Synthesis and Characterization of Vinyl Polymer−Silica Colloidal

M.; Armes, S. P.; Fairhurst, D.; Emmett, S.; Pigott, T.; Idzorek, G. Langmuir 1992, 8, 2178. ... (b) Stejskal, J.; Kratochvil, P.; Armes, S. P.; L...
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Langmuir 2000, 16, 6913-6920

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Synthesis and Characterization of Vinyl Polymer-Silica Colloidal Nanocomposites M. J. Percy, C. Barthet, J. C. Lobb, M. A. Khan, S. F. Lascelles, M. Vamvakaki, and S. P. Armes* School of Chemistry, Physics and Environmental Science, University of Sussex, Falmer, Brighton BN1 9QJ, U.K. Received March 21, 2000. In Final Form: June 5, 2000 Colloidal dispersions of polymer-silica nanocomposite particles were synthesized in high yield by homopolymerizing 4-vinylpyridine (4VP) in the presence of an ultrafine silica sol using a free-radical initiator in aqueous media at 60 °C. Copolymerization of 4VP with methyl methacrylate and styrene also produced colloidally stable nanocomposite particles, in some cases for comonomer feeds containing as little as 6 mol % 4VP. However, homopolymerization of styrene or methyl methacrylate in the presence of the silica sol did not produce nanocomposite particles in control experiments. Thus a strong acid-base interaction between the silica sol and the (co)polymer appears to be essential for nanocomposite formation. Transmission electron microscopy studies confirmed the presence of the ultrafine silica sols within the nanocomposite particles, which typically exhibited “currant-bun” particle morphologies. This is in contrast to the “raspberry” particle morphologies previously reported for conducting polymer-silica nanocomposite particles. The average silica contents and mean particle diameters of the vinyl (co)polymer-silica nanocomposites were surprisingly insensitive to the synthesis conditions, as judged by thermogravimetric analysis and disk centrifuge photosedimentometry studies, respectively. The latter technique also indicated that some of the copolymer-silica dispersions were appreciably flocculated, although the degree of dispersion could be improved by redispersion in alkaline media. 1H NMR spectroscopy studies on the extracted nanocomposites confirmed incorporation of the 4VP comonomer, with reasonable agreement between copolymer compositions and comonomer feeds being obtained. Aqueous electrophoresis measurements confirmed that the surface of the 4VP-silica particles is polymer-rich, which is consistent with their currant-bun morphology. Timeresolved photon correlation spectroscopy studies during nanocomposite formation showed that particle growth occurred rapidly, with particles reaching their final size after approximately 1 h. Doubling the 4VP monomer concentration at a fixed 4VP/silica ratio led to an increase in particle size from 150 to 220 nm.

Introduction In polymer nanocomposites the polymer chains are confined to nanoscale (1-10 nm) dimensions. Following pioneering work by Giannelis and co-workers,1,2 it is now recognized that these materials can exhibit unusual, even unique, properties3 which cannot be obtained simply by comixing the polymeric component with the inorganic phase.4,5 In many literature reports polymer nanocomposites are synthesized by creating or modifying the inorganic phase in the presence of preformed polymer chains. For example, Messersmith and Stupp6 prepared calcium aluminate in the presence of various water-soluble polymers and obtained “organoceramic” materials. In contrast, Mark and co-workers7 prepared monolithic * To whom correspondence should be addressed. (1) (a) Burnside, S. D.; Giannelis, E. P. Chem. Mater. 1995, 7, 1597. (b) Giannelis, E. P. Adv. Mater. 1996, 8, 29. (c) Krishnamoorti, R.; Giannelis, E. P. Macromolecules 1997 30, 4097. (d) Messersmith, P. B.; Giannelis, E. P. J. Polym. Sci., Part A: Polym. Chem. 1995, 33, 1047. (e) Messersmith, P. B.; Giannelis, E. P. Chem. Mater. 1994, 6, 1719. (2) (a) Messersmith, P. B.; Giannelis, E. P. Chem. Mater. 1993, 5, 1064. (b) Mehrotra, V.; Giannelis, E. P. Solid State Ionics 1992, 51, 115. (c) Mehrotra, V.; Keddie, J. L.; Miller, J. M., Giannelis, E. P. J. NonCryst. Solids 1991, 136, 97. (3) (a) Beecroft, L. L.; Ober, C. K. Chem. Mater. 1997, 9, 1302. (b) Tsai, H. L.; Schindler, J. L.; Kannewurf, C. R.; Kanatzidis, M. G. Chem. Mater. 1997, 9, 875. (4) Kojima, Y.; Usuki, A. A.; Kawasumi, M.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 983. (5) Kojima, Y.; Usuki, A.; Kuwasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.; Kamigaito, O. J. Mater. Res. 1993, 8, 1185. (6) Messersmith, P. B.; Stupp, S. I. J. Mater. Res. 1992, 7, 2599. (7) (a) Pu, Z. C.; Mark, J. E.; Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1997, 9, 2442. (b) Pu, Z. C.; Mark, J. E.; Jethmalani, J. M.; Ford, W. T. Polym. Bull. 1996, 37, 545.

