The Non-effect of Polymer-Network Inhomogeneities in Microgel Volume Phase Transitions: Support for the Mean-Field Perspective

Thermoresponsive polymer gels exhibit pronounced swelling and deswelling upon changes in temperature, making them attractive for applications in sensing and actuation. This volume phase transition can be discussed in terms of mean-field theoretical pictures to assess at which conditions it occurs continuously or discontinuously. However, this treatment disregards static nano-and micrometer-scale inhomogeneities in gel polymer networks, which are a common feature of these materials. To check for the impact of such structural complexity, droplet-based microfluidics are used to fabricate sub-millimeter-sized gel particles that exhibit critical compositions at the border between continuous to discontinuous volume phase transitions, along with determined static spatial polymer-network heterogeneity on the nanometer and micrometer length scales, which is characterized by low-field NMR. These different microgels are then used to study their swelling and deswelling volume phase transitions from a sub-millimeter perspective. In this investigation, microgel particles with similar content of crosslinker exhibit similar swelling and deswelling, independent of their extent of static polymer-network inhomogeneity, in agreement with mean-field theoretical predictions.