Hydrogels through emulsion templating: sequential polymerization and double networks

Abstract

Double-network (DN) hydrogels are a combination of two very different polymer networks. Here, porous hydrogels with DN structures were successfully synthesized in two sequential free-radical polymerization steps. The first network was templated within a high internal phase emulsion (HIPE) to yield a hydrogel polyHIPE (HG-PH). The double network hydrogel polyHIPEs (DN-PHs) combined a more rigid highly crosslinked, polyelectrolyte (poly(2-acrylamido-2-methylpropanesulfonic acid), PAMPS) first network with a more ductile, crosslinked, neutral (polyacrylamide, PAAm) second network. The DN-PHs were generated from the as-synthesized HG-PHs by imbibing an aqueous AAm solution followed by polymerization. HG-PHs with 70% porosity absorbed significantly more of the AAm solution than HG-PHs with 85% porosity. The density of the DN-PHs increased with increasing AAm concentration, resulting in an increase in the modulus and toughness of both the swollen and the dry DN-PHs. The DN-PHs based on a HG-PH with 85% porosity did not release their water and did not fail up to compressive strains of 70%, recovering their shapes upon the removal of stress. A comparison with inverse DN-PHs, synthesized using the reverse sequence of polymerization, demonstrated that the properties were strongly dependent both on the composition and on the sequence.