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Surface Activity of Myristic Acid in the Poly(methyl methacrylate)/Supercritical Carbon Dioxide System Katsuto Otake,*,† Masanori Kobayashi,‡ Yoshinobu Ozaki,‡ Satoshi Yoda,† Yoshihiro Takebayashi,† Tsutomu Sugeta,† Noriaki Nakazawa,† Hideki Sakai,‡ and Masahiko Abe‡ National Institute of Advanced Industrial Science and Technology, Nanotechnology Research Institute, Higashi 1-1-1, Tsukuba Central 5, Tsukuba, Ibaraki 305-8565, Japan, and Faculty of Science and Technology, Tokyo University of Science, Yamazaki 2641, Noda, Chiba 278-8565, Japan Received December 16, 2003. In Final Form: March 28, 2004 To confirm the surface activity of myristic acid in the dispersion polymerization of vinyl monomers in scCO2, the interfacial tension (IFT) at the polymer/supercritical carbon dioxide (scCO2) interface has been measured. For the IFT measurements, a high-pressure pendant drop apparatus was constructed. The IFT data was obtained by the axisymmetric drop shape analysis of melt polymer droplets formed at the tip of a capillary. The reliability of the apparatus was confirmed by measuring the IFT of polystyrene (PS)/ scCO2 and polypropylene (PP)/CO2 systems. The IFT of the poly(methyl methacrylate) (PMMA)/scCO2 system with and without myristic acid was also measured. The IFT decreased on addition of myristic acid. The magnitude of the IFT depression due to the myristic acid was comparable to that of PS/scCO2 systems with the block copolymer surfactant, PS-b-poly(fluorooctyl acrylate). The surface activity of the myristic acid was confirmed by the decrease of IFT.
1. Introduction In recent years, global scale environmental effects such as atmospheric warming, ozone layer depletion, and acid rain have become increasingly problematic. Therefore, severe pollution control is being carried out in many countries worldwide, and the conversion of conventional industrial processes to more environmentally friendly processes is being required (“green chemistry”). Among many such endeavors, researchers are looking to supercritical carbon dioxide (scCO2) as a possible alternative to the hydrocarbons frequently used as solvents and cleaning agents in industrial processes. ScCO2 is a nontoxic, noninflammable, inexpensive, and environmentally benign fluid. In addition, scCO2 has similar chemical properties to hexane, is inert toward radicals, and alleviates the need for the drying process since it is a gas under ambient conditions. One of the most promising industrial processes for using scCO2 is in polymerization processes.1-5 Unfortunately, scCO2 does not dissolve most polymers except at high pressures and temperatures.6 To use scCO2 as an alternative solvent in polymerization processes, new surfactants must be developed to increase polymer solubility and/or dispersivity. In the present stage of industrial development, only polymers that contain perfluoroalkyl side chains, siloxane* Corresponding author. Phone: +81-298-61-4567 or +81-29861-4819. Fax: +81-298-61-4567. E-mail:
[email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ Tokyo University of Science. (1) DeSimone, J. M.; Guan, Z.; Elsbernd, C. S. Science 1992, 257, 945. (2) DeSimone, J. M.; Maury, E. E.; Menceloglu, Y. Z.; McClain, J. B.; Romack, T. J.; Combes, J. F. Science 1994, 265, 356. (3) Kendall, J. L.; Canelas, D. A.; Young, J. L.; DeSimone, J. M. Chem. Rev. 1999, 99, 543. (4) Michel, U.; Resnick, P.; Kipp, B.; DeSimone, J. M. Macromolecules 2003, 36, 7107. (5) Beckman, E. J. J. Supercrit. Fluids, in press. (6) Kirby, C. F.; McHugh, M. A. Chem. Rev. 1999, 99, 565.
