pubs.acs.org/Langmuir © 2009 American Chemical Society
Controllable Synthesis of New Polymerizable Macrosurfactants via CCTP and RAFT Techniques and Investigation of Their Performance in Emulsion Polymerization Li Chen, Lili Yan, Qing Li, Caifeng Wang, and Su Chen* State Key Laboratory of Material-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, P. R. China Received July 21, 2009. Revised Manuscript Received November 6, 2009 We reported herein the synthesis of poly(methacrylic acid)-b-poly(butyl acrylate) (PMAA-b-PBA) block copolymers (surfmers) and their performance as novel polymerizable macrosurfactants in emulsion polymerization. The surfmers bearing terminal unsaturated carbon-carbon double bonds were first successfully designed and sythesized via catalytic chain transfer polymerization (CCTP) and radical addition-fragmentation polymerization (RAFT) techniques. The structures of surfmers were characterized by Raman spectra, nuclear magnetic resonance (1H NMR), and gel permeation chromatography (GPC). The critical micelle concentration of surfmers was determined. Subsequently, the surfmers were used as emulsifier to prepare polyacrylate latexes (PA-surf). The influence of the surfmer concentration as well as PMAA and PBA chain segment ratios of surfmer on their performance in emulsion polymerization was discussed thoroughly. The particle size, amount of coagulum, and stability against electrolyte solutions of the latexes were evaluated. Also, the relations between monomer conversion in emulsion polymerization, polymerization rate, emulsion particle size, surface tension, and reaction time were investigated, which showed some interesting information for the probable mechanism underlying this emulsion polymerization system. Atomic force microscopy (AFM) and attenuated total reflection Fourier transform infrared spectra (ATR FT-IR) were performed to investigate the surface morphology and component distribution of the latex films. The results show high efficiency of these surfmers in emulsion polymerization, suggesting that the resultant PMAA-b-PBA block copolymers act not only as the emulsifier but also as the stabilizer of monomer droplets as well as the so-called comonomer.
Introduction Functional polymer latexes prepared by emulsion polymerization have received considerable attention owing to their extensive applications in many fields including drug delivery, coatings, adhesives and cell separation, etc.1-6 In emulsion polymerization, the surfactant plays a crucial and versatile role in achieving stable latex with controllable particle size.7-9 However, most of the currently used surfactants are low molecular weight compounds, which will be associated with the polymeric particles through adsorption including weak H or σ bonding interactions.10 These physically bound surfactants will be desorbed from the latex particles during the latex production and storage stage to cause destabilization. During the film formation process, surfactants will further migrate toward the film interfaces and then have deleterious effects on final film properties including water sensitivity, adhesion, gloss, and blocking.11 *To whom correspondence should be addressed: e-mail
[email protected]. cn; Fax þ86-25-83172258.
(1) Reb, P.; Margarit-Puri, K.; Klapper, M.; M€ullen, K. Macromolecules 2000, 33, 7718–7723. (2) Chen, L.; Chen, S. Prog. Org. Coat. 2004, 49, 252–258. (3) Chen, S.; Chen, L. Colloid Polym. Sci. 2003, 282, 14–20. (4) Lee, C. H.; Chien, A. T.; Yen, H.; Lin, K. F. J. Polym. Res. 2008, 15, 331–336. (5) Korth, B. D.; Keng, P.; Shim, I.; Bowles, S. E.; Tang, C.; Kowalewski, T.; Nebesny, K. W.; Pyun, J. J. Am. Chem. Soc. 2006, 128, 6562–6563. (6) Missirlis, D.; Nicola, T.; Hubbell, J. A. Langmuir 2005, 21, 2605–2613. (7) Matahwa, H.; Mcleary, J. B.; Sanderson, R. D. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 427–442. (8) Prasath, R. A.; Ramakrishnan, S. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 3257–3267. (9) Cochin, D.; Laschewsky, A.; Nallet, F. Macromolecules 1997, 30, 2278–2287. (10) Asua, J. M.; Schoonbrood, H. A. S. Acta Polym. 1998, 49, 671–686. (11) Aramendia, E.; Mallegol, J.; Jeynes, C.; Barandiaran, M. J.; Keddie, J. L.; Asua, J. M. Langmuir 2003, 19, 3212–3221.
1724 DOI: 10.1021/la9037809
There are two solutions12 widely applied to reduce the undesirable effects of surfactant migration presently. One approach is the use of reactive surfactants that can connect covalently to the polymer material, preventing their desorption from the latex particle surface or migration onto the polymer film. These reactive surfactants could be a combination of a surfactant and an initiator,13,14 a combination of a surfactant with a transfer agent,15 or a combination of a surfactant and a monomer (polymerizable surfactant).16,17 The use of reactive surfactants has been shown to improve the water resistance and surface adhesion in comparison with conventional emulsifiers.18 The main limitation on this kind of polymerizable surfactant is related to the reactivity of the polymerizable unit to avoid the surfactant getting buried in the bulk of the polymeric particle. Toward the end of the reaction, high polymerizable surfactant incorporation should be achieved to avoid the presence of unreacted species in the final polymer eventually migrating through the film during the film formation.19 The other promising way involves the use of polymeric surfactants, i.e., macrosurfactants, which are more effective than the conventional low molecular weight ones. (12) Unzue, M. J.; Schoonbrood, H. A. S.; Asua, J. M.; Go~ni, A. M.; David, C.; Sherrington, K. S.; Goebel, K. H.; Tauer, K.; Sj€oberg, M.; Holmberg, K. J. Appl. Polym. Sci. 1997, 66, 1803–1820. (13) Taued, K.; Kosmella, S. Polym. Int. 1993, 30, 253–258. (14) Kusters, J. M. H.; Napper, D. H.; Gilbert, R. G.; German, A. L. Macromolecules 1992, 25, 7043–7050. (15) Vidal, F.; Guillot, J.; Guyot, A. Polym. Adv. Technol. 1994, 6, 473–479. (16) Jin, L. Q.; Liu, Z. L.; Xu, Q. H.; Li, Y. C. J. Polym. Sci., Part A: Polym. Chem. 2006, 99, 1111–1116. (17) Guyot, A.; Tauer, K. Adv. Polym. Sci. 1994, 111, 43–65. (18) Morizur, J. F.; Irvine, D. J.; Rawlins, J. J.; Mathias, L. J. Macromolecules 2007, 40, 8938–8946. (19) Schoonbrood, H. A. S.; Asua, J. M. Macromolecules 1997, 30, 6034–6041.
