Macromolecules 2009, 42, 5303–5309
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DOI: 10.1021/ma900709x
Hybrid Core@Soft Shell Particles as Adhesive Elementary Building Blocks for Colloidal Crystals C. Deleuze,† M. H. Delville,‡ V. Pellerin,† C. Derail,† and L. Billon*,† †
Institut Pluridisciplinaire de Recherche sur l’Environnement et les Mat eriaux Equipe de Physique et Chimie des Polym eres IPREM/EPCP, UMR 5254, Universit e de Pau et Pays de l’Adour H elioparc, 2 avenue Angot, 64053 PAU Cedex 9, France, and ‡Institut de Chimie de la Mati ere Condens ee de Bordeaux ICMCB, UPR 9048, Universit e de Bordeaux I, 87 Avenue du Docteur Schweitzer, 33608 PESSAC Cedex, France Received April 2, 2009; Revised Manuscript Received May 30, 2009
ABSTRACT: Hybrid materials have been developed through the synthesis of colloidal silica surrounded by elastomer polymer brushes. These core@shell materials have been achieved by the use of “nitroxide-mediated polymerization” (NMP) associated with a “grafting from” method. The living and controlled characteristics of the polymerization allow TEM observations of well-monodisperse core@shell nanohybrids. We present herein the synthetic process to create homogeneous smoothness particles surface associated with a strong adhesive property. The brush thickness is tuned in order to compare with molecular dynamic equations through the tailoring of the grafting density and the molecular weight. Finally, these particles were used as elementary adhesive building blocks and self-assembled by a dip-coating process to form a monolayered material. Its diffraction under light is described on a large-scale material as well as the elastomeric polymer brushes layer influence.
Introduction The investigation of advanced nanomaterials has aroused a new field of scientific interests in which the control of inorganic hybridization by organic matter should involve at large scale the management of new structural and physical properties.1 Under this perspective, spherical hybrid nanomaterials combining a hard core surrounded by a soft corona have attracted much attention.2 The effective contrast of properties between the two materials has already exhibited large possibilities of application as their use as chromatic separator,3 stimuli-responsive vector,4 novel biointerface,5 or model for condensed matter crystallization,6 tribology, and adhesion system.7 Nevertheless, to efficiently benefit from the original properties of the simply called “core-shell” hybrid materials, the perfect tailoring of the soft shell size or chemical property is totally required. Thanks to its high intrinsic controllable porosity, biocompatibility, hydroxyl surface functionality, and specific area,8 the use of monodisperse silica particles is constantly increasing, and hightech industries such as biotechnology/pharmaceuticals,9 rubber,10 or photonics11 are in a tremendous demand for such materials. Silica synthesis was first discovered by Kolbe,12 but it is only since the St€ober synthesis first described in 196813 that a strong scientific interest was dedicated to these well-defined silica nanoparticles as primary materials for core@shell structures. Much effort has been devoted to develop a wide variety of soft coating procedures.14 One range of advanced materials, owing to their capacity to form dense soft films on surface, has been pointed out, i.e., the so-called “polymer brushes”. Such a macromolecular organization consists of an assembly of polymer chains tethered by one end to a solid surface where the polymer chains are crowded and, if the grafting is dense enough, are forced to stretch away from the surface to avoid strong overlapping. One of the most efficient approaches to perform such a conformation *Corresponding author. r 2009 American Chemical Society
assembly is the “grafting from” process, where the initiator is covalently grafted on the substrate and chains grow from the surface during the polymerization step. This method is the only one which allows a wide range of grafting density, with as high as 1 polymer chain nm-2.15 Actually, the grafting density falls down for both the “grafting to” method using chemisorption hindered by the surface reactant diffusion and the physisorption method where subsequent desorption of the polymer brushes can take place. Recently, to tailor the chains growth from the silica surface, the use of controlled/living radical polymerization (CRP) method was undoubtedly required.16 So far, nitroxide-mediated polymerization (NMP) initiator was synthesized according to a versatile approach recently described by Billon et al.17,18 To contrast with the hard silica core, an elastomeric polymer, poly(butyl acrylate) (PBA), was chosen due to its low glass transition temperature (Tg ≈ -48.5 °C) and its low rubbery plateau modulus (G 0N ≈ 7.8 104 Pa).19 According to the Dahlquist criterion and validated experimentally by Creton et al.,20 this low value of rubbery plateau modulus confers to PBA natural tack properties at room temperature, which could induce a tacky behavior of the core@soft shell particles. The later ones will then be used as adhesive elementary building blocks and assembled to tailor