Polymer Nanocomposites: Dispersion of

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Langmuir 2005, 21, 6063-6072

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Gold Nanoparticle/Polymer Nanocomposites: Dispersion of Nanoparticles as a Function of Capping Agent Molecular Weight and Grafting Density Muriel K. Corbierre,† Neil S. Cameron,†,§ Mark Sutton,‡ Khalid Laaziri,‡ and R. Bruce Lennox*,† Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montre´ al, Que´ bec H3A 2K6, Canada, and Department of Physics, McGill University, Montre´ al, Que´ bec H3A 2T8, Canada Received November 15, 2004. In Final Form: February 25, 2005 The dispersion of polymer-covered gold nanoparticles in high molecular weight (MW) polymer matrixes is reported. Complete particle dispersion was achieved for PS125-Au in the polystyrene (PS) matrixes studied (up to and including Mn ) 80 000 g/mol). PS19-Au, on the other hand, exhibits complete dispersion in a low MW PS matrix (Mn ) 2000 g/mol) but only partial dispersion in higher MW matrixes (up to 80 000 g/mol). Similarly, PEO45-Au is fully dispersed in a low MW poly(ethylene oxide) (PEO) matrix (Mn ) 1000 g/mol) but only partially in a higher MW PEO matrix (Mn ) 15 000 g/mol). Wetting of the polymer-Au brushes by the polymer matrix is associated with dispersibility. Theory predicts that, for dense polymer brushes, wetting is achieved when the MW of the polymer brush equals (and is greater than) that of the polymer matrix. The observed partial dispersion of the PS19-Au and PEO45-Au nanoparticles in matrixes whose MW is greater than the brush MW is attributable to the existence of a high volume fraction of voids within the brush. These voids arise from the unique geometry of the nanoparticle surface arising from the juxtaposed facets of the gold nanoparticle. PS125-Au brushes are wetted by PS matrixes whose degree of polymerization is larger than 125, probably because of their lower grafting density on the gold core or the high fraction of void volumes caused by the facets on the gold cores. Dispersion thus occurs when the matrix MW is greater than that of the brush.

Introduction Functionalized metal nanoparticles have been the subject of intense activity for the past decade. The field was greatly expanded by Schiffrin and co-workers, who introduced new synthetic procedures providing fairly monodisperse thiol-stabilized gold nanoparticles (RSAu).1,2 Murray and co-workers have further increased the number and complexity of the RS-Au nanoparticles by using place-exchange reactions with a wide variety of thiols.3,4 Accessing the many interesting electronic, electrical, and spectroscopic properties of metal nanoparticles5-9 in * Author to whom correspondence should be addressed. E-mail: [email protected]. † Department of Chemistry, McGill University, the FQRNT Center for Self-Assembled Chemical Structures, and the FQRNT Re´seau des Mate´riaux de Pointe. ‡ Department of Physics, McGill University, and the FQRNT Re´seau des Mate´riaux de Pointe. § Current Address: IMI-NRC, 75 de Mortagne Blvd., Boucherville, QC J4B 6Y4, Canada. (1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (2) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655-1656. (3) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212-4213. (4) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782-3789. (5) Storhoff, J. J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 1959-1964. (6) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856-2859. (7) Demaille, C.; Brust, M.; Tsionsky, M.; Bard, A. J. Anal. Chem. 1997, 69, 2323-2328. (8) Ziolo, R. F.; Giannelis, E. P.; Weinstein, B. A.; O’Horo, M. P.; Gamguly, B. N.; Mehrotra, V.; Russell, M. W.; Huffman, D. R. Science 1992, 257, 219-223. (9) Beecroft, L. L.; Ober, C. K. Chem. Mater. 1997, 9, 1302-1317.

a stable nanocomposite polymer matrix is a timely endeavor. However, attractive as such nanocomposites may be, the process of blending or dispersing nanoparticles in a polymer matrix so that they remain isolated (i.e., dispersed) has proven to be problematic. Three general methods to prepare metal nanoparticle-polymer composites have been reported.9-21 The first involves the in situ synthesis of nanoparticles in the polymer matrix. This is usually achieved by reducing a metal salt already present in the matrix15-17 or by evaporating the metal at the heated surface of the matrix.18-20 The second approach consists of polymerizing the matrix around the nanoparticles.21 This approach, however, produces polydisperse polymer matrixes. In situ synthesis of nanoparticles also tends to yield polydisperse and uncontrolled nanoparticle sizes. Furthermore, both these techniques produce undesired species in the matrixes, either from the poly(10) Caseri, W. Macromol. Rapid Commun. 2000, 21, 705-722. (11) Fogg, D. E.; Radzilowski, L. H.; Blanski, R.; Schrock, R. R.; Thomas, E. L. Macromolecules 1997, 30, 417-426. (12) Fogg, D. E.; Radzilowski, L. H.; Dabbousi, B. O.; Schrock, R. R.; Thomas, E. L.; Bawendi, M. G. Macromolecules 1997, 30, 8433-8439. (13) Tsutsumi, K.; Funaki, Y.; Hirokawa, Y.; Hashimoto, T. Langmuir 1999, 15, 5200-5203. (14) Corbierre, M. K.; Cameron, N. S.; Sutton, M.; Mochrie, S. G. J.; Lurio, L. B.; Ru¨hm, A.; Lennox, R. B. J. Am. Chem. Soc. 2001, 123, 10411-10412. (15) Mayer, A. B. R. Mater. Sci. Eng., C 1998, 6, 155-166. (16) Selvan, S. T.; Spa¨tz, J. P.; Klok, H.-A.; Mo¨ller, M. Adv. Mater. 1998, 10, 132-134. (17) Watkins, J. J.; McCarthy, T. J. Chem. Mater. 1995, 7, 19911994. (18) Meldrum, A.; Haglund, R. F., Jr.; Boatner, L. A.; White, C. W. Adv. Mater. 2001, 13, 1431-1444. (19) Sayo, K.; Deki, S.; Hayashi, S. Eur. Phys. J. D 1999, 9, 429-432. (20) Deki, S.; Sayo, K.; Fujita, T.; Yamada, A.; Hayashi, S. J. Mater. Chem. 1999, 9, 943-947. (21) Lee, J.; Sundar, V. C.; Heine, J. R.; Bawendi, M. G.; Jensen, K. F. Adv. Mater. 2000, 12, 1102-1105.

