408
Langmuir 2007, 23, 408-413
Surface ATRP of Hydrophilic Monomers from Ultrafine Aqueous Silica Sols Using Anionic Polyelectrolytic Macroinitiators Cong-Duan Vo,* Andreas Schmid, and Steven P. Armes* Dainton Building, Department of Chemistry, The UniVersity of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.
Kenichi Sakai and Simon Biggs School of Process, EnVironmental and Materials Engineering, UniVersity of Leeds, Leeds LS2 9JT, U.K. ReceiVed October 12, 2006. In Final Form: NoVember 24, 2006 A convenient two-step route was developed to prepare new anionic ATRP macroinitiators from near-monodisperse poly(2-hydroxyethyl methacrylate) precursors by partial esterification with 2-bromoisobutyryl bromide, followed by esterification of the remaining hydroxyl groups using excess 2-sulfobenzoic acid cyclic anhydride. These new macroinitiators can be electrostatically adsorbed onto ultrafine cationic Ludox CL silica sols; subsequent surface polymerization of various hydrophilic monomers in aqueous solution at room temperature afforded a range of polymergrafted ultrafine silica sols. The resulting sterically stabilized particles were characterized by dynamic light scattering, transmission electron microscopy, aqueous electrophoresis, FTIR spectroscopy, and elemental microanalyses.
Introduction In recent years, there has been increasing interest in surfaceinitiated polymerization. In most cases, free radical polymerization chemistry has been used, which is very tolerant of monomer functionality.1,2 Of particular interest is the use of controlled/ living radical polymerization (e.g., atom-transfer radical polymerization (ATRP),3-10 reversible addition fragmentation transfer polymerization (RAFT),11 and nitroxide-mediated radical polymerization (NMRP)6) on planar and colloidal substrates because this allows much better control over molecular weight and molecular weight distribution.9b However, surface modifica* To whom correspondence should be addressed. E-mail: s.p.armes@ sheffield.ac.uk;
[email protected]. Fax: +44-114-222-9346. (1) (a) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 592. (b) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 602. (2) (a) Prucker, O.; Ru¨he, J. Langmuir 1998, 14, 6893. (b) Sto¨hr, T.; Ru¨he, J. Macromolecules 2000, 33, 4501. (c) Fan, X.; Xia, C.; Advincula, R. C. Langmuir 2003, 19, 4381. (3) Matyjaszewski, K.; Miller, P. J.; Skula, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Macromolecules 1999, 32, 8716. (4) (a) Jones, D. M.; Huck, W. T. S. AdV. Mater. 2001, 13, 1256. (b) Jones, D. M.; Brown, A. A.; Huck, W. T. S. Langmuir 2002, 18, 1265. (c) Osborne, V. L.; Jones, D. M.; Huck, W. T. S. Chem. Commun. 2002, 1838. (5) (a) Huang, W.; Kim, J.-B.; Bruening, M. L.; Baker, G. L. Macromolecules 2002, 35, 1175. (b) Huang, W. X.; Baker, G. L.; Bruening, M. L. Angew. Chem., Int. Ed. 2001, 40, 1510. (6) (a) Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597. (b) Hussemann, M.; Malmstrom, 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. (7) (a) Perruchot, C.; Khan, M. A.; Kamitsi, A.; Armes, S. P.; von Werne, T.; Patten, T. E. Langmuir 2001, 17, 4479. (b) Chen, X.; Randall, D. P.; Perruchot, C.; Watts, J. F.; von Werne, T.; Patten, T. E.; Armes, S. P. J. Colloid Interface Sci. 2003, 257, 56. (8) (a) Guerrine, M. M.; Charleux, B.; Vairon, J.-P. Macromol. Rapid Commun. 2000, 21, 669. (b) Carrot, G.; Diamanti, S.; Manuszak, M.; Charleux, B.; Vairon, J.-P. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 4294. (c) Kizhakkedathu, J. N.; Brooks, D. E. Macromolecules 2003, 36, 591. (d) Jayachandran, K. N.; Takacs-Cox, A.; Brooks, D. E. Macromolecules 2002, 35, 4247. (e) von Natzer, P.; Bontempo, D.; Tirelli, N. Chem. Commun. 2003, 1600. (f) Zheng, G.; Sto¨ver, H. D. H. Macromolecules 2003, 36, 1808. (9) (a) von Werne, T.; Patten, T. E. J. Am. Chem. Soc. 1999, 121, 7409. (b) von Werne, T.; Patten, T. E. J. Am. Chem. Soc. 2001, 123, 7497. (c) Farmer, S. C.; Patten, T. E. Chem. Mater. 2001, 13, 3920. (10) (a) Xiao, D. Q.; Wirth, M. J. Macromolecules 2002, 35, 2919. (b) Honigfort, M. E.; Brittain, W. J. Macromolecules 2003, 36, 3111.
