Phase Behavior of the Poly [hexyl (meth) acrylate]− Supercritical

Experimental cloud-point data to 200 °C and 2200 bar are measured for binary and ternary mixtures of poly(hexyl acrylate)−CO2−hexyl acrylate and ...
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Ind. Eng. Chem. Res. 2004, 43, 1543-1552

1543

Phase Behavior of the Poly[hexyl (meth)acrylate]-Supercritical Solvents-Monomer Mixtures at High Pressures Hun-Soo Byun,* Jeong-Gon Kim, and Jun-Seok Yang Department of Chemical Engineering, Yosu National University, Yosu, Chonnam 550-749, South Korea

Pressure-composition isotherms are obtained for carbon dioxide (CO2)-hexyl acrylate and CO2hexyl methacrylate systems at 40, 60, 80, 100, and 120 °C and pressure up to 189 bar. The CO2-hexyl acrylate and CO2-hexyl methacrylate systems exhibit type I phase behavior with a continuous mixture critical curve. Three-phase, liquid-liquid-vapor equilibrium was not observed at these conditions. The experimental results for CO2-hexyl acrylate and CO2-hexyl methacrylate mixtures are modeled using the Peng-Robinson equation of state. A good fit of the data is obtained with the Peng-Robinson equation of state using two adjustable parameters for CO2-hexyl acrylate and CO2-hexyl methacrylate systems. Experimental cloud-point data to 200 °C and 2200 bar are measured for binary and ternary mixtures of poly(hexyl acrylate)CO2-hexyl acrylate and poly(hexyl methacrylate)-CO2-hexyl methacrylate systems. Also, the cloud-point curves show the binary mixtures for poly(hexyl acrylate) in supercritical ethylene, propane, propylene, butane, 1-butene, chlorodifluoromethane, and dimethyl ether. The phase behavior for the system poly(hexyl methacrylate)-CO2-hexyl methacrylate is measured in changes of the pressure-temperature slope and with cosolvent concentrations of 0, 9.2, 14.4, and 32.4 wt %. With 43.8 wt % hexyl methacrylate to the poly(hexyl methacrylate)-CO2 solution significantly changed, the phase behavior curve takes on the appearance of a typical lower critical solution temperature (LCST) boundary. The cloud-point curves for the poly(hexyl acrylate)CO2-0, 5.0, 12.8, and 20.7 wt % hexyl acrylate system change the P-T curve from the upper critical solution temperature region to the LCST region as the hexyl acrylate concentration increases. Adding 33.9 wt % hexyl acrylate to the poly(hexyl acrylate)-CO2 solution significantly changes the phase behavior. The cloud-point curve takes on the appearance of a typical LCST region. Introduction The phase behavior of polymers in the supercritical fluids is an important role in most polymerization processes, polymer production, processing technologies, material development, and industrial application.1-5 Also, the design and operation of a separation process requires knowledge of phase behavior experimental data. In particular, high-pressure phase equilibrium data for binary mixtures containing supercritical carbon dioxide (CO2) will be needed to design and operate processing plants, industrial application, and supercritical fluid extraction.6,7 Recently, we have demonstrated that it is possible to dissolve a polar (meth)acrylate polymer in supercritical CO2 over a large temperature range at modest pressure if a (meth)acrylate monomer is used.8,9 A liquid monomer can greatly enhance polymer solubility in a given solvent for several different reasons. If the solvent is highly expanded, the addition of a dense liquid monomer reduces the free-volume difference between the polymer and the solvent.10 Also, because the cosolvent has most of the same physicochemical properties of a repeat unit of the polymer, energetically favorable solvent-polymer interactions are expected to expand the single-phase region.11 Interpreting the effect of a cosolvent added to a supercritical fluid solvent is slightly more complicated because increasing the system pressure reduces the freevolume difference between the solvent and the polymer * To whom correspondence should be addressed. Tel.: +8261-659-3296. Fax: +82-61-653-3659. E-mail: [email protected].

and increases the probability of interaction between polymer, solvent, and cosolvent segments.12 The highpressure, polymer-supercritical fluid solvent-cosolvent studies reported in the literature show that cloud points monotonically decrease in pressure and temperature with the addition of a polar cosolvent as long as the cosolvent does not form a complex with the polar repeat units in the polymer.12-14 In these cases, the cosolvent effect is directly related to the polar forces of attraction contributed by the cosolvent and to the increase in solvent density resulting from the addition of a liquid cosolvent to a supercritical fluid solvent. As a general rule, the cloud-point curve of a mixture consisting of a polar component, in this case the poly(acrylate), and a much less polar component, here CO2, exhibits a negative slope in pressure-temperature space. The interchange, which characterizes the balance of polymer segment-CO2 cross-interactions relative to polymer segment-segment and CO2-CO2 self-interactions, is very temperature-sensitive because of the strong polar interactions experienced between polymer segments. At the temperatures where entropic effects are expected to dominate, the location of the cloud-point curve is a reflection of the free-volume difference between the dense polymer and the expanded CO2 rather than the balance of intermolecular interactions.8,15 Experimental phase behavior data of small amount for binary CO2-hexyl acrylate and CO2-hexyl methacrylate systems are obtained to complement the poly(hexyl acrylate)-CO2-hexyl acrylate and poly(hexyl methacrylate)-CO2-hexyl methacrylate studies pre-

10.1021/ie030724u CCC: $27.50 © 2004 American Chemical Society Published on Web 02/12/2004