poly(methyl acrylate)/SiO2 nanocomposites by dispersing surface-modified silica particles in methyl acrylate, followed by polymerization of the monomeric continuous phase. In contrast, there has been relatively little work on colloidal polymer nanocomposites. Mann and co-workers8,9 have described the synthesis of magnetite-ferritin and CdS-ferritin nanoparticles via a biomimetic approach. In the 1970s Iler and co-workers at DuPont described the synthesis of uniform spherical polymer-silica composites with average diameters ranging from 500 nm up to 20 µm in the patent literature.10 These materials were readily prepared by step copolymerization of either urea or melamine with formaldehyde in aqueous media in the presence of an ultrafine silica sol. Since microporous silica particles were obtained on pyrolysis of the polymeric binder it is clear that, with the benefit of hindsight, the polymersilica precursor particles were indeed nanocomposites. In 1992 we reported11 the serendipitous synthesis of colloidally stable polyaniline-silica particles. In retrospect, this route bears many similarities to the work by Iler and co-workers described above, although in our case the “binder” component was a conducting polymer prepared using redox chemistry and the average diameters of the resulting polyaniline-silica particles were some(8) Wong, K. K. W.; Douglas, T.; Gider, S.; Awschalom, D. D.; Mann, S. Chem. Mater. 1998, 10, 279. (9) Wong, K. K. W.; Mann, S. Adv. Mater. 1996, 8, 928. (10) (a) Kirkland, J. J. U.S. Patent No. 3,782,075, 1974. (b) Iler, R. K.; McQueston, H. J. U.S. Patent No. 4,010,242, 1977. (11) Gill, M.; Mykytiuk, J.; Armes, S. P.; Edwards, J. L.; Yeates, T.; Moreland, P. J. Mollett, C. J. Chem. Soc., Chem. Commun. 1992, 108.

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what smaller (typically 300 nm). A subsequent small X-ray scattering study confirmed12 that these polyaniline-silica particles were true nanocomposites. In the last 6 years this work was extended to include another conducting polymer, polypyrrole.13-17 Transmission electron microscopy studies confirmed that the polyaniline-silica18 and polypyrrole-silica13-17 nanocomposites each had “raspberry” particle morphologies. The surface compositions of these particles were silica-rich, as judged from both X-ray photoelectron spectroscopy19 and aqueous electrophoresis measurements.20 However, the reaction conditions required for the preparation of conducting polymers do not allow efficient nanocomposite syntheses at high solids. The polymerization of pyrrole (and aniline) is very exothermic and prone to side reactions, such as overoxidation of the conjugated polymer backbone. Thus conducting polymer-silica nanocomposites can only be prepared as relatively dilute dispersions (typically no more than 3 wt % solids). In addition, acidic solutions (pH < 2) are required and the chemical oxidant, e.g. FeCl3 or (NH4)2S2O8, reacts stoichiometrically (rather than catalytically) with the monomer; hence, the ionic strength of the reaction solution is relatively high. These conditions exclude most ultrafine inorganic oxide sols from acting as dispersants.15 Indeed, in retrospect, the peculiarly high tolerance of silica sols toward electrolyte-induced flocculation21 was probably crucial to the success of our conducting polymer-silica nanocomposite syntheses. In contrast, the polymerization of vinyl monomers via free radical chemistry offers several potential advantages in the context of colloidal nanocomposite syntheses. Vinyl polymerizations can be carried out at much higher monomer concentrations in aqueous media over a wide range of solution pH. Furthermore, only low concentrations of the free radical initiator are required; hence, the ionic strength of the solution can be minimized. These synthetic advantages, together with the availability of a wide range of low-cost vinyl monomers, led us to extend our nanocomposite syntheses to include vinyl monomers. Our initial results have been summarized in a preliminary communication.22 Herein we report full details of the synthesis and characterization of a series of (co)polymer-silica colloidal nanocomposites by the free-radical (co)polymerization of vinyl monomers (see Figure 1). This discovery substantially broadens and extends the scope of “surfactant-free” syntheses of colloidal polymer nanocomposites. Experimental Section (1) Nanocomposite Syntheses. Homopolymerization. The homopolymer/silica nanocomposites were prepared by free radical polymerization of 4-vinylpyridine (4VP) in the presence of 20 nm (12) Terrill, N. J.; Crowley, T.; Gill, M.; Armes, S. P. Langmuir 1993, 9, 2093. (13) Maeda, S.; Armes, S. P. J. Colloid Interface Sci. 1993, 159, 257. (14) Maeda, S.; Armes, S. P. J. Mater. Chem. 1994, 4, 935. (15) Maeda, S.; Armes, S. P. Chem. Mater. 1995, 7, 171. (16) Maeda, S.; Armes, S. P. Synth. Met. 1995, 73, 151. (17) Flitton, R.; Johal, J.; Maeda, S.; Armes, S. P. J. Colloid Interface Sci. 1995, 173, 135. (18) (a) Gill, M.; Armes, S. P.; Fairhurst, D.; Emmett, S.; Pigott, T.; Idzorek, G. Langmuir 1992, 8, 2178. (b) Stejskal, J.; Kratochvil, P.; Armes, S. P.; Lascelles, S. F.; Riede, A.; Helmstadt, M.; Prokes, J.; Krivka, I. Macromolecules 1996, 29, 6814. (19) Maeda, S.; Gill, M.; Armes, S. P.; Fletcher, I. W. Langmuir 1995, 11, 1899. (20) Butterworth: M. D.; Maeda, S.; Johal, J.; Corradi, R.; Lascelles, S. F.; Armes, S. P. J. Colloid Interface Sci. 1995, 174, 510. (21) Healey, T. W. In The Colloid Chemistry of Silica; Bergna, H. E., Ed.; ACS Symposium Series No. 234; American Chemical Society: Washington, DC, 1994; pp 147-159. (22) Barthet, C.; Hickey, A. J.; Cairns, D. B.; Armes, S. P. Adv. Mater. 1999, 11, 408.

