Transparent, Healable Elastomers with High Mechanical Strength and

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Transparent, Healable Elastomers with High Mechanical Strength and Elasticity Derived from Hydrogen-Bonded Polymer Complexes Yan Wang, Xiaokong Liu, Siheng Li, Tianqi Li, Yu Song, Zhandong Li, Wenke Zhang, and Junqi Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08636 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 14, 2017

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ACS Applied Materials & Interfaces

Transparent, Healable Elastomers with High Mechanical Strength and Elasticity Derived from Hydrogen-Bonded Polymer Complexes

Yan Wang, Xiaokong Liu, Siheng Li, Tianqi Li, Yu Song, Zhandong Li, Wenke Zhang and Junqi Sun* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China. *Email: [email protected]

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Abstract: It is challenging to develop healable elastomers with combined high mechanical strength and good elasticity. Herein, a simple strategy to develop high-performance elastomers that integrate high mechanical strength, enormous stretchability, good resilience, and healability is reported. Through simply complexing poly(acrylic acid) and poly(ethylene oxide) based on hydrogen-bonding interactions, transparent composite materials that perform as elastomers are generated. The as-prepared elastomers exhibit mechanical strength (true strength at break) and toughness (fracture energy) as high as 61 MPa and 22.9 kJ/m2, respectively, and can be stretched to >35 times their initial length and are able to restore their original dimensions following the removal of stress. Meanwhile, the elastomers are capable of healing physical cuts/damages in a humid environment because of the reformation of the reversible hydrogen bonds between the polymer components. The high mechanical strength of the elastomers is ascribed to the high degree of polymer chain entanglements and multiple hydrogen-bonding interactions in the composites. The reversible hydrogen bonds, which act as cross-linkages, facilitate the unfolding and sliding of the polymer chains in the composites, thereby endowing the elastomers with good elasticity and healability. Furthermore, flexible conductors with water-enabled healability were developed by drop-casting Ag nanowires on top of the elastomers.

Keywords: polymeric materials, elastomers, polymer complexes, self-healing materials, hydrogen-bonding interactions


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Introduction Elastomers, represented by natural or synthetic rubber, have been utilized for centuries in diverse areas ranging from the automotive industry to household and healthcare supplies, due to their beneficial properties such as strength, toughness, elasticity, and so forth.1-5 Mechanical strength enhancements and elasticity improvements have become eternal topics for the development of high-performance elastomers.6-10 Elastomers are generally derived from cross-linked polymer networks that hold polymer chains together via intermolecular forces, including covalent and non-covalent bonds.1-16 The cross-linkages endow elastomers with both mechanical strength and elasticity, which respectively ensure the deformation of elastomers under a given stress without fracture and their subsequent return to their original dimensions following the removal of stress. However, high mechanical strength of elastomers always comes at the price of low elasticity in terms of their stretchability and resilience, and vice versa, because enhanced mechanical strength relies on high degree of cross-linking that in turn suppresses elasticity.7-10,17-19 Creton et al. developed high-strength and tough polyacrylate elastomers with true stress at break and toughness (fracture energy) as high as 29 MPa and 5.0 kJ/m2, respectively, by using a triple network covalent cross-linking strategy.9 Meanwhile, the elastomers allow a maximum elongation of ca. 2.6 times their initial length. Bao et al. obtained highly stretchable elastomers that can be elongated to ca. 27 times (at the stretching speed of 2 mm/min) their original length by cross-linking poly(dimethylsiloxane) (PDMS) chains via relatively weak coordination bonds. Such elastomers have a typical tensile strength of ca. 0.25 MPa.10 Therefore, it is a big challenge to reconcile the performance of elastomers with both high mechanical strength and good elasticity. Furthermore, it has been an increasing demand nowadays to impart self-healing capabilities to synthetic materials, aiming to extend their lifetime and sustainability.20-31


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Employing reversible non-covalent interactions or dynamic covalent bonds as cross-linkages that can be reformed after break, a number of self-healing/healable elastomers have been developed from deliberately synthesized polymers.7,8,10,14,32-37 However, ongoing effort is still required to develop self-healing/healable elastomers that simultaneously integrate more enhanced mechanical strength and elasticity. Moreover, it is also highly desirable to explore simple strategies that benefit the mass production of high-performance elastomers with healability based on cost-effective materials. Recently, our group and others reported on self-healing/healable polymer films that are capable of healing scratches/cuts based on the layer-by-layer (LbL) assembly of polymers with complementary non-covalent interactions.38-45 The healability of the polymer films results from the reversibility of the non-covalent bonds and the high mobility of polymer chains under external stimuli. The LbL assembled polymer films are virtually polymer complexes formed at the interfaces of solid substrates and polymer solutions.46-51 It is accordingly envisioned that complexation of polymers in bulk based on non-covalent interactions can also lead to crosslinked polymer composites with healability.52,53 Meanwhile, polymer complexation in bulk will favor higher degree of polymer chain entanglements in the resultant composite materials, as compared to solid films formed at the interfaces of solid substrates and polymer solutions, especially when polymers with high molecular weights are involved. The promoted entanglements of polymer chains in a cross-linked polymer network is believed to be beneficial for enhancing the mechanical strength of the materials.54,55 Moreover, the reversible noncovalent bonds in the polymer complexes can dynamically break and reform upon external stress, thereby facilitating the unfolding and sliding of the polymer chains during deformation and thus endowing the composite materials with good elasticity.10 In light of the above-described


