Polymer nanoparticle composites (PNC) with ultra high loading of nanoparticles (> 50%) have been shownto exhibit markedly improvedstrength, stiffness, and toughness simultaneously comparedto the neat systems of their components. Recent experimental studies on the effect of polymerfill fraction in these highly loaded PNCs reveal that even at low polymerfill fractions, hardness and modulusincreasesignificantly. In this work, we aim to understand the origin of these performance enhancements by examining the dynamics of both polymer and nanoparticles (NP) undertensile deformation. We perform molecular dynamics (MD) simulations of coarse-grained, glassy polymerin random-close-packed nanoparticle packings with a varying polymerfill fraction. We characterize the mechanical properties of the PNC systems, compare the NP rearrangement behavior, and study the polymer segmental and chain-level dynamics during deformation belowthe polymer glass transition. Our simulation results confirm the experimentally-observed increase in modulusat low polymerfill fractions, and weprovide evidence that the source of mechanical enhancementis the polymer bridging effect.

Nanocrystal gels can be responsive, tunable materials, but designing their structure and properties is challenging. By using reversibly bonded molecular linkers, gelation can be realized under conditions predicted by thermodynamics. However, simulations have offered the only microscopic insights, with no experimental means to monitor linking leading to gelation. We introduce a metal coordination linkage with a distinct optical signature allowing us to quantify linking in situ and establish structural and thermodynamic bases for assembly. Because of coupling between linked indium tin oxide nanocrystals, their infrared absorption shifts abruptly at a chemically tunable gelation temperature. We quantify bonding spectroscopically and use molecular simulation to understand temperature-dependent bonding motifs, revealing that gel formation is governed by reaching a critical number of effective links that extend the nanocrystal network. Microscopic insights from our colorimetric linking chemistry enable switchable gels based on thermodynamic principles, opening the door to rational design of programmable nanocrystal networks.

Optical properties of nanoparticle assemblies reflect distinctive characteristics of their building blocks and spatial organization, giving rise to emergent phenomena. Integrated experimental and computational studies have established design principles connecting the structure to properties for assembled clusters and superlattices. However, conventional electromagnetic simulations are too computationally expensive to treat more complex assemblies. Here we establish a fast, materials agnostic method to simulate the optical response of large nanoparticle assemblies incorporating both structural and compositional complexity. This many-bodied, mutual polarization method resolves limitations of established approaches, achieving rapid, accurate convergence for configurations including thousands of nanoparticles, with some overlapping. We demonstrate these capabilities by reproducing experimental trends and uncovering far- and near-field mechanisms governing the optical response of plasmonic semiconductor nanocrystal assemblies including structurally complex gel networks and compositionally complex mixed binary superlattices. This broadly applicable framework will facilitate the design of complex, hierarchically structured, and dynamic assemblies for desired optical characteristics.

We use simulations to capture ductile-to-brittle transition in glassy oligomer pillars, characterize the effects on mechanical, structural, and dynamical response during deformation. We present quantitative metrics to distinguish failure modes.