Structure and Dynamics of Polyethylene Glycol Polymer Brushes Revealed by All-Atom Molecular Dynamics Simulations

All-atom molecular dynamics (MD) simulations of polymer brushes composed of polyethylene glycol (PEG) chains grafted onto a planar substrate were conducted to investigate the structure and dynamics of PEG brushes in both the dried and hydrated states. In the dried state, MD simulations of a typical crystalline polymer brush, such as polyethylene (PE), were also performed for comparison at a high temperature that maintains the melt state of both PEG and PE. The results revealed that the dried PEG brush predominantly adopts a looped structure rather than a stretched configuration, primarily due to the gauche effect of the PEG chains. The effects of grafting density and degree of polymerization on the brush structure are also discussed. In the hydrated state, the looped structure observed in the dried state is largely preserved. However, the freely moving terminal atoms of PEG chains, which are stabilized near the substrate, are partially pulled toward the water phase due to hydrogen bonding with water molecules. The diffusion dynamics of the terminal atoms in the substrate-facing region of the PEG brush are somewhat restricted due to brush packing, whereas those in the water-facing region are enhanced as the terminal atoms penetrate into the water phase.