Phase separation critical temperature

Trichlorodifluoroethane (HCFC-122) is a co-spin agent, which lowers the cloud-point pressure. The cloud-point pressure means the pressure at which a single phase liquid solution begins to phase separate. At temperatures above the critical point, there cannot be any liquid phase present and therefore a single phase, supercritical solution phase separates into a polymer-rich/spin fluid-rich, two-phase gaseous dispersion. 

Miscible blends of poly(vinyl methyl ether) and polystyrene exhibit phase separation at temperatures above 100 C as a result of a lower critical solution temperature and have a well defined phase diagram ( ). This system has become a model blend for studying thermodynamics of mixing, and phase separation kinetics and resultant morphologies obtained by nucleation and growth and spinodal decomposition mechanisms. As a result of its accessible lower critical solution temperature, the PVME/PS system was selected to examine the effects of phase separation and morphology on the damping behavior of the blends and IPNs. 

To achieve desirable macroscopic properties, one would like to control the degree of miscibility of a blend. Heat treatment/annealing is a simple technique to modify a phase structure of a blend. Morphological changes induced by heat treatment can also affect NMR observable (see Section 10.3.2.1). Furthermore, as shown in Section 10.2.1, several blends exhibit a lower critical solution temperature (LCST) phase diagram. Such a blend phase-separates at temperatures above its LCST temperature. The compositional fluctuation during the phase-separation process is examined in Section... 

The carbon dioxide/water biphasic system is an example of binary mixtures consisting of components with widely separated critical temperatures. The critical properties of the pure compounds are given in Table 1. The typical phase diagram for such mixtures can be complex, including the possibility for areas of three-phase coexistence (LEV). For applications in biphasic catalysis, however, the key parameters to be discussed are solubility and cross-contamination, mass transfer, and chemical changes. 

The critical point obtained from the Flory-Huggins equation can well explain the critical condition for phase separation upon temperature drop. This critical point is... 

Consider the phase diagram for a binary polymer-solvent mixture in which the solvent is roughly the size of one monomer unit (segment). Starting from a one phase mixture, a decrease in temperature can lead to separation into polymer-rich and polymer-lean phases at the upper critical solution temperature (UCST) phase boundary. In general a polymer solution also phase separates as temperature is increased to the lower critical solution temperature (LCST) phase boundary [40]. In near critical and supercritical fluids, the driving force for phase separation at the LCST phase boundary is the difference in the compressibility of polymer and solvent, which becomes large... 

Liquid—Liquid Phase Separation, in contrast to solid-liquid phase separation, lowering temperature can induce liquid-liquid phase separation of a polymer solution with an upper critical solution temperature and when the crystallization temperature of the solvent is sufficiently lower than the phase separation temperature. In an equilibrium phase diagram of a polymer solution, the spin-odal curve divides the liquid-liquid phase separation region into two regions a thermodynamically metastable region (between the binodal and spinodal) and a thermodynamically unstable region (enclosed by the spinodal) (Fig. 11). Above the... 

In solutions, depending on the temperature, the various components can segregate into separate phases. For simplicity, we shall only consider binary mixtures. The phenomenon is similar to the critical phenomenon in a liquid-vapor transition in that across one range of temperature the system is in one homogeneous phase (solution) but across an another range of temperature the system becomes unstable and the two components separate into two phases. The critical temperature that separates these two ranges depends on the composition of the mixture. This can happen in three ways, as illustrated by the following examples. 

Fig. 13.17. Phase separation in polymer-solvent systems predicted by Flory-Huggins theory. The system exhibits UCST behavior that is, phase separation at temperatures below a critical point C. Coexistence curves for two different molecular weights are shown.

S, L, G curves upward and intersects the gas liquid line to form critical end points Ug and Ub. No liquid phase exits between the temperatures of and U,. The three-sided region is a region of gas-liquid phase separation. At temperatures between this region and U, the solid phase of component C] is in equilibrium with C2 rich gas phase. 

