Cooperative Chemical Coupling and Physical Lubrication Effects Construct Highly Dynamic Ionic Covalent Adaptable Network for High-Performance Wearable Electronics

Open AccessCCS ChemistryRESEARCH ARTICLES21 Jun 2022Cooperative Chemical Coupling and Physical Lubrication Effects Construct Highly Dynamic Ionic Covalent Adaptable Network for High-Performance Wearable Electronics Lijie Sun, Hongfei Huang, Qingbao Guan, Lei Yang, Luzhi Zhang, Benhui Hu, Rasoul Esmaeely Neisiany, Zhengwei You and Meifang Zhu Lijie Sun State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Institute of Functional Materials, Research Base of Textile Materials for Flexible Electronics and Biomedical Applications (China Textile Engineering Society), Shanghai Engineering Research Center of Nano-Biomaterials and Regenerative Medicine, Donghua University, Shanghai 201620 , Hongfei Huang State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Institute of Functional Materials, Research Base of Textile Materials for Flexible Electronics and Biomedical Applications (China Textile Engineering Society), Shanghai Engineering Research Center of Nano-Biomaterials and Regenerative Medicine, Donghua University, Shanghai 201620 , Qingbao Guan State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Institute of Functional Materials, Research Base of Textile Materials for Flexible Electronics and Biomedical Applications (China Textile Engineering Society), Shanghai Engineering Research Center of Nano-Biomaterials and Regenerative Medicine, Donghua University, Shanghai 201620 , Lei Yang State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Institute of Functional Materials, Research Base of Textile Materials for Flexible Electronics and Biomedical Applications (China Textile Engineering Society), Shanghai Engineering Research Center of Nano-Biomaterials and Regenerative Medicine, Donghua University, Shanghai 201620 , Luzhi Zhang State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Institute of Functional Materials, Research Base of Textile Materials for Flexible Electronics and Biomedical Applications (China Textile Engineering Society), Shanghai Engineering Research Center of Nano-Biomaterials and Regenerative Medicine, Donghua University, Shanghai 201620 , Benhui Hu Key Laboratory of Clinical and Medical Engineering, School of Biomedical Engineering and Informatics, Nanjing Medical University, Nanjing 211166 , Rasoul Esmaeely Neisiany Department of Materials and Polymer Engineering, Faculty of Engineering, Hakim Sabzevari University, Sabzevar 9617976487 , Zhengwei You *Corresponding authors: E-mail Address: email protected E-mail Address: email protected State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Institute of Functional Materials, Research Base of Textile Materials for Flexible Electronics and Biomedical Applications (China Textile Engineering Society), Shanghai Engineering Research Center of Nano-Biomaterials and Regenerative Medicine, Donghua University, Shanghai 201620 and Meifang Zhu *Corresponding authors: E-mail Address: email protected E-mail Address: email protected State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Institute of Functional Materials, Research Base of Textile Materials for Flexible Electronics and Biomedical Applications (China Textile Engineering Society), Shanghai Engineering Research Center of Nano-Biomaterials and Regenerative Medicine, Donghua University, Shanghai 201620 https://doi.org/10.31635/ccschem.022.202202037 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Covalent adaptable networks (CANs), which combine the benefits of traditional thermosets and thermoplastics, have attracted considerable attention. The dynamics of reversible covalent bonds and mobility of polymer chains in CANs determine the topological rearrangement of the polymeric network, which is critical to their superior features, such as self-healing and reprocessing. Herein, we introduce an ionic liquid to dimethylglyoxime-urethane (DOU)-based CANs to regulate both reversible bond dynamics and polymer chain mobility by cooperative chemical coupling and physical lubrication. Small-molecule model experiments demonstrated that ionic liquids can catalyze dynamic DOU