tion was required in all cases, which usually requires either siloxane chemistry (for silica) or thiol compounds (for gold substrates). We are particularly interested in using surface ATRP to graft water-soluble polymers onto colloidal inorganic oxide particles such as silica sols to produce stimulus-responsive particles. Unfortunately, commercial silica sols are usually available as aqueous dispersions, hence it is somewhat problematic to introduce siloxane-based ATRP initiators because the bromoesterbased siloxanes are normally rather hydrophobic in character. Moreover, dried ultrafine sols are usually difficult to redisperse because of their propensity for irreversible aggregation. In view of these problems, Chen and Armes recently designed a cationic ATRP macroinitiator for the surface ATRP of a range of hydrophilic methacrylic monomers from anionic colloidal silica sols.12 Unlike the siloxane or thiol surface chemistries favored by other research groups,1,2,7,9b,13a,b this new approach simply requires the electrostatic adsorption of the macroinitiator to coat the surface of the particles; this “priming” process is fast, efficient, and can be achieved at room temperature in aqueous solution. Although most colloidal and planar surfaces have anionic character, certain inorganic oxides such as alumina exhibit cationic surface charge over a wide pH range. Thus, the analogous anionic ATRP macroinitiator is required for surface ATRP from such materials. For colloidal substrates, it is also desirable that the mean degree of polymerization (Dp) of the polyelectrolytic macroinitiator is kept relatively low to avoid bridging flocculation, which is a particular problem for ultrafine sols.12b However, for surface ATRP from planar substrates such constraints do not apply. Indeed, it is expected that higher Dp’s will lead to stronger anchoring of the macroinitiator at the surface. Thus, anionic ATRP macroinitiators with both high and low Dp values are desirable. (11) (a) Baum, M.; Brittain, W. J. Macromolecules 2002, 35, 610. (b) Li, C.; Han, J.; Ryu, C. Y.; Benicewicz, B. C. Macromolecules 2006, 39, 3175. (12) (a) Chen, X.; Armes, S. P. AdV. Mater. 2003, 15, 1558. (b) Chen, X.; Armes, S. P.; Greaves, S. J.; Watts, J. F. Langmuir 2004, 20, 587. (13) (a) Brown, A. A.; Khan, N. S.; Steinbock, L.; Huck, W. T. S. Eur. Polym. J. 2005, 41, 1757. (b) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. ReV. 2004, 33, 14.
10.1021/la063003j CCC: $37.00 © 2007 American Chemical Society Published on Web 12/15/2006
Letters
Langmuir, Vol. 23, No. 2, 2007 409
Table 1. Summary of Particle Size, Monomer Conversion, and Adsorbed Amount of Polymer Grafted onto Ludox CL by Surface ATRP at 20 °C in Protic Media Using an Anionic Macroinitiator. polymer loadingb sample
target DP
pristine Ludox CL macroinitiator-coated Ludox CL PMPC-grafted Ludox CL PGMA-grafted Ludox CL PKSPMA-grafted Ludox CL PDMA-grafted Ludox CL PNaStS-grafted Ludox CL
100 100 100 200 100
reaction medium
H2O H2O H2O 1:1 IPA/H2O 1:1 CH3OH/H2O
reaction time (h)
3 3 3 24 24
monomer conversion (%)a
%
mg‚m-2
particle diameter (nm)c
67 61 67 28 22
12 43 49 25 29 21
1.00 3.6 4.1 2.1 2.4 1.7
69 166d 134 195 189 179 157
a As determined by 1H NMR spectroscopy (monomer depletion monitored relative to DMF, which was used as an internal standard). b Elemental microanalyses were performed after removing nongrafted polymer in solution by three centrifugation/redispersion cyles. c As determined from aqueous solution using dynamic light scattering. d This is an initial value. The particle size increased up to several micrometers on standing.
