Living Anionic Surface-Initiated Polymerization (LASIP) of a Polymer

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Living Anionic Surface-Initiated Polymerization (LASIP) of a Polymer on Silica Nanoparticles Qingye Zhou, Shuangxi Wang, Xiaowu Fan, and Rigoberto Advincula* Department of Chemistry, University of Alabama at Birmingham, Birmingham, Alabama 35294

Jimmy Mays* Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600 Received November 7, 2001. In Final Form: February 7, 2002 To demonstrate living anionic surface-initiated polymerization (LASIP) on silica nanoparticles, the initiator precursor 1,1-diphenylethylene (DPE) was functionalized with alkyldimethylchlorosilane and grafted onto silica particle surfaces. n-BuLi was used to activate the DPE, which allowed the anionic polymerization of the styrene monomer to proceed in benzene solution. A high-vacuum reactor was used to allow polymerization from the surface of silica particles under anhydrous solution conditions. The dispersion of the DPE functionalized silica particles showed a distinct red color indicating an activated nanoparticle-DPE-n-BuLi complex suitable for anionic polymerization. The degree and mechanism of polymerization were determined based on characterization of the grafted and detached polystyrene chains using thermogravimetric analysis, size exclusion chromatography, NMR, and Fourier transform infrared spectroscopy. In addition, atomic force microscopy and X-ray photoelectron spectroscopy were used to characterize the polymer-coated nanoparticles. The importance of activation of the grafted initiator, control of aggregation, and removal of the excess n-BuLi for high molecular weight formation is emphasized. While the polydispersities are broader compared to those obtained by solution polymerization of a free initiator, a living anionic polymerization mechanism is still observed.

Introduction Grafting polymers onto solid surfaces is attracting more and more interest due to their important applications ranging from colloidal stabilizers1,2 to nanocomposite materials.3,4 To tailor the surface properties of these inorganic particle materials, a number of model surface grafting techniques have been used on flat surfaces including the formation of molecular monolayers.5 One of the most reported methods for modifying inorganic surfaces with polymers involves the physisorption of homopolymers or block copolymers.6 However, such adsorbed layers are susceptible to removal, for example, by exposure to a thermodynamically good solvent. Furthermore, it is often difficult to precisely control the physical structure of such adsorbed layers. Thus, adhesion between the polymer chains and the substrate may be greatly enhanced if the chains are tethered to the inorganic surfaces by covalent bonds. One approach involves the reaction of a preformed end-functionalized polymer with appropriate reactive sites on the inorganic surfaces.7 The disadvantage of this method of grafting is that the chains, * Authors to whom correspondence should be addressed. R. Advincula([email protected])andJ.Mays([email protected]). (1) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1992, 100, 31. (2) The Effect of Polymers on Dispersion Properties, Tadros, T. F., Ed.; Academic Press: London, 1982. (3) Alexandre, M.; Dubois, P. Mater. Sci. Eng. 2000, 28, 1. (4) Giannelis, E. P.; Krishnamoorti, R.; Manias, E. Adv. Polym. Sci. 1999, 138, 108. (5) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991. (6) Fleer, G. J.; Cohen-Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993. (7) Koberstein, J.; Laub, C. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1999, 40 (2), 126.

which are attached at the beginning of the reaction, sterically shield the remaining reactive sites on surfaces.8 On the other hand, surface-initiated polymerization (SIP) promotes polymerization of monomers from initiator sites already attached to surfaces.9 Thus, it appears that the latter approach is a more promising and versatile method for preparing “polymer brushes”. Polymer brushes by SIP can be prepared using free radical9 and cationic polymerizations,10 ring-opening metathesis polymerization (ROMP),11 atom transfer freeradical polymerization (ATRP),12,16 polymerizations using 2,2,6,6-tetramethyl-1-piperidyloxy (TEMPO),13 and anionic polymerization.14,21 All of these methods have been shown to be suitable for polymerizing different types of monomers. However, living anionic polymerization should show the best possibility of control of the overall polymer architecture for block, graft, and uniform brush lengths in SIP. Although some theoretical predictions have been used to calculate the molecular weight of polymer brushes grown by SIP on flat surfaces,15 a detailed characterization (8) Balazs, A.; Lyatskaya, Y. Macromolecules 1998, 31, 6676. (9) Prucker, O.; Ruhe, J. Langmuir 1998, 14, 6893. (10) Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 243. (11) Weck, M. Jackiw, J. J.; Rossi, R. R.; Weiss, P. S.; Grubbs, R. H. J. Am. Chem. Soc. 1999, 121, 4088. (12) Ejaz, M.; Yamamoto, S.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macromolecules 1998, 31, 5934. (13) Husseman, M.; Malmstrom, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Genoit, d. G.; Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P.; Hawker, C. J. Macromolecules 1999, 32, 1424. (14) (a) Zhou, Q.; Nakamura, Y.; Inaoka, S.; Park, M.; Wang, Y.; Mays, J.; Advincula, R. Polym. Mater. Sci. Eng. Prepr. (Am. Chem. Soc.) 2000, 82, 291. (b) Advincula, R.; Zhou, Q.; Nakamura, Y.; Inaoka, S.; Park, M.-K.; Wang, Y.; Mays, J. Polymer Nanocomposites: Synthesis, Characterization, and Modeling; ACS Symposium Series Vol. 804; American Chemical Society: Washington, DC, 2002; p 39. (15) Milner, S. T. Science 1991, 252, 905.