based polymers, specially designed poly(ether-carbonate) copolymers, and their block copolymers have been found to be effective for dispersion polymerization.3,7 Unfortunately, polymeric surfactants become impurities that can be difficult to remove in successive polymer processing steps and are expensive, especially in the case of fluorinated polymers. De Simone et al. proposed the removal of the surfactants by supercritical extraction; however, it adds new process steps to the industrial production system, and the complete removal of the surfactant is difficult. In our previous studies,8-12 we found that copolymerization of carboxyl acid vinyl monomers such as acrylic acid (AA) and methacrylic acid (MAA) with methyl methacrylate (MMA) forms a fine white polymer powder in scCO2. The MAA and AA worked as surfmers (surfactant + monomer). It was also found that aliphatic carboxyl acids such as myristic acid (MA) could take on a surfactantlike nature in scCO2. Copolymerization of other vinyl monomers such as styrene (St), butyl acrylate (BMA), and ethyl acrylate (EA) with these acid monomers and dispersion polymerization with aliphatic carboxylic acids in scCO2 also gave fine white polymer particles. The amount of surfmers or additives needed to stabilize the polymer particle was about 5 mol % for the case of MAA and 10-30 mol % for AA and MA. Even the poly(AA) acted as a polymeric surfactant. The surfactant-like nature of these compounds might be explained as the effects of (7) Sarbu, T.; Styranec, T.; Beckman, E. J. Nature 405, 165. (8) Mizuguchi, K.; Otake, K.; Sako, T.; Sugeta, T.; Yoda, S.; Takebayasi, Y.; Nakazawa, N.; Kamizawa, C. Proceedings of 5th International Symposium on Supercritical Fluids, Atlanta, GA, 2000. (9) Otake, K.; Sako, T.; Sugeta, T.; Yoda, S.; Takebayasi, Y.; Nakazawa, N.; Kamizawa, C.; Mizuguchi, K. Proceedings of 5th International Symposium on Supercritical Fluids, Atlanta, GA, 2000. (10) Sugeta, T.; Otake, K.; Yoda, S.; Takebayasi, Y.; Nakazawa, N. Proceedings of 7th Meeting on Supercritical Fluids, Neith, France, 2000. (11) Kobayashi, M.; Otake, K.; Mizuguchi, K.; Sakai, H.; Abe, M. J. Jpn. Soc. Color Mater. 2002, 75, 371. (12) Kobayashi, M.; Otake, K.; Yoda, S.; Takebayashi, Y.; Sugeta, T.; Nakazawa, N.; Sakai, H.; Abe, M. Proceedings of 6th International Symposium on Supercritical Fluids, Versailles, France, 2003.
10.1021/la0363783 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/17/2004
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Figure 1. Experimental apparatus.
specific interactions between carbonyl oxygen atom and CO2,13 and static repulsion of hydroxyl groups. In this study, aimed at a quantitative understanding of the surface activity of the carboxyl acid group, interfacial tension (IFT) measurements were conducted with poly(methyl methacrylate) in the presence of MA. 2. Experimental Section Materials. Polystyrene (PS, Mw ) 309312, Mw/Mn ) 1.804) and poly(methyl methacrylate) (PMMA, Mw ) 89230, Mw/Mn ) 2.302) were purchased from Wako Pure Chemical Co. Ltd. Poly(propylene) (PP, Chisso Co. Ltd., FH3400, foam grade, Mw ) 985000, Tm ) 438 K) was graciously provided by the Japan Steel Works. Polymers were used after freeze-comminution. The myristic acid (MA) used as surfactant was also supplied by the Wako Pure Chemicals Co. Ltd. and used without further treatment. Carbon dioxide (99.99%) was purchased from Tomoe Shokai. It was filtered and dried with molecular sieves 3A prior to use. Apparatus and Procedures. For the IFT measurements, the pendant drop method14 was employed. Figure 1 shows a schematic representation of the experimental apparatus constructed in this study. The apparatus consisted of a high-pressure variable volume viewing cell having two sapphire windows (Tama Seiki Co. Ltd.), a constant temperature air bath (Yamato DN410H), a CO2 feed system, and an image analysis system. The internal volume of the viewing cell was 48 cm3. A stainless steel tube (1.59 mm in o.d. and 0.38 mm in i.d.) was used as a capillary. Prior to setup, its opening was cut and mirror polished to allow formation of polymer droplets. The accuracy of temperature and pressure measurements were (0.1 K and (0.01 MPa, respectively. In the experiments, freeze-comminuted polymer powder was placed in the cylinder and evacuated. The temperature was raised to the experimental conditions and held constant overnight. CO2 was introduced into both the viewing cell and the cylinder. After several hours of equilibration, a 1-10 MPa of pressure difference was induced between the viewing cell and the sample cylinder to form a drop of melt polymer at the tip of the capillary. The image of the drop was taken by a CCD camera and analyzed by the selected plane (SP) method14 and axisymmetric drop shape analysis (ADSA) method.15 In the pendant drop measurements, the IFT is calculated from the drop shape, at a point in time just before release from the capillary tip. Under these conditions, IFT is in equilibrium with (13) Kazarian, S. G.; Vincent, M. F.; Bright, F. V.; Liotta, C. L.; Eckert, C. A. J. Am. Chem. Soc. 1996, 118, 1729. (14) Ambwani, D. S.; Fort, T., Jr. In Surface and Colloid Science; Good, R. J., Stromberg, R. R., Eds.; Plenum Press: New York, 1979; Vol. 11, Chapter 3, pp 31-91. (15) Kwok, D. Y.; Cheung, L. K.; Park, C. B.; Neumann, A. W. Polym. Eng. Sci. 1998, 35, 757.