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hypothesis, by simply complexing high-molecular-weight poly(acrylic acid) (PAA) and poly(ethylene oxide) (PEO), we generated healable elastomers that integrate high mechanical strength, high toughness, and good elasticity. The elastomers derived from the hydrogen-bonded PAA-PEO complexes not only exhibit high transparency and water-facilitated healability, but also exhibit mechanical strength (true strength at break) and toughness (fracture energy) as high as 61 MPa and 22.9 kJ/m2, respectively. Moreover, the elastomers can be stretched up to 35 times (at the stretching speed of 2 mm/min) their initial length and are able to restore their original shapes following the removal of stress. Beyond the numbers of reports focusing on the nanoscale LbL assembled PAA/PEO films,45,56-58 this is the first known report dedicated to the PAA-PEO complexes-derived bulk materials of elastomers with integrated high mechanical strength, enormous stretchability, good resilience, and healability. The preparation simplicity of the PAA-PEO elastomers and their high mechanical and healing performances give them a significant potential for practical applications. Results and Discussion Preparation and Characterization of the PAA-PEO Elastomers. Scheme 1 presents the preparation process of the elastomers based on the complexation of PAA and PEO in the bulk. Following the mixing of aqueous solutions (100 mL, 4 mg/mL, pH 2.5) of PAA and PEO with a pre-designed ratio through peristaltic pumps at the flow rates of 5 mL/min under stirring, precipitation was formed due to the hydrogen-bonding interactions between the polymers. The precipitation was then collected via centrifugation, followed by compression molding for 2 days via two pieces of hydrophobized glass slides. A transparent rubber-like sheet of PAA-PEO composite was finally obtained after the sample was further dried in air. As listed in Table 1, the PAA-PEO composites with different compositions were obtained by complexing high-


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molecular-weight PAA (Mw ca. 450,000) and PEO of different molecular weights (Mw ca. 600,000, 300,000, or 100,000) at varied PAA/PEO monomer molar ratios. The as-prepared composites are denoted by the abbreviations listed in Table 1. For example, PAA1.4/450kPEO1.0/600k represents the PAA-PEO composite with a monomer molar ratio of 1.4:1 between PAA (Mw of 450k) and PEO (Mw of 600k), which was calculated based on the 1H NMR analysis (Figure S1, Supporting Information). The PAA-PEO composites prepared at the pH of 2.5 are highly transparent and did not exhibit PEO crystalline peaks when analyzed by X-ray diffraction (XRD, Figure S2 and S3, Supporting Information), suggesting that PAA and PEO are homogeneously complexed in the composites without phase separation. In a control experiment, PEO crystalline peaks are recorded by XRD in the PAA-PEO mixtures prepared from aqueous PAA and PEO solutions at a pH of 7.5 (Figure S3, Supporting Information). At pH 7.5, minimal hydrogen-bonding interactions between PAA and PEO can occur, which cannot lead to the formation of homogeneous PAA-PEO complexes but results in phase separation in the PAAPEO mixtures. The hydrogen-bonding interactions between PAA and PEO in PAA1.4/450kPEO1.0/600k composites are confirmed by the Fourier transform infrared (FTIR) spectroscopy (Figure S4).[59] Moreover, the as-prepared PAA-PEO composites can be dissolved in a 0.1 M NaOH aqueous solution in which PAA is deprotonated. This result indicates that there are no covalent or permanent crosslinks in PAA-PEO composites. The thermal transition properties of the PAA-PEO composites with different compositions were measured by differential scanning calorimetry (DSC) to determine their glass transition temperature (Tg). As shown in Figure 1a and 1b, Tg of the PAA-PEO composites increases with larger proportion of PAA and/or higher molecular weight of PEO. Except that the PAA2.0/450kPEO1.0/600k composite exhibits a Tg around room temperature (i.e., 24.5 ºC), the Tg values of all