bond exchange. Ionic liquid also breaks the hydrogen bonds between polymeric chains, thereby increasing their mobility. As a combined result, the activation energy of the dissociation of the dynamic network decreased from 110 to 85 kJ mol−1. Furthermore, as a functional moiety, the ionic liquid imparts new properties to CANs and will greatly expand their applications. For example, the consequent conductivity of resultant ionic CAN (iCAN) has demonstrated a great power to build high-performance multifunctional wearable electronics responsive to multiple stimulations including temperature, strain, and humidity. This study provides a new design principle that simultaneously uses the chemical and physical effects of two structural components to regulate material properties enabling novel applications. Download figure Download PowerPoint Introduction Generally, thermosets are preferred over thermoplastics in high-demand applications where mechanical and thermal stability are important.1,2 The crosslinked molecular structure of thermosets is responsible for their superior mechanical and thermal performance. However, these advantages come at the cost of certain limitations; for example, thermosets are non-melting, insoluble, and unsuitable for reprocessing and recycling.3 The use of nonrecyclable materials inherently results in the depletion of raw materials and the generation of waste after use, thereby negatively impacting the environment and economy.4 Therefore, new polymeric materials that provide thermoplastic recyclability without reducing the structural stability of thermosets are in the spotlight of materials research.5 Covalent adaptable networks (CANs), which combine the benefits of traditional thermosets and thermoplastics, have shown great promise in various applications from materials with superior features including self-healability, recyclability, and programmable systems.6–11 In the CANs structure, the dynamic covalent bond is a core unit, and its dynamic exchange is closely associated with the macroscopic rheological, mechanical properties, self-healing, and reprocessing abilities of CANs. Thus, a series of dynamic covalent structures, such as urethane, boronic ester, and imine bonds,8,12,13 have been developed to construct CANs. In addition to the chemical structure, the catalyst moiety is another key factor that controls the exchange rate of dynamic covalent bonds.14 However, most catalysts are toxic and additives without other functions in the CANs. Furthermore, both dynamics of the reversible covalent bonds and chain mobility of the polymer matrix determine the dynamic behavior of the CANs.15 However, the latter factor has not been thoroughly studied. Despite great potential, there is yet no suitable way to combine these factors to modulate the properties of CANs. Herein, we propose a new efficient strategy to modulate the dynamic behavior of CANs by combining the dynamics of reversible bonds and the mobility of polymer chains. A functional small-molecule additive, an ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, EMITFSI), was introduced into CANs based on a newly designed dynamic dimethylglyoxime-urethane (DOU) group.16–20 The ionic liquids have three cooperative functions: (1) A chemical coupling effect. The ionic liquid can couple with a dynamic DOU group and serve as a small-molecule catalyst that effectively promotes dynamic oxime-urethane exchange. (2) A physical lubrication effect. The ionic liquid is well integrated with the polymeric network via extensive hydrogen bonding. The hydrogen bonds break the interaction between the polymer chains of the CANs, which increases their mobility. The combined effects of chemical coupling and physical lubrication decrease the activation energy of the dissociation of the dynamic covalent network, which can efficiently tune the topological rearrangement and resultant properties of the CANs, including processability, self-healing, and recyclability. (3) A conductive component. The ionic liquid EMITFSI imparts ionic conductivity to the material to yield an ionic CAN (iCAN),21 which is expected to have a significantly expanded application scope.22–27 Experimental Methods Materials A 50∶50 mixture of 