10.1021/la047193e CCC: $30.25 © 2005 American Chemical Society Published on Web 05/27/2005

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merization or the reduction steps. The third method used to prepare metal nanoparticle-polymer composites was recently reported from our laboratories,14 as well as by Thomas11,12 and Hashimoto.13 This method involves the incorporation of pre-made particles into a pre-made polymer matrix, with the use of a common blending solvent. Thomas et al. reported the incorporation of CdSe and (CdSe)ZnS nanoparticles stabilized with small molecules containing a phosphine group into the phosphine-containing microdomains of a phase-separated block copolymer. The nanoparticles typically seemed to aggregate into the phosphine-containing microdomains. Hashimoto et al. used a similar technique to incorporate palladium nanoparticles stabilized with poly(2-vinyl pyridine) (P2VP) and poly(2-vinyl pyridine)-b-polyisoprene (P2VP-b-PI) into a microphase-separated P2VP-b-PI matrix. P2VP-stabilized nanoparticles were localized at the center of the P2VP lamellar microdomains, whereas P2VP-b-PI-stabilized nanoparticles were localized in the P2VP lamellae at the interface with the PI microdomain. In both cases, phase-separation forces as well as the preferential miscibility between the stabilizing molecule and one of the blocks of the matrix are dominant factors. Work from our laboratories involves the blending and dispersion of polystyrene (PS)-stabilized gold nanoparticles in a homo-PS matrix. In this case, the dominant factor determining the dispersion of the nanoparticles is the interaction between the matrix and the nanoparticle capping agent.14 Such a process involving the blending of pre-made nanoparticles into pre-made polymer matrixes allows for full synthetic control of both the nanoparticle and the matrix. Unwanted byproducts are also removed as a result of the additional purification steps in this procedure. A wide range of nanocomposites can be prepared this way. Both the nanoparticles and polymers used may be highly monodisperse, especially if the matrixpolymer is synthesized by a living polymerization technique (e.g., anionic polymerization) and if post-synthesis fractionation of the nanoparticles is undertaken.22,23 In a previous study, we attempted to incorporate tetradecanethiol-stabilized gold nanoparticles (C14-Au) into PS and poly(dimethylsiloxane) matrixes.14 Nanoparticle aggregates of up to several hundreds of nanometers in diameter resulted. Reasoning that nanoparticles decorated with a ligand whose chemical nature is the same as that of the matrix (favorable χ parameter) would be less prone to aggregation, we showed in the first instance that this is true for PS125-Au nanoparticles in PS matrixes of molecular weight (MW) up to 80 kg/mol.14 Chemical matching, however, is not the only factor governing the compatibility between a polymer brush and a polymer matrix. Brush wetting or dewetting by a polymer matrix (leading to nanophase separation of the composite and aggregation) is known to also be a function of the brush grafting density, the MW of the brush, and the MW of the matrix, as well as the presence of additional solvent.24-36 We demonstrate here how the combination of these factors, (22) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428-433. (23) Cowie, J. M. G. Polymers: Chemistry and Physics of Modern Materials, 2nd ed.; Blackie Academic and Professional: London, 1994; Vol. 1. (24) de Gennes, P. G. Macromolecules 1980, 13, 1069-1075. (25) Shull, K. R. J. Chem. Phys. 1991, 94, 5723-5738. (26) Shull, K. R. Macromolecules 1996, 29, 2659-2666. (27) Martin, J. I.; Wang, Z.-G. J. Phys. Chem. 1995, 99, 2833-2844. (28) Grest, G. S. J. Chem. Phys. 1996, 105, 5532-5540. (29) Aubouy, M.; Fredrickson, G. H.; Pincus, P.; Raphael, E. Macromolecules 1995, 28, 2979-2981. (30) Milner, S. T. Science 1991, 251, 905-914.

Corbierre et al. Table 1. Physical Characteristics of the Polymers Used in This Study polymer PS20 PS440 PS760 PS125-SH PS19-SH PEO25 PEO340 PEO45-SH

Mna (g/mol) 2000b 46 400b 80 000b 13 300b 2000b 1000c 15 000c 2100c

D.P.d

PIe

19 438 755 125 19 23 341 48

1.11b 1.05b 1.03b 1.7b 1.2b 1.41c 1.3c 1.05c

a Number-average MW. b From GPC. c From the supplier. Average degree of polymerization, calculated from Mn (GPC or manufacturer’s data). e Polydispersity index.

d

in addition to the geometry of the gold core, leads either to dispersion, partial aggregation, or complete aggregation of polymer-capped gold nanoparticles in a matrix of the same polymer species. The dispersion of PS-Au nanoparticles in PS matrixes and PEO-Au nanoparticles in PEO matrixes is studied. Characterization using a combination of transmission electron microscopy (TEM), UVvisible (UV-vis) spectroscopy, and small-angle X-ray scattering (SAXS) leads to a detailed understanding of the factors which determine the miscibility of polymercapped nanoparticles in polymer matrixes. Experimental Section Nanoparticle Preparation and Characterization. The thiol-terminated PS samples (PS125-SH, Mn ) 13 300 g/mol, and PS19-SH, Mn ) 2000 g/mol) and the PS matrixes (PS20, Mn ) 2000 g/mol; PS440, Mn ) 46 400 g/mol; and PS760, Mn ) 80 000 g/mol) were prepared by anionic polymerization in our laboratories. The subscript represents the number-averaged degree of polymerization (DPav). Characterization using gel permeation chromatography (GPC), 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, and matrix-assisted laser desorption/ ionization time-of-flight (MALDI-TOF) spectroscopy was undertaken. Details of the polymerization procedure can be found in a previous report.37 The PEO samples were purchased from J. T. Baker (PEO25, Mn ) 1000 g/mol) and Matheson, Coleman & Bell (PEO340, Mn ) 15 000 g/mol). The thiol-terminated poly(ethylene glycol) (PEO45-SH, Mn ) 2000 g/mol) was purchased from Polymer Source, Inc. (Dorval, Canada), and was used as received. Subsequent MALDI-TOF revealed the presence of a large fraction of polymer disulfides, and 1H NMR spectroscopy indicated the presence of some undisclosed impurities in these PEO45-SH samples. Tetradecanethiol (C14-SH) was purchased from Aldrich (Milwaukee, U.S.A.). The salient physical characteristics of the polymer samples are summarized in Table 1. The detailed preparation procedure of the gold nanoparticles was derived from the method of Yee et al.38 and can be found in a previous report.37 Briefly, HAuCl4‚3H2O and the appropriate thiol were dissolved in freshly distilled tetrahydrofuran (THF) and were subsequently reduced with lithium triethylborohydride (“Superhydride”, 0.1 M, in THF, Aldrich). The initial thiol/Au ratios were varied depending on the thiol MW. Gold nanoparticles stabilized with tetradecanethiol (C14-Au), poly(ethylene glycol)thiol (PEO45-Au), and two different PS-thiols (PS125-Au and (31) Milner, S. T.; Witten, T. A.; Cates, M. E. Macromolecules 1988, 21, 2610-2619. (32) Borukhov, I.; Leibler, L. Macromolecules 2002, 35, 5171-5182. (33) Currie, E. P. K.; Norde, W.; Cohen Stuart, M. A. Adv. Colloid Interface Sci. 2003, 100-102, 205-265. (34) Cerda`, J. J.; Sintes, T.; Toral, R. Macromolecules 2003, 36, 14071413. (35) Lindenblatt, G.; Scha¨rtl, W.; Pakula, T.; Schmidt, M. Macromolecules 2000, 33, 9340-9347. (36) Gohr, K.; Scha¨rtl, W.; Willner, L.; Pyckhout-Hintzen, W. Macromolecules 2002, 35, 9110-9116. (37) Corbierre, M. K.; Cameron, N. S.; Lennox, R. B. Langmuir 2004, 20, 2867-2873. (38) Yee, C. K.; Jordan, R.; Ulman, A.; White, H.; King, A.; Rafailovich, M.; Sokolov, J. Langmuir 1999, 15, 3486-3491.

