Entropic and Enthalpic Control of Interfacial Nanoparticle Jamming in Immiscible Polymers

The localization of polymer-grafted nanoparticles (GNPs) at the interface between immiscible polymers can significantly alter their phase separation kinetics. However, modifying the role of GNP surface activity has not been explored in this context. Using <i>in situ</i> atomic force microscopy and grazing-incidence X-ray photon correlation spectroscopy, we reveal how subtle variations in the entropic and enthalpic interactions between the GNPs and the blend polymers can result in dramatic differences in phase separation kinetics. We use a bilayer geometry of polystyrene (PS, molecular weight ≈ 50 kDa) and poly(vinyl methyl ether) (PVME, molecular weight ≈ 80 kDa), with GNPs bearing PS chains of different molecular weights incorporated in the PS layer. While the GNPs grafted with shorter PS chains (3 kDa) should prefer the interface due to entropic effects, the gold GNP cores have a strong enthalpic preference for the PVME phase (even though the PS grafts themselves do not enthalpically prefer the PVME). As a result, these GNPs do not localize and jam the interface and blends do not show arrested phase separation. In contrast, GNPs grafted with longer PS chains (20 kDa), which do not experience such favorable enthalpic effects with the PVME due to screening of the enthalpic interactions between the core and PVME, are interfacially localized leading to a strong arrest of phase separation. These results establish that the coevolution of GNP segregation and phase separation is the decisive factor in stabilizing polymer blends, offering a new design principle for GNP-polymer composites under nonequilibrium conditions.