Exchange of the Bound Polymer Layer on Silica Nanoparticles

It is now commonly accepted that a bound polymer layer (BL) naturally forms when a polymer melt is mixed with nanoparticles in the limit of favorable interactions. What is unclear is the temporal persistence of this BL – its very name implies that this layer is expected to be irreversibly adsorbed. We use contrast variation methods in conjunction with small angle neutron scattering to probe this issue in the canonical case of poly(2-vinylpyridine) mixed with 14 nm diameter silica nanoparticles. We find that there is essentially no long-term reorganization of the bound layer at 150 °C, but apparently a rapid reduction of the BL thickness at 175 °C. We believe that the dramatic temperature dependence arises from the polyvalency of the binding of a P2VP chain to a NP – that is the fact that each P2VP chain is adsorbed to the NP through multiple monomers. Thus, while the adsorption-desorption process of a single segment is an activated process that occurs over a broad temperature range, the cooperative nature of requiring multiple segments to desorb converts this into a sharp process that occurs over a relatively narrow temperature range.