Raman Spectroscopy Reveals Phase Separation in Imine-Based Covalent Adaptable Networks.

The introduction of dynamic covalent bonds into cross-linked polymer networks enables the development of strong and tough materials that can still be recycled or repurposed in a sustainable manner. To achieve the full potential of these covalent adaptable networks (CANs), it is essential to understand-and control-the underlying chemistry and physics of the dynamic covalent bonds that undergo bond exchange reactions in the network. In particular, understanding the structure of the network architecture that is assembled dynamically in a CAN is crucial, as exchange processes within this network will dictate the dynamic-mechanical material properties. In this context, the introduction of phase separation in different network hierarchies has been proposed as a useful handle to control or improve the material properties of CANs. Here we report-for the first time-how Raman confocal microscopy can be used to visualize phase separation in imine-based CANs on the scale of several micrometers. Independently, atomic force microscopy (AFM) confirmed the phase-separated domains inside the polymer. Remarkably, the materials were found to undergo phase separation despite being built up from miscible monomers, which arguably should yield homogeneous materials. We found that the phase separation not only affected the appearance of the material but-more notably-also had a noticeable effect on the thermal-mechanical properties of the material: CANs (of equal aliphatic/aromatic monomer composition) that displayed phase separation had both a higher crossover temperature (