Interaction-Limited Aggregation: Fine-Tuning the Size of pNIPAM Particles by Association with Hydrophobic Ions

Area of Science:

• Polymer Chemistry
• Physical Chemistry
• Materials Science

Background:

• Poly(N-isopropylacrylamide) (pNIPAM) exhibits a lower critical solution temperature (LCST) in water, leading to phase separation above this point.
• Controlling polymer aggregation is crucial for developing advanced materials and applications.

Purpose of the Study:

• To investigate the formation of stable poly(N-isopropylacrylamide) (pNIPAM) clusters above the LCST.
• To understand the role of sodium tetraphenylborate (NaPh4B) in stabilizing these clusters.
• To explore the potential for dynamic control over polymer microparticle formation.

Main Methods:

• Experimental investigation of pNIPAM chain behavior in water with NaPh4B.
• Physical modeling at the mesoscopic level.
• Atomistic molecular dynamic simulations.

Main Results:

• Sodium tetraphenylborate (NaPh4B) induces stable pNIPAM clusters above the LCST.
• Hydrophobic tetraphenylborate anions (Ph4B-) impart an effective negative charge to pNIPAM chains, promoting electrostatic repulsion.
• Cluster size shows a non-monotonic dependence on salt concentration.
• The interplay between hydrophobic attraction and electrostatic repulsion governs cluster formation and prevents macroscopic phase separation.

Conclusions:

• Weak associative anion-polymer interactions, driven by hydrophobicity, are significant in stabilizing polymer clusters.
• The balance between attractive hydrophobic and repulsive electrostatic forces offers a method for dynamic control of polymer microparticle formation.
• This research provides insights into preventing macroscopic phase separation and creating well-defined polymer microparticles.