Direct visualization of pH-dependent evolution of structure and dynamics in microgel suspensions

Area of Science:

• Soft Matter Physics
• Colloid Science
• Polymer Chemistry

Background:

• Microgel suspensions exhibit complex phase behavior.
• Tuning interparticle interactions is key to controlling suspension properties.

Purpose of the Study:

• Investigate the structural dynamics and kinetic arrest pathways of poly(N-isopropylacrylamide-co-acrylic acid) (p(NIPAm-co-AAc)) microgel suspensions.
• Elucidate the role of pH-tuned interparticle interactions on suspension solidification.

Main Methods:

• Utilized 3D confocal microscopy for direct visualization.
• Employed image analysis and particle tracking for quantitative analysis.
• Studied aqueous suspensions of fluorescently labeled p(NIPAm-co-AAc) microgel particles across varying pH and concentrations.

Main Results:

• At low pH, particle attraction drives crystallization and kinetic arrest, characterized by homogeneous nucleation and nucleation-limited growth kinetics.
• A significant lag time, dependent on particle concentration, precedes crystal domain growth.
• At high concentrations and low pH, suspensions form disordered solids due to constrained relaxation.
• At neutral pH, suspension dynamics are solely concentration-dependent, forming disordered soft glassy solids at high concentrations.

Conclusions:

• pH is a critical parameter for controlling the solidification pathways of p(NIPAm-co-AAc) microgel suspensions.
• Interparticle attraction at low pH promotes crystallization-driven arrest, distinct from the concentration-driven arrest at neutral pH.
• The findings provide insights into the fundamental mechanisms governing the formation of solid-like states in colloidal suspensions.