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Synthesis of Well-Defined, Polymer-Grafted Silica Particles by Aqueous ATRP C. Perruchot, M. A. Khan, A. Kamitsi, and S. P. Armes* School of Chemistry, Physics and Environmental Science, University of Sussex, Falmer, Brighton, East Sussex BN1 9QJ, U.K.
T. von Werne and T. E. Patten* Department of Chemistry, University of California at Davis, One Shields Avenue, Davis, California 95616-5295 Received February 21, 2001. In Final Form: May 29, 2001 Surface-initiated atom transfer radical polymerization (ATRP) of various hydrophilic methacrylate monomers on submicrometer-sized silica particles in aqueous media at 20 °C leads to polymer-grafted silica particles whose colloid stability depends on the nature of the grafted polymer. These new organicinorganic hybrid particles have been extensively characterized by thermogravimetry, elemental microanalyses, FT-IR spectroscopy, dynamic light scattering, scanning electron microscopy, and X-ray photoelectron spectroscopy. They are expected to be interesting model colloids for evaluating theories of steric stabilization.
Introduction In recent years there has been increasing interest in surface-initiated polymerization.1-8 In most cases this involves free radical polymerization chemistry, which is particularly tolerant of monomer functionality.1,2 Of particular interest is the use of controlled/living free radical polymerization [e.g. either nitroxide-mediated3,4 or atom transfer radical polymerization (ATRP)5] in such polymer-grafting reactions, since this allows better control over the target molecular weight and molecular weight distribution. In most cases, planar surfaces have been examined, but there is an increasing number of reports describing the use of high surface area colloidal substrates. Notably, von Werne and Patten have described the ATRP of MMA and styrene from the surface of colloidal silica particles.7 Similarly, Charleux and co-workers have reported the use of surface-modified latex particles for the subsequent polymerization of 2-hydroxyethyl acrylate and (2-(methacryloyloxy)ethyl)trimethylammonium chloride.8 However, in both cases high temperatures and long reaction times were utilized and, even under these * To whom correspondence should be addressed. (1) (a) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 602. (b) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 614. (2) (a) Biesalski, M.; Ru¨he, J. Macromolecules 1999, 32, 2309. (b) Sto¨hr, T.; Ru¨he, J. Macromolecules 2000, 33, 4501. (3) Husseman, M.; Malmstro¨m, 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. (4) Shah, R. R.; Merreceyes, D.; Hussemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597. (5) (a) Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Macromolecules 1999, 32, 8716. (b) Sedjo, R. A.; Mirous, B. K.; Brittain, W. J. Macromolecules 2000, 33, 1492. (c) Bo¨ttcher, H.; Hallensleben, M. L.; Nuβ, S.; Wurm, H. Polym. Bull. 2000, 44, 223. (d) Mandal, T. K.; Fleming, M. S.; Walt, D. R. Chem. Mater. 2000, 12, 3481. (6) Tanaka, M.; Sudo, A.; Sanda, F.; Endo, T. Chem. Commun. 2000, 2503. (7) (a) von Werne, T.; Patten, T. E. J. Am. Chem. Soc. 1999, 121, 7409. (b) von Werne, T.; Patten, T. E. ACS PMSE Prepr. 2000, 82 (1), 294. (8) Manuszak-Gerrini, M.; Charleux, B.; Vairon, J.-P. Macromol. Rapid Commun. 2000, 21, 669.
conditions, conversions of monomer to polymer were relatively low. Recently, we have reported that ATRP can be conducted in aqueous media. Under these conditions, the rate of polymerization of a wide range of hydrophilic methacrylates is unusually rapid under remarkably mild conditions.9-12 For example, methoxy-capped oligo(ethylene glycol) methacrylate (OEGMA) is polymerized to conversions exceeding 95% within 25 min at 20 °C. In view of these results, we decided to explore the feasibility of using aqueous ATRP for the synthesis of model polymer-grafted silica particles. In addition to the prospect of more efficient polymerizations, it was anticipated that such well-defined, polymer-modified silica particles might have interesting applications. For example, their colloid stability should be governed by the aqueous solution properties of the grafted hydrophilic polymer chains. Herein we report our preliminary results in this area. All polymerizations were conducted at ambient temperature in either aqueous or mixed aqueous media (see Table 1). The colloidal silica particles were prepared by the Sto¨ber method13 and were then surface-functionalized with 3-(dimethylethoxysilyl)propyl-2-bromoisobutyrate, which acted as the ATRP initiator. These procedures have been fully described previously.7 Relatively high degrees of polymerization (Dp) were targeted compared to those of conventional ATRP because thick polymer brushes were desired. To improve the living character of aqueous ATRP in this high Dp regime, the catalyst/initiator molar ratio was increased from 1 to either 10 or 20 (see Table 1). In a typical synthesis protocol, the silica particles were dispersed in water and the hydrophilic methacrylate monomer was dissolved along with the Cu(I)X/2bipy (X ) Cl or Br) ATRP catalyst in a second aqueous solution. On mixing these two solutions, a deep brown coloration (9) Wang, X.-S.; Lascelles, S. F.; Jackson, R. A.; Armes, S. P. Chem. Commun. 1999, 1817. (10) Wang, X.-S.; Jackson, R. A.; Armes, S. P. Macromolecules 2000, 33, 255. (11) Wang, X.-S.; Armes, S. P. ACS Polym. Prepr. 2000, 41 (1), 484. (12) Wang, X.-S.; Armes, S. P. Macromolecules 2000, 33, 6640. (13) Philipse, P.; Vrij, A. J. Colloid Interface Sci. 1989, 128, 121.
