When Does Silica Exchange Occur between Vinyl ... - ACS Publications

Jul 23, 2010 - Martin W. Murray, Kenneth A. Murray, and Neal S. J. Williams. AkzoNobel .... servation: addition of excess sterically stabilized poly(2...
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When Does Silica Exchange Occur between Vinyl Polymer-Silica Nanocomposite Particles and Sterically Stabilized Latexes? Jennifer A. Balmer, Elise C. Le Cunff, and Steven P. Armes* Dainton Building, Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, United Kingdom

Martin W. Murray, Kenneth A. Murray, and Neal S. J. Williams AkzoNobel, Wexham Road, Slough, Berkshire SL2 5DS, United Kingdom Received May 26, 2010. Revised Manuscript Received July 5, 2010 The redistribution of silica nanoparticles between “core-shell” polymer-silica nanocomposites and sterically stabilized latexes is investigated using a combination of electron microscopy, disk centrifuge photosedimentometry (DCP), and X-ray photoelectron spectroscopy (XPS). Facile exchange of silica nanoparticles occurs on addition of sterically-stabilized polystyrene (or poly(2-vinylpyridine)) latex to polystyrene-silica (or poly(2-vinylpyridine)-silica) nanocomposite particles previously prepared by heteroflocculation. In contrast, no silica exchange occurs after such a latex “challenge” if similar polymer/silica nanocomposite particles are prepared via in situ polymerization. Silica redistribution can be confirmed by post mortem electron microscopy studies, which are facilitated if the original nanocomposite and latex particles differ sufficiently in their mean diameters. Ideally, XPS requires a unique elemental marker for the nanocomposite particle cores, which become progressively more exposed if silica exchange occurs. DCP is a particularly convenient in situ technique for assessing whether or not silica exchange has occurred. If no silica exchange occurs, there is little or no change in the nanocomposite and latex size distributions. On the other hand, silica redistribution always results in a larger mean particle diameter for the (partially) silica-coated latex particles relative to the original bare latex. In addition, incipient flocculation is typically observed after silica exchange. Like electron microscopy, DCP studies are aided if there is a significant difference in particle diameter between the original polymer-silica nanocomposite particles and the added latex. Moreover, silica redistribution can be prevented for heteroflocculated polymer-silica nanocomposite particles under certain conditions. For example, although silica exchange is observed at pH 10 when adding sterically-stabilized polystyrene (or poly(2-vinylpyridine)) latex to heteroflocculated poly(2-vinylpyridine)-silica particles, it does not occur at pH 5. Presumably, this is due to greater electrostatic attraction between the cationic P2VP cores and the anionic silica nanoparticles at this lower pH.

Introduction Colloidal nanocomposite particles are of considerable and growing interest to both academic and industrial scientists.1-3 Potential applications for these nanocomposite particles include photonic devices,4 synthetic mimics for cosmic dust,5 and smart Pickering emulsifiers.6 One recent successful commercial application is the use of film-forming nanocomposite particles in highperformance exterior architectural coatings.7 We have recently developed two distinct routes for the preparation of “core-shell” polymer-silica nanocomposite particles with relatively high silica incorporation efficiencies (see Figure 1). The first route involves the in situ aqueous emulsion polymerization of a vinyl monomer *Corresponding author. E-mail: [email protected]. (1) Balmer, J. A.; Schmid, A.; Armes, S. P. J. Mater. Chem. 2008, 18, 5722. (2) Wang, T.; Keddie, J. L. Adv. Colloid Interface Sci. 2009, 147-48, 319. (3) Zou, H.; Wu, S. S.; Shen, J. Chem. Rev. 2008, 108, 3893. (4) Mitzi, D. B. Chem. Mater. 2001, 13, 3283. (5) Burchell, M. J.; Willis, M. J.; Armes, S. P.; Khan, M. A.; Percy, M. J.; Perruchot, C. Planet. Space Sci. 2002, 50, 1025. (6) Fujii, S.; Read, E. S.; Binks, B. P.; Armes, S. P. Adv. Mater. 2005, 17, 1014. (7) Tiarks, F.; Leuninger, J.; Wagner, O.; Jahns, E.; Wiese, H. Surf. Coat. Int. 2007, 90, 221. (8) Dupin, D.; Schmid, A.; Balmer, J. A.; Armes, S. P. Langmuir 2007, 23, 11812. (9) Schmid, A.; Tonnar, J.; Armes, S. P. Adv. Mater. 2008, 20, 3331. (10) Schmid, A.; Armes, S. P.; Leite, C. A. P.; Galembeck, F. Langmuir 2009, 25, 2486. (11) Schmid, A.; Scherl, P.; Armes, S. P.; Leite, C. A. P.; Galembeck, F. Macromolecules 2009, 42, 3721.

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using a cationic azo-initiator in the presence of an ultrafine silica sol.8-11 Well-defined poly(styrene-co-n-butyl acrylate)/silica nanocomposite particles with typical silica contents of 24-39 wt % were obtained using this one-pot surfactant-free formulation.11 An alternative route involves the physical adsorption (or heteroflocculation) of ultrafine silica particles onto preformed latex particles in aqueous solution. Nanocomposite particles were readily obtained by adding silica sol to a poly(ethylene glycol) methacrylate-stabilized poly(2-vinylpyridine) [PEGMA-stabilized P2VP] latex.12 There are two possible mechanisms for such nanocomposite formation. First, from the viewpoint of polymer thermodynamics, the favorable enthalpic interaction between the chemically grafted PEGMA stabilizer chains and the silica surface13-16 provides a driving force for heteroflocculation. Consideration of colloid stability mechanisms provides an alternative explanation. Sterically stabilized latexes remain stable on close approach since the steric repulsive force offsets the ever-present van der Waals attractive force operating between (12) Balmer, J. A.; Armes, S. P.; Fowler, P. W.; Tarnai, T.; Gaspar, Z.; Murray, K. A.; Williams, N. S. J. Langmuir 2009, 25, 5339. (13) Cosgrove, T.; Griffiths, P. C.; Lloyd, P. M. Langmuir 1995, 11, 1457. (14) Cosgrove, T.; Mears, S. J.; Thompson, L.; Howell, I. Adsorption studies on mixed silica-polymer-surfactant systems. In Surfactant Adsorption and Surface Solubilization; 1995; Vol. 615, pp 196. (15) Cosgrove, T.; Mears, S. J.; Obey, T.; Thompson, L.; Wesley, R. D. Colloids Surf., A 1999, 149, 329. (16) Flood, C.; Cosgrove, T.; Howell, I.; Revell, P. Langmuir 2006, 22, 6923.

