Stiffer but More Healable Exponential Layered Assemblies with Boron

Publication Date (Web): September 20, 2016 ... covalent bonding; on the other hand, during a water-enabled self-healing process, these two-dimensional...
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Stiffer but More Healable Exponential Layered Assemblies with Boron Nitride Nanoplatelets Xiaodong Qi,† Lei Yang,‡ Jiaqi Zhu,‡ Ying Hou,*,† and Ming Yang*,† †

Key Laboratory of Microsystems and Micronanostructures Manufacturing and ‡Center for Composite Materials and Structures, Harbin Institute of Technology, 2 Yikuang Street, Harbin 150080, China S Supporting Information *

ABSTRACT: Self-healing ability and the elastic modulus of polymeric materials may seem conflicting because of their opposite dependence on chain mobility. Here, we show that boron nitride (BN) nanoplatelets can simultaneously enhance these seemingly contradictory properties in exponentially layerby-layer-assembled nanocomposites as both surface coatings and free-standing films. On one hand, embedding hard BN nanoplatelets into a soft hydrogen bonding network can enhance the elastic modulus and ultimate strength through effective load transfer strengthened by the incorporation of interfacial covalent bonding; on the other hand, during a waterenabled self-healing process, these two-dimensional flakes induce an anisotropic diffusion, maintain the overall diffusion ability of polymers at low loadings, and can be “sealing” agents to retard the out-of-plane diffusion, thereby hampering polymer release into the solution. A detailed mechanism study supported by a theoretical model reveals the critical parameters for achieving a complete self-healing process. The insights gained from this work may be used for the design of high-performance smart materials based on other twodimensional fillers. KEYWORDS: layer-by-layer assembly, nanocomposites, mechanical properties, self-healing, BN nanoplatelets

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one thing that may be overlooked is that the high diffusion ability of polymers in these films also implies their release into water due to the concentration gradient,52 which may affect their self-healing properties. In addition, the dynamic nature of e-LBL films also comes with the compromise of their mechanical properties, which are generally much weaker than their more compact linear counterparts. A key mechanism for plants as well as many other living beings to self-repair is the fast self-sealing process, which can reduce water loss by mechanical deformation of cells and the formation of a drought layer of dead cells induced by the oxidative burst.53 Nanosheets such as clays,40 graphene oxide,27 alumina,41 and layered double hydroxide54 tend to align themselves to the substrates during LBL assembly, resulting in hybrid structures with excellent barrier properties.55 Even for rapidly growing e-LBL films, a preferred orientation of nanosheets can be detected despite a less defined structure.56,57 We reason that the anisotropic structure of these twodimensional flakes may enable them as “sealing” agents in eLBL films, reducing the unwanted polymer release during water-induced self-healing. The insertion of hard nanosheets or

he design of artificial structures with self-repair ability allows the emergence of smart functional materials,1−10 blurring the boundary between living beings and manmade objects.11−16 A great challenge that still exists in materials science is how to produce stiffer but more healable structures.17−19 From the technical aspect, such materials can be more robust during processing and withstand more load before the self-healing mechanism needs to be activated, allowing for longer service time.15,20,21 The successful design concept for achieving such a task may also help address the fundamental dilemma to synergistically improve conflicting properties such as stiffness and healing with opposite dependence on molecular dynamics.22−25 An interesting biomimetic approach is layer-by-layer (LBL) assembly26−39 with a great potential for replicating hierarchical inorganic−organic hybrid structures in nature.40,41 Of particular interest is exponential LBL (e-LBL) assembly, which can produce micrometer thick films much more efficiently compared with traditional linear LBL growth.42,43 The intrinsic dynamic bonding between different constituents in e-LBL films has allowed the emergence of the self-healing ability activated by water, providing a useful tool to fabricate self-healable surface coatings against other three-dimensional structures.44−51 Similar to other systems, self-healing of e-LBL films depends on the high mobility of polymers.46 However, © 2016 American Chemical Society

Received: July 6, 2016 Accepted: September 20, 2016 Published: September 20, 2016 9434

