Polymers with a Coiled Conformation Enable Healing of Wide and

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Polymers with a Coiled Conformation Enable Healing of Wide and Deep Damages in Polymeric Films Yan Wang, Miao Zheng, Xu Wang, Siheng Li, and Junqi Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10277 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 19, 2018

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ACS Applied Materials & Interfaces

Polymers with a Coiled Conformation Enable Healing of Wide and Deep Damages in Polymeric Films Yan Wang,a, ‡ Miao Zheng,a, ‡ Xu Wang,b Siheng Lia and Junqi Sun*a a

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin

University, Changchun 130012, P. R. China. b

National Engineering Research Center for Colloidal Materials, School of Chemistry and

Chemical Engineering, Shandong University, Jinan 250100, P. R. China. *Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

The development of efficient methods to trigger high mobility of polymer chains to migrate across the damaged areas is key for healing of wide damages in intrinsic healable polymeric films deposited on solid substrates. Herein, we establish a facile strategy for the fabrication of polymeric films with superhigh healing capability by controlling conformational transition of the polymer chains in polymeric films. The alternately spin-coated poly(acrylic acid) (PAA)/polyurethane (PU) films with coiled PU can heal cuts with a width of 6 times the thickness of the PAA/PU films in the presence of ethanol. In contrast, the same PAA/PU films with stretched PU or those films with coiled PU but without conformational transition from a coiled state to a stretched state fail to heal cuts. The conformational transition of PU from a coiled state to a stretched state in PAA/PU films triggered by ethanol enables a long-distance migration of PAA and PU polymers to heal wide mechanical damages.

Keywords: Self-healing, supramolecular chemistry, layer-by-layer assembly, polymers, thin films


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INTRODUCTION Self-healing/healable polymer materials capable of healing damages autonomically or under the assistance of external stimulus have drawn significant attentions since the late 1970s, when the temperature-dependent recovery phenomenon of deformed viscoelastic polymers was first observed by Wool.1 These materials that have excellent performances in extending service life, enhancing reliability and reducing raw material consumption, can be categorized into extrinsic2,3 and intrinsic4,5 types, depending on whether or not externally incorporated healing agents are required for the healing performance.6-13 Different from extrinsic healable materials that rely on healing agents, intrinsic healable materials provide a straightforward way to heal structural damages and restore functions through the reversibility of dynamic covalent bonds or noncovalent interactions including Diels-Alder reaction4, oxetane ring open and re-crosslink14, metal-ligand binding15,16, and hydrogen bonds.17,18 The healing process of intrinsic healable polymer materials mainly consists of two steps of (i) bringing the separate parts together and (ii) rebuilding the reversible interactions.6 Therefore, the healing capability of intrinsic healable materials is determined by the mobility of polymer chains and the capability to reform reversible interactions. The re-contact of separated parts of intrinsic healable bulk materials can be easily achieved by artificial assistant, therefore facilitating the healing process.5,17,19 In contrast, for intrinsic healable polymer films deposited on solid substrates, the strong binding force from the underlying substrates significantly restricts the migration of molecular components across the damaged regions to heal wide damages.20 Therefore, high mobility of polymer chains is an essential prerequisite for healing of wide damages in intrinsic healable polymer films. Polymeric composite films prepared by layer-by-layer (LbL) assembly technique21-23 have shown great promise to restore their structures and functions after mechanical damages.24-27 Most


