5680
Ind. Eng. Chem. Res. 2008, 47, 5680–5685
Dispersion Polymerization of Methyl Methacrylate using Poly(HDFDMA-co-MMA) as a Surfactant in Supercritical Carbon Dioxide Jungin Shin,† Kyung Shil Oh,† Won Bae,‡ Youn-Woo Lee,† and Hwayong Kim*,† School of Chemical and Biological Engineering and Institute of Chemical Processes, Seoul National UniVersity, San 56-1, Shilim-dong, Gwanak-Gu, Seoul 151-744, Korea, and R&D Institute, Miwon Commercial Co., Ltd., 405-3, Moknae-Dong, Ansan-Si, Kyonggi 425-100, Korea
New surfactants, random copolymers of heptadecafluorodecyl methacrylate (HDFDMA) and methyl methacrylate (MMA), were synthesized at 343.2 K and 30 MPa in supercritical carbon dioxide (scCO2) with varying HDFDMA/MMA ratios. 1H NMR and FT-IR were used to confirm the structure and to determine the composition of the resulting copolymers. The phase behavior of poly(HDFDMA-co-MMA) in CO2 was obtained by means of a variable volume view cell, which will be used to select the polymerization condition. After that, dispersion polymerization of methyl methacrylate (MMA) in scCO2 was performed using 2,2′azobis(isobutyronitrile) (AIBN) as the initiator and poly(HDFDMA-co-MMA) with various MMA mole fraction as a surfactant. The new surfactant, poly(HDFDMA-co-MMA), whose MMA content is lower than 12.3 wt %, shows comparable capacity to produce PMMA particles compared to poly(HDFDMA) surfactant. The resulting PMMA particles, such as those produced using poly(HDFDMA) homopolymer as a surfactant, could also be produced with poly(HDFDMA-co-MMA) up to 12.3 wt % MMA. 1. Introduction Supercritical carbon dioxide (scCO2) has been used in polymer fields for heterogeneous or homogeneous polymerization, polymer fractionation, hazardous additive extraction from polymer, functional ingredient impregnation, polymer dyeing, microcellular polymer manufacturing, polymer coating, etc.1 CO2 in polymer processes has recently been a particular focus of research and development in both academia and industry since it has relatively mild critical conditions (Tc ) 304.25 K; Pc ) 7.38 MPa) and it is the second least expensive solvent after water.2 Amorphous fluoropolymers and silicones are highly soluble in CO2. To take advantage of the useful properties of CO2 as a polymerization medium, CO2-philic molecules have been generated. Pioneering research using supercritical fluid as the polymerization medium was started by Desimone et al.,3 who performed homogeneous free radical polymerization of 1,1dihydroperfluorooctyl acrylate using scCO2 as the polymerization medium. They also reported heterogeneous dispersion polymerization of poly(methyl methacrylate) in scCO2 using PFOA [poly(1,1-dihydroperfluorooctyl acrylate)] as a surfactant.4 Thereafter, several kinds of polymeric or oligomeric materials that could be used as surfactants to stabilize particles in scCO2 were studied. Block or graft copolymers that have both CO2philic parts and CO2-phobic parts in a polymeric molecule have been used as surfactants.5 An ambidextrous surfactant that can stabilize in both CO2 and water phases with different mechanisms has also been proposed.6 Furthermore, poly(dimethylsiloxane) monomethacrylate7 and carboxylic acid-terminated perfluoroether8 have been used as surfactants. Various semifluorinated block and random copolymers have also been synthesized as surfactants.9–12 * To whom correspondence should be addressed. Tel: +82-2-8807406. Fax: +82-2-888-6695. E-mail:
[email protected]. † Seoul National University. ‡ Miwon Commercial Co., Ltd.