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Figure 1. (a) Schematic drawing for the formation of B−O−C bonds at the edges of BN nanoplatelets. (b) Atomic force microscopy image of BN1μm dispersed in water using PVA145k (insets: thickness profiles and optical images of PVA145k−BN1μm dispersion showing Tyndall effect). B1s X-ray photoelectron spectra for BN1μm dispersed (c) directly in water and (d) using PVA145k (the peaks are fitted using a Gaussian profile).

nanoplatelets in the dynamic phase may also bring multiphase design with the interplay of different chemical bonds similar to that used by nature,20,58,59 affording superior mechanical properties. In this work, we show that boron nitride (BN) nanoplatelets can simultaneously improve the mechanical properties (stiffness/strength) and self-healing ability of e-LBL films, namely, poly(vinyl alcohol) (PVA)/tannic acid (TA), as both surface coatings and free-standing films. The increase of elastic modulus and ultimate strength can be attributed to the construction of a hard−soft interface effective for load transfer. More interestingly, their plate-like structures with impermeable nature can reduce the out-of-plane diffusion so that polymers releasing into the solution can be suppressed due to the “sealing” effect. The overall diffusion ability of polymers is maintained at low filler concentrations with an improved inplane diffusion. A theoretical model based on mass transfer was used to correlate different experimental parameters with the self-healing behavior. The proposed mechanism can be generalized for other e-LBL-based self-healing systems.

PVAs with two different molecular weights (Mw = 145 000 and 47 000 g/mol), denoted as PVA145k and PVA 47k, respectively, were used to disperse BN nanoplatelets with two different sizes, namely, BN0.5μm and BN1μm (Supporting Information, Figure S1) in water. As similar results were obtained for different combinations, only the data for dispersing BN1μm using PVA145k are shown here, which should represent the typical dispersion results and mechanism. A 2 h bath sonication was found to be enough to produce a stable dispersion of PVA145k-stabilized multilayer BN1μm nanoplatelets with thicknesses of ca. 10 nm (Figure 1b and its inset in the top corner and Figure S1e,f). These dispersions (0.3 mg/mL) are stable for up to 1 week and show a typical Tyndall effect (the inset in the bottom corner in Figure 1b). In order to investigate the dispersion mechanism, B1s X-ray photoelectron spectra of BN1μm nanoplatelets dispersed in water with or without PVA145k were compared. For BN1μm nanoplatelets dispersed directly in water, B1s spectra can be fitted by two curves using a Gaussian profile (Figure 1c). The main peak with a binding energy of ca. 190.1 eV can be attributed to B−N bonds, and the other one at ca. 191.2 eV can be assigned to B−OH bonds, which is consistent with IR spectra (Supporting Information, Figure S2). However, for BN1μm nanoplatelets dispersed using PVA145k, an additional peak around 192.3 eV was observed, which can be associated with the presence of B−O−C bonds. It is likely the peripheral B−OH groups can react with PVA145k through an etherification reaction, which can be further confirmed by IR spectra (Figure S2). Besides the covalent grafting at the edge sites, PVA145k may also form hydrogen bonds with BN1μm nanoplatelets, further promoting the dispersion. Thermogravimetric analysis (TGA) showed that there are ca. 6% PVA145k grafted on BN1μm nanoplatelets (Figure S3). Dispersing BN nanoplatelets using PVA can be

RESULTS AND DISCUSSION BN Nanoplatelet Dispersion with PVA. BN with exceptional mechanical, electrical, and thermal properties has been extensively used for the synthesis of polymer nanocomposites.60−62 Our particular reason for such a choice is based on the recognition of the presence of B−OH groups on BN nanoplatelets in water63 that may readily react with PVA, forming ether bonds (Figure 1a). This is important not only because it may help disperse BN nanoplatelets but also because the strong covalent bonds allow for a better synergy between inorganic phase and polymers.40 9435