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Table 2. Physical Characteristics of the Gold Nanoparticles Studied nanoparticle

dcorea (nm)

C14-Au PS125-Au PS19-Au PEO45-Au

3.9 (0.5) 6.2 (1.7) 4.4 (1.2) 3.6 (2.0)

% organic/ % Aub 14.4/85.6 56.5/43.5 50.0/50.0 29.3/70.7

dnanoparticlec (nm)

λmaxd (nm)

λmaxe (nm)

n.a. 40 14 30

509f 523f 517f 515g

∼565 525 ∼540 ∼575

a Apparent diameter of the circumsphere of the truncated octahedral gold cores, from TEM, with the standard deviations in brackets. Several hundred nanoparticles were measured for the determination of the average diameter and standard deviation. Only symmetrical nanoparticles were taken into account. b Weight percent, from TGA. c Hydrodynamic diameter of the core plus shell nanoparticle in H2O (PEO45-Au) or THF (PSn-Au), from DLS. d Maximum absorbance of the gold surface plasmon resonance from dilute solutions, measured by UV-vis spectroscopy. e Maximum absorbance of the gold surface plasmon resonance of aggregated multilayers of particles on quartz substrates, measured by UVvis spectroscopy. f Measured in chloroform (CHCl3). g Measured in water.

PS19-Au) were prepared. The resulting gold nanoparticles were characterized by TEM, UV-vis spectroscopy, thermogravimetric analysis (TGA), dynamic light scattering (DLS), and 1H NMR. The physical characteristics most relevant to this study are summarized in Table 2. Nanocomposite Preparation. The PS nanocomposites were prepared by first dissolving both the PS matrix and the nanoparticles (C14-Au or PS-Au) each in 1-2 mL of a common solvent (chloroform, toluene, or THF) and then mixing the two solutions. The solutions were stirred at room temperature for several hours using a magnetic stirrer. The solvent was slowly evaporated under ambient conditions until the viscosity increased substantially. A 1.5 mm × 0.1 mm NiChrome ribbon, formed into the shape of a loop (∼2 mm diameter), was dipped several times into the solution. Sufficient time (∼20 min) elapsed between dips to allow most of the solvent to evaporate. Several days at ambient temperature and then under vacuum (first at room temperature, then at 50 °C) elapsed before the samples were released from the mold and microtomed for TEM. The calculated initial nanocomposite composition was 0.1% of Au in the matrix (v/v). The PS samples used as a host matrix were designated as PS20 (Mn ) 2000 g/mol), PS440 (Mn ) 46 400 g/mol), and PS760 (Mn ) 80 000 g/mol). The PEO45-Au/PEO samples were prepared by dissolving both the PEO matrix and the PEO45-Au nanoparticles in solvent (H2O or ethanol) and then mixing the two solutions. The solution was stirred for several hours at room temperature. The solvent was slowly evaporated at room temperature and then under vacuum for several days. For PEO matrix solubility reasons, we were limited to low MW PEO samples. Low MW PEO samples (ca. < PEO300), however, are too soft to be effectively microtomed for TEM characterization. SAXS characterization was therefore used for these samples. To this end, two composite samples were prepared: PEO45-Au in PEO25 (0.1%, v/v, Au/matrix) and PEO45-Au in PEO340 (0.1%, v/v, Au/matrix). Nanocomposite Characterization. TEM. The PS-Au/PS and C14-Au/PS nanocomposites were microtomed with a diamond knife at room temperature. The PEO45-Au/PEO340 nanocomposites were microtomed with a diamond knife at -196 °C. The microtomed sections were deposited on Formvar-coated 400 mesh copper grids for TEM observation. TEM experiments were performed on a JEOL JEM-2000 FX at an acceleration voltage of 80 kV. A large number of composite sections was examined in TEM for each nanocomposite sample. Both photographic negatives and digital images using a Gatan Bioscan chargecoupled device (CCD) camera (model 792) were obtained. The CCD camera was interfaced with a PC running Digital Micrograph software (Gatan, Inc.). The negatives were scanned using an Epson 1200 Photo scanner with a negative adapter at a resolution of 300 dpi. SigmaScan Pro 4.0 (SPSS, Inc.) was used to measure calibrated interparticle distances in the composites and to evaluate the number of particles per unit volume.