Experimental Section Materials. 2-Hydroxyethyl methacrylate (HEMA) and glycerol monomethacrylate (GMA) were kindly donated by Cognis Performance Chemicals (Hythe, U.K.) and used as received. 2-(Dimethylamino)ethyl methacrylate (DMA) was purchased from Aldrich and purified by passing it through a basic alumina column. 2-(Methacryloyloxy)ethyl phosphorylcholine (MPC, >99%) was kindly donated by Biocompatibles (U.K.) and was used as received. Silica gel 60 (0.063-0.2 mm) was obtained from E. Merck (Darmstadt, Germany). Benzoylated cellulose dialysis tubing (molecular weight cutoff 1200-2000 Da) was acquired from Sigma. D2O, CD3OD, CDCl3, and DCl were obtained from Goss Scientific (U.K.). Acetone, dichloromethane, methanol, tetrahydrofuran (all HPLC grade), and triethylamine (TEA) were from Fisher. Acetone was dried over molecular sieves (3 Å, 8-12 mesh) before use. 4-Dimethylaminopyridine (DMAP) was supplied by Lancaster and was used as received. 2-Sulfobenzoic acid cyclic anhydride (SBA), potassium 3-sulfopropyl methacrylate (KSPMA), sodium 4-styrenesulfonate (NaStS), and all other reagents were supplied by Aldrich and were used as received. The deionized water used in these experiments was generated using an Elga Elgastat Option 3 water purification apparatus. The ME-Br ATRP initiator was synthesized according to a previously reported protocol.14 Ludox CL was purchased from Aldrich and was used as received. The cationic surface charge of this sol is due to a thin overlayer (ca. 1 nm) of alumina. According to Aldrich, Ludox CL has a nominal specific surface area of 230 m2 g-1, whereas we determined its specific surface area by the BET adsorption isotherm method to be around 120 m2 g-1. The particle density was determined to be 2.07 g cm-3 by helium pycnometry, hence the mean particle diameter calculated from our BET value was approximately 24 nm. However, DLS studies indicated an intensity-average particle diameter of 69 nm for these Ludox CL particles, which suggested partial flocculation of this sol (Table 1). Syntheses. 1. Synthesis of Anionic ATRP Macroinitiators. Synthesis of PHEMA Precursors. A mixture of HEMA (40.00 g, 307 mmol), ME-Br (4.30 g, 15.4 mmol; target Dp ) 20), and bpy (4.80 g, 30.7 mmol) was degassed with nitrogen for 0.5 h while stirring at 20 °C. Nitrogen-degassed methanol (40 mL) and Cu(I)Cl (1.52 g, 15.4 mmol) were added to this mixture, and polymerization commenced at 20 °C. After 5 h, the reaction was terminated by exposure to air. 1H NMR analysis showed that the monomer conversion had reached 98%. The reaction mixture was then diluted with methanol and passed through a silica gel column to remove the catalyst. The resulting poly(2-hydroxyethyl methacrylate) (PHEMA20) solution was concentrated under reduced pressure and precipitated into a 1:1 v/v n-hexane/ethyl acetate mixture to remove any unreacted HEMA monomer. The resulting solid was then dried under vacuum to produce a white polymer (yield 41.50 g, 94%). The same protocol was repeated using a mixture of HEMA (40.00 g, 307 mmol), ME-Br (1.07 g, 3.84 mmol; target Dp ) 80), Cu(I)Cl (0.38 (14) Bories-Azeau, X.; Armes, S. P.; van den Haak, H. J. W. Macromolecules 2004, 37, 2348.
