LiCl/dimethylacetamide system

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]. 

Miyamoto et al. [165] observed a more uniform acetylation among different hydroxyl groups in LiCl dimethylacetamide (DMAC) as compared to heterogeneous reactions (Table 3). Cellulose dissolved in DMSO-PF is known to form methylol derivatives, especially for the 6-OH group. Acetylation of cellulose in this system [174-176] was shown to occur preferentially at the methylol hydroxyl group generated in situ. 

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... 

Mechanical properties are important for the practical use of blend fibers. Usually, the poor compatibility of the component polymers may result in extremely low tenacity of the blend fibers. Literatme reports [140] that the tenacity of cellulose/chitosan blend films increased up to a 20% chltosan content, which was explained by the occurrence of specific interactions between cellulose and chitosan molecules. Improvement of tenacity and of the initial modulus of blend fibers may therefore be attributed to the presence of the interactions between cellulose and chitosan molecules in the fibers. Also, the cellulose and polyacrylonitrile (PAN) molecules form miscible blend pairs in the dimethylacetamide-LiCl solvent system, their miscibility being due to the specific interactions between a hydroxyl group of cellulose and a nitrile group of PAN. 

In concluding this section, we should touch upon phase boundary concentration data for poly(p-benzamide) dimethylacetamide + 4% LiCl [89], poly(p-phenylene terephthalamide) (PPTA Kevlar)-sulfuric acid [90], and (hydroxy-propyl)cellulose-dichloroacetic acid solutions [91]. Although not included in Figs. 7 and 8, they show appreciable downward deviations from the prediction by the scaled particle theory for the wormlike hard spherocylinder. Arpin and Strazielle [30] found a negative concentration dependence of the reduced viscosity for PPTA in dilute Solution of sulfuric acid, as often reported on polyelectrolyte systems. Therefore, the deviation of the Ci data for PPTA in sulfuric acid from the scaled particle theory may be attributed to the electrostatic interaction. For the other two systems too, the low C] values may be due to the protonation of the polymer, because the solvents of these systems are very polar. 

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... 

For the direct method special solvent systems are employed without chemical modification of the cellulose chains. Some examples are LiCl/DMAc (lithium chloride/N,N-dimethylacetamide), DMSO/TBAF (dimethyl sulfoxide/tetra-n-butylammonium fluoride). 

Two cellulose solvents, cadoxen (133) and the more recently discovered. A(iV-dimethylacetamide/LiCl system (134), have shown good promise for use in the SEC analysis of cellulose. The use of these two solvents is described here. In addition, the cellulose solvent systems based on iron-sodium tartrate (8) and DMSO-paraformaldehyde (47,110) have had limited use for the SEC analysis of cellulose. 

The closed SLC is exemplified in Figure 14(b) by a class B system when sites are internally compensated and no further growth accompanies the formation of the mesophase. The behavior of the closed SLC is thus indistinguishable from that of a molecular LC (Figure 14(a)). Relevant cases are DNA [146], adequately described by the theory of the molecular LC (Section II.C.2), and poly(p-benzamide) (PBA) in AA -dimethylacetamide/LiCl solutions. An assembly of seven PBA molecules with a side-by-side shift of one fourth the molecular length was detected in both isotropic and lyotropic solutions. Even the axial ratio of the assembly ( 104) was undistinguishable from the axial ratio ( 100) of molecularly dispersed PBA [147]. 

Another example refers to the transition from the liquid-crystalline to the crystalline state. While studying the phase equilibrii mi in the system poly (p -benzamide), dimethylacetamide (+LiCl), a diagram was obtained which is reproduced in Fig. 7- The compositions of the coexisting phases (isotropic and anisotropic) were determined experimentally. While conducting measurements in the region above lOO C, it was found that on holding the system for one or two days at this temperature both phases pass irreversibly into the gel state, which is accompanied by a... 

Figure 7. Phase diagram of the system poly( -henzamide)-N,N-dimethylacetamide(+ LiCl) .

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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