1544 Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004

Figure 1. Schematic diagram of the high-pressure experimental apparatus used in this study.

sented here because there are no literature phase behavior data available on this mixture. The primary purpose for obtaining CO2-hexyl acrylate and CO2-hexyl methacrylate systems is to determine whether CO2 and hexyl acrylate or hexyl methacrylate form multiple phases in the pressure-temperature-composition regions explored in the poly(hexyl acrylate)-CO2-hexyl acrylate and poly(hexyl methacrylate)-CO2-hexyl acrylate studies. The experimental data of CO2-hexyl acrylate and CO2-hexyl methacrylate systems are fitted to the Peng-Robinson equation of state,16 and the phase behavior for this binary solvent mixture is calculated at elevated operating temperatures and pressure. The second focus of this work is to present the determination of the impact of hexyl acrylate and hexyl methacrylate cosolvents on the phase behavior of the poly(hexyl acrylate)-CO2 and poly(hexyl methacrylate)-CO2 systems. Given that CO2 has been considered a desirable reaction medium for free-radical polymerizations,2 the phase behavior for these ternary poly(hexyl acrylate)-supercritical CO2-hexyl acrylate mixtures provides the information needed on the regions where homogeneous polymerization can occur in the presence of excess monomer. Rindfleisch et al.17 have demonstrated that the poly(butyl acrylate)-CO2 cloud-point curves are almost vertical at ∼1100-2700 bar at high temperatures. Byun and McHugh8 have demonstrated that poly(butyl methacrylate)-CO2 phase behavior curves present the upper critical solution temperature (UCST) curve at ∼1400-2000 bar at below 240 °C. The intermolecular interactions between CO2 and a poly(hexyl acrylate) or poly(hexyl methacrylate) repeat unit are expected to be similar in strength to those between CO2 and a poly(hexyl acrylate) or poly(hexyl methacrylate) repeat unit. Hence, the difference in phase behavior is attributed to the different degrees of chain flexibility for these two polymers, which implies a more unfavorable conformational entropy of mixing for poly(octadecyl acrylate).9 The key issue is how to account for the intra- and intersegmental interactions of many segments of the polymer relative to the small number of segments in a solvent molecule.

Experimental Section Figure 1 shows a schematic diagram of the experimental apparatus used for pressure-composition isotherms for the CO2-hexyl acrylate and CO2-hexyl methacrylate mixtures18,19 and obtained cloud-point curves for poly(hexyl acrylate)-CO2-hexyl acrylate and poly(hexyl methacrylate)-CO2-hexyl methacrylate ternary mixtures.20,21 The bubble-point, dew-point, and cloud-point curves are obtained with a high-pressure, variable-volume cell described in detail elsewhere.18,19 Cloud points are measured for the polymer solutions at a fixed poly(hexyl acrylate) and poly(hexyl methacrylate) concentration of 5.0 ( 0.5 wt %, which is typical of the concentrations used for polymer-supercritical fluid solvent studies.21 Polymer is loaded into the cell to within (0.002 g, and then the cell is purged with nitrogen, followed by CO2, to ensure that all of the air is removed. Liquid hexyl acrylate and hexyl methacrylate are injected into the cell to within (0.002 g using a syringe, and CO2 is transferred into the cell gravimetrically to within (0.004 g using a high-pressure bomb. The polymer-solvent-monomer mixture in the cell is heated to the desired temperature and pressurized until a single phase is achieved. The mixture (polymersolvent-monomer system) is maintained in the onephase region at the designed temperature for at least 30 min so that the cell can reach thermal equilibrium. The pressure is then slowly decreased until the solution becomes cloudy. The cloud-point pressure is defined as the point at which the solution becomes so opaque that it is no longer possible to see the stir bar in the solution. After a cloud point is obtained, the solution is recompressed into a single phase and the process repeated. The mixture is compressed to the desired pressure with an internal piston displaced with water in a highpressure generator (HIP Inc., model 37-5.75-60). The pressure of the mixture is measured with a Heise gauge (Dresser Ind., model CM-108952; 0-3450 bar, accurate to within (3.5 bar). Because the measurement is made on the water side of the piston, a small correction (∼1

Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 1545 Table 1. Critical Temperatures, Critical Pressures, Critical Densities, Polarizabilities, Dipole Moments, and Quadrupole Moments of the Solvents Used in This Study22-26 solvent

Tc (°C)

Pc (bar)

Fc (g/cm3)

R × 1025 (cm3)

µ (D)

Q (esu‚cm2)

