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Dynamics of Pressure-Induced Phase Separation in Polymer Solutions. The Dependence of the Demixing Pressures on the Rate of Pressure Quench in Solutions of Poly(dimethylsiloxane) in Supercritical Carbon Dioxide Jintong Li,† Ming Zhang, and Erdogan Kiran* Department of Chemical Engineering, University of Maine, Orono, Maine 04469-5737
The dynamics of pressure-induced phase separation (PIPS) is important to processes that use near-critical or supercritical fluids. In this study, controlled pressure quench experiments were conducted in a 5 wt % solution of poly(dimethylsiloxane) (Mw ) 93 700; Mw/Mn ) 2.99) in supercritical carbon dioxide to determine the demixing pressures and its dependence on the rate of pressure change. Pressure quench rates were changed from extremely slow (ca. 0.008 MPa/s) to extremely rapid (ca. 500 MPa/s), and the system behavior was documented by real time recording of the changes in the transmitted light intensity, temperature, and pressure during the quench using a specially designed experimental system. The experiments were conducted on both the lower and the upper critical solution temperature (LCST and UCST) branches of the liquid-liquid-phase separation boundary. Apparent demixing pressures strongly depend on the rate and the depth of the pressure quench imposed and shift to significantly higher pressures with an increase in the rate of quench if the initial temperature of the homogeneous solution is in the range where the system behavior is of the UCST type. The opposite is observed if the initial temperature is in the range where the system behavior is of the LCST type, and the demixing pressures shift to lower values. These are explained in terms of the actual pressure-temperature path the system follows because of significant cooling effects that accompany deep quenches. Introduction Supercritical fluid based synthesis, modification, and processing of organic and inorganic materials is a rapidly growing area of research with industrial significance.1-12 This is driven not only by the desire to replace conventional solvents with environmentally less harmful fluids such as carbon dioxide but also by the desire to take full advantage of the processing flexibility that comes along with the pressure-tunable properties of supercritical fluids. Even though pressure-induced phase separation (PIPS) is an integral process step in supercritical fluid based processes, the dynamic aspects of this step are not well documented. It is only recently that some publications have appeared which describe experimental procedures and discuss some of the consequences of pressure quench rate and quench depth on the time evolution of new phase formation and growth in systems subjected to PIPS.3,13-19 Poly(dimethylsiloxane) (PDMS) is an important polymer in that it can be completely dissolved in supercritical carbon dioxide at reasonable pressures. It is now well-known that the phase diagram for this system displays both LCST (lower critical solution temperature) and UCST (upper critical solution temperature) branches.20-22 As such it is an ideal system to investigate the consequences of different pressure quench rates * To whom correspondence should be addressed. Tel: 207581-2286. Fax: 207-581-2323. E-mail:
[email protected]. † Present address: Department of Chemical Engineering, University of Massachusetts, Amherst, MA.
or depth of penetration into the region of immiscibility starting from conditions where the initial solution may be displaying UCST or LCST type behavior. Traditionally, the demixing pressures in polymer solutions are determined for the purpose of establishing phase boundaries. For this purpose, pressure is reduced slowly and the cloud point of the system is determined visually or by measuring changes in either the transmitted light or scattered light intensities that accompany phase separation. However, the phase boundary determined from these measurements does not represent the actual demixing pressures a system may display if subjected to very rapid decompression from a homogeneous condition. Rapid pressure quenches are commonly encountered in applications such as (a) spray coating, (b) particle or fiber formation by rapid expansion of supercritical fluid solutions, or (c) in-situ impregnation of matrixes as in textile dying or pharmaceuticals processing. It is desirable to know the dynamics of these processes and the demixing pressures that are actually experienced. The present paper describes a systematic study in which a 5 wt % solution of PDMS in supercritical carbon dioxide has been first subjected to very slow pressure reductions (at about 0.50 MPa/min) in the temperature range from 303 to 373 K to determine the “thermodynamic” phase boundary. Then, at two temperatures (313 and 348 K) that are on the UCST and LCST branches of the phase diagram, the solution was subjected to a series of rapid pressure quenches, with rates approaching 30 000 MPa/min, to determine the “dynamic” or the apparent phase boundary. The quenches were started
10.1021/ie990392m CCC: $18.00 © 1999 American Chemical Society Published on Web 10/12/1999
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Figure 1. Schematic diagram of the experimental system. MC ) main dissolution cell; VC ) small volume scattering cell; CP ) circulation gear pump; AAV ) air-actuated valve; PT ) pressure and temperature sensors; PD ) photodetector; VVA ) variablevolume attachment housing movable piston; LVDT ) piston position sensor; M-PG ) motorized pressure generator; V ) valves; SP ) polymer loading port.