10.1021/la015670c CCC: $22.00 © 2002 American Chemical Society Published on Web 03/22/2002

Living Anionic Surface-Initiated Polymerization

by determining the molecular weight and polydispersity is not available. This is due in part to the small amounts of polymer generated on these flat surfaces. Determining these properties is paramount for the complete understanding of the initiation process, the mechanism of polymerization, and the presence of side reactions. This difficulty can be overcome by conducting the polymerization on small particle surfaces in which the collective total surface area is large. In fact, there is a growing interest in this polymerization process applied to nanoparticle surfaces.16 Ruhe and co-workers17 have demonstrated this rationale by attaching azobisisobutyl nitrile (AIBN) monofunctional chlorosilane initiator on silica particles for free radical polymerization of styrene and obtained samples for the complete characterization of molecular weight and grafting density of the polymer brush in contrast to another work on flat surfaces.9 Similar work on grafting to particles has been done by Suter and co-workers18 who tethered an azo initiator by ion exchange to mica. A styrene-like initiator is another choice in free radical polymerization to grow polymer brushes from clay surfaces.19 Sogah and co-workers20 used living free radical polymerization to grow polystyrene from clay surfaces. Recently, Patten and co-workers reported the use of ATRP to polymerize polystyrene and poly(methyl methacrylate) on silica nanoparticles.16 Anionic polymerization is the most versatile method to make well-defined architectures of polymers. It has been employed to grow polymer brushes from various small particles such as silica gels,21 graphite and carbon black,22 and also flat surfaces.14,23,24 We have recently reported our initial results on living anionic polymerization on clay nanoparticles.25 Difficulties can arise due to the effect of moisture and other impurities on anionic polymerization, especially with polymerization from clay surfaces. Other problems can be present. For example, a major limitation of the prior work by Oosterling et al.21 is that the choice of tert-butyllithium (t-BuLi) as the initiator for styrene polymerization from silica in the presence of toluene is inefficient. t-BuLi is a very slow and inefficient initiator for styrene in hydrocarbon solvents, yielding broad molecular weight (MW) distributions and higher than expected MWs.26 They used a 6-fold excess of t-BuLi and assumed that the molecular weights of grafted and nongrafted polymers were the same. They were not able to investigate the differences between free polymerization in the solution and confined polymerization on the surface. Yet, despite these difficulties, anionic polymerization remains attractive for the synthesis of complex and welldefined macromolecular architectures.27 (16) von Werne, T.; Patten, T. J. Am. Chem. Soc. 2001, 123, 7497. (17) Prucker, O.; Ruhe, J. Macromolecules 1998, 31, 592. (18) Meier, L. P.; Shelden, R. A.; Caseri, W. R.; Suter, U. W. Macromolecules 1994, 27, 1637. (19) Akelah, A.; Moet, A. J. Mater. Sci. 1996, 31, 3589. (20) Weimer, M. W.; Chen, H.; Giannelis, E. P.; Sogah, D. Y. J. Am. Chem. Soc. 1999, 121, 1615. (21) Oosterling, M. L. C. M.; Sein, A.; Schouten, A. J. Polymer 1992, 33, 4394. (22) Tsubokawa, N.; Yoshihara, T.; Sone, Y. Colloid Polym. Sci. 1991, 269, 324. (23) Zhou, Q.; Wang, S.; Fan, X.; Pispas, S.; Sakellariou, G.; Hadjichristides, N.; Mays, J.; Advincula, R. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 2001, 42, 59. (24) Quirk R.; Mathers R. Polym. Bull. 2001, 6, 471. (25) Zhou, Q.; Fan, X.; Xia, C.; Mays, J.; Advincula, R. Chem. Mater. 2001, 13, 2465. (26) (a) Morton, M.; Fetters, L. J. Rubber Chem. Technol. 1975, 48, 359. (b) Hadjichristidis, N.; Iatrou, H.; Pispas, S.; Pitsikalis, M. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3211. (27) For a recent review, see: Pitsikalis, M.; Pispas, S.; Mays, J.; Hadjichristidis, N. Adv. Polym. Sci. 1998, 135, 1.

