Understanding the Topology Freezing Temperature of Vitrimer-Like Materials through Complementary Structural and Rheological Analyses for Phase-Separated Network

Vitrimers are sustainable cross-linked polymers characterized by an associative bond exchange mechanism within their network. A well-known feature of vitrimers is the Arrhenius dependence of the viscosity or relaxation time. Another important aspect is the existence of a topology-freezing temperature (<i>T</i>_v), which represents a transition between the viscoelastic solid state and the malleable viscoelastic liquid state. Various methods, including viscosity-temperature plots and temperature-ramp creep (or dilatometry), have been proposed for determining the <i>T</i>_v. In this study, we complementarily employ X-ray scattering-based structural analysis and rheological analysis to assign <i>T</i>_v in phase-separated vitrimer-like materials undergoing trans-<i>N</i>-alkylation bond exchange. Note that the <i>trans</i>-<i>N</i>-alkylation progresses via the dissociative bond exchange pathway, whereas our previous studies demonstrated that the temperature-dependence of relaxation time followed the Arrhenius dependence, which was the reason for the classification as a vitrimer-like material. Specifically, we identify <i>T</i>_v as the temperature at which an anomalous increase in domain distance occurs during the rubbery state in the structural analysis. In the rheological analysis, <i>T</i>_v corresponds to the transition temperature marking the shift from the Williams-Landel-Ferry dependence to the Arrhenius dependence in the shift factors used to create master curves for frequency sweep rheology data. Importantly, both methods yield nearly the same <i>T</i>_v, validating the accuracy of the proposed assignment and, thus, providing valuable insights into the specific properties of vitrimers.