Highly Transparent, Nanofiller-Reinforced Scratch ... - ACS Publications

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Highly Transparent, NanofillerReinforced Scratch-Resistant Polymeric Composite Films Capable of Healing Scratches Yang Li,† Shanshan Chen,† Xiang Li, Mengchun Wu, and Junqi Sun*

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State Key Laboratory of Supramolecular Structure and Materials, International Joint Research Laboratory of Nano-Micro Architecture Chemistry (NMAC), College of Chemistry, Jilin University, Changchun 130012, PR China. †Y.L. and S.C. contributed equally.

ABSTRACT Integration of healability and mechanical robustness is challenging in the

fabrication of highly transparent films for applications as protectors in optical and displaying devices. Here we report the fabrication of healable, highly transparent and scratch-resistant polymeric composite films that can conveniently and repeatedly heal severe damage such as cuts of several tens of micrometers wide and deep. The film fabrication process involves layer-by-layer (LbL) assembly of a poly(acrylic acid) (PAA) blend and branched poly(ethylenimine) (bPEI) blend, where each blend contains the same polyelectrolytes of low and high molecular weights, followed by annealing the resulting PAA/bPEI films with aqueous salt solution and incorporation of CaCO3 nanoparticles as nanofillers. The rearrangement of low-molecular-weight PAA and bPEI under aqueous salt annealing plays a critical role in eliminating film defects to produce optically highly transparent polyelectrolyte films. The in situ formation of tiny and well-dispersed CaCO3 nanoparticles gives the resulting composite films enhanced scratch-resistance and also retains the healing ability of the PAA/bPEI matrix films. The reversibility of noncovalent interactions among the PAA, bPEI, and CaCO3 nanoparticles and the facilitated migration of PAA and bPEI triggered by water enable healing of the structural damage and restoration of optical transparency of the PAA/bPEI films reinforced with CaCO3 nanoparticles. KEYWORDS: layer-by-layer assembly . nanofillers . polyelectrolytes . self-healing . transparent films

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elf-healing materials have gained particular interest in recent years because self-healing provides a new route to safer, longer lasting, and more reliable artificial materials. Upon being damaged, selfhealing/healable materials can repair damaged structures and functionalities using the resources inherently available in the materials automatically or with the assistance of external stimuli.1
6 Inspired from natural healing processes, many selfhealing/healable materials have been successfully fabricated, which is revolutionizing the design of artificial materials for more convenient maintenance and improved safety.7
19 The focus of self-healing materials is moving from restoring mechanical and structural properties to multiple healing functions.5 Artificial materials capable of healing superhydrophobicity,20
23 electrical conductivity,24
28 corrosion resistance,18,29
31 LI ET AL.

antifouling function,31
33 electrical capacitor28,34 and so forth have been fabricated by either extrinsic or intrinsic self-healing methods. Among the various self-healing materials, the fabrication of transparent film capable of healing scratches is still in its infancy despite their importance as protective and functional layers in optical and displaying devices.35 Scratches on transparent films change the light transmission and lead to blurring or distortion of the displayed images.36 Transparent-film-covered devices cannot be used without restoring the original transparency by repairing these scratches. Therefore, an ideal self-healing/ healable transparent film requires being highly transparent, scratch-resistant, and capable of repairing severe damage such as deep and wide cuts and scratches. Among the few attempts to fabricate healable transparent films,37
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* Address correspondence to [email protected]. Received for review June 15, 2015 and accepted September 22, 2015. Published online September 22, 2015 10.1021/acsnano.5b03629 C 2015 American Chemical Society

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on the mobility of the polyelectrolyte chains in the presence of water, which is governed by the configurations of the polyelectrolytes in the films. To enhance the healability of the polyelectrolyte films, the polyelectrolytes are manipulated into a highly coiled configuration to achieve high mobility due to the following: (i) the coiled polyelectrolytes screen their interactions with neighboring polyelectrolytes bearing opposite charges to produce polyelectrolyte films of weakened interactions; and (ii) high mobility is achieved when the polyelectrolytes change from a coiled to an extended configuration. In our previous work, we demonstrated the fabrication of water-enabled polyelectrolyte films by exponential LbL assembly of branched poly(ethylenimine) (bPEI) and poly(acrylic acid) (PAA), which can rapidly and repeatedly heal cuts several tens of micrometers wide and deep with exposure to water.13 However, because of the highly coiled bPEI and PAA, the as-prepared bPEI/PAA films have a very rough surface and are not qualified as transparent films. It is problematic to choose between transparency and healability in water-enabled healable polyelectrolyte films. Optically transparent bPEI/PAA films with less coiled polyelectrolyte configurations were fabricated by sacrificing healability of the resulting film, but these transparent bPEI/PAA films can only heal shallow scratches of a few micrometers wide and deep.38 Moreover, the healable polyelectrolyte films are not scratch-resistant because of the weak interactions between the oppositely charged polyelectrolytes.13,38 It remains a challenge to fabricate highly transparent and scratch-resistant films capable of healing severe damage. In this work, we report the fabrication of a highly transparent, scratch-resistant and healable polymeric composite film that has the ability to heal severe damages while simultaneously presenting unprecedented Young's modulus and hardness compared with previously reported intrinsic self-healing/healable materials. The dilemma between transparency and healability of the polymeric composite films is solved by LbL assembly of bPEI blend and PAA blend with each blend contains the same polyelectrolytes of high and low molecular weights. The high mobility of the low-molecularweight bPEI and PAA flattens the bPEI/PAA films in aqueous NaCl solution and renders the resultant films highly transparent. Upon in situ formation of CaCO3 nanoparticles, transparent and healable composite films with largely enhanced mechanical properties are obtained, which are scratch-resistant and capable of healing cuts several tens of micrometers wide and deep with exposure to water.

