Reduced Graphene Oxide-Reinforced Polymeric Films with Excellent

Jul 10, 2017 - In this study, we exploit the reversible host–guest interactions between nanofillers and the matrix polymer films and report the fabr...
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Reduced Graphene Oxide-Reinforced Polymeric Films with Excellent Mechanical Robustness and Rapid and Highly Efficient Healing Properties Zilong Xiang, Ling Zhang, Yixuan Li, Tao Yuan, Wenshi Zhang, and Junqi Sun* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China S Supporting Information *

ABSTRACT: The fabrication of nanofiller-reinforced intrinsic healable polymer composite films with both excellent mechanical robustness and highly efficient healability is challenging because the mobility of the polymer chains is suppressed by the incorporated nanofillers. In this study, we exploit the reversible host−guest interactions between nanofillers and the matrix polymer films and report the fabrication of intrinsically healable, reduced graphene oxide (RGO)-reinforced polymer composite films capable of conveniently and repeatedly healing cuts of several tens of micrometers wide. The healable films can be prepared via layer-by-layer assembly of poly(acrylic acid) (PAA) with complexes of branched poly(ethylenimine) grafted with ferrocene (bPEI-Fc) and RGO nanosheets modified with βcyclodextrin (RGO-CD) (denoted as bPEI-Fc&RGO-CD). The as-prepared PAA/bPEI-Fc&RGO-CD films are mechanically robust with a Young’s modulus of 17.2 ± 1.9 GPa and a hardness of 1.00 ± 0.30 GPa. The healing process involves two steps: (i) healing of cuts in an oxidation condition in which the host−guest interactions between bPEI-Fc and RGO-CD nanosheets are broken and the cuts on the films are healed; and (ii) reconstruction of host−guest interactions between bPEI-Fc and RGO-CD nanosheets via reduction to restore the original mechanical robustness of the films. KEYWORDS: host−guest systems, layer-by-layer assembly, materials science, nanofillers, self-healing materials noncovalent bonds such as hydrogen bonding,6,24 π−π stacking,5,25 electrostatic interaction,6,8,9 metal−ligand interaction,26,27 host−guest interaction,28,29 and so forth have been employed to fabricate intrinsic healable materials with improved healing efficiency and enhanced mechanical properties. The healing process of intrinsic healable materials can be accomplished through two steps: (i) the intimate contact of polymers at the broken surfaces that is achieved via migration of polymer chains near the damage, and (ii) rebuilding of dynamic covalent bonds or noncovalent interactions at the contact interface. The mobility of the polymer chains plays a key role in determining the healability of intrinsic healable materials because the reversible interactions can be rebuilt only when the polymers can migrate across the damaged area and

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elf-healing/healable materials that can repair damage on themselves automatically or via external stimuli have received extensive research attention because they can decrease maintenance costs and improve product safety, lifetime, and reliability.1−6 In recent decades, various strategies have been developed to fabricate self-healing/healable materials that restore their structures, mechanical properties, and functions.4−15 These strategies can be generally categorized into extrinsic and intrinsic methods based on whether or not externally added healing agents are utilitized in the healing process.1,7 The combination of extrinsic and intrinsic methods has also been employed for the fabrication of self-healing/ healable materials.16 In contrast to extrinsic healable materials, intrinsic healable materials achieve healability through the reversibility of dynamic covalent bonds and noncovalent interactions and can undergo multiple rounds of healing at the same spot.4,6 Dynamic covalent bonds such as disulfide bonds,17,18 boroxine bonds,19 radical dimerization reactions,20,21 and cycloaddition reactions22,23 and reversible © 2017 American Chemical Society

Received: April 30, 2017 Accepted: July 10, 2017 Published: July 10, 2017 7134

DOI: 10.1021/acsnano.7b02970 ACS Nano 2017, 11, 7134−7141

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Figure 1. (a) The chemical structures of bPEI-Fc, PAA, and RGO-CD. (b) Schematic illustration of the fabrication and healing process of the (PAA/bPEI-Fc&RGO-CDm)*n films.

