Surface-Initiated Polymerization as an Enabling Tool for

Review pubs.acs.org/cm

Surface-Initiated Polymerization as an Enabling Tool for Multifunctional (Nano-)Engineered Hybrid Materials Chin Ming Hui,† Joanna Pietrasik,†,§ Michael Schmitt,‡ Clare Mahoney,‡ Jihoon Choi,‡,∥ Michael R. Bockstaller,*,‡ and Krzysztof Matyjaszewski*,† †

Chemistry Department, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States

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ABSTRACT: Surface-initiated atom transfer radical polymerization (SI-ATRP) has become an indispensable tool for engineering the structure and properties of polymer/ inorganic and polymer/organic interfaces. This article describes the progress and challenges that are associated with the application of SI-ATRP to precisely control the molecular characteristics of polymer chains tethered to nanoparticle surfaces and explores the properties and potential applications of the resulting particle brush materials. Even for the conceptually most “simple” particle brush systemsthat is, spherical particles uniformly grafted with amorphous nonpolar polymersthe complex superposition of interactions as well as time- and length-scales related to particle core and tethered chains provides a rich and largely unexplored parameter space for the design of novel functional materials. The application of the particle brush approach to the development of materials for applications ranging from photonic inks and paints to advanced high “k” dielectrics for energy storage and advanced nanocomposite materials with improved optical, mechanical, or transport characteristics is discussed. KEYWORDS: nanocomposite, particle brush, ATRP, nanostructure, controlled radical polymerization

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INTRODUCTION The modification of surfaces by tethering of polymeric chains (either by primary or secondary chemical bonding) has become ubiquitous in the engineering of the physicochemical properties and/or biochemical functionalities of material interfaces.1 For example, polymer brushesa term first used by de Gennes to describe the molecular conformation of densely tethered polymer chainshave gained technological relevance as means to control the wetting, corrosion, or tribological characteristics of materials.1d,2 More recently, polymer brushes have also emerged as enabling technology in areas ranging from biomedical devices and energy generation and storage to separation and environmental remediation.3 In general, the properties of polymer brushes intimately depend on the conformation and, hence, on the distance between the tethered chains.1c,2f,4 Two regimes are distinguished depending on the respective conformation of the grafted chains: First, the sparse grafting regime with D > 2RG in which chains assume a relaxed conformation (also called “mushroom” structure) and, second, the dense grafting regime for D < 2RG in which excluded volume interactions give rise to stretched chain conformations (akin to a “molecular brush”). Here, D denotes the distance between tethered chains and RG the chains’ radius of gyration. The effect of brush architecture on the structure and properties of polymer brushes has been the subject of intense research, and the reader is referred to ref 1c for an in-depth review of the physics of polymer brush systems.1c It should be noted that the classification of polymer brushes into unique and distinct © XXXX American Chemical Society

conformational regimes based on grafting density is applicable only to macroscopic surfaces. This is because for nanostructured surfaces, i.e., surfaces with topographical features comparable to the polymer chain dimension, the curvature of the surface implies a systematic variation of the polymer segment density and thus of the chain conformation within the brush. The relation between surface curvature and chain conformation is illustrated in Scheme 1 that depicts the increase of chain crowding with increasing distance from the surface in case of concave surfaces corresponding to, for example, inner tubular geometries, while chain crowding decreases for convex surfaces, such as the surface of colloidal particles. The range of parameters that determines the conformation of densely tethered chains in case of nanostructured surfaces presents intriguing opportunities for the development of innovative hybrid materials that capitalize on the synergism between material constituents and surface geometry as well as the characteristics of the surface-tethered chains. Understanding of the complex relationship between the structure, composition, and physical properties of nanostructured polymer brush materials and methodologies for the viable synthesis of nanostructured polymer brush materials is needed Special Issue: Celebrating Twenty-Five Years of Chemistry of Materials Received: July 15, 2013

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dx.doi.org/10.1021/cm4023634 | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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distribution of tethered chains in case of inhomogeneous graft layers, and the morphology of grafted chains in the solid state as well as the potential coupling to external fields. A subset of the parameter space that is available for the design of particle brush materials is illustrated in Scheme 2. The synthesis of particle brush materials with precisely controlled structural attributes presents a formidable challenge. An important driving force for the surge of interest in particle brush materials has been the advances of graf ting-f rom polymerization techniques during the past 15 years that have provided a range of versatile and efficient methodologies to precisely control the structure and composition of tethered polymer chains across a wide range of grafting densities.1a Surface-initiated controlled radical polymerization (SI-CRP) has been particularly successfully applied to the synthesis of polymer brushes due to the combination of versatility and high level of control of molecular parameters such as composition, dispersity, and chain connectivity. Among the most relevant forms of SI-CRP are surface-initiated atom transfer radical polymerization (SI-ATRP), surface-initiated nitroxide-mediated polymerization (SI-NMP), and surface-initiated reversibleaddition fragmentation chain transfer polymerization (SIRAFT)the reader will find comprehensive review articles describing the application of these techniques to the synthesis of (planar) polymer brushes in the literature.1a,e,f,6 More recently, SI-CRP techniques have been applied to facilitate precise control of the architecture of particle brush materials. While the relevant parameters mimic those of planar surfaces, additional challenges arise, for example, due to the need to retain stability of particle dispersions during polymerization

Scheme 1. Illustration of the Effect of Surface Curvature on Conformation of Grafted Chainsa

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Increasing degree of polymerization results in relaxation (or stretching) of chains grafted on convex (or concave) surfaces.

to harness these opportunities. This article portraits recent developments in a particular subset of nanostructured polymer brush materials, i.e., polymer-grafted colloidal systemsin the following called “particle brushes”that have received special attention in the context of functional material design and that exemplify the opportunities for material engineering that are afforded by control of the polymer graft characteristics.5 These opportunities derive from the vast space of interacting parameters that can be leveraged to control and modulate the structure and properties of particle brush-based materials. Relevant parameters include the particle brush architecture, i.e., the density, molecular weight (MW) and molecular weight distribution (MWD), and connectivity and composition of grafted chains as well as the surface curvature, the spatial

Scheme 2. Examples of Controlled Macromolecular Architectures Prepared by ATRP and Illustration of the Parameter Space for Tailoring the Structure and Properties of Polymer-Grafted Particles Afforded by SI-ATRP

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reactions or due to the different reactivity of nanoscale surfaces as compared to their macroscopic counterpart.7 The objective of this Review is to provide a status report on synthetic strategies focusing on well-defined particle brush materials and to showcase examples of recent research that illustrate the potential for innovative material technologies based on particle brush materials. The structure of this article is as follows: in the first part, we discuss synthetic strategies for the control of structural parameters of particle brush materials. Here, our emphasis will be on SI-ATRP thatalbeit not being the best solution for every problemhas the potential to provide refined control of the full range of structural parameters of particle brush materials. Subsequently, we will survey recent research that demonstrates the potential of particle brush materials to spur new material science and technologies.

