Transparent self-healing polyurethane networks were firstly prepared by in situ polymerization of PTMG as a soft segment structure, HDI as a hard segment structure and stannous octanoate as a catalyst. As shown in Fig. 2a, optical images depicted the high transparency of colorless PTMG based polyurethane networks. The synthesized self-healing polyurethane elastomers were characterized by Fourier transform infrared spectroscopy (Fig. 2b). Specifically, the absorption band at 3300 cm− 1 was caused by the stretching vibration of N-H. The asymmetric and symmetric stretching vibrations of C-H corresponded to characteristic absorption peaks at 2849 cm− 1 and 2940 cm− 1, respectively. The stretching vibration region of the C = O group was at 1720 cm− 1. The disappearance of the peak at 2260–2280 cm− 1 (N = C = O stretching band) in the spectrum proved that the -NCO of HDI reacted completely with -OH to form polyurethane. Figure 2c showed the XRD spectrum of the polyurethane film. It was observed that after the addition of BP, the composite materials still displayed a distinct amorphous structure without significant peaks of fillers.
Figure 2d showed the DSC curves of the hybrid network films. As can be seen from the graphs, all five samples presented one melting peak around 20°C, which belonged to the crystalline melting temperature of PTMG. However, in the XRD results, only amorphous peaks were observed, which was mainly due to the coexistence of the crystalline and amorphous regions. In the reduced temperature crystallization curve of the polyurethane film (Fig. S1), the crystallization temperature of strontium aluminate containing 5% was 4.2°C, higher than that of BP content of 3%. This was due to the fact that the rare earth strontium aluminate acting as a heterogeneous nucleator, allowing the sample to crystallize at higher temperatures. In addition, the thermal stability of the PTMGBP composite films was analyzed, as shown in Fig. 2e, the thermogravimetric analysis (TGA) curves of the PTMGBP films demonstrated two different stages of weight loss, at 290–360°C and 360–450°C, respectively. The first stage originated from the decomposition of the hard segment of the PTMGBP networks, while the second stage originated from the decomposition of the soft segment, indicating the thermal stability of the PTMGBP films. More detailed descriptions of the thermal properties were presented in the supplementary information (Fig. S2 and Table S1).
In order to investigated the chain migration and viscoelastic behavior of polyurethane films, the temperature dependence of storage modulus (G') and loss modulus (G") was evaluated by dynamic mechanical analysis. In Fig. 2f, the PTMGBP1 material exhibited a higher storage modulus, which was probably due to its denser hydrogen bonding of crosslinking. At around − 90°C, the modulus started to decrease, indicating the onset of soft segment movement. At temperatures above − 65°C, the modulus of the material dropped sharply, which meant that the hydrogen bonds began to dissociate. When the temperature was raised to 30°C, the sample films softened and could not retain their shape due to the dissociation of most of the intermolecular hydrogen bonds. In Fig. 2g, loss tangent delta (tanδ) of the polyurethane films was obtained over the temperature range − 100 ~ 50°C using DMA. The polyurethane films exhibited a typical tanδ pattern with a single peak designated as the glass transition temperature (Tg). The Tg of the PTMGBP1 film was approximately − 58°C, indicating a highly elastic state at room temperature. When the BP content was increased to 5%, the glass transition temperature of the PTMGBP5 film increased slightly to -56°C.
The hydrophilicity of the films was furtherly tested, as shown in Fig. 2h, the contact angle gradually increased as the BP content increased, mainly due to the fact that strontium aluminate itself was hydrophobic. The solvent resistance of the films was tested and the results were shown in Fig. 2i. The gel content gradually decreased with increasing BP content, but still exceeded a minimum of 80%, indicating a high degree of cross-linking of the polymer matrix. In addition, its degree of swelling did not exceed 160% at the highest, indicating a good resistance to solvents (Fig. S3). The effect of different BP contents on the tensile properties of the films was investigated, with the tensile strength gradually increased as the strontium aluminate content rose over the same strain range (Fig. S4).
The phase morphology of hybrid network films was observed by AFM. The AFM images in Fig. 3a showed that the PTMGBP films had a distinct microphase separation structure, which was formed by the aggregation of soft segments (PTMG, dark region) and hard segments (H-bonded aggregates, bright region). The PTMGBP1 film with the highest content of hard segments exhibited the most pronounced microphase separation. Moreover, the healing process of PTMGBP1 was observed using an optical microscope, as shown in Fig. 3b, the size of the cracks gradually decreased over time and eventually healed completely.
