An intelligent polyurethane material with self-healing at sub-zero temperatures enabled by copolymerization and dual dynamic bonds

2.1. Reagents

HTPB (liquid, -OH: 0.75 mmol/g, 99 %) and HTPE (liquid, -OH: 0.68 mmol/g, 99.95 %) was obtained from Liming Research & Design Institute of Chemical Industry Co., Ltd., Luoyang. N, N-dimethylformamide (DMF, liquid, ≥ 99.8 %) and dibutyl phthalate (DBP, liquid, 99.5 %) were obtained from Damao Chemical Reagent Factory, Tianjin. Isophorone diisocyanate (IPDI, liquid, 99 %) and triphenylbismuthine (TPB, powder) were obtained from Macklin Biochemical Technology Co., Ltd, Shanghai. 1,4-Butanedio (BDO, liquid, ≥ 99 %) was purchased by Jining Huakai Resin Co., Ltd. 2-amino-4-hydroxyl-6-methyl pyrimidine (UPy, 98 %) and b is (2-hydroxyethyl) disulfide (HEDS, liquid, 90 %) were obtained from Aladdin Co., Shanghai.

2.2. Samples preparation

First, 2.4 g of HTPB was added into a 150 ml three-necked flask and dried at 60 °C overnight. Next, 0.49 g of IPDI, 0.39 g of DBP, and 1.8 mg of TPB in 3 ml of DMF were placed in the flask. The mixture was mixed for 2 h at 60 °C to acquire prepolymer, then copolymerized with 0.6 g of HTPE. After that, 0.32 g of HEDS was dropped gradually and fully reacted with the copolymer at 60 °C for 2 h. Thereafter, 0.12 g of UPy dissolved in 2 ml of DMF was added and was mixed for another 2 h. Ultimately, the mixture was poured into a Teflon mold and cured for 24 h at 60 ℃ in a vacuum oven and coded as HUEPU. For comparison, three control groups were established, namely PU (p-HTPB, the chain extender is BDO), EPU (without HEDS and UPy, the chain extender is BDO), and HUPU (without HTPE).

2.3. Characterization

Temperature sweeps, frequency sweeps, and stress relaxation experiments were performed on the prepared polyurethanes using a dynamic thermomechanical analyzer. In the temperature sweep, the frequency was set to 1 Hz, the heating rate was 3 °Cmin^-1, and the temperature range was –90-60 °C. For the frequency sweep and stress relaxation experiments, the temperatures were set to 25 °C, –20 °C, and –40 °C, the frequency range of the frequency sweep was 0.1-100 Hz, and the strain in the stress relaxation experiment was set to 50 %. Fourier transform infrared spectroscopy (FTIR) is measured by NicoletiS10 with a wave number range of 600-4000 cm^-1and a resolution of 4 cm^-1. According to GB/T 528-92, tensile experiments were characterized on the Instron 5982 at a rate of 100 mmmin^-1. The results were averaged by three measured data at least. The test section sizes of the samples are 12×2×2 mm. In the self-repairing test, the polyurethanes were cut into two halves and kept at a certain temperature for 12h after contact. After that, the tensile test was conducted via the Instron 5982 universal testing machine at 100 mmmin^-1 to record elongation at break (${\epsilon }_b$). The self-repairing efficiency was defined as Eq. (1):

3.1. Design and characterizations of materials

The innovation of this design lies in the combination of copolymerization and dynamic bonding into the polyurethane to prepare polyurethane materials with excellent mechanical performances which can be self-repaired at sub-zero temperatures. In this study, polyurethane was prepared by a four-step method. First, HTPB reacted with IPDI to generate -NCO-terminated prepolymers. Then, it is copolymerized with HTPE, and then HEDS is reacted with the mixture to introduce disulfide bonds. Finally, the addition of UPy consumes excess -NCO groups and introduces quadruple hydrogen bonds. The chemical structure of polyurethane was characterized by FTIR. Fig.1 showed FTIR spectra of prepolymer and HUEPU. The spectrum of the prepolymer has obvious characteristic peaks at 1714 cm^-1, 3312 cm^-1, 1322 cm^-1, and 1525 cm^-1, which corresponds to the N-H and C = O stretching vibrations of the urethane bond, the C-N stretching vibration and the N-H bending vibration, respectively [10-12]. The prepolymer possessed an evident absorption peak at 2256 cm^-1, which is due to the terminal -NCO group, and the peak disappeared after reacting with UPy, which means the successful synthesis of HUEPU.

Fig. 1FTIR spectra of HEPU and HUEPU

3.2. Dynamic thermomechanical analysis

From the temperature sweep curves (Fig. 2(a, b)), it can be seen that the glass transition temperature (Tg) of HUEPU was in the range of –77.8~–63.8 °C, while that of PU was in the range of –82.1~–66.8 °C. The Tg of the two were not much different, and both were at ultra-low temperatures. Fig. 3(b) is the stress relaxation curves of HUEPU polyurethanes. It can be seen that the relaxation time (2.3 min) at –20 °C was close to that at 25 °C (1.0 min), which indicates that the molecular chain mobility of the polyurethane is still good at –20 °C. However, the relaxation time at –40 °C was 30.7 min, which indicates that the molecular chain movement is greatly hindered at this time. This is due to the introduction of the HTPE long chain, which improves the fluidity of the polyurethane molecular chain. In addition, the rapid exchange reaction of disulfide bonds and coordination bonds also has an important contribution.

Fig. 2Temperature scan curves of a) PU and b) HUEPU

Fig. 3a) Frequency scan curves and b) stress relaxation curves of HUEPU

3.3. Mechanical and self-healing properties

Fig. 4(a-c) showed the stress-strain curves of four polyurethanes at 25 ℃, –20 ℃, and –40 ℃, respectively. Fig. 4(d) showed the elongation at break and tensile strength of HUEPU polyurethane. It can be concluded from Fig. 4 that at three temperatures, the tensile strength of PU was the highest, while the elongation at break was the lowest because its cross-linking network is irreversible. While HUEPU showed the highest elongation at break, followed by HUPU and EPU, which can be attributed to HEDS and UPy providing a large number of dynamic reversible disulfide bonds and quadruple hydrogen bonds for the polyurethane network [13]. At the same time, copolymerization can further reduce the cross-linking density of the polyurethane network, improve the flexibility of the molecular chain, and thus improve the mechanical properties [14].

Fig. 4Mechanical properties images of four polyurethanes

To test the self-repairing performance of specimens, all the polyurethane specimens were cut into two halves by a knife, and after being contacted without external force, they were placed in a temperature box for repairing for a certain time, and then their mechanical properties were tested (Fig. 5). The PU and EPU had almost no self-healing ability because of their irreversible crosslinking as shown in Fig. 5(a). With the addition of HEDS and UPy, the self-repair ability of HUPU was significantly improved. This can be due to the rapid exchange reactions of the quadruple hydrogen bonds and disulfide bonds, which enabled the polyurethane network to be re-crosslinked. The introduction of HTPE improves the fluidity of the molecular chain of polyurethane and further promotes the exchange reaction of dynamic bonds. Therefore, HUEPU shows the highest self-repairing performance. More importantly, the elongation at break of the repaired HUEPU at –40 ℃ and –20 ℃ reaches 998 % and 1285 %, which is 70.1 % and 77.9 % of that of the original sample, which is quite meaningful [15].

Fig. 5a) Self-repairing performance curves of four polyurethanes, b) c) self-repairing performance curves of HUPU and HUEPU, d) self-repair efficiencies of HUEPU at different temperatures