Miscibility

Miscibility of a polymer in a near-critical fluid depends on the temperature, pressure, polymer concentration, molecular weight and molecular weight distribution, the polymer type and the nature of the solvent fluid under consideration. Several comprehensive reviews have recently appeared. 15-9 The results of high-pressure miscibility studies are often reported in the form of pressure-temperature demixing curves that are typically based on data generated in a variable-volume view cell. The polymer + fluid mixture corresponding to a target polymer concentration is first brought into the one-phase homogeneous conditions, and then the pressure is slowly decreased. The incipient phase separation condition is visually or optically noted through the windows of the view-cell. Depending upon the polymer + solvent system, the shape of these demixing curves differ. The demixing curves corresponding to liquid-liquid phase separation can be generalized and grouped into the types that are shown in Figure 2. In these diagrams the curves a, b, and c represent three different concentrations and for each concentration the region above each curve is the homogeneous one-phase region. Below each curve is the two-phase region. In Figure 2A, at a given pressure, the system that may be in the two-phase region may become one-phase with increasing temperature (typical of systems showing upper critical solution temperature - UCST), but with further increase in temperature may again enter two-phase region (typical of system showing lower critical solution temperature -LCST). At lower pressures, the two- phase regions may persist at some concentrations resulting in an hour-glass shaped region of immiscibility in the temperature-composition plane. In Figure 2B one-phase regions are entered upon an increase in temperature. The reverse is the case in Figure 2C. The scenario in Figure 2D is not very common and represents the possibility of entering a two phase region first and then becoming one phase at higher temperatures for some pressures which would lead to an island of immiscibility in the temperature-composition plane.

Temperature Temperature Temperature Temperature

Figure 2. Demixing pressures for different concentrations a, b, c. Below each curve is the two-phase region. Depending upon the pressure, the system may display both UCST and LCST or hour-glass shaped region of immiscibility (A); only UCST (B); only LCST (C), or an island of immiscibility (D) in the temperature-composition plane. (See Figure 3).

Temperature Temperature Temperature Temperature

Figure 2. Demixing pressures for different concentrations a, b, c. Below each curve is the two-phase region. Depending upon the pressure, the system may display both UCST and LCST or hour-glass shaped region of immiscibility (A); only UCST (B); only LCST (C), or an island of immiscibility (D) in the temperature-composition plane. (See Figure 3).

Figure 3 shows the temperature-composition diagrams that would result from the constant pressure cuts in Figure 2A, which demonstrates the progressive shift from hour-glass shaped region of immiscibility to observation of both UCST and LCST branches and the eventual widening of the miscible region as pressure is increased. We have recently noted110 the striking similarity between the shape of the P-T demixing curves and the resulting T-x phase diagrams and the temperature dependence of the Flory interaction parameter % for polymer solutions and the resulting T-x phase diagrams.11 This analogy between pressure and Flory % parameter is helpful in understanding the role the pressure plays in influencing the polymer solvent interactions.

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