(381h) Evaluating Reprocessability of Polymer Networks: Flory-Stockmayer Analysis

Authors: 
Li, L., Northwestern University
Chen, X., Northwestern University
Jin, K., Northwestern University
Torkelson, J. M., Northwestern University
Reprocessable polymer networks can undergo structure rearrangement through dynamic chemistries under proper conditions, making them promising candidates for recyclable crosslinked materials, e.g. tires. See, for example, “Recyclable Crosslinked Polymer Networks via One-Step Controlled Radical Polymerization”, Adv. Mater. 2016, 28, 6746-6750. This research field has been focusing on various chemistries. However, there has been lacking of a fundamental theory explaining the relationship between the abundancy of dynamic linkages and polymer network reprocessability. Based on the classical Flory-Stockmayer analysis on network gelation, we developed a theory for reprocessable polymer networks to quantitatively predict the critical condition of being reprocessable. Our analysis indicates that it is unnecessary for all linkages between crosslinks to be dynamic to make the resulting network reprocessable. As long as there is no percolated permanent network in the system, the material can fully rearrange and thus be reprocessable. To experimentally validate our theory, we also used a thiol-epoxy network model system with various dynamic linkage compositions. The stress relaxation behavior and reprocessing of resulting materials support our theoretical prediction: only 50 % of linkages between crosslinks need to be dynamic for a tri-arm network to be reprocessable. Therefore, this analysis provides the first theoretical criterion for designing and evaluating reprocessability of polymer networks that contain both dynamic and permanent linkages between crosslinks.