Modeling Equilibrium Swelling of a Dual pH- and Temperature-Responsive Core/Shell Hydrogel

Abstract

The equilibrium swelling of a dual stimuli-responsive core/shell hydrogel is studied by a thermodynamic model. This hydrogel shows thermo-sensitivity as well as pH-sensitivity. The model captures the inhomogeneous swelling of core/shell hydrogels and also, accounts for temperature and pH sensitivity. The predictions of this model are verified with the swelling experiments of a core/shell microgel comprising poly N-isopropyl acrylamide (pNIPAM) and acrylic acid (AAc). The model calculates the equilibrium swelling within the ionic core and the neutral shell. Simulation results can reproduce the equilibrium swelling-temperature curves of this microgel at different pH values considering the delay in the volume phase transition temperature (VPTT) of the ionic polymer gel (pNIPAM-co-AAc) in the core. Two transition points are found in the equilibrium swelling behavior of the hydrogel akin to the VPTTs of the core and shell domains at high pH values of bath solutions. Likewise, the degree of ionization in the core domain is predicted to have a two-step transition behavior corresponding to the VPTTs of the core and shell domains at high pH values of bath solutions. It is shown that the equilibrium swelling of the ionic core is mainly influenced by the electrostatic repulsion between bound charges rather than the ionic pressure. Furthermore, it is determined that the maximum radial stress occurs at the core/shell interface and reaches its maximum value about the VPTT of the core.