Published on Web 07/23/2010

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Figure 1. Schematic representation of the preparation of nanocomposite particles by (a) the in situ polymerization route [unmodified Bindzil 2040 silica is used where the monomer is 2-vinylpyridine and glycerol-modified Bindzil CC40 silica is used where the monomer is styrene] and (b) the heteroflocculation route. The first route produces particles with high colloidal stability, whereas the second route tends to lead to some degree of flocculation as judged by disk centrifuge photosedimentometry.

Figure 2. Schematic representation of the effect of adding sterically stabilized latex to a nanocomposite dispersion. In (a) the polymer/silica nanocomposite particles are prepared by in situ polymerization and silica redistribution does not occur, whereas in (b) the polymer-silica particles are prepared by heteroflocculation and rapid silica redistribution can take place under certain conditions.

particles. On the other hand, charge-stabilized sols are mutually repulsive due to the unfavorable overlap of their respective electrical double layers. However, no repulsive force is generated by the overlap of an electrical double layer with adsorbed polymeric stabilizer chains; thus, heteroflocculation might be anticipated on mixing these two types of otherwise colloidally stable dispersions. This heteroflocculation approach to nanocomposite particle preparation allows facile variation of the latex core,17 and optimized protocols can produce a nanocomposite dispersion with no excess silica sol.12 Herein, nanocomposites prepared by the “in situ polymerization” route will be denoted as polymer/silica particles, and those prepared by the “heteroflocculation” route will be designated as polymer-silica particles. Recently, we reported an unprecedented and fascinating observation: addition of excess sterically stabilized poly(2-vinylpyridine) [P2VP] latex to a colloidal dispersion of P2VP-silica nanocomposite particles prepared by heteroflocculation leads to the facile redistribution of the silica nanoparticles such that partial coverage of all the P2VP particles is achieved.18 It was also observed that silica redistribution does not occur upon mixing latex with the equivalent P2VP/silica nanocomposite particles synthesized by in situ 2VP polymerization (see Figure 2). It was suggested that, for the latter polymer/silica nanocomposites, the silica particles are more strongly adsorbed onto (or embedded into) the latex cores than for the polymer-silica nanocomposite particles prepared by heteroflocculation. In the current work, we present disk centrifuge photosedimentometry and X-ray photoelectron spectroscopy as additional characterization techniques that are complementary to the electron microscopy and small-angle X-ray scattering methods described previously. We also further explore the conditions under which silica redistribution does and does not occur for a wide range of binary mixtures of nanocomposite particles and latexes and briefly discuss the implications for potential industrial colloidal nanocomposite formulations.

through basic alumina columns to remove inhibitor, and then stored at -25 °C prior to use. 2,20 -Azobis(isobutyramidine) dihydrochloride (AIBA; Aldrich), 2,20 -azobis(isobutyronitrile) (AIBN; BDH), 4,40 -azobis(4-cyanovaleric acid) (ACVA; Aldrich), ammonium persulfate (APS; Aldrich), and Aliquat 336 (Nþ[(CH2)7CH3]3CH3Cl-; Aldrich) were used as received. The Bindzil 2040 and Bindzil CC40 aqueous silica sols were supplied by Eka Chemicals (Bohus, Sweden) as 40 wt % aqueous dispersions at pH 10 and pH 7, respectively. Monomethoxycapped poly(ethylene glycol) methacrylate (PEGMA, supplied as a 50 wt % aqueous solution by Cognis Performance Chemicals, Hythe, UK) had a mean degree of polymerization of 45 and an Mw/Mn of 1.10. Deionized water (obtained from an Elgastat Option 3A water purifier) was used in all experiments. Latex Syntheses. All P2VP latexes, the AIBA-initiated PEGMA-stabilized PS latex, and the APS-initiated charge-stabilized PS latex (see Table 1) were prepared by aqueous emulsion polymerization. In a typical P2VP latex synthesis, the PEGMA stabilizer (1.00 g of 50.0 wt % aqueous PEGMA solution) and the cationic Aliquat 336 surfactant (0.50 g) were dissolved in water (38.45 g) in a 100 mL single-necked round-bottomed flask. A comonomer mixture of 2VP (5.00 g) and DVB cross-linker (0.050 g) was then added. The flask was sealed with a rubber septum, and the aqueous solution was degassed at ambient temperature using five evacuation/nitrogen purge cycles. The degassed solution was stirred at 250 rpm using a magnetic stirrer and heated at 60 °C with the aid of an oil bath. After 20 min, the initiator solution (0.050 g of AIBA dissolved in 5.0 g of water) was added. The polymerizing solution turned milky white within 10 min, and stirring was continued for 24 h at 60 °C. The same protocol was followed for synthesis of the PEGMA-stabilized PS latex using an AIBA initiator (except that Aliquat 336 was omitted and styrene was used in place of the 2VP/DVB comonomer mixture) and also for the charge-stabilized PS latex using APS initiator (where both the Aliquat 336 and the PEGMA were omitted). Both the AIBN-initiated PEGMA-stabilized PS latex and the ACVA-initiated PEGMA-stabilized PS latex were prepared by dispersion polymerization in a 9:1 methanol/water mixture.19 PEGMA stabilizer (1.00 g) was dissolved in 44.0 g of a 9:1 methanol/water mixture in a 100 mL three-necked roundbottomed flask fitted with a condenser and nitrogen inlet. This solution was purged with nitrogen for 30 min before being heated to 70 °C under a nitrogen blanket. AIBN or ACVA initiator (0.050 g) was dissolved in styrene (5.00 g) and injected into the

Experimental Details Materials. 2-Vinylpyridine, divinylbenzene (80% 1,4-divinyl isomer), and styrene were purchased from Aldrich, passed in turn (17) Greenwood, P.; Lagnemo, H. U.S. Patent No. 10683350, 2004. (18) Balmer, J. A.; Mykhaylyk, O. O.; Fairclough, J. P. A.; Ryan, A. J.; Armes, S. P.; Murray, M. W.; Murray, K. A.; Williams, N. S. J. J. Am. Chem. Soc. 2010, 132, 2166.

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(19) Fujii, S.; Iddon, P. D.; Ryan, A. J.; Armes, S. P. Langmuir 2006, 22, 7512.

