Cholesteric phase lyotropic polymers

Several natural10 and synthetic (e.g., polyisocyanates11) polymers form lyotropic cholesterics with the appropriate solvent also micellar systems formed by amphiphilic molecules and water, if chirality is introduced by either using a chiral amphiphile or adding a chiral dopant, can give cholesteric phases.12... 

Recently, a promising theoretical treatment was introduced by Ferrarini et al.22 which, in selected cases, leads to the effective calculation of the helical sense and pitch of the induced cholesteric phases.23 Attempts to relate the cholesteric handedness of lyotropic cholesterics to the helical sense of the polymers were first reported by Sato and co-workers.11... 

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. 

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. 

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). 

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... 

Until this point, the discussion has dealt with relatively dilute polymer solutions at low temperature, where chain conformation has been extensively studied. The knowledge of the structure of concentrated systems is of utmost importance in respect to swelling of initially dry polymers. In fact, lyotropic mesostructures with one-dimensional order (cholesteric phase) have been observed in recent years for most of the water-soluble cellulose derivatives [23-25]. The HPC-water system has been extensively investigated and Fortin and Charlet [25] have, for instance, established the phase diagram for the 0-100% polymer weight fraction range (Figure 3). 

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. 

Among the several liquid crystal polymers that have been studied in recent years those containing intrinsically chiral elements with a prevalent chirality hold a particular position. Some of these, in fact, by virtue of their structural characteristics, assume a spatial array with nematic planes stacked in a superhelical structure characterized by a prevalent screw sense and are known as cholesteric phase. This kind of order can be controlled by either concentration in solution (lyotropic systems) or temperature in bulk (thermotropic systems). 

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 ... 

Under certain conditions, stiff rod-like helical polymers can spontaneously form lyotropic or thermotropic cholesteric liquid crystal (TChLC) phases. 

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. 

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. 

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. 

As for low molecular weight surfactants, the superstructures are assumed to be formed by micellar aggregates [126], But it seems that the formation of lyotropic liquid crystals is supported by the additional presence of thermotropic mesogens [87,122-124,126], Lamellar, hexagonal, cubic and even nematic and cholesteric mesophases were reported for binary systems, the latter being exceptional. Lyotropic mesophases were also observed in non-aqueous solvents [240,400,401,405], If polymerizable surfactants are studied, not only the phase diagram but also the types of mesophases observed for the monomer and the polymer may be different. 

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. 

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)... 

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 ... 

This robust synthetic approach has frequently been used to produce LSCEs in the last few years as it works well for side chain as well as main chain elastomers. It is also applicable for polymer networks of different LC phase structures (smectic, cholesteric, lyotropic hexagonal) as long as they exhibit prolate chain conformations. We will therefore give a more detailed description in the following paragraphs. 

In conclusion, electric field effects in liquid crystals is a well-developed branch of condensed matter physics. The field behavior of nematic liquid crystals in the bulk is well understood. To a certain extent the same is true for the cholesteric mesophase, although the discovery of bistability phenomena and field effects in blue phases opened up new fundamental problems to be solved. Ferroelectric and antiferroelectric mesophases in chiral compounds are a subject of current study. The other ferroelectric substances, such as discotic and lyotropic chiral systems and some achiral (like polyphilic) meso-genes, should attract more attention in the near future. The same is true for a variety of polymer ferroelectric substances, including elastomers. 

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