Thermotropic cellulose liquid crystals

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... 

IX Cellulosic Liquid Crystals Table 5. Cellulose derivatives forming thermotropic LC mesophases. ... 

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... 

Since this initial observation the field has expanded rapidly and there are numerous reports of cellulose derivatives that form lyotropic liquid crystals. Some of them form botii lyotropic and thermotropic liquid crystals. Gray (2) has tabulated various cellulose derivatives reported to form liquid crystals prior to early 1982. 

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. 

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... 

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). 

K. Shimamura, J.L. White, and J.F. Fellers, "Hydroxypropyl cellulose, A Thermotropic Liquid Crystal," Univ. of Tenn. PATRA Report Mo. 158, 1980. 

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. 

Alderman N, Mackley M (1985) Optical textures observed during the shearing of thermotropic liquid-crystal polymers. Faraday Discuss Chem Soc 79 149-160 Antoun S, Lenz RW, Jin I (1981) Liquid crystal polymers. IV. Thermotropic polyesters with flexible spacers in the main chain. J Polym Sci Polym Chem Ed 19 1901-1920 Baek SG, Magda JJ, Cementwala S (1993) Normal stress differences in liquid crystalline hydroxypropyl cellulose solutions. J Rheol 37 935-945 Barnes HA (2003) A review of the rheology of filled viscoelastic systems. In The British Society of Rheology, pp 1-36... 

More recently El-Waki et al. reported the synthesis and characterization of a series of 4-alkyoxybenzoyloxypropyl cellulose (ABPC-n) samples, obtained via the esterification of hydroxypropyl cellulose (HPC) with 4-alkoxybenzoic acid bearing different numbers of carbOTi atoms (n= 1, 2, 3, 4, 5, 7, 8, 12). In this series the authors noted that only for the higher alkoxy chain lengths (7, 8 or 12) a thermotropic liquid crystal behavior was observed however all the esters of HPC within the series presented lyotropic behavior (El-Wakil et al. 2010). 

Hou H, Reuning A, Wendorff JH, Greiner A (2000) Tuning of the pitch height of thermotropic cellulose esters. Macromol Chem Phys 201(15) 2050-2054 Huang B, Ge JJ, Li Y, Hou H (2007) Aliphatic acid esters of (2-hydroxypropyl) cellulose—effect of side chain length on properties of cholesteric liquid crystals. Polymer 48(l) 264-269 Isogai A, Saito T, Fukuzumi H (2011) TEMPO-oxidized cellulose nanofibers. Nanoscale 3(l) 71-85... 

Cellulose derivatives and polypeptides are well known to exhibit lyotropic liquid crystal properties in a large variety of solvents and only recently have been found to possess, additionally, thermotropic characteristics. 

B. Polyesters Based on R)-3-methyladipoyl Residue Other than the systems based on cellulose and polypeptide derivatives treated previously, (i )-3-methyladipate polyesters containing two-phenyl mesogenic residues are almost the only optically active main chain thermotropic liquid crystal polymers described in the literature. 

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