Inspired biologically, stretchable ionic conductors with the ion-conducting nature and sensory functions have been widely applied into soft ionotronic devices1-6, such as stretchable touch panels7,8, actuators and sensors9,10, ionotronic diodes and transistors11, triboelectric nanogenerators12-14 and others. The majority of current ionic conductors, such as hydrogels and ionogels2,14-20, come in many flavors with diverse capabilities and limitations. In these systems, large amount of liquid provides free ions mobile environment and covalent crosslinked network contributes to the mechanical strength. However, the presence of liquid leads to poor thermal and electrochemical stability and mechanical deleterious effect21-23, covalent crosslinked network results in the irreversibility of the polymer structure24,25, thereby generating the canonical conflict between ionic conductivity, self-healing capability and mechanical performance, and becoming unfavorable for the flexible and wearable ionotronic devices. Many researchers have committed to breaking the aforementioned trade-off and constructing versatile ionic conductive elastomers26-29. Typical strategy is designing novel polymer molecular structures.
Materials properties depend on its molecular structure. High mechanical strength is mainly derived from the frozen covalent crosslinked network, in which the chain segmental motion is restricted. However, high stretchability requires fewer crosslinking sites and more free mobile chain segments. Employment of supramolecular non-covalent chemistries30-32 or dynamic covalent bonds33-35 as reversible crosslinks and sacrificial bonds endow the polymer structure with the reversibility feature, providing materials with self-healing capacity and recyclability to extend their service life and improve their reliability and durability36,37. Ions transport in the liquid-free polymer systems relies on the polymer polarity and the segmental motions38,39. These different properties originating from different molecular mechanism are generally mutually exclusive 40-43. Therefore, it is a long-standing challenge to achieve the combination of high strength and high toughness with self-healing capacity and recyclability into a given synthetic ionic conductive elastomers. Most of the as-reported liquid-free ionic conductive elastomers with mechanical versatility and self-healability were obtained by introducing supramolecular hydrogen-bonding into the designed ionic conductive polymer networks28,40-46. Jia et al. recently synthesized a novel liquid-free ionic conductive elastomer (ICE) hosting lithium (Li+) cations and associated anions via lithium bonds and hydrogen bonds, which features high strength and toughness, self-healing behavior, quick self-recovery, 3D-printability, as well as thermal stability and optical transparency29. However, the structural characteristic makes this novel ICE unrecyclable and difficult to repair macroscopic damages.
The compatibility among the ionic conductivity, self-healing and mechanical properties in the polymer electrolytes has been addressed through several polymer engineering strategies. The most eminent strategy is based on the hard-soft dual-phase block copolymer, in which hard block (polystyrene, PS) contributed to mechanical strength and the soft block (polyethylene oxide, PEO) was responsible for ion transport47-49. In addition, nanoscale-phase separation strategy has been proposed to avoid the occurrence of contradictory properties50. Guan et al. proposed a phase-separated structure to settle the conflict between mechanical and self-healing ability, in which the polystyrene provided the increasing modulus, and the terminated amide groups were in charge of self-healing mission51. Then, Bao et al. introduced the supramolecular design into a polyurethane (PU) network to overcome the conflict between mechanical robustness and ionic conductivity 52. Moreover, many design strategies including the combination of supramolecular H-bonding interactions and metal ligand bonds, phase separation, and dynamic hard domains were also achieved53-56.
In this work, inspired by polymer electrolytes for solid-state lithium-ion batteries, and combining with dynamic supramolecular engineering, we design a novel dynamic supramolecular ionic conductive elastomers (DSICE) via “phase-locked” strategy, wherein “locking soft phase” polyether backbone conducts lithium-ion (Li+) transport and the synergistic interaction of dynamic disulfide metathesis and stronger supramolecular quadruple hydrogen bonds in the hard phase contributes to the self-healing capacity and mechanical versatility. The well-designed DSICE possesses good ionic conductivity, high optical transparency, superior mechanical robustness and toughness, excellent autonomous self-healing ability and favorable recyclability. With these desirable traits, we have demonstrated its application for a flexible ionic conductive substrate and a stretchable touch sensor.
