Lyotropic liquid crystalline behavior

Scheme 6.6. Procedures for the preparation of poly(aryl amide) polymers. The carboxylic acid terminated polymers exhibited lyotropic liquid crystalline behavior.

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. 

Lyotropic liquid crystalline behavior of rodlike peptides was reported for the first time by Robinson, C. Ward, J. C. Nature 1957, 180, 1183. 

Self-assembly and molecular recognition of some biomolecules lead to lyotropic liquid-crystalline behavior with water [5]. These molecular complexes derived from biomolecules could be more important in view of biofunction. Nucleic acids such as DNA and RNA, which self-organize supramolecular complexes, show lyotropic mesophases [5]. Complexes of nucleic acid and cationic amphiliphic molecules form mesomorphic molecular arrangements, which can be used for drug delivery such as gene therapy [6]. 

In the following sections, the concept of MJLCPs is first described, followed by discussions on the synthesis, die Uquid crystallinity, and chain properties of representative MJLCPs. Amorig the pcdymers are the newty itynthesized poty-2,5-bi8(4-methoxybenzamido)styiene (PA-1) and its homologues. The lyotropic liquid crystalline behavior of PA-1 and other MJLCPs is also reported for die first time in psqier. 

Due to the wide spectrum of behavior exhibited by this class of compounds, the discussion is broken up into the major areas of surfactant behavior, namely, thermotropic liquid crystal formation, microemulsions, lyotropic liquid crystalline behavior, reverse micellar systems, and dilute solution. This spectrum of available modes of self-assembly facilitates their use in an increasing range of applications. 

Polyisocyanates are comprised of only amide bonding in the main chain and maintain their helical rod structures, so they normally show liquid crystallinity or high crystallinity. For instance, the helix conformation (12/5) of poly(hexylisocyanate) (PHIC) and its high crystallinity in the solid state are well observed by X-ray scattering (Wu et al., 1992). PHIC displays rod structures with a very long persistence length of 50-60 nm in the solution state (Bur and Fetters, 1976). It also shows lyotropic liquid-crystalline behavior at high concentrations (Aharomi, 1979 Aharoni and Walsh, 1979). 

Another feature of surfactant-water systems is that they can also aggregate into lyotropic liquid crystalline phases when Intermicellar interactions are significant. Typically, non-Newtonian behavior is usually found for these liquid crystalline phases. For the 3LDA0/ISDS mixed system, all evidence suggests that they do form liquid crystalline phase. 

LCP phases are subdivided into thermotropic or lyotropic. Lyotropic liquid crystals are formed by macromolecules that show liquid crystalline behavior in solution. This behavior is strongly concentration dependent. Thermotropic liquid crystals are molecules that show liquid crystalline behavior above the melting point of their crystallites. 

The appearance of tubular myelin-like structures in swollen lecithin was observed by light microscopy well before the systematic investigation of liposomes [351-352]. Similarly, it was also demonstrated some time ago that the addition of calcium ions converted phospholipid liposomes to cochleate cylinders [353]. Subsequent studies have, however, revealed that the system is extremely complex. For example, examination of the phase-transition behavior of synthetic sodium di-n-dodecyl phosphate [(C12H2sO)2PO2Na+ or NaDDP] and calcium di-n-dodecyl phosphate [Ca(DDP)2] showed the presence of many diverse structures [354]. In particular, hydrated NaDDP crystals were shown to form lyotropic liquid-crystalline phases which transformed, upon heating to 50 °C, to myelin-like tubes. Structures of the tubes formed were found... 

Lyotropic liquid crystals they display liquid crystalline behavior when mixed with another material in the right concentration (typically a solvent). They can also be a mixture of more than two components (e.g., cell membranes). 

The rigid rod chains in para-aramids tend to form so-called liquid crystals when they are dissolved in polar solvents or heated to certain temperatures. The polymers showing liquid crystalline behavior in melts are called the thermotropic liquid crystalline polymers, and those showing similar properties in solution are called the lyotropic liquid crystalline polymers. These liquid crystals exist in three distinct phases according to their specific structures (Fig. 12.25). 

Siekmeyer, M. Steinmeier, H. Zugenmaier, P. Structural investigations and phase behavior of a ternary lyotropic liquid-crystalline cellulosic system cellulose tricarbanilate/3-chlorophenyl-urethane of cellulose/triethylene glycol monomethyl ether. Makromol. Chem. 1989, 190 (5), 1037-1045. 

At higher concentrations block copolymers form lyotropic liquid-crystalline phases. Their range of stability can depend strongly on temperature. In aqueous solutions polyethylene oxide (PEO) is usually the soluble block. An increase of temperature reduces the solubility of the PEO block which can result in phase transitions into different phases. Most of the present knowledge on lyotropic phase behavior of block copolymers was obtained from studies of Pluronics,i.e., poly(ethyleneoxide-h-propyleneoxide-h-ethyleneoxide) (PEO-PPO-PEO) [31]. Phase diagrams of block copolymers with shorter chains resemble those of low-molecular surfactants. 

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