Molecule nematogenic

Fig. 18. Isobaric phase diagram of a LC main chain polyester and a small molecule nematogen. From ref. 55-...

Binary mixtures of a flexible polymer and a rigid rod-like molecule (nematogen or liquid crystal) play an important role in electro-optical devices, such as light shutters and displays. Since the miscibility or phase separation controls the performance of the materials, the phase behavior and phase separation kinetics have been of fundamental and practical interests. Liquid crystalline domains dispersed in a polymer matrix are called polymer dispersed-liquid crystals (PDLCs), or polymer-stabilized liquid crystals (PSLCs), where the polymer forces the liquid crystals to phase separate into droplets surrounding by the polymer matrix [2]. Practically, there are many ways to create PD LCs by mixing polymers and liquid crystals the emulsion method [37] and phase separation method [38], including polymerization-, thermally-, and solvent-induced phase separations. The reader is referred to text books [1, 2] for details of PDLC and a review [39] for the rheological and mechanical properties. 

We introduce here the phase behavior of a flexible polymer and a low molecular weight liquid crystal molecule (nematogen) of n, < 3.64. The conditions for binodal... 

In order for dipole—dipole and dipole-iaduced dipole iateractioas to be effective, the molecule must coataia polar groups and/or be highly polarizable. Ease of electronic distortion is favored by the presence of aromatic groups and double or triple bonds. These groups frequently are found ia the molecular stmcture of Hquid crystal compouads. The most common nematogenic and smectogenic molecules are of the type shown ia Table 2. 

In 1978, Bryan [11] reported on crystal structure precursors of liquid crystalline phases and their implications for the molecular arrangement in the mesophase. In this work he presented classical nematogenic precursors, where the molecules in the crystalline state form imbricated packing, and non-classical ones with cross-sheet structures. The crystalline-nematic phase transition was called displacive. The displacive type of transition involves comparatively limited displacements of the molecules from the positions which they occupy with respect to their nearest neighbours in the crystal. In most cases, smectic precursors form layered structures. The crystalline-smectic phase transition was called reconstitutive because the molecular arrangement in the crystalline state must alter in a more pronounced fashion in order to achieve the mesophase arrangement [12]. 

Note 3 Molecules which constitute nematogens are not strictly cylindrically symmetric and have their orientational order given by the Saupe ordering matrix which has elements 5 aP = (3< a P> - 5ap)/2, where la and ip are the direction cosines between the director and the molecular axes a and P, 5ap is the Kronecker delta and a, p denote the molecular axes X, Y, Z. 

Note 4 The constituent molecules of a nematogen are rarely rigid and their orientational order is strictly defined, at the second-rank level, by a Saupe ordering matrix for each rigid sub-unit. 

Actually, these considerations are confirmed by experiments 82). The systems investigated are shown in Table 8, No. 4, which are induced cholesteric polymer systems. The nematogenic host molecules of benzoic acid phenyl esters are linked via spacers of different length m (m = 3, 4, 5, 6) to the polymer backbone. The polymers are converted to polymers having a cholesteric phase by the chiral cholesteryl derivative, which is also linked to the polymer backbone (copolymer). 

The next refinement of the model takes into account that the shape of most molecular species differs from being rod-like typical nematogenic molecules are given in Table 4.6-1. The resulting behaviour of such a bi-axial molecule is often associated with hindered rotation, however it can also be understood from a rigid-body model where different moments of inertia lead to oscillations of different angular amplitudes in spite of identical (thermal) excitation and identical repulsive forces (Korte, 1983). This can be summarized by order parameters defined as above but referring to one of the two shorter. 

In this liquid crystal phase, the molecules have non-symmetrical carbon atoms and thus lose mirror symmetry. Otherwise optically active molecules are doped into host nematogenic molecules to induce the chiral liquid crystals. The liquid crystals consisting of such molecules show a helical structure. The most important chiral liquid crystal is the cholesteric liquid crystals. As discussed in Section 1.2, the cholesteric liquid crystal was the first discovered liquid crystal and is an important member of the liquid crystal family. In some of the literature, it is denoted as the N phase, the chiral nematic liquid crystal. As a convention, the asterisk is used in the nomenclature of liquid crystals to mean the chiral phase. Cholesteric liquid crystals have beautiful and interesting optical properties, e.g., the selective reflection of circularly polarized light, significant optical rotation, circular dichroism, etc. 

The main limitation of ECCB-type methods is that they become inefficient as the strength of intramolecular interactions increases. They cannot be efficiently applied, for example, to stiff nematogenic chains and to chain molecules with rigid bond angles (a model commonly adopted for alkanelike molecules). In those instances, different methods are necessary. The next section describes one such method. 

As an example of an application of the mean field method we shall consider the theory of the dielectric anisotropy of the nematic phase. The low frequency dielectric anisotropy of a molecule is determined by two factors (i) the polarizability anisotropy which for the elongated molecules of nematogenic compounds always makes a positive contribution (i.e., a... 

Optical anisotropies characteristic of nematogenic molecules, comprising a linear array of aromatic groups, appear to play an important role in thermotropic LCs. Anisotropies of the polarizability tensors of the interacting molecules enhance parallel molecular alignment, providing a basis for the Maier-Saupe [1959, 1960] theory of the nematic LC state. The internal energy per mole is defined as... 

Polymer chains with fully rigid conformation are rare in nature. Usually, the nematogenic chain molecules are able to adopt a variety of conformations, some more extended and rodlike than others. Occurrence of liquid crystallite in such... 

All the data show that the flexocoefficients for the many different single components as well as multicomponent mixtures of nematogens with monomeric rod-like molecules are 10 C/m, the measurements being made well below the nematic-isotropic transition temperature. This agrees well with the estimate made on the basis of simple physical considerations, viz., as the ratio of an elementary charge over a molecular dimension. 

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