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MATERIALS AND INTERFACES Effect of Concentration and Degree of Saturation on RESS of a CO2-Soluble Fluoropolymer Andre Blasig,† Chunmei Shi,‡ Robert M. Enick,‡ and Mark C. Thies*,† Center for Advanced Engineering Fibers and Films, Department of Chemical Engineering, Clemson University, Clemson, South Carolina 29634, and Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
RESS experiments were performed on the semicrystalline fluoropolymer poly(heptadecafluorodecyl acrylate) [poly(HDFDA); Mw ) 254 000, Mw/Mn ) 2.95] from solutions in carbon dioxide, with both concentration and degree of saturation being varied in a systematic manner. Phasebehavior measurements were carried out beforehand to locate the liquid-liquid equilibrium phase boundaries and thus establish the degree of saturation for a given concentration and preexpansion temperature and pressure. Concentrations of 0.5-5 wt % polymer were rapidly expanded through a short orifice (L/D ) 5.08) at preexpansion temperatures and pressures corresponding to unsaturated, saturated, and supersaturated solutions. The results indicate that the size of the polymer product is controlled by the degree of saturation (S); however, the morphology of the product is controlled primarily by the concentration, not by S. Only at some intermediate concentration, a threshold concentration where fiber formation first starts to occur, does S play a significant role. Thus, for example, micron-sized particles are produced by rapidly expanding a supersaturated solution of 0.5 wt % poly(HDFDA) in CO2, whereas continuous, submicron fibers are produced by the rapid expansion of an unsaturated solution containing 5 wt % of the polymer. Introduction The formation of structure is of interest for a variety of industries, including textiles (e.g., fibers), packaging (e.g., films), and pharmaceuticals (e.g., particles for drug release). Although conventional techniques exist for producing these morphologies, new structure-forming techniques are of interest for the development of products with new or improved properties. The technique known as RESS (rapid expansion of supercritical solutions) is the subject of this study. In RESS, a mixture of a supercritical solvent and a nonvolatile solute is rapidly expanded across a throttling device, such as an orifice1-3 or a capillary.4-6 Along the expansion path, the solvent density dramatically decreases from liquidlike to gaslike densities, causing the solute to precipitate. This rapid decrease in density leads to both uniform conditions and very high supersaturation ratios in the postexpansion environment. Consequently, RESS is being investigated as a micronization technique for the production of submicron- and micron-sized particles5,7,8 and thin films.4,9 When polymers are processed by RESS, fibers can also be formed.10-12 Several research groups have investigated the effect of RESS processing conditions, such as nozzle geometry and preexpansion temperature and pressure, * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: (864) 656-3055. Fax: (864) 6560784. † Clemson University. ‡ University of Pittsburgh.
on the product morphology and size of the polymers produced. Lele and Shine13 concluded that, if precipitation of a polymer-rich phase occurs late in the nozzle, the time available for structure formation is short, on the order of 10-6 s, and particles are formed. On the other hand, if a polymer-rich phase precipitates upstream of the nozzle, the time available for structure formation is long, on the order of seconds, and fibers are formed. Lele and Shine associated the two time scales with the phase behavior of the solution upstream of the nozzle and concluded that particles are formed from unsaturated solutions and fibers from saturated, two-phase solutions. Mawson et al.11 subsequently performed RESS on solutions of CO2 and poly(TA-N), a semicrystalline, CO2-soluble fluoropolymer. Solutions containing 0.5 and 2.0 wt % polymer were expanded through both a capillary and an orifice, and the preexpansion temperature was varied such that both homogeneous and two-phase solutions were rapidly expanded. Good agreement with the proposed mechanism of Lele and Shine was observed. Aniedobe and Thies12 investigated the rapid expansion of cellulose acetate from near-critical and supercritical methanol solutions. The maximum polymer concentrations were significantly higher (i.e., up to 14 wt %) than those previously investigated. Rapid expansion was initiated by the flow of fresh solvent into the initial polymersolvent solution in the view cell, so the polymer concentration during expansion was not accurately known. Nevertheless, the authors observed a consistent transition from continuous fibers to films consisting of small
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Figure 1. Molecular structure of poly(HDFDA).
