Mesophase separation

Bulk processes of carbonization can be "stopped" at an appropriate stage and mesophase separated from the remaining pitch by solvent extraction. Spheres of mesophase can then be isolated. However, from the viewpoint of specialised industrial usage, e.g. spinning /moulding (not traditional coke-making) such mesophase can be said to be "overcooked". It is not readily soluble in generally available solvents it does not represent a minimum in its visco-elastic properties. 

In those days, pitch chemistry had not advanced sufficiently to understand fundamentally the foregoing phenomena. The preparation of mesophase pitch with low softening point (the so-called "soft mesophase pitch") was based on direct experiment. Nevertheless through extensive and serious efforts, it became possible to prepare soft mesophase pitches from naphtha tars, decant oils from fluidized catalytic crackers (FCC), atmospheric-reduced crude oils, and other pitch-like materials. A typical example of these preparation procedures is the following. A purified FCC or naphtha pitch is heated at 400°C for one hour under methane to convert the pitch to a mesophase content of 23.6% (27,28). The mesophase separated by sedimentation has a softening point of 226°C it is spun at 320°C, and the fiber is stabilized in air and finally carbonized by rapid heating at 100 to 1600°C/min (29). 

TABLE II. Ratio of Theoretical to Experimental Volume Fractions at Mesophase Separation in the High Molecular Weight Limit... 

Volume fractions of polymer at mesophase separation as function of axial ratio of rod-like or semirigid chain, (fi and a, the volume fractions of polymer in coexisting isotropic and anisotropic phases, respectively, are calculated on the basis of Odijk s theory with q = 1200 A and d = 18 A. The metastable volume fraction, Vn, calculated on the basis of X = L/d from Flory s lattice model is also shown for comparison. Filled circles are experimental data points for PBG in nitrobenzene. The vertical bar on the abscissa is the axial ratio at which L q for this polymer in this solvent. 

FIGURE 2. Width of blphasic zone expressed as (i >a -persistence lengths, Np, in the polymer chain. 

FIGURE 3. Order parameter, S, at mesophase separation from Khokhlov-Semenov-Odijk theory as a function of Np. 

The comparison of copolymers SEO and SCL has shown that the nature of the crystallizable blocks governs both the number of mesophases in the copolymers and the number of folds of the crystallizable chains In a solvent for the crystallizable block as well as in a solvent for the amorphous block, SEO and BEO copolymers dependent on temperature exhibit two mesophases separated by the melting of the PEO chains. SCL copolymers contain mesophases only in a solvent for the amorphous blocks and these mesophases disappears at the melting temperature of the PCL chains, where the two types of blocks become compatible For SEO copolymers containing less than 50% PEO, the number of folds of the PEO chains is determined by the molecular weight of polystyrene which remains the dominating factor at higher PEO contents. For SCL copolymers on the contrary, the number of folds of the PCL chains increases with the PCL content of the copolymer ... 

Figure 47 shows the qualitative behavior of this free energy density. A crucial feature is that the renormalized distance xR corresponds still to the inverse scattering intensity S-l(q) at q = q. Since xocxocl/T in simple polymers, the nonlinear relation between x and xR then implies a nonlinear relation between xR and 1/T. Thus while Leibler s theory [43] predicts a linear variation of S" (q ) with 1/T (near the temperature where S-1(q ) should vanish for f = 1/2), the fluctuation effects of Helfand and Fredrickson [58] imply a curved variation of S l(q ) with 1/T. Such a curved variation indeed is found both in experimental data [317-323] and simulations [325, 328], see Figs. 43b, 48. Of course, due to finite size problems in the simulation one cannot as yet detect the small jump singularity that signals the mesophase separation transition in the experiment (Fig. 48). 

In ternary systems, too, where the anion- and cation-active association colloids are replaced by a non-ion-active colloid of the polyoxyethylene type, similar conditions were found, with different mesophases separated by two-phase and three-phase regions. For instance, in the alkylpoly-oxyethylene-oleic acid-water system all five of known mesophases E, D, F, C, and B, are found as well as two extremely stiff, completely transparent gels, which are optically isotropic (19, 21). Their x-ray patterns, which are similar but differ from the other phases on essential points, have so far resisted satisfactory interpretation. These mesophases, Ii and I2, occur in different parts of the system it is obvious that one structure is the reverse of the other—thus parallel to E and F. The structure would possibly be one of spheres with the densest cubical packing. 

Finally, the subject of the present review, the reaction-induced phase separation, is present in several other areas of the polymer field. For example, composition fiuctuation effects in chain copolymerization can be sufficient to induce liquid-liquid phase separation or mesophase separation within the reaction medium [150]. This has a bearing in bulk and emulsion polymerizations. 

Cloud-Point Curve, Shadow Curve, and Mesophase Separation in Solutions... 

On the basis of these assumptions the change in free energy was calculated for the process going from a disordered mixture to the mesophase separated system, with the contribution of polydispersity neglected. Phenomenological models are widely in use [65, 87]. However, such calculations invoke many premises, which include the shape of the microdomains, the special nature of the interface, etc. They were so extensively reviewed in monographs and symposium proceedings that we will not concerned with them in this paper (see Ref. [65] for primary sources). 

Finally, the coexistence curves are calculated by the same method as developed for random copolymer solutions, Eqs. (44)-(52), with Inf and Infa replaced by Eqs. (184). The spinodal limit of mesophase separation may be calculated by solving the condition in which all second-order parameters in AF [94,99] vanish. 

Xap = The locations of the spinodal limit of mesophase separation changes remarkably if the polydispersity in rg increases. Fig. 16. The symmetry condition chosen (%a = Xap) predicts a linear spinodal limit between the homogeneous random phase (H) and the microphase region (M), a result which has already been found by Hong and Noolandi [99]. Generally, polydispersity expands the... 

SIM PLE Spinodal-decomposition-induced macro and mesophase separation plus extraction by rinsing... 

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