Thermotropic cellulose derivatives

These thermotropic cellulose derivatives are of course of interest from the viewpoint of their structure and properties and might be considered for such applications as chiroptical filters. However, they are unlikely to be considered for fiber formation and certainly not for regenerated fibers, as essenti dly they are ethers of cellulose and desubstitution woiild be difficult. Pawlowski et al. (I2fi) prepared a series of cellulose derivatives, namely phenylacetoxy, 4-meflioxyphenyl-acetoxy-, and p-tolylacetoxy cellulose and tnmethylsilyl cellulose that... 

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

Figure 24. Dependence of side-group length N on L-and for thermotropic cellulose derivatives trialkyl cellulose , cellulose trialkonates A, trialkylesters of CMC (no mesophase) (adapted from [3]).

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). 

Figure 24. Dependence of side-group length N on T and for thermotropic cellulose derivatives trialkyl...

Wu X, Wang L, Yu H, Huang Y (2005) Effect of solvent on morphology of electrospinning ethyl cellulose fibers. J Appl Polym Sci 97 1292-1297 Wiistenherg T (2014) Hydroxypropylcellulose. In Wiistenberg T (ed) Cellulose and cellulose derivatives in the food industry. Wiley, Weinheim, pp 319-342 Yamagishi T, Fukuda T, Miyamoto T, Watanabe J (1988) Thermotropic cellulose derivatives with flexible substituents. Polym Bull 20(4) 373-377... 

Many cellulose derivatives form Hquid crystalline phases, both in solution (lyotropic mesophases) and in the melt (thermotropic mesophases). The first report (96) showed that aqueous solutions of 30% hydroxypropylceUulose [9004-64-2] (HPC) form lyotropic mesophases that display iridescent colors characteristic of the chiral nematic (cholesteric) state. The field has grown rapidly and has been reviewed from different perspectives (97—101). 

LC polyesters belong to the class of thermotropic main-chain LCPs, which also comprises polymers such as polycarbonates, polyethers, polyphenylenes, polyester-imides, polymers containing azo- or azo V-oxide linking groups, some cellulose derivatives, and polypeptides such as po 1 y (y - be n zy 1 -1. - g 1 u tamate). Both from the academic and industrial points of view, polyesters are by far the most important representatives of this class of polymers. 

Since this initial observation the field has expanded rapidly and there are numerous reports of cellulose derivatives that form lyotropic liquid crystals. Some of them form botii lyotropic and thermotropic liquid crystals. Gray (2) has tabulated various cellulose derivatives reported to form liquid crystals prior to early 1982. 

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. 

Quite recently, a series of N-alkyl substituted imidazolium salts has been evaluated for additive effects on the mesomorphic behavior and ensuing optical properties of HPC aqueous solutions, followed by characterization of the thermotropicity of novel cellulose derivatives with such an ionic liquid structure in the side-chains [193]. 

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. 

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. 

One of the main features of nonionic water-soluble cellulose derivatives is that they exhibit, like some other polyethers, an inverse solubility-temperature behavior, i.e. there is phase separation on heating above the so-called lower critical solution temperature (LCST). The temperature at which a polymer-rich phase separates is normally referred to as the cloud point (CP). For ideal solutions, this temperature corresponds to the theta-temperature. Actually, for some derivatives, the cloud point may be preceded, if the concentration is not too low, by a sol-gel transformation with an increase in viscosity and possibly formation of liquid crystals (see Sect. 3.5). As it will be seen later, this reversible thermotropic behavior may be detrimental to the performance of the derivatives or can be advantageneously utilized to develop applications. 

The numerous cellulose derivatives commercially available, displaying varying second order transition, molecular mass, viscosity, lyophilicity, thermotropic properties, have considerable potential. Significant advances have been made in the collection of fundamental data, but the interpretation of this information is somewhat more difficult than for synthetic polymers, mostly because of the heterogeneity of the materials, which exhibit a broad molecular mass distribution and an uneven substitution. 

Cellulose and oligocellulose derivatives have been studied and a variety of thermotropic LC phases established, i.e., cholesteric and colunmar structures for cellulose derivatives, discotic columnar and smectic-type for the oligomers, depending on the side group and the main-chain lengths. A list of compounds exhibiting thermotropic me-sophases is presented in Table 5. 