CO2 ethylene propane propylene n-butane 1-butene dimethyl ether CHClF2

31.0 9.2 96.7 91.9 152.1 146.4 126.8 96.2

73.8 50.4 42.5 46.2 38.0 39.7 53.0 49.7

0.469 0.217 0.217 0.236 0.228 0.234 0.258 0.522

26.5 42.3 62.9 62.6 81.4 82.4 52.2 44.4

0.0 0.0 ∼0.08 0.37 -0.0 0.34 1.3 1.4

-4.3 1.5 1.2 2.5

bar) is added to account for the pressure required to move the piston. The temperature in the cell is measured using a platinum-resistance thermometer (Thermometrics Corp., class A) connected to a digital multimeter (Yokogawa, model 7563; accurate to within (0.005%). The system temperature is typically maintained to within (0.2 °C below 200 °C. The mixture inside the cell is viewed on a video monitor using a camera coupled to a borescope (Olympus Corp., model F100-038000-50) placed against the outside of the sapphire window. Light is transmitted into the cell with a fiberoptic cable connected at one end to a high-density illuminator (Olympus Optical Co., model ILK-5) and at the other end to a borescope. Mole fractions are accurate to within (0.0025. Cloud points are measured and reproduced at least twice to within (2.8 bar and (0.3 °C. Bubble-, dew-, and criticalpoint transitions for the CO2-hexyl acrylate and CO2hexyl methacrylate mixtures are measured and reproduced at least twice to within (0.5 bar and (0.2 °C. CO2-hexyl acrylate and CO2-hexyl methacrylate mole fractions have an estimated accumulation error of less than (0.8% except for the data point at 1.0 mol % hexyl acrylate and hexyl methacrylate, which has a slightly higher estimated error of (1.5%. Materials CO2 (99.8% minimum purity) was obtained from Daesung Oxygen Co. and used as received. Poly(hexyl acrylate) (Mw ) 90 000) and hexyl methacrylate (99.9% purity) were obtained from Scientific Polymer Products, Inc., and used as received. Poly(hexyl methacrylate) (GPC: Mw ) ∼400 000) and hexyl acrylate (99.9% purity) used in this work were obtained from Aldrich Co. and Polysciences Inc., respectively. To prevent hexyl acrylate and hexyl methacrylate polymerization, 2,6di-tert-butyl-4-methylphenol (Aldrich; 99% purity) was used as an inhibitor at a concentration of 0.005 times the amounts of hexyl acrylate and hexyl methacrylate. Because poly(hexyl acrylate) was supplied in a toluene solution, the polymer solution was placed under vacuum for at least 10 h by a rotary evaporator (Tamato Scientific Co., model RE-47) for toluene removal. Results and Discussion The properties of the solvents used in this study, shown in Table 1,22-26 cover a broad spectrum of potential intermolecular interactions. For the poly(hexyl acrylate)-solvent systems, CO2, ethylene, propylene, propane, butane, 1-butene, and dimethyl ether were used as solvents. For the poly(hexyl methacrylate)-solvent systems, propylene, propane, butane, chlorofluoromethane (CHClF2), and dimethyl ether were used as solvents. Ethylene has the critical temperature, critical pressure, and polarizability. However, ethylene has a

2.5

other interactions quadrupole moment polar moments of complexes with HA polar moments of complexes with HA hydrogen bonds with HA hydrogen bonds with HA

quadrupole moment, which enables ethylene to interact favorably with the polar hexyl acrylate groups. Propane and propylene also have similar critical properties and polarizabilities, but propylene possesses both dipole and quadrupole moments, which enable it to favorably interact with the hexyl acrylate groups in the polymer. Hence, with propane and propylene, the effect of solvent polarity can be studied independently of polarizability effects. On the other hand, a comparison of propane and butane, or propylene and 1-butene, yields information of the effect of the solvent size and, hence, polarizability on the phase behavior independent of polarity effects. Like propane, n-butane does not possess an appreciable dipole moment and, therefore, it interacts with the polymer only through dispersion and induction forces. 1-Butene, like propylene, interacts with the hexyl acrylate segments through quadrupolar and dipolar forces. Dimethyl ether is a Brønsted base (proton acceptor), which can form strong hydrogen bonds with the hexyl acrylate repeat units in the poly(hexyl acrylate) polymers. Phase Behavior of CO2-Hexyl Acrylate and CO2-Hexyl Methacrylate Mixtures. Bubble-, dew-, and critical-point data for the CO2-hexyl acrylate and CO2-hexyl methacrylate systems are measured and reproduced at least twice to within (0.3 bar and (0.2 °C for a loading of the cell. The mole fractions are accurate to (0.002. CO2-hexyl acrylate and CO2-hexyl methacrylate mole fractions for the solubility isotherms at 40-120 °C have an estimated accumulated error of less than (1.0%. Table 2 and Figure 2 present the CO2-hexyl acrylate system experimental data obtained in this study. Table 3 and Figure 3 show the phase behavior data and pressure-composition (P-x) curve for the CO2-hexyl methacrylate mixture. Figure 2 shows the experimental pressure-composition (P-x) isotherms at 40, 60, 80, 100, and 120 °C and pressure up to 181 bar. Figure 3 presents the experimental phase behavior at 40-120 °C and pressure up to 189 bar. The P-x isotherms shown in Figures 2 and 3 are consistent with those expected for a type I system,6,27 where a maximum occurs in the continuous mixture critical curve. Three-phase liquid-liquid-vapor (LLV) equilibrium was not observed at these conditions. The behavior exhibited by the CO2-hexyl acrylate and CO2-hexyl methacrylate systems is consistent with that of the CO2-butyl acrylate,15 CO2-butyl methacrylate,8 and CO2-octadecyl acrylate9 systems. Note that the bubble-point portions of the P-x isotherms for the CO2hexyl acrylate and CO2-hexyl methacrylate systems are convex toward lower pressures, which means that CO2 is very miscible in the hexyl acrylate and hexyl methacrylate rich liquid phase. The lower solubility of CO2 in hexyl acrylate is likely due to the steric hindrance of the hexyl chain that prevents facile complex formation