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importantly, precise controls of chemical compositions, film structures and polymer chain conformations can be conveniently realized in LbL-assembled films to enable the fabrication of various functional healable films and allow for the fundamental research to understand the healing mechanisms.23 Since 2010, our group has reported several types of LbL-assembled healable films capable of repairing structural damages20 and restoring functions including transparency,28 superhydrophobicity,29-31 electrical conductivity,32,33 antifouling34 and frostresisting abilities.35 These studies have stimulated the fabrication of a large variety of LbLassembled self-healing/healable films and bulk materials composed of oppositely charged polyelectrolyte complexes.36-39 Although the mechanism for healing of LbL-assembled films is clearly demonstrated,28,34 the factors governing the healing process to enable healing of polymeric films with wide and deep damages is far from being well exploited. The clarification of this issue is highly important not only for the fabrication of self-healing/healable films deposited on solid substrates, but also for self-healing/healable bulk polymer materials. In our previous study, we demonstrated that the LbL-assembled poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) films with coiled chain conformations can act as a humidity-responsive layer to drive a walking device carrying a load 120 times heavier than the actuator to walk steadily on a ratchet substrate when the environmental humidity is alternatively altered.40 Moreover, micron-sized honeycomb-like pores can be generated in the LbL-assembled PAA/PAH films having coiled chain conformations after being treated in acidic aqueous solution in contrast to the nano-sized pores in the PAA/PAH films with stretched chain conformations.41,42 These results suggest that polymer chains with coiled conformations can have high chain mobility in response to external stimuli. Therefore, polymeric films with coiled polymer conformations may provide an efficient pathway for the fabrication of intrinsic healable


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polymer films with superhigh healing capability because high chain mobility is required for the healing process. Herein, we present a facile strategy to explore polymers with coiled conformations for the fabrication of LbL-assembled polymer films capable of healing wide damages. The healable films are comprised of alternately deposited positively charged polyurethane (PU) with coiled conformations and PAA. The transition of PU from a coiled state to a stretched state triggered by ethanol enables extensive migration of PU and PAA chains to heal cuts with a width of 6 times the thickness of the PAA/PU films. RESULT AND DISSCUSSION To demonstrate our idea, positively charged PU (chemical structure, Figure 1a) having a coiled conformation in water dispersion is LbL-assembled with PAA to fabricate the healable PAA/PU films. The spherical PU aggregates have an average diameter of 246 ± 79 nm, which is acquired by transmission electron microscopy (TEM) (Figure S1a). Because the large PU nanoparticles in water strongly scatter visible light, the PU water dispersion looks ivory-white (Figure 1b). In contrast, the PU ethanol/water mixture solution with the same PU concentration is transparent (Figure 1b), wherein the ethanol content in the mixture solvent is Xethanol = 98% (Xethanol represents the molar concentration of ethanol in the ethanol/water mixture). Based on the different solution behaviors of PU in water and ethanol, it is speculated that PU is in a compact coiled state in water and relative stretched in ethanol (Figure 1c). To verify this speculation, the evolutions of PU particle size and solution transmittance as a function of Xethanol were systematically investigated by dynamic light scattering (DLS) measurements. In the Xethanol range of 0%-36%, each DLS curve of PU dispersions shows a single peak. The hydrodynamic diameter of the PU nanoparticles in water is 217 ± 72 nm (Figure S2), which acquires reasonable agreement with the TEM result. The hydrodynamic diameter of the PU nanoparticles in water


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shows no obvious change after being stored in a refrigerator with a temperature of 4 °C for 1 year, indicating the good stability of PU water dispersion. The hydrophobic segments of PU collapse to form the cores and the hydrophilic groups are located in the shells. In this way, PU exists in a compact coiled state in water. Addition of ethanol to water dispersions of PU decreases the solvent polarity and increases the solubility of hydrophobic segments of PU. The hydrodynamic diameter of PU nanoparticles increases with increasing Xethanol (Figure 1d), indicating that the hydrophobic segments in PU start to swell and the PU chains become stretched. When Xethanol is over 36%, the DLS curves of PU in ethanol/water mixtures show multiple peaks (Figure S2), suggesting that the hydrophobic interactions in PU are mostly broken and PU adopts a more stretched conformation with increasing contents of ethanol. Figure 1e shows that the transmittance at 550 nm for PU ethanol/water solutions increases slowly with increasing ethanol content in the initial stage (Xethanol < 36%), followed by a sharp rise (Xethanol 36%-60%) and then a plateau (Xethanol > 60%). The sharp rise of transmittance at 550 nm marks the transition of PU chains from a coiled conformation to a more stretched conformation when Xethanol is higher than ~36% in PU ethanol/water solutions. These results also endorse that PU exists in a relatively coiled conformation in water and a stretched conformation in ethanol. TEM and atomic force microscopy (AFM) images of PU cast from PU water dispersion and ethanol/water (Xethanol = 98%) mixture solution exhibit particle-like (Figure S1a and S3a) and discontinuous film-like (Figure S1b and S3b) structures, respectively, confirming that conformations of PU in solutions can be retained when deposited on solid substrates or in films. DLS curves indicate that the hydrodynamic diameter of PAA in water and in ethanol is 40 ± 2 nm and 46 ± 3 nm, respectively (Figure S4). These results show that PAA has very similar


