Non-equilibrium processes in polymer colloids
We have interests in the physical properties of colloids, with particular expertise in non-equilibrium processes, such as drying.
Colloids, consisting of particles in a fluid, are used to deposit layers for a wide range of applications, such as adhesives, inks, pharmaceuticals, agricultural treatments, cosmetics and paints. The Group has more than two decades of experience in studying the multi-step process by which colloidal polymer particles in water create a dry polymer film. This process, which is called latex film formation, is illustrated below.
When water evaporates, the average distance between particles decreases, until the particles are close packed. Under the right conditions, mono-sized particles will pack into a face-centred cubic (FCC) structure. A reduction in the surface energy in combination with capillary forces generated by the water menisci in between the particles provide a driving force for the spherical particles to be deformed.
In an FCC structure, each particle has 12 neighbours, and the 12 contact points are flattened over time. If the particles are “soft” enough, they will eventually fill all available space when they create rhomboid dodecahedra. When the voids between the particles disappear, light is no longer scattered from the interfaces, and the optical clarity of the film increases. Cross-sections of a latex film sometimes show the hexagonal array of particles in the (111) plane deformed to create a honeycomb pattern. Over time, polymer chains diffuse across the particle boundaries and a uniform film results.
This is a confocal optical microscopy image of a dried coating made from a mixture of large particles (fluorescently labelled red) and small particles (labelled green). The resolution of the image does not enable individual particles (on the order of 100 nm) to be seen. However, it is apparent that the large and small particles have segregated into layers, with the small green particles forming a layer on the top. We find similar structure formation in the computer simulations, illustrated to the right. We found that swelling the small particles caused the particles to jam together early in the process, preventing the segregation into layer.
This pair of computer simulation snaphots show the region of a drying coating just below the descending water/air interface - as evaporation proceeds the water/air interface drops pushing the particles before it. The model coating contains a mixture of small (gold) and karge (red) particles. In the left-hand snapshot taken early in drying, the large and small particles are still mixed - as they were at the start of drying. In the right-hand snapshot taken later on, the large and small particles have stratified into a layer of small particles on top of a layer of mixed small and large particles.
This nanocomposite film was created from core-shell particles in which the shells crosslinked during film formation. The shells created an elastic network that extended through the film. In this AFM phase image, the elastic shells (appearing bright) create a honeycomb network.