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method37 has been proven to be technically difficult in fabricating healable films with satisfactory transparency.35 The advantage of the capsule-based method is that it can produce healable films with enhanced mechanical properties. However, the capsules, which contain healing agents and release them upon damage, usually have sizes of several to several tens of micrometers and strongly scatter visible light. The capsule-based method has to compromise between healability and transparency, with large capsules producing films of enhanced healability but largely decreased transparency. Furthermore, films fabricated by the capsule-based method can usually only undergo one damage/healing event at the same damage spot because of extinction of healing agent.1,2 Different from the extrinsic self-healing materials fabricated by the capsule-based method, intrinsic self-healing materials repair damage through the reversibility14 of noncovalent interactions9
13,17 and dynamic covalent bonds.8,15,16 Without the need for the externally incorporated healing agents contained in microcapsules, transparent intrinsic self-healing bulk materials have been successfully fabricated by preventing the formation of aggregations that are too large to scatter visible light.40,41 The intrinsic self-healing materials can achieve repeated healings of damage in the same spot without the issue of extinction of healing agent.3
6 Therefore, the intrinsic self-healing method provides a practical way of fabricating transparent and healable polymeric films.38,39 Until now, most intrinsic selfhealing/healable materials were soft and rubbery42 since, on the one hand, these materials are usually composed of soft building blocks of polymers and organic components and on the other, the reversible and dynamic nature of the noncovalent interactions and dynamic bonds tends to produce soft materials with high mobility to facilitate the healing process.9,12,16 Efforts have been devoted to fabricating intrinsic healable materials with enhanced mechanical properties by introducing hard components into the materials.10,15,17,39,43
45 However, these materials with Young's modulus lower than ∼300 MPa are not sufficiently rigid to resist scratches and other physical damages. The layer-by-layer (LbL) assembly of oppositely charged polyelectrolytes provides a useful method for the fabrication of water-enable intrinsic healable films.13,38,39 The advantages of LbL assembly lie in its wide choice of building blocks, substrate-independent deposition process and the capability of precise control over film compositions and structures.46
53 With the stimulus of water molecules, the interactions between oppositely charged polyelectrolytes in the healable films are weakened and the mobility of the polyelectrolyte chains is enhanced to achieve healing of structural damages.13 The healability of LbLassembled polyelectroyte films significantly depends

RESULTS AND DISCUSSION LbL Assembly of Polyelectrolyte Films. Aqueous solutions of polyelectrolyte blends containing the same polyelectrolytes of low and high molecular weights are VOL. 9

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ARTICLE Scheme 1. Schematic illustration of the fabrication of healable, highly transparent, and scratch-resistant CaCO3A-(PAA450kþ1.8k/bPEI750kþ25k)*n film. For clarity, PAA (Mw ≈ 450 000 g 3 mol
1) and bPEI (Mw ≈ 750 000 g 3 mol
1) are not presented in the scheme. (1) As-prepared (PAA450kþ1.8k/bPEI750kþ25k)*n film; (2) annealed (PAA450kþ1.8k/bPEI750kþ25k)*n film, which is denoted as A-(PAA450kþ1.8k/bPEI750kþ25k)*n film; (3) A-(PAA450kþ1.8k/bPEI750kþ25k)*n film after incorporation of CaCO3 nanoparticles, which is denoted as CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*n film; and (4) CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*n film capable of healing deep cuts with exposure to water.

used for the fabrication of LbL-assembled polyelectrolyte films to facilitate their structural tailoring. The fabrication of the healable, highly transparent, and scratch-resistant polymeric composite films on substrates predeposited with a layer of poly(diallyldimethylammonium chloride) (PDDA) is shown in Scheme 1, which comprises the LbL assembly of PAA450kþ1.8k and bPEI750kþ25k, the film annealing in aqueous NaCl solution, and the incorporation of CaCO3 nanoparticles. The LbL assembly of PAA450kþ1.8k and bPEI750kþ25k produces exponentially grown (PAA450kþ1.8k/bPEI750kþ25k)*n films based on electrostatic and hydrogen bonding interactions between carboxylic acid/carboxylate groups of PAA and amine/protonated amine groups of PAA as the main driving force. As shown in Figure 1a, the thickness of the (PAA450kþ1.8k/bPEI750kþ25k)*n film increases exponentially in the initial 10 deposition cycles, and thereafter the film thickness increases nearly linear with an increment of approximately 1.5 μm per deposition cycle. In a control experiment, the LbL-assembled (PAA450k/PEI750k)*n films also exhibit an exponential deposition behavior followed by rapid linear film deposition. Comparison of the deposition behaviors of the (PAA450kþ1.8k/bPEI750kþ25k)*n and the (PAA450k/ PEI750k)*n films shows that the addition of low-molecular weight PAA and bPEI produces an obvious influence on the first 10 cycles of film deposition but this influence is gradually offset by increasing film deposition to about 30 cycles. The (PAA450kþ1.8k/ bPEI750kþ25k)*30 and (PAA450k/PEI750k)*30 films have a thickness of 31.2 ( 0.5 μm and 32 ( 0.8 μm, respectively. Previous studies indicate that the exponential LI ET AL.

