[SY-H4] Design principles for high modulus and toughness of assembled hairy nanoparticles
Matrix-free polymer-grafted nanocrystals, called assembled hairy nanoparticles (aHNPs), can significantly enhance the thermomechanical performance of nanocomposites by overcoming nanoparticle dispersion challenges and achieving stronger interfacial interactions through grafted polymer chains. However, effective strategies to improve both the mechanical stiffness and toughness of aHNPs is lacking given the general conflicting nature of these two properties and the large number of molecular parameters involved in design of aHNPs. Here, we propose a computational framework that combines multi-response Gaussian process metamodeling and coarse-grained molecular dynamics simulations to establish design strategies for achieving optimal mechanical properties of aHNPs within a parametric space. Taking poly(methyl methacrylate) grafted to high-aspect ratio cellulose nanocrystals as a model nanocomposite, our multi-objective design optimization framework reveals that the polymer chain length and grafting density are the main influencing factors governing the mechanical properties of aHNPs, in comparison to the nanoparticle size and the polymer-nanoparticle interfacial interactions. In particular, the Pareto frontier marking the upper-bound of mechanical properties within the design parameter space can be achieved when the weight percentage of nanoparticle is above around 60% and the grafted chains exceed the critical length scale governing transition into the semi-dilute brush regime. We show that theoretical scaling relationships derived from the Daoud-Cotton model capture the dependence of the critical length scale on graft density and nanoparticle size. Our established modeling framework provides valuable insights into the mechanical behavior of these hairy nanoparticle assemblies at the molecular level and allows us to establish guidelines for nanocomposite design.