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Oct 29, 2009 - Chinlun Huang, Tyler Tassone, Kendra Woodberry, Dan Sunday, and David L. Green*. Department of Chemical Engineering, University of ...
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Impact of ATRP Initiator Spacer Length on Grafting Poly(methyl methacrylate) from Silica Nanoparticles Chinlun Huang, Tyler Tassone, Kendra Woodberry, Dan Sunday, and David L. Green* Department of Chemical Engineering, University of Virginia, 102 Engineers Way, Charlottesville, Virginia 22904 Received May 29, 2009. Revised Manuscript Received October 5, 2009 We quantified the impact of the carbon spacer length (CSL) of immobilized alkoxysilanes initiators on grafting poly(methyl methacrylate) (PMMA) from the surfaces of monodisperse silica nanoparticles. PMMA was grafted using surface-initiated atom transfer radical polymerization (SI-ATRP), a facile technique to produce well-controlled polymer brushes. The polymerizations were carried out in environmentally friendly 4:1 (v/v) methanol-water solutions at room temperature. Monoethoxysilane initiators of 3, 11, and 15 carbon spacer lengths were synthesized and characterized with 1H NMR and 13C NMR. The initiators were then used to modify the surfaces of monodisperse silica nanoparticles in methyl isobutyl ketone, producing dense initiator monolayers with site densities between 1.8-3.6 initiators/nm2. PMMA was subsequently grafted from the functionalized nanoparticles using both CuCl and CuBr catalysts. We found that polymerizations performed with CuBr were uncontrolled, whereas those with CuCl were controlled. PMMA graft densities ranged between 0.10-0.43 polymers/nm2, which increased with the initiator carbon spacer length (CSL). Interestingly, longer CSLs make nanoparticle surfaces hydrophobic, causing nanoparticle aggregation in methanolwater solutions. Our results indicate that surface hydrophobicity correlates to increases in PMMA graft density through the adsorption of hydrophobic MMA monomers on initiators with longer CSLs. Thus, to augment PMMA graft densities, a subtle balance must be struck between enabling particle stability and increasing MMA adsorption in methanol-water solutions.

1. Introduction Our focus is to attach alkoxysilane initiators to the surface of silica nanoparticles and to quantify the impact of the carbon spacer length (CSL) of initiators on grafting poly(methyl methacrylate) (PMMA) from nanoparticle surfaces. The “grafting from” reactions, in which methyl methacrylate (MMA) is polymerized into PMMA brushes, was carried out with surfaceinitiated atom transfer radical polymerization (SI-ATRP) in methanol-water solutions. The first step in SI-ATRP from silica involves the synthesis and attachment of alkoxysilane (or chlorosilane) initiators from which polymer chains are grown through reversible reactions with a transition metal-ligand catalyst. The controlled nature of SI-ATRP and its intolerance of impurities permit the grafting of polymer brushes of well-controlled molecular weight (10 000-500 000 g/mol), composition (styrenes, acrylates, lactides, etc.), and graft density (0.3-1.0 chains/nm2) in a range of solvents (p-xylene, anisole, tetrahydrofuran, etc.).1-9 The living nature of SI-ATRP is demonstrated subsequent to terminating the reaction by reinitiating polymer growth, which often occurs in the presence of other monomers to form complex *Corresponding author. E-mail: [email protected]. (1) Audouin, F.; Blas, H.; Pasetto, P.; Beaunier, P.; Boissiere, C.; Sanchez, C.; Save, M.; Charleux, B. Macromol. Rapid Commun. 2008, 29, 914–921. (2) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. Rev. 2004, 33, 14–22. (3) Faucher, S.; Zhu, S. Macromol. Rapid Commun. 2004, 25, 991–994. (4) Grimaud, T. Macromolecules 1997, 30, 2216–2218. (5) Husseman, M.; Malmstr€om, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P.; Hawker, C. J. Macromolecules 1999, 32, 1424–1431. (6) Miller, P. J.; Matyjaszewski, K. Macromolecules 1999, 21, 8760–8767. (7) Ohno, K.; Morinaga, T.; Koh, K.; Tsujii, Y.; Fukuda, T. Macromolecules 2005, 38, 2137–2142. (8) von Werne, T.; Patten, T. J. Am. Chem. Soc. 2001, 123, 7497–7505. (9) von Werne, T.; Patten, T. J. Am. Chem. Soc. 1999, 121, 7409–7410.

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structures (random, block, or branch polymers).4,5,10-15 SIATRP has been carried out from various forms of silica, including particles, wafers, and mesoporous materials. These materials are used in various applications such as electronics, sensors, catalysts, drug delivery, colloid stabilization, and chromatography.5,16-24 Moreover, grafting PMMA is highly desired due to its excellent optical, electronic, surface, and mechanical properties as well as its chemical and thermal stability.25,26 To our knowledge, however, no peer-reviewed study has been published on the impact of the carbon spacer length (CSL) of an alkoxysilane or chlorosilane initiator on SI-ATRP from any form of silica, much less grafting PMMA from silica nanoparticles. (10) Chatterjee, U.; Jewrajka, S. K.; Mandal, B. M. Polymer 2005, 46, 10699– 10708. (11) Li, Y.; Armes, S. P.; Jin, X.; Zhu, S. Macromolecules 2003, 36, 8268–8275. (12) Oikonomou, E. K.; Pefkianakis, E. K.; Bokias, G.; Kallitsis, J. K. Eur. Polym. J. 2008, 44, 1857–1864. (13) Vo, C.-D.; Schmid, A.; Armes, S. P. Langmuir 2007, 23, 408–413. (14) Vogel, B. M.; DeLongchamps, D. M.; Mahoney, C. M.; Lucas, L. A.; Fischer, D. A.; Lin, E. K. Appl. Surf. Sci. 2008, 254, 1789–1796. (15) Wang, D.; Peng, Z.; Liu, X.; Tong, Z.; Wang, C.; Ren, B. Eur. Polym. J. 2007, 43, 2799–2808. (16) Daniel, M.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (17) Ejaz, M.; Yamamoto, S.; Ohno, K.; Tsujii, Y.; Fukuda, T. Marcomolecules 1998, 31, 5934–5936. (18) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid Science and Surface Chemistry; Marcel Dekker: New York, 1997 (19) Shenhar, R.; Norsten, T. B.; Rotello, V. M. Adv. Mater. 2005, 17(6), 657– 669. (20) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18–52. (21) Trindade, T.; O’Brien, P.; Pickett, N. L. Chem. Mater. 2001, 13, 3843–3858. (22) Tsujii, Y.; Ohno, K.; Yamamoto, S.; Goto, A.; Fukuda, T. Adv. Polym. Sci. 2006, 197, 1–45. (23) Radhakrishnan, B.; Ranjan, R.; Brittain, W. J. Soft Matter 2006, 2, 386– 396. (24) Zhu, H.; Wu, S.; Shen, J. Chem. Rev. 2008, 108, 3893–3957. (25) Daraboina, N.; Madras, G. Ind. Eng. Chem. Res. 2008, 47(18), 6828–6834. (26) Tanaka, K.; Fujii, Y.; Atarashi, H.; Akabori, K.-i.; Hino, M.; Nagamura, T. Langmuir 2008, 24, 296–301.