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Figure 1. Reaction scheme for the formation of vinyl polymersilica nanocomposite particles by the free-radical polymerization of 4VP at 60 °C in the presence of an ultrafine silica sol. Table 1. Summary of 4VP/SiO2 Colloidal Nanocomposite Synthesesa monomer concn (w/v %)

initial silica concn (w/v %)

particleb diameter (nm)

silicac content (%)

silica aggregation efficiency (%)

particle densityd (g cm-3)

5 10 5 10

8 8 16 16

130-150 160-220 130-160 150-190

38 34 37 36

35 60 19 39

1.49 1.42 1.45 1.47

a Reaction conditions: polymerizations at 60 °C for 24 h using 1% ammonium persulfate initiator based on 4VP. b Determined from TEM studies. c As determined by thermogravimetry. d Measured by helium pycnometry.

silica sol (Nyacol 2040; supplied as a 40 w/w % aqueous dispersion). The 4VP (5.0 or 10 mL) was added to 100 mL of an aqueous solution at approximately pH 10 containing either 8.0 or 16.0 g of the 20 nm silica sol (see Table 1) in a three-necked round-bottom flask fitted with a mechanical overhead stirrer. The reaction vessel was purged with nitrogen and an aqueous solution of ammonium persulfate initiator (1 wt % based on monomer) was added to the reaction vessel, with vigorous stirring. Each polymerization was performed at 60 °C under nitrogen and allowed to proceed for 24 h. Essentially no pH drift was detected during polymerization; presumably this is due to the self-buffering nature of the 4VP monomer/polymer. The milky-white dispersions were purified by four centrifugation-redispersion cycles, with each successive supernatant being decanted and replaced with deionized water. Care was taken to avoid excessive centrifugation rates and times since these would result in the unwanted sedimentation of the excess silica sol in addition to the nanocomposite particles. In one experiment 4VP was homopolymerized at 25 °C using a 1:1 ammonium persulfate-tetramethylethylenediamine initiator system.23 Copolymerization. Nanocomposite particles were synthesized by copolymerizing 4VP in turn with the following four monomers: styrene (St); methyl methacrylate (MMA); n-butyl methacrylate (BuMA); n-butyl acrylate (BuA). In a typical synthesis, MMA (3.95 mL) and 4VP (1.05 mL) were added to an aqueous silica sol (95 mL total volume, 8.0 g silica) at pH 10 and (23) Odian, G. Principles of Polymerisation; Wiley and Sons: New York, 1981.

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Table 2. Summary of Data for Various Copolymer-Silica Nanocomposite Particles Prepared by Copolymerizing Various Vinyl Monomers with 4VPa entry no.

comonomer type

4VP in comonomer feed (mol %)

colloidally stable dispersion?

particle diameterb (nm)

silica contentd (wt %)

particle densitye

4VP in copolymer (mol %)f

1 2 3 4 5 6

styrene styrene styrene MMA MMA MMA

6 10 21 4 10 20

yes yes yes no yes yes

180-230c 220-280 140-170

8 10 27

1.15 1.17 1.31

7 12 32

160-190 210g

35 54

1.49 1.59

10 23

a Reaction conditions: 5.0 mL total monomer volumes, 8.0 g silica, 100 mL total support volume, and all polymerization at 60 °C for 24 h. b As measured by TEM. c Some particle deformation. d As determined by thermogravimetry (20 °C/min in air). e As measured by helium pycnometry. f Determined by NMR analysis of the extracted copolymer. g Determined by PCS.