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conformation in water and in ethanol. Therefore, we believe that the PAA conformation can have a very limited or negligible influence on the healing behavior of the (PAA/PU)*n-water films. To investigate the relationship between polymer conformation and film healability, PU dispersed in water with a coiled conformation was alternately deposited with negatively charged PAA by a spin-assisted LbL assembly technique

to fabricate (PAA/PU)*n-water films (n

presents the number of film deposition cycles, water means PAA and PU are dissolved/dispersed in water) (Scheme 1). The driving forces for the film fabrication are electrostatic and hydrogenbonding interactions between the quaternary ammonium/amide groups of PU and the carboxylic acid/carboxylate groups of PAA. The thickness of the (PAA/PU)*n-water films with different deposition cycles measured by a stylus profiler is shown in Figure 2a. The (PAA/PU)*n-water films exhibit a rapidly linear growth behavior with an increment of approximately 172 nm per deposition cycle. The rapid growth of the (PAA/PU)*n-water films is attributed to the large sizes of coiled PU aggregates, which increase the surface roughness of the PU layer to enable the deposition of a large amount of PAA in each deposition cycle. After 50 deposition cycles, the thickness of the (PAA/PU)*50-water films reaches ~8.6 µm. AFM image in Figure 2b indicates that the surface of the (PAA/PU)*50-water film is rough, with a root mean square (RMS) roughness of 556 ± 42 nm within a measured area of 90 × 90 µm2. The large RMS roughness of the (PAA/PU)*50-water film also confirms that coiled PU nanoparticles are deposited within the (PAA/PU)*50-water films. Because of the hydrogen-bonding interactions within the amide groups of PU and the hydrophobic interactions, the PU particles can overcome lateral shear force during the spin-coating process and retain a compact coiled conformation in the (PAA/PU)*nwater films. Scanning electron microscopy (SEM) was employed to monitor the damage-healing process of the (PAA/PU)*50-water films. A ~40 µm wide cut penetrated to the substrate was


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made on the film using a scalpel (Figure 2c). The closure of cut was observed after immersing the damaged film in ethanol for 5 min (Figure 2d). The complete healing of the cut was achieved after 10 min immersion in ethanol (Figure 2e). However, the cut in Figure 2c cannot be healed by immersing the film in water even for 60 min, despite that the width of the cut decreased to ~29 µm (Figure 2f). Both water and ethanol can act as plasticizers to enhance the mobility of the polyelectrolytes in (PAA/PU)*50-water films. However, ethanol can trigger the PU conformation transition from a coiled state to a stretched one, endowing the films with high mobility to heal damages. While in water, the PU chains remain in a coiled state, which does not contribute to the healing. It is worth noting that the (PAA/PU)*50-water films can heal cuts with a maximum width of ~51 µm penetrated to the substrate, which is 6 times of the film thickness (Figure S5). Therefore, the (PAA/PU)*50-water films possess high chain mobility to heal large damages in the presence of ethanol. The LbL-assembled (PAA/PU)*50-water films can heal cuts for at least 5 times in the same damaged regions (Figure S6). Moreover, after 5 cycles of damage-healing process, the Young’s modulus of the healed (PAA/PU)*50-water films shows no obvious difference with that of the pristine films. To further verify that coiled PU is critically important to enable healing of cuts in (PAA/PU)*50-water films, (PAA/PU)*n-ethanol films (n presents the number of film deposition cycles, and ethanol means that PU and PAA are dissolved in ethanol) with a stretched PU conformation by using PU ethanol solution were fabricated by a spin-assisted LbL assembly technique. The (PAA/PU)*n-ethanol films grow in a linear way with an increment of approximately 52 nm per deposition cycle (Figure 3a), which is only one-third of the growth speed of the (PAA/PU)*n-water films. After 100 deposition cycles, the thickness of the (PAA/PU)*100-ethanol film reaches ~5.2 µm. The (PAA/PU)*100-ethanol film is relatively flat