Published on Web 10/29/2009

DOI: 10.1021/la901918v

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Successful polymerizations from nanoparticles typically require the covalent attachment of initiators, which form monolayers that foster particle dispersion and mitigate polymer bridging. Covalently attached monolayers of alkoxysilane initiators, which consist of an alkoxysilane anchoring group, an aliphatic alkyl spacer, and an ATRP initiator group, are usually synthesized in a three-step procedure. The first step involves the esterifcation of a halogenated ATRP initiator group, such as 2-bromoisobutyryl bromide, to the carbon spacer, an unsaturated alcohol. The number of carbons in the unsaturated alcohol determines the CSL. The second step involves forming the initiator through a hydrosilylation reaction that binds the unsaturated alcohol to an alkoxysilane, leaving it attached to an aliphatic alkyl chain. The third step entails anchoring the initiator to silica through the alkoxysilane that can be mono-, di-, or trifunctional, indicating the numbers of alkoxy groups on the silicon atom in the initiator. The numbers of alkoxy groups are speculated to condense to an equivalent number of -OH groups on silica; thus initiators should be covalently attached to silica through single, double, or triple site attachment. Several reviews have been published covering a range of initiators and solvents used in SI-ATRP.23,27,28 Inferring from the reviews, most nanoparticles are functionalized with short initiators, consisting of spacers of only three carbon repeat units. In contrast, on flat surfaces polymers are most often grafted from initiators with a spacer of 11 carbon repeat units. During reactions from interfaces, small amounts of free polymers, or polymers in solution, grow to the same molecular weights as those on the surface. More specifically, few studies exist on the SI-ATRP of PMMA in polar solvents such as in methanol-water mixtures in which small amounts of water can speed reaction rates above that in nonpolar solvents. Polymerizations in the presence of water are often referred to as aqueous ATRP, which is highly desirable as the solvents are significantly less hazardous to health and environment than those used in the organic synthesis. Further, aqueous SI-ATRP of PMMA occurs at room temperature, mitigating the thermal polymerization of MMA, which is negligible at ambient temperature.29-32 Decreasing temperature also permits the modification of heat sensitive surfaces. In contrast, the SI-ATRP of PMMA in nonpolar solvents, which has been well studied, require higher temperatures (70-110 °C) and is carried out for longer periods (12-42 h).7-9,23,27,28 The harsh conditions encountered in nonpolar solvents justify the use of aqueous SI-ATRP in grafting PMMA from silica nanoparticles, because water improves not only the control of MMA polymerization in polar solvents but also particle stability. Moreover, the addition of water improves the solubility of MMA, a hydrophobic monomer, in protic solvents such as methanol, ethanol, or isopropanol. However, the impact of particle stability on the SI-ATRP of PMMA in polar solvents has not been explored. This includes elucidating the impact of the CSL of the ATRP initiator on grafting PMMA as quantified by the PMMA graft density and the initiation efficiency in tandem with the graft and free polymer molecular weights. Researchers have so far exploited the narrow solubility window of polar cosolvents to graft PMMA from flat silicon wafers or gold surfaces, which have only been carried out from 11 CSL (27) Advincula, R. J. Dispersion Sci. Technol. 2003, 24(3-4), 343–361. (28) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677–710. (29) Jones, D. M.; Brown, A. A.; Huck, W. T. S. Langmuir 2002, 18, 1265–1269. (30) Jones, D. M.; Huck, W. T. S. Adv. Mater. 2001, 13(16), 1256–1259. (31) Kim, J.-B.; Bruening, M. L.; Baker, G. L. J. Am. Chem. Soc. 2000, 122, 7616–7617. (32) Bao, Z.; Bruening, M. L.; Baker, G. L. Macromolecules 2006, 29, 5152– 5258.

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ATRP initiators. For example, Kim et al. reported the first SI-ATRP of PMMA on gold at room temperature from a ω-mercaptoundecyl bromoisobutyrate monolayer in solutions of 4:1 (v/v) methanol-water.31 Bao et al. subsequently controlled the graft density of PMMA from both gold and silica substrates by diluting the active initiators with inactive molecules, where the initiator density was quantified by assuming a maximum coverage of 4 molecules/nm2.32 Inferring from their results, high PMMA graft densities of σn = 0.60 polymers/nm2 were obtained on undiluted initiator monolayers, which are comparable to graft densities obtained in nonpolar solvents. Further, using NMR, they did not detect polymers in solution, indicating that thermal polymerization of PMMA was negligible at room temperature and no chain transfer occurred from the surface into solution.32 Huck and co-workers also used aqueous ATRP in a 4:1 (v/v) methanol-water cosolvent with CuBr to pattern the surfaces of gold wafers with PMMA from ω-mercaptoundecyl bromoisobutyrate monolayers.29,30 They found that water greatly increased the polymerization rate without a loss of control as determined by measuring the brush thickness with ellipsometry. Moreover, they did not observe the formation of free polymers in solution.30 They also studied how changing the initiator density affected the SI-ATRP of PMMA by varying the surface concentrations of undecanthiol and ω-mercaptoundecyl bromoisobutyrate on gold.29,30 Similar to Bao, Bruening, and Baker, Huck and coworkers found that the growth of PMMA was well-controlled as the brush thickness decreased in proportion of the initiator surface coverage, indicating that the PMMA chains grow at the same rate, but are spaced further apart. Inferring from their results, PMMA graft densities varied from σn = 0.030.20 polymers/nm2 for initiator surface coverages between 10100%. To our knowledge, the only published study that exists on the aqueous SI-ATRP of PMMA from silica nanoparticles is that of Zhang and co-workers who grafted PMMA from 10 μm particles functionalized with (3-(2-bromopropionyl)propyl)triethoxysilane, a 3 CSL initiator.33 The catalyst CuCl was used with CuBr2 to control polymerization. In contrast, several groups have found with aqueous ATRP, methacrylates will polymerize faster with CuBr, but with less control than CuCl, which yields slower polymerization rates but better control.34-38 For example, Wang and co-workeres found that brominated initiators had a faster initiation than chlorinated initiators in the ATRP of methoxy-capped oligo(ethylene glycol) methacrylate (OEGMA) in water at 20 °C.36 Edmundson and Huck in the SI-ATRP of poly(glycidyl methacrylate) as well as Jewrajka et al. in the ATRP of PMMA also found that using CuBr led to a higher PDI as compared to using CuCl.34,38 They state that the reason for increased control when using CuCl as the catalyst was due to the fact that C-Cl bonds are stronger than C-Br bonds.34,38 Huck and co-workers have found that the faster polymerization rates can lead to less dense polymer graft layers.37 Thus, while many have studied the grafting of PMMA from silica, none have systematically investigated the effect of initiator CSL on grafting PMMA from silica using aqueous ATRP. (33) Zhang, K.; Li, H.; Zhang, H.; Zhao, S.; Wang, D.; Wang, J. Mater. Chem. Phys. 2006, 96, 477–482. (34) Edmundson, S.; Huck, W. T. S. J. Mater. Chem. 2004, 14, 730–734. (35) Chatterjee, U.; Jewrajka, S. K.; Mandal, B. M. Polymer 2005, 46, 1575– 1582. (36) Wang, X.-S.; Armes, S. P. Macromolecules 2000, 33, 6640–6647. (37) Cheng, N.; Azzaroni, O.; Moya, S.; Huck, W. T. S. Macromol. Rapid Commun. 2006, 27, 1632–1636. (38) Jewrajka, S. K.; Chatterjee, U.; Mandal, B. M. Macromolecules 2004, 37, 4325–4328.