10.1021/la0102758 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/26/2001
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Table 1. Summary of the Target Dp’s, Particle Densities, Actual Polymer Contents, Hydrodynamic Particle Diameters, and Surface Compositions of Polymer-Grafted Silica Particles Prepared by Surface-Initiated ATRP in Aqueous Media at 20 °C sample code
sample description
1
SiO2-init
2 3 4 5 6 7 8
SiO2-init-OEGMA SiO2-init-OEGMA SiO2-init-OEGMA SiO2-init-MEMA SiO2-init-SEM SiO2-init-HEMAf SiO2-init-GMAf
target Dp
250 500 1000 500 500 500 500
catalyst
CuCl CuCl CuCl CuBr CuBr CuBr CuBr
catalyst/initiator molar ratio
particle densitya (g cm-3)
20 20 20 10 10 10 10
1.896d 2.023e 1.856d 1.821d 1.747d 1.902e 1.967e 1.942e 1.922e
polymer content by TGAb (%)
avg particle diameter by DLSc (nm)
C/Si atomic ratio by XPS 1.4
5.37 6.52 8.20 8.62 5.32 5.63 3.35
356 318 370d 397d 443d 397e 560e 480e,f 381e
5.1 5.7 8.1 6.3 4.6 g g
Γ (mg m-2)
6.4 7.8 10.0 10.1 6.0 6.4 3.7
a Determined by helium pycnometry (Micromeritics Accupyc 1330). b Determined by thermogravimetric analysis; sample heated to 800 °C in air at 20 °C min-1 (Perkin-Elmer TGA-7). c Determined by dynamic light scattering (Malvern Instruments ZetaMaster). d 356 nm silica particles were used in these experiments. e 318 nm silica particles were used in these experiments. f In 50/50 methanol/water solution. g Not analyzed.
Figure 1. Reaction scheme for the synthesis of polymer-grafted silica particles via aqueous ATRP.
developed, an exotherm was immediately observed, and the stirred solution became progressively more viscous, indicating that polymerization was occurring (Figure 1). After approximately 2 h at 20 °C, the polymerization was terminated by exposure to air; this caused the reaction solution to turn blue, indicating oxidation of Cu(I) to Cu(II). This blue solution was centrifuged at 3000 rpm for 10 min, which led to sedimentation of the polymer-grafted silica particles. The supernatant, which contained unreacted monomer and residual ATRP catalyst, was discarded and replaced with deionized, distilled water. The particulate sediment was redispersed with the aid of an ultrasonic bath, and after two more centrifugationredispersion cycles, the off-white polymer-grafted silica particles were deemed sufficiently pure for characterization purposes. The amount of grafted polymer on the silica particle surface was assessed by FT-IR spectroscopy, elemental microanalyses, and thermogravimetry. Estimated polymer contents from the latter two techniques are summarized in Table 1 for various hydrophilic methacrylates. By increasing the target degree of polymerization (Dp), higher polymer contents were obtained, as expected. Polymer contents ranged from 3.3 to 8.6% by mass, with the best results being obtained for OEGMA and 2-(N-morpholino)ethyl methacrylate (MEMA), which is another wellbehaved hydrophilic methacrylate under aqueous ATRP conditions.14 On the basis of SEM studies (see Figure 2a), we estimate the specific surface area of the precursor silica sol to be around 10 m2 g-1. Hence, these polymer loadings correspond to relatively high Γ values of 3.7-10.1 mg m-2, which suggests that the polymer chains adopt extended brush conformations at the silica surface. The characteristic carbonyl band due to the polymer chain is clearly
detected by FT-IR spectroscopy. Dynamic light scattering studies were carried out on the original colloidal silica particles and also on selected polymer-grafted silica particles. In each case, the intensity-average hydrodynamic diameter of the polymer-grafted silica particles was larger than that of the uncoated silica particles (see Table 1). Furthermore, the hydrodynamic thickness of the polymer overlayer increased monotonically as the target degree of polymerization was increased, as expected. Figure 2 depicts scanning electron micrographs of the colloidal silica particles before and after the surfaceinitiated polymerization of OEGMA. In Figure 2a the silica particle morphology is confirmed to be spherical and nearmonodisperse, as expected. In Figure 2b these silica particles are all well-separated, which suggests that each silica particle is coated with a relatively thick outer layer of polyOEGMA chains. Absorbance versus temperature (turbidimetry) experiments on two types of polymer-grafted silica particles were conducted on dilute aqueous dispersions at pH 8. The results are depicted in Figure 3. As expected, the polyOEGMA-silica particles remained colloidally stable on heating from 20 °C up to 65 °C, because the polyOEGMA chains remain hydrophilic over this temperature range. In contrast, the polyMEMA-silica particles exhibited a large increase in absorbance (indicating the onset of particle aggregation) commencing at around 34 °C, which corresponds approximately to the known LCST (cloudpoint) of the polyMEMA chains. Bu¨tu¨n et al. have recently determined the precise relationship between cloud point and mean degree of polymerization for polyMEMA in dilute aqueous solution at pH 8.15 Using these data we estimate that the actual mean Dp of the surface-grafted polyMEMA chains is around 160, compared to the target
(14) Malet, F. L. G.; Billingham, N. C.; Armes, S. P. ACS Polym. Prepr. 2000, 41 (2), 1811.
