Diffusion of Bottlebrush Polymer-Grafted Nanoparticles: The Role of Matrix Molecular Weight

Nanoparticle diffusion in a polymer matrix is known to be sensitive to the surface chemistry and size of the nanoparticles, and this study explores the effect of grafted polymers on the nanoparticles. We report the diffusion coefficients of nanoparticles grafted with bottlebrush polymers in melts of linear polymers. These bottlebrush polymer-grafted nanoparticles (PGNs) are comprised of large silica NPs (radius 80 nm) grafted with various backbone degrees of polymerization and fixed polystyrene side chains (4.2 kDa) to give bottlebrush molecular weights of 80–388 kDa. The polymer matrix is polystyrene (100 or 425 kDa), and nanoparticle diffusion was studied at 170 °C. After annealing a trilayer sample, time-of-flight secondary ion mass spectrometry (ToF-SIMS) measured the cross-sectional PGN concentration profiles and extracted diffusion coefficients. The PGNs with high bottlebrush molecular weights relative to the matrix polymer display core–shell diffusion behavior, wherein the shell thickness is comparable to the grafted bottlebrush thickness. Surprisingly, when the bottlebrush molecular weight is smaller than the matrix polymer, the PGNs diffuse 10–100 times faster than predicted, which we attribute to a local decrease in viscosity associated with disentanglement of the matrix polymer near the large nanoparticles.