Published on Web 11/24/2009
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Appropriate steric stabilization can be achieved by adsorbing neutral polymers on particle surface.20-24 For this purpose, block (A-B or A-B-A) and graft (BAn) surfactants (where B is the anchor chain and A is the stabilizing chain) are commonly used,25-29 which now can be available conveniently by living polymerization including anionic, cationic, group transfer polymerization (GTP), and atom transfer radical polymerization (ATRP).30-33 Even though those new living polymerization methods are promising, there still remain many practical problems, such as conversion, color, catalyst removal, or lowtemperature reaction circumstances. Over the past two decades, catalytic chain transfer polymerization (CCTP) and radical addition-fragmentation transfer (RAFT) polymerization techniques have been well established as powerful synthetic routes to obtain high-purity functionalized block polymers. Relative low molecular weight macromonomers with terminal double bond are readily available via CCTP in the presence of small amounts of catalytic chain transfer agent, Co(II) complex.34-37 These macromonomers can be employed further as chain transfer agent in the RAFT process to prepare block copolymer containing terminal unsaturated carbon-carbon double bonds.38-40 Herein, in view of the above consideration, we have made full use of the characteristics of CCTP and RAFT, designing and sythesizing structure-controllable PMAA-b-PBA block copolymers containing terminal unsaturated carbon-carbon double bonds as polymerizable macrosurfactants to stabilize the resultant latex in PMAA/PBA emulsion polymerization. These resultant surfactants possess advantages of both polymerizable surfactants and macrosurfactants: first, the steric effect of these higher macromolecular weight surfactants can dramatically improve the stability of latexes; second, the surfactants show good compatibility with polymer matrix because of their similar structures; additionally, these polymerizable sufactants via CCTP techniques containing unsaturated carbon-carbon double bonds in end groups can react with acrylic monomers with covalent bond, preventing the migration of the surfactants during the (20) Eastoe, J.; Summers, M. Chem. Mater. 2000, 12, 3533–3537. (21) Tadros, T. F. Polym. J. 1991, 23, 683–696. (22) Capek, I. Adv. Colloid Interface Sci. 2002, 99, 77–162. (23) Ortega-Vinuesa, J. L.; Martı´ n-Rodrı´ guez, A.; Hidalgo-Alvarez, R. J. Colloid Interface Sci. 1996, 184, 259–267. (24) Romero-Cano, M. S.; Martı´ n-Rodrı´ guez, A.; Chauveteau, G.; de las Nieves, F. J. J. Colloid Interface Sci. 1998, 198, 273–281. (25) Kukula, H.; Schlaad, H.; Tauer, K. Macromolecules 2002, 35, 2538–2544. (26) Bijsterbosch, H. D.; Cohen, S. M. A.; Fleer, G. J. J. Colloid Interface Sci. 1999, 210, 37–42. (27) Nestor, J.; Esquena, J.; Solans, C.; Levecke, B.; Booten, K.; Tadros, T. F. Langmuir 2005, 21, 4837–4841. (28) Tadros, T. Adv. Colloid Interface Sci. 2003, 104, 191–226. (29) Esquena, J.; Domı´ nguez, F. J.; Solans, C.; Levecke, B.; Booten, K.; Tadros, T. F. Langmuir 2003, 19, 10463–10467. (30) Pradel, J. L.; Boutevin, B.; Ameduri, B. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3293–3302. (31) Miura, Y.; Nakamura, N.; Taninguchi, I. Macromolecules 2001, 34, 447– 455. (32) Brouwer, H. D.; Schellekens, M. A. J.; Klumperman, B.; Monteiro, M. J.; German, A. L. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3596. (33) Freal-Sainson, S.; Save, M.; Bui, C.; Charleux, B.; Magnet, S. Macromolecules 2006, 39, 8632–8638. (34) Yang, S.; Li, Q.; Chen, L.; Chen, S. J. Mater. Chem. 2008, 18, 5599–5603. (35) Lu, Z.; Wang, J. Y.; Li, Q.; Chen, L.; Chen, S. Eur. Polym. J. 2008, 45, 1072– 1079. (36) Wang, C. F.; Cheng, Y. P.; Wang, J. Y.; Zhang, D.; Hou, L. R.; Chen, L.; Chen, S. Colloid Polym. Sci. 2009, 287, 829–837. (37) Gridnev, A. A.; Ittel, S. D. Chem. Rev. 2001, 101, 3611–3660. (38) Haddleton, D. M.; Maloney, D. R.; Suddaby, K. G.; Clarke, A. Polymer 1997, 38, 6207–6217. (39) Krstina, J.; Moad, G.; Rizzardo, E.; Winzor, C. L. Macromolecules 1995, 28, 5381–5385. (40) Kristina, J.; Moad, C. L.; Moad, G.; Rizzardo, E.; Berge, C. T.; Fryd, M. Makromol. Symp. 1996, 111, 13–23.
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Article
filming of latexes and resulting in high performance of end-use products. Even though lots of efforts have been devoted to the synthesis of polymeric or polymerizable surfactant, the preparation of polymerizable macrosurfactant still remains relatively unexplored. In addition, it is still not fully understood with respect to the mechanism of film formation and morphology of film-air interface. Therefore, the another objective of this work is to compare the distribution of a conventional surfactant with that of as-prepared polymerizable macrosurfactant prepared by CCTP and RAFT in dried latex film. Also described are the possible mechanism of emulsion polymerization stabilized by surfmers and film formation of latexes.