2,4′- and 4,4′-diphenylmethane diisocyanate (MDI) was provided by Wanhua Chemical (Yantai, China). Poly(ethylene glycol adipate) diols (PEGAD, Mw ∼1000) were purchased from Jining Baichuan Chemical Co., Ltd. (Jining, China) and dried for 2 h under vacuum at 110 °C before use. Dimethylglyoxime (DMG, 98%) was supplied by Sinopharm Chemical Reagent (Shanghai, China). EMITFSI (≥99%) was obtained from the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. Glycerol was purchased from Sigma-Aldrich (Shanghai, China). Dibutyltin dilaurate (DBTDL, 95%) was purchased from Aladdin (Shanghai, China). Phenethyl isocyanate, acetone oxime, and p-tolyl isocyanate were purchased from J&K (Shanghai, China). Deuterated solvents for nuclear magnetic resonance (NMR) analysis were supplied by Cambridge Isotope Laboratories, Inc. (Shanghai, China). All other solvents were purchased from Sinopharm Chemical Reagent (Shanghai, China). All reagents were used as received without further purification unless otherwise noted. Preparation of the CAN and iCANs PEGAD (2 g, 2 mmol), DMG (93.6 mg, 0.8 mmol), glycerol (74.4 mg, 0.8 mmol), and the required equivalent of EMITFSI were dissolved in 4 mL acetone in a glass vessel equipped with a magnetic stirrer at 40 °C. MDI (1 g, 4 mmol) and DBTDL (7 mg) were added, and the mixture was left to react for 30 min. The reaction mixture was then poured into a polytetrafluoroethylene mold and reacted at 40 °C for 12 h before further curing in a vacuum oven at 60 °C for another 24 h to produce DOU-based crosslinked polyurethane (DOU-CPU) and iCANs. The DOU-CPU without the ionic liquid is denoted as CAN, and the DOU-CPU samples with x = 20, 40, and 60 wt % ionic liquid (with respect to the weight of DOU-CPU) are denoted as iCAN-x. Synthesis of compound AB Acetone oxime (compound A, 0.73 g, 10 mmol) was dissolved in acetone (5 mL), and then p-tolyl isocyanate (compound B, 1.33 g, 10 mmol) was added to react at 40 °C under a nitrogen atmosphere for 4 h. After the reaction, a white precipitate was generated at −20 °C and recrystallized from acetone three times. After drying at room temperature under a vacuum, compound AB was obtained. Characterization and measurement Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker AVANCE 600 NMR spectrometer (Bruker, Germany). The attenuated total reflectance Fourier-transform infrared (FTIR) spectra were recorded on a Thermo Scientific Nicolet 8700 spectrometer (Thermo Fisher Scientific, USA). A Jasco V-630 UV–visible spectrophotometer (JASCO, Japan) was used to measure the optical transmittance. The mechanical properties were investigated using an MTS E42 tensile machine with a 100-N load cell. Uniaxial tensile tests were performed at a stretching speed of 50 mm min−1 unless otherwise noted. Impedance spectroscopy was performed using a CHI670E electrochemical analyzer. The conductivity was calculated using the equation σ = L/AR, where L is the length of the iCAN, A is the iCAN cross-sectional area, and R is the bulk resistance. Thermogravimetric analysis (TGA) was performed on a TG 209 F1 thermogravimetric analyzer (NETZSCH, Germany) from 40 to 600 °C at a heating rate of 10 °C min−1 under nitrogen atmosphere. Differential scanning calorimetry (DSC) studies were performed on a DSC-822 differential scanning calorimeter (Mettler Toledo, Switzerland) at a heating rate of 10 °C min−1 under nitrogen atmosphere. The morphology and elemental distribution of the samples were characterized using field-emission scanning electron microscopy (Hitachi SU-8010, Japan) and energy-dispersive X-ray spectroscopy (Oxford Inca X-Max, UK). Stress–relaxation tests of CAN and iCAN samples (thickness of 1 mm) were then performed on a TA Instruments Discovery HR-2 rheometer (TA, USA) with a 25-mm plate–plate geometry by applying 5% strain at a constant gap. Results and Discussion The design and chemical structure of the proposed iCAN is schematically illustrated in Figure 1a. The iCAN is formed through the extensive noncovalent interactions between the EMITFSI cation–anion pairs and DOU-CPU elastomer.28 The H atoms of the –NH groups in the polymer chains and of the imidazolium cation act as hydrogen-bond donors, while the strongly electronegative