Study of the equilibrium swelling of poly(methyl methacrylate-co-n-butyl methacrylate) immersed in water via MD simulation

Fully atomistic molecular dynamics (MD) simulations are employed to study the swelling behavior of poly(methyl methacrylate-co-n-butyl methacrylate) (P(MMA-co-n-BMA)) immersed in water. The chemical potential of water is calculated using the thermodynamic integration method to estimate the equilibrium swelling (ES) point. The solvation free energy, radial distribution functions (RDFs) and water structures are investigated to study the swelling process. The simulation results indicate an average water uptake of approximately 26.96 wt%. The van der Waals (VDW) interaction and carbonyl oxygens play critical roles in the swelling process. Hydrophilic-hydration water preferentially forms and binds with carbonyl oxygen atoms through hydrogen bonding when water molecules permeate into P(MMA-co-n-BMA). As the water content increases, hydrophobic-hydration water and intermediate water occupy more of the interior space away from the carbonyl oxygens. The amount of hydrophilic-hydration water remains constant once the swelling reaches equilibrium, which implies the saturation of the hydrogen bonding interaction between the carbonyl oxygen atoms and water. The results are significant for the structural design of nanocapsule shells for the controlled release of antifoulant.