Low - temperature self - healing polyurethane adhesives via dual synergetic crosslinking strategy

: Damage to polymer adhesives is one of the most common reasons for structural integrity damage of com - posite solid propellants. The introduction of self - repairing technology into the adhesive is expected to solve this pro - blem. However, at low temperatures, the self - repairing and mechanical properties of the materials are greatly impaired, thereby limiting the application of self - repairing adhesives in composite solid propellants. In this study, based on the dual synergistic crosslinking strategy, a poly - urethane adhesive exhibiting excellent self - healing and mechanical properties at low temperatures was success - fully prepared. The adhesive exhibited high self - repairing e ﬃ ciency and ultra - long elongation at break at low tem - peratures. Speci ﬁ cally, at a low temperature of − 40°C, the self - repair e ﬃ ciency was over 70% and the elongation at break was over 1,400%, which were much higher than the results of the control group. Moreover, the strength was comparable to that of the control group. This polyurethane adhesive shows excellent self - healing and mechanical properties at low temperatures and is expected to provide the strong self - healing ability and mechanical properties for composite solid propellants, alleviating the problem of structural integrity damage.


Introduction
The composite solid propellant grain accounts for more than 90% of the total mass of a solid engine, and its performance is the key to determining the quality of a solid engine (1)(2)(3). However, the composite solid propellant is considered the weakest part of the whole propulsion system. It is a type of polymer energetic composite based on polymer binder and is filled with metal combustion agents, oxidizer powder, and other combustion auxiliaries (4)(5)(6), and its properties are highly dependent on the temperature. As a result, it shows weak mechanical properties at low temperatures (7). However, due to extreme conditions, such as temperature shock and complex loads, in the process of production, storage, transportation, and use, the propellant can easily suffer from micro-cracks and even structural damage (1), which pose a serious threat to the safety and service life of the system. At present, the occurrence of structural damage cannot be avoided, and once damage occurs, it is difficult to repair or replace it, which causes serious waste of resources and great security risks. The research and development of a composite solid propellant binder with a selfrepairing function is expected to solve this problem.
Self-repair means that a material can repair its damage and maintain its structural integrity (8,9). Depending on their self-repairing performance, damaged materials can restore their original state and properties, improving their safety and durability thereby prolonging their life. Accordingly, in recent years, self-repair technology has been studied in various fields, especially in polymer materials (10)(11)(12). According to the repair mechanism, self-repairing materials can be divided into external (13)(14)(15)(16)(17) and intrinsic (18)(19)(20)(21)(22) self-repairing materials. White et al. (23) used a ruthenium catalyst to encapsulate a dicyclopentadiene (DCPD) repair agent in microcapsules and embedded them in an epoxy resin matrix; after the rupture of the microcapsule, the capillary diffuses the repair agent to the damaged area, polymerizes through contact with the embedded catalyst, and then repairs the crack. This method can achieve a repair efficiency of up to 75% but the number of repairs is very small. Mei et al. (24) combined dipyridine with the main chain of polydimethylsiloxane (PDMS). By exploiting the excellent fluidity of PDMS and the coordination ability of Pt 2+ and dipyridine, they cross-linked each other to obtain self-repairing linear elastomers. Self-healing can be achieved by optimizing the strength of the cross-linking interaction; when the elastomer is damaged, it can heal at room temperature without any repair agent or external stimulation. Liu et al. (17) designed a new type of self-healing polyurea-carbamate by combining the adhesive protein structure of mussels with dynamic aromatic disulfide bonds. The dissociation energy of the aromatic disulfide bond is low and the free radical is stable, so the exchange reaction can be conducted under mild conditions. The results showed that the self-healing rate of the polyurea ester at room temperature was 98.4% after only 6 h and that after 30 min at 60°C was 90%. From the above results, compared with external self-repairing materials, intrinsic self-repairing materials do not need additional repairing agents and catalysts; rather, they rely on the movement and entanglement of molecular chains and dynamic reversible bonds to achieve self-repairing. As such, intrinsic self-repairing technology is more widely used.
However, some studies have shown that in intrinsic self-repairing materials, the introduction of self-repairing properties will lead to a decrease in mechanical properties because the strength of dynamic bonds is weaker than that of ordinary covalent bonds. For example, Tavakolizadeh et al. (25) designed a new type of hydrogel with high strength and rapid self-repair ability, which can self-repair in 15 min at room temperature, and the self-healing efficiency can reach 85%. However, its tensile strength is very low, and only 0.027 MPa. In addition, the mechanical and self-repairing properties of polymer materials will decrease significantly at lower temperatures. This is because the molecular chain of polymer materials is greatly hindered at low temperatures, and its migration ability is significantly reduced, which is very disadvantageous to the exchange reaction of dynamic reversible bonds, leading to a decrease or even loss of self-repair ability. For example, Li et al. (26) prepared a polymer with high stretchability and self-repair ability that can achieve healing at −20°C, but the repair efficiency is low (68 ± 2%) and the repair time is too long (72 h). Moreover, its tensile strength at −20°C is only 0.23 MPa. Sun et al. (27) synthesized a polyurea elastomer that can quickly repair surface scratches at room temperature within 100 s. However, when the temperature dropped to 0°C, the elastomer could only partially heal, even after 72 h. Moreover, its mechanical properties are poor at 0°C, and its tensile strength is only approximately 0.27 MPa. Therefore, the design of a self-repairing composite solid propellant adhesive must overcome the disadvantages of self-repair and weak mechanical properties at low temperatures, as well as the performance balance between them.
This article proposes an effective method for preparing polyurethane adhesives with excellent low-temperature self-repair and mechanical properties based on a double synergistic crosslinking strategy. Through copolymerization and double dynamic bond cross-linking, the molecular chain fluidity of polyurethane adhesive was improved, and disulfide bonds and highly dynamic reversible coordination bonds were introduced, providing the necessary conditions for low-temperature self-repairing properties and high extensibility. In this work, the selfrepair and mechanical properties of the prepared polyurethane adhesive at low temperatures were characterized in many aspects.