come into intimate contact.6,8,19 To guarantee that the polymer chains have a sufficiently high mobility, intrinsic healable materials are generally comprised of soft building blocks containing dynamic covalent bonds or weak noncovalent interactions, which usually leads to soft and rubbery materials.6,25,30 Improving the mechanical robustness of the intrinsic healable materials will suppress the mobility of the polymer chains and decrease their healability. Therefore, the contradiction between mechanical robustness and healability is a major hurdle to the fabrication of intrinsic healable materials that have excellent mechanical robustness and rapid, highly efficient healing properties. The homogeneous incorporation of stiff nanofillers into polymer matrices is a practically useful method to improve the mechanical robustness of the polymer materials.5,9,31,32 The strong interactions between the nanofillers and polymers can effectively transfer and distribute stress and significantly enhance the mechanical performance of the polymer composites. However, the healability of the nanofillerreinforced polymer composite materials is largely decreased because the strong interactions between the nanofillers and polymers can suppress the mobility of the polymer chains. As a result, nanofiller-reinforced polymer composites require more time to heal than systems without nanofillers.5,9 In many cases, these reinforced polymer composites will lose their capacity to heal.33 For example, Walther and co-workers have demonstrated that after incorporation of nanoclays into highly dynamic and self-healing polymers bonded by quadruple hydrogen-bonding motifs, the nacre-mimetic polymer/nanoclay composite films can no longer heal themselves. Our strategy for resolving the conflict between high mechanical robustness and excellent healability in intrinsic healable materials is to homogeneously disperse nanofillers within intrinsic healable polymers in which the interactions between nanofillers and the polymers are reversible and can be dissociated/associated on demand. When the nanofiller-reinforced polymer composites are mechanically damaged, an external stimulus is employed to dissociate the interactions between nanofillers and polymers. This stimulus allows the polymer composites to be healed like soft intrinsic healable materials without nanofillers. After healing, another stimulus is applied to restore the original mechanical robustness of the polymer composites by

reconstructing the associations between nanofillers and polymers. Compared with intrinsic healable bulk materials, the fabrication of intrinsic healable film materials deposited on solid substrates is much more challenging because the strong interactions between the films and the underlying substrates restrict the migration of polymers across the damaged regions.6,9 Of the various film preparative methods, layer-bylayer (LbL) assembly, which involves the sequential deposition of species with complementary noncovalent interactions, is an effective method to fabricate functional films with wellcontrolled mobility of polymer chains.34−37 By enhancing the mobility of the polymer chains, we and others have successfully fabricated several intrinsic healable polymer films using LbL assembly technique.8,9,38−41 However, the mechanical robustness of these LbL-assembled healable films needs to be largely enhanced to match their excellent healability. The conflict between enhancing the mechanical robustness and improving the healability also exists in LbL-assembled intrinsic healable polymer films. In this work, we develop an innovative and effective strategy for the fabrication of polymeric composite films with both excellent mechanical robustness and rapid and highly efficient healability. The approach takes advantage of the reversible host−guest interactions between nanofillers and LbLassembled polyelectrolyte films. Reduced graphene oxide (RGO) modified with β-cyclodextrin (CD) (denoted as RGO-CD) can complex with branched poly(ethylenimine) grafted with ferrocene groups (Fc) (denoted as bPEI-Fc) based on host−guest interactions to form bPEI-Fc&RGO-CD complexes (Figure 1a). The bPEI-Fc&RGO-CD complexes are LbL assembled with PAA to fabricate PAA/bPEI-Fc&RGOCD composite films with well-dispersed RGO nanosheets, which significantly enhance the mechanical robustness of the resulting films (Figure 1b). The host−guest interactions between RGO-CD nanofillers and bPEI-Fc polyelectrolytes can be broken on demand by oxidation of bPEI-Fc to bPEIFc+,28 which facilitates the mobility of the polymer chains and endows the PAA/bPEI-Fc&RGO-CD films with rapid and highly efficient healing capacity. After the damage heals, the host−guest interactions between RGO-CD and bPEI-Fc can be reconstructed via reduction of bPEI-Fc+ to bPEI-Fc to restore the original mechanical robustness of the films. 7135

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RESULTS AND DISCUSSION Fabrication of bPEI-Fc&RGO-CD Complexes. The host−guest interactions between Fc and CD are employed to fabricate healable polymeric films with enhanced mechanical properties because these interactions are highly reversible and controllable by oxidation/reduction reactions. The chemical structures of bPEI-Fc and RGO-CD are shown in Figure 1a, and the details of their synthesis are described in the Supporting Information (Figure S1). The 1H NMR spectrum (D2O) of bPEI-Fc shows that the grafting density of Fc is ∼2.0 mol % of −NH 2 groups from the bPEI (Supporting Information, Figure S2). CDs were covalently linked on GO nanosheets through the reaction of amino-β-CDs with epoxide and carboxyl groups present on the basal planes and sheet edges of GO (Supporting Information, Figures S3 and S4). The GO-CD nanosheets are further reduced to RGO-CD by hydrazine hydrate to eliminate the carboxyl and carboxylate groups on the GO nanosheets. The reduction step can largely decrease the electrostatic interactions between GO nanosheets and bPEI in bPEI-Fc&RGO-CD complexes. As shown in Figure 2a, the GO nanosheets have a smooth paper-like