1546 Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 Table 2. Experimental Isotherms Data for the Hexyl Acrylate-CO2 System Obtained in This Study hexyl acrylate mole fraction

hexyl acrylate mole fraction

P (bar)

transitiona

0.663 0.569 0.556 0.463 0.415 0.413 0.350 0.333 0.287 0.269 0.247

25.6 31.5 32.2 38.5 42.5 42.4 49.7 51.8 57.0 56.9 61.9

BP BP BP BP BP BP BP BP BP BP BP

0.663 0.569 0.556 0.463 0.415 0.350 0.333 0.287 0.269 0.247

31.8 40.4 42.1 50.5 57.2 65.5 67.3 71.6 75.1 80.9

BP BP BP BP BP BP BP BP BP BP

0.663 0.569 0.556 0.463 0.415 0.333 0.287 0.269 0.247 0.190

38.6 49.0 50.7 63.6 73.0 84.9 95.9 97.6 104.3 117.0

BP BP BP BP BP BP BP BP BP BP

0.663 0.569 0.556 0.463 0.415 0.333 0.287 0.269 0.247

42.1 55.8 58.0 76.1 84.8 101.8 112.8 118.1 129.8

T ) 100 °C BP 0.190 BP 0.161 BP 0.135 BP 0.105 BP 0.089 BP 0.078 BP 0.056 BP 0.036 BP 0.014

143.1 147.6 155.9 158.4 159.9 161.0 158.8 152.1 129.0

BP BP BP BP BP BP CP DP DP

0.663 0.569 0.556 0.463 0.415 0.333 0.287 0.269 0.247

45.2 63.4 66.9 90.2 98.8 117.3 130.4 135.7 146.1

T ) 120 °C BP 0.190 BP 0.161 BP 0.135 BP 0.105 BP 0.089 BP 0.078 BP 0.056 BP 0.036 BP 0.014

158.1 169.3 174.1 181.0 179.5 178.6 174.8 166.1 111.4

BP BP BP BP BP BP CP DP DP

P (bar)

transitiona

T ) 40 °C 0.190 0.161 0.135 0.105 0.089 0.078 0.056 0.036 0.014 0.007

67.0 70.0 72.8 76.0 76.6 78.3 78.8 79.2 80.3 82.4

BP BP BP BP BP BP BP BP BP BP

T ) 60 °C 0.190 0.161 0.135 0.105 0.089 0.078 0.056 0.036 0.014 0.007

88.7 94.8 98.6 103.0 105.5 105.9 105.9 108.3 105.2 100.7

BP BP BP BP BP BP CP DP DP DP

T ) 80 °C 0.161 0.135 0.105 0.089 0.078 0.056 0.036 0.014 0.007

122.1 127.2 131.2 133.4 133.9 133.9 130.3 123.8 107.2

BP BP BP BP BP CP DP DP DP

a BP is a bubble point, CP is a critical point, and DP is a dew point.

between CO2 and the acrylate carbonyl oxygen. Moreover, the strength of a CO2-hexyl acrylate complex is expected to be lower than that of a CO2-hexyl methacrylate complex because the electron-donating character of hexyl acrylate is less than that of hexyl methacrylate because of its larger molar volume. Kazarian et al.28 also show CO2 complexes with the carbonyl oxygen in poly(vinyl acetate); this complex is slightly stronger than the one with poly(hexyl methacrylate) and leads to a higher solubility. The high solubility of CO2 in the hexyl methacrylate rich liquid is probably

Figure 2. Comparison of experimental data (symbol) for the CO2hexyl acrylate system with calculations (solid line) obtained with the Peng-Robinson equation of state with kij equal to -0.045 and ηij equal to -0.036.

due to the formation of a weak complex between hexyl methacrylate and CO2. Kazarian et al.28 show that the carbon atom of CO2 acts as an electron acceptor that complexes with the carbonyl oxygen of poly(hexyl methacrylate); hence, the same behavior can be reasonably expected for CO2 with a hexyl methacrylate monomer. The experimental data obtained in this work are modeled using the Peng-Robinson equation of state. The equation of state is briefly described here. The Peng-Robinson equation of state16 is used with the following mixing rules:

P)

a(T) RT V - b V(V + b) + b(V - b)

(1)

R2Tc2 a(T) ) 0.45724 Pc

(2)

RTc b ) 0.07780 Pc

(3)

∑i ∑j xixjaij

(4)

amix )

aij ) (aiiajj)1/2(1 - kij) bmix )

(5)

∑i ∑j xixjbij

(6)

bij ) 0.5(bii + bjj)(1 - ηij)

(7)

These two binary interaction parameters were determined by regression experimental data with the PengRobinson equation of state. The objection function (OBF) and root-mean-square relative deviation (RMSD) percent of this calculation were defined as follows: N

OBF )

∑i

(

RMSD (%) )

)

Pexp - Pcal Pexp

2

× 100 xOBF ND

(8)

(9)

ND in eq 9 means the number of data points. We used

Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 1547 Table 3. Phase Behavior of the Hexyl Methacrylate-CO2 System Obtained in This Study hexyl hexyl methacrylate methacrylate mole P mole P fraction (bar) transitiona fraction (bar) transitiona 0.665 0.431 0.420 0.359 0.285 0.213 0.208 0.195