Originally ATRP was carried out at relatively high catalyst concentration to compensate for unavoidable radical termination because every act of termination irreversibly consumed an equivalent amount of a catalyst. Therefore, the catalyst concentration was in the range of 0.1−1 mol % vs monomer.11 However, during the last several years, it became possible to reduce the amount of catalysts to a few parts per million (ppm) in the presence of various reducing agents.12 These processes are shown at the bottom part of Scheme 3. In ARGET ATRP (activators regenerated by electron transfer) reducing agents such as ascorbic acid, sugars, tin(II) octanoate, sulfites, or amines can be employed.13 In ICAR ATRP (initiators for continuous activator regeneration), conventional radical initiators are used.13b SARA ATRP (supplemental activators and reducing agents) is based on zerovalent metals.14 Electrochemistry, that can be run under either potentiostatic or galvanostatic conditions, is used in eATRP.15 Some CuII complexes can be photoreduced by irradiation at the ligand− metal charge transfer region (usually at λ < 450 nm).16 In all these systems, the amount of catalyst can be significantly diminished to ppm level. However, decrease of catalyst concentration beyond a certain value, for lower activity catalysts, may be accompanied by increase of dispersity of formed polymers, according the second equation in Scheme 3. This opens the possibility of synthesis of polymers with designed dispersity that can have novel morphological features. For example, block copolymers with one segment of narrow and the other of broad MWD phase-separate into bicontinuous morphology of hexagonally perforated lamellae rather than typical cylinders at ca. 1/2 volume ratio.17 If the contribution of radical termination would be very small, in principle, it should be possible to reduce catalyst concentration even without reducing agents. Since kp/kt values are 30 times larger in polymerization of acrylates than those of methacrylates, there is much less termination, and lower amount of Cu catalyst could be used.18 It is also possible to increase kp/kt ratios at higher temperature and also at higher pressure.19 Termination events in systems with multifunctional initiators often result in gel formation, but macroscopic gelation can be avoided in polymerization in confined systems such as miniemulsion.20 One of the advantages of ATRP is a possibility to precisely control polymer architecture, not only in terms of molecular weight (MW) and molecular weight distribution (MWD) but also in terms of the shape or topology of polymer chains, exemplified by linear, cyclic, branched (also regular combs or stars), networks, and (hyper)branched polymers; chain composition, as in segmented copolymers (blocks and grafts), as well as gradient, periodic, and statistical copolymers; and chain functionality, represented by mono- or difunctional telechelics, macromonomers, linear polymers with various side groups, multifunctional (hyper)branched polymers or stars prepared from functional monomers or initiators but also capping agents, as well as via various postpolymerization techniques.7,21 Some examples of these structures were shown in Scheme 2 (see above). Fundamentals of SI-ATRP. SI-ATRP is the predominant SI-CRP method to prepare polymeric/inorganic hybrid materials.1a,e,f,6 SI-ATRP utilizes the same mechanism as a conventional ATRP from an untethered initiator, which involves initiation, propagation, activation/deactivation, and termination processes. However, there are several differences due to the existence of the functionalized inorganic surface.

I. ENGINEERING THE MOLECULAR STRUCTURE AND FUNCTION OF (NANO)MATERIAL INTERFACES BY SI-ATRP Atom transfer radical polymerization (ATRP) is among the most efficient and robust controlled radical polymerization techniques.8 It provides access to previously inaccessible polymeric materials and various hybrids.6,7,9 As shown in Scheme 3, in ATRP alkyl halide initiators/dormant species Scheme 3. General Scheme for ATRP and Low-ppm CuATRP

(Pn−X) react with activators, typically complexes of CuI halides with various N-containing ligands, to reversibly form propagating radicals (Pn•) and deactivators, complexes of CuII halides.10 The dormant species in this ATRP equilibrium can be polymer chains able to grow in one or many directions or polymers attached to functional colloidal particles, surfaces, biomolecules, etc. The upper right part of Scheme 3 shows a typical dependence of polymerization rate (Rp) and dispersity (Mw/Mn) on kinetic parameters and reagent concentrations. Rp depends on the propagation rate constant kp, the ratio of the rate constants of activation (ka) and deactivation (kda), and the concentrations of involved reagents. Dispersity decreases for faster deactivation (catalysts with a higher value of kda and at higher deactivator concentrations). The dispersity decreases with the monomer conversion (conv) and is lower for higher molecular weight (smaller [Pn−X]0).10a C



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Scheme 4. Examples of Anchoring Groups for Surface Modification (a) and a Strategy To Control Grafting Density via Inactive (“dummy”) Initiators (b)

possibility of forming a network of silsesquioxanes (e.g., Cl3− Si−, (MeO)3−Si−, (EtO)3−Si−) could permit generation of multiple attachments to the surface, providing better stability, compared to single bond attachment. Modification of metal (oxide) surfaces should follow Pearson’s hard/soft−acid/base (HSAB) principle.27 Thus, hard−hard (ionic character) and soft−soft (covalent character) acid−ligand interactions are more stable than the opposite approaches. For example, phosphoric acid, carboxylic acid, catechol, or acyl halides are suitable functional groups to modify an Fe surface and form a stable Fe−O bond. The properties of the resulting hybrid materials can be finely tuned by controlling the grafting density.1e,28 Generally, initiator density is controlled during the synthetic procedures employed for the functionalization of the surface, thereby controlling the final grafting density. Scheme 4b illustrates a typical procedure employed for tuning grafting density by conducting the surface modification with the mixture of active and inactive (“dummy”) tetherable ATRP initiators to control the concentration of initiating sites on the surface.28c,29 Alternatively, ATRP initiators can be partly destroyed by high energy irradiation.30 Similar to conventional ATRP initiators used in bulk/ solution polymerization, the appropriate ATRP initiating functionality on a tetherable initiator should be selected to ensure a rate of activation at least as fast as the rate of propagation. This is a prerequisite to provide high initiation efficiency and narrow MWD. Generally, the activation rate constant increases with initiator substitution from primary to tertiary and increases with better radical stabilizing groups; CN > Ph > C(O)OR.31 Propagation. As noted above, some untethered (sacrificial) initiators are often added, when grafting from flat surface systems. The MW and MWD from these untethered polymer chains can be used to provide approximate values for the MW and MWD of tethered polymer chains and, hence, graft density for a specific polymer layer thickness.1e However, both higher/ lower MW and different26,32 or similar33 MW and MWD of the tethered polymers relative to untethered polymers were reported. Crowding and confinement effects and other issues associated with catalyst and monomer diffusion during the growth of polymer chains from surfaces were modeled.34 Scheme 5 illustrates the decrease of accessible volume

First, initiators are immobilized onto the surface by coupling techniques prior to polymerization.22 Second, since the chains grow from the surface the rate of propagation could be limited due to the need for diffusion of monomer to the chain ends, affecting polymerization kinetics.23 Third, the addition of sacrificial initiator24 or deactivator23 is required in SI-ATRP from flat surfaces, because of the slow spontaneous generation of deactivators from the very low concentration of surfacebound initiator. If the deactivator concentration is low, then no reversible deactivation occurs and irreversible radical termination should dominate. On a flat surface, this may result only in on-surface termination,25 but as predicted by Flory’s gelation theory, the occurrence of only ca. 0.1% of termination in an interparticle fashion could produce macroscopic gels in SIATRP from nanoparticles (NPs) with thousands tethered chains.20 The selection of an inorganic surface for SI-ATRP depends on the targeted application. SI-ATRP has been applied to various substrates, including silica, germanium, alumina, titanium oxide, iron oxides, gold, and quantum dots, as well as a range of organic and biological materials in order to fulfill different needs.1a Inorganic substrates with different surface geometries have been selected to fit different applications. However, different surface geometries may affect SI-ATRP in different ways. As noted above, the concentration of tethered initiator on a flat substrate is usually very low and it does not provide enough polymer for MW and MWD measurement by size exclusion chromatography (SEC). On the other hand, although convex systems, e.g., functionalized nanoparticles, have much higher initiator/grafting density and can provide enough polymer for SEC, the higher radical concentration can cause macroscopic gelation.20 In concave systems, the growing chains will experience confinement effects which could reduce the level of control over MW and MWD if the reaction conditions are not appropriately selected.26 Initiation. SI-ATRP has been applied to many different inorganic substrates,1a and some examples are illustrated in Scheme 4. Among these substrates, silica and metals (oxides) are the most commonly reported surfaces for SI-ATRP, due to their market availability and novel physical properties. The silanol (Si−OH) functionalities on the silica surface can easily react with related reactive groups (e.g., Cl−Si−, MeO−Si−, EtO−Si−) which form stable siloxane bonds. In addition, the D



Review

Scheme 5. Illustration of Confinement Effecta in the SIATRP System

a

Scheme 6. Schematic Illustration of the Population of Dormant (blue dots) and Radical Species (red dots) in a Typical SI-ATRP Systema

Accessible volume for grafted chains is highlighted in pink.

