Wall-Spring Thermostat: A Novel Approach for Controlling the Dynamics of Soft Coarse-Grained Polymer Fluids at Surfaces

Abstract

The rheological properties of polymer composites depend on the interfacial interactions between solid fillers and a polymer fluid. In highly coarse-grained (hCG) models, where one coarse-grained segment represents multiple monomeric repeat units, the solid surface of a filler appears smooth on the hCG scale. Thus, special simulation techniques are required to control the single-chain dynamics and friction at the solid–fluid contact. We devise a simulation strategy─the wall-spring (WASP) thermostat─where transient bonds are formed between the solid surface and the polymer segments, based on a grand canonical Monte Carlo (MC) algorithm. These transient bonds mimic strong, specific interactions of the polymer segments with the solid. The attraction, induced by the transient bonds, can be compensated with a permanent, analytically known potential such that static properties do not differ from the system without WASPs. The single-chain and collective dynamics of the polymer fluid at the surface can be tailored by the areal density of transient bonds and their lifetime. The WASP thermostat allows us to capture dynamic heterogeneities at surfaces, such as those quantified by the non-Gaussian behavior of the van Hove self-correlation of polybutadiene at silica surfaces, obtained by atomistic simulations. The parametrized hCG model enables us to explore the dynamics of polymers at solid surfaces for a wide range of molecular weights. We study the Navier-slip boundary condition and demonstrate that both the slip length and the position of the hydrodynamic boundary increase like the polymer’s end-to-end distance, Re. Since both lengths are approximately equal, the velocity profile vanishes close to the narrow interface between polymer melt and solid.