Transparent Polymeric Films Capable of Healing Millimeter-Scale Cuts

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 13073−13081

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Transparent Polymeric Films Capable of Healing Millimeter-Scale Cuts Wenjin Guo, Xiang Li, Fuchang Xu, Yang Li,* and Junqi Sun State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, PR China

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ABSTRACT: Transparent polymeric films have been successfully integrated with selfhealing capabilities. However, these films can only heal damages in the scale of several to several tens of micrometers, thereby greatly limiting their practical applications. The present study reports the fabrication of transparent polymeric films capable of healing millimeter-scale cuts by incorporating hydrogen-bonding units into zwitterionic polymer films, which are cross-linked by electrostatic interactions. The intermolecular interactions in the resulting films are greatly reduced when the films absorb water as a result of the reversibility of hydrogen-bonding and electrostatic interactions, thereby promoting the flowability of the film materials. Thus, the transparent films can heal 7.9 mm wide cuts and recover their damaged transparency following exposure to water. Furthermore, owing to their strong binding affinity to water molecules, the healable transparent films can effectively clean up oil fouled on dry films following rinsing with water. The combination of hydrogen bonding and electrostatic interactions provides a new means of design for transparent films with enhanced healing capabilities and an extended service life. KEYWORDS: self-healing material, transparent film, oil-cleaning film, macroscale, multiple interactions

1. INTRODUCTION Clear and transparent polymeric films are essential in electronic and optical devices as protective and functional layers.1,2 During daily use, transparent polymeric films can be damaged by abrasion, cutting, or impacting, thereby resulting in image blurring and decreased transparency. Thus, one of the most challenging aspects of the fabrication of transparent polymeric films is the enhancement of their robustness and scratch resistance while maintaining a good degree of transparency.3 Although the robustness of transparent polymeric films can be reinforced by incorporating inorganic nanofillers3,4 or rigid chain polymers,5,6 scratches inevitably appear on the films after long-term use. Moreover, enhancing the robustness of transparent polymeric films greatly challenges the repair and replacement of damaged films. Considering the aforementioned facts, the development of an alternative method to increase the durability of transparent polymeric films is highly desirable but remains a great challenge. In nature, living organisms possess the ability to self-heal damaged tissue and functions, thus increasing their chances of survival.7 Inspired by this fascinating ability, man-made selfhealing materials capable of repairing damage have been successfully fabricated to prolong material life span and enhance the reliability of various materials, such as concrete, metal, plastic, and so forth.8−14 In general, self-healing materials can be categorized into extrinsic and intrinsic self-healing materials depending on whether an external healing component is required during the healing process.8,10 Growing research interest on the fabrication of self-healing functional materials © 2018 American Chemical Society

has recently increased the fabrication of artificial materials capable of healing damaged antifogging ability,15−17 corrosion resistance,18,19 superhydrophobicity,20−22 conductivity,23−25 and so forth.26−29 Especially, transparent films have been successfully integrated with self-healing capabilities, thereby providing a novel and effective means of increasing the durability of transparent films.16,17,30−36 For instance, Jackson et al. reported the fabrication of extrinsic self-healing transparent films by dispersing dibutyl phthalate plasticizer-loaded microcapsules in polymethylmethacrylate.30 Such films are capable of healing cracks with the help of the plasticizer that was released from the broken microcapsules. However, it is very difficult to execute multiple healing at the same damaged area on capsule-based self-healing transparent films given the presence of plasticizer depletion. To solve this problem, intrinsic self-healing transparent films capable of repeatedly healing damage through reversible noncovalent interactions have since been fabricated.15,17,31,33,36−39 For instance, Wang and co-workers have fabricated intrinsic self-healing transparent films by layer-by-layer assembly of branched polyethylenimine (bPEI) and poly(acrylic acid) (PAA).31 Benefiting from the reversibility of electrostatic interactions between the amino groups of bPEI and the carboxyl groups of PAA, the resulting bPEI/PAA multilayer films can repeatedly heal damage in the same area with the assistance of water. Furthermore, Chen and Received: February 4, 2018 Accepted: March 23, 2018 Published: March 23, 2018 13073

DOI: 10.1021/acsami.8b02124 ACS Appl. Mater. Interfaces 2018, 10, 13073−13081

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Chemical structures of MSA and AMPS. (b) Schematic mechanism of the TiO2 NP-catalyzed gelation process. (c) Schematic of the fabrication process of the healable transparent PMSA-co-AMPSn films.