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the other composites are well below room temperature, which is a typical characteristic of elastomers used at room temperature.1-16 PAA homopolymer (Tg of 106 ºC) has a much higher Tg than the PEO homopolymer (Tg of ca. -67 ºC), which results in the higher Tg of the PAA-PEO complexes with higher proportion of PAA. The PAA-PEO complexes made from PEO with higher molecular weight will feature a higher degree of polymer chain entanglements, which relatively suppress the mobility of the polymer chains and thus lead to the higher Tg of the PAAPEO complexes. Note that the DSC curves of the PAA-PEO composites do not show phase transition corresponding to the glass transition of PAA or PEO homopolymer, which also verifies the absence of phase separation between PAA and PEO in the composites because of the hydrogen-bonding interactions between the polymers. Mechanical Properties of the PAA-PEO Elastomers. The mechanical properties of various PAA-PEO composites, as summarized in Table 2, were measured by tensile tests at the stretching speed of 100 mm/min using the obtained rubber-like sheets (gauge length 14 mm, width 5 mm, thickness 0.5 mm) as specimens at ambient conditions (ca. 30% relative humidity (RH), 25 ºC). Figure 2a shows the stress-strain curves of the PAA-PEO composites with different PAA/PEO ratios at identical PAA and PEO molecular weights (see Table 1), wherein the tensile strength (i.e., nominal strength at break) and moduli of the PAA-PEO composites exhibit an increase following the increase of the PAA proportion (Figure 2a, Table 2). This result is in good agreement with the fact that a larger proportion of PAA results in a higher Tg for the PAA-PEO composites (Figure 1a), which leads to increased rigidity of the composites at room temperature. Although the PAA2.0/450k-PEO1.0/600k composite exhibits the highest tensile strength and modulus, its stress-strain curve shows a well-defined yield point at a very low strain of 15% (i.e., 1.15 times its initial length). This result signifies that the PAA2.0/450k-PEO1.0/600k


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composite deforms plastically/non-reversibly when elongated beyond the yield point, which does not follow the performance of elastomers that should deform elastically within a high elongation. Relatively, the PAA1.4/450k-PEO1.0/600k and PAA1.0/450k-PEO1.0/600k composites exhibit stress-strain behaviors of classical elastomers, allowing large elastic deformation of 850% strain for the former and 900% for the latter. Compared to PAA1.0/450k-PEO1.0/600k, the PAA1.4/450k-PEO1.0/600k composite exhibit much higher tensile strength and modulus, while no significant difference (i.e., 850% vs 900% strains) was observed between their stretchability (Figure 2a, Table 2). Subsequently, the mechanical properties of the PAA-PEO composites were further studied by fixing the PAA/PEO monomer molar ratio at 1.4:1 but varying the molecular weight of PEO. As shown in Figure 2b and Table 2, the tensile strength and moduli of the PAA-PEO composites decrease with lower PEO molecular weight, while the opposite trend is observed for their stretchability. This result can be explained by the fact that polymers with higher molecular weights give rise to higher degree of polymer chain entanglements in the resultant complexes, which reasonably reinforces the composite material but suppresses its stretchability. Taken together, among all the prepared PAA-PEO composite elastomers (the PAA2.0/450k-PEO1.0/600k composite is not counted), the PAA1.4/450k-PEO1.0/600k composite exhibit the highest mechanical strength given its tensile strength of 6.4 MPa and modulus of 7.1 MPa. Note that the tensile strength (i.e., nominal strength at break) of the PAA1.4/450k -PEO1.0/600k composite corresponds to a true strength at break as high as 61 MPa (Figure S5), which is ca. 2 times greater than that of the previously reported triple-network polyacrylate elastomers.9 Meanwhile, the PAA1.4/450kPEO1.0/600k composite also exhibits satisfactorily large stretchability with strain at break as high as 850% (i.e., 9.5 times its initial length) at the stretching speed of 100 mm/min. Figure 2c depicts the dependence of the stress-strain behaviors of the PAA1.4/450k-PEO1.0/600k composite on


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the stretching speed applied in the tensile tests. The tensile strength of the sample decreases but its stretchability significantly increases with the decrease of the stretching speed, which is also a typical feature of elastomers. This is because a lower stretching speed gives more time for the reformation of the broken hydrogen-bonds in the PAA1.4/450k-PEO1.0/600k composite, leading to improved deformability/stretchability. The largest strain at break of the PAA1.4/450k-PEO1.0/600k composite can be as high as 3400% (i.e., 35 times its initial length) at the stretching speed of 2 mm/min, which is comparable to that of the highly stretchable PDMS elastomers (27 times its initial length at the same stretching speed) with tensile strength of ca. 0.25 MPa reported by Bao et al.10 Although PAA and PEO are hydrophilic, the influence of environmental humidity on the mechanical properties of the PAA1.4/450k-PEO1.0/600k composites is very limited. The PAA1.4/450kPEO1.0/600k composite incubated in an ~70% RH environment only exhibits a slightly decreased tensile strength (5.6 MPa vs. 6.4 MPa) and a slightly increased strain at break (940% vs. 850%) compared to the same composite incubated at an ~30% RH environment (Figure S6). Meanwhile， the stress-strain curve of the PAA1.4/450k-PEO1.0/600k composite that was soaked in water for 48 h and then re-dried under an environment of ~ 30% RH overlaps with that of the freshly prepared PAA1.4/450k-PEO1.0/600k composite measured under an environment of ~ 30% RH (Figure S7). Therefore, the PAA1.4/450k-PEO1.0/600k composite elastomers are suitable for practical application because they are stable in water and their mechanical properties only exhibit a limited dependence on environmental humidity. Furthermore, the tensile properties of the PAA1.4/450k-PEO1.0/600k composite are also temperature-dependent. As shown in Figure S8, the tensile strength and moduli of the composite decrease but the strain at break increases with temperature. This result can be explained by the fact that the polymer chain mobility and


