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The design of nanoparticles (NPs) that respond to external stimuli like pH, temperature, and electric or magnetic fields has found immense interest in various fields of nanotechnology like nanomedicine, drug delivery, and cancer therapy. Nanoparticles grafted with polymeric ligands have been extensively used as building blocks in the directed self assembly of nanoparticles. These moieties not only assemble into various morphologies but also respond to a wide range of external stimuli. In this work, we have used coarse grained molecular dynamics simulations to understand the stimuli-responsive behavior of assemblies of NPs grafted with oppositely charged polyions (PGNs) in the presence of salt and polyelectrolytes. At low grafting density, a transformation from ring morphology to form dimers/strings/dispersed NPs was observed upon addition of divalent/trivalent salts. NPs grafted with longer grafts showed higher stability to remain as rings compared to shorter grafts. The change in NP morphology was a direct consequence of preferential interaction of the polyaion grafts with the oppositely charged salt ions compared to the oppositely charged grafts on the NPs. At fixed salt valency, the size of the salt ion, concentration and molecular connectivity played a crucial role in the stimuli responsive behavior of polyion grafted NPs in solutions. Further, in the presence of polyelectrolytes, these transitions occurred at lower monomer valency due to the stronger electrostatic interactions between the grafted chains and oppositely charged free polyelectrolytes in solutions. Disordered and ordered aggregates assemblies formed at higher grafting density were broken into smaller NP assemblies in the presence of salt. Drug encapsulation studies in the presence of salt and polyelectrolytes were performed on model drug moieties in order to demonstrate the potential use of the modelled stimuli responsive nanoparticle assemblies in drug delivery applications.

Relative surface affinity between polymers and colloids is leveraged in many applications like filtration, adhesion, bio-sensing, etc. The surface affinity is governed by both enthalpic (relative interactions between the species and surface) and entropic (excluded volume) effects. Neglecting enthalpic effects, i.e. for purely athermal systems, entropy is the only driving force that controls the surface affinity of the species in a binary or multi-component mixture. Many intensive (relative size of colloids, chain length, equilibrium bond angle, chain flexibility) and extensive (confinement, temperature) factors can dramatically change the entropy of the system and thus enhance or decrease the surface affinities of the constituent species. In this article, we use coarse grained metropolis Monte Carlo simulations to delineate the role of these factors in entropic surface segregation in a binary mixture of polymers and colloids. At low number densities, excluded volume effects are negligible and we do not observe any entropic driven surface segregation. Therefore our system of interest is binary polymer–colloid mixtures at moderate to high number densities where excluded volume effects are predominant. Our results indicate that for flexible polymer chains, the surface is always enriched with colloids compared to polymers and this effect is enhanced for longer polymer chains. The configurational entropy of the flexible polymers is significantly reduced near the surface and therefore they prefer to stay in the bulk (away from the surface). However this behavior can be completely reversed by introducing a large degree of confinement and making the chains relatively rigid (less flexible). Our results show that polymer segregation of long stiff chains in slit pore geometry is driven by nematization near the surface while looping of polymers is observed under a large degree of confinement. We observe that for longer polymer chains with an equilibrium bond angle (θ = π), both confinement and chain stiffness enhance the surface segregation of polymers relative to colloids. However, the segregation behavior within a confinement is dependent on the polymer chain length. The surface segregation of polymers is dramatically decreased for chains with equilibrium bond angles independent of chain length and flexibility due to excluded volume effects and inefficient packing near the surface.