particles as the polymer concentration decreased over the course of an experiment. In summary, although previous work has provided insight into how RESS processing variables affect structure formation, the picture is still incomplete. In particular, the effect of polymer concentration (and its interrelationship with the degree of saturation that exists in the solution to be expanded) has yet to be adequately explored. In this work, we report on the rapid expansion of the fluoropolymer poly(heptadecafluorodecyl acrylate), or poly(HDFDA), from supercritical CO2 solutions. Although this particular polymer is probably too expensive to be of direct commercial interest, we seek to better understand the processability of highly CO2-soluble polymers that are being developed.14 RESS experiments were chosen such that both the concentration and degree of saturation could be varied in a systematic manner. Experimental Section Materials. Carbon dioxide was obtained from Air Products (cooling grade, 99.99% purity). Poly(HDFDA) was synthesized via solution polymerization from the monomer 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyl acrylate (Aldrich). For a typical experiment, 10 g of the heptadecafluorodecyl acrylate monomer and 5 mg of initiator (AIBN) were transferred into a 50-mL ampule. The inhibitor present in the monomer (100 ppm of monomethyl ether hydroquinone) was removed beforehand with an inhibitor removal column (Aldrich). Ten milliliters of trifluorotoluene was then added to the ampule, yielding a transparent solution. The reaction mixture was subsequently purged with argon for 5 min, and the ampule was sealed with a flame. The ampule was then placed in a silicone oil bath at 333 K, and the polymerization was allowed to proceed for 24 h. The resultant polymer solution was then cooled to ambient temperature, removed from the ampule, and washed three times with 300 mL of methanol. The unreacted monomer dissolved in the methanol while the poly(HDFDA) precipitated out of solution. The polymer was recovered by filtration, followed by overnight drying under vacuum. The yield of the poly(HDFDA) product, a white powder, was typically 80-85%; its structure is shown in Figure 1. The polymer was determined to be semicrystalline by X-ray powder diffraction (Scintag Inc., model XGEN 4000), to have a melting point of 78 °C and no glass transition temperature above ambient by differential scanning calorimetry (Mettler Toledo Inc., model DSC 820), and to be stable up to 275 °C by thermal gravimetric analysis (Mettler Toledo, model TGA/SDTA 851). The molecular weight (Mw ) 254 000 g/mol) and polydispersity (Mw/Mn ) 2.95) of poly(HDFDA) were determined by American Polymer Standards Corp. (Mentor, OH) using gel permeation chromatography (GPC). The mobile phase, 0.1 M sodium trifluoroacetate in hexafluoroisopropanol, was pumped at a flow rate of 1 mL/ min at 30 °C through a single, mixed-bed column
Figure 2. Apparatus for measuring polymer-solvent phase behavior: P1 and P2, calibrated Heise pressure gauges; T1, thermocouple; T2, calibrated PRT.
(American Polymer Standards, cat. no. AM Gel Linear/ 10) and a refractive index detector. GPC results are based on a reference calibration curve obtained from poly(methyl methacrylate) standards that ranged in molecular weight from 10 000 to 2 million Da. Phase-Behavior Measurements. Because previous work demonstrated that the degree of saturation is a key processing variable in RESS, liquid-liquid equilibrium (LLE) phase transitions (commonly called cloud points) were measured before the RESS experiments to locate the phase boundaries. Figure 2 shows a schematic of the apparatus that was used to perform both cloudpoint and liquid-liquid-vapor measurements. Its central feature is a variable-volume view cell (1.59 cm i.d. × 27.9 cm length) that can be operated to 690 bar and 200 °C. The cell, supplied by McHugh and co-workers,15 has a piston that separates a pressurized working fluid from the mixture of interest, which is observed through a single view port with a borescope (Olympus Ind., model R100-038-000-50) and light generator (Olympus, model ILK-5). A CCD camera (Olympus, model XC-999) connected to the borescope displays the contents of the cell on a monitor, and a VCR is used for recordings. A pressure generator (High Pressure Equipment (HiP), model 81-5.75-10), which uses water as the working fluid, is used to move the cell piston and compress or expand the solution of interest to the desired pressure. Unless otherwise noted, all tubing has dimensions of 1.59 mm o.d. × 0.762 mm i.d. A constant-temperature environment for the cell is provided by a nitrogenpurged oven designed and constructed at Clemson University. A nitrogen-purged cooling jacket was built into the oven wall to maintain the borescope below its estimated maximum operating temperature of 80 °C. The oven is equipped with a rotating permanent magnet (Dexter Corp., model H-109) that provides mixing in the cell via a magnetic stir bar. For a typical experiment, the view cell was charged with 0.003-0.8 g of polymer to an accuracy of (0.0005 g as determined gravimetrically. After the cell had been purged with low-pressure CO2, a syringe pump (Isco, model 500HP) was used to deliver 12-34 g of the solvent CO2 into the view cell. The mass of CO2 charged into the cell was calculated with the assistance of a PVT equation of state16 and is accurate to within a (3% deviation. The temperature of the CO2 inside the syringe
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Figure 3. Apparatus for performing RESS experiments: P1, calibrated Heise pressure gauge; P3, pressure gauge; T1, thermocouple; T2, calibrated PRT; T3, thermocouple.