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Article Scheme 1. Synthesis of ATRP Initiators That Vary in Carbon Spacer Length (CSL) Based on the Number of Carbons Present in the Unsaturated Alcohol, where n = CSL-2a

Figure 1. SEM of unmodified St€ ober silica nanoparticles (radius

R = 23 ( 1 nm).

Consequently, our goal is to quantify the impact of initiator CSL on grafting PMMA in methanol-water solutions from silica nanoparticles. To accomplish this goal, well-characterized monoethoxysilane initiators with 3, 11, and 15 CSLs were synthesized and attached to monodisperse silica nanoparticles. PMMA with a target Mn = 50 000 g/mol was subsequently grafted from silica nanoparticle surfaces using either CuCl or CuBr catalysts in methanol-water solutions. Specific attention was placed on elucidating how the stability of the initiator-functionalized nanoparticles in methanol-water solutions affects the SI-ATRP of PMMA as quantified by the graft density (σn), initiation efficiency (σi/σn), graft and free polymer molecular weights (Mn’s), and their polydispersities (PDI’s). Emphasis was also placed on selecting the catalyst that enabled control over PMMA polymerization. Overall, the knowledge gained from quantifying the effect of the initiator CSL on grafting PMMA from nanoparticle surfaces can then be used to tailor nanoparticles for various applications.

2. Experimental Methodology 2.1. Materials. Dimethylethoxysilane (DMES) was pur-

chased from Gelest. ω-Pentadecalactone was purchased from Spectrum Laboratory Products. Methyl methacrylate (MMA) was treated with alumina to remove the inhibitor. Tetrahydrofuran (THF) was filtered with a 0.2 μm nylon filter prior to use. Deionized water was purified to a resistivity of 18 MΩ. All other chemicals were used as received from Fisher Scientific. 2.2. Silica Nanoparticle Synthesis. St€ober synthesis, or the ammonia catalyzed reactions of tetraethylorthosilicate (TEOS), was used to synthesize monodisperse silica nanoparticles of R = 23 ( 1 nm from [TEOS] = 0.50 M, [NH3] = 0.41 M, and [H2O] = 1.17 as calculated by an empirical correlation. The sizes of the nanoparticles were determined by SEM in Figure 1.39-42 2.3. Alkoxysilane ATRP Initiator Synthesis. SI-ATRP of PMMA from silica requires the immobilization of halogenated alkoxysilane initiators on the substrate surface. The alkoxysilane initiators were synthesized in two steps, as shown in Scheme 1, where the quantity n = CSL-2 is the carbon spacer length minus two. First, a brominated precursor was synthesized through an esterification reaction that couples an unsaturated alcohol to a brominated acid, such as 2-bromoisobutryl bromide. Subsequently, the brominated precursor was used to synthesize the alkoxysilane initiator, which is formed through a hydrosilylation (39) Bogush, G. H.; Tracy, M. A.; Zukoski, C. F., IV J. Non-Cryst. Solids 1988, 104, 95–106. (40) Green, D. L.; Jayasundara, S.; Lam, Y.-F.; Harris, M. T. J. Non-Cryst. Solids 2003, 315, 166–179. (41) Green, D. L.; Lin, J. S.; Lam, Y.-F.; Hu, M. Z.-C.; Schaefer, D. W.; Harris, M. T. J. Colloid Interface Sci. 2003, 266, 346–358. (42) St€ober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62–69.

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a Bromination occurred in dichloromethane for the 3 CSL and 11 CSL precursors, whereas bromination was carried out in ethyl acetate for the 15 CSL precursor.

