Probing the glass transition in reversible cross-linked polymer composites.

Understanding the nature of glass transition is still a great challenge. Glass transition is widely observed in many glassy materials; however, it has never been unambiguously observed in reversible cross-linked polymer, which is an ideal model of the percolation process. Herein, we report the synthesis of a reversible cross-linked polymer incorporated with four-armed Diels-Alder (DA) dynamic covalent bonds, and the robust experimental observation of percolation-induced glass transition in this reversible four-armed cross-linked polymer (DAMF1). Temperature-modulated differential scanning calorimetry (TMDSC) experiment results clearly revealed the presence of a glass transition along with an endothermic or exothermic peak associated with DA/retro-DA (RDA) reaction related to the reconstitution/disassociation of the DAMF1's four-armed cross-linked network. C variable-temperature solid-state NMR experiments further confirmed the DA/RDA reaction during glass transition at a molecular level. The above experimental results provide a direct experimental evidence for the recently developed percolation model of glass transition, which provides new insights into the nature of glass transition.