(73c) Designing the Phase Behavior and Microstructure of Nanocrystal Linker Gels

Gels made from networks of nanocrystals (NCs) linked by physical or chemical bonds are promising candidates for soft materials with tunable optical and rheological properties. A standard route to produce a gel is to rapidly cool a suspension of isotropically attractive particles, with gelation resulting from kinetic arrest during spinodal decomposition. However, such gels are inherently nonequilibrium, and the gel’s properties inevitably change as it ages. Alternatively, “equilibrium” gels with open, homogeneous structures that are resilient to aging can be created by restricting the number of bonds that form between particles. Here, we report on equilibrium gelation controlled by the addition of a secondary “linker” macromolecule that mediates bonding between NCs. The phase diagrams of such mixtures were predicted using Wertheim’s thermodynamic perturbation theory and compared to molecular dynamics simulations. Good agreement was obtained between the predictions and simulations, with the spinodal region depressed to lower NC densities by increased linker length at fixed linker-to-NC number ratio. The simulations further revealed that the linker length modulates the gel microstructure, which in turn controls its optical activity and rheology. We propose a strategy to simultaneously design the phase behavior and microstructure of the gel using blends of linkers with different lengths.