IX Cellulosic Liquid Crystals Table 5. Cellulose derivatives forming thermotropic LC mesophases. ... 

Besides being at the origin of lyotropic phases, cellulose derivatives can also originate thermotropic liquid crystalline phases without solvent. This behavior is an indication that lateral chains act as solvent, or plasticizer, increasing the mobility of the polymer backbone. 

Many other cellulose derivatives were studied and, among them, acetoxyproylcellu-lose (APC) was found to develop a thermotropic cholesteric phase as well as a lyotropic phase, in several organic solvents, at room temperature. Gray et at [18] prepared this cellulose derivative by the acetylation of hydroxypropylcellulose (a schematic of the chemical reaction is shown in Figure 8.3). 

On the other hand, literature data show [16] that different cellulose derivatives which form liquid crystalline solutions in organic solvents may also form cholesteric thermotropic phases in the absence of a solvent—with spontaneous molecular orientation and cholesteric reflection, such as 2-acetoxypropyl cellulose, 2-hydroxypropyl cellulose, the trifluoroacetate ester of hydroxypropyl cellulose, the propanoate ester of hydroxypropyl cellulose, the benzoate ester of hydroxypropyl cellulose, 2-ethoxypropyl cellulose, acetoacetoxypropyl cellulose, trifluoroacetoxypropyl cellulose, the phenylac-etate and 3-phenylpropionate of hydroxypropyl cellulose, phenylacetoxy, 4-methoxy-phenylacetoxy, p-tolylacetoxy cellulose, trimethylsilyl cellulose, trialkyl cellulose, cellulose trialkanoate, the trialkyl ester of (tri-o-carboxymethyl) cellulose, 6-o-a-(l-methylnaphthalene)-2,3-o-pentyl cellulose, etc. Moreover, the suspensions of cellulose crystallites spontaneously form the chiral nematic phase. The formation of mesophase suspension of cellulose crystalHtes varies from one type of cellulose to another, being influenced, in the formation of the chiral nematic phase, by the mineral acid selected... 

Banded texture is generally observed in relaxed polymer liquid crystal solutions or melts after shearing or annealing of the melts of the thermotropic polymer liquid crystal. For the cholesteric liquid crystalline phase of cellulose derivatives in crosslinkable solvents, the banded texture can be fixed by crosslinking. When polymerizable solvents were used for the preparation of cholesteric liquid crystalline composites films, the... 

Chiral mesophases can be obtained from sugars by several strategies. Many cellulose derivatives show thermotropic and lyotropic cholesteric phases [16]. Peracylated sugars can be used as chiral dopants for discoid nematic phases [17]. Also classical cholesteric and ferroelectric phases can be obtained from carbohydrate-based compounds [18]. In this case, chiral oxa-heterocycles are prepared from sugars. Figure 4.8 shows a chiral twin compound prepared from mannitol [19]. 

Wu, C.C., Gu, Q.C., Huang, Y., and Chen, S.X. (2003) The synthesis and thermotropic behaviour of an ethyl cellulose derivative containing azobenzene-based mesogenic moieties. Liq. Cryst., 30, 733-737. 

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... 

Most cholesteric cellulose derivatives form the right-handed helical stmcture. However the occurrence of helical sense inversion, induced by temperature, was also reported for thermotropic oligomeric cellulose derivatives (Yamagishi et al. 1988). The flexible side-chain not only assists in the melting and the orientation of the cellulose backbone, due to an increase in the mobility of the latter, but also plays an important role in the formation of helical stractures in the cholesteric mesophases (Yamagishi et al. 2006). 

Despite the drawbacks of thermotropic HPC ether and esters interesting optical properties were observed, e.g., iridescent acetoxypropylceUulose (APC) (Tseng et al. 1981), lower-mass HPC (Shimamura et al. 1981) and HPC trilluoroacetate films (Gray 1983), when heated, can be obtained. The ability that cellulose derivatives show to form thermotropic phases is an indication that the lateral chains can play the role of the solvent by increasing the distances between the main chains of the molecules they help the mobility of the macromolecules without the presence of a solvent. 

The production of thermotropic APC cellulose derivatives micro fibers is reported (Canejo et al. 2008) for the electrospinning of APC. These were obtained from a lyotropic solution of APC at room temperature. Scanning Electron Microscopy (SEM) observations showed that the APC electrospun fibers exhibit a spontaneous twist along their axis. 

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