Figure 2. (a) Thickness of the PMSA, PMSA-co-AMPS0.1, and PMSA-co-AMPS0.2 films as a function of mixture solution volumes. (b) Top-view and (c) cross-sectional SEM images of the PMSA-co-AMPS0.1 film. (d) Transmission spectra of the PMSA, PMSA-co-AMPS0.1, and PMSA-co-AMPS0.2 films. The inset shows the photo image of the as-prepared PMSA-co-AMPS0.1 film. (e) FTIR spectra of the PMSA, PMSA-co-AMPS0.1, and PMSA-coAMPS0.2 films. (f) Young’s moduli of the PMSA, PMSA-co-AMPS0.1, and PMSA-co-AMPS0.2 films in a 40% RH environment.

long distance to the damage area because of the strong intermolecular interactions in the film. Previous studies have revealed that the strength of polymer chain intermolecular interactions greatly influences the healing efficiency of self-healing bulk materials.14,45 Here, we propose that the healing ability of self-healing transparent films can be greatly increased by regulating the strength of the intermolecular interactions in the films. To prove this concept, transparent films capable of healing millimeter-scale cuts are fabricated by incorporating hydrogen-bonding units into zwitterionic polymer films. The intermolecular interactions of the resulting transparent films are greatly weakened when films absorb water because of the weak strength of the hydrogenbonding interactions, thereby greatly improving the mobility of the film materials. As a result, the millimeter-scale cuts on the films can be completely healed by the transportation of film materials through a long distance to the damaged area. Moreover, because of the strong water-binding ability of zwitterionic polymer films, oil fouled on the dry films can be readily removed by rinsing the films in water.

co-workers demonstrated that scratch resistance of the selfhealing transparent films can be greatly promoted by the in situ synthesis of calcium carbonate (CaCO3) nanoparticles (NPs) in the bPEI/PAA multilayer films.33 Despite all of that success, the development of self-healing transparent films remains in its infancy because almost all previously reported self-healing transparent films only heal damages in the scale of several to several tens of micrometers.17,31 Considering that commonly encountered damages on transparent films are in the macroscopic scale, the promotion of the healing ability of self-healing transparent films is of practical significance.40 Although the healing of wide-open cracks in the millimeter scale has been achieved previously in self-healing bulk materials,41−43 the fabrication of transparent films capable of healing millimeter-scale cuts remains a considerable challenge.44 This is because of the following reasons: (i) the limited film materials near the damage area owing to the thin thickness of the film (usually tens to hundreds of micrometers) and (ii) insufficient transportation of film materials through a 13074


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Figure 3. (a) AFM and (b) optical images of the scratched PMSA-co-AMPS0.1 film (1) before and (2) after healing in a 90% RH environment for 30 min. (c) Transmission spectra of the as-prepared (black), scratched (blue), and healed (red) PMSA-co-AMPS0.1 films. (d−f) Optical microscopy images of the scratched (d) PMSA, (e) PMSA-co-AMPS0.1, and (f) PMSA-co-AMPS0.2 films before (top images) and after (bottom images) healing in a 90% RH environment. The corresponding healing speed of the films is marked in each panel.

2. RESULTS AND DISCUSSION 2.1. Fabrication of PMSA-co-AMPS n Films. We fabricated healable transparent films by the random copolymerization of [2-(methacryloyloxy)ethyl]dimethyl-(3sulfopropyl)ammonium hydroxide (MSA) and 2-acrylamide2-methylpropanesulfonic acid (AMPS) at feed molar ratios of 9:1 and 8:2 (Figure 1a), respectively. The resulting copolymers are denoted as PMSA-co-AMPSn, where n represents the feed molar fraction of AMPS. The feed molar fractions of MSA are higher than 0.8 to ensure that the electrostatic interactions are maintained as the dominant cross-linking sites in the resulting films. In this study, titanium dioxide (TiO2) NPs were used to catalyze the random copolymerization reactions of MSA and AMPS because of their excellent hydrophilicity and biocompatibility. As shown in Figure 1b, TiO2 NPs can generate radicals under ultraviolet (UV) irradiation (365 nm),46 thereby trigging the copolymerization of MSA and AMPS. The resulting PMSAco-AMPS0.1 and PMSA-co-AMPS0.2 copolymers were characterized by 1H-NMR spectroscopy. As shown in Figure S1a,b, the 1H-NMR signals corresponding to the −CH2− proton of PMSA-co-AMPSn copolymers appear as broad peaks ranging from 0.84 to 1.32 ppm, whereas the signals corresponding to the olefinic proton at 6.18, 6.09, 6.06, 5.72, and 5.63 ppm disappear after UV irradiation for 2 h, confirming the successful synthesis of PMSA-co-AMPS0.1 and PMSA-co-AMPS0.2 copolymers catalyzed by TiO2 NPs. To fabricate the PMSA-co-AMPSn-based films, we casted mixture solutions of MSA, AMPS, TiO2 NPs, and poly(ethylene glycol) diacrylate (PEGDA) on glass substrates and subsequently UV-irradiated the solutions in a sealed vessel filled with nitrogen for 2 h. Hydrogel films were generated after the

