Compatibilizing Immiscible Polymer Blends with Sparsely Grafted Nanoparticles

The use of nanoparticles (NPs) to compatibilize immiscible polymer blends remains an ongoing challenge requiring a high level of control over the NP dispersion and localization. Here, we show that silica NPs sparsely grafted with long polystyrene (PS) chains are surfactant-like because they permit core-core, core-matrix, and corona-matrix interactions. When placed at an immiscible polymethyl methacrylate (PMMA)-PS interface, the silica core strongly interacts with one component (PMMA), while the corona mixes with the other (PS). These carefully designed NPs are demonstrated to be efficient stabilizers, even outperforming block copolymers. While such surfactant-like behavior is evident, and understood on the basis of existing ideas, a new concept that we leverage is how the shape of the interfacial free energy profile is affected by the surface grafting density. For an optimally chosen grafting density and graft chain length, we find a nearly symmetric free energy profile as a function of the NP contact angle at the interface, ensuring that the NPs are strongly localized in a region where they are the most efficient in terms of stabilization.