Cholesteric Liquid Crystals have many applications as electro-optic materials in thin film devices in much the same way as nematic liquid crystals do. The presence of a polymer network formed at low polymer concentrations provides similar advantages in enhancing the stability of the structure, aiding in the return of the liquid crystal director orientation to the desired stable configuration, reducing the switching time, and helping to determine and maintain the poly-domain size.
After introducing cholesteric textures and some of their optical properties (in the regime where the pitch of cholesteric helix is in the range of visible wavelengths of light, not the infrared), we will consider some representative devices.
|Just as in the case of the nematic mesophase, cholesterics placed between two parallel surface treated plates form a planar homogeneous texture in the absence of an external electric field. The axis of the helix formed by the director lies normal to the plates, with the director of the molecules adjacent to the plates parallel to the rubbing direction. In the figure, the wedge-shaped symbols represent layers of planes and indicate their average director orientation.|
|If a small electric field is applied
normal to the plates in the example above, the molecules
experience a torque which, as we have seen in the nematic
mesophase, tends to align the director normal to the
plates, eventually producing the homeotropic
texture. However in the cholesteric mesophase, due to
interactions with adjacent molecules and with anchoring
effects near the plates, a focal conic texture is formed,
A random distribution of helical axes is characteristic of the focal conic texture. In this case, incident light is weakly scattered in all directions.
In the planar texture, cholesteric liquid crystals display selective reflection. The reflected intensity plot over the visible spectrum shown below illustrates this, with maximum reflection occurring at a wavelength equal to the pitch, which may be pre-selected by choices of materials. This represents the reflection of one handedness of circularly polarized light due to periodic variations of the index of refraction of the material. The other handedness is transmitted through the cholesteric material.
Please note, the focal conic texture scatters light of all frequencies.
|If the electric field is increased above a threshold value, the helical structure of the focal conic texture is untwisted and the director becomes perpendicular to the plates, forming the homeotropic texture. In this state, shown in the diagram, the material is transparent to incident light.|
Polymer networks are formed in cholesterics during the initial stages of film preparation by combining a small quantity of reactive monomer and photoinitiator with the cholesteric liquid crystal molecules, just as in the case of the nematics. In fact, the process can be identical, except that a small amount of chiral dopant is added to produce the desired cholesteric pitch. After the desired texture is established through the combination of surface preparations and applied field, ultraviolet light is used to photopolymerize the sample. The morphology of the resulting polymer network mimics the textures of ordinary cholesteric mesophases. Such a network is schematically illustrated by the blue lines in the figure below (shown in the focal conic texture). These polymer stabilized cholesteric textures are sometimes referred to by the acronym PSCT.
|In the presence of polymer networks, the liquid crystal material is broken up into small domains referred to as polydomains which do not necessarily coincide with the boundaries between regions of common helicity in the focal conic texture. Even in the planar texture, polydomains have slightly different orientations relative to each other, as noted earlier.|
Studies of the network formed in cholesterics have been reported by several groups including the ALCOM researchers mentioned in the discussion of stabilization of nematics (Rajaram, 1996 and Dierking, 1997). Just as in the case of nematics, after removal of the liquid crystal material, SEM and special optical microscopy have shown a fiber-like anisotropic network whose morphology has been influenced by the presence of the liquid crystal, including evidence of the helical cholesteric texture. In turn, the network influences the structure of the focal conic state and stabilizes initial states. Factors controlling morphology, such as liquid crystal texture, monomer concentration, photopolymerization temperature, UV intensity and exposure time have been explored and discussed in those references.