Effect of Temperature on the Kinetics of Nanocluster Nucleation and Growth in Polymeric Media
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Effect of Temperature on the Kinetics of Nanocluster Nucleation and Growth in Polymeric Media Oz Gazit1 and Rina Tannenbaum2,3 1 Chemical Engineering, Technion, Haifa, 32000, Israel 2 Technion, Haifa, 32000, Israel 3 Material Science and Engineering, Georgia Institut of Technology, Atlanta, GA, 30332 ABSTRACT The synthesis of metal nanoclusters in a polymeric environment has been shown to yield nearly monodisperse particles, whose size is controlled by the strength of the polymer/metal interactions. Although the phenomenon is quite general, little is known regarding the mechanism by which the polymer controls nanocluster size. Previous studies of the kinetics of nanocluster growth in polymeric melts above the glass transition temperature (Tg) showed that the nanoparticle size is set by the critical cluster size rather than the rate of metal precursor transport. In this paper we examined the kinetics of ironoxide (Fe2O3) nanocluster formation in two polymer melts below their Tg: polystyrene (PS) and poly(methyl methacrylate) (PMMA). We found that the rate of particle formation is nucleation-controlled for both PS and PMMA, similarly to the same process above Tg, while the growth stage is diffusion limited. Transmission electron microscopy (TEM) revealed needlelike particles morphology with larger average dimensions as the temperature is elevated towards polymer matrix Tg. The stabilizing effect of the polymer chains on the growth of the nanoparticles was dominant when the synthesis was performed in solution or in the melt, allowing the polymer to adsorb on the cluster. However, below its Tg the chains were not free to move and conform to the nanocluster surface, and hence, capping occurred when the cluster growth limitation was imposed by the geometric confines set by the voids in the polymer matrix. INTRODUCTION A common type of nanocomposite materials is that of inorganic clusters dispersed in a polymeric medium.1-5 In-situ synthesis of metal nanosclusters in a polymeric media has been shown to produce clusters with a narrow size distribution and uniform morphology in a wide range of polymer and metal systems 5-10 Cluster size, shape and spatial distribution was found to depend on such parameters as polymer type, polymer concentration and MW, and the temperature of decomposition. 5-8 The fact that conducting the synthesis of many different types of metal clusters in a wide variety of polymers (either solution or in bulk) leads to uniform cluster populations suggests that the control mechanism is not polymer-specific; rather, it should arise from the macromolecular nature of the media. Previous studies suggested that the polymer controls cluster growth by a ‘capping mechanism’, where polymer adsorption on the metal surface hinders transport of precursor atoms and thus limits growth. 8 However, while this mechanism can explain the non-existence of particles that are larger than the optimal size set by the ‘cap’, it cannot address the lack of smaller particles. Most studies of metal cluster synthesis in
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