Revisiting crosslinking density effects on pNIPAM microgel properties: Size, electrophoretic mobility, and transition temperatures

Abstract

Poly(N-isopropylacrylamide) (pNIPAM) microgels exhibit a reversible thermoresponsive behavior, undergoing a volume phase transition. This property makes pNIPAM microgels highly appealing for diverse applications, including drug delivery, tissue engineering, and sensors, where temperature-triggered changes in size, charge, and mechanical properties are advantageous. However, a plethora of data available in the literature regarding the relationship between the crosslinking density and the above-mentioned properties of pNIPAM microgels necessitates a consolidation and re-examination. This study aims to address two key objectives: (1) elucidate the relationship between the crosslinking density and size/electrophoretic mobility of pNIPAM microgels, building upon existing knowledge, and (2) examine the influence of crosslinking density on transition temperatures, particularly the electrokinetic transition temperature, which is not well explored and understood. To achieve these objectives, we synthesized 20 batches of pNIPAM microgels using two distinct synthesis routes: 18 batches via conventional one-pot synthesis, with triplicate replicates for six crosslinking densities, and two batches of pNIPAM microgels via semi-batch synthesis, with a duplicate replicate for one crosslinking density. These microgels were characterized using a combination of dynamic light scattering to determine the size and thermoresponsive behavior, electrophoretic light scattering to analyze electrophoretic mobility, and atomic force microscopy to evaluate the structural morphology and assess stiffness. The insights from the characterization techniques enhance our understanding of how the crosslinking density influences the physical and electrokinetic properties of pNIPAM microgels, potentially creating a pathway for rational design of microgels tailored for specific applications.