Experimental Section Materials. Methacrylic acid (MAA) and butyl acrylate (BA) were purchased from Aldrich, vacuum-distilled, and deoxygenated by a stream of high-purity nitrogen for at least 2 h prior to use for CCTP (they were not purified in emulsion polymerization). 2-Propanol, methyl methacrylate (MMA), and sodium hydrogen carbonate were purchased from Aldrich and used as received. Azobis(isobutryonitrile) (AIBN) and 2,2-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (VA-044) were recrystallized from ethanol twice and used as the initiator. CO 436 and CO 897 surfactant were provided by Jiangsu Richu Chemical Limited Co. The cobalt catalyst bis(aqua)bis((difluoroboryl)dimethylglyoximato)cobalt(II) (CoBF) was prepared according to the literature41 and further confirmed by elemental analyses (Anal. Calcd for C8H12N4O4B2F4Co 3 2H2O (%): C, 22.84; N, 13.32; H, 3.83. Found: C, 22.86; N, 13.40; H, 3.89). Synthesis of PMAA Macromonomer via CCTP. An amount of 150 g of deionized water, 0.3 g of azo-initiator VA-044, and 13 mg of CoBF dissolved in 2 mL of acetone were added to a 500 mL flask equipped with a magnetic stirrer under oxygen-free conditions by six freeze-pump-thaw cycles. The flask was consecutively evacuated and purged with nitrogen for six times and then heated at about 55 °C with continuous stirring. An amount of 74 g of MAA mixed with 7.5 mg of CoBF was added over about 1 h. The reaction was left at 55 °C for another hour and then stopped by ice-water immediately. The MAA macromonomer was isolated by vacuum evaporation of water. Synthesis of PMAA-b-PBA Block Copolymers (Surfmers) via RAFT. A solution of MAA macromonomer (10 g), BA monomer (6.9 g), and AIBN initiator (0.34 g) in 40 g of 2-propanol was transferred into a 250 mL flask equipped with a cold water condenser and a magnetic stirrer. Before sealing, the contents were carefully degassed by five freeze-pump-thaw cycles. The polymerization was carried out in a water bath at 80 °C for 5 h, and the reaction was stopped by immersing in an ice-water bath. The product was purified by vacuum evaporation of 2-propanol at room temperature. Emulsion Polymerization of MMA-BA-MAA. A stable monomer pre-emulsion was prepared by the addition of the monomers to the vigorously stirred aqueous solution of surfactant (traditional surfactants or sufmers), according to the recipe of Table 1. The high-speed agitator maintains a speed of 1000 rpm in 10 min. The initial reactor charge was heated to 65 °C under a nitrogen phase stream, and then the nitrogen stream was stopped and an aqueous solution of potassium persulfate was injected, followed by the addition of 10% of the monomer pre-emulsion. The mixture was allowed to react for about 30 min; when this charge had polymerized, reflux stopped and temperature normally reached 80 °C. Then, the remainder of the monomer preemulsion was gradually added over a period of 3 h at 80 °C. The initiator solution was added simultaneously via a separate line (41) Bakac, A.; Espenson, J. H. J. Am. Chem. Soc. 1984, 106, 5197–5202.
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Chen et al. Table 1. Recipes for Preparation of Polyacrylic Latexes (PA Latexes) PA-CO
initial charge water surfmer (60:40) surfmer (70:30) surfmer (50:50) CO-436 NaOH NaHCO3 monomer pre-emulsion water surfmer (60:40) surfmer (70:30) surfmer (50:50) CO-436 CO-897 MMA BA AA NMA initiator solution water potassium persulfate
20
PA-surf 1 20 0.5
PA-surf 2 20 1.0
PA-surf 3 20 1.5
PA-surf 4
PA-surf 5
20 0.5
20
PA-surf 6 20
1.5 0.2 0.3
0.15 0.5 0.3
0.1 1.0 0.3
0.05 1.5 0.3
30
30
30
30
0.6 1.2 20 30 1 2
0.45 0.9 20 30 1 2
0.3 0.6 20 30 1 2
0.15 0.3 20 30 1 2
10 0.165
10 0.165
10 0.165
10 0.165
0.05 1.5 0.3
0.5 0.3 30 1.5
1.5 0.05 1.5 0.3
30
30
20 30 1 2
0.15 0.3 20 30 1 2
0.15 0.3 20 30 1 2
10 0.165
10 0.165
10 0.165
over 15 min to improve monomer conversion, and then the reaction mixture was maintained for 1 h at 80 °C. Finally, the latex was cooled and filtered. Film Formation. The film surfaces were prepared by spincoater the corresponding solution onto clean glass substrates and dried at room temperature.
content and final conversion were calculated by the following formulas:44
Characterization. Characterization of PMAA Macromonomers and PMAA-b-PBA Block Copolymers. FT-Ra-
where W0 is the weight of the Petri dish and W1 and W2 are the total weight of latex and Petri dish before and after drying to a constant weight, respectively.
man spectroscopy was performed on a NXR FT-Raman module by sharing interferometer installed in the FT-IR bench. The Raman optics system is comprised of Nd:YVO4 laser operating at 1064 nm, sample holders, an InGaAs detector, and a CaF2 beam spliter. Spectra of fine powder samples pressed in a suitable sample holder were then collected with a laser power of 1.0 W, a mirror velocity of 0.3165 cm s-1, and 128 scans at a resolution of 8 cm-1. The nuclear magnetic resonance (NMR) spectra were observed in Bruker DAX 500 after dissolution of the samples in deuterated N,N-dimethylformamide (DMF). Molecular weight distributions were analyzed by gel permeation chromatography (GPC) using a Waters 1515 isocratic pump, a Waters 717 plus autosampler, a column set consisting of three Waters Styragel columns (7.8 300 mm) HR 4, HR 3, and HR 1, and a Waters 2414 differential refractive index detector. Tetrahydrofuran (TEDIA, HPLC grade)42 was used as eluent at 0.6 mL/min. Calibration of the GPC equipment was carried out43 with narrow polystyrene standards (Shodex Standard, peak molecular weights range 1200-538 000). Measurements of the surface tension of the as-prepared block copolymers aqueous solution were carried out on a Kruss DSA 100 tensiometer and recorded several times to ensure an internal reproducibility for each solution. Critical micellar concentrations were determined by looking for the discontinuity on the surfactant concentration vs surface tension curve. Latex Characterization. During the emulsion polymerization, samples were withdrawn from the reactor at regular sampling times (polymerization was stopped by the addition of hydroquinone) to analyze the solid content, the instantaneous conversion, particle size, and surface tension of the latex. Solid content and final conversion were measured by gravimetric analysis. A certain quantity of emulsion was cast into a Petri dish and dried to a constant weight in a dry oven at 75-85 °C. Solid (42) Saito, R.; Yamaguchi, K. Macromolecules 2003, 36, 9005–9013. (43) Okubo, M.; Sugihara, Y.; Kitayama, Y.; Kagawa, Y.; Minami, H. Macromolecules 2009, 42, 1979–1984.
