Multifunctional polyurethane elastomers with high mechanical robustness and exceptional crack tolerance performance based on bi-incompatible soft segments and dynamic coordination bonds

Abstract

Elastomers with high strength and super toughness are widely used in aerospace, flexible electronics, biomedical, and other fields. However, the development of healable and recyclable elastomers with high strength, high toughness, and excellent crack resistance remains a significant challenge. In this work, we successfully synthesized a multifunctional silicone-based elastomer (SiPUU-HPA@Fe) with outstanding mechanical properties, self-healing capabilities and recyclability by combining biocompatible polycarbonate (PCDL) segments, flexible polydimethylsiloxane (PDMS) segments and metal coordination bonds. Owing to the presence of phase separation structures and dynamic coordination interaction, the optimized SiPUU-HPA₂@Fe_1/3 elastomer exhibits a high tensile strength of 45.5 MPa, an ultrahigh toughness of 412.5 MJ m⁻³, and an exceptional fracture energy of 179.3 kJ m⁻². Additionally, the SiPUU-HPA₂@Fe_1/3 elastomers can be healed and recycled to regain their original mechanical strength and integrity under heating. Furthermore, the SiPUU-HPA₂@Fe_1/3 elastomer demonstrates excellent antibacterial properties and no cytotoxic effects. Finally, a soft SiPUU-HPA₂@Fe_1/3/Li based strain sensor was developed by combining SiPUU-HPA₂@Fe_1/3 with conductive ionic lithium bis(trifluoromethane)sulfonimide (LiTFSI) and demonstrated remarkable sensing capability to diverse human body motions. This research provides ideas for the design of polyurethane elastomers with high mechanical properties and multiple functions, and the elastomers are expected to be used in emerging fields such as biomedical and flexible electronics.

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1 Introduction

In recent years, thermoplastic polyurethane (TPU) has been widely applied in soft robotics, biomedical materials, and flexible electronics due to its good biocompatibility, easy processing ability and tunable mechanical properties.^1–10 Polysiloxane elastomers are an ideal substrate for the preparation of flexible electronic and biomedical devices, owing to their excellent biocompatibility, remarkable flexibility, high gas permeability, and exceptional thermal and oxidative stability.^11–15 However, the non-polar and weak intermolecular interactions of PDMS lead to poor mechanical strength in silicone-based elastomers.^16,17 At present, enhancing the strength, toughness and crack resistance of silicone-based elastomers remains a huge challenge that significantly limits their application range and service life. In addition, endowing elastomers with self-healing ability and recyclability can not only increase their service life but also reduce waste generation and environmental pollution.^18–22 Therefore, the development of elastomers that exhibit high mechanical strength, toughness, exceptional crack resistance, recyclability, and self-healing properties is essential for building a sustainable society.

Although developing a self-healing PDMS elastomer that combines high strength and toughness poses significant challenges, researchers have proposed various strategies to address the issue. For example, Wang et al. successfully synthesized a thermoplastic silicone-based elastomer by integrating the synergistic effects between bi-incompatible soft segments and multi-scale hydrogen bonds.¹⁹ The resulting silicone-based elastomer exhibited a tensile strength of 8.0 MPa, an elongation at break of 1910%, a high toughness of 67.8 MJ m⁻³, and an exceptional fracture energy of 31.8 kJ m⁻². Sun et al. developed a healable and recyclable polyurethane elastomer with high strength (52.4 MPa), good toughness (363.8 MJ m⁻³) and exceptional fracture energy (192.9 kJ m⁻²) by incorporating dynamic hierarchical domains into a polymer matrix.²³ Furthermore, the elastomers can be efficiently healed and recycled to regain their original mechanical properties and integrity, owing to the reversibility of the dynamic hierarchical domains and hydrogen and coordination bonds. Recently, Wang et al. synthesized a multifunctional polyurethane elastomer by employing PCDL and PDMS as soft segments and incorporating dynamic boron–oxygen and dynamic disulfide bonding into the polymer chain segments.²⁴ This innovative design endowed the elastomer with outstanding mechanical properties, such as a tensile strength of 16.3 MPa, a remarkable elongation at break of 3300%, a toughness value of 278.8 MJ m⁻³, and a fracture energy of 234.9 kJ m⁻². Therefore, the development of silicone-based elastomers with high mechanical strength and toughness, excellent tear resistance, self-healing and recyclability through rational molecular structure design remains a significant challenge.

In this work, a multifunctional polyurethane elastomer is developed through the introduction of two thermodynamically incompatible components, i.e., poly(dimethylsiloxane) (PDMS) and polycarbonate (PCDL), and metal coordination bonds. Owing to the synergistic effect of phase separation and hydrogen and coordination bonds, the optimized SiPUU-HPA₂@Fe_1/3 elastomer demonstrates exceptional mechanical properties including a tensile strength of 45.5 MPa, an ultrahigh toughness of 412.5 MJ m⁻³, and an exceptional fracture energy of 179.3 kJ m⁻². Meanwhile, based on the reversibility and dynamic characteristics of hydrogen and coordination bonds, the SiPUU-HPA₂@Fe_1/3 elastomer can be healed and recycled to regain its original mechanical strength and integrity. Moreover, the PU elastomer exhibited good biocompatible and antibacterial properties, demonstrating its great potential for biomedical applications. Finally, its potential in flexible electronic sensors was demonstrated by combining the SiPUU-HPA₂@Fe_1/3 elastomer and lithium bis(trifluoromethane)sulfonimide (LiTFSI).

