Overcoming the Achilles' Heel of Dynamic Vitrimer Networks: Modification and Application of Flory-Stockmayer Theory to Minimize Creep

Vitrimers are a promising class of dynamic polymer networks, but they have an Achilles’ heel: above Tg, vitrimers exhibit significant creep under conditions where permanently cross-linked networks exhibit little or no creep. Vitrimers can be designed with strongly suppressed creep and excellent reprocessability by incorporating a subcritical fraction of permanent cross-links. This fraction of permanent cross-links, which has little or no effect on reprocessability, is defined by the gelation point of only permanent cross-links leading to a percolated permanent network. Modifying Flory–Stockmayer theory, we developed a simple theory that predicts an approximate limiting fraction. To test our theory, we designed vitrimers with controlled fractions of permanent cross-links. Our experiments support our theoretical prediction: when the fraction of permanent cross-links is subcritical, the vitrimer can be reprocessed with full recovery of cross-link density. In particular, with a predicted limiting fraction of 50 mol %, a vitrimer system designed with 40 mol % permanent cross-links achieved full property recovery associated with cross-link density after reprocessing and ~70% creep reduction relative to a similar vitrimer without permanent cross-links.