Unveiling the rheological behavior of poly(styrene-co-N-isopropylacrylamide) microgel aqueous suspensions

Poly(N-isopropylacrylamide)-based microgels are soft and deformable colloids, which can swell and shrink in response to external stimuli such as temperature or pH, making them suitable for various industries. Herein, poly(styrene-co-N-isopropylacrylamide) microgels with varying softness were synthesized through the doping of different proportions of styrene into their network structure. Rheological tests revealed that the deformability, surface charge, and wettability of microgels were precisely regulated through adjustments to styrene doping levels. As the molar ratio of styrene increased, the range of the linear viscoelastic region regarding stain did not exhibit a consistent change. The microgel suspensions exhibited a pronounced viscous-like behavior, as indicated by their frequency dependence. Additionally, the concentration or volume fraction of microgels significantly influenced the phase distribution in the phase diagram. At lower concentrations, the microgel suspensions typically underwent a colloidal gel-to-liquid-to-glass transition, with the liquid-like regime expanding as the styrene doping level increased. At higher microgel concentrations, the volume phase transition shifted to a direct colloidal gel-to-glass transition, particularly at high styrene levels. This study also revealed a strong correlation between the power-law viscoelastic exponents and the loss tangent during the entire phase transition, offering valuable insights for structure and rheological properties of poly(N-isopropylacrylamide)-based microgels.