Cholesteric cellulosics

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). 

Greiner A., Hou H., Reuning A., Thomas A., Wendorff J.H., Zimmermann S., Synthesis and opto-electronic properties of cholesteric cellulose esters. Cellulose, 10, 2003, 37. 

Most cholesteric cellulose derivatives form the right-handed helical stmcture. However the occurrence of helical sense inversion, induced by temperature, was also reported for thermotropic oligomeric cellulose derivatives (Yamagishi et al. 1988). The flexible side-chain not only assists in the melting and the orientation of the cellulose backbone, due to an increase in the mobility of the latter, but also plays an important role in the formation of helical stractures in the cholesteric mesophases (Yamagishi et al. 2006). 

Gray DG (1994) Chiral nematic ordering of polysaccharides. Carbohydr Pol5fm 25(4) 277-284 Greiner A, Hou H, Reuning A, Thomas A, Wendorff JH, Zinunermami S (2003) Synthesis and opto electronic properties of cholesteric cellulose esters. Cellulose 10(l) 37-52 Habibi Y, Lucia LA, Rojas OJ (2010) Cellulose nanocrystals chemistry, self-assembly, and applications. Chem Rev 110(6) 3479-3500... 

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). 

The study of mesophases of cellulose and cellulose derivatives is an active field which has expanded rapidly since the initial observation of liquid crystms of hydroxy-propyl cellulose in 1976. There are two areas that warrant turther investigation recent observations regarding the influence of solvent and/or substituents on the cholesteric helicoidal twist await a theoretical explanation there is a lack of careful studies to permit a theoretical treatment of the behavior of ordered celltdose phases. To date, no applications have been developea where the unusual properties of cellulose derivatives are utilized. 

Solvent viscosity vs, concentration plots for cellulose dissolved in TFA-CH2CI2 (70/30, v/v) do not exhibit a maximum (1I,S1) in contrast to the typicid behavior of polymer liquid crystal solutions. This same behavior is exhibited by other cellulose-solvent systems (52,fiQ). Conio et al. (59) si gest that due to the close proximity of the cholesteric mesophase to its solubility limit, it is only observed in a metastable condition. 

Cholesteric lyotropic mesophases of cellulose in LiCl-DMAC solutions at 1(>-15% (w/w) concentration have been observed by Ciferri and coworkers (19.59.61.62) and McCormick et al. (63). LiCl/DMAC ratios between 3/97 and 11/89 (w/w) were used. LiCl-DMAC does not degrade cellulose and does not react with the polymer (59). It does form a complex with the OH CToups on cellulose which is believed to result in dissolution (62). Optical rotary dispersions are negative, indicating the superhelicoidal structure has a left-handed twist. 

Solutions of cellulose in NH3/NH4SCN (27 73 w/w) are liquid crystalline at concentrations from 10-16% (w/w) depending on the cellulose molecular weight (64). Optical rotations of the solutions indicate the mesophase is cholesteric with a left-handed twist. The solvent does not react with cellulose. Recently, Yang (60) foimd that cellulose (D.P. 210) formed a mesophase at 3.5% (w/w) concentration at a NH3/NH4SCN of 30 70 (w/w). 

Werbowyj and Gray (79) examined the relationships between the cholesteric pitch and optical properties of HPC in water, CH3COOH and CH3OH. The reciproc pitch varied as the third power of the HPC concentration. Optical rotatory dispersion results show HPC has a right-handed superhelicoidal structure regardless of structure. As will be discussea below, a change in solvent can reverse the handedness of other cellulose derivatives. 

Bheda et al. ( ) showed that cellulose triacetate forms a mesophase in dichloroacetic acid. Navard and Haudin (18) examined the thermal behavior of liquid crystalline solutions of CTA in TFA. Navard et al. (23) studied the isotropic to anisotropic transitions of solutions of cellulose triacetate in TFA using differential scanning calorimetry. Navard and Haudin (S2) studied the mesophases of cellulose and cellulose triacetate calorimetrically. Navard et al. (83) report similar studies. Meeten and Navard (97) showed the twist of the cholesteric helicoidal structure of CTA and secondary cellulose in TFA is left-handed. 

