Effects of bond exchange reactions and relaxation of polymer chains on the thermomechanical behaviors of covalent adaptable network polymers.

Abstract Bond exchange reaction (BER) induced stress relaxation in covalent adaptable network (CAN) polymers can be leveraged to realize the reshaping, thermoforming, and functional shape changing of cross-linked polymers. However, there is no existing study to reveal how the BERs and viscoelastic relaxation of polymer chains would affect the thermomechanical behaviors of CANs individually or synergistically. In this work, we synthesized polyimine-based CANs with BER activation temperature close to the network glass transition temperature. A thermo-viscoelastic constitutive model based on the Bernstein-Kearsley-Zapas (BKZ) theory was established to separate the contributions of BERs from those of chain relaxation. Assisted by the model, we investigated the effects of BERs and chain relaxation on the energy dissipation in the loading-unloading cycle, the degree of stress relaxation, as well as the dynamic responses of CANs under different thermal-temporal conditions. The findings in this paper provide theoretical guidance to design and fabrication of CAN based devices or structures. Graphical abstract Image 1 Highlights • BER and relaxation of polymer chains dominate the energy dissipation of CANs at high and low temperatures, respectively. • These two relaxation mechanisms are coupled at medium temperature, whose boundary depends on the applied strain rate. • A second peak in the tan δ curve is corresponding to the damping effects of BER on the polymer networks. • As decreasing the frequency of dynamic loads or the BER activation energy, the second peak shifts to a lower temperature. [ABSTRACT FROM AUTHOR]