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Figure 2. QCM results for the growth of first 10 bilayers of (a) PVA145k/TA, PVA145k-BN0.5μm/TA, and PVA145k-BN1μm/TA and (b) PVA47k/TA, PVA47k-BN0.5μm/TA, and PVA47k-BN1μm/TA. Cross-sectional scanning electron microscopy (SEM) images of (c) (PVA145k/TA)50 and (d) (PVA145k-BN1μm/TA)50. The red lines in (c,d) indicate the film thickness (university logo used with permission). High-resolution crosssectional SEM images of (e) (PVA145k-BN0.5μm/TA)50 and (f) (PVA145k-BN1μm/TA)50. The arrows indicate the presence of BN nanoplatelets with a good dispersion and preferred in-plane alignment in the polymer matrix.

accumulation and later rapid buildup of materials is similar to the growth patterns of many e-LBL films.42,43,46,57 Importantly, the film growth rate is decreased by the addition of BN nanoplatelets, with the larger one having a more obvious effect (Figure 2a,b). Accordingly, it appears that (PVA145k/ TA)50 (where 50 represents the number of bilayers) is ca. 80 μm thick (Figure 2c) compared to ca. 40 μm for (PVA145kBN1μm/TA)50 (Figure 2d). Similarly, the thickness of (PVA47kBN1μm/TA)50 (ca. 20 μm) is also one-half of that of (PVA47k/ TA)50 (ca. 40 μm) (Figure S4). The thickness reduction can be attributed to the presence of BN nanoplatelets, which can block polymer diffusions in and out of the film during material deposition.75 This finding is, however, different from previous work showing that the introduction of clay nanosheets by linear growth has a minimal effect on the rate of exponential growth.56,57 It is likely that direct incorporation of BN nanoplatelets during e-LBL assembly may result in more effective inhibition of the out-of-plane diffusion. Such an effect can be further enhanced by their good dispersion in and preferred alignment along the in-plane direction of the film (Figure 2e,f). In addition, the presence of BN nanoplatelets also affected the transparency of the film: for example, compared with (PVA145k/TA)50 (inset in Figure 2c), (PVA145k-BN1μm/

more suitable for e-LBL assembly than previous attempts using polar organic solvents64,65 or basic additions.66,67 e-LBL Assembly. The stable dispersion of PVA-stabilized BN nanoplatelets (PVA-BN) provides an opportunity to incorporate them directly into e-LBL films, different from previous work which achieved the incorporation of clays using an additional linear growth cycle.56,57 Such a difference makes our approach more time-efficient and may vary the kinetics of film growth. Here, PVA was combined with TA, which not only carries multiple hydrogen bonding donors/acceptors but also provides antibacterial and antioxidant activities,68−70 an important characteristic for self-defensive coatings.68,71 The pH of both PVA and TA solutions were kept at 2 to maximize the hydrogen bonding interactions, which are the main driving force for film growth, similar to previous studies.72−74 The growths of PVA/TA and PVA-BN/TA multilayers were monitored by quartz crystal microbalance (QCM). With the number of bilayers increasing, the frequency of QCM at the stable state nonlinearly decreased for all of the films (Figure 2a,b). As the mass coverage increment is proportional to the negative frequency change, a nonlinear or exponential growth of multilayered films was achieved. The slow initial 9436

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Figure 3. Load−displacement curves for (a) (PVA145k/TA)50, (PVA145k-BN0.5μm/TA)50, and (PVA145k-BN1μm/TA)50 and (b) (PVA47k/TA)50, (PVA47k-BN0.5μm/TA)50, and (PVA47k-BN1μm/TA)50. (c) Stress−strain curves for free-standing (PVA145k/TA)50, (PVA145k-BN0.5μm/TA)50, and (PVA145k-BN1μm/TA)50. (d) Load−displacement curves obtained from the crack area after self-healing for (PVA47k-BN1μm/TA)50 and (PVA145k-BN1μm/TA)50.

