Alignment layer symmetry

Monoclinic and triclinic crystals have the lowest number of symmetry elements, and the smectic C phase also has a monoclinic symmetry. Therefore, the symmetry of alignment layers with monoclinic and triclinic crystal structures resembles the symmetry of the smectic C phase. It is plausible to think that the epitaxial growth of a smectic C liquid crystal proceeds most smoothly on alignment layers which resemble their crystallographic structures [12,13]. 

To obtain good initial alignment, varieties of polyimide materials for the alignment layer were investigated. The symmetry of the polymer molecules is critically important. In Table 6.2.1 [11], the molecular structures of polyimide materials, the evaluation of the alignment state and the contrast ratios of AFLCDs using the polyimides are shown. 

Because of the analytical complications involving the stiffnesses Ai6, A26, D g, and D26, a laminate is sometimes desired that does not have these stiffnesses. Laminates can be made with orthotropic layers that have principal material directions aligned with the laminate axes. If the thicknesses, locations, and material properties of the laminae are symmetric about the middle surface of the laminate, there is no coupling between bending and extension. A general example is shown in Table 4-2. Note that the material property symmetry requires equal [Q j], of the two layers that are placed at the same distance above and below the middle surface. Thus, both the orthotropic material properties, [Qjjlk. of the layers and the angle of the principal material directions to the laminate axes (i.e., the orientation of each layer) must be identical. 

We start with the ground state (°), fi(° defined by the simple shear flow y(°), Fig. 17. The principal effect is, as expected, the appearance of a small tilt of the director from the layer normal (flow alignment), predominantly in z direction (Fig. 18). Note that the configuration of layers is also modified by the shear (Figs. 19 and 20), i.e., the cylindrical symmetry is lost. This is analogous to the shear-flow-induced undulation instability of planar layers (wave vector of undulations in the... 

---