Elucidation of chain dispersity effect on structure and mechanical properties of brush particle solids

The modification of nanoparticles with polymer chains has emerged as a versatile platform to enable particle building blocks that can be assembled into functional materials with controlled microstructure and enhanced physical properties, such as modulus, toughness, dielectric strength or thermal conductivity. Often, a combination of high inorganic loading and modulus is desired. This contribution will present brush chain dispersity as a ‘design parameter’ that enables the concurrent enhancement of Young’s modulus and fracture toughness, while also facilitating increased inorganic loadings. Brush particle model systems with continuous variation of the chain dispersity were synthesized using modified atom transfer radical polymerization and films were characterized using a combination of indentation, dynamic mechanical analysis as well as small angle scattering, electron imaging and tomography. Structure factor and image analysis in bulk and thin films reveal that microstructure order remains unaffected even in the limit of high dispersity (D ~ 2). The fracture toughness of brush particle solids is found to strongly increase with chain dispersity. This is interpreted to be a consequence of entanglement formation between high molecular chains in disperse brush systems. Analysis of the craze density in brush materials is used to establish a quantitative link between dispersity and entanglement formation.