Unexpected strengthening effect in brush particle-based hybrid materials with intermediate grafting density*

Surface functionalization of nanoparticles with polymers has emerged as a versatile platform to enable hybrid building blocks that can be assembled into functional nanocomposites with controlled microstructure and enhanced mechanical properties. However, the dense functionalization by conventional method presents a barrier to the utilization of this concept due to the limited inorganic content. This contribution will present a systematic analysis of the role of brush architecture on the elastic properties of brush particle films. Material systems with continuous variation of the grafting density as well as bimodal graft architectures were synthesized using modified atom transfer radical polymerization and films were characterized using indentation and thermal analysis. The results reveal that chain interdigitation plays a dominant role for the modulus and strength of the films. In particular, the results demonstrate an ‘optimum degree of functionalization’ to maximize cohesive interactions as a consequence of the enhanced dispersion interactions in interdigitated brush particle assemblies. The results are used to postulate design guidelines for the synthesis of brush particles capable of forming hybrid materials with enhanced performance.