Figure 1. Profile of electron density of the core-shell model representing the nanoparticle: Au core and polymer (PS or PEO) shell. F0, Fshell, and Fc represent the electron densities of the matrix, the shell, and the gold core. R and Rc are the total radius of the nanoparticle (core + shell) and the core radius, respectively. UV-Vis Spectroscopy. Films of PS-Au/PS, C14-Au/PS, and PEO45-Au/PEO (0.1% VAu/V matrix) were drop-cast from toluene (PS composites) or ethanol/H2O (PEO composites) solution inside UV-vis quartz cuvettes and allowed to dry slowly in a nearsaturated solvent atmosphere for several days. Spin-coated films of C14-Au/PS, PS-Au/PS, and PEO45-Au/PEO340 composites (0.1% VAu/Vmatrix) were also prepared on thin quartz slides. Spincoating speeds ranging from 3000 to 5000 rpm on a Laurell benchtop spin-coater were used. The solvents were chloroform for the C14-Au/PS composites, toluene for the PS-Au/PS composites, and water/ethanol for the PEO45-Au/PEO composites. Spin-coated films of multilayers of single C14-Au nanoparticles in chloroform and of PS125-Au and PS19-Au in toluene (in the absence of a matrix) were also prepared at 3000 rpm. PEO45-Au multilayers were obtained by first drop-casting a solution in H2O/THF on a quartz slide and then allowing the solvent to evaporate at room temperature. UV-vis spectra (300-800 nm) of the composite films and of the “aggregated” multilayers of single particles were obtained using a Varian Cary 50 spectrophotometer. SAXS. SAXS measurements of the solutions and composites were carried out on the COSAXS station at beam line 8-ID of the Advanced Photon Source (APS). The beam energy was 7.66 keV produced by a 72-pole undulator. A silicon mirror and germanium monochromator selected a relative energy bandwidth of ∆E/E ≈ 3.3 × 10-4 full width at half-maximum. The beam size (20 µm × 20 µm) was defined by a pair of precision crossed collimating slits 65 m from the undulator source. The setup includes one pair of guard slits 165 mm before the sample. The resulting flux on the sample was ∼3 × 109 photons per second. The sampleto-detector distance, R, was 802 mm for high Q measurements (from 0.03 to 0.3 Å-1) and R ) 2802 mm for low Q measurements (from 0.003 to 0.04 Å-1). The diffraction patterns were recorded on an SCX-TE phosphor CCD with an active area of 1152 pixels × 1242 pixels. The exposure times were varied from 5 to 100 s. The scattering cross section per unit volume, dΣ/dΩ, was obtained by dividing the number of counts detected by the number of incident photons, the solid angle subtended by the detector, and the thickness of the sample. To model the SAXS data, a core-shell model is used. The gold nanoparticles consist of a gold core with a polymer shell (PS or PEO) dissolved in solution or in bulk polymer. A plot of the density for this model is shown in Figure 1. To account for the polydispersity of our samples the Schulz distribution39 is used. The scattering cross section per unit volume is expressed40 as

dΣ Np 2 ) r0 〈|F(QR)|2〉S(Q) dΩ V

(1)

where Np is the number of particles in the sample, V is the volume (39) Schulz, G. V. Z. Phys. Chem. 1939, B43, 25-46. (40) Kotlarchyk, M.; Chen, S.-H. J. Chem. Phys. 1983, 79, 24612469.

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Table 3. Parameters Used in the Calculation of the Scattering with the Core-Shell Modela sample

PS125-Au

PS19-Au

PEO45-Au

dAu (nm) Z φ (%) FAu - Fshell (e-/nm3)

5.8 14.5 0.075 4093

4.4 14.5 0.1 4093

7.6 32.5 0.1 4041

a d Au ) 2RAu is the diameter of the Au core, and φ is the volume fraction of Au in the matrix (in percent). The electron densities of the polymers were calculated using the formula FmNeNa/M, where Fm is the mass density, Ne is the number of electrons in the monomer, Na is Avogadro’s number, and M is the molar mass of the monomer.

of the sample, r0 denotes the Thomson radius (cross section for forward scattering from one electron), S(Q) is the structure factor accounting for spatial correlation of the particles, and 〈|F(QR)|2〉 is

〈|F(QR)|2〉 )

∫ F (QR) f (R) dR ∞

2

z

0

(2)

The Schulz distribution is

fz(R) )

[ ]

1 z+1 Γ(z + 1) R0

(z+1)

[( )]

exp -

z+1 R R0

where R0 is the average radius of the particle and z is a parameter that measures the width of the size distribution [σ2 ) 〈R0〉2/(z + 1)]. F is the form factor for a spherical particle with a shell defined as

4 4 F(QR) ) πRAu3(FAu - Fshell)2j(QRAu) + πR3(Fshell 3 3 F0)2j(QR) (3) Here, RAu is the radius of the gold core, R is the total radius of the particle (core + shell), F0 is the electron density of the matrix, and j(x) is

sin(x) - x cos(x)

j(x) ) 3

x3

(4)

Assuming that RAu is a fixed fraction of R and that fz represents the size distribution of the overall nanoparticles (core + shell), eq 2 can be calculated analytically. The thicknesses of the samples were determined from measured transmissions and calculated absorption coefficients. For the nanoparticles used in this measurement, the electron density of the shell is similar to that of the matrix, so the shell term is only a small correction to the scattering. The scattering cross sections for the model were calculated using the values from Table 3 supplemented by the shell dimensions deduced from Table 2. The value for Z was determined from the ratio of average particle sizes to the width of the distribution determined by the TEM data. The modelderived curves are plotted in Figure 2 as the dotted lines and correspond to dispersed nanoparticles in the polymer matrix.41 Figure 2 shows the absolute cross section per unit volume for C14-Au, PS125-Au, PS19-Au, and PEO45-Au nanocomposites, each in several matrixes. A visual assessment of the absolute cross sections shows that the normalization of the detected intensities is valid to within 20%. Because intensities of each type of particle are consistent across the different matrixes as (41) In the experimental curves, a peak at high Q indicates complete aggregation of the nanoparticles (e.g., curves in Figure 2d). However, in the absence of such a peak, extensive experimental intensity deviation from the model curves on the entire Q range measured is also an indication of a fraction of nanoparticle aggregation within the composite sample (e.g., the black curve in Figure 2a). On the other hand, a slight increase in normalized intensity only at low Q compared to that of the model curve (e.g., the red curve in Figure 2a) may indicate a very small amount of aggregation or the presence of bubbles within the sample. It is, therefore, absolutely not possible to rule out the presence of a very small fraction of aggregated nanoparticles (as in Figure 2a, red curve).

