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Dispersion polymerization in supercritical carbon dioxide using comb-like fluorinated polymer surfactants having different backbone structures
J. of Supercritical Fluids 55 (2010) 381–385
Contents lists available at ScienceDirect
The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Dispersion polymerization in supercritical carbon dioxide using comb-like fluorinated polymer surfactants having different backbone structures Byoung Gak Kim a,b , Jungin Shin a , Eun-Ho Sohn a , Jae-Seung Chung a , Won Bae c , Hwayong Kim a , Jong-Chan Lee a,∗ a b c
Department of Chemical and Biological Engineering, Seoul National University, 559 Gwanangno, Gwanak-gu, Seoul, 151-744, Republic of Korea Center for Fuel Cell Research, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Sungbuk-gu, Seoul, 136-791, Republic of Korea R&D Institute, Miwon Commercial Co., Ltd., 405-3, Moknae-Dong, Ansan-Si, Kyonggi, 425-100, Republic of Korea
a r t i c l e
i n f o
Article history: Received 26 January 2010 Received in revised form 1 July 2010 Accepted 1 July 2010 Keywords: Supercritical carbon dioxide Dispersion polymerization Comb-like fluorinated polymer Backbone structure
a b s t r a c t Comb-like fluorinated polymers with different backbone structures, poly(heptadecafluorodecyl acrylate) (PA-Rf ), poly[oxy[(2-perfluorooctylethylene)thiomethyl]ethylene] (PEO-Rf ), and poly[p[[(perfluorooctylethylene)thio]methyl]styrene] (PS-Rf ), were used as surfactants in dispersion polymerization to examine the effect of backbone structure on the formation of polymer particles. Dispersion polymerization of monomers with different polarities using these comb-like fluorinated polymer surfactants in CO2 showed that PEO-Rf containing a polar oxyethylene backbone was an effective surfactant for the dispersion polymerization of a polar monomer, such as N-vinyl-2-pyrrolidone, whereas PA-Rf was effective for less polar monomers, such as methyl methacrylate and N-vinyl caprolactam. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Supercritical carbon dioxide (sc CO2 ) has great potential as an alternative to common volatile organic solvents because it is inexpensive, environmentally benign, nontoxic, and has tunable properties. These properties render sc CO2 a useful medium compared to other organic solvents [1–3]. Over the past decade, there have been extensive studies into its use as a solvent for polymerization. Heterogeneous polymerization techniques, including dispersion polymerization, have employed CO2 as a solvent because it is poor solvent for most polymers but a good one for the monomers. Comb-like fluorinated polymers are useful surfactant systems for dispersion polymerization in sc CO2 [4,5]. For example, poly(1,1-dihydroperfluorooctyl acrylate) has been used to prepare poly(methyl methacrylate) particles through the dispersion polymerization of methyl methacrylate in sc CO2 [4,5]. Other polymer particles, such as poly(styrene), poly(divinylbenzene), poly(vinyl acetate), and poly(acrylonitrile) particles were also prepared from the corresponding monomers by dispersion polymerization in sc CO2 using comb-like poly(acrylate) derivatives as surfactants [4,6–14]. In these polymerization systems, the hydrocarbon backbone region of the surfactant plays a key role in anchoring the
∗ Corresponding author. Tel.: +82 2 880 7070. E-mail address: [email protected] (J.-C. Lee). 0896-8446/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2010.07.001
surface of the hydrocarbon polymeric particle. However, there are only a few reports on the effect of the backbone structure of the comb-like fluorinated polymer on the formation of polymer particles through dispersion polymerization in sc CO2 [15]. In this study, three different monomers with different polarities, such as methyl methacrylate, N-vinyl-2-pyrrolidone, and N-vinyl caprolactam, were polymerized in sc CO2 using three different comb-like fluorinated polymers, such as poly(heptadecafluorodecyl acrylate) (PA-Rf ), poly[oxy[(2-perfluorooctylethylene)thiomethyl]ethylene] (PEO-Rf ), and poly[p-[[(perfluorooctylethylene)thio]methyl]styrene] (PS-Rf ), as surfactants. As the chemical structure of the side groups of these comb-like fluorinated polymers is identical, the effect of the backbone structures on the formation of polymer particles with different polarities could be studied.
2. Experimental 2.1. Materials Carbon dioxide (min. 99.99%) was purchased from Korea Industrial Gases. 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-Heptadecafluorodecyl acrylate (min. 97%, CAS No. 27905-45-9) was obtained from Aldrich. PA-Rf was synthesized by the homogeneous radical solution polymerization of 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10heptadecafluorodecyl acrylate in sc CO2 using a polymerization apparatus (Fig. 1) described in the literature [16].
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the cell from a high-pressure generator. A magnetic stirring bar in the cell helped the mixture to reach equilibrium rapidly. Once the solution reached a single phase, pressure was decreased slowly until the cloud point appeared. This procedure was repeated several times until the fluctuations in the transition pressure were minimized to within ±0.03 MPa.