2 Experimental

2.1. Materials

Octamethylcyclotetrasiloxane (D₄, > 99.5%), 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane (AMM, 97%), tetramethylammonium hydroxide pentahydrate (TMAH, 97%), 2-amino-1,3-propanediol (98%), salicylaldehyde (98%), isophorone diisocyanate (IPDI, 99%), lithium bis(trifluoromethane)sulfonimide (LiTFSI, 99%) and dibutyltin dilaurate (DBTDL) were obtained from Shanghai Macklin, China. Tetrahydrofuran (THF, 99.5%) and N,N-dimethylformamide (DMF, 99.5%) were obtained from Shanghai Macklin and dried using 3 Å molecular sieves before use. Polycarbonate diol (PCDL) with a molecular weight of 2000 g mol⁻¹ was obtained from Shandong Liduo Chemical Co., Ltd (Shandong, China) and was evaporated under reduced pressure at 110 °C for 3 h before use. Escherichia coli (E. coli), Staphylococcus albus (S. albus) and mouse fibroblast cells (L929) were provided by South China Agricultural University.

2.2. Synthesis of SiPUU-HPA@Fe elastomers

2.2.1 Synthesis of SiPUU-HPA. The SiPUU-HPA elastomer was prepared by a two-pot method, and the corresponding synthesis route is shown in Scheme 1a. Taking the synthesis of SiPUU-HPA₂ as an example, PCDL (12 g, 6 mmol) and NH₂-PDMS-NH₂ (7.5 g, 1.5 mmol) were dissolved in 100 ml THF. Then, the solution containing PCDL and NH₂-PDMS-NH₂ was heated to 60 °C in an argon atmosphere. IPDI (3.33 g, 15 mmol) and DBTDL (0.06 g) were added to the solution. This solution was continuously stirred at 60 °C for 12 h to obtain the pre-polymer solution. Subsequently, HPA (1.56 g, 7.5 mmol) was charged into the above solution. The reaction mixture was stirred for 12 h at 60 °C under a nitrogen atmosphere. After the reaction, the reaction solution was poured into H₂O/methanol (v/v = 3/2) to precipitate the product which was washed three times. The resulting polymers were collected by filtration and then dried under vacuum at room temperature. The dried samples were re-dissolved in THF, and the polymer products were poured into a mold and then stored at 50 °C for 24 h. The synthetic procedure for the other SiPUU-HPA_x elastomers was similar to that of SiPUU-HPA₂, following the recipes detailed in Table S1.

Scheme 1 (a) Synthesis route of an SiPUU-HPA elastomer. (b) Schematic diagram of the preparation process of the SiPUU-HPA@Fe elastomer. (c) Schematic diagram of the segments and dynamic bonds in the SiPUU-HPA@Fe elastomer.

FTIR (ATR, cm⁻¹): 3330 (–NH), 2962 (–CH₃), 1640 (C=O in urea), 1694 (C=O in urethane), 1552 (N–H), 1731 (C=O in PCDL), 1625 (C=N in HPA), 1247 (C–O in PCDL), 1017 (–Si–O–Si–), 960 (C–O in PCDL), 797 (–Si–CH₃).

¹H NMR (600 MHz, CDCl₃, δ, ppm): 12.88 (s, Ph–OH in HPA), 8.39 (s, –CH=N in HPA), 7.30–7.35 (m, Ph–H in HPA), 6.89–6.95 (m, Ph–H in HPA), 3.68–3.86 (m, –CH₂–O in HPA), 3.35–3.49 (m, –NCH(CH₂)₂ in HPA), 4.05–4.15 (m, –O–CH₂–(CH₂)₄–CH₂–O– in PCDL), 1.58–1.75 (m, –O–CH₂–CH₂–(CH₂)₂–CH₂–CH₂–O– in PCDL), 1.24–1.45 (m, –O–(CH₂)₂–CH₂–CH₂–(CH₂)₂–O– in PCDL), −0.05–0.11 (m, –Si(CH₃)₂ in PDMS), 2.86–2.96 (m, –CH₂–N in PDMS), 1.45–1.52 (m, –CH₂–CH₂–N in PDMS), 0.51 (s, –Si(CH₃)₂–CH₂– in PDMS), 0.84 (s, –C(CH₃)₂), 1.06 (s, –C–CH₃ in IPDI), 2.04 (m, –CH– in IPDI), 2.91 (m, –CH–CH₂–N in IPDI).

2.2.2 Synthesis of the SiPUU-HPA₂@Fe_x elastomer. The schematic illustration of the preparation of SiPUU-HPA₂@Fe_x is shown in Scheme 1b. Taking SiPUU-HPA₂@Fe_1/3 as an example, the SiPUU-HPA₂ elastomer was initially dissolved in DMF until the sample was completely dissolved. The solution containing SiPUU-HPA₂ was then heated to 60 °C under continuous stirring. Subsequently, a DMF solution of FeCl₃ (the molar ratio of Fe³⁺ ions to HPA groups is 1/3) was slowly added to the above solution, and the mixture was stirred at 60 °C for 4 h. After the coordination reaction was completed, the solution was poured into a mold. The SiPUU-HPA₂@Fe_1/3 elastomer was finally obtained by drying at 80 °C under vacuum for 24 h. Additionally, other SiPUU-HPA₂@Fe_x elastomers, i.e., SiPUU-HPA₂@Fe_x (where the subscript “x” represents the molar ratio of Fe³⁺ ions to HPA groups in SiPUU-HPA₂), were synthesized using the above same preparation method.