Molecular Design and Preparation of DSICE
Thermoplastic polyurethane (TPU) system with distinct two-phase morphology is well known to possess fine-tuned structures and microphase separation of soft segments and hard segments. In view of this specific structure of TPU combing with Li-ion transport mechanism, the soft phase polyether was employed to associate/dissociate Li+ and the counterparts and transport ions, while the dynamic disulfide metathesis (S-S) and stronger supramolecular quadruple hydrogen bonds (H-bonds) in the hard phase domains was used to regulating the self-healing capacity and mechanical properties. In the case of keeping the structure and function of the soft phase fixed and regulating that of the hard phase, we define this as “soft phase-locked” strategy, as shown in Fig. 1. Bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) with large anion group, high ionic conductivity, good solubility, and electrochemical and thermal stability was chosen as conductive lithium salt. The soft phase was chosen as polytetramethylene ether glycol (PTMEG, Mn = 2000 g mol-1), in which the loosely coordinating O-Li+ interaction can contribute to higher ion conductivity57,58. The hard phase featuring dynamic disulfide metathesis (S-S) and strong supramolecular quadruple hydrogen bonding (H-bonds) was chosen as aliphatic bis(2-hydroxyethyl) disulfide (HEDS) and cyclic 2-ureido-4-pyrimidinone (UPy), wherein HEDS mainly contribute to the self-healing capability, and UPy is used for mechanical enhancement. Multiple dynamic bonds including disulfide metathesis (S-S), cooperative crosslinking H-bonds (UPy-UPy) and anti-cooperative crosslinking H-bonds (urethane-urethane, urea-urea, or urea-urethane) spontaneously form a dynamic supramolecular polymer network.
First, transparent and colorless dynamic supramolecular elastomers (DSE) were successfully synthesized via condensation polymerization. To systematically study the synergistic interaction of dynamic disulfide metathesis (HEDS) and supramolecular crosslinking quadruple H-bonds (UPy) on the DSE properties, a series of DSEs, which was denoted as DSE-0, DSE-1, DSE-2, DSE-3, was synthesized through increasing the content of UPy from 0% to 30% (mol %). The preparation procedure is shown in Fig. S1. PTMEG-based prepolymer was first synthesized with two equivalents of hydrogenated 4,4’-methylenediphenyl diisocyanate (HMDI) and a certain amount of dibutyltin dilaurate (DBTDL) as a catalyst, followed by chain extension using bis(2-hydroxyethyl) disulfide (HEDS) and 2-ureido-4-pyrimidone (UPy) in a given ratio. Compared to hexamethylene dissocyanate (HDI) and isophorone dissocyanate (IPDI), HMDI with the alicyclic isocyanates has large steric hinderance preventing crystallization, resulting in completely amorphous transparent colorless materials. Then, a certain amount of LiTFSI was introduced into the optimal DSE polymers for fabricating dynamic supramolecular ionic conductive elastomers (denoted as DSICE).
Characterization of DSE
Fig. 1a showed the schematic diagram of the as-synthesized DSE macromolecules. 1H NMR proved the successful synthesis of DSE, as indicated by the characteristic peaks of PTMEG, HMDI, HEDS and UPy segments in polymeric backbones (Fig. S2). Fourier transform infrared (FTIR) spectra and Raman spectra further confirmed the successful preparation of DSE. The N=C=O disappearance of the peaks at 2260 cm-1 and the increasing peaks at ~1660 cm-1 and ~1695 cm-1 associated with H-bonded C=O in urea and urethane in the FTIR spectra (Fig. S3) indicate that the diisocyanate monomers were completely converted into urethane or urea moieties and the increasing amount of UPy in the DSE polymers. The peaks at 510 cm-1and 640 cm-1 in the Raman spectra (Fig. S4) belong to the υ(S-S) and υ(C-S), respectively, suggesting that disulfide bonds have been successfully introduced into the DSE systems. The molecular weight and polymer dispersity index (PDI) of all DSE samples were presented in Fig. S5 and Table S1.
