Ternary phase separation during spin casting

Lead Research Organisation: Durham University
Department Name: Chemistry


Spin coating is a widely used tool in the manufacture of very smooth and very thin polymer films. For example, it is routinely used on a massive scale for making integrated circuits and colour filters for liquid crystal displays. It is likely to be a centrally important processing step in the emerging field of polymer electronics. The most common situation is the spin coating of a polymer solution (a polymer dissolved in a solvent). A flat disc, which has been flooded with the solution, is rotated at speeds of up to 10,000 revolutions per minute. This results in a uniform coating of solution. The solvent then rapidly evaporates to leave a very smooth and high quality film of pure polymer. A more complicated situation, that has great technological potential, occurs when two polymers are spun from a common solvent. Most chemically different polymers do not like to mix, but if diluted enough in a solvent favourable to both polymers a mixed solution forms. However, the two polymers will separate from each other as the solvent evaporates. Significantly, this can result in a wide variety of microstructures in the final films. These can range from two well defined layers to situations in which lateral phase separation takes place on a variety of length-scales, from nanometres to many micrometres. In thin films of, for example, semiconducting polymers used in light emitting diodes and photovoltaic devices, the device efficiency depends critically on the microstructure. Hence, if one understands how to control the microstructure, one can achieve an optimised film in a single, inexpensive processing step.


10 25 50
publication icon
Souche M (2009) Phase equilibria in polymer blend thin films: a Hamiltonian approach. in The Journal of chemical physics

publication icon
Souche M (2009) Interfacial instability in bilayer films due to solvent evaporation. in The European physical journal. E, Soft matter

Description Blending different polymers is an excellent way to produce materials with new properties and is used to make products ranging from car tyres to composites for aerospace applications. Typically different polymers do not like to mix at a molecular level with phase separation taking place during processing. If the phase separation can be controlled to result in structures with appropriate length scales, the resultant material can be designed to have properties that are greater than the sum of the components. Over the past few years, there has been an increase in the use of blends of conducting polymers as the active layers in both organic LEDs and photovoltaics. In these applications, the blend is often a thin-film, being of the order of 100 nm thick. Since the structures that arise from phase separation are typically of the order of tens of nanometers, surface interactions play an important role in controlling and directing the phase separation process throughout the entire film.

We developed models for finding the equilibrium profile in thin films and how fluid flow couples to solvent evaporation to determine how structure in thin films evolves
Exploitation Route The model formed the basis for a PhD thesis and 4 further publications extending the ideas to more general cases of greater relevance to industrial applications.
Sectors Aerospace, Defence and Marine,Energy