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with a RMS value of 240 ± 28 nm (Figure 3b), which is much lower than that for the (PAA/PU)*50-water films with a similar thickness. The slow growth speed and flat surface morphology originate from the stretched and flexible PU in ethanol solution. PU is more stretched and easily collapsed in the (PAA/PU)*100-ethanol films than that in the (PAA/PU)*50water films. A cut with a width of ~41 µm was made on the (PAA/PU)*100-ethanol films (Figure 3c). The damaged (PAA/PU)*100-ethanol film was immersed in ethanol and the change of the cut was monitored by SEM. After incubation in ethanol for 20 min, the width of the cut slightly decreased to ~39 µm (Figure 3d). The (PAA/PU)*100-ethanol film failed to heal the cut even when the incubation time in ethanol increased to 24 h. Compared with the (PAA/PU)*50water films that can heal ~51 µm wide cuts within 10 min, the (PAA/PU)*100-ethanol films show no healing performance because the PU exists in a stretched state and cannot be further stretched in ethanol to facilitate migration of PU and PAA to the damaged area. When the (PAA/PU)*100-ethanol films with stretched PU was immersed in water for 20 min, the width of the cut significantly increased from ~41 µm to ~173 µm (Figure 3e), meaning that the stretched PU became coiled after incubation in water. This transition was further confirmed by the fact that the thickness of the water-incubated (PAA/PU)*100-ethanol film near the cut increased from ~5.2 µm to ~6.4 µm (Figure 3f). Therefore, the healing capability of the PAA/PU films is indeed related to the polymer conformations in films. The coiled PU has a high mobility to enable healing of wide damages in the (PAA/PU)*50-water films. In contrast, the stretched PU possesses a poor mobility, significantly decreasing healing capacity of the (PAA/PU)*100ethanol films. The mechanical properties of the (PAA/PU)*50-water films under dry conditions, and incubated in ethanol and water were measured by nanoindentation.28 The Young’s modulus of


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the (PAA/PU)*50-water films under dry condition (30 °C and a relative humidity of ~20%) is measured to be 1.7 ± 0.6 GPa (Figure 4a). Because healing of the damaged (PAA/PU)*50-water films is conducted in ethanol under a static state, the storage modulus at a low frequency of 1 Hz, which is 3.5 ± 0.8 MPa, is regarded as the modulus of the films during healing process (Figure 4b). The significant decrease in modulus from 1.7 GPa under dry condition to 3.5 MPa in ethanol demonstrates that the absorbed ethanol swells the (PAA/PU)*50-water films. Ethanol can break the hydrophobic interactions in coiled PU nanoparticles so that the PU chains can transform from a coiled state to a stretched state. This transition generates huge migration of PU and PAA polymers and imparts the (PAA/PU)*50-water films with a good swellability to heal wide cuts. In contrast, the storage modulus of the (PAA/PU)*50-water films in water is 7.8 ± 0.2 MPa, which is higher than the modulus of the films in ethanol and is an indication of a relative low degree of swelling because of the strong hydrophobic interactions of PU in water. When the damaged (PAA/PU)*50-water films are immersed in water, the hydrophobic interactions in PU nanoparticles significantly restrict the transition of PU chains from a coiled conformation to a stretched conformation, which further hinders the migration of the films to the damaged regions to heal cuts. To further verify that conformation transition of PU plays an important role in the healing process, the chain mobility of the polymers in the (PAA/PU)*n-water films when immersed in ethanol and water was measured using a fluorescence recovery after photobleaching (FRAP) method. The FRAP method is frequently utilized for measuring the lateral mobility of polymers in films by calculating their lateral diffusion coefficient (D).43, 44 PU is difficult to be labelled by fluorescent dyes due to the lack of reactive groups. As an alternative, PAA was conjugated with Lucifer yellow cadaverine (noted as PAA@LYC) to investigate its lateral mobility in


