Characterizing the Areal Density and Desorption Kinetics of Physically Adsorbed Polymer in Polymer Nanocomposite Melts

The interfacial regions between nanoparticles (NPs) and polymers in polymer nanocomposites (PNCs) underlie enhanced properties, and the temporal stability of these bound polymer layers is necessary for extended control on PNC performance. Using ion scattering techniques and poly­(2-vinyl pyridine) (P2VP) mixed with 26 nm silica NPs, we investigate the lifetime of the bound polymer layer by separating and directly measuring the fraction of free polymer and polymer adsorbed to attractive NPs entirely in the melt state. By annealing thin PNC films deposited on bulk polymer matrices, free polymer from the PNC rapidly diffuses into the underlying matrix while the spontaneously formed bound polymer in the melt remains with the NPs. By correlating the fraction of bound chains with the NP surface area, our analysis shows that bound polymer chains extend ∼R g from the NP surface into the melt. The calculated average NP surface area occupied by adsorbed chains in the melt is much smaller than predicted for an isolated chain or measured in an NP–polymer solution. The bound polymer fraction decreases as a function of annealing time and decreases more rapidly at higher temperatures and for lower molecular weights. This work demonstrates that ion scattering methods can quantitatively measure the chain-scale structure and dynamics of polymers bound to NPs in the melt state. This new information provides fundamental insights and enables the design of PNCs with greater thermal stability during fabrication and use.