Miscibility, Phase Separation, and Phase Settlement Dynamics in

Article pubs.acs.org/IECR

Miscibility, Phase Separation, and Phase Settlement Dynamics in Solutions of Ethylene−Propylene−Diene Monomer Elastomer in Propane + n‑Octane Binary Fluid Mixtures at High Pressures Erdogan Kiran,*,† John C. Hassler,† and Rakesh Srivastava‡ †

Department of Chemical Engineering, Virginia Tech, Blacksburg, Virginia 24061, United States Dow Chemical Company, Midland, Michigan 48667, United States

‡

ABSTRACT: Miscibility and phase separation of ethylene−propylene−diene monomer (EPDM) elastomer at a polymer concentration of 11.7 wt % in n-octane (61.9 wt %) + propane (26.4 wt %) have been explored over a wide range of temperatures up to 187 °C and pressures up to 200 bar. Experiments were carried out using a new system that allows assessment of volumetric properties, thermodynamic phase boundaries, phase settlement times, and the mechanistic details of phase separation all in the same instrument. The system involves a novel, dual-piston variable-volume cell with two pairs of sapphire windows. One pair of the windows is used for the measurement of the transmitted light intensities, while the other pair is used for the measurement of scattered light intensities from a He−Ne laser source. Piston positions are continuously recorded with two dedicated linear variable differential transformers, which allow continuous determination of the solution densities as a function of pressure at a given temperature. The solution displayed lower critical solution type behavior with the liquid−liquid (LL) demixing pressures increasing from about 40 bar at 125 °C to about 135 bar at 187 °C. The LL phase separation was found to proceed by spinodal decomposition, and the phase settlement times recorded by monitoring the transmitted light intensities in the polymer-rich and polymer-lean phases showed strong dependence on the temperature at which phase separation was induced. They were particularly long at temperatures below 186 °C. Isothermal compressibilities were found to decrease with pressure, but increase with temperature, essentially doubling in value from about 0.00042 to 0.00092 bar−1 in going from 125 to 187 °C at 100 bar.

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carbon dioxide,5 miscibility of polyisoprene in ethane, ethylene, propane, propylene, or dimethyl ether,6 miscibility of block copolymers of polystyrene with polybutadiene or polyisoprene in propane or in carbon dioxide + pentane mixtures,7a,b grafting of methyl methacrylate onto natural rubber in supercritical CO2,8 phase behavior of cross-linked polyisoprene rubber in supercritical carbon dioxide,9 dielectric behavior of polyisoprene in carbon dioxide,10 surface modification of polybutadiene in carbon dioxide,11 phase behavior of polybutadiene in toluene + CO2 mixtures,12 and depolymerization of polyisoprene in supercritical THF.13 Miscibility of ethylene-based copolymers in subcritical and supercritical propane has also been studied14−18 with a more recent article reporting on the foaming of poly (ethylene-co-octene) rubber in supercritical carbon dioxide.19 Prior publications on the miscibility and phase separation dynamics of EPDM in fluid mixtures are extremely rare. One study reports on the miscibility of 8 wt % EPDM in hexane plus 20 or 40 wt % propene and small amounts of (± 0.5 wt %) of diene monomer (ENB) over a temperature range of 45−245 °C and pressure range from 30 to 190 bar.20 The focus of this early study was on the possible use of liquid−liquid extraction as a method for purification of rubber solutions, and incorporation of a countercurrent extraction column in the ethylene−propylene copolymer production process. It was

INTRODUCTION In this Article, we present new data on the miscibility and phase separation behavior of ethylene−propylene−diene monomer (EPDM) elastomer in binary fluid mixtures of propane and noctane. EPDM is a commercially important elastomer that is used in a wide range of applications, from door and window seals in vehicles to water-resistant roofing membranes in houses. It is reported to be more stable than the conventional elastomers based on butadiene or isoprene.1 Further, in EPDM, while the propylene and ethylene segments are saturated, the incorporation of the diene monomer provides the unsaturated groups that can be cross-linked. As such, a range of graft copolymers can be produced or specialty blends can be formulated,2 which are typically employed to improve the impact strength of commodity thermoplastics such as polystyrene or polypropylene. In the synthesis of high impact polystyrenes (HIPS), using EPDM in lieu of polybutadiene offers advantages that arise from its stability. Also, polypropylene has been shown to be cross-linkable with EPDM in supercritical propane to form thermoplastic vulcanizates, which are a form of interpenetrating networks.3 EPDM and polypropylene can also be physically blended with the aid of supercritical fluids such as carbon dioxide.4 Unlike the vast literature that is available on the phase behavior and processability of glassy or semicrystalline thermoplastics in dense fluids, similar studies with elastomers are not extensive and have been mostly limited to polyisoprene or polybutadiene. Examples of these include solubility of modified polybutadiene and polyisoprene in supercritical © 2013 American Chemical Society

