Isotropic states, nematics

By the same procedure are obtained the corresponding biphenyl isocyanide derivatives (Figure 7.20) [18]. Now, the free isonitriles are already liquid crystals displaying nematic and SmA phases with a short range of existence at moderate temperatures (40-85 °C), while the complexes show a marked increase in the melting points and also an expansion ofthe range of existence of the mesophase (up to 140 °C N and SmA phases). The exception is the shortest iodo-derivative, which is not a mesogen. Most of the complexes decompose into the isotropic state (above 220 °C). The biphenyl moiety increases the polarizability anisotropy compared to the phenyl and hence facilitates liquid crystal behavior. 

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. 

As discussed in section 7.1.6.4, semidilute solutions of rodlike polymers can be expected to follow the stress-optical rule as long as the concentration is sufficiently below the onset of the isotropic to nematic transition. Certainly, once such a system becomes nematic and anisotropic, the stress-optical rule cannot be expected to apply. This problem was studied in detail using an instrument capable of combined stress and birefringence measurements by Mead and Larson [109] on solutions of poly(y benzyl L-glutamate) in m-cresol. A pretransitional increase in the stress-optical coefficient was observed as the concentration approached the transition to a nematic state, in agreement of calculations based on the Doi model of polymer liquid crystals [63]. In addition to a dependence on concentration, the stress-optical coefficient was also seen to be dependent on shear rate, and on time for transient shear flows. 

Liquid crystals can be in the smectic, nematic, or isotropic states. In the smectic liquid crystalline state there is a long-range order in the direction of the long axis of the molecules. These molecules may be in single- or bilayer conformation, have molecular axis normal or tilted to the plane of the layer, and frozen or melted chains. In the nematic liquid crystalline state the molecules are aligned side by side but not in specific layers. The isotropic liquid crystalline state is more or less a liquid state, but where clusters with short-range order persist (Small, 1986, pp. 49-51). 

For low concentrations v—that is, small Uo—the only solution is the trivial one, rfr = const. = 1 /(47t), corresponding to the isotropic state. For a high enough value of Uo, there is in addition to the isotropic solution a stable nontrivial solution corresponding to a nematic state with 5 > 0. Of these two solutions, the one with the lowest free energy corresponds to the equilibrium state. As Uq increases, the lowest free-energy state changes from the isotropic to the nematic. 

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... 

Figure 10.22 A sequence of images of disclinations in 5CB at various times t after a pressure jump Ap = 4.7 MPa, sufficient to induce a transition from an isotropic state at 3.6 MPa and 33°C to a nematic state. The field of view is 360 pm. (Reprinted with permission from Chuang et al. 1991 Copyright 1991. American Association for the Advancement of Science. ...

The transition from the isotropic to the nematic state is marked by a decrease in the shear viscosity (Hermans 1962 Gillmor et al. 1994) (see Figs. ll-4a and 11-5). This is not surprising, since molecules can slide past each other more readily in the nematic state than in the isotropic state. However, since the nematic is accessed from the isotropic state by an... 

To avoid these problems, model thermotropic LCPs are often studied whose melting temperatures are low enough to access both the nematic and isotropic states without chemical reactions, and which have side groups that suppress crystallite formation (Kim and Han 1993 Chang and Han 1997). 

Spontaneous orientation of the molecules occurs on transition to the nematic state. Experiments of Spencer and Berry [36] on polymerization kinetics of poly (p-phenylene benzo bis thiazole) (PBT) in polyphosphoric acid, in which an isotropic to nematic transition occurred during polymerization, surprisingly, showed no sudden change in the polymerization rate. The reasons for this are not apparent. An interesting result of the study is that the polymerization reaction may be diffusion controlled even in the nematic phase. 

The study by Percec, Tomazos and Willingham (15) looked at the influence of polymer backbone flexibility on the phase transition temperatures of side chain liquid crystalline polymethacrylate, polyacrylate, polymethylsiloxane and polyphosphazene containing a stilbene side chain. Upon cooling from the isotropic state, golymer IV displays a monotropic nematic mesophase between 106 and 64 C. In this study, the polymers with the more rigid backbones displayed enantiotropic liquid crystalline behavior, whereas the polymers with the flexible backbones, including the siloxane and the polyphosphazene, displayed monotropic nematic mesophases. The examples in this study demonstrated how kinetically controlled side chain crystallization influences the thermodynamically controlled mesomorphic phase through the flexibility of the polymer backbone. 

Fig. 9.8 The change in sample length observed when heating the LSCE from nematic to isotropic. Relative length A = L/Lisotropic of an LSCE versus reduced temperature Tred = (/.isotropic = length ofthe sample in the isotropic state, T = temperature, = phase transformation temperature).

The uniform anisotropic structure of a nematic LSCE is directly reflected in the thermal expansion behaviour. When heating the LSCE from the nematic state into the isotropic state a strong reduction of the sample length along the optical axis is observed (Eigure 9.8). This process directly indicates... 

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