Laterally branched mesogen

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). 

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.). 

Fig. 13 Schematic 2D representation of an end-group dendrimer of second generation with a 4-fold core connectivity (Nc = 4), and a ternary branch multiplicity (Nb = 3). The mesogen can be attached terminally or laterally to yield a end-on or b side-on LCDs...

In laterally alkyl substituted mesogens, each branch in a terminal or lateral substituent further decreases the mesophase stability dramatically (e.g. 2g, 2h, 3b, 6c, 6d, 7 and lOh). This fact is important in the synthesis of optically active compounds. 

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