irradiation of the mixture solutions because of the electrostatic interactions among the MSA units and the hydrogen-bonding interactions among the AMPS units (Figure 1b). Afterward, the resulting translucent PMSA-co-AMPSn gel films were dried in ambient conditions [25 °C, 40% relative humidity (RH)] for 24 h to generate healable transparent PMSA-co-AMPSn films (Figure 1c). According to Figure 2a, the thickness of the PMSA-co-AMPSn films increased linearly following an increase in the volume of the mixture solutions casted on the glass substrates, thereby providing a convenient means for controlling the thickness of the films. The top-view scanning electronic microscopy (SEM) image indicates that the PMSAco-AMPS0.1 film with a thickness of ∼53 μm has a smooth surface (Figure 2b). According to the atomic force microscopy (AFM) test, the PMSA-co-AMPS0.1 film has a root-mean-square (rms) roughness of 1.63 nm. In addition, the cross-sectional SEM image shows that the PMSA-co-AMPS0.1 film is compact and flat (Figure 2c). Benefiting from their smooth surfaces and compact structures, the PMSA-co-AMPSn films are transparent and clear (Figure 2d). According to the inset of Figure 2d, the glass covered with the PMSA-co-AMPS0.1 film was transparent, and the details of the flowers behind the glass can be clearly seen. We further used Fourier transform infrared (FTIR) spectroscopy to characterize the PMSA-co-AMPSn and PMSA films (Figure 2e). For the PMSA film, the bands located at 1726 and 1480 cm−1 are assigned to the stretching vibration of CO and the C−H stretching vibration of −N+(CH3)2−, respectively. The bands located at 1193 and 1039 cm−1 are corresponding to the asymmetric and symmetrical stretching vibrations of OSO, respectively. Upon the addition of the AMPS units, a band corresponding to the bending vibration of 13075


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Figure 4. Optical (left) and microscopy (right) images of the (a, b) damaged PMSA-co-AMPS0.1, (c, d) PMSA-co-AMPS0.2, and (e, f) PMSA films (a, c, e) before and (b, d, f) after healing in a 90% RH environment. (g) Maximum widths of the fully healable cuts on the PMSA, PMSA-co-AMPS0.1, and PMSA-co-AMPS0.2 films.

