The connection between network structure and terminal relaxation of unentangled vitrimer melts: A dynamic...

Vitrimers are flowable cross-linked polymer networks that have drawn significant attention as a platform for developing novel polymer materials. Here, we utilize a dynamic cross-linked Gaussian-strand model to investigate network structure-viscoelasticity relationships for unentangled vitrimer melts. The terminal relaxation of model vitrimers depends on the number of dynamic linkages and the strand-length distribution. Because the short-term dynamics are coarse-grained, the stress relaxation modulus curves of the model vitrimers exhibit well-defined plateaus. This is consistent with experimental observations, as Rouse dynamics usually occurs far faster than the relaxation of networks in most cases. For uniform model vitrimers, the distribution of relaxation times moves toward longer times as the dynamic-linkage number increases, extending the spread and changing the shape of relaxation curves. The links between the longest relaxation time and zero-shear viscosity with the dynamic-linkage number are further examined. The ratio between the maximum loss modulus and plateau modulus is also affected by the dynamic-linkage number and strand-length distribution. The intrinsic exchange reaction kinetics and segmental mobility determine the topological reshuffle of model vitrimers. The temperature dependence of their terminal relaxation is also discussed in order to gain a better grasp of their characteristic viscoelasticity.