Cybotactic nematic mesophases

Secondary nematic structure was found by Blumstein and cowork-gj.gi29,243 during the X-ray investigation of polyester prepared from 4,4 -hydroxy(2,2 -methyl)azoxybenzene and dodecanedioic acid. The diffraction pattern of the nematic shows the development of enhanced order characteristic of smectic C phase the first of the meridional arcs splits up into four spots. This phenomenon is incompatible with the classical definition of the nematic phase and suggests an additional order of macromolecules within cybotactic groups for which a structural model was proposed. Cybotactic nematic mesophases were also found for the copolyesters of the same series containing spacers with an even number of methylene units, but ordinary nematic phases were identified for the members with odd-numbered spacers. ... 

As in the case of linear polymers (see Qiapter 5), a comparative analysis of comb-shaped LC polymers should be conducted by comparing samples with DP above the critical value, i.e., using samples in which is not dependent on DP. In addition, the effect of DP on the type of mesophase which they can form was recently found on the example of acrylic polymers with cyanobiphenyl groups. The low-molecular-weight fractions of the polymer (DP = 20-70) only produce a nematic or cybotactic nematic type of mesophase, while the formation of a smectic mesophase is characteristic of fractions with a higher molecular weight (cf. Fig. 6.7a). 

In these materials, the phase sequence is accounted for by a change in the nature and/or degree of antiparallel correlations. This leads, in effect, to the mesogenic unit changing to a different supramolecular species with a different anisotropy and showing a different mesophase, the effect being driven by temperature. Thus, the supramolecular nature of some systems is dynamic. This dynamic nature is further evidenced by the phenomenon of cybotacticity, in which localized clusters that may be, for example, SmA or SmC in nature, are found in nematic phases. Such behavior is now the subject of intense speculation as proposals... 

---