intensity of the G band is lower than the D band. After being modified with CD, the intensity ratio of the D band to the G band of GO-CD increases from 1.08 to 1.22, indicating an increase in disorder degree in graphitic structures after modification of the amino-CDs on the GO nanosheets. After the reduction step, the intensity ratio of the D band to the G band of RGO-CD slightly increases to 1.30, implying further elimination of carboxyl and carboxylate groups via reduction.42 Thermogravimetric analysis (TGA) was used to measure the weight ratio of CD in RGO-CD (Figure 2d). The RGO nanosheets have an ∼11.3% weight loss after being heated to 700 °C, which is attributed to the decomposition of residual groups on the basal planes and sheet edges of RGO nanosheets. In contrast, the obvious weight loss of RGO-CD from 250 to 700 °C was mainly assigned to the decomposition of CD modified on the RGO nanosheets. The weight ratio of CD in RGO-CD is calculated to be ∼45.1 wt % based on the final weight loss of CD and RGO at 700 °C. The RGO-CD nanosheets can complex with bPEI-Fc to form bPEI-Fc&RGO-CD complexes based on host−guest interaction between CD and Fc (Figure 1b). Five types of bPEIFc&RGO-CD complexes were prepared with feed mass ratios of RGO-CD to bPEI-Fc being 0:1, 0.02:1, 0.04:1, 0.08:1, and 0.15:1. For simplicity, they were denoted as bPEI-Fc&RGOCDm, with m representing the mass ratio of RGO-CD to bPEIFc (m = 0, 0.02, 0.04, 0.08, and 0.15, respectively). Taking the bPEI-Fc&RGO-CD0.04 complexes for instance, cross-sectional atomic force microscopy (AFM) analysis indicates that the thickness of the RGO nanosheets increases from ∼2 nm to ∼3 nm after being mixed with bPEI-Fc (Supporting Information, Figure S5). This result confirms the successful complexation of bPEI-Fc with RGO-CD. To prove that the host−guest interactions between Fc and CD are stable and can be broken upon oxidation of Fc, the bPEI-Fc&RGO-CD0.04 complexes are drop-casted onto silicon substrates, and the resulting films were released from silicon substrates and immersed in water or 70 mM H2O2 solution. The bPEI-Fc&RGO-CD0.04 complex film is stable in water for more than 24 h, but dissolves in 70 mM H2O2 solution within 1 min (Supporting Information, Figure S6). The complex film is stable in water because the host−guest interactions can hold bPEI-Fc and RGO-CD together and prevent their dissolution in water. However, when Fc is oxidized to Fc+ by H2O2, the host−guest interactions of Fc and CD are broken, leading to disassembly of the bPEI-Fc&RGOCD complexes. These results confirm that the host−guest interactions between Fc and CD are the main driving forces for the formation of bPEI-Fc&RGO-CD complexes. The host− guest interactions between bPEI-Fc and RGO-CD can be broken on demand by the oxidation of Fc. LbL Assembly of (PAA/bPEI-Fc&RGO-CDm)*n Films. Positively charged bPEI-Fc&RGO-CDm complexes can be alternately assembled with polyanion PAA for the fabrication of (PAA/bPEI-Fc&RGO-CDm)*n films with controlled thickness (where n represents the number of deposition cycles, and a half number means PAA being the outmost layer). Electrostatic and hydrogen-bonding interactions between bPEI and PAA are the main driving forces for the construction of (PAA/bPEIFc&RGO-CDm)*n films. The film deposition process was investigated by measuring the thickness of the (PAA/bPEIFc&RGO-CDm)*n films with different contents of RGO-CD via a surface profilometer. Figure 3a indicates that the (PAA/ bPEI-Fc&RGO-CD0.04)*n films exhibit an exponential deposition behavior in the initial 8 deposition cycles. Thereafter, there

Figure 2. (a, b) AFM images and cross-sectional analysis of GO (a) and RGO-CD nanosheets (b). (c) Raman spectra of GO, GO-CD, and RGO-CD nanosheets. (d) TGA curves of CD, RGO, and RGOCD nanosheets. TGA was measured at a heating rate of 10 °C min−1 under N2 atmosphere.

structure with a thickness of ∼1 nm. After reacting with the amino-CDs and being reduced by hydrazine, the RGO-CD nanosheets still exhibit a similar structure and surface roughness as the GO nanosheets. However, the thickness of the RGO-CD nanosheet increases to ∼2 nm (Figure 2b), demonstrating the successful modification of GO nanosheets with CDs. Figure 2c presents the typical Raman spectrum of GO with a D band at ∼1330 cm−1 and the G band at ∼1590 cm−1. The relative 7136

