Tailoring the morphology of emulsion-templated porous polymers

Abstract

Methods with which to tailor the morphology of polystyrene-based emulsion-templated (PolyHIPE) materials are presented. Increasing the temperature of the aqueous phase used to prepare the parent emulsion leads to an increase in average void and interconnect size in the resulting porous material. Additionally, the presence in the aqueous phase of small quantities of organic additives that are capable of partitioning between the two emulsion phases also affects the morphology of the porous material obtained. The additives examined were tetrahydrofuran (THF), methanol and poly(ethylene glycol) (PEG), all of which were found to increase the average void and interconnect diameters. It is suggested that THF and, to a lesser extent, PEG enhance Ostwald ripening, resulting in emulsion coarsening over time. Evidence for this was gleaned from NMR experiments to determine the rates of water diffusion in each emulsion. However, methanol was shown not to affect the rate of water diffusion. An alternative mechanism by which methanol could affect the emulsion stability is by depleting surfactant from the interface. However, higher levels of surfactant in emulsions containing methanol did not have a significant effect on morphology. To explain this, we suggest that methanol may result in depletion of surfactant from the emulsion interface, however additional surfactant serves not only to replace this depleted surfactant but also to increase the number of w/o micelles in the continuous phase. These facilitate transport of water between droplets, thus negating the effect of replacing the surfactant lost from the interface.