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flexibility of the composite is promoted at elevated temperatures, which decreases the mechanical strength but improves the stretchability of the composite. Besides tensile strength and modulus, resistance to fracture is another important feature that is used for the evaluation of the mechanical performance of an elastomer. Fracture energy, which indicates the minimum energy required to break the material containing a crack, is a quantitative way of describing the toughness of a material to resist fracture in the presence of a crack.9,10, 61,62 Herein, the fracture energy of the PAA1.4/450k-PEO1.0/600k composite was measured to be as high as 22.9 kJ/m2 via the tensile tests on a single-edge-notched sample (Figure S9). It can be found that the fracture energy of the PAA1.4/450-PEO1.0/600k composite is ca. 4 times higher than that of the previously reported polyacrylate elastomers toughed by the triple-network covalent crosslinking strategy.9 The origin of such a high toughness (fracture energy) of the PAA1.4/450kPEO1.0/600k composite can be attributed to the dynamic feature of the hydrogen-bonding interactions that cross-link PAA and PEO in the composite. When the sample is stretched under external stress, the hydrogen bonds as cross-linkages can dynamically break and reform, thereby effectively dissipating the energy to resist the propagation of a crack. As shown in Movie S1, the crack in the single-edge-notched sample longitudinally deforms but exhibits little lateral propagation during stretching until the occurrence of the sample break, showing good resistance towards the fracture of the cracked sample. Apart from stretchability, resilience is also an important parameter to evaluate the elasticity of an elastomer. The resilience of the PAA1.4/450k-PEO1.0/600k composite was assessed via the cyclic tensile tests. When the sample was successively stretched up to a 200% strain and released without rest between the stretch-release cycles, hysteresis was detected between the following and previous cycles (Figure 3a). Namely, the tensional stresses of the sample measured in the


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second cycle is significantly lower than those measured in the first one and a 14% residual strain was detected after the first stretch-release cycle. Moreover, the hysteresis of tensional strength becomes inconspicuous and the residual strain reaches a plateau of 26% over 5 stretch-release cycles (Figure 3a and the inset). However, the hysteresis of tensional strength and residual strain can be significantly diminished by elongating the rest intervals between the stretch-release cycles (Figure 3b). A 60 min-rest makes the sample completely recover to its original dimension and almost restore its original stress-strain performance (Figure 3b and the inset). Figure 3c and Movie S2 show that the PAA1.4/450k-PEO1.0/600k composite can recover to its original shape even after being stretched to a ~650% strain, demonstrating its excellent stretchability and resilience. Moreover, the PAA1.4/450k-PEO1.0/600k composite after being stretched to even 35 times its initial length can also recover to its original dimension after 48 h following the removal of stress (Figure S10). The as-demonstrated good elasticity of the PAA1.4/450k-PEO1.0/600k composite elastomer is attributed to the reversible hydrogen bonds that cross-link the components. The reversible hydrogen bonds can dynamically break and reform before and after stretching, restoring their original mechanical properties following the removal of stress. To further understand the captivating elasticity of the PAA-PEO composite at the nanoscale level, atomic force microscopy (AFM)-based force spectroscopy61,62 was employed to study the molecular interactions within the hydrogen-bonded PAA-PEO composite. In the experiment, a bare AFM tip was driven to randomly pick up and stretch a polymer chain (also possible to be a bundle of polymer chains) from the composite and then relax it prior to the detachment of the polymer chain(s) from the AFM tip. To minimize the influence of capillary force (between the AFM tip and the sample surface in air) on the experimental result, the force spectroscopy measurements were conducted in a liquid (i.e., amyl acetate) environment, in which the PAA-