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This paper reports our detailed investigation on the formation of covalently attached polystyrene on silica nanoparticle surfaces by living anionic surface-initiated polymerization (LASIP). To investigate living anionic polymerization on particle surfaces, we have utilized 1,1diphenylethylene (DPE) with a chlorosilane end group as an initiator precursor attached to silica nanoparticles. The DPE group avoids the potential for “two-dimensional polymerization” on the surface since DPE cannot propagate. Thus, in principle, each DPE group should initiate growth of one polymer chain after activation with n-BuLi. Using a high-vacuum reactor with a modified filter unit, anionic polymerization was used to form polystyrene “grafted from” the Si nanoparticles. The composition and physical and thermal properties of the nanocomposite material were analyzed. After the polystyrene was detached from the particle surface, the MW, the polydispersity, and the structural properties of the polymer were also analyzed. Interesting insights on the polymerization phenomena on nanoparticle surfaces were obtained. Experimental Section Materials. Smooth silica spherical nanoparticles (Aerosil A200, Degussa) with a specific surface area of 175-225 m2/g and a 12-20 nm particle diameter were dried overnight at 120 °C under vacuum. Benzene (Fisher, >99%) and tetrahydrofuran or THF (Aldrich, 99.9%) were purified according to the anionic polymerization standards, as described in the literature.26 Styrene (Aldrich, 99%) was stirred with CaH2 overnight at room temperature and degassed three times on the vacuum line. The “roughly” purified styrene was treated with dibutylmagnesium (MgBu2, Aldrich, 1.0 M in heptane) for a few hours and then finally distilled into ampules. n-Butyllithium (n-BuLi) was prepared by the reaction of 1-chlorobutane with lithium powder in hexane under vacuum. Methanol was treated with CaH2 overnight and degassed three times on the vacuum line before distillation into ampules. Characterization. Number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity indices (Mw/Mn) were determined by size exclusion chromatography (SEC) relative to calibration with polystyrene standards in THF. SEC in THF (flow rate, 1 mL/min; Waters Styragel 100, 500, 103, 104, and 105 Å columns) was obtained by using a Waters 510 pump with a Waters 410 differential refractometer detector. A Bruker ARX-300 proton nuclear magnetic resonance (1H NMR) instrument was used to obtain 1H NMR and 13C NMR spectra in CDCl3. A Mettler Toledo TG 50 thermal gravimetric analysis (TGA, 20°/min, N2) unit was employed to determine the densities of DPE initiator precursor and bound polymer. The phase transition temperatures were determined by a Mettler Toledo DSC 30 differential scanning calorimeter (10°/min, N2). The solutions or suspensions of cleaved polymer, silica with bound polymer, and silica with DPE were dropped to a KBr IR window. The IR spectra were obtained from a Bruker FTIR from 4000 to 600 (cm-1) wavenumbers. A quartz crystal microbalance (QCM), Maxtek Inc., with a PM-740 frequency counter was employed to measure the density of the DPE initiator precursor self-assembled monolayer (SAM). The quartz crystal electrode was modified for SAM formation of the silane initiator.30 Ellipsometric thickness measurements were performed using the Multiskop ellipsometer (Optrel GmbH, Germany) with 632.8 nm He-Ne laser beam as the light source. Both delta and psi values (thickness data) were measured and calculated using integrated specialized software that came with the instrument. Atomic force microscopy (AFM) imaging was performed in air using a PicoScan system (Molecular Imaging) equipped with an 8 × 8 µm scanner. Magnetic-ac (Mac) mode (a noncontact mode) was used for all of the AFM images. A Mac lever, a silicon nitride based cantilever coated with magnetic film, was used as an AFM tip. The polymer bound nanoparticles were spin-casted to freshly cleaned Si wafer substrates. X-ray photoelectron spectroscopy (XPS) was done on a Kratos Axis 165 Multitechique Electron Spectrometer system. The samples for analysis were prepared by pressing dried powder samples as a thick layer of ∼1 mm on a depressed stainless steel