growth of the LbL-assembled polyelectrolyte films originates from the “in-and-out” diffusion of at least one kind of polyelectrolytes during the film deposition process.48,50 PAA conjugated with lucifer yellow cadaverine (LYC) (PAA-LYC) and bPEI conjugated with fluorescein isothiocyanate (FITC) (bPEI-FITC) of low and high molecular weights were deposited as the outmost layers to investigate their diffusion into (PAA450kþ1.8k/bPEI750kþ25k)*30 and (PAA450kþ1.8k/ bPEI750kþ25k)*30.5 films by confocal laser scanning microscope (CLSM). As shown in Figure 1b and 1c, PAA450k-LYC and PAA1.8k-LYC can diffuse throughout the entire (PAA450kþ1.8k/bPEI750kþ25k)*30 films. The ability of PAA450k-LYC and PAA1.8k-LYC to diffuse into PAA450kþ1.8k/bPEI750kþ25k films shows no difference in the CLSM images. The CLSM images in Figure 1d and 1e show that, due to the branched structure,13 bPEI750kFICT and bPEI25k-FICT can diffuse to a limited depth of the films when they are deposited as the top layers. The diffusion depth of bPEI25k is twice as deep as bPEI750k. Therefore, the exponential deposition of the (PAA450kþ1.8k/bPEI750kþ25k)*n films can also be explained by the “in-and-out” diffusion mechanism. For example, for the diffusion of PAA, the PAA diffused into the PAA450kþ1.8k/bPEI750kþ25k film and diffused out when the film was immersed into the aqueous bPEI dipping solution, which led to more bPEI deposition in the current layer than in the former layer. This means the PAA450kþ1.8k/bPEI750kþ25k film undergoes accelerated growth with increased film deposition cycles. When the “in-and-out” diffusion of the PAA and bPEI polyelectrolytes reaches equilibrium, the film deposition gradually transforms from exponential to rapid VOL. 9

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Figure 1. (a) Thickness of (PAA450kþ1.8k/bPEI750kþ25k)*n and (PAA450k/bPEI750k)*n films as a function of the number of deposition cycles. (b, c) CLSM images of a (PAA450kþ1.8k/ bPEI750kþ25k)*30 film with an outmost layer of PAA450k-LYC (b) and PAA1.8k-LYC (c). (d, e) CLSM images of a (PAA450kþ1.8k/ bPEI750kþ25k)*30.5 film with an outmost layer of bPEI750k-FITC (d) and bPEI25k-FITC (e). The scale bars are 40 μm.

linear deposition mode. The diffusion results also show that, at least for bPEI, polyelectrolytes with lower molecular weight have higher mobility in LbLassembled polyelectrolyte films. Film Annealing in Aqueous NaCl Solution. Transmission spectrum shows that the as-prepared (PAA450kþ1.8k/ bPEI750kþ25k)*30 film is transparent in the visible region, with a transmittance of ∼95.9% at 550 nm (Figure 2a, curve 1 and 2b). Despite its satisfactory transmittance in the visible region, the glass covered with the as-prepared (PAA450kþ1.8k/bPEI750kþ25k)*30 film blurred the view of the plant, which had a distance of 2 m behind the film (Figure 3a). The scanning electron microscopy (SEM) image in Figure 3b shows that the as-prepared (PAA450kþ1.8k/bPEI750kþ25k)*30 film has a rough surface, with a root-mean-square (RMS) roughness of 86 nm within an area of 20  20 μm measured by atomic force microscopy (AFM). The film surface is full of undulating structures of a few micrometers that change the path of light propagation through the film leading to blurred images. Therefore, film annealing in aqueous salt solution was performed to eliminate the rough and aggregated structures in (PAA450kþ1.8k/bPEI750kþ25k)*30 films to make them highly transparent. After immersing the LI ET AL.

Figure 2. (a) Transmission spectra of different kinds of PAA/bPEI films. (PAA450kþ1.8k/bPEI750kþ25k)*30 (curve 1), (curve 2), CaCO3A-(PAA450kþ1.8k/bPEI750kþ25k)*30 A-(PAA450kþ1.8k/bPEI750kþ25k)*30 (curve 3), (PAA450k/ bPEI750k)*30 (curve 4), and A-(PAA450k/bPEI750k)*30 (curve 5) films. b) Transmittance of different kinds of PAA/ bPEI films at 550 nm. (PAA450kþ1.8k/bPEI750kþ25k)*30 (2), CaCO3(1), A-(PAA450kþ1.8k/bPEI750kþ25k)*30 A-(PAA450kþ1.8k/bPEI750kþ25k)*30 (3), (PAA450k/bPEI750k)*30 (4), and A-(PAA450k/bPEI750k)*30 (5) films. The transmission spectra were recorded with only one side of the glass substrates covered with polyelectrolyte films and bare glass substrates were used as reference.