a

(highlighted in pink) when the curvature of a substrate changes, for the same brush thickness (red arrow) and graft density. Simulations suggested that MW and MWD could be affected when the crowding or grafting density increased.23,35 The experimental results show that in some cases concurrent growth from the surface and in solution provides very similar polymers, but in other cases MW of polymers formed by SI-ATRP are smaller than in solution. This could be due to particular choice of initiators, catalysts, and reaction conditions, such as solvent, targeted MW, temperature, concentration, etc. In the case of convex surfaces (e.g., spherical NPs), confinement effect is less of a concern and tethered chains generally have the same MW and MWD as the untethered chains.1e Exchange Reactions. Control over the concentration of the deactivator in a SI-ATRP is crucial to regulate the lifetime of the radical and provide a controlled/living character to the polymerization. The equation in Scheme 3 quantitatively shows how the control of MWD can be affected by the concentration of deactivator. During the polymerization from curved surfaces, there should be enough spontaneously formed deactivator via a radical termination process to ensure appropriate rate of exchange. However, during the growth from flat surfaces, concentration of tethered initiators is very small, and a sufficient amount of the deactivator cannot be generated. One approach to resolve this problem is to add sacrificial initiator which can generate deactivator during the polymerization.24 Another preferred method is to add a deactivator at the beginning of the reaction.23 Results of simulations of these two methods in SI-ATRP show that the addition of deactivator at the beginning of polymerization provides instantaneous control over the polymerization that is retained during the entire polymerization. It provides a more uniform growth, higher grafting density, and thicker films.25b When sacrificial initiators are used, deactivators accumulate during the polymerization but their concentrations are low at the beginning of the reaction, resulting in inferior control. The decay of initiator concentration by termination reactions decreased the grafting density, and the excessive consumption of monomer by sacrificial initiators decreased the thickness growth rate. Thus, the initially added deactivator provides a better controlled process and thicker films. Termination. Termination in SI-ATRP decreases the chain end functionality on all kinds of surfaces and may produce macroscopic gelation for growth from nanoparticles. The termination mechanism during grafting from a flat surface is complex.25b A typical surface-initiated polymerization system is illustrated in Scheme 6, in which the hypothetical

Bimolecular termination happens only when a dormant chain end close to a radical is activated.

grafting density is ca. 1 chain/nm2 and radical concentration is 10−6 chains/nm2. These values correspond to ca. 1 and 1000 nm interchain and inter-radical distance, respectively. Hence, direct bimolecular chain termination between radicals separated by 1000 nm is impossible. Instead, termination could occur only when an activator activates a dormant chain adjacent to an existing radical. Then, the two radicals should terminate instantaneously, via a so-called “migration effect”.25b Therefore, it has been proposed that the termination rate coefficient should be proportional to the catalyst concentration, meaning that a high catalyst concentration may cause more termination.25b Since nanoparticle (convex) systems have a very high surface area and a high number of initiating sites per particle, the consequence of interparticle termination reactions during grafting from NP could be the formation of a macroscopic gel. A NP with R0 = 10 nm has a surface area ca. 1200 nm2 and over 1000 initiation sites. Thus, according to Flory’s gelation theory,20 the macroscopic gel point occurs when only 2/1000 = 0.2% of the chains terminate in an interparticle fashion. The concentration of terminated chains can be diminished when the reaction is conducted under conditions that reduce the concentration of radicals, e.g., the polymerization is conducted at a slower rate, up to a limited monomer conversion or to a reduced targeted degree of polymerization.18,36 However, this is still challenging for monomers with low kp/kt ratio (e.g., styrene or MMA).37 High dilution and low monomer conversion may not be desired. Recent advances in preparation of hybrid particles by using SI-ATRP in mini-emulsion or under high pressure can overcome this problem. Compartmentalization of the particles effectively entraps the multifunctional species, such as stars and NPs, in each miniemulsion droplet, and they are not able to react between droplets and percolate to form a macroscopic gel. Interparticle termination within a droplet can lead to the formation of small amounts of linked dimers and trimers.20 When ATRP is carried out under high pressure, the propagation rate constants increase and the termination rate constants decrease, resulting in a relatively fast polymerization with retained control over polymer architecture without sacrificing MW or conversion.38 In such a way, at 6 kbar pressure, hybrid particles with PMMA or PBMA grafts with Mn > 106 were prepared even at room temperature in a relatively short time. The reaction process is illustrated in Scheme 7. E



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Scheme 7. Schematic Illustration of (a) the Procedure of High Pressure AGET SI-ATRP and (b) the SEC Traces of the Chain Extension of PMMA (Mn = 1 610 000) with Methacrylate (MA) Block Copolymer (Mn = 2 400 000)38a

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Reprinted with permission from ref 38. Copyright 2011 Wiley-VCH.

Scheme 8. Elements of Control in SI-ATRP

distribution of chains, binary (miktoarm) brushes that can be separated to form Janus particles, and also brushes with a thin cross-linked layer next to the surface of nanoparticle. The addition of a reducing agent allows the reaction to be started from oxidatively stable CuII species while high pressure significantly increased the kp/kt ratio which facilitated higher MW and allowed the production of poly(methyl methacrylate) (PMMA) with MW > 106 and PMA with MW > 1.5 × 106. The samples of PMMA40 and PS41 prepared under 6 kbar pressure from untethered initiator produced polymers with MW ca. 106 in a few hours at room temperature. The high-pressure SIATRP applied to NP systems also yielded well-defined brushes with very high MW.38 The MWD and dispersity of grafted chains can be fine-tuned by varying [CuIILnX], according to the equation in Scheme 3. This is very easily achieved in ARGET, ICAR, or eATRP processes. However, sometimes broad MWD or even bimodal distribution may be desired. NPs with long polymer chains can be dispersed better even in high MW matrices and could

Some additional side reactions can also affect MW and MWD in hybrid systems. They may include not only termination but also transfer or self-initiation. The effect of thermal-self-initiation (TSI) of styrene in SI-ATRP from silica NPs was recently reported.39 During SI-ATRP of styrene from silica NPs, the TSI of St generated additional untethered polymer chains in solution. These untethered chains, analyzed by SEC, had lower MW and broader MWD than chains attached to NP, because they were continuously generated by TSI. The selection of a more active Cu catalyst (Cu/PMDETA vs Cu/dNbpy) and a lower polymerization temperature (70 °C vs 90 °C) resulted in a reduction of the fraction of unattached PS generated by TSI from 17.2% to 4.2%. Control Parameters. Several critical aspects of polymeric/ inorganic hybrid materials can be controlled by the appropriate selection of inorganic substrates. Major tools of macromolecular engineering such as MW, MWD, topology, functionality, and composition, are shown in Scheme 8. They also include various copolymers as well as brushes with bimodal F



Review

from the surface by SI-ATRP, and a second polymer was grafted onto the remaining amino groups. Polymeric organic/inorganic hybrid materials can also serve as templates to prepare novel nanostructured carbons, in which the inorganic materials are employed as structural templates and polymeric materials serve as the nanocarbon precursor. Thus, polyacrylonitrile (PAN) was grafted from convex silica nanoparticles55 and also from mesoporous silica materials.56 The substrates were used as the sacrificial phase allowing, after graphitization, the preparation of well-defined nanostructured carbons. An alternative approach employed poly(ethylene oxide) (PEO)/PAN diblock copolymers that were used directly as supramolecular templates for mesoporous silica and as precursors for mesoporous carbons.57 Poly(styrene-r-acrylonitrile) (PSAN) copolymers58 and carbonyl cross-linked polystyrene (PSt)58 can replace PAN and can also be used as carbon precursors. The thermally decomposable St units in PSAN generated additional micropores, and the carbonyl crosslinked PSt was successfully converted to carbon. The nanostructured carbons with N-preserved sites can be used for membranes suitable for gas adsorption, filtration, and separation or as electronic materials. The excellent control over the architecture, composition, and distribution of polymer grafts, facilitated by SI-ATRP (and other SI-CRP techniques), provides new opportunities to tailor the interaction, assembly, and properties of particle brush-based materials. This gives rise to intriguing opportunities for novel scientific insights into the physics of nanostructured hybrid materials but also the development of novel functional materials. Current research on particle brush materials can be divided into two major threads, depending on whether the particle brush takes the role as exclusive building block or as additive for the target material system. The first thread includes the use of polymer−graft modification to control the interaction and assembly of particle brush systems into novel functional materials, ranging from processable multicomponent nanoparticle and colloidal assemblies with tailored microstructures to advanced, one component, polymer hybrid materials. The second thread takes advantage of the high level of control of particle/polymer interactions, facilitated by precision graft modification, to realize the preparation of novel particle brush/polymer nanocomposite materials with tailored microstructure and enhanced properties. The following two sections will highlight recent results that illustrate the potential for novel scientific insights into the physics of hybrid materials, facilitated by the precision control of polymer graft characteristics as well as the potential for transformative material technologies based on particle brush materials.