g, 3.84 mmol), and bpy (1.20 g, 7.68 mmol). 1H NMR analysis indicated complete monomer conversion after 17.5 h (no vinyl peaks could be detected at δ 6.0 and 5.5). The reaction mixture was diluted with methanol, passed through a silica gel column to remove the catalyst, concentrated under reduced vacuum, and precipitated into excess water. The purified white PHEMA80 was dried under vacuum (yield 25.60 g, 87%). Partial Esterification of PHEMA with 2-Bromoisobutyryl Bromide. PHEMA20 (30.00 g, 230 mmol of hydroxyl groups) was dissolved in dried acetone (72 mL) followed by the addition of TEA (2.92 g, 28.8 mmol) and DMAP (3.56 g, 28.8 mmol) at 20 °C. The mixture was then cooled in an ice bath, and 2-bromoisobutyryl bromide (BIBB) (7.27 mL, 57.6 mmol, 25 mol % relative to the hydroxyl groups of PHEMA20) was added dropwise under dry nitrogen. The temperature was allowed to rise to 20 °C, and the reaction mixture was stirred for an additional 24 h. The solvent was then removed under vacuum. The isolated crude solid was redissolved in methanol and subsequently precipitated in excess deionized water (twice). The resulting solid was finally dried under vacuum to produce the partially esterified precursor (yield 27.2 g, 74%). The degree of esterification was determined by 1H NMR analysis. The PHEMA80 precursor was partially esterified with BIBB using the same protocol described above, targeting a mean degree of esterification of 30% (yield 25.0 g, 76%). Synthesis of the Anionic Macroinitiator from the Partially Esterified Precursor. The partially esterified precursor (16.00 g, 81.70 mmol of OH residues) was dissolved in anhydrous THF (200 mL), followed by the addition of TEA (34.27 mL, 246 mmol) and SBA (45.28 g, 246 mmol) under nitrogen. The reaction was allowed to proceed for 3 days at 20 °C, and the solvent was removed under vacuum. The isolated crude product was dissolved in deionized water, and this aqueous solution was purified by dialysis, replacing successive mother liquors with either water or saturated aqueous NaCl, prior to freeze drying overnight to obtain an off-white anionic macroinitiator (yield 22.2 g, 67%). The same protocol was used to esterify the remaining hydroxyl groups in the BIBB-esterified PHEMA80, except that the crude copolymer was precipitated into excess diethyl ether prior to dialysis (yield 35.0 g, 74%). 2. Electrostatic Adsorption of the Macroinitiator and SurfaceInitiated Polymerization from Macroinitiator-Coated Sols. The anionic ATRP macroinitiator was electrostatically adsorbed onto the aqueous Ludox CL sol, followed by the in situ surface ATRP of hydrophilic monomers (Scheme 2, Table 1). Typically, the anionic macroinitiator (precursor Dp ) 21) (1.00 g, 0.5817 mmol of Br) was first dissolved in 82.33 g of degassed deionized water. Ludox CL (16.67 g, 35 wt %) was added to this aqueous solution, and the pH (initially around pH 4) was adjusted to 7 using 5.4 g of a 0.50 M NaOH solution. The dispersion was stirred for 1 h at 20 °C under a nitrogen purge to allow electrostatic adsorption to occur. Subsequently, bpy (90.8 mg, 0.582 mmol) and MPC (1.717 g, 5.82 mmol, target Dp ) 100) were added to 10.54 g of the macroinitiatorcoated sol. The resulting dispersion was purged with nitrogen for 30 min at 20 °C. Then Cu(I)Br (41.7 mg, 0.2908 mmol) was added to start the polymerization at ambient temperature. The dispersion
410 Langmuir, Vol. 23, No. 2, 2007
Letters
Scheme 1. Synthesis of Near-Monodisperse Poly(2-hydroxyethyl methacrylate) via ATRP and the Two Sequential Esterification Steps Used to Prepare Anionic ATRP Macroinitiatorsa
a
Each reaction step was conducted at 20 °C.
