Polymers, liquid crystalline lyotropic type

Academic and industrial interest in liquid-crystalline polymers of the main-chain type has been stimulated by certain special properties shared by lyotropic and thermotropic systems that exhibit a nematic phase. Although these special properties affect both the processing into fibres and other shaped articles and the physical behaviour of the products, the product behaviour is at least partly attributable to the novel processing behaviour. 

Experiments by Muller et al. [17] on the lamellar phase of a lyotropic system (an LMW surfactant) under shear suggest that multilamellar vesicles develop via an intermediate state for which one finds a distribution of director orientations in the plane perpendicular to the flow direction. These results are compatible with an undulation instability of the type proposed here, since undulations lead to such a distribution of director orientations. Furthermore, Noirez [25] found in shear experiment on a smectic A liquid crystalline polymer in a cone-plate geometry that the layer thickness reduces slightly with increasing shear. This result is compatible with the model presented here as well. 

Liquid crystallinity can be attained in polymers of various polymer architectures, allowing the chemist to combine properties of macromolecules with the anisotropic properties of LC-phases. Mesogenic imits can be introduced into a polymer chain in different ways, as outhned in Fig. 1. For thermotropic LC systems, the LC-active units can be connected directly to each other in a condensation-type polymer to form the main chain ( main chain liquid crystalline polymers , MCLCPs) or they can be attached to the main chain as side chains ( side chain liquid crystalline polymers , SCLCPs). Calamitic (rod-Uke) as well as discotic mesogens have successfully been incorporated into polymers. Lyotropic LC-systems can also be formed by macromolecides. Amphiphihc block copolymers show this behavior when they have well-defined block structures with narrow molecular weight distributions. 

It was, however, observed that such systems under appropriate conditions of concentration, solvent, molecular weight, temperature, etc. form a liquid crystalline solution. Perhaps a little digression is in order here to say a few words about liquid crystals. A liquid crystal has a structure intermediate between a three-dimensionally ordered crystal and a disordered isotropic liquid. There are two main classes of liquid crystals lyotropic and thermotropic. Lyotropic liquid crystals are obtained from low viscosity polymer solutions in a critical concentration range while thermotropic liquid crystals are obtained from polymer melts where a low viscosity phase forms over a certain temperature range. Aromatic polyamides and aramid type fibers are lyotropic liquid crystal polymers. These polymers have a melting point that is high and close to their decomposition temperature. One must therefore spin these from a solution in an appropriate solvent such as sulfuric acid. Aromatic polyesters, on the other hand, are thermotropic liquid crystal polymers. These can be injection molded, extruded or melt spun. 

Liquid crystalline compounds are remarkable because of their ability to show spontaneous anisotropy and readily induced orientation in the liquid crystalline state. When polymers are processed in the liquid crystalline state, this anisotropy may be maintained in the solid state and can readily lead to the formation of materials of great strength in the direction of orientation. A particularly important example of the use of this property for polymers is in the formation of fibers from aromatic polyamides which are spun from shear oriented liquid crystalline solutions Solutions of poly(benzyl glutamate) also show characteristics of liquid crystalline mesophases, and both of these types of polymers are examples of the lyotropic solution behaviour of rigid rod polymers which was predicted by Flory... 

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. 

Chen, W. (1996) in Liquid-Crystalline Polymer Systems (ed. A.I. Isayev, T. Kyu and S.Z.D. Cheng), American Chemical Society, Washington, chapter 15], and liquid crystals (that is, condensed phases in which molecules exhibit orientational order and varying degrees of positional order [2]). The distinctions among these three mesophase types will be maintained throughout this chapter liquid crystalline systems which are thermotropic (that is, induced by changes in the temperature or pressure of a sample) rather than lyotropic (that is, induced by addition of an isotropic liquid as solvent ) will be emphasized. 

Polymeric liquid crystals can be classified into either of two types, thermotropic and lyotropic, according to their formation conditions, or three types, main-chain, side-chain and complex liquid crystalline polymer (MCLCP, SCLCP and CLCP,... 

In the phrase liquid-crystalline, the crystalline adjective refers to the faa that these materials are sufSdentiy ordered to diffract an X-ray beam in a way analogous to that of normal crystalline materials. On the other hand, the liquid part specifies that there is frequently sufSdent disorder for the material to flow like a liquid. liquid crystals can be divided into thermotropic, that exhibit a phase transition with change of temperature, and lyotropic, that exhibit phase transition as a function of both temperature and concentration of the LC molecules in a solvent. Both low molecular wdght materials and polymers " can show liquid crystallinity. In the case of polymers, it frequently occurs in very stiff chains such as the Kevlars and other aromatic polyamides. It can also occur with flexible chains, however, and it is these flexible chains in the elastomeric state that are the focus of the present discussion. LC networks of flexible chains have the following three properties (1) they can be extensively deformed (as described for elastomers throughout this book), (2) the deformation produces alignment of the chains, and (3) alignment of the chains is central to the formation of LC phases. Elastomers of this type have been the subject of numerous studies, as described in several detailed reviews. ... 

By Heck-type coupling liquid crystalline rigid-rod polymers containing [1,3-(diethynyl)cyclobutadiene] cyclopentadienyl moieties were prepared [202]. One example is the reaction of the diethynyl derivative 41 with a 2,5-diodothiophene 42 to the polymer 43 [equation (21)] which show lyotropic nematic phases. 

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