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.

Surface-active particles at immiscible polymer/ polymer interfaces can provide unparalleled stability against droplet coalescence. However, they often deteriorate the fracture toughness (G c ) of the interface because their rigid cores act as stress concentrators. Here, we draw on the knowledge developed for the interfacial strengthening mechanisms of block and random copolymers to design analogous particle-based systems. We use silica nanoparticles "grafted-from" with poly(styrene-r-methyl methacrylate) (PS-r-PMMA) chains to strengthen PS/PMMA interfaces. In this manner, the silica cores suppress droplet coalescence, while the PS-r-PMMA grafts entangle with the homopolymers and transmit stress across the interface. Interestingly, we show that G c for the interfaces compatibilized with these particle brushes can exceed that of the interfaces compatibilized with ungrafted copolymer analogues. Rheology experiments attribute this phenomenon to increased connectivity between the entanglement points in these hybrid particle brush systems.

There has been significant interest in polymer nanocomposites (PNCs) because of their promising property enhancements – however, achieving these improvements are contingent on controlling the nanoparticle (NP) dispersion state in the polymer matrix. The inherent incompatibility between a hydrophilic filler and a hydrophobic polymer is the ultimate barrier for obtaining controlled NP spatial dispersion. We can mediate this unfavorable interaction by incorporating moieties on the chains that interact favorably with the NP surface, but preparation strategies typically used to prepare nanocomposites, e.g., casting from a common solvent or melt blending, frequently result in (far) out‐of‐equilibrium NP dispersion states with the degree of non‐equilibrium character sensitively dependent on the particular preparation technique employed. This is due to the energy landscape of even a simple PNC material being very highly complex with many local equilibria and large barriers. Most theoretical studies and our current understanding are based on the assumption that an equilibrium description of the nanocomposite applies, i.e., the system attains its global minimum state. However, different preparation conditions can place the system in different local basins and the extremely slow relaxation times of these systems to go from the “as cast” state to the equilibrium state imply that nanocomposites can end up in out‐of‐equilibrium conditions, including states where properties might be favorable. These points, which are elaborated in this review, speak to the importance of preparation and annealing conditions in the NP dispersion state and hence the properties of this novel class of materials. This topic is the focus of this chapter.

We systematically investigate the effects of adding ungrafted PMMA versus silica-grafted PMMA (PMMA-g-SiO2) on the crystallization kinetics and the mechanical properties of PEO. In the grafted analogue, PMMA chains were grown from the surface of 14 nm diameter silica nanoparticles at a grafting density of ∼0.42 chain/nm2. The molecular weight of the PMMA was varied systematically (20–85 kDa), and the extent of confinement was regulated by varying the PEO fraction. While the PEO crystallinity and the crystal growth rate in the ungrafted and grafted systems track each other, the grafted systems exhibit enhanced crystallization kinetics, most likely because of faster chain dynamics. Additionally, the grafted systems have higher nucleation rates, especially for shorter PMMA chains. These differences in crystallization kinetics are likely due to the dry-brush zone near the silica surface, where graft chains do not interpenetrate with the matrix. In the context of mechanical properties, both additives enhance the PEO modulus and toughness, with the PMMA-g-SiO2 filled materials undergoing their brittle-to-ductile transition at lower loadings. Surprisingly, there is little difference between the two additives in how they affect PEO crystallization and mechanical propertiesthis is likely due to the high grafting densities where the core is effectively shielded from the PEO matrix.