The same enhancement of elasticity is observed for PS/PVME blends at temperatures above the critical temperature of phase separation. For temperatures lower than 140 C, the components are totally miscible and the linear viscoelastic behaviour of these blends is typical of an homogeneous polymer melt. 

In considering the process of cellulose acetate dissolution in chloroform during experimental investigation of polymer dissolution in low-molecular solvents, Papkov et al. [126] were the first to distinguish several states of the phase structure formation in polymer systems with limited compatibility and some low-molecular blends. The solution was transparent at the selected initial temperature and concentration. However, if the temperature goes below the critical point (its position depends on concentration) the system shows turbidity phase disintegration (transition, separation) occurs. In this case an emulsion is formed. It is characterized by the number and size distribution of constituent particles which depends on phase separation mechanism, temperature decrease rate and etc. 

Phase separation in this way is most effective if the light key component is significantly above its critical temperature. If a component is above its critical temperature, it does not truly condense. Some, however, dissolves in the liquid phase. This means that it is bound to have an extremely high K value. 

Remember that the hump which causes the instability with respect to phase separation arises from an unfavorable AH considerations of configurational entropy alone favor mixing. Since AS is multiplied by T in the evaluation of AGj, we anticipate that as the temperature increases, curves like that shown in Fig. 8.2b will gradually smooth out and eventually pass over to the form shown in Fig. 8.2a. The temperature at which the wiggles in the curve finally vanish will be a critical temperature for this particular phase separation. We shall presently turn to the Flory-Huggins theory for some mathematical descriptions of this critical point. The following example reminds us of a similar problem encountered elsewhere in physical chemistry. 

For the phase separation problem, the maximum and minima in Fig. 8.2b and the inflection points between them must also merge into a common point at the critical temperature for the two-phase region. This is the mathematical criterion for the smoothing out of wiggles, as the critical point was described above. 

By combining Eqs. (8.42), (8.49), and (8.60), show that Vi°(52 - 5i) = (l/2)RTj., where T. is the critical temperature for phase separation. For polystyrene with M = 3 X 10, Shultz and Floryf observed T. values of 68 and 84°C, respectively, for cyclohexanone and cyclohexanol. Values of Vi° for these solvents are abut 108 and 106 cm mol", respectively, and 5i values are listed in Table 8.2. Use each of these T. values to form separate estimates of 62 for polystyrene and compare the calculated values with each other and with the value for 62 from Table 8.2. Briefly comment on the agreement or lack thereof for the calculated and accepted 5 s in terms of the assumptions inherent in this method. Criticize or defend the following proposition for systems where use of the above relationship is justified Polymer will be miscible in all proportions in low molecular weight solvents from which they differ in 5 value by about 3 or less. 

By combining Eqs. (8.60) and (8.115), the following relationship is obtained between the critical temperature for phase separation and the degree of polymerization ... 

Supercritical fluids can be used to induce phase separation. Addition of a light SCF to a polymer solvent solution was found to decrease the lower critical solution temperature for phase separation, in some cases by mote than 100°C (1,94). The potential to fractionate polyethylene (95) or accomplish a fractional crystallization (21), both induced by the addition of a supercritical antisolvent, has been proposed. In the latter technique, existence of a pressure eutectic ridge was described, similar to a temperature eutectic trough in a temperature-cooled crystallization. 

The separation of Hquid crystals as the concentration of ceUulose increases above a critical value (30%) is mosdy because of the higher combinatorial entropy of mixing of the conformationaHy extended ceUulosic chains in the ordered phase. The critical concentration depends on solvent and temperature, and has been estimated from the polymer chain conformation using lattice and virial theories of nematic ordering (102—107). The side-chain substituents govern solubiHty, and if sufficiently bulky and flexible can yield a thermotropic mesophase in an accessible temperature range. AcetoxypropylceUulose [96420-45-8], prepared by acetylating HPC, was the first reported thermotropic ceUulosic (108), and numerous other heavily substituted esters and ethers of hydroxyalkyl ceUuloses also form equUibrium chiral nematic phases, even at ambient temperatures. 

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