Rotational viscosity molecular structure dependence

The switching time r of a TN cell depends mostly on the rotational viscosity which can be influenced by molecular design, and on the elastic splay constant Kp, the correlation of with molecular structure remains quite elusive. 

Viscosity, especially rotational viscosity (yi), plays a crucial role in the LCD response time. The response time of a nematic LC device is linearly proportional to yi [45]. The rotational viscosity of an aligned liquid crystal depends on the detailed molecular constituents, structure, intermolecular association, and temperature. As the temperamre increases, viscosity decreases rapidly. Several theories, rigorous or semi-empitical, have been developed in an attempt to account for the origin of the LC viscosity [46,47]. However, owing to the complicated anisotropic attractive and steric repulsive interactions among LC molecules, these theoretical results are not completely satisfactory [48,49]. 

The zero shear viscosity of flexible linear polymers varies experimentally with and theoretically with [20]. Due to the highly restricted rotational diffusion, the viscosity of TLCPs is much more sensitive to the molecular weight than that of ordinary thermoplastics as discussed in section 3. Doi and Edwards predicted that the viscosity of rod-like polymers in semi-dilute solutions scales with A/ [see Equation (12)] [2]. Such a high power dependence of viscosity on the molecular weight has been experimentally observed both for lyotropic LCPs [14,15] and for TLCPs [16-18]. The experimental values of the exponent range from 4 to 7 depending on the chemical structure, the chain stiffness, and the domain or defect structure of the liquid crystalline solution or melt. The anisotropicity of the liquid seems to have little effect on the exponent. A slightly smaller exponent for the nematic phase than for the isotropic phase (6 in the nematic phase versus 6.5 in the isotropic... 

In this chapter, the binary mixture of GB particles of different aspect ratios has been studied by molecular dynamics simulation. The composition dependence of different static and dynamic properties has been studied. The radial distribution function has been found to show some interesting features. Simulated pressure and overall diffusion coefficient exhibit nonideal composition dependence. However, simulated viscosity does not show any clear nonideality. The mole fraction dependence of selfdiffusion coefficients qualitatively signals some kind of structural transition in the 50 50 mixture. The rotational correlation study shows the non-Debye behavior in its rank dependence. The product of translational diffusion coefficient and rotational correlation time (first rank) has been found to remain constant across the mixture composition and lie above the stick prediction. 

Early work on NMR of polymers in dilute solution was reviewed by Heatley(29). It was already clear in that early review that relaxation times of dilute polymers were independent of polymer molecular weight, at least for molecular weights above a few to ten thousand, and were nearly independent of polymer concentration for concentrations up to 100-150 g/1 or so. A revealing exception to this rule was provided by polymers plausibly expected to rotate as nearly rigid bodies, for which Ti continued to depend on M up to much larger M. From these observations, it was plausibly inferred that local chain motions are primarily responsible for the observed relaxation times. Dependences of Ti on solvent temperature and viscosity were concluded to scale linearly with solvent viscosity, at least in most systems, a matter treated in more detail below. Heatley also considers correlations between Ti and chain structure. 

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