Vo and Zugenmaier (105) determined the pitch of cellulose tricarbanilate (CTC, D.P. = 100) in 2-pentanone and methyl ethyl ketone (MEK) and ethyl cellulose (EC) in glacial acetic acid as a function of temperature, concentration, solvent, and degree of polymerization. The pitch of the helicoidal structure of CTC/MEK and CTC/2-pentanone is right-handed but EC in glacial acetic acid is left-handed. This is the first report that the substituent will influence the sense of the cholesteric superhelicoidal structure. 

Ritchey et al. (113) showed the introduction of trifluoroacetate groups at the unsubstituted hydroxyls of cellulose acetate causes a reversal in handedness of the cholesteric structure. Likewise the introduction of an aceto group in acetox3rpropyl cellulose changes the twist (116). 

Guo and Gray (114) foimd that acetylation of the imsubstituted groups in ethyl cellulose changes the sense of the helicoidal cholesteric twist from leff-handed to right-handed in either CHCI3 or m-cresol. 

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. 

Liquid-crystalline solutions and melts of cellulosic polymers are often colored due to the selective reflection of visible fight, originating from the cholesteric helical periodicity. As a typical example, hydroxypropyl cellulose (HPC) is known to exhibit this optical property in aqueous solutions at polymer concentrations of 50-70 wt%. The aqueous solution system is also known to show an LCST-type of phase diagram and therefore becomes turbid at an elevated temperature [184]. 

Aqueous suspensions of cellulose microcrystalhtes obtained by acid hydrolysis of native cellulose fibers can also produce a cholesteric mesophase [ 194]. Sulfuric acid, usually employed for the hydrolysis, sulfates the surface of the micro crystallites and therefore they are actually negatively charged. Dong et al. performed some basic studies on the ordered-phase formation in colloidal suspensions of such charged rod-like cellulose crystallites (from cotton filter paper) to evaluate the effects of addition of electrolytes [195,196]. One of their findings was a decrease in the chiral nematic pitch P of the anisotropic phase, with an increase in concentration of the trace electrolyte (KC1, NaCl, or HC1 of < 2.5 mM) added. They assumed that the electric double layer on... 

There have been a lot of studies of cholesteric films and gels in order to exploit their potential as specific optical media and as other functional materials. Most of the preparations were achieved by modification or improvement of previous attempts to immobilize the cholesteric structure of cellulose derivatives into the bulky networks either by crosslinking of cellulosic molecules with functional side-chains in the liquid-crystalline state [203], or by polymerization of monomers as lyotropic solvents for cellulose derivatives [204-206],... 

There have been continuing works by Suto et al. [213-217] on the preparation of cholesteric solid films of hydroxypropyl cellulose (HPC) crosslinked preferably with glutaraldehyde. Coloring conditions [213,217], swelling [215, 216], and tensile-creep behavior [214] of the crosslinked HPC films were examined. Also, the permselectivity of O2/N2 gasses for such liquid-crystalline cellulosic films was reported to become greater than that for the corresponding amorphous ones [218]. 

Before and after the works described above, contributions to the design and fabrication of similar multicomponent films or gels of cholesteric character, mainly based on HPC, EC, or their derivatives were also made [202, 219-224], Some of these [219,220,224] dealt with shear-deformed network systems preserving a unique banded structure, so that the disappearance and recovery of the optical anisotropy could be controlled thermo-reversibly. Special mention should be made of the successful preparation of two novel classes of solid materials maintaining cholesteric liquid-crystalline order. One consists of essentially pure cellulose only, and the other is a ceramic silica with an imprint of cellulosic chiral mesomorphy. 

Scheme 8 Cellulose urethane derivatives, and their mixing partners to make cholesteric liquid crystals [233]...

Fig. 15 Schematic representation of the photoisomerization effect on a cholesteric meso-phase based on azo-chromophore-containing cellulose derivative molecules. (Quoted from [233] with an adequate modification)...

---