Codeposition strategies for improved layer stability in bio-inspired catechol-containing adhesives

Catechol-containing copolymers are a class of polymers often encompassing polydopamine and are considered as biomimetic candidates for high-performance adhesives. However, improvements in adhesive layer stability are needed for high-performance applications to enhance the cohesive strength of the adhesive layers. In this paper, we address this challenge by introducing multiple strategies, including thermal treatment and codeposition. We synthesize a copolymer with adhesive dopamine methacrylamide (DOMA), segmental "filler" methyl methacrylate (MMA), and ATRP-initiator 2-(2-bromoisobutyryloxy)ethyl methacrylate (BIEM). This copolymer is spin coated on silicon wafers, annealed at increasing temperatures, and deposited in combination with (3-aminopropyl)triethoxysilane (APTES), iron chloride (FeCl3), or copper chloride (CuCl2) on silicon and polyethylene substrates. The layers are characterized using ellipsometry, atomic force microscopy (AFM), and Fourier transform infrared (FTIR) spectroscopy. Subsequently, we demonstrate the grafting of a zwitterionic polysulfobetaine methacrylate (PSBMA) layer from the BIEM moiety of the codeposited films. Annealing at 200 degrees C crosslinks the polymer so that almost no physisorbed material remains to be washed off, resulting in enhanced layer stability. Reinforcement through co-deposition shows that using APTES and FeCl3 improved the copolymer layer stability to allow succesful grafting of PSBMA layers from the surface - which copolymer films without codeposition could not achieve. Thus, we demonstrate two pathways based on lessons learned from mussel-based adhesion that can be used to improve PDA-based film formation.