Chemical Engineering Seminar Series: Prof. Nirmalya Bachar

 How to control self-assembly of polymer-grafted nanoparticles

It is now well-accepted that hydrophilic nanoparticles (NPs) lightly grafted with polymer chains self-assemble into a variety of superstructures when placed in a hydrophobic homopolymer matrix or in a small molecule solvent. Currently, it is thought that a given NP sample should only assemble into one kind of superstructure depending on the relative balance between favorable NP core−core attractions and steric repulsion between grafted polymer chains. Surprisingly, we find that each sample shows the simultaneous formation of a variety of NP-assemblies, e.g., well-dispersed particles, strings, and aggregates. We show through the generalization of a simple geometric model that accounting for the distributions of the NP core size and the number of grafted chains on each NP (which is especially important at low coverages) and allows us to quantitatively model the aggregate shape distribution. We also develop a model that builds on the work of Daoud and Cotton to explore a wide parameter space of PGN with dispersity simultaneously in core size and grafted chain length. We demonstrate that dispersity in core size is the dominant factor affecting the self-assembled solution structure of PGN aggregates. I will also present a first principle multiscale model which can capture the evolution of nanoparticle size, which explains how to arrest the particle growth. Our work suggests the importance of focusing on synthetic strategies for control of core-size dispersity to control aggregate structure in PGN.