Dimethylacetamide/LiCl

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. 

Fig. 10. The concentration dependence of viscosity for a PBA solution in dimethylacetamide (-(-LiCl) C is the critical concentration of the transition into the liquid crystalline state, U is the maximum viscosity at the point of the liquid crystalline transitions (according to >)...

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. 

In addition to the necessity for high-temperature SEC, each of the above solvents has its own drawbacks, such as adsorptive effects with phenol derivatives, column blocking, and carcinogenicity with hexamethylphos-phoramide and corrosion of stainless steel by the dimethylacetamide-LiCl mixture. 

Some clear solutions of cellulose, however, are in fact micellar. Clear, apparently purely physical solutions of cellulose are formed in 7V-methyl-morpholine A -oxide (NMNO) and anhydrous dimethylacetamide-LiCl, but they can give rise to anomalously high DP values, because of association of the glucan chains in so-called fringe micelles , in which a group of chains associate chain-to-chain but the ends are largely random chain (Figure 4.34b and c). 

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. 

Terbojevich M., Cosani A., Bianchi E., Marsano E., Solution behaviour of chitin in dimethylacetamide/LiCl, in in... 

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]. 

MAR Marsano, E., Tamagno, M., Bianchi, E., Tetbojevich, M., and Cosani, A., Cellulose-polyaciylonitrile blends 1. Thermodynamic interaction parameters and phase diagram in dimethylacetamide-LiCl, Polym. Adv. Technol, 4, 25, 1993. 

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. 

Solubility in Non-Agueous Solvents and Their Admixtures with Aqueous Organic Bases. Only optically isotropic solutions have been obtained with polyhydrazides in such solvents as dimethyl sulfoxide-LiCl or dimethylacetamide-LiCl. 

The following experiments may serve as examples of the described type of transition. An isotropic solution of poly( -benzamide) in N,N-dimethylacetamide (+LiCl) with a concentration of one per cent is far below the critical concentration for the formation of the liquid-crystalline state. Mixing of such a solution with an excess of nonsolvent (acetone) results in the formation of a precipitate... 

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) .

Extension to the case of liquid crystals formed by supramolecular assem-bhes, the supramolecular liquid crystals (SLC), entails consideration of the contact or intraparticle interactions that stabilize the assembled units. Figure 17 schematizes the transition from the isotropic to the nematic phase for molecularly dispersed polymers (A), and for closed (B) and open (C) supramolecular assem-bhes. If, as discussed in Section I.B, no further association —> dissociation equilibria accompany the transition of closed assemblies, their liquid crystalhne behavior is indistinguishable from that of a MLC. The relevant axial ratio will be determined by the geometry of the assembly with no need to otherwise account for contact interaction. Among the examples are DNA [88] and some synthetic polymers [89,90]. For instance, poly(p-benzamide)(PBA) in NJ I-dimethylacetamide/ LiCl isotropic or nematic solutions may be represented by an assembly of seven PBA molecules with a side-by-side shift of Ath the molecular length. In this... 

Figure 9.4 Micrograph of lyotropic solution of poly(chloro-l,4-phenylene terephthalamide) in dimethylacetamide-LiCl in a quiescent state, where droplets of liquid-crystalline phase are dispersed in a dark background of isotropic phase. (Reprinted from Morgan, Macromolecules 10 1381. Copyright 1977, with permission from the American Chemical Society.)...

MCQ microcrystalline cellulose DMF, dimethylformamide DMA, dimethylacetamide LiCL, lithium chloride BC, bacterial cellulose. ... 

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