The microstructure of DSE samples was systematically investigated. Fig. 2a presented small angle X-ray scattering (SAXS) profile plots of DSE specimens. It can be observed that the single broad peak intensity increases with the increasing UPy motifs, indicating more prominent accumulation of the hard segments and the increasing microphase separation domain size on the order of 1 to 3.7 nm in the materials. Both the electron density contrast between the two phases and the period lengths of the system increase as the content of UPy motifs in the hard phase increases, as shown in Fig. S6. This can demonstrate that the strong hydrogen bonding of UPy provides a stronger driving force for microphase separation35. The microphase separated structure of DSE systems was further certified by atomic force microscopy (AFM), as shown in Fig. 2b. The AFM images showed the separation of soft phase (dark areas) and hard phase (bright areas) that are even-distributed and the increasing aggregation of the hard phase with the increasing UPy groups. On account of the microphase separation of the soft and hard domains in the nanometer dimension and the amorphous system, the DSE samples exhibit excellent transparency, as shown in Fig. 2c. The thickness of the DSE films was ~500 µm and the transmittance is higher than 90% in the visible light region. The differential scanning calorimetry (DSC) traces for DSE-0 to DSE-3 was shown in Fig. 2d. It can be seen that the glass transition temperature (Tg) of all DSE samples keeps a very low constant around -76 °C that is the Tg of PTMEG, suggesting that the lower Tg of DSE polymers derive from the local motions of the soft PTMEG domain and is independent of the UPy content in the DSE backbones. The result fits with “phase-locked” strategy, wherein “locking soft phase” PTMEG realizes Li-ion conduction in the DSE polymer systems. This would be discussed later. The thermal performance of DSE was further studied by thermal gravimetric analysis (TGA), which displayed that the four DSE samples was thermally stable up to 275 °C in Fig. S7.
Achieving both strength and self-healing through the synergistic interaction between HEDS and UPy
The mechanical properties of the resulting DSE materials were evaluated by tensile testing with an extension speed of 100 mm cm-1 at room temperature. Fig. 3a showed the tensile stress-strain curves of DSE samples, suggesting that all DSE materials exhibit representative elastic behavior because they do not display yielding phenomena during elongation. Based on a reasonable structure design, DSE materials possess impressive mechanical properties, such as superior tensile strength, stretchability and toughness, which vary greatly depending on the amount of UPy crosslinks embedded in the polymer backbones. An increase in UPy content contributes to a high improvement in tensile strength and toughness and a slight decrease in stretchability. Without UPy crosslinks, DSE-0 displayed highest stretchability of 2884.58% and weaker tensile strength of 9.62 MPa. When the UPy content increased to 30 mol %, DSE-3 exhibited optimal mechanical performance with a maximum stress up to 42.60 MPa and a high stretchability of 1630.53%. As calculated toughness in Fig. S8, DSE-3 possessed highest toughness of 259.9 MJ m-3. The high stretchability and toughness may be derived from the increasing strong quadruple H-bonding UPy motifs as physical crosslinking in the polymer backbones. In the unstretched state, the interchain loops lead to the folding of the polymer backbones. When stretched, a considerable chain extension occurs due to the extensive soft segments and the broken weak H-bonding interactions. The reversible nature of UPy moieties and dynamic S-S bonds under further stretching would bring about the opening of interchain rings and the unfolding and sliding of the polymeric backbones, thus improving the stretchability and toughness of DSE (Fig. 1a). Fig. 3b showed consecutive cyclic tensile behavior of DSE-3, which was stretched to 300% for five successive loading-unloading cycles. The first cycle exhibited a large hysteresis loop, then, the hysteresis loops of the subsequent cycles 2-5 slightly decreased with the increase of cycles and nearly overlapped with some signs of hysteresis, manifesting the continuous reorganization of dynamic supramolecular interactions. These observations demonstrate that the UPy cross-linking dissociation caused by mechanical strain is a mechanism of strain energy dissipation, a key feature of high toughness. DSE was observed to dissipate mechanical energy during stretching, as manifested by the large hysteresis in the cyclic stress-strain curves. Although different strain (100%, 300%, 500%) was applied on the toughest sample (DSE-3), it still exhibited a remarkable hysteresis and an apparent residual strain and the hysteresis increasing progressively with an increasing extension, which is due to the fact that the breakage of S-S bonds and multiple H-bonds cannot recover to their original state in the limited time (Fig. S9).