Figure 4. Geometry of nozzle used in RESS experiments.
pump was estimated by measuring the temperature in the line exiting the pump using a type-T thermocouple (T1). After the charging of CO2 and polymer had been completed, the view cell was allowed to reach a steadystate temperature. The contents of the cell were then compressed with the pressure generator under continuous mixing with the stir bar until the pressure was such that a homogeneous solution was obtained. To determine cloud-point pressures, the pressure was incrementally decreased until a cloudy solution was obtained. In this work, the cloud-point pressure was defined as the pressure at which the piston inside the view cell first became invisible as a result of the cloudiness of the mixture as the pressure was being decreased. The onset of liquid-liquid-vapor (LLV) equilibrium occurred at even lower pressures and was defined as the pressure at which a vapor phase first became visible upon decreasing pressure. The pressure inside the view cell was determined indirectly by measuring the pressure of the working fluid; the pressure drop across the piston was found to be less than 0.7 bar. A calibrated 0-5000 psig pressure gauge, accurate to (0.5 bar, was used (labeled P2; Heise, model CM-57927). The temperature in the view cell (T2) was measured with a calibrated platinum resistance thermometer (Burns Engineering, model WSPOG141/2-5C) and is accurate to (0.06 °C. Finally, as a safety measure, rupture disks were placed downstream of both the syringe pump and the pressure generator to prevent accidental overpressuring of the system. RESS Experiments. A schematic of the RESS apparatus is shown in Figure 3. Note that the phasebehavior and RESS apparatuses share the same view cell, syringe pump, and borescope setup. However, in the RESS configuration, the syringe pump was connected to both ends of the view cell, as it was used for the delivery of CO2 both as a solvent and as a working fluid. The laser-drilled nozzle (Tavex America) had a diameter of 50 µm and an aspect ratio of 5.08 and was located in a section of tubing with dimensions 0.318 cm i.d. × 0.635 cm o.d. × 18 cm length that served as the preexpansion section (see Figure 4). Two heating tapes were used to obtain a constant temperature upstream of the nozzle. One of the heating tapes was wrapped around the preexpansion tubing upstream of the nozzle and powered by an on-off controller. The other heating tape was wrapped around the lower end of the tubing and the nozzle and was controlled by a PID controller
(Omega Engineering, model 76000). The sensing element for the controller was a 1.59-mm-o.d., type-T thermocouple (T3 in Figure 3) located inside the preexpansion tubing about 2 mm upstream of the nozzle (see Figure 4). As discussed by Weber and Thies,17 the temperature must be measured as close as possible to the nozzle entrance if the true preexpansion temperature is to be obtained. A 4-mm-long, 120° fraction of a ceramic ring (Macor) was used as a spacer between the thermocouple and the inner wall of the tubing (see Figure 4) to ensure that the fluid temperature (rather than the wall temperature) was measured. With this arrangement, the preexpansion temperature was controlled to within (2 °C. The preexpansion pressure was controlled by the syringe pump to within (0.35 bar and measured with a calibrated 0-10 000 psig pressure gauge (labeled P1; Heise, model CM-109834) located upstream of the nozzle; the pressure measurements were accurate to (1.0 bar. A 0.5-µm filter (Alltech Associates, model 9200) was used to prevent clogging of the nozzle due to impurities. A second pressure gauge (P3; McDaniel Controls) was used to verify that the pressure had not changed significantly due to possible plugging of the filter. For a typical RESS experiment, the view cell was charged with 0.1-0.9 g of polymer and 9-32 g of CO2 solvent in the same manner as for the phase-behavior measurements. (The cell was maintained at ambient temperatures at all times, so the oven was not used.) Next, the contents of the cell were compressed with CO2 (now used as the working fluid) from the syringe pump to the desired pressure. After the mixture had been stirred magnetically for ∼10 min at the desired preexpansion pressure, a homogeneous solution was obtained. Then, pure CO2 was delivered from the pump and expanded through the nozzle, bypassing the view cell by means of the three-way valve (see Figure 3), all while maintaining a constant pressure on the working-fluid side of the view cell. This pure-fluid expansion continued until steady state (i.e., a constant preexpansion temperature and pressure) was achieved at the nozzle, which usually took about 10 min. To initiate actual RESS experiments, the pure CO2 flow was diverted exclusively to the working-fluid side of the view cell by means of the three-way valve, and the valve on the process side of the view cell was opened, pushing the polymer-solvent solution out of the cell and through the nozzle. The solution was expanded into a 500-mL glass reaction kettle maintained at ambient pressures.