reaction that links an alkoxysilane, such as dimethylethoxysilane (DMES), to the unsaturated end of the precursor. The alkoxysilane anchors the ATRP initiator to the -OH groups on the silica surface. A monosiloxane initiator was selected because monosiloxane initiators create a smoother and more uniform surface. This increased smoothness and uniformity is attributed to the monosiloxane end group of the initiator, which attaches to -OH groups on the silica surface through a single site attachment. By varying the number of carbons in the unsaturated alcohol of the brominated precursor, three alkoxysilane ATRP initiators were synthesized, a three CSL initiator (3-(2-bromoisobutyryl)propyl)dimethylethoxysilane (3-BIDS), an 11 CSL initiator (11(2-bromoisobutyryl)undecyl)dimethylethoxysilane (11-BIDS), and a 15 CSL initiator (15-(2-bromoisobutyryl)pentadecyl)dimethylethoxysilane (15-BIDS). The 15 CSL initiator is new, while previously, von Werne and Patten synthesized the 3 CSL initiator8,9 and Brittain and co-workers made the 11 CSL.43,44 2.3.1. Pentadec-14-en-1-ol Synthesis. To synthesize the 15 CSL initiator, (15-(2-bromoisobutyryl)pentadecyl)dimethyl ethoxysilane (15-BIDS), the unsaturated alcohol, pentadec-14en-1-ol was first synthesized because it was not commercially available. Pentadec-14-en-1-ol is used to synthesize the 15 CSL precursor, which is hydrosilylated to form 15-BIDS. As a result, Scheme 2, included in the Supporting Information, was developed to synthesize pentadec-14-en-1-ol based on the work of Collier and co-workers, Hostetler and co-workers, and Rai and Basu.45-47 2.3.2. 15-BIDS Synthesis. The 15 CSL initiator, 15-BIDS, was synthesized by first making the 15 CSL precursor, pentadec14-enyl-2-bromo-2-methylpropanoate by first dissolving pentadec-14-en-1-ol (0.237 M) and TEA (0.237 M) in ethyl acetate. To this solution was added dropwise 2-bromoisobutryl bromide (0.355 M). After approximately 24 h, the reaction was washed with DI water and a saturated sodium bicarbonate solution, dried with magnesium sulfate, and concentrated to yield the precursor. 1 H NMR (300 MHz, CDCl3): δ (ppm) 1.24-1.35 (20H), 1.65 (2H), 1.90 (6H), 2.04 (2H), 4.14 (2H), 4.91 (1H), 4.98 (1H), 5.77 (1H); 13C NMR (300 MHz, CDCl3): δ (ppm) 29.3-31.2, 32.9, 34.9, 56.1, 66.4, 114.3, 139.4, 171.9. 15-BIDS was synthesized through the hydrosilylation of its precursor to dimethylethoxysilane with Karstedt’s catalyst. The hydrosilylation reaction was then heated at reflux until there were no longer any peaks present at approximately 114.3 and 139.4 ppm on 13C NMR scans. Those peaks correspond to the (43) Akgun, B.; Boyes, S. G.; Granville, A. M.; Brittain, W. J.; Foster, M. D. Polym. Prepr. 2003, 44, 514–515. (44) Wang, Y.; Hu, S.; Brittain, W. J. Macromolecules 2006, 39, 5675–5678. (45) Collier, T. L.; Hwang, Y.; Ramasamy, R.; Sciacca, R. R.; Hickey, K. T.; Simpson, N. R.; Bergmann, S. R. J. Nucl. Med. 2002, 43, 1707–1714. (46) Hostetler, E. D.; Fallis, S.; McCarthy, T. J.; Welch, M. J.; Katzenellenbogen, J. A. J. Org. Chem. 1998, 63, 1349–1351. (47) Rai, A. N.; Basu, A. Org. Lett. 2004, 6, 2861–2863.

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Article double bond on the end of the precursor, and that bond is broken when the initiator is synthesized. Once the reaction was complete, the product was concentrated and 1H NMR and 13C NMR scans were taken. The reaction scheme is shown in Scheme 1 where n = 13. 1H NMR (300 MHz, CDCl3): δ (ppm) 0.17 (6H), 1.15 (3H), 1.17 (24H), 1.19 (2H), 1.84 (2H), 1.89 (3H), 3.65 (2H), 4.13 (2H); 13 C NMR (300 MHz, CDCl3): δ (ppm) -1.9, 16.5, 18.7, 23.3, 25.9-33.6, 45.0, 55.8, 58.3, 66.1, 191.6.

2.4. ATRP Initiator Functionalization of Silica Nanoparticles. A major goal for this work was to coat the monodisperse silica nanoparticles with a covalently attached monolayer of the alkoxysilane initiators so that PMMA could be grafted from the surface. The surface modification was carried out in a solvent that enables the electrostatic stabilization of the bare silica nanoparticles without competing with the alkoxysilane initiators for the OH surface sites. To achieve this goal, the silica nanoparticles were functionalized in methyl isobutyl ketone (MIBK), which has a dielectric constant of ε =13.1 (T = 20 °C), which is high enough to ionize the hydroxyl groups on the nanoparticle surface, but the butyl substituent on MIBK likely reduces its affinity for silica surface. The impact of using methyl ethyl ketone (MEK) and a MEK/TEA solution to attach the alkoxysilane initiators to silica were also investigated. In the Results and Discussion section (section 3), it is shown that higher initiator site densities (σi) are obtained when surface functionalization reactions were carried out in MIBK with minimal solvent adsorption as quantified with elemental analysis. The functionalization of the silica nanoparticles with the ATRP initiators was carried out by placing a sonicated suspension of silica nanoparticles (30 g) in either MIBK or MEK (400 mL) in a round-bottom flask, which was then placed in a oil bath and magnetically stirred. Then, 3-BIDS, 11-BIDS, or 15-BIDS were added to the flask, which was then connected to a condenser and heated at reflux for at least 24 h. After the condensation of the alkoxysilane initiators to the silica nanoparticle surface, the nanoparticles were washed into hexane with five centrifugation cycles. Subsequently, the nanoparticles were dried and analyzed with elemental analysis to determine the initiator site density (σi). 2.5. SI-ATRP of PMMA from Silica Nanoparticles. To graft PMMA from silica, all reagents except the copper salt were placed initially in a Schlenk flask. The reagents in the flask include the ATRP initiator functionalized nanoparticles (0.5 g), 2,20 bypyridine (bipy, 1.846 g, 11.8 mmol), MMA (25 mL, 0.233 mol), MeOH (20 mL, 0.494 mol), and DI water (5 mL, 0.278 mol). The flask was then tightly sealed, and the contents were then completely frozen by placing the flask in a liquid-nitrogen bath. Subsequently, the flask was opened to vacuum to remove dissolved gases, and upon the equilibration of the vacuum as indicated by a low nonchanging pressure reading, the Schlenk flask was resealed and the contents were remelted. This freeze/ pump/thaw procedure was repeated until the bubbling of gases through the solution was not observed upon melting. The flask was subsequently charged with nitrogen, the contents refrozen, and the copper salt (3.94 mmol, 0.390 g of CuCl, or 0.565 g of CuBr) was then added. The reaction was run for a designated amount of time (2 h for CuBr and 6 h for CuCl), and subsequently, the reaction was quenched by exposing it to air, and the sample was centrifuged and resuspended to separate the free and graft polymers. To separate the free polymer, the samples were centrifuged at 13 000 rpm for 30 min, the supernatant was precipitated in methanol, and the pellet was resuspended in acetone until no free polymer was found in the supernatant. The free polymer was then dried and analyzed with GPC. The grafted particles and polymers were analyzed with GPC, elemental analysis, SEM, and DLS.

