Understanding asymmetry effects at low grafting density on the self-assembly of polyion grafted nanoparticles

Grafting of spherical nanoparticles (NPs) with polymeric ligands has been an effective way of controlling the dispersion state of NPs either in the matrix or in solution. Despite the fast evolving synthesis techniques, it is still experimentally difficult to precisely control the position of tethers on the surface of NPs. At low grafting density, due to the surface anisotropy, a wide range of assemblies could be achieved depending on the position of the tethers on the NP surface. This may pose a challenge in interpreting data from scattering and electron micrographs. In the current manuscript, we use coarse-grained molecular dynamics simulations to study the effect of graft angles (positions) on the self-assembly of NPs grafted with oppositely charged polyions at low grafting density driven by coulombic interactions. Our study shows that the NPs grafted with oppositely charged polyions self-assemble into a wide range of morphologies like dimers, rings, strings, coils, tetramers and aggregates depending on the angle between the grafts. Furthermore, the morphologies obtained in the case of semi-flexible and rigid-chains as tethers are more sensitive to the graft positions (angles) compared to that of flexible chains. Molecular dynamics studies carried out at low grafting densities of the system comprising an equimolar mixture of nanoparticles grafted with polyions at varying grafting angles showed that the polyions grafted at low/high grafting angles preferentially interacted with oppositely charged polyion grafts tethered at low/high graft angles.