Fig. 1. Schematic diagram of the polymerization apparatus. (P = pressure gauge; T = temperature gauge; PR = pressure regulator)
(PA-Rf : Mn = 10,300, Mw = 20,300) PEO-Rf was synthesized by a reaction of polyepichlorohydrin and sodium 1H,1H,2H,2Hperfluorodecanthiolate in 1-butyl-3-methyl-imidazolium chloride, [bmim][Cl], (Mn = 16,600, Mw = 17,600) [17,18]. PS-Rf was prepared by free radical polymerization in toluene as reported earlier (Mn = 20,300, Mw = 25,100) [19,20]. Number and weight-average molecular weights of all fluorinated polymer surfactants used were determined using size-exclusion chromatography. 1,1,2Trifluorotrichloroethane and poly(dimethylsioxane)s were used as an eluent and a standard, respectively. Methyl methacrylate (MMA, Junsei Chemical, min. 99.5%), N-vinyl-2-pyrrolidone (NVP, Aldrich, min. 99%) and N-vinyl caprolactam (NVCL, Aldrich, min. 98%) were pretreated on a alumina column to remove the inhibitor, and dissolved oxygen was removed by nitrogen purging. 2,2 -Azobisisobutyronitrile (AIBN, Junsei Chemical, min. 98%) was recrystallized from methanol. 2.2. Apparatus and procedure 2.2.1. Phase behavior measurement The cloud point curves were obtained with a variable volume view cell apparatus (Fig. 2). The window of this apparatus was made of sapphire to observe the cloud point. This apparatus and procedure have been described in detail elsewhere [21–23]. The cloud point data of the comb-like fluorinated polymer + CO2 was obtained from a polymer solution at a fixed polymer concentration of 1.0 wt.% in CO2 using standard procedure. Initially, the comblike fluorinated polymer was loaded into the cell and the cell was purged carefully with an inert gas and CO2 . CO2 was then added to the cell using a high-pressure bomb. After injecting the comb-like fluorinated polymer + CO2 mixture, the solution was compressed to the desired operating pressure by replacing a piston fitted within
2.2.2. Dispersion polymerization Dispersion polymerization of MMA, NVP and NVCL was carried out in a 30 mL SUS 316 reactor, and the inner phase change was observed through an observation window. CO2 was supplied from a gas booster pump (Maxpro Technologies, Model DLE 751). A 300 mL reservoir between the pump and reactor was used to minimize fluctuations from the pump and maintain a stable feed. The pressure was measured using a Bourdon tube pressure gauge (WIKA, type 213.53.063, accuracy class 1.0). The temperature was measured using a K (CA) type thermocouple and indicator (Hanyoung Electronics Inc. model DX-7) (accuracy 0.05 K). A PTFE-coated magnetic stirring bar was used to agitate the reaction mixture. An amount of 2.0 g of the monomer (MMA, NVP and NVCL), 0.02 g AIBN (1.0 wt.% relative to the monomer) and 0.2 g comblike fluorinated polymer (PA-Rf , PEO-Rf and PS-Rf ) as the surfactant (10.0 wt.% relative to the monomer) were placed in a 30 mL reactor. The reactor was then purged several times with CO2 to remove air and charged with a known amount of CO2 at room temperature. Polymerization was performed for 24 h at 343.2 ± 0.5 K and 30 ± 0.5 MPa. After polymerization, the reactor was cooled to below 283 K. Vapor/liquid phase separation occurred, and CO2 was vented slowly from the vapor phase through two glass traps. Glass traps filled with methanol and cooled with ice water were used to prevent discharge of the unreacted monomer to the atmosphere during CO2 venting. Finally, the resulting polymer was precipitated and washed in methanol to remove the unreacted monomer. The particle morphology and size were characterized by Field Emission Scanning Electron Microscope (FE-SEM) (Joel5410LV). The number-average particle size and particle size distribution (PSD) were measured using an image analyzer, TDI Scope EyeTM version 3.1 with SEM images. The number-average (Dn ) and weightaverage (Dw ) particle diameters were calculated from the following equations.
N Dn =
i=1
di
N
(1)
N Dw =
d4 i=1 i d3 i=1 i
N
(2)
where di is the diameter of particle i, and N is the total number of particles measured in the SEM images. The polydispersity index (PDI), which indicates the particle size distribution (PSD), was defined as Dw /Dn . 3. Results and discussion 3.1. Chemical structure of comb-like fluorinated polymers and phase behavior of comb-like fluorinated polymer + CO2 binary system
Fig. 2. Schematic diagram of the variable volume view cell apparatus. (1) Camera; (2) light source; (3) borescope; (4) fast response PRT; (5) view cell; (6) magnetic stirrer; (7) air bath; (8) digital thermometer; (9) digital pressure transducer; (10) pressure gauge; (11) hand pump; (12) computer monitor; (13) trap.
Fig. 3 shows the chemical structure of the three comb-like fluorinated polymers used in this study. All the polymers had the same CO2 -philic side groups, Rf (–(CF2 )7 CF3 ), whereas they had different backbone structures, such as acrylate, ethylene oxide, and styrene for PA-Rf , PEO-Rf , and PS-Rf , respectively. PA-Rf and PS-Rf were prepared by free radical polymerization from the corresponding fluorinated monomers and PEO-Rf was obtained from a polymer analogous reaction of polyepichlorohydrin with sodium
B.G. Kim et al. / J. of Supercritical Fluids 55 (2010) 381–385
383
Table 1 Particle size and particle size distribution of PMMAa , PVPb and PVCL.c . Surfactantd
Sample
Dn (m)e
PSDf
Morphology
g
PMMA PVCL PVP
11.28 0.74 NA
1.22 1.04 NA
Spherical (discrete) Spherical (discrete) Irregular (aggregate)
PEO-Rf h
PMMA PVCL PVP
NA 0.62 0.92
NA 1.16 1.11
Irregular (aggregate) Spherical (aggregate) Spherical (discrete)
PS-Rf i
PMMA PVCL PVP
0.71 0.76 0.81
1.14 1.41 1.07
Spherical (aggregate) Spherical (aggregate) Spherical (aggregate)
PA-Rf
a
Fig. 3. Chemical structure of comb-like fluorinated polymer surfactants.