2.3. Preparation of the SiPUU-HPA₂@Fe_1/3/Li_x film

SiPUU-HPA₂@Fe_1/3/Li_x was fabricated by mixing SiPUU-HPA₂@Fe_1/3 and LiTFSI with different ratios, where x represents the ratio of LiTFSI mass to SiPUU-HPA₂@Fe_1/3 mass. Taking the synthesis of SiPUU-HPA₂@Fe_1/3/Li_80% as an example, a DMF solution of SiPUU-HPA (5.0 g) was mixed with a DMF solution of FeCl₃ (the molar ratio of Fe³⁺ ions to HPA groups is 1 : 3). After stirring for 4 h at room temperature, LiTFSI (4.0 g, the mass ratio of LiTFSI to SiPUU-HPA₂@Fe_1/3 is 80%) was added into the above DMF solution, which was further stirred for another 4 h. Finally, the homogeneous solution was poured into a PTFE mold and then dried at 60 °C for 48 h to obtain the SiPUU-HPA₂@Fe_1/3/Li_80% elastomer.

3 Results and discussion

3.1. Preparation and characterization of the SiPUU-HPA@Fe elastomer

The Schiff-base ligand HPA, which contains imine groups and phenol groups, demonstrates good biocompatibility. When a metal–ligand interaction involves multiple bonds, it forms a chelation bond and is stronger. Under the synergistic coordination of N and O atoms, coordination bonds easily form with various metal ions such as Fe³⁺ and Zn²⁺. In this work, focusing on potential biomedicine and wearable device applications, we chose biosafe PDMS and PCDL as soft segments, along with biologically relevant Fe³⁺ ions.

The chemical structure of the HPA monomer was characterized by ¹H NMR, as shown in Fig. S1. In the ¹H NMR spectrum, the resonance peak at 8.49 ppm was assigned to the imine bond (CH=N), confirming the successful reaction between the aldehyde of salicylaldehyde and the amino group of 2-amino-1,3-propanediol. In addition, the peak area ratio of the protons in HPA aligned with the theoretical calculations. These results indicate that the HPA monomer is successfully prepared.

The SiPUU-HPA@Fe elastomer is a multiblock polymer composed of PDMS and PCDL cross-linked with hydrogen bonds and coordination bonds between the HPA groups and Fe³⁺ ions. The synthesis routes and schematic diagram of the SiPUU-HPA@Fe elastomer are shown in Scheme 1. First, isocyanate-terminated PDMS and PCDL prepolymers were synthesized through the reaction of IPDI with a mixture of PCDL and PDMS. Then, the synthesized NCO-PDCL-NCO and NCO-PDMS-NCO were reacted with the HPA monomer to obtain SiPUU-HPA. Finally, the SiPUU-HPA@Fe elastomers were synthesized by adding FeCl₃ into the THF solution of SiPUU-HPA under stirring. In terms of GPC analysis, the GPC trace of SiPUU-HPA_x elastomers displayed a unimodal peak. Among them, the number-average molecular weight (M_n) of SiPUU-HPA₂ was calculated to be 82.5 × 10³ g mol⁻¹ with a polydispersity index (PDI) of 1.64 (Fig. S2 and Table S2). As shown in the ¹H NMR spectrum of SiPUU-HPA₂, the characteristic peaks of the PCDL, PDMS, IPDI and HPA segments in the polymer chain were clearly observed (Fig. S3).

The FTIR and EDS analysis verified the chemical structures and elemental compositions of SiPUU-HPA₂@Fe elastomers. As shown in Fig. 1a, the coordination bond between HPA groups and Fe³⁺ ions in the SiPUU-HPA₂ elastomer was confirmed through FTIR analysis. In the FTIR spectrum of SiPUU-HPA₂, the peak at 1625 cm⁻¹ is attributed to the stretching vibration of C=N in the HPA groups. This peak shifts to 1592 cm⁻¹ in the FTIR spectrum of SiPUU-HPA₂@Fe_1/3, indicating the formation of coordination bonds between the Fe³⁺ ions and the HPA groups. The EDS mapping results showed that Si, O and Fe were uniformly distributed in the SiPUU-HPA₂@Fe_1/3 elastomer (Fig. 1b), indicating the homogeneous structure of the elastomer. This is beneficial for the formation of dynamic metal coordination between Fe³⁺ ions and HPA groups in the cross-linked network. Furthermore, the formation of coordination between Fe³⁺ ions and HPA groups was confirmed by XPS analysis. As shown in Fig. 1c, the Fe 2p XPS spectrum presented two signals corresponding to Fe 2p_1/2 and Fe 2p_3/2 at 725.6 and 710.8 eV, respectively.^25,26 In addition, Fe signals were also found in the XPS fitting of N 1s and O 1s, which are located at 399.6 eV and 532.3 eV, respectively (Fig. 1d and e).^27,28 These results demonstrate that the SiPUU-HPA₂@Fe elastomer was successfully synthesized containing dynamic coordination bonds and hydrogen bonds. In addition, the thermal stability of the SiPUU-HPA₂@Fe_1/3 elastomer was characterized by TGA tests, as shown in Fig. S4. The T_{d, 5%} values of SiPUU-HPA₂ and SiPUU-HPA₂@Fe_1/3 elastomers are 295 °C and 282 °C, respectively, indicating excellent thermal resistance properties. The residual carbon content of the SiPUU-HPA₂@Fe_1/3 elastomer is higher than that of the SiPUU-HPA₂ elastomer, which may be attributed to the formation of Fe-containing compounds during the thermal degradation process.

