Polymer liquid crystals nematic phases

The synthesized in Ref [136] were the block-copolymers containing the links of terephthaloyl-bis( 2-oxybenzoate) and polyarylate. It was found that such polymers had liquid-crystal nematic phase and the biphase... 

The systematic synthesis of non amphiphilic l.c.-side chain polymers and detailed physico-chemical investigations are discussed. The phase behavior and structure ofnematic, cholesteric and smectic polymers are described. Their optical properties and the state of order of cholesteric and nematic polymers are analysed in comparison to conventional low molar mass liquid crystals. The phase transition into the glassy state and optical characterization of the anisotropic glasses having liquid crystalline structures are examined. 

The isotropic-to-nematic transition is determined by the condition [1 — (2/3)TBBWBB/k T] = 0 whereas the spinodal line is obtained when the denominator of XAA is equal to zero. These conditions are evaluated in the thermodynamic limit (Q = 0) in Fig. 7 for a Maier-Saupe interaction parameter Web/I bT = 0.4xAb and for NA = 200, N = 800, vA = vB = 1. When the volume fraction of component A(a) is low, the isotropic-to-nematic phase transition is reached first whereas at high < >A the spinodal line is reached first. In the second case, the macromolecules do not have a chance to orient themselves before the spinodal line is reached. This RPA approach is a generalization of the Doi et al. [36-38] results (that were developed for lyotropic polymer liquid crystals) to describe thermotropic polymer mixtures. Both approaches cannot, however,... 

FIGURE 5.3 Schematic representation of (a) nematic phase and (b) smectic phase for main-chain liquid crystalline polymers, showing the director as the arrow. The relative ordering is the same for side-chain-polymer liquid crystals. 

Recently very interesting results have been reported on the behaviour of polymer/liquid crystal membrane consisting of bisphe-nol A polycarbonate (PC) and of N-ethoxybenzylidene A -n-butyl aniline (EBBA). This substance shows a crystal-nematic transition at 304° K and nematic-to-isotropic phase transition at 355° K. 

During steady shear flow at higher rates, a liquid crystalline system is considered to flow as an oriented continuous phase, which is shown schematically in Fig. 5(top). When the flow is ceased, molecular orientation relaxes, while liquid crystalline structure is re-formed under the influence of the wall and dis-clination points. Such structural changes must be reflected in optical data, as shown above. To interpret the optical data for a nematic polymer liquid crystal, a model for structural re-forma-tion including relaxation of molecular orientation will be proposed. 

In this chapter we shall only be concerned with electro-optic and thermo-optic switching effects in thermotropic side-chain polymer liquid crystals. We will consider briefly the synthesis and structure of such compounds and show how the nematic, cholesteric and smectic phases arise. Since the optical properties of each of these phases are different, and may be altered depending on alignment within the phase, each gives rise to different electro-optic effects. If these are coupled to the use of dye additives or substituents, then it will be realized that a wide range of electro-optic devices based on dichroism or fluorescence as well as birefringence or scattering power may be fabricated. These will be considered and discussed in terms of their performance and potential applications. Finally, possible uses of polymer-low molar mass liquid crystal solutions will be considered in terms of electro-optic device applications. 

The initial research on electro-optic phenomena in side-chain polymer liquid crystals concentrated on systems that exhibited nematic phases so that a ready comparison could be made with low molar mass mesogens. Such measurements have established that electro-optic devices are feasible and have allowed elastic constants to be deduced from applications of the continuum theory. This theory, originally derived for low molar mass nematic liquid crystals, defines a relationship for the free energy density F in terms of the elastic constants (/ ) and the director n such that ... 

BoamS MI, Viertler K, Wewerka A, Stelzer F, Christianen PCM, Maan JC (2003) Magnetic-field-induced changes of the isotropic-nematic phase transition in side-chain polymer liquid crystals. Phys Rev E 67 050701(R)... 

There are three principal methods of fabricating PDLC (to disperse liquid crystals in polymer matrix) phase separation, encapsulation and permeation. In the phase separation method, liquid crystal materials, prepoljnner (monomer or oligomer) and photoinitiator (curing agent) are mixed, and then polymerization is brought about by heating [2] or UV irradiation [3]. During the polymerization, the liquid crystal material phase separates from the solution, and liquid crystal droplets and a polymer matrix are formed. By measurement of the nematic-isotropic transition temperature, the purity of the liquid crystal in the droplets can be checked. In the case that the polymerization-induced phase separation is not complete and unreacted prepolymer is dissolved in the liquid crystal droplets, the... 

For a numerical calculation, we introduce the temperature parameter t defined by t = llxi=k TfUo). We then have four parameters characterizing our systems itp, the number of segments on a flexible polymer n, the number of segments on a liquid crystal Xa. the attractive interaction (Maier-Saupe) parameter between liquid crystals x = lA, the polymer-liquid crystal interaction (Flory-Huggins) parameter whose the origin is the dispersion forces. We here define the nematic interaction parameter a = Xa/X-From Eq. (3), we can obtain the values of the order parameter S(t, < ) related to a certain temperature r and concentration (p. The nematic phase appears at fo(l- ) = 4.55 in Eq.(3) [15]. We then obtain the nematic-isotropic transition (NIT) temperature (tni) as a function of the polymer concentration 0 ... 

On the basis of the molecular theory combining both the excluded volume interactions and the orientation-depen-dent attractive interactions between liquid crystals, we have theoretically studied the phase transitions of mixtures of a flexible polymer and a liquid crystal. We have found that (1) our theory can qualitatively explain the observed phase diagrams not only in the polymer-liquid crystal systems but also in the solutions of rigid rod-like molecules, (2) gel immersed in a liquid-crystal solvent can undergo the second-order volume transition due to the nematic ordering of the external solvent. 

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