Vitrimers Designed Both To Strongly Suppress Creep and To Recover Original Cross-Link Density after Reprocessing: Quantitative Theory and Experiments

Vitrimers form a promising class of dynamic polymer networks, but they have an Achilles’ heel: elastomeric vitrimers exhibit significant creep under conditions where permanently cross-linked, elastomeric networks exhibit little or no creep. We demonstrate that vitrimers can be designed with strongly suppressed creep and excellent reprocessability by incorporating a substantial yet subcritical fraction of permanent cross-links. This critical fraction of permanent cross-links, which has little or no detrimental effect on reprocessability, is defined by the gelation point of only permanent cross-links leading to a percolated permanent network. Via a modification of classic Flory–Stockmayer theory, we have developed a simple theory that quantitatively predicts an approximate limiting fraction. To test our theory, we designed vitrimers with controlled fractions of permanent cross-links based on thiol–epoxy click chemistry. We characterized the rubbery plateau modulus before and after reprocessing as well as stress relaxation of our original vitrimers. Our experimental results strongly support our theoretical prediction: as long as the fraction of permanent cross-links is insufficient to form a percolated permanent network, 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 as well as 65–71% creep reduction (for both original and reprocessed samples) relative to a similar vitrimer without permanent cross-links. In contrast, a system with 60 mol % permanent cross-links could not be reprocessed into a well-consolidated sample, nor did it recover full cross-link density; it failed by breaking at early stages of creep tests. The ability to predict an approximate limiting fraction of permanent cross-links leading to enhanced creep resistance and full reprocessability represents an important advance in the science and design of vitrimers.