Lyotropic Liquid-Crystalline Cellulose Derivatives

Lyotropic LC cellulose derivatives are formed in highly concentrated solutions (-20-70 wt%). As general goals for studying these mesophases, the following incentives might serve. 

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Lyotropic liquid crystalline cellulose derivatives exhibit unique optical properties because of their helicoidal supramolecular structure.The chiro-optical properties of the helicoidal structure can be described by a pitch p (or its inverse, the twist p ) p = 2o/fi, where is the reflection wavelength and h is the mean refractive index of a sheet, and the corresponding handedness of the twist right-handed helicoidal structure being assigned to a positive pitch p > 0) and left-handed helicoidal structures to a negative pitch p < The nematic mesophase can be... 

Zugenmaier, P. Polymer solvent interaction in lyotropic liquid crystalline cellulose derivative... 

Cellulose and its derivatives can form liquid crystalline solutions in a variety of organic solvents. Most of the lyotropic liquid crystalline phases derived from these compoxmds are cholesteric. Since the flow occurs in a shear field, the chiral nematic structure is transformed into a nematic phase. Nevertheless, shear phase orientation can be destroyed when the applied force is removed. This phenomenon is caused by the driving force that makes the liquid crystal form a supramolecular helical structure with thermodynamic stability [70]. The mesophase has a supramolecular helical structure, whose cellulose molecules are inclined at a small angle, which varies from one layer to another. 

Since Robinson [1] discovered cholesteric liquid-crystal phases in concentrated a-helical polypeptide solutions, lyotropic liquid crystallinity has been reported for such polymers as aromatic polyamides, heterocyclic polymers, DNA, cellulose and its derivatives, and some helical polysaccharides. These polymers have a structural feature in common, which is elongated (or asymmetric) shape or chain stiffness characterized by a relatively large persistence length. The minimum persistence length required for lyotropic liquid crystallinity is several nanometers1. 

There have been a lot of studies of cholesteric films and gels in order to exploit their potential as specific optical media and as other functional materials. Most of the preparations were achieved by modification or improvement of previous attempts to immobilize the cholesteric structure of cellulose derivatives into the bulky networks either by crosslinking of cellulosic molecules with functional side-chains in the liquid-crystalline state [203], or by polymerization of monomers as lyotropic solvents for cellulose derivatives [204-206],... 

Gray, D.G. Harkness, B.R. Chiral nematic mesophase of lyotropic and thermotropic cellulose derivatives. In Liquid Crystalline and... 

A typical example of a thermotropic liquid crystalline polymers is the polyesters and the mesogen substituted polysiloxane. The aromatic amide, the super strength fiber known commercially as Kevlar belongs to the lyotropic liquid crystalline polymers. The other important lyotropic liquid crystalline polymers are poly(7-benzyl-L-glutamate), abbreviated as PBLG, cellulose derivatives, the tobacco mosaic virus, etc. 

Liquid crystalline derivatives of the polysaccharide cellulose are characterized by wormlike chains with modest persistence lengths in the order of around ten nanometers [175] and modest axial ratios [166,270]. When the lyotropic liquid crystallinity of hydroxypropyl cellulose was first discovered [251], it was... 

Besides being capable of forming lyotropic liquid crystalline phases, cellulose derivatives can also originate thermotropic mesophases in the absence of any solvent. 

There are also anisotropic solutions formed by hydrophilic polymers such as cellulose derivatives, collagen, a-helical polypeptides, and nucleic acids (DNA and RNA), but, as far as I am aware, there are no lyotropic liquid crystalline phases analogous to the thermotropic nematics, in which small solute molecules are free to move individually. 

There are now numerous examples of cellulose derivatives that form both lyotropic and thermotropic mesophases. Of course, cellulose itself is unlikely to form a thermotropic liquid crystalline phase because it decomposes prior to melting. 

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). 

The physical properties of fibers produced from precursor liquid crystalline solutions are generally superior to those obtained from the corresponding isotropic solutions. Probably the most well-known commercial fiber derived from a lyotropic system is Kevlar , produced by Du Pont. Cellulose fibers have not yet been produced commercially from mesophase solutions using the direct solvent route [15]. Tencel , the commercialized cellulose fibers by... 

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, literature [90] states that at room temperature, acetoxypropyl cellulose exhibits both chiral nematic phases—the lyotropic and the termotropic one. When subjected to specific conditions of shear flow, the cellulose derivative cholesteric liquid crystal suffers transformations, such as cholesteric helix and cholesteric-to-nematic transition. The films prepared from anisotropic solutions of termotropic acetoxypropyl cellulose in an isotropic solvent exhibit anisotropic mechanical properties, generated by the molecular orientation of the solution under shear stress. Thus, liquid crystalline solutions give rise to films with anisotropic mechanical properties the films are brittle when stretched parallel to the shear direction and ductile when stretched perpendicular to it. 

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