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PEO composite was quite stable. Figure 4a shows the measured force-extension curves associated with 3 cycles of the successive stretching-relaxation process, during which the polymer chain(s) was stretched to a further extension in the following cycle relative to the previous one. The force curves corresponding to the different rounds of steady stretching processes (highlighted via a rectangle in the figure) are almost coincident with each other, suggesting that the stretched polymer chain(s) can restore its original state after multiple cycles of stretching-relaxation (Figure 4b). This result verifies the reversibility of the hydrogenbonding interactions in the PAA-PEO composite, which leads to the good elasticity and also healability (will be discussed later) of the PAA1.4/450k-PEO1.0/600k elastomers. Healability of the PAA-PEO Elastomers. Benefiting from the reversible nature of the hydrogen-bonding cross-linkages, the elastic PAA-PEO composites also exhibit water-facilitated self-healing properties. As shown in Figure 5a-c, a PAA1.4/450k-PEO1.0/600k elastomer was cut into two pieces and then they were brought into contact and incubated in a humid environment with RH above 90% at room temperature, wherein the cut pieces self-healed together over time. The success of healing can be verified by the large strain of the 1-h healed sample up to 190% without break (Figure 5d), despite a minor scar being observable on the sample (Figure 5c). The sample was better healed with longer healing times, and the 24-h healed sample completely restores its original mechanical performance with the stress-strain curve almost coincident with that of the intact one (Figure 5e). Note that the cut/damaged PAA1.4/450k-PEO1.0/600k composite can also be self-healed under ambient RH (e.g., 30%-50%), while a longer healing time (e.g., over 7 days) is required as compared to conditions at an elevated RH. The healability of the PAA1.4/450k-PEO1.0/600k elastomers results from the reformation of the hydrogen bonds between the polymer components when the separated parts were once again brought into contact. The


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feasibility of the hydrogen bonds (between PAA and PEO) reformation in the elastomers after break can also be revealed by the AFM-based force spectroscopy measurements that are discussed above. In this aspect, the high humidity enables the permeation of water into the PAAPEO elastomers, thereby facilitating the motion of the polymer chains across the damaged region and accelerating the healing process. As a proof-of-concept, the PAA-PEO composite elastomers were applied to develop flexible conductors with healability by drop-casting polyvinylpyrrolidone (PVPON)-decorated Ag nanowires (AgNWs) on top of the PAA1.4/450k-PEO1.0/600k elastomer (Figure 6a).63 The good elasticity and high mechanical performance of the PAA1.4/450k-PEO1.0/600k elastomers are anticipated to endow the as-prepared conductor with the capability of maintaining its conductance after undergoing multiple and high degree of deformation. The hydrogen-bonding interactions between the pyrrolidone groups of the PVPON-decorated AgNWs and the carboxylic acid groups in the PAA1.4/450k-PEO1.0/600k elastomers enable strong adhesion between the two layers. As shown in Figure 6b, the AgNWs with diameters of ca. 90 nm randomly stack on the PAA1.4/450k-PEO1.0/600k elastomer in a non-directional manner and formed high-density of wire-wire junctions, which are beneficial for the high conductance of the resultant conductor. The cross-sectional scanning electron microscopy (SEM) image of the AgNWs/PAA1.4/450kPEO1.0/600k conductor indicates that the AgNWs layer adheres well onto the PAA1.4/450kPEO1.0/600k elastomer (Figure 6c). The sheet resistance of the AgNWs/PAA1.4/450k-PEO1.0/600k conductor is as low as ca. 0.41 Ω/sq. As expected, the as-obtained bilayer-conductor is highly flexible and its conductivity can be well maintained even after the sample is bent up to a bending angle of 140º (Figure 6d and the inset), despite a very slight increase (ca. 4%) in its resistance. Furthermore, the resistance of the conductor increases by only ca. 5% after the sample is bent for


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3,000 times at a bending angle of 120º (Figure 6e), which is benefited from the good resilience and high toughness of the PAA1.4/450k-PEO1.0/600k elastomer. More intriguingly, after 3,000 times of bending, the conductivity of the conductor can be completely recovered by simply blowing the conductor with a humid air flow (RH > 90%) for 1 min (Figure 6e), which results from the water-facilitated healability of the PAA1.4/450k-PEO1.0/600k elastomer. The as-prepared flexible conductor can be further applied as a water-enabled self-healing conductor in a circuit. As shown in Figure 6f(i), the flexible conductor is conductive enough to transmit electricity in a circuit to light a light-emitting-diode (LED). The LED immediately went off after the conductor was cut into two pieces (Figure 6f(ii)). Intriguingly, by bringing the two separated conductor pieces into contact and dropping appropriate amount of deionized water on the contacted region, the conductor can be healed together in 2 min. Meanwhile, its conductance is immediately recovered after the water is dried with air flow, as evidenced by the phenomenon that the LED is re-lit following the healing process (Figure 6f(iii)). Moreover, the conductance of the healed conductor can also be well maintained when the sample is bent (Figure 6f(iv)). The waterenabled healing capability of the as-prepared conductor can be further evinced by the SEM images of the cut and healed samples, which indicate that the cut and separated AgNWs layer is well-connected following the healing process (Figure 6g). In addition, the as-prepared flexible and healable conductor maintains its conductance even after multiple cycles (e.g., 9 cycles) of the cutting-healing processes and exhibits a negligible conductivity decrease (Figure 6h). Therefore, it is believed that the as-prepared flexible and healable conductor is promising to be practically applied as electric wires in flexible electric devices with extended lifespans and reliable functions. Conclusions


