Linking mesogenic properties

In comparison to a conventional polymer and l.c. mentioned above, we will now discuss the PVT behavior of a l.c. side chain polymer, which has linked mesogenic moieties as side chains, and is very similar to the previous monomer. The experimental results are shown in Fig. 5. It is obvious, that the phase behavior of the l.c. polymer differs from that of a 1-l.c. and amorphous polymer. At high temperature we observe a transformation from the isotropic polymer melt into the l.c. phase, indicated by the jump in the V(T) curve. At low temperatures no crystallisation is observed but the bend in the curves signifies a glass transition. Obviously the phase behaviour is determined by the combination of l.c. and polymer properties. 

A very recent synthesis of a thermotropic copolyimide contains an AT-(car-boxyphenyl) trimellitimide ester unit linked to a p-aminobenzoic acid via an amide linkage, which in turn is linked by an alkane diol unit back to the trimellitimide unit [86]. This is probably the first example of a copoly(ester-amide-im-ide) with mesogenic properties identified. Its MI score is 9.6. 

Overall, p-carborane interacts with the linking group L in a way similar to a saturated system such as cyclohexane. As a consequence, the choice of the linking group L has a relatively small impact on mesogenic properties of the compound. However, the bulk of the /7-carborane diminishes the effectiveness of the carboxamide group in stabilization of the mesophases by obstructing the formation of the intermolecular H-bonds. 

Nagamine, T. Januszko, A. Ohta, K. Kaszynski, P. Endo, Y. The effect of the linking group on mesogenic properties of three-ring derivatives of p-carborane and biphenyl, Liq. Cryst. 2008, 35, 865-884. 

The realization of nematic side chain polymers implies the possibility of the existence of cholesteric side chain polymers, presuming the mesogenic molecules, which are linked to the backbone, are chiral. For these polymers it is of interest, whether the polymer fixation influences the helical twist and therefore the optical properties of the cholesteric phase. This will be discussed in 2.3.2.2. 

With these three different examples it has been demonstrated that the systematics observed for the polymorphism of m-l.c. s is also valid for the side chain polymers, provided that a flexible spacer connects the rigid mesogenic moieties to the polymer main chain. Deviations from this behavior are observed, when the mesogenic moieties are directly linked to the backbone. Under these conditions, normally no liquid crystalline behavior is to be expected, according to the model considerations mentioned in Chap. 2.1. Some examples, however, proved l.c. properties for such systems, which are characterized by two striking properties Very high glass transition temperatures and only smectic structures even in case of short substituents... 

Dynamic mechanical experiments, where the material is periodically strained, are common methods to characterize the visco-elastic behavior of elastomers by measuring the storage modulus G and loss modulus G". G is a measure for the maximal, reversibly stored energy for a periodical deformation and G" is proportional to the dissipated energy for the oscillation cycle. It is obvious to investigate, whether the l.c. state of the l.c. elastomers influences the dynamic mechanical properties and whether different modes of linking the mesogenic moieties to the backbone can be detected. 

The concept of local perturbations of the director around nanoparticles, often linked to homeotropic anchoring to the nanoparticle surface, is a concept often brought forward in discussions of thermal, optical and electro-optic properties of nanoparticle-doped nematic liquid crystals, which adds a slightly different perspective to the invisibility of smaller particles in aligned nematics. This appears to be of particular relevance for particles coated with either hydrocarbon chains or pro-mesogenic as well as mesogenic units. 

Chien and Cada [42] have prepared optically active and photoactive SCLC copolymers, 15, with the 4-alkoxyphenyl-4 -alkoxycinnamate chromophore, with the intention of creating LC polysiloxane networks that could be used to prepare macroscopically oriented organic ferroelectric polymers for electro-optical devices. Optical activity was introduced into the polymer by the use of a chiral spacer. Those copolymers which were mesogenic exhibited properties characteristic of a Sc. phase. UV-irradiation of thin films of the polymers in their mesomorphic states at 90°C, led to a loss of the IR absorption at 1635 cm-1 that is due to the cinnamate double bond, and to cross-linking. Long-term irradiation led to... 

In another approach crystallizable side groups like TCNQ and phtha-locyanine were attached to the polymer chain, as well as mesogenic groups leading to liquid crystalline properties. Also side groups with transition metals have been linked in view of macromolecular catalysts, electronically conducting properties, etc. All these functional polymers are at an exploratory stage in laboratories. 

Complementary dendritic hexamers based on a central scaffold made up of linked pentaerythritol and tri(hydroxymethyl)amino methane units have been found to also exhibit liquid-crystalline properties. This star-shaped scaffold was used to create supermolecules containing two different hemispheres, referred to thereafter as Janus supermolecular Hquid crystals (Fig. 76) [323,324]. One of the hemisphere contains three cyanobiphenyl end-groups, whereas the other lobe consists of three chiral phenyl benzoate mesogenic moieties laterally attached. The type of mesophase observed (N ... 

Another type of unsaturated mesogenic structure, which is based on a structure in between an azo- and vinylene-type, is that in which a Schiff base unit links two phenylene groups. This unit is similar in geometry to the three previous mesogenic units, the azo, azoxy and stilbene groups The only reported example of this structure, however, was Polymer 8 with a decamethylene spacer, and it was quite similar in properties to the polymers just discussed. It also possessed a smectic phase, and the mesogenic unit had terminal carboxy groups. 

In addition to number density and orientation of attachment, the degree to which the mesogenic units are decoupled from the central structure is important. The shorter the linking unit, the more likely the material will act as a single supermolecular entity, whereas the longer the spacer the more likely it is that the properties of the individual mesogenic groups will dominate the overall properties of the material. 

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