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Table 1. Summary of the Various Polymer Latexes and Polymer-Silica Nanocomposites Used in These Silica Redistribution Studies and Their Corresponding Particle Diameters As Judged by Dynamic Light Scattering

entry

sample description

monomer type

initiator type

initiator charge

PEGMA contenta (wt %)

adsorbed amount of PEGMAb (mg m-2)

particle diameterc (nm)

PDIc

1 2 3 4 5 6 7 8 9

P2VP latex 2VP AIBA cationic 3.0 1.3 216 0.04 P2VP latex 2VP AIBA cationic 1.5 1.4 453 0.04 P2VP latex 2VP AIBA cationic 1.8 2.3 616 0.07 charge-stabilized PS latex styrene APS anionic 667 0.05 PS latex styrene AIBA cationic 4.4 1.8 237 0.02 PS latex styrene AIBN neutral 2.8 1.3 260 0.05 PS latex styrene ACVA anionic 3.8 1.6 240 0.07 PS/silica nanocomposite styrene AIBA cationic 333 0.06 P2VP/silica nanocomposite 2VP AIBA cationic 200 0.03 a PEGMA contents were determined from 1H NMR spectra recorded in CD2Cl2 for either linear polystyrene latexes or linear P2VP latexes of approximately equivalent diameter to the DVB cross-linked P2VP latexes shown in the table. b Adsorbed amount of PEGMA calculated assuming that all of the stabilizer is located at the surface of the latex particles, a P2VP latex density of 1.17 g cm-3, and a PS latex density of 1.05 g cm-3. c Hydrodynamic diameter and polydispersity index, as judged by dynamic light scattering.

reaction vessel. The polymerizing solution turned milky-white within 10 min, and stirring was continued for 24 h at 70 °C. All latex dispersions were purified by several centrifugation/ redispersion cycles where the supernatant was carefully decanted before being replaced with fresh deionized water, and the sedimented particles were redispersed with the aid of mechanical rollers. This protocol was used to remove residual monomer, excess Aliquat 336 surfactant, and nongrafted PEGMA stabilizer (where appropriate). Purification was continued until the serum surface tension was close to that of pure water (71 ( 1 mN m-1), as measured using a surface tensiometer (Kr€ uss K10ST instrument).

Preparation of Polymer-Silica Nanocomposite Particles by Heteroflocculation. The appropriate volume of a 1.0% w/v aqueous latex dispersion was added to a known volume of a 1.0% w/v aqueous silica sol, such that the number of silica particles per latex was equivalent to monolayer silica coverage of the latex surface as reported earlier.12 For example, 5.00 mL of the 453 nm P2VP latex dispersion (1.0% w/v) was added to 1.22 mL of 1.0% w/v aqueous silica sol. The pH of each dispersion was adjusted using KOH or HCl prior to mixing. These mixtures were homogenized using a vortex mixer at 2000 rpm for 10 s before being allowed to equilibrate on a roller mixer for a minimum of 1 h at 20 °C.

Preparation of Polymer/Silica Nanocomposite Particles by in Situ Polymerization. A typical polystyrene/silica nanocomposite synthesis was conducted as follows. The appropriate amount of the aqueous silica sol (5.4 g of aqueous dispersion, which is equivalent to 2.0 g of dry silica) was diluted with water (37.6 g) and placed in a round-bottomed flask containing a magnetic stirrer bar, followed by the addition of styrene monomer (5.0 g) The flask was sealed with a rubber septum, and the aqueous solution was degassed at ambient temperature using five evacuation/nitrogen purge cycles. The degassed solution was stirred at 250 rpm using a magnetic stirrer and heated to 60 °C in an oil bath. AIBA initiator (50.0 mg; 1.0 wt % based on monomer) was dissolved in 4.0 g of degassed water and added after 20 min. Polymerization was allowed to proceed for 24 h at 60 °C. The resulting milky-white colloidal dispersion was purified by repeated centrifugation-redispersion cycles (5000 rpm for 30 min) with each successive supernatant being carefully decanted and replaced with fresh deionized water. Redispersion of the sedimented particles was achieved with the aid of a mechanical roller. This purification protocol was repeated for up to five cycles until TEM studies confirmed that all excess silica sol had been removed. A typical poly(2-vinylpyridine)/silica nanocomposite synthesis was conducted as follows. The aqueous silica sol (5.0 g of aqueous dispersion, which is equivalent to 2.0 g of dry weight of silica) was mixed with deionized water (40.0 g) in a 100 mL single-necked round-bottomed flask, and the initiator (50.0 mg of AIBA dissolved in a further 5.0 g of water) was added. The flask was 13664 DOI: 10.1021/la102127v

sealed with a rubber septum, and the aqueous solution was degassed at ambient temperature using five evacuation/nitrogen purge cycles. The degassed solution was stirred at 250 rpm using a magnetic stirrer and then heated to 60 °C with the aid of an oil bath. After 20 min at this temperature, the comonomer mixture (4.95 g of 2VP and 0.050 g of DVB) was added. The polymerizing solution turned milky-white within 15 min, and stirring was continued for 24 h at 60 °C. The resulting colloidal dispersion was purified by several centrifugation/redispersion cycles where the supernatant was carefully decanted before being replaced with fresh deionized water, and the sedimented particles were redispersed with the aid of mechanical rollers. As for the PS/silica nanocomposite synthesis, this purification protocol was repeated for up to five cycles until TEM studies confirmed that all excess silica sol had been removed. Silica Redistribution Studies. Prior to mixing, the solution pH of both the latex and nanocomposite dispersions was adjusted by the addition of either KOH or HCl to obtain a final solution pH of either 5 or 10. All redistribution experiments were conducted in dilute aqueous solution (typically 1.0% w/v), with latexes being added to preformed nanocomposite particles such that the nanocomposite/latex total surface area ratio was 1:1. For example, 1.44 g of a 1.0% w/v dispersion of the AIBA-initiated PEGMA-stabilized PS latex (entry 5 in Table 1) was added to 3.00 g of a 1.0% w/v dispersion of the PS/silica nanocomposite (see entry 8 in Table 1). All nanocomposite/latex mixtures were homogenized using a vortex mixer at 2000 rpm for 10 s before being allowed to equilibrate on a roller mixer for a minimum of 1 h at 20 °C. Dynamic Light Scattering (DLS). Studies were conducted at 25 °C using a Malvern Zetasizer Nano ZS instrument equipped with a 4 mW He-Ne solid-state laser operating at 633 nm. Backscattered light was detected at 173°, and the mean particle diameter was calculated over 30 runs of 10 s duration from the quadratic fitting of the correlation function using the StokesEinstein equation. All measurements were performed in triplicate on highly dilute aqueous dispersions. Disk Centrifuge Photosedimentometry (DCP). DCP analyses were conducted using a CPS disk centrifuge model 24000. Particle densities were determined by helium pycnometry (see below) prior to DCP analysis. A density gradient was constructed that ranged from 12% to 4% sucrose solutions in deionized water (or 8% to 2% sucrose solutions for samples containing PS latex) and allowed to stabilize for 20 min. A 377 nm poly(vinyl chloride) latex calibration standard was injected prior to the analysis of each sample. Run times were between 3 and 15 min with the centrifugation rate being typically 21 000-23 000 rpm. Helium Pycnometry. The solid-state densities of the latex particles and nanocomposites were determined using a helium pycnometer (Accu Pyc 1330 instrument, Micrometrics). Samples were freeze-dried under vacuum prior to measurement. Langmuir 2010, 26(16), 13662–13671