Figure 2. SAXS data obtained from Au nanoparticles. In all cases the circles correspond to the calculated cross sections from the core-shell model described in the text. (a) PEO45-Au (offset by 105). The matrixes are PEO340 (Mn ) 15 000 g/mol; black solid) and PEO25 (1000 g/mol; red dashed). (b) PS125-Au (offset by 103), with matrixes PS760 (Mn ) 80 000 g/mol; black solid), PS440 (Mn ) 46 400 g/mol; red dashed), and PS20 (Mn ) 2000 g/mol; blue dotted). (c) PS19-Au (offset by 102) with matrixes PS20 (Mn ) 2000 g/mol; blue dotted), PS19-SH (red dotted) and toluene (green dotted-dashed). (d) C14-Au, with matrixes PS440 (pink dashed-dotted) and PS760 (black solid). well as between the different particles, the errors are mainly due to thickness and absorption measurements. For the PS125-Au nanocomposites, the calculated scattering uses parameters from the TEM data. However, to get good agreement in overall intensity, a volume fraction of 0.075% was used for all curves involving these particles (instead of the value of 0.1% derived from average TEM/TGA measurements). For the other samples, good agreement with experimental intensities results when the TEM/TGA-derived volume fraction values are used. In the case of PEO45-Au nanocomposites, a larger Au nanoparticles diameter (7.6 nm) is necessary to describe the SAXS data. This is consistent with the bimodal particle size (averaged about 2.5 and 7.5 nm diameters) observed in TEM images. X-ray scattering tends to emphasize larger particles, thus, biasing the average size. Temperature. The effect of elevated temperature on the stability of a PS125-Au/PS760 composite sample was studied. Different samples were heated to 100 and 145 °C for 1 h each, or at 145 °C for 18 h. Microtomed sections of the heated samples were then examined using TEM.

Results and Discussion The dispersion of ligand-capped nanoparticles in both solvent and solid matrixes is quite complex, given the interplay between (i) ligand conformation and dynamics when chemisorbed at one end, (ii) ligand chain packing on a faceted surface, and (iii) solvent (or melt) diffusion into the ligand layer. To delineate the importance of each of these factors, we first discuss features of the nanoparticles and then discuss the features of the resulting nanocomposites. Nanoparticles: The Polymer Brush at the Faceted Surface. High-resolution TEM images of the nanoparticle samples reported here suggest that the gold cores are

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Table 4. Gold Nanoparticle Physical Characteristics Derived from Experimental Data nanoparticle

compositiona,b x/yc

σb,d (chains/nm2)

footprintb (Å2/chain)

C14-Au PS125-Au PS19-Au PEO45-Au

180/1250 97/5000 177/1800 40/1000

4.35 0.94 3.45 1.15

23 106 29 87

a Average number of thiols and Au atoms per nanoparticle, calculated from TEM and TGA data, assuming regular truncated octahedral gold cores. b Only symmetrical nanoparticles were taken into account, because the assumption of regular truncated octahedral gold cores is needed for the determination of these results. c x ) number of thiol molecules per nanoparticle; y ) number of gold atoms per nanoparticle. d Chain grafting density, calculated from the average gold nanoparticle composition and the core surface area (TEM data), assuming regular truncated octahedral gold cores.

faceted.37 From the micrographs we infer that the gold cores are truncated octahedra as per Whetten and coworkers.22 The nanoparticle composition was calculated using TGA data (% gold/thiol), and the average diameter of the gold cores was determined from TEM images. The assumption that the gold cores are regular truncated octahedra was used in the calculation of particle composition and ligand footprint data. The gold core diameters estimated from TEM images were assumed to be the diameters of the circumsphere of the truncated octahedra. The thiol grafting densities on the gold nanoparticles were calculated using this presumed geometry and the TGAderived elemental analyses.37 Table 4 summarizes some calculated physical characteristics for the gold nanoparticles used here. The C14-Au nanoparticles (core diameter 3.9 ( 0.5 nm) are capped with tetradecanethiol (C14-SH) and have a thiol grafting density of 4.35 chains/nm2. The PEO45-Au (core diameter 3.6 ( 2.0 nm) sample is capped with thiolterminated PEO chains (DPav ) 45, Mn ) 2100 g/mol), and the grafting density is 1.15 chains/nm2. PS125-Au (core diameter 6.2 ( 1.7 nm) is capped with thiolterminated PS chains (DPav ) 125, Mn ) 13 300 g/mol), and the grafting density is 0.94 chain/nm2. Finally, PS19Au (core diameter 4.4 ( 1.2 nm) is capped with thiolterminated PS chains (DPav ) 19, Mn ) 2000 g/mol), and the grafting density is 3.45 chains/nm2. The composition of the nanoparticles is a function of both the core size and the bulkiness of the ligand, given that a polymeric ligand will generally have a much larger footprint than an n-alkanethiol. A detailed discussion of the composition and properties of these nanoparticles is reported elsewhere.37 The morphology of the gold nanocrystal core is particularly important when considering the structure of the thiol ligand capping layer. Because the polymer chains are adsorbed to the facets of the truncated octahedra, the capping layer should be a two-dimensional brush. However, the voids between adjacent facets of the gold core provide large free volumes that can be filled by the chains. The void volume fraction present at the core facets and edges is a function of the core size and brush thickness and is largest for thicker brushes and smaller core sizes.37 C14-Au/Matrix Nanocomposites. As described above, nanocomposites were prepared by dissolving the nanoparticles and the polymer matrix in a common solvent and then mixing the two solutions for several hours. The solvent was allowed to slowly evaporate at room temperature and ambient pressure. Residual solvent was subsequently removed under vacuum. Some samples were drop-cast as a viscous solution before allowing the solvent to evaporate, while other samples were prepared by

Figure 3. A: TEM micrograph of a 75 nm section of the C14Au/PS440 nanocomposite prepared by drop-casting. B: UV-vis spectra comparing the plasmon band of C14-Au in solution and of drop-cast and spin-coated films of the C14-Au/PS440 nanocomposite.

