A Sunlight-Degradable Autonomous Self-Healing Supramolecular

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A Sunlight-Degradable Autonomous Self-Healing Supramolecular Elastomer for Flexible Electronic Devices Xiaohong Li, Hanzhi Zhang, Ping Zhang, and You Yu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00832 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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Chemistry of Materials

A SunlightSunlight-Degradable Autonomous SelfSelf-Healing Supramolecular Elastomer for Flexible Electronic Devices Xiaohong Li, Hanzhi Zhang, Ping Zhang and You Yu* Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710069, China. ABSTRACT: Preparing an autonomous self-healing supramolecular elastomer with sunlight-degradability is still a challenging task in related fields. In this work, we report a supramolecular elastomer by using the classic host-guest complexation of visible-light-photolabile picolinium-contained adamantanes (Ad) and β-cyclodextrin nanogels (β-CD). The as-synthesized elastomer possesses merits of high mechanical strength, excellent stretchability (>1500% strain), efficient self-healing (>85% @ 60 min), ultrastability against electrolytes and photodegradation properties, implying versatile applications in flexible and stretchable electronics. As proofs-of-concept, self-healable strain and pressure sensors using conductive elastomers are firstly fabricated, which feature exceptionally high sensitivity (e.g., 0.1% in capacitance @ 0.2 kPa) and fast response to detect human body motions. A degradable and flexible supercapacitor is also fabricated using the conductive elastomer as the flexible matrix. Remarkably, both the elastomer and this supercapacitor can be degraded upon the exposure to sunlight irradiation in 48 h at very mild conditions. Therefore, it is anticipated that such a novel strategy and the asprepared supramolecular elastomer can inspire further applications in the multidisciplinary fields of materials science, electronics, etc.

1. INTRODUCTION Degradable polymers have been attracting much interest over the past decade and demonstrated many applications in a wide spectrum of fields which span from drug and cell deliveries1, 2, tissue engineering3, actuators and chemical sensors4-6 to “green” electronics7-10. Basically, the degradation behavior of these polymers relies on the cleavage processes of labile bonds within chains. In contrast to covalent polymers, supramolecular polymers are synthesized via dynamic and reversible bonds11-14 that not only possess the conventional properties, but also feature new functions such as self-healing and stimuli-responsiveness15-19. Importantly, the performance of these polymers can be altered with exogenous aids, e.g., heat, light, pH or chemicals20-25, therefore making them selective candidates for smart and multiresponsive systems. Among these stimuli aforementioned, light is undoubtedly a clean, noncontact and controllable stimulus to spatially tune properties of supramolecular polymers at mild conditions. For instance, light is used for (i) triggering the inclusion complexation of azobenzene and macrocyclic compounds12, 26-29, disengagement of the metal-ligand motifs30-34, and (ii) removing the screening moieties to disassemble polymers35-37. In particular, light at long wavelengths (>400 nm) is recognized as safer stimuli when comparing to those undesirable high-energy light sources (UV light or γ ray). Furthermore, the abundant visible light can be obtained from sunlight (~43%), which is low-cost, energy-saving and readily available for degrading products at large scales. Thus, rational design of supramolecular materials with the capacity of sunlight degradation (or

responsiveness) is highly desirable. Yet, how to simply synthesize visible-light and even sunlight degradable building blocks is still a challenging task in this field. N-alkyl-picolinium esters (NAPEs) are typical visiblelight-photolabile groups that have been demonstrated in our group for the fabrication of surface charge reversible monolayers, photodegradable poly(ionic liquid)s and biocompatible coatings by anchoring them onto substrates or polymeric chains38-40. Upon light irradiation, C-O bonds in such esters are chemically cleaved via a photo-induced electron transfer process with mediators, and the properties, e.g., surface charge status and the ability for cell proliferation are tuned simultaneously. Inspired by the unique photodegradable property in these works, we aim to apply NAPEs in tough supramolecular elastomers for flexible and stretchable electronics that are autonomously self-healable and efficiently sunlight-degradable at mild conditions. To meet this requirement, we design herein a sunlightdegradable, autonomous self-healing supramolecular elastomer (SDSE) using the host-guest complexation of photolabile adamantanes (Ad) and β-cyclodextrin nanogels (βCD). As shown in Figure 1, this elastomer is composed of three major components, namely (i) visible-lightphotolabile picoliniums38-40, (ii) host-guest complexation of Ad/β-CD and (iii) polyacrylamide (PAAm). The introduction of (i) and (ii) into (iii) network provides the SDSE with excellent flexibility and stretchability, rapid self-healing property and photodegradability. Furthermore, the combination of the covalently crosslinked structure of β-CD nanogels and inclusion complexation of Ad/β-CD favors for preparing PAAm elastomers with moderate mechanical

