H

Concentration Concentration Concentration Concentration

Figure 3. Temperature-composition phase diagrams in polymer solutions that display liquid-liquid demixing pressures of the type shown in Figure 2A. . Shaded areas are the two-phase regions. From left-to-right the system shows progressively improving degree of miscibility as the system moves from one showing an hour glass-shaped region of immiscibility ( A and B) to one displaying upper and lower critical solution behavior with widening region of miscibility (C and D) with increased pressure.

A well known example of a polymer - solvent system showing behavior like Figure 2A is the poly(dimethylsiloxane) solutions in supercritical carbon dioxide.10 Figure 4a shows the demixing pressures for this system for concentrations in the range from 0.06 to 5.15 % by weight. Figure 4b is the corresponding pressure-composition diagram generated at 350 K. These diagrams provide complete information on the minimum pressures needed to achieve complete miscibility for a given polymer concentration at a given temperature. A common feature for many polymer solutions is the relatively flat nature of the P-x diagrams especially for polymers with somewhat broad molecular weight distributions.

Temperature. K Ccnctrtrakn, wt%

Figure 4. A (left). Demixing pressures for solutions of poly (dimethylsiloxane) (MW = 94,300 and MW/MN = 3) in supercritical carbon dioxide at different concentrations. B (right) Pressure-composition phase diagram at 350 K.

Temperature. K Ccnctrtrakn, wt%

Figure 4. A (left). Demixing pressures for solutions of poly (dimethylsiloxane) (MW = 94,300 and MW/MN = 3) in supercritical carbon dioxide at different concentrations. B (right) Pressure-composition phase diagram at 350 K.

Another example of demixing pressures is shown in Figure 5. The figure shows the behavior of polyethylene (M, = 121,000; PDI = 4.43) in n-pentane and in pentane + carbon dioxide mixtures.12 In pure pentane (Figure 5A), the system behavior is similar to Figure 2C and shows LCST type behavior. In this figure, the vapor pressure curve for pentane is also included. Most of the miscibility data shown in the figure corresponds to the sub-critical temperatures for pentane. Figure 5B demonstrates how the solvent quality can influence the observed behavior. The figure shows the demixing pressures for a nominal 3 % by wt polyethylene solution in the binary fluid mixtures of pentane and carbon dioxide. The polymer phase behavior in such ternary systems depends strongly on the composition of the solvent fluid. As shown, even though polyethylene shows an LCST type behavior in pure pentane and dissolves at modest pressures, the behavior shifts gradually to a UCST type behavior in pentane + carbon dioxide mixtures with increasing carbon dioxide content, and complete miscibility conditions require very high pressures.112

Figure 5. A(left) Demixing pressures in solutions of polyethylene (MW= 121,000; PDI= 4.43) in n-pentane at different concentrations. Figure shows the vapor pressure curve for n-pentane also. B(right). Demixing pressures in 3 % solution of the same polyethylene sample in binary fluid mixtures of pentane and carbon dioxide.

Figure 5. A(left) Demixing pressures in solutions of polyethylene (MW= 121,000; PDI= 4.43) in n-pentane at different concentrations. Figure shows the vapor pressure curve for n-pentane also. B(right). Demixing pressures in 3 % solution of the same polyethylene sample in binary fluid mixtures of pentane and carbon dioxide.

Miscibility in fluid mixtures containing carbon dioxide is of particular importance in applications that aim to reduce the use of traditional organic solvents in polymer processing. 113 For example, use of acetone may be reduced in processing of cellulose acetate by using acetone + carbon dixoide fluid mixture.13 A different type of ternary mixtures that is also of particular interest are the mixtures of two different polymers in one common high-pressure fluid. A limiting case of this is the behavior of mixtures of fractions of the same polymer with different molecular weights.14 Recent studies in our laboratory have shown that mutuall incompatible polymers can be mixed and solution blended in supercritical fluids115 Some subtle but significant effects are however encountered. A particularly interesting case is the miscibility of mixtures of isotactic polypropylene with polyethylene which are mutually incompatible. Both of these polymers are individually soluble in n-pentane at relatively low pressures (i.e., at pressures less than 15 MPa). Their mixtures however, depending upon the actual compositions involved, may require pressures around 50 MPa and higher for complete miscibility in n-pentane.115

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