The Effect of Shear on the Evolution of Morphology in High Internal Phase Emulsions Used as Templates for Structural and Functional Polymer Foams

Polymer foams have broad application as structural and functional materials in a variety of technological applications. Polymerization of monomers present in the continuous phase of water-in-oil high internal phase emulsions (HIPEs) provides a unique processing route for fabricating highly porous polymeric foams known as polyHIPEs. The microstructure of the polyHIPEs is related to the morphology of the HIPE from which it is templated. Shear-induced microstructural changes within the HIPE result from the combination of droplet breakup and coalescence induced by shear or thermal diffusion. HIPEs were prepared from a two-component oil phase consisting of 2-ethylhexyl acrylate (2-EHA) and ethylene glycol dimethacrylate monomers. Imaging of the HIPEs revealed shifts in the mean and polydispersity of the droplet size distribution with repeated shearing. Isothermal, oscillatory time-sweep chemorheological studies, conducted in the linear viscoelastic regime, were used to gain insights about the kinetics of polymerization as well as the morphological changes due to the thermal-induced droplet coalescence. The compressive modulus of wet polyHIPEs was found to increase with decreasing cell size and to decrease when coalescence occurred within the precursor HIPE. Thermal gravimetric analysis revealed an enhancement in the 2-EHA composition within the polymer compared to the original oil phase, presumably due to a difference in solubilities of the two monomer components into the aqueous phase.