Figure 3. (a, b) Photograph (a) and SEM image (b) of the asprepared (PAA450kþ1.8k/PEI750kþ25k)*30 film. (c, d) Photograph (c) and SEM image (d) of the annealed (PAA450kþ1.8k/PEI750kþ25k)*30 film. The photographs were taken with the plants 2 m behind the films.

(PAA450kþ1.8k/bPEI750kþ25k)*30 films in aqueous NaCl solution (0.6 M) at room temperature for 20 min and thoroughly rinsing them with deionized water, the transparency of the annealed films was improved, with the transmittance at 550 nm increasing from ∼95.9% to ∼99.1% (Figure 2a, curve 2 and 2b). As observed, the most significant change after annealing is that the image VOL. 9

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of a plant placed 2 m behind the A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film (where the prefix “A” denotes annealed) is clear (Figure 3c). The A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film looks crystal clear. The SEM image in Figure 3d confirms that the surface of the A-(PAA450kþ1.8k/bPEI750kþ25k)*30 film is smooth. The RMS roughness of the A-(PAA450kþ1.8k/bPEI750kþ25k)*30 film is 6.1 nm according to the AFM measurements. The transparency of the as-prepared (PAA450k/ bPEI750k)*30 films without low-molecular-weight PAA and bPEI is almost the same as that of the (PAA450kþ1.8k/bPEI750kþ25k)*30 films in the visible region (Figure 2a, curve 4 and 2b). As a comparison, the (PAA450k/bPEI750k)*30 film was also annealed in 0.6 M NaCl solution for 20 min. The transparency of the A-(PAA450k/bPEI750k)*30 film slightly increased, with a transmittance increment of 0.3% at 550 nm (Figure 2a, curve 5 and 2b). Furthermore, the surface morphology of the A-(PAA450k/bPEI750k)*30 film, which is also rough with undulating structures, shows negligible change compared with the as-prepared (PAA450k/bPEI750k)*30 film. The plant behind the as-prepared and annealed (PAA450k/bPEI750k)*30 film is difficult to see (Supporting Information, Figure S1). These results indicate that annealing in aqueous NaCl solution produces almost no influence on the morphology and transparency of the (PAA450k/bPEI750k)*30 film. In aqueous PAA and bPEI dipping solutions, there exist coiled and aggregated PAA and bPEI, in particular for PAA and bPEI of large molecular weights. During the exponential LbL deposition of the PAA/bPEI films, the deposition of coiled and aggregated PAA and bPEI is a rapid process but their chain rearrangement process is quite low and even requires external energy input. Without sufficient chain rearrangement to eliminate aggregations, structurally inhomogeneous films with defects are produced.54 There are strong electrostatic and hydrogen bonding interactions between PAA and bPEI of different molecular weights in (PAA450kþ1.8k/bPEI750kþ25k)*30 films. When the as-prepared (PAA450kþ1.8k/bPEI750kþ25k)*30 films are immersed into aqueous NaCl solution, the electrostatic/ hydrogen bonding interactions between PAA and bPEI are weakened, with the charges on PAA and bPEI being screened with counter Naþ and Cl
ions.55 Therefore, the mobility of PAA and bPEI is facilitated in aqueous NaCl solution. However, for the same polyelectrolytes existing in the same environment, their mobility is significantly dependent on their molecular weights. In the (PAA450kþ1.8k/bPEI750kþ25k)*30 films, PAA450k and bPEI750k are highly entangled while PAA1.8k and bPEI25k are only slightly entangled or their entanglement can be neglected altogether. The diffusion coefficient of the entangled polymers can be described with the Reptation model,56 which is inversely proportional to the square of the degree of polymerization. For the nonentangled polymers with short chains, their

Figure 4. TEM image (a), TGA curve (b), SEM image (c), and photograph (d) of the CaCO3-A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film. The photograph was taken with the plant 2 m behind the films.