Poly(methyl methacrylate). Poly(N-vinyl-2-pyrrolidone). Poly(N-vinyl caprolactam). d Reaction conditions: 2.0 g of monomer, 1.0 wt.% of AIBN, 10.0 wt.% of surfactant, 343.2 ± 0.5 K and 24 h with stirring. e Determined by FE-SEM image. f Particle size distribution (= Dw /Dn ). g Poly(heptadecafluorodecyl acrylate). h Poly[oxy[(2-perfluorooctylethylene)thiomethyl]ethylene]. i Poly[p-[[(perfluorooctylethylene)thio]methyl]styrene]. b c
1H,1H,2H,2H-perfluorodecanthiolate. The weight-average molecular weights (Mw ) of polymers ranged from 18,000 to 25,000 and the polydispersity indices of polymers were <2.0. Although it is well known that the molecular weight of the polymer affects the polymer solubility in sc CO2 , these differences in molecular weight can be expected to have only a small effect on the polymer solubility in sc CO2 [3]. A typical lower critical solution temperature (LCST) phase behavior was observed from the cloud point curves of the comb-like fluorinated polymer + CO2 binary systems (Fig. 4). The cloud point pressures of PA-Rf and PEO-Rf in CO2 were similar, whereas that of the PS-Rf + CO2 system was approximately 4 MPa larger than those of the other comb-like fluorinated polymers + CO2 systems. Since all the polymers have the same side chain groups, this different solubility behavior can be attributed to their different backbone structures. The higher solubility of PA-Rf and PEO-Rf compared with that of PS-Rf can be explained by the intermolecular interactions between CO2 and the polymer chains in PA-Rf and PEO-Rf . For example, the specific interactions between the carbonyl oxygen in the ester group (PA-Rf has the ester groups) and CO2 was reported by Nelson and Borkman [24], and the intermolecular interaction between the oxide groups (PEO-Rf have oxyethylene backbone) and CO2 has been reported by others [25,26]. The cloud point pressure of PA-Rf appears to be slightly smaller than that of PEO-Rf . It is possible that the interactions between the carbonyl oxygen in the ester group are larger than those between the oxide groups and CO2 , or that there are slight differences in the molecular weights and molecular weight distributions. PS-Rf , which has a styrene backbone, does not contain any CO2 -philic groups, such as carbonyl or oxide groups, which in turn might give PS-Rf a higher cloud point
Fig. 4. Cloud point curves for comb-like fluorinated polymer + CO2 system.
than the other polymers in this study. Nevertheless, all the polymers in this study were quite soluble in CO2 and could be used in dispersion polymerization reactions in sc CO2 (343.2 K and 30 MPa). 3.2. Dispersion polymerization of MMA, NVP and NVCL in sc CO2 The three comb-like fluorinated polymers shown in Fig. 3 were used as a surfactant in dispersion polymerization in sc CO2 . The hydrocarbon backbone can anchor the surface of the hydrocarbon polymeric particle and the long fluorinated alkyl groups soluble in the sc CO2 phase can prevent the aggregation of polymer particles. Since the three polymers have an identical side chain, the difference in anchoring ability of their backbone regions is believed to affect the resulting polymer particles in the dispersion polymerization of MMA, NVCL, and NVP. Fig. 5 shows scanning electron micrographs of PMMA (poly(methyl methacrylate)), PVP (poly(N-vinyl-2-pyrrolidone)), and PVCL (poly(N-vinyl caprolactam)) particles. Table 1 lists the particle size, particle size distribution and morphology. An attempt was made to polymerize these monomers in CO2 without a fluorinated polymer surfactant for comparison but only aggregated non-spherical polymer particles were obtained. This means that our comb-like fluorinated polymers are quite effective surfactants in dispersion polymerization. Others have reported that discrete spherical polymer particles with a narrow size distribution were obtained for most monomers when poly(heptadecafluorodecyl methacrylate) having methyl groups in the backbone region was used as a surfactant for dispersion polymerization in sc CO2 [4,27]. Methyl groups in the backbone of comb-like fluorinated polymer show better anchoring ability than ester, acetate, and hydroxyl group in the fluorinated polymer [27,28]. It is because primary mechanism by which comb-like fluorinated polymer are effective is through either chemical grafting or physical adsorption to the surface of the growing polymer colloid (steric stabilization). Poly(heptadecafluorodecyl methacrylate) can be used for dispersion polymerization of various monomers ranging from polar monomers to non-polar monomers due to this steric stabilization effect of methyl groups in the backbone region [29,30]. When PA-Rf was used, however, dispersed particles were obtained from the polymerization of MMA and NVCL, as shown in Fig. 5(a-1) and (a-2), whereas irregular aggregated particles were obtained from the polymerization of polar NVP monomer, as shown in Fig. 5(a-3). It means that physical adsorption could be not effective due to the little tendency of fluorinated acry-
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Fig. 5. SEM images of (a) PMMA (poly(methyl methacrylate)); (b) PVP (poly(N-vinyl-2-pyrrolidone)); (c) PVCL (poly(N-vinyl caprolactam)) particle polymerized using (1) PA-Rf (poly(heptadecafluorodecyl acrylate)); (2) PEO-Rf (poly[oxy[(2-perfluorooctylethylene)thiomethyl]ethylene]); (3) PS-Rf (poly[p[[(perfluorooctylethylene)thio]methyl]styrene]) as a surfactant (magnification; (a-1) (b-1) × 1000, (a-2,3) (b-2,3) (c-1,2,3) × 5000).