Received: Revised: Accepted: Published: 1806

November 28, 2012 January 9, 2013 January 14, 2013 January 14, 2013 dx.doi.org/10.1021/ie303249m | Ind. Eng. Chem. Res. 2013, 52, 1806−1818

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematic representation of phase separation in a polymer solution displaying LCST. Diagrams on the left illustrate lowering of the LCST with a decrease in pressure (P3 < P2 < P1) showing how a point at A with a polymer weight fraction of wo at homogeneous conditions at P1 may then find itself in the two-phase domain at P2 or P3 and undergo phase separation. The cartoons on the right illustrate the dynamics of the phase settlement process showing the bubbles of polymer-lean phase rising in the more viscous polymer-rich phase with time as the phases settle to their eventual equilibrium phase compositions of w1 and w2.

pressure, but can equally be achieved by increasing the amount of nonsolvent, such as carbon dioxide, or a light hydrocarbon such as propane. The phase settlement may be fast or slow depending upon the facility with which the respective phases migrate. The diagram illustrates the possible entrapment of the polymer-lean phase and its slow rise to the upper region from a viscous polymer-rich bottom phase. Temperature of operation, the mode of phase separation (i.e., nucleation and growth vs spinodal decomposition), and the quench depth (i.e., whether liquid−liquid or liquid−liquid−vapor phase boundary is crossed during the quench) are among factors that may influence the outcomes with respect to the dynamics of phase separation and phase settlement. Clearly, after phases settle, the amount of solvent to be removed by evaporation from the polymer-rich phase with polymer fraction w2 will be much less than it would be for the initial homogeneous solution, leading to the possibilities for energy efficient separation pathways. Indeed, as has been reported in the literature, the devolatilization especially from solution polymerization processes is energy intensive as the solvent can amount to about 90 wt % of the solution.20 High-pressure miscibility and phase separation of EPDMs in mixtures of propane and higher alkanes such as hexane or octane are thus important and of direct relevance to the synthesis, separation, and purification as well as post processing of these polymers. In this study, we have focused on their phase behavior and phase separation dynamics in binary fluid mixtures of n-octane (as the solvent) and propane (as the nonsolvent) at a nominal concentration of 12 wt %. It is important to note that elastomeric mixtures tend to become very viscous as concentrations are increased, and using a simple magnetic stirring bar often becomes ineffective in bringing about efficient mixing of the solutions. Furthermore, one must also be concerned with wall effects because viscous solutions have a tendency to cling on surfaces, which may adversely influence the observations of phase settlement times.23 The present study has therefore been carried out using a new experimental system with an efficient mixing arrangement to handle even very viscous solutions with minimal wall effects. The system is a dual-piston, variable-volume cell with an internal size and configuration that reduces the wall effects and allows a full, unobstructed view of the cell interior. The system incorporates two pairs of optical windows, one pair with a long, and the other pair with a very short path length for measurements of both the transmitted and the scattered light intensities in the same system. This allows the assessment of not only the phase boundaries, but also for determining the

reported that with incorporation of a single demixing step it was possible to obtain a bottom slurry concentration of about 35 wt % at pressures in the vicinity of the LV phase boundary, reported to be equivalent to a polymer separation efficiency of 85−95%. Separation of solvents from polymers is indeed an important problem in the synthesis of rubbers such as polybutadiene, polyisoprene, and ethylene−propylene copolymers as they are produced via solution polymerization and the solvent must eventually be removed.21,22 Simple evaporation is a costly operation as these polymerizations are carried out at relatively low concentrations to maintain low viscosities in the solutions. An alternative to simple evaporation is to take advantage of the LCST behavior in polymer solutions, and utilize the high temperature liquid−liquid phase separation that may be induced by either a pressure reduction or a temperature increase. The boundary may also be altered by changing the initial mixture composition by altering the nonsolvent content. As has been discussed in the literature for the case of polybutadiene + n-hexane system, increasing the concentration of the polymer in the polymer-rich phase via liquid−liquid phase separation and settlement greatly reduces the penalty that would otherwise be paid for the total solvent removal by vaporization.23 In earlier studies on ethylene−propylene copolymer systems, recognition of the sensitivity of LCST to fluid composition had led to a novel approach to lower the LCST by using a combination of dissolved gas with a relatively poor solvent to achieve separation of the polymer from the solution at temperatures not far from the polymerization temperatures.22 It should be noted that lowering the LCST also helps reduce the risk of degrading the polymer. In this context, detailed studies on the LCST behavior of EPDM with ethylene contents in the range 40−46 wt % and diene contents in the 0− 9 wt % range were explored in a mixed hexane−methyl pentane−methylcyclopentane solvent system.22 Further work exploring these concepts involved the LCST behavior of ethylene−propylene copolymers in mixtures of hexane with methane, carbon dioxide, as well as ethylene and propylene.24 Separation of polymers by solution polymerization that employs manipulation of LCST is continuing to be of academic interest25 and of significant industrial relevance.26,27 Figure 1 illustrates schematically how a solution with a polymer weight fraction of wo at homogeneous conditions at A can undergo phase separation upon lowering the LCST, by reducing the pressure from P1 to P2 or P3, leading to formation of a polymerlean and a polymer-rich phase with compositions of w1 and w2. The lowering of LCST is readily achieved by reducing the 1807