1726 DOI: 10.1021/la9037809
solid content ðwt%Þ ¼
conversion ðwt%Þ ¼
W2 - W0 100% W1 - W0
½W3 solid content 100% -W4 100% W5
where W3 is the total weight of all the materials put in a flask in each polymerization, W4 is the weight of materials that cannot volatilize when drying, and W5 is the total weight of monomers. The amount of coagulum was measured by collecting coagulum on reactor wall and stirrer and by filtering the latex (mesh 63). It was presented as weight of coagulum per total weight of monomer added. The coagulation ratio was used to evaluate the polymerization stability: coagulation ratio ðwt%Þ ¼ ðWc =Wm Þ 100% where Wc is the weight of dried coagulate and Wm is the total weight of all the monomers. The larger the value of coagulation ratio, the worse is the polymerization stability. The particle sizes of emulsions were measured by using DLS (Zetasizer 3000HSA, Malvern Instruments). Scanning electron microscope (SEM) observation was obtained using a Hitachi S4800 scanning electron microscope. Transmission electron microscopic (TEM) observation was performed with a JEOL JEM2010 transmission electron microscope. The sample was placed on a copper grid that was left to dry before transferring into the TEM sample chamber. The latex stability against electrolytes was determined by adding 10 mL of 5 wt % calcium chloride solution to 10 mL of latex. After 24 h at room temperature, we examined them visually for coagulation or precipitation. The latex fails if coagulation takes place. Characterization of Latex Films Properties. Attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectra were collected using a Nicolet 6700 FT-IR single-beam spectrometer. Atomic force microscopy (AFM) topographies were obtained using a Digital Instruments PSI AutoCP-Research (44) Chen, Y. J.; Zhang, C. C.; Chen, X. X. Eur. Polym. J. 2006, 42, 694–701.
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Scheme 1. Schematic Representation of Synthesis of PMAA-b-PBA Block Copolymers and Their Application as Polymerizable Macrosufactants in MMA-BA-AA Emulsion Polymerization
Figure 1. Raman shift spectra of (a) PMAA prepared via free radical polymerization (solvent: H2O; reaction time: 5 h) and (b) via CCTP (solvent: H2O; reaction time: 1 h); (c) PMAA-b-PBA block copolymer prepared via CCTP and RAFT (solvent: 2-propanol; reaction time: 5 h). Inset: 1H NMR (300 MHz, DMF) of PMAA prepared via CCTP with the water resonance reduced by solvent suppression. p: vinyl protons in the product; m: vinyl protons in the monomer (solvent: DI water; reaction time: 1 h). AFM equipped with an E-scanner. Tapping mode silicon nitride cantilevers TESP with nominal spring constants of 20-100 N/m and nominal resonance frequencies of 200-400 kHz were employed. A piece of freshly cleaved mica (ca. 5 mm 5 mm) was used as a substrate of film preparation. To minimize possible contamination of the surface by ambient air, each sample was freshly prepared just before the AFM experiments. Water contact angle was measured using a contact angle meter type DSA 100 (Kruss, Germany).
Results and Discussion In this study, we first report the synthesis of PMAA-b-PBA block copolymers and their application as polymerizable macrosufactants in emulsion polymerization (Scheme 1). The synthesis of polymerizable surfactant started from the PMAA macromonomers Langmuir 2010, 26(3), 1724–1733
synthesized via CCTP in which CoBF acted as chain transfer agents (CTAs).37 Subsequently, the PMAA-b-PBA block copolymers (surfmers) were synthesized via RAFT technique in which PMAA macromonomers acted as macro-CTAs.37-40 Finally, the as-prepared surfmers bearing terminal unsaturated carboncarbon double bonds were used as polymerizable macrosurfactant in emulsion polymerization to prepare the poly(MMABA-AA) latexes. Synthesis and Characterization of PMAA Macromonomers and PMAA-b-PBA Block Copolymers. The evidence for successful preparation of PMAA macromonomer and PMAA-b-PBA block copolymers with unsaturated end vinyl groups via CCTP and RAFT can be demonstrated by NMR measurements and Raman spectra. As seen in the inset of Figure 1, vinyl groups in the end of PMAA macromonomer are determined by 1H NMR spectroscopy from the region 5.5-6.5 ppm. In the expanded inset, “p” shows the expected vinylic ω-end group in PMAA macromonomer and “m” indicates trace monomer in the end product isolated by evaporation of water. It would be worthy to indicate that it is very difficult and even impossible to remove all residual MAA monomer in the product. And thus the weak peaks “m” would still exist. An analogous result is also obtained in the literature.45 Moreover, the chemical structures of the PMAA macromonomer prepared via CCTP can be characterized by Raman spectra. Figure 1 shows Raman spectra of PMAA macromonomer via CCTP (Figure 1b) and PMAA-b-PBA block copolymer via RAFT (Figure 1c) as well as corresponding PMAA prepared by free radical polymerization for comparison (Figure 1a). The Raman shift which appeared at 1640 cm-1 in Figure 1b,c can be assigned to the unsaturated carbon-carbon double bonds in end groups of the PMAA macromonomer, which is consistent with the result of 1H NMR. These results indicate that we have successfully synthesized the PMAA macromonomer and PMAA-b-PBA block copolymers with terminal unsaturated double bonds via CCTP and RAFT techniques. (45) Haddleton, D. M.; Depaquis, E.; Kelly, E.; Kukulj, D.; Morsley, S. J. Polym. Sci., Polym. Chem. 2001, 39, 2378.