from the same initial pressure but were carried out to reach different final pressures or at different rates. The results show that the temperature change that accompanies the pressure change leads to significant differences in the observed demixing pressures and leads to opposite effects on the UCST and LCST branches. Experimental Section Materials. PDMS (Mw ) 93 700 and Mw/Mn ) 2.99) was obtained from Scientific Polymer Products and used as received. Experimental System. Figure 1 is a schematic of the experimental system. Its design and operational features have been previously described.13,15 Briefly, it consists of two high-pressure cells of different volumes that are part of a loop. The small cell (VC) is an optical cell with flat windows positioned at different angles. The large cell (MC) is a variable-volume view cell that also functions as the polymer loading chamber. The dynamic changes in the pressure in the optical cell are monitored by a dedicated pressure transducer (PCB Piezotronics) which has a response time of 2 µs. The temperature in the optical cell is monitored using an unshielded thermocouple (Omega model IRCO-005) to minimize thermal lag and follow the temperature changes to the greatest extent possible in the time scale of pressure changes. The response time for the thermocouples (about 90 ms) is not as fast as that for the pressure transducers. The two cells, the circulation pump, and all of the connecting lines and valves between the cells are housed in a heated oven. The temperature is controlled from room temperature to 473 K with a resolution of 0.1 K. The overall accuracies of pressure and temperature readings are 0.14 MPa and 0.5 K, with resolutions of 0.017 MPa and 0.1 K, respectively. After the polymer and the solvent are loaded, the pressure is increased with the movement of the piston inside the main cell (MC) using the motorized pressure generator (M-PG). The contents of the two-cell loop are mixed and circulated with a gear pump (CP) until homogeneous conditions are obtained. Then at a given temperature, the pressure is lowered (either slowly or rapidly) while measuring the changes in the transmitted light intensity in the optical cell (VC) as a function of time. An argon-ion laser (wavelength ) 514 nm) is used
Figure 2. Change in transmitted light intensity with pressure in a 5 wt % solution of PDMS in carbon dioxide at 313 and 348 K. The pressure reduction rate is in the range of 0.5-0.7 MPa /min.
as the light source. For slow pressure quenches, the pressure in the system is reduced by moving the piston in the variable-volume view cell through the motorized movement of the pressure generator. For rapid pressure quenches, the two cells are first isolated from each other by closing the valves V3 and the air-actuated valve AAV in the loop. The pressure in the large cell is then lowered by moving the piston to a new position. This is followed by computer-controlled opening of the air-actuated valve which initiates a very rapid pressure drop in the optical cell. To conduct a different quench with a different depth of penetration, or with a different initial pressure, the valves between the cells are opened and the system is rehomogenized by circulation. Then the cells are isolated, the pressure in the large view cell is lowered to a new value, and the quench is triggered once again with opening of the air-actuated valve. Results and Discussion Figure 2 shows the change in the transmitted light intensity with pressure during slow pressure reduction (in the range of 0.5-0.7 MPa/min) at 348 and 313 K starting from an initial pressure of about 39 MPa. The demixing pressures are identified from these plots as the pressures corresponding to the point where the transmitted light intensity starts to display a marked decrease. By similar determinations conducted at different temperatures, the phase boundary shown in Figure 3 was determined. The region above the curve is the one-phase homogeneous region, while the region below represents the two-phase region. As shown, the demixing pressures initially decrease and then increase with increasing temperature. At temperatures below about 310 K, the system shows UCST type behavior. In this domain, at a pressure above about 31 MPa, one-phase regions are entered with an increase in temperature, whereas at temperatures above 310 K, the system shows LCST behavior, and two-phase regions are now entered with an increase in temperature. Figure 4 shows the variation of transmitted light intensity and pressure with time for the case where the solution is subjected to a rapid pressure quench from 41.4 to 24 MPa with a ∆P/∆t of about 18 000 MPa/min at 313 K. Figure 5 is a different quench from 41.5 to 31 MPa with a rate of about 9300 MPa/min at 348 K. A series of quench experiments have been conducted at different rates at these temperatures. The results are
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Figure 3. Variation of the demixing pressure with temperature for a 5% solution of PDMS in carbon dioxide. The region above the curve is the one-phase region.