On account of the synergistic interaction between dynamic covalent disulfide metathesis (HEDS) and reversible supramolecular quadruple H-bonds (UPy)59,60 in the hard domain, DSE is expected to confer self-healing capability at room temperature. To visualize the excellent self-healing property of DSE materials, Fig. 3c showed that the scratch on the DSE-2 film was observed and finally faded away within 12 h at room temperature. Meanwhile, DSE-2 was chosen to evaluate the full-cut self-healing capability of DSE materials. The original dumbbell DSE-2 was fully cut into two pieces and then put them into contact at ambient condition for different times and at 60 °C for 2 h. Fig. 3d depicts the representative stress–strain curves for the original and recombined films at room temperature for different healing times and at higher temperature 60 °C for 2 h with the tensile rate of 100 mm min–1. The self-healing DSE-2 can be able to reach a 3.07 MPa tensile strength and 604.76% tensile strain, along with 14.76% self-healing efficiency of toughness at room temperature for 2 h. Upon increasing healing time, the ultimate self-healing stress can reach almost 92.76% and the strain mostly overlapped with that of the original sample after 48 h. The self-healing process can be accelerated by raising temperature, while a healing temperature of 60 °C for 2 h brought about the recovery efficiency of 79.80% tensile strength and 89.40% stretching strain. The observed temperature-dependent self-healing performance is attributed to the easier occurrence of dynamic disulfide bonds exchange reaction and the easier recombination of multiple H-bonding interactions in multiphase polymer chains at higher temperature. The self-healing ability of the DSE systems mainly depends on dynamic disulfide bonds and multiple H-bonding interactions between polymer chains (Fig. 1b).
To gain a deeper insight into the bulk performance of DSE materials, time-temperature superposition (TTS) rheology experiments were carried out. Fig. 3e showed the shear modulus of the DSE materials from 10-10 to 103 rad s-1. It can be observed that the modulus for the rubber plateau is similar to all DSE. At the crossover frequencies (ωc) between the storage (G′) and loss (G″) modulus is the location where G′(ωc) = G″(ωc), that is, the relaxation time (1/τf) of chain segments, the DSE polymers experience a transition from viscous state to rubber state. At low frequency, G″ is higher than G′, indicative of the viscosity behavior is predominant. As the frequency increases, G′ increases faster than G″, meaning that the elastomer property is ascendant at the ω>ωc region. The increasing amount of UPy in the DSE backbones from 0 to 30% results in a higher rubber plateau and longer chain relaxation time, which corresponds with the increasingly strong UPy H-bonding crosslinking density. Accordingly, the crossover points of G′ and G″ of DSE move to higher frequency, indicating that DSE-3 is more “rubber-like” than DSE-0 at higher frequency. These results demonstrate that DSE materials mainly exhibit elastic behavior at room temperature, which is consistent with the lower Tg of DSE. As shown in Fig. S10, DMA curves presented temperature dependences of the storage modulus (G′) and the loss factor (tan δ) for the DSE materials. The remarkable drops in G′ relating to the relaxation of soft segments were all distinct and the four strong relaxation peaks appeared in the tan δ curves, which could be assigned to Tg55. At temperature above Tg, a second continuous decrease in the storage modulus G′ occurred for DSE samples. This phenomenon was also reported by Kim et al.61, manifesting that the hard phase domains have a tendency to rearrange at room temperature, which is beneficial to self-healing process.
