Pressure-induced phase separation

Figure 1. Schematic representation of the influence of various factors on the miscibility and phase separation. The shaded areas represent the two-phase regions. The arrows show the paths for phase separation, while the reverse directions would be the paths for miscibility. From left-to-right Temperature-induced phase separation solvent-induced phase separation reaction (i.e. polymerization) -induced phase separation (the two-phase regions are entered with increasing degree of polymerization (DP)) and pressure-induced phase separation. < ) = polymer concentration, S2 and SI solvent and nonsolvent PI andP2 = polymer 1 and polymer 2.

E. Kiran, Polymer miscibility and kinetics of pressure-induced phase separation in near-critical and supercritical fluids. Chapter 6 in Supercritical Fluids. Fundamentals and Applications, E. Kiran, P. G. Debenedetti and C. J. Peters, Eds., Kluwer Academic Publishers, Dordrecht, The Netherlands (2000). 

W. Zhuang and E. Kiran, Kinetics of pressure-induced phase separation (PIPS) from polymer solutions by time-resolved light scattering. Polyethylene + n-pentane, Polymer, 39,2903-2915 (1998). 

Y. Xiong and E. Kiran, High-pressure light scattering apparatus to study pressure-induced phase separation in polymer solutions. Rev. Sci. Instrum., 69, 1463-1471 (1998). 

K. Liu and E. Kiran, Pressure-induced phase separation (PIPS) in polymer... 

LIU Liu, K. and Kiran, E., Pressure-induced phase separation in polymer solutions kinetics of phase separation and crossover fiom nrrcleation and growth to spinodal decomposition in solutions of polyetltylene in n-perrlane, Macromo/ecM/ex, 34, 3060, 2001. 

SHI Shibayama, M., Isono, K., Okabe, S., Karino, T., arrd Nagao, M., SANS study on pressure-induced phase separation of poly(N-isopropylacrylanride) aqueous solutions and gels. Macromolecules, 37, 2909, 2004. 

LIJ Li, J., Zhang, M., and Kiran, E., Dynamics of pressure-induced phase separation in polymer solutions. The dependence of the demixing pressure on the rate of pressure quench in solutions of poly(dimethylsiloxane) in supercritical carbon dioxide, Ind Eng. Chem. Res., 38, 4486, 1999. 

Spinodal decomposition is of fundamental importance in processes involving phase separation of polymers in near- and supercritical fluids [145]. Pressure-induced phase separation (PIPS) has recently been used [4], with a novel experimental apparatus [146] that permits the imposition of rapid and controlled multiple pressure quenches, to study spinodal decomposition of near- and off-critical mixtures of a polymer and a compressed solvent following deep quenches into the unstable region. Spinodal decomposition is also important in SAS, in situations where the mass transfer pathway leads to penetration into the unstable region [76,147,148]. It can also be important in RESS involving polymeric solutes [35]. Experimental aspects of spinodal decomposition and the kinetics of phase separation in polymer solutions in near-critical fluids are discussed in the chapter by E. Kiran in this volume. 

POLYMER MISCIBILITY AND KINETICS OF PRESSURE -INDUCED PHASE SEPARATION IN NEAR-CRITICAL AND SUPERCRITICAL FLUIDS... 

References 1 through 7 are comprehensive reviews, and references 8 through 21 are some more specific papers that provide perspectives of these diverse applications. Another chapter in the present volume [2] has been specifically devoted to the polymerization and polymer modification reactions in near-critical and supercritical fluids. The present chapter is focused on miscibility and the kinetics of pressure-induced phase separation that are central to many of the applications. 

The final part of the chapter presents selected examples of microporous polymeric materials that are produced by pressure-induced phase separation. Examples include polyolefins and cellulosic polymers. 

Among these various phase separation techniques, pressure-induced phase separation is particularly important since pressure changes can be brought about uniformly and very fast throughout the bulk of a solution. This would not be so in other techniques due to for example heat (in TIPS) and mass transfer (in SIPS) limitations. The technique therefore opens up new opportunities for formation of microstructured materials with potentially more uniform morphologies. It is also important to recognize temperature, solvent, reaction, or field-induced phase separation may all be carried out at elevated pressures if so desired, as such all modes of phase separation methods are of interest when working with near-critical or supercritical fluid systems. 

Some of the basic features of pressure-induced phase separation have already been discussed in connection with Figure 5. The figure demonstrates three different paths corresponding to three different polymer concentrations in which the system pressure has been reduced from an initial pressure of Pi to a final pressure of Pf. Paths AB and A B take the system to the metastable regions, while path A B takes the system inside the spinodal envelop. Once the phase separation is complete, the two phases that coexist in equilibrium will have the same phase compositions (specified by the binodals) irrespective of which path has been followed. Phase volumes of polymer-rich and polymer-lean phase will however depend on the path. What is not described in Figure 5 is the transient structures that form along these paths. These are demonstrated in Figure 16. 

KINETICS OF PRESSURE-INDUCED PHASE SEPARATION. Crossover from Nucleation and growth to Spinodal Decomposition... 

Figure 21. Examples of microporous polymer materials produced by pressure-induced phase separation showing closed-cell and open-cell morphologies.

Liu, K. and Kiran, E. (1999) Kinetics of pressure-induced phase separation (PIPS) in solutions of polydimethylsiloxane in supercritical carbon dioxide Crossover from nucleation and growth to spinodal decomposition. J. Supercrit. Fluids, 16, 59-79. 

Liu, K. (1999) Kinetics of pressure-induced phase separation in polymer solutions by time and angle-resolved light scattering, M.Sc. Thesis, University of Maine, Orono, Maine, USA ( Thesis Advisor E. Kiran)... 

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