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sample holder with inert adhesive.28 This procedure and thickness ensured that only the nanocomposite sample was probed. A monochromatic Al KR X-ray source (1486.6 eV) was applied to excite the photoemission at q ) 0 (normal to the surface). Survey scans were collected from 0 to 1400 eV to obtain elemental composition information. Synthesis of 4-Bromo-DPE (1). To a 500 mL round-bottomed flask fitted with a dry nitrogen inlet septum, methyltriphenylphosphonium iodide (31 g, 76 mmol) was suspended in dry THF under a nitrogen atmosphere. To the suspension was added n-BuLi (31 mL of 2.5 M in hexane, 76 mmol) at room temperature with stirring. The mixture became dark red, and the solution was stirred for a half-hour. 4-Bromobenzophenone (20 g, 76 mmol) was then added via a syringe over 30 min with vigorous stirring at room temperature. After this addition, the mixture became yellow and was stirred overnight at room temperature under a nitrogen atmosphere and then diluted with 150 mL of chloroform and 150 mL of dilute hydrochloric acid aqueous solution (0.01 N). The organic phase was collected, washed, and dried over MgSO4. The solvent was removed by rotary evaporation, and the resultant residue was purified by chromatography on silica gel using n-hexane as the eluent to yield 16.8 g of 4-bromo-DPE as a yellow oil. The product could also be purified by distillation. 1H NMR (CDCl ): δ 7.44 (2H, d, J ) 8.7 Hz, Ar-H), 7.30 (5H, 3 m, Ar-H), 7.19 (2H, d, J ) 8.7 Hz, Ar-H), 5.44 (2H, d, J ) 3.0 Hz, CdCH2). 13C NMR (CDCl3, 300 MHz): 149.5, 141.4, 140.9, 131.8, 130.4, 128.8, 128.7, 128.4, 122.3, 115.2. Synthesis of 4-(11′-Undecenyl)-DPE (2). Activated magnesium turnings (1.5 g, 51.8 mmol) and a small iodine particle were placed in a three-necked flask equipped with a reflux condenser and purged with nitrogen. A portion of a solution of 1-bromo-11-undecene (12 g, 51.8 mmol) in 20 mL of dry diethyl ether was added to the flask. Following Grignard reaction initiation, the rest of the 1-bromo-11-undecene solution was added over 30 min. After refluxing for 2 h, the Grignard reagent was transferred to an addition funnel and was slowly added to a mixture of 40 mg of 1,3-bisdiphenylphosphinopropane nickel(II) chloride or Ni(dppe)Cl2 and 5 g (19.3 mmol) of 4-bromo-DPE in 20 mL of diethyl ether. After stirring at room temperature for 12 h, the mixture was acidified with dilute HCl solution and extracted with diethyl ether three times. The combined organic extracts were washed with saturated aqueous Na2CO3 and dried over anhydrous Na2SO4. Distillation under reduced pressure gave 3.92 g of 4-(undec-9-enyl)-DPE as a colorless oil. 1H NMR (CDCl3): δ 7.34-7.32 (5H, m, Ar-H), 7.24 (2H, d, J ) 8.7 Hz, Ar-H), 7.13 (2H, d, J ) 8.5 Hz, Ar-H), 5.44-5.41 (1H, m, -CHd ), 5.40 (2H, d, J ) 3.0 Hz, dCH2), 5.03-5.01 (2H, m, dCH2). 2.61 (2H, t, J ) 6.6 Hz, -CH2-), 2.05-1.28 (m, 16H, -(CH2)8-). 13C NMR (CDCl3, 300 MHz): 150.4, 143.0, 142.2, 139.7, 139.1, 128.8, 128.6, 128.5, 128.0, 114.6, 114.1, 36.1, 34.3, 33.1, 31.9, 29.9, 29.8, 29.7, 29.6, 29.4. Synthesis of Monochlorosilane (3). 4-(10′-Undecenyl)-DPE (2) (1.73 g, 5.94 mmol) was dissolved in 10 mL of dry toluene followed by addition of 851.5 mg (9 mmol) of chlorodimethylsilane and three drops of the catalyst platinum(0)-1,3-divinyl-1,1,3,3tetramethyldisiloxane complex in xylene under a N2 atmosphere (Scheme 1). The mixture was heated overnight under a N2 atmosphere at 50-60 °C, and the solvent and unreacted chlorodimethylsilane were removed under vacuum. 1H NMR analysis indicated that all 4-(10′-undecenyl)-DPE (2) was completely consumed. Yield, 1.82 g. 1H NMR (CDCl3): δ 7.34-7.32 (5H, m, Ar-H), 7.24 (2H, d, J ) 8.7 Hz, Ar-H), 7.13 (2H, d, J ) 8.5 Hz, Ar-H), 5.40 (2H, d, J ) 3.0 Hz, dCH2), 2.61 (2H, t, J ) 6.6 Hz, -CH2-), 2.05-0.71 (m, 16H, -(CH2)10-), 0.29 (s, 6H, -Si(CH3)2-). Immobilization of the Initiator Precursor. The immobilization of the DPE derivative on the silica surface was done by mixing excess DPE derivative with dried toluene and pyridine and dispersing silica nanoparticles with vigorous stirring for several days in a glovebox.23,29 After the reaction of chlorodimethylsilane with hydroxyl on the silica surfaces, the excess DPE was removed by an alternate dissolution and centrifugation (28) Erdem, B.; Hunsicker, R.; Simmons, G.; Sudol, E. D.; Dimonie, V.; El-Aasser, M. Langmuir 2001, 17, 2664. (29) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1999, 15, 3759.

Zhou et al. Scheme 1. Schematic Diagram of Initiator Precursor Synthesis (DPE) Functionalized with Silane Coupling Agent and Alkyl Spacer

Scheme 2. Immobilization of DPE Initiator Precursor and Activation by the Addition of n-BuLia

a

Monomer is eventually added and results in polymerization.

