Evolution of a Radical-Triggered Polymerizing High Internal Phase Emulsion into an Open-Cellular Monolith

The time and manner by which initially discrete cells in a high internal phase emulsion (HIPE) become highly interconnected by windows during/after polymerization (polyHIPE) remain a controversy. Water-in-oil HIPE with styrene/divinylbenzene as the oil phase is one of the most widely studied systems. Here, new evidence is presented on the evolution of polyHIPE with the aid of a macrosurfactant and new instruments. The macrosurfactant has a branched polyethylenimine as the hydrophilic head and polystyrene chains as the hydrophobic tail. It can stabilize HIPE within a wide range of dosages and lead to different polyHIPE morphologies, all of which are proven to be of open-cellular monolith. At a low macrosurfactant dosage, the windows that interconnect the cells are sparse but large and can be effectively detected with a 3D optical microscope (3DOM) and a 2D photothermal infrared (2D PTIR) spectrometer in an open environment. 3DOM detection supports the claim that windows form during HIPE polymerization. Tests involving the extraction of pre-colored water droplets of the polymerizing HIPE also lend support to the formation of an open-cellular structure during polymerization. 2D PTIR provides evidence on phase separation on the surface once covered by the macrosurfactant, which favors window development.