Mesophase formation, cellulosics

Investigators of cellulosic liquid crystals have two main motivations to study mesophase formation primarily from a scientific viewpoint or a technolomcsd vie oint. The main focus of the latter has been on the potential of preparing high strength/high modulus regenerated cellulose fibers. Another potentim use of cellulosic liquid crystal derivatives is as chiroptical filters (S,lfi). 

Figure 6. Minimum cellulose concentration for mesophase formation as determined by solvent composition DP, 210 storage time at 25 C, 30...

In the current study, the aggregated anisotropic phase occurred in solutions prepared from acid hydrolyzed cellulose of dp 35. The higher minimum cellulose concentration for mesophase formation was observed in cellulose solutions richer in NH4SCN (see Figure 6). In these aspects, the cellulose/NH3/NH4SCN system resembles the DMAC/LiCl/cellulose system. 

The former prediction is now discussed. In the cellulose/NH3/NH4SCN system, the observation that the minimum cellulose concentration for mesophase formation increased with increasing NH4SCN in the solvent up to approximately 75.5%, indicates that this specific solvent composition may be regarded as a good solvent composition. 

An increase in minimum cellulose concentration for mesophase formation with increase in NH4SCN concentration up to 75.5%,... 

Following the first observation of mesophase formation of HPC/H2O, the rheology of this system has been widely investigated. Rheological studies of other cellulosic mesophases, e.g., cellulose, ethylcellulose, and cellulose tricarbanilate, have also been reported. 

The primary factors governing mesophase formation for cellulose derivatives is not only chain stiffness, but also the type and degree of substitution, the molar mass of the polymer, as well as the solvent and the temperature [103]. Among the water-soluble cellulose biopolymers, HPC is still the most investigated derivative (it forms stable and easy to handle mesophases) and as such will... 

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... 

Over the years, the NH3/NH4SCN system has been extensively studied. The research topics covered include the effects of solvent composition, mesophase formation, and fiber formation from isotropic and anisotropic solutions [20,21]. The authors primary interest was in determining the intimate mechanism of dissolution of cellulose in the solvent. In the two papers involving NMR in its various forms, this matter was studied and the results were... 

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). 

Marsano E, Bianchi E and Ciferri A (1984) Mesophase formation and poljoner compatibility. 2. Cellulose acetate/(hydroxypropyl)cellulose/diluent system. Macromolecules 17 2886-2889. 

CON Conio, G., Bianchi, E., Ciferri, A., Tealdi, A., and Aden, M.A., Mesophase formation and chain rigidity in cellnlose and derivatives. 1. (Hydroxypropyl)cellulose in dimethylacetamide, A/ocro/wo/ecM/es, 16, 1264, 1983. 

One of the main properties of cellulose derivatives is the fact that they can originate, under suitable conditions, liquid crystalline phases (mesophases). For each derivative, the solvent used and the critical concentration needed for the formation of a lyotropic phase depend on the type of lateral chain the interaction solvent/lateral chain is a key factor in the formation of a mesophase. Some cellulose derivatives never form a meso-phase with certain solvents and, in some cases, the liquid crystalline phase only forms after shearing [7-9] due to the alignment promoted by the flow of the molecules [10]. 

In this context, according to the Hterature data on the behavior of cellulose solutions [77], polar solvents or acids favor mesophase formation. 

At room temperature, phase separation of high-molecular-weight cellulose-based polymers indicates mesophase formation in a variety of solvents, at critical concentrations in the range of 30 to 50% w/w [75,77]. These critical concentrations ... 

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