dipping a wire in the viscous solution several times and allowing the solvent to evaporate between each dip. Both methods lead to a similar degree of dispersion of the nanoparticles. The C14-Au nanoparticles systematically aggregate in PS matrixes. The presence of aggregates in the samples can be qualitatively monitored by the development of a blue color in the dried C14-Au/PS samples. Each solution of C14-Au/PS/solvent is initially an intense red, and the blue color appears upon solvent removal. The aggregation of C14-Au in PS (0.1% VAu/Vmatrix) is, however, more thoroughly assessed using TEM images of microtomed sections of the composites, by UV-vis measurements of drop-cast and/or spin-coated films on quartz cuvettes, and by SAXS measurements. TEM allows for the coincident observation of aggregates and the complete absence of individual nanoparticles (Figure 3). Examination of several samples reveals aggregates which are tens to hundreds of nanometers in size. The aggregates are reasonably homogeneous in size within a given sample. UV-vis spectroscopy is often used to detect gold nanoparticle aggregation. This method relies on the fact that the plasmon band, characteristic of small gold nanoparticles, is red-shifted and broadened when gold nanoparticles come into close contact. The plasmon band maxima of a nanoparticle dispersed in solution lies between 510 and 525 nm (Table 2). This range is typical of gold nanoparticles larger than 2 nm. The plasmon band position is broadly dependent upon nanoparticle type, particle size, and polydispersity, as well as the dielectric

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Figure 4. UV-vis spectroscopy of the plasmon band of gold nanoparticles in solution and the plasmon band of multilayers of gold nanoparticles spin-coated onto a quartz slide.

constants of the shell and of the solvent.42 Table 2 also shows the plasmon bands of multilayers of spin-coated gold nanoparticles in the absence of matrix on a quartz surface. This sample corresponds to deliberate aggregation. The resulting close proximity of C14-Au nanoparticles leads to a red-shift and broadening of the plasmon band peak arising from the coupling of the individual plasmon bands (Figure 4). UV-vis spectroscopy reveals that a redshifted, broad absorbance band caused by the aggregation of the nanoparticles occurs in the case of the drop-cast C14-Au/PS nanocomposite samples (Figure 3). The maximum of the plasmon band of C14-Au in a chloroform solution is 509 nm, for example, and is broadened and shifted to 560 nm for the C14-Au/PS composites. Spincoated films also exhibit a red-shifted plasmon band, but the rapid evaporation of the solvent prevents the aggregation of all the nanoparticles in a given sample. This results in a lesser broadening and a lesser red-shift of the plasmon band compared to the drop-cast films (Figure 3). The small peak at 510 nm in the UV-vis spectrum in each of the C14-Au/PS composite samples is attributable to the plasmon band of individual particles, suggesting that isolated particles exist in the films. A similar trapping of CdS nanoparticles within a polymer matrix occurs under conditions of rapid evaporation of the solvent.12 The bulk samples prepared for TEM studies correspond to these drop-cast films. However, individual nanoparticles were (42) Kreibig, U.; Vollmer, M. In Optical Properties of Metal Clusters, 2nd ed.; Kreibig, U., Ed.; Springer-Verlag: New York, 1995.

not observed in the TEM samples. The occurrence both of the individual particle plasmon band and of the broadened red-shifted band in the spectra might also originate from coexisting excitation modes.43-47 SAXS is a useful tool for studying colloid systems because the intensity of the scattered X-rays is determined by particle sizes and the structure factor characteristic of the system. An indication of any aggregation of the particles can be inferred using the structure factor. The (43) Theoretical calculations suggest that two different excitation modes (longitudinal and transverse) have to be considered in the case of a pair of small, touching gold nanoparticles. The transverse in-phase oscillation appears at roughly the same position as the plasmon band of single nanoparticles, whereas the longitudinal in-phase oscillation appears at a strongly red-shifted position. The difference between the energies of the longitudinal and the transverse modes decreases with the size of the particles, and the effect is strongest for nanoparticles in contact.42,44 Although it is predicted theoretically, the peak corresponding to the transverse in-phase oscillation usually does not appear in the experimental spectra of close, aggregated small gold nanoparticles.45-47 Because the particles in the present study are covered with stabilizing ligands, the gold cores are not in direct contact when aggregation occurs. Moreover, this theoretical treatment was applied to monodisperse, relatively large nanoparticles (diameter 10 nm) and very small aggregates. Our system, on the other hand, consists of small particles forming very large aggregates (several hundreds of nanoparticles). (44) Quinten, M.; Kreibig, U. Surf. Sci. 1986, 172, 557-577. (45) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425-5429. (46) Schmitt, J.; Ma¨chtle, P.; Eck, D.; Mo¨hwald, H.; Helm, C. A. Langmuir 1999, 15, 3256-3266. (47) McConnell, W. P.; Novak, J. P.; Brousseau, L. C., III; Fuierer, R. R.; Tenent, R. C.; Feldheim, D. L. J. Phys. Chem. B 2000, 104, 89258930.

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Figure 5. TEM micrograph of a section of a C14-Au/ hydrocarbon wax composite.

plots in Figure 2d illustrate the cross section per unit volume for C14-Au nanoparticles in PS samples. A qualitative assessment of the data shows that the C14Au nanoparticles in all the PS matrixes have a strong scattering peak at Q ) 0.12 Å-1, confirming the formation of large aggregates in this system observed by TEM. C14-Au nanoparticles were incorporated into a hydrocarbon wax matrix (mp ∼ 70 °C, 0.1% VAu/Vmatrix) by adding a toluene solution of C14-Au nanoparticles to melted wax (ca. 70 °C). The solution was stirred at 70 °C for 1 h and was then cooled to room temperature. Although individual nanoparticles were observed within the wax matrix in TEM micrographs (Figure 5), the samples were highly inhomogeneous. Some regions contain no nanoparticles, while others are concentrated in nanoparticles. As the wax solidifies, it is probable that the nanoparticles in these samples experience a trapping process similar to that described in our polymer matrix case. It is difficult, therefore, to draw conclusions about the extent (or lack) of the dispersion of the C14-Au nanoparticles in a hydrocarbon wax matrix. UV-vis spectroscopy of a thin film could not be performed on this type of sample due to the opacity of the wax. SAXS at room temperature may help determine the extent of aggregation in this type of sample. This system is of interest for further study because it is a small molecule analogue to the polymeric system studied here (i.e., PS-Au nanoparticles dispersed in PS matrixes). PEO45-Au/PEO Nanocomposites. Samples of PEO45-Au in PEO25 (0.1% VAu/Vmatrix) are too soft to microtome for TEM characterization. However, the pink color of the solvent-free composite suggests that there is little or no nanoparticle aggregation in the sample. The absence of aggregation41 of the nanoparticles is quantitatively confirmed by SAXS measurements (red dashed curve in Figure 2a). PEO45-Au in PEO340 was, however, amenable to microtoming. TEM of several cross sections reveals some individual nanoparticles within the matrix, although the small number of sections obtained by microtoming and the opacity of the samples do not allow for a definitive description of the entire sample (Figure 6). UV-vis spectroscopy of a very thin film was possible despite the opacity of the sample. The resulting plasmon band, with a maximum at 520 nm (compared to the solution maximum plasmon band of 515 nm), is consistent with extensive dispersion of the nanoparticles within the host matrix (Figure 6). SAXS experiments, however, establish that some aggregation in the PEO45-Au/PEO340 sample in fact occurs (black solid curve in Figure 2a). PS125-Au/PS Nanocomposites. Several PS-Au/PS nanocomposites were prepared to study the effect of the host matrix MW, the effect of the thiol-ligand MW, and the effect of thiol grafting density on the stability of the composites.