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indicate that noncomplexed bPEI-Fc-FITC and PAA-LYC can diffuse throughout the entire (PAA/bPEI-Fc&RGOCD0.04)*45 and (PAA/bPEI-Fc&RGO-CD0.04)*45.5 films. The “in-diffusion” enables deposition of an excess amount of bPEI-Fc, bPEI-Fc&RGO-CD, and PAA. Meanwhile, the “outdiffusion” of these polyelectrolytes and polyelectrolyte complexes can complex many oppositely charged polyelectrolytes to enable rapid deposition of (PAA/bPEI-Fc&RGOCD0.04)*n films. The other LbL-assembled PAA/bPEIFc&RGO-CDm films with RGO-CD contents in bPEIFc&RGO-CDm complexes being 0, 0.02, 0.08, and 0.15 all exhibit exponential growth behaviors similar to (PAA/bPEIFc&RGO-CD0.04)*n films. As shown in Figure 3d, the thickness of the (PAA/bPEI-Fc&RGO-CDm)*45 films decreases with increasing RGO-CD content in bPEI-Fc&RGO-CDm complexes. The RGO-CD nanosheets loaded in the (PAA/bPEIFc&RGO-CDm)*n films can block the diffusion of the polyelectrolytes within the films. Therefore, the increase in RGO-CD in bPEI- Fc&RGO-CDm complexes leads to (PAA/ bPEI-Fc&RGO-CDm)*n films with lower deposition speed. Figure 3d and Figure S7 show the TGA curves of (PAA/bPEIFc&RGO-CDm)*45 films with various RGO-CD contents. These films have a similar thermal behavior as (PAA/bPEI)*45 films except that the residuals at 700 °C are higher. By comparing the residual weights of (PAA/bPEI-Fc&RGOCDm)*45 films with (PAA/bPEI)*45 film at 700 °C, the mass ratios of RGO in the (PAA/bPEI-Fc&RGO-CDm)*45 films are calculated to be about 1.7%, 3.4%, 5.3%, and 8.5%, respectively, at m values of 0.02, 0.04, 0.08, and 0.15. Mechanically Reinforced (PAA/bPEI-Fc&RGO-CDm)*45 Films. The mechanical properties of the (PAA/bPEIFc&RGO-CDm)*45 films were investigated by nanoindentation. A Berkovich diamond indenter with a radius ≤20 nm was utilized to measure the Young’s moduli and hardness of the films in air with ∼25% relative humidity (RH) at 25 °C using the “G-Series continuous stiffness measurement (CSM) standard hardness, modulus, and tip cal” method.6,9,41 The Young’s moduli and the hardness of the (PAA/bPEI-Fc&RGOCD0.04)*45 and (PAA/bPEI-Fc)*45 films as a function of indentation depth are shown in Figure 4a. To eliminate the influence of film surface and the underlying substrates, the Young’s moduli and the hardness in the plateau regions with indentation depth between ∼300 to 400 nm are used to calculate the “real” Young’s moduli and hardness of the films. The changes of Young’s moduli and hardness of the (PAA/ bPEI-Fc&RGO-CDm)*45 films as a function of RGO-CD contents in bPEI-Fc&RGO-CDm complexes are shown in Figure 4b. Compared with (PAA/bPEI-Fc)*45 films, the

Figure 3. (a) Thickness of (PAA/bPEI-Fc&RGO-CD0.04)*n films as a function of the number of film deposition cycles. Inset in (a) is the cross-sectional SEM image of a (PAA/bPEI-Fc&RGOCD0.04)*45 film. (b) CLSM image of a (PAA/bPEI-Fc&RGOCD0.04)*45 film with an outmost layer of PAA-LYC. (c) CLSM image of a (PAA/bPEI-Fc&RGO-CD0.04)*45.5 film with an outmost layer of bPEI-Fc-FITC&RGO-CD0.04 complexes. (d) Thickness of (PAA/bPEI-Fc&RGO-CDm)*45 film as a function of mass ratio of RGO-CD to bPEI-Fc in bPEI-Fc&RGO-CDm complexes. (e) TGA curves of (PAA/bPEI-Fc&RGO-CDm)*45 films with m being 0, 0.04, and 0.15, respectively.

is a rapid and nearly linear growth with an increment of ∼630 nm per deposition cycle. The PAA/bPEI-Fc&RGO-CD0.04 films reach a thickness of 28.0 ± 0.4 μm after 45 deposition cycles. This is consistent with the thickness measured from cross-sectional SEM images of (PAA/bPEI-Fc&RGOCD0.04)*45 films (inset in Figure 3a). Based on previous studies, the exponential growth of LbL-assembled polyelectrolyte films mainly originates from the “in-and-out” diffusion of the polyelectrolytes during film deposition process.43,44 PAA grafted with lucifer yellow cadaverine (LYC) (denoted as PAALYC) and the complexes labeled with fluorescein isothiocyanate (FITC) (denoted as bPEI-Fc-FITC&RGO-CD0.04) were deposited as the outmost layer of the (PAA/bPEI-Fc&RGOCD0.04)*45 and (PAA/bPEI-Fc&RGO-CD0.04)*45.5 films to investigate the diffusion of the polyelectrolytes within the films. The cross-sectional confocal laser scanning microscopy (CLSM) images of (PAA/bPEI-Fc&RGO-CD0.04)*45/PAALYC and (PAA/bPEI-Fc&RGO-CD 0.04 )*45.5/bPEI-FcFITC&RGO-CD0.04 films presented in Figure 3b and 3c clearly

