Tunable nanopatterns formed by polymer brushes

Abstract

Tunable nanopatterns formed by polymer brushes A molecular dynamics simulation study of polymer brushes is presented. When exposed to a poor solvent, or when dried out in air, polymer brushes can undergo “constrained dewetting” and collapse into a nanopatterned layer. By changing the chemical environment of the polymer, the pattern can be switched on or off reversibly. Coarse-grained molecular dynamics was used to investigate the morphology and dynamics of these nanopatterns, and their influence on fluid flow and nanoparticle formation. Evaporation of a good solvent film from polymer brushes with a range of grafting densities was simulated in order to study the pattern morphology as a function of solvent content. The pattern type, dynamics, and size and number of features depended on the total amount of adsorbed material, including polymer and solvent. The result suggests the possibility for the use of polymer brushes as surfaces with reversibly tunable nanopatterns. Nonequilibrium molecular dynamics simulations were used to study the boundary condition of fluid flow over a polymer brush. In a good solvent the stagnation length for flow within the brush scaled with the chain spacing D as D-2/3, while the physical height scaled as D2. In a poor solvent the patterns formed by constrained dewetting created variations in the liquid velocity near the surface, suggesting a possible application as a switchable nanofluidic mixer. A simple model relates the boundary condition at the patterned surfaces to the height and polymer surface coverage. The patterns formed by constrained dewetting could be used as a template for surface nanoparticle formation to create surfaces with useful optical and electronic properties. Simulations showed that when a solution of molecular species was evaporated from a polymer brush, the constrained dewetting of the polymer could impart control over the nucleation of nanoparticles. The nanoparticle size and distribution depended on the concentration and solubility of the dissolved species.