Mechanism of hierarchical structure formation of polymer/nanoparticle hybrids

Exploiting the assembly of metallic nanoparticles (NPs) in their hybrids with polymers is an important task for creating new properties or functionalities via collective interactions or dynamics of the constituents. In this work, we present a detailed study of the process and mechanism of the reaction-induced hierarchical assembly of NPs in the hybrid wherein the metallic Pd NPs were synthesized and their numbers in the system increased with time by reducing the metal precursor, Pd(acac)₂, originally dissolved uniformly in the solution of poly(2-vinylpyridine) (P2VP) with benzyl alcohol (BA) as a solvent and reduction agent. The time-resolved small-angle X-ray scattering (SAXS) experiment using synchrotron radiation revealed that the structural evolution process from the beginning of NP formation to the establishment of a fractal structure built up by the clusters of Pd NPs was constituted of four distinct stages, Stages 1-4, governed by the increasing overall NP volume fraction (β_overall) and hence by the increasing reduction time. At Stage 1, the NPs were uniformly distributed in the matrix of P2VP and BA with negligible interparticle interaction due to the low particle volume fraction (β_overall ≤ ca. 3 × 10^-4). The structural evolution advanced to Stage 2 when β_overall was increased above 3 × 10^-4. In this stage, the NPs experienced the sticky hard sphere (SHS) type attractive interaction, and as a consequence the NPs underwent a phase separation into a particle-poor phase (Phase I) and a particle-rich phase (Phase II) strikingly even in such a small β_overall. The net interparticle attraction created the dynamic aggregates distributed uniformly within Phase II. As the β_overall continued to increase to ca. 4.7 × 10^-4, the structural development entered Stage 3, in which the dynamic aggregates started to form a higher-order organization, generating a larger-scale heterogeneity in Phase II. The structure developed at Stage 3 served as a precursor directing the subsequent formation of the large-scale mass-fractal network built up by the static clusters of NPs within Phase II at Stage 4. The particles within the clusters were proposed to be bridged by the P2VP chains due to pyridine-Pd coordination interaction.