Chiral polymeric mesogens

Spontaneous resolved two chiral domains are formed in equal probability. In other words, enantiomeric excess (ee) is zero. We now ask whether ee can be controlled or not. The answer is yes. Several methods used in bent-shaped mesogenic phases will be introduced. The direct method is of course an addition of chiral dopant. Actually this has been shown to be a viable method [6, 61]. Use of chiral surface is also effective [62], By using polyimide with chiral side chains at both substrate surfaces, imbalance of two chiral domains (10% ee) has been achieved. Another method using macroscopic helical structure was demonstrated by Jakli et al. [63]. They used a nonchiral polymer network, which was formed in the N phase. After the polymerization, N compounds were washed out, then bent-core mesogens were introduced. Because of the polymer helical fibers, bent-core mesogen shows a chiral domain. 

The ease of forming the smectic mesophase by this class of side-group type liquid crystalline polymers has rendered a great possibility in synthesizing polymeric chiral smectic materials useful in non-linear optics, transducers, pyroelectric detectors and display devices (Chapter 6). The first polymer forming a chiral smectic-C phase was synthesized by Shibaev et al. (1984). It has a polymethacrylate main chain, a long polymethylene spacer, and a mesogenic unit attached at the end with a chiral moiety (polymer (3.60)). Since then, a lot of polymers with chiral mesophases have been synthesized and studied (Le Barny and Dubois, 1989). 

It is worthy to note at this point that the polymerization of mesogenic monomers with chiral moieties does not necessarily result in polymers with the chiral liquid crystalline phase. For example, Finkelmann (1982) reported that homopolymers of chiral monomers yielded only smectic mesophases. On the other hand, copolymerization of chiral monomers with different spacers or of a chiral monomer and a nematic monomer has been proven effective and convenient for synthesizing chiral liquid crystalline polymers. 

The texture of polymeric chiral liquid crystalline phases. The chiral liquid crystalline phases include the chiral smectics and the chiral nematic or cholesteric phase. Poly(7-benzyl-L-glutamate) and derivatives of cellulose are popular examples of polymers that form a chiral mesophase. Side-chain type copolymers of two chiral monomers with flexible spacers of different, lengths and copolymers of one chiral and the other non-chiral mesogenic monomers may also form a cholesteric phase (Finkelmann et al., 1978 1980). In addition, a polymeric nematic phase may be transformed to a cholesteric phase by dissolving in a chiral compound (Fayolle et al., 1979). The first polymer that formed a chiral smectic C phase was reported by Shibaev et al. (1984). It has the sequence of phase transition of g 20-30 Sc 73-75 Sa 83-85 I with the Sc phase at the lower temperature side of Sa- More examples of Sc polymers are given by Le Barny and Dubois (1989). 

The polymeric complexes derived from 4-nitro- and cyanostilbazoles also show a smectic A phase up to about 200 °C [78a]. Polysiloxane complexes 32 also exhibit thermally stable smectic A or C mesophases [79-81]. These carboxyl-functionalized polymers and stilbazoles are miscible in a whole range of composition and show stable mesomorphic behavior [26, 79]. The introduction of the chiral stilbazole for the formation of a mesogenic complex leads to the induction of ferroelectricity [80]. Polymeric complex 33 exhibits a chiral smectic C phase, while no ferroelectricity is observed for each of single components. The value of spontaneous polarization for 33 x — 0.43, n = 5) is 21.0 nC/cm at 112 °C. The hydrogen bonding between the carboxylic acid and... 

Mesogenic groups can be incorporated into polymeric systems [7]. This results in materials of novel features like main chain systems of extraordinary impact strength, side-chain systems with mesogens which can be switched in their orientation by external electric fields or—if chiral groups are attached to the mesogenic units—ferroelectric liquid crystalline polymers and elastomers. The dynamics of such systems depends in detail on its molecular architecture, i.e. especially the main chain polymer and its stiffness, the spacer molecules... 

In Part I we present a survey of the work done in the preparation and characterization of synthetic and semi-synthetic chiral thermotropic liquid crystal polymers. For convenience, we have grouped the polymeric materials (until now reported) according to the nature of the repeat unit and relevant position of the mesogen, side chain and main chain polymers. In Part II we report on the results obtained in our laboratories on optically active thermotropic polyesters containing mesogenic aromatic dyads or triads based on / -oxybenzoic acid. 

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