enhance mechanical properties by entanglement, while those with short chains can screen the particles and prevent their aggregation.42 Such systems were recently prepared by SIRAFT technique.43 Controlling Polymer Architecture. ATRP is a versatile method that allows polymerization of a wide range of monomers with controlled polymer composition and architecture.7 Thus, SI-ATRP has been applied to grow block,23,37 statistical,44 and gradient45 copolymers from various inorganic substrates. In addition to predetermined control over chemical composition, polymers with different topologies were also prepared by SI-ATRP. They include (hyper)branched and cross-linked brush grafted from/onto surfaces. Hyperbranched polymer hybrids were prepared by grafting a self-condensing AB* inimer, which had both polymerizable acrylic group and an initiating group, from silica particles via ATRP.46 This hyperbranched hybrid has a 3D globular structure which has an increased number of chain end functionalities formed during the synthesis. A miktoarm hybrid system was composed of two types of polymer chains and created surfaces responding to various solvents.47 Miktoarm systems have been prepared by grafting onto48 and grafting from.49 These methods provided limited grafting density. Higher density can be reached using asymmetric Y-shaped difunctional initiators and sequentially grafting two brushes via SI-ATRP and SI-NMP.50 Another approach was the use of a two-step reverse ATRP method, in which the polymerization was initiated from azo-initiators attached to the surface, forming brushes with different composition.51 The chemical composition and grafting density of the polymer brushes was controlled through temperature and time of decomposition of the tethered initiators during the synthesis of the first generation of grafted chains. A cross-linked brush architecture was prepared via homo- or copolymerization of a difunctional cross-linker with or without an additional monomer. A one-pot SI-ATRP grafting of BA and a dimethacrylate-based cross-linker from gold NPs was reported.52 The higher reactivity of the dimethacrylate crosslinker enabled the formation of a thin cross-linked polymer shell around the surface of the Au-NP, before the growth of linear polymer chains from the in situ formed shell. This crosslinked polymer shell served as a robust protective layer and prevented detachment of polymer chains as seen with chains weakly anchored to Au via thiol groups. It provided the hybrid Au-NPs with excellent thermal stability at elevated temperatures (i.e., over 24 h at 110 °C) with preserved plasmon resonance effect. Polymeric/inorganic hybrids may exhibit new properties, due to the ability to control and precisely tune the chemical nature and architecture of the tethered brushes by SI-ATRP. They can serve as a platform for the synthesis of a range of other microstructured and functional materials. For example, the surface hydrophilicity can be tuned by pretreatment with different solutions in the case a gradient copolymer grafted system.45 The polymer brush reorganization was confirmed by NMR in binary brush systems prepared using the Y-shape initiators.50 The miktoarm brushes could self-segregate and form Janus particles.53 The preparation of stimuli-responsive Janus particles by sequential SI-ATRP and grafting onto was reported.54 In the first step, amino groups were introduced onto the surface of silica nanoparticles. The coated particles were then assembled around wax colloidosomes, and the exposed faces of the NPs were selectively modified by deposition of ATRP initiators. A first polymer was grafted

II. PARTICLE BRUSH MATERIALSFROM ENGINEERED COLLOIDS TO ONE-COMPONENT COMPOSITE MATERIALS The objective of the present section is to illustrate the potential of particle brush materials as a platform for developing novel functional materials. Our discussion is limited to “material solids”, i.e., particle brush materials without a second component, such as a solvent or matrix, and this choice is made to distinguish this emerging field of research from the more “classical” area of polymer-stabilized colloidal dispersions. The opportunities for designing novel functional materials derive from the vast parameter space that is available for tailoring the interactions between particle brushes, their selfassembly, and hence the properties of particle brush-based G



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materials. Examples of control parameters were summarized in Scheme 2, above. Despite their complexity, a common feature of particle brush materials is that properties are intimately related to the conformation of the tethered chains. The latter can be (approximately) described on the basis of mean field models that analyze the implication of chain crowding on the structure and interactions of tethered chains. The conclusions of these models have proven to be powerful predictors of the properties of particle brushes and their assembly structures, in particular in the case of nonpolar and amorphous polymer grafts. Thus, before proceeding, a brief review of basic mean field scaling models that describe the structure and interactions in particle brush systems will be provided. Structure of Particle Brushes and Its Role in Particle Brush Interactions. The complexity that is associated with the wide range of interactions and geometries, as well as relevant time- and length-scales, renders the understanding of the static and dynamic properties of particle brushes an ongoing challenge.4a However, important aspects of both the structure and interactions in particle brush materials can be evaluated on the basis of a scaling model that was first derived by Daoud and Cotton (DC) to describe the structural transitions in topologically related star polymers and recently extended to particle brush systems.59 According to the DC model the structure of particle brushes can be categorized into two regimes: the concentrated particle brush (CPB) regime in which excluded volume interactions give rise to stretched chain conformations and the semidilute particle brush (SDPB) regime in which chains assume a relaxed chain conformation. A critical distance rc = R0(σ*)1/2(ν*)−1 is introduced to determine the transition between the two brush regimes; here R0 denotes the particle core radius, σ* = ρsa2 the reduced grafting density, ν* = ν/(4π)1/2 the reduced excluded volume parameter, ρs the grafting density, and a the length of a repeat unit.59b Hence, particle brushes are considered to be within the CPB regime if the total particle radius is less than the critical radius, i.e., R = R0 + h < rc, where h = hCPB + hSDPB denotes the brush height, and otherwise within the SDPB regime. The distinct conformational regimes that are predicted on the basis of the DC model are illustrated in Figure 1 for the case of a spherical particle brush. The distinct chain conformational regimes that are predicted by the DC model are expected to give rise to a characteristic change of the scaling exponent “x” that relates the brush height with the degree of polymerization of tethered chains (h ∼ Nx) as the brush changes from the CPB to the SDPB regime. Evaluation of the hydrodynamic radius of PS- and PMMAgrafted silica particle brushes in good solvents indeed confirm a transition of the scaling coefficient from x ≅ 1 in the CPB to x ≅ 0.6 in the SDPB regime.59b,60 Electron imaging analysis of particle brush monolayers reveals that the major conclusions of the DC model also apply to the solvent-free melt state of particle brushes.28b,61,62 This is illustrated in Figure 1b which depicts the dependence of the reduced interparticle distance d/ 2rc (with d = 2h) on the reduced degree of polymerization N/ Nc (where Nc denotes the critical degree of polymerization for the CPB → SDPB transition) for a wide range of PS- and PMMA-grafted silica particle brush systems. The figure demonstrates that for particle brushes of different size and composition the curves may be collapsed onto a single master curve that displays a transition of the scaling coefficient from x = 0.8 in the CPB to x = 0.52 in the SDPB at d ≅ 2rc. Note that the scaling exponent x ≅ 0.5 confirms near ideal characteristics

Figure 1. Panel a: Illustration of the particle brush model according to extended Daoud-Cotton model. In the vicinity of the particle surface, chains assume stretched conformations (CPB regime, hCPB ∼ N0.8); as particle brush size exceeds the critical radius (r > rc), chains assume relaxed conformations (SDPB regime, hSDPB ∼ N0.5). Panel b: Dependence of the reduced brush height (d/dc) on the reduced degree of polymerization of surface-grafted chains (N/Nc) determined from analysis of electron micrographs of particle monolayers (PS and PMMA grafted SiO2 particle systems; R0 ≅ 7.7 (abbreviated as 8), 29.6 (30) ,and 56.9 (60) nm; grafting density ρs ≅ 0.5 nm−2). The CPB → SDPB transition is determined from the depicted trend as Nc = 250, 1280, and 1850 for particles with radius 7.7 nm, 29.6 nm, and 56.9 nm, respectively. Different colored regions indicate the predicted CPB and SDPB regimes (DC model). Good agreement between experimental and predicted scaling relation is observed for all particle systems. Panel c: Illustration of “effective interaction models”. Particle brushes within the CPB regime interact by hard-sphere type potentials; particles with chains in the SDPB interact in a similar manner to star polymer systems (see text for more information).

