Lamellar discotics

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... 

This formation of nematic, lamellar and columnar phases within a single class of molecule leads to the idea that they can be considered as a missing link between calamitic and discotic mesogens. 

Chandrasekhar S 1998 Columnar, discotic, nematic and lamellar liquid crystals Their structures and physical properties Handbook of Liquid Crystals Vol 2B. Low Molecular Weight Liquid Crystals I ed D Demus, J Goodby, G W Gray, H-W Spiess and V Vill (New York Wiley-VCH)... 

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]. 

In certain cases, nematic (orientational but not positional) order may be observed in phases of (relatively small) micellar rods (canonic) or disks (discotic) [177], which are associated with lower temperature hexagonal and lamellar phases, respectively [178]. (See Fig. 17.) The nematic isotropic phases were thought to be built up of discrete aggregates of different shapes, but the presence of continuous aggregates has also been recently suggested in this case [178]. Some systems (e.g., potassium laurate-decanol-water) form biaxial nematic phases. In this case, the micelles are believed to be neither rods nor disks but rather to... 

Two other di-substituted species were examined by Ohta and coworkers. The tetra alkoxyl-containing 5,15-bis(3,4-didodecyloxyphenyl)porphyrin displayed a discotic lamellar mesophase with a melting point of 60.4 °C and clearing point of 200.7 °C (Figure 23). Four crystalline phases and one undetermined mesophase were delineated for 5,15-bis(4-didodecyloxybiphenyl)porphyrin between initial crystalline phase transitions at 40.1 °C to melting to the mesomorphic material at 430.9 °C (Figure 24). The material was observed to decompose at 450.5 °C. ... 

Dithiolate compounds are similar to the carboxylates, with sulfur atoms taking the place of the oxygen atoms in the polar head. Each of the tetrakis(alkyldithiolato)dinickel(II) complexes ((101) R = C H2 +i, n = 4- ) shows a monotropic lamellar mesophase between 80and 90°C, which has been described as discotic lamellar (Dl) phase (see Figure 22). In the case where R is a branched chain, an enantiotropic Colh and a monotropic Dl mesophases have been found for tetrakis( 1 -ethylpentyldithiolato)dinickel(II) complex. ... 

Other 5,15-disubstituted porphyrins were described by Ohta and co-workers, but most do not fit the label polycatenar and are described in the section on discotic materials (Section 1.9.12). However, one derivative did possess a more conventional tetracatenar structure (131) and was reported as showing the so-called discotic lamellar phase. While it does not contain a central metal, it is included here for completeness. 

DLCs generally exhibit two types of phases nematic phase and columnar phase (Fig. 7.1). Discotic nematic phases are rarely observed while columnar phases are commonly found. In the nematic phase, the molecules possess only orientational ordering however in the columnar phase, the disc-like molecules spontaneously self-assemble into infinite one dimensional (ID) stacks, which in turn self-organize to various two dimensional (2D) liquid crystalline lattices. The hexagonal columnar phase is the most commonly encountered phase in DLCs. Some discotic compounds are also known to exhibit lamellar and cubic phases. 

POM characterization revealed that Poly-2 has a batonnet-like texture from 125 to 94 °C and a fan-shaped texture from 94 to 80 °C during the cooling process. On the other hand, Poly-3 has a sandy texture from 142 to 112 °C and a batonnet-like texture from 112 to 93 °C during the cooling process. XRD measurements revealed that Poly-2 has a discotic nematic (No) phase at higher LC temperatures and a discotic lamellar (Dl) phase at lower LC temperatures. Poly-3 showed a Nq phase and a hexagonal columnar (CoL) phase at higher and lower LC temperatures, respectively. The phase transition temperatures of Poly-2 and Poly-3 are summarized in Table 11.2. 

Fig. 11.8 Schematic representation of LC PMPs after magnetically forced alignment. The a discotic lamellar (Dl) structure of Poly-2 and the b hexagonal columnar (Colh) structure of Poly-3 at room temperature. Reprinted with permission from [13]. Copyright 2009, Elsevier...

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