Mesophases of Cellulose

In a further step, the Onsager and lattice model were adjusted to account for some shortcomings by the Kyoto group and applied for the description of the molecular mass dependence of thermotropic cellulosics [3]. 

Only a few solvents are known to dissolve cellulose completely, and solid cellulose decomposes before melting. Therefore, it is difficult to study the mesophase behavior of cellulose. Chanzy et al. [32] reported lyotropic mesophases of cellulose in a mixture of jV-methyl-morpholine-Af-oxide and water (20-50%), but were unable to determine the nature of the mesophase. Lyotropic cholesteric mesophase formation in highly concentrated mixtures of cellulose in trifluoroa-cetic acid + chlorinated-alkane solvent [33] and in ammonia/ammonium thiocyanate solutions [34] has been studied, and although poor textures were obtained in the polarizing microscope, high optical rotatory power has been measured in an optical rotation (ORD) experiment, which could be fitted to the de Vries equation [Eq. (3)] for selective reflection beyond the visible wavelength region and was taken as proof of a lyotropic chiral nematic phase. 

Most of the investigations to obtain LC cellulose were undertaken to achieve high-performance films or fibers from anisotropic solutions. The development of stable cellulose LiCl/dimethylacetamide (DMAC) systems led to an attempt to produce anisotropic solutions [36, 37]. Evidence was found of mesophase formation at 10-15% by weight depending on the salt concentration, with some problems due to limited solubility at high concentration ( 15%). Measurements of the persistence length of cellulose in a dilute solution of this system indicate that the cellulose chains are stiffer than those of cellulose derivatives [38], and therefore lower the critical concentration for 

Cellulose/NMMNO (iV-methyl-morpholine-iV-oxide) [32, 40, 41] Cellulose/TFA +chlorinated alkanes (1,2-dichloroethane, CH2CI2) [33] right-handed 

Presently known lyotropic LC celluloses are collected in Table 1. 

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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. 

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. 

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. 

Lyotropic Mesophases of Cellulose in the Ammonia—Ammonium Thiocyanate Solvent System... 

The lyotropic mesophases of cellulose and cellulose derivatives were first observed only relatively recendy (1-3). It is of interest to note that Flory in his now classical papers (44) i icted in 1956 that cellulose or cellulose derivatives should exhitnt liquid crystal behavior. Since Werbowyj and Gray (I) first reported mesophases of hydroxylpropyl cellulose in water, the field has expanded rapidly (for reviews see References 6 and 7). Undoubtedly, the activity in this area originates from a desire to prepare fibers or films of cellulose or cellulose derivatives with supoior properties as well as to understand the purely scientific aspects of the systems. 

Cholesteric lyotropic mesophases of cellulose in dimethylacetamide-LiCl solutions have been observed by Ciferri and coworkers (9-11). While cellulose/TFA-CH2Q2 mesophases have positive optical rotations, the cellulose/ LiCl/DMAC mesophases have negative rotations. 

Recently, cellulose and its derivatives attracted researchers attention again. Besides the work by Aharoni who confirmed the appearance of the liquid crystalline state for cellulose acetate mentioned above, a number of other works appeared in which liquid crystalline state was established for hydroxypropyl cellulose . Finally, Chanzy et al. have obtained the results indicating the possible transition to the mesophase of cellulose itself, and not only its derivatives. 

Chen, Y.S. Cuculo, J.A. Lyotropic mesophase of cellulose in ammonia/ammonium thiocyanate solution. J. Polym. Sci. A Polym. Chem. 1986, 24 (9), 2075-2084. 

Yang, K.S. Theil, M.H. Cuculo, J.A. Lyotropic mesophases of cellulose in the ammonia-ammonium thiocyanate solvent system. Effects of system composition on phase types. ACS Symp. Ser. 1989, 384, 156-183. 

McCormick [6] discovered that Af,Af-dimethylacetamide (DMAc) (Figure 10.3) and lithium chloride (LiCl) would dissolve the cellulose. He and his coworkers also observed cholesteric lyotropic mesophases of cellulose in this solvent system [7,8], which formed at cellulose... 

The original cellulose "solvents" (e.g. cupraammonium-hydroxide, ferric tartaiate) do not yield true solutions. Rather, complexes with cellulose are involved. More recently, several new solvents including N-methyi-morpholine-N-oxide HjO, LiCl/DMAC, liquid NH,/NH.,SCN, trifluoroacetic acid/chlorinated alkanes have been developed and liquids crystalline solutions (lyotropic mesophases) of cellulose have been obtained in them (5). 

Lithium chloride (LiCl)/N,N-dimethylacetamide (DMAc) was employed as a solvent for cellulose by McCormick et al. [83]. Turbak and coworkers were the first to spin cellulose fibers from this solvent system and studied the process extensively [84]. Patel and Gilbert were the first to report the lyotropic mesophase of cellulose in mixtures of trifluoroacetie acid (TFA) and chlorinated alkanes, such as 1,2-dichloroethane and methylene chloride [85]. Other solvents proposed for cellulose include ammonia (NH3)/ammonium thiocyanate (NH4SCN), calcium thiocyanate (Ca(SCN)2)/water, zinc chloride (ZnCl)/water, sodium hydroxide (NaOH)/urea [86], NaOH/thiourea [87, 88], and phosphorie aeid [89, 90]. 

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