Cross-Linker Control of Vitrimer Flow

Vitrimers are a class of covalent adaptable networks (CANs) that undergo topology reconfiguration via associative exchange reactions, enabling reprocessing at elevated temperatures. Here, we show that the use of an associative mechanism additionally enables decoupling of stiffness and stress relaxation. Guided by calculated activation barriers, we prepared a series of cross-linkers with varying reactivity for the conjugate addition–elimination of thiols in a PDMS vitrimer, and demonstrate modulation of stress relaxation rate while maintaining constant stiffness. Surprisingly, despite a wide range of stress relaxation rates, we observe that the flow activation energy of the bulk material is independent of the cross-linker structure. Superposition of storage and loss moduli from frequency sweeps can be performed for different cross-linkers, indicating the same exchange mechanism. We show that we can mix different cross-linkers in a single material in order to further modulate the stress relaxation behavior.