Smectic liquid crystals rheology

Figure 5.132 illustrates that mesophases can also be identified by rheological properties. The crystals of PTFE show a highly anisotropic flow parallel to the molecular axes, a property often found in smectic liquid crystals. As soon as the molecules start to melt, however, the shear-stress increases abruptly because of the chain-entanglement that occurs during melting. On a microscopic basis, PTFE... 

Yonezawa Juniki, Martin Stephen M., Macosko Christopher W., et al. Rheology and morphology of smectic liquid crystal/polymer blends. Macromolecules. 37 no. 17 (2004) 6424-6432. 

For a nematic polymer in a transition region from LC to isotropic state, maximal viscosity is observed at low shear rates j. For a smectic polymer in the same temperature range only a break in the curve is observed on a lgq — 1/T plot. This difference is apparently determined by the same reasons that control the difference in rheological behaviour of low-molecular nematics and smectics 126). A polymeric character of liquid crystals is revealed in higher values of the activation energy (Ef) of viscous flow in a mesophase, e.g., Ef for a smectic polymer is 103 kJ/mole, for a nematic polymer3 80-140kJ/mole. 

Liquid crystals can be classified into lyotropic and thermotropic systems. The rheology of thermotropic liquid crystals is less documented, but in general, nematic liquid crystals were found to show Newtonian flow, whereas smectic and cholesteric liquid crystals demonstrated more or less pronounced plug flow. Plug flow is typical for non-Newtonian, shear thinning liquids. ... 

On a microscopic scale just as for low molecular weight liquid crystals, nematic and cholesteric mesophases have been identified. Whether the smectic structure actually exists for polymeric systems is still open to question. However, it seems to be the macroscopic structure which accounts for the rheological properties of LCP. 

The shear resonance behavior of the other homological members (5CB and 8CB) follows the general trends described already for 6CB. We need, however, more data to conclude for the existence of any effects of the liquid crystal state (nematic or smectic) on the rheological parameters. 

Here ri r]c is the relevant Miesovicz viscosity, and v is the velocity of the sphere. The situation is quite different for semi-infinite and finite barriers, where permeation is necessarily involved in the flow, leading to undulation of layers and arrays of defects. Since it is very difficult to trap and stabilize a sphere vdth a size smaller than the film thickness, as far as we know, the flow of smectics aroimd a sphere has never been studied experimentally. However, experiments when finite particles are moving in the liquid crystal medium, which is at rest far from the particles has been carried out very recently, and it was indeed found that the flow of beads in smectic A and smectic C liquid crystals is purely viscous at sufficiently high speeds. Such technique is analogous to the one-bead micro-rheology ° developed recently to monitor the mechanical properties of viscoelastic soft materials, especially biological systems. i... 

In conclusion, we note that although many problems in the theory of liquid-crystalline ordering in polymer systems have already been solved, this area is still in the initial stage of development on the whole. Among the most important directions of further research (cf. [139] for more detail) are the rheology of thermotropic polymer liquid crystals, the theory of liquid-crystalline elastomers, the statistical physics of the surface in liquid-crystalline polymers, the theory of smectic ordering in polymer systems, and the kinetics of phase transitions in liquid-crystal polymCTS. 

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