Joints that connect thermoplastic polymer matrices (TPMs) and metals, which are obtained by comolding, are of growing importance in numerous applications. The overall performance of these constructs is strongly impacted by the TPM–metal interfacial strength, which can be tuned by tailoring the surface chemistry of the metal prior to the comolding process. In the present work, a model TPM–metal system consisting of poly(methyl methacrylate) (PMMA) and titanium is used to prepare comolded joints. The interfacial adhesion is quantified by wire pullout experiments. Pullout tests prior to and following surface modification are performed and analyzed. Unmodified wires show poor interfacial strength, with a work of adhesion ( G a ) value of 3.8 J m –2 . To enhance interfacial adhesion, a biomimetic polydopamine (PDA) layer is first deposited on titanium followed by a second layer of a poly(methyl methacrylate- co -methacrylic acid) (P(MMA- co -MAA)) copolymer prior to comolding. During processing, the MAA moieties of the copolymer thermally react with PDA, forming amide bonds, while MMA promotes the formation of secondary bonds and molecular interdigitation with the PMMA matrix. Control testing reveals that neither PDA nor the copolymer provides a substantial increase in adhesion. However, when used in combination, a significant increase in adhesion is detected. This observation indicates a pronounced synergistic effect between the two layers that strengthens the PMMA-titanium bonding. Enhanced adhesion is optimized by tuning the MMA-to-MAA ratio of the copolymer, which shows a maximum at a 24% MAA content and a greatly increased G a value of 155 J m –2 ; this value corresponds to a 40-fold increase. Further growth in the G a values at higher MAA contents is hindered by the thermal cross-linking of MAA; MAA contents above 24% restrict the formation of secondary bonds and molecular interdigitation with the PMMA chains. Our results provide new design principles to produce thermoplastic–metal comolded joints with strong interfaces.

Advanced high‐performance structural applications require the right materials in the right place and suitable interface engineering. However, poor adhesion in harsh environmental conditions frequently challenge material interfaces. An example is the moisture sensitivity of titanium‐poly ether ketone ketone (PEKK) interfaces. Here, this work offers a high‐performance composite adhesive system, which combines strong adhesion and high interfacial toughness, particularly when used in metal‐polymer bonding. This system includes aminopropyl triethoxy silane (APTES)–polydopamine (SiPDA) layers, which can be formed on the titanium surface before the joining process with carbon fiber‐reinforced PEKK (C/PEKK). Adhesion between PEKK and titanium is evaluated before and after hot/wet conditioning using mandrel peel tests. This work discovers that applying thin SiPDA layers not only results in a remarkable rise in the interfacial fracture toughness but also provides durable bond stability after hot/wet conditioning. These findings indicate that polydopamine‐based coatings show great potential to achieve stable interfaces for the next generation of high‐performance metal‐polymer hybrid materials.

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.

Control over adhesion at interfaces from strong bonding to release between thermoplastic polymers (TPs) and metal oxides is highly significant for polymer composites. In this work, we showcase a simple and inexpensive method to tune adhesion between a TP of growing interest, poly(lactic acid) (PLA), and two commercial metal alloys, based on titanium and stainless steel. This is realized by coating titanium and stainless steel wires with polydopamine (PDA), thermally treating them under vacuum at temperatures ranging from 25 to 250 °C, and then comolding them with PLA to form pullout specimens for adhesion tests. Pullout results indicate that PDA coatings treated at low temperatures up to a given threshold significantly improve adhesion between PLA and the metals. Conversely, at higher PDA annealing temperatures beyond the threshold, interfacial bonding gradually declines. The excellent control over interfacial adhesion is attributed to the thermally induced transformation of PDA. In this work, we show using thermogravimetric analysis, X-ray photoelectron spectroscopy, Fourier transform infrared, and 13 C solid-state NMR that the extent of the thermal transformation is dependent on the annealing temperature. By selecting the annealing temperature, we vary the concentration of primary amine and hydroxyl groups in PDA, which influences adhesion at the metal/PLA interface. We believe that these findings contribute to optimizing and broadening the applications of PDA in composite materials.