Figure 1. Schematic diagram of the vacuum polymerization reactor having several compartments: (A) ampule containing styrene; (B) ampule containing n-BuLi; (C) ampule containing MeOH. procedure, which was repeated more than 10 times. The final density was obtained after no weight change could be seen with washing by thermogravimetric monitoring. Thermogravimetric analysis (TGA) results showed that 6.9% (w/w) DPE was attached on the silica surface. The process for immobilization of DPE onto silica and polymerization of styrene is shown in Scheme 2. Polymerization Procedure. The reaction system for surfaceinitiated anionic polymerization is shown in Figure 1. The ampules containing n-BuLi (or sec-BuLi), styrene, and MeOH were first sealed onto the reactor. A 0.5 g portion of the DPEcoated silica particles was put into the reactor, which was then attached to the vacuum line. The reactor was heated to 120 °C

Living Anionic Surface-Initiated Polymerization Scheme 3. Surface Grafting of the Initiator and Polymerization Scheme on the Surface of the Si Nanoparticle

Langmuir, Vol. 18, No. 8, 2002 3327 extractable polymer.18 The procedure was continued until no change in the amount of bound polymer was observed with washing by TGA monitoring. The material which could not be extracted was considered as bound polymer. The remaining solids were dried at 50 °C in a vacuum oven for a few days. The first supernatant was precipitated by MeOH and dried for further tests. The material containing bound polymer was stirred with HF in THF or refluxed with 30% KOH aqueous/alcohol/THF to cleave off bound polymer. After centrifugation, TGA was run to compare the weight loss before and after cleaving bound polymer. The supernatant was dried in a vacuum oven for a few days and dissolved in chloroform-d for NMR and THF for SEC and Fourier transform infrared (FTIR) spectroscopy characterization.

Results and Discussion

using a silicone oil bath and evacuated for at least 8 h. Benzene and a small amount of THF were distilled into the reactor through the vacuum line, and the reactor was sealed off. The break-seal to the n-BuLi ampule was broken to add the initiator (we usually used more than 5-fold excess of the theoretical equivalent needed to initiate DPE) into the reactor. The initiation was allowed to proceed for at least 6 h with stirring so as to make sure all DPE was activated (Scheme 3). It is very important to dry the particles at high enough temperatures. Room temperature is not adequate to remove trace impurities such as H2O, which will result in broadened molecular weight distributions.25 TGA was employed to monitor the weight change of the DPE derivative with temperature, which showed that no decomposition of the initiator precursor occurred during drying. This is another advantage of the DPE derivative precursor on silica for surface-initiated polymerization. The DPE-silica can be dried completely without the double bond reacting. Differential scanning calorimetry (DSC) can be used to monitor the absence of this reaction. The filter was used to allow separation of solvent and other soluble material from silica particles. This reactor design made it possible to investigate the effect of free (unattached) initiator in the reacting system. Following the initiation of DPE, a red solution was seen when the solvent was poured into the collection flask and the functionalized Si particles were collected on the filter. Unbound DPE and unreacted n-BuLi have a very high solubility in benzene and will go through the filter to the collection flask. The same procedure was repeated twice, distilling the benzene back to the reactor and separating silica from solvent. After several washings, a very pale yellow or colorless solvent indicated that some unattached DPE still existed (n-BuLi does not show color). Styrene was added to the reactor by breaking the break-seal in the ampule. The reaction was allowed to occur for more than 2 days. At this point, the polymerization was complete but a light red color remained. MeOH was used to terminate the polymerization resulting in the loss of the red color. Extraction of Insoluble Silica and Bound Polymer. The polymerization samples were poured into centrifuge tubes and centrifuged for more than 1 h. The supernatant was decanted into a large volume of methanol to precipitate dissolved polymer. Fresh toluene was added to centrifuge tubes containing the insoluble fraction and stirred for an hour followed by 5 min of sonication and then removal of the supernatant. This treatment was repeated more than 10 times to make sure all soluble polymer was separated from insoluble bound polymer. This procedure has been demonstrated to be an efficient method to remove all

