Dual pH-triggered physical gels prepared from mixed dispersions of oppositely charged pH-responsive microgels

Abstract

pH-Responsive microgels are crosslinked polymer particles that swell when the pH approaches the pK_a of the polyacid or polybase chains. In this study we investigate for the first time pH-triggered physical gel (soft particle glass) formation from concentrated mixed dispersions of heteroaggregated pH-responsive microgels. The two microgels studied are poly(ethyl acrylate/methacrylic acid/1, 4-butanediol diacrylate) (referred to as PEAMAA) and poly(2-vinylpyridine/divinylbenzene) (referred to as PVP). The anionic PEAMAA particles are alkali-swellable and the cationic PVP particles are acid-swellable. Concentrated homodispersions formed physical gels at either low pH (PVP) or high pH (PEAMAA). The hard sphere (latex) particle size ratio (R = d_PEAMAA/d_PVP) was 0.15. Uniquely, the PEAMAA/PVP microgel heterodispersion allowed R to be varied by an order of magnitude (from 0.04 to 0.61) at a fixed particle number ratio simply by adjusting the solution pH. Concentrated PEAMAA/PVP heterodispersions formed physical gels at both low and high pH. Dynamic rheology measurements showed that at low pH the presence of collapsed PEAMAA particles greatly increased the storage modulus (G′) and also the critical yield strain (γ_c) of the mixed PEAMAA/PVP gels compared to the parent PVP homoparticle gel. A remarkably high G′ value of 0.12 MPa was obtained for a mixed PEAMAA/PVP gel containing 90 vol.% water. Our new, versatile method for preparing pH-responsive dispersions that form physical gels at two different pH values could lead to dual-action injectable gels for soft tissue repair.