Li-ion transport mechanism of DSICE
To study the “phase-locked” strategy, DSICE, denoted as DSICE-10 to DSICE-40, was created by dissolving different amount of LiTFSI from 10 wt.% to 40 wt.% into the DSE polymers and casting a film (Fig. S11). Ion transport properties were investigated through electrochemical impedance spectroscopy (EIS) and DSC traces. Fig. 4a showed the ionic conductivity of DSICE specimens. Experimentally, 35 wt.% LiTFSI was chosen to obtain the maximum value, up to 3.77 × 10-3 S m-1 at 30 °C, calculated by the equation σ = L/SR, where L corresponds to the thickness of DSICE samples, S corresponds to the effective overlap area, and R corresponds to the bulk resistance (Fig. S12). DSICE exhibited higher conductivity with the increasing temperature, as shown in Fig. S13, which can be attributed to more intense movements of polymer chains and ions at higher temperature. Li-ion transport behavior in the DSE polymers was demonstrated in Fig. 4b and Fig. S14, which presented that the ionic conductivity for DSICE with the same content 35 wt.% LiTFSI in the DSE-0~3 polymers remains relatively constant as the amount of quadruple UPy H-bonding increases. The similarity in the ionic conductivities of the DSE samples suggests that the Li-ion conduction is governed by the soft PTMEG segments and irrelevant to the hard UPy motifs. Further investigations confirmed the Li-ion transport mechanism. Fig. 4c depicts the Tg-dependent Li-ion transport behavior. The elevated Tg temperature along with the increasing amount of LiTFSI is due to the restricted movement of polymer chains caused by the coordination of Li+ with the soft PTMEG segments. This strongly suggests that Li+ dissociation environment in the DSICE materials derive from the soft PTMEG segments and the UPy groups do not interfere in the Li+ dissociation and transport, that is, all the DSE macromolecules dissociate and transport Li+ similarly52,62. These observations and results demonstrate that the “locking soft phase” realizes Li-ion transport and provides ionic conductivity.
Mechanical properties and thermal stability of DSICE
The mechanical properties of DSICE is an important consideration for the application of the flexible ionic devices. Addition of LiTFSI salt causes a certain decrease in the mechanical properties of DSICE, which may be due to the large size of the TFSI anions interfering with chain packing and thus preventing aggregation of the UPy domains52,63. Fig. 4d was the photographs of an DSICE-30 test specimen before and after stretching to 2000%. The stress-strain curves of DSICE, inside which was the local enlarged view, were showed in Fig. 4e. It can be observed that all DSICE exhibit exceptional strength and stretchability, of which stretching strength can reach up to 27.83 MPa and stretch strain can be more than 2000% of the original length. As the amount of LiTFSI increases, the mechanical strength decreases and the stretch strain increases. This is due to the loose polymer chains stacking caused by the increasing TFSI anions. Fig. 4f showed toughness of DSICE, of which DSICE-10 was maximum 164.36 MJ m-3 and DSICE-30 reached 79.20 MJ m-3. Combing with ionic conductivity, the preferable choice for the high-performance DSICE is DSICE-30 with 30 wt.% LiTFSI, which competes with the highest reported ionic conductors5,8,9,26-29,46,47, as shown in Table S2. In addition, Fig. S15 exhibits excellent thermal stability of DSICE, of which possess high decomposition temperature, up to 230 °C. The operating temperature of DSICE-30 was assessed by rheological measurements upon a frequency sweep, as shown in Fig. S16. When the temperature is low, such as 30 °C, G′ is much higher than G″, indicative of the perfectly elastic response of the materials. The crossover point of G′ and G″ appears at the sweep temperature over 60 °C, suggesting that the viscous response starts to happen. Furthermore, the crossover point moves to the high-frequency direction as the sweeping temperature increases to 90 °C, meaning that the obvious viscous behavior occurs in the DSICE system. In order to make DSICE behavior more elastically, it is suggested that the operating temperature do not exceed 90 °C.
Autonomous self-healing capability and recyclable performance of DSICE
The self-healing capability in mechanical performance was investigated. To visualize the excellent self-healing abilities of the DSICE samples, the dumbbell-shaped DSICE-30 film colored blue and red with standard 12 mm × 2 mm rectangular and a thickness of 0.5 mm was cut into two pieces, respectively, then put any two colored pieces into contact and subsequently self-healed quickly at ambient temperature. After self-healing for 5 min, the jointed sample can be bent, twisted and even stretched to 100%, 400%, 800% of the original specimen, as shown in Fig. 5a and Movie S1. The optical microscope image of the self-healed sample was shown in Fig. S17, which displayed seamless combination of two cut-off pieces. The self-healing effect on mechanical performance was quantitatively evaluated by uniaxial tensile experiments. Dumbbell-shaped DSICE-30 specimens were bisected, and then recombined under different conditions. Fig. 5b presented the typical stress-strain curves of DSICE-30 specimens after from 30 min to 6 h healing time at ambient temperature. When the healing time was over 6 h, the stretch strength and strain of self-healed samples could recover to 10.13 MPa and 2172.64 %, respectively, which were almost coincident with original specimens, suggesting that DSICE were endowed with excellent autonomous self-healing capability. which ascribed the critical contribution of the reversible nature of dynamic S-S bonds and supramolecular H-bond motifs. From the point of view of molecular level, the dynamic bonds existing on or near damaged area could promote the chains exchange reaction once contact occurs. Meanwhile, the low Tg of DSICE (< -45 °C) makes them in a high elastic state and enhances the movement of polymer chains at room temperature, which could facilitate self-healing.