Scheme 2. Reaction Scheme Depicting the Electrostatic Adsorption of the Anionic ATRP Macroinitiator onto a Cationic Ludox CL Sol and the Subsequent Surface ATRP of Various Hydrophilic Methacrylic Monomersa
a For example, 2-(methacryloyloxy)ethyl phosphorylcholine [MPC], glycerol monomethacrylate [GMA], potassium 3-sulfopropyl methacrylate [KSPMA], 2-(dimethylamino)ethyl methacrylate [DMA], and sodium 4-styrenesulfonate [NaStS].
became dark brown, and its viscosity increased rapidly as soon as Cu(I)Br was added. The polymerization was terminated after the desired reaction time by exposure to air. Monomer conversion was determined by 1H NMR analysis using DMF as an internal standard. The PMPC-grafted silica particles were purified by three centrifugation cycles at 18 000 rpm for 1 h and redispersed with the aid of an ultrasonic bath, with successive supernatants being replaced with deionized water. This protocol removed the soluble ATRP catalyst, unreacted MPC monomer, and any soluble MPC homopolymer that formed in the reaction solution. In most cases, the visual appearance of the purified polymer-grafted silica sols was white, which, on the basis of our previous experience with ATRP-synthesized polymers, indicated that their copper contents were below 10 ppm. However, with the anionic PKSPMA and PNaStS polymer chains, the final product had a distinctly greenish tinge, which suggested a residual copper level of a few hundred parts per million. Such contamination is due to the fact that the Cu(I)/2bpy catalyst is cationic and hence acts as a counterion for the anionic polyelectrolyte chains. Characterization. NMR Spectroscopy. All 1H NMR spectra were recorded in D2O, CD3OD, CDCl3, or d5-pyridine using a Bruker AC 250 MHz spectrometer. GPC Analyses of Homopolymers and Corresponding Precursors. Molecular weights were determined by DMF GPC at 70 °C using
three PL gel 10 µm MIXED-B columns in series with a Viscotek TriSEC model 302 refractive index detector. The mobile phase contained 10 mmol of LiBr and was used at a flow rate of 1.0 mL min-1. The GPC columns were calibrated using 10 near-monodisperse poly(methyl methacrylate) homopolymer standards (Mp ) 2000-300 000 g mol-1). The data were analyzed using Viscotek TriSEC 3.0 software. Dynamic Light Scattering (DLS). DLS measurements were carried out at 20 °C using a Brookhaven BI-200SM goniometer equipped with a BI-9000AT digital correlator and a solid-state laser (125 mW, λ ) 532 nm) at a fixed scattering angle of 90°. The intensityaverage hydrodynamic diameter (Dh) and polydispersity index (µ2/ Γ2) of the particles were evaluated from cumulants analysis of the experimental correlation functions. Unless otherwise stated, 0.10 w/v % aqueous copolymer solutions were used for all measurements. Elemental Microanalysis. Microanalytical contents of samples were determined using a Perkin-Elmer 2400 CHNS/O series II elemental analyzer, which uses a combustion method in a pure oxygen environment to convert the accurately weighed sample into gaseous CO2, H2O, N2, and SO2 (if present). These gases were separated using a chromatographic column and measured as a function of their thermal conductivity. In practice, only the carbon contents were used to determine the adsorbed amounts of polymer.
Letters
Langmuir, Vol. 23, No. 2, 2007 411
Figure 1. GPC traces obtained for (A) PHEMA20 (Mn ) 7100 g mol-1, Mw/Mn ) 1.20) and (B) PHEMA80 (Mn ) 24 400 g mol-1; Mw/Mn ) 1.23) using DMF GPC at 70 °C calibrated with nearmonodisperse poly(methyl methacrylate) standards.