diffusion coefficient is inversely proportional to the degree of polymerization, as described by the Rouse model.56 Therefore, with a reasonable approximation, we can see that the mobility of PAA1.8k and bPEI25k is significantly higher than that of PAA450k and bPEI750k during the film annealing in aqueous NaCl solution. This conclusion is also validated by the higher diffusion of bPEI25k than bPEI750k in (PAA450kþ1.8k/bPEI750kþ25k)*30.5 films (Figure 1d and 1e). The inability of PAA450k and bPEI750k to rearrange their chains explains why the A-(PAA450k/bPEI750k)*30 films retain their original structure (Figure S1). In contrast, the high mobility of PAA1.8k and bPEI25k enables chain rearrangement, which eliminates the defects resulting from the film deposition process. Therefore, structurally homogeneous (PAA450kþ1.8k/ bPEI750kþ25k)*30 films with smooth surface are obtained upon annealing in aqueous NaCl solution (Figure 3d and Scheme 1). Transparent PAA/bPEI Films Reinforced with CaCO3 Nanoparticles. CaCO3 nanoparticles are incorporated into the A-(PAA450kþ1.8k/bPEI750kþ25k)*30 films, which are denoted as CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*30 films for simplicity, to enhance their mechanical properties. CaCO3 nanoparticles were chosen because they are colorless, and a small amount of well-dispersed CaCO3 nanoparticles can significantly reinforce the host polymer materials.57,58 The CaCO3 nanoparticles were incorporated by sequential immersing the A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film in aqueous CaCl2 solution for 10 min and Na2CO3 solution for 5 min in which calcium ions coordinated with carboxylic acids in PAA and then reacted with carbonate to produce CaCO3 nanoparticles (Scheme 1). It is confirmed that 5 min immersion in aqueous Na2CO3 solution is sufficient for the formation of CaCO3 nanoparticles within the A-(PAA450kþ1.8k/bPEI750kþ25k)*30 films (Supporting Information, Figure S2). The transmission electron microscopy (TEM) image in Figure 4a shows that the CaCO3 nanoparticles of 7
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in the CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*30 film. The CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*30 film was scratched from glass substrates and collected for thermogravimetric analysis (TGA) measurements. The TGA curve in Figure 4b shows that the decomposition of the PAA/bPEI matrix film and CaCO3 nanoparticles occurs between 300 and 600 °C and 600
700 °C, respectively.58 The weight content of the CaCO3 nanoparticles in the CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*30 film is calculated to be ∼5.3 wt % according to the TGA curve. The incorporation of CaCO3 nanoparticles produces no influence on the surface morphology and transparency of the resultant film. The SEM image in Figure 3c indicates that the CaCO3-A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film has a very smooth surface. The RMS roughness of the CaCO3-A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film is 6.3 nm within a measured area of 20  20 μm. Furthermore, the CaCO3-A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film is still highly transparent in the visible region, with a transmittance of 98.6% at 550 nm (Figure 2a, curve 3 and 2b). Like the A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film, the image of the plant 2 m behind the CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*30 film can be clearly seen (Figure 4d). The Young's moduli and hardness of the A-(PAA450kþ1.8k/bPEI750kþ25k)*30 film before and after incorporating CaCO3 nanoparticles were measured by nanoindentation. A Berkovich diamond indenter with radius e20 nm was used to measure the Young's moduli and hardness of the films in air with ∼20% RH at 20 °C by the “G-Series continuous stiffness measurement (CSM) Standard Hardness, Modulus and Tip Cal” test method. Figure 5a and 5b display the Young's moduli and hardness of the A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 films before and after incorporating CaCO3 nanoparticles as a function of indentation depth. The Young's moduli in the plateau region (∼200 to 300 nm) are considered to be the “real” Young's moduli of the films because in this region the influence of the film surface and the underlying substrates are avoided (Figure 5a).38,59,60 The Young's moduli of the A-(PAA450kþ1.8k/bPEI750kþ25k)*30 and CaCO3A-(PAA450kþ1.8k/bPEI750kþ25k)*30 films are measured to be 7.8 ( 1.9 and 17.6 ( 2.8 GPa, respectively. In the same principle, hardness in the plateau region with indentation of 300
400 nm is “real” hardness, which is averaged to be 0.28 ( 0.1 and 0.95 ( 0.15 GPa for (PAA450kþ1.8k/bPEI750kþ25k)*30 and CaCO3-A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 films, respectively. The incorporation of CaCO3 nanoparticles significantly enhances the Young's modulus and hardness of the CaCO3A-(PAA450kþ1.8k/bPEI750kþ25k)*30 films by a factor of 2.3 and 3.4 compared with those of A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 films, respectively. The Fourier transform infrared (FT-IR) spectrum of the CaCO3A-(PAA450kþ1.8k/bPEI750kþ25k)*30 film shows that there exists a strong coordination interaction between the

Figure 5. (a, b) Typical Young's moduli (a) and hardness (b) of the A-(PAA450kþ1.8k/PEI750kþ25k)*30 films before (red curves) and after (blue curves) incorporation of CaCO3 nanoparticles as a function of the penetration depth.

CaCO3 nanoparticles and PAA/bPEI matrix films (Supporting Information, Figure S3). The homogeneously dispersed CaCO3 nanoparticles prevent stress concentration while the tiny CaCO3 nanoparticles have a significantly increased surface area, which further enhances the interaction of CaCO3 nanoparticles with the PAA/bPEI matrix films.58 The combination of homogeneous dispersion, nanometer size, and strong coordination interactions contributes to the greatly enhanced Young's modulus and hardness of the CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*30 films. The CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*30 films have an enhanced ability to resist scratches because of their largely enhanced Young's modulus and hardness and smooth surface. The scratch resistance of the CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*30 films was evaluated by repeatedly rubbing the films with a cylindrical metal.58 During the rubbing test, a pressure of 12 kPa was applied on the cylindrical metal with its bottom covered with a piece of ramie cloth. After 1000 rubbing cycles, the CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*30 film was slightly damaged with the appearance of a few of ∼200 nm deep and a few micrometerswide grooves (Figure 6a and 6b). The CaCO3A-(PAA450kþ1.8k/bPEI750kþ25k)*30 film before and after rubbing shows negligible transmittance change in the visible region (Figure 6d, curve 1 and 2). In contrast, after being rubbed for 1000 cycles, the A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film was heavily damaged, leading to a dense array of grooves as wide as 30 μm and as deep as 2 μm (Figure 6a and 6c). Furthermore, VOL. 9

