Effect of polymer network inhomogeneity on the volume phase transitions of thermo- and pH-sensitive weakly charged microgels

Abstract

Environmentally sensitive polymer gels exhibit pronounced swelling and deswelling upon changes in temperature and pH, which has been discussed in mean-field pictures to assess at which conditions it occurs continuously or discontinuously. However, such treatment disregards nanometer-scale inhomogeneities of the distribution of cross-links and pH-sensitive groups in the polymer gel networks. To check for the impact of such inhomogeneity, we use droplet-based microfluidics to fabricate submillimeter-sized gel particles with either homogeneous-random or inhomogeneous-arranged polymer cross-linking and/or acrylic acid (AAc) group densities. These particles are then used to study their volume phase behavior from macroscopic and microscopic perspectives. Our results show that a systematic variation of the spatial distribution of ionizable groups inside the gels notably impacts the continuity and critical temperature of their volume phase transitions. This is remarkable, because even though our samples are just weakly charged, we observe effects similar to earlier findings made for strong polyelectrolytes.