Revealing the Differences Between Ultrasmall Nanoparticles and Solvents upon Mixing with Entangled Polymers

Ultrasmall nanoparticles (NPs) exhibit solvent-like characteristics, such as induced disentanglement. However, previous studies have observed that adding ultrasmall NPs can either slow down or not affect polymer segmental relaxation. Therefore, the similarities and differences between ultrasmall NPs and solvents when mixed with entangled polymers remain elusive. This study used polyhedral oligomeric silsesquioxane (POSS) NPs with a diameter of 3 nm as model ultrasmall NPs. Using single-molecule fluorescence tracking and microrheology measurements, we investigated their effects on the diffusion and relaxation of polyethylene glycol (PEG) relative to solvents and large silica NPs. Furthermore, coarse-grained molecular dynamics (CGMD) simulations were employed to elucidate the underlying mechanisms. Compared to solvents, we found that athermal POSS NPs more effectively promote chain disentanglement while exhibiting a negligible plasticizing effect. In addition, ultrasmall NPs retain their particulate characteristics and can form an NP network at high volume fraction to constrain the movement of polymer chains─a distinct effect compared to that of solvent molecules. We expect these findings to provide insight into the role of ultrasmall NPs in entangled polymer composites.