2.6. Characterization Methods. 2.6.1. Gel Permeation Chromatography (GPC): Graft and Free Polymer MW. Prior to GPC analysis, the graft PMMA polymer was etched from the silica nanoparticles with hydrofluoric acid (HF). Etching was accomplished by adding to a plastic vial, the 13354 DOI: 10.1021/la901918v

Huang et al. PMMA-g-silica particles (250 mg), toluene (10 mL), phase transfer catalyst Aliquot 336 (75 mg), and a 2% aqueous solution of HF (15 mL), sealing the vial and shaking the contents. Etching was allowed to take place for several hours, at which point the shaking was stopped and the aqueous and organic phases were allowed to separate. The organic phase was then removed, washed with a saturated sodium bicarbonate solution to neutralize residual HF, and the polymer was precipitated in methanol and prepared for GPC analysis. Both the free and graft polymers were prepared for GPC analysis in the same way subsequent to silica etching. To remove residual copper, the polymer was precipitated in methanol and redissolved in acetone until the polymer was white and the solution was colorless. The precipitated polymer was then dried under vacuum at 50 °C, and approximately 5-10 mg of the polymer was weighed into a glass vial, to which 1-2 mL of THF was added to dissolve the polymer. Subsequently, 1 mL of the dissolved polymer solution was removed with a syringe, filtered with a 0.2 μm nylon filter to remove particulates, and the sample was analyzed with GPC using THF as the eluent at a flow rate of 1 mL/min. GPC was used to determine the number-averaged molecular weight (Mn), weight-averaged molecular weight (Mw), and the molecular weight distribution, or polydispersity index (PDI = Mw/Mn) of the graft and free polymers. These parameters were determined by calibrating the elution times of the graft and free polymers against polystyrene standards. Further, we verified that repeated precipitations did not change the Gaussian shapes of the GPC curves and the elution times of the free and graft polymers.

2.6.2. Elemental Analysis: ATRP Initiator and PMMA Graft Densities. The initiator and polymer graft densities, σi and

σn, respectively, were calculated from elemental analysis which was used to determine the carbon and nitrogen content of the sample. The quantity σi was determined using eqs 2-4 below, whereas σn was calculated similarly, except with a change in some parameter. For example, the mass of the ATRP initiator or the graft PMMA polymer (M) was calculated from the following equation: 0:01ð%C total - %Cbase ÞMtotal  M ¼  MWcarbon - 0:01ð%Cbase Þ MW

ð1Þ

in which Mtotal is the total mass of the sample and %Ctotal is the total percentage of carbon in a sample of either initiator-functionalized or PMMA-grafted silica nanoparticles. For the initiatorfunctionalized nanoparticles, %Cbase is the percentage of carbon from the umodified silica nanoparticles. In contrast, %Cbase is the percentage of carbon from either the 3-BIDS, 11-BIDS, or 15-BIDS functionalized nanoparticles, depending on the monolayer from which PMMA was grafted onto silica. The quantity, MWcarbon/MW, represents the ratio between total molecular weight of all carbons present in either an ATRP initiator or a MMA monomer (MWcarbon) with respect to the molecular weight of that molecule (MW). The initiator and PMMA graft densities (σi and σn) were determined by   M NA ð2Þ σx ¼ Asilica MW where NA is the Avogadro number. When the subscript x equals that of initiator (x = i), M and MW are the mass and molecular weight, respectively of the initiator. To calculate σn, the subscript equals that of the grafted polymer (x = n) and M and MW are those for the graft polymer. The surface area of silica (Asilica) is determined by Asilica ¼

4πR2 ðMtotal - MÞ 3ðMtotal - MÞ ¼ ð4=3ÞFsilica R3 Fsilica R

ð3Þ

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where R is the geometric radius of the particle, Fsilica = 1.85 g/cm3 is the mass density of silica, and Msilica is the mass of the silica. SEM was used to confirm that the geometric particle sizes of the unmodified and initiator-functionalized nanoparticles are essentially the same, which was also confirmed through hydrodynamic particle size measurements with DLS. Ultimately, the initiator efficiency, or σi/σn, was quantified to determine the effectiveness in grafting PMMA from silica nanoparticle surfaces in methanol-water solutions.

2.6.3. Dynamic Light Scattering (DLS): Particle Size Determination. A DLS from PhotoCor instruments was used to determine the size of the 3-BIDS, 11-BIDS, and 15-BIDS modified silica nanoparticles as well the PMMA-g-silica nanoparticles in acetone. All suspensions were sonicated to create uniform dispersions prior to DLS measurement. Subsequently, the hydrodynamic radius of the particles was calculated from the photon-photon autocorrelation function, from which the particle diffusion coefficient (De) was determined using a third-order cumulant analysis. The hydrodynamic radius (Rh) was determined from De through the Stokes-Einstein relation in eq 4. Rh ¼

kB T 6πηDe

ð4Þ

where kB is the Boltzmann constant, T the absolute temperature, and η the solution viscosity. The PMMA hydrodynamic brush thickness, L, was determined by subtracting the size of the either the 3-BIDS, 11-BIDS, or 15-BIDS modified particles from that of the brush-grafted nanoparticles. All polydispersity indices, the second-order cumulant from the autocorrelation function, were below 0.07 indicating that all particles (i.e., those functionalized and grafted) were of uniform size and completely dispersed.

3. Results and Discussion 3.1. Synthesis and Characterization of the ATRP Initiators. Several research groups have synthesized various ATRP initiators that are attached to silica nanoparticle surfaces.23,27,28 Typically, the alkoxysilane ATRP initiators are either ethoxysilane or chlorosilane derivatives with short CSLs, such as 3BIDS,8,9 which may not stabilize the particles against van der Waals aggregation during the covalent attachment of the initiator or at the early stages of SI-ATRP reaction. To help resolve this issue, two additional initiators, an 11 CSL and a 15 CSL initiator, or 11-BIDS43,44 and 15-BIDS, respectively, were synthesized. These molecules both have a monoethoxysilane anchoring group, permitting the formation of stable siloxane bonds (Si-O-Si) through single site attachment with OH groups on the silica surface. The use of ethoxysilane derivatives enables the modification of silica in both protic and aprotic solvents of high polarity in which electrostatic interactions can stabilize silica nanoparticles. In addition, the longer CSLs present in the 11-BIDS and 15-BIDS initiators should form thicker alkoxysilane layers that prevent the close contact of silica nanoparticles, mitigating aggregation. The objective of this study is to determine the impact of the longer CSL initiators on grafting PMMA from silica surfaces in 4:1 (v/v) methanol-water solutions. The monoethoxysilane derivatives with ATRP initiating sites were synthesized in the two-step reaction described in Scheme 1. In short, an unsaturated alcohol with 3, 11, or 15 carbons was acylated with 2-bromoisobutyryl bromide to obtain ATRP precursors of 3, 11, or 15 CSL. Subsequently, the respective aryl groups of these molecules were hydrosilylated with dimethylethoxysilane in the presence of Karstedt’s catalyst to form the SI-ATRP initiators. The successful synthesis of 15-BIDS, a new molecule, was confirmed with 1H NMR. To this end, the complete hydrosilylaLangmuir 2009, 25(23), 13351–13360