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Figure 3. Disk centrifuge particle size distributions obtained for latexes before (solid line) and after (dashed line) addition of silica sol for (a) AIBA-initiated PEGMA-stabilized P2VP latex (entry 2 in Table 1) [note the shift in apparent particle size and incipient flocculation that occurs under these conditions] and (b) anionic charge-stabilized PS latex (entry 4 in Table 1). In the latter case, the anionic silica sol does not adsorb onto the surface of the anionic PS latex, and thus the two traces are identical.

Transmission Electron Microscopy (TEM). Samples were prepared by drying a drop of a dilute dispersion onto a carboncoated copper grid. Analyses were conducted using a Philips CM100 electron microscope operating at 100 kV. 1 H NMR Spectroscopy. Both PS and P2VP latex particles were freeze-dried before being dissolved in CD2Cl2. 1H NMR spectra were recorded using a 250 MHz Bruker Avance DPX 250 spectrometer. PEGMA-stabilized P2VP latexes prepared using 1.0 wt % DVB cross-linker have slightly narrower particle size distributions than PEGMA-stabilized P2VP latexes prepared without cross-linker.20 Thus, all experiments detailed in this work were conducted using cross-linked PEGMA-stabilized P2VP latexes. Determination of the PEGMA contents of such latexes proved problematic, since the relatively high solution viscosity of the corresponding swollen microgels dispersed in CD2Cl2 led to substantial NMR line broadening and overlapping of peaks. However, dissolution of the equivalent linear (non-cross-linked) latexes in CD2Cl2 gave well-resolved 1H NMR spectra, which allowed their PEGMA stabilizer contents to be estimated.20 Fortunately, linear P2VP latexes have similar DLS diameters to those prepared using DVB cross-linker, so their PEGMA contents were expected to be comparable.12 Aqueous Electrophoresis. These measurements were conducted in the presence of 1 mM KCl using a Malvern Zetasizer Nano ZS instrument. The zeta potential was calculated from the electrophoretic mobility using the Smoluchowski relationship. The solution pH was adjusted by the addition of either HCl or KOH using a Malvern MPT-2 autotitrator. X-ray Photoelectron Spectroscopy (XPS). The surface compositions of selected particle dispersions were examined using a Kratos Axis Ultra DLD X-ray photoelectron spectrometer operating at a base pressure of ∼10-8 Torr. Samples were prepared by drop-casting dilute dispersions onto indium foil. A monochromatic Al X-ray source (10.0 mA, 15 kV) was used. The step size was 1.0 eV for the survey spectra (pass energy = 160 eV) and 0.1 eV for the core-line spectra (pass energy = 20 eV).

Results and Discussion Preparation of Nanocomposite Particles by Heteroflocculation. Our initial studies for the heteroflocculation route to polymer-silica nanocomposites focused on determining the ideal conditions for the preparation of “core-shell” P2VP-silica (20) Dupin, D.; Fujii, S.; Armes, S. P.; Reeve, P.; Baxter, S. M. Langmuir 2006, 22, 3381.

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particles.12 It was established that pH 10 was a suitable pH for the adsorption of silica sol onto PEGMA-stabilized P2VP latex. An experimental packing efficiency of P = 69 ( 4% was determined when using 20 nm silica particles in combination with both 463 and 616 nm latexes.12 Thus, it is possible to calculate the optimum amount of silica sol required to form “core-shell” nanocomposite particles with little or no excess silica. For simplicity in the present work, full monolayer coverage of the latex by the silica nanoparticles will be denoted by θ = 1.00. In our recent communication,18 small-angle X-ray scattering (SAXS) was utilized to characterize “core-shell” P2VP-silica nanocomposite particles prepared by heteroflocculation. This in situ technique allowed us to discount the possibility that the silica redistribution observed by electron microscopy were simply due to drying artifacts. However, such SAXS studies can only be conducted at a synchrotron facility; thus in the current work we evaluated disk centrifuge photosedimentometry (DCP) as a more readily accessible in-house analytical technique. Comparison of DCP weight-average particle size distributions of latex particles before and after the addition of silica sol can determine whether or not nanocomposite particles have been formed by heteroflocculation. For example, Figure 3a shows DCP data obtained for a 453 nm AIBA-initiated PEGMA-stabilized P2VP latex (entry 2 in Table 1) before and after the addition of the 20 nm silica sol. Upon addition of silica, the narrow particle size distribution of the P2VP latex broadens and the mean particle diameter increases, which is consistent with the formation of P2VP-silica nanocomposite particles. There appears to be some incipient flocculation of the nanocomposite particles, which may be at least partly due to the sucrose-rich DCP spin fluid. Unfortunately, the DCP software only allows a single density to be selected. If heteroflocculation occurs, the latex and nanocomposite populations necessarily have two different densities, so an accurate weight-average diameter can only be obtained for one population. In our experiments we elected to use the latex density for data analysis. Hence the observed size distribution for the nanocomposite population is subject to a significant systematic error. This approach leads to an oversizing of the nanocomposite particles, which fortuitously enables better discrimination between the two populations. It should be noted that, for consistency, all mean diameters for the latex and nanocomposite particles cited throughout this article refer to intensity-average diameters determined by dynamic light scattering (DLS). DOI: 10.1021/la102127v