of segments within the SDPB regimethis will become an important feature in the context of the discussion of particle brush solids and one-component composites (see below). Figure 1b also highlights an important aspect that renders the DC model a valuable tool for material chemists to predict and design the properties of particle brush materials. In particular, the model facilitates the determination of the respective segment length of the stretched (CPB) and relaxed (SDPB) regions of tethered chains based on purely geometric parameters. Sinceas will be shown belowthe structure H



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the balance of attractive and repulsive interactions as well as particle size disparity, diffusion dynamics, or process kinetics.69 While significant progress has been achieved in controlling the formation of complex and long-range ordered particle assembly structures, many challenges remain for the scalable integration of particle solids into device architectures. One challenge is associated with the brittle nature of particle solids that promotes crack formation during fabrication and processing.70 The brittle characteristics of particle solids are a consequence of the weak and short-ranged interactions between particles within the array that limits the resistance of the materials to crack formation and fracture. Nanoindentation studies on a range of short-ligand coated semiconductor nanocrystals reveal that interactions in particle solids are dominated by short-ranged dispersion interactions between ligands that are grafted to the particle surface.71 The weak interactions limit the toughness of particle solids to values of Kic ≈ 50 kPa m1/2, where Kic denotes the plane strain fracture toughness. The brittle nature of particle solids contrasts with the rather high fracture toughness of polymer materials, which is a consequence of the entanglement of polymeric chains.72 The similarity in terms of interaction between particle brushes in the SDPB regime and star polymers motivates research to harness chain entanglement in SDPB systems for production of particle brush solids with polymer-like mechanical properties and processability. The potential for property enhancement is illustrated in the “Ashby chart” shown in Figure 2 that compares the relevant range of mechanical properties.

and interaction in particle brush materials sensitively depends on the competing influence of stretched and relaxed segments, the DC model provides valuable information for the design of particle brush materials with tailored properties. Here a comment should be made about the shortcomings of the DC approach. In particular, the model rests on the assumption of equal distance of the tethered chain ends from the surface of the particle. This limitation (among others) has motivated more elaborate studies that reveal a more complex relationship between composition and structure of particle brush systems.2g,4a,60a,63 For example, while the existence of distinct scaling regions has been confirmed, the transition between these regions is found to be gradual and to depend on the degree of polymerization of surface-grafted chains. In general, the DC model has been argued to become less applicable with increasing particle size and grafting density. However, despite its shortcomings, the DC model has been able to capture experimental trends and observations within experimental error. This “apparent validity” might be understood as the result of the effect of corrections being minor (within the tested range of particle architectures) or as a consequence of the inherent heterogeneity of brush materials that blurs measured trends.64 Consequently, the “apparent validity”, the DC model has been widely applied in the literature to interpret the structure and properties of particle brush materials. A second limitation that should be noted is the restriction of the DC model to spherical particle systems. While similar arguments should also apply to nonspherical shapes such as discs or cylinders, more complex chain conformational transitions are expected for nonspherical shapes due to the variation of surface curvature. The effect of particle brush architecture on the interactions and dynamic properties of particle brush systems bears similarities to star polymers that have been extensively studied during the past 20 years.65 Recent theoretical studies reveal that densely grafted polymer chains effectively act to increase the hard-sphere radius of particles.66 Strong repulsive interactions in densely grafted particle brush systems have been argued to arise from excluded volume interactions and the entropic penalty associated with the overlap of crowded surface-grafted chains.66b Combined static and dynamic light scattering of solutions of PS-grafted silica particle brushes above the overlap concentration confirm the approximately hard-sphere type interaction within the CPB regime.67 This is in contrast to the SDPB regime where brush-overlap and chain entanglement is observedin analogy to soft star polymer systems.67 Figure 1c illustrates the analogy between CPB/SDPB particle brushes and “hard sphere”/“star polymer” systems. Particle Brush Solids. Research in the area of “particle brush solids” is fundamentally motivated by the opportunities that arise to control the structure, properties, and processability of particulate materials that are provided by SI-CRP. Particle solidsi.e., solid forms of assembled nano- or colloidal particlesplay an important role in technological applications of nanoparticle or colloidal materials. Examples include the fabrication of photonic crystal materials by colloidal assembly techniques or the fabrication of light harvesting layers in solar cells from quantum dot film structures.68 Important parameters governing and controlling the properties of periodic particle assembly structures are the symmetry and dimensions of the unit cell as well as the type and density of defects within the particle array. Both are generally thought to be determined by

Figure 2. Comparison of strength and toughness of materials across a range of distinct material classes. Particle solids are located in the lower left corner indicating low strength and toughness. The arrow indicates the expected potential for property enhancement by means of polymer graft modification.

The effect of polymer-graft modification on the mechanical characteristics of particle solids has been evaluated for the example of PS- and PMMA-grafted silica particle systems using nanoindentation.28b Figure 3 depicts the dependence of the fracture toughness on the degree of polymerization of surfacegrafted chains. A significant increase of the film toughness is observed for brushes in the SDPB regime. The increase in film toughness concurs with the formation of crazes during fracture of the particle brush solid as is revealed by the electron micrographs shown in the inset of Figure 3. The formation of crazes is a direct confirmation of the presence of chain I



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within the array. Retaining this capability in particle brush solids is therefore a desirable goal. Analysis of the order parameter in particle brush monolayers reveals that the propensity for order formation depends on the competing influences of the brush’s effective hard-sphere component and the relaxed segments within the SDPB regime.73 The latter have been argued to fill in the interstitial space in the ordered lattice formed by “effective hard spheres” constituted of the particle core and segments within the CPB regime.74 The proposed role of stretched and relaxed segments on the formation of ordered assembly structures is illustrated in Figure 4.

Figure 3. Dependence of reduced fracture toughness Kic/Kic0 on the degree of polymerization N of grafted chains for PS-grafted SiO2 particle systems with R0 ≅ 7.7 nm and ρs ≅ 0.5 nm−2 measured by nanoindentation, Kic0 is the reference value of fracture toughness determined for high molecular weight PS with Mn = 300 000. The fracture toughness increases by about 500% at the CPB → SDPB transition, indicated by transition between differently colored regions. The dotted line is introduced to guide the eye. Insets show transmission electron micrographs of cracks formed during deformation of particle monolayers in CPB and SDPB regime, the stress direction is indicated by white arrows, and the scale bars are 100 nm. The high crack density in case of CPB systems confirms brittle fracture characteristics while the formation of crazes in SDPB systems rationalizes the higher fracture toughness. The schematic (top figure) illustrates distinct crack propagation characteristics in CPB and SDPB regimes.