tion of the precursor, pentadec-14-enyl-2-bromo-2-methylpropanoate, to 15-BIDS occurs upon disappearance of the protons attached the to the double bonded carbons at locations a, b, and c of the precursor in Figure 2a. These peaks are nonexistent in Figure 2b, indicating that the monoethoxysilane anchoring group was attached to the precursor to form 15-BIDS. The syntheses of the 3-BIDS and 11-BIDS initiators were confirmed in a similar fashion. 3.2. Attachment of ATRP Initiators to Silica Nanoparticle Surfaces. SI-ATRP of PMMA from silica first requires the attachment of dense, covalently attached monolayers of alkoxysilane initiators. Condensation reactions of the alkoxysilane initiators onto silica are often carried out in nonpolar organic solvents, such as THF, in which unmodified particles can aggregate. In contrast, initiators can be attached in polar solvents such as methanol or ethanol, which provide particle stability through electrostatic stabilization, but can also lead to lower initiator site densities as the alcohol can outcompete the alkoxysilane for the silica surface. Since the optimization of the initiator attachment was desired, surface modifications were carried out in MEK and MIBK as well as with MEK and triethylamine (TEA). TEA was used to further deprotonate the silica surface, allowing for an increase in the alkoxysilane initiator surface concentration. Elemental analysis was used to quantify the initiator site density (σi) of the 3-BIDS, 11-BIDS, and 15-BIDS modified nanoparticles that were functionalized in MEK, MEK with TEA, and MIBK. In these reactions, σi was governed by the initiator concentration in solution as quantified by the initiator to surface site ratio, [I]/[S], which is the ratio of the initiator concentration, [I], to the number of hydroxyl groups on the silica surface, [S]. The number of -OH surface groups was calculated by assuming that the surface density of OH groups on silica was 5 OH/nm2 and multiplying this density by the surface area of silica in the reaction bath.48,49 Based on these calculations, the [I]/[S] ratio was varied between 1.5 and 8.0. The initiator site densities in Figure 3 are grouped by the solvents in which the silica nanoparticles were functionalized: MEK, MEK with TEA, or MIBK. The unmodified silica nanoparticles contained a small fraction of carbon (approximately 4.2%), which is consistent with a previous NMR study.50 The small amount of carbon is due to the presence of the hydrolysis products of TEOS, e.g., its singly, doubly, and triply hydrolyzed monomers,40,41 as the St€ober reaction does not completely proceed to completion, resulting in a small number of ethoxy groups in the inorganic silica matrix. The percent carbon from the unmodified silica nanoparticles was taken into account when determining σi in eq 2. Initially, an initiator ratio of 1.5 in MEK was used based on previous research in which the same initiator ratio was used to attach 3-BIDS to silica in THF.8,9 However, the same initiator ratio in MEK produced a low σi of 0.97 initiators/nm2 for 3-BIDS and 0.73 initiators/nm2 for 11-BIDS. These initiator densities are undesirable as they led to low PMMA graft densities, ranging from σn = 0.01-0.07 polymers/nm2 in this study. To increase the initiator site density, the initiator ratio was increased to 5 and 1 mol equiv of TEA was added because TEA can deprotonate the surface hydroxyl groups, enabling the (48) Suratwala, T. I.; Hanna, M. L.; Miller, E. L.; Whitman, P. K.; Thomas, I. M.; Ehrmann, P. R.; Maxwell, R. S.; Burnham, A. K. J. Non-Cryst. Solids 2003, 316, 349–363. (49) Vansant, E. F.; Van Der Voot, P.; Vrancken, K. C. Characterization and Chemical Modification of the Silica Surface; Elsevier: Amsterdam, 1995. (50) van Blaaderen, A.; Kentgens, A. P. M. J. Non-Cryst. Solids 1992, 149, 161– 178.

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Figure 2. 1H NMR scans of the (a) 15 CSL ATRP precursor, pentadec-14-enyl-2-bromo-2-methylpropanoate, and (b) 15 CSL ATRP initiator, (15-(2-bromoisobutyryl)pentadecyl) dimethylethoxysilane (15-BIDS).

attachment of alkoxysilane initiators.51,52 In addition, researchers have found that brush growth of polymers on surfaces modified in the presence of TEA are less rough and grow more uniformly with time.1,34,51 Figure 3 shows that σi increases with TEA addition at an initiator ratio of 5 for the 3-BIDS and 11-BIDS initiators. However, functionalizations carried out in the presence of TEA had a higher %N (%N = 0.6-1.0%), which could be attributed to the adsorption of TEA. Therefore, experiments were per(51) Carrot, G.; Diamanti, S.; Manuszak, M.; Charleux, B.; Vairon, J.-P. J. Polym. Sci. A: Polym. Chem. 2001, 39, 4294–4301. (52) Kiselev, A. V.; Kuznetsov, B. V.; Lanin, S. N. J. Colloid Interface Sci. 1979, 69, 148–156.

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formed with MIBK based on the rationale that reducing the polarity of the solvent would reduce its interaction with silica, thus lowering the competition between the solvent and the alkoxysilane initiators. Additionally, MIBK has a higher boiling point than MEK, and thus, in MIBK, the surface functionalizations can proceed at a higher temperature, favoring the condensation of the ATRP alkoxysilane initiators onto silica. A high σi is obtained in MIBK with an initiator ratio of 8 without the adsorption of nitrogen as the %N was below the detection limit of the elemental analyzer. As a result, all successive particles were functionalized in MIBK with an initiator ratio of 8 because these conditions provided the best balance between Langmuir 2009, 25(23), 13351–13360

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Figure 3. Initiator graft density (σi) as a function of the initiator to surface site ratio ([I]/[S]) and the carbon spacer length (CSL) of the alkoxysilane ATRP initiators.

