A simulation-based comparative study on the reaction-controlled terminal relaxation of associative and dissociative...

Covalent adaptable networks (CANs) are polymer networks that engage in chemical reactions. Their dynamic covalent linkages permit topology fluctuations, making them processable. Here, we demonstrate the reaction-controlled terminal relaxation of unentangled CANs by using a mesoscopic coarse-grained single-chain model based on Gaussian strands. The association dynamics is incorporated to reproduce the features of reversible or bond-exchange reactions in CANs. With this model, the dependence of terminal relaxation on cross-ink density [i.e., the number of associated stickers (Nas) for this model] is comparatively studied for dissociative and associative CANs, in terms of stress-relaxation behavior, plateau modulus, as well as terminal relaxation times. Both dissociative and associative model CANs exhibit plateau moduli and exponential terminal relaxations. Their slow and fast relaxation modes are of different Nas dependences, inducing the stress-relaxation curves to undergo a change in shape with Nas. The temperature dependence of terminal relaxation is also examined for both model CANs by considering the kinetics of intrinsic reaction and segmental motion. The engagement of segmental motion forces the horizontal shift factor of time-temperature superposition (TTS) to depart from the Arrhenius-like equation. For dissociative model CANs, the shape of stress-relaxation curve changes with temperature, causing the TTS principle not to hold.