Polymer-Supercritical Fluid Phase Diagrams

The effect of dissolved CO2 on the miscibility of polymer blends and on phase transitions of block copolymers has been measured with spectroscopy and scattering (40). The shifts in phase diagrams with CO2 pressure can be pronounced. Polymer blends may be trapped kinetically in metastable states before they have time to phase separate. Metastable polymer blends of polycarbonate (PC) and poly(styrene-cn-acrylonitiile) were formed with liquid and supercritical fluid CO2 in the PCA process, without the need for a surfactant. Because of the rapid mass transfer between the CO2 phase and the solution phase, the blends were trapped in a metastable state before they... 

Figure 15.3 Effect of gas content on phase behavior of a polymer-solvent system (a) pressure-temperature diagram at constant concentration and (b) temperature-composition diagram at constant pressure. Source Adapted with permission from ter Horst MH, Behme S, Sadowski G, de Loos TW. J Supercrit Fluids 2002 14 181 [7]. Copyright 2002 Elsevier.

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... 

A number of equation of state theories have been used to model phase behavior of polymers in supercritical fluids. For example the lattice-fluid theory of Sanchez and Lacombe[4U 42] includes holes on the lattice in order to model compressibility. The lattice-fluid theory has been applied to model phase behavior of both homopolymers and copolymers in supercritical fluids[32, 38, 43, 44]. The statistical associating fluid theory (SAFT)[43,45-48] and corresponding state models[49] have also been employed to model compressible polymer-solvent mixtures. Figure 1 gives the pressure-concentration phase diagram for poly(dimethyI siloxane) in CO2 modeled with the lattice-fluid equation of state[50]. 

Saeki, S. (1997) Calculation of the phase diagram of polymer solution over supercritical fluid region based on the Patterson-Delmas theory of corresponding state. Fluid Phase Equilib. 136,87-92. 

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