Shear, director orientations structure/orientation

Muller e.a. [116] observed that the orientation of transient band structures upon shearing of an oriented monodomain was perpendicular to initial monodomain director, and independent of the direction of shear They did not determine whether the orientation of band structures formed upon cessation of shear was perpendicular to the initial monodomain director or to the shearing direction. We suspect that it will prove to be the latter. 

Smectic elastomers, due to their layered structure, exhibit distinct anisotropic mechanical properties and mechanical deformation processes that are parallel or perpendicular to the normal orientation of the smectic layer. Such elastomers are important due to their optical and ferroelectric properties. Networks with a macroscopic uniformly ordered direction and a conical distribution of the smectic layer normal with respect to the normal smetic direction are mechanically deformed by uniaxial and shear deformations. Under uniaxial deformations two processes were observed [53] parallel to the direction of the mechanical field directly couples to the smectic tilt angle and perpendicular to the director while a reorientation process takes place. This process is reversible for shear deformation perpendicular and irreversible by applying the shear force parallel to the smetic direction. This is illustrated in Fig. 2.14. 

The interpretation of the double-layer structure of the electrospxm APC fibers is based on the fact that APC solutions subjected to shear show a transition from cholesteric to nematic [104]. Calculation of the Reynolds number shows that the flow inside the needle is laminar. The velocity profile is parabolic and, since the solution becomes nematic, the director should align parallel to walls in the center of the flow, away from the walls of needle, there is some reorientation due to the flow and the director makes an angle with the flow direction. As the solution exits the needle, a rapid evaporation of the solvent freezes the orientation of the director in the different areas of the cross-section of the fiber. 

The microstructure of an ER system definitely determines how this system behaves under an electric field. Figure 22 shows the shear stress of oclylcynaobiphenyl vs. the electric field at shear rate 329.5 s " and various temperatures. As indicated in the literature [70-73], Oclylcynaobiphenyl is a liquid crystal material, and has a phase transition from the smectic to the nematic phase at 306.72 K and from the nematic to the isotropic phase at 313.95 K. With the increase of temperature from 306.6 K to 312.8 K, oclylcynaobiphenyl may have the different structures marked as a to b [73]. The ER property of oclylcynaobiphenyl should depend on how the director is orientated in the fields, fhe shear stress passes through a maximum value when the liquid crystal material is in the smectic phase state. Once the material is in tlie nematic phase state, the ER effect becomes weak and saturates at the electric field strength above 0.7 kV/mm. 

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