Lyotropic side-chain polymer liquid

Lyotropic Side Chain Polymer Liquid Crystals... 

SYNTHESIS OF LYOTROPIC SIDE CHAIN POLYMER LIQUID CRYSTALS... 

Direct comparisons between hydrocarbon and siloxane main chains are few for lyotropic side chain polymer liquid crystals. Pietschmann et al. have recently synthesised polymers of 1,3-diols which differ in the nature of the polymer backbone. DSC and optical polarizing microscopy were used to construct the phase diagrams. These were compared with the phase diagrams obtained for the analogous low-molar-mass compounds. The structures of the polyacrylate (I) and polymethacrylate (II) based materials are shown below ... 

It is very often the commercial interest in novel materials which stimulates the growth in their study and eventual exploitation. This is certainly true in the case of thermotropic liquid crystals and their application in electro-optic displays. Indeed, the production of high-strength, high-modulus fibres has seen a wealth of interest in lyotropic main chain polymers. The use of lyotropic side chain polymers has, by comparison, been less well publicized. This is not to say that there are no applications. Alkyl polyoxyethylene surfactants attached to polysiloxane polymers have found uses in many personal care products such as liquid soaps, shampoos, skin creams, and hair mousses. Unfortunately the physical properties of these and other similar materials have been closely guarded secrets and the amount of information available in the literature is low. The limited data which does exist, however, provides us with some interesting structure - behaviour relationships. 

Finkelmann, H., Lehmann, B. and Rehage, G. Phase behaviour of lyotropic liquid crystalline side chain polymers in aqueous solutions. Colloid Polymer Sci. 260, 56 (1982)... 

Liquid crystals (LCs) are described as a fluid phase that flows like a liquid and is oriented in a crystalline manner. LCs are divided into two types thermotropic LCs, where the LC phase transition is dependent on temperature or lyotropic LCs, where the LC phase transition occurs as a function of solvent concentration. To introduce liquid crystallinity to conjugated polymers, LC moieties can be introduced to the polymer side chains for side chain-type liquid crystallinity. On the other hand, designing conjugated polymers with rigid main chain structures with flexible alkyl side chains for solubility enables main chain-type liquid crystallinity. 

In the following, attention is focused only on some results obtained for solutions of thermotropic PLCs, and particularly on those of side group polymer liquid crystals (SGPLCs) since main chain polymer liquid crystals (MCPLCs) are normally difficult to dissolve and at least are insoluble in non-protonated solvents. The inclusion of lyotropic systems would be beyond the scope of this chapter and also would give no contribution to solving the question of how the properties of solutions of PLCs differ from those of non-LC polymers with similar molecular design. 

Conoscopy provides an extremely sensitive method with which to determine the degree of biaxiality. By the early 1990 s, conoscopic measurements had already indicated the presence of phase biaxiality in a nematic side-on liquid crystalline side-chain polymer [9]. However, the method s sensitivity is also its weak point because surface effects may induce optical biaxiality in an actual uniaxial system. For this reason, deuterium NMR was used to confirm phase biaxiality in a liquid crystalline polymer system similar to the one investigated with conoscopy by Leube [11-13]. Due to the fairly high viscosity of the polymeric samples, the tilt experiment, employed by Yu and Saupe to show phase biaxiality in a lyotropic liquid crystal [4], was used. The results obtained in this way are in good agreement with observations of optical textures in a biaxial cholesteric copolymer [16], where phase biaxiality disturbs the smooth optical periodicity of the cholesteric phase structure. 

More recent work has shown that the distinction between lyotropic and thermotropic polymer liquid crystals need not be so rigidly defined. If the amphiphilic side chains have an element of rigidity built into them, using for example a biphenyl group, then some of the mesophases formed closely resemble those of thermotropic side chain polymers. Furthermore, polymers which produce lyotropic liquid crystals may well form mesophases in the absence of a solvent, should the molecular structure favour them. (Note that many surfactants, particularly ionic amphiphiles, also form thermotropic mesophases.) Some specific examples will be discussed in due course. 

Both thermotropie and lyotropic liquid crystal polymers exhibit eharacteristic micro-strueture features [9,10]. Anisometrieal monomers such as rods or disks are conneeted to ehains in an appropriate manner. These anisometrieal monomers are considered the mesogens and may be part of main ehain LCP, side chain LCP, or of both types together (Fig. 6). Flexible spacers of nonmesogenic character are located between the mesogens. A sufficient flexibility is a prerequisite for the liquid crystal formation with an increase in temperature or solvent concentration. 

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