colloidal crystal materials. For all those reasons, we developed core@shell PBA-grafted silica particles which exhibit highly smooth monodispersed shape and well self-organized polymer-grafted corona. To the best of our knowledge, none of the previous efforts have been able to show the tuning of such a uniform polymer coating on silica using surface-initiated nitroxide-mediated polymerization (SINMP) and TEM (Figure 1). Two simple methods based on the control of the grafting density, on the one hand, and of the molecular weight (MW) on the other, are herein described to finely tune size-controlled core@shell hybrid materials. The hybrid core@shell particles elaboration is eventually followed by an illustration of the ability of these adhesive elementary building blocks to strongly interact Published on Web 06/19/2009
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self-assemble in a monolayer and create a highly ordered twodimensional colloidal crystal presenting fascinating optical properties. Experimental Section The production of silica beads is provided by the St€ ober bottom-up synthesis. Improved by the use of constant addition growth process of Nozawa et al.,21 this method supplies us with monodispersed spherical amorphous beads from a few nanometers to about 2 μm of diameter. The surface-initiated polymerization (SIP) was conducted in bulk using nitroxide-mediated polymerization (NMP) with a monomolecular initiator. Such process is also called surface-initiated nitroxide-mediated polymerization (SINMP). The efficiency of the SINMP to elaborate inorganic/organic hybrid materials has been demonstrated for a different particle shape and size, i.e., with micrometer-sized mica particles22 or carbon nanotubes.23 Such a versatile initiator is synthesized in two steps. First, an “in situ thermo-dependent trapping of carbon radicals” of the CdC monomer double bond of an alkoxysilane derivative and its consecutive grafting onto the silica surface has been developed in our previous works to elaborate initiator-grafted silica particles (SiP). The procedure is described in more details by Inoubli. In a second step, a solution of 2 wt % of SiP is mixed with a determined ratio of butyl acrylate (BuA) purchased from Aldrich and free initiator 2-methyl-2[N-tert-butyl-N-(diethoxyphosphoryl-2,2-dimethylpropyl)aminoxy]propionic acid (MAMA) provided by Arkema as BlockBuilder (Scheme 1). The use of free initiator in the media volume is
Deleuze et al. required to form through the Fischer effect enough counter radical to displace the dissociation equilibrium to the dormant species and control the propagation of the grafted polymer chains on silica particles.24 No excess of counter radical is added to keep the reaction under control; the low constant temperature and the Fischer effects are sufficient.25 The complete volume is degassed for 15 min under nitrogen in a glass necked flask in ice bath and then heated at 110 °C. The novelty of the approach proposed here is to control both the grafting initiator density and the size of the grafted macromolecule chains in order to tailor and tune the shell thickness of the core@soft shell particles. The noncovalently bounded polymer chains formed in the bulk are removed by several successive acetone washings and centrifugations. The number of washings is determined by a UV-2450 Shimadzu UV-vis spectrophometer analyses which evaluates the free polymer concentration contained in the centrifugation supernatant. From 10 to 20 washings are classically required to obtain an absorbance close to zero. According to previous results, the chemical properties and macromolecular dimensions of both the free polymer chains and polymer brushes are close (see Supporting Information).17,18,26 Such a behavior also permits an analysis of the free extracted polymers by size exclusion chromatography (SEC) and proton nuclear magnetic resonance (1H NMR) to respectively determine the molecular weights and the conversion of the grafted polymer chains in the grafted monolayer. SEC analyses were performed on a GPCV 2000 Waters Alliance system, equipped with a capillary refractive index meter and a differential viscosimeter detector. MWs are expressed as the absolute value obtained by viscosimetry. 1H NMR has been performed with a Bruker 400 MHz instrument in CDCl3. Analysis of the shape and the monodispersity of raw and produced core@shell particles was performed by transmission electron microscopy (TEM) with a JEOL JEM-2000 FX transmission electron microscope, using an accelerating voltage of 200 kV at room temperature. For the particle self-assembly into a large material, a glass slide of (75 25 1 mm) is previously treated by a piranha solution (70/30 vol % H2SO4/H2O2). Then, the substrate is dipped into a core@shell particles dispersion maintained under stirring to avoid sedimentation. After several minutes to reach equilibrium, the substrate was withdrawn at constant high speed of 2 mm s-1. The substrate was dried at ambient temperature. Highly ordered arrays of adhesive elementary building blocks are analyzed without metallization through an environmental scanning electronic microscope (ESEM) Electroscan E-3.
