Morphologies of polymer chains adsorbed on inorganic nanoparticles in a polymer composite as revealed by atomic-resolution electron microscopy

Polymers adsorbed on nanoparticles (NPs) play essential roles in determining the physical properties of polymer nanocomposites. Herein, polymer chain structures (morphologies) on NPs and their determining factors were investigated. A model polymer composite comprising a polystyrene (PS) matrix, gold NPs (GNPs), and a small amount of poly(4-iodostyrene) (P4IS) was prepared. The polymer chain morphologies of P4IS adsorbed on GNPs inside the PS matrix were visualized by using annular dark-field scanning transmission electron microscopy, which enabled us to observe clearly the iodine atoms in P4IS. This observation revealed that the P4IS polymer chains were adsorbed on GNPs with a unique morphology wherein 1–2 molecular layers of P4IS surrounded a GNP, and the rest of the P4IS was aggregated at the side of the GNP. The P4IS aggregates were estimated to be composed of single polymer chains. Adsorption of P4IS onto GNPs in a PS matrix was determined to be energetically viable via molecular dynamics simulations. Furthermore, the preferred morphologies of the P4IS polymer chains on the GNPs were analyzed based on the conformational entropy of the polymer chains. The characteristics of polymer chains adsorbed on inorganic NPs were clarified in this study, and they are essential in controlling the performance of polymer nanocomposites at the molecular level. The polymer chain morphologies of poly(4-iodostyrene) (P4IS) adsorbed on gold nanoparticles (GNPs) inside a polystyrene matrix were visualized using annular dark-field scanning transmission electron microscopy. The P4IS polymer chains adsorbed on GNPs exhibited a unique morphology wherein 1–2 molecular layers of P4IS surrounded a GNP, and the rest of the P4IS was aggregated at the side of the GNP. Enthalpy gain for the surrounding layer via adsorption and entropy gain for the remaining polymer chains via aggregation was considered the reason why the morphology was preferred.