35.0 56.6 57.4 62.9 66.5 75.4 75.0 76.9

T ) 40 °C BP 0.167 BP 0.139 BP 0.098 BP 0.079 BP 0.053 BP 0.042 BP 0.031 BP 0.016

78.8 80.1 82.7 83.8 85.5 85.5 85.1 85.6

BP BP BP BP BP BP BP BP

0.665 0.431 0.420 0.359 0.285 0.213 0.208 0.195

43.5 73.4 74.8 82.1 90.7 101.9 102.8 105.6

T ) 60 °C BP 0.167 BP 0.139 BP 0.098 BP 0.079 BP 0.053 BP 0.042 BP 0.031 BP 0.016

107.7 110.0 114.0 115.5 116.2 116.4 116.1 113.3

BP BP BP BP BP BP DP DP

0.665 0.431 0.420 0.359 0.285 0.213 0.208 0.195

51.4 86.7 88.6 97.2 113.1 125.1 126.9 130.8

T ) 80 °C BP 0.167 BP 0.139 BP 0.098 BP 0.079 BP 0.053 BP 0.042 BP 0.031 BP 0.016

134.8 139.7 144.1 145.5 145.8 144.9 140.2 130.4

BP BP BP BP BP DP DP DP

0.665 0.431 0.420 0.359 0.285 0.213 0.208 0.195

59.0 99.8 101.3 113.4 128.4 146.1 148.5 151.8

T ) 100 °C BP 0.167 BP 0.139 BP 0.098 BP 0.079 BP 0.053 BP 0.042 BP 0.031 BP 0.016

160.0 163.2 166.7 166.9 165.5 160.7 158.3 142.4

BP BP BP BP CP DP DP DP

0.665 0.431 0.420 0.359 0.285 0.213 0.208 0.195

66.5 115.9 117.7 129.8 149.0 165.8 167.8 172.1

T ) 120 °C BP 0.167 BP 0.139 BP 0.098 BP 0.079 BP 0.042 BP 0.031 BP 0.016 BP

181.0 186.6 189.3 186.6 177.9 170.0 133.5

BP BP BP DP DP DP DP

a BP is a bubble point, CP is a critical point, and DP is a dew point.

Marquardt29 to optimize OBF. All isotherms were included for calculation. The expression for the fugacity coefficient using these mixing rules is given by Peng and Robinson16 and is not reproduced here. Table 4 lists the pure-component critical temperatures, critical pressures, and acentric factors for CO2, hexyl acrylate, and hexyl methacrylate used with the Peng-Robinson equation of state. The critical properties of hexyl acrylate and hexyl methacrylate are obtained by Lydersen’s method in the group contribution.22 The vapor pressures were calculated by the Lee-Kesler method.22 Figure 4 presents a comparison of the CO2-hexyl acrylate experimental results with the calculated data obtained using the Peng-Robinson equation of state at a temperature of 80 °C. The values of the optimized parameters (bubble-point data ) 15, RMSD ) 4.24%) of the Peng-Robinson equation of state for the CO2-

Figure 3. Comparison of experimental data (symbol) for the CO2hexyl methacrylate system with calculations (solid line) obtained with the Peng-Robinson equation of state with kij equal to 0.030 and ηij equal to -0.040.

Figure 4. Comparison of the best fit of the Peng-Robinson equation of state to the CO2-hexyl acrylate system at 80 °C. Table 4. Pure-Component Critical Properties with the Peng-Robinson Equation of State22,23 component

Mw

Tc (°C)

Pc (bar)

acentric factor

CO2 hexyl acrylate hexyl methacrylate

44.01 156.2 170.3

31.1 371.3 409.0

73.9 27.4 25.5

0.2250 0.6043 0.4536

hexyl acrylate system are kij ) -0.045 and ηij ) -0.036. A reasonable fit of the data is obtained over most of the composition range even if no binary interaction parameters are used. However, if two mixture parameters (kij ) -0.045 and ηij ) -0.036) are used, the fit of the experimental results is significantly better. We compared the experimental results with the calculated P-x isotherms at temperatures of 40, 60, 100, and 120 °C for the CO2-hexyl acrylate system using the optimized values of kij and ηij determined at 80 °C. Therefore, Figure 2 shows a comparison of the experimental results with the calculated P-x isotherms at temperatures of 40, 60, 100, and 120 °C for the CO2-hexyl acrylate system using the adjusted values of kij and ηij determined at 80 °C. A good fit of the data is obtained with the Peng-Robinson equation of state using two adjustable mixture parameters for the CO2-hexyl acrylate system. However, if two mixture parameters, kij ) -0.045 and ηij ) -0.036, are used, the fit of the experimental results is significantly better. RMSD at five temperatures for the CO2-hexyl acrylate system was 5.07% of the bubblepoint number 82. Figure 3 shows a comparison of the experimental data with the calculated results at 40, 60, 80, 100, and 120

1548 Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004

Figure 5. Pressure-temperature diagram for the CO2-hexyl acrylate system. The solid lines and solid circles represent the vapor-liquid lines and the critical points for pure CO2 and hexyl acrylate. The open squares are critical points determined from isotherms measured in this work. The dashed lines represent calculations obtained using the Peng-Robinson equation of state with kij equal to -0.045 and ηij equal to -0.036.

Figure 7. Effect of hexyl acrylate on the phase behavior of the poly(hexyl acrylate)-CO2-x wt % hexyl acrylate system, where x equals 0 (open triangles), 5.0 (open squares), 12.8 (closed circles), and 20.7 (closed squares) and the lines mean only a bridge between symbols. Table 5. Experimental Cloud-Point Data for the Poly(hexyl acrylate) (PHA)-CO2-Hexyl Acrylate (HA) System Measured in This Study T (°C)

Figure 6. Pressure-temperature diagram for the CO2-hexyl methacrylate system. The solid lines and solid circles represent the vapor-liquid lines and the critical points for pure CO2 and hexyl methacrylate. The open squares are critical points determined from isotherms measured in this work. The dashed lines represent calculations obtained using the Peng-Robinson equation of state with kij equal to 0.040 and ηij equal to -0.030.