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(PAA/PU)*n-water when the films were incubated in ethanol and water. The FRAP measurements were carried out on [(PAA/PU)*20/(PAA@LYC/PU)*5/(PAA/PU)*2]-water films, where the outmost (PAA/PU)*2 film acts as a protective layer to prevent dissolution of the PAA@LYC from the films as its re-adsorption onto the films can interfere with the FRAP measurements. The films were first photobleached by scanning a defined circular area with a radius of 50 µm for 10 min. Subsequently, the fluorescence microscopy images were captured every 30 min to calculate the fluorescent recovery of the bleached area. Figure 5 shows the fluorescence

recovery

results

of

the

photobleached

area

in

the

[(PAA/PU)*20/(PAA@LYC/PU)*5/(PAA/PU)*2]-water films after different incubation times in ethanol and water. The fluorescence recovery speed of the films in ethanol is faster than that in water. According to literatures,43, 44 the lateral diffusion coefficient of PAA@LYC in ethanol (Dethanol) can be calculated as Dethanol = R2/4t1/2

(1)

where R is the radius of the bleaching spot, t1/2 is the half-times of fluorescence recovery. As R is 50 µm and t1/2 in ethanol is ~323 min, Dethanol is calculated to be ~3.22×10-10 cm2/s. In contrast, the fluorescent recovery in the [(PAA/PU)*20/(PAA@LYC/PU)*5/(PAA/PU)*2]-water films is less than 10% even after 450 min of incubation in water, meaning that PAA@LYC can hardly diffuse when the films are incubated in water. The difference in fluorescence recovery behaviors demonstrates that PAA@LYC has a much fast diffusion in ethanol-incubated (PAA/PU)*n-water films than in water-incubated films. In the [(PAA/PU)*20/(PAA@LYC/PU)*5/(PAA/PU)*2]water films, the diffusion of PAA@LYC is highly related to the diffusion of PU, disclosing that PU has a higher mobility in ethanol-incubated films than in water-incubated films. When the films are immersed in ethanol, PU transforms from a compact coiled conformation to a stretched


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one to obtain high mobility. The high mobility of PU further promotes the migration of PAA and significantly enhance the healing capacity of the (PAA/PU)*n-water films in ethanol. In contrast, as a poor solvent for PU, water cannot induce the transition of PU from a coiled conformation to a stretched one, and fails to endow PU with high mobility. The FRAP experiments undoubtedly prove that ethanol-induced transition of PU chains from a coiled conformation to a stretched one endows the (PAA/PU)*n-water films with a superhigh healing capacity. CONCLUSIONS In summary, we have demonstrated a novel strategy for the fabrication of healable polymeric films capable of healing wide and deep cuts. The LbL-spin-coated (PAA/PU)*n-water films with coiled PU show excellent healability and can heal cuts as wide as 6 times of the film thickness. The excellent healability of the films originates from the compactly coiled PU that can transform to a stretched state to trigger a long-distance migration of polymers when exposed to ethanol. Compared with new kinds of noncovalent interactions and dynamic covalent bonds, conformation of polymers is usually neglected in design of self-healing/healable polymer materials. This study discloses that polymer conformation is also a key factor to govern the healing capacity of polymer films in which the polymer migration is restricted by the underlying substrates. A large degree of conformation transition of polymers enables a long-distance migration of polymers through the damaged area to heal large damages. The strategy developed in this study is highly useful for the fabrication of self-healing/healable polymer films with excellent healing capacity. Novel self-healing/healable polymer materials capable of healing large damages under the stimuli such as temperature, ionic strength, pH conditions, and so forth can be fabricated through reasonable control over the conformation of polymers and their transition.