an aqueous solution of ammonium persulfate initiator (1.0% based on comonomer) was added to this vigorously stirred solution. Polymerizations were performed at 60 °C for 24 h under nitrogen before cooling to room temperature. The resulting milky-white colloidal dispersions were purified by repeated centrifugation/ redispersion cycles as described above. The various 4VP comonomer feed ratios and total comonomer volumes investigated are summarized in Table 2. Copolymers were extracted from the nanocomposites by treating approximately 5 mg of the dried particles with CDCl3. The NMR tubes were maintained in an ultrasonic bath at 25 °C for 2 h to aid copolymer dissolution. Thermogravimetric analyses of the remaining silica sol confirmed quantitative extraction of the (co)polymers. A shorthand notation is used in this paper to describe the nanocomposites. Thus, “4VP/SiO2” denotes a homopoly(4-vinylpyridine)/silica nanocomposite. Similarly, “90:10 MMA-4VP/ SiO2” denotes a methyl methacrylate-4-vinylpyridine copolymersilica nanocomposite prepared using a comonomer feed comprising 90 mol % methyl methacrylate and 10 mol % 4-vinylpyridine. (2) Nanocomposite Characterization. Particle Size Analysis. Disk centrifuge photosedimentometry (DCP; Brookhaven Instruments) was used to obtain the weight-average particle size distribution of the nanocomposite particles. Solidstate particle densities were measured by helium pycnometry. The centrifugation rate was in the 3000-8000 rpm range, and the dispersions were assumed to have the same scattering characteristics as polystyrene in water. Typical DCP runs lasted 30 min. Particle growth was monitored using photon correlation spectroscopy (Malvern 4700 instrument equipped with a 75 mW argon ion laser; all measurements were carried out at 25 °C at a fixed angle of 90° on highly diluted aliquots taken from the reaction solution). Transmission electron microscopy (TEM) studies were performed using either a Hitachi 7100 or a Zeiss EM 902 instrument operating at 80 kV. Dilute dispersions were dried onto carbon-coated copper grids. Chemical Composition. Thermogravimetric analyses were performed with a Perkin-Elmer TGA-7 instrument. The nanocomposite powders were heated to 800 °C at a scan rate of 20 °C min-1 in air, and the observed mass loss was attributed to the quantitative pyrolysis of the (co)polymer component. The silica residues were corrected for loss of surface moisture. Proton NMR spectra of the extracted copolymers and homopolymer reference materials were recorded in CDCl3 (16 scans per spectrum) at 293 K on either a Bruker ACP 250 or a Bruker DPX 300 instrument. The NMR spectrum of the polystyrene homopolymer was recorded in CD2Cl2 to avoid peak overlap with the solvent signal at δ 7.25 due to residual CHCl3 in CDCl3. CHN microanalyses were carried out at an external analytical laboratory (Medac Ltd., Egham, Surrey, U.K.). Aqueous electrophoresis. Aqueous electrophoresis data were obtained using a Malvern Instruments Zetamaster instrument. The zeta potential, ζ, was calculated24 from the electrophoretic mobility (u) using the Smoluchowsky relationship, ζ ) ηu/, where it is assumed that κa . 1 (where η is the solution viscosity,  is the dielectric constant of the medium, and κ and a are the Debye-Hu¨ckel parameter and the particle radius, (24) Hunter, R. J. Foundations of Colloid Science; Clarendon Press: Oxford, U.K., 1989; Vol. 2.

respectively). The solution pH was adjusted by the addition of NaOH or HCl.

Results and Discussion Background. In the past decade or so there have been significant advances in the encapsulation of inorganic oxide particles with polymers. This technology is of considerable interest to the paint industry, since more efficient dispersion of pigment particles within latex films can lead to improved coating properties (e.g. better gloss and opacity). Thus Caris and co-workers have used titanate-based coupling agents to coat titania particles with poly(methyl methacrylate).25 In related work, Furusawa et al. used a polymeric stabilizer, hydroxypropylcellulose, to assist their synthesis of “core-shell” silicapolystyrene composite particles.26 In the context of the present work, it is interesting to note that the use of an anionic surfactant in addition to the hydroxypropylcellulose led to the formation of “raspberry” composite particles of around 200-300 nm diameter which contained many silica particles per composite particles. However, the average silica content of the particles was not determined. Similarly, Hergeth et al. claimed to encapsulate silica particles within poly(vinyl acetate) latexes by carrying out the emulsion polymerization of vinyl acetate in the presence of ultrafine quartz particles (26 nm Suprasil) using an anionic surfactant.27 Again, no details concerning the particle size or silica content of the composite particles were provided. The aim of the present work was to investigate the feasibility of synthesizing nanostructured vinyl polymersilica particles, preferably via a “surfactant-free” route. The first vinyl monomer evaluated for nanocomposite syntheses was 4VP. This monomer was selected for two reasons. First, since the surface of the ultrafine silica sol is acidic, it seemed likely that using a basic monomer would lead to a strong acid-base interaction and hence promote nanocomposite formation (in this context we note that the adsorption of pyridine onto silica substrates is well-documented28). Second, poly(4-vinylpyridine) is waterinsoluble in neutral or basic media; thus judicious selection of the solution pH should lead to precipitation of this polymer onto the silica particles, hopefully resulting in the formation of colloidally stable nanocomposite particles. (25) Caris, C. H. M.; Kujpers, R. P. M.; van Herk, A. M.; German, A. L. Makromol. Chem. Macromol. Symp. 1990, 535, 35-36. (26) Furusawa, K.; Kimura, Y.; Tagawa, T. J. Colloid Interface Sci. 1986, 109, 69. (27) (a) Hergeth, W. D.; Schmutzler, K.; Wartewig, S. Makromol. Chem. Macromol. Symp. 1990, 1, 123. (b) Hergeth, W. D.; Steinau, U. J.; Bittrich, H. J.; Schmutzler, K.; Wartewig, S. Prog. Colloid Polym. Sci. 1991, 85, 82. (28) (a) Curthoys, G.; Davydov, V. Y.; Kiselev, A. V.; Kiselev, S. A.; Kuznetsov, B. V. J. Colloid Interface Sci. 1974, 48, 58. (b) Kno¨zinger, H. Surf. Sci. 1974, 41, 339.