N−H appears at 1547 cm−1. Moreover, the band located at 1039 cm−1 gradually shifts to a lower wavenumber following an increase in the feed molar fraction of the AMPS, thereby confirming that the hydrogen-bonding interactions have been successfully introduced into the PMSA-co-AMPSn films as cross-linking sites.47 Mechanical properties of the PMSA and PMSA-co-AMPSn films were tested by nanoindentation. As shown in Figure 2f, the Young’s moduli (E) values of the PMSA, PMSA-coAMPS0.1, and PMSA-co-AMPS0.2 films in a 40% RH environment are 5.1 ± 0.5, 5.2 ± 0.3, and 5.1 ± 0.4 GPa, respectively, indicating that the introduction of the AMPS units cannot influence the robustness of the resulting films. In addition, the adhesion of the PMSA and PMSA-co-AMPSn films was tested by the cross-tape test (ASTM D 3359 standard). After tape test, no detachment of the films was observed, indicating that the PMSA and PMSA-co-AMPSn films have the highest level of adhesion (5B) (Figure S2). The excellent adhesion of the PMSA and PMSA-co-AMPSn films can be attributed to the strong electrostatic interactions between the copolymers and the substrates and the cross-linked structure of the films. 2.2. Promoted Healing Ability of the PMSA-co-AMPSn Films. The PMSA-co-AMPSn films exhibited water-facilitated healing abilities because of the reversibility of hydrogenbonding and electrostatic interactions. We first tested the healing ability of the PMSA-co-AMPS0.1 film by manually rubbing the film with a piece of 2000-grit sandpaper. Numerous grooves appeared on the scratched PMSA-co-AMPS0.1 film (Figure 3a-1) and resulted in severe light scattering (Figure 3b1). Therefore, the transparency of the PMSA-co-AMPS0.1 film dramatically decreased following the abrasion test (Figure 3c), as observed in the decreased transmittance at 550 nm (T550) from 96.8% to 65.9%. Interestingly, after being placed in a 90% RH environment for 30 min, the scratched PMSA-co-AMPS0.1 film became transparent and clear (Figure 3b-2). The AFM image indicates that the scratches on the healed PMSA-coAMPS0.1 film disappeared and the rms roughness of the film decreased from 87.9 to 1.25 nm (Figure 3a-2). In addition, the transparency of the healed PMSA-co-AMPS0.1 film was fully recovered and exhibited a T550 of 96.7% (Figure 3c). By contrast, the scratches on the PMSA-co-AMPS0.1 film cannot be healed even after storing the film in a 40% RH environment for 2 weeks. These results indicate that water plays a crucial role in the healing of the PMSA-co-AMPSn films. The healing speeds of the PMSA and PMSA-co-AMPSn films were characterized by optical microscopy (Figure 3d−f), where the healing speed is defined as the time required by the film to

fully recover its damage. According to Figure 3d, the scratched PMSA film required ∼60 min to fully heal the scratches on its surface. In contrast, the scratches on the PMSA-co-AMPS0.1 film were completely removed after the damaged film was placed in a 90% RH environment for 30 min (Figure 3e), thereby suggesting that the integration of the AMPS units into the PMSA film can greatly increase the healing speed of the film. In addition, the healing speed of the PMSA-co-AMPSn films increase with increasing AMPS content. For instance, the healing speed of the PMSA-co-AMPS0.2 film was only 10 min (Figure 3f). These results indicate that the integration of the hydrogen-bonding units into the polyelectrolyte-based films can greatly promote their healing speed. Apart from promoting the healing speed, the introduction of hydrogen-bonding units allows the PMSA-co-AMPSn films to heal the millimeter-scale mechanical damage. According to Figure 4a, a millimeter-scale cut that can be clearly seen from a distance was made on the PMSA-co-AMPS0.1 film by cutting and removing a part of the film with a knife. According to the optical microscopy image, the cut has a width of 1.0 mm and exposes the glass substrate. To heal the cut, the damaged PMSA-co-AMPS0.1 film with a thickness of 53.6 μm was dipped in water for 2 s and then placed in a 90% RH environment. Surprisingly, the cut disappeared after healing for 5 h (Figure 4b). In addition, the optical microscopy image in Figure 4b indicates that the cut was healed completely, thereby validating the excellent self-healing ability of the PMSA-co-AMPS0.1 film. Moreover, increasing the content of AMPS in the PMSA-coAMPSn films can speed up the cut-healing process. According to Figure 4c,d, a cut with a width of 1.1 mm was made on the 55.2 μm thick PMSA-co-AMPS0.2 film and was completely healed in 1 h. In a controlled experiment, a cut with a width of 1.1 mm was made on the PMSA film with a thickness of 50.1 μm (Figure 4e). However, the damaged PMSA film was unable to heal the cut (Figure 4f) even after 24 h healing in a 90% RH environment. The optical microscopy image in Figure 4f demonstrates that the width of the cut exhibited a slight reduction to 0.9 mm after healing, thereby indicating that the AMPS units in the PMSA-co-AMPSn films are indispensable to the realization of healing millimeter-scale cuts. More importantly, the maximum widths of the fully healable cuts on the PMSA-co-AMPSn films are influenced by the AMPS content of the films. According to Figures 4g and S3, the PMSA-co-AMPS0.1 and PMSA-co-AMPS0.2 films are capable of healing 3.2 mm wide and 7.9 mm wide cuts, which are ∼60 and ∼143 times larger than the thickness of the films, respectively. By contrast, the PMSA film can only heal a cut with a width of 13076


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Figure 5. (a) Young’s moduli of the healed area on the PMSA, PMSA-co-AMPS0.1, and PMSA-co-AMPS0.2 films in a 40% RH environment. (b) Young’s moduli of the PMSA, PMSA-co-AMPS0.1, and PMSA-co-AMPS0.2 films in a 70% RH environment. (c) Storage moduli of the wet PMSA, PMSA-co-AMPS0.1, and PMSA-co-AMPS0.2 films.

