Cellulose LiCl/DMAC

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. 

B. Morgenstem and H. W. Kammer, Solvation in cellulose-LiCl-DMAc solutions, Trends Polym. Sci., 4 (1996) 87-92. 

Cellulose LiCl/DMAc, NMMO/water Crystallization, oxidation to produce oxidized cellulose fiber [128]... 

Another activation treatment, suitable for most celluloses (although with great variation of the time required, 1 to 48 h) is polar solvent displacement at room temperature. The polymer is treated with a series of solvents, ending with the one that will be employed in the derivatization step. Thus, cellulose is treated with the following sequence of solvents, before it is dissolved in LiCl/DMAc water, methanol, and DMAc [37,45-48]. This method, however, is both laborious, needs ca. one day for micro crystalline cellulose, and expensive, since 25 mL of water 64 mb of methanol, and 80 mb of DMAc are required to activate one gram of cellulose. Its use may be reserved for special cases, e.g., where cellulose dissolution with almost no degradation is relatively important [49]. 

The relative importance of the hafide anion - HO - Cell interactions can be inferred from application of the Taft-Kamlet-Abboud equation to the UV-Vis absorbance data of solvatochromic probes, dissolved in cellulose solutions in different solvent systems, including LiCl/DMAc and LiCl/N-methyl-2-pyrrolidinone [96]. According to this equation, the microscopic polarity measured by the indicator, Ej (indicator), in kcalmol is correlated with the properties of the solvents by Eq. 1 ... 

Finally, dissolution of non-activated cellulose in LiCl/DMAc, and in ionic liquids has been accelerated by microwave irradiation [72,103,104], although the effect of microwave heating on the DP of the polymer has not been investigated. This last point is relevant in view of the fact that ILs are heated with exceptional efficiency by microwaves [105], so that care must be taken to avoid excessive localized heating that can induce chain degradation of the polymer during its dissolution. 

As discussed above, solvatochromic data for cellulose solutions in liCl-DMAc indicate that Cl - HO - Cell hydrogen bonding is more important for dissolution than li-cellulose interactions. If decrystalUzation is rate Umiting, and considering that the eqiulibria shown in Eqs. 7 and 8 occur prior to decrys-... 

A comment on the properties of the base employed in reactions that involve the formation of the Vilsmeier-Haack adduct is in order, because several derivatives of cellulose are obtained by this route. Preparation of Cell-Tos has been attempted in LiCl/DMAc, by reacting the polymer with TosCl/base. Whereas the desired product was obtained by employing triethy-lamine, use of pyridine (Py) resulted in the formation of chlorodeoxycellu-lose. In order to explain these results, the following reaction pathways have been suggested [147] ... 

Recently, use of LiCl/DMAc and LiCl/l,3-dimethyl-2-imidazolidinone as solvent systems for acetylation of cellulose by acetic anhydride/pyridine has been compared. A DS of 1.4 was obtained the substituent distribution in the products synthesized in both solvents was found to be the same, with reactivity order Ce > C2 > C3. Therefore, the latter solvent system does not appear to be better than the much less expensive LiCl/DMAc, at least for this reaction. It appears, however, to be especially efficient for etherification reactions [178]. It is possible, however, that the effect of cellulose aggregation is more important for its reaction with the (less reactive) halides than with acid anhydrides this being the reason for the better performance of the latter solvent system in ether formation, since it is more efficient in cellulose dissolution. 

Fig. 8 Use of oxalyl chloride in the synthesis of cellulose esters in LiCl/DMAc, from [200]...

The imminium chloride formed was transformed, in-situ, into the corresponding carboxyhc acid derivative, this was added to a solution of cellulose in LiCl/DMAc. Palmitic, stearic, adamantane-1-carboxylic, and 4-nitrobenzoic acids were employed. The DS of the corresponding esters increased as a function of increasing the ratio oxalyl chloride/AGU. The solubihty of the products obtained in aprotic solvents was tested GPC results have indicated negligible degradation of the polymer [200]. 

The solubility of chitin is remarkably poorer than that of cellulose, because of the high crystallinity of chitin, supported by hydrogen bonds mainly through the acetamido group. Dimethylacetamide containing 5-9% liCl (DMAc/IiCl), and N-methyl-2-pyrrohdinone/LiCl are systems where chitin can be dissolved up to 5%. The main chain of chitin is rigid at room temperature, so that mesomorphic properties may be expected at a sufficiently high concentration [67,68]. 

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. 

Slow Thermal Endwise Peeling of Cellulose in DMAc/LiCl. 176... 

McCormick et al (12) observed that cellulose concentrations of 10% (w/w) and above in 9% LiCl/DMAC appear lyotropic after slight shearing, but a pure anisotropic phase was not observed even in 15% (w/w) cellulose solutions. 

Chen and Cuculo (13) found solutions of cellulose in a mixture of liquid aimnonia/NH4SCN (27 73 w/w) are liquid crystalline at concentrations between 10 and 16% w/w depending on the cellulose molecular weight. Optical rotations of the solutions indicate the cellulose mesophase is cholesteric. As in the case of LiCl/DMAC solutions, the optical rotations were negative. 

Advances of the past three decades, however, have produced alternatives to the heavy metal-based cellulose solvents. Prominent among them are dimethylacetamide/LiCl (DMAc/LiCl) N,MMN-0 hydrate (NMMO) tetrabutylammonium fluoride/DMSO (TBAF/DMSO) and potassium thiocyanate/DMSO [48]. While many of these solvents have gained significant popularity among laboratory chemists, only the amine oxide solvent, N,MMNO, has achieved industrial practicality. This will be discussed in the section on regenerated cellulose fibers. Several other important physical properties of cellulose are given in O Table 4. 

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