Interfacial Behavior of PNIPAM Brushes under Shear and Thermal Stimuli: The Role of Grafting Density.

The interface between grafted poly(N-isopropylacrylamide) (PNIPAM) and fluid plays an important role in drug delivery, responsive nanomaterials, and separation technologies. However, under external shear, the transport at the interface is regulated by both the thermoresponsive behavior and grafting densities. This study combines equilibrium and nonequilibrium molecular dynamics simulations to investigate the synergistic effects of grafting density, temperature, and shear flow on the structural and dynamic properties of PNIPAM brushes in methanol-water solutions. The results reveal that higher grafting densities promote the formation of rigid and stable polymer matrices, which, in turn, exhibit reduced sensitivity to thermal and shear stimuli. This behavior can be attributed to the structural transitions occurring at the polymer-fluid interface, which are primarily driven by hydrogen-bond reorganization. Specifically, water forms structured yet temperature-sensitive hydration shells, whereas methanol forms disordered but thermally stable solvation layers due to steric and dynamic effects. The interplay between temperature and polymer chain dynamics further influences the system's behavior. Increasing the temperature enhances polymer chain mobility, which reduces the effective viscosity of the fluid. However, a higher grafting density counteracts this effect by preserving interfacial viscosity under shear conditions. These findings offer molecular-level insights into solvent-polymer interactions at complex interfaces and inform the design of PNIPAM-functionalized materials with tunable thermoresponsive and flow-adaptive properties for advanced applications.