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Preliminary experiments were very encouraging. In the presence of an ultrafine silica sol at pH 10, the free radical polymerization of 4VP at 60 °C for 24 h led to a milky white dispersion, with no evidence of any macroscopic precipitation.22 On the other hand, a control experiment in the absence of any silica sol led to quantitative precipitation of the water-insoluble poly(4-vinylpyridine), with no colloid formation. This clearly demonstrates the crucial role played by the silica sol in producing colloidally stable dispersions. In retrospect our initial choice of 4VP was somewhat fortuitous, since attempted nanocomposite syntheses with related vinyl monomers such as 2-vinylpyridine (2VP) were unsuccessful.22 The superior performance of 4VP is most likely due to its higher basicity. We determined the pKa of 4VP monomer to be 5.62 by titration, which is consistent with literature values.29,30 On the other hand, we found the pKa of 2VP to be 4.92 (compared to a literature value of 5.19). Thus 2VP is significantly less basic than 4VP; hence, a weaker interaction would be expected between the acidic surface sites on the silica particles and this monomer. It is also possible that the 2VP is less easily adsorbed onto the silica sol due to steric constraints.31 These results suggest that a basic vinyl monomer is a necessary, but not sufficient, condition for nanocomposite formation. Moreover, 4VP is clearly a preferred monomer for these nanocomposite syntheses. Homopolymer Nanocomposites. The starting point for the present study was the systematic variation of the synthesis conditions and their effect on the particle size and silica content of the resulting 4VP/SiO2 nanocomposite particles. Our results are summarized in Table 1. A set of four nanocomposite syntheses were carried out at two 4VP and two silica concentrations (see Table 1) in order to examine the effect of varying these synthesis parameters on the properties of the 4VP/SiO2 particles. Each of these four formulations produced stable dispersions with a polymer yield of greater than 95% in all cases. However, after repeated centrifugation-redispersion cycles, dynamic light scattering studies suggested some degree of flocculation compared to that observed for the reaction solutions. TEM studies indicated that the mean particle size tended to be slightly larger at higher monomer/silica ratios, but overall both the silica content and particle diameter of the nanocomposite particles proved surprisingly insensitive to the synthesis conditions. Since the final silica content of the particles did not depend strongly on the initial concentration of the silica sol, this leads to large differences in the silica aggregation efficiency, with lower efficiencies being obtained at higher concentrations of silica sol. Nanocomposite particle densities generally agreed quite well with the theoretical densities calculated from their silica contents and the densities of the respective components (the densities of poly(4-vinylpyridine) and the ultrafine silica sol were determined by helium pycnometry to be 1.20 and 2.17 g cm-3, respectively). TEM was used to investigate the particle morphology of the nanocomposites. Previous studies confirmed12-20 that conducting polymer-silica nanocomposites generally have a distinctive raspberry particle morphology, which is particularly pronounced at higher silica contents (>50% by mass). The surface roughness is caused by surfaceadsorbed 20 nm silica particles protruding from the particle surface. In contrast, the 4VP/silica nanocomposites described in the present work have a “currant-bun” (29) Pietrzyk, A.; Wilby, R.; McDaniel, D. J. Org. Chem. 1957, 22, 83. (30) Ellam, G. B.; Johnson, C. D. J. Org. Chem. 1971, 36, 2284. (31) Kagel, R. O. J. Phys. Chem. 1970, 74, 4518.

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Figure 2. Transmission electron micrographs of (a) 4VP/SiO2 nanocomposites and (b) 90:10 St-4VP/SiO2 nanocomposites. Note the “currant-bun” morphology due to the darker ultrafine silica sol within the interior of the particles in (a). In contrast, (b) suggests a “raspberry” morphology.

particle morphology, with the ultrafine silica sol being located primarily within the interior of the particles rather than at their surface (see Figure 2). Thermogravimetric analyses indicated silica contents of approximately 3438% by mass. Similar particle compositions were indicated by elemental microanalyses (the carbon and nitrogen contents of the nanocomposite were compared to that of 4VP homopolymer prepared by precipitation polymerization in the absence of the silica sol). Disk centrifuge photosedimentometry (DCP) studies indicate that the particle size distributions of 4VP/SiO2 nanocomposites were generally unimodal and are skewed slightly to larger particle diameters. DCP can also be used to assess the degree of dispersion.14,18 The DCP analysis of an aqueous dispersion of a 4VP/SiO2 nanocomposite at a solution pH of 11 and 6 is shown in Figure 3. A relatively narrow size distribution is obtained under alkaline conditions, but at pH 6 a much broader size distribution is observed. This indicates significant flocculation at this pH, which corresponds approximately to the isoelectric point of this dispersion (see later). We believe that the colloid stability of these nanocomposite particles is largely

Vinyl Polymer-Silica Colloidal Nanocomposites

Figure 3. Particle size distributions obtained using disk centrifuge photosedimentometry for an aqueous dispersion of a 4VP/SiO2 nanocomposite at (a) dispersed at pH 11 and (b) partially flocculated at pH 6 (the isoelectric point of this dispersion).

Figure 4. Evolution of particle diameter during the formation of 4VP/SiO2 nanocomposites at 60 °C, as determined by dynamic light scattering. Reaction conditions: 5.0 mL 4VP, 8.0 g SiO2 ([); 10.0 mL 4VP, 16.0 g SiO2 (0).

dictated by the silica sol component, but there is also the likelihood of additional charge stabilization arising from the anionic chain ends created by the persulfate initiator. To investigate the kinetics of particle formation, the reaction mixture was sampled at regular intervals and analyzed using photon correlation spectroscopy. This technique is highly biased toward the larger nanocomposite particles and is relatively insensitive to the remaining nonaggregated small silica particles. Thus it is ideally suited for monitoring the onset of particle formation. Figure 4 depicts the evolution of particle growth during a typical nanocomposite synthesis. The nanocomposite particles reach their maximum size after approximately 1 h, which appears to correspond to the time required for 98% monomer conversion. Interestingly, this time period appears to be independent of the 4VP concentration at a given 4VP/SiO2 ratio. The final solids contents of the four 4VP/SiO2 nanocomposites summarized in Table 1 ranged from 7 to 12 wt %, which is significantly higher than the 3 wt % solids achieved for the conducting polymer-silica nanocomposite syntheses reported previously.12-20 However, our preliminary experiments using monomer-starved conditions (i.e. slowly drip-feeding 4VP monomer into the reaction solution over a period of 1 h) suggests that the vinyl polymer-silica nanocomposites can be successfully synthesized at least 20% solids and possibly higher.32 Furthermore, addition of tetramethylethylenediamine to the persulfate initiator enables nanocomposite syntheses to be carried out at room temperature.33 (32) Percy, M. J.; Armes, S. P. Unpublished results. (33) Barthet, C.; Armes, S. P. Unpublished results.

