Small-molecule nematics

The degree of orientational order in a uniaxial nematic is given by the order parameter S, defined by Eq. (2-3). S is zero in the isotropic state, and it approaches unity for hypothetically perfect molecular alignment (i.e., all molecules pointing in the same direction). In single-component small-molecule nematics, such as MBBA, S varies with temperature from 5 0.3 at Tni, the nematic-isotropic transition temperature, to S 0.7 or so at lower... 

The magnitudes of the viscosities (the a ,- s) for a single small-molecule nematic can differ from one another by an order of magnitude or more. As a result, the fluid s resistance to flow depends strongly on the directions of the flow and the flow gradient relative to the nematic director. In a shearing flow, the viscosities o 2 and o 3 determine director torques in the orientations shown in Fig. 10-7b and 10-7c. If the director is oriented in the flow direction... 

Textures exist in small-molecule nematics, but are much more persistent in LCPs because of the high viscosities of the latter. One can estimate that the annealing time tg for spontaneous disappearance of a disclination texture from a nematic layer of thickness h is... 

Significant shifts in S are not expected to occur in small-molecule nematics unless the shear rate is extraordinarily high. For polymeric nematics, however, molecular relaxation times T are typically 0.001-10 sec, or even higher, and therefore molecular elastic effects are produced at shear rates y r = 0.1-1000 sec L Thus, the order parameter S is significantly distorted away from that of equilibrium when the Deborah number De (discussed in Section 3.6.2.1.1) is of order unity or greater, where... 

At low enough shear rates, polymeric nematics ought to obey the same Leslie-Ericksen continuum theory that describes so well the behavior of small-molecule nematics. The main difference is that polymers have a much higher molecular aspect ratio than do small molecules, which leads to greater inequalities in the the numerical values of the various viscosities and Frank constants and to much higher viscosities. 

Under steady shearing, these trapped disclinations should play the role of an anchoring condition, much like the role solid walls play in the flow properties of small-molecule nematics. A scaling analysis of this problem in Section 10.2.5 gives an equation, (10-28), for the steady-state shear viscosity for flow between surfaces with strong, homeotropic anchoring ... 

The structure of the talk is as follows. After a very brief general description of liquid crystals (LC s), the rheology of small molecule nematic LC s Is summarized. The experimental observations are discussed In light of the predictions of the Leslle-Ericksen (L-E) theory. Including the question of orientation... 

Fig. 4. Because these three elastic constants are usually of similar magnitude for small-molecule nematics, one often refers to a single elastic constant, K, for the material. For polymeric materials, on the other hand, the three elastic constants can be very different and are indeed found to be very different experimentally [7]. For instance, in order to have a splay distortion, there must be a net excess of tails over heads of molecules, defined by the molecular orientation along the splay direction. In a polymeric system in which the molecular length is large, the density of chain ends is small, so splay becomes more and more energetically expensive with increasing molecular length. This and many other issues associated with polymeric liquid crystals are reviewed by Meyer [6].

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