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Table 2. GPC Data of PMAA Macromonomer and PMAA-b-PBA Block Copolymers samples
Mn (103)
Mw (103)
PDI
PMAA PMAA-b-PBA (PMAA:PBA = 70:30 wt %) PMAA-b-PBA (PMAA:PBA = 60:40 wt %) PMAA-b-PBA (PMAA:PBA = 50:50 wt %)
1.6 2.0 3.0 3.5
2.2 2.8 4.3 5.7
1.4 1.4 1.4 1.6
Figure 3. Surface tensions vs concentrations of PMAA-b-PBA solution (PMAA:PBA = 60:40 (wt:wt), surfmer:NaOH = 1:1 (wt:wt), T = 25 °C). Table 3. Influence of Block Copolymer Content on the Latex Propertiesa runs
Figure 2. GPC profiles of the PMAA via CCTP and PMAA-bPBA via RAFT (solvent: isopropanol; reaction time: 5 h): (a) PMAA, (b) PMAA:PBA = 50:50 (wt:wt), (c) PMAA:PBA = 60:40 (wt:wt), and (d) PMAA:PBA = 70:30 (wt:wt).
PMAA prepared by free radical polymerization usually has high molecular weight and uncontrollable structure. In the presence of CoBF, PMAA macromonomer with Mn = 1.6 103 and PDI = 1.4 was fabricated successfully (Table 2 and Figure 2), which indicates that CCTP is an effective method to product controllable polymers with low molecular weight and narrow molecular weight distribution. The as-prepared PMAA bearing terminal double bond was then used as macro-CTAs for the chain extension with butyl acrylate to prepare a series of PMAA-b-PBA block copolymers via the RAFT technique. The molecular weight of the block copolymers PMAA-b-PBA with different weight ratio of PMAA and PBA still remains relatively low (Table 2 and Figure 2), which demonstrates the “livingness” of the PMAA and a very high reinitiation efficiency of the macrochain-transfer agent.46 The most important characteristics of surfactant used in the synthesis of polymeric latex by means of emulsion polymerization are surface tension and the critical micelle concentration (cmc). At the cmc, some properties of the aqueous surfactant solution show discontinuity, such as surface tension, density, and turbidity.12 Herein, surface tension values of the as-prepared block copolymers aqueous solution were monitored as a function of their surfactant concentration in order to determine the cmc. To improve the solubility of surfmers, sodium hydroxide was added to the suspension of surfmers in water with a weight ratio of 1:1 to convert them into the corresponding sodium carboxylates. Figure 3 shows the surface tension of a series of PMAA/PBA block copolymers with different PMAA and PBA chain segment ratios as a function of their concentration. The surface tension remarkable reduces when the block copolymer concentration decreases, owing to the absorption of block copolymer molecules on the air/ water interface. The break of the curve is indicative of the cmc, (46) Li, C. Z.; Benicewicz, B. C. Macromolecules 2005, 38, 5929–5936. (47) Chang, Y. D.; Lee, Y. D.; Karlsson, O. J.; Sundberg, D. C. Polymer 2000, 41, 6741–6747.
1728 DOI: 10.1021/la9037809
surfmer:traditional surfactant (wt %)
viscosity (mPa 3 s)
electrolyte coagulum stability (wt %)
PA-CO 0:4 80.2 pass 0.374 PA-surf 1 1:3 335.5 pass 0.362 PA-surf 2 2:2 372.5 pass 0.096 PA-surf 3 3:1 250.7 pass 0.080 PA-surf 4 4:0 fail 14 a The total amount of surfactant was 4 wt % of monomer weight and PMAA:PBA = 60:40.
above which the surface tension remains constant.47 The values of surface tension of surfmers are round 36 mN/m at the cmc, which are about 30 mN/m lower than that of the deionized water used in this case. The lowering of the water surface tension and the micelle formation clearly reveal the amphiphilic character of the synthetic block copolymers.48 As shown in Figure 3, independent of the hydrophilic block lengths, samples appeared to have similar cmc values near 5 g/L, in agreement with the results of the literature.49,50 The results suggest that the obtained PMAA/ PBA block copolymers are potential candidates for surfactant used in emulsion polymerization reactions.51 Characterization and Analysis of PA Latexes. Stability tests were performed on the latexes to investigate the effect of the amount of block copolymer on the latex stability, and the results are presented in Table 3. In these latexes, the weight ratio of PMAA to PBA was fixed at 60:40, the total surfactant weight was 4% of monomer weight, and the weight ratio of surfmer and traditional surfactant was 0:4, 1:3, 2:2, 3:1, and 4:0. Under ambient conditions stable latexes were obtained for the majority of runs, except the latex from PA-surf 4. It can be seen that the amount of coagulum of latexes decreases significantly from 0.374 to 0.080 wt % with the weight ratios of surfmer surfactant to traditional surfactant varying from 0:4 to 3:1. Two reasons could be responsible for this high stability: one is this surfmer surfactant has good compatibility with polymer matrix owing to their similar structure, and the other is the steric effect of surfmer with high molecular weight. However, when only surfmer is used as the (48) Chang, C. J.; Kao, P. C. Polymer 2006, 47, 591–601. (49) Zhang, J. G.; Dubay, M. R.; Houtman, C. J.; Severtson, S. J. Macromolecules 2009, 42, 5080–5090. (50) Wong, C. L. H.; Kim, J.; Roth, C. B.; Torkelson, J. M. Macromolecules 2007, 40, 5631–5633. (51) Uzulina, I.; Zicmanis, A.; Graillat, C.; Claverie, J.; Guyot, A. Macromol. Chem. Phys. 2001, 202, 3126–3135.
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Article Table 4. Influence of Different Segment Composites of Block Copolymers on the Latex Propertiesa
run
PMAA/PBA (wt:wt)
viscosity (mPa 3 s)
electrolyte stability
coagulum (wt %)
contact angle of film (deg)
PA-surf 6 50:50 3356.25 pass 1.96 91 PA-surf 3 60:40 250.67 pass 0.08 101 PA-surf 5 70:30 177.25 pass 0.07 93 a The total amount of surfactant was 4 wt % of monomer weight, and the weight ratio of surfmer to traditional surfactant was 3:1 wt:wt.