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Figure 5. Cloud-point curves for poly(HDFDA) in CO2 at 25, 50, 70, and 100 °C; LLV data at 25 °C are also given.
Figure 6. Pressure vs composition diagram for the poly(HDFDA)-CO2 system at 25 °C in the dilute solution region. Vapor pressure for pure CO2 is from Angus et al.16
The gasified CO2 was withdrawn from the kettle, and the polymer precipitate was collected onto an aluminum SEM specimen stage covered with carbon tape. Two samples per run were collected for about 1 min each; one sample was collected about 1 cm and the other about 10 cm downstream of the nozzle. The samples were then gold-coated (Anatech LTD, model Hummer X), and the size and morphology of the precipitate were determined by scanning electron microscopy (SEM; Hitachi, models SEM S3500N and FESEM S4700). Following each experiment, the lines of the RESS apparatus were flushed with CO2 for more than 30 min to remove any residual polymer. In addition, the filter was cleaned for several hours in an ultrasonic bath immersed in a beaker filled with acetone and then dried with CO2. Results and Discussion Phase-Behavior Measurements. Cloud-point pressures for poly(HDFDA) in carbon dioxide are shown in Figure 5 for polymer concentrations and temperatures ranging from 0.01 to 6 wt % and from 25 to 100 °C, respectively. The maximum cloud-point pressure, which for this polydisperse system is not the mixture critical pressure but the precipitation threshold,18 occurred at between 2 and 4 wt % for all temperatures measured. Mawson et al.11 also measured cloud-point pressures for this system at temperatures up to 70 °C. [Poly(HDFDA) and poly(TA-N) are different names for the same polymer.] The cloud-point results of the two groups are similar, with ours being displaced to slightly higher pressures (e.g., 10 bar higher at 70 °C). Judging from this information, one would presume that our polymer has a higher average molecular weight, but this information was not reported by Mawson et al.11 We also made additional measurements at highly dilute concentrations to clarify the phase behavior in this region; as can be seen in Figures 5 and 6, the cloud-point pressures remain essentially constant until very low polymer concentrations ( 1. Here, we use the term “supersaturated” loosely, as the solution would not actually be expected to be in a homogeneous, metastable, supersaturated state, but rather would have split into two phases, one polymer-rich and one solvent-rich, as the initially homogeneous solution was heated to the desired preexpansion temperature (see Figures 5 and 7). Other researchers have varied S upstream of the nozzle by changing either pressure at constant temperature or temperature at constant pressure. In this work, both temperature and pressure were used to vary S. From the measured cloud-point curves, preexpansion conditions for RESS were chosen. For the experimental design, broad ranges of operating temperatures and pressures were selected, with unsaturated, saturated, and supersaturated conditions being equally weighted (see Figure 8). For each of the nine RESS conditions selected, polymer concentrations of 0.5, 2, and 5 wt % were investigated to give a total of 27 experimental runs; several of these runs were duplicated to check reproducibility. An upper temperature limit of 100 °C was selected so that any fibers formed would have adequate time to solidify before entering the transonic portion of the free-expansion jet, where turbulence forces become significant. One of our concerns was that partial plugging of the nozzle with polymer could itself alter product morphology and lead to incorrect interpretations of the results. Therefore, flow-rate tests were conducted beforehand with pure CO2 to establish the rates that should be obtained for an unobstructed orifice. As an additional check, flow rates for pure CO2 were calculated with a one-dimensional model for subsonic flow of a supercritical solution through a nozzle.17 Good agreement between measurement and calculation was obtained. During actual RESS experiments with poly(HDFDA), solution flow rates were monitored; those few experiments that exhibited reduced flow rates due to partially obstructed orifices were terminated and rerun.
