The effect of polarity on the molecular exchange dynamics in imine-based covalent adaptable networks

Abstract

Covalent adaptable networks (CANs) are a rising type of polymeric materials that consist of covalently crosslinked polymer chains, but with the inclusion of dynamic covalent bonds, and that can perform bond exchange reactions under equilibrium control. The dynamic behaviour of these exchange reactions within a polymer matrix has been established to be a key parameter in the control of the material properties. Therefore, in order to fully control the macroscopic material properties of CANs, understanding the underlying molecular exchange processes of these dynamic covalent bonds is essential. In this work, we studied the effect of polarity in polyimine-based CANs, and considered not only the network response itself, but also the – so far often overlooked – effect on the exchange dynamics. By combining results from kinetic studies and material analysis we were firstly able to show a distinct correlation between the presence of polar domains in the molecular structure and the thermal and dynamic mechanical properties of the materials. More importantly, the presence of polar domains also greatly affected the exchange kinetics of the dynamic imine bonds. On the molecular level, we showed that the imine exchange could be greatly enhanced (up to 20 times) when polar groups were present near the reactive imine species. As a result, in our polymer materials we established a tuneable range of phase transition temperatures from glass-to-rubber and rubber-to-liquid over roughly 100 °C as a result of either presence or absence of polar groups in the polymer matrix. Furthermore, detailed analysis in the stress relaxation behaviour of the polyimine materials revealed three relaxation processes, which we could attribute to the relaxation in network topology, to the imine exchange on a local level, and to the imine exchange as a result of diffusion through the polymer network. From this analysis we were also able to illustrate the effect of polarity on the polymer network to each of the three relaxation mechanisms.