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

Abstract

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 °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.