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ARTICLE Figure 6. (a) Photographs of the CaCO3-A-(PAA450kþ1.8k/PEI750kþ25k)*30 film (left) and A-(PAA450kþ1.8k/PEI750kþ25k)*30 film (right) after 1000 cycles of rubbing. (b, c) Magnified photographs and AFM images of the scratches in CaCO3-A-(PAA450kþ1.8k/ PEI750kþ25k)*30 (b) and A-(PAA450kþ1.8k/PEI750kþ25k)*30 (c) films. (d) Transmission spectra of CaCO3-A-(PAA450kþ1.8k/ PEI750kþ25k)*30 and A-(PAA450kþ1.8k/PEI750kþ25k)*30 films. Pristine CaCO3-A-(PAA450kþ1.8k/PEI750kþ25k)*30 film (curve 1); scratched CaCO3-A-(PAA450kþ1.8k/PEI750kþ25k)*30 film before (curve 2) and after healing in deionized water (curve 3); scratched A-(PAA450kþ1.8k/PEI750kþ25k)*30 film.

the transmittance of the rubbed (PAA450kþ1.8k/ bPEI750kþ25k)*30 film significantly decreased, with its transmittance at 550 nm decreasing from ∼99.1% to ∼63.8% (Figure 6d, curve 4). The rubbing test confirms that, with homogeneous dispersion of tiny CaCO3 nanoparticles, the CaCO3-A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film gains enhanced ability to resist scratches. Meanwhile, the adhesion of the CaCO3A-(PAA450kþ1.8k/bPEI750kþ25k)*30 films on PDDA-modified glass substrates was measured by the cross-cut tape test following ASTM D 3359 specifications. Optical microscope image shows that no detachment of the cross-cut films occurs after the tape test (Supporting Information, Figure S4). Therefore, the CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*30 films has a 5B adhesion on the substrate, which is the highest level in ASTM standards. The excellent adhesion of the CaCO3A-(PAA450kþ1.8k/bPEI750kþ25k)*30 films on PDDA-modified substrates is attributed to the strong interaction of the bottom layer of PAA with PDDA-modified substrates and high Young's modulus and hardness of the films. Convenient Healing of the CaCO3-A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 Films. The as-prepared (PAA450kþ1.8k/ bPEI750kþ25k)*30 film can heal a cut ∼100 um wide that penetrates to the underlying substrate by immersing the film in deionized water for 30 min (Figure S5a and S5b). The (PAA450kþ1.8k/bPEI750kþ25k)*30 film has the same maximum cut-repairing capacity as that of the (PAA450k/bPEI750k)*30 film. The A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film can also heal a cut ∼100 um wide LI ET AL.

with exposure to deionized water (Figure S5c and S5d). The introduction of the low-molecular-weight PAA25k and bPEI1.8k and the annealing step only insignificantly alter the healability of the polyelectrolyte films. The CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*30 films retain the ability of the PAA450kþ1.8k/bPEI750kþ25k films to heal scratches and cuts in spite of the enhanced mechanical properties enabled by the incorporated CaCO3 nanoparticles. Shallow scratches made on the CaCO3A-(PAA450kþ1.8k/bPEI750kþ25k)*30 films after 1000 rubbing cycles can be readily healed by immersing the films in deionized water for 10 min. After healing in water, the rubbed CaCO3-A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film is fully restored to its original transparency (Figure 6d, curve 3). The healing of the shallow scratches can be repeated multiple times in the same region of the CaCO3-A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film, which is similar to our previously reported transparent and healable bPEI/PAA films. However, different from the transparent bPEI/ PAA films which can only heal shallow scratches of several micrometers wide and deep, the CaCO3A-(PAA450kþ1.8k/bPEI750kþ25k)*30 films can heal cuts of several tens of micrometers wide and deep. As displayed in Figure 7a, a cut of ∼80 μm wide was made on the CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*30 film using a scalpel, which exposed the underlying substrate. After immersing in deionized water for 30 min, the cut on the film completely disappears, confirming full healing of the cut (Figure 7b). In regards to the maximum width of cuts that can be healed, the healability VOL. 9

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of the CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*30 film is decreased ∼20% compared with the A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film. As a result of its intrinsic healing nature and excellent healing ability, the CaCO3A-(PAA450kþ1.8k/bPEI750kþ25k)*30 film can achieve multiple healings of deep and wide cuts in the same region. The CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*30 film was damaged with an ∼80 μm cut and then healed in deionized water, and the cutting/healing process was repeated 5 times. The SEM image in Figure 7c shows that the cuts in the CaCO3-A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film is finely healed even after 5 cycles of cutting/healing process (Supporting Information, Figure S6). In the healed region, grooves ∼100 nm deep and several tens micrometers wide are observed in AFM image (Supporting Information, Figure S7). However, owing to their smooth surface and shallow depth, these grooves can cause negligible light scattering. The energy-dispersive X-ray spectroscopy (EDS) map in Figure 7d indicates that the distribution of Ca2þ ions on the healed region after 5 cycles of cutting/healing process shows no difference with the nondamaged part of the CaCO3-A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film, furthering confirming the successful healing of the cuts. The in situ fabricated CaCO3 nanoparticles have strong coordination interactions with the transparent PAA450kþ1.8k/bPEI750kþ25k films. The healing process in deionized water does not lead to leaching of CaCO3 nanoparticles from PAA450kþ1.8k/ bPEI750kþ25k films. Therefore, the original mechanical properties, i.e., high Young's modulus, hardness, and scratch-resistance are retained even for the repeatedly healed CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*30 films. The healing mechanism for CaCO3-A-(PAA450kþ1.8k/ bPEI750kþ25k)*n films follows that of our previously reported healable polyelectrolyte films.13 When immersed in deionized water, the adsorbed water molecules interact with carboxylic acid and amine LI ET AL.

