Unusual High-Frequency Mechanical Properties of Polymer Grafted Nanoparticle Melts

We use Brillouin light scattering to characterize the high-frequency mechanical response of polymer-grafted nanoparticle (GNP) melts where the grafted chain molecular weight and grafting density is systematically varied at fixed NP size (diameter, D=16 nm). These measurements parallel recent work (Nano Lett. 2019, 19, 2715) using much larger D=114 nm NPs. The longitudinal sound velocity (and the engineering moduli) in the D=16 nm GNPs follow the effective medium, Wood’s law, in the limit of low NP loadings,φ, but display a steep increase φ>0.05 (shorter grafts). These results are rationalized by the fact that long-enough grafted brushes have a dry zone in the vicinity of the NP core, surrounded by an outer interpenetrated zone. We find that all the modulus results from NPs of different D, but with sufficiently high grafting to ensure the existence of a dry brush layer, fall on an apparently universal curve. These materials have three distinctly different mechanical response regions - the inorganic core, the inner dry region and an outer region which is similar to a neat polymer melt. This conclusion clearly illustrates the critical need to understand the mechanical properties of the dry region if we need to have a quantitative description of these GNP-based systems.