Spontaneous Phase Separation in Polymer Solution by Janus Nanoparticle Compatibilizers: An Atomistic Molecular Dynamics Study.

We perform atomistic molecular dynamics simulations in order to investigate the structural and dynamic properties of Janus gold nanoparticles within diverse solvent environments, specifically pure polyethylene, pure water, and mixed polyethylene/water matrices. The nanoparticles, approximately 7 nm in diameter and almost totally surrounded by (211) surfaces at the thermodynamic limit, are functionalized with hydrophobic polyethylene chains on one hemisphere and hydrophilic poly-(ethylene oxide) chains on the other. In mixed systems, the Janus nanoparticle acts as an effective stabilizer, directing and stabilizing phase separation in the solvent, where water and polyethylene concentrate around their respective preferred functionalized hemispheres. The resulting interface between the two matrix components was measured at approximately 0.82 nm in thickness. The study characterizes these systems through density profiles, radii of gyration, and second-rank bond order parameters. Regarding the local dynamics of the polymer matrix, the simulations reveal that while polyethylene chains maintain a random orientation in the bulk, they adopt a predominantly parallel orientation relative to the nanoparticle surface when within a distance of 3.2-3.9 nm. Additionally, while the radius of gyration of matrix chains remains stable across systems, the relative shape anisotropy increases significantly in the mixed environments. These molecular-level insights into structural and dynamic organization underscore the potential of gold Janus nanoparticles to control interfacial behavior and stabilize immiscible blends in nanotechnology and biomedicine.