Figure 4. Panel a: Illustration of the proposed effect of chain conformational regimes on order formation in particle brush systems. Hard-sphere type interactions in the CPB regime result in closepacked particle arrangement of effective hard spheres, constituted of particle core and stretched segments; relaxed segments fill in the interstitial space. As the volume occupied by relaxed polymer segments exceeds the available interstitial space, the array becomes increasingly disordered. Panels b and c: Representative transmission electron micrographs of particle monolayers of PS-grafted SiO2 particles (R0 = 56.9 nm, ρs ≅ 0.5 nm−2) revealing more ordered array structure for N = 600 (b) compared to N = 2200 (c). Scale bars are 200 nm.

entanglements in SDPB particle brush solids and thus further supports the assumption of polymer-like interactions in the SDPB regime. An interesting conclusion for the design of “tough” particle brush solids can be derived from the data shown in Figure 3. If chain entanglement is assumed to be a prerequisite for craze formation, a minimum degree of polymerization for tethered chains to facilitate toughening can be defined on the basis of the DC model as Nmin = 2Ne + [a−1(rc − R 0)]1/ x

The presumed role of the distinct chain conformational regimes, illustrated in Figure 4, allows the definition of an (approximate) upper limit of the degree of polymerization of tethered chains for retaining ordered array structures. By equating the volume occupied by segments within the SDPB regime with the available interstitial volume within the lattice formed by effective hard spheres, the threshold degree of polymerization for close-packed structures follows as

(1)

where Ne is the entanglement segment length that depends on the type of grafted polymer and x is the scaling exponent in the CPB regime; see Figure 1b.28b An important feature of colloidal particles is the ability of the particle system to organize, under appropriate conditions, into long-range ordered array structures in which new properties can arise due to coupling and interaction between the particles

Nmax = ϕvoid(C′)−1 + [a−1(rc − R 0)]1/ x

(2)

where ϕvoid denotes the available void space and C′ = C(4π) R0ρsa3 with the constant C (of the order unity) accounting for the number of particles per unit cell volume. On the basis of eq 2, particle brushes are expected to form ordered structures analogous to hard sphere systems if N < Nmax. Although the J



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example, the competing influence of the driving forces for order formation of the hard core to maximize configurational entropy and of the relaxed SDPB segments to maximize chain configurational entropy could induce structural transitions of particle brush solids from more close-packed lattices, such as face-centered cubic, fcc, to lattices with more uniform interstitial size, such as body-centered cubic, bcc. Indeed, numerical simulations on star polymer systems would suggest fcc−bcc type transitions in particle brush superlattice structures with increasing length of surface-bound ligands.66a The superposition of competing driving forces and interactions could provide a rich energy landscape that could facilitate a new class of “reconfigurable” hybrid materials that can be triggered to switch between distinct structural states by minor external stimulus. Further opportunities are presented by consideration of mixed particle brush systems. Similar to binary nanocrystal systems that have been shown to organize into superstructures that bear analogy to intermetallic phases, provided an appropriate size ratio of particles are selected, permitting hierarchical superstructures to be formed from binary particle brush systems.75 Indeed, experimental results on the structure formation in athermal bimodal particle brush systems have shown small particle brushes to selectively populate the interstitial spaces within a lattice formed by larger particle brushes.76 It will be interesting to further explore the relevance of soft repulsive potentials, for example, by systematic variation of the degree of polymerization of surface-grafted chains, on the order formation in multicomponent particle brush systems. Blending of particle brushes with distinct polymer composition could further facilitate tailoring of attractive, neutral or repulsive particle brush interactions and facilitate dynamic structural transitions by modulating the contribution of polymer− polymer interactions, for example, via the variation of temperature. In the above examples, particle brush building blocks were assumed to be spherical and interactions isotropic. An even richer parameter space to control the organization of particle brushes becomes available by breaking the symmetry of interactions between individual particle brushes. This might be accomplished in several ways: First, by the use of nonspherical particle shapes to achieve nonisotropic steric interactions. For example, theoretical simulations on singletethered nanorod systems suggest the formation of a “block copolymer-type” hierarchy of structures depending on the composition of hybrid nanorods. At present, it remains unclear whether similar conclusions would apply to uniformly grafted nanorod systems.77 Second, “structuring” multicomponent polymer graft layers into distinct “regions” of the respective polymer types thereby induces nonisotropic interactions that correlate with the geometry of the graft distribution. The perhaps most well-known example are Janus-type particle brushes in which two distinct types of tethered chains are segregated into opposing hemispherical regions. Theoretical predictions as well as experimental studies on hard-sphere type Janus particles suggest a hierarchy of assembly structures depending on the strength of interactions, geometry, and dynamics of the system.53b An example is provided in Figure 6a that depicts the association processes leading to “chiral strand structures” in case of electrostatically interacting Janus systems.78 Conceptually, similar organization processes might be expected for Janus-type particle brush architectures. For example, Figure 6b illustrates a subset of the predicted assembly structures of single-grafted “tadpole” particle systems, which

above arguments can only be considered to provide (at best) a semiquantitative guideline to predict and interpret the characteristics of particle brush materials, there is an interesting hypothesis that follows from the above discussion. In particular, eqs 1 and 2 suggest particle brush systems to be capable of forming both ordered and tough array structures if the degree of polymerization of tethered chains is within the range Nmin < N < Nmax, where Nmin is the minimum DP to achieve toughening as discussed above. This condition is illustrated in the highlighted region of Figure 5a, which depicts the predicted

Figure 5. Panel a: Plot of Nmin and Nmax as function of particle size for ρs ≅ 0.5 nm−2 according to eqs 1 and 2 in the text. Highlighted in yellow is the predicted range of particle brush architectures that combine craze formation (and hence increased toughness) with the capability of forming long-range ordered particle brush arrays. Panel b: Image of thick film (t ∼ 0.5 mm) of PS-grafted SiO2 particles (R0 ≅ 56.9 nm, ρs ≅ 0.5 nm−2, N = 2200) revealing strong iridescence. Inset shows flexural stability of film during bending. Panel c: Transmission electron micrograph (TEM) of particle bilayer revealing regular superstructure. Particles in the bottom layer form a hexagonal array, and particles in the top layer are centered on interstitials of the bottom layer. The observed structure is commensurate with cubic or hexagonal close packing in the solid state. Panel d: TEM of particle monolayer upon deformation revealing multiple craze formation during fracture.

trend lines representing eqs 1 and 2 calculated for a grafting density ρs = 0.5 nm−2. The predicted combination of structural and mechanical properties is substantiated by the analysis of PS-grafted silica particles with R0 ≅ 56.6 nm, ρs = 0.5 nm−2, and N = 2200 (60SiO2−S2200). Figure 5b illustrates the formation of flexible and free-standing “photonic” films fabricated by rapid solution casting. Figure 5c,d reveals the origin of the optical and mechanical properties of the 60SiO2−S2200 solid film. In particular, Figure 5c depicts a TEM image of a bilayer film of 60SiO2−S2200 revealing a high degree of regularity of the particle brush solid that is consistent with the observed uniform optical iridescence. The formation of crazes during the deformation of 60SiO2−S2200 films (see Figure 5d) confirm the presence of polymer-like chain entanglement that also rationalizes the films’ mechanical flexibility; see inset of Figure 5b. The fortuitous combination of structure formation and mechanical properties could render particle brushes, with appropriate architecture, interesting candidates for the synthesis of photonic inks and paints. Several intriguing questions arise in the context of order formation of polymer-grafted hard sphere systems. For K



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and microphases observed in surfactant or block copolymertype systems.79 A third strategy to expand the range of microstructures is by grafting of polymer chains that superimpose a secondary organization process on the particle brush assembly. For example, recent simulations of the organization of single-chain block copolymer-grafted particles suggest that the microphase separation process that is characteristic of the block copolymer graft candepending on the composition dictate the symmetry of the particle assembly (see Figure 6c).80 Finally, polarity of the inorganic particle core can be harnessed to break the symmetry of interactions and facilitate the assembly of particle brushes into low-symmetry structures. This concept was utilized, for example, to facilitate the formation of particle-string as well as “folded-string” structures in assemblies of ferromagnetic particle brush systems (see Figure 6d).81 An interesting extension of this concept is to use soft polymer grafts to facilitate dynamic realignment of particle cores while retaining the overall crystal structure of “yolk− shell” particles.82 Conceptually this approach might enable the design of materials in which “internal reconfiguration” and change of associated properties occurs without affecting the materials’ external dimension. Figure 6 illustrates examples of the diverse range of microstructures that are attainable by symmetry-breaking of particle brush interactions. The examples shown in Figure 6 illustrate how the various structural and kinetic constraints in polymer-grafted particle systems can give rise to a range of hierarchical organized structures that can be selectively realized by appropriate design of the tethered chains. Because of the complexity that is associated with the wide range of energy, time, and length scales involved in the organization of “nanostructured hybrid particles” further progress in the theoretical understanding of these complex systems will play an important role to guide future synthetic and experimental efforts. An intriguing and complementary perspective on particle brush assembly structures is provided by considering the “composite material” attributes of particle brush materials. In this case, the grafted polymer is viewed as the primary material component of the particle brush while the inorganic core plays the role of augmenting the properties of the polymer matrix that is constituted by the grafted chains. Thus the “composite” perspective becomes a more “natural” interpretation of particle brush materials in the limit of long-chain grafts that impart distinctive polymer-like properties to the material. One-Component Nanocomposite Materials. From a microstructure perspective, i.e., considering only the distribution of phases, particle brush solids are nanocomposite materials comprised of inorganic filler dispersed within a polymeric matrix. However, since in the case of particle brushes the polymer matrix is tethered to the inorganic phase a particle brush-based nanocomposite is thermodynamically a onecomponent material. This is in contrast to “classical” polymer nanocomposites thatin their most “simple” formare binary blends of a polymer matrix and filler particles. The general concept of one-component composite materials is illustrated in Figure 7a. The one-component nature of particle brush composite materials affords several advantages such as the absence of miscibility gaps that are typical for binary, nonisomorphous systems and improved control of the composite morphology even at high inorganic content. The potential for innovative material applications that derives from these attributes fuels research in both the fundamental structure−property relations of particle brush-based composite

