Clustering and Nanoscale Vitrimeric Behavior in Telechelic Polybutadiene Model Vitrimers

Despite the promise of vitrimeric materials applications, there are many aspects that remain poorly understood. To advance the fundamental knowledge of these materials, model systems of well-controlled composition are needed. Here we report the synthesis of polybutadiene-based model vitrimers, which are further investigated with respect to their structure and dynamic properties. Four vitrimers with different cross-linking concentrations were obtained by modifying the molecular weight of the precursor functionalized polybutadiene precursors. A tetrafunctional aromatic cross-linker was used in all cases. Due to the nature of the cross-linker, the resulting vitrimers exhibit fluorescence indicative of cross-linker association. The structural investigation by small-angle X-ray diffraction showed that the cross-linkers cluster together to form very stable cylindrical structures linked by polybutadiene chains. The dynamic behavior of the vitrimers, as studied by differential scanning calorimetry and broadband dielectric spectroscopy, can be interpreted by the presence of such a cluster structure, where already a few degrees above the glass transition there are significant fluctuations of the dynamic covalent bonds, which remain essentially limited to the intracluster level─i.e., confined in the interphase between the cross-linkers’ clusters and the polymer. This results in rubber-like vitrimeric materials in which the mechanical stability is maintained by the cluster network as long as it is not perturbed by the application of high mechanical stresses, in stark contrast to recently investigated polyisoprene-based model vitrimers with flexible triamine cross-linkers, which also exhibit cluster structure.