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EXPERIMENTAL SECTION Materials. Cationic PU water dispersion with a concentration of 35 wt% was obtained from HEPCE CHEM Co., Ltd. as a gift. The molecular weight (Mw) of PU was ~92 kDa. Various concentrations of PU solutions/dispersions were prepared by adding required amount of deionized water and/or ethanol and stirring for at least 2 h before use. Poly(ethylenimine) (PEI, Mw ca. 750 kDa) and PAA (Mw ca. 450 kDa) were purchased from Sigma-Aldrich. Lucifer yellow cadaverine (LYC) was obtained from Invitrogen. LYC-labelled PAA (named PAA@LYC) was prepared according to our previous method.[18] All chemicals were used without further purification. Ethanol was purchased from Sinopharm Chemical Reagent Co., Ltd. Fabrication of PAA/PU Films. Silicon or glass substrates were immersed in piranha solution (30% H2O2/98% H2SO4, 1:3, v/v) and heated until no bubbles were released. A newly cleaned silicon or glass substrate was first immersed in an aqueous PEI solution (1 mg/mL) for 20 min to obtain a positively charged surface. The (PAA/PU)*n-water films were fabricated by alternately spin-coating PAA and PU on a PEI-modified substrate loaded on a commercial spin coater (Laurel Technologies, Model WS-400BZ-6NPP/LITE). A droplet of 1 mL of PAA (4 mg/mL) aqueous solution was dropped on a PEI-modified substrate which was loaded in a spin coater, spun at 500 rpm for 5 s and followed by spun at 1500 rpm for 1 min, to form a layer of PAA. Immediately the substrate was rinsed by 1 mL of deionized water and dried while spinning to remove physically absorbed polyelectrolytes. A droplet of 1 mL of PU (1 wt%) aqueous dispersion was spun at 500 rpm for 5 s and then spun at 1500 rpm for 1 min, followed by the same rising step with PAA. These depositing and rinsing steps were repeated until desired number of film deposition cycles was achieved. The (PAA/PU)*m-ethanol films were fabricated


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following a similar procedure with the (PAA/PU)*n-water films by replacing the deionized water by ethanol for preparing all the polyelectrolyte solutions and rinsing solutions. Instruments and Characterization. TEM samples of PU aggregates were obtained by dispersing highly diluted PU water dispersion on a TEM micro-grid with a carbon film and then measured under a JEM-2100F transmission electron microscope at an acceleration voltage of 200 kV. Digital photographs were captured with a Canon SX40 HS camera. DLS studies were carried out on a Malvern Nano-ZS zetasizer. The measurements were conducted at a scattering angle of θ = 173° using a He-Ne laser with a wavelength of 633 nm at room temperature. The UV-vis transmission spectra were recorded on a Shimadzu UV-2550 spectrophotometer. The film thickness was measured with a Dektak 150 surface stylus profilometer using a 5 µm stylus tip with 3 mg stylus force. The AFM images were obtained with a commercial instrument (Veeco Company Nanoscope IV) in tapping mode under ambient environment. The SEM images were obtained on a JEOL JSM 6700F field emission scanning electron microscope. Optical micrographs were taken using an Olympus BX-51 optical microscope. Mechanical properties of the films were studied according to our previous method[26,33] using an Agilent Nano Indenter G200 equipped with an XP-style actuator, and the continuous stiffness measurement (CSM) method was used. Young’s modulus and hardness were measured using a Berkovich diamond tip with a relative humidity of ∼20% at 30 °C. The storage moduli of the films in ethanol and water were measured with a flat-ended cylindrical tip made of diamond by the “G-Series XP CSM flat punch complex modulus” method. The photobleaching of samples was conducted by scanning a defined area on samples for 10 minutes with the range of 380-405 nm laser lines at a maximum laser power using an Olympus FV1000 confocal laser scanning microscope. An Olympus BX-51 microscope equipped with fluorescence light sources transmitted excitation wavelengths in the


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range of 530-550 nm for the observation of fluorescence recovery on the photobleached area in (PAA/PU)*n-water films. Data are presented as means ± standard deviation (n = 3) unless otherwise stated.


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ASSOCIATED CONTENT Supporting Information. AFM and TEM images of PU cast from water dispersion and the ethanol/water mixture, hydrodynamic size distributions of PU in water and in the ethanol/water mixture, and PAA in water and in ethanol, microscopy images of the (PAA/PU)*50-water films with different wide cuts before and after healing in ethanol for 10 min, and microscopy images of the (PAA/PU)*50water film after multiple healing processes. 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. ‡These two authors contributed equally. Funding Sources This work was supported by the National Natural Science Foundation of China (NSFC grant 21225419) and the National Basic Research Program (2013CB834503). Notes The authors declare no competing financial interest. ABBREVIATIONS