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As a control experiment, heteroflocculation between the anionic 20 nm silica sol and an anionic charge-stabilized polystyrene latex (entry 4 in Table 1) was attempted at pH 10. Because of electrostatic repulsion, it was not expected that colloidal nanocomposite particles would be obtained in this case. Indeed, Figure 3b shows that there is little or no difference between the particle size distributions for the charge-stabilized polystyrene latex before and after addition of the silica sol. Latex and silica simply form a binary mixture of noninteracting particles; the much smaller silica nanoparticles cannot be analyzed using the same DCP conditions (spin rate, run times) required for the latex particles and hence remain undetected. Preparation of polymer-silica nanocomposite particles is not confined to the addition of silica to a PEGMA-stabilized P2VP latex; PEGMA-stabilized polystyrene (PS) can also be readily used as the core of these “core-shell” particles. The extension of the heteroflocculation route to include PS-silica particles also allows evaluation of the effect of the underlying latex surface charge on the adsorption of the silica particles. Thus cationic AIBA, neutral AIBN, and anionic ACVA initiators were used to synthesize PEGMA-PS latexes based on protocols previously reported by Fujii et al.19 The AIBA-initiated PEGMA-PS latex was prepared by aqueous emulsion polymerization using 10 wt % PEGMA and had an intensity-average diameter of 237 nm by DLS (see Table 1). The other two PEGMA-PS latexes were prepared by dispersion polymerization in a 9:1 methanol/water mixture using 20 wt % PEGMA. The DLS diameters were 260 and 240 nm for the AIBN- and ACVA-initiated latexes, respectively (see Table 1). PEGMA contents of the three purified latexes were determined by 1H NMR to be 4.4 wt % for the AIBAinitiated PS, 2.8 wt % for the AIBN-initiated PS, and 3.8 wt % for the ACVA-initiated PS. These values correspond to adsorbed amounts of 1.8, 1.3, and 1.6 mg m-2, respectively, for the chemically grafted PEGMA stabilizer chains (see Table 1). The appropriate amount of 20 nm silica sol in order to ensure full monolayer silica coverage (θ = 1.00; i.e., a silica packing efficiency of P = 69%12) was added to a 1.0 % w/v dispersion of each PS latex at approximately pH 10. Weight-average particle size distributions of the colloidal particles obtained before and after addition of silica (obtained by DCP) are shown in Figure 4. As discussed earlier, for mixtures in which there is a latex-silica interaction the mean particle diameter always increases after silica addition, indicating the formation of core-shell nanocomposite particles. For those mixtures where there is no attractive interaction between PS latex and silica, there is little or no difference between the DCP particle size distributions obtained before and after addition of the silica sol to the PS latex. Thus PS-silica nanocomposite particles are obtained on adding silica to PS latexes prepared using either the cationic AIBA initiator or the neutral AIBN initiator, but nanocomposite particles are not obtained when the PS latex was prepared using the anionic ACVA initiator. All three PS latexes contain similar adsorbed amounts of PEGMA stabilizer (see Table 1). However, their underlying surface charge differs significantly due to the differing nature of the initiator fragments. Redistribution of Silica Nanoparticles between Latexes. Previous investigation of the spontaneous redistribution of silica nanoparticles between latexes involved the addition of PEGMAstabilized P2VP latex particles to P2VP-silica nanocomposite particles prepared using the same P2VP latex core.18 Silica redistribution was also observed between P2VP latexes of differing diameters, which offered the advantage of using the latex diameter as a “label” in electron microscopy studies.18 In the present study, using two different P2VP latexes also proved useful 13666 DOI: 10.1021/la102127v

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Figure 4. Disk centrifuge particle size distributions obtained before (solid line) and after (dashed line) addition of a 20 nm silica sol at pH 10 for (a) AIBA-initiated PEGMA-stabilized PS latex (entry 5 in Table 1), (b) AIBN-initiated PEGMA-stabilized PS latex (entry 6 in Table 1), and (c) ACVA-initiated PEGMA-stabilized PS latex (entry 7 in Table 1). In the latter case, the anionic silica sol does not adsorb onto the surface of the anionic PS latex, and thus the two traces are almost identical.

for observing silica redistribution by DCP. Thus Figure 5a shows disk centrifuge particle size distributions obtained for a bare 216 nm P2VP latex and a P2VP-silica nanocomposite prepared by coating a 616 nm P2VP latex with silica. Figure 5b shows a binary mixture of the 216 nm P2VP latex and this nanocomposite, such that the final mean silica surface coverage θ = 0.50. The observed change in the position and shape of the smaller latex population (assigned to the 216 nm P2VP latex prior to mixing) confirms that silica redistribution has occurred. Thus both latexes now possess Langmuir 2010, 26(16), 13662–13671

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Figure 5. Disk centrifuge particle size distributions obtained for (a) a 216 nm PEGMA-stabilized P2VP latex (solid line) and a P2VP-silica nanocomposite (dashed line) prepared by coating a 616 nm PEGMA-stabilized P2VP latex with silica; (b) a binary mixture of these two latex and nanocomposite dispersions at pH 10; (c) a 237 nm PEGMA-stabilized PS latex (solid line) and P2VP-silica nanocomposite (dashed line) prepared by coating a 453 nm PEGMA-stabilized P2VP latex with silica; (d) a binary mixture of these two latex and nanocomposite dispersions at pH 10. In (b) and (d) the marked change in the appearance of the smaller latex population confirms that silica redistribution has occurred in both cases. This was subsequently confirmed by electron microscopy studies.

Figure 6. Transmission electron micrographs obtained for (a) PS-silica nanocomposite particles prepared by coating a 237 nm PEGMAstabilized PS latex with the 20 nm silica sol [such that the silica surface coverage θ = 1.00]; (b) a 453 nm PEGMA-stabilized P2VP latex; (c) a binary mixture of the PS-silica nanocomposite particles shown in (a) and the 453 nm P2VP latex [such that the final mean silica surface coverage θ = 0.50]. Clearly, the silica particles partially cover both latex cores, confirming that silica redistribution has occurred in this case.

partial silica shells and are somewhat flocculated. Again, given the software limitations of the DCP instrument, we chose to use the latex density for analysis. This inevitably means that the original nanocomposite population prior to silica redistribution is always oversized. Moreover, the size distribution for the final partiallycoated nanocomposite particles is also subject to a (smaller) systematic error. Nevertheless, this approach emphasizes any change in the original latex population caused by silica adsorption, since this inevitably leads to both an increase in mean Langmuir 2010, 26(16), 13662–13671

weight-average particle diameter and an increase in particle density, as well as incipient flocculation. In principle, if the original nanocomposite merely coexisted with the added latex, then no change should be observed in either population. This hypothesis is readily validated (see later). Thus our DCP analysis provides a highly sensitive, albeit qualitative, means of assessing whether silica redistribution has occurred. This approach can also be extended to include silica redistribution between P2VP and PS latex cores. P2VP latexes of varying DOI: 10.1021/la102127v

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Figure 7. Transmission electron micrographs obtained for (a) 333 nm PS/silica nanocomposite particles prepared by in situ polymerization of styrene in the presence of an ultrafine silica sol; (b) a 237 nm PEGMA-stabilized PS latex; (c) a binary mixture of the PS/silica nanocomposite particles shown in (a) and the 237 nm PEGMA-stabilized PS latex. Clearly, little or no silica redistribution has occurred in this case.

