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.

The effect of polymer-graft modification on the structure formation and mechanical characteristics of inorganic (silica) nanoparticle solids is evaluated as a function of the degree of polymerization of surface-grafted chains. A transition from 'hard-sphere-like' to 'polymer-like' mechanical characteristics of particle solids is observed for increasing degree of polymerization of grafted chains. The elastic modulus of particle solids increases by about 200% and levels off at intermediate molecular weights of surface-grafted chains, a trend that is rationalized as a consequence of the elastic modulus being determined by dispersion interactions between the polymeric grafts. A pronounced increase (of about one order of magnitude) of the fracture toughness of particle solids is observed as the degree of polymerization of grafted chains exceeds a threshold value that is similar for both polystyrene and poly(methyl methacrylate) grafts. The increased resistance to fracture is interpreted as a consequence of the existence of entanglements between surface-grafted chains that give rise to energy dissipation during fracture through microscopic plastic deformation and craze formation. Within the experimental uncertainty the transition to polymer-like deformation characteristics is captured by a mean field scaling model that interprets the structure of the polymer shell of polymer-grafted particles as effective 'two-phase' systems consisting of a stretched inner region and a relaxed outer region. The model is applied to predict the minimum degree of polymerization needed to induce polymer-like mechanical characteristics and thus to establish 'design criteria' for the synthesis of polymer-modified particles that are capable of forming mechanically robust and formable particle solid structures.

The propensity of particle brush materials to form long-ranged ordered assembly structures is shown to sensitively depend on the brush architecture (i.e., the particle radius as well as molecular weight and grafting density of surface-bound chains). In the limit of stretched chain conformations of surface-grafted chains the formation of regular particle array structures is observed and interpreted as a consequence of hard-sphere-type interactions between polymer-grafted particles. As the degree of polymerization of surface-grafted chains increases beyond a threshold value, a reduction of the structural regularity is observed that is rationalized with the increased volume occupied by relaxed polymer segments. The capacity of polymer grafts to increase or decrease order in particle brush assembly structures is interpreted on the basis of a mean-field scaling model, and "design criteria" are developed to help guide the future synthesis of colloidal systems that are capable of forming mechanically robust yet ordered assembly structures.

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The emergence of flexible and stretchable electronic components expands the range of applications of electronic devices. Flexible devices are ideally suited for electronic biointerfaces because of mechanically permissive structures that conform to curvilinear structures found in native tissue. Most electronic materials used in these applications exhibit elastic moduli on the order of 0.1–1 MPa. However, many electronically excitable tissues exhibit elasticities in the range of 1–10 kPa, several orders of magnitude smaller than existing components used in flexible devices. This work describes the use of biologically derived heparins as scaffold materials for fabricating networks with hybrid electronic/ionic conductivity and ultracompliant mechanical properties. Photo-cross-linkable heparin–methacrylate hydrogels serve as templates to control the microstructure and doping of in situ polymerized polyaniline structures. Macroscopic heparin-doped polyaniline hydrogel dual networks exhibit impedances as low as Z = 4.17 Ω at 1 kHz and storage moduli of G′ = 900 ± 100 Pa. The conductivity of heparin/polyaniline networks depends on the oxidation state and microstructure of secondary polyaniline networks. Furthermore, heparin/polyaniline networks support the attachment, proliferation, and differentiation of murine myoblasts without any surface treatments. Taken together, these results suggest that heparin/polyaniline hydrogel networks exhibit suitable physical properties as an electronically active biointerface material that can match the mechanical properties of soft tissues composed of excitable cells.

