Anisotropic solution

Many ceUulosic derivatives form anisotropic, ie, Hquid crystalline, solutions, and cellulose acetate and triacetate are no exception. Various cellulose acetate anisotropic solutions have been made using a variety of solvents (56,57). The nature of the polymer—solvent interaction determines the concentration at which hquid crystalline behavior is initiated. The better the interaction, the lower the concentration needed to form the anisotropic, birefringent polymer solution. Strong organic acids, eg, trifluoroacetic acid are most effective and can produce an anisotropic phase with concentrations as low as 28% (58). Trifluoroacetic acid has been studied with cellulose triacetate alone or in combination with other solvents (59—64) concentrations of 30—42% (wt vol) triacetate were common. 

This equation describes the orientation around a common axis called the director of the domain. For perfectly parallel orientation (P2) equals 1. The orientation of the directors in the solution is described by the order parameter PD. The overall orientational order of the anisotropic solution is given by... 

As the pol3nner concentration is increased a highly anisotropic solution is formed. 

Bianchi et al. (19) spun fibers from isotropic and anisotropic solutions of cellulose (D.P. 290) in LiCl (7.8%)-DMAC solutions. The fiber mechanical properties increased throimh the isotropic-anisotropic transition with elastic moduli as high as 22 GPa (161 g/d) being obtained. 

Navard and Haudin (100) examined the rheology of HPC-acetic acid solutions. The anisotropic solutions were strongly viscoelastic. 

Figure 5. Regions of stability of isotropic and anisotropic solutions of 150 bp DNA, calculated according to Stigter 22L) The light band corresponds to the coexistence region for fully charged DNA, the dark band to DNA with 76% of charge neutralized by counterion condensation. The salt/DNA concentration regions where the gelation and ordinary-extraordinary transitions were studied are indicated by brackets.

A major impetus was given to work, both academic and industrial, in the field of lyotropic systems by the development by duPont of commercial fibres having exceptionally high tensile strength and modulus through use of nematic anisotropic solutions of relatively rigid-chain aromatic polyamides. The earliest product to appear, Fibre B, was based upon poly (p-benzamide) (I)10), but was replaced by the fully commercial product, Kevlar, based upon poly (p-phenylene terephthalamide) (II) U). Arenka, from Akzo, also has the latter chemical repeating unit. 

Poly(terephthaloyl hydrazide) forms anisotropic solutions in concentrated solutions of some quarternary ammonium hydroxides such as tetramethyl ammonium hydroxide 33). These solutions contain typically about 10% each of the organic base and the polymer. Aromatic polyhydrazides such as poly(chloroterephthaloyl hydrazide) and various co-polyhydrazides also form anisotropic solutions in some of the solvents used for PPT, such as 100% sulphuric acid and fluorosulphonic acid 34). 

Finally, its worthwhile remarking that this method can be applied to the study of EET in isotropic and anisotropic solutions, and that we have recently extended it to the treatment of EET at gas-liquid and liquid-liquid interfaces or membranes [28],... 

FIG. 16.36 Rheological behaviour of isotropic vs. anisotropic solutions. All graphs are double logarithmic. 

Dopes like these exhibit mesomorphic behaviour they are solid at room temperature, but at higher temperature become less viscous and show optical anisotropy. If heated further, the clearing temperature (Td) is reached at which a phase transformation takes place from an anisotropic solution to an isotropic solution. With increasing polymer concentration both Ta and the melting point of the dope (T ) increase. Fibres of the highest quality (tenacity) are obtained by spinning at temperatures between and Ta, but as low as possible, The spun filament is solidified in a coagulating bath, preferably at temperatures below 5 °C. 

Fig. 2. T- phase diagram for the solution of rods obtained in9) (T temperature, polymer volume fraction in the solution). / isotropic solution, II anisotropic solution, III phase separation region...

The so-called polarizable continuum model (PCM) offers a unified and well sound framework for the evaluation of all these contributions both for isotropic and anisotropic solutions. In PCM, the solute molecule (possibly supplemented by some strongly bound solvent molecules, to include short-range effects such as hydrogen bonds) is embedded in a cavity formed by the envelope of spheres centered on the solute atoms. The procedures to assign the atomic radii and to form the cavity have been described in detail together with effective classical approaches for evaluating K vand ,... 

Liquid crystals are usually excellent solvents for other organic compounds. Non-mesomorphic solute molecules may be incorporated into liquid-crystalline solvents without destruction of the order prevailing in the liquid-crystalline matrix. The anisotropic solute-solvent interaction leads to an appreciable orientation of the guest molecules with respect to the axis of preferred solvent alignment. The consequences may be useful as shown by the use of liquid crystals as anisotropic solvents for spectroscopic investigations of anisotropic molecular properties [166]. Ordered solvent phases such as liquid crystals have also been used as reaction media, particularly for photochemical reactions cf. for example [111, 155, 163] and Section 5.5.9. 

The solution behavior of polymers has been intensively investigated in the past. Dilute solutions, where polymer-polymer interactions may be excluded, have become the basis for the characterization of the primary structure of macromolecules and their dimensions in solution. Besides this "classical" aspect of macromolecular science, interest has focussed on systems, where - due to strong polymer/polymer interactions - association of polymers causes supermolecular structures in homogeneous thermo-dynamically-stable isotropic and anisotropic solutions or in phase-separated multi-component systems. The association of polymers in solutions gives rise to unconventional properties, yielding new aspects for applications and multiple theoretical aspects. 

The molecular weight dependence of the critical concentration for the establishment of uniformly anisotropic solutions of PBG is shown in Table I for various solvents that we have examined. Volume fractions ij>) of polymer quoted in this compilation correspond to the B-point in the nomenclature of Robinson (28-29). The B-point differs from the A-point, a lower concentration where the anisotropic phase just begins to form and is in equilibrium with isotropic polymer solution. 

The rate of degradation is higher in the isotropic solutions (e.g., 14 and 19% (w/w)) than in the anisotropic solutions (Table VII). This would suggest that the mesophases are less accessible and therefore less susceptible to attack by TFA than the isotropic regions. 

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