Time-Temperature-Transformation, Temperature-Conversion-Transformation, and Continuous-Heating-Transformation Diagrams of Reversible Covalent Polymer Networks

Time-temperature-transformation (TTT), temperature-conversion-transformation (TxT), and continuous-heating-transformation (CHT) diagrams are studied for a set of reversible elastomeric and thermosetting covalent networks, based on Diels–Alder (DA) furan–maleimide cycloaddition reactions using different concentrations of furan and maleimide functional groups. Microcalorimetry, modulated temperature differential scanning calorimetry, and dynamic rheometry are used as experimental tools in combination with kinetic modeling. The DA kinetics, based on two parallel equilibrium reactions for endo and exo cycloadducts, are optimized for the set of reversible networks cured between 20 and 90 °C. Each simulated isoconversion line in TTT and CHT, in contrast with irreversible networks, shows a totally different shape with a horizontal asymptotic limit at the high-temperature side, Tcure, corresponding to the DA equilibrium conversion xeq at Tcure. It is also proven that all gelation lines are isoconversion lines and that each gel conversion can be predicted by the Flory–Stockmayer equation. Moreover, the slight differences in the endo–exo kinetics and equilibrium constants lead to a predicted superposition and a double asymptotic behavior of the isoconversion lines in TTT and CHT. As a consequence, two subsequent gelation/degelation events can occur during nonisothermal curing, as shown in the CHT diagram. These phenomena are experimentally confirmed for one of the reversible covalent networks.

Publication year:2021

Keywords:reversible covalent bonding, gelation, de-gelation, vitrification

BOF-keylabel:yes

IOF-keylabel:yes

BOF-publication weight:6

Authors:Regional

Authors from:Higher Education

Accessibility:Open