Biphasic gap

The addition of water to solutions of PBT dissolved in a strong acid (MSA) causes phase separation in qualitative accord with that predicted by the lattice model of Flory (17). In particular, with the addition of a sufficient amount of water the phase separation produces a state that appears to be a mixture of a concentrated ordered phase and a dilute disordered phase. If the amount of water has not led to deprotonation (marked by a color change) then the birefringent ordered phase may be reversibly transformed to an isotropic disordered phase by increased temperature. This behavior is in accord with phase separation in the wide biphasic gap predicted theoretically (e.g., see Figure 8). The phase separation appears to occur spinodally, with the formation of an ordered, concentrated phase that would exist with a fibrillar morphology. This tendency may be related to the appearance of fibrillar morphology in fibers and films of such polymers prepared by solution processing. 

The biphasic gap appears to be little affected by the polydispersities of these polymers, contrary to predictions of theory cf. seq. 

Concomitantly with the partitioning of solute species between the coexisting phases, broadening of the biphasic gap (measured by the concentration difference between the two phases) is predicted the more so the greater the disparity... 

Predictions of theory for rods with axial ratio x = 100 are shown by the curves in Fig. 8. Here % is plotted as ordinate against the volume fractions Vp and Vp in the coexisting phases the ordinate may alternatively, be regarded as an (inverse) measure of temperature. The narrow biphasic gap is httle affected by the interactions for negative values of %, as was noted above. If, however, % is positive, a critical point emerges at = 0.055. For values of % immediately above this critical limit, the shallow concave curve delineates the loci of coexisting anisotropic phases, these being in addition to the isotropic and nematic phases of lower concentration within the narrow biphasic gap on the left. At x = 0.070 the compositions of two of the phases, one from each of the respective pairs, reach the same value. Three phases coexist at this triple point. 

Diphasic and segregation phenomena [ 127 -131 ] at the nematic-to-isotropic transition reveal the multicomponent nature of the main-chain polymers as due to the wide molar mass distribution. The thermodynamic width of the biphasic gap was delineated by annealing the samples inside the apparent biphasic gap, giving a result... 

The effect of an elongational flow field on a nematic solution is, according to Panar, rather similar to the observed effects of a magnetic field." Calculations by Khokhlov and Semenov, and Maissa et al, show that an external field does shift the isotropic-nematic transition to lower polymer concentrations and that the biphasic gap disappears at sufficiently high strain rates. According to calculations by Bahar and Erman, the effect of an elongational flow on a system of rod-like particles is pronounced when the system is close to or in the biphasic state, but the effect is of minor importance when the quiescent solution is wholly nematic. However, no experimental proof of these predictions is yet available. 

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