Synthesis of HEPU-Zn
First, 2.4 g of HTPB was added to a 100 mL three-necked flask and dried overnight at 60°C in a vacuum environment. Then, 1.8 mg of TPB (catalyst), 0.39 g of DBP (plasticizer), and 0.49 g of IPDI (curing agent) were thoroughly mixed, added to the flask, and stirred at 60°C under nitrogen protection for 2 h. Next, 0.6 g of HTPE was mixed well with 1 ml of DMF, and the mixture was added to the flask to conduct a copolymerization reaction for 1 h. Next, 0.27 g of HEDS (chain extender) was gradually added to the mixture, followed by stirring for 2 h to obtain the ligand. Finally, 58.6 mg of ZnCl 2 was fully dissolved in 2 mL of THF and added to the mixture, reacted at 25°C for 12 h, poured into a polytetrafluoroethylene mold, and dried in a vacuum oven at 60°C for 24 h to evaporate the solvent and obtain the sample (labeled as HEPU-Zn). In addition, three control groups were fabricated, namely, the PU (HTPB was crosslinked with IPDI, and BDO was used as the chain extender instead of HEDS), EPU (without HEDS and ZnCl 2 , and BDO was used as the chain extender instead of HEDS), and HEPU (without ZnCl 2 ) groups. In our experiments, the ratio of the isocyanate group to the hydroxyl group (R-value) of each group was 1.

