Discotic physical properties

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)... 

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. 

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]. 

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

Triphenylene (TP) derivatives have been described as the work horses in the field of DLCs [11]. It is the most studied discotic core system [20, 21]. TP (1), see Scheme 4.1, belongs to the polycyclic aromatic hydrocarbon (PAH) group and has been known in the chemical literature for more than a century. This trimer of benzene was isolated from the pyrolytic products of benzene by Schultz who named it as triphenylene [22]. It can also be isolated from coal tar. Trimerization of cyclohexanone followed by dehydrogenation has been used to generate TP in the early twentieth century and its various chemical and physical properties were studied [22]. 

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. 

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. 

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. ... 

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

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