Demixing temperature, critical

The course of the elaidinization reaction can be followed graphically by means of the critical demixing temperature of the oil, using, for example, aniline as a solvent. According to Dahmen the rise in the critical demixing temperature is approximately proportional to the degree of elaidinization (cf. Fig. 85). 

In Fig. 85 the critical demixing temperature of isomerized olive is plotted against the percentage of elaidinized oil (calculated on the amount of elaidinic acid that can... 

Fig. 85. Elaidinization of olive oil. Critical demixing temperatures /od p cl ted against...

The course of the elaidinization reaction can be followed by dilatometric measurements (11), consistency determinations (12), by means of critical demixing temperatures, using aniline or triacetin as a solvent (9), or, more directly, by infrared spectrophotometry (13). 

Chu and co-workers (IJO, 111) have studied the photon correlation function of light scattered by solutions of narrow molecular weight fractions of PS (Mn = 397,000) in cyclohexane near the critical demixing temperature Tc- Their data fit Equations 82 and 83 with y = 1.26 0.08, y = 0.77. The correlation lengths were in the range of 100-250A, and Vs = 0.62, vr = 0.58. For (T — Tc) < 0.1°, deviations from the critical exponent concept were discovered which may result from macro-molecular polydispersity or configurational fluctuations 111)-... 

The difference between the volume fraction of the solute at the maximum of the cloud-point curve and the critical volume fraction can be used as a measure of the polymolecularity. The same holds for the difference between the maximum cloud-point temperature and the critical demixing temperature. 

These approximations (1-3) give the correct critical exponents near the critical demixing temperature, like Pi,2 - Pc oc Tc - oc Tc- or interface... 

Wolf Jend, 1977, Possibilities for determination of enthalpies and volumes of mixing from molecular-weight dependence of critical demixing temperatures and pressures, demonstrated for system trans-decahydronaphthalene-pelystyrene, Makromol. Chem, Vol. 178, No. 6, PP. 1811-1822 1979, Pressure and temperature-dependence of the viscosity of polymer-solutions in the region of phase-separation. Macromolecules, Vol. 12, No. 4, PP. 7i2-7 >7... 

Macrophase separation after microphase separation has been observed in an AB block copolymer/homopolymer C blend (Hashimoto et al 1995). Blends of a PS-PB starblock copolymer (75wt% PS) and PVME homopolymer were prepared by solvent casting. Binary blends of PS and PVME exhibit a lower critical solution temperature (LCST), i.e. they demix at high temperatures. The initial structure of a 50% mixture of a PS-PB diblock and PVME shown in Fig. 6.20(a) consists of worm-like micelles. Heating led to macrophase separation as evident... 

There is, however, a mass transfer problem of demixing at lower temperatures caused by high viscosities. Concentrated polymer solutions tend to take hours to form two distinct liquid phases. A solution to this problem is the use of the lower critical solution temperature. Because of their thermodynamic nature, all polymer-solvent mixtures tend to form two liquid phases ( LL ) with low viscosities, at higher temperatures (LCST) as depicted in Figure 3. 

Interestingly, in the buffered solutions of PDMAEMA stars with 0.1 M NaCl, addition of small amounts of multivalent counterions can induce an upper critical solution temperature (UCST), i.e., demixing upon lowering the temperature (see... 

Figure 4. Demixing temperatures for Lennard-Jones polymer in Lennard-Jones solvent at P, = 2 as a function of polymer concentration for 16-mers (circles) and 64-mers (triangles). Temperature and pressure units are relative to the critical solvent properties. The Lennard-Jones interaction potential energy is cut at 2.5a and shifted [80].

Certain principles mnst be obeyed for experiments where liquid-liquid equilibrium is observed in polymer-solvent (or snpercritical flnid) systems. To understand the results of LLE experiments in polymer solutions, one has to take into acconnt the strong influence of polymer distribution functions on LLE, because fractionation occnrs dnring demixing. Fractionation takes place with respect to molar mass distribution as well as to chemical distribution if copolymers are involved. Fractionation during dentixing leads to some effects by which the LLE phase behavior differs from that of an ordinary, strictly binary mixture, because a common polymer solution is a mnlticomponent system. Clond-point cnrves are measnred instead of binodals and per each individnal feed concentration of the mixtnre, two parts of a coexistence cnrve occnr below (for upper critical solution temperatnre, UCST, behavior) or above the clond-point cnrve (for lower critical solution temperature, LCST, behavior), i.e., produce an infinite nnmber of coexistence data. 

---