Controlling nanoparticle aggregation <i>via</i> gel point shift in the <i>in situ</i> photochemical synthesis of plasmonic epoxy-based nanocomposites

Plasmonic metal/polymer nanocomposites are hybrid materials that integrate the intrinsic properties of metal nanoparticles (NPs) with the mechanical strength and thermal stability offered by the polymer matrix. These materials can be fabricated using <i>in situ</i> photochemical synthesis, in which both the metal NPs and the polymer matrix are generated simultaneously in a one-pot process. While this approach is rapid and simple, the ability to control the organization of particles within the polymer matrix remains a challenge. This is a key factor for the preparation of plasmonic materials with well-tuned properties. In this study, we demonstrate that the gel point of a crosslinking epoxy matrix can serve as an effective tool to control the aggregation process of <i>in situ</i> generated silver NPs. The epoxy matrix was formulated as a copolymerization reaction of a diepoxy monomer (diglycidyl ether of bisphenol A, DGEBA) with a monoepoxy monomer (phenyl glycidyl ether, PGE). By varying the DGEBA/PGE ratio, we were able to precisely shift the gel point of the matrix, during the simultaneous formation of silver NPs. This behavior was monitored using dynamic rheology. SAXS analysis, combined with UV-visible spectroscopy and transmission electron microscopy (TEM), allowed us to demonstrate that shifting the gel point toward higher conversions leads to a dimerization process of primary silver NPs, resulting in a significant increase in the plasmonic response of the material. Based on the experimental evidence presented, the dimerization mechanism is discussed.