Investigation of Covalent Adaptable Networks (CANs)Based on Dynamic Acylhydrazone Bonds FeaturingShape-Memory Ability

Covalent adaptable networks (CANs) are based on reversible crosslinks, which are present within the polymer network structure. Due to these reversible moieties, CANs feature interesting properties/abilities such as self-healing after damage, shape-memory behavior and recyclability, respectively. Here, we report the synthesis of dynamic polymer networks based on acylhydrazones as the reversible crosslinker. For this purpose, the acylhydrazone crosslinker was copolymerized with different alkyl methacrylates (i.e., butyl methacrylate or 2-ethylhexyl methacrylate). Hereby, two different types of networks were prepared. Firstly, networks containing only the reversible crosslinker and, secondly, networks featuring both a reversible crosslinker and an additional permanent covalent crosslinker were synthesized. The reversibility of the arylhydrazone moiety was studied by Raman spectroscopy as well as stress relaxation measurements. Latter method was also utilized to determine the activation energy of the acylhydrazones within the polymer networks. Furthermore, the shape-memory behavior of the polymer networks was studied in detail revealing excellent shape-recovery properties with strain fixity rate (R f) above 95% for almost all polymer networks and also strain recovery rate (R r) above 90%. Finally, the recyclability of the polymer networks was studied indicating that the intelligent properties, such as the shape-memory behavior is still completely available after the recycling process. For this purpose, we performed four recycling cycles, and the shape-memory parameters were analyzed using TMA method after each. We were able to quantify the recyclability behavior, as well as the margin of error for strain fixity (R f) and strain recovery (R r) for the polymer networks over four cycles.