Role of softness on transition temperatures for pNIPAM microgels

Abstract: Poly(N-isopropylacrylamide) (pNIPAM) microgels are renowned for their thermoresponsive behavior, exhibiting a distinct volume phase transition (VPT) upon temperature changes. This study investigates the influence of microgel softness, controlled by varying the crosslinking density during synthesis via free radical polymerization (FRP), on the difference between the volume phase transition temperature (VPTT) and the electrokinetic transition temperature (ETT). These transition temperatures mark the points at which the microgel size and surface charge, respectively, undergo significant alterations in response to temperature. Here, we investigate this phenomenon, employing dynamic light scattering (DLS) and electrophoretic light scattering (ELS) measurements to characterize the size and electrophoretic mobility response of pNIPAM microgels with different crosslinking densities as a function of temperature. By analyzing the observed trends in the difference between the transition temperatures, we aim to develop a hypothesis that provides a deeper physical understanding of the microgel structure and its relationship to transition temperatures. This investigation thus sheds light on the intricate interplay between microgel structure and its thermoresponsive behavior, offering insights for the design and optimization of pNIPAM microgels for future applications.