STRUCTURE, PROPERTIES, AND POTENTIAL APPLICATIONS OF POLYDOPAMINE MATERIALS

Abstract

Polydopamine (PDA) as a novel polymer material has attracted much attention in recent years owing to its unique universal adhesive behavior and easy fabrication through self-assembly. Its monomer form (dopamine, DA) is composed of catechol and amine, which both contribute to the adhesive properties. Since 2007, PDA has been investigated extensively by materials research communities. Application wise, most recent researches focused on utilizing PDA as a surface chemistry modifier and secondary platform. Moreover, by heat treating layer assembled PDA film in an inert or reductive environment, PDA will carbonize and transform into a conductive form, cPDA. It has been found that cPDA has a comparable property to reduced graphene oxide (rGO). The hypothesis is that cPDA also process a layered structure with interlayer distances similar to rGO. Furthermore, with amine groups presents in dopamine, cPDA is believed to be N-doped rGO after carbonization. However, even with a decade of research on this topic, the structure of PDA has not yet been fully understood. In our work, the structural evolution of PDA and cPDA with different heat treatment temperature is investigated by Raman spectroscopy and neutron diffraction, finding the nanocrystal carbon growth respective to temperature. Carbon crystallization also explained the electrical conductivity increase from our measurement. Furthermore, with catechol groups in DA, PDA is capable of forming coordination bonds with metal ions. These bonds will pin the metal ions within PDA and form a metal-PDA complex (M-PDA). In the second part of our work, the effect of doping to structure and properties was investigated by TEM and AFM. We found the thickness of the doped film is thinner than undoped film, which indicates the crosslinking mechanism of PDA is affected by the metal ion dopant. In addition, the pinned metal in M-PDA matrix tends to be reduced into its metal phase after annealing in a protective environment. These finding has also explained the properties change in the thin film and lead us to further investigation on the mechanism of the metal reduction. In TEM, metal nanoparticles are found reduced from M-PDA complex and remain attached under irradiation of electrons. The abundance of electrons in TEM directly supplies the reduction of metal cations and forms metal nanoparticles. With different metal cation, the behavior and final products are vastly different in size and shape. Heating M-PDA powder or film is also a valid way to synthesis self-supported metal nanoparticles which has potential applications in catalysis. The performance of the synthesized catalysts was tested for hydrogen generation in acid solution. This research works forms the third and fourth part of my study. The last part of this study includes the mechanical properties of pristine PDA and Cu-PDA with and without annealing. Finding that increased annealing temperature and metal ion coordination increases Young’s modulus.