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Figure 7. (a, b) SEM images of the CaCO3-A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film with an 80 μm wide cut (a) and after healing in water (b). (c) SEM image and SEM-EDS elemental map for Ca (d) of CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*30 film after 5 cutting/healing cycles.

groups in CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*n films, which weakens the electrostatic and hydrogen bonding interactions within the films. Water molecules can also weaken the coordination interaction of CaCO3 nanoparticles with the PAA450kþ1.8k/bPEI750kþ25k films, which means water molecules act as plasticizers to make the CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*n films soft and flowable. As a result of decreasing interfacial energy created by the damage, PAA, bPEI polyelectrolytes, and CaCO3 nanoparticles migrate to the damaged region and fill the scratches and cuts, where they reform electrostatic, hydrogen bonding and coordination interactions to complete the healing process. Therefore, the reversibility of electrostatic, hydrogen bonding, and coordination interactions and the facilitated migration of PAA and bPEI polyelectrolytes under the assistance of water enable healing of transparent and scratch-resistant CaCO3A-(PAA450kþ1.8k/bPEI750kþ25k)*n films. It is worth mentioning that a higher extent of PAA/bPEI dissociation is required for annealing of the (PAA450kþ1.8k/ bPEI750kþ25k)*n films than for healing of CaCO3A-(PAA450kþ1.8k/bPEI750kþ25k)*n films. This is proven by the fact that the (PAA450kþ1.8k/bPEI750kþ25k)*30 film after healing in deionized water still has a rough surface similar to the as-prepared (PAA450kþ1.8k/ bPEI750kþ25k)*30 film. As an intrinsically healable film without an externally incorporated healing agent, the CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*n film has an extremely high Young's modulus of 17.6 ( 2.8 GPa and hardness of 0.95 ( 0.15 GPa. These values are not only higher than those of previously reported intrinsic healing materials, but also higher than daily used nonhealable plastic protective films.61 For instance, the Young's modulus and hardness of a commercially available Samsung screen protector film, which is nonhealable, (RG Brand Screen Guard for Samsung I569/S5660, ∼ 130 μm thick) are 4.6 ( 0.1 GPa and 0.41 ( 0.02 GPa, respectively, as measured by nanoindentation. The CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*n films are mechanically robust, scratch-resistant, highly optical transparent, and healable. The integration of healability and high mechanical robustness suggests the application of the CaCO3-A-(PAA450kþ1.8k/ bPEI750kþ25k)*n films as protectors for optical and displaying devices. CONCLUSIONS In conclusion, we have demonstrated the fabrication of conveniently healable, highly transparent, and scratch-resistant polymeric composite films that involves LbL assembly of PAA and bPEI blends with low- and high-molecular-weight PAA or bPEI, annealing the polyelectrolyte films with aqueous salt solution and incorporation of CaCO3 nanoparticles as nanofillers. The low-molecular-weight PAA and bPEI facilitate the rearrangement of polyelectrolytes under aqueous VOL. 9

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modulus and hardness as intrinsic healable materials. The integration of healability and scratch-resistance ensures the long-term and reliable applications of CaCO3-A-(PAA450kþ1.8k/bPEI750kþ25k)*n films as protectors for optical and display devices. The LbL-assembly, which is independent of substrate type and morphology, enables the convenient deposition of the CaCO3A-(PAA450kþ1.8k/bPEI750kþ25k)*n films on various substrates of large areas. This advantage will definitely extend the application of healable, optically transparent, and scratch-resistant CaCO3-A-(PAA450kþ1.8k/ bPEI750kþ25k)*n films. Furthermore, this work provides an avenue for the fabrication of other kinds of robust and healable polymer composite films and materials through the combination of different types of polyelectrolytes and nanofillers.

MATERIALS AND METHODS

was for 5 min to remove the adsorbed inorganic ions. Finally, the films were dried with N2 flow. Incorporation of CaCO3 Nanoparticles. The annealed (PAA450kþ1.8k/ bPEI750kþ25k)*n films were immersed in aqueous CaCl2 solution (10 mg mL
1) for 10 min followed by water washing for 1 min. Then, the films were transferred to aqueous Na2CO3 solution (5 mg mL
1) for 5 min followed by rinsing for another 1 min. Finally, the films were dried with N2 flow. Film Characterization. UV
vis transmission spectra were recorded with a Shimadzu UV-2550 spectrophotometer. Film thicknesses were determined by a Dektak 150 surface profiler using a 5 μm stylus tip with a 3 mg stylus force. The digital images of the films were captured by a Sony Cyber-shot DSCH10 camera in macro mode. SEM images were obtained using an XL30 ESEM FEG scanning electron microscope. AFM images were taken using a commercial instrument, Veeco Company Nanoscope IV. Diffusion of the dye-labeled polyelectrolytes was characterized by obtaining cross-sectional images of the film with an Olympus Fluoview FV1000 confocal laser scanning microscope. EDS measurements were conducted on an EDAX Genesis 2000 X-ray microanalysis system attached to an XL30 ESEM FEG SEM. TEM observations were made on a JEM-2100F microscope, operating at an acceleration voltage of 200 kV. TGA measurements were performed on a Pyris Diamond TG/DTA thermogravimeter (PerkinElmer, USA) with a heating rate of 5 °C min
1 under an air atmosphere. The mechanical properties of films were tested using an Agilent Nano Indenter G200 with the CSM method and XP-style actuator at ambient conditions (20% relative humidity (RH), 20 °C). Measurements were obtained at least ten different sites for each film. The detailed measurements were made according to our previous publication. Conflict of Interest: The authors declare no competing financial interest.

