The incorporation of nanoparticles into polymers constitutes a powerful strategy for introducing new optical, electrical, and magnetic functionalities into the polymers and for enhancing their mechanical properties. We are working on many different aspects of nanoparticle-polymer composites. Our main focus is on understanding how shaped, polymer-grafted nanoparticles interact with each other and how one could manipulate such interactions to make the particles assemble into higher-order structures relevant to plasmonic, photovoltaic, and shock mitigation applications. A key feature of this work is the use of advanced Monte Carlo methods to compute free energies and phase diagrams relevant to assembly. Our efforts have led to a remarkably simple strategy, involving changes in the length of grafted chains, for tuning the interparticle orientation of silver nanocubes between face-to-face and edge-to-edge configurations (Fig. 4) that has been experimentally demonstrated by the group of Dr. Andrea Tao.
The ability to characterize higher-order structures formed by nanoparticle assembly is critical for predicting and engineering the properties of advanced nanocomposite materials. We are developing an automated quantitative image analysis tool that will allow researchers to analyze electron microscopy images of nanocomposites and obtain a range of structural properties of nanoparticle clusters as they assemble into higher-order structures (Fig. 5). The first version of this software, named particle image characterization tool or PICT, is coded in MATLAB R2012b and is available for download from the MATLAB Exchange Server. To gain further insights into the assembly mechanism, we have developed a computational approach that will allow researchers to recover key dynamic parameters of nanoparticle assembly from the analysis of static, disjointed microscopy images of nanoparticle composites.
In another project, in collaboration with the groups of Dr. Sia Nemat-Nasser and Dr. Zhibin Guan, we are developing new and improved elastomeric composite materials for mitigating and/or redirecting blast-induced stress waves over a range of frequencies and energies. Our group is using a range of computational methods to provide molecular-level insights into energy dissipation and redirection within polymer nanocomposites as a function of polymer architecture and nanoparticle composition. Some of our ongoing work includes developing high-resolution coarse-grained models of polyurea, the polymer currently used for the above application, and examining its viscoelastic properties and shock response (Fig. 6) using equilibrium and nonequilibrium molecular dynamics simulations to dissect the molecular origins of its superior dissipative properties. In a related project, we are collaborating with Dr. Darren Lipomi to develop molecular design rules for the design of bulk heterojunctions for photovoltaics with superior mechanical and electronic properties.