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strength. Also, the conductivity of SDSE can be significantly improved by loading electrolytes into its porous network. As proofs-of-concept, this supramolecular elastomer shows versatile applications in flexible electronic devices such as self-healable sensors and a degradable supercapacitor. Therefore, it is anticipated that such a novel strategy and SDSE can inspire further applications in the multidisciplinary fields of materials science and multiresponsive systems, etc. 2. RESULTS AND DISCUSSION Figure 1 shows the typical procedure to fabricate SDSE. In the fabrication, the picolinium-containing Ad was synthesized by the esterification of Ad-COOH with 4-pyridinemethanol and sequential N-alkylation with the bromine-terminated acrylic ester (Figure S1). After the inclusion complexation with epichlorohydrincross-linked β-CD nanogels (size: ~100 nm, Figure S2), SDSE was obtained via the traditional radical copolymerization of the Ad/β-CD complexes and AAm monomers. The effects of the molar ratio of β-CD:Ad:AAm and monomer concentration on the mechanical property of SDSE were first evaluated. Note that the β-CD content in SDSE was roughly calculated from the weight of nanogels used, and it was approximately 60% of the total molar amount of β-CD and epichlorohydrin in nanogels41. As shown in Figure S3a-d, control experiments showed that a combination of β-CD, Ad and AAm (e.g., 3:1:19, molar ratio) was necessary for fabricating uniform SDSE samples. While maintaining the total concentration of monomers at 0.64 M, varying the molar ratio of β-CD, Ad and AAm from 3:1:19 to 6:1:19 resulted in the decreased stress of the SDSE sample by more than half of the original value at the same strain (Figure S3e and S4a). Further adjustment of the molar ratio to 3:5:19 only resulted in viscous liquids (Figure S3f). The reason for these results was attributed to that the unmatched molar ratio of β-CD, Ad and AAm lacked efficient cross-linking points, leading to the decrease of its mechanical strength and even no SDSE elastomer obtained. Moreover, when increasing the

Figure 1. Schematic illustration for the preparation, selfhealing and photodegradation of SDSE. (i) Inclusion complex-

ation and polymerization. (ii) Breaking and self-healing. (iii) Visible -light or sunlight-degradation.

concentration of monomers (in total) from 0.64 M to 2.54 M, the mechanical property of the resultant SDSE was significantly improved with the maximum stress increased from several kPa to ~90 kPa and the strain increased from ~800% to >1600% (Figure S4a). Such an excellent mechanical property of the PAAm-based hydrogels at the level of kPa can be assigned for the fact that a tough double-crosslinked network was formed in the SDSE by covalently cross-linked β-CD nanogels and the reversible host-guest complexation of Ad and β-CD (Figure 1)42, 43. To achieve comparable results, if not specially mentioned, all of SDSE samples were prepared at the optimized concentration of monomers (2.54 M) with the molar ratio of β-CD:Ad:AAm at 3:1:19. Tensile-stress of the SDSE fabricated under the optimized recipe was further studied. Figure S4b showed that SDSE exhibited excellent stretchability and the performance highly depended on the stretching speed. For example, when the stretching speed increased from 20 mm min-1 to 60 mm min-1, the maximum stress increased from 45 kPa to 67.5 kPa but the elongation decreased by 25% (~1600% vs ~1200%). When continuously increasing the strain from 25% to 200%, a fully reversible behavior of the SDSE was observed (Figure S4c). However, further increasing the strain from 200% to 300% resulted in a more

