The Molecular Stiffening of Polymers Ionically Grafted on the Surface of Molecular Clusters

Polymer nanocomposites (PNCs) have been broadly applied in various industries for the synergy of nanoparticles (NPs)' functions and polymers' flexibility; nevertheless, the study of NP/polymer interfacial structures is still rare. Herein, the cation-terminated polystyrene (PS) is ionically associated with a 2.9 nm anionic molecular cluster, {Mo₁₃₂}, and the obtained hairy NPs are applied as model systems for the studies of structures and relaxation dynamics of the absorbed polymer layers. The M _w of PS is systematically varied from 1.1 to 9.8 k, while structural analysis from X-ray and light scattering confirms the morphologies of the fabricated hairy NPs. The grafted polymers show significantly slower segment dynamics up to two orders of magnitude compared to pristine ones due to the surface nanoconfinement effect, and the PNCs demonstrated enhanced glass transition temperatures and mechanical strengths. Moreover, dynamics heterogeneity of the grafted polymers can be probed in broadband dielectric spectrum measurements: The segments close to the {Mo₁₃₂} core exhibit slower relaxation dynamics compared to the segments far from the surface of {Mo₁₃₂} due to the varied confinement condition. This work provides microscopic understanding of the interface structure and dynamics in PNCs and sheds lights on the unique role of ultrasmall particles in mechanical strengthening.