Figure 3. Aqueous electrophoresis data obtained for (0) an aluminacoated silica sol (Ludox CL), ()) the corresponding sol coated with the electrostatically adsorbed anionic ATRP macroinitiator, (4) PMPC-grafted Ludox CL with a target degree of polymerization of 100, and (O) PKSPMA-grafted Ludox CL with a target degree of polymerization of 100.
Figure 2. Assigned 1H NMR spectrum recorded in D2O for the anionic macroinitiator derived from the PHEMA20 precursor.
Figure 4. FTIR spectra recorded for (A) the pristine Ludox CL, (B) the anionic macroinitiator-coated Ludox CL, and (C) a PMPC-grafted Ludox CL sol with a target degree of polymerization of 100.
Surface Area Analysis. Brunauer-Emmett-Teller (BET) surface area measurements were performed using a Quantachrome Nova 1000 instrument using dinitrogen gas as an adsorbate at 77 K. The area per molecule for dinitrogen was taken to be 16.2 Å2. Helium pycnometry measurements were conducted using a Micromeritics AccuPyc 1330 instrument. The mean particle diameter of the Ludox CL sol was calculated using As ) 6/FD, where As is the BET specific surface area, F is the particle density, and D is the particle diameter. The particle size of Ludox CL determined from BET measurement was in reasonable agreement with that estimated from TEM images. FTIR Spectroscopy. FTIR spectra of the original Ludox CL, the macroinitiator-coated Ludox CL and the polymer-grafted Ludox CL samples were recorded from KBr disks using a Perkin-Elmer Spectrum GX FTIR spectrometer and Spectrum V2.0 software. Four scans were accumulated for each spectrum, and the spectral resolution was 4 cm-1.
1H
Results and Discussion Two near-monodisperse PHEMA homopolymer precursors with target Dp’s of 20 and 80 respectively were synthesized via ATRP using the protocol described by Weaver and co-workers.15 Their molecular weights were assessed using DMF GPC calibrated with poly(methyl methacrylate) standards (Figure 1). The PHEMA20 precursor had an Mn of 7100 g mol-1 and an Mw/Mn of 1.20, whereas the PHEMA80 precursor had an Mn of 24 400 g mol-1 and an Mw/Mn of 1.23. End-group analysis by
NMR also confirmed that the actual Dp of the PHEMA20 precursor had been achieved within experimental error, suggesting relatively high initiator efficiencies in these ATRP syntheses. These two PHEMA precursors were then esterified with the desired amount of 2-bromoisobutyryl bromide (Scheme 1). The mean degrees of esterification were calculated to be 19% for the PHEMA20 and 27% for the PHEMA80 from 1H NMR spectra (CD3OD). The remaining hydroxyl groups in these partially esterified copolymers were derivatized under mild conditions using excess 2-sulfobenzoic acid cyclic anhydride (SBA) to produce strong anionic polyelectrolytic macroinitiators.16 1H NMR analysis (Figure 2) confirmed that essentially full esterification was achieved and the final anionic macroinitiators were both water-soluble, as expected. The low-molecular-weight anionic macroinitiator was electrostatically adsorbed onto the Ludox CL sol at pH 7 for (15) Weaver, J. V. M.; Bannister, I.; Robinson, K. L.; Bories-Azeau, X.; Armes, S. P.; Smallridge, M.; McKenna, P. Macromolecules 2004, 37, 2395. (16) During the preparation of this Letter, Advincula and co-workers reported that poly(methyl methacrylate) chains can be grown from an anionic ATRP macroinitiator based on a weak polyacid [poly(acrylic acid)] adsorbed onto 400 nm polystyrene particles as a final layer of a layer-by-layer cationic/anionic polyelectrolyte coating. (See Fulghum, T. M.; Patton, D. L.; Advincula, R. C. Langmuir 2006, 22, 8397-8402.) However, the new macroinitiators described in the present work are strong, rather than weak, polyelectrolytes. Hence, they should remain highly anionic (and therefore strongly adsorbed onto the Ludox CL sol) regardless of the solution pH.