Figure 2. Self-healing and photodegradation properties of SDSE. (a) Demonstrating the self-healing behavior of the damaged SDSE: the red samples were stained with rhodamine for visually differentiating. (b) Stress-strain curves of the pristine and self-healed SDSE with different healing time (samples size was: 20 mm × 4 mm × 2 mm). (c) Plotting the fracture stress and healing efficiency against different healing times. Rheology measurements of SDSE (d) as a function of strain

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Chemistry of Materials (0.1%~1000%) and (e) in alternate step strain (strain 1% vs 800%, 25 oC).

pronounced hysteresis where the reversible behavior no longer existed. Fatigue test of the SDSE was also carried out to confirm its potential applications in flexible and stretchable devices. The fatigue test (from 0% to 200% strain) in Figure S4d indicated that reversible behavior of the SDSE was still observable after 20 cycles of strainstress experiment with only 10% decrease in stress. All of the above experiments suggested that the SDSE was a potential candidate for making flexible and stretchable devices. As-fabricated SDSE also possessed rapid self-healing property that is majorly brought by the host-guest complexation of Ad/β-CD. When Ad/β-CD disassembled, SDSE was broken. At rest, the complexes formed again, leading to an autonomous self-healing behavior without any external intervention. This speculation was confirmed by 2D NOESY NMR characterization and the visibly completive reaction with better guest molecules. The strong hostguest interaction between Ad and β-CD that was observed in Figure S5 indicated the formation of Ad/β-CD complexes. When adding adamantane carboxylic acid sodium salt (Ad-COONa), the sample was gradually dissolved in water because of lacking sufficient crosslinking points in SDSE (Figure S6). Compared with other reversible noncovalent interactions, Ad is bound to β-CD with a higher association constant of ~105 M-1 at room temperature, which favored the preparation of rapid self-healing materials with moderate mechanical strength29, 44, 45. Moreover, the interference from other guest molecules such as electrolytes on Ad/β-CD was found to be neglectable46. To evaluate its self-healing ability, two SDSE samples were cut into two pieces with a razor blade and put together for a pre-determined time (Figure 2a). For easy tracking of the self-healing effects, one sample was stained with rhodamine. After contacting the samples for 10 min, two separate samples healed and formed into one single piece. The mechanical experiments showed that the healing process only took 5 min to achieve ~65% self-healing efficiency, which was defined as the ratio of maximum strains of the healed and pristine samples ((Strainmax,healed/Strainmax,pristine)*100%). Importantly, the healed sample could be stretched, where ~1000% strain and 23% of the maximum fracture stress (20 kPa) as compared to the pristine sample exhibited (Figure 2b and c). With a prolonged healing time of 12 h, the self-healing efficiency and the maximum strain reached >95% and 1600%, respectively, which were very close to that of the pristine SDSE. The rheology measurement was performed to further character such a superior self-healing process (Figure 2d): It was found that the storage (G') and loss modulus (G") curves intersected at the strain of 500%, implying that the state of SDSE was between solid and fluid near this critical point. When increasing the strain to 1000%, the G' dramatically decreased from ≈2400 Pa to ≈140 Pa due to the disruption of Ad/β-CD complexes. Figure 2e showed that when the larger strain (800%) and small strain (1%) were

alternatively applied, the G' quickly restored the initial value, revealing a rapid self-healing behavior of SDSE.

Figure 3. (a) Digital images of degraded SDSE with different irradiation time. It is worthy of note that colorless xylene

was used for sealing the samples to avoid the water evaporation and swelling in SDSE during the degradation experiments. (b) The compressing and releasing test of the freshprepared and partially degraded SDSE samples in (a). (c) The maximum stress vs predetermined irradiation time. (d) The sample before and after being treated with sunlight irradiation.

