# Bio-Based Aromatic Polyesters Reversibly Crosslinked via the Diels-Alder Reaction
## 3 Results and Discussion
The T\({}_{\text{g}}\) of the material will become closer to that of the uncrosslinked polymer as less crosslinker is added. Interestingly, when reducing the amount of crosslinker even further to the ratio of 1:10 MAL:FUR, the softening point of DPA-fur also dropped below the T\({}_{\text{g}}\) of the uncrosslinked polymer. The aromatic groups present in DPA-fur are likely involved in \(\pi\)-\(\pi\) stacking interactions, which are disturbed by the presence of small amounts of crosslinker. Apparently, the \(\pi\)-\(\pi\)-interactions have a larger influence on the softening point than the small amount of crosslinker present. Figure 5: DMTA measurements of polymers containing various maleimide to furan ratios. Finally, the softening point observed for the 1:1 MAL:FUR DPA-fur network was much closer to the theoretical RDA temperature for furane and maleimide groups (e.g., 150 [35]), which could be an indication of a more stable crosslink. Furthermore, as the material remains crosslinked for a larger range of temperatures (e.g., decrosslinking occurs at higher temperatures), this should provide a larger window of application for the resulting material than the PFS- and PK50-based materials. Another way of influencing the thermal behavior is the use of a different crosslinker [36]. When the aromatic bismaleimide was substituted by an aliphatic one containing a C12 chain between the maleimide groups, a decrease in the softening point by 10 \({}^{\circ}\)C was observed (Figure 7). These results are comparable to the findings reported by [PERSON] et al. where a drop of approximately 10 \({}^{\circ}\)C was reported, as well for the 1:2 ratio of MAL:FUR when substituting the aromatic bismaleimide for an aliphatic one containing a C\({}_{2}\)OC\({}_{2}\)OC\({}_{2}\) ether bridge [38]. This behavior likely comes from the increased flexibility of the aliphatic bismaleimide compared to its aromatic counterpart. This increased flexibility should result in a less rigid network. Furthermore, as suggested by the gelation experiments, the aliphatic crosslinker should be able to participate in backbiting coordination, limiting its availability for the formation of crosslinks. Independent of the exact mechanism, the observed results indicate that in the case of the present polymer, the amount and structure of the crosslinker might be conveniently used to manipulate the mechanical behavior of the end product. In order to assess the mechanical reworkability and thermal recovery of the material, cyclical DMTA measurements were performed. The samples were heated to 160 \({}^{\circ}\)C and successively cooled back to room temperature (20 \({}^{\circ}\)C) for four cycles (Figure 8). Figure 6: Softening temperature of various DA systems for varying MAL:FUR ratios (0:1 refers to the uncrosslinked polymer). During cooling, the material was expected to re-form the DA crosslinks that had been broken at high temperatures by the RDA reaction (which is dominant at these elevated temperatures). If the DA reaction is performed consistently, the material should display the same mechanical properties in all subsequent cycles. A relatively large change in properties was observed between the first and second cycle; this is consistent with the shift observed in DSC, and the explanation given of a kinetically favored orientation (endo vs. exo) still holds true. However, after each consecutive cycle, the softening point of the material increased. If the only equilibrium involved is the changing from an endo to exo isomer, the difference should not be as large as that observed. Most likely, a more permanent change occurs in the material under these circumstances. A well-known side reaction of  
Figure 7: DMTA measurements of aromatic (**top**) and aliphatic (**bottom**) crosslinkers. the Diels-Alder adduct of furan and maleimide is the formation of an aromatic ring via the elimination of water [35].