Polymer nematics

The Freedericksz transition discussed in 3.4.1 may be called a homogeneous transition since the distortion occurring above the threshold is uniform in the plane of the sample. In low-molecular-weight nematics, which as a rule have relatively small elastic anisotropy k i kjj 2 22), it is the homogeneous transition that is generally observed. Some polymer nematics, however, are known to exhibit high elastic anisotropy - an example is a racemic mixture of poly-y-benzyl-glutamate (PEG) which has k Jk =11.4 and k /k = 13.0 - and in such cases more complex types of field-induced deformations are possible. ... 

For a concise review of Freedericksz transition in polymer nematics, see U. 

F. Lonberg and R.B. Meyer, New ground state for the splay-Preedericksz transition in a polymer nematic liquid crystal, Phys. Rev. Lett. 55(7), 718-721, (1985). doi 10.1103/PhysRevLett.55.718... 

Based on the director distribution we can derive the electrooptical response of a nematic liquid crystal cell (such as birefringence), rotation of the polarization plane of the incident light, total internal reflection, absorption, or some other important characteristics of the cell. In this chapter we will consider in detail these particular features of the electrooptical phenomena in uniform structures. Special attention will be paid to their possible applications. Electrooptics of the isotropic phase and polymer nematics, including Polymer Dispersed Liquid Crystals (PDLC), are also discussed. 

Let us now briefly describe electrohydrodynamic instabilities in polymer nematics. The first observation of the Kapustin-Williams domains in nematic polymers were reported in [117, 118]. The qualitative picture of the phenomenon is, in fact, the same as that for the conventional nematics (domains perpendicular to the initial director orientation in a planar cell, typical divergence of the threshold voltage at a certain, critical frequency, etc.). The only principal difference is a very slow dynamics of the process of the domain formation (hours for high-molecular mass compounds [117]). The same authors have observed longitudinal domains in very thin samples which may be referred to as the flexoelectric domains [5-14] discussed in Section 5.1.1. 

FIGURE 5.23. Threshold of Kapustin-Williams domains in polymer nematics versus frequency. 

Keywords Chain conformation Cholesteric Elastomer Liquid crystal Liquid crystal polymer Nematic Polymer networks Smectic... 

Twist relaxation of splay and bend is a general phenomenon in materials with small K2. Chiral structures can occur in defective nematic samples even when there is no azimuthal anchoring at all. Twisted brushes observed by Press and Arrott in textures of lens-shaped nematic droplets floating on the water surface are one example [15]. Another well-known illustration of twist relaxation is the periodic pattern of stripes that occur in the geometry of splay Frederiks transition in polymer nematics with a small (less than 0.33) ratio K2/K [16]. A field applied normally to the planar nematic cell causes stripe structures composed mostly of twist rather than the uniform splay response observed in regular materials. 

Figure 6.21 Domain order parameter (S) as a function of reduced temperature (Tj being the isotropization temperature, the temperature at which the nematic melt is in equilibrium with the isotropic melt) for a small-molecule- and a side-chain polymer nematic. Drawn after data from Finkelmann and Rehage (1984).

Figure 6.39 shows one of the important signatures of nematics, namely their low viscosity. Thermotropic liquid-crystalline polymers such as Vectra are of very low viscosity and complicated, tortuous moulds are readily filled. A generalized shear rate-viscosity curve for liquid-crystalline polymers (nematics) is shown in Fig. 6.40. Shear thinning occurs in both regions I and III. Some nematics only show parts of this curve.

High gain of light in photoconducting polymer-nematic liquid crystal hybrid stmctures. Opt. Commun., 187, 257-261. 

Several types of spontaneous periodic director pattern yield information about elastic coefficients. Static stripe textures, as described by Lonberg and Meyer [45], appear in polymer nematics if the twist/splay ratio below the critical value of 0.303. Calculations of director fields and the influence of elastic constants and external fields on the appearance of these periodic patterns have been performed by several authors (e.g. [49-51]). In nematic cells with different anchoring conditions at the upper and lower cell plates (hybrid cells), other types of striped texture appear these are similar in nature, but involve different director deformations and elastic coefficients. For a description of various types of static periodic texture and their relationship to elastic coefficients see, for example, Lavren-tovich and Pergamenshchik [52]. In thin hybrid aligned films, a critical thickness is observed below which the director align-... 

Side-chain liquid crystalline polymers (SCLC polymer nematic, S, or amorphous)... 

There are three additional specific problems that have to be addressed in any molecular theory for polymer nematics. The first one is a possible effect of the Frank elasticity in these systems. It was shown [3] that the Frank and molecular polymer elasticity for LCEs have well-separated space scales, with their crossover, the characteristic scale I evaluated ash = s/K/G [3]. Here, G > 10 dyn/cm is a... 

An interesting feature of polymer nematics was discovered in studying the orientation of some polymethacrylate polymers and cross-linked LC elastomers based on polysiloxanes [57]. Most nematic polymers form an optically positive, uniaxial, homeotropic structure under the effect of a mechanical Held such polymers have a positive birefringence (An > 0), like most low-molecular-weight liquid crystals. 

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