Figure 6. A: TEM micrograph of a microtomed section of a PEO45-Au/PEO340 nanocomposite (0.1% VAu/Vmatrix) showing the dispersion of the nanoparticles in the matrix. B: UV-vis spectrum (i) of the plasmon band of the PEO45-Au nanoparticles in a PEO45-Au/PEO340 nanocomposite thin film and (ii) of the plasmon band of PEO45-Au nanoparticles in water, added for comparison.

PS125-Au nanoparticles were incorporated into a series of PS matrixes (PS20, PS440, and PS760). Each composite was studied by TEM and UV-vis spectroscopy, and in some cases SAXS was used. TEM observation of microtomed sections showed a remarkably uniform dispersion of the nanoparticles within each matrix tested and a complete absence of aggregates. The average edge-to-edge distance between the colloids is 60 nm48 in the present case (Figure 7). The volume fraction of gold cores in the PS matrix, measured by TEM, is about 0.001. This corresponds to the initial solution volume fraction. UVvis spectra of thin films of PS125-Au particles in PS20, PS440, and PS760 (Figure 7) exhibit a plasmon band at 523 nm, as per that in chloroform solution (Table 2). These UV-vis results do not, however, confirm the absence of aggregation since aggregated PS125-Au particles in the absence of a matrix also display a plasmon band around 525 (Figure 4). (48) Because of the thickness of the microtomed samples (between 60 and 85 nm), it is difficult to precisely measure the distance between particles in a two-dimensional plane. The average distance can, therefore, only be estimated. A simple geometrical treatment leads to the conclusion that an “apparent” distance of 60 nm between two colloids in an x-y plane can actually be 96 nm if one colloid is located 75 nm “deeper” in the z axis of the plane.

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Figure 7. A: TEM micrographs of microtomed sections of PS125-Au in PS matrixes of differing MWs. B: UV-vis spectra comparing the plasmon band of PS125-Au in solution with the plasmon band of the nanoparticles dispersed in thin films of differing MWs.

Figure 8. TEM sections of PS125-Au/PS760 composites heated at (A) 100 °C (1 h), (B) 145 °C (1 h), and (C) 145 °C (18 h).

The extent to which UV-vis spectroscopy reports aggregates of gold nanoparticles in polymer matrixes is dependent upon the steric bulk of the nanoparticle ligand shell. Although nanoparticle aggregation has clearly occurred in the PS125-Au (no matrix) case (Figure 4), the PS125 ligand enforces a particle-particle distance which exceeds that necessary for coupling of the plasmon bands. Because the average interparticle distance measured in TEM images (15 nm for PS125-Au37) is larger than the largest core diameter in this sample, no coupling of the plasmon bands is expected.42 UV-vis spectroscopy is clearly an appropriate tool for detecting aggregation of nanoparticles only when the capping ligand is small enough to allow the distance between the particles to be on the order of or smaller than a core diameter distance.42 SAXS measurements of the composites do, however, confirm that the nanoparticles were indeed thoroughly dispersed in all three PS matrixes (Figure 2b).41 The PS125-Au/PS760 composite was heated to several temperatures over a variety of time frames to study the

effect of temperature on the nanoparticle dispersion. The resulting samples were characterized by TEM imaging (Figure 8). The composites are quite stable at elevated temperatures, even above the glass transition temperature (Tg) of the matrix (ca. 100 °C). When heated at 100 °C for 1 h, the composites showed no sign of aggregation even though segmental chain motion in the matrix will have been initiated. When heated at 145 °C for 1 h, the nanoparticles remain dispersed within the matrix. After 18 h at 145 °C, some nanoparticles have coalesced. In general, however, the nanoparticles remain dispersed within the matrix at moderately high temperatures, indicating that this type of nanocomposite is highly resistant to changes caused by aggregation. PS19-Au/PS Nanocomposites. The PS19-Au nanoparticles, whose polymer ligand is much shorter than in the PS125-Au case, were also incorporated in the PS matrixes (PS20, PS440, PS760). Both TEM and UV-vis spectra exhibit signs of partial aggregation in the case of PS440 and PS760 and complete dispersion in the case of

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Figure 9. A: TEM micrographs of microtomed sections of PS19-Au in PS matrixes of differing MWs. B: UV-vis spectra comparing the plasmon band of PS19-Au in solution with the plasmon band of the nanoparticles dispersed in thin films of differing MWs.

PS20 (Figure 9). The plasmon band of PS19-Au in a chloroform solution arises at 517 nm, as it does for the PS19-Au/PS20 composite. SAXS experiments also confirm the dispersion of PS19-Au in the PS20 matrix (Figure 2c).41 The green dotted-dashed curve in Figure 2c is of nanoparticles dispersed in a good solvent (toluene) and has similar scattering cross sections as obtained for the particles in the polymer matrixes. For data above Q ) 0.01 Å-1, the model describes the data well and confirms that the main scattering in this region is dominated by the Au core. However, the PS19-Au/PS760 composite had a red-shifted plasmon band (maximum ca. 535 nm), indicative of some aggregation. The refractive indices of PS (1.55-1.59) and of toluene (1.49) are very similar. Traces of residual solvent in the composite most probably would not cause the difference in the position of the plasmon band. Also, the presence of a higher MW PS (from the matrix) in the vicinity of the gold core is not the cause for the slight shift in the plasmon band, since the difference in the indices of refraction between the PS samples used in this study is negligible. TEM confirms that isolated, small aggregates coexist with a predominance of dispersed nanoparticles. PS19-Au/PS440 composites show the same type of incomplete aggregation as shown in Figure 9. Conditions for Aggregation: Brush De-Wetting. The presence of aggregates within the PS19-Au/PS440 and PS19-Au/PS760 composites is probably due to a partial de-wetting by the polymer matrix of the short, densely grafted PS19-SH at the surface of the gold core. The wetting of a polymer brush by a homopolymer matrix of the same chemical nature depends on several parameters

