Layer-by-Layer assembled polyelectrolyte coatings of PAA and PDADMAC for anticorrosive applications

The layer-by-layer (LbL) technique enables the fabrication of nanostructured coatings by alternately depositing oppositely charged polyelectrolytes onto substrates. By incorporating suitable molecules between the layers, functional coatings with anticorrosive properties can be developed. In this work, LbL-assembled films incorporating the corrosion inhibitor 4(5)-methylimidazole (4-IM) were fabricated using poly(diallyldimethylammonium chloride) (PDADMAC) and poly(acrylic acid) (PAA) on glass substrates at pH 3, 6, and 9. Before multilayer assembly, the electrostatic complexation between PAA and PDADMAC was studied using samples at different charge ratios (Z_−/+), where negatively charged PAA solutions were introduced into positively charged PDADMAC at pH 3, 6, and 9. Zeta potential (ZP) measurements, light scattering, and visual inspection across a broad range of charge ratios (Z_−/+ ~ 0.1–10) confirmed interactions at these pH values. Additionally, ZP values for pure solutions of the individual components guided the deposition sequence. The resulting multilayer structure consisted of an eight-layer assembly with the sequence PDADMAC / PAA / PDADMAC / 4-IM / PDADMAC / 4-IM / PDADMAC / PAA, where 4-IM was incorporated into the fourth and sixth layers by using 4-IMI immersion solution at a concentration of 0.25 mol·L⁻¹. Since the polymers interacted at pH 3, 6, and 9, the LbL assembly was subsequently investigated at these pH values using the dip-coating method. Each deposition step involved a 10-minute immersion in the respective pH-adjusted polymer solution (20 mmol·L⁻¹; pH 3, 6, and 9), followed by a 30-second rinse in pH-adjusted ultrapure water and a 5-minute drying period under airflow (<0.2 psi) at 25 °C. The release profiles of 4-MI were monitored using UV-Vis spectroscopy in solutions at pH 3, 6, and 9. The results demonstrated a release profile dependent on the pH at which the multilayers were assembled, indicating that release behavior is influenced by both the assembly conditions and the environmental pH during release. The highest release concentrations (~20 ppm) were observed for multilayers assembled at pH 9, while films prepared at pH 3 or 6 released smaller amounts of 4-IM. Overall, the findings suggest the successful incorporation of the corrosion inhibitor into the multilayers, demonstrating that the combination of the LbL technique with selected polyelectrolytes is a promising strategy for developing functional coatings with anticorrosive properties. However, as the assembly includes a weak polyelectrolyte whose charge varies with pH, both deposition and release conditions must be carefully considered to optimize performance.