late polymers to adsorb to the very polar surface. On the contrary, in the dispersion polymerization using PEO-Rf with a more polar and basic backbone, uniform dispersed particles were obtained from polar monomer NVP as shown in Fig. 5(b-2) and (b-3), whereas nonuniform particles with irregular shapes were obtained from the less polar monomer, MMA, as shown in Fig. 5(b-1). This indicates that polar-polar or non-polar-non-polar interaction is predominant when the steric stabilization is ineffective. Mostly aggregated particles were obtained in dispersion polymerization using PS-Rf with non-polar groups in the backbone region, even though most of the particles appear to be quite uniform, as shown in Fig. 5(c-1)–(c-3). It is possible that the less polar styrene backbone, that does not contain any polar and/or basic functional groups, has weaker anchoring ability that might cause the aggregation. The polymerization condition (at 343.2 ± 0.5 K and 30 ± 0.5 MPa, for 24 h) using PS-Rf might not be the optimum condition for the preparation of polymer particles. However, the same polymerization conditions were used in all the cases to allow a direct comparison, even though more uniform dispersed particles might be obtained using PS-Rf under different conditions. The results in Fig. 5, which were obtained when all the comb-like fluorinated polymers were soluble in sc CO2 , clearly show the effect of the backbone structures of the comb-like fluorinated polymers on the formation of polymer particles. PEO-Rf with a more polar backbone can only disperse polar polymer particles (PVP), whereas PA-Rf can disperse less polar polymer particles (PMMA and PVCL). 4. Conclusion poly[oxy[(2Poly(heptadecafluorodecylacrylate) (PA-Rf ), perfluorooctylethylene)thiomethyl]ethylene] (PEO-Rf ), and poly[p-[[(perfluorooctylethylene)thio]methyl]styrene] (PS-Rf ) with a CO2 -philic tail (–(CF2 )7 CF3 ) and different backbone struc-
ture were used as the surfactant for the dispersion polymerization of the monomer with different polarities, such as methyl methacrylate, N-vinyl-2-pyrrolidone, and N-vinyl caprolactam. The polarity of the polymer backbone determines the dispersion ability of the polymer surfactants. When PEO-Rf was used, discrete particles were obtained from a polar N-vinyl-2-pyrrolidone, whereas PA-Rf produces such discrete particles from less polar monomers, such as methyl methacrylate and N-vinyl caprolactam. Mostly aggregated particles were obtained in the case of PS-Rf with non-polar groups in the backbone region, even though spherical particles had formed. Acknowledgements This work was financially supported by a grant (M2009010025) from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy (MKE), Republic of Korea. This work was also supported by Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean government (MEST) (grant code: R2009-007711) and a grant from Construction Technology Innovation Program (CTIP) funded by Ministry of Land, Transportation and Maritime Affairs (MLTM) of Korean government. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.supflu.2010.07.001. References [1] J.M. DeSimone, Practical approaches to green solvents, Science 297 (2002) 799–803.
B.G. Kim et al. / J. of Supercritical Fluids 55 (2010) 381–385 [2] K.P. Johnston, Block copolymer as stabilizers in supercritical fluids, Current Opinion in Colloids & Interface Science 5 (2000) 351–356. [3] C.F. Kirby, M.A. McHugh, Phase behavior of polymers in supercritical fluid solvents, Chemical Review 99 (2) (1999) 565–602. [4] J.M. DeSimone, E.E. Maury, Y.Z. Menceloglu, J.B. McClain, T.J. Romack, J.R. Combes, Dispersion polymerization in supercritical carbon dioxide, Science 265 (1994) 356–359. [5] J.L. Kendall, D.A. Canelas, J.L. Young, J.M. DeSimone, Polymerizations in supercritical carbon dioxide, Chemical Review 99 (2) (1999) 543–564. [6] H. Shiho, J.M. DeSimone, Dispersion polymerization of acrylonitrile in supercritical carbon dioxide, Macromolecules 33 (2000) 1565–1569. [7] M.R. Giles, R.M.T. Griffiths, A. Aguiar-Ricardo, M.M.C.G. Silva, S.M. Howdle, Fluorinated graft stabilizers for polymerization in supercritical carbon dioxide: the effect of stabilizer architecture, Macromolecules 34 (2001) 20–25. [8] H. Shiho, J.M. DeSimone, Dispersion polymerization of glycidyl methacrylate in supercritical carbon dioxide, Macromolecules 34 (2001) 1198–1203. [9] D.A. Canelas, D.E. Betts, J.M. DeSimone, Dispersion polymerization of styrene in supercritical carbon dioxide: importance of effective surfactants, Macromolecules 29 (1996) 2818–2821. [10] D.A. Canelas, J.M. DeSimone, Dispersion polymerizations of styrene in carbon dioxide stabilized with poly(styrene-b-dimethylsiloxane), Macromolecules 30 (1997) 5673–5682. [11] H. Shiho, J.M. DeSimone, Preparation of micron-size polystyrene particles in supercritical carbon dioxide, Journal of Polymer Science Part A: Polymer Chemistry 37 (1999) 2429–2437. [12] H. Shiho, J.M. DeSimone, Dispersion polymerization of 2-hydroxyethyl methacrylate in supercritical carbon dioxide, Journal of Polymer Science Part A: Polymer Chemistry 38 (2000) 3783–3790. [13] A.I. Cooper, W.P. Hems, A.B. Holmes, Synthesis of highly cross-linked polymers in supercritical carbon dioxide by heterogeneous polymerization, Macromolecules 32 (1999) 2156–2166. [14] D.A. Canelas, D.E. Betts, J.M. DeSimone, M.Z. Yates, K.P. Johnston, Poly(vinyl acetate) and poly(vinyl acetate-co-ethylene) latexes via dispersion polymerizations in carbon dioxide, Macromolecules 31 (1998) 6794–6805. [15] A.I. Cooper, Polymer synthesis and processing using supercritical carbon dioxide, Journal of Material Chemistry 10 (2000) 207–234. [16] J. Shin, W. Bae, B.G. Kim, J.-C. Lee, H.-S. Byun, H. Kim, Phase behavior of a ternary system of poly[p-perfluorooctyl-ethylene(oxy, thio, sulfonyl)methyl styrene] and poly[p-decyl(oxy, thio, sulfonyl)methyl styrene] in supercritical solvents, Journal of Supercritical Fluids 48 (2008) 1–7. [17] B.G. Kim, E.-H. Sohn, J.-S. Chung, S.-Y. Kwak, J.-C. Lee, Solubilization and polymer analogous reactions of polyepichlorohydrin in ionic liquids, Journal of Applied Polymer Science 114 (2009) 132–138.