Analysis of DPE Functionalization on Surfaces. To obtain uniform and well-controlled polymer brushes on surfaces, it is important to have a controlled composition and density of the initiator precursor monolayer. We had used QCM to characterize the adsorption properties of the DPE onto a flat gold surface modified for silane coupling.30 A frequency change of ∆F ) -1.44 (Hz) indicated a thickness of 1.7 nm for the DPE assuming a density of 1 g/cm3 using the Sauerbrey equation.31 This is the thickness of the DPE-substituted SAM on a flat substrate, which is close to the length of 11 CH2 chains and the DPE group. This was verified by ellipsometric measurements on gold and Si wafers with a surface thickness of about 1.7-2.0 nm. Thus, the surface density of an ideal DPE monolayer can be calculated to be about 2.7 DPE molecules/nm2 coupled on a flat surface. In our experiments, we determined a much lower density of DPE on silica nanoparticle surfaces with a value of 0.7 DPE molecules/nm2 by using TGA. This is close to the 0.8-1.6 initiator molecules/nm2 value as reported in the literature.17 The nature of adhesion on the modified gold, Si wafer, and silica nanoparticles may lead to very different densities of DPE on surfaces. Another reason could be geometric effects. Prucker and Ruhe17 mentioned that not all SiOH groups on the silica surface can react with the chlorosilane and the number of silane groups attached to the surface is limited by the steric demands of the dimethylchlorosilyl groups.29 Activation of the DPE Initiator. It is important to make sure all DPE on silica surfaces is initiated by n-BuLi.21 Thus, much excess n-BuLi was used. However, this requires removal of excess initiator. Oosterling and co-workers21 polymerized styrene from both bound and free initiator without removing the excess and assuming the same molecular weight of grafted and nongrafted polymer. The free initiator can strongly affect polymerization results if the propagation rates are different for surface-bound initiator and free initiator.23 We thus designed a reactor to remove as much as possible of the excess initiator in order to investigate the effect of free and unattached initiator in the system on polymerization initiated from surfaces. Another advantage of this specially designed reactor is the capacity to directly see the evidence of living polymerization from surfaces (colored complex of the anions). This has also been demonstrated in our recent paper on living anionic polymerization from clay surfaces.25 When DPE was activated by n-BuLi, the suspension of DPE-silica nanoparticles in benzene showed a color change from pale yellow to orange/red, which indicated the evidence of living DPE initiated by n-BuLi. The red (30) Taylor, D.; Morgan, H.; D’Silva, C. J. Phys. D: Appl. Phys. 1991, 24, 1443. (31) Sauerbrey, G. Z. Phys. 1959, 155, 206.

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Table 1. Summary of Polymerization Results

samplea

monomer/initiator (w/w) g styrene/g Si nanoparticleb

Mn (c) [Mw/Mn]

Mn (s) [Mw/Mn]

S-7 SP-3 SP-2 SP-4

21.89 17.28 16.10 15.23

397 000 [1.79] 135 000 [1.19] 101 000 [1.21] 91 000 [1.25]

433 000 [2.02] 126 000 [1.14] 190 000 [1.18] 133 000 [1.20]

a In all cases, styrene was used as the monomer, n-BuLi was used to activate DPE, and THF was used as an additive. b The g styrene/g Si gel represents the [Mo/Io] ratio typically used to describe monomer and initiator composition in conventional solution polymerization. In this case, ∼6.9% of the Si nanoparticle weight is composed of the DPE initiator.

color could come from DPE anions either on the silica surface or in solution. After pouring into the collection bottle through the filter-equipped reactor, most of the excess n-BuLi should be separated from DPE-silica gel. We still obtained a very light yellow color, which indicated that residual unattached DPE initiated in solution is still present even after the first washing. However, the color was eliminated after distilling benzene back to the reactor vessel and separating the solution from silica. Thus, we concluded that the color from the first separation was mainly due to unattached DPE. The silica nanoparticles formed a red suspension and stayed on the filter during the separation procedures of free initiator and initiated DPE-silica gel. This is direct evidence of activated anionic DPE attached to the silica nanoparticles. We also found that high temperature with a vacuum condition is very important to dry the DPE-silica nanoparticle completely because trace impurities such as H2O may result in a broadened molecular weight distribution or even terminate the polymerization. This is actually the main difficulty reported in most literature associated with anionic polymerization from surfaces.21,23,25 The advantage of DPE is that it allowed us to remove impurities at 120 °C under high vacuum without side reactions on the double bond as evidenced by DSC.23,25 From the DSC trace of pure DPE and DPE-coated silica nanoparticles, we did not see any exothermic peak resulting from double bond reactivity of DPE until above 500 °C. This demonstrates that DPE is a thermally stable initiator precursor allowing us to dry the reactor and DPE-coated silica gel thoroughly. Anionic Polymerization. The results of characterization for both free polymers (formed with unattached initiator in solution) and cleaved polymers (formed on Si particle surfaces) are summarized in Table 1. A small amount of THF was always used to reduce the aggregates of n-BuLi and speed the reaction of n-BuLi with DPE. However, too much THF will not only cleave the polymer chain from Si nanoparticle surfaces by reacting with the Si-O-Si bonds21 but also react with BuLi.32 Thus, the addition of THF to the reaction system was minimized and was used mainly for reducing the aggregates of n-BuLi and increasing initiating efficiency. From Table 1, the molecular weight of the free polymer, Mn (s), is generally higher than that of bound polymer, Mn (c), and showed a slightly narrower polydispersity [Mw/Mn]. These results are very similar to those from our studies on anionic polymerization from clay25 where a much higher molecular weight free polymer was obtained due to limited diffusion of monomer to activated sites inside interlayers. In this case, however, the low molecular weight bound polymer could be caused by confined ends of polymer on the surfaces, which decrease the mobility of free living (32) Jung, M. E.; Blum, R. B. Tetrahedron Lett. 1977, 43, 3791.

