Lyotropic cholesteric liquid crystalline polymers

To form cholesteric liquid crystalline polymers, one either polymerizes cholesteric monomers or mixes low molecular mass cholesteric liquid crystals with polymers. In the latter case, two components may be mixed homogeneously or in such a way that the polymers act as a matrix while the small molecular mass cholesteric liquid crystals are in droplets, known as the polymer-dispersed liquid crystals (PDLC) (Doane et al., 1988) or the nematic curvilinear aligned phase (NCAP) (Fergason, 1985). In addition, there are many polymers in nature exhibiting the cholesteric phase such as PBLG, cellulose, DNA, etc. 

The more concentrated cholesteric polymers in solution have small pitch. No satisfactory theory has been presented to describe the relationship between 

Experiments were carried out on poly-peptides with various side chain length in a solvent. The substitution of poly-peptides affects the pitch (Yoshida, 1972). Usually, the larger substitute has the greater pitch. The pitch varies with the substitutes of poly(L-glutamate) in such sequences 

It is demonstrated that the original chiral structure in the cholesteric phase is frozen-in after the solvent is removed. 

An interesting experiment was performed (Tsutsui and Tanaka, 1980, 1981). PBLG and PLG was placed in the solvent propenoic acid and 

---

Other important lyotropic cholesteric liquid crystalline polymer are the cellulose esters. Cellulose is a natural polymer. Their chemical formula is shown in Figure 6.24. 

In addition to the above mentioned lyotropic cholesteric liquid crystalline polymers composed of rigid polymers, there is a diversity of thermotropic cholesteric liquid crystalline polymers which consist of a flexible chain incorporated with a mesogenic and chiral units. The thermotropic cholesteric liquid crystalline polymers are classified into two categories main chain and side chain. 

Stiff rod-like helical polymers are expected to spontaneously form a thermotropic cholesteric liquid crystalline (TChLC) phase under specific conditions as well as a lyotropic liquid crystal phase. A certain rod-like poly(f-glutamate) with long alkyl side chains was recently reported to form a TChLC phase in addition to hexagonal columnar and/or smectic phases [97,98]. These properties have already been observed in other organic polymers such as cellulose and aromatic polymers. 

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. 

Predictions (a), (b) and (c) find abundant verification in experiments on lyotropic solutions of liquid crystalline polymers. a-Helical polypeptides dissolved in various solvents exhibit separation of a cholesteric phase at concentrations in close agreement with the equation above. " Degraded DNA dissolved in aqueous solution likewise induces the formation of a nematic phase above a well-defined concentration " that is in good agreement with the equation above. In both instances, the ratio of the volume fractions in the two phases is about 1 3-1-4, in satisfactory agreement with theory. Observations on the onset of phase separation in solutions of polyaramides are also in approximate agreement with theoretical predictions. Further predictions of the theory are as follows ... 

Since Robinson [1] discovered cholesteric liquid-crystal phases in concentrated a-helical polypeptide solutions, lyotropic liquid crystallinity has been reported for such polymers as aromatic polyamides, heterocyclic polymers, DNA, cellulose and its derivatives, and some helical polysaccharides. These polymers have a structural feature in common, which is elongated (or asymmetric) shape or chain stiffness characterized by a relatively large persistence length. The minimum persistence length required for lyotropic liquid crystallinity is several nanometers1. 

Rod-like 6, adopting an almost 73 helical conformation, belongs to a unique set of stiff polymers, exhibiting both TchLC and lyotropic liquid crystallinity. Indeed, experiments demonstrated that solutions of 6 became cholesteric at high concentrations [99]. The isotropic-biphasic phase boundary concentration increases as the molecular weight is increased. This increase has been described theoretically using the molecular parameters determined from dilute solution data. 

Liquid crystals are broadly classified as nematic, cholesteric and smectic (I)- There are at least nine distinct smectic polytypes bearing the rather mundane labels smectic A, B, C,... I, by the chronological order of their discovery. Some of the smectics are actually three-dimensional solids and not distinct liquid-crystal phases at all. There are three t s of liquid crystals. Thermotropic liquid-crystal phases are those observed in pure compounds or homogeneous mixtures as the temperature is changed they are conventionally classified into nematic, cholesteric, and smectic phases in Fig.2. Lyotropic liquid-crystal phases are observed when amphiphilic molecules, such as soaps, are dissolved in a suitable solvent, usually water. Solutions of polymers also exhibit liquid-crystalline order, the polymeric phases. Most of our knowledge about liquid crystals is based on the thermotropic phases and much of this understanding can be transferred to elucidate polymeric and lyotropic phases. 

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. 

The papers presented in this symposium give some indication of the wide variety of polymers which are now known to form liquid crystalline phases Polymeric liquid crystals are usually classified according to the mesophase structure e g., nematic, cholesteric, smectic A, etc ). However, these classes are quite broad For example, the cholesteric lyotropic phases formed by synthetic polypeptides in suitable solvents differ markedly from the cholesteric thermotropic phases formed from silicone polymers with cho-lesteryl ester side chains. In particular, the driving forces behind the formation of the mesophases are quite different for these two examples, being essentially due to chain stiffness in the first case and to anisotropic dispersion force interactions in the second case It may therefore be useful to classify polymeric liquid crystals according to the polymer chain structure ... 

Many cellulose derivatives form lyotropic liquid crystals in suitable solvents and several thermotropic cellulose derivatives have been reported (1-3) Cellulosic liquid crystalline systems reported prior to early 1982 have been tabulated (1). Since then, some new substituted cellulosic derivatives which form thermotropic cholesteric phases have been prepared (4), and much effort has been devoted to investigating the previously-reported systems. Anisotropic solutions of cellulose acetate and triacetate in tri-fluoroacetic acid have attracted the attention of several groups. Chiroptical properties (5,6), refractive index (7), phase boundaries (8), nuclear magnetic resonance spectra (9,10) and differential scanning calorimetry (11,12) have been reported for this system. However, trifluoroacetic acid causes degradation of cellulosic polymers this calls into question some of the physical measurements on these mesophases, because time is required for the mesophase solutions to achieve their equilibrium order. Mixtures of trifluoroacetic acid with chlorinated solvents have been employed to minimize this problem (13), and anisotropic solutions of cellulose acetate and triacetate in other solvents have been examined (14,15). The mesophase formed by (hydroxypropyl)cellulose (HPC) in water (16) is stable and easy to handle, and has thus attracted further attention (10,11,17-19), as has the thermotropic mesophase of HPC (20). Detailed studies of mesophase formation and chain rigidity for HPC in dimethyl acetamide (21) and for the benzoic acid ester of HPC in acetone and benzene (22) have been published. Anisotropic solutions of methylol cellulose in dimethyl sulfoxide (23) and of cellulose in dimethyl acetamide/ LiCl (24) were reported. Cellulose tricarbanilate in methyl ethyl ketone forms a liquid crystalline solution (25) with optical properties which are quite distinct from those of previously reported cholesteric cellulosic mesophases (26). 

---