Characterization
The structure of the coordination bonds was characterized using a Horiba Labram Raman spectrometer (Horiba Company) equipped with an Ar laser source (excitation wavelength = 633 nm, scanning range = 200-800 cm −1 ). The absorption spectra in the wavelength range of 400-4,000 cm −1 were analyzed using Fourier transform infrared spectroscopy (FTIR) with an infrared spectrometer (NICOLETiS10) at a resolution of 4 cm −1 . In accordance with the GB/T528-92 standard, the tensile tests at −40°C and −20°C were performed at a speed of 100 mm·min −1 on an Instron 5982 material testing machine, and the results were averaged from at least three measured data. Optical microscopic images of the sample healing process were obtained using an AOSVI polarizing microscope (CM2000-3M100). For the self-healing experiments, the samples were cut into two pieces, contacted without an external force at each set temperature (−20°C or −40°C), and left to repair at this temperature for 12 h. Subsequently, they were tested on an Instron 5982 material testing machine. The tensile experiments were performed at each temperature at a speed of 100 mm·min −1 , and the tensile strength (σ b ) and elongation at break (ε b ) values were recorded. The selfhealing efficiency based on the strength and elongation at break (η σ and η ε , respectively) was defined according to Eqs 1 and 2, respectively. In addition, frequency scanning tests of the original and repaired samples were conducted using a dynamic thermomechanical analyzer (DMA, Q800, TA Instruments) to further characterize the low-temperature self-healing performance of HEPU-Zn. The frequency scanning range was 0.1-100 Hz, and the temperature was set to −20°C or −40°C. In the stress relaxation experiments, the temperature was set to 25°C, −20°C, or −40°C, and the strain was set to 50%. The relaxation time (τ) of the polyurethane network was defined as the time required for the stress-relaxation modulus to reach 37% of its original value and was plotted on a curve: 3 Results and discussion

Material design and structural characterization
The main purpose of this study was to develop a polyurethane adhesive with superior self-healing and mechanical performances at low temperatures. To achieve this objective, dynamic bond crosslinking and copolymerization strategies were simultaneously applied to the polyurethane network (Figure 1). Initially, HTPB and IPDI were used as raw materials to synthesize long chains terminated with isocyanate groups. After the chains were copolymerized with HTPE, HEDS was added to form HEPU ligands. Finally, HEPU ligands were crosslinked with Zn 2+ to obtain polyurethane (HEPU-Zn). The introduction of HTPE into HTPB effectively reduced the crosslinking density, which benefited the molecular chain mobility of polyurethane (28). In addition, HEDS can provide disulfide bonds to molecular chains. On the one hand, because the bond length of disulfide bonds is longer than that of C-C bond, the potential barrier of the molecular chain moving around the S-S axis is lower; on the other hand, the long molecular chain is decomposed into multiple-chain segments through the exchange reaction of disulfide bonds, which reduces the activation energy of the whole chain motion, thus improving the fluidity of the molecular chain and promoting the self-repair process. Moreover, the ZnCl 2 formed Zn-coordination bonds with the ligands, where the zinc ions have an alternating configuration, and transformation between the tetrahedral and octahedral structure is possible. This endowed the zinc coordination bonds with high dynamic exchange characteristics and provided excellent self-repairing properties to polyurethane, especially at low temperatures (29). Figure 2a shows the Raman spectra of HEPU-Zn, where the characteristic peaks at 245 and 319 cm −1 correspond to the stretching vibrations of Zn-N and Zn-O coordination bonds, respectively, indicating the presence of Zn-ligand bonds. The characteristic peak at approximately 626 cm −1 is due to the tensile vibration of the S-S covalent bond (30,31), suggesting the successful introduction of disulfide bonds into HEPU-Zn. Figure 2b shows the FTIR spectra of EPU, HEPU, and HEPU-Zn. Compared with the HEPU spectrum, new absorption peaks at 1,526 and 520 cm −1 are observed for HEPU-Zn, which are attributed to the stretching vibrations of Zn-N and Zn-O, respectively (29); this further indicates the presence of Zn-ligand bonds. In addition, the spectrum of EPU has a prominent absorption peak at 2,257 cm −1 , which is due to the excess of -NCO groups. However, HEPU and HEPU-Zn show no characteristic -NCO peak, which indicates that -NCO is completely consumed and the disulfide bonds are successfully introduced. From the above characterization results, it is concluded that the HEPU-Zn polyurethane samples containing Zn-coordination and disulfide bonds were successfully synthesized. Figure 3 shows the tensile stress-strain curves of the original and repaired PU samples, HEPU and HEPU-Zn  Figure 3c. The PU adhesive has no self-repairing ability at −20°C and −40°C, which is mainly due to its irreversible cross-linking network (32)(33)(34). After introducing copolymerization and disulfide bonds into the PU adhesive, the self-repair performance of HEPU at low temperatures is still very low because the disulfide bonds tend to be stable at low temperatures and its dynamic exchange capacity is essentially lost (12). Therefore, although the introduction of HTPE improves the fluidity of the molecular chain, self-repair cannot be realized without the reversible exchange reaction of dynamic bonds. After introducing Zn 2+ into HEPU, the self-repairing property of the HEPU-Zn adhesive at low temperatures is significantly enhanced. Specifically, the η ε and η σ values of HEPU-Zn are as high as approximately 79.3% and 88.6% at −20°C and 72.6% and 87.8% at −40°C, respectively. This can be attributed to the following factors. First, the introduction of HTPE reduces the crystallization of the adhesive and improves the fluidity of the molecular chain. Second, although the disulfide bond cannot have a dynamic exchange reaction at low temperatures, its bond length is greater than that of C-C, which reduces the potential barrier of the molecular chain around the S-S axis (35)(36)(37)(38), improving the dynamic characteristics of the adhesive network. Finally, although the dynamic characteristics of the Zn-urethane coordination bond are affected by low temperatures, its dynamic exchange ability is still strong and its dynamic exchange reaction helps reduce the energy barrier of the molecular chain slip, giving the adhesive network good low-temperature self-repair performance.