atoms, including F, O, and N in the TFSI anion and C=O in the polymer chains, act as hydrogen-bond acceptors.29,30 In this study, linear DOU-based polyurethane (DOU-PU) was used as a model to investigate the interactions between the components. The 1H NMR spectra in Figure 1b show that the peaks of the H atoms of the imidazolium cation in EMITFSI shifted after the addition of PU, indicating the formation of hydrogen bonds between EMITFSI and the PU chains.31 The 19F NMR spectra in the Supporting Information Figure S1 indicate the formation of hydrogen bonds between the CF3 group and PU chains. We further investigated the role of the molecular interactions between EMITFSI and PU chains using FTIR spectroscopy. The four vibrational bands located at 1051, 1139, 1177, and 1347 cm−1, corresponding to the S–N–S and O=S=O symmetric stretch and CF3 and O=S=O asymmetric stretch, respectively, from the TFSI anion, shifted to higher wavenumbers in the iCANs ( Supporting Information Figure S2). With an increase in ionic liquid content of the iCANS, the C=O and N–H (in the PU chains) stretching vibrations shifted from 1726 and 3339 cm−1 to 1730 and 3352 cm−1, respectively (Figure 1c). This indicates that the hydrogen bonds between the carbamate and carbonyl groups of the polymer were replaced by interactions between the PU chains and ionic liquid,32 as revealed by the 1H NMR and FTIR spectra. Thus, the ionic liquid is expected to serve as a lubricant. The DSC results provided more direct evidence of the lubricating effect.33 As shown in Figure 1d, the glass transition temperature (Tg) of the CAN decreased from −18.8 to −58.4 °C upon the addition of 40 wt % EMITFSI. TGA results indicated that the prepared iCANs exhibited excellent thermal stability with thermal decomposition temperatures of 265–288 °C (Figure 1e). Moreover, the strong interactions of EMITFSI with the PU chains facilitated its effective dispersion in the PU network and prevented it from leaking out of the structure. Elemental mapping of the iCAN-40 (40% loaded ionic liquid) cross section showed a uniform spatial distribution of C, O, N, F, and S elements ( Supporting Information Figure S3), thereby revealing the even distribution of EMITFSI in the CAN without detectable phase separation. Furthermore, the high miscibility of the ionic liquid with the polymer made the iCAN composite highly transparent,34 with an average transmittance of over 97% for a 1-mm-thick film under visible-light wavelengths of 400–800 nm (Figure 1f). As shown in Supporting Information Figure S4, the high transparency was maintained during stretching. Figure 1 | Design and characterization of iCANs. (a) Schematic illustration of the iCAN showing the chemical structure of DOU-PU and interaction between the EMITFSI and polymer matrix. (b) 1H NMR spectra of EMITFSI, DOU-PU, and email protectedEMITFSI composite in acetone-d6. (c) FTIR spectra of EMITFSI, CAN, and the iCANs (iCAN-20, iCAN-40, and iCAN-60). (d) DSC and (e) TGA curves of the CAN and iCANs. (f) Transmittance of iCAN-40 with a film thickness of 1 mm. Inset: photograph of iCAN-40 (scale bar: 2 cm). Download figure Download PowerPoint The ionic liquid formed hydrogen bonds with the PU network, which effectively increased the mobility of the polymer chains and tuned the mechanical properties of the CAN. Uniaxial and cyclic tensile tests were performed to investigate the mechanical properties of the iCANs with different ionic liquid contents. The CAN, had a tensile strength of 8.42 ± 0.98 MPa and maximum extensibility of 675 ± 112%, with a relatively high Young's modulus (3.6 ± 1.2 MPa) (Figure 2a). However, a hysteresis loop was observed during the cyclic tensile test, indicating the poor elasticity of the CAN (Figure 2b). This was because the hydrogen bonds between the PU chains hampered the resilience of the CAN after stretching.35 With the introduction of the ionic liquid EMITFSI, the hydrogen bonds between the PU chains were extensively replaced by hydrogen bonds between EMITFSI and the PU chains, while the covalent crosslinking maintained the structural stability of PU networks, leading to an elastic recovery. Therefore, the iCANs exhibited a significantly reduced hysteresis