particle stability and the formation of dense alkoxysilane monolayers on silica. Based on these conditions, the initiator site density decreases as the carbon spacer length increases. For example, in MIBK, σi = 3.64 ( 0.03, 2.23 ( 0.03, and 1.80 ( 0.02 initiators/ nm2 for 3-BIDS, 11-BIDS, and 15-BIDS, respectively. These values are similar to those reported in the literature for initiator functionalized silica nanoparticles.7,8 Ultimately, σi, or inversely the area per molecule, σi-1, decreases (or increases for σi-1) with increasing CSL as the alkoxysilane initiator occupies more space on the silica surface. SEM images of the 3-BIDS, 11-BIDS, and 15-BIDS functionalized particles were similar to those of the unmodified silica nanoparticles in Figure 1 as the surface modification did not change the size of the silica nanoparticles from R = 23 ( 1 nm, indicating that the initiator monolayers are too small to be imaged by SEM, which is expected. This was also confirmed with DLS as the average Rh ≈ 23 nm. 3.3. Stability of ATRP Initiator Functionalized Nanoparticles. Visual observations were used to examine the stability of the ATRP initiator functionalized silica nanoparticles in solutions of 4:1 (v/v) methanol-water solutions. These studies permit us to determine how the initiator CSL impacts particle stability, and then allow us to connect how the stability of the functionalized colloids affects the grafting of PMMA from the initiator monolayers in methanol-water solutions. Figure 4 shows vials that contain the 3-BIDS, 11-BIDS, and 15BIDS initiator-coated nanoparticles in 4:1 (v/v) methanol-water solutions after 10 days. The 3-BIDS and 11-BIDS coated nanoparticles remained uniformly dispersed and were stable, whereas the 15-BIDS coated nanoparticles were unstable and settled out in a matter of minutes. These results indicate that the silica nanoparticles functionalized with the 3-BIDS and 11-BIDS initiators are hydrophilic, whereas the nanoparticles coated with the 15BIDS initiators are hydrophobic. Thus, it appears that the Langmuir 2009, 25(23), 13351–13360

Article

Figure 4. Functionalized silica nanoparticles in 4:1 (v/v) methanol-water solutions. Vials 3, 11, and 15 contain 3-BIDS, 11-BIDS, and 15-BIDS functionalized silica nanoparticles, respectively. Silica nanoparticles with 3-BIDS and 11-BIDS initiators are stable after 10 days, whereas silica nanoparticles with 15-BIDS are not stable and settle out of suspension in a matter of minutes.

initiator-functionalized nanoparticles are both electrostatically and sterically stabilized for CSLs below 11 carbon repeat units. However, increasing the ATRP spacer length to 15 carbons appears to tip the balance of the unfavorable interactions between the carbon spacer and the methanol-water solvent, resulting in nanoparticle aggregation as the 15-BIDS initiators would prefer to interact with themselves as opposed to the solvent. Increasing the CSL of the initiator causes the immobilized monolayer to change from being hydrophilic to hydrophobic, which should impact its interactions with hydrophobic MMA monomers and affect the grafting of PMMA from silica nanoparticle surfaces. 3.4. SI-ATRP of PMMA from Silica Nanoparticle Surfaces. A major focus is to quantify how the CSL of the ATRP initiator impacts the grafting of PMMA from the surface of silica nanoparticles using aqueous SI-ATRP in 4:1 (v/v) methanolwater solutions at ambient temperature. In addition, these reactions were carried out in the presence of either a CuBr or CuCl catalyst to determine which catalyst enables better control over the grafting reaction. The reaction times with CuBr and CuCl were varied to achieve a target number average molecular weight (Mn) of approximately Mn ≈ 50 000 g/mol. Thus, the polymerizations catalyzed with CuBr were carried out for two hours, while the polymerizations catalyzed with CuCl were carried out for six hours. The effectiveness of varying the CSL and catalyst type on SI-ATRP of PMMA was determined with the following techniques: (1) GPC, to quantify the molecular weights and polydispersity of the graft and free polymers; (2) elemental analysis (EA), to determine the PMMA brush graft density; and (3) DLS and SEM, to determine particle size and brush graft thickness of the PMMA-g-silica nanoparticles. DOI: 10.1021/la901918v

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Figure 5. Representative GPC results for the graft (N) and free (P) PMMA polymers produced from the CuCl-catalyzed SI-ATRP of MMA from functionalized silica nanoparticles. Polymerization conditions: [MMA] = 4.66 mM, [CuCl] = 0.079 mM, and [2,2-bipyridine] = 0.236 mM in 4:1 (v/v) methanol/water solutions.

The GPC results of the SI-ATRP reactions of PMMA from silica are shown in Figure 5. The retention times are shown on the abscissa from which the number average molecular weight (Mn), mass average molecular weight (Mw), and the polydispersity index (PDI = Mw/Mn) can be determined. In addition, each figure displays the chromatographs of the free and graft polymers for two separate but identical polymerization experiments. Based on the overlaps of the chromatographs in each figure, the SIATRP reactions are repeatable and the Mn of the graft and free polymers are identical. Thus, we do detect some free polymer; however, the amount is very small, on the order of milligrams, which is much lower than the initial amount of MMA monomer (23 g) in the reaction. Based on the gram quantities of graft polymer harvested from the large total surface area of the silica nanoparticles (35 m2), most of the monomer is polymerized from the nanoparticle surface. Consequently, either a small amount of free polymer grows in solution to the same Mn of the graft polymer during the aqueous SI-ATRP of PMMA in 4:1 (v/v) methanol/water solutions or a small number of chains (e.g., 1 in 1000) detach from the silica nanoparticles during the washing steps. Further, based on the GPC results in Figure 5, the reactions catalyzed by CuBr were uncontrolled leading to PMMA graft layers with a fairly large polydispersities, ranging between PDI = 2.2-2.5. In contrast, the reactions with CuCl were controlled, as indicated by a fairly low polydispersity, ranging between PDI = 1.3-1.4. The ability to control aqueous SI-ATRP reactions of PMMA with CuCl and not CuBr has also been confirmed by various other researchers.34-38 Wang et al. found that brominated initiators had a faster initiation than the chlorinated initiator.36 For example, under the same conditions, the brominated initiator reached a conversion of 90% in 15 min, whereas it took more than 10 h for the chlorinated initiator under identical conditions for ATRP of OEGMA in water at 20 °C.36 Edmundson and Huck as well as Chatterjee et al. also found that using CuBr led to a higher 13358 DOI: 10.1021/la901918v

PDI as compared to using CuCl.34,35 These findings are supported by the results of our research since it took six hours for the CuCl mediated polymerizations to reach the same Mn as the CuBr catalyzed polymerizations did in two hours. Huck and co-workers have also found that the fast polymerization rates lead to less dense graft layers.37 Our results also support this as uncontrolled SI-ATRP reactions with the CuBr catalyst result in lower polymer graft densities (σn) and lower initiation efficiencies (σn/σi) of PMMA on silica, which is observed in Figure 6a across the 3-BIDS, 11-BIDS, and 15-BIDS initiator monolayers on which the graft densities for the CuBr system remain consistently below that in the CuCl reactions. Moreover, the hydrodynamic brush thicknesses are greater with the CuBr mediated polymerizations, as indicated in Figure 6b. The larger hydrodynamic thicknesses are due to the longer polymer chains present in the more polydisperse PMMA graft layers. The impact of increasing the brush thickness with respect to the CuCl catalyzed polymerizations are shown in the SEM micrographs of Figure 7. SEM images for the PMMA-g-silica particles reacted in the presence of CuBr were similar to those in Figure 7, but were not shown for the sake of brevity. Figure 7 demonstrates that greater brush thicknesses are linked to more polymer being grafted to the nanoparticles, causing them either to stick together or form a sheet. Sheet formation is favored when each core-shell PMMA-g-silica particle consist mostly of the polymer brush as supported the comparison of the hydrodynamic brush thickness for the CuCl polymerization from 15-BIDS monolayer (Rh = 294 nm) to the radius of the silica core (R = 23 nm), yielding a particle softness (L), or the ratio of the brush thickness to particle size of L = 12.8. Sheet formation, indicative of thicker brushes, occurred at lower initiator CSL for the PMMA-g-silica nanoparticles catalyzed with CuBr. Dramatic increases in brush thickness are also associated with longer initiator CSL, as shown in the SEM micrographs of Figure 7 and the DLS measurements in Figure 6b. For graft Langmuir 2009, 25(23), 13351–13360