Results and Discussion
Figure 1. TEM images of the core@soft shell PBA monolayer-grafted micrometer silica particles synthesized by SINMP (scale bar: 50 nm).
As expected for a controlled living nitroxide radical polymerization without counter radical excess, the MW and the ratio ln[BuA]0/[BuA] vary in a linear way with the conversion and time t2/3, respectively (see Supporting Information). This controlled
Scheme 1. Surface-Initiated Nitroxide-Mediated Polymerization (SINMP) Process
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Figure 2. TEM images of core@shell of SiO2@polymer hybrids based on spherical silica particles of various sizes.
Figure 3. TEM images of the adhesion between soft elastomeric layers of the hybrid core@shell particles (scale bars = 500, 200, and 100 from left to right).
process of the chains growth can be used to predefine the polymer chains dimension and thus by extrapolation the monolayer thickness. Hybrid Core@Soft Shell Particles. We succeeded in synthesizing with control SINMP, core@shell hybrid particles, with hard silica cores from 100 nm to 2 μm of diameter surrounded by a 10-100 nm polymer brushes thicknesses (Figure 2). In the latter example (Figure 2c), a controlled process yielding MW of 350 000 g mol-1 (degree of polymerization DPn=2700 and polydispersity index PDI=1.4) was absolutely required to create 100 nm polymer brushes. Such a thickness could theoretically lead in high solvation conditions (i.e., with a good solvent of the PBA) to the formation a swollen monolayer up to 600 nm. Indeed, in the case of flexible chains as polyacrylate, the Kuhn segment length can be estimated to 0.25 nm, leading to a thickness of 600 nm if chains are totally extended or stretched (Lchain = DPn 0.25 nm; see also eq 1b). The vacuum conditions in the electronic microscope can also be associated with a “bad” solvent, where the chains are then collapsed and not stretched. Under these analytical conditions, the direct observation of the polymer chains stretch S is calculated around 10-20%, i.e., corresponding to the ratio of the experimental thickness value to a fully extended chain Lchain. This value is comparable to the previous ones obtained by ellipsometry on PBA brushes-grafted silicon wafer in air.27 Moreover, the low observable limit of the monolayer thickness measurement was estimated around 5 nm with a standard deviation of ( 2 nm by TEM. Nevertheless, this analytical technique and this synthetic procedure are attractive to characterize and synthesize homogeneous, dense, and thick polymer brush-grafted silica particles. The soft elastomeric monolayer of such core@shell hybrid particles is potentially able to tune and optimize the interparticle interactions thanks to adhesion properties of the soft elastomeric monolayer covalently grafted all around the silica beads. Thus, as presented in Figure 3, we can clearly observe the autoadhesion between layers. However, one can also expect and not exclude that chain interpenetration takes place
during solvent evaporation “helping” self-aggregation and organization of particles to create colloidal crystal. Moreover, Shull et al. have demonstrated the attractive interactions due to brush/brush interactions in the case of block copolymer micelles, when the polymer shell is compatible with the bulk.20 Such behavior could be at the origin of the meniscus formation associated with the adhesion forces between particles. Finally, this PBA monolayer acts as “cement” between colloidal spherical silica particles which reinforces the interparticle interactions by adhesion of polymer brushes. Here, we can recall that, thanks to the rheological properties, the PBA is commonly used to elaborate and formulate pressuresensitive adhesive (PSA).19 High-Density Polymer Brushes Conformation. The influence of the grafting density σ (chains nm-2) on the polymer brushes behavior is crucial. Indeed, the polymer brushes through σ can present dilute, semidilute, or dense behavior, which impacts their future conformation in particular their thickness and their interdigitation properties.28 In the case of nanoparticles, its determination is not trivial and is directly correlated to the specific surface value, i.e., the particle diameter. One of the first aims is to determine the number of initiator groups suitable to be grafted on the surface. In fact, grafting