Solid–Liquid-Hysteresis Materials Based on Reversible Covalent Cross-Linking of Liquid Monomers

When a two-component liquid resin is cured, it usually ends up in the solid state and cannot be transferred to the liquid state again. This work introduces a polymeric material, which cross-links covalently at ambient temperatures, but can still be liquefied again on demand. The mechanism is based on the well-known concept of reversible cross-links, e.g., via Diels-Alder-reactions, which allow de-cross-linking at elevated temperatures. The presented material, however, differs from known variants by the fact that the non-cross-linked state is liquid at ambient conditions. Thus, the cross-linked solid material can be reversibly transferred to the liquid state again by heating. In contrast to classical melting, the material remains in the liquid state after cooling for a prolonged time. It therefore shows a behavior that is reminiscent of a melting hysteresis but is rather based on a superposition of thermodynamics and kinetics. At room temperature, cross-linking occurs rather slowly, while it is massively accelerated at raised temperatures. In contrast to supercooled liquids with a physical melting hysteresis, materials with a herein described "chemical liquefication hysteresis" are attractive for multiple applications, e.g., recycling, adhesives, coatings, or even polymeric bulk materials.