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[18] B.G. Kim, E.-H. Sohn, K. Cho, J.-C. Lee, Semifluorinated side groups poly(oxyethylene) derivatives with extremely low surface energy: synthesis, characterization, and surface property, European Polymer Journal 44 (2008) 2912–2919. [19] J. Shin, W. Bae, Y.-W. Lee, H. Kim, Kinetics for the free radical solution polymerization of heptadecafluorodecyl (meth)acrylate in supercritical carbon dioxide, Korean Journal of Chemical Engineering 24 (4) (2007) 664–669. [20] B.G. Kim, J.-S. Chung, E.-H. Sohn, S.-Y. Kwak, J.-C. Lee, Comb-like fluorinated polystyrenes having different side chain interconnecting groups, Macromolecules 42 (2009) 3333–3339. [21] W. Bae, H. Shin, H. Kim, High pressure behavior of carbon dioxide + heptadecafluorodecyl acrylate and carbon dioxide + heptadecafluorodecyl methacrylate systems, Physical Chemistry Chemical Physics 6 (2004) 2295–2300. [22] J. Shin, Y.-W. Lee, H. Kim, W. Bae, High-pressure phase behavior of carbon dioxide + heptadecafluorodecyl acrylate + poly(heptadecafluorodecyl acrylate) system, Journal of Chemical Engineering Data 51 (2006) 1571–1575. [23] J. Shin, W. Bae, Y.-W. Lee, H. Kim, High-pressure phase behavior of carbon dioxide + heptadecafluoro-1-decanol system, Journal of Supercritical Fluids 44 (2008) 260–265. [24] M.R. Nelson, R.F. Borkman, Ab initio calculations on CO2 binding to carbonyl groups, Journal of Physical Chemistry A 102 (1998) 7860–7863. [25] I. Pasquali, J.-M. Andanson, S.G. Kazarian, R. Bettini, Measurement of CO2 sorption and PEG 1500 swelling by ATR-IR spectroscopy, Journal of Supercritical Fluids 45 (2008) 384–390. [26] T. Sarbu, T. Styranec, E.J. Beckman, Non-fluorous polymers with very high solubility in supercritical CO2 down to low pressures, Nature 405 (2000) 165– 168. [27] S. Kwon, K. Lee, H. Kim, Y.-W. Lee, W. Bae, Synthesis of biocompatible and biodegradable polymer particles in supercritical carbon dioxide, Colloid Polymer Science 286 (2008) 1181–1191. [28] H.M. Woods, C. Nouvel, P. Licence, D.J. Irvine, S.M. Howdle, Dispersion polymerization of methyl methacrylate in supercritical carbon dioxide: an investigation into stabilizer anchor group, Macromolecules 38 (8) (2005) 3271– 3282. [29] H. Shiho, J.M. DeSimone, Dispersion polymerization of acrylonitrile in supercritical carbon dioxide, Macromolecules 33 (5) (2000) 1565–1569. [30] K.L. Harrison, K.P. Johnston, I.C. Sanchez, Effect of surfactants on the interfacial tension between supercritical carbon dioxide and polyethylene glycol, Langmuir 12 (11) (1996) 2637–2644.
Contents lists available at ScienceDirect
The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Dispersion polymerization in supercritical carbon dioxide using comb-like fluorinated polymer surfactants having different backbone structures Byoung Gak Kim a,b , Jungin Shin a , Eun-Ho Sohn a , Jae-Seung Chung a , Won Bae c , Hwayong Kim a , Jong-Chan Lee a,∗ a b c
Department of Chemical and Biological Engineering, Seoul National University, 559 Gwanangno, Gwanak-gu, Seoul, 151-744, Republic of Korea Center for Fuel Cell Research, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Sungbuk-gu, Seoul, 136-791, Republic of Korea R&D Institute, Miwon Commercial Co., Ltd., 405-3, Moknae-Dong, Ansan-Si, Kyonggi, 425-100, Republic of Korea
a r t i c l e
i n f o
Article history: Received 26 January 2010 Received in revised form 1 July 2010 Accepted 1 July 2010 Keywords: Supercritical carbon dioxide Dispersion polymerization Comb-like fluorinated polymer Backbone structure
a b s t r a c t Comb-like fluorinated polymers with different backbone structures, poly(heptadecafluorodecyl acrylate) (PA-Rf ), poly[oxy[(2-perfluorooctylethylene)thiomethyl]ethylene] (PEO-Rf ), and poly[p[[(perfluorooctylethylene)thio]methyl]styrene] (PS-Rf ), were used as surfactants in dispersion polymerization to examine the effect of backbone structure on the formation of polymer particles. Dispersion polymerization of monomers with different polarities using these comb-like fluorinated polymer surfactants in CO2 showed that PEO-Rf containing a polar oxyethylene backbone was an effective surfactant for the dispersion polymerization of a polar monomer, such as N-vinyl-2-pyrrolidone, whereas PA-Rf was effective for less polar monomers, such as methyl methacrylate and N-vinyl caprolactam. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Supercritical carbon dioxide (sc CO2 ) has great potential as an alternative to common volatile organic solvents because it is inexpensive, environmentally benign, nontoxic, and has tunable properties. These properties render sc CO2 a useful medium compared to other organic solvents [1–3]. Over the past decade, there have been extensive studies into its use as a solvent for polymerization. Heterogeneous polymerization techniques, including dispersion polymerization, have employed CO2 as a solvent because it is poor solvent for most polymers but a good one for the monomers. Comb-like fluorinated polymers are useful surfactant systems for dispersion polymerization in sc CO2 [4,5]. For example, poly(1,1-dihydroperfluorooctyl acrylate) has been used to prepare poly(methyl methacrylate) particles through the dispersion polymerization of methyl methacrylate in sc CO2 [4,5]. Other polymer particles, such as poly(styrene), poly(divinylbenzene), poly(vinyl acetate), and poly(acrylonitrile) particles were also prepared from the corresponding monomers by dispersion polymerization in sc CO2 using comb-like poly(acrylate) derivatives as surfactants [4,6–14]. In these polymerization systems, the hydrocarbon backbone region of the surfactant plays a key role in anchoring the
∗ Corresponding author. Tel.: +82 2 880 7070. E-mail address: [email protected] (J.-C. Lee). 0896-8446/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2010.07.001
