Fundamentals of the temperature-dependent dynamics in dynamic covalent polymer networks (DCPNs): What controls the apparent activation energies of stress relaxation and viscous creep in associative versus dissociative DCPNs?

The apparent activation energy (Ea) or temperature (T) dependence of DCPN stress relaxation, and thus the dynamics of reprocessability, often varies across DCPN matrices with the same dynamic crosslinker. A fundamental understanding of the dynamics associated with DCPN stress relaxation, viscous creep, and reprocessability has eluded researchers. Du Prez summarized this point (Polym. Chem. 2020, 11, 5377) by stating, "No straightforward prediction of relaxation times or activation energies (of DCPNs) is possible at this stage." We undertook studies to understand DCPN dynamics. DCPN dynamic chemistries fit into two categories: associative (increasing T leads to increasingly active exchange at constant crosslink density) and dissociative (crosslink density decreases with increasing T at which dissociation is active). By focusing on polymers with known alpha- and beta-relaxation dynamics (reported in the 1950's by Ferry) and using crosslinkers with known bond dissociation energies (BDEs), we achieved the following understanding: (1) in dissociative DCPNs, the stress relaxation and viscous creep Eas are the same within error and equal to the crosslinker BDEs (i.e., in the same T range, Ea is independent of DCPN matrix); (2) with strictly/predominantly associative DCPNs, the stress relaxation/viscous creep Ez is that of the polymer backbone alpha-relaxation (cooperative segmental mobility), which depends on the polymer matrix in the DCPN. Exceptions to these rules will also be discussed.