Non-Equilibrium Phase Behavior of Immiscible Polymer-Grafted Nanoparticle Blends

Preferential attraction of polymer chains to the substrate [i.e., poly(methyl methacrylate) (PMMA) on the hydroxyl-terminated Si substrate] typically results in the initial phase separation of spin-cast immiscible linear polymer blends [i.e., polystyrene (PS)/PMMA] with a characteristic interfacial microstructure depending on their molecular weight. A formation of the undesired microstructure in those blends is inevitable because of the thermodynamic force driving their phase separation combined with relatively rapid dynamics in solution. In contrast, the polymer ligands, which are grafted from nanoparticles, are capable of limited segmental interactions in the presence of segmental contacts of chemically distinct chains as well as hindered mobility by interpenetration (or entanglement) and thus exhibit a homogeneous but non-equilibrium phase behavior. Here, the microstructure of the blends consisting of immiscible polymer-grafted nanoparticles (PGNPs) was identified by their neutron reflectivity, in which the scattering length density was controlled by tethering deuterated PS on silica NPs. We demonstrate that the single-phase homogeneous microstructure is attributed to a significantly reduced particle dynamics arising from the cooperative motion (or interpenetration) of polymer ligands during vitrification of PGNP films despite a relatively high degree of segregation N chi(S/MM)(A). Furthermore, the slower segmental interactions of polymer ligands promote the thermal stability of the PGNP blends in the kinetically quenched non-equilibrium state. This suggests a crucial role of polymer ligands to determine the relevant properties relying on their microstructures in a wide range of blending approaches utilizing nanoparticles and polymers.