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Composites that show visible light transmittance, UV absorption, and moderately high refractive index, based on poly(methyl methacrylate) (PMMA) and zinc oxide (zincite, ZnO) nanoparticles, were prepared in two steps. First, surface-modified ZnO nanoparticles with 22 nm average diameter were nucleated by controlled precipitation via acid-catalyzed esterification of zinc acetate dihydrate with pentan-1-ol. The surface of growing crystalline particles was modified with tert-butylphosphonic acid (tBuPO 3 H 2 ) in situ by monolayer coverage. Particle size and graft density of -PO 3 H 2 on the particle surface were controlled by the amount of surfactant applied to the reaction solution. Second, the surface-modified particles were incorporated into PMMA by in-situ bulk polymerization. Free radical polymerization was carried out in the presence of these particles using AIBN as initiator. Volume fraction (φ) of the particles was varied from 0.10 to 7.76% (0.5 to 30 wt %). Although the particles are homogeneously dispersed in monomer, segregation of the individual particles upon polymerization was observed. Optical constants of the films ca. 2.0 µm including absorption and scattering efficiencies, indices of refraction, and dispersion constants were determined. The absorption coefficient at 350 nm increases linearly with ZnO, obeying Beer's law at low particle contents. However, it levels off toward a value of about 5000 cm -1 and shows a negative deviation at high concentrations because of aggregation of the individual particles. Waveguide propagation loss coefficients of the composite films were examined by prism coupling. A steep increase of the loss coefficient was found with a slope of 52 dB cm -1 vol % -1 as the volume fraction of the particle increases. The refractive index of the composites depends linearly on volume fraction of ZnO and varies from 1.487 to 1.507 (φ ) 7.76%) at 633 nm. The dispersion of refractive index was found to be consistent with Cauchy's formula. IntroductionThe development of polymer-based composites which exhibit various optical functionalities such as high/low refractive index, tailored absorption/emission properties, or strong optical nonlinearities attracts great interest because of the potential optoelectronic applications. 1,2 More specifically, it was pointed out that such composite materials could be applied as transparent substrate or flexible functional layers of optoelectronic devices which require high transparency in the visible range of the optical spectrum. 3 Replacing the conventional substrates made up of inorganic glasses by polymer-based materials could provide a number of advantages, as the polymer composites have milder processing conditions and better impact resistance, can be made flexible, and the optical parameters can be tailored. These composites are typically obtained by the incorporation of functional inorganic particles into a transparent polymer matrix. 3 While the polymeric component provides processability, flexibility, and transparency, the inorg...