Kinetics of Polymerization in High Internal Phase Emulsions

The close-packed polyhedral microstructure of high internal phase emulsions (HIPE) allows for the dispersed phase to occupy over 74% of the system volume, which is well beyond the packing limit of hard spheres. In the case of water-in-oil HIPE systems, polymerization of monomers present in the continuous phase yields open porous foams, known as polyHIPEs. In this work, the evolution of dynamic moduli and microstructure of HIPE systems containing 94.4% water in 2-ethylhexyl acrylate (EHA) and ethylene glycol dimethylacrylate (EGDMA) was studied. Isothermal, oscillatory time sweeps in the linear viscoelastic regime were used to follow the kinetics of polymerization. Due to the solid-like behavior of uncured HIPEs, methods reported in the literature for obtaining the gel-point during polymerization were not successful. However, the kinetics of different stages of curing including (i) induction period, (ii) polymerization, and (iii) final equilibrium can be quantified by rheological measurements of both the storage and loss moduli.  A transition is observed in the loss modulus versus reaction time curve and is attributed to the formation of a percolating covalently bonded network at that point during the polymerization. The final equilibrium moduli can be correlated with the degree of reaction conversion for poly(HIPE)s.