Poly(ethylene oxide) (PEO) hydration is an important factor determining brush properties and application efficacy. Using atomistic molecular dynamics simulations, we investigate hydration, static, and dynamic properties of PEO brushes on gold surface as functions of grafting density, σ. We found that at lower grafting densities PEO is well-hydrated except for the surface vicinity where we observe PEO adsorption substituting for the water layering. In pure THF or mixed THF/water solvent, there is no PEO adsorption on gold surface with water preferentially residing inside the PEO layer and THF forming a thin layer near the gold surface. In aqueous solutions, an increase in the grafting density results in PEO stretching away from the surface, the loss of hydration, and a steplike density profile with the brush height satisfying the expected σ1/3 scaling. We show that PEO hydration in the brush is the same as in aqueous solutions of the corresponding water content. While the residence time of hydrogen-bonded water increases, the hydration shell and PEO tail mobility are found to decrease substantially with an increase in grafting density, indicating that these properties associated with protein inhibition capability may be compromised in a dense brush.
Polymers hydrogen-bonding with solvent represent an important broad class of polymers, properties of which depend on solvation. Using atomistic molecular dynamics simulations with the OPLS/AA force field we investigate the effect of hydrogen bonding on PEO conformation and chain mobility by comparing its behavior in isobutyric acid and aqueous solutions. In agreement with experimental data, we found that in isobutyric acid PEO forms a rather rigid extended helical structure, while in water it assumes a highly flexible coil conformation. We show that the difference in PEO conformation and flexibility is the result of the hydrogen bond stability and overall solvent dynamics near PEO. Isobutyric acid forms up to one hydrogen bond per repeat unit of PEO and interacts with PEO for a prolonged period of time, thereby stabilizing the helical structure of the polymer and reducing its segmental mobility. In contrast, water forms on average 1.2 hydrogen bonds per repeat unit of PEO (with 60% of water forming a single hydrogen bond and 40% of water forming two hydrogen bonds) and resides near PEO for a noticeably shorter time than isobutyric acid, leading to the well-documented high segmental mobility of PEO in water. We also analyze PEO conformation, hydrogen bonding and segmental mobility in binary water/isobutyric acid solutions and find that in the phase separated region PEO resides in the isobutyric-rich phase forming about 25% of its hydrogen bonds with isobutyric acid and 75% with water. We show that the dynamics of solvation affects the equilibrium properties of macromolecules, such as conformation, and by mixing of hydrogen bond-donating solvents one can significantly alter both polymer conformation and its local dynamics.
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