Stoichiometric effects on bulk stress relaxation to enhance reprocessability in covalent adaptable networks†

Abstract

Achieving maximum crosslinking density is often thought to be crucial for high performance and mechanical strength in conventional thermoset covalent adaptable networks (CANs), although some network defects can enhance properties such as toughness and reprocessability. Controlling functional group stoichiometry and crosslinking density is therefore key to better understanding and programming the properties and reprocessability of CANs. Despite efforts to systematically control CAN properties, many previous studies show inconsistent trends in stress relaxation rates, hindering a clear understanding of the factors affecting reprocessability. We show that stoichiometry is a critical factor influencing bulk stress relaxation for a dissociative type of CAN based on the aza-Michael addition reactions utilizing triacrylate monomer and two polymeric amines (Jeffamine and polyethyleneimine, PEI). Relative functional group reactivity was characterized by Fourier transform infrared spectroscopy, and four different stoichiometries were analyzed. Thermal reprocessing demonstrated the stability of mechanical properties over multiple cycles, notably in stoichiometrically balanced systems, consistent with the general principle that mechanical properties and crosslinking density reach a maximum with stoichiometric equivalence. These findings underscore the importance of careful stoichiometric design in tailoring viscoelastic properties and mechanical behavior in dissociative type CAN systems, offering insights into polymer material design for applications requiring dynamic properties and reprocessability.