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Copolymerization of 4-Vinylpyridine with Other Monomers. Given the successful nanocomposite syntheses with 4VP, we considered whether other polymersilica interactions were sufficient to promote colloidal nanocomposite formation. Accordingly, attempts were made to prepare St/SiO2 and MMA/SiO2 nanocomposites.22 A stable colloidal dispersion was obtained for the homopolymerization of styrene, but subsequent thermogravimetric analysis confirmed the absence of silica. Thus only a conventional charge-stabilized polystyrene latex had been formed rather than the desired St/SiO2 nanocomposite. In the MMA experiment, only macroscopic precipitation occurred, with little or no colloid formation. Thus it was concluded that neither hydrophobic interactions (polystyrene) nor hydrogen bonding (PMMA) between the polymer and silica components was sufficient to promote nanocomposite formation. Given that nanocomposites could be easily prepared by homopolymerizing 4VP in the presence of a silica sol and the corresponding homopolymerizations of St and MMA were unsuccessful, a second question now had to be addressed. Could colloidally stable nanocomposites be prepared by copolymerization of 4VP with other comonomers such as St or MMA? As an initial “proof of concept” experiment, 4VP (21 mol %) was copolymerized with styrene under similar conditions to those used for the homopolymerization of 4VP. A colloidally stable milkywhite dispersion was obtained, and thermogravimetric analysis confirmed a silica content of ca. 27% for the resulting nanocomposite particles. Transmission electron microscopy studies indicated a mean number-average particle diameter of around 155 nm. Styrene is a relatively cheap monomer feedstock, whereas 4VP is relatively expensive. Thus this is a potentially important result for eventual commercial applications of these nanocomposites. Next, the effect of varying the St/4VP comonomer feed on the properties of the nanocomposite particles was investigated. If one inspects Table 2, it is apparent that, in the case of St, only a relatively low proportion of 4VP is sufficient to promote the formation of stable nanocomposites. Particles formed from different proportions of St comonomer were characterized. When the amount of 4VP is reduced from 20% to 10% in the initial monomer feed, there is an increase in mean nanocomposite diameter from 155 to 250 nm. It is noteworthy that the opposite trend was reported by Wang et al.34 These workers investigated the synthesis of St-4VP copolymer latexes via emulsion polymerization in the absence of silica sol35 and reported that the mean latex diameter increased as the proportion of 4VP in the comonomer feed was increased. However, it appears that the concentration of persulfate initiator was not held constant while varying the 4VP comonomer feed in these experiments. Furthermore, in our case there is no doubt that the presence of the 4VP comonomer is crucial for nanocomposite formation: it adsorbs strongly onto the silica particles, which leads to its incorporation within the growing copolymer particles. Thus it is perhaps unsurprising that decreasing the 4VP content leads to larger nanocomposite particles. At 6 mol % 4VP the final particle diameter is around 200 nm, which at first sight appears to be inconsistent with the trend of increasing particle diameter at lower 4VP concentration. However, some coagulum was observed in this formulation, which suggests that we are approaching the minimum proportion of 4VP in the feed for successful nanocomposite formation (34) Wang, Y.; Feng, L.; Pan, C. J. App. Polym. Sci. 1999, 74, 1502. (35) Attempted copolymerisation of 4-vinylpyridine and styrene in the absence of silica at the same monomer concentration and pH produced a stable colloids solution.

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Figure 5. Aqueous microelectrophoresis data for MMA-4VP/ SiO2 nanocomposite particles synthesized with increasing 4VP content. Data for the 20 nm silica sols are included as a comparison. Key: (]) 20 nm Nyacol 2040 silica sol; (2) 10 mol % 4VP; (9) 20 mol % 4VP; (+) 100 mol % 4VP.

