Grafting Density as a Design Parameter for Dynamics and Flow of Neat Poly(dimethylsiloxane)-Grafted Silica...

Polymer-grafted nanoparticles (PGNs) represent a versatile class of hybrid materials that combine the processability of polymers with the stability and functionality of inorganic fillers. In this work, we synthesized poly(dimethylsiloxane) (PDMS)-grafted silica nanoparticles (NPs) below the entanglement molecular weight (M_e) with controlled graft densities to investigate how grafting density governs viscoelasticity, flow, and thermal response. While previous studies of PDMS-grafted silica have primarily examined behavior in PDMS melts, little is known about the intrinsic properties of PGNs in their neat, matrix-free state. Rheology and time-temperature superposition measurements reveal that while high-grafting-density PGNs behave similarly to unentangled PDMS melts, medium and low-grafting-density systems exhibit viscoelastic-solid dynamics, with the onset of a storage modulus plateau and shear-thinning behavior. The terminal flow relaxation time slows by many orders of magnitude and the activation energy increases 3-fold with decreasing grafting density. This appears to be due to a shift from a dry concentrated brush at high grafting density to an interpenetrated semidilute polymer brush at low grafting density. While the effect of grafting density on flow behavior is dramatic, its effect on polymer chain dynamics is minor, with a slight increase in the PDMS glass transition temperature occurring at high grafting density. These results demonstrate grafting density as a key design parameter for tuning PGN flow, mechanical reinforcement, and thermal stability.