dx.doi.org/10.1021/ie303249m | Ind. Eng. Chem. Res. 2013, 52, 1806−1818


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Figure 2. Overall schematic of the experimental system. IL = illuminator; SW = sapphire window; MI = mixer impeller; MP = movable pistons 1 and 2; LVDT/PPS = piston position sensor 1 and 2; FSC = fast scan camera; MPGN = motorized and coupled pressure generators; PGN = pressure generator; He−Ne= helium−neon laser; FOS = fiber optic sensors; Itr = transmitted light intensity; Isc = scattered light intensity, V = valve; B = balance, FTV = fluid transfer vessel, LP = high-pressure liquid pump.

mode of phase separation being “nucleation and growth” or “spinodal decomposition”. Another feature of the experimental system is the flexibility to continuously monitor the positions of the pistons, which allows real-time continuous determination of the solution densities as pressures and/or temperatures are altered. This Article describes the effectiveness of this experimental system in investigating the EPDM solutions. The findings show that at 12 wt % these solutions are close to their critical polymer concentration, and pressure-induced quench proceeds via spinodal decomposition. It is further shown that phase settlement and the approach to final equilibrium is a very slow process but can be accelerated by increasing the temperature to lower the viscosity.

of the variable-volume parts. As will be explained in the following paragraph, this feature of the pistons permits bringing about a volume translation of the cell content across the mixer for efficient mixing, and also refreshing the solution in the narrow gap between the windows for the scattered light measurements. Each piston is connected to an extension rod with a magnetic core. Their positions are monitored with two LVDT sensors and piston position read out units (PPS 1 and 2). From the initial positions of the pistons and recordings of their positions with a change in temperature or pressure, the change in volume is assessed. Using the initial total mass loading, which is known, the densities of the solutions then are determined in real-time in a continuous manner following procedures similar to those reported in our earlier publications.28,29 Even though not shown in the schematic diagram, the pressure and the temperature of the solution in the cell are measured with a Dynisco transducer that is mounted on the top side of the cell between the sapphire windows dedicated to the transmitted light measurements. Both the main cell body and the variable volume parts are equipped with 16 symmetrically distributed cartridge heaters (not shown in the diagram) to control the temperature of the solution in the entire cell and the piston cavities. The impeller that is magnetically coupled is driven with a motor with adjustable speed. The main cell is equipped with an inlet port to charge a fluid from a fluid transfer vessel (FTV), and a discharge port connected to the exit valve E. There is an additional port (not shown in the diagram) to introduce or discharge fluids from the mixer-cap positioned at the top of the cell. The overall system involves three pressure generators to bring changes in the positions of the pistons. Two of the pistons (MPGN) are coupled and motorized for synchronized motion of the pistons so that when one piston is moved inward toward the mixer in its respective variable volume part, the other piston is moved outward. This capability permits translation of the entire solution across the mixer impeller to ensure complete mixing and homogenization at any given pressure without causing a change in the system pressure or internal volume. The third pressure generator (PGN), which is hand-operated, is used to change the pressure in the cell by moving either one or both of the pistons. These options of moving the pistons are carried out by opening and closing a set of dedicated valves that are positioned on the lines going to the

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EXPERIMENTAL SYSTEM Figure 2 shows a schematic diagram of the experimental system. The main cell body is a 7.62 cm × 7.62 cm × 15.24 cm stainless-steel block, which has been machined to have a 2.54 cm × 2.54 cm × 15.24 cm internal cavity that runs across the full length of the cell. The square-shaped 2.54 cm × 2.54 cm cross-section of the cavity permits flat mounting of two sapphire observation windows (SW) with a 2.54 cm separation distance and permits an unobstructed view of the entire height of the cell cross-section. In addition to these windows for direct visual observation or the measurement of the transmitted light intensities (Itr), the cell body accommodates two other windows, which are separated by a very short distance (60 μm) to allow the measurement of the scattered light intensities (Isc) in the same solution. A magnetically coupled mixer impeller (MI) is mounted in the cell cavity in the space between the windows for transmittance and scattered light measurement. The impeller is away from the windows and does not in any way block the view area of the observation or laser light paths. On each side of the main cell body, there is a variable volume part (VVP) of equal volume. These parts, each of which accommodates a movable piston (MP 1 and MP 2), have internal cavities with 2.54 cm diameter circular cross sections. The movable pistons are designed in such a way that they can enter the main cell cavity and come close to the mixer impeller from the visual observation sapphire window side, and can come close to the window holder extension inside the cell for the scattered light measurements from the other side, while the O-ring of each piston is still remaining in the circular cavity 1808