gravitational forces. In the case of the SP method, the IFT can be obtained from the maximum (equatorial) diameter and diameter of the selected plane of the droplet. On the other hand, in the ADSA method, IFT is calculated from all coordinates of the drop shape and is more accurate than the SP method. Thus, the IFT was first calculated by the SP method, and subsequently calculated by the ADSA method with the IFT obtained by the SP method as an initial value. If the IFT by the SP and ADSA methods were in agreement to within (5%, the value obtained by the ADSA method was employed as the IFT. It should be noted that to form the polymer melt droplet, 1 day to several weeks were required, depending on the viscosity of the melt polymer. Recently, Jaeger et al.16 measured the IFT of PS/CO2 and Taki et al.17 measured that of PP/CO2 by the pendant-drop method. They fixed a polymer drop on the tip of a metal rod under ambient conditions, mounted the rod in a high-pressure vessel, and measured the IFT. In each of these cases, there is the possibility that equilibrium between gravitational forces and IFT is not achieved. For comparison, experiments with the fixed drop method were also conducted with PMMA. PMMA was dissolved in toluene to form a concentrated solution (50 wt %). The viscous PMMA solution was fixed at the end of a stainless steel wire (0.1 mm in diameter), placed in the viewing cell, and dried in vacuo for 1 day at experimental temperature. CO2 was introduced into the viewing cell, and the IFT was measured. The IFT measurements with MA were conducted with a MA concentration of 0.4 g/L at 15 MPa and 363.15 K, which is lower than the saturation concentration.18 The MA (19 mg) was sealed in the viewing cell prior to the measurement and dissolved in scCO2 before the IFT measurements. In the experiments, to prevent change in the composition of the scCO2/myristic acid mixtures, the pressure was changed by adjusting the piston position inside the variable volume view cell. IFT Calculation. The CCD image of the drop was analyzed by the SP method14 and ADSA method15 as described above. In the calculation, the density difference of the polymer phase and scCO2 phase must be known. In this study, the density of the polymer was calculated by the Sanchez-Lacombe equation of state (S-L EOS)19-21 with kij’s determined from experimental data. Parameters used in the calculation are summarized in Table 1.22-28 In the table, T is the temperature at which the IFT (16) Jaeger, Ph. T.; Eggers, R.; Baumgartl, H. J. Supercrit. Fluids 2002, 24, 203. (17) Taki, K.; Murakami, T.; Oshima, M. Proceedings of 35th Autumn Meeting of The Society of the Chemical Engineering of Japan (in Japanese), I202, Kobe, 2002. (18) Maheshwari, P.; Nikolov, Z. L.; White, T. M.; Hartel, R. J. Am. Oil. Chem. Soc. 2001, 78, 827. (19) Sanchez, I. C.; Lacombe, R. H. J. Phys. Chem. 1976, 80, 2352. (20) Sanchez, I. C.; Lacombe, R. H. J. Phys. Chem. 1976, 80, 2568. (21) Sanchez, I. C.; Lacombe, R. H. Macromolecules 1978, 11, 1145. (22) Sato, Y.; Takishima, S.; Masuoka, H. Private communication.
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Table 1. Parameters for Sanchez-Lacomb Equation of State T*/K 24
(1) CO2 (2) PS 23,27 (2) PPa 25,28 (2) PMMA22,26 a
208.9 + 0.459 ‚T - 7.56 × 739.9 690.6 742.0
10-4‚T2
P*/MPa
F*/g‚cm-3
K12
720.3 387.0 300.7 488.3
1.580 1.108 0.8856 1.2498
0.32977-1.0700 × 10-3‚T 0.60399-1.9000 × 10-3‚T 0. 175 1-7.04 × 10-4‚T
Assumed to be 100% amorphous.
