Interfacial properties of oxidized polystyrene and its interaction with water

All-atom molecular dynamics simulations have been carried out to study the wetting of atactic polystyrene (aPS) thin films by water droplets. The effect of oxidation of the aPS surface on the contact angle has been studied as a function of oxygen concentration. Oxidation of aPS has been achieved by randomly replacing with oxygen the ortho and/or meta hydrogens on the aromatic rings within 1 nm of the aPS surface until the desired concentration of oxygen is reached. The simulated contact angle is found to decrease monotonically with increasing degree of oxidation, consistent with recent experimental results. The number of hydrogen bonds between water molecules and polystyrene at the interface is found to monotonically increase with oxygen concentration. By use of a modified Good-Girafalco-Fowkes-Young equation, the contribution of nondispersion interactions, γsl(P), to the interfacial energy at the aPS/water interface has been determined as a function of the degree of oxidation. The values of γsl(P) extracted appear to follow a quadratic dependence on oxygen concentration of the aPS surface. The roughness of the polystyrene surface appears to be independent of oxygen concentration when the polystyrene is exposed to vacuum, and it appears to increase slightly when it is in contact with water. The orientational ordering of the phenyl rings at the polystyrene surface exhibits no dependence on oxygen concentration for polystyrene in vacuum. However, the ordering appears to decrease slightly with increasing oxygen concentration when the polystyrene is in contact with water.