under these conditions. As the proportion of 4VP in the comonomer feed is reduced, the silica content of the nanocomposite particles decreases. In light of the above discussion this seems perfectly reasonable. The reduced nanocomposite particle densities determined by helium pycnometry are also consistent with these lower silica contents. To further expand the range of vinyl polymer-silica nanocomposites produced, copolymerizations were attempted with methyl methacrylate (MMA), n-butyl acrylate (BuA), and n-butyl methacrylate (BuMA) at various initial comonomer feeds of 4VP. Colloidally stable MMA4VP/SiO2 nanocomposites were obtained at both 20 and 10 mol % 4VP in the initial monomer feed. However, when the proportion of 4VP was reduced to 4 mol %, a stable colloidal dispersion was not obtained. As with the St4VP copolymerizations, the mean particle diameter of the MMA-4VP/SiO2 nanocomposites increases as the proportion of 4VP in the feed is reduced. Similarly, stable colloidal dispersions of BuA-4VP/SiO2 and BuMA-4VP/SiO2 were also obtained at 4VP comonomer feeds of 15% or higher. The film-forming properties of these low-Tg nanocomposite particles will be reported in due course.36 Aqueous electrophoresis is a well-established method for assessing the surface composition of colloidal particles. This method has been successfully applied to conducting polymer-silica nanocomposite particles by Butterworth et al.20 In the present work we were initially concerned that acidic media would cause protonation of the 4VP residues and hence dissolution of the polymeric binder. However, to our surprise, preliminary experiments confirmed that the nanocomposites remained colloidally stable even at pH 2. Thus ζ potential vs pH measurements were recorded for a 4VP/SiO2 nanocomposite, two MMA4VP/SiO2 nanocomposites, and also the original ultrafine silica sol; these data are shown together in Figure 5. The ultrafine silica sol exhibits a negative ζ potential across the whole pH range, as expected. In contrast, the ζ potential curve for the 4VP/SiO2 nanocomposite has a classic “S” shape, with an isoelectric point at approximately pH 6. This indicates that the basic 4VP residues are located at the surface of the nanocomposite particles and strongly influence the electrophoretic response at low pH, where they are protonated and hence cationic. This hypothesis is also consistent with the electrophoretic data obtained for the two MMA-4VP/SiO2 nanocomposites. Here the isoelectric points are both shifted to much lower pH, which reflects the reduced surface concentrations of the basic 4VP residues.37 These observations are in striking contrast to the results of Butterworth et al.,20 who reported that the electrophoresis curves obtained for a series of conducting (36) Amalvy, J.; Percy, M. J.; Armes, S. P. Manuscript in preparation.

Figure 6. Series of 1H NMR spectra for poly(4-vinylpyridine), polystyrene, and the 90:10 St-4VP statistical copolymer extracted from the 90:10 St-4VP/SiO2 nanocomposite.

polymer-silica nanocomposites were superimposable on that of a silica sol. Thus, these latter nanocomposites behaved essentially like silica sols; there was little or no evidence for any electrophoretic contribution from the conducting polymer component. These apparently contradictory findings can be readily explained in terms of the differing particle morphologies. The conducting polymer-silica nanocomposites examined by Butterworth et al.20 had a raspberry particle morphology and had silicarich surface compositions as judged by X-ray photoelectron spectroscopy (XPS).17,18 However, the homopolymer-silica nanocomposites described in the present work have a currant-bun particle morphology (see Figure 2a), which suggests that their surface is probably polymer-rich, rather than silica-rich. XPS studies are currently in progress in order to quantify the surface compositions of these vinyl (co)polymer-silica nanocomposites.38 To verify that copolymerization had occurred, the organic component of selected dried nanocomposites was extracted using CDCl3 at room temperature. The proton NMR spectrum of the polymeric extract obtained from a St-4VP copolymerization, along with St and 4VP homopolymer reference materials, is shown in Figure 6. The (37) We are aware that the electrophoretic behaviour of conventional (co)polymer latexes prepared from persulfate initiators is strongly influenced by the surface sulfate groups derived from the initiator fragment. Since sulfate is a strong acid, this usually leads to negative ζ potentials over a rather wide range of pH. In the present system, the electrophoretic component from the anionic sulfate groups cannot be distinguished from that due to the anionic silanol groups due to the inorganic silica component. In other words, the sulfate groups are probably located at the surface of the (co)polymer-silica nanocomposite particles but their presence is not easily verified. (38) Percy, M. J.; Barthet, C.; Armes, S. P.; Watts, J. F.; Greaves, S. J. Unpublished results.

Vinyl Polymer-Silica Colloidal Nanocomposites

NMR spectrum of the extract is assigned as follows: the signal at δ 7.9-8.3 is due to the two aromatic protons adjacent to the nitrogen atom in the 4VP residues (the other two aromatic protons associated with the pyridine ring are obscured by the aromatic signals due to the St comonomer). The integrated intensity of this signal was compared to that of the aliphatic backbone protons which are common to both the 4VP and St residues. This analysis indicated a 4VP content of approximately 12 mol % for the extracted copolymer, which is in reasonably good agreement with an initial comonomer feed of 10 mol %. On the basis of the respective monomer reactivity ratios,39 near-ideal behavior would be expected for the copolymerization of 4VP with St by free radical chemistry. Similarly, the NMR spectrum (not shown) of the extracted copolymer obtained from an 80:20 MMA-4VP/SiO2 nanocomposite also indicated the presence of the MMA comonomer (a signal at δ 3.2-3.7 was assigned to methoxy protons). In this case, comparison of this peak integral with the two aromatic signals due to the 4VP residues at δ 8.2-8.6 and δ 6.6-7.1 indicated a 4VP content of 23 mol %. Again, bearing in mind the relative monomer reactivity ratios, this value is in reasonable agreement with the comonomer feed. Similar results were obtained for the NMR analyses of several other extracted copolymers. Mechanism of Nanocomposite Particle Formation. The 4VP monomer is only sparingly soluble in water at pH 10, even at 60 °C. Since the emulsifying properties of ultrafine sols such as silica are well-documented,40 it is quite likely that the ultrafine silica sol acts as an emulsifier for the 4VP monomer droplets prior to polymerization. Whether this emulsification is important for the formation of colloidally stable nanocomposites particles is less clear. In addition to its (partial) dissolution and emulsification, a significant fraction of the 4VP (around 10-20%) is adsorbed onto the surface of the acidic silica sol prior to (co)polymerization. Using UV spectroscopy to monitor 4VP depletion from aqueous solution, we estimate an extent of adsorption of approximately 0.1 mg m-2 for this monomer on the ultrafine silica sol. This observation is also consistent with the study of the adsorption of the closely related poly(2-vinylpyridine) onto silica recently reported by Biggs and Proud.41 There are two obvious questions here. First, is preadsorption of the 4VP monomer a prerequisite for successful nanocomposite formation? Second, what role (if any) does surface polymerization play in nanocomposite formation? In this context, it is interesting to note that Mao and Fung have recently proposed that preadsorption of an acidic monomer (maleic acid) may be important in the formation of copolymer-coated alumina particles.42 Clearly the situation is even more complicated for the copolymerization syntheses, since hydrophobic comonomers such as styrene are much less soluble in aqueous media than 4VP and the locus of the copolymerization is by no means clear. To summarize, the apparent simplicity of these nanocomposite syntheses is deceptive: in reality many fundamental questions remain unresolved and the mechanism of particle formation is very poorly understood. Most other reports describing the synthesis of organicinorganic hybrid particles usually require (i) surface pretreatment of the inorganic component and (ii) addition of polymers or surfactants, which act as binders or (39) Brandrup, D., Immergut, E. H., Eds. Polymer Handbook 2nd ed.; John Wiley and Sons: New York, 1975. (40) Levine, S.; Bowen, B. D.; Partridge, S. J. Colloids Surf. 1989, 38, 325. (41) Biggs, S.; Proud, A. D. Langmuir 1999, 13, 7202. (42) Mao, Y.; Fung, B. M. Chem. Mater. 1998, 10, 509.

