Chevron layer structure

This description of FLC switching behavior is simplified for the sake of clarity. A small minority of FLC materials behave as described these are termed bookshelf materials in the field. For most FLCs, the formation of a chevron layer structure driven by layer shrinkage at the SmA-SmC transition changes the picture in complex ways. A discussion of this issue, which is not a chirality phenomenon, is outside the scope of this chapter. 

The origin of the chevron layer structure is explained by the discrepancy between the layer spacing of the SmC phase and that of the high temperature phase, which is usually the SmA phase. The layer spacing in the SmA phase, dA is fixed at the surfaces, and it decreases to do in the SmC phase because the molecules decline at the tilt angle Q from the smectic layer normal as shown in Fig. 5.1.6. This figure leads to a simple relation,... 

The chevron layer structures of several SSFLCs from various FLC materials exhibiting different optical molecular tilt angles have been precisely investigated, and a correlation between the layer tilt angle of the chevron structure and the optical molecular tilt angle was confirmed. 

The molecular orientational states of the SSFLCs have been analyzed by polarizing microspectroscopy and optical simulation. The X-ray studies indicated that the chevron layer structure is determined by the bulk properties of the FLC, but the molecular orientation in the smectic layer is strongly influenced by the surface properties. The effect of surface pretilt angle on the molecular orientation and the optical properties of SSFLCs have been studied by the optical simulation based on the molecular orientational models. 

Fig. 5.1.14 Cl and C2 states, distinguished by the relationship between the direction of the chevron layer structure and the direction of the surface pretilt, as shown in (a) and (b). The tilting direction of the chevron layer structure is confirmed by the direction of the zigzag defects, as shown in (b). (c) Smectic layer models of the Cl and C2 states.

Figure 5.1.21 shows the coordinate systems of the director in the chevron layer structure, where n is the director, c is the c-director, and p is the spontaneous polarization vector. It is assumed that the tilt angle 9 and the layer tilt angle S are constant, and the azimuthal angle depends only on the cell thickness, direction Y. The director is expressed as... 

Fig. 5.1.27 Summary of the orientational states in SSFLCs with a chevron layer structure.

Fig. 6.1.3 Bookshelf and chevron layer structures, (a) Bookshelf layer structure, (b) Cl orientation of a chevron layer structure, (c) C2 orientation of a chevron layer structure.

The first is AC-field treatment. In this method, an initial chevron layer structure is turned into a bookshelf layer structure by applying a strong low frequency AC field. The FLC material used must possess a high Ps because the interaction between the electric field and Pg induces a torque that changes the layer structure from chevron to bookshelf. 

Chevron layer structure with zigzag defects). 

Fig. 6.1.4 Changes in texture induced by AC-field treatment, (a) Texture I. Initial virgin texture with chevron layer structure and zigzag defects, (b) Textures II. A rooftop texture is observed after the application of an AC field of medium strength, (c) Texture HI. A quasi-bookshelf layer structure with stripe domains is observed after the apphcation of a strong AC field.

In case of strong planar and azimuthal anchoring when the director is fixed at the surfaces, typically a so-called chevron texture forms, where the layers have a kink in the middle of cells. The formation of the chevron layer structure leads to defects, where regions of opposite kink directions meet. 

A.R. MacGregor, A Method for Computing the Optical Properties of a Smectic C Liquid Crystal Cell with a Chevron Layer Structure, J. Mod. Optics 37, 919-935 (1990). 

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