Effect of Graft Density on Structure and Entanglements of Model Polymer-Grafted Nanoparticle Monolayers

Neat polymer-grafted nanoparticles (PGNs) are matrix-free nanoparticle polymer composites, consisting of an inorganic core and surface grafted polymer chains. PGNs on a surface can form robust monolayers with regular interparticle spacing, of interest for a range of applications such as flexible electronic or optical devices. We use coarse-grained molecular dynamics simulations to study chain conformations and nanoscale structure in hexagonally packed PGN monolayers as a function of graft density. We analyze entanglements, including calculating their lifetimes and categorizing by whether they connect different nanoparticles. As graft density increases, we observe decreasing interpenetration of chains on neighboring PGNs, which decreases the number of interparticle entanglements per chain, and causes localization of these entanglements in interstitial regions. We also find local alignment of chains normal to the NP surface at high graft density, which is related to increased intraparticle entanglement density near the surface. We expect that understanding these relationships, and generally connecting experimentally tunable parameters to molecular-scale structure and overall material properties, can provide insight into optimal design of future materials.