Assembly of copolymer functionalized nanoparticles: a Monte Carlo simulation study

Functionalizing nanoparticles with copolymer ligands is an attractive method to tailor the assembly of the nanoparticles. We use Monte Carlo simulation to demonstrate how the monomer sequence in the grafted copolymers is a tuning parameter to control assembly of nanoparticles, and the shapes, sizes and structures of the assembled nanoclusters. We have studied spherical nanoparticles grafted with AB copolymers with alternating or diblock sequences, and a range of monomer chemistries by varying strengths of like-monomer (A-A and/or B-B) attractive interactions in the presence of either relatively strong or negligible unlike-monomer (A-B) repulsive interaction. In the presence of negligible A-B repulsions the alternating sequence produces nanoclusters that are relatively isotropic regardless of whether A-A or B-B monomers are attractive, while the diblock sequence produces nanoclusters that are smaller and more compact when the block closer to the surface (A-A) is attractive and larger loosely held together clusters when the outer block (B-B) is attractive. In the presence of strong A-B repulsions the alternating sequence leads to either particle dispersion or smaller clusters than those at negligible AB repulsions; for the diblock sequence strong and negligible A-B repulsions exhibit similar cluster characteristics. Additionally, diblock copolymer grafted particles tend to assemble into anisotropic shapes despite the isotropic grafting of the copolymer chains on the particle surface. Particle size and graft length balance enthalpic gain and entropic losses coming from inter-grafted particle contacts and/or inter-and intra-grafted chain contacts within the same grafted particle, and in turn dictates the shape and size of the cluster. For constant graft length and when A-A attractions are stronger than B-B attractions, diblock copolymer grafted particles form long "caterpillar-like" structures with large particle diameters, and short nanowires with small particle diameters. In the dilute concentration regime a small increase in the particle concentration does not change the cluster characteristics confirming that the structure within a cluster is primarily governed by the copolymer functionalization imparting a "valency" to the nanoparticle "atom". This work illustrates how copolymer functionalization and tuning the grafted copolymer sequence could be an exciting new route experimentalists can take to tailor self-assembly of nanoparticles into target nanostructures.