Evolving glass transition of curing networks based on Diels-Alder reversible bonds studied by dielectric relaxation spectroscopy

Description

Studies using Broadband Dielectric Spectroscopy (BDS) to explore reversible covalent network formation are notably limited, especially for systems based on Diels-Alder (DA) cycloadditions, despite the extensive use of this technique for irreversible network curing. Such dynamic networks based on thermo-reversible DA reactions hold promise as self-healing materials due to their ability to break and reform DA, leading to repeatable healing cycles and enhanced lifetime.
In this context, this study [1] employs BDS to systematically investigate a dynamic network formed via the thermoreversible DA reaction between a 4-functional furan-functionalized Jeffamine and a 2-functional maleimide. The focus is put on the investigation of the evolution during the network cure of the cooperative segmental dynamics as sensed by the dielectric - relaxation by measurement in (non-)isothermal conditions between 10-1 and 107 Hz.
In a first place, the relaxation times () associated with the -relaxation are determined by Havriliak-Negami fits [2] of dielectric spectra measured upon heating, immediately after partial isothermal network cure at 55 °C for different reaction times. The Vogel-Fulcher-Tammann (VFT) law is then fitted to  data only including temperatures without noticeable kinetic changes. Subsequently, a more general approach is attempted by fitting a unique set of VFT parameters for all experiments with a Vogel temperature (TV) for each experiment to take into consideration the degree of cure. Lastly, this approach was extended to describe all experimental relaxation traces, with and without including effects of the reaction by introducing a kinetic model to predict the cure conversion (Figure 1) [3].
Building upon this comprehensive characterization, a novel technique for continuous cure monitoring is proposed and validated via simulation of a system curing isothermally at 55°C. This technique enables real-time computation of the dynamic glass transition temperature (Tg) from the relaxation time (τα), utilizing only three VFT parameters obtained from the global fitting procedure. Such an approach might stimulate new applications of BDS-based cure monitoring.

Periode	2 sep 2024
Evenementstitel	The 12th Conference on Broadband Dielectric Spectroscopy and its Applications (BDS2024)
Evenementstype	Conference
Locatie	Lisbon, Portugal
Mate van erkenning	International