Local Structure and Relaxation Dynamics in the Brush of Polymer-Grafted Silica Nanoparticles

Polymer chains are grafted to nanoparticle (NP) surfaces for a variety of purposes, including altering NP solubility or dispersion within a polymer matrix. At high grafting densities, high molecular weight polymers adopt two primary conformations on the NP surface. Polymer chains near the NP core are stretched in the concentrated polymer brush region (CPB). Farther away from the core, polymer chains are less confined and the conformation becomes more ideal in the semi-dilute polymer brush region (SDPB). Using a combination of small-angle neutron scattering (SANS) and neutron spin echo (NSE) spectroscopy, we directly characterized both the structure and dynamics of the CPB and SDPB on poly (methyl acrylate) (PMA) grafted SiO₂ NPs by selectively deuterating each region separately. Analysis of SANS measurements using a new core-chain-chain (CCC) model confirmed that the portion of the polymer chains in the CPB region are stretched, and transitions to a more ideal conformation in the SDPB region. From NSE, we found the dynamics in the CPB region were found to be much slower than the SDPB region across all length scales, and followed the Zimm model.