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In summary, we have developed a simple and practical strategy for the preparation of highperformance healable PAA-PEO elastomers that exhibit optimally integrated properties of high mechanical strength, high toughness, enormous stretchability and good elasticity. Such PAAPEO elastomers were achieved by simply complexing easily available PAA and PEO of high molecular weights based on their hydrogen-bonding interactions. The as-obtained elastomers allow to be stretched up to 35 times their initial length and can restore their original dimensions following the removal of stress, while their tensile strength and toughness (fracture energy) can reach as high as 6.4 MPa (i.e., true strength at break of 61 MPa) and 22.9 kJ/m2, respectively. These performances benefit from the reconciled cross-linking characteristics in the hydrogenbonded PAA-PEO elastomers. On one hand, the PAA-PEO elastomers feature high degree of cross-linking due to the high-degree polymer chain entanglements and multiple hydrogenbonding interactions between PAA and PEO, giving rise to the high mechanical strength of the elastomers. On the other hand, the reversible hydrogen-bonds as cross-linkages in the elastomers can dynamically break and reform in accordance with the external stress, endowing the elastomers with enormous stretchability and good resilience. Meanwhile, the reversible hydrogen-bonding cross-linkages also endow the elastomers with water-facilitated self-healing properties due to their capability of reformation after break. Based on the healable PAA-PEO elastomers, flexible AgNWs/PAA-PEO conductors with water-enabled self-healing capability were further developed. We believe that the as-prepared healable elastomers are highly promising for application in practice due to their high performance and preparation simplicity. The demonstrated polymer complexation concept is also expected to be applicable to a wide range of polymer systems for the development of high-performance healable elastomers. Experimental Section


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Materials. PAA (Mw ca. 450,000), PEO (Mw ca. 100,000, 300,000, and 600,000), PVPON (Mw ca. 40,000) and 1H,1H,2H,2H-perfluorooctyltriethoxysilane were purchased from SigmaAldrich. Ethylene glycol, and AgNO3 were purchased from Beijing Chemical Reagents Company. The Ag nanowires decorated with PVPON were synthesized according to a previous report.63 Deionized water was used for all the experiments. Preparation of the PAA-PEO Composites. The PAA-PEO composites with different compositions were prepared via the procedure illustrated in Scheme 1. Typically, aqueous solutions (100 mL, 4 mg/mL, pH 2.5) of PAA and PEO were mixed through peristaltic pumps at the flow rates of 5 mL/min under stirring with a PAA/PEO volume ratio of 1.3:1, 1:1, or 0.65:1, which corresponds to the feed monomer molar ratio between PAA and PEO of 0.8:1, 0.6:1, or 0.4:1, respectively. The resultant precipitation was collected via centrifugation, followed by compression molding for 2 days via two pieces of glass slides at a pressure of ca. 15 kPa applied using balancing weights. To facilitate the detachment of the sample from the glass slides following the molding process, the glass slides were hydrophobized via 1H,1H,2H,2Hperfluorooctyltriethoxysilane prior to use according to a previously reported protocol.64 A transparent rubber-like sheet of the PAA-PEO composite was finally obtained after the sample was further dried in air at room temperature. Preparation of the Flexible Conductors Based on the PAA-PEO Composite. Typically, the flexible conductor was prepared by drop-casting 1 mL of ethanol suspension of Ag nanowires (8.3 mg/mL) on top of a piece of a PAA1.4/450k-PEO1.0/600k composite sheet with dimension of 15×6×0.5 mm3, after which the sample was dried under ambient conditions.


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Characterization. The 1H NMR spectra were recorded on a Bruker 500 MHz spectrometer. The XRD patterns were collected on a Rigaku diffractometer (R-AXIS Rapid). FTIR spectra were collected on a Brucker VERTEX 80V instrument. The UV-vis transmittance spectra were recorded by a UV-2550 spectrophotometer. The tensile tests were implemented on a universal testing machine (Shimadzu AG-I 1 kN) under ambient conditions (ca. 30% RH, 25 ºC). The SEM images were obtained by a JEOL JSM 6700F field emission scanning electron microscope. The DSC measurements were performed on a Netzsch DSC-204 differential scanning calorimeter at the heating rate of 5 °C/min. The sheet resistance was measured on a four-point probe apparatus (RTS-8, Four Probe Tech., Guangzhou, China). The I-V curves of the circuit were measured by a computer-controlled sourcemeter (Keithley 2400, USA). Digital photographs and movies were captured using a Canon SX40 HS camera. The AFM-Based Force Spectroscopy Tests. The AFM-Based force spectroscopy experiments were performed on a NanoWizardII BioAFM (JPK instrument AG, Berlin, Germany) by using the Si3N4 AFM tips (Bruker) at room temperature. The spring constant of the cantilevers were calibrated using the thermal noise method, and the measured values were around 0.06 N/m. Prior to the experiment, a piece of PAA1.4/450k-PEO1.0/600k composite sample was incubated in amyl acetate for 24 h. The force spectroscopy experiments were conducted in amyl acetate and the AFM tip was driven to randomly pick up a polymer chain or a bundle of polymer chains from the PAA1.4/450k-PEO1.0/600k composite. Once the polymer chain(s) was picked up, cyclic stretching-relaxation experiments were performed at a pulling/relaxation speed of 1,000 nm/s with a waiting period of 3 s between the stretching-relaxation cycles.