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mixer motor, the pressure−temperature sensor, and the light source and fiber optic sensors for transmittance measurements. Figure 4 is a close-up of the window for visual observations and

back sides of the piston cavities. Ethanol is used as the piston back-pressure fluid. Initially ethanol is charged to the pressure generators by opening valves V1−V7. Pressure in the cell then is adjusted with hand operated pressure generator PGN by closing V1, V6, and V7. To bring about volume translation at a given pressure, V1, V2, and V3 are closed and V4, V5, V6, and V7 are kept open. Operating the motorized pressure generator then brings about the coordinated motion of the pistons; as one piston moves inward, the other moves out. The pressure generators are equipped with limit switches that when one piston reaches its maximum inward or outward positions, the direction of the rotation of the motor is switched, and the piston movement directions are revered. This is a churn-like motion that moves the entire solution across the mixer impeller without a change in the system pressure or internal volume, which assures very efficient mixing that becomes extremely important when working with solutions of very high viscosities. Three HPLC-type high-pressure liquid pumps are used to charge the liquid solvent (n-octane) and the nonsolvent (propane) to the cell and to charge the pressurizing fluid (ethanol) to the pressure generators. Propane is first pumped from a large tank to a small high-pressure fluid transfer vessel (FTV). For n-octane, a simple liquid bottle is used as a transfer vessel. The transfer vessels loaded with n-octane or propane are then used to charge the view-cell. The amount of n-octane or propane that is charged to the system is measured with a dedicated high capacity (16 kg) balance (Sartorius) with an accuracy of 0.1 g. Transmitted light intensities (Itr) are measured at three vertical positions across the sapphire windows using fiber optic sensors (FOS) that allow the documentation of the dynamics of phase settlement in the top, middle, and bottom regions of the solution as the polymer-lean and polymer-rich phases evolve after inducing phase separation. The scattered light intensity patterns (Isc) generated from a He−Ne laser source are projected on a screen for visual observations and/or for recording with a fast-scan (DALSA) camera (FSC). Figure 3 is a partial photograph of the system showing the view-cell, along with its variable-volume parts, LVDT sensors,

Figure 4. Close-up of the sapphire windows for visual observation, and of the fiber optic probe lines (black cables) and their holding stand for measurement of the transmitted light intensity at different positions across the sapphire window. The photographs also show the settling of a polymer-rich phase at two different times after phase separation.

for simultaneously recording the transmitted light intensities at different positions across the window. The photograph also shows the appearance of a polymer solution at different settlement times after imposing phase separation. Figure 5 shows two photographs capturing the scattered light pattern from a He−Ne laser source during phase separation. The figure illustrates the formation of a spinodal ring and its evolution, which is the fingerprint of systems undergoing spinodal decomposition. A dedicated computer records the pressure, temperature, transmitted light intensities, and the positions of the pistons in real time at adjustable sampling times. Typical sampling rate is 3 readings per second, but in long-time measurements employed for monitoring phase settlement times, the sampling rate is reduced depending upon the anticipated observation length. Temperatures and pressures are accurate to ±0.5 °C and 1 bar, respectively. The maximum internal volume of the cell including the variable volume parts with pistons all the way out is 112 cm3. The minimum internal volume when pistons are all the way in is 61 cm3. Densities are determined within 1% accuracy.

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MATERIALS The elastomeric polymer EPDM 3745 (Mw = 150 000; Tg = −44 °C; Tm = 41 °C; Mooney viscosity = 45) was kindly provided by Dow Chemical Co. It contains 70% ethylene and 0.5% diene monomer ethylene norbornene (ENB). Viscosity is expressed in Mooney units that are the common processability measures in the rubber industry. Mooney viscosity provides a measure of plasticity in units of torque measured on an arbitrary scale on a disk in a vessel that contains the elastomeric polymer at 100 °C and rotates at two revolutions per minute.30 The solvent n-octane with a purity of 98 wt % was purchased from Sigma-Aldrich. Propane, obtained from Scott Specialty Gases, was instrument grade with 99.5 wt % purity. The polymers and the solvents were used as received.

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RESULTS AND DISCUSSION a. Phase Separation Pressures, Densities, and Phase Settlement Times. Experiments were carried out with 11.7 wt % polymer solution in n-octane (61.9 wt %) + propane (26.4 wt %) at the nominal temperatures of 125, 150, 173, 185, and 187 °C.

Figure 3. Photograph of the main cell, showing the variable volume parts (VVP), piston position sensors (LVDT), mixer motor, pressure/ temperature sensor, illuminator, and the fiber optic sensor lines for transmitted light intensity measurements. 1809



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Figure 5. Photographs taken with the fast scan camera showing the projections of the scattered He−Ne laser light intensity patterns on a screen as a spinodal ring forms and evolves in the polymer solution undergoing liquid−liquid phase separation via spinodal decomposition.

Figure 6. Pressure and temperature history over 12 000 s during and after reducing the pressure at 187 °C from 185 to 85 bar at a rate of 1.5 bar/s.

