*3.4. Thermal Reversibility of PET-DA-PU*

To study the thermal reversibility of PET-DA-PU, NMR was employed to investigate the changes in 1H NMR spectroscopy after the heat treatment in Figure 7 [40]. It is well known that BMI and furan prepolymer formed in the retro-DA reaction of PET-DA-PU under heat treatment. As marked in Figure 7, the chemical shift at 3.17 ppm (labeled as a), 4.93 and 4.96 ppm (labeled as b), 6.54 ppm (labeled as c), 6.63ppm (labeled as d), 5.24 ppm (labeled as e), 7.14 ppm (labeled as g), 7.35 ppm (labeled as f) decreased visibly, and the new chemical shifts at 7.25 ppm (labeled as a'), 7.28 ppm (labeled as f'), 7.15 ppm (labeled as g') owing to BMI, and chemical shifts at 7.69 ppm (labeled as b'), 6.46 ppm (labeled as c'), 6.54 ppm (labeled as d'), 5.09 ppm (labeled as e') owing to furan prepolymer appeared after heat treatment [41–44]. It is indicated that DA bonds in PET-DA-PU de-bonded under heat treatment.

**Figure 6.** SEM images of freeze-fractured (**a**,**b**) and tension-fracture (**c**,**d**) surface of the PET-DA-PU gels. (wrinkles were marked as red square frame, and ravines were marked as blue circle frame).

**Figure 7.** 1H NMR spectra of PET-DA-PU during the heating procedure at 100 ◦C for 20 min. (PET-DA-PU was marked as black, and PET-DA-PU after heat treatment was marked as red, '\*' for DMSO-d6).

To investigate the efficiency of retro DA reaction under heat treatment quantitatively, the changes in 1H NMR spectroscopy of DA diol were also studied [45]. As shown in Figure 8, the chemical shifts at 5.18 ppm (labeled as a), 4.08 and 3.77 ppm (labeled as b), 6.57 ppm (labeled as c), 5.0 ppm (labeled as d), 3.02 and 3.18 ppm (labeled as e), 7.35 ppm (labeled as f), 7.14 ppm (labeled as g) owing to DA diol were significantly decreased after heat treatment, and the new chemical shifts at 5.19 ppm (labeled as a'), 4.38 ppm (labeled as b'), 6.27 and 6.38 ppm (labeled as c'), 7.57 ppm (labeled as d') owing to frufuryl alcohol, and 7.36 ppm (labeled as e'), 7.26 ppm (labeled as f'), 7.15 ppm (labeled as g'), 4.03 ppm (labeled as h') owing to BMI were observed. Based on the changes in the 1H NMR spectroscopy peak areas of DA diol after heat treatment, conversion x can be calculated from the following formula:

$$\chi = \frac{\mathbf{A\_{c'}} + \mathbf{A\_{c''}}}{\mathbf{A\_{c}} + \mathbf{A\_{c'}} + \mathbf{A\_{c''}}} \tag{1}$$

Ac, Ac', Ac" integral areas of peaks c, c', c", respectively.

**Figure 8.** 1H NMR spectra of DA diol during the heating procedure at 100 ◦C for 20 min. (DA diol was marked as black, and DA diol after heat treatment was marked as red, '\*' for DMSO-d6).

According to the formula, the conversion of DA diol was 70% after 20 min heating treatment at 100 ◦C. It demonstrated that DA bonds in PET-DA-PU can degrade into BMI and furan prepolymer in a short time via retro DA reaction, and it is good benefit for repairing of PET-DA-PU. All of these results indicate that PET-DA-PU has a good thermal reversibility.