TA has made their free-standing films very brittle. On the other hand, multilayers consisting of PVA145k can be easily peeled off from the substrate due to the highly entangled structure of PVA145k, which can largely retain the flexibility. It was found that Young’s modulus is increased from 1.28 ± 0.031 GPa for (PVA145k/TA)50 to 2.03 ± 0.039 and 3.43 ± 0.044 GPa for (PVA145k-BN0.5μm/TA)50 and (PVA145k-BN1μm/TA)50, respectively (Figure 3c and Table S2). The ultimate strength is improved from 49.14 ± 2.35 MPa to 79.74 ± 1.97 and 106.97 ± 1.54 MPa, respectively (Figure 3c and Table S2). The different modulus from those obtained by nanoindentation may indicate the anisotropy of these films due to the use of lowdimensional fillers.77 The Young’s modulus and ultimate strength of (PVA145k-BN1μm/TA)50 (3.43 ± 0.044 GPa and 106.97 ± 1.54 MPa) outperform those of clay-reinforced e-LBL films,56 representing the highest values among these rapidly growing multilayers.78 The presence of BN nanoplatelets in the hydrogen bonding network formed by PVA and TA, in principle, is a multiphase system.25 Previous study used “sacrificial” noncovalent bonds to improve mechanical properties of covalent self-healing materials and found that the stiffness is dominantly controlled by the covalent phase.20 Here, the covalent grafting of PVA on BN nanoplatelets (Figure 1), which should have a strong impact on load transfer, was used to improve the mechanical properties of the dynamic phase. Similar to the nanofiller effect,27,40,56,57 the larger the aspect ratios of nanoplatelets, a more effective load transfer can be expected, which can result in more obvious increases of modulus and ultimate strength (Figure 3 and Tables S1 and S2).79 The much improved

TA)50 is less transparent due to the light scattering effect of BN nanoplatelets (inset in Figure 2d). TGA results showed that there are similar amounts of BN nanoplatelets (ca. 2.5%) in the films with different combinations (Figure S5). It seems that the incorporation of BN nanoplatelets is dominantly controlled by the interactions between PVA and TA, and the molecular weights and particle sizes play a less important role. Mechanical Properties. The mechanical properties of these multilayers were first tested by nanoindentation. The micrometer scale thickness of these films has eliminated the substrate effect during the test, providing meaningful data for evaluation. It was found that the addition of BN nanoplatelets has resulted in an improvement of both elastic modulus and hardness (Figure 3a,b and Table S1) dependent on their lateral sizes. For example, the elastic modulus of (PVA145k/TA)50 is increased from 8.6 ± 0.060 GPA to 9.5 ± 0.015 and 12.9 ± 0.450 GPa for (PVA145k-BN0.5μm/TA)50 and (PVA145k-BN1μm/ TA)50, respectively (Table S1). Meanwhile, the hardness is improved from 0.25 ± 0.003 GPA to 0.38 ± 0.003 and 0.60 ± 0.039 GPa, respectively (Table S1). A similar trend was observed for films consisting of PVA47k (Figure 3b and Table S1), which is consistent with earlier reports on e-LBL films with nanofillers such as clays56,57 and calcium carbonate.76 These values are also comparable with other e-LBL films such as polyethylenimine/poly(acrylic acid) multilayers (PEI/ PAA).44,46 As the nanoindentation result typically relates to the out-ofplane mechanical properties, we also tried to use the tensile test to investigate the in-plane characteristics. However, the high density of hydrogen bonding cross-links between PVA47k and 9437

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Figure 4. Optical and SEM images of (a) (PVA145k/TA)50, (b) (PVA145k-BN1μm/TA)50, (c) (PVA47k/TA)50, and (d) (PVA47k-BN1μm/TA)50 coatings with a 50 μm wide cut through the film before and after immersion in water for different lengths of time.

the damaged region within 10 min (Figure 4d). Similar properties have been found for (PVA47k-BN0.5μm/TA)50 (Figure S6b). The scratch/healing cycle can be repeated at the same place for (PVA145k-BN1μm/TA)50 up to eight times (Figure S7). However, for (PVA47k-BN1μm/TA)50, at least 12 cycles can be easily achieved (Figure S8). It is likely that the free diffusing PVA145k supplied through the exchange with immobilized polymers is limited by the highly entangled structure, and as a result, its concentration may become insufficient for a full recovery after the repeated consumptions in filling the scratch as well as the continuous release into the solution during the contact with water. On the other hand, PVA47k with a higher exchange rate can maintain a high concentration of free polymers with better diffusion ability, allowing the complete self-healing for many more cycles. As an additional measure to evaluate the self-healing performance, we took the advantage of nanoindentation to check if the healed surface has similar mechanical properties with unscratched parts. This is important because a morphology recovery does not necessarily mean a property regeneration. It was found that, after the self-healing process, the elastic modulus of the recovered area can reach 12.1 ± 0.615 GPa for (PVA145k-BN1μm/TA)50 and 10.0 ± 0.517 GPa