Fluoroacrylate and fluoroether polymers are highly soluble in CO2, but they are very expensive. Therefore, the design and development of a CO2-philic, cost-effective surfactant is very important. In this work, to minimize the amount of expensive perfluoroalkyl acrylate monomer used and to improve the polymerphilic properties, we synthesized random copolymers of heptadecafluorodecylmethacrylate(HDFDMA)andmethylmethacrylate (MMA) in scCO2 with varying HDFDMA/MMA ratios via solution polymerization. 1H NMR and FT-IR were used to confirm the structure and to determine the composition of the resulting copolymers. Phase behavior in a CO2 system was measured using a variable volume cell to determine the solubility at polymerization conditions. After that, dispersion polymerization of methyl methacrylate (MMA) in scCO2 was performed using AIBN as the initiator and poly(HDFDMA-co-MMA) as the surfactant. The resulting PMMA particles were compared with previous results from our group for PMMA particles that were polymerized using poly(HDFDMA) homopolymer as the surfactant. 2. Experimental Section 2.1. Materials. Methyl methacrylate (MMA, Junsei Chemical, minimum 99.5%) and 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10heptadecafluorodecyl methacrylate (HDFDMA, Aldrich, minimum 97%) were pretreated on a alumina column to remove inhibitor (MEHQ, methyl ether of hydroquinone), and dissolved oxygen was removed through nitrogen purging. 2,2′-Azobis(isobutyronitrile) (AIBN, Junsei Chemical, minimum 98%) was recrystallized from methanol. 2.2. Preparation of poly(HDFDMA-co-MMA) in Supercritical CO2. Free-radical solution copolymerization of HDFDMA and MMA was carried out in a 30 mL SUS 316 reactor with two windows on both sides (Figure 1). CO2 was supplied from a gas booster pump (Maxpro Technologies, model DLE 75-1). We used a 300 mL reservoir between the pump and reactor to minimize fluctuation from the pump and to maintain a stable feed. Pressure was measured with a pressure transducer (Data
10.1021/ie070995v CCC: $40.75 2008 American Chemical Society Published on Web 07/01/2008
Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5681
Figure 1. Schematic diagram of the polymerization apparatus (P ) pressure gauge; T ) temperature gauge; PR ) pressure regulator). Table 1. Solution Polymerization of Poly(HDFDMA-co-MMA) in scCO2 sample F115 FM1 FM2 FM3 FM4 FM5 FM6 FM7 a
MMA (wt %)a
AIBN (wt %)a
T (K)
P (MPa)
0.0 1.8 5.0 7.7 12.3 18.7 25.4 28.9
1.0
343.2 ( 0.5
0 ( 0.5
phase
homogeneous
heterogeneous
Weight percent of MMA to monomers (4 g; HDFDMA and MMA).
Figure 3. Monomer-copolymer composition curve in the copolymerization of HDFDMA with MMA (f1: mol % of MMA in the comonomers; F1: mol % of MMA in the copolymer).
Figure 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.
Instruments, model AB/HP, accuracy (0.25%) and indicator (Laurel Electronics, L20010WM1). Temperature was measured with a K(CA)-type thermocouple (accuracy (0.05 K) and indicator (Hanyoung Electronics, model DX-7). A PTFE-coated magnetic stirring bar was used to agitate the reaction mixture. 4 g of monomer with varying HDFDMA/MMA ratios and about 0.04 g of AIBN (1.0 wt % relative to the total monomer) were introduced to the reactor. The reactor was then purged with CO2 several times to remove air and charged with a known amount of CO2 at room temperature. Polymerization was performed at 343.2 ( 0.5 K and 30 ( 0.5 MPa for 24 h. Table 1 shows the free-radical solution random copolymerization conditions of HDFDMA and MMA. After polymerization was complete, the reactor was cooled to below 283 K. Vapor/liquid phase separation occurred, and CO2 was slowly vented from the vapor phase through two glass traps. To prevent discharge of unreacted monomer to the
atmosphere during CO2 venting, the glass traps were filled with methanol and cooled with ice water. Finally, the resulting polymer was precipitated and washed in methanol to remove unreacted monomer. 