Effect of Sterics and Degree of Cross-Linking on the Mechanical Properties of Dynamic Poly(alkylurea-urethane) Networks

Dynamic covalent networks are polymer networks that contain a dynamic covalent bond which allows them to be reprocessable, remoldable, and recyclable as well as exhibit crack healing or stress-relaxation properties. A key component of these materials is the nature of the dynamic covalent bond, which in addition to chemical composition and architecture can be used to dramatically alter the physical properties of these networks. The aim of this study is to understand the impact of steric hindrance of N-alkyl substituents and network connectivity in poly(alkylureaurethane) dynamic network films. In these materials, the dynamic bond is the hindered alkyl urea moiety, whose dynamic behavior is dictated by the sterics of the alkyl substituent. Networks were prepared by the noncatalyzed curing reaction of aminoethanol compounds of varying substituents with a trifunctional isocyanate cross-linker and varying amounts of a monofunctional capping agent. Thermomechanical properties and FTIR studies show the impact of hindered urea bond sterics on the reaction conversion, network connectivity, and therefore the relaxation of the dynamic networks. Stress relaxation analysis show the vitrimer-like behavior of these dynamic networks only when the degree of cross-linking is maintained by high reaction conversion (high equilibrium constant of the dynamic bond). These results give some insights into the design and properties of dynamic covalent networks and how the nature of dynamic bonds can be used to impact their properties.