Internal hydrogen bonding of imines to control and enhance the dynamic mechanical properties of covalent adaptable networks

Abstract

Covalent Adaptable Networks (CANs) have the potential to replace current thermoset materials, as the dynamic covalent bonds in CANs enable reprocessability and recycling of crosslinked polymer networks. A current limitation of CANs is, however, that they are generally susceptible towards creep, as the bond exchange reactions facilitate stress relaxation. In this work, we propose the use of internal hydrogen bonding in polyimine CANs as an efficient tool to enhance creep resistance (at operating temperature) and further control the dynamic mechanical properties, while still enabling malleability at elevated temperatures. We are able to show on a small-molecule level that ortho-substituted hydroxy groups on aromatic imines stabilise the dynamic covalent imine bonds as a result of the internal hydrogen bond that is formed between the hydroxy and imine group. Furthermore, we show that polyimine CANs with incorporated ortho hydroxy groups have significantly enhanced material properties, as can be seen in the glass transition temperature (Tg), elastic modulus (G’), creep resistance and solvent resistance. While we also consider additional steric and electronic effects that might have arisen due to installation of the hydroxy groups, we find that the stabilising effect of the internal hydrogen bond is primarily dictating the material performance.