(324a) Cross-Linker Control of Vitrimer Stress Relaxation

The ability to design materials with specified properties, precise performance and environmentally friendly impacts is a necessary and emerging field in polymer science and engineering. Of these materials, covalent adaptable networks (CANs) are especially attractive since these thermosets and elastomers contain bonds with exchangeable chemistries. Under a specific stimulus such as light, pH, or heat, these bonds can internally rearrange, imparting the ability to reprocess and heal the material. Specifically, vitrimers, which are CANs that possess covalent bonds that undergo associative exchanges, are attractive as they enable reprocessing at elevated temperatures without altering cross-linking density. Here, we prepare a series of cross-linkers with differing reactivities in a PMDS vitrimer that relies on the conjugate addition–elimination of thiols. We show that small structural modifications to cross-linkers in the material enables control over the rate of stress relaxation while preserving the same exchange mechanism. The overall relaxation rate trends in the material span over four orders of magnitude while maintaining constant stiffness throughout. The overall stress relaxation differences can be furthermore related to computed small molecule activation barriers. Finally, we show that we can mix different cross-linkers in a single material in order to further control the relaxation behavior.