Vitrimer Transition Phenomena from the Perspective of Thermal Volume Expansion and Shape (In)stability

Abstract

Vitrimers are covalently cross-linked polymers that can be reshaped and recycled, triggered by the temperature. Well above the canonical glass transition temperature Tg, they gain malleable, thermoplastic-like properties; below, they behave as thermosets. The responsible molecular mechanism is enabled by dynamic covalent bond exchange. However, the underlying physics of the transition from thermoplastic to thermoset properties is rather unclear and the subject of current scientific debate. In this work, we address the questions of what extent the temperature-dependent malleability of vitrimers is reflected in the thermal expansion behavior and, conversely, to what extent the thermal expansion behavior is suited to characterize the respective creep behavior of vitrimers. It will be shown that at least in the case of the used model vitrimer, only the canonical glass transition is able to stop the vitrimer to undergo irreversible shape changes and that the often-discussed topology freezing temperature of vitrimers Tv is purely operational.