Figure 7. Phase settlement time after reducing the pressure from 185 to 85 bar at 187 °C, as recorded by the transmitted light intensities over 12 000 s from different positions across the sapphire. The solid (red) curve is the Itr recorded by the middle, while the dashed (green) curve is the Itr recorded by the bottom, and the (blue) curve between is the Itr recorded by the top fiber optic probes, respectively, as depicted in the diagram on the left, and shown in the photograph in Figure 4.

In bringing about the miscibility, the volume translation approach using the motorized coupled-pressure generators was indispensable. The coordinated motion of the pistons pushing the viscous content of the cell back-and-forth across the impeller ensured effective mixing, eventually leading to homogeneous conditions with the manipulation of pressure and temperature. Starting from the homogeneous conditions, at a given temperature, the pressure was lowered to cross the liquid−liquid, or both the liquid−liquid and, further down, the liquid−liquid−vapor phase boundaries. In some of the experiments, the pressure was immediately increased back to check the reproducibility of the phase separation pressures before reducing the pressure again to a level where final settlement times were monitored.

Figures 6−10 show the details of one of these experiments conducted at 187 °C. Pressure was reduced from 185 to 85 bar and held there for 12 000 s (3.3 h). The rate of pressure reduction was about 1.5 bar/s. The pressure history over the 12 000 s and during the expanded 200 s is shown in Figure 6. The temperature stability over the 12 000 s time interval was remarkably high as is illustrated in the figure. Figure 7 shows the change in the transmitted light intensities as recorded from the top, middle, and bottom regions of the sapphire observation window over the total time interval of 12 000 s. [Because of the nature of the light source that was used, the higher relative intensity of transmitted light corresponds to the middle fiber optic probe, the next highest corresponds to the top fiber optic probe, and the lowest intensity corresponds to the bottom fiber optic probe.] The transmitted light 1810



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Figure 8. Variation of the transmitted light intensities with pressure (left) and variation of the transmitted light intensity from the middle fiber optic probe and of the temperature with pressure (right) at 187 °C.

Figure 9. (a) Change in the positions of the movable pistons MP1 and MP2 during the pressure reduction process; and (b) variation of density with pressure at 187 °C.

intensities all first undergo a rapid reduction due to phase separation, but then, as the phases settle, they slowly increase. For this solution, more than 1000 s was required for the phases to settle, with a practical settlement time of about 2500 s. Figure 8 shows the variation in the transmitted light intensities with pressure. The rapid reduction around 130 bar is associated with the liquid−liquid (LL) phase separation. With further decrease in pressure to 85 bar, the transmitted light intensity remains low, but over time at this final pressure, they increase back toward their initial values, as the phases settle. Figure 8 also includes a combined plot of the variation of the transmitted light intensity (from the middle probe) and of temperature with pressure. Temperature remains essentially unchanged, which further illustrates its stability. Figures 9a shows the change in the positions of the pistons during this experiment as pressure was lowered. Figure 9b shows the change in density with pressure for the solution evaluated from the initial total mass loading and the volume calculated from the piston position readings shown in Figure 9a as a function of time or pressure. Densities decrease with pressure as expected, but more distinctly, at a certain pressure, the decrease in density continues without further reduction in pressure, which is indicative of crosssing the liquid−liquid− vapor (LLV) phase boundary. Thus, the combination of the change in the transmitted light intensity and the change in density as a function of pressure provides a broad description of the phase behavior of the solution, by showing the P/T conditions where the LL and LLV boundaries are crossed, which are reflected by the sharp decrease in transmitted light intensity at LL, and continuing decrease in density without a change in pressure at LLV. This is illustrated in Figure 10. Figures 11−14 show the behavior for this solution when subjected first to a pressure reduction from about 210 to 95 bar,

Figure 10. Variation of the transmitted light intensity as recorded with the middle fiber optic probe and the variation of the solution density with pressure at 187 °C.

which was then followed by a pressure increase back to 210 bar at the nominal temperature of 187 °C. Figure 11 shows that the actual temperature recorded was around 186.7 °C. As was also shown in the figure, the transmitted light intensities that decrease during the time interval corresponding to phase separation upon decrease in pressure recover immediately within the next time interval during which the pressure is increased and the system returns to its homogeneous conditions. Figure 12 shows the change in the transmitted light intensity, and the change in temperature as a function of pressure during this cyclic operation where, as can be deduced from Figure 11, pressure was decreased and then brought back to its initial value at about 1 bar/s. Figure 12 shows the high degree of reproducibility in decreasing and increasing directions of pressure. Only a small hysteresis loop is diplayed in the transmitted light intesities, which is unavoidable given the dynamic nature of the experiments, and the nature of the 1811



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Figure 11. Change in pressure, temperature, and transmitted light intensities when solution is subjected to a pressure reduction, which is then followed by an increase in pressure at 186.7 °C.

Figure 12. Variation in transmitted light intensities with pressure (left) and variation in temperature with pressure (right) during the cyclic pressure reduction and increase at 186.7 °C.