Associated Content


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Supporting Information. 1H NMR spectra of the PAA-PEO composites with different compositions (Figure S1); UV-vis transmission spectrum and a digital image of the PAA1.4/450k-PEO1.0/600k composite sheet (Figure S2); XRD spectra of the PAA powder, PEO powder, PAA1.4/450k-PEO1.0/600k composite prepared at a pH of 2.5 and PAA-PEO mixture prepared at a pH of 7.0 (Figure S3); FTIR spectra of the PAA1.4/450k-PEO1.0/600k composite and PAA (Figure S4); true stress-strain curves of PAA-PEO composites with different compositions (Figure S5); water contents, humidity- and temperature-dependent tensile properties of PAA1.4/450k-PEO1.0/600k composites (Figure S6, S7 and S8); fracture energy of the PAA1.4/450kPEO1.0/600k composite (Figure S9); the resilience performance of the PAA1.4/450k-PEO1.0/600k composite after being stretched to 35 times its initial length (Figure S10). Movie S1, stretching of the single-edge-notched PAA1.4/450k-PEO1.0/600k composite sample; Movie S2, cyclic stretching and relaxtion of the PAA1.4/450k-PEO1.0/600k composite sheet. This material is available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding Author *E-mail [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by the National Basic Research Program (2013CB834503) and the National Natural Science Foundation of China (NSFC Grants 21225419).


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Notes The authors declare no competing financial interest. Abbreviations PAA, poly(acrylic acid); PEO, poly(ethylene oxide); XRD, X-ray diffraction; FTIR, Fourier transform infrared; DSC, differential scanning calorimetry; Tg, glass transition temperature; RH, relative humidity; AFM, atomic force microscopy; PVPON, polyvinylpyrrolidone; AgNWs, Ag nanowires; SEM, scanning electron microscopy; LED, light-emitting-diode; PDMS, poly(dimethylsiloxane); layer-by-layer, LbL. References (1) Drobny, J. G. Handbook of Thermoplastic Elastomers, 2nd ed; William Andrew Publishing: Norwich, 2014. (2) Ikeda, T.; Mamiya, J.; Yu, Y., Photomechanics of Liquid-Crystalline Elastomers and Other Polymers. Angew. Chem. Int. Ed. 2007, 46 (4), 506-528. (3) Rogers, J. A.; Someya, T.; Huang, Y., Materials and Mechanics for Stretchable Electronics. Science 2010, 327(5973), 1603-1607. (4) Serrano, M. C.; Chung, E. J.; Ameer, G. A., Advances and Applications of Biodegradable Elastomers in Regenerative Medicine. Adv. Funct. Mater. 2010, 20 (2), 192-208. (5) Brochu, P.; Pei, Q., Advances in Dielectric Elastomers for Actuators and Artificial Muscles. Macromol. Rapid Commun. 2010, 31 (1), 10-36.


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Figures

Scheme 1. Schematic illustration of the preparation process of the PAA-PEO elastomers based on the hydrogen-bonding complexation between PAA and PEO.


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Table1. Summary of the formula for the preparation of the PAA-PEO composites and their compositions measured by 1H NMR. Composite abbreviations

PAA Mw (g/mol)

PEO Mw (g/mol)

Monomer molar ratio (Feed, PAA/PEO)

Monomer molar ratio (Measured, PAA/PEO)

PAA2.0/450k-PEO1.0/600k PAA1.4/450k-PEO1.0/600k PAA1.0/450k-PEO1.0/600k PAA1.4/450k-PEO1.0/300k PAA1.4/450k-PEO1.0/100k

450k 450k 450k 450k 450k

600k 600k 600k 300k 100k

0.8:1.0 0.6:1.0 0.4:1.0 0.6:1.0 0.6:1.0

2.0:1.0 1.4:1.0 1.0:1.0 1.4:1.0 1.4:1.0


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Figure 1. (a, b) DSC spectra of the PAA-PEO composites prepared from various PAA/PEO ratios with identical PAA and PEO molecular weights (a) and the same PAA/PEO ratio at varying PEO molecular weights (b). The compositions of the PAA-PEO composites are specified by the abbreviations (defined in Table 1) shown in the figures.


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Table2. Summary of mechanical properties of the PAA-PEO composites with different compositions, which were measured by the tensile tests at the stretching speed of 100 mm/min. The data shown here are the average values with standard-deviation derived from 5 measurements on different samples.