a thin molecular initiator layer around spherical particle involves its very low molar content in the media. To quantify the initiator amount on the SiP, thermogravimetric analysis (TGA) and elementary analysis (EA) are techniques usually used to give weight and molar contents, respectively. However, they are limited by their own detection threshold. We have statistically calculated an acceptable value of detection around 0.3% molar rate for carbon elementary analysis and 2% weight rate for TGA. These values have been estimated to obtain a satisfying reproducibility of the calculated grafting density for a same sample batch. Below these values, the quantification could not be significant with the detection threshold of our apparatus. For instance, the silica surface presents high rate of hydroxyl functionalities from 5 to 6 OH nm-2. Each trimethoxysilane reagent will statistically react with two of these hydroxyl
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Table 1. Influence of the Silica Diameter on the Estimated Grafted Initiator Amount calculated content of initiator silica beads diameter (nm)
molar %
weight %
1000 500 250 50
0.05 0.11 0.32 1.63
0.55 1.11 2.9 13.3
groups and even less due to the steric hindrance. These data were taken into account when we made calculation based on a simple sphere model to determine the presence of a monolayer on the silica surface with different diameters (Table 1). Beyond this average value, it is possible to quantify a number of grafted molecules; however, the grafting density value will be difficult to express in molecules nm-2. The reason is that the initiator tends to first functionalize the particle by forming a monolayer and for higher concentrations reacting with a remaining functional group forming a multilayer. Then, the grafting density per nm2 given by the previous techniques is absolutely not relevant anymore. In many ways, even if the particles are small enough to quantify the initiator grafting density (i.e., below 250 nm), the determination of the polymer chain grafting density is not obvious. Indeed, the initiator efficiency could vary with its reactivity or its steric congestion. The efficiency could be estimated to vary from 30% to 80% during polymerization.29 To overcome this issue, we propose to study the controlled living radical kinetics with regards to the thickness of the polymer layer observed by TEM. This technique can also give rise to the determination of the grafting density values and an estimated value of the stretch S, i.e., the rate of extension relative to a fully extended chain (eqs 1a and 1b).27,30 hFNa σ ¼ ð1aÞ Mn S ¼
h DPn l
ð1bÞ
where h is the film thickness (nm), F the polymer density (g cm-3), Mn the number-average molecular weight (g mol-1), Na the Avogadro number, DPn the polymerization degree, and l the Kuhn segment length of the chain (0.25 nm). The brush conformation of our polymer chain is deduced by the validation of the h g (DPn)1/2l equation, and the dry structure is revealed in vacuum by a constant 10-20% stretch value S of the maximal polymer length (see Supporting Information for examples). In order to verify eq 1a, we create a constant variation of the brushes thickness by only controlling two parameters: MW and σ. The first method consists in the surface saturation with the initiator concentration to keep constant the grafting density σ and tune the MW. The second method fixes the MW and adjusts the initiator concentration in an unsaturated regime to modulate the grafting density. In both cases, the main idea is to check the evolution of the polymer monolayer thickness of the core@shell hybrid particles by modification of one of the parameters and controlling all the other ones. Tailoring Thickness h through MW in the Saturated Regime. According to Waddell et al.,31 the occupied area of a common alkoxysilane molecule is about 0.5 nm2, which means that to build a perfect initiator monolayer, 2 108 molecules or about 30 μmol m-2 of silanes is required. Such a high value should exclude the presence of a dilute or semidilute collapsed brushes into mushroom or globule which
Figure 4. (1) TEM images of shell thickness h (scale bar: 50 nm) with (a) 27, (b) 45, (c) 51, (d) 54, (e) 75, and (f) 91 kg mol-1 MW and (2) linear variation of h with MW for 1 μm (square) and 500 nm (triangle) colloidal silica particles.