°C for the CO2-hexyl methacrylate system. These isotherms are calculated using the optimized values (bubblepoint data ) 13, RMSD ) 1.73%) of kij equal to 0.030 and ηij equal to -0.040 determined at 80 °C in the same way as above (Figure 4). RMSD at five temperatures for the CO2-hexyl methacrylate system was 3.50% of the bubble-point number 66. The calculated mixture critical curve is type I, in agreement with the experimental results. Figures 5 and 6 show the mixture critical curve for the CO2-hexyl acrylate and CO2-hexyl methacrylate systems predicted by the Peng-Robinson equation of state. The calculated mixture critical curve is type I, in agreement with the experimental observations. As shown in Figures 5 and 6, the solid lines represent the vapor pressure for pure CO2,22,23 hexyl acrylate,22 and hexyl methacrylate.22 The solid circles represent the critical point for pure CO2, hexyl acrylate, and hexyl methacrylate. The upper part of the dashed line is single phase (fluid), and the lower part is two phase (vaporliquid). The solid squares are the mixture critical points determined from isotherms measured in this experiment. The dashed lines represent the calculated value obtained using the Peng-Robinson equation of state.

P (bar)

T (°C)

P (bar)

125.3 126.2 130.3

4.9 wt % PHA + 0.0 wt % HA 2553.4 132.5 1932.8 141.7 1743.1 161.9

85.1 89.3 110.7

5.1 wt % PHA + 5.0 wt % HA 2246.6 129.7 1343.1 155.5 1077.6

983.5 902.4

44.3 45.2 45.9 55.8 71.9

5.0 wt % PHA + 12.8 wt % HA 1408.6 91.9 1115.5 112.2 1022.4 131.6 805.5 151.4 705.9

679.3 681.0 691.4 702.1

39.1 54.7 71.9 91.9

5.4 wt % PHA + 20.7 wt % HA 316.6 112.6 338.6 136.1 375.5 155.6 417.2

460.4 496.6 516.6

1598.3 1450.0 1186.2

Phase Behavior of the Poly(hexyl acrylate)CO2-Hexyl Acrylate System. Table 5 and Figure 7 show the cloud-point behavior of the poly(hexyl acrylate)-CO2-hexyl acrylate mixture obtained in this study. The phase behavior of the poly(hexyl acrylate)CO2-0.0 wt % hexyl acrylate mixture presented a negative slope with dissolution at a temperature of 160 °C and a pressure of 2500 bar. With 5.0 wt % hexyl acrylate added to the solution, the cloud-point curve exhibits a UCST region type phase behavior with a negative slope. With 12.8 wt % hexyl acrylate in solution, the cloud-point pressure remains virtually constant at 700 bar over a temperature range from 70 to 150 °C. The phase behavior of the poly(hexyl acrylate)-CO2-12.8 wt % hexyl acrylate mixture exhibits the U-LCST type phase behavior from positive slope at low pressures to negative slope, which rapidly increase at 70 °C. If 20.7 wt % hexyl acrylate is added to the solution, the cloud-point curve exhibits lower critical solution temperature (LCST) type phase behavior of a positive slope at 40-160 °C. The phase behavior of the poly(hexyl acrylate)-CO2-20.7 wt % hexyl acrylate system shows 1.8 bar/°C in a positive slope. The effect of the hexyl acrylate cosolvent on the phase behavior is similar to that observed for the poly(butyl acrylate)CO2-butyl acrylate.1

Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 1549

Figure 8. Phase behavior of the poly(hexyl acrylate)-CO2-33.9 wt % hexyl acrylate system obtained in this study. Open squares represent fluid f liquid + liquid transitions, closed circles represent fluid f liquid + vapor transitions, and closed squares represent LLV data.

Figure 9. Effect of hexyl methacrylate on the phase behavior of the poly(hexyl methacrylate)-CO2-x wt % hexyl methacrylate system, where x equals 0 (open squares), 9.2 (open triangles), 14.4 (open inverse triangles), and 32.4 (closed circles) and the lines mean only a bridge between symbols.

Table 6. Experimental Cloud-Point, Bubble-Point, and LLV Data for the Poly(hexyl acrylate) (PHA)-CO2-Hexyl Acrylate (HA) System Measured in This Study

Table 7. Experimental Cloud-Point Data for the Poly(hexyl methacrylate) (PHMA)-CO2-Hexyl Methacrylate (HMA) System Measured in This Study

T (°C)

P (bar)

transition

T (°C)

P (bar)

transition

T (°C)

CP CP

156.7 158.3 160.2 164.6

5.1 wt % PHMA + 0.0 wt % HMA 2208.6 172.9 2070.7 194.8 1953.4 198.9 1805.2

BP

111.6 114.7 117.5

5.6 wt % PHMA + 9.2 wt % HMA 1794.8 131.1 1612.1 151.4 1508.6 171.5

1227.6 1058.6 1022.4

82.3 84.1 88.1 92.9

4.5 wt % PHMA + 14.4 wt % HMA 1825.9 113.3 1541.7 132.2 1336.2 152.8 1205.2 176.2

