Lyotropic polymer molecular structures

Unfortunately, there is no report on the detailed physical characterization of these polymers. Such information as unidirectional twist angle and form optical rotation, as well as their dependence on chemical structures and temperature, can be very useful in further understanding the molecular orientations of the polymers in the cholesteric phase. In contrast, a number of studies have been made on the physical-chemical properties of cholesteric lyotropic polymer systems, especially polypeptides. 

The most important development in lyotropic liquid crystalline polymers after Kevlar is probably the synthesis of poly [benzo(l,2-d 4,5-d ) bisthiazole-2,6-diyl]-l,4-phenylene, or for short, poly(p-phenylene benzo-bisthiazole) ( PBT Wolfe and Loo, 1980 Wolfe et al., 1981), and the closely related poly [benzo(l,2-d 5,4-d )bisoxazole-2,6-diyl]-l,4-phenylene or poly(p-phenylene benzobisoxazole) ( PBO Helminiak and Arnold, 1977 Wolfe and Arnold, 1981). Both PBT and PBO are lyotropic liquid crystalline and can be spun into fibers with mechanical properties even superior to that of Kevlar fibers. The molecular structures of these polymers are shown in Figure 5.2. 

A basic understanding of the structure and behavior of liquid-crystalline cellulosics has yet to evolve. From a conceptual point of view, the chirality of the cellulosic chain is most sensitively expressed in the super-molecular structure of the cholesteric phase, which may be described by the twisting power or the pitch. At present, no information is available about domains or domain sizes (correlation lengths) of supermo-lecular structures. The chirality in the columnar phases has not been addressed at all. The principal problem, i.e., how does chirality on a molecular or conformational level promote chirality on the supermolecular level, has not been solved. If this correlation were known, it would enable the determination of the conformation of cellulosic chains in the mesomorphic phase and the development of models for the polymer-solvent interactions for lyotropic systems. On the other hand, direct probing of this interaction would provide a big leap towards an understanding of lyotropic phases. 

Observed structures of a lyotropic material are classified into three categories nematic, smectic, and cholesteric. Nematic and cholesteric mesophases can be readily identified by microscopic examination. The existence of a smectic mesophase is not well defined and is only suggested in some cases. Solvent, solution concentration, polymer molecular weight, and temperature all affect the phase behavior of lyotropic polymer solutions. In general, the phase transition temperature of a lyotropic solution increases with increasing polymer molecular weight and concentration. It is often difficult to determine the critical concentration or transition temperature of a lyotropic polymer solution precisely. Some polymers even degrade below the nematic isotropic transition temperature so that it is impossible to determine the transition temperatures. Phase behavior is also affected by the polymer molecular conformation and intermolecular interactions. 

The solution properties of these materials are unusual. They form optically anisotropic solutions in both amide and acid solvent systems over quite wide ranges of concentration and polymer molecular weight. In other words they are among the few known examples of synthetic polymers which can form lyotropic liquid crystals. (That is to say liquid crystals formed by the action of a solvent.) The usual example quoted in this context is poly(y-benzyl-L-glutamate) which forms cholesteric mesomorphic solutions in certain organic solvents. The helical structure adopted by the polypeptide in these solvents behaves as a rigid rod and it is... 

Although instances of lyotropic PLCs predate studies of thermotropic PLCs, as they involved solutions of comparatively esoteric species — virus particles and helical polypeptides — studies of these liquid crystals were isolated to a few laboratories. Nevertheless, observations on these lyotropic PLCs did stimulate the first convincing theoretical rationalizations of spontaneously ordered fluid phases (see below). Much of the early experimental work was devoted to characterizing the texture of polypeptide solutions. (23) The chiral polypeptides (helical rods) generate a cholesteric structure in the solution the cholesteric pitch is strongly dependent on polymer concentration, dielectric properties of the solvent, and polymer molecular weight. Variable pitch (<1 - 100 pm) may be stabilized and locked into the solid state by (for example) evaporating the solvent in the presence of a nonvolatile plasticizer.(24)... 

In the entire field of liquid-crystal polymers, an important place has been assigned to the analysis of the molecular structure of polymers capable of exhibiting lyotropic or thmnotropic mesomorphism. The study of the conformation of the macromolecules, the features of their structure determined by the structure of the monomeric unit and the molecular weight, and the degree of flexibility of the polymer molecule showed that the nature of polymeric mesomorphism is determined on the molecular level [1, 2]. 

MOLECULAR STRUCTURE AND CONFORMATION OF THE MACROMOLECULES OF LYOTROPIC POLYMERS... 

The molecular structure of L64 is shown in Figure 7.2. The triblock polymer is EO13PO30EO13, first a hydrophilic block, then a long hydrophobic block and then again the hydrophilic block. The mesoscale simulations proceed by calculating the lyotropic phases in a narrow concentration range 50-70% [21]. From accurate experimental data [22-24], in this interval four phases are known, and the structure factors have been measured micellar phase, cylindrical or hexagonal phase, a bicontinuous phase and a lamellar phase. 

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. 

The brief data presented in this chapter concerning the initial steps of structure formation in LC polymer solutions, are significant from two viewpoints. On the one hand, the study of these processes provides quantitative information about the molecular parameters and IMM of LC polymers, which is the basis for the understanding and prediction of physico-chemical behaviour of polymeric liquid crystals in bulk. On the other hand, understanding of the features of intramolecular structure formation in dilute solution, reveals broad prospects for the investigation of the formation of lyotropic LC systems of polymers with mesogenic side groups, which is in its infancy 195). 

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. 

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