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stabilizers. For example, Furusawa et al. utilized hydropropylcellulose as a binder in the aqueous emulsion polymerization of styrene in order to promote the formation of polystyrene/SiO2 composite particles. More recently, Bourgeat-Lami and Lang have reported the preparation of silica-encapsulated polystyrene latexes via nonaqueous dispersion polymerization in alcoholic media.43 In this case poly(N-vinylpyrrolidone) was added to the polymerization and acted as a steric stabilizer. The silica sols were prepared by the Sto¨ber method and were relatively large (50-629 nm diameter) compared to the 20 nm silica sols used in the present study. More importantly, these Sto¨ber silicas had to be pretreated with vinyl-functionalized siloxanes in order to achieve efficient encapsulation of the silica within the polystyrene latex. In contrast, our nanocomposite syntheses are conducted in aqueous media in the absence of any polymeric stabilizer or surfactant. Furthermore, no surface pretreament of the silica sol was necessary: (co)polymerization of 4VP is alone sufficient to produce colloidally stable nanocomposites. Thus the vinyl (co)polymer-silica nanocomposite syntheses described herein represent a new paradigm for the preparation of surfactant-free polymer colloids in aqueous media. It is worth emphasizing that the ultrafine silica sol which acts as the “particulate” dispersant is cheap, nontoxic and already used on a commercial scale in industry. Only relatively low levels of the 4VP auxiliary monomer are required and the copolymerization route appears to be rather general. In future work we intend to evaluate the properties of film-forming vinyl (co)polymer-silica particles based on acrylic monomers. If successful, these composite particles are expected to lead to tough, abrasionresistant, transparent nanocomposite coatings. Furthermore, we note that this synthetic route may be applicable to other ultrafine sols such as magnetite, zirconia, tin(IV) oxide, etc. If colloidal nanocomposites can be prepared using low-Tg comonomers in combination with an ultrafine tin(IV) oxide sol, there is the intriguing possibility of achieving transparent conductive coatings. Conclusions In summary, a facile route to colloidal polymer nanocomposites via the free radical (co)polymerization of vinyl monomers in the presence of an ultrafine silica sol is reported. We believe that this approach represents a new paradigm in the synthesis of “surfactant-free” polymer colloids. A strong acid-base interaction between the (co)polymer and the silica sol appears to be a prerequisite for nanocomposite formation: this is achieved using 4-vinylpyridine as a (co)monomer. The resulting nanocomposite particles have relatively narrow size distributions, mean particle diameters of 150-250 nm and silica contents of 8-54%. Stable colloidal dispersions can also be prepared at room temperature, and preliminary results suggest that formulations at high solids (>20%) are feasible. TEM studies indicate a “currant-bun” particle morphology, in which most of the silica particles are encapsulated within the 4VP homopolymer. This interpretation is consistent with aqueous electrophoresis data. Nevertheless, it is emphasized that the presence of the silica sol is essential for the formation of colloidally stable nanocomposite dispersions, at least for the homopolymerization of 4VP. Acknowledgment. EPSRC is acknowledged for a postdoctoral research grant (GR/M22017) for M.J.P. C.B. (43) (a) Bourgeat-Lami, E.; Lang, J. J. Colloid Interface Sci. 1998, 197, 293. (b) Bourgeat-Lami, E.; Lang, J. J. Colloid Interface Sci. 1999, 210, 281.

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thanks the EC Training and Mobility of Researchers (TMR) scheme for a Marie Curie postdoctoral fellowship. We thank Dr. Heckmann, Dr. Wiese, and Mr Heiter of BASF, Ludwigshafen, Germany, for their kind assistance

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in obtaining the TEM images. A reviewer is thanked for his/her helpful comments. LA0004294