2.3. Phase Behavior of CO2 + Poly(HDFDMA-co-MMA) System. Figure 2 shows a schematic diagram of a typical variable volume view cell apparatus used to obtain phase behavior data at high pressure. The experimental apparatus has been described in detail elsewhere.13,14 Cloud point data for CO2 + poly(HDFDMA-co-MMA) were obtained by following proper procedures and measured at a fixed polymer concentration of 5.0 ( 0.5 wt %, which is the typical concentration used for polymer + supercritical solvent studies.3 First, poly(HDFDMA-co-MMA) was loaded into the cell, and the cell was carefully purged with carbon dioxide. CO2 was added to the cell using a high-pressure bomb. After injection of the CO2 + poly(HDFDMA-co-MMA) mixture was complete, the solution was compressed to a single phase by a piston fitted within the cell using water pressed with a high-pressure generator (High Pressure Equipment Co., model 62-2-10). A magnetic stirring bar in the cell helped the mixture reach equilibrium rapidly. The pressure of the solution was adjusted while measuring the pressure of the water with a digital pressure transducer (Paroscientific, model 43KR-HHT-101, accurate to 0.01% of reading) and pressure indicator (Paroscientific, model 730). The temperature was measured with a PRT-type thermometer (Hart Scientific, model 5622-32SR, accuracy of (0.045 K) fixed to the surface of the cell and displayed by an indicator (Hart Scientific, model 1502). The temperature of the cell was
5682 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 Table 2. Dispersion Polymerization of PMMA in scCO2a sample
surfactant [MMA (wt %)b]
MPSc (µm)
Mw d
PDIe
PSDf
P1 PC1 PC2 PC3 PC4 PC5 PC6 PC7
F115 (0.0) FM1 (1.8) FM2 (5.0) FM3 (7.7) FM4 (12.3) FM5 (18.7) FM6 (25.4) FM7 (28.9)
6.35 6.21 6.24 7.78 7.16 3.84 NA NA
94 000 73 000 71 000 74 000 83 000 61 000 51 000 45 000
3.23 1.95 2.05 2.12 2.15 2.24 2.27 2.22
1.19 1.21 1.11 1.10 1.12 1.72 NA NA
a Reaction conditions: 2.0 g of MMA, 1.0 wt % of AIBN, 10.0 wt % of surfactant, 343.2 ( 0.5 K and 30 ( 0.5 MPa and 24 h with stirring. b Weight percent of MMA to monomer (4 g; HDFDMA and MMA). c Mean particle size was determined by FE-SEM. d Determined by GPC. e Polydispersity index. f Particle size distribution.
Table 3. Effect of Surfactant Concentration on the Dispersion Polymerization of PMMA in scCO2a sample PC8 PC9 PC10
surfactant (FM4) (wt %)b
MPS (µm)
Mw
PDI
PSD
5.0 10.0 15.0
9.75 7.16 7.10
60 000 83 000 75 000
2.11 2.15 1.99
1.13 1.12 1.12
a Reaction conditions: 2.0 g of MMA, 1.0 wt % of AIBN, 343.2 ( 0.5 K and 30 ( 0.5 MPa and 24 h with stirring. b Weight percent of surfactant to monomer.
Figure 4. 1H NMR spectrum of poly(HDFDMA-co-MMA) with various MMA ratios.
2.4. Dispersion Polymerization of MMA in scCO2. We used the same experimental apparatus and equipment as in our previous work.15 For dispersion polymerization, 2.0 g of the monomer, about 0.02 g of AIBN (1.0 wt % relative to the monomer), and 0.2 g of poly(HDFDMA-co-MMA) as the surfactant (10.0 wt % relative to the monomer) were placed in a 30 mL reactor. Polymerization was performed at 343.2 ( 0.5 K and 30 ( 0.5 MPa for 24 h. The remainder of the experimental procedure is the same as when solution polymerization was performed. 2.5. Polymer Characterization. Comonomer ratios incorporated in the copolymer were determined by 1H NMR spectroscopy (Bruker, 300 MHz, 3:2 mixture of CDCl3 and CFC113 as a solvent). Particle morphology and size were characterized by FE-SEM (Joel5410LV). The number-average particle size and the particle size distribution (PSD) were measured with an image analyzer (TDI Scope Eye ver 3.1) with SEM images and determined by measuring the diameter of 100 particles.16 Number-average (Dn) and weight-average (Dw) particle diameters were calculated from the following equations:4 N
∑d
i
Dn )
i)1
(1)
N N
∑d
4
∑d
3
i
Dw ) Figure 5. Comparison of cloud point curves for CO2 + poly(HDFDMAco-MMA) system and CO2 + poly(HDFDMA) system.
maintained and measured to within (0.1 K. Once the solution reached single phase, the pressure was slowly reduced until the cloud point appeared.13,14 This procedure was repeated several times until the fluctuation of transition pressure was minimized. The status inside the cell was projected onto a monitor using a camera (Veltek International, model CVC5520) with a borescope (Olympus Corp., model R100-038-000-50).
i)1 N
(2)
i
i)1
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) that indicates the particle size distribution (PSD) is defined as Dw/Dn. 3. Results and Discussion 3.1. Preparation of Poly(HDFDMA-co-MMA) in Supercritical CO2. The copolymeric surfactants were synthesized by free-radical solution polymerization of HDFDMA and MMA
Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5683
Figure 6. SEM images of PMMA particles polymerized with different surfactants, 10.0 wt % to MMA: (a) F115, (b) FM1, (c) FM2, (d) FM3, (e) FM4, (f) FM5, (g) FM 6, and (h) FM7.