mechanical properties also benefit from the good dispersion of BN nanoplatelets in the polymer matrix as ensured by the LBL assembly process (Figure 2e,f). Self-Healing Properties as Surface Coatings. The noncovalent intermolecular interactions (dynamic hydrogen bonding) involved here imply that our films may be responsive to external stimuli and re-form upon damage.80 To investigate the self-healing properties of our films, a 50 μm wide scratch was made through the film on the glass substrate. Water was used to initiate the self-healing process. (PVA145k/TA)50 was not able to fully heal the scratch even after 90 min (Figure 4a). A close scanning electron microscopy (SEM) observation clearly showed the shallow surface around the scratch area with rough textures (Figure 4a). Interestingly, after exposure to water for 10 min, (PVA145k-BN1μm/TA)50 film can heal the scratch much better than (PVA145k/TA)50, and after 90 min, a nearly complete healing can be achieved (Figure 4b). Similar to (PVA145k-BN1μm/TA)50, (PVA145k-BN0.5μm/TA)50 also shows a much improved self-healing performance compared with that of (PVA145k/TA)50 (Figure S6a). (PVA47k/TA)50 shows better self-healing ability compared to that of (PVA145k/TA)50; however, a full recovery is still not possible (Figure 4c). Encouragingly, (PVA47k-BN1μm/TA)50 can absolutely self-repair 9438

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Figure 5. (a−c) Cross-sectional fluorescent optical images of different films with PVA-c as the top layer: (a) (PVA145k/TA)50, (b) (PVA145kBN0.5μm/TA)50, and (c) (PVA145k-BN1μm/TA)50 after their immersion into PVA-c solutions for 5 min. (d) UV−vis spectra of the solutions after immersion of (PVA145k/TA)50, (PVA145k-BN0.5μm/TA)50, and (PVA145k-BN1μm/TA)50 in 50 mL of water for different lengths of time. The arrows in (a−c) indicate the thicknesses of the films.

Figure 6. (a) Steady viscosity as a function of shear rate for 10% PVA solution with 2% addition of BN nanosheets; fluorescent optical images showing the shifts of the boundaries between fluorescent and nonfluorescent parts for (b) (PVA145k/TA)50, (c) (PVA145k-BN0.5μm/TA)50, and (d) (PVA145k-BN1μm/TA)50, which are first partially immersed in PVA-c solutions for 5 min and then completely soaked in water for 14 h.

dynamic nature of hydrogen bonding, which can dissociate under appropriate conditions so that polymers can diffuse into the scratch area and re-form stable complexes to heal the surface. Previous studies have shown that e-LBL films can selfheal upon exposure to water due to the high diffusion ability of

for (PVA47k-BN1μm/TA)50 (Figure 3d and Table S1). These values essentially reached the level before being damaged (Table S1). Mechanism for the Enhancement of Self-Healing Performance. The self-healing properties originate from the 9439