412 Langmuir, Vol. 23, No. 2, 2007
Letters
Figure 5. TEM images of (A) the pristine Ludox CL sol, (B) the anionic macroinitiator-coated Ludox CL, (C) PKSPMA-grafted Ludox CL, and (D) PMPC-grafted Ludox CL. The grafted polymers both have a target Dp of 100.
subsequent polymer grafting via surface ATRP (Scheme 2). A modest excess (18%) of macroinitiator was employed, hence there is a minor nonadsorbed fraction in the continuous phase. Elemental and thermogravimetric analyses indicated that the adsorbed amount of anionic ATRP macroinitiator on the cationic Ludox CL corresponded to around 1.0 mg m-2, which is characteristic of polyelectrolyte adsorption.17 However, the addition of macroinitiator led to an increase in aggregation of the sol, as judged by the increase in turbidity. DLS studies showed that these aggregates had an initial intensity-average diameter of around 166 nm (Table 1), but this size increased up to several micrometers on standing for 0.5 h. Thus, significant flocculation occurs despite the relatively low degree of polymerization of the anionic macroinitiator. A similar degree of weak aggregation was also observed by Chen and Armes when adsorbing a cationic macroinitiator onto ultrafine anionic silica particles.12 TEM images of both the pristine sol and the anionic macroinitiatorcoated sol are shown in Figure 5a and b. Although sample preparation artifacts cannot be excluded, these images are also consistent with additional aggregation of the Ludox CL sol. Aqueous electrophoresis studies confirmed that surface charge reversal occurred on addition of the anionic macroinitiator to the cationic sol, as expected. For example, the zeta potential of the original Ludox CL sol was +40 mV at pH 6, whereas the (17) Fleer, G. J.; Cohen-Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman and Hall: London, 1993.
macroinitiator-coated sol had a zeta potential of -29 mV at the same pH (Figure 3). The surface ATRP of various hydrophilic methacrylic monomers such as 2-(methacryloyloxy)ethyl phosphorylcholine (MPC), glycerol monomethacrylate (GMA), potassium 3-sulfopropyl methacrylate (KSPMA), 2-(dimethylamino)ethyl methacrylate (DMA), and sodium 4-styrene sulfonate (NaStS) from these anionic macroinitiator-coated sols was evaluated. Table 1 summarizes the reaction conditions, overall monomer conversions, polymer loadings, and DLS particle diameters obtained in either aqueous or alcohol/water mixtures at 20 °C. Polymer-grafted particles were purified by removing spent ATRP catalyst, unreacted monomer, and any nonadsorbed graft copolymer via three centrifugation-redispersion cycles. Both DLS (Table 1) and TEM studies (Figure 5) suggest that the polymer-grafted particles become somewhat less flocculated compared to the anionic macroinitiator-coated sols (Figure 5). The higher degree of dispersion of the polymer-grafted sols compared to that of the macroinitiator-coated sol is also indicated by their differing sedimentation behavior. For example, a much higher centrifugation rate (20 000 rpm for 2.5 h) was required to fully sediment the PMPC- or PKSPMA-grafted sol compared to that required to sediment the flocculated macroinitiator-coated sol (8000 rpm for 0.5 h). The pristine Ludox CL had an isoelectric point (IEP) at around pH 8.7 (Figure 3). Adsorption of the anionic macroinitiator lowers this IEP to around pH 4.5: thus, the macroinitiator-coated sol
Letters
has anionic character at around neutral pH, which corresponds to the conditions used for the surface ATRP syntheses. The various grafted polymer layers on the Ludox CL particles also had a substantial influence on their electrophoretic behavior (Figure 3). For example, the neutral zwitterionic PMPC chains shield the net anionic surface charge of the macroinitiator-coated Ludox CL sol very effectively, with only very low zeta potentials ((5 mV) being observed over a wide pH range. Similar results were also obtained for the nonionic PGMA-grafted Ludox CL sol (not shown). In contrast, a grafted layer of a strong polyacid such as PKSPMA rendered the particles highly anionic from pH 2 to 11. These data suggest that relatively thick and/or dense polymer layers are grown on the Ludox CL particles. DLS