As a crucial criterion for degradable polymers, the photodegradation property of the SDSE was also systematically investigated. When exposing it to visible light (~452 nm, 30 mW cm-2), sample color changed from light yellow to orange and a gel-to-sol transition was observed with the increase of the irradiation time. If prolonging the irradiation time to 64 h, an aqueous solution of the degraded products was obtained, indicating a full photodegradation of SDSE (Figure 3a and Supporting Video). Furthermore, since SDSE was compressible, the degradation process could be tracked by measuring its maximum compression stress after certain time intervals of light irradiation (Figure 3b). It was found that the maximum stress of the partially degraded samples gradually decreased from 3 to 0.25 kPa after 18 h irradiation (Figure 3c). These results indicated that SDSE was visible-light degradable at mild conditions. To explain the photodegradation mechanism of SDSE, a similar model which only consisted of Ad was synthesized. The 1H-NMR spectra of the model Ad and degradation products showed that the C-O bond in Ad was photocleaved but the adamantane group was not destroyed (Figure S7). This result implied that the inclusion complex of Ad/β-CD was still inside the degraded SDSE, and the photodegradability of the SDSE was majorly brought by the cleavage of picolinium molecules. Figure S8 showed that the reduction potential of this picolinium-contained

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Ad was about -1.4 eV. Only Ru(II) in the degrading system featured the characteristic absorption at visible-light region (λmax= 452 nm) (Figure S9). Moreover, control experiments in Figure S10 implied that such a degradation process must be carried out via the catalysis of Ru(II)/ASC with visible light irradiation. On the basis of these results, we can deduce that the mechanism of the degradation behavior in the SDSE was assigned to the photocleavage of CO bonds via a mediated electron transfer process with Ru(II)/ASC when they are under the exposure to visible light (Figure S11)38-40. More importantly, we were wondering if the photodegradation process could be performed with the irradiation of low-cost sunlight for potentially industrial applications. To that end, SDSE samples were exposed to the mimicked sunlight from a commercial white LED bulb instead of blue light (452 nm). Same photodegradation behavior was observed (Figure 3d), indicating that even sunlight was potentially suitable for degrading SDSE-based electronic devices. Originally, the SDSE is non-conductive. However, the conductivity of SDSE can be significantly improved by soaking the dried SDSE samples in aqueous solutions such as Na2SO4, KCl, LiCl, NaCl, H3PO4 and BMPyr, due to the swelling mechanism of PAAm-based hydrogels in these aqueous electrolytes. To screen out any electrolytes that were incompatible with the SDSE, effects of different electrolytes on the mechanical and optical properties of SDSE were investigated. As shown in Figure S12, after being immersed in aqueous solutions of electrolytes for 12 h, all of the samples became swollen, conductive and transparent (except in Na2SO4 and KCl). But the tensile-strain tests showed that introducing these foreign electrolytes would harden the conductive SDSE to result in less stretchable SDSE (Figure S13). Among these conductive samples, the H3PO4-loaded SDSE possessed good transparency, comparative conductivity and mechanical properties. Therefore, H3PO4 was selected as the electrolyte for preparing conductive SDSE in the following studies. Typically, the H3PO4-loaded SDSE can be knotted and power a commercial LED light after being damaged and healed (Figure 4a and b), indicating potential applications in flexible electronic devices. As a proof-of-concept, a selfhealable resistance sensor using the H3PO4-loaded SDSE was prepared to detect human motions. We first attached the sensor onto a joint of the index finger where the resistance change of the sensor at four different bending gestures was measured (Figure 4c). It was found that the resistance increased as the finger bent, due to the large tensile strains applied to the sensor. When the strains were released, the resistance of the sensor almost went back to the original value. Notably, the sensor that was damaged and healed showed similar performance as that of the pristine one due to its excellent self-healing ability of the SDSE (Figure 2). In addition, such a stretchable SDSE sensor was also able to recognize small deformation when it was under the sound vibrations of different spoken Chinese words such as “hello”, “thanks” and “bye-bye”, and the continuous words of “hello”, respectively (Figure S14a and b). However, the resistance sensitivity (SR) of the sensor was 0.08 kPa-1, and slight changes in resistance (