Dynamic covalent networks have emerged as a promising class of materials with applications ranging from reprocessable materials to the thermal stabilization of biologics [1-2]. Their mechanical properties are largely controlled by the thermodynamic and kinetic properties of the bonds [3]. However, a quantitative understanding of some of the classic properties of polymer networks, such as elasticity and the gelation threshold, is still lacking in the case of dynamic polymer networks. In this work, we investigate model networks formed by 4-arm PEG stars linked by dynamic covalent bonds. We show that their elasticity is determined by the proportion of formed bonds, p. We observe striking discrepancies between our experimental results and previously developed models concerning the elasticity and the gelation threshold. We show that the ability of the network to rearrange after formation allows it to maximize its entropy. By considering the entropy associated with the connectivity of the stars, independent of the conformational entropy of the chains, we show that dynamic polymer networks adopt a different internal structure than their permanent counterparts. This allows us to explain the delayed gelation and, combined with previously published models, to quantitatively predict their elasticity. We obtain similar results with networks formed by 8-arm PEG stars, showing the generality of our approach [4]. We further validate this approach experimentally, highlighting that differences in the gel point and mechanics between dynamic and permanent networks are caused by variations in their internal structure. To achieve this, we triggered bond exchange in otherwise permanent covalent networks: depending on p we observe liquefaction (p = 0.6) or stiffening of the network (p = 0.85), and these changes persist after the bonds returned to their permanent state. This implies that reprocessable materials based on dynamic bonds can be permanently altered by reprocessing, even without damage to the bonds or fraction of formed bonds. Overall, our results underscore the critical role of network entropy, distinct from chain entropy, in defining the properties of dynamic polymer networks.
[1] Webber, M.J., Tibbitt, M.W., Nat. Rev. Mat., 2022, 7, 541–556
[2] Marco-Dufort, B. et al., Sci. Adv., 2022, 8, eabo0502
[3] Marco-Dufort, B. et al., J. Am. Chem. Soc., 2020, 142, 36, 15371–15385
[4] Cousin, L. et al., in preparation