diameter can be readily prepared by adjusting the amount of PEGMA stabilizer, Aliquat 336 surfactant, and AIBA initiator used for 2VP polymerization;12,20 thus an appropriate latex can be selected to ensure good resolution between the P2VP and PS latexes in the DCP analysis. Figure 5c shows DCP data obtained for a 237 nm AIBA-initiated PEGMA-stabilized PS latex and a P2VP-silica nanocomposite prepared by coating a 453 nm P2VP latex with silica. Figure 5d shows a binary mixture of the 237 nm latex and this nanocomposite, such that the final mean silica surface coverage θ = 0.50. The observed change in the position and shape of the smaller latex population again confirms that silica redistribution has occurred. Transmission electron microscopy (TEM) images of this system before and after silica redistribution are shown in Figure S1. These studies confirm that the 237 nm PS latex becomes coated with silica on mixing with the nanocomposite particles. Silica exchange is also observed when PS-silica nanocomposite particles prepared using the 237 nm PS latex (Figure 6a) are mixed with the 453 nm P2VP latex (Figure 6b). Silica redistribution is clearly evident in the TEM image of the binary mixture (Figure 6c): the silica particles now partially coat both the PS and the P2VP latex cores. Characteristic time scales determined by time-resolved SAXS for (i) silica adsorption onto latexes and (ii) silica redistribution will be reported in detail in a future paper. Briefly, these unpublished observations confirm that silica redistribution occurs within a few seconds and strongly suggests that this phenomenon is due to interparticle collisions caused by Brownian motion. Since the PEGMA chains are relatively short (∼5 nm) compared to the silica nanoparticle diameter (20 nm), the latter are likely to be only weakly adsorbed on the latex surface. If the enthalpy of silica adsorption is sufficiently low relative to the mean thermal energy of the particles, entropy alone should drive silica redistribution. As discussed earlier, the in situ polymerization of monomer in the presence of an appropriate ultrafine silica sol provides an alternative well-documented route to the preparation of polymer/ silica nanocomposite particles.8-11 Unlike the polymer-silica particles prepared by heteroflocculation, these polymer/silica nanocomposites are not susceptible to silica redistribution upon addition of bare latex, since the polymer/silica interaction is significantly stronger in this case. Silica etching experiments8 previously conducted using 50% NaOH on the P2VP/silica nanocomposite resulted in P2VP particles with “golf-ball”-like morphologies; the observed indentations indicated that the original silica particles were partially embedded in the surface of the underlying latex cores. In the case of the PS/silica nanocomposites, similar etching experiments revealed no surface indentations.10 However, solid-state NMR studies suggest that there 13668 DOI: 10.1021/la102127v

Figure 8. Disk centrifuge particle size distributions obtained for (a) 237 nm PEGMA-stabilized PS latex (solid line) and PS/silica nanocomposite particles (dashed line) prepared by in situ polymerization of styrene in the presence of an ultrafine 20 nm silica sol; (b) a binary mixture of this latex and the nanocomposite dispersion shown in (a). Very little change in the latex peak at lower particle diameter can be observed, indicating that silica redistribution has not occurred in this case.

is intimate contact between the silica surface and the polystyrene chains, which in turn suggests a specific interaction between these two components.21 Figure 7a shows a TEM image of a 333 nm PS/silica nanocomposite prepared by in situ polymerization of (21) Lee, D.; Balmer, J. A.; Schmid, A.; Tonnar, J.; Armes, S. P.; Titman, J. J., Langmuir, accepted for publication.

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Table 2. XPS Characterization of P2VP-Silica Nanocomposite Particles (Prepared by Heteroflocculation) and P2VP/Silica (Prepared by in Situ Polymerization) before and after Mixing with AIBA-Initiated PEGMA-Stabilized PS Latexa sample description

C atom %

N atom %

O atom %

Si atom %

N/Si atomic ratio

Bindzil 2040 silica sol 1.1 61.0 37.9 PEGMA-P2VP latex (entry 2, Table 1) 84.3 8.9 6.5 PEGMA-PS latex (entry 5, Table 1) 90.1 9.9 PS-silica nanocomposite (by heteroflocculation) 31.2 43.9 24.9 P2VP/silica nanocomposite (by in situ polymerization) 46.8 4.3 30.0 18.6 0.23 ( 0.02 P2VP/silica mixed with PS latex 74.0 1.7 14.7 9.1 0.19 ( 0.02 P2VP-silica nanocomposite (by heteroflocculation) 31.1 3.3 40.2 25.3 0.13 ( 0.01 P2VP-silica mixed with PS latex 60.7 3.3 23.0 13.0 0.25 ( 0.03 a There is little or no change in the surface N/Si atomic ratio for the P2VP/silica nanocomposite particles (entry 9, Table 1) before and after addition of PS latex, indicating that silica redistribution does not occur in this case. In contrast, the surface N/Si atomic ratio for the P2VP-silica nanocomposite particles prepared by heteroflocculation increases significantly after addition of the PS latex, confirming that some silica particles were transferred from the nanocomposite particles to the PS latex.

styrene in the presence of an ultrafine silica sol (entry 8 in Table 1; the full characterization of such nanocomposite particles has been published elsewhere10) and Figure 7b shows a TEM image of a 237 nm AIBA-initiated sterically stabilized PS latex. Figure 7c shows a TEM image obtained for a binary mixture of the PS/silica nanocomposite particles and the PS latex: it is clear that little or no silica redistribution has occurred in this case. DCP studies also confirmed that no silica exchange takes place. Figure 8a shows the disk centrifuge data obtained for both the PS latex and PS/silica nanocomposite particles prepared by in situ polymerization; Figure 8b shows the DCP data obtained for a binary mixture of these two samples. Very little change in the smaller latex population is observed, which indicates that silica has not transferred from the PS/silica nanocomposite particles onto the PS latex. The same PS/silica nanocomposite particles can also be mixed with a PEGMA-stabilized P2VP latex, and once again little or no silica redistribution is observed (see Figure S2). It should be emphasized that the silica sol used for the preparation of these PS/silica nanocomposite particles (glycerolmodified silica; Bindzil CC4022) has a somewhat different surface character to that used for the preparation of the polymer-silica nanocomposites (unmodified silica; Bindzil 2040). Nevertheless, we are confident that the precise nature of the silica sol is not critical for the phenomenon of silica redistribution. For example, P2VP/silica nanocomposite particles (entry 9 in Table 1) can be prepared by in situ polymerization of 2VP in the presence of an unmodified Bindzil 2040 silica sol using a cationic azo-initiator.8 When mixed with either the 237 nm AIBA-initiated PEGMAstabilized PS latex or a 453 nm PEGMA-stabilized P2VP latex, no silica exchange was observed (see Figures S3 and S4). Thus it appears that there is a genuine and significant difference in the strength of the interaction between the polymer core and the latex shell for the two distinct nanocomposite preparation routes. Redistribution experiments where a PEGMA-stabilized PS latex is added to either P2VP-silica or P2VP/silica nanocomposite particles are particularly well suited to analysis by X-ray photoelectron spectroscopy (XPS). XPS has a typical sampling depth of 2-5 nm and has excellent interelement resolution. It is usually preferable to exploit the presence of unique elemental markers when using XPS to analyze the surface compositions of colloidal nanocomposites.23 In previous XPS studies of P2VP/ silica8 and P2VP-silica12 nanocomposites, the N 1s signal due to the pyridine rings has been used as an elemental marker for the P2VP component, while the Si 2p signal serves as a marker for the (22) Greenwood, P.; Lagnemo, H. Int. Patent WO2004/035474A1, 2004. (23) Percy, M. J.; Amalvy, J. I.; Barthet, C.; Armes, S. P.; Greaves, S. J.; Watts, J. F.; Wiese, H. J. Mater. Chem. 2002, 12, 697.

