Liquid crystalline solution concentration

Solutions of cellulose in NH3/NH4SCN (27 73 w/w) are liquid crystalline at concentrations from 10-16% (w/w) depending on the cellulose molecular weight (64). Optical rotations of the solutions indicate the mesophase is cholesteric with a left-handed twist. The solvent does not react with cellulose. Recently, Yang (60) foimd that cellulose (D.P. 210) formed a mesophase at 3.5% (w/w) concentration at a NH3/NH4SCN of 30 70 (w/w). 

If one follows the solution viscosity in concentrated sulfuric acid with increasing polymer concentration, then one observes first a rise, afterwards, however, an abrupt decrease (about 5 to 15%, depending on the type of polymers and the experimental conditions). This transition is identical with the transformation of an optical isotropic to an optical anisotropic liquid crystalline solution with nematic behavior. Such solutions in the state of rest are weakly clouded and become opalescent when they are stirred they show birefringence, i.e., they depolarize linear polarized light. The two phases, formed at the critical concentration, can be separated by centrifugation to an isotropic and an anisotropic phase. A high amount of anisotropic phase is desirable for the fiber properties. This can be obtained by variation of the molecular weight, the solvent, the temperature, and the polymer concentration. 

The bis(2-ethylhexyl) sodium sulfosuccinate system was initially investigated because its structure of liquid crystalline solution phases and mechanism of solubilization with water had been reported by Rogers and Winsor (10). In our studies, we substituted methanol for water. Table I lists critical micelle concentrations for bis(2-ethylhexyl) sodium sulfosuccinate, triethylammonium linoleate and tetradecyldimethylammonium linoleate in methanol and 2-octanol at 25°C. Literature references for critical micelle concentrations in methanol are sparse, and it has even been suggested that in polar solvents such as ethanol, either micellization does not occur or, if it does, only to a small degree (4). The data of Table I show that micellization occurs in methanol at low concentrations. 

Other structural variations on the rigid-rod PBZXs have encompassed a variety of changes that affect the backbone geometry. Deviation from 180° para-catenation has been investigated by a number of researchers for improved processability. Solution properties are of particular interest in an effort to determine concentration effects on the ability to form liquid crystalline solutions. Most notable backbone deviations have been the ABPBT, ABPBO and ABPBI systems which are characterized by catenation angles of 162°, 150°, and 150° respectively. They are classified as extended chain systems because of the unrestricted rotation between the repeat units. The polymer backbone can... 

Many ccllulosic derivatives form anisotropic, i.e.. liquid crystalline, solutions, and cellulose acetate and triacetate are no exception. Various cellulose acetate anisotropic solutions have been made using a variety ol solvents. The nature of Ihe polymer -solvent interaction determines the concentration at which liquid crystalline behavior is initiated. The better the interaction, the lower the concentration needed to form the anisotropic, birclringenl polymer solution. Strong organic acids, eg, trifluoroac etic acid, are most effective and can produce an anisotropic phase with concentrations as low as... 

Liquid-crystalline solutions and melts of cellulosic polymers are often colored due to the selective reflection of visible fight, originating from the cholesteric helical periodicity. As a typical example, hydroxypropyl cellulose (HPC) is known to exhibit this optical property in aqueous solutions at polymer concentrations of 50-70 wt%. The aqueous solution system is also known to show an LCST-type of phase diagram and therefore becomes turbid at an elevated temperature [184]. 

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. 

It was, however, observed that such systems under appropriate conditions of concentration, solvent, molecular weight, temperature, etc. form a liquid crystalline solution. Perhaps a little digression is in order here to say a few words about liquid crystals. A liquid crystal has a structure intermediate between a three-dimensionally ordered crystal and a disordered isotropic liquid. There are two main classes of liquid crystals lyotropic and thermotropic. Lyotropic liquid crystals are obtained from low viscosity polymer solutions in a critical concentration range while thermotropic liquid crystals are obtained from polymer melts where a low viscosity phase forms over a certain temperature range. Aromatic polyamides and aramid type fibers are lyotropic liquid crystal polymers. These polymers have a melting point that is high and close to their decomposition temperature. One must therefore spin these from a solution in an appropriate solvent such as sulfuric acid. Aromatic polyesters, on the other hand, are thermotropic liquid crystal polymers. These can be injection molded, extruded or melt spun. 

The steady-state didiroic ratio of liquid crystalline solutions of TOLG (Fig. 3) increa s with external field strength and the asymptotic value is 4.5—4.7, regardless of the polymer concentration for comfdetely birefiringent solutions (23). It may be safe to s that all the polymer molecules are parallel or neady parallel within molecular a regates (31). Therefore, the value of 7 for the partide is tentatively assumed... 

Fig. 14. Extinction aiigle as a function of shear rate for liquid crystalline solutions of polyf-y-ethylgbitamatels. The numbers near the lines represent the polymer concentration in vol%. Lines, same as in F 13...

Iv) Liquid crystalline solutions (v > v ) in which the molecules spontaneously organize into an orientationally ordered phase (nematic) comprising domains in which the orientation of the molecules is along a single direction, but the centre of mass positions are random. An estimate for the critical concentration for transition to the nematic phase is V l = A15L b [13]. 

The values of pitch for lyotropic AEC solutions in AA (50% w/w) are given in Table 1, where AEC-2 is a pure AEC with medium DA and AEC-3 is the mixture of EC and fully acetated AEC, which has the same average DA as AEC-2. However, in the liquid crystalline solutions, even at the same concentration, AEC-2 and AEC-3 have different pitch and handedness. This phenomenon was also observed in the lyotropic AEC/chloroform system. The difference in chiro-optical properties may come from the complex interactions of multiple chiral centers present in each repeating unit of the cellulose chain, not from simple racemic mixtures as in the PBG system. 

When an electric field is applied to the liquid crystalline solution of polypeptide, the proton signal of a solvent molecule such as methylene bromide or methylene chloride splits into a doublet however, the center signal is still observed in the initial position, even in the steady state (Fig. 7). The origin of the splitting wfll be mentioned in Section V, and let us pay attention only to this center signal here. This signal corresponds to disordered solvent molecules which are free from the action of molecular fields caused by the oriented molecular clusters. These free solvent molecules are more in evidence in the ratio in less concentrated (but fuUy birefringent) liquid crystalline solution. When measured soon after the removd of the external field (the... 

Fig. 9. The NMR spectra observed after a liquid crystalline solution is placed in the magnetic field of a high resolution NMR spectrometer (14 kG) for PELG (DP 1500) in CH2CI2. Polymer concentration, 14.0 voi%. The numbers near the curves give the number in minutes...

Already in 1965, the formation of lyotropic liquid crystalline solutions of poly-(p-aminobenzoic acid) in concentrated sulfuric acid was observed. This work was the basis for the commercialization of high strength, high modulus, and heat-resistant poly-(p-phenylene terephthalate) (PPDT) fibers under the trade name Kevlar (Dupont) and Twaron (Akzo) (Fig. 15) [31]. PPDT is typically prepared by the polycondensation of terephthaloyl chloride and 1,4-pheny-lenediamine in tertiary amidic solvents such as A-methylpyrrolidinone or dimethylacet-... 

Fig. 4.7 A sketch of concentration regimes of a polymer solution (a) dilute polymer solution — coils do not overlap, c

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