Thermotropic mesophase ferroelectricity

To understand how chirality is expressed, it is important to first describe the different thermotropic mesophase assemblies which can be formed by chiral discotics. Even though expression of chirality has been observed in thermotropic mesophases, the chiral expression occurs in a rather uncontrolled manner, and systems which are suitable for applications, for example, easily switchable columns/ferroelectric discotic liquid crystals, consequently have not yet been developed. Hence, the assembly of discotics in solution has received considerable attention. Supramolecular assemblies of discotic molecules in solution are still in their infancy and have not yet found commercial application, but they are of fundamental importance since they allow a detailed and focused investigation of the specific interactions that are required to express chirality at higher levels of organization. As such, the fundamental knowledge acquired from supramolecular assemblies in solution might formulate the design criteria for thermotropic chiral discotic mesophases and provide the necessary tools for the creation of functional systems. 

Cholesterol itself, not being a mesogen, induces high spontaneous polarization in thermotropic smectic C matrices [52]. Moreover, it can induce ferroelectric properties even in lyotropic mesophases [53, 54]. Other biologically active molecules also induce Pg values [55]. 

Chiral mesophases can be obtained from sugars by several strategies. Many cellulose derivatives show thermotropic and lyotropic cholesteric phases [16]. Peracylated sugars can be used as chiral dopants for discoid nematic phases [17]. Also classical cholesteric and ferroelectric phases can be obtained from carbohydrate-based compounds [18]. In this case, chiral oxa-heterocycles are prepared from sugars. Figure 4.8 shows a chiral twin compound prepared from mannitol [19]. 

Up to now the most extensively investigated chiral lyotropics are the chiral nematics (the historical but not appropriate term cholesteric is avoided in this chapter). The first evidence on this kind of mesophase has been reported by Radley and Saupe in 1978 [9]. Two studies on a lyotropic phase proposed to be analogous to the thermotropic Blue Phase have been published [10], one has been retracted [11] if there is any analogy, it is small. Blinov et al. reported on a nonaqueous chiral lamellar phase with ferroelectric properties experimental evidence for ferroelectricity in lyotropics is difficult to gain because of high electric conductivity and mostly nonuniform sample orientation. Nevertheless, the existence of piezoelectricity was interpreted as a manifestation of ferroelectricity [12]. 

Liquid crystals are wonderful materials. In addition to the solid crystalline and liquid phases, liquid crystals exhibit intermediate phases where they flow like liquids, yet possess some physical properties characteristic of crystals. Materials that exhibit such unusual phases are often called mesogerrs (i.e., they are mesogenic), and the various phases in which they could exist are termed mesophases. The well-known and widely studied ones are thermotropics, polymeries, and lyotrop-ics. As a function of temperatirre, or depending on the corrstituerrts, concentration, substituents, and so on, these liqirid crystals exist in many so-called mesophases— nematic, cholesteric, smectic, and ferroelectric. To understand the physical and optical properties of these materials, we will begin by looking into their constituent molecules. ... 

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