Figure 6. Illustration of the effects of symmetry breaking on structure formation in particle brush-type materials. Panel a: Illustration of the kinetic pathways for the organization of Janus colloids into hierarchical superstructures. Modification of solvent quality triggers growth of “chiral strands” from aggregate precursors. Reprinted with permission from ref 78. Copyright 2011 American Association for the Advancement of Science. Panel b: Predicted formation of microdomain-type structures in single-homopolymer-grafted nanoparticle systems (“tadpole” particles) in concentrated solution. Depending on solvent quality and volume fraction of polymer component the formation of lamellar, cylindrical, and spherical microdomain structures is observed. Adapted with permission from ref 79. Copyright 2003 American Chemical Society. Panel c: Predicted organization of singe-block copolymergrafted nanoparticles in the melt. A gradual (triblock terpolymer-like) transition between microstructures from close-packed particles in pure “A” to close-packed particles in pure “B” matrix is observed with increasing volume fraction of “B” component in the AB-type block copolymer graft. Adapted with permission from ref 80. Copyright 2010 American Chemical Society. Panel d: Transmission electron micrographs of string patterns formed by dipolar (ferromagnetic) PS-grafted Co-nanoparticles (R0 ≈ 9 nm). Chain-folded lamellae-type arrangements are observed for high density particle string arrangements. Reprinted with permission from ref 81. Copyright 2007 American Chemical Society.

could be range of that are highlight

seen as a special case of the Janus architecture. The microstructures (lamellar, cylindrical, and spherical) observed depending on the particle composition the similarity between “tadpole” particle assemblies L



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blends have been highlighted in the literature: First, the absence of aggregation phenomena renders particle brush materials intrinsically dispersed independent of the inorganic content. This is an important aspect since thermodynamic stable dispersion is difficult to accomplish in binary particle/polymer blends.80,62 The intrinsic separation of the inorganic phase supports optical transparency and prevents the formation of percolative pathways. Particle brush materials are thus being considered as a platform for high refractive index polymer lenses or as high “k” dielectrics in applications such as separators in supercapacitors that require resistance to electric breakdown at high inorganic content. The ability to “assemble” uniform hybrid materials from particle brush building blocks either from the solution or the melt state in conjunction with the potential for efficient cross-linking, for example, by introducing reactive sites into the tethered chains furthermore make particle brush materials an interesting material platform for additive manufacturing processes of hybrid structures. A second distinguishing feature of particle brush-based onecomponent composite materials is the high level of control over structural parameters such as particles nearest neighbor distance or coordination number that results from the interaction between the tethered polymer chains. The ability to achieve nanocomposite materials with high degree of internal order is demonstrated in Figure 7d which depicts the microstructure of a one-component composite system of PSgrafted silica particles revealing short-ranged particle order within the film.84 This should be contrasted with the microstructure of a corresponding binary composite composed of PS-grafted silica particles in PS that reveals random particle distribution (see Figure 7e).84 The high degree of regularity of the materials’ microstructure renders the mechanical properties of the one-component composite directionalthus demonstrating a wider range of tunability of material properties as compared to isotropic binary nanocomposites.84 Interestingly, one-component composites (based on densely grafted particle brushes) were also observed to exhibit increased resistance to fracture as compared to equivalent binary composite systems a feature that was interpreted to be a consequence of the longer range of correlations between entanglement points as compared to the binary composite.28b Many important questions will need to be addressed to further exploit the potential of one-component particle brush composites for innovative hybrid material applications. For example, what is the interrelation between parameters such as the grafting density, molecular weight, and dispersity of grafted chains and the mechanical properties of the composite, melt and solution processability, or structure formation in the solid state? Most studies on one-component composites involve studies on spherical inorganic particles with a size of the order of the radius of gyration of the grafted chainswhat is the relevance of particle size and shape on the above-mentioned parameters? Finally, all systems reported to date involve glassy polymer matriceshow will the constraints imparted by the chain grafting interact to determine structure formation in semicrystalline or liquid-crystalline graft polymer matrices? Further opportunities might arise from the use of low-Tg graft polymers, where Tg denotes the glass transition temperature, that could provide a path toward dynamically responsive one component composites. The latter could find use, for example, as solventless liquid hybrid materials with high inorganic content for thermal interface applications or high thermal conductivity coolants in heat exchange systems.

Figure 7. Panel a: Comparison of nanocomposite fabrication schemes: (1) by blending of matrix polymer and particle fillers (binary composite) or (2) by tethering the matrix polymer to the surface of particles (one-component composite). Panels b and c: image of PSgrafted TiO2 (N ≈ 950) based one-component composite before (b) and after (c) deformation. Large ductility indicates typical polymer-like deformation. Reprinted with permission from ref 83. Copyright 2010 American Chemical Society. Panel d: Transmission electron micrograph of cross-section of one-component composite formed by PSgrafted silica particles (R0 = 7.7 nm, ρs ≅ 0.5 nm−2, N = 660) revealing short-range order of particles within film (inorganic content is ϕSiO2 ≈ 0.05). Image width is 1 μm. Panel e: TEM of cross-section of binary composite formed by PS-grafted silica particles (R0 = 7.7 nm, ρs ≅ 0.5 nm−2, N = 150) in PS (N = 490) revealing random dispersion of particles within film (ϕSiO2 ≈ 0.05). Image width is 1 μm. Images shown in Panel d and e are reprinted with permission from ref 84. Copyright 2011 American Chemical Society.

materials as well as functional material applicationsthe reader is referred to ref 5 for a more comprehensive perspective on this emerging field. The analogies of bulk particle brush materials and conventional polymer nanocomposite materials are illustrated by the polymer-like deformation characteristics of PS-grafted TiO2 nanoparticle systems shown in Figure 7a,b.83 The similarities in mechanical properties point to intriguing opportunities to leverage the vast range of technologies that has been developed to process polymer materials, such as extrusion, molding, or roll-to-roll, to the processing of particle brush materials. Several important distinguishing features, with regard to the development of innovative functional materials, of the one-component composite approach as compared to binary polymer/particle M