110 μm, which is only ∼2 times larger than the thickness of the films. These results indicate that the healing ability of the PMSA-co-AMPSn films increases with increasing AMPS content. Subsequently, the mechanical properties of the healed area on the healed PMSA, PMSA-co-AMPS0.1, and PMSA-coAMPS0.2 films were tested by nanoindentation. As shown in Figure 5a, the E values of the healed area on the healed PMSA, PMSA-co-AMPS0.1, and PMSA-co-AMPS0.2 films in a 40% RH environment are 5.1 ± 0.3, 5.2 ± 0.3, and 5.1 ± 0.5 GPa, respectively, which are similar to those of the as-prepared films. These results suggest that the material composition of the healed area is similar to that of the undamaged area. 2.3. Healing Mechanism of the PMSA-co-AMPSn Films. To understand the healing mechanism of the PMSA-co-AMPSn films, swelling ability of the films was investigated. After being dipped in water for 2 s and placed in a 90% RH environment for 1 h, the thickness of the PMSA, PMSA-co-AMPS0.1, and PMSA-co-AMPS0.2 films increased 130%, 141%, and 156%, respectively. However, we noticed that the PMSA, PMSA-coAMPS0.1, and PMSA-co-AMPS0.2 films cannot swell horizontally because of the strong confinement of the substrates. Therefore, we conclude that swelling is not the main reason for the healing of millimeter-scale cuts on the PMSA-co-AMPSn films. Subsequently, influence of moisture on the mechanical properties of the PMSA and PMSA-co-AMPSn films was studied. As shown in Figure 5b, the E values of the PMSA, PMSA-co-AMPS0.1, and PMSA-co-AMPS0.2 films in a 70% RH environment slightly decrease to 3.9 ± 0.2, 3.2 ± 0.1, and 2.9 ± 0.2 GPa, respectively. Softening of the PMSA-co-AMPSn films is attributed to the hygroscopicity of the films. Nevertheless, the E values of the PMSA-co-AMPSn films in a 70% RH environment are higher than those of the polypropylene (PP) or polyethylene terephthalate (PET) films (2.5 ± 0.3 and 2.6 ± 0.3 GPa, respectively, Figure S4). Moreover, adhesion of the PMSA, PMSA-co-AMPS0.1, and PMSA-co-AMPS0.2 films in a 70% RH environment was unchanged (Figure S5). Therefore, the mechanical properties of the PMSA-co-AMPSn films in ambient conditions are comparable to those commercial plastics. The PMSA and PMSA-co-AMPSn films became highly soft after being dipped in water and placed in a 90% RH environment. Thus, instead of E, storage moduli (E′) of the wet films were measured by nanoindentation.31 Because healing of the films was conducted under a static condition, E′ values obtained at 1 Hz were used to characterize the mechanical properties of the wet films. As shown in Figure 5c, E′ values of the wet PMSA, PMSA-co-AMPS0.1, and PMSA-co-AMPS0.2 films were measured to be 0.89 ± 0.03, 0.43 ± 0.05, and 0.08 ± 0.007 MPa, respectively. The dramatic decrease of the moduli of the wet films indicates that water can greatly increase the mobility of film materials. More importantly, the E′ values

of the PMSA-co-AMPSn films decreased with increasing AMPS content, suggesting that the introduction of AMPS units can further promote the mobility of film materials with the presence of water. In addition, the change of the intermolecular interactions in the PMSA-co-AMPSn films before and after absorbing water was characterized by FTIR spectroscopy (Figure S6). Upon absorption of water, a band corresponding to the O−H stretching of water appears at 3565 cm−1, confirming that water molecules are incorporated into the PMSA-co-AMPS0.1 film. Meanwhile, the band corresponding to the symmetrical stretching vibrations of OSO shifts from 1037 to 1039 cm−1, indicating that the hydrogen-bonding interactions in the wet PMSA-co-AMPS0.1 film are broken by water. Moreover, the FTIR spectrum of the redried PMSA-coAMPS0.1 film is similar to that of the original dry film, suggesting the re-formation of hydrogen-bonding interactions after removing water from the film. On the basis of the results mentioned above, the healing mechanism of the PMSA-co-AMPSn films can be explained as follows (Scheme 1): When placed in a highly humid Scheme 1. Schematic of the Healing Mechanism of the PMSA-co-AMPSn Films

