Time Scale-Dependent Structure-Property Relationships in Dynamic Imine-Benzoxazine Networks

Dynamic polymer networks, or vitrimers, incorporate labile cross-links that enable topological rearrangement of the network via associative bond exchange, a process accelerated by the application of strain and heat. The linear viscoelasticity of vitrimers is governed by the behavior of the network at both short and long time scales, influenced by both local segmental motions and the kinetics of the dynamic exchange mechanism itself. Herein, we investigate the linear viscoelasticity of imine-containing benzoxazine (iBOX) networks, the majority of which possess glass transition temperatures (T _g) exceeding 100 °C and resemble nondynamic thermosets used in structural applications. Nine iBOX monomers were synthesized by the condensation reaction of an aldehyde-functionalized benzoxazine precursor with a series of difunctional amines. Subsequent cationic ring-opening polymerization produced iBOX vitrimer networks with varying poly-(propylene oxide) (PPO), poly-(ethylene oxide) (PEO), and poly-(ethylene) (PE) backbone structures, each represented by three different molecular weights between cross-links. Time-temperature superposition (TTS) was applied to small-amplitude oscillatory shear (SAOS) and stress relaxation data to elucidate the influence of polymer architecture on viscoelastic response. The occurrence of multiple relaxation processes with distinct temperature dependences necessitated separate shift factors for short- and long-time dynamics, each exhibiting an Arrhenius temperature dependence and yielding different estimates of apparent activation energy (E _a). Notably, the structural variations lead to nonuniform changes in E _a across time scales, which exemplifies the complexity of tuning exchange kinetics in dynamic polymer networks.