studies are consistent with this interpretation, although it is difficult to calculate reliable polymer layer thicknesses from the data shown in Table 1 because of the uncertainties introduced by the macroinitiator-induced flocculation prior to polymerization. FTIR studies confirmed the presence of the grafted polymer chains (Figure 4). The upper spectrum is of the pristine Ludox CL sol. A weak band due to the electrostatically adsorbed anionic macroinitiator is discernible at 1725 cm-1 (ester carbonyl stretch). The polymerization of MPC led to a significant increase in the intensity of this band. The grafted polymer loadings on the Ludox CL sol were determined by carbon microanalysis, and these data are summarized in Table 1. Although substantial amounts of polymer (21-49% by mass) can be grafted onto the ultrafine Ludox CL sol, the estimated adsorbed amounts are not so high as to be certain that the grafted chains are in the “brush” regime, as opposed to the “mushroom” regime. As expected, higher monomer conversions, and hence higher polymer loadings, were obtained in purely aqueous solution compared to alcohol/water mixtures, even though the latter syntheses were conducted for longer reaction times. However, it is likely that the addition of alcohol leads to improved living character.15 In all syntheses, a catalyst/initiator molar ratio of 5 was employed, and the target degree of polymerization per grafted chain was either 100 or 200. In view of the incomplete conversions achieved (Table 1), these target degrees of polymerization are unlikely to be attained, but it is perhaps noteworthy that ATRP conducted in purely aqueous solution often suffers from relatively low initiator efficiencies (sometimes as low as 50%), which will tend to offset the incomplete conversions. Weakly basic monomers such as DMA can be successfully polymerized in a 1:1 isopropyl alcohol/water mixture, but gelation
Langmuir, Vol. 23, No. 2, 2007 413
always occurred if this polymerization was conducted in purely aqueous media. However, polymerization of quaternized cationic monomers such as [2-(methacryloyloxy)ethyl]trimethylammonium chloride from these anionic macroinitiator-coated sols was unsuccessful even in alcohol/water mixtures. In this case, macroscopic precipitation was always observed during the polymerization. Presumably, this is due to massive electrostatic attraction between the growing cationic polymer chains and the anionic surface of the macroinitiator-coated sol. Similar problems were encountered by Chen and Armes when polymerizing anionic sodium 4-styrenesulfonate from the cationic surface of a macroinitiator-coated silica sol.12b Future work will focus on evaluating these new anionic macroinitiators for surface ATRP from planar surfaces using the higher-molecular-weight anionic macroinitiator described herein. One cationic substrate of particular interest is sapphire because this material is approximately contrast matched by D2O. Thus, sapphire offers a potentially decisive advantage for neutron reflectivity studies, because high-quality data can be obtained from protonated (rather than deuterated) polymer chains.
Conclusions The esterification of hydroxylated polymers provides a convenient route to new anionic ATRP macroinitiators. These sulfonated macroinitiators were electrostatically adsorbed onto an ultrafine aqueous cationic sol (Ludox CL) and subsequently used to graft water-soluble polymer chains via surface ATRP of nonionic, zwitterionic, and anionic hydrophilic methacrylic monomers in protic media at ambient temperature. The resulting polymer-grafted sols were characterized using electron microscopy, aqueous electrophoresis, dynamic light scattering, FTIR spectroscopy, and elemental microanalyses. However, it was not possible to polymerize a quaternized monomer using this anionic macroinitiator approach; presumably this is due to unfavorable electrostatic interactions. Acknowledgment. EPSRC is gratefully acknowledged for a postdoctoral fellowship for C.-D.V. (GR/S60419). S.P.A. is the recipient of a 5-year Royal Society-Wolfson Research Merit Award. We thank Dr. F. Ganachaud for first alerting us to the commercial availability of the SBA reagent. LA063003J
