Cellulose derivatives, liquid crystalline properties

An effort to combine the thermotropic liquid-crystalline properties of selected cellulose derivatives with the ion-conductive characteristics of poly(ethylene oxide) (PEO) has currently been pursued [234,235]. TPEO-CELL shown in Scheme 7 is a representative sample. 

Pawlowski W.P, Gilbert R.D., Pomes R.E., Purrington S.T., The liquid-crystalline properties of selected cellulose derivatives, J. Polym. Sci. B Polym. Phys., 26, 1988, 1101-1110. 

Liquid crystalline properties of cellulose and its derivatives can be exploited to produce biomimetic materials or all-cellulosic-based composites with enhanced mechanical properties. These materials will be the focus of this chapter. 

Chemical modification of natural polymers such as starch, dextran, cellulose and proteins represents an attractive alternative route to totally synthetic polymers for producing biodegradable polymers. Early modification of polysaccharides resulted in hydrophobic materials such as cellulose acetate and cellulose nitrate. Both are degradable by microorganisms. Hydroxyalkylcelluloses should be of interest because of their liquid crystalline properties.The only question is whether their biodegradability is unified. Polysaccharides react with small carboxylic acids to produce derivatives that are biodegradable. " ... 

The first part of the book discusses formation and characterization of the microemulsions aspect of polymer association structures in water-in-oil, middle-phase, and oil-in-water systems. Polymerization in microemulsions is covered by a review chapter and a chapter on preparation of polymers. The second part of the book discusses the liquid crystalline phase of polymer association structures. Discussed are meso-phase formation of a polypeptide, cellulose, and its derivatives in various solvents, emphasizing theory, novel systems, characterization, and properties. Applications such as fibers and polymer formation are described. The third part of the book treats polymer association structures other than microemulsions and liquid crystals such as polymer-polymer and polymer-surfactant, microemulsion, or rigid sphere interactions. 

Discovery of the formation of liquid crystalline solutions by cellulosics in the mid-1970s has resulted in attempts to develop new cellulosics products with properties superior to those of conventional cellulosic. Following the first observation of mesophases formed in aqueous solutions of hydroxypropyl cellulose (HPC), a variety of other cellulose derivatives have been reported to form liquid crystals. Liquid crystalline solutions of cellulose and its derivatives provide a potential route to high-modulus and high-tenacity cellulosic fibers, films, and other high-performance products. 

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... 

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... 

Cellulose is the most abundant biopolymer on earth. It can be used in different applications, namely in the form of fibers, and cellulose can be converted into numerous cellulose derivatives. Cellulose micro- and nanofibers have been the subject of intense research in the field of composites. Cellulose derivatives can show liquid crystalline chiral nematic phases, which can be used for the production of diverse composite systems. All-cellulosic composites based on liquid crystalline cellulosic matrices reinforced by cellulose micro- and nanofibers can show enhanced mechanical properties due to fiber orientation induced by the liquid crystalline matrix. Cellulose-based fibers electrospun from liquid crystalline phases can develop different structures, which are able to mimic the shape of plant tendrils on the nano- and microscale, opening new horizons for ceDulosic membrane applications. 

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]. 

Cellulose is a fascinating biopolymer that has always been used in the production of textile fibers. Due to environmental concerns intense research has been conducted in the past decades in order to substitute traditional carbon or glass fibers used in the production of composites with eco-friendly cellulose fibers. The research in cellulose-based biocomposites is now focused on the concept of self-reinforced nanocomposites. In this sense all-cellulose composites have been investigated showing mechanical properties comparable or even better than those of traditional composites. Cellulose and its derivatives may also show liquid crystalline mesophases, which can be used to produce new and biomimetic materials with distinctive mechanical and optical properties. Most likely, enhanced mechanical properties will be obtained in all-cellulose nanocomposites by taking full advantage of the orientational order, when both the matrix and the fibers are in a liquid crystalline state. 

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. 

White JL, Dong L, Han P, Laun HM (2004) Rheological properties and associated structural characteristics of some aromatic polycondensates including liquid-crystalline polyesters and cellulose derivatives. Int Union Pure Appl Chem 76(ll) 2027-2049 Wiberg G, Hillborg H, Gedde UW (1998) Assessment of development and relaxation of orientation in a sheared thermotropic liquid crystalline copolyester. Polym Eng Sci 38 1278-1285 Wilson TS, Baird DG (1992) Transient elongational flow behavior of thermotropic liquid crystalline polymers. J Non-Newt Fluid 44 85-112... 

The chemical modification of cellulose can be an effective way to facilitate the processing of ceUulosic materials. It was observed that adding lateral chains to the cellulosic main chain increases the number of solvents in which the cellulose derivative is soluble and that a high degree of substitution, or non-uniform substitution, prevents crystallization and improves solubility. Processing cellulose derivatives from the liquid crystal phase opens the door to the production of materials with new properties and several works are reported in literature concerning the processing of cellulosic derivatives from the liquid crystalline phase. 

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