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PAA, poly(acrylic acid); PU, polyurethane; PAH, poly(allylamine hydrochloride; DLS, dynamic light scattering; TEM, transmission electron microscopy; AFM, atom force microscopy; SEM, scanning electron microscopy; RMS, root mean square; FRAP, fluorescence recovery after photobleaching; PEI, poly(ethylenimine); LYC, lucifer yellow cadaverine; CSM, continuous stiffness measurement. REFERENCES (1) Wool, R. P. Material Response and Reversible Cracks in Viscoelastic Polymers. Polym. Eng. Sci. 1978, 18, 1057-1061. (2) White, S. R.; Sottos, N.; Geubelle, P.; Moore, J.; Kessler, M.; Sriram, S.; Brown, E.; Viswanathan, S. Autonomic Healing of Polymer Composites. Nature 2001, 409, 794-797. (3) Shchukin, D. G. Container-based Multifunctional Self-Healing Polymer Coatings. Polym. Chem. 2013, 4, 4871-4877. (4) Chen, X.; Dam, M. A.; Ono, K.; Mal, A.; Shen, H.; Nutt, S. R.; Sheran, K.; Wudl, F. A Thermally Re-mendable Cross-Linked Polymeric Material. Science 2002, 295, 1698-1702. (5) Li, L.; Yan, B.; Yang, J.; Huang, W.; Chen, L.; Zeng, H. Injectable Self-Healing Hydrogel with Antimicrobial and Antifouling Properties. ACS Appl. Mater. Interfaces 2017, 9, 9221–9225. (6) Blaiszik, B. J.; Kramer, S. L. B.; Olugebefola, S. C.; Moore, J. S.; Sottos, N. R.; White, S. R. Self-Healing Polymers and Composites. Annu. Rev. Mater. Res. 2010, 40, 179-211. (7) Karimineghlani, P.; Palanisamy, A.; Sukhishvili, S. A. Self-Healing Phase Change Salogels with Tunable Gelation Temperature. ACS Appl. Mater. Interfaces 2018, 10, 14786-14795.


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FIGURES

Figure 1. (a) Chemical structure of PU. (b) PU in water (left) and in ethanol/water (Xethanol=98%) mixture (right). (c) Schematic illustration of PU in water and in ethanol. (d) Hydrodynamic diameters of PU nanoparticles in ethanol/water mixture with different molar concentrations of ethanol. (e) Transmittance at 550 nm of PU ethanol/water mixture with different molar concentrations of ethanol. PU concentration is 1 wt% for b, d, and e.


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Scheme 1. Schematic illustration of the fabrication of (PAA/PU)*n-water films by spin-assisted LbL assembly. (PAA/PU)*n-water films with desired thickness can be fabricated by repeating steps 1 to 4 in a cyclic fashion.


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Figure 2. (a) Thickness of the (PAA/PU)*n-water films as a function of the number of film deposition cycles. (b) AFM height image of a (PAA/PU)*50-water film. (c-e) SEM images of a (PAA/PU)*50-water film with a ~40 µm wide cut before (c) and after healing in ethanol for 5 min (d) and 10 min (e). (f) The same film in c after being immersed in water for 60 min.


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Figure 3. (a) Thickness of the (PAA/PU)*m-ethanol films as a function of the number of film deposition cycles. (b) AFM height image of a (PAA/PU)*100-ethanol film. (c, d) SEM images of a (PAA/PU)*100-ethanol film with a ~41 µm wide cut before (c) and after (d) being immersed in ethanol for 20 min. (e, f) SEM image (e) and cross-sectional SEM image (f) of the film in c after being immersed in water for 20 min. Inset is the cross-sectional SEM image of the as-prepared (PAA/PU)*100-ethanol film before being immersed in ethanol.

Figure 4. (a) Young’s modulus of a (PAA/PU)*50-water film as a function of indentation depth at 30 °C and RH of ~20%. (b) Storage modulus of a (PAA/PU)*50-water film under the frequency of 1 Hz in ethanol and water.


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Figure 5. Time-dependent fluorescence recovery of the [(PAA/PU)*20/(PAA@LYC/PU)*5/(PAA/PU)*2]-water films in the photobleached circle areas when incubated in ethanol and water.


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Table of Contents Graphic


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Polymers with a Coiled Conformation Enable Healing of Wide and

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