Figure 9. SEM micrographs of RESS products obtained from a 0.5 wt % poly(HDFDA) solution: (a) Tpre ) 30 °C, Ppre ) 248 bar, S < 1; (b) Tpre ) 65 °C, Ppre ) 138 bar, S > 1.
RESS Morphologies. Typical RESS morphologies collected about 10 cm downstream of the nozzle are presented in Figures 9-11, with the operating conditions that were used also being shown in Figure 8. At unsaturated conditions and 0.5 wt % polymer, the precipitate is spherical and mainly submicron in size (Figure 9a). However, at supersaturated conditions and 0.5 wt %, larger, micron-sized particles are formed (Figure 9b). The same increase in product size as we move from unsaturated to supersaturated solutions is seen for the rapid expansion of 2 wt % polymer solutions in Figure 10a and b. However, another trend also appears, as S is now seen to also affect product morphology. In particular, fibers become the dominant morphology for supersaturated solutions (S > 1), but particles still predominate when S < 1. At still higher polymer concentrations (5 wt %), S no longer affects product morphology significantly, as fibers become the dominant morphology (see Figure 11). However, S continues to affect product size, as submicron-sized fibers are produced when S < 1 (Figure 11a) and micron-sized fibers when S > 1 (Figure 11b). Incidentally, increased concentration was also seen to increase product size, but its effect was much smaller than the effect of S. It is important to note that changes in pressure and temperature without concomitant changes in the degree
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Figure 10. SEM micrographs of RESS products obtained from a 2 wt % poly(HDFDA) solution: (a) Tpre ) 30 °C, Ppre ) 248 bar, S < 1; (b) Tpre ) 100 °C, Ppre ) 248 bar, S > 1.
of saturation or concentration had no effect on either product size or morphology. Thus, referring to Figure 8, for the three runs with S < 1 (i.e., above the cloudpoint curve) and a fixed concentration, essentially the same results were obtained. The same is true for the corresponding three runs with S > 1. Detailed information on all 27 experimental runs is presented elsewhere.20 No impact on product morphology was observed when precipitate was collected 1 cm downstream of the nozzle (compared to the 10 cm results shown above). Finally, for all six of the experiments that were duplicated, identical results were obtained. The observed relationship between RESS processing conditions and product morphology for the poly(HDFDA)-CO2 system is summarized in Figure 12; both the cloud-point curve (darker surface) and the boundary between particle and fiber formation (lighter surface) are shown. Here, we see that only at intermediate concentrations is product morphology controlled by S, as proposed by Lele and Shine.13 However, at the highest and lowest concentrations, concentration, and not S, controls the product morphology (as indicated by the fiber-particle surface becoming vertical). In Figure 13, the observed relationship between RESS processing conditions and product size is summarized; here, we see that the cloud-point curve (darker surface) and the
Figure 11. SEM micrographs of RESS products obtained from a 5 wt % poly(HDFDA) solution: (a) Tpre ) 30 °C, Ppre ) 331 bar, S < 1; (b) Tpre ) 100 °C, Ppre ) 248 bar, S > 1.
Figure 12. Relationship between concentration, degree of saturation, and product morphology for the poly(HDFDA)-CO2 system as shown on a pressure-temperature-concentration diagram.
boundary between product sizes (lighter surface) coincide. Thus, supersaturated solutions (S > 1) always give the larger product sizes (be they fibers or particles), and the unsaturated (S < 1) solutions result in submicronsized precipitates. The observed results are consistent with our understanding of (1) the kinetics of phase separation and (2) nucleation and growth theory as follows. When moving
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the likelihood that condensation and coagulation can take place as the rapid expansion process occurs within the nozzle. Shear forces are then responsible for oriented rather than isotropic growth. Because of the different modes of structure formation, lower minimum concentrations are required for fiber formation when a supersaturated vs an unsaturated solution is being expanded (see Figure 12). Conclusions
Figure 13. Relationship between concentration, degree of saturation, and product size for the poly(HDFDA)-CO2 system as shown on a pressure-temperature-concentration diagram.