environment or immersed in water, the PMSA-co-AMPSn films absorb a large amount of water because of their good affinity to water. The absorbed water can weaken the hydrogenbonding and electrostatic interactions in the films and act as plasticizers to increase the mobility of polymer chains. In addition, because the hydrogen-bonding interactions between the AMPS units are more susceptible to water than the electrostatic interactions between the MSA units, increasing the AMPS content can reduce the intermolecular interactions in the wet films and thereby further promotes the flowability of film materials. This is proven by the fact that E values of the PMSA-co-AMPSn films in a 70% RH environment and E′ values of the wet PMSA-co-AMPSn films decrease with the increase of AMPS content. Driven by decreasing interfacial energy caused by the damage, film materials in the wet PMSA-co-AMPSn films migrate to the damaged area through a long distance and fill the millimeter-scale cuts. Finally, when the films are placed in ambient conditions, hydrogen-bonding and electrostatic 13077


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Figure 6. Time-sequence images of cleaning the (a) 20 μL hexadecane and (b) 20 μL colza oil-fouled PMSA-co-AMPS0.1 films in water. (c) Timesequence images of cleaning the 20 μL hexadecane-fouled HA/bPEI multilayer film in water. (d,e) Oil-cleaning behavior of the damaged PMSA-coAMPS0.1 film (d) before and (e) after healing.

Irgacure(R)-2959-catalyzed PMSA-co-AMPS0.1 film, which has a thickness of 49.7 μm, cannot be completely removed by rinsing in water because of the good oleophilicity of the residual photoinitiator (Figure S7c). According to previous studies, the excellent oil-cleaning ability of the PMSA-co-AMPSn films originates from the zwitterionic MSA units, which have strong binding affinities to water molecules.49 When the oil-fouled PMSA-co-AMPSn films are immersed in water, oil molecules at the edge of the oil layers that are spread on the films are gradually displaced by water molecules, thereby resulting in the complete dewetting and detachment of the oil droplet. Apart from restoring their damaged transparency, PMSA-coAMPSn films are also capable of recovering their damaged oilcleaning ability. As shown in Figure 6d, a 20 μL hexadecane droplet was dropped on a PMSA-co-AMPS0.1 film on which a ∼1 mm-wide cut was made with a knife. When the hexadecanefouled film was immersed in water, the hexadecane layer rapidly shrunk into a droplet but firmly pinned itself to the cut because of the good oleophilicity of the glass substrate. Subsequently, the damaged PMSA-co-AMPS0.1 film was healed in a 90% RH environment for 5 h and dried in a desiccator. Interestingly, the oil-cleaning ability of the healed film was completely restored (Figure 6e), thereby suggesting that the ability to heal mechanical damage is crucial for the restoration of the oilcleaning ability of the films. Furthermore, unlike polymeric films that are cross-linked by hydrogen-bonding interactions, the PMSA-co-AMPSn films are stable in water because of electrostatic interaction stabilization. For instance, the weights of the PMSA-co-AMPS0.1 and PMSA-co-AMPS0.2 films slightly decreased by 0.3% and 1.7%, respectively, after 20 cycles of fouling and cleaning test. Moreover, the oil-cleaning ability of the PMSA-co-AMPS0.1 film was well-retained after storing the film in ambient conditions for 8 months. In contrast, the weight of the PAMPS film decreased by ∼2.7% after two cycles of fouling and cleaning test. These results indicate that the integration of the hydrogen-bonding units into the polyelectrolyte-based transparent film can greatly enhance its healing ability, while maintaining a good stability of the film.

