Porous polymeric foams are well-known materials and have been produced by a wide variety of techniques ranging from leaching soluble fillers through gas-blowing to phase separation. Although these materials are widely used, their structure is often irregular and difficult to control. However, a novel method for producing porous materials with a more regular structure has been developed based on high internal phase emulsions (HIPE). These foams are called emulsion-derived foams and are commercially known as Polyhipe® (Unilever) polymers.
If one takes a liquid oil, introduces water and a suitable emulsifier with agitation, a dilute emulsion is produced with small water droplets dispersed within the oil phase, about the consistency of milk. If the amount of the water (i.e., the internal or pore forming) phase is increased, a concentrated emulsion will be formed and the consistency of the mixture changes into a more viscous fluid. As further water is introduced, a limit is reached where the droplets are so prevalent that they approximate a close-packed structure (see cover of this issue); beyond this point the droplets must distort to accommodate any additional internal phase. For a collection of monodispersed spheres this packing limit corresponds to 74% by volume and is the accepted definition of a high internal phase emulsion, i.e., an emulsion with an internal phase greater than 74%.4 The structure of the emulsion is now analogous to soap bubbles, with thin films surrounding and separating the drops. Note also that, compared to “standard” emulsion polymerization, this is the exact reverse or “inverse” of that process. In the standard route, the dispersed oil phase is polymerized to yield a collection of polymeric spheres; with inverse emulsion, the continuous phase is polymerized, yielding a rigid matrix.