Molecular Shapes of Liquid Crystal Dimers

Fig. 4a,b. The molecular shape of a an even b an odd membered liquid crystal dimer with the spacer in the all-trans conformation... 

During recent decades the molecular theory of flexoelectricity in nematic liquid crystals was developed further by various authors. " In particular, explicit expressions for the flexocoefiicients were obtained using the molecular-field approximation taking into account both steric repulsion and attraction between the molecules of polar shape. The influence of dipole-dipole correlations and molecular flexibility was later considered. Recently flexoelectric coefficients have been calculated numerically using the mean-field theory based on a simple surface intermolecular interaction model. This approach allows us to take into consideration the real molecular shape and to evaluate the flexocoefiicients for mesogenic molecules of different structures including dimers with flexible spacers. 

Consider now a tetramer that has a structure similar to the dimer and trimer, " see Figure 17. The addition of the extra mesogenic unit should have the effect of producing an overall molecular shape that is tubular. Thus, it should be more unlikely that this material will exhibit a syncUnic mesophase over an anticlinic organization. Interestingly, the melting point is lower than either the dimer or the trimer however, the transition temperature to the liquid is very similar. The similarity of the liquid crystal to liquid transition temperatures suggests that the... 

The data presented here have excluded nematic polymer liquid crystals. These present particular difficulties for the conventional methods of measurement, which require well-aligned samples. A magnetic resonance technique has been developed which is particularly suited to the measurement of the elastic and viscous properties of liquid crystalline polyma s, and this technique and specimen results are described in Datarevicw 8.3 in this volume [40]. Thore have been some measurements on the precursors to polymers i.e. dimers and trimers (see Section C3), and these limited results do indicate the importance of molecular shape in determining the elastic properties. In particular, the extended even dimer has elastic constants almost twice those of the bent odd dimer. Thus in polymer liquid crystals, it is clear that the chain configuration will have a dramatic influence on the elastic properties. 

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