Main chain PLCs

Among the several hundreds of PLCs reported in papers and patents, thermotropic longitudinal PLCs [1] comprise a very important category. Longitudinal PLCs, formerly called main-chain PLCs, are those whose backbones consist of rod-like mesogens connected parallel to the backbone. Most thermotropic longitudinal PLCs are aromatic LC... 

The acoustic absorption can also be obtained from ultrasonic measurements. However, no work has been done on longitudinal or other main chain PLCs. There is only one study [17] of the ultrasonic absorption of a comb PLC, the result of which will be discussed briefly. 

To attain a deeper understanding of the elastic properties of thermotropic main chain PLCs, we briefly discuss the chemical composition and physical structure of some commercially available PLCs (Vectra A950, Vectra B950, Vectra B900 and HBA/IA/HQ) for which ultrasonic modulus data are available. Vectra A950 is a copolyester consisting of 73 mol% p-hydroxybenzoic acid (HBA) and 27 mol% 2,6-hydroxynaph-... 

Unlike main chain PLCs, the mesogenic units in side chain PLCs are located not in the polymer backbone but in the side chains. The dynamics of a side chain PLC is therefore determined by the behavior of both the backbone and the mesogenic groups as well as the coupling between them. Neutron scattering studies [43,44] reveal that the polymer... 

Fig. 5.20. Some simple topologies of polymer liquid crystals (PLCs) derived from calamltic and discotic mesogenic cores main-chain PLCs, copolymers with core and flexible spacer alternating side-chain PLCs with mesogenic cores attached by flexible spacers to the main chain of a conventional polymer and dendritic (or star-shaped) stmctures with mesogens emanating from a central core via flexible spacers.

Thermotropic PLC s evolved in academic research laboratories by incorporating known monomeric liquid crystals into polymer chains. From such studies two types of PLC s have been developed 1) side-chain polymers with variable flexibility in the main chain, and 2) semi-flexible linear polymers. In the former the monomeric mesogen appears as a pendant sidechain attached to the main chain by a flex-... 

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. 

Rigid and semi-rigid main chain systems, historically the first synthetic PLCs studied, are treated with emphasis on structural, morphological, and mechanical properties. More recent flexible main chain systems are discussed, focusing mainly on structure-property relationships. 

There are many other examples of the dramatic effects that small groups and their positions along the main chain of an MLC can have on phase behavior. Demonstrations of both induction and destruction of liquid crystallinity have been documented when even small groups like methyl are added [148-150] they can affect packing arrangements by influencing conformations of single molecules and electrostatic interactions between molecules. Unfortunately, the manifestations of these perturbations on MLCs and PLCs are not easily related in many cases. 

Some studies of longitudinal ion-containing PLCs involve complexed poly(ethylene oxide) backbone segments [55,56] and ionic groups in the main chain [57-59]. The PLCs in references 57-59, in particular, will... 

Ringsdorf and coworkers [71] have shown that it is possible to induce liquid crystalline phases, namely discotic-columnar mesophases, by doping amorphous polymers containing disk-shaped electron donors, in either the side chain or the main chain, with a low molar mass electron acceptor, as shown in Figure 3.31. The resulting complexes can be considered as diskcomb PLCs and disk PLCs, respectively [5]. The electron-rich moiety is a triphenylene unit and the electron acceptors are fluorenone derivatives. When 20-25 mol% of 2,4, 7-trinitrofluorenone (TNF) is added to the side chain polymethacrylate or polyacrylate... 

Complexes of polymeric acceptors and monomeric donors are also effective in inducing discotic PLCs. This was shown, in particular, for a main chain polyester into which TNF was incorporated, mixed with a triphenylene derivative [73]. In order to avoid phase separation, the donor concentration must exceed the acceptor concentration by a mole ratio of at least 3 1. It was also demonstrated that the alkyl spacer length in the main chain can be critical to the definition and stability of the discotic mesophase obtained. As illustrated in Figure 3.32, spacer lengths which are either too short or too long can inhibit the incorporation of the polymeric acceptor into the donor columns, the... 

In the following, attention is focused only on some results obtained for solutions of thermotropic PLCs, and particularly on those of side group polymer liquid crystals (SGPLCs) since main chain polymer liquid crystals (MCPLCs) are normally difficult to dissolve and at least are insoluble in non-protonated solvents. The inclusion of lyotropic systems would be beyond the scope of this chapter and also would give no contribution to solving the question of how the properties of solutions of PLCs differ from those of non-LC polymers with similar molecular design. 

There have been very few studies of the acoustic absorption of PLCs, probably because of the difficulty in interpreting the data. To date, absorption measurements have been carried out mainly to reveal the relaxation peak associated with the glass transition in side chain PLCs. 

We recall that the first liquid crystals, which were monomer LCs (MLCs), were discovered by Friedrich Reinitzer in 1888 [23] while the first book about them appeared 20 years later [24]. While much has been done in the last century, both MLCs and PLCs remain fascinating object to study and to apply. We have seen several results of MD simulations of PLCs. A tacit assumption was made above all along the simulated PLCs were all longitudinal, that is with LC sequences in the main chain and oriented along the chain backbone. Various other classes of PLCs have been synthesized by chemists [18,25,26]. These include orthogonal, with the LC sequences also in the backbone but perpendicular to it. There exist as well various kinds of combs, and even polymers with three-dimensional LC units. MD simulations should be able to elucidate mechanical behavior of such PLCs as well. This is particularly noteworthy since the promise made in section 15.1 to provide simulation results unobtainable from experiments appears to have been kept. 

While numerous examples of differences between PLC classes shown in Figure 1 can be found in the literature, we shall quote here only one but an instructive example. When we have a melt of a longitudinal polymer, class, in a LC state, the phase flows easily, hence the viscosity is low. Heating such a material above its clearing temperature (= the transition from an LC to the isotropic liquid phase) results in a viscosity increase. Thus, in contrast to decent liquids to which we are used, here a temperature increase does not lower the viscosity. However, if we now switch to the second class, or orthogonal polymers, the above statements do not apply. PLCs do not flow easily as LC phases, nor does heating above the clearing temperature result in a viscosity increase. This example illustrates also the fact how inadequate was the old classification of PLCs into main-chain and side-chain ones. 

The sequences in PLC chains which cause the LC character can be of different shapes elongated (represented in the following by rectangles), approximately spherical (which will be represented by discs), or stars. The LC sequences can be placed in the main chain, or in side chains, or in both. To survey existing and possible structures, a comprehensive classification of PLCs based on their molecular structures was developed [25] and subsequently amplified [9,27] a recent version is shown in Table 41.1. 

Before the classification in [25] was proposed, one talked about main-chain and side-chain PLCs. It is clear from looking at classes such as a, p, -y, and that the name main-chain is far from sufficient, since it includes... 

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