Tensile strengtha) Composite abbreviations (MPa) PAA2.0/450k-PEO1.0/600k 8.10±0.32 PAA1.4/450k-PEO1.0/600k 6.38±0.30 PAA1.0/450k-PEO1.0/600k 4.15±0.28 PAA1.4/450k-PEO1.0/300k 3.64±0.11 PAA1.4/450k-PEO1.0/100k 1.33±0.06

True strength at breaka) (MPa) 70.86±0.47 60.45±0.73 38.44±0.52 42.32±0.59 18.90±0.36

Strain at break (%) 783.4±30.5 858.6±34.3 934.2±43.4 1068.4±52.6 1385.8±67.4

Young’s Modulus (MPa) 52.46±2.42 7.02±0.33 2.32±0.14 1.76±0.10 0.96±0.07

a)

Note that “tensile stress” is the stress determined by the instantaneous load acting on the original cross-sectional area of the sample, while “true strength” is the stress determined by the instantaneous load acting on the instantaneous cross-sectional area of the sample.


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Figure 2. (a, b) Stress-strain curves of the PAA-PEO composites prepared from varied PAA/PEO ratios with identical PAA and PEO molecular weights (a) and the same PAA/PEO ratio at varying PEO molecular weights (b). The compositions of the PAA-PEO composites are specified by the abbreviations (defined in Table 1) shown in the figures. The dimensions of the samples between the two clamps used for stretching in the tensile tests were 14×5×0.5 mm3 and the tensile tests were performed at the stretching speed of 100 mm/min. (c) Stress-strain curves of the PAA1.4/450k-PEO1.0/600k composite, which were measured at the stretching speed varied from 2, 10, 25, 50, to 100 mm/min.


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Figure 3. (a, b) Stress-strain curves of the PAA1.4/450k-PEO1.0/600k composite, measured via the cyclic tensile tests that were conducted without (a) or with (b) rest between each stretch-release cycle. The stress-strain curves corresponding to the first, second, sixth, and tenth cycle of tensile tests are presented in (a) and the rest intervals in (b) between each test cycle were varied from 0, 30, to 60 min. Insets: Residual strains of the sample, determined after each cycle of the tensile test, as a function of the number of test cycles (a) and rest intervals between each test cycle (b). The dimensions of the samples between the two clamps used for stretching in the tensile tests were 14×5×0.5 mm3 and the tensile tests were performed at the stretching speed of 100 mm/min. (c) Photos of the PAA1.4/450k-PEO1.0/600k composite sheet, captured before (i) and after (ii) being stretched to a 650% strain, and photos of the stretched sample, captured immediately (iii) and 16 h (iv) after the stress removal. Note that the two ends of the sample are contaminated by the white powders on the gloves and become opaque during stretching.


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Figure 4. (a) Stretching and relaxation curves obtained during the first, second, and third cycles of stretching and relaxing the same polymer chain(s) that was picked up by the AFM tip from the PAA1.4/450k-PEO1.0/600k composite. (b) Schematic illustration of the configuration of the polymer chain as it was stretched out of the PAA-PEO composite and then relaxed. Note that the real situation can be more complicated than what is presently illustrated.


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Figure 5. (a, b) Photos of the PAA1.4/450k-PEO1.0/600k composite sheet captured before (a) and after (b) being cut into two pieces. Photos of the 1-h healed PAA1.4/450k-PEO1.0/600k sample that was previously cut into two pieces, captured before (c) and after (d) being stretched to a 175% strain. (e) Stress-strain curves of the intact and 1-h, 6-h, 12-h, and 24-h healed samples that were previously cut into two pieces. The dimensions of the samples between the two clamps used for stretching in the tensile tests were 14×5×0.5 mm3 and the tensile tests were performed at the stretching speed of 100 mm/min. The healing process was conducted by bringing the two separated pieces together and then incubating the sample in an environment with RH above 90% at room temperature.


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Figure 6. (a) Photo of the flexible conductor fabricated by drop-casting AgNWs on top of a PAA1.4/450k-PEO1.0/600k composite sheet. (b, c) Top view (b) and cross-sectional (c) SEM images of the as-fabricated conductor. (d, e) The electric resistance ratio between the bent sample and the original sample as a function of the bending angle (d) and number of bending times at the bending angle of 120º (e). Inset of (d): Schematic illustration of the configuration of the asfabricated flexible conductor and the determination of the bending angle (θ). The point of (e, Healed) represents the electric resistance ratio between the previously bent (i.e., 3,000-times bending) sample and the original sample, while the previously bent sample was treated with humid air flow (RH >90%) for 1 min. (f) A series of photos demonstrating that the conductance of the as-fabricated flexible conductor that is connected in an LED-integrated circuit can be wellrestored after being cut and then healed. (g) Top view SEM images of the as-fabricated conductor after being cut and healed. (h) I-V curves of the series circuit, in which the asfabricated flexible conductor and a resistor with resistance of 20 Ω were connected, measured using the intact sample or the sample that underwent multiple cutting-healing cycles (e.g., 3, 6, and 9 cycles).


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TOC Graphics


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Transparent, Healable Elastomers with High Mechanical Strength and

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