Figure 4. Change in transmitted light intensity and pressure with time in a 5% PDMS solution in carbon dioxide during a very rapid quench starting at an initial pressure of 41.4 MPa at 313 K. The pressure reduction rate is 18 200 MPa/min.
Figure 5. Change in transmitted light intensity and pressure with time in a 5% PDMS solution in carbon dioxide during a rapid quench starting at an initial pressure of 41.5 MPa at 348 K. The pressure reduction rate is 9300 MPa/min.
compared in Figures 6 and 7 as variation of transmitted light with pressure during quench. These figures show the change in transmitted light intensity with pressure for different quench rates at 348 and 313 K that start from about a common initial pressure but end at different final pressures. At 348 K, when the initially homogeneous solution at about 44 MPa is subjected to fast pressure reductions
Figure 6. Dependence of the change in transmitted light intensity with pressure on the rate of pressure reduction. The initial solution is 348 K and about 44 MPa.
Figure 7. Dependence of the change in transmitted light intensity with pressure on the rate of pressure reduction. The initial solution is 313 K and about 42 MPa.
Figure 8. Schematic representation of the pressure quench pathways in the UCST (A0-D0) and LCST (A-D) branches of the phase diagram.
with deeper quenches, the observed demixing pressures are progressively shifted to lower pressures. As shown in Figure 6, the demixing pressures decrease from about 35 to 31 MPa. Completely opposite trends are observed at 313 K, where solutions subjected to rapid pressure quenches show progressively higher demixing pressures with quench depth. As demonstrated in Figure 7, the demixing pressures increase from about 30 to 37 MPa. These results demonstrate the dynamic nature of the observed phase boundary and the consequences of the path followed. Figure 8 provides a physical picture for
Ind. Eng. Chem. Res., Vol. 38, No. 11, 1999 4489 Table 1. Dependence of the Observed Demixing Pressure on the Rate of Pressure Change and Its Comparison with the Demixing Pressure That Would Correspond with the Temperature Change That Takes Place during Quencha Ti (K)
Pi (MPa)
Pf (MPa)
∆P/∆t (MPa/min)
∆Tobs (K)
Pdemix,obs (MPa)
Pdemix(Ti-∆T) (MPa)
313
39.0 41.4 41.4 41.1 39.0 44.1 44.1
28.0 25.1 24.1 22.8 30.2 31.7 29.8
0.67 16 320 18 180 27 900 0.522 9300 12 240
∼0 5.5 7.2 9.5 ∼0 7.0 7.4
30.7 35.2 36.4 38.4 34.8 33.2 32.2
30.7 33.0 35.0 38.4 34.8 33.4 32.2
348
a T ) initial temperature before quench. P ) initial pressure. P ) final pressure reached after the imposed quench. ∆P/∆t ) pressure i i f change rate. ∆Tobs ) temperature change experimentally measured. Pdemix,obs ) apparent demixing pressure assigned from transmitted light vs pressure curves obtained in a given quench. Pdemix(Ti-∆T) ) demixing pressure corresponding to the new temperature read from slow pressure change experiments (from Figure 3).
at 313 K the observed demixing pressures are slightly higher, whereas at 348 K they are slightly lower, both of which suggest that the temperature drops experienced in the system may have been greater than that can be recorded in real time. Response time even for unshielded thermocouples are not fast enough to follow the temperature changes as rapidly as the fast-responding pressure sensors can follow the pressure changes. Further Comments
Figure 9. Variation of the temperature in the scattering cell during a rapid quench starting at an initial pressure and temperature of 41.4 MPa and 313 K. The pressure reduction rate is 18 200 MPa/min.
