Linear and Nonlinear Viscoelasticities of Dissociative and Associative Covalent Adaptable Networks: Discrepancies and Limits

Abstract

Covalent adaptable networks (CANs) can be classified into dissociative (Diss-CANs) and associative (Asso-CANs) networks according to the exchange mechanism of covalent bonds. We simulate the exchange reaction by the hybrid Monte Carlo–molecular dynamics (hybrid MC/MD) algorithm, aiming to discover the connection and difference between Diss-CANs and Asso-CANs in viscoelasticity behavior. In the linear regime, a major difference originating from the cross-linking density is reflected in the pre-exponential factor τs 0 of the characteristic relaxation time τs. For nonlinear rheology, Diss-CANs show a faster shear thinning behavior under steady shear, while Asso-CANs have a stronger strain hardening under the shear rate start-up. The physics behind the phenomenon results from the different chain conformations and configurations related to the exchange mechanism. Compared with Diss-CANs, the inability for sticker dissociation of Asso-CANs generates a slower relaxation under shear, leading to less chain orientation and tumbling. Meanwhile, we find that multiscale relaxation times obtained from linear viscoelasticity (LVE) can be crucial limits in nonlinear applications for associative polymers (APs). Our work strongly deepens the understanding of APs in terms of both linear and nonlinear viscoelasticities.

Linear viscoelasticity, nonlinear rheology and applications of polyethylene terephthalate vitrimers

“…Consequently, PET vitrimers show more obvious nonlinear characteristics as shown in Figure 9. It is worth noting that the effect of orientation is negligible in vitrimers without dissociation, and all PET vitrimers show rigid network under extensional flow 35 . In summary, the ductility of PET vitrimers would decrease with increase in crosslinking degree, resulting in smaller maximum strain.…”

“…A predicted overshoot in viscosity growth which is similar to ionomers was not observed at higher extensional rate of 0.3 s À1 due to edge rupture. 35 It is obvious that 811-1.5 underwent melt fracture at edge as shown in the inserted pictures in Figure 10A. In dissociated ionomers, η þ at long time slightly hardens above the LVE curve at higher extensional rates, exhibits an overshoot, and then approaches the zero-shear viscosity η 0 .…”

“…The creep resistance and crosslinking degree were investigated by Qiu's work, confirming excellent processability of PET vitrimers 20 . The theory and simulation study with regard to dynamic network structures have been investigated in detail by our group 32–35 . However, rheology behavior of PET vitrimers in experiments such as different mechanisms in the whole relaxation process, especially nonlinear rheology of vitrimers, has not been explored rigorously so far.…”

“…It is worth noting that the effect of orientation is negligible in vitrimers without dissociation, and all PET vitrimers show rigid network under extensional flow. 35 In summary, the ductility of PET vitrimers would decrease with increase in crosslinking degree, resulting in smaller maximum strain. Obviously, the extensional behavior of materials can be tuned by the load of crosslinking agent (network density).…”

“…20 The theory and simulation study with regard to dynamic network structures have been investigated in detail by our group. [32][33][34][35] However, rheology behavior of PET vitrimers in experiments such as different mechanisms in the whole relaxation process, especially nonlinear rheology of vitrimers, has not been explored rigorously so far. Though this PET system is not perfect for research of vitrimer dynamics due to difficulty in characterization of detailed chemical structure, high-performance PET with excellent processability has an enormous potential in industry.…”