Viscoelastic Properties of Polyelectrolyte Multilayers Swollen with Ionic Liquid Solutions.

Area of Science:

• Materials Science
• Polymer Science
• Physical Chemistry

Background:

• Polyelectrolyte multilayers (PEM) are fabricated using layer-by-layer assembly.
• Ionic liquids (ILs) can dope PEM films through swelling.
• Understanding the mechanical properties of IL-doped PEM is crucial for their applications.

Purpose of the Study:

• To investigate the mechanical properties of PEM films doped with ionic liquids.
• To analyze the effect of ionic liquid concentration on film thickness, viscosity, and elastic modulus.
• To differentiate the behavior of ILs from inorganic salts in PEM swelling.

Main Methods:

• Fabrication of PEM using layer-by-layer assembly.
• Swelling PEM films with varying concentrations of water-soluble ionic liquids (1-Ethyl-3-methylimidazolium chloride and 1-Hexyl-3-methylimidazolium chloride).
• Utilizing dissipative quartz crystal microbalance (QCM-D) to measure frequency and dissipation shifts.
• Applying a Kelvin-Voigt model to determine film thickness, viscosity, and elastic modulus.

Main Results:

• PEM thickness changed monotonically with increasing IL concentration.
• Viscosity and elastic modulus exhibited non-monotonic changes with IL concentration.
• Three distinct concentration regimes were identified for IL-induced changes in viscoelastic properties.
• This behavior differs from swelling with inorganic salts like NaCl.

Conclusions:

• Ionic liquids induce unique viscoelastic responses in polyelectrolyte multilayers.
• The observed regimes are attributed to a balance between hydrophobic interactions and electrostatic screening.
• These findings provide insights into the structure-property relationships of IL-doped PEM.