Polymeric mesogen

Figure 14.26 Discotic polymeric mesogens (a) columnar hexagonal (b) sandic nematic.

The study of liquid crystalline materials by X-ray diffraction has until recently been confined to low molecular weight compounds because of the lack of availability of suitable polymeric mesogens. Based on the observations by X-ray diffraction made with small molecules as reviewed by Azaroflf a system for identifying the type of mesophase has been developed by DeVries and this system is now being applied to polymeric liquid crystals. 

Structure-property relations in small molecule liquid crystals have been the object of systematic investigation for some time and are at least qualitatively well understood. The question naturally arises as to whether these relationships can be transferred to polymeric mesogens. At present the answer seems to be a tentative yes, at least to the level of a first approximation. A few... 

Flory and Ronca[126,127] predicted that asymmetric molecules with aspect ratios higher than 6.417 can act as a mesogett and mesogens with different aspect ratios also form liquid crystals. According to the theoretical predictions, an anisotropic solittion of single phase composed of polymeric mesogens with different aspect ratios is prepared, which was experimentally verified[128,129]. The theoretical predictions further suggest that miscible mesomorphic polyblends may be prepared. Takayanaki et a/.[56,130] exantined the phase transition of... 

Subsequent intramolecular oxidative coupling with either vanadium(V) oxyfluo-ride/boron trifluoride diethyl ether etherate or thallmm(III) oxide/trifluoracetic acid resulted in the corresponding phenanthrene-9,10-diones 1259. It has been shown that oxygen functionalities at the 3-, 3 -, 4-, and 4 -positions are necessary for coupling to occur. These substituted benzils and phenanthrene-9,10-diones constitute precursors for ligands in the field of discotic metallomesogens or polymeric mesogens. 

These Twin dimers have recently received attention as monodisperse models for polymeric mesogens of similar constitution. ... 

Since 1979 a large voliune of work on these types of polymeric mesogens has appeared. This review, however, sought to cover only the first four years, the beginning years, of research into this intriguing aspect of polymer science. 

Fig. 2.31 The room temperature operational ID polymeric mesogen [Fe(Ci2-tba)3](CF3S03)2 arranges in disk-like columns with the iron ions stacked on top of each other in the middle of the columns. The spin state of the Fe ions depends on the distance d between the disks. At room temperature, d is relatively small and the LS state (singlet) is favored. On warming the distance d increases which favors the HS state (doublet) [53]...

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