High Internal Phase Emulsions (HIPEs) - Structure, Properties and Use in Polymer Preparation

High internal phase emulsions (HIPEs) are concentrated systems possessing a large volume of internal, or dispersed phase. The volume fraction is above 0.74, resulting in deformation of the dispersed phase droplets into polyhedra, which are separated by thin films of continuous phase. Their structure, which is analogous to a conventional gas-liquid foam of low liquid content, gives rise to a number of peculiar and fascinating properties including high viscosities and viscoelastic rheological behaviour. Like dilute emulsions, HIPEs are both kinetically and thermodynamically unstable; nevertheless, it is possible to prepare metastable systems which show no change in properties or appearance over long periods of time. Polymer materials can easily be prepared from HIPEs if one or the other (or both) phases of the emulsion contain monomeric species. This process yields a range of products with widely differing properties. Additionally, as the concentrated emulsion acts as a scaffold or template, the microstructure of the resultant material is determined by the emulsion structure immediately prior to polymerisation. In this review, the structure, properties, stability and applications of highly concentrated emulsions will be discussed in the first section. Following this, the use of HIPEs to generate novel polymer materials will be the focus of the second part.