Figure 13. Variation in the positions of the movable pistons and density with change in pressure during the cyclic pressure reduction and increase at 186.7 °C.

process being phase separation during the pressure reduction step and dissolution during the increasing pressure stage. Figure 13 shows the change in the piston positions, and the densities that are determined along the pressure decrease and increase directions. Clearly in this experiment, pistons moved in a more complex manner, but when processed to generate the densities, the reproducibility of the density in pressure decrease and increase directions is remarkable, and the directional difference, if there is, is essentially not visible. The density data show that, in this experiment, the final pressure reduction did not take the solution into its three-phase LLV state, as it does not display a portion with continuing decrease in density without change in pressure. Figure 14 shows the combined plot of transmitted light intensity from the middle fiber optic sensor and the variation of density as a function of pressure, which help identify the demixing and remixing pressures associated with the LL phase separation and miscibility, and the corresponding densities at 186.7 °C.

Figure 14. Combined plot showing the variation of transmitted light intensity as recorded by the middle fiber optic probe, and density as a function of pressure during the cyclic pressure reduction and increase at 186.7 °C.

Figure 15 illustrates another experiment with a cyclic change in pressure, this time from 100 to 22 bar and then back to 100 bar conducted at a lower temperature. The temperature−time and the pressure−time histories show that temperature was at 1812



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Figure 15. Pressure/temperature/transmitted light intensity−time history and variation of transmitted light, temperature, piston positions, and density with pressure during the cyclic pressure reduction and increase at 124.8 °C.

124.8 °C and the pressure change rate was about 2 bar/s. Similar to the cyclic experiment conducted at 186.7 °C discussed above, there is high degree of reproducibility in the transmitted light intensity and density values along the pressure reduction and increase directions. Here, however, variation of density with pressure as displayed in the last diagram in Figure 15 and also in Figure 16 shows that the system enters its LLV

region at 24 bar, and the presence of the vapor phase leads to the observation of a somewhat larger hysteresis loop at the low pressure range, which possibly arises from the differences in the dynamics of the bubble formation versus bubble collapse with the direction of the pressure change. Figure 16 is the combined summary plot showing the change in transmitted light intensity and density with pressure, displaying the pressures where the LL and LLV transitions take place at this temperature. Figures 17 shows the results for the experiment conducted at 150 °C in which the pressure was reduced from about 130 to about 50 bar at a rate of about 1.5 bar/s and held there for 1500 s. During this path, the solution crosses both the LL and the LLV boundaries as displayed by the variation of transmitted light intensities and the density as a function of pressure. As displayed by the transmitted light (Itr) versus time plot, phase settlement at this temperature is extremely slow, with essentially no significant recovery in the transmitted light intensities with time, which is in contrast to the phase settlement dynamics shown in Figure 7 after phase separation at 187 °C. b. Comparative Discussions. Phase Boundaries. Figure 18 is a comparative plot of the densities at different temperatures discussed in the foregoing sections. The figure includes additional data that were generated at 125.5 and 173.2

Figure 16. Combined plot showing the variation of transmitted light intensity as recorded by the middle fiber optic probe, and density as a function of pressure during the cyclic pressure reduction and increase at 124.8 °C. 1813



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Figure 17. Pressure/temperature/transmitted light−time history, and variation of transmitted light, temperature, piston positions, and density with pressure during pressure reduction from 130 to 50 bar at 150 °C.

°C. The density data at 125.5 and 124.8 °C are very similar, and likewise the data at 186.7 °C replicate the data at 187 °C well. The isotherms are separated in a regular manner in accordance with the temperature differences. Once again, it should be emphasized that these curves that appear as continuous curves represent thousands of data points that are generated in the present experimental system from real-time continuous recording of the piston positions. These plots help identify the LLV conditions as the pressure where density changes continue without a change in pressure. Figure 19 is a comparative plot of the transmitted light intensities as recorded by the middle fiber optic probe as a function of pressure at these temperatures. Here, to make the comparisons easier, transmitted light intensities have been normalized with respect to the initial homogeneous solution

value for each temperature, and the pressure range has been selected to take the system only below its LL transition. In this diagram, the pressure below which the transmitted light intensity starts to decrease from its initial base value in the homogeneous solution conditions represents the liquid−liquid phase separation boundary at each temperature. Table 1 is a summary of the incipient LL and LLV phase separation conditions that have been assigned on the basis of the density and transmittance data shown in Figures 18 and 19. (It should be pointed out that if one were to select the LL phase separation pressures as the pressure where the steep change in transmitted light intensity occurs, which is sometimes done in the literature, the LL demixing pressures would be lower and be 36, 74, 106, and 123 bar at 124.8, 150, 173.2, and 187 °C, respectively.) The values given in Table 1 are plotted in 1814



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Figure 18. Comparison of the variation of density with pressure in 11.7 wt % solution of EPDM 3745 in n-octane (61.9 wt %) + propane (26.4 wt %) at different temperatures [from left to right: 124.8, 125.5; 150, 173.2; 186.7, 187 °C].

Figure 20. LL and LLV phase boundaries in 11.7 wt % solution of EPDM 3745 in n-octane (61.9 wt %) + propane (26.4 wt %).

