Linear viscoelasticity, nonlinear rheology and applications of polyethylene terephthalate vitrimers

To enhance mechanical properties and processing performance of poly(ethylene terephthalate) (PET), it was upcycled to processable PET vitrimers with different crosslinking degrees by introducing dynamic network. The thermodynamics and linear viscoelasticity of PET vitrimers were explored by non-isothermal crystallization, isothermal sweep, frequency sweep and stress relaxation after incorporation of network. In particular, rheology experiments are sensitive to network structure and bond exchange mechanism in vitrimers. The pseudo-master curves show that relaxation processes are composed of three characteristic regions: Rouse-type relaxation of network strands, rubbery plateau and terminal relaxation of network, which is consistent with reversible gelation (RG) model. Two distinct (flow and chemical reaction) activation energies, are obtained by time-temperature superposition principle due to different temperature dependences of two relaxation behaviors. In addition, nonlinear rheology of PET vitrimers was investigated by extensional flow and start-up shear at the same Weissenberg number, and obvious strain hardening behavior were observed in all vitrimers. However, vitrimers with different crosslinking density exhibited distinct strain hardening trends as increase of extensional rate, corresponding to the ductility of material. On the basis of kinetics study, self-repairing and welding properties are further quantitatively explored for industrial applications.