Mesogenic groups macromolecule conformation

Schematically the arrangement of a macromolecule with side-chain mesogenic groups in the liquid crystalline state is shown in Fig. 13. Flexible spacers give the mesogenic group its mobility. It is of interest to note that in such liquid crystals all positional mobility of the mesogen is based on conformational motion of the flexible spacer and backbone. Restricting this mobility either prohibits ordering, or freezes the order into the glassy state.

More recently, a systematic study of the properties of side-chain LC dendrimers has been undertaken on these PPI dendrimers and also on PAMAM systems by Serrano et al. [211]. In these dendrimers, the mesogenic group is connected to the dendritic scaffold by an imine linkage. In all cases, it was found that the enthalpic gain of the mesogenic units arranged as in a classical liquid crystalline mesophase dominates over the entropic tendency of the dendrimer core to adopt a globular isotropic conformation. The flexibility of the dendritic PAMAM and PPI cores allows the macromolecule to adopt... 

Also for polymer solutions, the properties depend on interactions between solvent and dissolved components and vary strongly with concentration and temperature. Therefore, beside the molecular analysis, there is the important question for the understanding of the relationship between structure and properties, whether and how PLCs in a solution can be distinguished from those containing non-liquid crystalline polymers with a similar molecular architecture. Another interesting question is, to what extent the conformation of the macromolecules in solution is influenced by interactions between mesogenic groups. As a consequence of that, the hydro- and thermodynamic properties of the solution should also be affected. 

The conformational properties of macromolecules with mesogenic groups in the main chain were studied in a series of studies by VJ. Tsvetkov et al. for alkylene aromatic polyethers [21, 100-103] with a different structure and length of the flexible spacers [101-103] and for some copolymers containing mesogenic fragments in the main chain [100]. The synthesis and features of formation of the mesophase in these polymers are described in [104, 105] and in Chapter 5 of the present book. The orientational order was also studied by IR spectroscopy for alkylene aromatic polyethers in [106, 107]. 

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.). 

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