Figure 2. Drop shape and interfacial tension of PS at 423.15 K and 30MPa. measurements were conducted. The density of the scCO2 was calculated by equations proposed by Span et al.29 assuming that the polymer is not dissolved by the scCO2 phase. We also assumed that the MA does not dissolve in the polymer phase.
3. Results and Discussion Reliability of the Experimental Apparatus. To confirm the reliability of the apparatus, the IFT of the PS/scCO2 and PP/scCO2 systems were measured. As described in the Experimental Section, in the pendant drop measurements, IFT is calculated from the drop shape just before its release from the capillary tip. At that point, the IFT is in equilibrium with gravitational forces. Figure 2 shows the IFT of PS and images of the droplet taken at various stages of drop release at 423.15 K and 30 MPa. In this study, the IFT was measured from the drop just before its detachment (Figure 2, part (c) or (d)). Figure 3a shows the pressure dependence of the IFT of the PS/scCO2 system. The results of Harrison et al.30,31 (Mw ) 2250, Mw/Mn ) 1.2) and Jaeger et al.16 (Mw ) 150000) are shown for comparison. The IFTs at ambient conditions were calculated by an equation proposed by Wu.32 In the (23) Sato, Y.; Yurugi, M.; Fujiwara, K.; Takishima, S.; Masuoka, H. Fluid Phase Equilib. 1996, 125, 129. (24) Wang, N.-H.; Hattori, K.; Takishima, S.; Masuoka, H. Kagaku Kogaku Ronbunshu (in Japanese) 1991, 17, 1138. (25) Sato, Y.; Sorakubo, A.; Takishima, S.; Masuoka, H. Proceedings of 9th APCChE Congress and CHEMECA 2002, Christchurch, New Zealand, 2002. (26) Rodgers, P. A. J. Appl. Polym. Sci. 1993, 48, 1061. (27) Sato, Y.; Takikawa, T.; Takishima, S.; Masuoka, H. J. Supercrit. Fluids 2001, 19, 187. (28) Sato, Y.; Yurugi, M.; Yamabiki, M.; Takishima, S.; Masuoka, H. J. Appl. Polym. Sci. 2001, 79, 1134. (29) Span, R.; Wagner W. J. Phys. Chem. Ref. Data 1996, 25, 1509. (30) Harrison, K. L. Ph.D. Thesis, University of Texas, 1996. (31) Harrison, K. L.; Johnston, K. P.; Sanchez, I. C. Langmuir 1996, 12, 2637. (32) Wu, S. J. Macromol. Sci. Macromol. Chem. 1974, C10, 1.
figure, lines are provided to aid following the data trends. Our results agreed well with Jaeger et al. except in the low-temperature region. In the low-temperature region, the viscosity of the polymer increases and it becomes difficult to measure the IFT properly. The extremely low IFT reported by Harrison et al. is probably due to the low molecular weight of the PS used in their study. From the figure, it is clear that the IFT decreases with increasing pressure. It is well-known that the physical properties of supercritical fluid systems often correlate with density rather than with pressure. At the same time, the IFT may depend on the solubility of CO2 in the polymer because dissolution of scCO2 weakens the interaction between polymer segments, resulting in swelling.33,34 Recently, O’Neill et al. related the solubility of a polymer in scCO2 to the surface tension of the pure polymer.35 Figure 3b shows the density dependence, and Figure 3c shows the solubility dependence of the IFT of the PS/CO2 systems. In the figures, solid lines represent the thirdorder polynomial least-squares fit of all IFT data obtained in this study. The solubility was calculated by the S-L EOS using the parameters shown in Table 1. As shown in the figures, all data obtained could be presented on one single line, which means that the IFT of PS/scCO2 systems may be possibly predicted using either the density or the solubility of scCO2. Comparing our results with those of Jaeger et al., it can be seen that our data are somewhat higher than those in the literature. As mentioned above, the results reported (33) Miller-Chou, B. A.; Koenig, J. L. Prog. Polym. Sci. 2003, 28, 1223. (34) Drohmann, C.; Beckman, E. J. J. Supercrit. Fluids 2002, 22, 103. (35) O’Neill, M. L.; Cao, Q.; Fang, M.; Johnston, K. P.; Wilkinson, S. P.; Smith, C. D.; Kerschner, J. L.; Jureller, S. H. Ind. Eng. Chem. Res. 1998, 37, 3067.