interactions gradually re-form with the evaporation of water, completing the healing process. 2.4. Oil-Cleaning Ability of the PMSA-co-AMPSn Films. In practical applications, transparent films are readily contaminated by oily materials, which are hard to remove and can result in blurry images. Therefore, the ability to clean oily contaminants is crucial in maximizing the life span of transparent films. Benefiting from the strong water-binding ability of the MSA units,48 the PMSA-co-AMPSn films are capable of cleaning up oily contaminants in water. As shown in Figure 6a and Movie S1, a 20 μL hexadecane droplet dyed with Nile red was dropped on a dry PMSA-co-AMPS0.1 film. The hexadecane droplet spread into a thin film because of the high surface energy of the PMSA-co-AMPS0.1 film. Interestingly, when the hexadecane-fouled PMSA-co-AMPS0.1 film was immersed in water, the hexadecane layer rapidly shrank into a droplet and detached from the film in 1.8 s without any residue. Apart from hexadecane, the PMSA-co-AMPS0.1 film is capable of cleaning up viscous oil, such as colza oil, peanut oil, corn oil, and so forth. For instance, a droplet of colza oil (20 μL) fouled on the dry PMSA-co-AMPS0.1 film can be completely removed following film immersion in water for 2.5 s (Figure 6b and Movie S2). Moreover, the oil-cleaning abilities of the PMSA and PMSA-co-AMPS0.2 films were similar to that of the PMSAco-AMPS0.1 film, thereby indicating that the introduction of the AMPS units has negligible influences on the oil-cleaning ability of the PMSA-co-AMPSn films (Figure S7a,b). By contrast, a 20 μL hexadecane droplet fouled on a hydrophilic hyaluronic acid (HA)/bPEI multilayer film, which was fabricated according to the previous literature,17 could not be cleaned up in water despite the fact that the HA/bPEI multilayer film is superoleophobic in water (Figure S8). As shown in Figure 6c, the hexadecane layer slightly dewetted into a hemispherical droplet and firmly pinned itself on the film. Moreover, replacing TiO2 NPs with Irgacure(R) 2959 (2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone), a commonly used photoinitiator, can greatly impair the oil-cleaning ability of the resulting film. A 20 μL hexadecane droplet fouled on the 13078


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Characterization of the resulting films can be seen in Figures S9 and S10. 4.5. Nanoindentation Test. The mechanical properties of the films were measured on an Agilent Nano Indenter G200. To test the E values of the films under dry conditions (40% and 70% RH, 24 °C), a Berkovich diamond indenter was used. The test method was set as “GSeries CSM Standard Hardness, Modulus and Tip Cal”. To eliminate the influence of the substrate, the E value in the plateau region was considered to be the “real” E value of the film.31 To test the E′ values of the wet films, a flat-ended cylindrical punch was used. The test method was set as “G-Series XP CSM Flat Punch Complex Modulus”. 4.6. Characterization. Thicknesses of the films were measured on a surface stylus profilometer (Dektak 150) with a 5 μm stylus tip in ambient conditions. The UV−Vis transmission spectra were recorded on a Shimadzu UV-2550 spectrophotometer by using a bare glass as a reference. Digital images and videos were captured by a Canon PowerShot SX40 HS camera. The SEM images of the films were captured by a field-emission scanning electron microscope (JEOL JSM 6700F). The AFM images of the films were taken on a commercial instrument (Veeco Nanoscope IV) in tapping mode. Optical microscopy images were obtained on an Olympus BX-51 optical microscope. The FTIR spectra were obtained on a VERTEX 80v FTIR spectrometer. Contact angle measurements were performed on a DSA30 drop shape analysis system (Krüess).

3. CONCLUSIONS In summary, we have fabricated transparent polymeric films that are capable of healing millimeter-scale cuts via the TiO2catalyzed random copolymerization of MSA and AMPS. The healing ability of the PMSA-co-AMPSn films originates from the water-assisted long-distance migration of the film materials and the reversibility of hydrogen-bonding and the electrostatic interactions. Because of the weak strength of the hydrogenbonding interactions, increasing the content of AMPS units can further reduce the intermolecular interactions among the polymer chains in water, thereby enhancing the mobility of the film materials. Thus, the healing ability of the PMSA-coAMPSn films can be mediated by the AMPS content in the resultant films. Specifically, the PMSA-co-AMPS0.2 film is capable of healing a cut with a width of 7.9 mm, whereas the PMSA film can only heal a cut with a width of 110 μm. Furthermore, the PMSA-co-AMPSn films can readily clean up oily contaminants in water because of the strong water-binding ability of the zwitterionic MSA units. In addition, the PMSA-coAMPSn films can conveniently restore their damaged transparency and oil-cleaning ability because of their promoted healing ability, thereby prolonging the life span and reliability of the films. This study paves a new avenue for the design of healable transparent films that are capable of healing millimeterscale damages to meet the requirements of practical applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02124. 1 H-NMR spectra of the PMSA, PMSA-co-AMPSn, and Irgacure(R)-2959-catalyzed PMSA-co-AMPS0.1 copolymers; adhesion tests of the PMSA and PMSA-coAMPSn films in 40 and 70% RH environments; optical microscopy images of healing 110 μm wide, 3.2 mm wide, and 7.9 mm wide cuts on the PMSA, PMSA-coAMPS0.1 and PMSA-co-AMPS0.2 films, respectively; Young’s moduli of the PP, PET, and Irgacure(R)-2959catalyzed PMSA-co-AMPS0.1 films; FTIR spectra of the dry, wet, and redried PMSA-co-AMPS0.1 film; oil cleaning ability of the PMSA, PMSA-co-AMPS0.2, and Irgacure(R)-2959-catalyzed PMSA-co-AMPS0.1 films; and contact angle of a hexadecane droplet on the HA/bPEI multilayer film in water (PDF) Detachment of a 20 μL hexadecane droplet fouled on a PMSA-co-AMPS0.1 film when immersed in water (AVI) Detachment of a 20 μL colza oil droplet fouled on a PMSA-co-AMPS0.1 film when immersed in water (AVI)