along the RESS process path from a homogeneous solution into the two-phase region, nuclei of a second phase are formed within the original phase. These nuclei then grow either by condensation (precipitation of matter from the original phase onto the nuclei) or by coagulation (merging of two second-phase volumes). The time scale for formation of a new phase (via nucleation, condensation, and coagulation) depends on the driving force for phase separation, that is, on the degree of supersaturation. Zhuang and Kiran21 showed that polymer-solvent phase separation can occur very rapidly (i.e., on the order of milliseconds) if the pressure reduction into the two-phase region is rapid enough. Thus, when a solution for which S > 1 is rapidly expanded, the time for phase separation is on the order of seconds. (For our experiments, the time required for the solution to flow through the preexpansion section is 2-4 s.) As a result, the nominally supersaturated solution has already phase separated before entering the nozzle, and polymer-rich droplets have already formed and even coalesced within the solvent-rich phase upstream of the nozzle. Relatively large, micron-sized structures are formed when such a two-phase mixture is expanded. On the other hand, if RESS is carried out such that the solution is unsaturated at preexpansion conditions, nucleation will not occur until a significant pressure drop has occurred, that is, inside the nozzle or possibly not even until the free jet. Particles have on the order of only 10-6 s to form a critical nucleus (with a size on the order of 10 nm) and grow by condensation and coagulation17 until the formed structures solidify when the temperature drops below the melting point of the polymer-rich phase. With so little time available for growth, submicron structures are formed. Our explanation as to why concentration is the controlling variable for product morphology is as follows. When a supersaturated solution is expanded, a higher polymer concentration in the starting mixture results in even larger volumes of the polymer-rich phase. At low polymer concentrations (e.g., 0.5 wt %), polymerrich volumes are still relatively small, and, when sheared within the nozzle, they remain in droplet form. (Nevertheless, some of the particles shown in Figure 9b are elongated, indicating the tendency for fiber drawing.) At 2 and 5 wt %, the polymer-rich volumes are larger and are drawn into fibers by the shear flow (see Figures 10b and 11b). For the expansion of an unsaturated solution, a higher polymer concentration increases
Rapid expansion of the semicrystalline fluoropolymer poly(HDFDA) in CO2 was performed, with both concentration and degree of saturation being varied in a systematic manner. In contrast to previous workers, we extended our investigation to relatively high polymer concentrations, that is, up to 5 wt %, conditions that are of more commercial interest. With these new results, we propose a modification of the generally accepted mechanism of polymer morphology in RESS first proposed by Lele and Shine13 and later confirmed by Mawson et al.11 We concur that the size of the polymer product is controlled by degree of saturation and that this relationship is valid over all concentrations that have now been investigated by researchers (i.e., from 0.05 to 5 wt %). However, the morphology of the polymer product is controlled primarily not by the degree of saturation, but rather by the concentration. Only at intermediate concentrations near the threshold concentration for fiber formation does the degree of saturation play a significant role in morphology. For our polymer and experimental RESS setup, this threshold concentration was about 2 wt % polymer; for other types of polymers and experimental setups, the threshold concentration would be expected to occur at higher or lower concentrations. For example, Lele and Shine13 and Mawson et al.11 were able to produce fibers from saturated solutions at lower concentrations. We maintain that, if one performed RESS experiments with their systems for unsaturated solutions at some higher (i.e., above the threshold) polymer concentration, continuous submicron fibers would be produced. Finally, the fact that RESS product morphologies are dependent on both the type of polymer and its concentration in solution suggests that a more fundamental property of the polymer (or of the polymer-solvent system) needs to be examined before product morphologies in RESS can be adequately predicted. Acknowledgment This work was supported primarily by the ERC Program of the National Science Foundation under Award EEC-9731680. X-ray powder diffraction, DSC, and TGA analyses were performed by students in Prof. Ya-Ping Sun’s group in the Department of Chemistry at Clemson University. The authors thank Mr. Chris Norfolk for his assistance with the phase-behavior measurements and Dr. Markus Weber for his helpful discussions on RESS processing. Literature Cited (1) Mohamed, R. S.; Debenedetti, P. G.; Prud’homme, R. K. Effects of Process Conditions on Crystals obtained from Supercritical Mixtures. AIChE J. 1989, 35, 325-328. (2) Lele, A. K.; Shine, A. D. Morphology of Polymers Precipitated from a Supercritical Solvent. AIChE J. 1992, 38, 742-752.