Figure 21. Comparison of phase settlement times in 11.7 wt % solution of EPDM 3745 in n-octane (61.9 wt %) + propane (26.4 wt %) after phase separation at different temperatures. Figure 19. Comparison of the variation of normalized transmitted light intensities with pressure in 11.7 wt % solution of EPDM 3745 in n-octane (61.9 wt %) + propane (26.4 wt %) at different temperatures [from left to right: 124.8, 125.5; 150, 173.2; 184.7, 186.7, 187 °C].

recorded by the middle fiber optic probe at several different temperatures after inducing LL (except LLV at 187 °C) phase separation. The plot shows that at temperatures below 186 °C, the phase settlement is extremely slow. At 187 °C, as was noted earlier also, phase settlement is still not very rapid, taking more than 2000 s. The long settlement times displayed with this solution at temperatures especially below about 186 °C point to the importance of conducting these types of measurements and highlight the challenges in developing effective separation strategies that may be practical. Visual observations during the various stages of settlement time indicated that at low temperatures once the polymer-rich phase starts to develop, as was depicted in Figure 1 earlier, any droplet of polymer-lean phase that is trapped in the more viscous polymer-rich phase takes a long time to coalesce and/or rise to the top phase. This is a transport limitation that arises from the high viscosities encountered. The entrapment of the lean-phase in the rich phase can also be appreciated in the photographs shown in Figure 4. There is extensive literature on the significance of coalescence and coalescence-induced coalescence in the late stages of phase separation.31,32 In general, this may involve hydrodynamic flows that induce other coalescence to occur, or it may occur via diffusion when droplets merge due to the

Table 1. Phase Separation Pressures and Temperatures in EPDM (11.7 wt %) + n-Octane (61.9 wt %) + Propane (26.4 wt %) temp (oC)

LL boundary pressure (bar)

LLV boundary pressure (bar)

124.8 125.5 150 173.2 184.7 186.7 187

40 45 78 120 130 135 135

24 25 53 72 nda nd 90

a

nd = not determined.

the P−T coordinates in Figure 20. The region to the left of the LL boundary is a homogeneous one-phase liquid domain; the region between the LL and LLV boundary is the liquid−liquid two-phase domain; and the region below the LLV boundary is the liquid−liquid−vapor three-phase domain for this solution. Phase Settlement Dynamics. Figure 21 is a comparative plot of the phase settlement dynamics in terms of the change in the normalized transmitted light intensities with time as 1815



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possible in the present system and the consequences of employing higher quench rates were not explored at this time and will be considered in future investigations. Isothermal Compressibility. Near continuous density data that are generated with the present system allow fitting the density versus pressure data to polynomial equations, which can then be differentiated to evaluate the isothermal compressibility of the solutions from

diffusion of molecules arising from concentration gradients, or may involve a process in which domains change their shape and touch each other leading to further coalescence. In very viscous systems such as the solutions in the present study, the long time behavior is most likely dominated by the diffusion process, which is slow, and also possibly influenced by the changing shape effects of the domains. The challenges in bringing about phase separation by gravity if the polymer-lean (dilute) phase nucleates in the polymer-rich (concentrated) phase has been discussed in an earlier publication on solutions of polybutadiene.23 As has been noted in that study, separation by gravity proceeds by the coalescence and/or rise of the dilute phase, which is an extremely slow process due to the very high viscosity of the polymer-rich phase. It has been further noted that process may even be completely halted if the polymer-rich phase becomes a swollen gel. To avoid these limitations, one needs to lower the viscosity or prevent the tendency to form a gel. One obvious approach to lower the viscosity is to increase the temperature. The penalty associated with increasing the temperature is the higher pressures that are initially required to form homogeneous conditions because of the LCST nature of the system. The alternative approach is to bring about the phase separation by spinodal decomposition, which would take the solution into its thermodynamically unstable domain. Under spinodal decomposition conditions, polymer-lean and polymer richphases should initially form cocontinuous domains. Literature suggests that this may allow a “drainage path” for the dilute, polymer-lean phase and accelerate the settlement time. As will be discussed later in this Article, the EPDM solution that we are reporting undergoes its liquid−liquid phase separation by spinodal decomposition. The long settlement times do emphasize that even though the phase separation may be initiated and proceeded by spinodal decomposition, the eventual coarsening of the phases and settlement under the pressure-quench rates that have been employed are slow and are greatly influenced by the viscosity of the medium. As shown in Figure 21, the settlement times at experiments conducted at higher temperatures were much shorter. It should be noted that in these experiments the rate of pressure reductions was not high, being in the range of 1−2 bar/s. If higher rates were to be employed, the settlement times would likely be shorter. A prior work conducted with 12.3 wt % solution of polybutadiene in n-hexane system reports23 that at 170 °C and about 90 bar, if pressure drop rates were less than about 10 bar/s, gravity separation was not significant. This is consistent with the observations in the present study. Gravity settlement was significant at pressure quench rates in the range of about 15−60 bar/s, and became very rapid for rates greater than 60 bar/s (achieved by venting to atmosphere) along with vapor nucleation. A pressure quench rate of about 280 bar/s led to settlement times of around 30 s showing that vapor nucleation enhances the settlement dynamics. In Figure 21, the settlement dynamics at 187 °C was generated by crossing the LLV boundary, which appeared to accelerate the settlement of the phases as suggested in the literature. In the present experimental system, pressure reduction rates depend on the rate at which pressure generators can be rotated and the rate at which pistons can move. The pressure reduction range also depends on the initial loading in the cell and the temperatures that fix the internal volume at a given pressure and the maximum volume expansion (thus pressure reduction) that can be imposed. However, the maximum quench rates that are