Figure 4. Particle size distributions of PA emulsion by using different content of surfmers, PMAA:PBA = 60:40 (wt:wt).
surfactant, the latex stability deteriorates markedly and the amount of coagulum reaches an unacceptable level of 14%. This could be due to the fact that the surfmer is less surface reactive than conventional surfactant. As shown in Figure 3, The surface tension of the surfmer aqueous solution can be decreased to about 40 mN/m at most, much higher than the surface tension level of about 20-30 mN/m52 using a conventional surfactant, and hence the only use of the surfmer is not reactive enough to stabilize the latexes. Therefore, using surfmer together with a little conventional surfactant to stabilize the latex would be a better choice. The latex stability against electrolytes was determined by the addition of 10 mL of 5 wt % calcium chloride solution to 10 mL of latex. After 24 h at room temperature, no visual coagulation or precipitation was observed for all latexes except PA-surf 4. The effect of the weight ratio of PMAA to PBA on the latex stability was investigated. In this procedure, the total surfactant weight was fixed on 4% of monomer weight, the weight ratio of surfmer to traditional surfactant was 3:1, and the weight ratios of PMAA to PBA vary from 50:50 to 70:30. As PMAA content increases, the amount of coagulum of latex reduces obviously (Table 4), which can be attributed to the absorption of COOwith negative charge on the surface of latex particles. Hereby, the same kind of charge repulses each other, resulting in the good stability of latex particles. When the PMAA content reaches 70 wt % in the block copolymer, the amount of coagulum of latex is only 0.07% and emulsion viscosity is 177.25 mPa 3 s. However, emulsion viscosity and the amount of coagulum of latex increase significantly with the decrease of the PMAA content. Additionally, the ratio of PMAA to PBA in surfmers has obvious influence (52) Dahanayake, M.; Cohen, A. W.; Rosen, M. J.; Hua, X. Y. J. Phys. Chem. 1982, 86, 541–545.
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on the wettability of the latex film. Theoretically, the more the content of PBA hydrophobic group, the higher the contact angle (CA) value. Accordingly, the CA value of PA-surf 3 film (PBA = 40 wt %) is 101°, 8° higher than that of PA-surf 5 film (PBA = 30 wt %). However, the CA value of PA-surf 6 film (PBA = 50 wt %) is only 91°, less than that of PA-surf 3 film (Table 4), attributing to the lower charge density and latex stability. The size of the latex particles and their size distributions were studied by dynamic light scattering (Figure 4) and TEM (Figure 5). Both traditional surfactant and surfmer surfactants give latexes with narrow particle size distributions. The average particle size of the PA latex using traditional surfactant latex (PACO) is 315.3 nm (Figure 4a), which is in good agreement with the result obtained from TEM micrograph (Figure 5a). The dispersion index of particles is only 0.0473, implying narrow size distribution of latex particles. Slightly larger particles are formed for the latexes with the use of as-prepared sufmer (Figure 4b-d). The molecular weight of the block copolymer is much larger than that of traditional surfactant, resulting in the different sizes of micelles. When the mass of surfmer is 3% of the total monomer mass, the particle size of the latex increases to 528.8 nm (Figure 4d). The TEM microphotograph (Figure 5b) also reveals the well distributed ball-like latex particles with average particle size of about 500 nm. The wettability of the PA hybrids prepared by the two different surfactant systems and the effect of the block copolymer content on the hydrophobicity of the hybrids films were also investigated. The film surfaces were prepared by spin-coating the corresponding solution onto clean glass substrates and dried at room temperature. Figure 6 shows the plot of water contact angle against block copolymer content along with the shapes of water droplets on the surface of the samples. Compared with the contact DOI: 10.1021/la9037809
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Figure 5. TEM micrographs of (a) PA-CO and (b) PA-surf 3 (emulsifier: 4 wt %; surfmer: 3 wt %; PMAA:PBA = 60:40 wt: wt; T: 80 °C; t: 260 min). Figure 7. Curves of (a) polymerization conversion vs reaction time and (b) polymerization rate vs reaction time (emulsifier: 4 wt %; surfmer: 3 wt %; PMAA:PBA = 60:40 wt:wt; T: 80 °C; t: 260 min).
Figure 6. Water contact angles of latex films vs content of surfmers (emulsifier: 4 wt %; PMAA:PBA = 60:40 wt:wt; T: 80 °C; t: 260 min).
angle (CA, 76°) on the native PA-CO film, the CA values of the PA-surf hybrid films increase significantly. The CA value increases from 79° to 101° when the block copolymer concentration varies from 1 to 3 wt %. This fact reveals the mechanism we proposed above. That is, conventional physically bound surfactant can be desorbed from the latex particles, creating hydrophilic domain at the film’s surfaces, where the hydrophilic group of surfactant arrays toward the air in order and is unfavorable to the water resistance of film. The surfmers with terminal double bond can further copolymerize with acrylic monomers by covalent bond, preventing the migration of these surfactants onto the surfaces of latex films and achieving high water contact angle value. Investigation into the Mechanism of Emulsion Polymerization Using Block Copolymer as Surfactant. In order to pursue the mechanism of emulsion polymerization using asprepared block copolymer as surfactant, the changes in monomer conversion, particle size and surface tension were studied by periodic sampling every 10 min during the process of emulsion polymerization. A conversion rate-time curve, particle size-time curve, and surface tension-time curve are shown in Figures 7 and 8. The curve of polymerization rate vs reaction time was obtained by derivation of the conversion vs time curves, as shown in Figure 7b. On the basis of the above experimental data and analysis, the mechanism of surfmer stabilized emulsion polymerization is proposed. First, stoichiometric block copolymers (surfmers), in which one segment PMAA is hydrophilic group and the other long carbon chain PBA is hydrophobic group with terminal double bonds, are dissolved in deionized water by adding 1730 DOI: 10.1021/la9037809
Figure 8. Curves of (a) surface tension of emulsion vs reaction time and (b) particle size of emulsion vs reaction time (emulsifier: 4%; surfmer: 3%; PMAA:PBA = 60:40 wt:wt; T: 80 °C; t: 260 min).