Figure 4. (a) Young’s moduli and hardness of (PAA/bPEI-Fc&RGO-CD0.04)*45 and (PAA/bPEI-Fc)*45 films as a function of penetration depth at 25 °C and 25% RH. (b) Relationship of Young’s modulus and hardness of (PAA/bPEI-Fc&RGO-CDm)*45 films with the mass ratio of RGO-CD to bPEI-Fc in bPEI-Fc&RGO-CDm complexes. 7137

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Figure 5. (a) SEM images of a (PAA/bPEI-Fc&RGO-CD0.04)*45 film with an ∼70 μm wide cut. (b) The film in (a) after being immersed in water for 180 min. (c) The film in (a) after being immersed in 70 mM H2O2 solution for 10 min. (d) Changes in storage modulus (at a frequency of 1 Hz) of a (PAA/bPEI-Fc&RGO-CD0.04)*45 film over five cycles of oxidation in H2O2 solution and reduction in GSH solution.

In contrast, the (PAA/bPEI-Fc&RGO-CD0.04)*45 films with an ∼70 μm wide cut that penetrates to the underlying substrates failed to completely heal the cut after being immersed in water for 3 h (Figure 5a and 5b). This result demonstrates that the mobility of the bPEI-Fc polyelectrolytes within the (PAA/ bPEI-Fc&RGO-CD0.04)*45 films is significantly suppressed by the RGO nanofillers because of their strong host−guest interactions. 2 8 However, the (PAA/bPEI-Fc&RGOCD0.04)*45 films can completely heal a cut of ∼70 μm wide after being immersed in 70 mM H2O2 solution for 10 min (Figure 5c and Figure S9). Subsequent immersion of the (PAA/bPEI-Fc&RGO-CD0.04)*45 films into aqueous glutathione (GSH) solution (70 mM) for 30 min restores the original robustness of the films. The mechanism of the oxidation/reduction-assisted healing of the (PAA/bPEIFc&RGO-CD0.04)*45 films is illustrated in Figure 1b. The healing process includes cut healing in an oxidation condition and restoration of the original mechanical properties of the films via a reduction reaction. When the (PAA/bPEI-Fc&RGOCD0.04)*45 films with cuts are immersed in H2O2 solution, the host−guest interactions between bPEI-Fc and RGO-CD are broken because of the oxidation of bPEI-Fc to bPEI-Fc+. As a result, the restriction exerted upon bPEI-Fc polyelectrolytes by RGO-CD nanosheets is eliminated. Meanwhile, the absorbed water within the (PAA/bPEI-Fc&RGO-CD0.04)*45 films can markedly weaken the electrostatic and hydrogen-bonding interactions between bPEI-Fc+ and PAA. Therefore, the mobility of the bPEI-Fc and PAA polyelectrolytes is significantly enhanced in diluted H2O2 solution. The bPEI-Fc and PAA polyelectrolytes with high mobility can migrate across the damaged area to fill the cuts, and the electrostatic and hydrogen-bonding interactions can be reformed upon drying the films. In this way, cuts in (PAA/bPEI-Fc&RGO-CD0.04)*45 films are healed. After healing of cuts, the (PAA/bPEIFc&RGO-CD0.04)*45 films are transferred into aqueous GSH solution The bPEI-Fc+ is reduced to bPEI-Fc in GSH solution, and the host−guest interactions of bPEI-Fc and RGO-CD are reconstructed to restore the original mechanical robustness of the (PAA/bPEI-Fc&RGO-CD0.04)*45 films. The oxidation/reduction-assisted healing mechanism of the (PAA/bPEI-Fc&RGO-CD0.04)*45 films can be verified by

