Thermotropic phase, cellulosic

Besides being at the origin of lyotropic phases, cellulose derivatives can also originate thermotropic liquid crystalline phases without solvent. This behavior is an indication that lateral chains act as solvent, or plasticizer, increasing the mobility of the polymer backbone. 

On the other hand, literature data show [16] that different cellulose derivatives which form liquid crystalline solutions in organic solvents may also form cholesteric thermotropic phases in the absence of a solvent—with spontaneous molecular orientation and cholesteric reflection, such as 2-acetoxypropyl cellulose, 2-hydroxypropyl cellulose, the trifluoroacetate ester of hydroxypropyl cellulose, the propanoate ester of hydroxypropyl cellulose, the benzoate ester of hydroxypropyl cellulose, 2-ethoxypropyl cellulose, acetoacetoxypropyl cellulose, trifluoroacetoxypropyl cellulose, the phenylac-etate and 3-phenylpropionate of hydroxypropyl cellulose, phenylacetoxy, 4-methoxy-phenylacetoxy, p-tolylacetoxy cellulose, trimethylsilyl cellulose, trialkyl cellulose, cellulose trialkanoate, the trialkyl ester of (tri-o-carboxymethyl) cellulose, 6-o-a-(l-methylnaphthalene)-2,3-o-pentyl cellulose, etc. Moreover, the suspensions of cellulose crystallites spontaneously form the chiral nematic phase. The formation of mesophase suspension of cellulose crystalHtes varies from one type of cellulose to another, being influenced, in the formation of the chiral nematic phase, by the mineral acid selected... 

Despite the drawbacks of thermotropic HPC ether and esters interesting optical properties were observed, e.g., iridescent acetoxypropylceUulose (APC) (Tseng et al. 1981), lower-mass HPC (Shimamura et al. 1981) and HPC trilluoroacetate films (Gray 1983), when heated, can be obtained. The ability that cellulose derivatives show to form thermotropic phases is an indication that the lateral chains can play the role of the solvent by increasing the distances between the main chains of the molecules they help the mobility of the macromolecules without the presence of a solvent. 

Many cellulose derivatives form Hquid crystalline phases, both in solution (lyotropic mesophases) and in the melt (thermotropic mesophases). The first report (96) showed that aqueous solutions of 30% hydroxypropylceUulose [9004-64-2] (HPC) form lyotropic mesophases that display iridescent colors characteristic of the chiral nematic (cholesteric) state. The field has grown rapidly and has been reviewed from different perspectives (97—101). 

There are now numerous examples of cellulose derivatives that form both lyotropic and thermotropic mesophases. Of course, cellulose itself is unlikely to form a thermotropic liquid crystalline phase because it decomposes prior to melting. 

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. 

Lyotropic Phases. Lyotropic cellulosic mesophases can be observed in a large variety of solvents with derivatives that can be thermotropic (ethylcellulose, hydroxypropylcellulose, acetoxypropylcellulose, etc.) or not (cellulose acetate). 

Figure 2 shows an example of a phase diagram of a mixture consisting of a thermotropic polymer—ethyl cellulose (EC)--dissolved in an ordinary organic solvent—acetic acid. 

Gray et al. have reported that (acetoxypropyl)cellulose behaves as a thermotropic cholesteric liquid crystal below 164 °C. It has been also observed that some (hydroxy-propyljcellulose forms a thermotropic liquid crystalline phase at temperatures above 160 °C From these results together with our finding, we presume that rigid rod-like... 

One of the main features of nonionic water-soluble cellulose derivatives is that they exhibit, like some other polyethers, an inverse solubility-temperature behavior, i.e. there is phase separation on heating above the so-called lower critical solution temperature (LCST). The temperature at which a polymer-rich phase separates is normally referred to as the cloud point (CP). For ideal solutions, this temperature corresponds to the theta-temperature. Actually, for some derivatives, the cloud point may be preceded, if the concentration is not too low, by a sol-gel transformation with an increase in viscosity and possibly formation of liquid crystals (see Sect. 3.5). As it will be seen later, this reversible thermotropic behavior may be detrimental to the performance of the derivatives or can be advantageneously utilized to develop applications. 

Cellulose and some derivatives form liquid crystals (LC) and represent excellent materials for basic studies of this subject. A variety of different structures are formed, thermotropic and lyotropic LC phases, which exhibit some unusual behavior. Since chirality expresses itself on the configuration level of molecules as well as on the conformation level of helical structures of chain molecules, both elements will influence the twisting of the self-assembled supermolecular helicoidal structure formed in a mesophase. These supermolecular structures of chiral materials exhibit special optical properties as iridescent colors, and... 

Lyotropic cellulosics mostly exhibit chiral nematic phases, although columnar phases have also been observed. The molecules in the thermotropic state also form chiral nematic order, but it is sometimes possible to align them in such a way that a helicoidal structure of a chiral nematic is excluded. Upon relaxation they show banded textures. Overviews on lyotropic LC cellulosics are... 

Rod-like macromolecules or semi-flexible chains such as cellulosics with a certain rigidity may form thermotropic and, in highly concentrated solution with suitable interactions, lyotropic liquid-crystalline phases. 

Cellulose and oligocellulose derivatives have been studied and a variety of thermotropic LC phases established, i.e., cholesteric and colunmar structures for cellulose derivatives, discotic columnar and smectic-type for the oligomers, depending on the side group and the main-chain lengths. A list of compounds exhibiting thermotropic me-sophases is presented in Table 5. 

Ethoxypropyl cellulose [59], an ethyl ether of HPC forms excellent thermotropic and lyotropic mesophases, the lyotropic ones with acetonitrile, dioxane, and methanol. Both thermotropic and lyotropic systems exhibit cholesteric phases with a right-handed helicoidal supermolecular 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). 

Many other cellulose derivatives were studied and, among them, acetoxyproylcellu-lose (APC) was found to develop a thermotropic cholesteric phase as well as a lyotropic phase, in several organic solvents, at room temperature. Gray et at [18] prepared this cellulose derivative by the acetylation of hydroxypropylcellulose (a schematic of the chemical reaction is shown in Figure 8.3). 

Cellulose and its derivatives have the ability to behave both as thermotropic and lyotropic liquid crystals. As mentioned above, several specific phases of liquid crystals occurs, depending on the structure or combination of molecules. In the nematic phase, the molecules have only orientational ordering (making the liquid crystal phase less ordered), while in the smectic phase, the molecules have both orientational and positional ordering [75]. In addition, the optically active molecules can form a chiral nematic phase (or cholesteric phase). In this case, the molecules are helix-oriented generating some spectacular optical properties. 

Banded texture is generally observed in relaxed polymer liquid crystal solutions or melts after shearing or annealing of the melts of the thermotropic polymer liquid crystal. For the cholesteric liquid crystalline phase of cellulose derivatives in crosslinkable solvents, the banded texture can be fixed by crosslinking. When polymerizable solvents were used for the preparation of cholesteric liquid crystalline composites films, the... 

Chiral mesophases can be obtained from sugars by several strategies. Many cellulose derivatives show thermotropic and lyotropic cholesteric phases [16]. Peracylated sugars can be used as chiral dopants for discoid nematic phases [17]. Also classical cholesteric and ferroelectric phases can be obtained from carbohydrate-based compounds [18]. In this case, chiral oxa-heterocycles are prepared from sugars. Figure 4.8 shows a chiral twin compound prepared from mannitol [19]. 

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