lead to the formation of pinned micelles as observed with PBA-grafted silicon wafer. The formation of pinned micelles could only be observed when lateral microphase separation could occur and for low MW or low grafting density. Also observed by Zhao et al. on PS brushes,32 this polymer brushes conformation is comforted by an observation of a highly defined and smooth air/polymer interface due to the elaboration of a really dense polymer brushes and the intrinsic elastomer behavior of the PBA (Figure 2). After the description of the polymer layer structure, Figure 4 represents the variation of thickness of the dried polymer brushes with the MW under TEM vacuum. As Husseman et al. have also observed on PS layer-grafted silicon wafer by indirect ellipsometric measurement,33 the calculated linear variation of the shell thickness versus the MW, h µ Mnσ, has been verified (eq 1a). The average grafting density was extracted from the slope of the linear variation, h µ Mnσ. For this saturated regime, the grafting density deducted by TEM is estimated to 0.22 chain nm-2. This value is directly correlated with the value estimated by elemental analysis or TGA (around 0.23 chain nm-2). Herein, this value can only involve a “high density brush” conformation of the grafted polymers with high MW. This result is confirmed by the shape of the polymer layer. Indeed, a very homogeneous and regular polymer layer is observed for the both sets of experiments in this saturated regime. Two different silica core radii have been used to study the thickness evolution. The particle radius does not seem to have an influence on the final result. Silica core@shell particles with several diameters from 100 nm to 2 μm were also produced. We showed that for the same concentration of initiator in solution and various silica radii, we obtained the same grafting density values, resulting in a more than saturated surface, a multilayer of the initiator over the whole
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range size. This NMP process then allows a low average PDI (less than 1.3) and a high constant grafting density independent of the sample set. This method allows us a fine-tuning of the well-controlled polymer brushes thickness and shows very reproducible results to elaborate adhesive elementary building blocks. Tailoring Thickness h through the Polymer Chains Grafting Density σ in an Unsaturated Regime. Under the saturated regime, the lower the initiator concentration, the lower the grafting density is. The influence of the grafting density σ on h was checked with a 64 kg mol-1 MW polymer (SEC values of the free polymers) (eq 1a). The initiator concentration in solution was deliberately decreased down to the disappearance of the polymer brushes in TEM. Since the MW is constant, we should be able to plot the thickness variation versus the grafting density as a linear function (Figure 5). Despite the reachable lower density, no semidilute regime or mushroom transition was revealed due to the lateral collapse of the macromolecular chains to form “pinned
Figure 5. (1) TEM images of length brush h with (a) 0.09, (b) 0.11 (scale bar: 50 nm), (c) 0.15, (d) 0.18, (e) 0.25 chain nm-2 (scale bar: 100 nm). (2) Linear variation of h with the grafting density σ for a 1 μm silica colloidal particle.
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micelles”. Indeed, this specific transition is obtained when the grafting density values are too low and induce the lateral microphase separation observable by a noncontinuous and nonhomogeneous layer of homopolymer brushes.27 As far as we observed under our specific observation conditions, the elastomeric brushes remained homogeneous and very smooth at the interface (Figure 5-1). On the contrary, the core@polystyrene shell hybrid presents a low surface roughness (see Supporting Information). Such a behavior could be due to the intrinsic glassy behavior or “frozen” state of the PS after total solvent evaporation which forbids the polymer chain relaxation at room temperature. In Figure 5-2, we plotted the thickness variation versus the grafting density σ. As expected, the experimental variation is well fitted by the equation h µ Mnσ. A linear slope is obtained even for a grafting density value as low as 0.09 chain nm-2. The value of the slope is in good agreement with the expected MW value of the shell-grafted colloidal particles (≈61.7 kg mol-1). The use of CRP on particles allowed establishing a very precise MW and so at last minimizing the error. We have just presented the elaboration of elementary adhesive building blocks based on core@shell hybrid particles. We have also demonstrated the ability of this synthetic methodology to tune not only the size of the core but also the thickness of the polymer shell with the MW or the grafting density σ. A set of very homogeneous spherical objects was synthesized in order to use them as elementary adhesive building blocks and to self-assemble them in a monolayer material by dip-coating. Core@Soft Shell Particles as Self-Assembled Building Blocks in a Colloidal Crystal Material. These polymergrafted microparticles were deposited by dip-coating on a flat substrate to elaborate highly ordered monolayers as colloidal crystal materials (Figure 6). The self-assembling was setup by following a Dimitrov and Nagayama process.34 Technical and specific experimental details will be developed in a further publication, but nevertheless, this technique is well-known to require strict experimental conditions in term of primary material. Indeed, the core@shell polydispersity and the presence of aggregates are prohibitive to support further self-assembling. The first issue can be avoided by changing particle interactions; the second denotes the necessity to elaborate controlled core@soft shell particles of high quality, as we herein described. The behavior of dispersed silica particles is controlled by the Derjaguin, Landau, Vervey, and Overbeek (DLVO) potential which discerns three types of interactions depending of the interparticular distance: (i) the attractive long-range van der Waals, (ii) the always shortrange steric repulsion, and (iii) the long-range Coulombian interactions.35 The latter one just occurring in an electrolyte system, our core@shell system was only submitted to the
Figure 6. ESEM images of core@shell particles self-assembled in a monolayer by dip-coating (scale bars: 5, 2, and 0.5 μm from left to right, respectively).
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Figure 8. TEM images of self-assembled core@shell particles (scale bars: 350 nm and 1 μm, from left to right).