Young’s modulus and hardness of the (PAA/bPEI-Fc&RGOCDm)*45 films increase with increasing RGO-CD and reach a maximum Young’s modulus of 17.2 ± 1.9 GPa and a hardness of 1.00 ± 0.30 GPa for the (PAA/bPEI-Fc&RGO-CD0.04)*45 films, which are 1.7- and 2.4-times higher than (PAA/bPEIFc)*45 films. Thereafter, the Young’s modulus and hardness of the (PAA/bPEI-Fc&RGO-CDm)*45 films decrease with further increases in the RGO-CD in bPEI-Fc&RGO-CDm complexes. The Young’s modulus and hardness of the (PAA/bPEIFc&RGO-CD0.15)*45 films are even lower than those of (PAA/bPEI-Fc)*45 films. The mechanical robustness of the (PAA/bPEI-Fc&RGO-CD0.04)*45 films is comparable to some previously reported nacre-mimetic composite materials that do not have healing capacity.45−47 The mechanical reinforcement of the (PAA/bPEI-Fc&RGO-CD0.04)*45 films is owing to the homogeneous dispersion of RGO nanosheets within the films and the host−guest interactions between RGO-CD nanosheets and the PAA/bPEI-Fc films. The host−guest interactions between RGO-CD nanofillers and the films can effectively transfer the stress from the polymers to nanofillers and enhance Young’s modulus and the hardness of the films. However, when the ratio of RGO-CD in bPEI-Fc&RGO-CD complexes exceeds 0.04, the Young’s modulus and the hardness of the resulting (PAA/bPEI-Fc&RGO-CDm)*45 films decrease because the aggregation of RGO-CD nanofillers occurs (Supporting Information, Figure S8). Aggregation of RGOCD nanofillers produces defects within the films, resulting in a decrease in the film’s mechanical performance. With increasing RGO-CD content, aggregation of nanofillers becomes more obvious, and more defects are formed within the films. Therefore, the (PAA/bPEI-Fc&RGO-CD0.15)*45 films have the lowest Young’s modulus and hardness. Healing of the (PAA/bPEI-Fc&RGO-CD0.04)*45 Films. The healability of the (PAA/bPEI-Fc&RGO-CD0.04)*45 films with the highest Young’s modulus and hardness is investigated. In our previous study, we demonstrated that the LbLassembled PAA/bPEI films can heal cuts of several tens of micrometers wide after 5 min of immersion in water.6 Although LbL-assembled healable films comprised of CD-conjugated PEI and adamantine-conjugated PAA are fabricated, the resultant films are soft and easy to be healed in the presence of water.48 7138

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ACS Nano measuring the storage moduli of the films in water, H2O2 and GSH solutions. A flat-ended cylindrical punch made of diamond with a diameter of 108.5 μm was used to test the storage moduli of the (PAA/bPEI-Fc&RGO-CD0.04)*45 films in solutions according to the “G-Series XP CSM flat punch complex modulus” method.6,9,41 The storage moduli of the (PAA/bPEI-Fc&RGO-CD0.04)*45 films in water, H2O2, and GSH solution are plotted as a function of frequency (Supporting Information, Figure S10). The storage modulus under a low frequency of 1 Hz was selected to illustrate the changes in film mechanical properties in different solutions because healing of the (PAA/bPEI-Fc&RGO-CD0.04)*45 films was performed in undisturbed solutions. Dipping the (PAA/ bPEI-Fc&RGO-CD0.04)*45 films in water can lead to a significant decrease of modulus from 17.2 ± 1.9 GPa of dry state to ∼117.5 MPa in water. This is because the electrostatic and hydrogen interactions between bPEI-Fc and PAA can be largely broken by the absorbed water that acts as plasticizer to soften the films.6,9,41 A further decrease in the storage modulus to ∼34.3 MPa is observed after transferring the (PAA/bPEIFc&RGO-CD0.04)*45 films into the H2O2 solution. This result confirms that the host−guest interactions between RGO-CD and bPEI-Fc are broken. Therefore, bPEI-Fc and PAA of the (PAA/bPEI-Fc&RGO-CD0.04)*45 films in H2O2 solution have high mobility to facilitate the healing process. Further transferring the (PAA/bPEI-Fc&RGO-CD0.04)*45 films into GSH solution can restore the storage modulus to ∼110.6 MPa, confirming reconstruction of the host−guest interactions between bPEI-Fc and RGO-CD nanosheets. As shown in Figure 5d, alternate dipping of the (PAA/bPEI-Fc&RGOCD0.04)*45 films in H2O2 and GSH solutions leads to a periodic change in storage modulus, suggesting that the host− guest interactions between bPEI-Fc and RGO-CD nanosheets are highly reversible and can be repeated multiple times. This is important for multiple rounds of healing in (PAA/bPEIFc&RGO-CD0.04)*45 films. After GSH reduction, the storage modulus of the (PAA/bPEI-Fc&RGO-CD0.04)*45 films can be restored to more than 90% of its original value after five oxidation/reduction cycles. After one damage/healing cycle, the Young’s modulus and hardness of the healed (PAA/bPEIFc&RGO-CD0.04)*45 films can restore to 16.9 ± 2.2 GPa (∼98% of the original value) and 0.95 ± 0.26 GPa (∼95% of the original value), respectively. The reversible host−guest interactions between bPEI-Fc and RGO-CD endow the (PAA/ bPEI-Fc&RGO-CD0.04)*45 films with rapid and highly efficient healing functions. Meanwhile, the (PAA/bPEI-Fc&RGOCD0.04)*45 films can heal cuts up to ∼70 μm wide at the same location (within an ∼600 μm lateral region) at least five times with alternate immersion in 70 mM H2O2 and GSH solutions (Supporting Information, Figure S11). The (PAA/ bPEI-Fc&RGO-CD0.04)*45 films can heal a cut with a maximum width of ∼90 μm by sequentially dipping the films into H2O2 and GSH solutions (Supporting Information, Figure S12). Thicker (PAA/bPEI-Fc&RGO-CD0.04)*n films can heal wider cuts.

