| Summary: | 博士 === 國立清華大學 === 化學工程學系 === 103 === Using small angle X-tray scattering (SAXS), we elucidated the spatial organization of palladium (Pd) nanoparticles (NPs) in the polymer matrix of poly(2-vinylpyridine) (P2VP) and the nature of inter-nanoparticle interactions, where the NPs were synthesized in the presence of P2VP by the reduction of palladium acetylacetonate (Pd(acac)2). The experimental SAXS profiles of the hybrids formed after the completion of the reduction were analysed on the basis of a hierarchical structure model considering the following two types of interparticle potential: (i) hard-core repulsion only (i.e., the hard-sphere interaction) and (ii) hard-core repulsion together with an attractive potential well (i.e., the sticky hard-sphere interaction). The corresponding theoretical scattering functions, which were used for analysing the experimental SAXS profiles, were developed within the context of the Percus-Yevick closure and the Ornstein-Zernike equation in the fundamental liquid theory. The analyses revealed that existence of the attractive potential well is indispensable to account for the experimental SAXS profiles. Moreover, the morphology of the hybrids was found to be characterized by a hierarchical structure with three levels, where the primary NPs (level one), formed local clusters (level two), and these clusters aggregated to build up a large-scale mass-fractal structure (level three). The scattering function developed here is of general use for quantitatively characterizing the morphological structures of polymer/NP hybrids and in particular for exploring the interaction potential of the NPs on the basis of the fundamental liquid theory.
The time-resolved SAXS experiment using synchrotron radiation was further performed to reveal the mechanism of the hierarchical structure formation. We revealed that the structural evolution from the beginning of NP formation to the establishment of a fractal structure by the clusters of Pd NPs constituted of four distinct stages governed by the overall NP volume fraction (overall). At Stage (1), the NPs were uniformly distributed with negligible interparticle interaction due to low particle concentration (overall(1) ≤ 0.04). The structural evolution advanced to Stage (2) when overall was increased above 0.04 by chemical reduction. In this case, the NPs distributed over the matrix phase experienced the sticky hard sphere (SHS) interaction, and the hybrid was still located in the one-phase regime. Stage (3) was accessed as overall continued to increase to ca. 0.135; in this case, the hybrid underwent a phase separation through spinodal decomposition, yielding a particle-richer phase and a particle-poorer phase. When the NP volume fraction in the particle-richer phase exceeded ca. 0.2, the structural evolution entered Stage (4), where several NPs assembled to form local clusters driven by depletion interaction, and the clusters further aggregated to build up a fractal network with the mass fractal dimension of ca. 2.3.
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