Low-temperature self-healing performance
To further characterize the self-repairing performance of the HEPU-Zn adhesive at low temperatures, the original and repaired samples were tested by DMA frequency scanning, as shown in Figure 4a and b. It can be seen that the G′ and G″ values of the repaired HEPU-Zn samples at −20°C and −40°C are lower than those of the original samples,  which is thought to be due to the incomplete healing of cracks. It also shows that HEPU-Zn has good self-repairing ability at −20°C and −40°C. Subsequently, stress relaxation experiments were conducted at a constant temperature and deformation rate to evaluate the recombination ability of the HEPU-Zn adhesive network to reflect its healing ability.  A smaller τ value means that the recombination ability of the adhesive network is higher, which is beneficial to selfhealing (39). Figure 4c shows the stress relaxation curves of the HEPU-Zn adhesive at 25°C, −20°C, and −40°C. Because the molecular chain movement is blocked at low temperatures, the recombination ability of the polyurethane network is weakened, and the value of τ increases with decreasing temperature. It is worth noting that the τ values of HEPU-Zn at 25°C and −20°C are similar, which indicates that HEPU-Zn has a strong network recombination ability at −20°C. Although the τ value of HEPU-Zn at −40°C is higher than that at 25°C, its internal stress decreased from 100% to 37% after only 22.46 min, which demonstrates the strong low-temperature recombination ability of the HEPU-Zn adhesive network. This is consistent with the above selfrepairing performance test results. To more intuitively characterize the self-repairing ability of the HEPU-Zn adhesive, its repair process at −20°C and −40°C was observed under an optical microscope, and the tensile condition of the damaged HEPU-Zn after repair at −20°C and −40°C was demonstrated by the manual tensile test, as shown in Figure 5a and b. It can be observed under the optical microscope that after the adhesive was repaired at −20°C and −40°C, the cracks became less noticeable, which demonstrates the excellent self-repairing ability of HEPU-Zn at low temperatures. And, in the manual tensile test, the repaired HEPU-Zn at low temperatures (−20°C and −40°C) still shows strong stretchability. Figure 5c shows the self-healing mechanism of the HEPU-Zn adhesive. When the adhesive is damaged, there will be many reversible active groups on the fracture surface, which is the key to self-repair. When the fracture  surfaces are in contact with each other, with the help of excellent molecular chain fluidity, the active groups contact each other to form new coordination and disulfide bonds, leading to self-repair. In summary, the HEPU-Zn adhesive has excellent self-repairing properties at low temperatures.