976.6 901.0 871.4 859.3

61.0 81.6 100.0

5.0 wt % PHMA + 32.4 wt % HMA 501.7 120.1 507.6 142.0 519.3 161.4

524.5 528.3 493.8

5.0 wt % PHA + 33.9 wt % HA 50.6 62.3 82.9

110.4 149.7 213.5

Cloud-Point Transition CP 104.2 268.3 CP 123.6 306.6 CP

30.1 34.3

67.2 71.0

Bubble-Point Transition BP 40.7 80.4 BP

50.7

90.0

LLV Transition LLV 59.4

99.8

LLV

The slope of the 33.9 wt % hexyl acrylate curve is 2.7 bar/°C, which is very close to the slope found for the poly(butyl acrylate)-CO2-32.0 wt % butyl acrylate curve.15 These slopes are ∼40% greater than those observed for binary poly(isobutylene)-alkane mixtures reported by Zeman and Patterson,30 which is the result of the enhanced influence of hydrostatic pressure on the free-volume difference between poly(hexyl acrylate) and the CO2-hexyl acrylate mixture. The alkanes used by Zeman and Patterson30 are less compressible than the CO2-hexyl acrylate solutions used here. Even though the poly(hexyl acrylate)-CO2-hexyl acrylate curve extends to ∼40 °C, bubble points are observed, which implies that the cloud-point curve will intersect the LLV curve at a much lower temperature.6 Finally, it is noted that the effect of the hexyl acrylate cosolvent on the location of the cloud-point curve diminishes in a nonlinear fashion as increased amounts of hexyl acrylate are added to the solution. This diminishing returns effect is also seen in the poly(butyl acrylate)-CO2-butyl acrylate system. When 33.9 wt % hexyl acrylate is added to the poly(hexyl acrylate)-CO2 solution, the cloud-point curve shown in Figure 8 and Table 6 takes on the typical appearance of a LCST boundary. At 120 °C, the phase boundary shifted from 477 to 300 bar as the concentration of hexyl acrylate is increased from 20.7 to 33.9 wt %. The poly(hexyl acrylate)-CO2-33.9 wt % hexyl acrylate phase behavior curve intersects a liquid f liquid + vapor (LV) curve at ∼43 °C and ∼84 bar. Liquid and vapor phases coexist at pressures below this curve, and the LV curve switches to a liquid1 + liquid2 + vapor (LLV) curve at temperatures greater than about 43 °C. The initial slope of the poly(hexyl acrylate)-CO2-hexyl

P (bar)

T (°C)

P (bar) 1683.1 1462.1 1424.1

acrylate LCST curve at the lowest pressures is ∼3.3 bar/°C. The results obtained in this study demonstrate clearly that it is possible to obtain a single phase that extends over a modest pressure when operating with supercritical CO2 as long as sufficient amounts of free hexyl acrylate monomer are presented in the solution. Phase Behavior of the Poly(hexyl methacrylate)-CO2-Hexyl Methacrylate System. Table 7 and Figure 9 present the cloud-point behavior of the poly(hexyl methacrylate)-CO2-x wt % hexyl methacrylate system data obtained in this study. Poly(hexyl methacrylate) does dissolve in pure CO2 to a temperature of 199 °C and a pressure of 2208 bar. When 9.2 wt % hexyl methacrylate is added to the poly(hexyl methacrylate)-CO2-hexyl methacrylate solution, the cloudpoint curve exhibits UCST type phase behavior of a negative slope at a temperature range of 110-170 °C. With 14.4 wt % hexyl methacrylate added to the solution, the cloud-point curve exhibits an UCST type phase behavior of a negative slope. The cloud-point behavior appears virtually flat at ∼900 bar and a temperature range of 115-175 °C. Also at 125 °C, the cloud-point pressure of the poly(hexyl methacrylate)CO2-hexyl methacrylate system decreases by ∼400 bar with the first 9.2 wt % hexyl methacrylate added to the solution, and it decreases by another ∼400 bar with the

1550 Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004

Figure 10. Phase behavior of the poly(hexyl methacrylate)-CO243.8 wt % hexyl methacrylate system obtained in this study. Open squares represent fluid f liquid + liquid (L + L) transitions, closed circles represent fluid f liquid + vapor transitions, and closed squares represent LLV data.

Figure 11. Effect of the phase behavior of poly(hexyl acrylate) dissolved in supercritical propane, propylene, butane, and 1-butene. The concentration of the polymers in solution is ∼5 wt %.

Table 8. Experimental Cloud-Point, Bubble-Point, and LLV Data for the Poly(hexyl methacrylate) (PHMA)-CO2-Hexyl Methacrylate (HMA) System Measured in This Study T (°C)

P (bar)

transition

T (°C)

P (bar)