the observed phenomena. Here T1 and T2 represent two temperatures, one in the UCST and the other in the LCST branch of the phase diagram. Nearly isothermal pressure change may be achievable (the dashed paths A and A0 in the figure) if the pressure is reduced extremely slowly while either the system remains homogeneous and does not show major volume change or the heat effects associated with phase separation are not very large. When however pressure is reduced rapidly and the system is of highly compressible nature, there is a significant cooling effect that forces the system to follow a different path depending upon the end pressure imposed. These are shown as path B, C, and D or B0, C0, and D0. The figure demonstrates that if the initial temperature (T2) is in the LCST region, any cooling effect on the system will shift the apparent demixing pressure to a lower pressure, whereas if the initial temperature (T1) is in the UCST region of the system, the cooling effect will shift the observed demixing pressure to higher pressures. This is, in fact, what is observed in Figures 6 and 7. In the present experimental system, the actual temperature change during each quench is also recorded. Figure 9 shows such a plot for the pressure quench from 41.4 to 24 MPa at 313 K where a temperature drop of about 7 K is observed. In the rapid pressure quench experiments included in Figures 6 and 7, the recorded temperature drops have been in the range of 0-10 K. These are shown in Table 1. The table also shows the demixing pressures that would correspond to these temperature drops vis-a`-vis the observed values. The observed trends in the demixing pressures follow the temperature change closely. However, it is noted that
The significant cooling effects observed in the system are an outcome of the high compressibility of carbon dioxide. Deep pressure quench experiments in solutions of polyethylene in pentane for similar pressure drops were shown earlier to result in only about a 2 K change in temperature,13 whereas in PDMS + carbon dioxide mixtures, measurable temperature effects are observed even for shallow quenches in the range from 0.2 to 1 MPa.18 Pentane is significantly less compressible than carbon dioxide. For example, at 348 K, changing the pressure from about 22 to 32 MPa leads to a density change from about 0.69 to 0.80 g/cm3 in carbon dioxide and from 0.60 to 0.61 g/cm3 in n-pentane.23 The compressibility difference in pure carbon dioxide and npentane is reflected also in the pressure sensitivity of the density of solutions of PDMS in carbon dioxide and the density of polyethylene in n-pentane.20,24 The present results show clearly that in systems that display UCST behavior initiation of new phase formation during pressure reduction starts at pressures higher than anticipated from the equilibrium phase boundary prevailing at the initial temperature. In systems showing LCST, the initiation of new phase formation may be shifted to lower pressures. The temperature effects must therefore be adequately accounted for in the design of, for example, expansion units or nozzles for materials processing or recovery. Once initiated, further progress of the new phase growth and the related kinetics of phase separation depend on the mode of phase separation (i.e., nucleation and growth or spinodal decomposition depending upon the depth of penetration into the region of immiscibility) and a number of other factors that may also play a role. These include viscosity, interfacial tension, and hydrodynamic factors and viscoelastic effects such as the characteristic deformation times (which will be influenced by the rate of pressure quench) versus relaxation time for polymer chains.15,25 The crossover from nucleation and growth to spinodal decomposition with a progressive increase in the depth of penetration into the region of immiscibility and the associated kinetics in poly(dimethylsiloxane) + carbon dioxide systems are described in detail in another publication.18
