Polydopamine Chemistry: Adhesion Mechanism, Copolymerization, and Application

Abstract

Polydopamine (PDA), inspired by mussel adhesive proteins, has garnered significant interest for its ability to coat surfaces independently of their nature, using a simple and environmentally friendly process in basic aqueous solutions. However, understanding its binding and polymerization mechanisms remains challenging due to the numerous reactive sites of the dopamine monomer and the insoluble nature of the resulting polymer film. Additionally, PDA's short, rigid backbone differs from the long, flexible sequences found in mussel-binding proteins, which limits its effectiveness in achieving the conformal contact necessary for efficient adhesion. In this dissertation, I systematically investigate the roles of PDA's building blocks—poly(catechol), poly(catecholamine), and PDA—to enhance our understanding of their binding mechanisms. Contrary to initial expectations, we find that PDA's adhesion primarily arises from the solubility limits of catecholamine oligomers in aqueous environments, complemented by catechol's multiple binding modes. Notably, in the absence of amines, poly(catechol) remains in solution or forms minor suspensions without surface coating, emphasizing the crucial role of amines in facilitating insoluble aggregate formation for adhesion. We validate our findings by inducing poly(catechol) aggregation with quaternized poly(4-vinylpyridine) (qPVP), demonstrating adhesion upon agglomerate formation. Based on these insights, we develop a sequential self-polymerization strategy of phenolic compounds combined with alkanedithiol (ADT) crosslinkers for versatile surface-independent coating and functionalization. The resulting copolymer leverages phenol's diverse binding modes and ADT's flexible aliphatic chain for conformal substrate contact, with Michael addition reactions yielding solvent-resistant crosslinked polymer films. This approach successfully functionalizes various surfaces with phenolic monomers, overcoming stability issues encountered with conventional polydopamine derivatives. Furthermore, we achieve 5,6-dihydroxyindole (DHI)-free poly(catecholamine) (PCA) coating using 3,4-dihydroxybenzylamine hydrobromide, yielding smooth, uniform surfaces. The suppressed indole ring formation preserves primary amine groups, offering an alternative surface functionalization strategy, particularly for hydroxyl-containing surfaces. DHI-free PCA modification enhances the thermal conductivity of graphene-polymer composites, achieving a significant increase compared to untreated graphene controls. Overall, this work advances understanding of PDA's binding mechanisms and offers novel strategies for surface coating and functionalization with broad implications across material science and engineering. By elucidating the intricate processes involved in PDA adhesion and developing innovative coating techniques, this research opens avenues for the design of advanced materials with tailored properties and enhanced performance in various applications.