After 1 hour of healing at 60°C, the PTMGBP films showed different healing efficiencies. As shown in Fig. 3c, PTMGBP0 films stretched to 1200%, and they could present an elongation at break of 971%, after 1 hour of healing at 60°C, giving a healing efficiency of 80.9%. Similarly, as shown in Fig. 3d, the PTMGBP1 film stretched to 1149%, and they could present an elongation at break of 907% after 1 hour of healing at 60°C, with a healing efficiency of 78.9%. This was mainly due to the small amount of BP occupying a small portion of the crosslinking point, resulting in a reduction in urethane content, a reduction in hydrogen bonding and a reduction in healing efficiency. Different amounts of strontium aluminate had different effects on the healing efficiency of the films, but overall there was a trend for the healing efficiency to decrease as the BP content increased (Fig. S5-S7). In addition, we investigated the effect of PTMGBP1 films on self-healing efficiency at different temperatures and healing times. As shown in Fig. 3e, the elongation at break increased from 700–907% at 60°C for 15, 30, 45 and 60 min of healing, respectively. Also, the healing efficiency showed the same trend after healing at 45, 50, 55 and 60°C for 1 h respectively (Fig. S8 and S9). This was mainly due to the fact that longer healing temperatures and times provided opportunities for the re-formation of hydrogen and carbamate bonds.
Dynamic urethane bonding and hydrogen bonding supported the PTMGBP composite film with the ability to self-heal. In order to evaluated the influence of BP content on the healing performance of PTMGBP film, the tensile properties of the half-cut films were tested after healing at 60 ℃ for one hour. As shown in Fig. 4a, PTMGBP1 film can be stretched to 900% without fracture after healing. More interestingly in Fig. 4b, the PTMGBP1 film with a width of 10 mm and a thickness of 2 mm could still lift a weight of 6 kg with little deformation after healing, which was unbearable for traditional soft rubber. The SEM results in Fig. 4c indicated that the incision was significantly smaller after healing at 60 ℃ for one hour, but it was still not completely healed, which might be due to insufficient healing time or low healing temperature. As shown in Fig. 4d, the self-healing efficiency gradually increased from 60.9–78.9% after healing at 60°C for 15, 30, 45 and 60 minutes respectively, which corresponded to the smaller SEM incisions. Figure 4e demonstrated the mechanism of self-healing of PTMGBP films, where hydrogen bonds were re-formed under heating conditions.
As an initial demonstration, as shown in Fig. 5a, we have coated the PTMGBP1 film with a layer of gold to form a stretchable self-healing conductor that was connected to a 3 V battery, and a light emitting diode (LED), which can be observed to light up when connected. The LED was switched off when the hybrid conductor was cut into two pieces. After heating at 60°C for 20 min, the two conductor pieces could heal themselves and the LED can be illuminated again. This was much faster than the healing rate of some self-healing conductors reported so far, which took several hours to heal. Interestingly, the ester exchange provided the polyurethane the ability to remold and reprocess (Fig. 5b). The PTMGBP1 films were cut into many small pieces, injected them into customized molds, left them at 100°C for 2 hours. They were furtherly pressurized, resulted in 3D patterns or hollow structures such as the clover, pig and pancake shapes as shown in Fig. 5c. This was mainly due to the reconfiguration of the network at high temperatures through dynamic urethane bond exchange reactions, and the progressive increased in the probability of urethane bond transitions with increasing contact time, resulting in polyurethane remolding, suggesting that PTMGBP films could be used to prepare complex textures with 3D structures.
Under UV light (365 nm), the white BP powder showed blue luminescence (Fig. S10). The excitation spectrum of the strontium aluminate phosphor obtained from the 365 nm emission peak was obtained using a fluorometer (Fig. S11), while the emission spectrum of the phosphor excited at 450 nm was given (Fig. S12). It is worth noting that, as shown in Fig. 6a, after BP powders were embedded into the PU, the PTMGBP film showed white color in natural light and still emitted blue light in UV light (365 nm), its blue light did not diminish significantly during the stretching process, and it almost recovered its original shape after release, showing excellent flexibility and fluorescence effect. In Fig. 6b, the excitation peak was significantly enhanced between 300–400 nm, which was mainly due to the increase of the rare earth strontium aluminate content and the Eu2+ content in the luminescence center, and therefore the luminescence intensity. Figure 6c showed the emission spectrum of the PTMGBP films excited at 450 nm. The position of the emission peak did not change significantly, but the intensity of the emission peak at 450 nm increased with the increase of strontium aluminate content. Furthermore, Fig. 6d showed the CIE position of the fluorescence emission at 450 nm excitation, which was located at (0.1674, 0.0085).
Invisible fluorescent anti-counterfeiting patterns were usually used in the field of anti-counterfeiting printing and were not recognizable to the naked eye, but were visible under ultraviolet light. To demonstrate the anti-counterfeiting function of the film obtained by combining phosphor with PU, the words "ABCD" were scratched on the surface of the PTMGBP7 film, but these marks were barely recognizable to the naked eye under natural light and clearly visible under UV light. As shown Fig. 6e, the scratches disappeared almost completely after only 10 min at 60°C. The PTMGBP film in this article was white in sunlight, but emitted a blue light in UV light, making it tamper-resistant. This visible-light invisible and UV-visible polyurethane has attractive potential as an anti-tampering ink or coating.