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Figure 3. Interfacial tension of the PS/CO2 system. (a) Pressure dependence, (b) CO2 density dependence, and (c) CO2 solubility dependence.
Figure 4. Interfacial tension of the PP/CO2 system. (a) Pressure dependence, (b) CO2 density dependence, and (c) CO2 solubility dependence.
by Jaeger et al. were measured by the fixed drop method, and hence the IFT might not be in equilibrium with gravitational forces. Figure 4a shows IFT of PP/scCO2 systems. The results of Taki et al.17 (PP: Mitsubishi Chemical Co.Ltd., EA7A, foam grade, Mw ) 410000, Tm ) 433 K) are shown for comparison. Again, our results agreed with the literature data, but have somewhat higher values. Effects of temperature on the IFT of PP/CO2 is smaller than that of the PS/CO2 system. This will be due to the difference in solubility of CO2. As the solubility of the CO2 in PP is higher than that in PS,25 the effect of solubility on the IFT, or the cohesive energy density of PP/CO2 mixture,33,34 is expected to be smaller than PS. Similar to the case of PS/scCO2, as shown in Figure 4, parts (b) and (c), the IFT is well-correlated by density and solubility of scCO2. The lines in the figures are the third-order least-squares fit of the IFT obtained in this study. Surface Activity of Myristic Acid (MA). As described above, the IFT of PS/scCO2 and PP/scCO2 agreed well with the literature data. From this fact, the reliability of the apparatus was taken as proven, and the IFT of PMMA/ CO2 with and without MA was measured. Figure 5 shows the IFT of PMMA/scCO2 systems with and without MA at 363.15 K. In the figure, the theoretical IFT of the PMMA/scCO2 system calculated by Goel et al.36 (36) Goel, S. K.; Beckman, E. J. Polymer 1993, 34, 1410.
is also shown for comparison. Lines in the figure are for easier visualization. It is clear from the figure that the theoretical calculation by Goel et al. is not sufficient for the precise evaluation of the IFT. It is presumably due to the fact that their theory does not correctly account for the effect of the solubility of scCO2. In the figure, the open circles and open rectangles are the IFT values obtained by the fixed drop and capillary drop methods, respectively. The tendency that the IFTs measured by the fixed drop are somewhat lower than the capillary drop is the same as in the case of the PP/scCO2 system. From the figure, it is also clear that the existence of the MA lowers the IFT of the PMMA/scCO2 system to some extent. The degree of lowering of the IFT is comparable to the PS/PS-b-PFOR/scCO2 systems.30,31 It is well-known that the carbonyl oxygen has a specific interaction with the carbon atom of CO2.13 The surface activity of the MA presumably arises from this interaction between the carbonyl group and scCO2. The -OH of the carboxylic acid group may provide static repulsion, further stabilizing polymer particles during the polymerization in scCO2.3-7 In this study, the surface activity of MA in the PMMA/ scCO2 system was experimentally proven. The two factors described abovesthe surface activity of the carboxylic acid and the static repulsion of -OH groupssmay be the cause of the formation of fine polymer powder in the polymerization of vinyl monomers in scCO2. To our knowledge,
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Figure 5. Interfacial tension of PMMA/CO2 and PMMA/MA/CO2 systems at 363.15 K. (a) Pressure dependence, (b) CO2 density dependence, and (c) CO2 solubility dependence.
this is the first report proving the surface activity of carboxylic acid groups. 4. Conclusion An apparatus for IFT measurements of polymer/scCO2 systems was constructed and its reliability was confirmed by IFT measurements of PS/scCO2 and PP/scCO2 systems. The IFT measurements of PMMA/scCO2 systems with and without MA were conducted. The surface activity of MA in the PMMA/scCO2 system was experimentally proven for the first time. With this knowledge, it will be possible to build polymerization processes using scCO2 as an alternative solvent without resorting to expensive fluorinated surfactants. At the same time, the existence of
a universal relationship between solubility and/or density of CO2 and the IFT was experimentally shown. It will be important for processes such as supercritical foaming and other polymer processing processes that use CO2. Acknowledgment. This work was in part financially supported by Japan Chemical Innovation Institute, JCII, and New Energy and Industrial Technology Development Organization, NEDO, and Ministry of Economy, Trade and Industry, METI. We thank Dr. Yoshiyuki Sato of Tohoku University for his help with the density calculations. LA0363783
