Discotic polymers columnar phase

Recent work focuses on non-classical mesogenes which are built up by self-assembly. One example is a family of polymers containing disk-like groups which form no liquid crystalline phase, but ean act as an electron acceptor or donor. Charge transfer complexation with a complementary low molecular mass compound induces nematic or columnar discotic liquid crystalline order [153,154]. Figure 13 demonstrates this with the example of a polyester, bearing electron-rich tetra(alkoxy)tri-phenylene-units in the main chain, mixed with the electron deficient aromatic 2,4,7-trinitro-9-fluorenone (TNF). While the pure polymer shows a non-ordered isotropic melt, a columnar phase appears on addition of TNF. 

Thermotropic liquid crystals can then be furflier subdivided into high molecular mass, main and side-chain polymers [10] and low molecular mass, the latter class of compounds being one of the areas of this review. The phases exhibited by the low molecular mass molecules are then properly described with reference to the symmetry and/or supramolecular geometry of the phases, which are briefly introduced here and are discussed in more detail further below. Thus, the most disordered mesophase is the nematic (N), which is found for calamitic molecules (N), discoidal molecules (Nq) and columnar aggregates (Nc), among others. The more ordered lamellar or smectic phases (S) [11, 12] are commonly shown by calamitic molecules, and there exists a variety of such phases distinguished by a subscripted letter (e. g. Sa, Sb)- Columnar phases (often, if incorrectly, referred to as discotic phases) may be formed from stacks of disc-like molecules, or from... 

In general, for side chain liquid-crystalline polymers, macroscopic molecular alignment is not easy and therefore clear evidence of electronic charge carrier transport was confirmed first in liquid crystals with low molecular weight. In the 1990s, fast electronic conduction was verified in discotic columnar phases of triphenylene derivatives [79,80] and hexabenzocoronene derivatives [81,82] as well as smectic phases of 2-phenylbenzothiazole [83, 84] and 2-phenylnaphthalene derivatives [85], as shown in Fig. 14. Carrier... 

Fig. 1.1.10. Mesophases of discotic polymers (a) the hexagonal columnar phase. The diagram illustrates intercolumnar binding as well as intracolumnar back-folding of the main chain (Herrmann-Schonherr, Wendorff and Ringsdorf ) (b) the sanidic nematic phase composed of boards stacked parallel to one another (Herrmann-Schonherr et a/. ) (c) the columnar nematic phase (Ringsdorf...

We end this chapter with a very brief account of a new class of liquid crystal polymers, i.e. discotic polymers [118-126]. The basic monomer units are discotic mesogen-ic moieties, which are components of the polymer main chain itself or are attached to the polymer backbone as side groups. A few examples are shown as structures 14-16. Besides the columnar phase, some new types of mesophase have been identified. A... 

Figure 33. Mesophases of discotic polymers (a) hexagonal columnar phase (b) the board-like, or sanidic, nematic phase (c) the columnar nematic phase.

In discotics, the interdisk stacking interaction is several orders of magnitude stronger than the inter-columnar interactions, because of the phase separation induced by the side-chains whose van der Waals interactions are much weaker. Generally, long polymers are obtained via discotics that have strong and specific intermolecular interactions, either via a large aromatic core or via additional specific intermolecular... 

Discotic hydrogen bonded complex of l,3,5-tris(l-alkyl-l//-l,2,3-triazol-4-yl) benzene (TTB) and 10,12-pentacosadiynoic acid (PCDA) can form helical conjugated polymers in an asymmetric reaction field consisting of the lamellar columnar LC phase and CPUL (circular polarized ultraviolet light) stimulus (Fig. 5.15) [76]. 

Ringsdorf and coworkers [71] have shown that it is possible to induce liquid crystalline phases, namely discotic-columnar mesophases, by doping amorphous polymers containing disk-shaped electron donors, in either the side chain or the main chain, with a low molar mass electron acceptor, as shown in Figure 3.31. The resulting complexes can be considered as diskcomb PLCs and disk PLCs, respectively [5]. The electron-rich moiety is a triphenylene unit and the electron acceptors are fluorenone derivatives. When 20-25 mol% of 2,4, 7-trinitrofluorenone (TNF) is added to the side chain polymethacrylate or polyacrylate... 

Figure 23 The discotic arnphiphile 2,3,6,7,10,11 -hexa-(trioxoacetyl) triphenylene (cf. Table 3, polymer 15) Is the repeating unit in a columnar stack stabilized by solvophobic interactions in D2O. The temperature versus volume fraction phase diagram shows single columns occurring in the nematic phase (A/) at volume fraction as low as 0.2. Hexagonal columnar and eventually crystalline phases evolve upon increasing concentration. (From R. Hentschke, P.J.B. Edwards, N. Boden, and R. Bushby. Macromol. Symp. 81 361, 1994. With permission.)...

The phase diagram of the discotic arnphiphile based on triphenylene with polar side chains (2,3,6,7,10,1 l-hexa-(l,4,7-trioxoacetyl) triphenylene in D2O, Table 3, polymer 15) [152,157] is reported in Figure 23. Large columnar stacks formed at a volume fraction v 20% (RT) simultaneously with the isotropic nematic transition. The nematic phase was stable up to v 40%, when... 

This book does not intend to cover the whole field of thermotropic liquid crystalline (TLC) materials as it is extremely difficult to cover within a single book. Instead it presents a collection of Chapters written by experts on various exciting topics in the field. Properties of recently developed TLCs (such as banana-type, thiophene-based, and columnar TLCs), phase biaxiaUty, and novel polymeric TLCs are discussed In detail. Solid-state NMR studies to obtain atomistic-level structural and geometrical information of TLCs are presented. Synthesis of liquid crystalUne conjugated polymers, fast switching of nematic materials by an electric field, and photoconducting discotic systems are also presented. 

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