Figure 7. Photograph of the optical property of a monolayer of core@ shell particles self-assembly by dip-coating on glass slide (scale bar: 0.5 cm).
two first ones. Growing polymer brushes from particles surface increases the steric repulsion and as a result offers a better stability or sedimentation rate in solution to the system. Thus, the final quality of our 10% weight dispersion was highly improved by the synthesis of large brush thickness or shell. Deposited on a flat substrate by capillarity effect, the core@shell hybrids self-assemble in a welldesigned patterned monolayer which follows the facecentered-cubic (fcc) structure (Figure 6). In the case of the raw micrometer-sized silica beads, the absence of selforganization and adhesion between particles was observed when using the dip-coating process. Such behavior is also associated with the high content of cracks characteristic of a low adhesion force between the particles (see Supporting Information). Also called “photonic crystal”, the colloidal crystal array presents an ordered variation in refractive index and induces the formation of a band structure with forbidden regions and allowed energies. In other words, Bravay’s array, whose dielectric periodicity is of the same order of magnitude as the light wavelength, diffracts the incident beam following Bragg’s law.36 According to the crystallography properties, the core@shell array is only able to present a pseudo or stop gap, i.e., diffraction in preferential space propagation. Besides the highly ordered hexagonal pattern of these materials, we also observed color diffraction due to the regularity of such a structured surface of the colloidal crystal. This micrometer-sized patterned array, in the same range of the visible wavelength, decomposes the white as well as the sunlight and creates iridescence on the flat surface. This interferential optical phenomenon can be observed by a simple tilt of the coating. Such a behavior has been recently observed on a highly hexagonal ordered film based on honeycomb structures37 (Figure 7 and film in Supporting Information). As we described before, a more precise study can be achieved using spectrophotogoniometry.38 In a forthcoming paper, we will check the interferential optical phenomena and the decomposition of the white light which interferes with a structured surface for different and precise observation angles. Moreover, because of the close refractive index between silica (1.5) and PBA (1.45), the presence of a polymer brushes layer does not seem to disturb the dielectric periodicity air/grafted silica. This optical effect means that the void space between three silicon grafted spheres is still there and permits a high difference in refractive indices. This
kind of connection, previously discussed (see Figure 3), can be observed in Figure 8. Moreover, concerning the mechanical behavior, the soformed material offers on a large scale (4 2 cm in Figure 6) both flexibility and toughness which are difficult to obtain otherwise from raw silica particles. These mechanical properties are due to the presence of the tacky grafted polymer layer which creates strongly adhesion when silica particles are in contact in the dry state. Such a behavior is also illustrated in Figure 8 (as well as in Figure 3), where an adhesive zone is observed between two silica spheres (white circle in Figure 8 and Figure 3). An interesting macroscopic effect is also observed with a such small thickness which induces a high difference of behavior when using PS or PBA. With PS, the colloidal crystal is brittle whereas the PBAbased colloidal crystal is tough and flexible. This property will be rapidly described in a forthcoming paper. Conclusion We have presented a large range of feasible high-density PBA polymer brushes-grafted silica particles. Often characterized by ellipsometry, it is the first time as far as we know that direct images of core@shell deposit on a TEM grid have led to a direct analysis of the thickness of the polymer brushes. Thanks to a constant high grafting density and a controlled SINMP polymerization, a highly monodispersed shape and a smooth surface of core@shell hybrid materials were tailored. We also showed the linear variation of the dry polymer brushes length with the MW and the grafting density in either a saturated or an unsaturated regime and eventually determined an intermediate polymer grafting density. The intrinsic behavior of the PBA, as a tacky polymer, is directly observed by the autoadhesion of layers from different particles. Such core@shell hybrids were then used as elementary adhesive building blocks. The control of the core@shell monodispersity and very smooth surface of such spherical colloidal particles allows us to elaborate and to observe on a large scale of a few square centimeters well self-assembled polymergrafted silica particles in a monolayer leading to a 2D colloidal crystal with optical properties. Acknowledgment. We acknowledge the 2PSM Group of Research and the CNRS (Centre National Recherche Scientifique) for the financial support of C.D. PhD. We thank Dr. O. Borisov for discussions on polymer brushes theory and behavior. Groupement de Recherches de Lacq and ARKEMA is also acknowledged for the providing of the BlockBuilder initiator. TEM measurements were performed at CREMEM, University of Bordeaux 1. Supporting Information Available: Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.
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