Mesoporous materials are highly promising in electronics industry as low-dielectric-constant (low-k) on-chip insulating media. [1][2][3] In these applications, materials with closed (isolated) pores are preferred. 1,3 The synthesis of ordered mesoporous silicas and organosilicas with closed spherical mesopores of diameters 5-30 nm has been reported using poly(ethylene oxide)-polystyrene (PEO-PS) copolymer micelles as templates (porogens). [3][4][5][6] Herein, it is demonstrated that the formation of closed mesopores can be achieved through a simple thermal treatment in widely known ordered mesoporous silicas (OMSs) templated by readily available poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) Pluronic block copolymers (e.g, Pluronic F127). 7-10 Our results indicated that the thermal closure is likely to be a general property of copolymer-templated OMSs with spherical mesopores, provided that the pore entrance size is significantly smaller than the pore cage diameter.Recently, the synthesis of large-pore FDU-12 (LP-FDU-12) silica 11 with Fm3m (face-centered cubic, fcc) structure of connected spherical mesopores was reported. Our subsequent study showed 12 that LP-FDU-12 synthesized exclusively at low temperature (e.g., 15 °C), and some samples obtained after heating of the synthesis mixture at 100 °C, exhibited low, sometimes negligible, N 2 adsorption capacity at -196 °C after the removal of copolymer template via calcination for 5 h at 550 °C under air (see Figure 1). At first, the lack of accessible porosity seemed to indicate a collapse of the mesopore structure during calcination, but such a collapse was unlikely on the basis of earlier studies of thermal stability of OMSs. 13,14 Therefore, the existence of closed pores was envisioned and subsequently confirmed as follows. The decrease in calcination temperature to 450 °C afforded silica with pore size of ∼15 nm and appreciable adsorption capacity, whereas the increase to 640 °C reduced the adsorption capacity to virtually zero (see Figure 1

Demands to increase the stored energy density of electrostatic capacitors have spurred the development of materials with enhanced dielectric breakdown, improved permittivity, and reduced dielectric loss. Polymer nanocomposites (PNCs), consisting of a blend of amorphous polymer and dielectric nanofillers, have been studied intensely to satisfy these goals; however, nanoparticle aggregates, field localization due to dielectric mismatch between particle and matrix, and the poorly understood role of interface compatibilization have challenged progress. To expand the understanding of the inter-relation between these factors and, thus, enable rational optimization of low and high contrast PNC dielectrics, we compare the dielectric performance of matrix-free hairy nanoparticle assemblies (aHNPs) to blended PNCs in the regime of low dielectric contrast to establish how morphology and interface impact energy storage and breakdown across different polymer matrices (polystyrene, PS, and poly(methyl methacrylate), PMMA) and nanoparticle loadings (0-50% (v/v) silica). The findings indicate that the route (aHNP versus blending) to well-dispersed morphology has, at most, a minor impact on breakdown strength trends with nanoparticle volume fraction; the only exception being at intermediate loadings of silica in PMMA (15% (v/v)). Conversely, aHNPs show substantial improvements in reducing dielectric loss and maintaining charge/discharge efficiency. For example, low-frequency dielectric loss (1 Hz-1 kHz) of PS and PMMA aHNP films was essentially unchanged up to a silica content of 50% (v/v), whereas traditional blends showed a monotonically increasing loss with silica loading. Similar benefits are seen via high-field polarization loop measurements where energy storage for ∼15% (v/v) silica loaded PMMA and PS aHNPs were 50% and 200% greater than respective comparable PNC blends. Overall, these findings on low dielectric contrast PNCs clearly point to the performance benefits of functionalizing the nanoparticle surface with high-molecular-weight polymers for polymer nanostructured dielectrics.

The modification of the surface of nanoparticles with polymeric chains is ubiquitously used to engineer the physicochemical properties of nanoparticle fillers and to enable new material technologies based on polymer hybrid materials with controlled microstructure. The tethering of particles with polymeric chains of distinct (high and low) degree of polymerization (so-called "bimodal polymer grafts") has emerged as a particularly interesting strategy to combine the synergistic benefits of dense and sparse polymer grafts (i.e., good control of particle interactions facilitated by densely grafted polymer chains with the high inorganic content characteristic for sparsely grafted systems). In this contribution, surface-initiated atom transfer radical polymerization (SI-ATRP) is demonstrated to be a versatile tool that enables the synthesis of bimodal graft modifications with precise control of the degree of polymerization of the respective graft species. For the particular case of polystyrene-tethered silica particles, it was demonstrated that the presence of even small fractions of "long" chains provided an order-of magnitude increase of the mechanical toughness of particle films that is comparable to values found in densely tethered particle systems only in the limit of high degree of polymerization of tethered chains (and corresponding low inorganic content).