Physical discotics

Liquid crystals (LCs) are organic liquids with long-range ordered structures. They have anisotropic optical and physical behaviors and are similar to crystal in electric field. They can be characterized by the long-range order of their molecular orientation. According to the shape and molecular direction, LCs can be sorted as four types nematic LC, smectic LC, cholesteric LC, and discotic LC, and their ideal models are shown in Fig. 23 [52,55]. 

The polyaromatic mesophase (PA-MP) is a nematic, discotic, chemotropic liquid crystal. Owing to its high density (about 1.5 gcm ), its high carbon yield of about 90 %, and its thermoplasticity, it is unique as a precursor of structure carbons. An important application is the manufacture of high modulus (HM) and ultra-high modulus (UHM) carbon fibers [1]. By alloying with silicon, physical and chemical properties of the materials, such as strength, hardness and oxidation resistance, can be improved. These modified carbons were available by chemical vapor deposition (CVD) processes only up to now. The preparation by liquid phase pyrolysis is novel, economic, and thus opens a completely new field of applications. 

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)... 

Lyotropic liquid crystals occur abundantly in nature, being ubiquitous in living systems.Their structures are quite complex and are only just beginning to be elucidated. However, in this monograph we shall be confining our attention mainly to the physics of low molecular weight thermotropic liquid crystals and do not propose to discuss polymer and lyotropic systems in any further detail. In chapters 2-5, we deal with the nematic, cholesteric and smectic mesophases of rod-like molecules and in chapter 6 discotic systems. 

The subject of liquid crystals has now grown to become an exciting interdisciplinary field of research with important practical applications. This book presents a systematic and self-contained treatment of the physics of the different types of thermotropic liquid crystals - the three classical types, nematic, cholesteric and smectic, composed of rod-shaped molecules, and the newly discovered discotic type composed of disc-shaped molecules. The coverage includes a description of the structures of these four main types and their polymorphic modifications, their thermodynamical, optical and mechanical properties and their behaviour under external fields. The basic principles underlying the major applications of liquid crystals in display technology (for example, the twisted and supertwisted nematic devices, the surface stabilized ferroelectric device, etc.) and in thermography are also discussed. 

There are only two papers describing the physical properties of discotic LCEs [35, 36]. X-ray diffraction showed that a narrower azimuthal intensity distribution can be obtained when a second crosslinking step is done under an external load reflecting a macroscopic alignment parallel to the direction of the external stress [35]. X-ray diffraction experiments on monodomains (LSCE) show the occurrence of sharp maxima in the azimuthal distribution. Thermal expansion has also been studied for polydomain LCEs and for LSCEs in [36]. It is found that well inside the discotic phase, that is sufficiently far below the discotic-isotrop-ic transiton, the length parallel to the stress axis of both, poly- and monodomain samples, increases approximately linearly with increasing temperature as expected from the isobaric expansion of the network. 

The vast range of supramolecular polymeric materials also extends from the use of small molecules (e.g., double-sided discotics or multifunctional monomers with a discrete tether) to very large molecules (polymeric or dendritic). Another important characteristic of SPs is the timescale upon which the chains exist, which is defined by the rate of association/ dissociation of the monomers. Tme dynamic SPs must be reversible (breaking and recombining) on experimental time-scales (e.g., NMR timescale). A physical model developed by Cates and co-workers predias many viscoelastic properties of SPs as a funaion of the strength of the noncovalent interactions existing between monomers.Although initial studies focused on wormlike micelles, the model has been demonstrated to successfully describe the viscosity behavior of reversible, self-complementary UPy-based SPs. ... 

Chandrasekhar S and Ranganath GS (1990) Discotic liquid crystals. Research Progress in Physics 53 57-84. 

Figure 1.1(b) shows a typical discotic liquid crystal molecule [6]. It also has a rigid core and flexible tails. The branches are approximately on one plane. The space-filling model of the molecule is shown in Figure 1.1 (d). If there is no permanent dipole moment perpendicular to the plane of the molecule, it can be regarded as a disk in considering its physical behavior as shown in Figure 1.1(f) because of the fast rotation around the axis which is at the center of the molecule and perpendicular to the plane of the molecule. If there is a permanent dipole moment perpendicular to the plane of the molecule, it is better to visualize the molecule as a bowl, because the reflection symmetry is broken and all the permanent dipoles may point in the same direction and spontaneous polarization occurs. The flexible tails are also necessary, otherwise the molecules form a crystal phase where there is positional order. 

At present, the physical and electrooptical properties of discotics and biaxial nematics are under investigation. 

Another research group published numerous studies of syntheses and physical properties of LC-polyethers having linear chains, hyperbranched and cyclic structures or containing discotic mesogens [327-341]. All these different LC-polyethers were prepared via the normal Williamson ether synthesis involving the nucleophilic attack of a phenoxide ion onto a bromoalkane. For instance, numerous linear LC-polyesters were prepared from the mesogenic diphenols outlined in (214) and formulas (215a,b) [327,337]. 

In conclusion, electric field effects in liquid crystals is a well-developed branch of condensed matter physics. The field behavior of nematic liquid crystals in the bulk is well understood. To a certain extent the same is true for the cholesteric mesophase, although the discovery of bistability phenomena and field effects in blue phases opened up new fundamental problems to be solved. Ferroelectric and antiferroelectric mesophases in chiral compounds are a subject of current study. The other ferroelectric substances, such as discotic and lyotropic chiral systems and some achiral (like polyphilic) meso-genes, should attract more attention in the near future. The same is true for a variety of polymer ferroelectric substances, including elastomers. 

Chapter VIII Discotic Liquid Crystals Their Structures and Physical Properties. . . 749 S. Chandrasekhar... 

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