Coupled Dewetting and Phase Separation in Thin Film Binary Mixtures

This short project will initiate collaboration with Russell Composto at the University of Pennsylvania. We share a common interest in the evolution of microstructure in polymer materials. In addition to fostering UK/US links in polymer science, we will use the visit to plan a future collaborative research program in phase separation and dewetting in multi-component polymer mixtures. The collaboration will permit rapid and significant advances in the specific topic of phase separation and dewetting in thin-film mixtures. Our aim is to develop improved models and design new experiments that will enhance our ability to optimise processing conditions to achieve desired material properties. Polymeric thin films are being increasingly utilised in advanced materials applications, ranging from adhesives to plastic electronics. For many technologies, the thin films of interest are typically multi-component polymer blends. New properties, beyond those of the components, often arise due to the existence of a microstructure, which may have an associated length-scale, or even multiple length-scales, ranging from nanometres to microns. One method of forming microstructures is a step change in temperature that causes an initially miscible blend to become immiscible. The dynamics of the resultant phase separation process control the structures that develop. For example, the blend may phase separate spontaneously into a co-continuous structure with a preferred length-scale dominating, a process known as spinodal decomposition. The initial 'spinodal' length-scale can be controlled by the extent to which the temperature is changed; the greater the change the finer the length-scale.In thin films, another significant factor is whether a film spreads or forms isolated droplets on a surface. This depends on whether the interactions between the surface and the film are favourable. The strength of these interactions can also be controlled by temperature changes. At one temperature a film may favour being spread over a surface, whilst at a different temperature it may prefer to form droplets. The process by which a spread film becomes isolated droplets after a change in temperature is known as dewetting. The remarkable patterns of undulations in height that develop during dewetting also impact upon the microstructure of the film. The use of nanoparticles is also attracting interest, particularly to improve properties such as toughness, impermeabilty to gases and flame retardancy. Although devices based on blending functional nanoparticles with multiphase polymer blends are of increasing interest, control over their microstructure and properties is not yet possible because of a lack of understanding over the role of nanoparticle dispersion. The Composto group has been at the forefront of experiments aimed at developing an understanding of the combined processes of phase separation and dewetting in thin-film binary mixtures, and the PI has developed the first model to address the problem from a theoretical viewpoint. Recently, the Composto group has shown that their addition can result in microstructure evolution in blends being frozen due to 'jamming' the interface. In this project, using our collective expertise, we will consider the extent to which the existing model is able to explain recent observations, the ways in which the model can be improved, how it can be extended to describe the consequences of adding nanoparticles, and what new experiments will best test the new theories.