Melting lateral substituents

The disruption of chain regularity by the introduction of lateral substituents or kinks on repeating units is a supplementary means to decrease the melting temperature of aromatic polyesters.72 This is illustrated in Table 2.9, where the melting temperatures of unsubstituted and methyl-substituted aromatic-aliphatic and aliphatic acids are reported. Regularity disruptions often cause significant... 

The first chiral combined lc polymers prepared for this purpose showed the desired cholesteric and chiral smectic C phases only at high temperatures (8) (the melting point was always above 100°C). By using lateral substituents (see Figure 3) it is possible however to suppress the melting temperature and to obtain polymers with a glass transition temperature of about room temperature, without losing the cholesteric and chiral smectic C phases (9). 

As already mentioned, positional isomerism is important for the solubility and fusibility of aromatic LC polyesters. Consequently, polyesters made from symmetrical 2,5-disubstituted or 2,3,4,5-tetrasubstituted monomers should result in polymers that are less soluble and less fusible. This is in general the case with short lateral substituents. Ballauff and others reported that the series of poly( 1,4-phenylene-2,5-dialkoxy tereph-thalate)s with long flexible alkoxy side chains at the terephthalic moiety result in tractable LC polyesters [20] (Fig. 12). These polyesters exhibit a high degree of crystallinity with melting temperatures below 300 °C. Polyesters with short side chains (2350°C for m = 2... 

Figure 6.32 Effect of lateral substituent on isotropization temperature and melting temperature range. Drawn after data from Berger and Ballanf (1958).

Table 33. Decrease in the melting and clearing temperatures by the addition of lateral substituents.

Lateral substituents in core units have to be kept small in order to preserve liquid crystallinity and the most generally useful lateral substituent is fluoro. The special features of fluoro substitution, which separately or in combination may be useful, include the following (a) a small size which usually has only a small effect on nematic thermal stability (b) electron-attraction which may give compounds of negative dielectric anisotropy (c) the ability to diminish or destroy anti-parallel molecular association and increase positive dielectric anisotropies (d) a depression of melting point (e) a reduction in the prominence of ordered smectic phases, frequently, with the consequent relevation of a nematic phase and... 

However, a later study with more anisotropic-elongated acetylenes (86 M = Pt, Pd, R = Me, Et, Pr) indicated that larger phosphines were not well tolerated as mesomorphism was lost on the introduction of tripropylphosphine (Table 14)4 Increasing the size of the phosphine substituents considerably reduced the melting transition temperatures. Mesomorphism was absent in the analogous palladium complexes, decomposing rapidly above 100 °C due to their lower thermal stability compared to their platinum congeners. 

Through these anecdotal examples, a clear pattern emerges in which smectic-like phases can form when the crystallinity of the laterally associated perfluoroalkyl groups can be retained at temperatures where the (terminally modified) alkyl portions are melted. However, when a substituent is near the middle of a molecule, a much more complicated interplay between energies of association and steric and electronic interactions determines whether mesomorphism can occur. 

When compared with compound 63, the lateral fluoro substituent of compound 77 causes a dramatic fall in the melting point and a similar reduction in the j is seen these... 

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