Cellulose mesophases

In addition to the rigidity, steric effects and flexibility of side groups seems to Influence the formation and properties of cellulosic mesophases they allow (or not) the existence of a mesophase before crystallization, influence the temperature for onset of a mesophase, and contribute to the value of the cholesteric pitch. 

Lyotropic Phases. Lyotropic cellulosic mesophases can be observed in a large variety of solvents with derivatives that can be thermotropic (ethylcellulose, hydroxypropylcellulose, acetoxypropylcellulose, etc.) or not (cellulose acetate). 

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. 

Cellulose Mesophases. An anistopropic phase is formed in a 6% (w/w) solution of cellulose in TFA-CH2Q2 (60 40 sN) and remains anisotropic for at least 16 days (Table I). In TFA-CH2CI2 (70 30 l ) solutions birefringence was observed as... 

Efforts to make high-performance fibers and films from cellulosic mesophases have been made. For example, cellulose fibers produced from cellulosic mesophases show properties superior to those of commercially available fibers. Although these fibers are superior to commercial products, their physical properties are lower than theoretically predicted. This is in part because of that the ordered structures... 

Besides degree of substitution, nature of substituents, solvent, concentration, and temperature, other factors that change the polymer-solvent interactions can affect the pitch of lyotropic cellulosic mesophases. Doping inorganic salts " or small chiral molecules into the lyotropic mesophase changes the polymer-solvent interactions. As the results, the pitch of the chiral nematic mesophase changes accordingly. 

The microstructure and polymer-solvent interactions of lyotropic cellulosic mesophases can be derived from rheological studies. The lyotropic LCP solution is a complicated system and a wide range of unusual rheological phenomena have been observed. 

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. 

Shimamoto, S. Gray, D.G. A method to preserve the chiral nematic order of lyotropic ethylcellu-lose and (acetyl)(ethyl)cellulose mesophases in solid films. Chem. Mater. 1998,10 (6), 1720-1726. 

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). 

One of the best examples of a man-made fiber spun from a lyotropic liquid crystalline polymer is poly (p-phenylene terephthalamide) (PPTA, Kevlar ), which was commercialized by DuPont in 1972. The Kevlar fibers possess impressive mechanical properties. Some polysaccharides are able to form lyotropic liquid crystalline states. High performance fibers from liquid crystalline spinning of cellulose mesophases have gained much interest, and a considerable amoimt of work has been done on spinning cellulose, with some of the products commercialized. This will be further discussed below. 

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