Direct measurement of normal and shear forces between surface-grown polyelectrolyte layers

Area of Science:

• Surface science
• Polymer chemistry
• Physical chemistry

Background:

• Polyelectrolyte brushes are crucial in various applications, including coatings and biomaterials.
• Understanding forces between these layers is essential for controlling interfacial phenomena.
• Surface-initiated atom transfer radical polymerization (ATRP) offers a method for creating well-defined polymer brushes.

Purpose of the Study:

• To measure normal and shear forces between grafted polyelectrolyte layers in aqueous solutions.
• To investigate the influence of electrical double layers and polymer interactions on measured forces.
• To explore the effect of ionic strength on these forces.

Main Methods:

• Utilized the surface force balance (SFB) technique to probe forces between surfaces.
• Employed surface-initiated ATRP with a positively charged methacrylate monomer to grow polyelectrolyte layers on mica.
• Confirmed layer formation and thickness using X-ray reflectometry.

Main Results:

• Observed long-range electrostatic repulsion from electrical double layers and shorter-range repulsion upon contact of polyelectrolyte brushes.
• Determined swollen layer thicknesses to be between 15-40 nm.
• Found that adding sodium nitrate (10^-2–10^-1 M) screened electrostatic forces, reducing them to undetectable levels.
• Shear force measurements indicated friction possibly due to polymer chain bridging between surfaces.

Conclusions:

• Surface-initiated ATRP is effective for creating strongly grafted, high-density polyelectrolyte layers.
• Electrostatic interactions dominate forces at low ionic strength, while polymer chain overlap and bridging influence friction.
• The study provides insights into the interfacial behavior of polyelectrolyte brushes, relevant for material design and application.