Lyotropic polymer solutions

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 self-organizing nature of liquid crystal polymers is reflected in their complex flow behavior. The relaxation phenomenon of lyotropic polymer solution after shear cessation leads to band texture morphology that can be further induced to isotropic materials. Future research should focus on solvents influence on band size implicitly related to the induced pattern in polymers with different structures. Another aspect that could be explored is the imidization of patterned polyimide precursors and those conditions in which the texture is still maintained. 

Lyotropic polymer solutions evidence a special behavior at critical concentration, the solution is separated into two phases, one with a high concentration of liquid crystals, the other with low concentration and isotropic characteristics. At critical concentration, the viscosity of solutions increases sharply. 

Lyotropic polymer solutions exhibit a peculiar behavior above a critical concentration, they separate in two phases the highly concentrated phase is liquid crystaUine, whereas the least concentrated phase is isotropic. The higher the factor A, the lower the critical volume fraction at which phase separation occurs. 

Rigid backbone polymers are typically difficult to dissolve. A significant application of lyotropic polymer solutions is the nematic spinning dope... 

Figure 6 shows the phase diagrams plotting temperature T vs c for PHIC-toluene systems with different Mw or N [64], indicating c( and cA to be insensitive to T, as is generally the case with lyotropic polymer liquid crystal systems. This feature reflects that the phase equilibrium behavior in such systems is mainly governed by the hard-core repulsion of the polymers. The weak temperature dependence in Fig. 6 may be associated with the temperature variation of chain stiffness [64]. We assume in the following theoretical treatment that liquid crystalline polymer chains in solution interact only by hardcore repulsion. The isotropic-liquid crystal phase equilibrium in such a solution is then the balance between S and Sor, as explained in the last part of Sect. 2.2. 

Figures 7 and 8 display such plots for various lyotropic liquid-crystalline polymer systems, which range in q from 5.3 to 200 nm. As expected, most data points come close to the theoretical curve. This finding suggests that liquid crystallinity of stiff-chain or semiflexible polymer solutions has its main origin in the hard-core repulsion of the polymers.

Both polymer melts and polymer solutions sometimes form phases with orientational and positional ordci. Thermotropic polymer liquid crystals possess at least one liquid crystal phase between the glass-transition temperature and the transition temperature to the isotropic liquid. Lyotropic polymer liquid crystals possess at least one liquid cry stal phase for certain ranges of concentration and temperature. 

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). 

The phenomenology of physical organogels and jellies is extremely rich, and their comportments are similar in some aspects to those of both surfactants in solution (e.g., lyotropism and crystallization) and polymer solutions (6 (e.g.. swelling/shrinking behaviors and microscopic mass motion). Gels can be considered as being at the interface between complex fluids (i.e.. micellar systems) and phase-separated states of matter. The main properties and concepts appropriate to describe the gels and the basic principles of techniques for their study will be reviewed here. 

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. 

The phenomenon of phase separation in liquid crystalline polymer solutions has been the subject of recent interest. This phenomenon was noticed in a number of lyotropic liquid crystal... 

Solid formulations for sustained drug release may contain mesogenic polymers as excipients. The mesogenic polymers form a matrix, which is usually compressed into tablets. Some of the most frequently used excipients for sustained release matrices include cellulose derivatives, which behave like lyotropic liquid crystals when they are gradually dissolved in aqueous media. Cellulose derivatives such as hydroxy-propyl cellulose or hydroxy-propylmethyl cellulose form gel-like lyotropic mesophases in contact with water, through which diffusion takes place relatively slowly. Increasing dilution of the mesophase with water transforms the mesophase to a highly viscous slime and then to a colloidal polymer solution. 

The microstructure and polymer-solvent interactions of lyotropic cellulosic mesophases can be derived from rheological studies. The lyotropic LCP solution is a complicated system and a wide range of unusual rheological phenomena have been observed. 

Finally, there is considerable interest in polymeric assemblies both in solution and in liquid crystalline phases [87]. In a seminal report, Meijer and co-workers [49] have synthesized dimers of module 75 (e.g. 101) and shown that its solutions have rheological properties similar to those shown by normal polymer solutions (Fig. 25). In this regard, the high dimerization constant of 75 allows a high degree of polymerization at accessible concentrations. Likewise, Lehn has shown that 1 1 mixtures of 102 103 and 33 104 form supramolecular, polymeric, liquid crystalline phases (Fig. 25). The structure of 102 103 is believed to contain a triple helical superstructure [88], whereas rigid assembly 33 104 forms a lyotropic mesophase [89]. 

The most extensively studied lyotropic polymer is poly-y-benzyl-L-glutamate (PBLG). This polymer exists in solution as a rigid rod-like a-helix. A variety of solvents including dimethylformamide. 

It may be assumed that the decisive part in the appearance of lyotropic mesomorphism in a concentrated polymer solution is played by the parameter A which determines the degree of its intramolecular orientational axial order, Experimental data show that in contrast to low molecular weight liquid crystals, in this case, the mesomorphism is determined by the length of the part that is of rodlike shape rather than by the chain length as a whole. 

Nonequilibrium electro-optical properties of polymers with mesogenic side groups are also very distinctive. In alternating electric fields, low-frequency dispersion of the Kerr effect is observed. The range of dispersion depends on the molecular weight of the fraction, just as for lyotropic polymers. Fig. 9 shows the dependence of the relative value of the Kerr constant at the field frequency V and at v=0 on the field frequency for solutions of... 

Cellulose triacetate-trifluoroacetic acid cholesteric solutions - This kind of lyotropic polymer liquid crystals undergoes a mesomorphic-isotropic phase transition upon heating. The peak is well defined but very small The determination of N for this... 

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