## 3 Results and Discussion
The maximum of tan \(\delta\) also slightly shifts to a lower temperature. A similar material response upon varying the furan/maleimide ratios was also observed for other furan/maleimide systems [6; 26]. The stiffness of the polymers generally decreases with the cross-link density [6; 54; 55; 56]. The loss moduli (E\") for the various ratios are more or less similar at lower temperatures, though here also a ratio-dependent softening temperature is observed. The softening points of the different samples are 146 \({}^{\circ}\)C and 136 \({}^{\circ}\)C for PE-fur/Bism-2:1 and -4:1, respectively. The softening points are determined from the points of the maximum decrease of the storage moduli during the first cycles. In a previous publication describing [1] measurements on long-chain PE-fur/Bism networks, comparable softening points have been reported: 138 \({}^{\circ}\)C and 136 \({}^{\circ}\)C for furan/bismaleimide mixtures with 2:1 and 4:1 molar ratios, respectively. Thus, it can be concluded that the addition of a chain stopper has had a negligible effect on the stability of the polymer network. It must be noted here that these softening temperatures are in overall agreement with the rDA ones determined by DSC (see the Supplementary Materials for a comparison). The slight differences might be due to the broadness of the DSC peaks but are also the result of more profound reasons. Although the DSC rDA temperatures refer to a molecular conversion of DA adducts into the corresponding precursors under static conditions, the DMTA-derived ones refer to a dynamic system in which the chains are mechanically stressed. In particular, during DMTA, the rupture of the first DA adducts allows translation and rotation to polymer segments. This might in turn favor the rupture of other DA adducts in a cooperative manner. Figure 5: DMTA of various specimens of PE-fur/Bism (**a**) Storage modulus E’. (**b**) Loss modulus E”. (**c**) Damping factor tan \(\delta\). Only the heating steps are shown for clarity. A life cycle of the test specimens prepared from PE-fur/Bism shows newly compression-molded bars prepared according to the method described in the Experimental Section (Figure 6). After a four-cycle DMTA experiment (20 \({}^{\circ}\)C \(-\)160 \({}^{\circ}\)C \(-\)20 \({}^{\circ}\)C), an optical change in the appearance/shape of the bars was visible (Figure 6b); the test specimens buckled under the force exerted during the DMTA measurement. Furthermore, a change in color is observed, changing from dark yellow before the DMTA to brown afterward. Since no antioxidants had been added, this color change may have been caused by the oxidation of the material. Analysis of the material by NMR and IR however, showed no (significant) formation of side products or any indication of degradation (see the Supplementary Materials for FT-IR and NMR spectra). After measuring, the bars were reground into powder using a motorized hand grinder (Figure 6c). Ultimately, the reground powder was again processed into DMTA bars via the same compression-molding procedure (Figure 6d). These resulting, recycled PE-fur/Bism bars displayed an even darker brown color in comparison to the dark-yellow and light-brown colors seen in Figure 6a,b, respectively. Analysis of the reprocessed material still indicated no impurities. As noted, the coloring is likely due to the formation of very small amounts of highly colored products that are undetectable by \({}^{1}\)H-NMR. This life-cycle sequence of PE-fur/Bism test specimens emulates the ultimate goal in terms of thermoreversibility: recycling polyester resin material after usage. A temporary decrease in the storage and loss moduli is also visible during the first DMTA cycle of the recycled test bars due to the buckling of the test specimens (Figure 7), as a result of the sideways forces exerted by the DMTA machine sample holder. Figure 6: Bars prepared from PE-Fur/Bism (**a**) Hot-pressed PE-Fur/Bism bars. (**b**) PE-Fur/Bism bars after 4 DMTA cycles. (**c**) Reground PE-Fur/Bism bars after DMTA.