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III. PARTICLE BRUSH MATERIALS AS MULTIFUNCTIONAL FILLERS FOR HIGH PERFORMANCE NANOCOMPOSITE MATERIALS The grafting of polymer chains to the surface of nanoparticles is ubiquitously being used as means to stabilize dispersions of particle fillers within polymeric host media and thus plays a central role in the context of binary (or multicomponent) polymer nanocomposite materials.85,86 The latter present one of the most widely researched areas in the field of polymer materials due to the significant technological and economic relevance that derives from the multitude of material property advancements that have been facilitated by particle addition. A comprehensive discussion of this field is outside the scope of this article and the reader is referred to refs 85 and 87 for reviews of the field. The following section will highlight recent advances made in the field of binary polymer/particle nanocomposite materials that are motivated by the advancements of SI-CRP and particle brush synthesis. While the differentiation between “classical” polymer-grafted particle fillers and recently developed particle brush fillers appears artificial, the advancements in control of the particle brush architecture that are afforded by SI-CRP techniques have facilitated dramatic progress in the engineering of the microstructure and hence properties of polymer nanocomposite materials. For example, a systematic experimental and theoretical evaluation of the structure formation in quiescent PS-grafted SiO2/PS blend systems reveals a sensitive dependence of the composite microstructure on the particle brush architecture.88 The novel opportunities for composite structure engineering are perhaps best exemplified by the formation of string and sheet-like aggregate structures that have been observed in the sparse grafting limit (σ < 0.1 nm−2) and attributed to the competition between core−matrix and graft−matrix interactions. A morphology diagram and representative micrographs for a range microstructures observed in binary nanocomposites composed of PS-grafted silica particles dispersed in PS are depicted in Figure 8.89 The assembly of particle brush fillers into string or sheet-like aggregate structures offers new opportunities for tailoring the mechanical or transport characteristics in particle brush/ polymer blend systems beyond the limits of simple dispersed morphologies that are defined by homogenization theory. For example, particle brush superstructures could provide a means to facilitate conductive pathways, in case of string and sheet structures, or increased barrier properties, sheet structures, while retaining the benefits of otherwise compatible blend systems.90 A major challenge in the context of particle brush/polymer composite systems is the loss of optical transparency due to scattering of dispersed particle fillers. The scattering losses can be quantified in terms of the particle scattering cross-section that is, for small particle fillers, proportional to the square values of the particle volume V and dielectric contrast between particle (P) and matrix (M), i.e., Cscatt ∼ V2(εP − εM)2.91 Because of the large dielectric contrast in most organic/ inorganic material combinations, the dependence of the scattering cross-section on the square of the dielectric contrast typically implies strong scattering even in the case of small and well-dispersed particle fillers. SI-ATRP methodologies have been shown to provide an elegant solution to this problem by facilitating the formation of graft layers that act both to

Figure 8. Structure map representing the various particle dispersion states as a function of particle brush architecture. Electron micrographs depict respective examples of microstructures for the example of PSgrafted silica (R0 = 7.7 nm) in PS matrix: discrete aggregate structure (1), particle network (2), and sheet aggregate structure (3). Adapted with permission from ref 89. Copyright 2011 American Chemical Society.

compatibilize as well as index-match the particle brush to the matrix. Two criteria are relevant for the design of low-scattering particle brushes: First, the graft layer (S) has to be chosen such that the refractive index of the matrix (M) is intermediate between those of core (P) and shell (S), i.e., nP < nM < nS or nP > nM > nS, to allow for index-matching. Here ni denotes the refractive index of constituent “i”the refractive index is, for nonabsorbing materials, related to the dielectric constant via n2 = ε. Second, the graft layer has to facilitate compatibilization of the particle brush within the matrixthis generally will require attractive graft−matrix interactions.62 Once a proper choice of graft polymer has been made, effective medium models can be used to determine the target composition of the particle brush required for index-matching.28c,92 Experiments on solution dispersed PS-grafted silica particle systems in which the grafting density is systematically varied by mixing of reactive and inactive initiator sites show that composition rather than the detailed architecture determines the scattering of particle brush fillers.28c Examples of suitable particle brush compositions to facilitate index-matching and compatibilization for a wide range of matrix polymers have been suggested in the literature.28c Figure 9a illustrates the concept of index-matching for the example of poly(styrene-r-acrylonitrile) (SAN)-grafted silica particle (R0 ≅ 7.7 nm) dispersed in PMMA. Future progress in controlling the molecular characteristics of grafted chains (such as random or gradient copolymer compositions) is expected to further expand the opportunities to control the optical properties of particle brush composite materials. The extension of SI-CRP techniques to bimodal particle brush systemsi.e., particles grafted with a binary blend of short and long polymer chainsprovides a path toward stable (athermal) particle brush/polymer dispersions with high inorganic content.42a This approach is motivated by the idea to combine the screening effect of dense polymer grafts with the compatibilization effect of long-chain polymer grafts while concurrently maintaining a high inorganic content. One advantage of the “bimodal” compatibilization approach is that N



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polymer-grafted particulates with unprecedented control over the architecture of grafted chains. The high level of structural control that is facilitated by SI-CRP allows the synthesis of “particle brush materials” that derive novel properties from the complex superposition of interactions and dynamic processes related to core and tethered polymer chains. The opportunities to engineer hierarchically organized hybrid materials with precisely controlled microstructure render particle brush materials intriguing building blocks for innovative material systems that could have a transformative impact on a wide range of “soft material” technologies. For example, the presence of manifold structural and/or kinetic frustrations in organized particle brush systems could give rise to a range of nonequilibrium and metastable stateseach associated with distinct property characteristicsthat potentially could be accessed by interaction with the environment. To harness these opportunities for the development of interactive and multifunctional advanced materials will require concerted efforts in both the modeling and the characterization of particle brush materials as well as the development of synthetic routes to access the full parameter space outlined in Scheme 2 above. Further research in the area of particle brush synthesis should focus on the expansion of the range of monomer systems applicable to SI-CRP and the development of methodologies to control higher order characteristics such as the distribution of molecular weight or the distribution of repeats in gradient structures, as well as strategies to control the spatial distribution of tethered chains in multicomponent graft systems.

Figure 9. Effect of polymer graft modification on the structure and optical properties of binary particle/polymer blend systems. Panel a: Scheme illustrates alternative particle brush architectures, high grafting density/short chain, and low grafting density/long chain, with equal effective refractive index. Example shows particle compatibilization and effective index matching in case of SAN-grafted silica particles (R0 ≅ 7.7 nm, N = 12) embedded in PMMA (N ≈ 1000) matrix (15 vol % inorganic content). Electron micrograph shows dispersed morphology; the scale bar is 100 nm. Inset shows optical image of sample and refractive index line-up of core, shell, and matrix. Panel b: Particle compatibilization through “bimodal” polymer grafts yielding significant improvement of optical transparency. Short grafted chains screen-off matrix−core interactions while long grafted chains support mixing of tethered chains with the polymer matrix. Example shows optical images of samples corresponding to PDMS-grafted TiO2 particles (5 wt % inorganic content) dispersed in PDMS (N ≈ 1200) matrix: uniform graft layer (N ≈ 490) resulting in aggregation and opaque characteristics; bimodal graft layer (N1 ≈ 130, N2 ≈ 490) resulting in particle dispersion and optical transparency. Adopted with permission from ref 42a. Copyright 2013 American Chemical Society.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.R.B.). *E-mail: [email protected] (K.M.). Present Addresses §

(J.P.) Institute of Polymer and Dye Technology, Technical University of Lodz, ul. Stefanowskiego 12/16, 90-924 Lodz, Poland. ∥ (J.C.) Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104-6272, United States. Notes

compatibilization is facilitated for athermal particle brush/ matrix compositions; i.e., brush and matrix are chemically identical. The approach should be applicable to a wide range of particle and matrix compositions. The combination of high inorganic content with random particle dispersion is of particular interest in the context of refractive index engineering of polymer glasses. For example, TiO2 nanoparticles grafted with a bimodal blend of poly(dimethyl siloxane) (PDMS) can be used to realize TiO2/PDMS composite materials combining an inorganic content of 5% while retaining high optical clarity.42a The conceptual idea of the bimodal particle brush compatibilization approach as well as representative images of the TiO2/PDMS blends are depicted in Figure 9b. An alternative and potentially more versatile approach to multicomponent graft compositions could be provided by engineering the continuous molecular weight distribution of polymer graftsinitial strategies might follow the approach used to realize disperse homo- and block copolymers by ATRP.17b

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (via Grants CMMI-1234263, DMR-0969301, and DMR1006473) as well as the Air Force Office for Scientific Research (via Grant FA9550-09-1-0169). C.M. and M.R.B. acknowledge support through the NSF-IGERT program via NSF-0966227. C.M.H. and K.M. acknowledge National Science Foundation (via Contract DMR-0706265).

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REFERENCES

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CONCLUSIONS Recent progress in the area of surface-initiated controlled radical polymerization (SI-CRP) has enabled the synthesis of O



Review

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