4. EXPERIMENTAL SECTION 4.1. Materials. MSA, AMPS, Nile red, D2O, and PEGDA (Mn ≈ 700 g mol−1) were purchased from Sigma-Aldrich. The TiO2 NPs (5− 10 nm) were purchased from Aladdin. Hexadecane was purchased from Beijing Chemical Regents Company. Irgacure(R) 2959 was purchased from Tokyo Chemical Industry. Colza oil, peanut oil, corn oil, the PP film, and the PET film were purchased from a local market. All chemicals were used without further purification. Deionized water was used for film fabrication and oil cleansing. 4.2. Synthesis and Characterization of the PMSA and PMSAco-AMPSn Copolymers. Three kinds of mixed D2O solutions of AMPS/MSA with different molar ratios (total concentration was set as 0.7 mol L−1; feed molar fractions of AMPS were 0, 0.1, and 0.2), and TiO2 NPs (0.060 g L−1) were loaded in NMR tubes and were UVirradiated (365 nm, 250 W) for 2 h. Afterward, the resulting polymers/ copolymers were characterized by 1H-NMR spectroscopy (Bruker, AVANCE III 500) at 500 MHz. The 1H NMR spectra of the polymers/copolymers can be found in the Supporting Information. 4.3. Fabrication of the PMSA and PMSA-co-AMPSn Films. Three kinds of mixed aqueous solutions of AMPS/MSA with different molar ratios (total concentration was set as 3.5 mol L−1; feed molar fractions of AMPS were 0, 0.1, and 0.2), TiO2 NPs (0.50 g L−1), and PEGDA (0.0091 mol L−1) were carefully dropped on glass substrates and UV-irradiated (365 nm, 250 W) in a sealed vessel filled with nitrogen for 2 h. Afterward, the glass substrates covered with the gel films were dried in ambient conditions (25 °C, 40% RH) for 24 h, thereby resulting in transparent PMSA and PMSA-co-AMPSn films. To fabricate the PMSA and PMSA-co-AMPSn films for self-healing and oilcleaning tests, we controlled volumes of the mixture solutions that were dropped onto the glass substrates at 11 μL cm−2. 4.4. Fabrication of the Irgacure(R)-2959-Catalyzed PMSA-coAMPS0.1 Films. To fabricate the Irgacure(R)-2959-catalyzed PMSAco-AMPS0.1 films, mixed aqueous solution of AMPS (0.35 mol L−1), MSA (3.15 mol L−1), PEGDA (0.0091 mol L−1), and Irgacure(R) 2959 (0.0022 mol L−1) were carefully dropped on glass substrates and UV-irradiated (365 nm, 250 W) in a sealed vessel filled with nitrogen for 2 h, followed by drying in ambient conditions (25 °C, 40% RH) for 24 h. The amount of the casted solution was 11 μL cm−2.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yang Li: 0000-0003-4646-3695 Junqi Sun: 0000-0002-7284-9826 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (grant no. 21604029). 13079


Research Article


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ABBREVIATIONS [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)MSA ammonium hydroxide AMPS 2-acrylamide-2-methylpropanesulfonic acid; RMS root-mean-square RH relative humidity PEGDA poly(ethylene glycol) diacrylate bPEI branched polyethylenimine PAA poly(acrylic acid) PP polypropylene PET polyethylene terephthalate HA hyaluronic acid

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Transparent Polymeric Films Capable of Healing Millimeter-Scale Cuts

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