Coarse-grained molecular modelling of amphiphilic polymers at a water/air interface

Abstract

Previous atomic-level simulations have been shown to provide invaluable insight into the adsorption behaviour of amphiphilic polymers at a water-air interface. Neutron
reflectivity profiles generated from these simulations showed good agreement with experiment, particularly at low surface concentrations. Unfortunately, previous detailed atomistic simulations have failed to produce adequate results at high surface concentrations due to crowded configurations, which could not relax within the simulation times available. To tackle this problem, a coarse-graining (CG) technique, where the structure of the simulated molecule is simplified to a chain of beads, has been employed in this study. This provides for the simulation of larger time and length scales allowing for a more detailed study of the capture of polymer chains by a surface and the structure of surface layers.
The work presented in this thesis involves development of coarse-grained models for water and for poly(ethylene oxide) (PEO)/water systems, with the aim of reproducing the properties of key importance for the bulk and liquid/vapour interfacial states. These models are then used in the coarse-grained simulation studies of di-and trifluoro dendritic end-capped PEO at an air-water interface; the amphiphilic polymers that have been studied recently by neutron reflectivity experiments.
It is shown in this study that simulation of very large polymer chains comparable to that used in real experiments, is achievable using coarse-grained molecular dynamics.
Neutron reflectivity profiles generated from simulations of di- and trifluoro dendritic end-capped PEO materials at low polymer concentrations are in good agreement with experiment data. Simulations at high polymer concentrations
showed no evidence of a stretched brush structure, in accordance with experimental findings. It is shown from these simulations that there are polymers adsorbed to the interface by a combination of fluorocarbon ends and ethylene oxide segments,resulting in a rather at layer structure. At high surface concentrations of polymers, it proved possible to see the formation of polymer micelles in bulk water. The process of micelle capture by the surface and incorporation of the micelle contents into the surface, were also observed.