Low-temperature mechanical properties
The tensile strength and elongation at break can be obtained from the stress-strain curves of the original samples of PU, HEPU, and HEPU-Zn adhesives in Figure 3 at −20°C and −40°C, as shown in Figure 6a and b. It can be seen that the PU adhesive shows higher tensile strength and lower elongation at break at −20°C and −40°C. After introducing copolymerization and disulfide bonds into the PU adhesive, the elongation at break of the HEPU adhesive increases significantly but its tensile strength decreases. This is because the introduction of HTPE reduces the crystallization of the adhesive and improves the fluidity of the molecular chain. Thus, the segmented action of the disulfide bond and its longer bond length will further reduce the activation energy of the molecular chain movement and improve the dynamic characteristics of the adhesive network, resulting in decreased strength and increased elongation at break. After introducing Zn 2+ into HEPU, the elongation at break of the HEPU-Zn adhesive is further improved, and the elongation at break could still reach approximately 1,424.5% at −40°C. Moreover, its strength is also higher than that of HEPU. This can be attributed to the excellent dynamic exchange ability of the Zn-urethane coordination bond, which dissipates significant strain energy and increases the elongation at break. In addition, Zn 2+ can act as a cross-linking point to increase the cross-linking density of the adhesive network, thus improving the tensile strength. Figure 7 shows the DMA frequency scanning curves of the original samples of PU and HEPU-Zn binders at −20°C and −40°C, respectively. Generally, a higher G″/G′ ratio indicates better fluidity of polyurethane chains (40). In addition, the G″/G′ ratio of HEPU-Zn is significantly higher than that of PU at −20°C and −40°C, indicating that the molecular chain mobility of HEPU-Zn is higher at low temperatures, which is consistent with our previous conclusion. Based on the above analysis, the HEPU-Zn adhesive has excellent mechanical properties at low temperatures.
To further highlight the excellent self-repair and mechanical properties of the HEPU-Zn adhesive at low temperatures prepared in this study, the self-repairing temperature, repair time, self-repairing efficiency, elongation at break, and tensile strength of HEPU-Zn polyurethane are compared with those of other self-repairing materials, and the results are listed in Table 1. It can be seen that the properties of the HEPU-Zn polyurethane adhesive prepared in this study are comparably excellent. It can achieve self-repair at lower temperatures, and the self-repair efficiency can reach a higher level. Moreover, it has an ultra-long elongation at break and high tensile strength.

Conclusions
Based on the double synergistic crosslinking strategy, a kind of HEPU-Zn adhesive with excellent self-repair and mechanical properties at low temperatures was successfully prepared by introducing zinc-carbamate coordination and Table 1: Comparison of self-healing systems, self-healing temperature (T), self-healing time (t), and self-healing efficiency (η ε , η ε ), as well as the tensile strength (σ b ), and elongation at break (ε b ) of the polymers reported in this work and others

References
Dynamic bonds dynamic disulfide bonds into the polyurethane adhesive. The adhesive has strong self-repairing ability at low temperatures, with self-repairing efficiencies as high as approximately 79.3% and 72.6% at −20°C and −40°C, respectively. In addition, HEPU-Zn has an ultra-long elongation at break, as high as approximately 1,424.5% at −40°C, which is approximately 359.5% higher than that of the PU adhesive. Moreover, its tensile strength is comparable to that of PU. The binder is expected to endow the composite solid propellant with the strong self-repairing ability and mechanical properties, alleviating the problem of structural integrity damage.