Figure 2. Typical gel permeation chromatography results of free polymer and bound polymer, sample SP-3 (cleaved from silica nanoparticles).

ends. The geometric effects can also result in a lower molecular weight than that of the free polymer because of steric crowding outside the growing polymer active site.25 Aggregation of the particles may also play an important role. The broadened polydispersity of bound polymer had been theoretically predicted.33 Free polymers also show a broader MW distribution than that observed in conventional solution anionic polymerization. Our reactor allowed us to investigate the effects of unattached DPE and unremovable n-BuLi. As discussed in the Experimental Section, we often saw very light yellow colors in the collection flask. The unattached DPE was the only source for the color because n-BuLi is colorless in benzene. The free polymers could be initiated from either unattached DPE or n-BuLi. Combined with the aggregation effects on the solid silica nanoparticles, those factors may be the cause of the higher molecular weight distribution observed. Typical SEC traces are shown in Figure 2. Nevertheless, the molecular weight of bound polymer increases almost linearly with increasing amount of monomer (g styrene/g silica nanoparticles), SP-4 < SP-2 < SP-3 < S-7. Based on the results of Mn and the w/w (g styrene/g Si nanoparticles) (Table 1), these results affirm that the styrene polymerized in the solution by a living anionic polymerization mechanism. The densities of DPE and bound polymers can be determined by TGA (Figure 3). The TGA curve of DPE-coated silica nanoparticles presents a 336 °C decomposition transition with a long tail. This tail also showed up in the polymer-bound silica nanoparticle’s TGA curve, where the decomposition temperature of bound polymer increases to 420 °C compared to the 336 °C of the DPE-bound silica nanoparticle. The increase of decomposition temperature of bound polymer obviously comes from the polystyrene layer on silica surfaces. We believe that the tail of the TGA curve is caused by the delayed decomposition of DPE, which has a higher thermal stability. The DPE initiator precursor decomposes by a two-step procedure. The first decomposition happens at 336 °C and is attributed to the alkyl spacer, while the second decomposition at 460 °C is attributed to the DPE group. Thus, we conclude that the tail of TGA in Figure 3 for both curves comes from the DPE, which in the case of the polymer-coated particles could represent uninitiated (by n-BuLi) DPE during polymerization. This helps explain the low initiation efficiency observed. The DPE initiator (33) Wittmer, J.; Cates, M.; Jhoner, A.; Turner, M. Europhys. Lett. 1996, 33, 397.

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Figure 3. TGA traces of DPE-coated and polystyrene-coated silica nanoparticles under air conditions.

Figure 5. FTIR spectra of polystyrene-coated silica nanoparticles showing major peaks of importance.

Figure 4. Linear plot of the amount of bound polymer vs concentration of styrene monomer (g styrene/g silica nanoparticle).

precursor density on silica nanoparticles is about 0.59 mmol/g silica nanoparticles. A range of bound polymer from 60 to 234 g polystyrene/g silica nanoparticles would be expected if each site had a chain attached with 9.1 × 105 to 39.7 × 105 molecular weight. It is not surprising to see such a low amount of polymer grafted from silica surfaces. Oosterling and co-workers21 reported 0.27-0.87 g polystyrene/g silica nanoparticles with different molecular weight attached polymers synthesized by anionic polymerization on silica particle surfaces. Analogous results were also presented by Suter and co-workers18 who used free radical polymerization to make polymer brushes on mica surfaces. Only less than 10% of the polymer was grafted from mica surfaces, which indicates that a very small amount of initiator has initiated polymerization. In our previous publication,14 the use of 16% DPE versus 100% did not show any clear differences in either contact angle or thickness measurements when styrene was polymerized from a flat Si wafer. This indicates that the low initiation efficiency gives no advantage to 100% coverage of DPE on a surface. Again, the effect of aggregation may play an important role. Aggregation tends to reduce the available surface area for polymer grafting by preventing free access to initiator sites both by activation with n-BuLi or initiation of polymerization.25 We thus believe that confined ends of the initiator and crowded reactive ends will affect the initiation procedure and further polymerization resulting in broadened polydispersities. However, the amount of bound polymer still increases almost linearly with increasing monomer concentration (Figure 4), which is concrete evidence for anionic polymerization of styrene

