Polydopamine as Adhesion Promotor: The Effect of Thermal Treatment on the Performance of Poly(lactic acid) (PLA)-Metal Co-molded Joints

Molecular interactions in polymer/metal oxide interfaces are of paramount interest in polymer composite applications, including comolding of polymer-metal joints, additive manufacturing, and mold release. This study shows the potential of biomimetic polydopamine (PDA) layers to control polymer-metal adhesion covering a range from strong bonding to release for poly(lactic acid) (PLA) adhering to two metals of significant commercial importance, i.e., titanium (Ti) and stainless steel (SS). The results show that even though PLA bonds significantly weaker to Ti than to SS surfaces, both metals exhibit considerably higher and similar adhesion values following deposition of a PDA layer. In addition, a simple thermal annealing of the PDA-coated wires before the comolding process results in a sharp increase of the bonding strength at low annealing temperatures, followed by a gradual drop at higher annealing temperatures. This observation opens the possibility to provide control of adhesion in polymer-metal interfaces. As PDA forms strongly bound adhesive layers on a wide range of materials, this study proposes that the phenomenon described here can be successfully applied to surfaces other than metals, raising high expectations for future polymer composite applications.