Orientational dynamics thermotropic liquid crystals

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

We now discuss the orientational dynamics of mesogens. The orientational dynamics in the isotropic phase of thermotropic liquid crystals near the I-N transition have drawn much attention over the years [5-7, 33-40]. The focus was initially on the verification of the Landau-de Gennes (LdG) theory, which predicts a long-time exponential decay with a strongly temperature-dependent... 

In the quest for a universal feature in the short-to-intermediate time orientational dynamics of thermotropic liquid crystals across the I-N transition, Chakrabarti et al. [115] investigated a model discotic system as well as a lattice system. As a representative discotic system, a system of oblate ellipsoids of revolution was chosen. These ellipsoids interact with each other via a modified form of the GB pair potential, GBDII, which was suggested for disc-like molecules by Bates and Luckhurst [116]. The parameterization, which was employed for the model discotic system, was k = 0.345, Kf = 0.2, /jl= 1, and v = 2. For the lattice system, the well-known Lebwohl-Lasher (LL) model was chosen [117]. In this model, the particles are assumed to have uniaxial symmetry and represented by three-dimensional spins, located at the sites of a simple cubic lattice, interacting through a pair potential of the form... 

Davies, G.R. and Ward, I.M. (1988) Structure and properties of oriented thermotropic liquid crystal polymers in the solid state, in High Modulus Polymers (eds A.E. Zachariades and R.S. Porter), Marcel Dekker, New York, Chap. 2 Troughton, M.J., Davies, G.R. and Ward, I.M. (1989) Dynamic mechanical properties of random copolyesters of 4-hydroxybenzoic acid and 2-hydroxy 6-naphthoic acid. Polymer, 30, 58 Green, D.I., Unwin, A.P., Davies, G.R. and Ward, I.M. (1990) An aggregate model for random liquid crystalline copolyesters. Polymer, 31, 579. 

In addition to thermotropic nematic liquid crystals, others such as chiral, smectic and lyotropic liquid crystals have been investigated and their dynamics and orienting properties studied. The structure of the tilted phase of a chiral liquid crystal has been investigated by means of the line-shape... 

A common feature of different types of liquid crystal polymers (LCPs), e.g., thermotropic side-chain or main-chain (either stiff or with flexible spacers) polymers, is their slowed-down dynamics compared to low molecular weight liquid crystals (LCs). Often polymers can be quenched to a glassy state in which the liquid-crystalline order is preserved but motions are completely frozen out. Such liquid-crystalline glasses provide a unique opportunity to determine, in principle, the full orientational distribution function, whereas only its second moment is available from motionally averaged NMR spectra. Thus LCP studies have made fundamental contributions to LC science. 

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