Synergistic Effects of Particle Size and Molecular Weight on Chain Entanglements in Single-Polymer Nanoparticles

The polymer dynamics at the nanoscale exhibit significant deviations from their bulk materials, affecting both our fundamental understanding and practical applications of polymers. Despite growing applications and synthesis advancements of 3-dimensional confined polymeric nanomaterials, comprehensive studies of their dynamics remain scarce, mainly due to a lack of suitable characterization techniques. In this study, we employed temperature-controllable dark-field microscopy to visualize the thermally activated segmental and whole-chain motions of individual polystyrene nanoparticles (PSNPs), achieved by quantitatively tracking deformation-induced changes in their optical scattering intensity during heating. Single-particle measurement enables the determination of intrinsic deformation temperatures by eliminating average effects, inevitable interparticle interactions, and the potential influence of thermal equilibrium that commonly exists in conventional bulk measurements. Building on this foundation, a longer confinement length scale for whole-chain motion compared to segmental motion was observed in single PSNPs. This distinct size dependency further suggested that the particle size, in addition to molecular weight, also played a great role in influencing the entanglements within PSNPs. The synergistic effects of these two structural factors on chain entanglement were further mapped, revealing that the rubber plateau of single PSNPs began to narrow when the particle size decreased to approximately 20 times the radius of gyration of the constituent polymer chains. This study presents a platform for imaging the thermally activated chain mobility of 3D-confined polymeric nanoparticles, expanding single-particle scale insights into how nanoscale confinement affects polymer entanglements and providing guidance for their further applications.