The role of thermo-oxidative degradation on the dynamic behavior of disulfide-based epoxy vitrimers
Niklas Lorenz (TU Delft - Aerospace Engineering)
Andreas Klingler (University of Kaiserslautern-Landau)
William E. Dyer (TU Delft - Aerospace Engineering)
Santiago J. Garcia (TU Delft - Aerospace Engineering)
Baris Kumru (Vidyasirimedhi Institute of Science and Technology, TU Delft - Aerospace Engineering)
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Abstract
Vitrimers are a class of polymer networks that hold promise as recyclable thermosets with self-healing capabilities, enabled by dynamic molecular-level rearrangements. However, achieving the desired network rearrangements usually demands thermal treatments at elevated temperatures substantially above the glass transition temperature Tg while maintaining these harsh conditions for prolonged dwell times. Therefore, the present paper examines the effects of thermo-oxidative degradation on the dynamicity of a disulfide-based epoxy vitrimer. First, comparison with a non-disulfide-containing reference indicates that disulfide bond degradation is the predominant early-stage degradation mechanism. The thermo-oxidative degradation process was described using model-free kinetics fitted to thermogravimetric data, which was subsequently used to selectively control the degradation state of the vitrimer samples as a function of temperature and exposure time. FTIR identified the presence of a highly oxidized carbonyl surface layer, while DMTA confirmed a drop in the primary Tg[jls-end-space/]. Stress-relaxation testing indicates a temporary, favorable effect of decreased crosslink density: increased bond exchange rates, which in turn facilitate shorter dwell times for healing and shape reconfiguration. This manifests as shifts in the initiation of macroscopic flow, reducing the (re)processing temperature regime. In the long run, cleavage of the dynamic S-S crosslinks becomes predominant, adversely compromising the dynamic properties of these systems, as evidenced by incomplete relaxation and reduced macroscopic flow capabilities. These insights into the distinct effects of thermo-oxidative aging provide a critical foundation for evaluating the long-term viability after high-temperature exposure in an oxygen environment and have important implications for designing appropriate (re)processing regimes for disulfide-based epoxy vitrimers.
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