CD0.04)*45 films, in which the well dispersed RGO-CD nanosheets have reversible host−guest interactions with bPEIFc, are mechanically robust and have a Young’s modulus of 17.2 ± 1.9 GPa and a hardness of 1.0 ± 0.3 GPa. Because the host− guest interactions between RGO-CD nanosheets and the matrix PAA/bPEI-Fc films can be broken when bPEI-Fc is oxidized into bPEI-Fc+ in H2O2, the mechanically robust (PAA/bPEI-Fc&RGO-CD0.04)*45 films can reversibly and highly efficiently heal cuts of several tens of micrometers wide similar to films without RGO nanosheets. In the presence of GSH, the bPEI-Fc+ is reduced into bPEI-Fc, which rebuilds the host−guest interactions between RGO-CD nanosheets and PAA/bPEI-Fc films and restores the original mechanical robustness of the (PAA/bPEI-Fc&RGO-CD0.04)*45 films. Repeated healing of cuts at the same region of the (PAA/ bPEI-Fc&RGO-CD0.04)*45 films can be accomplished because of the highly reversible host−guest interactions between bPEIFc and RGO-CD governed by oxidation/reduction. The introduction of reversible host−guest interactions between RGO nanofillers and PAA/bPEI polyelectrolyte films solves the conflict between highly efficient healing capability and excellent mechanical robustness. This strategy for fabrication of healable and mechanically robust (PAA/bPEI-Fc&RGO-CD0.04)*45 films is believed to be applicable for the design of healable bulk polymeric composites with enhanced mechanical properties. Reversible interaction between nanofillers and polymer matrices is not limited to β-CD and ferrocene. Light responsive host−guest interactions are believed to be applicable to achieve healing of nanofiller-reinforced polymer composites. Therefore, this work provides a simple and practically useful strategy for the fabrication of intrinsically healable polymer composite materials with high healing efficiency and good mechanical robustness by introducing reversible interactions between nanofillers and polymer matrices.

EXPERIMENTAL SECTION Materials. All the chemicals were used as received unless otherwise specified. PAA (Mw ≈ 450,000 g mol−1), bPEI (Mw ≈ 750,000 g mol−1), N-hydroxysuccinimide (NHS), FITC, and 1-ethyl-3-(3(dimethylamino)propyl)carbodiimide hydrochloride (EDC) were obtained from Sigma-Aldrich. LYC was purchased from Biotium. βCD was purchased from Sinopharm Chemical Reagent Co., Ltd. and was purified by recrystallizations in water before use. Ferrocenecarboxaldehyde was obtained from Energy Chemical. Graphite with an average particle size of ∼50 μm and a purity of >99% was supplied from China Graphite Co., Ltd. GO was synthesized according to the Hummer method.49 PAA-LYC and bPEI-Fc-FITC were prepared following previously reported procedures.6 Synthesis of bPEI-Fc. bPEI-Fc was synthesized according to a literature method.50 Ferrocenecarboxaldehyde (0.10 g, 0.47 mmol) dissolved in methanol (1.0 mL) was added dropwise into bPEI (4.0 g) in methanol (100.0 mL). The resulting solution was stirred for 2 h in an ice−water bath. Then, NaBH4 (0.020 g, 0.53 mmol) was added into the solution. The mixture was continuously stirred for 1 h until its color turned from dark red to light yellow. Next, a rotary evaporator was used to remove most of the methanol. The residue was extracted with deionized water and washed with diethyl ether to remove the unreacted ferrocenecarboxaldehyde and ferrocenylmethanol. The resulting solution was dialyzed in a dialysis bag against deionized water for 1 week. Finally, pure bPEI-Fc product was obtained via lyophilization. Synthesis of RGO-CD. Amino-CD was synthesized according a literature method.51 GO (0.60 g) was dispersed in water (300 mL) by ultrasonication for more than 2 h. KOH (0.40 g, 7.0 mmol) and amino-CD (2.0 g, 1.8 mmol) were then added into aqueous GO dispersion, and the mixture was stirred in an 80 °C oil bath for 24 h.