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Article Table 1. Characteristics of PMMA Graft Polymers As a Function of ATRP Initiator Carbon Spacer Length (CSL) from CuBr or CuCl Catalyzed SI-ATRP, Including Number Average Molecular Weight (Mn), Polydispersity Index (PDI = Mw/Mn), Brush Thickness (h), Polymer Graft Density (σn), and Initiator Efficiency (σn/σi) Mn brush thickness, σn catalyst CSL (kg/mol) PDI h (nm) (polymers/nm2) σn/σi CuBr CuCl

Figure 6. (a) PMMA graft density (σn) as a function of initiator carbon spacer length (CSL). (b) Hydrodynamic brush thickness (h) of the PMMA graft layer as a function of initiator carbon spacer length (CSL). Lines drawn to aid viewing.

3 11 15 3 11 15

62 45 53 49 46 58

2.5 2.2 2.5 1.3 1.3 1.4

125 ( 4 200 ( 6 597 ( 30 45 ( 1 53 ( 1 294 ( 8

0.10 ( 0.00 0.18 ( 0.01 0.30 ( 0.00 0.15 ( 0.01 0.20 ( 0.00 0.43 ( 0.00

0.03 0.08 0.17 0.04 0.09 0.24

can be correlated to the instability of the initiator-functionalized nanoparticles, where the stability studies indicate that the nanoparticle surfaces become increasing hydrophobic with the initiator CSL in 4:1 (v/v) methanol-water solutions. Greater hydrophobicity of the functionalized nanoparticles favors the adsorption of MMA, a hydrophobic molecule. Thus, our results indicate that in methanol-water solutions longer initiator CSLs lead to the increased adsorption of MMA, enabling higher PMMA graft densities as well higher initiator efficiencies. Lower PMMA graft densities and initiation efficiencies were obtained from the 3-BIDS and 11-BIDS monolayers as shown in Figure 6b. These results reflect the subtle balance between enabling particle stability and increasing the adsorption of MMA in methanol-water solutions. While the shorter CSL initiators provide a hydrophilic surface, elevating particle stability, the hydrophilic nature of the surface most likely decreases the adsorption of MMA and may even cause the monomers to deplete from the 3-BIDS and 11-BIDS monolayers if the interactions between the monomers and the monolayers are exceedingly unfavorable. Similar low PMMA graft densities and poor initiation efficiencies were noted by Huck and co-workers from similar 11 CSL initiators on gold in 4:1 methanol-water solutions.29,30 Thus, to achieve higher PMMA graft densities and efficiencies from shorter CSL initiators in methanol-water solutions, our result suggest that greater optimization of the SI-ATRP reaction is needed. This optimization can result from developing initiators that encourage the adsorption of MMA in polar solutions.

4. Conclusions

Figure 7. SEM of PMMA-g-silica nanoparticles from CuCl catalyzed SI-ATRP. PMMA was grafted from the following monolayers (a) 3-BIDS, (b) 11-BIDS, and (c) 15-BIDS.

PMMA of target Mn = 50 000 g/mol, thicker graft layers arise from higher graft densities (Figure 6a) and grafting efficiencies with longer initiator CSLs. The increase of σn and σn/σi with CSL Langmuir 2009, 25(23), 13351–13360

The goal of this work was to quantify the impact of the alkoxysilane initiator carbon spacer length (CSL) on grafting PMMA from silica nanoparticles with SI-ATRP in methanolwater solutions at room temperature. Three monoethoxysilane initiators were synthesized that varied in CSL from 3, 11, and 15 carbon repeat units. The initiators were termed 3-BIDS, 11-BIDS, and 15-BIDS, respectively, in which the last initiator is a new molecule. The attachment of the ATRP initiators to monodisperse silica nanoparticles (radius, R = 23 nm) was optimized in methyl isobutyl ketone (MIBK), a solvent in which the nanoparticles were electrostatically stable. The initiator site densities varied between 1.8-3.6 initiators/nm2 with decreasing CSL, indicating that the silica nanoparticles were covered in a dense, covalently attached monolayer of ATRP initiators. The “grafting from” reactions were optimized to produce graft PMMA polymers of target Mn = 50 000 g/mol using either CuCl or CuBr catalyst. Results from GPC, DLS, and elemental analysis (EA) indicate that using CuCl instead of CuBr provides better control over the SI-ATRP reaction as graft PMMA possessed fairly narrow polydispersities between PDI = 1.3-1.4. In addition, a small amount of free polymer (on the order of milligrams) DOI: 10.1021/la901918v

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was detected in the methanol-water solutions during polymerization whose Mn matches that of the graft PMMA. PMMA graft densities (σn) and initiation efficiencies (σn/σi) increased strongly with initiator CSL, leading to thicker graft layers. The increase of σn and σn/σi with CSL was correlated to the instability of the initiator-functionalized nanoparticles, where visual particle stability studies indicate that the nanoparticle surfaces become more hydrophobic with initiator CSL in methanol-water solutions as the particles with longer CSL initiators aggregate in the alcoholic media. It was concluded that greater hydrophobicity of the functionalized nanoparticles favors the adsorption of hydrophobic MMA, leading to higher PMMA graft densities and initiation efficiencies. Thus, a subtle balance

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must be struck between enabling particle stability and increasing MMA adsorption to augment PMMA graft densities and initiation efficiencies in methanol-water solutions. Acknowledgment. This work was supported by the National Science Foundation (NSF) in the Division of Chemical, Bioengineering, Environment and Transport Systems (CBET-0644890). We gratefully acknowledge discussions with Ramon Espino and Mark Aronson. Supporting Information Available: Scheme 2 and additional experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2009, 25(23), 13351–13360