loop, indicating excellent resilience. Meanwhile, the iCANs possessed higher stretchability and lower Young's modulus than the CAN. For example, iCAN-40 showed a tensile strength and strain at break of 2.01 ± 0.31 MPa and 783 ± 67%, respectively (Figure 2a). The Young's modulus of iCAN-40 (440 ± 78 kPa) was similar to that of natural skin (140–600 kPa), indicating its mechanical potential as an ionic skin.36 Furthermore, the elasticity of the iCANs was quantitatively characterized in terms of the residual strain after the first loading–unloading cycle at a strain of 200% (Figure 2c). Compared with the CAN (24.65 ± 2.40%), the iCANs exhibited significantly better elasticity with much smaller residual strains. When the amount of loaded ionic liquid was further increased to 60% (iCAN-60), the residual strain (9.76 ± 1.11%) increased from that of iCAN-40 (5.16 ± 0.24%) because the excess ionic liquid increased the viscosity, instead of the elasticity, of the material. Moreover, in the tensile test, the elastic modulus of iCAN-40 was independent of the tensile rate (Figure 2d). The elastic modulus and tensile strength remained nearly constant with an increasing tensile rate from 10 to 150 mm min−1, which further confirmed the lubricating effect of the ionic liquid.37 In addition, iCAN-40 showed a rubber-like recovery during a fatigue cyclic tensile test at 200% strain for 200 cycles, and a low residual strain (∼8%) was maintained ( Supporting Information Figure S5). Excellent elasticity and durability are necessary for a functional sensor for long-term applications.38 Figure 2 | Mechanical properties of the CAN and iCANs. (a) Uniaxial and (b) cyclic tensile tests of the CAN, iCAN-20, iCAN-40, and iCAN-60. (c) Residual strains of the CAN and iCANs after the stretching and releasing cycle tensile test at 200% strain. (d) Tensile tests of iCAN-40 at different tensile rates. Download figure Download PowerPoint In the CANs structure, the dynamics of reversible covalent bonds determine the dynamic behavior of the polymeric network. In addition to the dynamic bond structure, the catalyst moiety is another key factor controlling the exchange rate of dynamic covalent bonds. Ionic liquids have been recognized as catalysts that facilitate a diverse range of chemical transformations.39 In addition to its physical effect on the polymer chains, we demonstrated that the ionic liquid can form chemical coupling with the dynamic DOU group to serve as a small-molecule catalyst. This activates the dynamic DOU bonds, facilitating the reprocessing and recycling properties of iCANs. To clarify the catalytic effect of the ionic liquid, the dynamic exchange reaction of oxime-urethane units at 90 °C was evaluated using small molecules models (Figure 3a). First, compound AB with an oxime-urethane unit was synthesized using acetone oxime ( A) and p-tolyl isocyanate ( B), and its structure was confirmed by 1H NMR ( Supporting Information Figure S6). The equilibrium exchange reaction between AB and phenethyl isocyanate ( C) in DMSO-d6 with or without the ionic liquid was investigated through in situ 1H NMR (Figure 3a). We monitored the ratio change of the proton peaks at 1.99 and 1.97 ppm that correspond to the CH3 protons in A. These proton peaks gradually decreased in intensity, while those at 1.94 and 1.92 ppm appeared and gradually intensified, which indicates the generation of compound AC (Figure 3b). To quantitatively evaluate the reaction equilibrium rate, the CH3 peak area of A was integrated. The conversion rate of AC is AC/(AB + AC), where AB and AC are the concentrations of AB and AC, respectively, at a certain time. The conversion rate of AC at 120 min and 90 °C was 42.7% in the presence of the ionic liquid, while that without the ionic liquid was only 34.5%. The rate of the exchange reaction in ionic liquid was significantly higher than that without the ionic liquid (Figure 3c). These results demonstrate that the ionic liquid can serve as a catalyst to promote dynamic oxime-urethane exchange. Figure 3 | The dynamic reaction of small model molecules of DOU group catalyzed by the ionic liquid. (a) Exchange reaction between model compounds AB and C produced AC and B at 90 °C. (b) In