Ind. Eng. Chem. Res., Vol. 41, No. 20, 2002 4983 (3) Domingo, C.; Berends, E.; Van Rosmalen, G. M. Precipitation of Ultrafine Organic Crystals from the Rapid Expansion of Supercritical Solutions over a Capillary and a Frit Nozzle. J. Supercrit. Fluids 1997, 10, 39-55. (4) Matson, D. W.; Fulton, J. L.; Petersen, R. C.; Smith, R. D. Rapid Expansion of Supercritical Fluid Solutions: Solute Formation of Powders, Thin Films, and Fibers. Ind. Eng. Chem. Res. 1987, 26, 2298-2306. (5) Charoenchaitrakool, M.; Dehghani, F.; Foster, N. R.; Chan, H. K. Micronization by Rapid Expansion of Supercritical Solutions to Enhance the Dissolution Rates of Poorly Water-Soluble Pharmaceuticals. Ind. Eng. Chem. Res. 2000, 39, 4794-4802. (6) Chernyak, Y.; Henon, F.; Harris, R. B.; Gould, R. D.; Franklin, R. K.; Edwards, J. R.; DeSimone, J. M.; Carbonell, R. G. Formation of Perfluoropolyether Coatings by the Rapid Expansion of Supercritical Solutions (RESS) Process. Part 1: Experimental Results. Ind. Eng. Chem. Res. 2001, 40, 6118-6126. (7) Kim, J.-H.; Paxton, T. E.; Tomasko, D. L. Microencapsulation of Naproxen Using Rapid Expansion of Supercritical Fluid. Biotechnol. Prog. 1996, 12, 650-661. (8) Tu¨rk, M. Formation of Small Organic Particles by RESS: Experimental and Theoretical Investigations. J. Supercrit. Fluids 1999, 15, 79-89. (9) Shim, J.-J.; Yates, M. Z.; Johnston, K. P. Polymer Coatings by Rapid Expansion of Suspensions in Supercritical Carbon Dioxide. Ind. Eng. Chem. Res. 1999, 38, 3655-3662. (10) Petersen, R. C.; Matson, D. W.; Smith, R. D. The Formation of Polymer Fibers from the Rapid Expansion of Supercritical Fluid Solutions. Polym. Eng. Sci. 1987, 27, 1693-1697. (11) Mawson, S.; Johnston, K. P.; Combes, J. R.; DeSimone, J. M. Formation of Poly(1,1,2,2-tetrahydroperfluorodecyl acrylate) Submicron Fibers and Particles from Supercritical Carbon Dioxide Solutions. Macromolecules 1995, 28, 3182-3191. (12) Aniedobe, N. E.; Thies, M. C. Formation of Cellulose Acetate Fibers by the Rapid Expansion of Supercritical Methanol
Solutions. Macromolecules 1997, 30, 2792-2794. (13) Lele, A. K.; Shine, A. D. Effect of RESS Dynamics on Polymer Morphology. Ind. Eng. Chem. Res. 1994, 33, 1476-1485. (14) Leitner, W. Green Chemistry: Designed to Dissolve. Nature 2000, 405, 129-130. (15) Meilchen, M. A.; Hasch, B. M.; McHugh, M. A. Effect of Copolymer Composition on the Phase Behavior of Mixtures of Poly(ethylene-co-methyl acrylate) with Propane and Chlorodifluoromethane. Macromolecules 1991, 24, 4874-4882. (16) Angus, S.; Armstrong, B.; De Reuck, K. International Thermodynamic Tables of the Fluid States3 Carbon Dioxide, 1st ed.; Pergamon Press: New York, 1994; Vol. 3. (17) Weber, M.; Thies, M. C. Understanding the RESS Process. In Supercritical Fluid Technology in Materials Science and Engineering: Synthesis, Properties, and Applications; Sun, Y.-P., Ed.; Marcel Dekker: New York, 2002; Chapter 13. (18) Folie, B.; Radosz, M. Phase Equilibria in High-Pressure Polyethylene Technology. Ind. Eng. Chem. Res. 1995, 34, 15011516. (19) Scott, R. L.; van Konynenburg, P. H. Static Properties of Solutions. van der Waals and Related Models for Hydrocarbon Mixtures. Discuss. Faraday Soc. 1970, 49, 87-97. (20) Blasig, A. Effect of Concentration and Degree of Saturation on RESS Product Morphology and Size. Ph.D. Dissertation, Clemson University, Clemson, SC, Dec 2002. (21) 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.
Received for review March 11, 2002 Revised manuscript received July 2, 2002 Accepted July 9, 2002 IE0201819
