Mesogenic elements

P.Meurisse, C.Noel, L.Monnerie, and B.Fayolle, Polymers with mesogenic elements and flexible spacers in the main chain Aromatic-aliphatic polyesters, Br.Polym.J. 13 55 (1981). 

A molecular chain of PDES contains no mesogenic elements still two columnar mesophases Xi and X2 exist above the melting point of the crystalline state. In cross-linked PDES elastomers stretching can induce an amorphous-mesophase transition. With increasing stress the transition shifts to higher temperature. Kinetically this transition follows a 2D growth model. 

Thermotropic Polymeric Liquid Crystals (PLCs) which are formed by regularly alternating mesogenic elements and flexible spacer groups in the main chain are currently the focus of intensive investigation The standard meso-gens, such as biphenyl, stilbene, azo or azoxybenzene derivatives, which form the core of low molecular mass liquid crystals (LMLCs), are used in synthesis of PLCs as well. 

This volume of Topics in Stereochemistry could not be complete without hearing about ferroelectric liquid crystals, where chirality is the essential element behind the wide interest in this mesogenic state. In Chapter 8, Walba, a pioneering contributor to this area, provides a historical overview of the earlier key developments in this field and leads us to the discovery of the unique banana phases. This discussion is followed by a view of the most recent results, which involve, among others, the directed design of chiral ferroelectric banana phases, which display spontaneous polar symmetry breaking in a smectic liquid crystal. 

One of the characteristic features of mesogenic molecules in uniaxial mesophases is their orientational order specified by a set of order parameters, which forms a symmetric and traceless order matrix S. For rigid molecules, the elements of order matrix Sa/Y defined by Saupe,52 are given by... 

If the mesogens are pendant to the polymer backbone, materials are obtained with special magnetic, electrical and optical properties. They provide for nonlinear optics (NLOs) applications in numerous optoelectronic elements. 

In a system based largely on self-assembly through mesogenic interactions such as the above example by Percec, it can also be beneficial to incorporate hydrogen bonding as well for an added element of organizational control. For instance, Frechet and coworkers demonstrated sequential, hierarchical self-assembly of various levels of order in a small molecule-based system which ultimately resulted in a hydrogen-bonded liquid crystalline network (Fig. 7.9) [53]. 

There is some doubt about the relevance of aspect ratio considered in isolation and as applied just to the empirically identified rigid mesogen. It can only really be applied to in-chain (or nematic unidirectional phases) but this is not applicable to discotic phases [61]. The examples quoted above also make it clear that flexible elements can be included in the mesogen. Strict definition of the mesogen is therefore difficult. 

Other attempts at synthesizing chiral smectic polymers have reached the conclusion that no general conclusion can be made so far as for what molecular structure will yield the desired phase. Again, all the structural elements including the main chain, the spacer, and the mesogenic unit are playing a part. Nevertheless, a longer spacer and more flexible main chain seem to favor the chiral Sc phase (Le Barny and Dubois, 1989). 

Organometallic liquid crystals of group 7 elements can be divided into three families, namely those based on halotricarbonylrhenium(l) complexes of a-diimines, tetracarbonylrhenium(l) amidines, and ordo-metaUated complexes of imine mesogens. 

A particular feature of the mesogens of group 8 elements is the extensive liquid-crystal chemistry of ferrocene and its derivatives, and as such this subject is given its own subsection below. 

In this way, some nickel and palladium compounds have been synthesized with the intention of obtaining chiral nematic (N ) and smectic (SmC ) phases. The compounds where the chiral element is situated in the chain of the monodentate ligand are not themselves mesogenic. The others are, and two of them have a modest twisting power in Merck MLC-6401 nematic solvent with a right-handed helix. 

In polymer 1, the Kerr effect coincides in sign with the Maxwell effect, which means that the orientation of the side chains in an electric field correlates with the direction of the main chain. However, both the small values of K and [ig / Ql] and the absence of dispersion of the Kerr effect show that the chains of polymers 1 and 6 exhibit no high orientational axial and orientational polar orders characteristic of polymers with mesogenic side groups. In these polymers (1 and 6), the Kerr effect is due to small scale chain motion the kinetic elements are much smaller in size and their mobility is much higher than, for example, for polymer 7. 

A final example of induced mesogenicity in a multicomponent system is the well studied, but less well understood, carbonaceous mesophases which are comprised of a myriad of unidentified molecules which are created in situ as petroleum pitches are heated to temperatures where chemical transformations occur [163]. The processes leading to a mesophase involve decreases in both the elemental weight fraction of hydrogen and the group fraction of aliphatic carbon atoms [164]. Model studies have demonstrated that the component molecules of these phases are fused, polycyclic aromatic molecules with disk-like shapes the exact structures of the components depend upon the natures of the precursor molecules which are heated [164-167]. All of the carbonaceous mesophases somewhat resemble discotic nematic phases [168]. At least some of them probably represent another example of liquid crystallinity induced by mixing molecular components which, when separated, are not mesogenic. 

For side chain PLCs there are two orienting molecular elements which can influence the elastic properties the mesogenic groups and the monomer units of the polymer backbone. The fact that is... 

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