Figure 9. SEM image of the PMAA-b-PBA copolymer in aqueous sodium hydroxide solution (PMAA:PBA = 60:40 wt:wt; PMAAb-PBA copolymer:NaOH = 1:1 (wt/wt)).
sodium hydroxide solution to improve the solubility of surfmers. To seek balance in the water, some hydrophobic groups of surfmers escape from water, and their hydrophilic groups still stay in the air-water interface to form the monolayer distribution on the surface of the solution, resulting in the reduction of surface Langmuir 2010, 26(3), 1724–1733
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Scheme 2. Schematic Representation of Film Formation Mechanism of PA-CO Latex (top) and PA-Surfmer Latex (bottom)
tension of aqueous solutions dramatically to 27 mN/m (Figure 8a). The other hydrophobic groups of surfmers close together and eventually form the micelle when the surfmer concentration exceeds the cmc. The SEM image of the block copolymer exhibits the morphologies of spherical or cylindrical in water and the diameter of micelles ca. 100-150 nm (Figure 9), similar to the value in the literature.53 After that, the monomer pre-emulsion and water-soluble initiator solution are gradually added simultaneously and separately. Monomer spreads to micelles, and decomposed radicals are also absorbed into the monomer-swollen micelles and start the polymerization. At the stage of beginning of polymerization the particle size of emulsion keeps about 250 nm (Figure 8b). As a rule, three major intervals in a classical emulsion polymerization were depicted. The first interval is named as nucleation interval (interval I). Micelle nucleation is the predominant process in this interval. As soon as the entire surfactant is adsorbed on the growing latex particles, the second interval begins (interval II) in which the nucleated particles grow through reacting with monomer which diffuses into them from the monomer droplets. These monomer droplets act as reservoirs, and the overall reaction speed is constant because it is governed by the diffusivity of the monomer. When the monomer droplets are consumed, the reaction speed decreases exponentially (interval III). As a comparison, the nonconventional behavior was observed in the surfmer-stabilized emulsion polymerization of methyl methacrylate, butyl acrylate, and methacrylic acid (seen in Figure 7). It is different from conventional emulsion polymerization: no constant rate period is observed, and only two steps exist; i.e., the rate of polymerization abruptly increased to a maximum (Rp,max) and thereafter decreased. The first interval is the particle nucleation interval (interval I). At the beginning, the monomer conversion and polymerization rate are all zero because of induction effect. After a short time initiation, the polymerization rate increases markedly. In the classical emulsion system, the micelles diameters are usually 4-5 nm, much smaller than monomer droplets sizes, resulting in the high number ratio of micelles to monomer droplets, and the probability of radicals entering into micelles is larger. However, in this surfmer-stabilized emulsion polymerization, the mean diameter of the micelle is 100-150 nm, which is the same order with the size of monomer droplets. Accordingly, the probability that the radicals enter micelles is nearly the same as that they enter monomer droplets. (53) Tian, Y. Q.; Chen, C. Y.; Cheng, Y. J.; Young, A. C.; Tucker, N. M.; Jen, A. K. Y. Adv. Funct. Mater. 2007, 17, 1691–1697.
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Figure 10. AFM topographic images of (a) the surface of asprepared PA-CO film, (b) the surface of PA-CO film after rinsing with water, (c) the surface of as-prepared PA-surf 6 film, and (d) the surface of PA-surf 6 film after rinsing with water.
The hydrophobic groups with terminal double bonds of sufmers copolymerize with monomers in the micelles, forming stable latex particles connected by covalent bond. In this case, the surfmers act as both the emulsifier and the stabilizer of the primary monomer droplets. On the basis of this analysis, we assume that the combination of micelle nucleation and droplet nucleation are the predominant process at the same time. At this stage, since the surfmers concentration is larger than the cmc value and the new micelles are continuously produced, the polymerization rate is dominated by the number of particles and increases remarkably. The particle sizes increases rapidly (Figure 8b). The sufmers distributed on the surface of solution were resolved and absorbed onto the growing latex particles to stabilize the latexes, resulting in the continuous enhancement of the surface tension to 41 mN/m (Figure 8a). This interval ends at the reaction rate maximum of about 60% conversion at 120 min. After the maximum rate, the reaction enters the rate decreasing period directly (interval II); no interval with constant reaction rate is discovered. At this time the DOI: 10.1021/la9037809
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Figure 11. ATR FT-IR spectra of PA-CO latex film (top) and PA-surf 4 latex film (bottom): (a) F-A interface; (b) F-S interface. R = 45°.
surface tension keeps at the constant maximum value (Figure 8a). Still, the nucleated particles grow continually to 650 nm through the residual monomers mutual reacting until the monomers are consumed at 180 min (Figure 8b). Characterization and Analysis of Latex Films. The mechanism of film formation of PA emulsion is discussed. The top of Scheme 2 shows the film formation mechanism of PA-CO latex stabilized by traditional surfactant. As conventional surfactants are only physically attached to the surface of polymer particles, surfactants can be desorbed from polymer particle and migrate toward the F-A interface of the film (F-A: near the air; F-S: near the glass substrate) during the film formation process, creating monolayer distribution of surfactant at the film’s interfaces.54 The bottom of Scheme 2 displays the film formation mechanism of PA-surfmer latex stabilized by as-prepared block copolymer surfactant. The surfactant moiety is bound covalently to the polymer material so that desorption from the latex particle surface or migration in the polymer film could be impeded. In order to gain an insight into this proposed mechanism of film formation of hybrid latexes, we used AFM and ATR FT-IR spectra to observe the morphology and distribution of the surface of latex film. Atomic force microscopy (AFM) was performed to observe the morphology of the surface of hybrid latexe films.55 Figure 10a is AFM images of the surface of PA-CO latex film. The topography map reveals the well-distributed and interfacedistinct sphere particles from the top surface of the PA-CO (54) Zhao, Y. Q.; Urban, M. W. Macromolecules 2000, 33, 7573–7581. (55) Yamamot, T.; Higashitani, K. Asia-Pac. J. Chem. Eng. 2008, 3, 239–249.