(15) (a) Bu¨tu¨n, V. Ph.D. Thesis, University of Sussex, U.K. (b) Bu¨tu¨n, V.; Billingham, N. C.; Armes, S. P. Polymer 2001, 42, 5993.
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Figure 2. Representative scanning electron micrographs of (a) the uncoated, near-monodisperse colloidal silica particles and (b) the purified polyOEGMA-grafted silica particles (polymer content is 8.2% by mass).
Figure 3. Absorbance versus temperature plots for (a) polyMEMA-grafted silica particles, showing a critical flocculation temperature of around 34 °C, and (b) polyOEGMA-grafted silica particles, showing no critical flocculation temperature over this temperature range.
Dp of 500. However, these data should be treated with caution, since an implicit assumption is that terminal attachment of the polyMEMA chains to the silica particles has no effect on their cloud point. Essentially complete redispersion of the flocculated silica particles occurs on
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cooling or on addition of acid, as judged by turbidimetry. Thus, the colloid stability of these new sterically stabilized silica particles is dictated by the thermodynamic solution properties of the attached polymer chains. In particular, surface polymerization leads to new thermoresponsive particles in the case of the polyMEMA-silica system. XPS analyses were carried out in order to determine the surface compositions of the uncoated and polymergrafted silica particles. In these experiments the Si 2p signal acts a marker for the silica particles and almost all of the C 1s signal is due to the surface-grafted polymer chains (a small fraction is also due to the ATRP initiator). The C/Si atomic ratios are summarized in Table 1. Increasing the initial target Dp in the OEGMA polymerizations leads to an increase in the C/Si atomic ratio, as expected. However, the silicon from the underlying silica particles is still detected even at the highest target Dp, which suggests either that the dried polymer thickness is less than the XPS analysis depth (approximately 2-10 nm) or that the polymer overlayer is patchy. In the case of the MEMA and SEM monomers, either nitrogen or sulfur, respectively, can be used as unambiguous elemental markers for the grafted polymer component. Further details will be published in due course. Although we are greatly encouraged by our preliminary findings, several synthetic problems remain to be overcome. Although polymerization occurs rapidly at 20 °C, monomer conversions are typically only 50-76%. These yields are substantially lower than those obtained for the same monomer in aqueous solution with a watersoluble ATRP initiator, which suggests that an unidentified termination mechanism may be operating in the surface ATRP syntheses. The reduced conversions may be due in part to the relatively high target Dp’s in these syntheses; in most ATRP literature reports the target Dp’s are usually no more than 100. Furthermore, there is some evidence to suggest that a significant fraction of the polymer chains become detached from the silica surface during the in situ polymerization. This may be due to hydrolysis of the single Si-O bonds by which the polymer chains are grafted to the silica surface. In future studies we will focus on the cleavage of the grafted polymer chains from the surface of the silica particles for GPC analysis in order to assess whether surface-initiated aqueous ATRP leads to reasonable control over molecular weight distributions, as anticipated. In summary, the surface-initiated polymerization of various hydrophilic methacrylates at the surface of silica nanoparticles proceeds reasonably efficiently via aqueous ATRP, even at ambient temperature. The polymer-grafted silica particles produced in this initial study are fascinating new “model” sterically stabilized colloids which are likely to prove attractive for both theoretical and experimental studies. The aqueous solution properties of the grafted polymer chains determine the colloid stability of the particles, as expected. Acknowledgment. EPSRC is thanked for a ROPA postdoctoral grant for C.P. (GR/M76683). This work was also supported by the NSF CAREER program (Grant DMR-9733786) and by the NSF IGERT program (Grant DGE-9972741). Prof. J. F. Watts’ group at the University of Surrey (Guildford, U.K.) are thanked for their assistance with the XPS studies, which will be published in full elsewhere. LA0102758