κT = (1/ρ)(∂ρ /∂P)T The density data shown in Figure 18 have been processed in the pressure range above the LLV boundary pressures. Table 2 Table 2. Density (ρ in g/cm3)−Pressure (P in bar) Correlations and the Correlation Coefficients T T T T

= = = =

124.8 °C 150 °C 173.2 °C 187 °C

ρ ρ ρ ρ

= = = =

−7 × 10−7P2 + 0.0004P + 0.5673 −10−6P2 + 0.0006P + 0.5300 −10−6P2 + 0.0007P + 0.4922 −10−6P2 + 0.0007P + 0.4765

R2 R2 R2 R2

= = = =

0.9974 0.9976 0.9989 0.9989

gives the density versus pressure correlation equations. The isothermal compressibility curves that have been generated from these correlations are shown in Figure 22. Compressibil-

Figure 22. Variation of isothermal compressibility with pressure of 11.7 wt % solution of EPDM 3745 in n-octane (61.9 wt %) + propane (26.4 wt %) at different temperatures [from bottom to top: 124.8, 150, 173.2, and 187 °C].

ities decrease with increasing pressure at each temperature, but increase with temperature at a given pressure. In going from 124.8 to 187 °C, compressibility is observed to essentially more than double in its value, going from about 0.00042 to 0.00092 bar−1 at around 100 bar. c. Mechanism of Phase Separation. Mechanistically, upon a pressure quench, the phase separation in a polymer solution will proceed by either the “nucleation and growth” or the “spinodal decomposition” mechanism. Nucleation and growth is the primary mechanism if the pressure quench takes the system from its homogeneous conditions into a metastable region of the phase envelope. Spinodal decomposition is the mode of phase separation if unstable domains are entered. Whether the phase separation proceeds by nucleation and growth or by spinodal decomposition is readily ascertained from the angular variation of the scattered light intensities and their time evolution.33−37 Spinodal decomposition is charac1816


Industrial & Engineering Chemistry Research terized by the formation of a spinodal ring, which leads to the observation of a maximum in the angular distribution of the scattered light intensities. In time, the spinodal ring becomes smaller but more intense. This is reflected by the scattered light intensities becoming higher, and the angular position of the maximum in the scattered light intensities moving to smaller angles. In systems undergoing phase separation by nucleation and growth, angular variation of scattered light intensities, while increasing with time, does not go through a maximum. The liquid−liquid phase separation in the 11.7 wt % EPDM 3745 solution that we are reporting was found to undergo phase separation by “spinodal decomposition”. Figure 23 shows

ACKNOWLEDGMENTS

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REFERENCES

We thank the Dow Chemical Co. for financial support. We also thank S. Takahashi for his help with fast scan camera and data processing, and Dr. J. M. Milanesio and H. Grandelli for their help in the early stages of instrumentation. R.S. would like to also thank Drs. P. Jain and J. D. Guzman for their discussions at Dow.

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Figure 23. Spinodal decomposition in 11.7 wt % solution of EPDM 3745 at 187 °C upon crossing the liquid−liquid phase boundary at 135 bar. Formation of spinodal ring and its time evolution and collapse.

the formation and evolution of the spinodal ring after crossing the LL phase boundary at 187 °C. The distinct scattering ring that forms becomes more intense with time but reduced in diameter. The ring collapses within about 10 s time interval. The figure shows the snapshots taken with a fast scan camera.

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CONCLUSIONS Solutions of EPDM elastomer in n-octane + propane fluid mixture display LCST-type behavior. The liquid−liquid phase separation proceeds by spinodal decomposition. Phase settlement dynamics depend on temperature, and settlement times become greatly reduced if phase separation is carried out at high temperatures. Continual recording of the piston positions with dedicated LVDTs allows the continuous determination of the solution densities from which the pressure/temperature conditions when LLV phase boundary is crossed are readily determined. Continuous assessment of density also permits the assessment of compressibilities with facility. Use of dual sapphire window sets with different distances of separation permits the optical assessment of the phase conditions. It also permits the determination of the mechanism of phase separation from time evolution of the scattered light intensity patterns.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (540) 231-1375. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 1817



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Miscibility, Phase Separation, and Phase Settlement Dynamics in

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