Selective Interfacial Interaction between Diblock Copolymers and Cobalt Nanoparticles

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David, Kasi
Tannenbaum, Rina
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In order to optimize the synthesis of metal nanoparticle-polymer systems, there are certain processes which must be understood. Perhaps the most important one is the selective interfacial interaction between the block copolymer and the growing metal nanoparticles. To investigate this interaction, four different approaches were taken. The first approach looked at the strength of interaction between the competing blocks of the copolymer and the metal nanoparticles surface. The second approach looked at the effect of polymer architecture on the metal nanoclusters. The third approach looked at the polymer composition and solvent effects on the phase behavior of the metal nanocluster-block copolymer nanocomposite. Finally, the influence of the metal precursor on the rate of the decomposition was examined. It was found that adsorbed layers of PS on the cobalt nanoparticles are completely displaced by PMMA when the solvent is a common good solvent. An adsorbed layer of only PMMA is also obtained through competitive adsorption from a common good solvent. However, in a selective solvent that is poor for PS, sequential adsorption leads to the formation of mixed layers. In homopolymer solutions, the cluster size reaches a minimum at a finite chain MW. In the case of diblock copolymers, the only parameter (for a fixed copolymer concentration) controlling the cluster size in suspensions of di-block copolymers is the molecular weight of one block, in this case PMMA, and is indifferent to other parameters including the molecular weight of the other block (PS) or the solvent quality. It was also found that the spatial distribution of the metal clusters synthesized in-situ coincided with the morphology dictated by thermodynamically-driven microdomain structure of the block copolymer. Moreover, the overall final morphology of the nanocomposite is locked into place while in solution, and fast solvent evaporation does not cause this morphology to change. Finally, results showed that the rate of nanocomposite synthesis occurred faster in the PS suspensions compared to PMMA, indicating that chemical bonds between PMMA and the cobalt nanoclusters slowed the thermal decomposition of the metal precursor. So the PMMA chains provided sites for nucleation, but did not necessarily aid particle growth.
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