from surfaces. It will be interesting to investigate the aggregation and solution properties of these polymercoated nanoparticles using light scattering (LS) experiments (both dynamic and static LS) and distinguish possible similarities with polymer micelles.34 Experimental protocols (solvent, concentration, temperature conditions, etc.) are currently being investigated in preparation for LS analysis. FTIR analysis confirmed the presence of polystyrene on silica surfaces. In Figure 5, the styrene C-C vibrations can be assigned to 1458 and 1496 (cm)-1, which were not present in the pure silica nanoparticle IR spectrum. The cleaved polystyrene IR spectrum is very analogous to the pure polystyrene spectrum in which 3061 and 3028 (cm-1) are assigned to polystyrene aromatic C-H stretching vibrations. The peaks at 2925 and 2852 (cm-1) belong to the aliphatic C-H stretch. Further evidence of polystyrene presence on the silica nanoparticle surface is given by NMR. After cleavage from silica gel, the detached polystyrene was centrifuged to separate from solid particles and purified by chromatography. NMR showed the broad proton signals characteristic of polystyrene with the aromatic ring protons at 7.04 ppm and CH2 at 1.28 ppm. AFM Images. The silica nanoparticles grafted with polystyrene were spin-coated to a freshly cleaned Si wafer substrate (spin coating 1 wt % of the polymer-coated Si nanoparticle solution in toluene on a silicon wafer at 2000 rpm for 90 s). Adhesion was sufficient for the polystyrenecoated silica nanoparticles but not for the uncoated Si nanoparticle starting material. This allowed us to observe the adsorbed polymer-coated nanoparticle at the surface as shown in Figure 6. The size distribution of the Si spheres is definitely affected by the polymer brush showing a glassified state for the polystyrene coating. The distribution of these particles in a dry and glassy state (of polystyrene) is broad as shown in the image. The diameter is noticeably thicker compared to the starting silica nanoparticle by several orders of magnitude, ca. 100500. This is due to the polymer coating and the formation of aggregates on the Si wafer surface. Wetting of the Si wafer film was affected by the presence of the thin layer (34) Mo¨ssmer, S.; Spatz, J. S.; Mo¨ller, M.; Aberle, T.; Schmidt, J.; Burchard, W. Macromolecules 2000, 33, 4791.

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Figure 7. XPS results on the (a) DPE-coated and (b) polymercoated silica nanoparticles.

sufficient XPS penetration depth on the nanoparticle surface. This is consistent with the low grafting density as determined by TGA, where the particles are composed of nonuniformly coated Si particles and its aggregates. The absence of Li peaks (54.9) indicates that no active anion is present and that all the Li salts have been removed by the extraction and washing procedure. Conclusions Figure 6. AFM images of the polymer-bound silica nanoparticle and aggregates on the Si wafer surface.

of polystyrene-coated nanoparticles on the surface increasing from 20° to 69° with the static contact angle measurements with H2O. It will be interesting to distinguish the aggregates from monoparticle domains as observed by AFM using in situ temperature-controlled experiments. It will also be interesting to correlate these results with future light scattering experiments. XPS Results. Our results showed the presence of the relevant C peak, representing the presence of the polystyrene on the surface of the nanoparticles. The XPS results for the DPE-coated nanoparticle are shown in Figure 7a indicating the presence of the C (284.5 eV) 1s peak as well as the Si (150 eV) 2s, (103 eV) 2p, and O (533 eV) 2s peaks.28 The C peak is observed to be significantly higher for the polymer-coated nanoparticle indicating a greater amount of carbonaceous polymer material. It is interesting to note the presence of relevant Si peaks (103.00 for SiOx) even with the polymer-coated nanoparticles, indicating the exposure of bare SiOx surfaces or

The DPE derivative with chlorosilane end groups was successfully attached on silica nanoparticle surfaces to serve as the initiator precursor. This was then initiated by n-BuLi with subsequent addition of monomer to make polymer brushes grafted from the nanoparticle. The DPE initiator precursor allows us to dry coated silica nanoparticles completely to avoid the effects of moisture residue on anionic polymerization. DPE also avoids the selfpolymerization on the surfaces. The molecular weight of free polymers, Mn (s), is higher than that of bound polymers, Mn (c), with a lower polydispersity. The appearance of a red color on the Si nanoparticles and the linear increase of molecular weight and the content of polymer on Si with increasing monomer-to-initiator ratio demonstrate that polymerization proceeded in accordance with the anionic polymerization mechanism. Unattached DPE, excess n-BuLi, and aggregation lowered the reproducibility of polymerization initiated from surfaces, which might be improved by a better filter-equipped reactor and the use of methods for breaking the aggregates during polymerization. Nevertheless, this work demonstrates the feasibility for living anionic surface-initiated polymerization (LASIP) on Si nanoparticle surfaces. Future work

Living Anionic Surface-Initiated Polymerization

involves investigating the application of this process to a variety of nanoparticles and block copolymer strategies with controlled parameters. Acknowledgment. Funding for this project from the Army Research Office (ARO) under DAAD19-99-1-0106 is gratefully acknowledged. Also, contributions on some

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of the analysis and synthesis by Yo Nakamura, Seiji Inaoka, Chuanjun Xia, and Mi-kyoung Park and technical support from Molecular Imaging and Maxtek Inc. are gratefully acknowledged. LA015670C