surface of the hydrocarbon polymeric particle. However, there are only a few reports on the effect of the backbone structure of the comb-like fluorinated polymer on the formation of polymer particles through dispersion polymerization in sc CO2 [15]. In this study, three different monomers with different polarities, such as methyl methacrylate, N-vinyl-2-pyrrolidone, and N-vinyl caprolactam, were polymerized in sc CO2 using three different comb-like fluorinated polymers, such as poly(heptadecafluorodecyl acrylate) (PA-Rf ), poly[oxy[(2-perfluorooctylethylene)thiomethyl]ethylene] (PEO-Rf ), and poly[p-[[(perfluorooctylethylene)thio]methyl]styrene] (PS-Rf ), as surfactants. As the chemical structure of the side groups of these comb-like fluorinated polymers is identical, the effect of the backbone structures on the formation of polymer particles with different polarities could be studied.
2. Experimental 2.1. Materials Carbon dioxide (min. 99.99%) was purchased from Korea Industrial Gases. 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-Heptadecafluorodecyl acrylate (min. 97%, CAS No. 27905-45-9) was obtained from Aldrich. PA-Rf was synthesized by the homogeneous radical solution polymerization of 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10heptadecafluorodecyl acrylate in sc CO2 using a polymerization apparatus (Fig. 1) described in the literature [16].
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the cell from a high-pressure generator. A magnetic stirring bar in the cell helped the mixture to reach equilibrium rapidly. Once the solution reached a single phase, pressure was decreased slowly until the cloud point appeared. This procedure was repeated several times until the fluctuations in the transition pressure were minimized to within ±0.03 MPa.
Fig. 1. Schematic diagram of the polymerization apparatus. (P = pressure gauge; T = temperature gauge; PR = pressure regulator)
(PA-Rf : Mn = 10,300, Mw = 20,300) PEO-Rf was synthesized by a reaction of polyepichlorohydrin and sodium 1H,1H,2H,2Hperfluorodecanthiolate in 1-butyl-3-methyl-imidazolium chloride, [bmim][Cl], (Mn = 16,600, Mw = 17,600) [17,18]. PS-Rf was prepared by free radical polymerization in toluene as reported earlier (Mn = 20,300, Mw = 25,100) [19,20]. Number and weight-average molecular weights of all fluorinated polymer surfactants used were determined using size-exclusion chromatography. 1,1,2Trifluorotrichloroethane and poly(dimethylsioxane)s were used as an eluent and a standard, respectively. Methyl methacrylate (MMA, Junsei Chemical, min. 99.5%), N-vinyl-2-pyrrolidone (NVP, Aldrich, min. 99%) and N-vinyl caprolactam (NVCL, Aldrich, min. 98%) were pretreated on a alumina column to remove the inhibitor, and dissolved oxygen was removed by nitrogen purging. 2,2 -Azobisisobutyronitrile (AIBN, Junsei Chemical, min. 98%) was recrystallized from methanol. 2.2. Apparatus and procedure 2.2.1. Phase behavior measurement The cloud point curves were obtained with a variable volume view cell apparatus (Fig. 2). The window of this apparatus was made of sapphire to observe the cloud point. This apparatus and procedure have been described in detail elsewhere [21–23]. The cloud point data of the comb-like fluorinated polymer + CO2 was obtained from a polymer solution at a fixed polymer concentration of 1.0 wt.% in CO2 using standard procedure. Initially, the comblike fluorinated polymer was loaded into the cell and the cell was purged carefully with an inert gas and CO2 . CO2 was then added to the cell using a high-pressure bomb. After injecting the comb-like fluorinated polymer + CO2 mixture, the solution was compressed to the desired operating pressure by replacing a piston fitted within
2.2.2. Dispersion polymerization Dispersion polymerization of MMA, NVP and NVCL was carried out in a 30 mL SUS 316 reactor, and the inner phase change was observed through an observation window. CO2 was supplied from a gas booster pump (Maxpro Technologies, Model DLE 751). A 300 mL reservoir between the pump and reactor was used to minimize fluctuations from the pump and maintain a stable feed. The pressure was measured using a Bourdon tube pressure gauge (WIKA, type 213.53.063, accuracy class 1.0). The temperature was measured using a K (CA) type thermocouple and indicator (Hanyoung Electronics Inc. model DX-7) (accuracy 0.05 K). A PTFE-coated magnetic stirring bar was used to agitate the reaction mixture. An amount of 2.0 g of the monomer (MMA, NVP and NVCL), 0.02 g AIBN (1.0 wt.% relative to the monomer) and 0.2 g comblike fluorinated polymer (PA-Rf , PEO-Rf and PS-Rf ) as the surfactant (10.0 wt.% relative to the monomer) were placed in a 30 mL reactor. The reactor was then purged several times with CO2 to remove air and charged with a known amount of CO2 at room temperature. Polymerization was performed for 24 h at 343.2 ± 0.5 K and 30 ± 0.5 MPa. After polymerization, the reactor was cooled to below 283 K. Vapor/liquid phase separation occurred, and CO2 was vented slowly from the vapor phase through two glass traps. Glass traps filled with methanol and cooled with ice water were used to prevent discharge of the unreacted monomer to the atmosphere during CO2 venting. Finally, the resulting polymer was precipitated and washed in methanol to remove the unreacted monomer. The particle morphology and size were characterized by Field Emission Scanning Electron Microscope (FE-SEM) (Joel5410LV). The number-average particle size and particle size distribution (PSD) were measured using an image analyzer, TDI Scope EyeTM version 3.1 with SEM images. The number-average (Dn ) and weightaverage (Dw ) particle diameters were calculated from the following equations.