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Figure 9. Zeta potential vs pH curves for (a) a 453 nm PEGMAstabilized P2VP latex and (b) the pristine 20 nm silica sol [Bindzil 2040].

silica component. However, the PS latex contains neither nitrogen nor silicon (see Table 2). The surface N/Si elemental ratios presented in Table 2 were estimated to have an uncertainty of around (10%.23 Mixing the nanocomposite particles with the PS latex should lead to no change in the N/Si ratio provided that silica redistribution does not occur. This negative result is indeed observed (within experimental error) for the addition of PS latex to the P2VP/silica nanocomposite: before latex addition the N/Si atomic ratio is 0.23 ( 0.02, and after latex addition this ratio is 0.19 ( 0.02. However, if some silica particles are transferred from the nanocomposite particles to the PS latex, the N/Si atomic ratio should increase as more of the underlying P2VP latex surface is exposed. The N/Si atomic ratio for the P2VP-silica nanocomposite prepared by heteroflocculation increases significantly from 0.13 ( 0.01 to 0.25 ( 0.03 after addition of the PS latex, thus again confirming that silica redistribution has occurred. These XPS data are in good agreement with the related DCP studies shown in Figure 5c and 5d (for the addition of PS latex to the P2VP-silica nanocomposite) and with the TEM images shown in Figures S1 and S3 (for the addition of PS latex to P2VP-silica and P2VP/ silica, respectively). Inhibition of Silica Transfer between Polymer-Silica Nanocomposites and Latex. As discussed above, there is a fundamental difference in the strength of the interaction between the polymer core and the latex shell for the in situ polymerization and heteroflocculation nanocomposite preparation routes. Facile redistribution of adsorbed silica nanoparticles occurs on addition of sterically stabilized latex to core-shell polymer-silica nanocomposite particles. However, no silica exchange occurs if coreshell polymer/silica nanocomposite particles are prepared via DOI: 10.1021/la102127v

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Figure 10. Transmission electron micrographs obtained for (a) P2VP-silica nanocomposite particles prepared at pH 5 by coating a 453 nm PEGMA-stabilized P2VP latex with 20 nm silica [such that the silica surface coverage θ = 1.00]; (b) a 237 nm PEGMA-stabilized PS latex; (c) a binary mixture of the P2VP-silica nanocomposite particles shown in (a) and the 237 nm PEGMA-stabilized PS latex prepared at pH 5 [such that the final mean silica surface coverage θ = 0.50]. Clearly, no silica redistribution has occurred under these conditions, thus indicating that silica redistribution can be controlled by adjusting the pH of the P2VP-silica dispersion.

Figure 11. Transmission electron micrographs obtained for (a) P2VP-silica nanocomposite particles prepared by coating a 453 nm PEGMA-stabilized P2VP latex with 20 nm silica [such that the silica surface coverage θ = 1.00]; (b) a 240 nm ACVA-initiated PEGMAstabilized PS latex; (c) a binary mixture of the P2VP-silica nanocomposite particles shown in (a) and the 240 nm PEGMA-stabilized PS latex [such that the final mean silica surface coverage θ = 0.50]. Clearly, no silica redistribution has occurred in this case.

in situ polymerization rather than by heteroflocculation. These observations are expected to have important implications in the context of commercial nanocomposite formulations for the coatings industry, since control over the spatial distribution of the silica nanoparticles within the final nanocomposite film is vital for ensuring optimal transparency.11 Thus it may be useful to identify appropriate conditions under which loss of silica from polymer-silica nanocomposite particles prepared by heteroflocculation can be prevented. In principle, if the interaction between the latex cores and the adsorbed silica particles could be strengthened, this should inhibit silica redistribution. Solution pH affects the surface charge on both a PEGMAstabilized P2VP latex and the silica sol (see Figure 9). Protonation of the 2VP residues at low pH leads to cationic (microgel) character for cross-linked P2VP latexes, and linear P2VP latex prepared in the absence of DVB cross-linker simply dissolves below pH 4.12,20 Heteroflocculation of the P2VP latex with the silica sol has been conducted at pH 10, where both the sol and the latex possess negative surface charge.12 At pH 5, the silica particles retain their anionic surface character, but the P2VP latex becomes cationic. Nanocomposite particles prepared at pH 5 were less colloidally stable than those prepared at pH 10,12 which is perhaps due to the fact that the silica sol itself is less stable at pH 5.24 Nevertheless, the interaction between the latex cores and the anionic silica particles could be considerably stronger at pH 5 due to the underlying cationic character of the P2VP latex. (24) Iler, R. K. The Chemistry of Silica; Wiley-Interscience: New York, 1979.

