Refractive Index Engineering of Polymer Nanocomposites Prepared by End-grafted Polymer Chains onto Inorganic Nanoparticl
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Refractive Index Engineering of Polymer Nanocomposites Prepared by End-grafted Polymer Chains onto Inorganic Nanoparticles Peng Tao1, Anand Viswanath2, Yu Li2, Atri Rungta2, Brian C. Benicewicz2, Richard W. Siegel1, and Linda S. Schadler1 1
Department of Materials Science and Engineering and Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute, Troy, NY 12180, U. S. A. 2 Department of Chemistry and Biochemistry and USC NanoCenter, University of South Carolina, Columbia, SC 29208, U. S. A. ABSTRACT Transparent polymer nanocomposites with high refractive index were prepared by grafting polymer chains onto TiO2 nanoparticles. Reversible addition-fragmentation chain transfer (RAFT) polymerization was used to prepare poly(methyl methacrylate) (PMMA) polymer brushes grafted from TiO2 nanoparticles. The refractive index of the hybrid material increased from 1.49 for neat PMMA to 1.6 by increasing the loading of TiO2 to 40 weight percent. UV-vis spectra showed that grafted particles had a transparency of more than 90% in the visible light range. The hybrid particles can be processed into transparent, high refractive index coatings and self-standing films. The grafted TiO2 nanoparticles can also be easily dispersed into a polymer matrix forming thick, robust transparent polymer nanocomposites. INTRODUCTION Refractive index is one of the most important and fundamental characteristic properties of optical materials. High refractive index, transparent materials have attracted much interest due to their increased potential integration into optical and optoelectronic applications. For instance, high refractive index coatings are used as brightness enhancing films and antireflection films on liquid crystal displays (LCDs). To improve light extraction efficiency of light emitting diodes (LEDs), transparent high refractive index encapsulant materials are urgently needed to enable LEDs to become the preferred lighting source [1-4]. However, most organic materials have a small adjustable refractive index range (1.3-1.7) [2]. To increase the refractive index of organic polymers, one strategy is to incorporate high refractive index inorganic materials and fabricate organic/inorganic hybrid materials. These hybrid materials combine the light weight, good processability and cost-effectiveness of the polymeric component, and the high refractive index of the inorganic components. Among these hybrid materials, TiO2 nanocomposites have been proposed as one of the promising candidates to achieve high refractive index and maintain high transparency because TiO2 nanoparticles have a high refractive index (2.45-2.7 for anatase and rutile phase respectively) and a very low absorption coefficient in the visible light range [3]. The challenge to commercialization of these high refractive index nanocomposites is the reduction in transparency that occurs due to agglomeration of the nanoparticles within polymer matrices. To solve this problem, great efforts have been made to modify the nanofiller surfaces using physical sonication in com
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