N Dn =
i=1
di
N
(1)
N Dw =
d4 i=1 i d3 i=1 i
N
(2)
where di is the diameter of particle i, and N is the total number of particles measured in the SEM images. The polydispersity index (PDI), which indicates the particle size distribution (PSD), was defined as Dw /Dn . 3. Results and discussion 3.1. Chemical structure of comb-like fluorinated polymers and phase behavior of comb-like fluorinated polymer + CO2 binary system
Fig. 2. Schematic diagram of the variable volume view cell apparatus. (1) Camera; (2) light source; (3) borescope; (4) fast response PRT; (5) view cell; (6) magnetic stirrer; (7) air bath; (8) digital thermometer; (9) digital pressure transducer; (10) pressure gauge; (11) hand pump; (12) computer monitor; (13) trap.
Fig. 3 shows the chemical structure of the three comb-like fluorinated polymers used in this study. All the polymers had the same CO2 -philic side groups, Rf (–(CF2 )7 CF3 ), whereas they had different backbone structures, such as acrylate, ethylene oxide, and styrene for PA-Rf , PEO-Rf , and PS-Rf , respectively. PA-Rf and PS-Rf were prepared by free radical polymerization from the corresponding fluorinated monomers and PEO-Rf was obtained from a polymer analogous reaction of polyepichlorohydrin with sodium
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383
Table 1 Particle size and particle size distribution of PMMAa , PVPb and PVCL.c . Surfactantd
Sample
Dn (m)e
PSDf
Morphology
g
PMMA PVCL PVP
11.28 0.74 NA
1.22 1.04 NA
Spherical (discrete) Spherical (discrete) Irregular (aggregate)
PEO-Rf h
PMMA PVCL PVP
NA 0.62 0.92
NA 1.16 1.11
Irregular (aggregate) Spherical (aggregate) Spherical (discrete)
PS-Rf i
PMMA PVCL PVP
0.71 0.76 0.81
1.14 1.41 1.07
Spherical (aggregate) Spherical (aggregate) Spherical (aggregate)
PA-Rf
a
Fig. 3. Chemical structure of comb-like fluorinated polymer surfactants.
Poly(methyl methacrylate). Poly(N-vinyl-2-pyrrolidone). Poly(N-vinyl caprolactam). d Reaction conditions: 2.0 g of monomer, 1.0 wt.% of AIBN, 10.0 wt.% of surfactant, 343.2 ± 0.5 K and 24 h with stirring. e Determined by FE-SEM image. f Particle size distribution (= Dw /Dn ). g Poly(heptadecafluorodecyl acrylate). h Poly[oxy[(2-perfluorooctylethylene)thiomethyl]ethylene]. i Poly[p-[[(perfluorooctylethylene)thio]methyl]styrene]. b c
1H,1H,2H,2H-perfluorodecanthiolate. The weight-average molecular weights (Mw ) of polymers ranged from 18,000 to 25,000 and the polydispersity indices of polymers were <2.0. Although it is well known that the molecular weight of the polymer affects the polymer solubility in sc CO2 , these differences in molecular weight can be expected to have only a small effect on the polymer solubility in sc CO2 [3]. A typical lower critical solution temperature (LCST) phase behavior was observed from the cloud point curves of the comb-like fluorinated polymer + CO2 binary systems (Fig. 4). The cloud point pressures of PA-Rf and PEO-Rf in CO2 were similar, whereas that of the PS-Rf + CO2 system was approximately 4 MPa larger than those of the other comb-like fluorinated polymers + CO2 systems. Since all the polymers have the same side chain groups, this different solubility behavior can be attributed to their different backbone structures. The higher solubility of PA-Rf and PEO-Rf compared with that of PS-Rf can be explained by the intermolecular interactions between CO2 and the polymer chains in PA-Rf and PEO-Rf . For example, the specific interactions between the carbonyl oxygen in the ester group (PA-Rf has the ester groups) and CO2 was reported by Nelson and Borkman [24], and the intermolecular interaction between the oxide groups (PEO-Rf have oxyethylene backbone) and CO2 has been reported by others [25,26]. The cloud point pressure of PA-Rf appears to be slightly smaller than that of PEO-Rf . It is possible that the interactions between the carbonyl oxygen in the ester group are larger than those between the oxide groups and CO2 , or that there are slight differences in the molecular weights and molecular weight distributions. PS-Rf , which has a styrene backbone, does not contain any CO2 -philic groups, such as carbonyl or oxide groups, which in turn might give PS-Rf a higher cloud point
Fig. 4. Cloud point curves for comb-like fluorinated polymer + CO2 system.
