Metal Ions in Polydopamine Coatings Enhance Polymer–Metal Adhesion

Strong bonding at polymer–metal oxide interfaces is of high importance for lightweight thermoplastic composite structures. However, interfacial adhesion in polymer–metal material systems often poses grand challenges in applications, and hence, tailoring the molecular interactions is necessary. Here, the interfacial adhesion between polycarbonate (PC) and titanium (Ti) is optimized by utilizing metal ion-containing polydopamine (M⁺PDA) produced using two methods, i.e., postdeposition and codeposition with respect to the PDA polymerization. M⁺PDA thin films were formed on the surface of titanium wires, which were then comolded with a PC matrix to form pullout samples in order to evaluate the interfacial energy of adhesion (G_a). For the postdeposition process, Fe³⁺-, Fe²⁺-, Co²⁺-, Ni²⁺-, Cu²⁺-, or Zn²⁺-containing PDA layers were evaluated. Fe³⁺PDA and Fe²⁺PDA coatings resulted in a significant increase of G_a between Ti and PC, while the other metal ions had an insignificant effect. For the codeposition process, Cu²⁺ was utilized, and CuPDA films of various Cu²⁺:DA ratios were evaluated. G_a values for these systems followed an increasing trend by increasing the Cu²⁺:DA ratio until a plateau was reached at a 1:1 value. The M⁺ content had no influence on the values of G_a, regardless of the deposition process utilized. In addition to the specific results to obtain strong adhesion at PC-Ti interfaces, by FTIR, AFM, and solid-state NMR, we also provide insights into hitherto unknown features regarding the surface morphology and chemistry of M⁺-containing PDA films.