Author: Selemon Bekele

Publisher:

Published: 2018

Total Pages: 0

ISBN-13:

All-atom molecular dynamics simulations have been carried out to study the wetting of model atactic polystyrene thin films by water droplets as a function of surface polarity, the structure and dynamics at the surface of polymer films, the properties of interfacial water molecules in contact with atactic polystyrene surfaces of varying polarity, and the spreading dynamics of water droplets of varying sizes on a completely wetting surface. The simulated contact angle of a water droplet on atactic polystyrene thin films is found to decrease monotonically with increasing degree of surface oxidation which is used as a measure of surface polarity. The number of hydrogen bonds between water molecules and the polystyrene at the interface is found to monotonically increase with polarity. The contribution of the non-dispersion interactions to the interfacial energy at the polystyrene/water interface has been determined as a function of surface polarity. The roughness of the polystyrene surface and the orientational ordering of the surface phenyl rings are found to be independent of surface polarity when the polystyrene is exposed to vacuum. Surface roughness appears to increase and orientational ordering of surface rings appears to decrease slightly with polarity when the polystyrene is in contact with water. Density profiles of polymer films as a function of distance relative to an instantaneous surface exhibit a structure indicative of a layering at the polymer/vapor interface similar to the typical layered structure observed at the polymer/substrate interface. Interfacial molecules at the polymer/vapor interface have a higher mobility compared to that in the bulk while the mobility of the molecules is lower at the polymer/substrate interface. Time correlation of the instantaneous polymer/vapor interface shows that surface fluctuations are strongly temperature dependent and are directly related to the mobility of polymer chains near the interface. Interfacial water molecules on a bare atactic polystyrene surface and those which do not make hydrogen bonds with the oxidized surfaces are found to have a faster dynamics and appear to have a universal water-water hydrogen bond relaxation time of about 5 ps. Diffusion coefficients and the relaxation times of the water molecules involved in hydrogen bonding with the surface show strong dependence on surface polarity with a hydrophobic to hydrophilic transition regime with contact angle in the range 40-50°. The spreading of water droplets on completely wetting surface is characterized by a bulk part of normal density sliding over a monolayer of high density water. The monolayer has a molecular dimension and moves ahead of the bulk part of the droplet. The monolayer motion exhibits two spreading regimes, each following a power law in time. The bulk part of the droplet initially spreads over the monolayer with increasing radius until a characteristic time t* where the monolayer changes from one power law behavior to another. For times after t* ~ 0.1- 0.5 ns, it shrinks while maintaining what appears to be a constant contact angle until it disappears altogether and only a monolayer of water remains on the substrate.