Understanding the Interfacial Exchange Reaction Kinetics and Rheological Behavior of Multiphase Vitrimers

Next-generation thermoplastic vulcanizates (TPVs) employ a multiphase architecture constructed from vitrimeric networks with distinct cross-linking densities, where accelerated interfacial network relaxation confers unique advantages during high-throughput reprocessing (e.g., extrusion). Nevertheless, the influences of disparity in cross-linking density between the original networks and of interphase broadening on the viscoelastic and interfacial relaxation behaviors of multiphase vitrimers remain poorly understood. Here, a model multiphase vitrimer system consisting of monodioxaborolane-grafted and bis-dioxaborolane-cross-linked styrene–butadiene rubber (SBR) was developed to systematically evaluate the effects of grafting density and cross-linking density on interfacial exchange kinetics. The investigation reveals that stress overshoot and interfacial relaxation are both governed by interphase broadening and cross-linking density contrast. When the contrast between the two original networks is significant, interfacial relaxation persists over long time scales of interfacial broadening, whereas the contrast is insignificant, and interfacial relaxation is limited to short time scale of interfacial broadening. Moreover, increasing interfacial exchange extent and dispersed phase cross-linking density both exacerbate irreversible structural damage during cyclic reprocessing. These findings imply that multiphase vitrimers designed to exploit interfacial relaxation for efficient extrusion reprocessing require judicious control of the component structure and processing/annealing time windows, providing theoretical guidance and design criteria for their optimization and applications.