Solvents nematic, liquid crystalline polymers

In contrast to polypeptides that have many possible conformations, poly(hexyl isocynate) is known to have a stiff rodlike helical conformation in the solid state and in a wide range of solvents, which is responsible for the formation of a nematic liquid crystalline phase.45-47 The inherent chain stiffness of this polymer is primarily determined by chemical structure rather than by intramolecular hydrogen bonding. This results in a greater stability in the stiff rodlike characteristics in the solution as compared to polypeptides. The lyotropic liquid crystalline behavior in a number of different solvents was extensively studied by Aharoni et al.48-50 In contrast to homopolymers, interesting new supramolecular structures can be expected if a flexible block is connected to the rigid polyisocyanate block (rod—coil copolymers) because the molecule imparts both microphase separation characteristics of the blocks and a tendency of rod segments to form anisotropic order. 

In the latter two phases backbones have the spindle-like conformation, i.e., the prolate shape with (R%) > R p), the characteristic of main chain liquid crystalline polymers. Important means of investigating the conformations of side chain liquid crystalline polymers include small angle neutron scattering from deuterium-labeled chains (Kirst Ohm, 1985), or small angle X-ray scattering on side chain liquid crystalline polymers in a small molecular mass liquid crystal solvent (Mattossi et al., 1986), deuterium nuclear resonance (Boeffel et al., 1986), the stress- or electro-optical measurements on crosslinked side chain liquid crystalline polymers (Mitchell et al., 1992), etc. Actually, the nematic (or smectic modifications) phases of the side chain liquid crystalline polymers have been substantially observed by experiments. 

More recently in the study of another series of mesogen-jacketed liquid crystalline polymers we found (Zhang et al, 1996) that the frozen-in nematic order (formed from evaporation of the solvent or when precipitated from solution) in the non-crystalline polymers will disappear when they are heated up to the temperature region of the glass transition. The DSC curves of these polymers shows again only the glass transition, however instead of a jump a broad endothermic peak appears. Obviously in such cases the glass transition and the transition of the nematic-to-amorphous are superimposed. 

The precipitation process is assumed to lead either from the lyotropic, nematic liquid crystalline state via predpitation with water under maintenance of ordered water-polymer layers to the crystal form II, or with other solvents (and also from low polymer concentrations with water) through disordered solvent-polymer structures to crystal form I In both crystal structures the ultimate polymer crystal layers are H-bonded in the aystallographic bc-plane (100), as shown in Fig. 6.2. In crystal form 1 the second molecular chain goes through the center of the unit cell (Pn or P2j/n space group, 2 chains per unit cell, monoclinic, pseudo-orthorhombic)... 

Analogous ochre-yellow polymers 4.26 that contain thiophene in the main chain (Mw= 11,300-35,300 PDI = 1.5-2.7) have also been prepared [61]. Upon slow concentration of their solutions in chlorinated solvents, such as chloroform, lyotropic nematic liquid-crystalline phases form. In one case, a lyotropic smectic state was identified. 

Polymer-dispersed liquid crystals (PDLCs) are pm-sized dispersions of nematic liquid-crystalline droplets within a polymeric matrix (1-3). They are finding intense interest for applications such as light switches in flat-panel displays and windows (3, 4). They are usually prepared by UV-induced phase separation and cross-linking of a prepolymer containing a compatible blend of liquid crystals (LCs), although solution-casting from a common solvent and cooling from the melt below the upper-critical-... 

Polymer P25/26 self-orders in solvent-cast films, with the hackhones parallel to the substrate and a strong solvent dependence of the degree of ordering (78). Spacings of 2.2-2.G nm are observed by x-ray diffraction, indicating interdigita-tion of the dendritic side chains. P25 6 also forms thermotropic nematic liquid crystalline phases. With optical microscopy, Schlieren textures are observed for thin films cast from solution. 

Achard et al. observed a biphasic region for mixtmes of low molar mass nematic solvents and some liquid crystalline polymers ... 

Liquid crystals and colloids have different structures, i.e., there exist colloids dispersed in liquid-crystalline media as either matrices or LC-droplets in an isotropic matrix, prepared in polymer-dispersed liquid crystals. Literature has described the method for the preparation of liquid-crystalline colloidal particles (i.e., photo-crosslinking of dispersion), which leads to large colloidal particles with a broad size distribution [153]. In the latter case, the matrix is hard (crosslinked polymer) and the LC-droplet is mobile. The perfectly inverted system for colloids in a LC-matrix is represented by the liquid-crystalline colloidal particles in a liquid-like isotropic matrix. These studies describe a system prepared by photopolymerization of a nematic liquid-crystalline monomer dispersion in a viscous solvent. The liquid crystalline colloidal particles are manipulated in electrical fields, due to their anisotropic properties. Generally, the anisotropic... 

The first commercially important liquid-crystalline polymer was Kevlar. Kwolek wrote in 1965 about anisotropic solutions of wholly aromatic polyamides in alkylamide and alkylurea solvents. This development led to Kevlar, i.e. ultra-oriented fibres of poly(p-phenylene terephthalamide) (Fig. 6.17). The solution of the polymer in concentrated sulphuric acid is nematic (the term nematic will be explained in section 6.5.3) and fibres of high modulus and high strength can be spun from the solution. 

Predictions (a), (b) and (c) find abundant verification in experiments on lyotropic solutions of liquid crystalline polymers. a-Helical polypeptides dissolved in various solvents exhibit separation of a cholesteric phase at concentrations in close agreement with the equation above. " Degraded DNA dissolved in aqueous solution likewise induces the formation of a nematic phase above a well-defined concentration " that is in good agreement with the equation above. In both instances, the ratio of the volume fractions in the two phases is about 1 3-1-4, in satisfactory agreement with theory. Observations on the onset of phase separation in solutions of polyaramides are also in approximate agreement with theoretical predictions. Further predictions of the theory are as follows ... 

LIQUID CRYSTALLINE POLYMERS IN NEMATIC SOLVENTS EXTENSION AND CONFINEMENT... 

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