Cellulose mesophase solutions

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... 

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. 

Yang, K.S. Cuculo, J.A. Formation and characterization of the fibers and films from mesophase solutions of cellulose in ammonia/ammonium thiocyanate solvent. Polymer 1992,33 (1), 170-174. 

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... 

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). 

Lyotropic liquid crystals are those which occur on the addition of a solvent to a substance, or on increasing the substance concentration in the solvent. There are examples of cellulose derivatives that are both thennotropic and lyotropic. However, cellulose and most cellulose derivatives form lyotropic mesophases. They usually have a characteristic "critical concentration" or "A point" where the molecules first begin to orient into the anisotropic phase which coexists with the isotropic phase. The anisotropic or ordered phase increases relative to the isotropic phase as the solution concentration is increased in a concentration range termed the "biphasic region." At the "B point" concentration the solution is wholly anisotropic. These A and B points are usually determined optically. 

Chanzy and Peguy (13) were the first to report that cellulose forms a lyotropic mesophase. They used a mixture of N-methyl-morpholine-N-oxide (MMNO) and water as the solvent. Solution birefringence occurred at concentrations greater than 20% (w/w) cellulose. The concentration at which an ordered phase formed increased as the cellulose D.P. decreased. The persistence length of cellulose in MMNO-H2O is not known but presumably it has an extended chain configuration in this solvent. Again the question arises as to what is the relevant axial ratio to be used for cellulose. This will be discussed further below. 

Solvent viscosity vs, concentration plots for cellulose dissolved in TFA-CH2CI2 (70/30, v/v) do not exhibit a maximum (1I,S1) in contrast to the typicid behavior of polymer liquid crystal solutions. This same behavior is exhibited by other cellulose-solvent systems (52,fiQ). Conio et al. (59) si gest that due to the close proximity of the cholesteric mesophase to its solubility limit, it is only observed in a metastable condition. 

Cholesteric lyotropic mesophases of cellulose in LiCl-DMAC solutions at 1(>-15% (w/w) concentration have been observed by Ciferri and coworkers (19.59.61.62) and McCormick et al. (63). LiCl/DMAC ratios between 3/97 and 11/89 (w/w) were used. LiCl-DMAC does not degrade cellulose and does not react with the polymer (59). It does form a complex with the OH CToups on cellulose which is believed to result in dissolution (62). Optical rotary dispersions are negative, indicating the superhelicoidal structure has a left-handed twist. 

Bheda et al. ( ) showed that cellulose triacetate forms a mesophase in dichloroacetic acid. Navard and Haudin (18) examined the thermal behavior of liquid crystalline solutions of CTA in TFA. Navard et al. (23) studied the isotropic to anisotropic transitions of solutions of cellulose triacetate in TFA using differential scanning calorimetry. Navard and Haudin (S2) studied the mesophases of cellulose and cellulose triacetate calorimetrically. Navard et al. (83) report similar studies. Meeten and Navard (97) showed the twist of the cholesteric helicoidal structure of CTA and secondary cellulose in TFA is left-handed. 

For a specific polymer, critical concentrations and temperatures depend on the solvent. In Fig. 15.42b the concentration condition has already been illustrated on the basis of solution viscosity. Much work has been reported on PpPTA in sulphuric acid and of PpPBA in dimethylacetamide/lithium chloride. Besides, Boerstoel (1998), Boerstoel et al. (2001) and Northolt et al. (2001) studied liquid crystalline solutions of cellulose in phosphoric acid. In Fig. 16.27 a simple example of the phase behaviour of PpPTA in sulphuric acid (see also Chap. 19) is shown (Dobb, 1985). In this figure it is indicated that a direct transition from mesophase to isotropic liquid may exist. This is not necessarily true, however, as it has been found that in some solutions the nematic mesophase and isotropic phase coexist in equilibrium (Collyer, 1996). Such behaviour was found by Aharoni (1980) for a 50/50 copolymer of //-hexyl and n-propylisocyanate in toluene and shown in Fig. 16.28. Clearing temperatures for PpPTA (Twaron or Kevlar , PIPD (or M5), PABI and cellulose in their respective solvents are illustrated in Fig. 16.29. The rigidity of the polymer chains increases in the order of cellulose, PpPTA, PIPD. The very rigid PIPD has a LC phase already at very low concentrations. Even cellulose, which, in principle, is able to freely rotate around the ether bond, forms a LC phase at relatively low concentrations. 

In the current study, the aggregated anisotropic phase occurred in solutions prepared from acid hydrolyzed cellulose of dp 35. The higher minimum cellulose concentration for mesophase formation was observed in cellulose solutions richer in NH4SCN (see Figure 6). In these aspects, the cellulose/NH3/NH4SCN system resembles the DMAC/LiCl/cellulose system. 

A solvent composition that would otherwise promote the development of a nematic phase, at higher cellulose concentrations yields solutions containing some cholesteric mesophase. 

Cholesteric lyotropic mesophases of cellulose in dimethylacetamide-LiCl solutions have been observed by Ciferri and coworkers (9-11). While cellulose/TFA-CH2Q2 mesophases have positive optical rotations, the cellulose/ LiCl/DMAC mesophases have negative rotations. 

The solution viscosity y5 concentration plots do not exhiUt a maximum in contrast to ical polymer liquid ciystal solutions. As noted in the introduction, this same behavior is exhibited by other cellulose-solvent systems (.9,14) and, as discussed in the introduction, suggests the mesophase is metastable. 

Thin films of the solutions between microscope slides were sheared by applying even pressure on a coverslip while sliding it approximately one cm. The anisotropy appeared to increase as measured by increases in the birefiingence. Solutions containing 10-16% (w/w) cellulose developed a threaded texture and the mesophases were stable with time and oriented in the direction of shear. These observations, while not definitive, suggested a cholesteric to nematic transition occurred on shearing. 

Solid formulations for sustained drug release may contain mesogenic polymers as excipients. The mesogenic polymers form a matrix, which is usually compressed into tablets. Some of the most frequently used excipients for sustained release matrices include cellulose derivatives, which behave like lyotropic liquid crystals when they are gradually dissolved in aqueous media. Cellulose derivatives such as hydroxy-propyl cellulose or hydroxy-propylmethyl cellulose form gel-like lyotropic mesophases in contact with water, through which diffusion takes place relatively slowly. Increasing dilution of the mesophase with water transforms the mesophase to a highly viscous slime and then to a colloidal polymer solution. 

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. 

The theory predicts that the handedness of cellulosic liquid crystalline solutions, designated by the sign of the pitch, depends not only on temperature (T) and on steric repulsion of the chain X), but also on an attractive interaction parameter, %, which depends on the nature of the solvent. The chiral forces are balanced when (x XkT) = 0. In this compensated condition, the pitch of the mesophase should become infinite, and the mesophase resembles a normal nematic phase. 

Chen, Y.S. Cuculo, J.A. Lyotropic mesophase of cellulose in ammonia/ammonium thiocyanate solution. J. Polym. Sci. A Polym. Chem. 1986, 24 (9), 2075-2084. 

Nishio, Y. Chiba, R. Miyashita, Y. Oshima, K. Miyajima, T. Kimura, N. Suzuki, H. Salt addition effects on mesophase structure and optical properties of aqueous hydroxypropyl cellulose solutions. Polym. J. 2002, 34 (3), 149-157. 

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