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 governing parameters controlling the miscibility of particle additives within polymeric host media are analyzed for the particular case of silica particle fillers embedded within a poly(methyl methacrylate) (PMMA) matrix. For athermal polymer-graft modification of particles (corresponding to equal chemical composition of graft and matrix polymer), compatibility is found to be a sensitive function of the degree of polymerization of graft and host polymer chains as well as the particle radius. In agreement with theoretical predictions, uniform particle dispersion is observed if the degree of polymerization of grafted chains is comparable to (or exceeds) the corresponding value of the polymer matrix. The resulting restriction to high degree of polymerization limits the accessible inorganic fraction that is attainable in athermal particle/polymer blends. In contrast, favorable interaction between grafted polymer chains and the polymeric host (as realized in the case of poly(styrene-r-acrylonitrile)-grafted particles embedded within PMMA matrix) is shown to facilitate thermodynamically stable and uniform particle dispersion across the entire compositional range even in the limit of large particle size, short grafted chains, and high molecular matrix chains. The synthesis of thermoplastic composite materials with inorganic fraction exceeding 50 vol % combining quantitative optical limiting within the UV frequency range and polymer-like mechanical properties is demonstrated.

The role of polymeric ligands on the optical transparency of polymer-matrix composites is analyzed by evaluating the effect of surface modification on the scattering cross-section of particle fillers in uniform particle dispersions. For the particular case of poly(styrene-r-acrylonitrile)-grafted silica particles embedded in poly(methyl methacrylate), it is shown that the tethering of polymeric chains with appropriate optical properties (such as to match the effective refractive index of the brush particle to the embedding matrix) facilitates the reduction of the particle scattering cross-section by several orders of magnitude as compared to pristine particle analogues. The conditions for minimizing the scattering cross-section of particle fillers by polymer-graft modification are established on the basis of effective medium as well as core-shell Mie theory and validated against experimental data on uniform liquid and solid particle dispersions. Effective medium theory is demonstrated to provide robust estimates of the "optimum polymer-graft composition" to minimize the scattering cross-section of particle fillers even in the limit of large particle dimensions (comparable to the wavelength of light). The application of polymer-graft modification to the design of large (500 nm diameter) silica particle composites with reduced scattering cross-section is demonstrated.

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Development of high refractive index glasses on the basis of commodity polymer thermoplastics presents an important requisite to further advancement of technologies ranging from energy efficient lighting to cost efficient photonics. This contribution presents a novel particle dispersion strategy that enables uniform dispersion of zinc oxide (ZnO) particles in a poly(methyl methacrylate) (PMMA) matrix to facilitate hybrid glasses with inorganic content exceeding 25% by weight, optical transparency in excess of 0.8/mm, and a refractive index greater than 1.64 in the visible wavelength range. The method is based on the application of evaporative ligand exchange to synthesize poly(styrene-r-acrylonitrile) (PSAN)-tethered zinc oxide (ZnO) particle fillers. Favorable filler-matrix interactions are shown to enable the synthesis of isomorphous blends with high molecular PMMA that exhibit improved thermomechanical stability compared to that of the pristine PMMA matrix. The concurrent realization of high refractive index and optical transparency in polymer glasses by modification of a thermoplastic commodity polymer could present a viable alternative to expensive specialty polymers in applications where high costs or demands for thermomechanical stability and/or UV resistance prohibit the application of specialty polymer solutions.