in scCO2 at 343.2 K and 30 MPa such as polymerization condition in previous study.14,17 As seen in Table 1, polymerization medium is homogeneous phase until the MMA ratio reaches 18.7 wt %. 1H NMR spectra of the copolymers were recorded, and the proportion of HDFDMA in the copolymers
was calculated. Figures 3 and 4 represent the results of 1H NMR measurements. The HDFDMA in the resulting copolymers is similar to the feed ratios of comonomers (Figure 3). Figure 4 shows 1H NMR spectra of poly(HDFDMA-co-MMA) polymerized as the MMA ratio changed before removal of monomer.
5684 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008
Figure 7. SEM images of PMMA particles polymerized with FM4 (a) 5.0, (b)10.0, and (c) 15.0 wt %.
Peak 4 represents CH2 of poly(HDFDMA), and peak 5 represents CH3 of PMMA; peak 4 decreased and peak 5 increased as the MMA ratio increased. 3.2. Phase Behavior of CO2 + Poly(HDFDMA-co-MMA) System. Phase behavior measurement plays an important role in determining experimental conditions for dispersion polymerization of PMMA in CO2. For dispersion polymerization in scCO2, monomer, initiator, and surfactant must be dissolved in scCO2 at given conditions (343.2 K and 30 MPa) when polymerization is beginning. Figure 5 shows the phase behavior of poly(HDFDMA-co-MMA) in CO2 using the variable volume view cell apparatus. The cloud point pressure of the copolymer is higher than that of the homopolymer, and in the case of poly(HDFDMA-co-MMA), the cloud point pressure increased as MMA ratio increased. Consequently, poly(HDFDMA-coMMA) can be used as a surfactant in CO2 at 343.2 K and 30 MPa because poly(HDFDMA-co-MMA) is soluble in CO2 under that conditions until the MMA ratio is 28.9 wt %. 3.3. Dispersion Polymerization of MMA in scCO2. Data for the dispersion polymerization of MMA in scCO2 are summarized in Tables 2 and 3. Figures 6 and 7 show scanning electron micrographs of the resulting PMMA particles. To investigate the effect of various HDFDMA/MMA ratios, PMMA was polymerized in scCO2 using the surfactants with eight different HDFDMA/MMA ratios, as shown in Table 2 and Figure 6. When the surfactant with MMA content of 12.3 wt % or lower are used in the polymerization, around 7 µm particles of PMMA with nearly homogeneous size distribution were produced. However, when the MMA content in the surfactant was 12.3 wt % or higher (entries PC1-PC7 in Table 2 and Figure 6f-h), the resulting polymers become broad in their size distributions. The weight-average molecular weight was obtained between 45 000 and 94 000. As the MMA content in the surfactants increased, the average molecular weight tends to decrease, except PC4.
As shown in Figure 7, dispersion polymerization of MMA was performed with various surfactant concentrations to examine the effect of the surfactant concentration on the morphology and the particle size. When the concentration of surfactant increased from 5.0 to 15 wt % to monomer, the average particle size decreased from 9.75 to 7.10 µm. It is also found that the morphology does not change with surfactant concentration in this study. 4. Conclusion In the order to find an economical surfactant which is working in scCO2, we modified poly(HDFDMA) by introducing MMA. The newly synthesized surfactants were tested in the dispersion polymerization of MMA in scCO2. It is found that the new surfactants, of which MMA content is 12.3 wt % or lower, can be used to produce ca. 7 µm spherical PMMA particles with a homogeneous size distribution. From these results, one can use the easily prepared new surfactants, which is cost-effective due to the low fluorine content, to other scCO2 processes. Acknowledgment This work was supported financially by the BK21 Project of the Ministry of Education, the National Research Laboratory (NRL) Program of Korea Institute of Science & Technology Evaluation and Planning, the Ministry of Commerce, Industry & Energy, and the Energy Management Corporation. Literature Cited (1) McHugh,M.A.;Krukonis,V.J.SupercriticalFluidExtractionsPrinciples of Practice, 2nd ed.; Butterworth-Heinemann: Oxford, 1994. (2) Kemmere, M. F.; Meyer, T. Supercritical Carbon Dioxide. In Polymer Reaction Engineering; Wiley-VCH: Weinheim, 2005. (3) Desimone, J. M.; Guan, Z.; Elsbernd, C. S. Synthesis of Fluoropolymers in Supercritical Carbon Dioxide. Science 1992, 257, 945.
Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5685 (4) Desimone, J. M.; Maury, E. E.; Menceloglu, Y. Z.; McClain, J. B.; Romack, T. J.; Combes, J. R. Dispersion Polymerizations in Supercritical Carbon Dioxide. Science 1994, 265, 356. (5) Lepilleur, C.; Beckman, E. J. Dispersion Polymerization of Methyl Methacrylate in Supercritical CO2. Macromolecules 1997, 30, 745. (6) Yates, M. Z.; Li, G.; Shim, J. J.; Maniar, S.; Johnston, K. P.; Li, K. P. Ambidextrous Surfactants for Water-Dispersible Polymer Powders from Dispersion Polymerization in Supercritical CO2. Macromolecules 1999, 32, 1019. (7) Gile, M. R.; Hay, J. N.; Howdle, S. M.; Winder, R. J. Macromonomer Surfactants for the Polymerization of Methyl Methacrylate in Supercritical CO2. Polymer 2000, 41, 6715. (8) Christian, P.; Howdle, S. M. Dispersion Polymerization of Methyl Methacrylate in Supercritical Carbon Dioxide with a Monofunctional Pseudo-Graft Stabilizer. Macromolecules 2000, 33, 237. (9) Lim, K. T.; Lee, M. Y.; Moon, M. J.; Lee, G. D.; Hong, S.-S.; Dickson, J. L.; Johnston, K. P. Synthesis and Properties of Semifluorinated Block Copolymers Containing Poly(ethylene oxide) and Poly(fluorooctyl methacrylates) via Atom Transfer Radical Polymerization. Polymer 2002, 43, 7043. (10) Hwang, H. S.; Kim, H. J.; Jeong, Y. T.; Gal, Y.-S.; Lim, K. T. Synthesis and Properties of Semifluorinated Copolymers of Oligo(ethylene glycol) Methacrylate and 1H,1H,2H,2H-perfluorooctylmethacrylate. Macromolecules 2004, 37, 9821. (11) Hwang, H. S.; Gal, Y.-S.; Johnston, K. P.; Lim, K. T. Dispersion Polymerization of Methyl Methacrylate in Supercritical Carbon Dioxide in the Presence of Random Copolymers. Macromol. Rapid Commun. 2006, 27, 121.
(12) Hwang, H. S.; Lee, W.-K.; Hong, S.-S.; Jin, S.-H.; Lim, K. T. Dispersion Polymerization of MMA in Supercritical CO2 in the Presence of Poly(poly(ethylene glycol) Methacrylate-co-1H,1H,2H,2H-perfluorooctylmethacrylate). J. Supercrit. Fluids 2007, 39, 409. (13) Bae, W.; Kwon, S.; Byun, H-S.; Kim, H. Phase Behavior of the Poly(vinyl pyrrolidone) + N-vinyl-2-pyrrolidone + Carbon Dioxide System. J. Supercrit. Fluids 2004, 30, 127. (14) Shin, J.; Bae, W.; Lee, Y.-W.; Kim, H. High Pressure Phase Behavior of Carbon Dioxide + Heptadecafluorodecyl Acrylate + Poly(heptadecafluorodecyl acrylate) System. J. Chem. Eng. Data 2006, 51 1571. (15) Oh, K. S.; Bae, W.; Kim, H. Dispersion Polymerization of N-vinylcarbazole using Siloxane-based and Fluorine-based Surfactants in Compressed Liquid Dimethyl Ether. Polymer 2007, 43, 1450. (16) Shin, B. H.; Bae, W.; Kim, H. Dispersion Polymerization of Crosslinked PMMA in Supercritical CO2. Stud. Surf. Sci. Catal. 2004, 153, 284. (17) Shin, J.; Bae, W.; Lee, Y.-W.; Kim, H. Kinetics for Free Radical Solution Polymerization of Heptadecafluorodecyl (meth)acrylate in Supercritical Carbon Dioxide. Korean J. Chem. Eng. 2007, 24, 664.
ReceiVed for reView July 23, 2007 ReVised manuscript receiVed May 19, 2008 Accepted May 20, 2008 IE070995V
