Effect of Softness and Charges on the Volume Phase Transition of Colloidal Microgels and Macrogels

Abstract

Microgels and macrogels, due to their stimuli-sensitive nature and tunability, are of great interest in both applications and fundamental research. However, their softness increases the number of degrees of freedom compared to hard colloidal particles and gives rise to a more complex behavior. So far, there is no generally accepted model to describe microgel interactions, especially for concentrated microgel suspensions. In this project, we delve into the study of the effect of counterions on the microgel periphery resulting from the use of ionic initiators in the synthesis. Using small-angle neutron scattering (SANS), we confirm the presence of counterion clouds at the particle periphery. More importantly, we observe that counterions can delocalize and control the swelling of the microgel. Using confocal microscopy, we show that delocalized counterions lead to noncentral many-body forces between microgels, controlling the elasticity of the crystals—a behavior also observed in metals and crystals of charged, hard colloids. Our results not only contribute valuable insights for developing a model to describe microgel interactions but also showcase the capability of SANS in studying soft matter. Furthermore, we explore the thermodynamic instability of macrogels using SANS, neutron imaging, and rheology. We conduct a detailed examination of the characteristics of the polymer-dense skin that forms on the gel surface upon rapid heating and arrests the gel in a metastable coexistence of swollen and deswollen phases. These results are valuable for developing hydrogels that exploit thermodynamic instabilities for shape actuation in various applications.