The reversible nature of dynamic S-S metathesis and supramolecular H-bonds in the polymer chains contributes to the recyclable performance of DSICE. Therefore, we further study the recyclable feature of DSICE through recycling in the THF solvent and reprocessing under compression molding conditions. Typically, Fig. 5c showed good recycling of DSICE, that is, DSICE can be fully dissolved into the THF solvent, recycled by casting the solution and drying, and then reprocessed into the desired specimens. Besides, the small pieces DSICE samples were hot-pressed in a mold applying a force of 0.5 MPa at 70 °C for 30 min, reprocessing into an integrate and coherent film, as shown in Fig. 5d. Interestingly, transparent and smooth films of DSICE were obtained even after three times reprocessing or recycling. To further reveal the mechanical properties of reprocessed DSICE, tensile tests were performed. As shown in Fig. S18, the reprocessed DSICE-35 showed slightly decreased stretching stress, which is attributed to the insufficient crosslinking of the UPy units after the recycling process.
DSICE-based flexible conductive substrate and stretchable touch sensor
Transparent DSICE with highly competitive properties (see Fig 5e and Table S2) can be used to fabricate flexible iontronic devices, such as ionic conductive substrate and stretchable touch sensor. DSICE-based conductive substrate was visualized via “LED-lights” experiment, as shown in Fig. 6a. We placed a “heart-shaped” pattern with 18 LED lights on the DSICE-30 substrate. As expected, the “heart-shaped” pattern with LED lights can be entirely lit in an electric field. When DSICE-30 was cut from the middle of the “heart-shaped” pattern, the LED lights on either side of the “heart-shaped” pattern could not be lit. Then the cut-off DSICE-30 was contacted and self-healed for 30 min at room temperature, on which the “heart-shaped” pattern with 18 LED lights would be lit up again, and the luminescence intensity did not decay, which demonstrates DSICE-based conductive substrate with self-healing capacity could extend the service life of devices.
Fig. 6b was the schematic diagram of DSICE-based touch sensor in four different states: original, touched, stretched, stretched and touched. Human body can act as an ionic conductor. When a finger touches the sensor, the human body become part of the circuit, the finger can conduct current and introduce new elements into the circuit, causing the change of the circuit characteristics8. As shown in Fig. 6c, 6d, when the sensor was stretched, the |Z| (impedance value) increased and the -ϕ (negative phase angle) changed slightly; when touched, the |Z| and -ϕ versus frequency curves showed significant discrepancy from original or stretched states, i.e., the |Z| exhibited a large peak in the frequency range of 100 Hz–1 MHz, the -ϕ changed a lot from positive value (~87°) to negative value (~-20°) in the same frequency range. Obviously, the -ϕ became negative values at low frequency range (10 kHz to 100 Hz), so the touched sensor exhibited inductance characteristics. Fig. 6e presented the differences in the complex plane in the Nyquist plots of the impedance spectra under the four different states. In order to detect signals from different stimulus we set a single frequency (f = 3 kHz), as shown in Fig. 6E, different stimulus appeared at different regions in impedance complex plane, meaning that the sensor responds very selectively to different stimulus. Fig. S19 depicted the impedance changes of DSICE-30 when stretched to different tensile elongation, as seen in Movie S2. And Fig. S20 depicted the impedance changes of DSICE-30 at different touched stimuli when stretched, as seen in Movie S3.