Mesophase formation requirements

In his classic paper, Flory predicted the phase behavior in solutions of rod-like particles (5). The resulting phase diagram related the solvent-solute interaction parameter %i ( -5) to the volume fraction, V2, for polymer rods with an axial ratio of 100. A positive Xi makes a positive or excess free energy contribution to mixing. Good solvents are characterized by small Xi values. Two of Flory s major predictions are that the minimum polymer concentration required for mesophase formation will increase as Xi decreases, sharply at first, then more gradually, and at certain Xi values two different anisotropic phases coexist. Our microscopical observations of conjugated phases may reflect the validity of the latter prediction. 

Admittedly, the existence of the postulated transient mesophase would still require structural confirmation. Even so, the taking of a sharp drop In viscosity as Indicator of mesophase formation has well established precedents In the liquid crystal field. Such Is e.g. the well documented effect In Kevlar referred to above (12) which In many respects has similarities to the presently discussed PE, except that Kevlar Is "mesogenlc" and can exist as stable liquid crystal under ambient conditions, while the mesophase In the flexible PE Is "virtual". The latter "virtual" phase only becomes "real" transiently, which suffices to dramatically affect the entire flow behavior of the material, the effect through which It Is being detected. 

The proof that an isotropic pitch can contain an appreciable mesogenic component can be obtained by removing a sufficient amount of the nonmesogenlc species. Volatilization has been successfully used to reduce the lower molecular weight nonmesogenlc species. However, appreciable chemical reaction occurs at the temperatures required to remove the less volatile nonmesogens. Hence, the chemical reactions may be the cause for mesophase formation rather than volatilization. Solvent fractionation of pitches circumvents this problem as the... 

The distributions required for mesophase formation are broad, but not specific. Both DHFI and TI distributions are different, yet they both form coalesced mesophase. 

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). 

To test the theories quantitatively, careful measurements on well-characterized fractions of polymer are required. The contour length, persistence length and chain diameter may be found from dilute solution measurements on fractions in a given solvent. The critical concentrations for mesophase formation in the same solvent may be calculated from the contour length, persistence length and chain diameter by means of the theories for freely jointed or worm-like chains mentioned above. Results have been reported for... 

The introduction of HPCF by Union Carbide in the USA initiated intensive research and development to improve the processability of mesophase pitch (MP). Riggs and Diefendorf in the US worked on a neo-mesophase pitch based on a solvent extraction technique. Yamada and co-workers discovered the pre-mesophase pitch [41,42] using hydrogenation followed by a rapid heat treatment, while workers at the Kyushu Industrial Research Institute in Japan hydrogenated an anisotropic pitch (preferably a coal tar pitch), which after heat treatment produced a dormant mesophase pitch [43,44], a process known as the Kyukoshi method and able to produce a type of carbon fiber intermediate between a GP and HP fiber. Mochida and co-workers used a Lewis acid, such as AICI3, for the catalytic polymerization of an isotropic pitch, but found that an excessive amount of catalyst was required to achieve mesophase formation. 

This period of history also saw the publication of work by Eaborn and Hartshome [32] on di-isobutylsilandiol, which generated a mesophase. This was a puzzling result at the time, as the molecular shape was inconsistent with views of the time that liquid crystal formation required rod-shaped molecules. Light would be shed on this only after the discovery of liquid crystal phases formed by disc-shaped molecules in the early 1970s. 

In recent years, studies of solutions of polymer blends and of copolymers have aroused a substantial theoretical and experimental interest. This is motivated by both numerous applications and more fundamental issues concerning the usefulness of the scaling and universality concepts to describe the thermodynamic properties and the phase transitions in these systems. In this lecture, chain interactions in dilute and semidilute solutions are reviewed and it is discussed how and when the interactions between chemically different monomers lead to a macroscopic phase separation in the case of ternary polymer A-polymer B- solvent systems and to a mesophase formation in diblock-copolymer solutions. The important conclusion is that due to both the overall monomer concentration fluctuations (excluded volume effects) and the composition fluctuations, the classical Flory theory often fails. This requires the use of the renormalization method and of scaling concepts to give a correct description of the phase diagrams and the critical phenomena observed in these complex systems. We give only here a brief outline, a complete review has been published elsewhere, ... 

The specification requirements for electrode binder pitch, eg, high C/H ratio, high coking value, and high P-resin content, effectively ruled out pitches from gasworks or low temperature tars. The cmde tar is distilled to a medium-soft pitch residue and then hardened by heating for several hours at 385—400°C. This treatment increases the toluene-insoluble content and produces only a slight increase in the quinoline-insoluble (Ql) material, the latter by the formation of mesophase. 

The prime requirement for the formation of a thermotropic liquid crystal is an anisotropy in the molecular shape. It is to be expected, therefore, that disc-like molecules as well as rod-like molecules should exhibit liquid crystal behaviour. Indeed this possibility was appreciated many years ago by Vorlander [56] although it was not until relatively recently that the first examples of discotic liquid crystals were reported by Chandrasekhar et al. [57]. It is now recognised that discotic molecules can form a variety of columnar mesophases as well as nematic and chiral nematic phases [58]. 

These micellar cubic mesophases require large surface curvature and low charge density. Their formation is thus favored by the use of surfactant molecules with large polar head group, and acidic conditions under which the charge density at the silicate/surfactant is always limited. The fact that this phase can be prepared with CTAB when PTES is present, suggests the existence of specific interactions between the phenyl groups and the polar head of the surfactant molecules. It was indeed reported that benzene molecules are preferably located at the hydrophilic-hydrophobic interface [29]. 

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