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latex film, referring to the polymer latex particle encapsulated by the surfactant. The resultant PA-surf 6 matrix also showed similar morphology with lots of homogeneous mounds in the film with the use of 3% block copolymer together with 1% traditional emulsifier as the stabilizer (Figure 10c). Whereafter, these PA-CO and PA-surf 6 latex films were immersed in water for 2 min and then shaken dry. The surface of the PA-CO film after rinsing with water was much rougher than that before (Figure 10b). Instead of clear polymer latex particle, many holes were left in the film. When the film is immersed in water, the free surfactant migrated onto the film surface will be dissolved in the water so that the holes topography is formed on the surface hybrid latex, while the topography of the PAsurf 6 film (Figure 10c,d) did not undergo a significant change before and after rinsing. Compared with the morphology in Figure 10c, the only change in Figure 10d is that the particle interface is a little obscure, and some pinholes are observed on the particle surface. The surfmers reacting with acrylic monomers via covalent bond will not migrate onto the surfaces of latex films. The surfactant anchored to the polymer particles cannot be dissolved in the water, so its latex particle morphology will not change. The pinhole-like defects emerged on the particle surface just because of the use of 1% traditional surfactant in this sample. The results proved the film formation mechanism proposed above and give a reasonable explanation that the contact angle of PA-surf 6 film is much higher than that of PA-CO film. The concentration and depth distribution of groups in the two interfaces of latex films were detected by attenuated total reflection infrared (ATR FT-IR) spectra which is a very useful tool for Langmuir 2010, 26(3), 1724–1733
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analysis of the surface of materials.54,56,57 Latex samples were cast onto glass slide and allowed to dry at 30 °C under static air in a temperature-controlled chamber for 30 min (when they were visually dry), and then they were placed in an oven and annealed in air at 90 °C for 30 min. After drying, the films were removed from the substrate. The F-A interface (near the air) and the F-S interface (near the glass substrate) of the film were analyzed by ATR FT-IR spectroscopy so as to compare the group distribution of surfactant in the air-film interface with that in the bulk. The top of Figure 11 presents the ATR FT-IR spectra of the F-A interface (a) and F-S interface (b) of PA-CO latex film. We chose the same angle of incidence 45° to eliminate the effect on the intensity of the ATR FT-IR spectrum. In order to more clearly distinguish the group distribution between the F-A interface and F-S interface of the film, the two spectra were superimposed together, as shown on the right of Figure 11. It has been noted that the characteristic peak of CdO at the wavenumber of 1731 cm-1 is nearly coincident between the two spectra, indicating the CdO group has the same intensity distribution in the interface of the film as that in bulk, which means the acrylate ester groups has a uniform distribution in the different depth of PA film. Whereas the intensity of the -CH2 vibration peak at 1453 and 1389 cm-1 and C-O-C bond stretching vibration peaks at 1234 and 1165 cm-1 in spectrum (a) is slightly stronger than those in spectrum (b), verifying the fact that the traditional surfactant migrated to the surface during the film formation process, which results in an uneven distribution of different depth of the film. However, in the PA-surf latex film, the ATR FT-IR spectrum (the bottom of Figure 11) of the F-A interface is in good agreement with that of F-S interface in the region 1000-3500 cm-1, which suggests the composition of the PA-surf film including sufactant and polyacrylate matrix is relatively uniform with depth from the surface of the PA film. The results show that covalent bond combination between the surfmers and acrylic monomers prevents the migration and accumulation of the emulsifiers during filming of the latex while traditional surfactants tend to migrate to the F-A interface of the film. (56) Bae, W. S.; Lestage, D. J.; Proia, M.; Heinhorst, S.; Urban, M. W. Biomacromolecules 2005, 6, 2615–2621. (57) Dreher, W. R.; Urban, M. W. Langmuir 2003, 19, 10254–10259.
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Conclusion A series of novel amphiphilic PMAA-b-PBA block copolymers (surfmers) bearing terminal unsaturated carbon-carbon double bonds have been fabricated successfully via CCTP and RAFT techniques. These versatile block copolymers were applied as polymerizable macrosurfactants in emulsion polymerization of methyl methacrylate, butyl acrylate, and methacrylic acid. For comparison, the latex samples prepared with traditional surfactants (CO 897 and CO 436) were also made with the same component. The results indicate that the higher performance latexes with good stability, lower coagulum, and excellent hydrophobic property could be achieved by using the as-prepared surfmers. The possible surfmer-stabilized emulsion polymerization mechanism was also investigated based on the studies of the relation rule among effect factors including emulsion polymerization conversion rate, polymerization rate, emulsion particle size, surface tension, and reaction time. Kinetics analysis shows that there is no constant rate stage during the whole process of polymerization, which indicates the mechanism of the combination of the micellar nucleation and droplet nucleation. Compared with traditional surfactant, the use of sufmers can significantly improve the contact angles (CA) values of latex film. The analyses of atomic force microscopy (AFM) and attenuated total reflection spectra (ATR FT-IR) reveal that covalent bond combination between the surfmers and acrylic monomers prevents the migration and accumulation of the emulsifiers usually occurring for traditional surfactants, and thus the as-prepared surfmers are well distributed through the latex film with different depth. The foregoing results demonstrate that the high efficiency of these surfmers in emulsion polymerization and suggests that the resultant PMAA-b-PBA block copolymers act not only as the emulsifier but also as the stabilizer of the monomer droplets as well as the so-called comonomer. Acknowledgment. This work was supported by Natural Science Foundations (NSFs) of China (Grants 20576053 and 20606016), National Natural Science Foundations of ChinaNSAF (Grants 10676013 and 10976012), “863” Important National Science & Technology Specific Project (Grant 2007AA06A402), and the NSF of the Jiangsu Higher Education Institutions of China (Grants 07KJA53009 and 09KJB530005) and Postdoctoral Foundation (Grant 200904501087).
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