including the brush MW and grafting density, the MW of the homopolymer matrix, and the presence of an additional solvent. Generally, dense brushes are not wetted by a polymer matrix when the brush chains are of lower MW than the matrix chains.24-26,32,49 Theory-derived phase diagrams suggest that there is a range of smaller brush grafting fractions (N-1 < σ < N-1/2, where σ is the grafting fraction and N is the number of units of the brush) where wetting by a higher MW polymer matrix is possible.32 The grafting fraction σ can be calculated from the experimental grafting densities (in chains/nm2) and from the monomer size a in a given solvent.50 For a sufficiently low PS brush grafting density, wetting by a higher MW PS matrix has been experimentally observed.51 Maas et al. showed, using atomic force microscopy, that PS brushes attached to a two-dimensional substrate can be wetted by a PS matrix of higher MW for a range of low brush grafting densities.51 The geometry of the crystalline nanoparticles used throughout this study is of particular relevance to rationalizing dispersibility as a brush wetting problem. The nanoparticle gold cores described here can be viewed as a collection of planar facets joined at corners and edges. Volumetric voids exist at every corner of the truncated octahedral gold core.37 These voids (or a low-density packing state suggested by this space-filling model) at the brush-gold interface can be filled in by the polymer (49) Reiter, G.; Auroy, P.; Auvray, L. Macromolecules 1996, 29, 21502157. (50) However, because a wide range of “a” values for PS are reported in the literature, we decided to not report a σ value herein. (51) Maas, J. H.; Fleer, G. J.; Leermakers, F. A. M.; Cohen Stuart, M. A. Langmuir 2002, 18, 8871-8880.

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chains from the matrix. Partial or complete wetting of the polymer brush by the matrix thus results. This wetting prevents the complete aggregation of the particles. In the case of the PS125-Au nanoparticles, aggregation was not detected in the PS matrixes studied. The PS125Au nanoparticle sample combines a relatively low grafting density (0.94 chains/nm2) and a high volume fraction of voids.37 Both factors most likely contribute to the complete dispersion of the PS125-Au nanoparticles in 80 000 g/mol PS matrixes. The PS19-Au nanoparticles, on the other hand, partially aggregate in PS matrixes of MW > 2000 g/mol (DPav ) 20). Given that the grafting density of the PS19 chains on the gold core is very high, theoretical studies suggest that the brush cannot be wetted by a polymer of higher MW than that of the brush.24-26,32,49 It is therefore surprising that the nanoparticle aggregation is not complete in this case. Under the same experimental conditions, the C14Au nanoparticles completely aggregate in the PS matrixes studied. We therefore can rule out slow aggregation kinetics being the source of incomplete aggregation in the case of PS19-Au in PS. The partial dispersion of PS19-Au nanoparticles in high MW PS is likely due to the presence of voids at the edges of the brushes which allow for partial wetting of the brush. The smaller particles in the sample, which contain a higher fraction of void volume, would be wetted to a greater extent than the larger nanoparticles. The smaller particles thus remain dispersed, and the larger particles aggregate in the PS matrix upon evaporation of the common solvent. Such a nanoparticle sizerelated brush property may also be the origin of the partial aggregation observed for the PEO45-Au particles in PEO340.52 It is interesting to ask how polymer chain packing on a faceted surface relates to a continuously smooth surface. Judging from the results of the few studies involving spherical surfaces,35 it appears that the chain packing is in fact quite different than at a faceted surface. Similar comparisons between the faceted three-dimensional surface of a nanoparticle and the semi-infinite two-dimen(52) In principle, TEM analysis can be used to verify that the smaller nanoparticles are dispersed and the larger ones make up the aggregates. However, TEM of about 70 nm thick composite sections does not allow for enough resolution to distinguish between 2 and 5 nm particles that are not necessarily in the same plane, and the aggregates, which consist of multilayers of nanoparticles, are generally too dense to distinguish individual particles.

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sional surface51 also reveal that chain packing is different in both cases. The faceted three-dimensional surface thus appears to be a rather unique surface on which to form polymer brushes, and a brush-matrix phase diagram involving faceted nanoparticles is distinct from twodimensional and spherical three-dimensional cases reported to date. Conclusions The dispersion of polymer-coated Au nanoparticles in polymer matrixes of the same chemical composition was studied. High density (of grafting) polymer brushes on Au yield partial dispersion in high MW matrixes and total dispersion in matrixes of smaller MW than the brush (PS19-Au, PEO45-Au). Although theoretical studies on two-dimensional brushes predict that highly dense brushes will not be wetted by high MW matrixes, the presence of a large volume fraction of voids in the brush, due to the truncated octahedral geometry of the Au core, apparently increases the matrix-nanoparticle compatibility. A less densely grafted brush on Au leads to the dispersion of PS125-Au nanoparticles in both high and small MW matrixes. This is probably due to the combination of a lower grafting density and the void volumes among the brushes favoring dispersibility. A combination of techniques (TEM, UV-vis spectroscopy, and SAXS) allows for the comprehensive characterization of the gold nanoparticle/polymer composites. Taken individually, some of these techniques sometimes fail to completely describe the composites. In particular, one has to be vigilant about UV-vis spectroscopy results since it is shown that Au nanoparticles stabilized by a bulky shell (PS125-Au) show no change in the plasmon band when either dispersed or aggregated. Blending pre-made particles with a pre-made matrix introduces the possibility of complete control and characterization of the particle sizes and matrix (MW and purity) prior to blending and, hence, of the nanocomposite itself. Acknowledgment. The authors thank Ms. J. Mui for TEM microtoming and Dr. A. Lihl for the MALDI measurements. Funding for this research was provided by NSERC (R.B.L., M.S.), Merck Frosst Canada (R.B.L.), and FCAR (R.B.L., M.S.). The APS is supported by the DOE under Contract No. W-31-109-Eng-38. LA047193E