CONCLUSIONS In summary, by exploiting the reversible interactions between nanofillers and the matrix polymer films, we have developed a novel strategy for the fabrication of nanofiller-reinforced intrinsic healable polymer composite films that have simultaneous excellent mechanical robustness and highly efficient healing properties. The LbL-assembled (PAA/bPEI-Fc&RGO7139

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ACS Nano After the dispersion was cooled to room temperature, EDC (0.17 g, 0.90 mmol), NHS (0.10 g, 0.90 mmol), and amino-CD (1.0 g, 0.85 mmol) were added to the reaction solution under strong agitation and sonication. The dispersion was stirred at room temperature for 24 h at pH 8.0. The as-prepared GO-CD was purified by centrifugation and resuspension in water three times. After that, GO-CD was reduced with 0.5 mL hydrazine monohydrate (98% aq.) at 95 °C for 12 h to obtain RGO-CD. The RGO-CD was further purified by repeated centrifugation and resuspension in water. The RGO-CD powders were obtained after drying under vacuum.51,52 Preparation of bPEI-Fc&RGO-CDm Complexes. The bPEIFc&RGO-CD0.04 complexes were prepared by dropwise addition of 50 mL of aqueous dispersion of RGO-CD (0.16 mg mL−1) into 50 mL aqueous solution of bPEI-Fc (4.0 mg mL−1) under strong stirring. The mixture was allowed to stir for 24 h at room temperature, and then pH of the mixture was adjusted to 9.5. The final concentrations of bPEI-Fc and RGO-CD in the complex dispersion were 2 mg mL−1 and 0.08 mg mL−1, respectively. These complexes were denoted as bPEI-Fc&RGOCD0.04 for simplicity. In the control experiments, aqueous dispersions of bPEI-Fc&RGO-CD 0.02 , bPEI-Fc&RGO-CD 0.08 , and bPEIFc&RGO-CD0.15 complexes were prepared similarly as the aqueous dispersion of bPEI-Fc&RGO-CD0.04 complexes. Film Fabrication. Silicon and glass substrates were cleaned by immersion in heated piranha solution (30% H2O2/98% H2SO4 = 1/3, v/v) for 1 h. After water rinse and N2 dry, the resulting substrates were immersed in an aqueous PDDA solution (1 mg mL−1) for 20 min to obtain a positively charged surface. The LbL deposition of PAA/bPEIFc&RGO-CD films can be described as follows: The PDDA-modified substrate was alternately dipped into aqueous PAA (2 mg mL−1, pH 3.5) solution and aqueous bPEI-Fc&RGO-CD dispersion (pH 9.5) for 15 min each with intermediate water washing in three water baths for 2, 1, and 1 min each. The deposition of PAA and bPEI-Fc&RGO-CD was repeated until the desired number of deposition cycles was obtained. In control experiments, the (PAA/bPEI-Fc)*n films were fabricated in a similar way as those of (PAA/bPEI-Fc&RGO-CD)*n films by replacing bPEI-Fc&RGO-CD complexes with bPEI-Fc. Instruments and Characterizations. 1H NMR spectra were recorded on a Bruker 500 Hz NMR spectrometer. AFM images were taken with a commercial instrument, Veeco Company Nanoscope IV, in tapping mode. TGA measurements were performed on a Q500 Thermogravimetric Analysis (TA Instruments, USA) with a heating rate of 10 °C min−1 under a N2 atmosphere. Raman spectra were recorded on a JY-T6400 Super LabRam-II spectrometer (Horiba Jobin Yvon, France). The source excitation laser wavelength is 532 nm. Film thickness was measured by a Dektak 150 surface stylus profilometer (Vecco) using a 5 μm stylus tip with 3 mg stylus force. Cross-sectional CLSM images were taken by an Olympus Fluoview FV1000 confocal laser scanning microscope. SEM images were obtained using a field emission scanning electron microscope (JEOL JSM 6700F). Mechanical properties of the films attached to silicon substrates were measured with an Agilent Nano Indenter G200 with an XP-style actuator and continuous stiffness measurement (CSM) method. Young’s modulus and hardness were measured with a Berkovich diamond indenter with a radius ≤20 nm at 25 °C and 25% RH by the “G-Series CSM standard hardness, modulus, and tip cal” test method. The storage modulus was measured in water with a flat-ended cylindrical diamond indenter by the “G-Series XP CSM flat punch complex modulus” test method. The details of the nanoindentation measurements are available in our previous publications.9,41

quency, and relevant AFM images, digital images, topview and cross-sectional SEM images and optical images (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Junqi Sun: 0000-0002-7284-9826 Notes

The authors declare no competing financial interest.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b02970. Detailed experimental procedures, 1H NMR of bPEI-Fc, TGA curves of (PAA/bPEI-Fc&RGO-CDm)*45 films with m being 0.02 and 0.08, storage moduli of a (PAA/ bPEI-Fc&RGO-CD0.04*45 film as a function of fre7140

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