Influence of Deposition Time and Aging on the Wettability, Zeta Potential and Adhesive Properties of Thin Polydopamine Films.

Ultrathin polydopamine (PDA) films have a high potential as bioinspired functional and adhesive coatings. While their preparation has been studied extensively, little attention has been paid yet to their stability and aging under ambient conditions. The study at hand closes this gap by showing that two important macroscopic surface properties, the wettability and zeta potential, change continuously even after the wet-chemical preparation has been finished. PDA films were prepared using a standard procedure and varying deposition times (10 min to 24 h). Longer deposition times led to a higher thickness and roughness, lower contact angles and a more alkaline behavior of the film. Storage under ambient conditions for up to 31 days increased the contact angle and shifted the pH-dependent zeta potential curves to lower pH values. This indicates ongoing chemical reactions that reduce the hydrophilicity and lead to a more acidic (or less alkaline) behavior of the amphoteric PDA surface. Despite these changes, the adhesive performance of the PDA films was not affected by the storage. Nickel layers obtained by electroless metallization of PDA-coated polyethylene plates show a good quality and adhesion independent of the deposition and storage time of the adhesive PDA film.