Fig. 1 (a) FTIR spectra of SiPUU-HPA₂ and SiPUU-HPA₂@Fe_1/3 elastomers. (b) The EDS maps of the surface of the SiPUU-HPA₂@Fe_1/3 elastomer. (c–e) Fe 2p, N 1s and O 1s XPS spectra of the SiPUU-HPA₂@Fe_1/3 elastomer.

3.2. Mechanical performance of the SiPUU-HPA@Fe elastomer

The mechanical properties of SiPUU-HPA_x elastomers were evaluated through a universal tensile test at a stretching speed of 100 mm min⁻¹. As shown in Fig. S5, the tensile strength of the SiPUU-HPA_x elastomers increased, whereas their elongation at break decreased as the HPA content increased. Among them, the SiPUU-HPA₂ elastomer had a tensile strength of 9.2 MPa and an elongation at break of 1489%. This finding can be attributed to the increase in the number of hydrogen bonds and physical crosslinking points between polymer chains as the HPA content increases. Consequently, the tensile strength of the SiPUU-HPA_x elastomers increased. However, an excess of physical crosslinking points limited the slippage of polymer chains, leading to a gradual decrease in the elongation at break of the elastomers.

The excellent mechanical properties of the SiPUU-HPA₂@Fe_x elastomers were evaluated via a uniaxial tensile test at a stretching speed of 100 mm min⁻¹ at room temperature. The stress–strain curves of the SiPUU-HPA₂@Fe_x elastomers are shown in Fig. 2a, while the mechanical properties are summarized in Table S3. All SiPUU-HPA₂@Fe_x elastomers are typical elastomers because no yielding phenomena during elongation were observed. When Fe³⁺ ions were introduced, the mechanical strength of the SiPUU-HPA₂@Fe_x elastomers gradually increased, while the elongation at break gradually decreased. This result demonstrates that the introduction of coordination interactions between Fe³⁺ ions and HPA groups increases the cross-linking density, significantly enhancing the mechanical properties of SiPUU-HPA₂@Fe_x elastomers.

Fig. 2 (a) Typical stress–strain curve of the SiPUU-HPA₂@Fe_x elastomers; (b) tensile strength and toughness histograms of the SiPUU-HPA₂@Fe_x elastomers; (c) comparison of toughness of SiPUU-HPA₂ and SiPUU-HPA₂@Fe_1/3 elastomers; (d) comparison of the tensile strength and toughness of SiPUU-HPA₂@Fe_1/3 with data from other elastomers.^4,22,29–37 (e) Schematic of the mechanism of enhanced mechanical properties of the SiPUU-HPA₂@Fe elastomer.

When the molar ratio of Fe³⁺ ions to HPA is 1 : 3, the resulting SiPUU-HPA₂@Fe_1/3 elastomer exhibits the highest tensile strength and toughness, which are 45.5 MPa and 412.5 MJ m⁻³, respectively (Fig. 2b). In contrast, the tensile strength of SiPUU-HPA₂ without Fe³⁺ ions was only 9.2 MPa and 58.1 MJ m⁻³, respectively (Fig. 2b). This is attributed to the addition of Fe³⁺ ions, which increases the cross-linking density and limits the mobility of polymer chains, thereby improving the mechanical strength of the SiPUU-HPA₂@Fe_x elastomer. Furthermore, the toughness of the SiPUU-HPA₂@Fe_1/3 elastomer is 7 times greater than that of the SiPUU-HPA₂ elastomer (58.1 MJ m⁻³) (Fig. 2c). To the best of our knowledge, the SiPUU-HPA₂@Fe_1/3 elastomer exhibits exceptional strength and toughness, surpassing those of the majority of polyurethane elastomers currently reported (Fig. 2d).^4,22,29–37

To evaluate energy dissipation capability of SiPUU-HPA₂@Fe elastomers, cyclic tensile tests were further performed. The cyclic tensile curves of the SiPUU-HPA₂@Fe_1/3 elastomer under different strains are depicted in Fig. 3a. The hysteresis loop gradually increased with increasing strain. As illustrated in Fig. 3b, the hysteresis areas at different strains were plotted as a function of the strain. The hysteresis area value represents the energy dissipation value. When the strain is below 300%, the hysteresis areas of the elastomer are small and the slope of the curve remains essentially constant. Within this range, elastic deformation mainly occurs, with an extremely small amount of hydrogen bonds and coordination bonds breaking. The hysteresis area rapidly increases when the strain exceeds 300%, indicating that energy can be effectively dissipated to enhance the toughness and crack tolerance of the elastomer. As illustrated in Fig. 2e, when the elastomer was stretched to small strains, the hydrogen bonds with lower bonding energy dissociated first. As the strain increased further, the coordination bond between Fe³⁺ and HPA groups, which has higher bonding energy, gradually dissociated.

Fig. 3 (a) Successive cyclic stress–strain curves of SiPUU-HPA₂@Fe_1/3 at different strains. (b) The energy loss value of SiPUU-HPA₂@Fe_1/3 at different strains. (c) The cyclic tensile curve of SiPUU-HPA₂@Fe_1/3 at a fixed strain of 500%. (d) Load–unload cycle curves for 500 cycles of the SiPUU-HPA₂@Fe_1/3 elastomer at a strain of 100% at a deformation rate of 100 mm min⁻¹.