than the other polymers in this study. Nevertheless, all the polymers in this study were quite soluble in CO2 and could be used in dispersion polymerization reactions in sc CO2 (343.2 K and 30 MPa). 3.2. Dispersion polymerization of MMA, NVP and NVCL in sc CO2 The three comb-like fluorinated polymers shown in Fig. 3 were used as a surfactant in dispersion polymerization in sc CO2 . The hydrocarbon backbone can anchor the surface of the hydrocarbon polymeric particle and the long fluorinated alkyl groups soluble in the sc CO2 phase can prevent the aggregation of polymer particles. Since the three polymers have an identical side chain, the difference in anchoring ability of their backbone regions is believed to affect the resulting polymer particles in the dispersion polymerization of MMA, NVCL, and NVP. Fig. 5 shows scanning electron micrographs of PMMA (poly(methyl methacrylate)), PVP (poly(N-vinyl-2-pyrrolidone)), and PVCL (poly(N-vinyl caprolactam)) particles. Table 1 lists the particle size, particle size distribution and morphology. An attempt was made to polymerize these monomers in CO2 without a fluorinated polymer surfactant for comparison but only aggregated non-spherical polymer particles were obtained. This means that our comb-like fluorinated polymers are quite effective surfactants in dispersion polymerization. Others have reported that discrete spherical polymer particles with a narrow size distribution were obtained for most monomers when poly(heptadecafluorodecyl methacrylate) having methyl groups in the backbone region was used as a surfactant for dispersion polymerization in sc CO2 [4,27]. Methyl groups in the backbone of comb-like fluorinated polymer show better anchoring ability than ester, acetate, and hydroxyl group in the fluorinated polymer [27,28]. It is because primary mechanism by which comb-like fluorinated polymer are effective is through either chemical grafting or physical adsorption to the surface of the growing polymer colloid (steric stabilization). Poly(heptadecafluorodecyl methacrylate) can be used for dispersion polymerization of various monomers ranging from polar monomers to non-polar monomers due to this steric stabilization effect of methyl groups in the backbone region [29,30]. When PA-Rf was used, however, dispersed particles were obtained from the polymerization of MMA and NVCL, as shown in Fig. 5(a-1) and (a-2), whereas irregular aggregated particles were obtained from the polymerization of polar NVP monomer, as shown in Fig. 5(a-3). It means that physical adsorption could be not effective due to the little tendency of fluorinated acry-
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Fig. 5. SEM images of (a) PMMA (poly(methyl methacrylate)); (b) PVP (poly(N-vinyl-2-pyrrolidone)); (c) PVCL (poly(N-vinyl caprolactam)) particle polymerized using (1) PA-Rf (poly(heptadecafluorodecyl acrylate)); (2) PEO-Rf (poly[oxy[(2-perfluorooctylethylene)thiomethyl]ethylene]); (3) PS-Rf (poly[p[[(perfluorooctylethylene)thio]methyl]styrene]) as a surfactant (magnification; (a-1) (b-1) × 1000, (a-2,3) (b-2,3) (c-1,2,3) × 5000).
late polymers to adsorb to the very polar surface. On the contrary, in the dispersion polymerization using PEO-Rf with a more polar and basic backbone, uniform dispersed particles were obtained from polar monomer NVP as shown in Fig. 5(b-2) and (b-3), whereas nonuniform particles with irregular shapes were obtained from the less polar monomer, MMA, as shown in Fig. 5(b-1). This indicates that polar-polar or non-polar-non-polar interaction is predominant when the steric stabilization is ineffective. Mostly aggregated particles were obtained in dispersion polymerization using PS-Rf with non-polar groups in the backbone region, even though most of the particles appear to be quite uniform, as shown in Fig. 5(c-1)–(c-3). It is possible that the less polar styrene backbone, that does not contain any polar and/or basic functional groups, has weaker anchoring ability that might cause the aggregation. The polymerization condition (at 343.2 ± 0.5 K and 30 ± 0.5 MPa, for 24 h) using PS-Rf might not be the optimum condition for the preparation of polymer particles. However, the same polymerization conditions were used in all the cases to allow a direct comparison, even though more uniform dispersed particles might be obtained using PS-Rf under different conditions. The results in Fig. 5, which were obtained when all the comb-like fluorinated polymers were soluble in sc CO2 , clearly show the effect of the backbone structures of the comb-like fluorinated polymers on the formation of polymer particles. PEO-Rf with a more polar backbone can only disperse polar polymer particles (PVP), whereas PA-Rf can disperse less polar polymer particles (PMMA and PVCL). 4. Conclusion poly[oxy[(2Poly(heptadecafluorodecylacrylate) (PA-Rf ), perfluorooctylethylene)thiomethyl]ethylene] (PEO-Rf ), and poly[p-[[(perfluorooctylethylene)thio]methyl]styrene] (PS-Rf ) with a CO2 -philic tail (–(CF2 )7 CF3 ) and different backbone struc-
ture were used as the surfactant for the dispersion polymerization of the monomer with different polarities, such as methyl methacrylate, N-vinyl-2-pyrrolidone, and N-vinyl caprolactam. The polarity of the polymer backbone determines the dispersion ability of the polymer surfactants. When PEO-Rf was used, discrete particles were obtained from a polar N-vinyl-2-pyrrolidone, whereas PA-Rf produces such discrete particles from less polar monomers, such as methyl methacrylate and N-vinyl caprolactam. Mostly aggregated particles were obtained in the case of PS-Rf with non-polar groups in the backbone region, even though spherical particles had formed. Acknowledgements This work was financially supported by a grant (M2009010025) from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy (MKE), Republic of Korea. This work was also supported by Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean government (MEST) (grant code: R2009-007711) and a grant from Construction Technology Innovation Program (CTIP) funded by Ministry of Land, Transportation and Maritime Affairs (MLTM) of Korean government. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.supflu.2010.07.001. References [1] J.M. DeSimone, Practical approaches to green solvents, Science 297 (2002) 799–803.
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