Mechanical Properties of Glassy Brush Particle Solids Featuring Chain Dispersity

The modification of nanoparticles with polymer chains has emerged as a versatile platform to enable hybrid particle building blocks that can be assembled into functional materials with controlled microstructure and enhanced mechanical or physical properties. Often, a combination of high inorganic loading and mechanical strength is desired. Brush chain dispersity is shown to be an effective design parameter to balance Young's modulus, fracture toughness, and inorganic loading with synthetic versatility. Poly(methyl methacrylate) brush particle model systems with similar grafting density but varied chain dispersity in the range of 1.1 < Đ < 2 were synthesized by varying catalyst concentration in surface-initiated atom transfer radical polymerization (SI-ATRP). Tensile testing revealed an increase of the toughness of brush particle solids with dispersity, whereas the elastic modulus was unaffected. The increased fracture resistance could be attributed to the positively skewed molecular weight distribution of grafted chains, which promoted chain entanglement and craze formation even for low-molecular weight brush chains (i.e., with an average degree of polymerization below the entanglement limit). This renders the "toughening effect" of chain dispersity particularly beneficial for improving the durability of low-molecular brush particle solids (i.e., brush particle solids with high inorganic content) that are susceptible to fracture.