Modified chitosan emulsifiers: small compositional changes produce vastly different high internal phase emulsion types

Abstract

High internal phase emulsions (HIPEs) are indisputably a core technology for various industries involving pharmaceuticals, food, cosmetics, and biologics but they usually require surfactants/co-surfactants to form, which is often undesired. More specifically, micro-HIPEs are thermodynamically stable, optically clear emulsions with droplet sizes in the range of around 1–100 nm that form spontaneously with little energy input but are rare. Mini-/macro-HIPEs have larger droplet sizes in the range of 50–500 nm and >500 nm, respectively, and typically require high energy input for emulsification. We have synthesized a series of chitosan-graft-oligoN-isopropylacrylamide-graft-oligolysine (CSNLYS) copolymers that act as both emulsifiers for HIPEs without needing extraneous surfactants as well as the matrix material of the resulting porous solid polyHIPE. By merely adjusting the length of the oligolysine graft from relatively long to medium to short, we can form either a micro-, mini- or macro-HIPE, respectively. These emulsions can then be solidified into porous polymers, polyHIPEs, simply by increasing the temperature by exploiting the copolymer thermo-responsiveness and then removing the solvents. These porous polyHIPE, particularly the ones from micro-HIPEs, have surface areas as high as 988 m² g⁻¹ and pore sizes below 200 nm.