Physical Polyurethane Hydrogels via Charge Shielding through Acids or Salts

Area of Science:

• Materials Science
• Polymer Chemistry
• Colloid Science

Background:

• Anionic polyurethanes are versatile polymers with potential applications in soft materials.
• Controlling the physical properties of hydrogels, such as stress-relaxation and recovery, is crucial for advanced material design.
• Existing gelation methods may have limitations in tunability and pH range.

Purpose of the Study:

• To develop anionic polyurethane-based physical hydrogels with tunable mechanical properties.
• To investigate a novel colloidal gelation mechanism for these hydrogels.
• To explore the influence of pH, salt concentration, and valence on gel properties.

Main Methods:

• Synthesis of anionic polyurethanes.
• Hydrogel formation via addition of acids, monovalent salts, or divalent salts.
• Characterization of gel properties including stress-relaxation, stress recovery, and Young's modulus.
• Investigation of the colloidal gelation mechanism involving charge repulsion and shielding.

Main Results:

• Tunable physical hydrogels were successfully formed from anionic polyurethanes.
• Gel properties, including stress-relaxation and stress recovery, were effectively controlled by pH, salt valence, and concentration.
• Young's moduli ranged from 10 to 140 kPa across acidic to neutral pH conditions (pH 4-5.5).
• A novel colloidal gelation mechanism based on charge repulsion and shielding was proposed and supported by experimental observations.

Conclusions:

• Anionic polyurethanes offer a versatile platform for creating physical hydrogels with precisely tunable mechanical properties.
• The proposed colloidal gelation mechanism provides a new understanding of hydrogel formation in this system.
• These tunable hydrogels have potential for broad applications in materials science and engineering.