To evaluate the elastic behavior of the SiPUU-HPA₂@Fe_1/3 elastomer, the cyclic tensile tests were conducted at a strain of 500%, as shown in Fig. 3c. The first cyclic curve of SiPUU-HPA₂@Fe_1/3 exhibited a significant hysteresis loop (with 11.58 MJ m⁻³ dissipated energy), indicating that significant energy dissipation occurred attributed to the rupture of the dynamic crosslinking interactions within the polymer chains. As the number of cycles increased, the energy dissipation area gradually decreased, primarily because the dynamic hydrogen bonding and coordination bond interactions could not be quickly and fully restored to their original states. With the extension of waiting time, the cyclic tensile curves and hysteresis area gradually recovered and nearly reached the original state after a waiting time of 120 min. These results indicate that the SiPUU-HPA₂@Fe_1/3 elastomer exhibits excellent elastic restorability, significantly improving the material's fatigue resistance under deformation. To evaluate the fatigue resistance of the SiPUU-HPA₂@Fe_1/3 elastomer, a cyclic tensile test was further conducted. The results of the 500 continuous cyclic stretching tests at a strain of 100% are shown in Fig. 3d. As the number of cycles increases, the maximum force tends to be stable, indicating that hydrogen bonds and coordination bonds have achieved a dynamic balance between breakage and rapid association. These results demonstrate that the SiPUU-HPA₂@Fe_1/3 elastomer has excellent fatigue resistance.

The presence of phase separation structures and strong dynamic coordination interactions could also effectively improve the crack tolerance of the elastomer. Crack tolerance is regarded as a critical mechanical property parameter for elastomers. The crack tolerance of the SiPUU-HPA₂@Fe elastomer was evaluated through notch stretching experiments. As shown in Fig. 4a, the notched SiPUU-HPA₂@Fe_1/3 elastomer can be stretched up to 11 times its original length, without rapid lateral propagation of the notch. This phenomenon could be primarily attributed to the dynamic dissociation and reassociation of hydrogen bonds and coordination bonds within the hard microdomains, resulting in a stick-slip motion of the polymer chains under external force. Consequently, the stress concentration at the notch tip is efficiently transmitted and dispersed into the entire polymer network, thereby effectively resisting the lateral propagation of the notch. Using the Greensmith method from the stress–strain curves, the fracture energy was calculated to be 179.3 kJ m⁻², which is 18 times greater than that of natural rubber (approximately 10 kJ m⁻²). Additionally, the notched SiPUU-HPA₂@Fe_1/3 elastomer exhibited a fracture strength of 13.7 MPa. In contrast, the fracture energy of the SiPUU-HPA elastomer was only 31.1 kJ m⁻², significantly lower than the values of the SiPUU-HPA₂@Fe_1/3 elastomer (Fig. 4b). Compared to most previously reported polyurethane elastomers, the SiPUU-HPA₂@Fe_1/3 elastomer demonstrated exceptional advantages in fracture energy (Fig. 4c).^{6,29,34,38–44} The above findings indicate that the elastomer exhibited excellent tear resistance, ensuring its safety in practical applications.

Fig. 4 (a) Notched tensile test curves of the SiPUU-HPA₂@Fe_1/3 elastomer. The inset images show the single-edge-notched elastomer at 1000% strain. (b) Comparison of fracture energy of SiPUU-HPA₂ and SiPUU-HPA₂@Fe_1/3 elastomers. (c) Comparison of the fracture energy of the SiPUU-HPA₂@Fe_1/3 elastomer with those of elastomers reported in the literature.^{6,29,34,38–44}

3.3. Microstructure and thermal properties of the SiPUU-HPA₂@Fe elastomer

To evaluate the relationship between the structure and properties of the SiPUU-HPA₂@Fe elastomer, the microstructure of the elastomer was characterized using XRD, DSC, SAXS and AFM. The crystallization behavior of the SiPUU-HPA₂@Fe_1/3 elastomer was evaluated by XRD and DSC. As shown in Fig. 5b, the SiPUU-HPA₂ and SiPUU-HPA₂@Fe elastomers display two distinct diffraction peaks at about 12° and 20°, corresponding to the PCDL and random PDMS blocks, respectively, suggesting an amorphous structure. According to the DSC spectrum analysis, the glass transition temperatures (T_{g, PCDL}) of the SiPUU-HPA₂ and SiPUU-HPA₂@Fe_1/3 elastomers are −31.6 °C and −30.4 °C, respectively (Fig. 5a). Compared to SiPUU-HPA₂, the T_{g, PCDL} of the SiPUU-HPA₂@Fe_1/3 elastomer decreased after coordination of Fe³⁺ ions, which could be attributed to the enhanced microphase separation. The introduction of Fe³⁺ ions affected the arrangement of polymer molecules, increasing the aggregation degree of the hard segment and consequently reducing the dispersion between hard and soft segments.^45,46

Fig. 5 (a) DSC curves, (b) XRD curves and (c) SAXS patterns of SiPUU-HPA₂ and SiPUU-HPA₂@Fe_1/3 elastomers. (d) AFM image of the SiPUU-HPA₂@Fe_1/3 elastomer.

