectric permittivity and effective polarization

“…However, the mechanism for the occurrence of the helical fracture is not clear, and it is difficult to estimate the electric field required to generate helical fractures at this state. As reported in our previous paper [9], we observed that the electric field, at which the helical fracture occurs, varies according to the enantiomeric excess ratio in the racemized mixtures. This implies that the material parameters such as the spontaneous polarization, pitch and the interlayer interactions might affect the emergence of the helical fracture.…”

“…The electric field for the emergence of the domain walls corresponds exactly to the step-like increase of the selective reflection peak. The domain walls have single sharp lines and propagate independently of each other indicating that these are π walls propagating along layers [9]. π walls always accompany with the helical fractures as shown in fig.…”

“…Moreover, the mechanism of the emergence of the fractures is not clear at this stage, though a possible model was suggested in our previous paper based on a special steric energy [9]. However, a break in the helix gives rise to an additional flexibility to explain various physical properties occurring during the unwinding.…”

“…However, the fixed pitch model implies that E c may not be a distinct field since the physical parameters reach saturated values gradually. Electro-optical measurements for the unwinding process support it [9]. In the future, we need to extend this model to include the non-homogeneous domain wall propagation.…”

“…This is called the diverging pitch model and is based on the continuum approximation, where the elastic energy is proportional to the square of the deformation of directors, hence the deformation by an (a) E-mail: jvij@tcd.ie external force should be spatially continuous for minimizing the elastic energy. However, we showed recently that a number of helical fractures are produced by a relatively low field in some FLCs [9]. This is to state that the continuity of the helix can easily be broken, and a number of helical fractures occur.…”