situ 1H NMR spectra of the mixture of AB and C with or without the ionic liquid. The proton peaks at 1.94 and 1.92 ppm appeared with gradually increasing intensity, which indicates the generation of compound AC. (c) Conversion rate of AC with or without the ionic liquid. Download figure Download PowerPoint Subsequently, we demonstrated that the combined effects of chemical coupling and physical lubrication in iCANs can significantly decrease the activation energy of the dissociation of the dynamic covalent network. This efficiently tunes the molecular network and resultant properties, including the processing, self-healing, and recycling performance of the CAN (Figure 4a). Consequently, the rheological properties of the iCANs were investigated by tests at different temperatures to the dynamic properties of the CAN and iCANs (Figure For the a strain of 5% was and the modulus was monitored as a of time. The CAN and iCANs showed from 110 to indicating the dynamic dissociation of their polymer Furthermore, based on the model for the was = = of the The decreased with increasing The of the CAN was at 110 while that of iCAN-40 was significantly reduced to at 110 °C ( Supporting Information Figure The temperature of the can by the ( = ( a R where is the is the and is the activation energy of The for the CAN kJ was much higher than those for kJ (Figure indicating that the ionic liquid can serve as a catalyst and to significantly the of the dissociation of the DOU The low to network that facilitated the self-healing and reprocessing of the iCANs. Furthermore, the self-healing of the CAN and iCAN-40 was investigated by into two and then the into After heating the samples for 12 h at 90 the tensile strength and strain at break of iCAN-40 were ± MPa and ± respectively, which were and of the (Figure while those of the CAN were only and respectively ( Supporting Information Figure the superior self-healing of iCAN-40 with that of the CAN. The FTIR spectra of the CAN and iCAN-40 samples at different temperatures are shown in Supporting Information Figure 120 the peak corresponding to the stretching of iCAN-40 had a higher than that of the CAN, indicating the catalytic effect of the ionic liquid in the dynamic dissociation of A catalytic of ionic liquid is proposed in Supporting Information Figure The dynamic dissociation of DOU The with carbonyl the of the units more the the formation of a hydrogen bond between the group of the oxime and of TFSI the more electronegative to facilitate its on the Figure 4 | Dynamic properties of the CAN and iCANs. (a) Schematic illustration of the reprocessing of the iCANs. (b) of the CAN and iCANs to the (c) of for the CAN and iCANs. (d) curves of iCAN-40 and (e) of the reprocessing of the polymer network by (scale bar: 2 cm). (f) FTIR spectra of iCAN-40 before and after reprocessing. curves of the Download figure Download PowerPoint of their use and has a Thus, the of thermosets is of great to the and of such of the highly dynamic chemical structure and mobility of the molecular network, the iCAN exhibited excellent recyclability. This was evaluated by the samples of iCAN-40 at 110 °C (Figure To the samples were to structural and mechanical The FTIR spectra showed that the chemical structure of iCAN-40 was maintained after the recycling (Figure tensile properties of iCAN-40, including Young's strain, and at were observed after the of recycling (Figure the iCAN exhibited excellent recyclability, which is highly for studies of CANs have on their dynamic Despite great potential, such as properties are to using CANs. properties are for their applications in such as wearable and energy the newly introduced structural an ionic liquid, provided high ionic conductivity to the Figure the iCAN-40 to a equipped with a As iCAN-40 was the of the gradually decreased to the increase in resistance. upon the the length of iCAN-40 and the of the were The ionic of iCAN-20, iCAN-40, and at °C were to and S respectively (Figure As the EMITFSI increased from to 60 wt the conductivity of the material increased by two of that from S to S Figure | The performance of (a) of of iCAN-40 and (scale bar: 2 cm). (b) spectra of the iCANs. (c) of iCAN-40 strain The in the to the