Materials. PDDA (Mw ≈ 100 000
200 000 g 3 mol
1), bPEI (Mw ≈ 750 000 g 3 mol
1 and 25 000 g 3 mol
1), PAA (Mw ≈ 450 000 g 3 mol
1 and 1800 g 3 mol
1), fluorescein isothiocyanate FITC, 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide, hydrochloride (EDC), and N-hydroxysuccinimide (NHS) were purchased from Sigma-Aldrich. LYC was obtained from Invitrogen. NaCl, CaCl2, and NaCO3 were purchased from the Beijing Chemical Reagents Company. All chemicals were used without further purification. bPEI-FITC and PAA-LYC were synthesized according to a literature method.13 In the reaction vessels, the fed mass ratios of FITC to bPEI and LYC to PAA were 1:30 and 1:15, respectively. Deionized water was used for film fabrication. The concentration of the polyelectrolyte aqueous solutions used for the LbL assembly was 4 mg mL
1. Polyelectrolytes with high molecular weight were mixed with the same polyelectrolytes of low molecular weight with a mass ratio of 1:1 to produce a polyelectrolyte blend. Aqueous bPEI solution contains bPEI with molecular weights of 750 000 and 25 000, which is denoted as bPEI750kþ25k (where k = 1000). Aqueous PAA solution contains PAA with molecular weights of 450 000 and 1800, which is denoted as PAA450kþ1.8k. Aqueous bPEI and PAA solutions without the addition of low-molecularweight bPEI and PAA are denoted as bPEI750k and PAA450k, respectively. The pH of the polyelectrolyte dipping solutions was adjusted with either 1 M HCl or 1 M NaOH. Fabrication of Polyelectrolyte Films. Silicon and glass slides were immersed in piranha solution (1:3 mixture of 30% H2O2 and 98% H2SO4) and heated until no bubbles were released. After being washed with water and dried with N2 flow, the cleaned substrate was immersed in PDDA solution (1 mg mL
1) for 20 min to obtain a positively charged surface. The PDDA-modified substrate was alternately dipped into aqueous solutions of PAA450kþ1.8k (pH 4.0) and bPEI750kþ25k (pH 9.5) for 15 min each time, with intermediate washing in three water baths for 1 min each to remove physically adsorbed polyelectrolytes. The deposition of PAA450tþ1.8t and bPEI750tþ25t was repeated to obtain (PAA450kþ1.8k/bPEI750kþ25k)*n films (where n refers to the number of film deposition cycles, and a half cycle denotes that PAA450kþ1.8k is the outmost layer). The film fabrication was conducted by a programmable dipping machine (Dipping Robot DR-3, Riegler & Kirstein GmbH) with no drying step being conducted until it was in the last layer. For comparison, (PAA450k/bPEI750k)*n films were fabricated in the same way as the (PAA450kþ1.8k/bPEI750kþ25k)*n films by using aqueous PAA450k (pH 4.0) and bPEI750k (pH 9.5) solutions as the dipping solutions. Film Annealing in Aqueous NaCl Solution. Annealing of the (PAA450kþ1.8k/bPEI750kþ25k)*n films were conducted by immersing the as-prepared films in an aqueous NaCl solution (0.6M) for 20 min. Then, the resultant films were transferred to deionized

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salt annealing to eliminate defects and obtain highly transparent A-(PAA450kþ1.8k/bPEI750kþ25k)*n films. The homogeneous incorporation of CaCO3 nanoparticles enhances the mechanical properties of the CaCO3A-(PAA450kþ1.8k/bPEI750kþ25k)*n films and renders them scratch-resistant. It is believed that the noncovalent interaction between the CaCO3 nanofillers and the PAA/bPEI matrix films plays a key role in retaining the healability of the A-(PAA450kþ1.8k/bPEI750kþ25k)*n films after incorporation of CaCO3 nanoparticles. This design principle can be used for the fabrication of other kinds of robust intrinsic healable materials. The CaCO3A-(PAA450kþ1.8k/bPEI750kþ25k)*n films have the ability to repeatedly heal severe damages such as deep and wide cuts of several tens of micrometers while simultaneously maintaining extremely high Young's

Acknowledgment. National Basic Research Program (2013CB834503), National Natural Science Foundation of China (NSFC grants 21225419 and 21221063). Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b03629. Optical and SEM images of the as-prepared (PAA450k/ PEI750k)*30 film and A-(PAA450k/PEI750k)*30 film. SEM images of (PAA450kþ1.8k/bPEI750kþ25k)*30 film and A-(PAA450kþ1.8k/ bPEI750kþ25k)*30 film before and after healing a cut. (PDF)

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