#### *3.5. Healing Behavior of PET-DA-PU Based Composites*

To evaluate the healing efficiency of PET-DA-PU in propellant, the PET-DA-PU/Al/Na2SO4 composite was prepared and measured by the universal testing machine. The PET-DA-PU/Al/Na2SO4 composite was prepared in a ratio of 1/0.5/0.5, and the obtained composite film was cut into dumbbell-shaped specimens and tested according to GB/T528-1998. The PET-DA-PU/Al/Na2SO4 composite film was cut off and heated at 100 ◦C for 20 min, then cooled down to 60 ◦C and kept for 48 h and the healed PET-DA-PU/Al/Na2SO4 composite

films were obtained. As shown in Figure 9 and Table 1, in comparison with the PET-DA-PU/Al/Na2SO4 composite (the tensile strength was 0.82 MPa, with an elongation at 138.8%), the tensile strength of repaired PET-DA-PU/Al/Na2SO4 composite was 0.72 MPa with an elongation at 113%. Hence, the healing efficiency of PET-DA-PU/Al/Na2SO4 composite was 87.8%, which was slightly lower than PET-DA-PU. It is due to the reduction in DA bond density in composite as to the introduction of Al and Na2SO4 and decrease the healing efficiency of the PET-DA-PU/Al/Na2SO4 composite. However, the PET-DA-PU/Al/Na2SO4 composite still has a high healing efficiency exceeding 85% and exhibits outstanding self-healing performance.

**Figure 9.** Mechanical property of PET-DA-PU/Al/Na2SO4 composite and its damaged and healed samples.

**Table 1.** Tensile testing of PET-DA-PU/Al/Na2SO4 composites before and after reparation.


The fracture morphologies of the PET-DA-PU/Al/Na2SO4 composite which were prepared by freeze-fracturing and tensile-fracturing were also observed by SEM. As shown in Figure 10a,b, the crack surface of the composite was smooth and the interface between the exposed particles and polymeric matrix was fuzzy, meaning that the interface adhesion property between solid filler and matrix is strong. However, a number of exposed particles and cracks appeared in Figure 10c,d, and voids were found between the exposed particles and polymeric matrix, meaning that the interface adhesion property between solid fillers and PET-DA-PU matrix became poor during the tensile process [46]. Generally, the cracks and voids also occurred in aging stages of polymeric binder. Therefore, the PET-DA-PU polymeric binder which can repair the micro-cracks within the matrix under heat treatment could prolong its service life.

**Figure 10.** SEM images of freeze-fractured (**a**,**b**) and tension-fracture (**c**,**d**) surface of the PET-DA-PU/Al/Na2SO4 composite. (the cracks and voids were marked as red arrows).

#### **4. Conclusions**

A Diels–Alder bond containing PET based linear polyurethane, PET-DA-PU, was synthesized using PET as raw material, DA diol as chain extender agent, and TDI as coupling agent. From ATR-FTIR, 1H NMR and 13C NMR results, the PET-DA-PU was synthesized successfully via prepolymer process. The DSC curves indicated that PET-DA-PU had a low glass transition temperature of −59 ◦C, which allows it to work in a low temperature environment. The evolution of cracks on PET-DA-PU was observed by POM, and the results indicated that the cracks on PET-DA-PU film completely disappeared in 9 min under the heat treatment at 100 ◦C. A tensile test was used to determine the self-healing efficiency by the recovery of the largest tensile strength after being damaged and healed; the self-healing efficiency of PET-DA-PU can reach 89.1% after 20 min heating treatment at 100 ◦C. NMR spectroscopy indicated that the efficiency of retro DA reaction of PET-DA-PU can reach up to 70% after 20 min heating treatment at 100 ◦C. SEM images were used to investigate the fracture morphologies of the PET-DA-PU film, and the results revealed the micro-cracks along with the blocky aggregated hard segments which were the important reasons for fracture. Moreover, the PET-DA-PU/Al/Na2SO4 composite was also prepared to simulate the propellant formulation, and its healing efficiency was 87.8% under the same heat treatment. In the SEM images of tensile-fractured PET-DA-PU/Al/Na2SO4 composite, exposed particles, cracks and voids were observed, meaning poor interface adhesion property between solid fillers and PET-DA-PU matrix. Consequently, PET may find its application as a novel self-healing binder in propellant formulations.

**Author Contributions:** Conceptualization, M.X. and N.L.; methodology, H.M.; software, X.L.; validation, N.L., X.L., J.D. and B.T.; formal analysis, J.D.; investigation, M.X.; resources, B.T.; data curation, N.L.; writing—original draft preparation, M.X.; writing—review and editing, N.L. and X.L.; visualization, J.D. and B.T.; supervision, H.M.; project administration, N.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.