Furthermore, the microphase separation structure of the SiPUU-HPA₂@Fe_1/3 elastomer was investigated by SAXS. In the SAXS spectrum, the SiPUU-HPA₂ and SiPUU-HPA₂@Fe_1/3 elastomers exhibit a broad scattering peak (Fig. 5c), suggesting a microphase separation structure. The q value of the SiPUU-HPA₂ elastomer is 0.153 nm⁻¹, and the periodicity (D) of the phase-separated domains was calculated to be 26.8 nm. After coordination with Fe³⁺ ions, the q value of the SiPUU-HPA₂@Fe_1/3 elastomer increases, while the periodicity of the phase-separated domains decreases from 41 nm to 32 nm. These results demonstrate that the phase separation becomes more evident and the distance between hard segments becomes smaller after coordination.⁴⁷ In brief, the incorporation of coordination bonds in hard segments draws the benzene rings closer to form more hydrogen bonds between adjacent carbamate bonds and modulates the microscopic phase structure of the elastomers. The phase separation of the elastomer was further analyzed using AFM. As shown in Fig. 5d and S6, both the SiPUU-HPA₂ elastomer and the SiPUU-HPA₂@Fe_1/3 elastomer exhibited a two-phase structure, where the bright regions refer to hard domains and the dark regions refer to soft domains. After coordination with Fe³⁺ ions, the phase separation became more pronounced, because the interface between the dark and bright regions in the SiPUU-HPA₂@Fe_1/3 elastomer was more distinct compared to that in the SiPUU-HPA₂ elastomer.

In conclusion, the remarkable mechanical properties of the SiPUU-HPA₂@Fe elastomer could be attributed to several factors: (1) the microphase separation domain functioned as a rigid nanofiller; (2) the coordination interactions between Fe³⁺ ions and HPA groups increased the energy barrier for chain separation from the domain; (3) the dynamic dissociation and recombination behavior of coordination bonds and hydrogen bonds significantly improved energy dissipation.

3.4. Healability and recyclability of the SiPUU-HPA₂@Fe elastomer

The viscoelastic properties of the SiPUU-HPA₂@Fe_1/3 elastomer at different temperatures were evaluated using rheological analysis. Fig. 6a and b illustrate the temperature-dependent oscillation rheology curve of SiPUU-HPA₂ and SiPUU-HPA₂@Fe_1/3 elastomers across a temperature range from 20 °C to 180 °C. As shown in Fig. 6a, when the temperature was raised to approximately 162 °C, the storage modulus (G′) intersected with the loss modulus (G′′), indicating that the viscous flow transition temperature of the SiPUU-HPA₂ elastomer was 162 °C. This phenomenon was further investigated in terms of frequency dependency. By applying small-amplitude oscillatory shear at different temperatures ranging from 20 °C to 120 °C, we constructed a master curve (reference temperature = 100 °C) for the SiPUU-HPA₂ elastomer, demonstrating the frequency dependency of G′ and G′′, as depicted in Fig. S7. A cross-point of G′ and G′′ at a critical shear frequency (ω_c) was observed for the SiPUU-HPA₂ elastomer, aligning with the results from temperature scanning. After the addition of Fe³⁺, the G′ of the SiPUU-HPA₂@Fe_1/3 elastomer decreases at a relatively slow rate and remains within a consistent order of magnitude as the temperature increases from 20 to 150 °C (Fig. 6b), demonstrating the excellent thermomechanical stability of the SiPUU-HPA₂@Fe_1/3 elastomer. When the temperature exceeds 150 °C, the G′ and G′′ decrease rapidly (Fig. 6b), indicating that a large number of the dynamic bonds in the elastomer are broken.⁴⁸ However, the SiPUU-HPA₂@Fe_1/3 elastomer remained in a solid state up to 180 °C because its G′ is always higher than its G′′ in the tested temperature range. Rheological frequency sweep curves of SiPUU-HPA₂@Fe_1/3 were also measured at different temperatures. As shown in Fig. 6c, the rheological tests demonstrate that the G′ of the SiPUU-HPA₂@Fe_1/3 elastomer is higher than its G′′ in the entire frequency spectrum. The stress-relaxation experiments further demonstrated the dynamic viscoelasticity of the SiPUU-HPA₂@Fe_1/3 elastomer, as depicted in Fig. 6d. After coordination with Fe³⁺ ions, the relaxation time of the SiPUU-HPA₂@Fe_1/3 elastomer increases at room temperature. This result indicates that the formation of metal coordination bonds enhances the interactions between molecular chains and reduces the efficiency of chain movement.

Fig. 6 (a) Temperature scanning curves of the SiPUU-HPA₂ elastomer. (b) Temperature scanning curves of the SiPUU-HPA₂@Fe_1/3 elastomer. (c) Master curves of frequency dependency of G′ and G′′ for the SiPUU-HPA₂@Fe_1/3 elastomer (reference temperature is 100 °C). (d) The stress relaxation curves of SiPUU-HPA₂ and SiPUU-HPA₂@Fe_1/3 elastomers.

Polymer materials generate a considerable amount of waste after use or damage, resulting in significant resource waste and long-term environmental pollution. Therefore, the SiPUU-HPA₂@Fe elastomers were designed to have a self-healing ability and recyclability. Taking SiPUU-HPA₂@Fe_1/3 as an example, the healing efficiency was studied at different temperatures and times. As shown in Fig. 7a, the healing efficiency of the SiPUU-HPA₂@Fe_1/3 elastomer gradually increases with the healing temperature or time. After being subjected to a temperature of 100 °C for 24 h, the tensile strength and elongation at break of the elastomer showed a self-healing efficiency of 88.3% and 95.7%, respectively (Fig. 7b). Moreover, Fig. 7a demonstrates that reducing the temperature significantly decreases the healing efficiency. The storage modulus (G′) of the SiPUU-HPA₂@Fe_1/3 elastomer at 25 °C and 100 °C is 0.54 MPa and 0.07 MPa, respectively (Fig. 6b). This finding indicates that the mobility of the polymer chains significantly increases when heated to 100 °C, which is attributed to the dynamic breaking of hydrogen bonds and coordination bonds and the high mobility of PDMS soft segments.²³

Fig. 7 (a) Stress–strain curves of the SiPUU-HPA₂@Fe_1/3 elastomer at different temperatures and times. (b) Healing efficiency of the SiPUU-HPA₂@Fe_1/3 elastomer at different temperatures and times. (c) Recycling stress–strain curves at different times via hot press. (d) A histogram of mechanical properties of the SiPUU-HPA₂@Fe_1/3 elastomer.

