Nematic monomers

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. 

Figure 3.10. Chemical structures of a side-on nematic monomer (8a) and a striated artificial muscle with a lamellar structure based on a nematic triblock copolymer, RNR (N nematic R rubber). Source Li et al., 2004.

Liquid crystalline textures in polymeric systems can be obtained in two different ways. One way consists in inducing a specific texture in a liquid crystalline monomer phase. Often the forces acting on the sample will be sufficiently large to stabilize the texture during the polymerization. A texture may be obtained in this way even in polymers which do not display liquid crystalline phases. In the case of polymer 3 the polymerization in the nematic monomer phase under the influence of a magnetic field led to a polymer sample with high orientational order on a macroscopic scale. 

A phase separation during polymerization occurred when a nematic monomer was polymerized to a smectic polymer, because the polymer was insoluble in the monomer. 

A homogenous polymerization was achieved by the polymerization of a nematic monomer to a nematic polymer where the temperature region of the liquid crystalline monomer phase overlapped the temperature region of the liquid crystalline polymer phase (see Table II). This polymerization process is influenced either by a precipitation or by a phase transition. 

If the same nematic monomer but various mesogenic cholesterol-containing monomers are used for copolymerization (copolymer series 6-9, 10-13, and 14-16), the optical properties of copolymers are only slightly affected. The dependence of on the content of cholesterol-... 

Figure 5. Chemical structure for the nematic monomer RM257 from EM...

In this line, one can take advantage of such intermolecular interactions to generate elastomeric materials, where no solvent is also present, with low response times. In order to describe properly the influence of the phenolic group in the optomechanical properties of the final photoactuator a co-elastomer, which contains the nematic monomer M40Me (85 % mol), the isotropic cross-linker VI (10 % mol) and the photo-active hydroxyazoderivative AZO-OH (5 % mol), will be considered... 

Lattice models have the advantage that a number of very clever Monte Carlo moves have been developed for lattice polymers, which do not always carry over to continuum models very easily. For example, Nelson et al. use an algorithm which attempts to move vacancies rather than monomers [120], and thus allows one to simulate the dense cores of micelles very efficiently. This concept cannot be applied to off-lattice models in a straightforward way. On the other hand, a number of problems cannot be treated adequately on a lattice, especially those related to molecular orientations and nematic order. For this reason, chain models in continuous space are attracting growing interest. 

However, the hydrogen-bonded mesogens that are of most interest in the context of this article are those elaborated initially by Kato and Frechet in the early 1990s [24-33]. In this approach, a pyridine, which may or may not have liquid crystal properties, was hydrogen bonded with a 4-substituted benzoic acid to form a new species with its own, distinct mesomorphism. For example, complex 9 shows a SmA phase that persists to 238 °C (n = 2, m = 4), while its free component pyridine is nematic to 213 °C the component benzoic acid is also nematic (as the H-bonded dimer) to 147 °C (although note that the notional monomer would not be liquid crystalline). 

To produce novel LC phase behavior and properties, a variety of polymer/LC composites have been developed. These include systems which employ liquid crystal polymers (5), phase separation of LC droplets in polymer dispersed liquid crystals (PDLCs) (4), incorporating both nematic (5,6) and ferroelectric liquid crystals (6-10). Polymer/LC gels have also been studied which are formed by the polymerization of small amounts of monomer solutes in a liquid crystalline solvent (11). The polymer/LC gel systems are of particular interest, rendering bistable chiral nematic devices (12) and polymer stabilized ferroelectric liquid crystals (PSFLCs) (1,13), which combine fast electro-optic response (14) with the increased mechanical stabilization imparted by the polymer (75). 

Figure 1. Chemical structures of the monomers used in this work. Shown are a) p-phenylene diacrylate (PPDA- melting point 89° C), b) 1,6-hexanediol diacrylate (HDDA- melting point 5° C, boiling point 316° C), and c) C6M, a liquid crystalline diacrylate (phase sequence Isotropic - 116° C -> Nematic -> 86° C - Crystalline.)...

Monomers- + Flexible -sheet ribbons Rigid fibrils and fibers Nematic Nematic ... 

One alternative approach is to use photoisomerisable chiral compounds where the E and Z isomers have different helical twisting powers, e.g. menthone derivatives. By incorporating co-polymers, prepared from menthone containing monomers and cyano esters (5.5), as dopants into nematic LC mixtures materials, e.g. a mixture of cyanobiphenyls and cyanoterphenyls (E7 available from Merck), colour change can be effected by irradiating with UV light (365 nm). The colour obtained is dependent... 

Polymer-dispersed liquid crystals (PDLCs) are made up of nematic liquid crystals dispersed in a solid continuous polymer matrix. These are prepared by mixing a reactive monomer into a non-polymerisable LC medium and then polymerising the reactive monomer to create a polymer matrix, at the same time capturing the LCs as dispersed droplets, greater than 1 pm in diameter, i.e. the wavelength of visible light.3 -33... 

Fig. 13. Temperature dependence of the absorption E of the polymer/dye mixture M3 (refer to Table 4) (k = 430 nm, c = 0.5 mol % dye, related to the monomer unit of the polymer, d = 10 pm, E and E are the absorption parallel and perpendicular to the director of the nematic phase,...

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