transition

4.7 wt % PHMA + 42.8 wt % HMA 80.4 101.3

131.0 192.1

Cloud-Point Transition CP 121.9 241.7 CP

CP

39.9 50.8

67.2 81.7

Bubble-Point Transition BP 60.2 92.1 BP

BP

78.5 82.8

112.2 119.2

LLV Transition LLV 100.0 LLV

139.8

LLV

addition of the next 18 wt %. The phase behavior curve with 32.4 wt % hexyl methacrylate has a slightly positive slope so that now the remnant of the sharp upturn in the cloud-point pressure is eliminated, which significantly expands the single-phase region. It is evident that the impact of the hexyl methacrylate cosolvent diminishes as the hexyl methacrylate concentration increases. Similarities are apparent between the phase behavior of the poly(hexyl methacrylate)-CO2-43.8 wt % hexyl methacrylate mixtures shown in Figure 10 (Table 8) and the phase behavior of the poly(hexyl acrylate)-CO233.9 wt % hexyl acrylate mixture shown in Figure 10. When 43.8 wt % hexyl methacrylate is added to the solution, the phase behavior curve exhibits LCST type cloud-point behavior with a positive slope. The poly(hexyl methacrylate)-CO2-hexyl methacrylate cloudpoint (LCST) curve intersects the LV curve at 75 °C and 110 bar with 43.8 wt % hexyl methacrylate. Liquid and vapor phases coexist at pressures below this curve. Note that the LV behavior curve switches to a LLV curve at greater than 75 °C. The slope of the poly(hexyl methacrylate)-CO2-hexyl methacrylate LCST curve, ∼2.7 bar/°C, is approximately 40% greater than that observed for binary poly(isobutylene)-alkane mixtures reported by Zeman and Patterson.30 Phase Behavior for Poly(hexyl acrylate)-Solvents and Poly(hexyl methacrylate)-Solvents Systems. Figure 11 shows the phase behavior curve of poly(hexyl acrylate) dissolved in supercritical propane, propylene, butane, and 1-butene. The cloud-point behavior for poly(hexyl acrylate)-propane, -propylene, -butane, and -1-butene systems exhibits LCST curves

Figure 12. Effect of the phase behavior of poly(hexyl acrylate) dissolved in supercritical CO2, ethylene, and dimethyl ether. The concentration of the polymers in solution is ∼5 wt %.

with a positive slope. The poly(hexyl acrylate)-alkane system was presented at a temperature range of 35-135 °C and a pressure of 240 bar. At 100 °C, the phase behavior boundary has shifted from 200 bar (propane) to 40 bar (butane), and this is due to the polarizability difference of propane (62.9 × 10-25 cm3) and butane (81.4 × 10-25 cm3). The poly(hexyl acrylate)-alkene system is shown at a temperature range of 60-150 °C and a low pressure of 230 bar. For the temperature at 110 °C, the cloud-point curve is reduced from 200 bar (propylene) to 30 bar (1-butene). As shown in Table 1, this seems to be due to polarizability and the dipole moment difference for propylene and 1-butene. The pressure difference for poly(hexyl acrylate)propane and poly(hexyl acrylate)-butane systems is considered to be due to the polarity factor. Figure 12 shows the impact of the phase behavior of poly(hexyl acrylate) in supercritical CO2, ethylene, and dimethyl ether. As shown in Figure 12, dimethyl ether has a dipole moment of 1.3 D22 and does not possess a quadrupole moment. The low cloud-point behavior of the poly(hexyl acrylate)-dimethyl ether system seems to be due to polarizability and the dipole moment. Experimental cloud-point behavior of the poly(hexyl acrylate)ethylene system shows a sharp plate of a pressure of ∼650 bar in a temperature range of 50-150 °C. Figure 13 shows the phase behavior for the poly(hexyl methacrylate)-supercritical propane, propylene, butane, dimethyl ether, and dichlorofluoromethane. The phase behavior curves for poly(hexyl methacrylate)propane and poly(hexyl methacrylate)-propylene mix-

Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 1551

Literature Cited

Figure 13. Effect of the phase behavior of poly(hexyl methacrylate) dissolved in five supercritical solvents. The concentration of the polymers in solution is ∼5 wt %.

tures are shown in Figure 13. While the cloud-point curve for the poly(hexyl methacrylate)-propane mixture shows a plate as the temperature is decreased, the poly(hexyl methacrylate)-propylene system decreases rapidly as the temperature is decreased. The pressure difference between these two systems is due to the polarity. The solubility curve for the poly(hexyl methacrylate)propane and poly(hexyl methacrylate)-butane mixtures is shown at a temperature range of 40-180 °C. Also, the phase behavior for the poly(hexyl methacrylate)dimethyl ether and poly(hexyl methacrylate)-chlorofluromethane systems shows a low pressure and has a dipole moment. Conclusions High-pressure phase behavior for the CO2-hexyl acrylate and CO2-hexyl methacrylate systems is shown at a temperature range of 40-120 °C and a pressure range of 25-189 bar. The P-x bubble-point curves are convex, which indicates that CO2 exhibits a high solubility in hexyl acrylate or hexyl methacrylate probably because of the formation of a weak complex between the carboxylic oxygen in hexyl acrylate or hexyl methacrylate and the carbon in CO2. The Peng-Robinson equation of state can be used with two adjustable parameters to calculate a reasonable representation of the phase behavior of the CO2hexyl acrylate and CO2-hexyl methacrylate systems. With a single, temperature-independent interaction parameter, quantitative agreement can be obtained between experimental data and the calculated phase behavior. The phase behavior data for the system poly(hexyl acrylate)-CO2-hexyl acrylate are measured with hexyl acrylate concentrations of 0.0, 5.0, 12.8, 20.7, and 33.9 wt %. This system changes the pressure-temperature slope of the phase behavior curves from the UCST region to the LCST region as the hexyl acrylate concentration increases. Cloud-point data are obtained for poly(hexyl methacrylate)-CO2-hexyl methacrylate mixtures and with hexyl methacrylate concentrations of 0.0, 9.2, 14.4, 32.4, and 43.8 wt %. Acknowledgment This work was supported by Grant R05-2003-00010478-0 from the Basic Research Program of the Korea Science & Engineering Foundation.

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Received for review September 19, 2003 Revised manuscript received December 17, 2003 Accepted January 5, 2004 IE030724U