Furthermore, the SiPUU-HPA₂@Fe_1/3 elastomer also demonstrated excellent recyclability due to the dynamic reversibility of metal coordination bonds and hydrogen bonds. As shown in Fig. 7c, the SiPUU-HPA₂@Fe_1/3 elastomer can be recovered by a hot-pressing process at 100 °C under a pressure of 10 MPa for 0.5 h. After three cycles of recycling, the stress–strain curves of the recycled SiPUU-HPA₂@Fe_1/3 elastomer nearly overlap with those of the original one (Fig. 7d), demonstrating its excellent recyclability and reprocessing performance. In summary, the material can maintain high mechanical performance even after three recycling processes, significantly reducing waste generation and consequently dissolving resource waste and environmental pollution.

3.5. Antibacterial properties and biocompatibility evaluation

Enhancing polyurethane materials with antimicrobial properties is essential in the practical application of medical devices to protect them from bacterial contamination and erosion. The antibacterial properties of the SiPUU-HPA₂@Fe_1/3 film were evaluated using representative Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus albus (S. albus). As shown in Fig. S8, the SiPUU-HPA₂@Fe_1/3 film exhibited excellent antibacterial performance, achieving antibacterial efficiencies of 97.82% and 99.99% against E. coli and S. albus, respectively. The Schiff-base ligand HPA contains imine groups and phenol groups, providing synergistic coordination of N and O atoms toward Fe³⁺ ions. The antibacterial mechanism of the SiPUU-HPA₂@Fe_1/3 elastomer can be summarized as follows: (1) after Fe³⁺ coordinates with the imine and phenolic hydroxyl groups of HPA, it disrupts the bacterial film structure and functions, hinders bacterial metabolism, and affects the metabolic processes, ultimately leading to bacterial apoptosis and exerting a bactericidal effect.⁴⁹ (2) Schiff base compounds, known for their remarkable antibacterial activity, interact with the phospholipid bilayer in bacterial cell membranes, causing disruption of membrane integrity and ultimately leading to bacterial death.⁵⁰ In summary, the SiPUU-HPA₂@Fe_1/3 film exhibits excellent antibacterial properties, attributed to the synergistic effects of Schiff base and metal–ligand bonds.

The hemocompatibility of the SiPUU-HPA₂@Fe_1/3 film was evaluated by in vitro hemolysis testing. As illustrated in Fig. 8c and S9, compared to both negative and positive control groups, the SiPUU-HPA₂@Fe_1/3 film extracts with different immersion durations did not cause significant erythrocyte rupture, and their hemolysis rate was less than 2%, which is far lower than the standard requirement level of 5%. These findings demonstrate that SiPUU-HPA₂@Fe_1/3 exhibits excellent blood compatibility under in vitro conditions. Furthermore, the cytocompatibility of the SiPUU-HPA₂@Fe_1/3 film was evaluated using MTT assay and live/dead staining. The fluorescence images of L929 cells cultured in a dish containing SiPUU-HPA₂@Fe_1/3 leachate for 24 h and 72 h are displayed in Fig. 8a. It was observed that there was no significant difference in cell density between the control group and the experimental group, suggesting that SiPUU-HPA₂@Fe_1/3 does not have a significant effect on cell proliferation. The MTT assay results depicted in Fig. 8b demonstrate that the cell survival rate exceeded 85% during both 1-day and 3-day culture periods, demonstrating that the synthesized SiPUU-HPA₂@Fe_1/3 film exhibits low cytotoxicity. These results indicate that SiPUU-HPA₂@Fe_1/3 demonstrates excellent cytocompatibility and good biocompatibility and is expected to be used in the biomedical field.

Fig. 8 (a) Morphology under a microscope of the cells cultivated after 24 h and 72 h. (b) Relative growth rate (RGR%) of the L929 cells cultured in the extracts of SiPUU-HPA₂@Fe_1/3. (c) Hemolysis rates of SiPUU-HPA₂ and SiPUU-HPA₂@Fe_1/3.

3.6. Application in strain sensors

Solid-state ionic conductive elastomers (SICEs) have effectively addressed the liquid leakage issues associated with gel-based ionic conductors due to their solid-state structure and exceptional environmental stability, making them key materials in flexible electronics. Ionic conductive polyurethane has advantages in terms of strength and toughness due to hydrogen bonds and microphase separation. However, achieving a balance between the mechanical properties and ionic conductivity of SICEs remains a significant challenge. In this work, a solid-state ionic conductive elastomer with good mechanical properties and high ionic conductivity was successfully prepared by introducing lithium salts into a SiPUU-HPA₂@Fe_1/3 elastomer utilizing metal–ligand coordination and hydrogen bonding interaction.

