Polydopamine as a promising materials platform for next generation thermoplastic-metal structural adhesives
Georgios Kafkopoulos is a PhD student in the department Materials Science and Technology of Polymers. (Co)promotors are prof.dr. G.J. Vancso and dr.ir. J. Duvigneau from the faculty of Science and Technology.
Inspired by catecholamine chemistry utilized by marine organisms to attach to surfaces under harsh environmental conditions, Messersmith and co-workers in 2007 introduced polydopamine (PDA). Polydopamine is formed by the oxidative polymerization of dopamine in alkaline aqueous solutions. Virtually any object immersed in the polymerizing solution is coated with a PDA film of a few tenths of nanometers thickness. Owing to its simplicity, versatility, and universal adhesion, PDA, over the past 16 years, has developed to be one of the most prominent methodologies to alter the surface chemistry of materials. However, the poor cohesive strength of the as-formed PDA coatings has hindered their application as structural adhesives. Within the framework of this thesis, a major opportunity for PDA to be applied as a structural adhesive is highlighted. This is based on the unique thermal transformation PDA undergoes upon exposure to elevated temperatures, which alters the chemical structure of PDA coatings and results in an enhancement of their mechanical performance. A field where the thermal transformation of PDA can be utilized in a beneficial manner is the thermal joining of thermoplastic polymers (TPMs) to metals. Efficiently joined TPM-metal hybrid structures are relevant for a number of applications ranging from biomedical to automotive and aerospace. Thermal joining is a process where the respective TPM and metal are brought into contact with each other under pressure and at temperatures above the melting point of the TPM, followed by a cooling step to form a hybrid structure. By thermally joining titanium to a number of thermoplastic matrices, i.e., poly(methyl methacrylate) (PMMA), poly(lactic acid), polycarbonate (PC), and poly(aryl ether-ketone-ketone) (PEKK), this thesis showcases the relevance of PDA for such applications. Within this context, PDA performs excellently as an adhesive interlayer between thermally joined TPMs and metals. This was accounted to three factors: 1) the intrinsic adhesion of PDA to titanium, 2) the enhancement of the cohesive strength of PDA via its thermal transformation due to the required processing temperatures, and 3) the numerous possibilities to tune the chemistry of PDA such as the addition of metal ions, crosslinkers or even simple thermal pretreatments, that do not only promote adhesion but also provide functionality such as resistance to environmental conditioning. Overall, this thesis aims to extend the application of PDA chemistry in the field of structural adhesives and provide new insights via practical case studies.
