Mesogenic side chain units

As illustrated in Figure 5.2, SCLCPs can also be generated by using a disc-like mesogenic side chain unit. The triphettylene imit is commotdy used to generate discotic mesophases, and when incorporated into a poly(siloxane), a discotic mesophase is exhibited (compound 37). 

This can be explained by the fact that in a polymer molecule (Fig. 78b) the longitudinal components of monomer unit dipoles mh are mutually compensated and the main part in the observed EB is played by normal components of monomer unit dipoles, mi, which can be parallel to the main chain of the macromolecule owing to its comb-like structure. In other words, in molecules of comb-like polymers containing mesogenic side chains, the orientations of the Mi components of the side group dipoles are correlated with each other. As a result, the macromolecule as a whole or part of it can exhibit a considerable dipole moment m in the direction of the main chain L (Fig, 78b). The existence of this dipole accounts for the orientation of the main chain in the field direction leading to negative EB. 

The addition of dimethylsiloxane units (dilution) leads to a further swelling of the polysiloxane sublayer without disturbing the preferential interactions between the mesogenic side chains too much, thus retaining the mesomorphic properties. 

In polymers 1 and 6, the mesogenic group is separated from the main chain by a flexible polymethylene spacer. For these polymers, the optical anisotropy and the Kerr constant are lower by one order of magnitude than for polymers with a similar structure of the mesogenic side chain but without a flexible unit. 

In addition to these main-chain liquid crystalline polymers, there are also side-chain liquid crystalline polymers, where the liquid crystalline nature arises from the presence of rigid straight side-chain units (called the mesogens) chemically linked to an existing polymer backbone either directly or via flexible spacer units. 

Fig. 1 Different attachmcait geometries for the synthesis of LCEs side chain elastomers with end-on (a) or side-on (b) attached mesogenic side chains and main chain elastomers with mesogenic units incorporated end-on (c) or side-on (d) into the polymer main chain...

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. 

It is well known for ordinary polyacrylics that the chain stiffness can be modified significantly by varying the alpha group. The influence of the flexibility of the main chain on the dynamics of the mesogen side chain was studied, on the one hand, by comparing the -process for acrylate, methacrylate and chloroacrylate main-chains with the same spacer (n = 6) and very similar side groups, and on the other by varying the amount of methacrylate units on the PA-coPMA/6/ COO/CN... 

The steric frustrations have also been detected in LC polymers [66-68]. For example, the smectic A phase with a local two-dimensional lattice was found by Endres et al. [67] for combined main chain/side chain polymers containing no terminal dipoles, but with repeating units of laterally branched mesogens. A frustrated bilayer smectic phase was observed by Watanabe et al. [68] in main-chain polymers with two odd numbered spacers sufficiently differing in their length (Fig. 7). 

In general terms, high molar mass liquid crystals are classified according to the location of the mesogenic unit in the polymer. Thus, they are either incorporated into the main chain (main-chain liquid crystal polymers - MCLCP Fig. 5A) or they are pendant from the main chain (side-chain liquid crystal polymers - SCLCP Fig. 5B). 

In this category, the units giving rise to the liquid-crystalline behavior can be in the backbone (as already discussed), in the side chains, or in both [172], Of considerable interest is the orientation of the mesogenic groups and the chain backbones to which they are attached [173,174], Frequently-studied backbones include siloxanes and acrylates [175,176], but a variety of other structures have also been studied, including amphiphilics [177]. 

Polymer containing mesogenic units in their main chains but not in side-chains. 

Polymer, the molecules of which have mesogenic units only in the side-groups or side-chains. 

Note 2 The structures as in Fig. 34 can also be used with the proviso that the side-group units are replaced by chains containing mesogens. 

In addition to the mesogen units contained within the polymer backbone, they can occur as side chains. These mesogen units can be introduced either through reaction with monomers that contain the mesogen unit or through introduction with already formed polymers. 

We conclude that the existence of side chain smectic LC-polymers is basically dependent on the length of the spacer. This length must be sufficiently large to allow rotation of segments in the mesogenic units as well as to allow a perfect separation of these units from the incompatible spacer-chain-complex. This hypothesis is qualitatively justified by finding analogous characteristics of WAXS-patterns as shown in this paper. 

It was also shown in 1975 that incorporation of rigid-rod and mesogenic units into the side-chain of acrylate or methacrylate polymers could lead to thermotropic products21). The properties of these polymers, which are not further considered in this review, differ in a number of respects from those of polymers whose thermotropic behaviour depends upon the constitution of their main chain. 

Considerations by V. Shibaev and N. Plate (see this issue) led to a similar conclusion. Investigations on comb like polymers26), where each monomer unit of the macromolecule carried a non-branched alkyl chain of m methylene groups, have shown that for m > 8 side chain crystallization takes place independently to the main chain conformation. Consequently, if mesogenic molecules are linked to the side chains, they should occupy a l.c. order without influence of the backbone conformation. Following these considerations, alkyl chain lengths m > 8 are necessary for the formation of the l.c. order. As shown later, however, nearly only smectic polymers are possible under these conditions (see Chap. 2.3.3.). 

A broad variety of l.c. polymers is conceivable because of the wide range of well known mesogenic molecules, e.g. tabulated in the book of Dcmus27), and the different types of polymers. Further variations are possible by copolymers or systems, where each monomer unit carries more than one mesogenic moiety ( en bloc systems28)). Furthermore the synthesis of linear, branched and crosslinked systems has to be mentioned. Because of this broad variety a manifold influence on the phase behavior of the systems via the chemical constitution is feasible. In the following chapter we will discuss some basic considerations on the phase behavior of l.c.-side chain polymers. 

---