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Figure 10 confirms that adding PEGMA-stabilized PS latex prepared using a cationic AIBA initiator (entry 5 in Table 1; aqueous dispersion adjusted to pH 5) to P2VP-silica nanocomposite particles (also at pH 5) results in a noninteracting binary mixture whereby all the silica particles remain adsorbed on the P2VP latex. Disk centrifuge data for this system are shown in Figure S5: the P2VP-silica nanocomposite is somewhat flocculated, but there is little or no change in the PS latex population. Similarly, no silica exchange was observed on adding PEGMAstabilized P2VP latex to a P2VP-silica nanocomposite dispersion prepared at pH 5 (data not shown). Thus these results suggest that silica redistribution can indeed be controlled by adjusting the pH of the P2VP-silica nanocomposite dispersion in order to ensure sufficient underlying cationic surface character to bind the anionic silica nanoparticles more tightly to the latex core. Lowering the solution pH appears to be a valid method for inhibiting silica redistribution between polymer-silica nanocomposite particles and sterically stabilized latexes. However, this approach can only be used in cases where adjusting the solution pH significantly affects the underlying latex surface charge. For example, silica redistribution is observed at pH 5 when adding a PEGMA-stabilized P2VP latex to PS-silica nanocomposite particles (see entry 5 in Table 1), presumably because the partially protonated P2VP latex has a greater cationic surface charge density at pH 5 than the AIBA-initiated PS latex cores. An alternative, more general, approach would be to ensure that any particles added to a polymer-silica nanocomposite dispersion have appropriate surface character that prevents silica Langmuir 2010, 26(16), 13662–13671

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Table 3. Summary of Whether Silica Exchange Occurs When Polymer-Silica Nanocomposite Particles Prepared by Heteroflocculation Are Challenged by the Addition of Latexa heteroflocculated nanocompositesb latex added

solution pH

P2VP-silica √ √ √

in situ nanocompositesb

PS-silica √ √ √ √

P2VP/silica

PS/silica

AIBA-initiated charge-stabilized P2VP 10   AIBA-initiated PEGMA-P2VP 10   AIBA-initiated PEGMA-PS 10   AIBA-initiated PEGMA-P2VP 5    c   AIBA-initiated PEGMA-PS 5  a Note that silica exchange is never observed for polymer/silica nanocomposite particles prepared by in situ polymerization (see entries 8 and 9 in Table 1) when these are challenged by latexes. b An AIBA initiator was used for all these nanocomposite syntheses. c TEM studies suggest that only partial silica redistribution occurs in this case.

adsorption. For example, as discussed earlier, nanocomposite particles are not obtained when silica sol is added to PEGMAstabilized PS latex prepared using an anionic azo initiator (ACVA). Therefore, it was expected that silica redistribution would not be observed when mixing polymer-silica nanocomposite particles with this PS latex. Figure 11 confirms this hypothesis: silica redistribution does not occur upon mixing a ACVA-initiated PEGMA-stabilized PS latex with P2VP-silica nanocomposite particles (see Figure S6 for the DCP data). Thus, it should be possible to avoid potential problems relating to silica redistribution within commercial nanocomposite coating formulations simply by ensuring that all other components have sufficient anionic surface character. All the results from the silica redistribution studies described in this work are summarized in Table 3. Clearly, silica exchange is never observed for polymer/silica nanocomposite particles prepared by in situ polymerization. Conversely, silica redistribution is always observed when polymer-silica nanocomposites prepared by heteroflocculation are challenged with an AIBA-initiated latex at pH 10. The results obtained at pH 5 are a little more complicated. At this pH, the P2VP particles are partially protonated but are still in their latex (rather than microgel) form. The isoelectric point for the PEGMA-stabilized PS latex is around pH 6.5 (data not shown); thus these particles have net cationic charge at pH 5. Silica redistribution is not observed when P2VP-silica nanocomposite particles are challenged with PS latex at pH 5. However, when PS-silica is challenged with the P2VP latex under the same conditions, silica redistribution is observed, presumably due to the difference in the underlying nature of the latex surface. The AIBA-initiated P2VP latex becomes more strongly cationic at pH 5 than the AIBA-initiated PS latex, as the 2VP residues become protonated (in addition to the influence of the underlying cationic AIBA initiator fragments). Thus there is a stronger interaction at pH 5 between the P2VP latex and the silica than there is between the PS latex and the silica. Redistribution experiments where polymer-silica nanocomposite particles were challenged with the same latex core used to prepare the nanocomposite also gave interesting results. Addition of the AIBA-initiated PEGMA-stabilized P2VP latex to the P2VP-silica nanocomposite did not result in silica redistribution at pH 5. In this case, the silica is electrostatically bound so tightly to the original latex that the challenging latex is unable to remove it. In contrast, when PS-silica nanocomposite particles are challenged with the AIBA-initiated PEGMA-stabilized PS latex at pH 5, our TEM studies suggest that only partial silica redistribution occurs and the binary particle mixture becomes flocculated (see Figure S7). DCP studies are consistent with this hypothesis (data not shown). Hence, it seems that this partial redistribution occurs because the silica is more tightly bound to the PS latex surface at

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pH 5 but not so tightly bound as to prevent silica redistribution completely (as in the case of the P2VP-silica nanocomposite).

Conclusions The preparation of “core-shell” polymer-silica nanocomposites by heteroflocculation has been extended to include the successful formation of colloidally stable polystyrene-silica particles. The effect of the type of initiator used to synthesize the precursor PEGMA-stabilized polystyrene latexes has been investigated: it was found that using cationic and neutral initiators led to nanocomposite formation whereas an anionic initiator only gave a noninteracting binary mixture of the original latex and the anionic silica sol. Presumably, electrostatic repulsion prevents heteroflocculation in the latter case. Disk centrifuge photosedimentometry has been demonstrated to be a very powerful and convenient technique for the in situ analysis of both the nanocomposite dispersions and the phenomenon of silica redistribution. Facile redistribution of adsorbed silica particles occurs on addition of sterically stabilized latex to “core-shell” polymersilica nanocomposite particles where the latex core is either polystyrene or poly(2-vinylpyridine). However, no silica exchange occurs if such polymer/silica nanocomposite particles are prepared via in situ polymerization rather than by heteroflocculation; presumably this reflects the stronger polymer/silica interaction achieved in such in situ syntheses. These results have been confirmed by electron microscopy and X-ray photoelectron spectroscopy in post mortem analyzes. The conditions under which silica redistribution can be prevented for heteroflocculated polymer-silica nanocomposite particles were also explored. It was found that silica exchange did not occur upon addition of bare PEGMA-stabilized polystyrene or poly(2-vinylpyridine) latex to poly(2-vinylpyridine)-silica particles at pH 5 due to the relatively strong electrostatic interaction between the cationic poly(2-vinylpyridine) cores and the anionic silica nanoparticles under these conditions. Finally, a more general approach is suggested: potential problems relating to silica redistribution in commercial nanocomposite formulations could be avoided by ensuring that all other components have anionic surface character. Acknowledgment. J.A.B. thanks AkzoNobel and EPSRC for an Industrial CASE studentship. Eka Chemicals (Bohus, Sweden) is thanked for donating the aqueous silica sols, and Dr. Tracie Whittle is thanked for conducting the XPS analysis. Supporting Information Available: Further transmission electron microscopy images and disk centrifuge data. This material is available free of charge via the Internet at http:// pubs.acs.org.

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