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Acknowledgment This research has in part been supported by a DuPont Educational Grant. Literature Cited (1) Kiran, E. Polymer formation, modification and processing in or with supercritical fluids. In Supercritical FluidssFundamentals for Applications: Kiran, E., Levelt Sengers, J. M. H., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1994; pp 541-588. (2) Kiran, E. Polymerization and polymer modifications in Near- and supercritical fluids. Proceedings of the International Meeting GVC-Fauchausschuss “High-Pressure Chemical Engineering”, Karlsruhe, Germany, March 3-5, 1999; Wisenschaftliche Berichte FZKA 6271, 1999; pp 5-14. (3) Kiran, E.; Zhuang, W. Miscibility and phase separation of polymers in near- and supercritical fluids. In Supercritical Fluidss Extraction and Pollution Prevention; Abraham, M., Sunol, A. K., Eds.; ACS Symposium Series 670; American Chemical Society: Washington, DC, 1997; pp 2-36. (4) Kendall, J. L.; Canelas, D. A.; Young, J. L.; DeSimone, J. M. Polymerizations in supercritical carbon dioxide. Chem. Rev. 1999, 99, 543-563. (5) Darr, J. A.; Poliakoff, M. New directions in inorganic and metal-organic coordination chemistry in supercritical fluids. Chem. Rev. 1999, 99, 495. (6) Reverchon, E. Supercritical antisolvent precipitation of micro- and nanoparticles. J. Supercrit. Fluids 1999, 15, 1. (7) Weidner, E. Powder generation by high-pressure spray processes. Proceedings of the International Meeting GVCFauchausschuss “High-Pressure Chemical Engineering”, Karlsruhe, Germany, March 3-5, 1999; Wisenschaftliche Berichte FZKA 6271, 1999; pp 225-230. (8) Dixon, D. J.; Luna-Barcenas, G.; Johnston, K. P. Microcellular microspheres and microballoons by precipitation with vaporliquid compressed fluid antisolvent. Polymer 1994, 35, 3998. (9) Yeo, S.; Debenedetti, P. G.; Radosz, M.; Gieas, R.; Schmnidt, H.-W. Supercritical antisolvent process for a series of substituted paralinked aromatic polyamides. Macromolecules 1995, 28, 1316. (10) Muth, O.; Hirth, Th.; Vogel, H. Polymer modification by supercritical impregnation. Proceedings of the International Meeting GVC-Fauchausschuss “High-Pressure Chemical Engineering”, Karlsruhe, Germany, March 3-5, 1999; Wisenschaftliche Berichte FZKA 6271, 1999; pp 39-41. (11) Watkins, J. J.; McCarthy, T. J. Polymerization of styrene in supercritical CO2-swollen poly(chlorotrifluoroethylene). Macromolecules 1995, 28, 4067. (12) Turk, M.; Helfgen, B.; Cihlar, S.; Schaber, K. Experimental and theoretical investigations of the formation of small particles from the rapid expansion of supercritical solutions (RESS). Proceedings of the International Meeting GVC-Fauchausschuss “High-Pressure Chemical Engineering”, Karlsruhe, Germany,
March 3-5, 1999; Wisenschaftliche Berichte FZKA 6271, 1999; pp 243-246. (13) Kiran, E.; Zhuang, W. A new experimental method to study kinetics of phase separation in high-pressure polymer solutions. Multiple Rapid Pressure Drop techniquesMRPD. J. Supercrit. Fluids 1994, 7, 1-8. (14) Kojima, J.; Nakayama, Y.; Takenaka, M.; Hashimoto, T. Apparatus for measuring time-resolved light scattering profiles from supercritical polymer solutions undergoing phase separation under high pressure. Rev. Sci. Instrum. 1995, 66, 4066. (15) Zhuang, W.; Kiran, E. Kinetics of pressure-induced phase separation (PIPS) from polymer solutions by time-resolved light scattering. Polyethylene + n-pentane. Polymer 1998, 39, 29032915. (16) Kiran, E. Kinetics of pressure-induced phase separation (PIPS) in polymer solutions. Proceedings of the 4th International Symposium on Supercritical Fluids, Sendai, Japan, May 11-14, 1997; pp 777-784. (17) Xiong, Y.; Kiran, E. A high-pressure light scattering apparatus to study pressure-induced phase separation in polymer solutions. Rev. Sci. Instrum. 1998, 69, 1463-1471. (18) Liu, K.; Kiran, E. Kinetics of pressure-induced phase separation (PIPS) in solutions of poly(dimethylsiloxane) in supercritical carbon dioxide: Crossover from nucleation and Growth to Spinodal Decomposition mechanism. J. Supercrit. Fluids 1999, 16, 59-79. (19) Xiong, Y.; Kiran, E. Kinetics of pressure-induced phase separation in polystyrene + methyl cyclohexane solutions at high pressures. Polymer, in press. (20) Xiong, Y.; Kiran, E. Miscibility, density and viscosity of polydimethylsiloxane in supercritical carbon dioxide. Polymer 1995, 36, 4817. (21) Dris, G.; Barton, S. W. Polymer adsorption from supercritical fluids. Proc. ACS Div. Polym. Mater. Sci. Eng. 1996, 74, 226. (22) Bayraktar, Z.; Kiran, E. Miscibility, phase separation and volumetric properties ion solutions of poly(dimethylsiloxane) in supercritical carbon dioxide. J. Appl. Polym. Sci., accepted for publication. (23) Kiran, E.; Po¨hler, H.; Xiong, Y. Volumetric properties of pentane + carbon dioxide at high pressures. J. Chem. Eng. Data 1996, 41, 158. (24) Kiran, E.; Gokmenoglu, Z. High-pressure viscosity and density of polyethylene solutions in n-pentane. J. Appl. Polym. Sci. 1995, 58, 2307. (25) Tanaka, H. Critical dynamics and phase separation kinetics in dynamically asymmetric binary fluids: New dynamic universality class for polymer mixtures or dynamic crossover. J. Chem. Phys. 1994, 100, 5323.
Received for review June 4, 1999 Revised manuscript received August 13, 1999 Accepted August 17, 1999 IE990392M
