Thermotropic cellulosics

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... 

In a further step, the Onsager and lattice model were adjusted to account for some shortcomings by the Kyoto group and applied for the description of the molecular mass dependence of thermotropic cellulosics [3]. 

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...

Hou H, Reuning A, Wendorff JH, Greiner A (2000) Tuning of the pitch height of thermotropic cellulose esters. Macromol Chem Phys 201(15) 2050-2054 Huang B, Ge JJ, Li Y, Hou H (2007) Aliphatic acid esters of (2-hydroxypropyl) cellulose—effect of side chain length on properties of cholesteric liquid crystals. Polymer 48(l) 264-269 Isogai A, Saito T, Fukuzumi H (2011) TEMPO-oxidized cellulose nanofibers. Nanoscale 3(l) 71-85... 

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. 

Navard and Zachariades (125) examined the optical properties of shear deformed trifluoroacetox3q)ropyl cellulose and observed band phenomena identical to that for thermotropic nematic copolyesters. Steinmeier and Zugenmaier (107) demonstrated that the phenylacetate... 

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. 

Further challenges need to be addressed for the functional development of liquid-crystalline cellulosics, coupled with the elucidation of fundamental aspects of their chiral nature. In addition, the potential thermotropicity of cellulose microcrystallites with the surfaces modified adequately and manipulation of the cholesteric sense (handedness) in the helically arranged molecular assembly are embryonic but fascinating subjects in this research field. 

Stiff rod-like helical polymers are expected to spontaneously form a thermotropic cholesteric liquid crystalline (TChLC) phase under specific conditions as well as a lyotropic liquid crystal phase. A certain rod-like poly(f-glutamate) with long alkyl side chains was recently reported to form a TChLC phase in addition to hexagonal columnar and/or smectic phases [97,98]. These properties have already been observed in other organic polymers such as cellulose and aromatic polymers. 

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). 

Figure 2 shows an example of a phase diagram of a mixture consisting of a thermotropic polymer—ethyl cellulose (EC)--dissolved in an ordinary organic solvent—acetic acid. 

Gray et al. have reported that (acetoxypropyl)cellulose behaves as a thermotropic cholesteric liquid crystal below 164 °C. It has been also observed that some (hydroxy-propyljcellulose forms a thermotropic liquid crystalline phase at temperatures above 160 °C From these results together with our finding, we presume that rigid rod-like... 

Isotropic polymeric systems as well as particulate systems might also show time-dependent moduli after cessation of flow. As long as the shear does not induce structure growth, the moduli always increase with time after flow. An increase of the moduli upon cessation of flow has also been reported for thermotropic PLCs (18) as well as for lyotropic solutions of hydroxy propyl cellulose in water (19) and in acetic add (20). The possibility of changing in either direction seems to be characteristic for mesomorphic materials. A fundamental theory for describing complex moduli does not exist for such materials. The present results, combined with the information about optical relaxation mentioned above, could be explained on the basis of reorientation of domains or defects. The different domains orient differently, even randomly, at rest whereas flow causes an overall orientation. Depending on the molecular interaction the flow could then cause an increase or decrease in moduli as recently suggested by Larson (21). 

Harrison, P. Navard, P. Cidade, M.T. Investigation of the band texture occurring in acetoxy-propyl cellulose thermotropic liquid crystalline polymer using rheo-optical, rheological, and light scattering techniques. Rheol. Acta 1999, 38 (6), 594-605. 

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 some derivatives form liquid crystals (LC) and represent excellent materials for basic studies of this subject. A variety of different structures are formed, thermotropic and lyotropic LC phases, which exhibit some unusual behavior. Since chirality expresses itself on the configuration level of molecules as well as on the conformation level of helical structures of chain molecules, both elements will influence the twisting of the self-assembled supermolecular helicoidal structure formed in a mesophase. These supermolecular structures of chiral materials exhibit special optical properties as iridescent colors, and... 

Lyotropic cellulosics mostly exhibit chiral nematic phases, although columnar phases have also been observed. The molecules in the thermotropic state also form chiral nematic order, but it is sometimes possible to align them in such a way that a helicoidal structure of a chiral nematic is excluded. Upon relaxation they show banded textures. Overviews on lyotropic LC cellulosics are... 

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