The mechanical properties of SiPUU-HPA₂@Fe_1/3/Li_x elastomers were evaluated through universal tensile experiments. As shown in Fig. S10, the tensile strength of SiPUU-HPA₂@Fe_1/3/Li_x elastomers decreased sharply with increasing LiTFSI content, which is attributed to the destruction of the polyurethane microstructure caused by the introduction of LiTFSI. In particular, the tensile strength of the SiPUU-HPA₂@Fe_1/3/Li_80% elastomer reached 4.3 MPa when the lithium salt was 80%. However, the Young's modulus of SiPUU-HPA₂@Fe_1/3/Li_80% exhibited a significant reduction compared with that of SiPUU-HPA₂@Fe_1/3 (Fig. 9a), suggesting that the incorporation of ionic liquid imparted enhanced flexibility to the elastomer, thereby rendering it more suitable as a substrate for flexible sensing applications. This property is particularly beneficial for skin-mounted sensing, as the softened material demonstrates improved conformability to the inherent softness of human skin. Subsequently, we investigated the ionic conductivity of the SiPUU-HPA₂@Fe_1/3/Li_x elastomer. As shown in Fig. S11, the amount of freely moving Li ions increased and the ionic conductivity of the films increased with increasing LiTFSI content. Among them, the SiPUU-HPA₂@Fe_1/3/Li_80% film had an ionic conductivity of 0.24 mS cm⁻¹. This could be attributed to the addition of LiTFSI, which enhanced the mobility of the chain segments to some extent, facilitating Li transport and thereby improving ionic conductivity of the elastomers. In summary, SiPUU-HPA₂@Fe_1/3/Li_80% exhibited excellent mechanical properties and electrical conductivity, making it an ideal material for assembling flexible sensors.

Fig. 9 (a) Stress–strain curves of the SiPUU-HPA₂@Fe_1/3/Li_80% elastomer. (b) Gauge factor (GF). (c) Response and recovery times of the strain sensor. (d) The response of the SiPUU-HPA₂@Fe_1/3/Li_80% strain sensor to finger bending. (e) Time-dependent relative resistance changes of the SiPUU-HPA₂@Fe_1/3/Li_80% strain sensor for real-time monitoring of a knee bent at different angles. (f) Variation in relative resistance during wrist flexion under different conditions.

The gauge factor (GF) value is a critical parameter for evaluating the sensitivity of a strain sensor. The slope of the fitting curve in Fig. 9b reveals that the GF of the SiPUU-HPA₂@Fe_1/3/Li_80% strain sensor was 1.72 within the 0–300% strain range, demonstrating its broad detection range and stable sensitivity. Meanwhile, the response and relaxation times of the SiPUU-HPA₂@Fe_1/3/Li_80% sensor were measured at 0.38 s and 0.31 s, respectively, demonstrating the rapid response behavior of the sensor (Fig. 9c). Owing to the outstanding sensing performance of SiPUU-HPA₂@Fe_1/3/Li across a wide strain range, the sensor can be used as a wearable soft sensor for monitoring human joint motion. The sensor was attached to a finger, and the ΔR/R₀ signals gradually changed as the finger bent, indicating that it can accurately recognize the degree of finger bending (Fig. 9d). Moreover, the sensor was also used to monitor the signals of knee bending. Fig. 9e shows the changes in ΔR/R₀ signals as the straightened knee gradually bent. The ΔR/R₀ signal increased with the increasing bending angle of the knee. When the knee was straightened again, the ΔR/R₀ signal returned to its original value. After being exposed to 25 °C for one month, the sensor maintains a consistent relative resistance change, demonstrating a long-term environmental stability (Fig. 9f). Additionally, we further investigated the sensing performance of SiPUU-HPA₂@Fe_1/3/Li_80% at low temperatures. It was observed that the sensing performance exhibited no significant changes at −20 °C, as shown in Fig. 9f. In summary, SiPUU-HPA₂@Fe_1/3/Li enables real-time monitoring of ΔR/R₀ signal variations, highlighting its potential for applications in flexible electronics.

4 Conclusions

In summary, we have successfully developed a thermoplastic silicone-based elastomer with outstanding mechanical performance, self-healing properties and recyclability by combining bi-incompatible soft segments and metal coordination bonds into the polymer chain segments. The presence of phase separation structures and strong dynamic coordination interactions significantly enhances its outstanding mechanical properties. The synthesized SiPUU-HPA₂@Fe_1/3 elastomer demonstrated a high tensile strength of 45.5 MPa, an ultrahigh toughness of 412.5 MJ m⁻³, and an exceptional fracture energy of 179.3 kJ m⁻². Additionally, the SiPUU-HPA₂@Fe_1/3 elastomers can be healed and recycled to regain their original mechanical strength and integrity due to the reversibility of hydrogen and coordination bonds. Furthermore, the SiPUU-HPA₂@Fe_1/3 elastomer exhibits excellent antibacterial properties and biocompatibility, demonstrating its significant potential for use in biomedical applications. Finally, a strain sensor was developed by combining SiPUU-HPA₂@Fe_1/3 with conductive ionic lithium bis(trifluoromethane)sulfonimide (LiTFSI), demonstrating its potential applications in wearable electronics. This study provides ideas for the design of multifunctional polyurethane elastomers and broadens the application of high-performance polyurethane elastomers in biomedical and flexible electronic fields.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author contributions

This manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Data availability

The data supporting this article have been included as part of the SI. Supplementary information: experimental part, including the synthesis and characterization of HPA and NH₂-PDMS-NH₂, characterization of SiPUU-HPA and SiPUU-HPA₂@Fe_1/3 elastomers, antibacterial and hemolysis performance of SiPUU-HPA₂ and SiPUU-HPA₂@Fe_1/3 elastomers, and mechanical properties and ionic conductivity of SiPUU-HPA₂@Fe_1/3/Li elastomers. See DOI: https://doi.org/10.1039/d5ta06699a.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (52073098), the Natural Science Foundation of Guangdong Province (2023A1515011264) and the Open Fund for Key Lab of Guangdong High Property and Functional Macromolecular Materials, China (20240010).

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