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ACS Nano polymers.46,81 However, there are still many open questions to answer: for example, why some e-LBL films can self-heal but others cannot, and how the interaction strength of polymers and their diffusion ability together influence the self-healing properties. A detailed mechanism study is much needed to answer these questions, which may also shed light on the key role of BN nanoplatelets for enhancing the self-healing performance. Polymer diffusion within e-LBL films can be monitored by attaching florescent dyes.46,57 However, this is nontrivial for PVA due to the lack of specific reactions with common organic dyes.82 So, instead, we partially carbonized PVA to obtain PVAcoated carbon quantum dots (denoted as PVA-c), which have shown strong blue luminescence, enough for a direct observation under fluorescence microscopy.83 This strategy should be applicable for many other investigations where fluorescent PVA is required. PVA-c was used as the top layer of e-LBL films, and its out-of-plane diffusion can be studied by looking at the cross sections. The results showed that the presence of BN nanoplatelets has apparently limited the out-ofplane diffusion of PVA-c (Figure 5a−c). The average out-ofplane diffusion rate of PVA145k has been reduced from 9.41 μm/min in (PVA145k/TA)50 to 7.06 μm/min in (PVA145kBN0.5μm/TA)50 and 4.17 μm/min in (PVA145k-BN1μm/TA)50. A similar trend was observed for films consisting of PVA47k (Figure S9a−c). This is consistent with the decrease of film growth rate in the presence of BN nanoplatelets (Figure 2a,b), which can be barriers for out-of-plane polymer diffusion and prevent the exposure of polymers to water through BN-to-BN gaps (Figure 2e,f). The inhibition of out-of-plane diffusion can be correlated with the retardation of polymer release into the solution through the large solid−liquid interface. This can be readily studied by absorption spectra due to the phenolic structure of TA. It was found that the characteristic absorption of TA can be detected in the solution after immersion of the films in water (Figure 5d and Figure S10). The absorption intensity of TA was increased after longer immersion as a result of more TA being released into the solution (Figure 5d and Figure S10). The absorption enhancement was, however, clearly impeded in the presence of BN nanoplatelets, with the larger one having more effective inhibition due to the more extensive diffusion path (Figure 5d and Figure S10). The above results mainly concern the out-of-plane diffusion of polymers; however, it is important to note that, within a twodimensional film, the diffusion can be in-plane and out-of-plane, both of which can contribute to the self-healing process. It is therefore important to estimate the overall diffusion ability. According to the Stokes−Einstein equation, the diffusion ability of the system is associated with its viscosity (η); we compared η of PVA solutions with and without the addition of BN nanoplatelets. The concentrations of PVA solutions were chosen to be 10%, which is far beyond the entanglement concentration, to ensure the abundant inter- and intrapolymer interactions,84 mimicking the condition in the swollen films. The shear rate was chosen from 1 to 100 1/s to eliminate the particle−particle contacts.85 As expected, PVA145k has η much higher than that of PVA47k, and a weak thinning was observed for PVA145k in the tested shear rate range (Figure 6a), similar to previous work.86 It was found that the addition of 2% BN nanoplatelets resulted in a slight decrease of η (Figure 6a). Typically, the addition of fillers can increase η due to the hydrodynamic reinforcement,87 but other results showed that the reduction of η was also possible when the filler

concentration is low due to the size effect88 or selective absorption.89 Here, the determining factor for the rheology of PVA solutions should be the amounts of hydrogen bonding within the system. We believe that the addition of BN nanoplatelets can, in one hand, immobilize PVA chains by confinement and behave as chain bridging sites which may contribute to the increase of η. On the other hand, PVA chains can be segregated by BN nanoplatelets, diminishing the interchain hydrogen bonding and thereby reducing η. This is also consistent with the blue shift of the −OH IR peak from PVA after the addition of BN nanoplatelets (Figure S2). As a result, at such a low filler concentration, the overall η does not change obviously. As water can result in highly swollen films, within which polymer diffusion can be compared with that in the solution,75 our rheology test may indicate that the diffusion ability of PVA during a water-enabled self-healing process can be maintained when small amounts of BN nanoplatelets are present. The result of rheology, however, seems counterintuitive because the out-of-plane diffusion has been limited by BN nanoplatelets. To understand this, we investigated the in-plane diffusion of PVA. In this case, the top layer of the films was partially coated with PVA-c, after which the whole film was immersed in water. The average in-plane diffusion rate can be estimated by comparing the boundary between the fluorescent and the nonfluorescent part before and after immersion. As it is more difficult to discern the boundary change than to just monitor the out-of-plane diffusion, a much longer immersion time was used to ensure a reliable reading. It was found that the addition of BN nanoplatelets resulted in a longer diffusion length (Figure 6b−d). The average in-plane diffusion rates of PVA145k in (PVA145k/TA)50, (PVA145k-BN0.5μm/TA)50, and (PVA145k-BN1μm/TA)50 are 8.33, 13.10, and 17.85 μm/min, respectively. Similar results were observed for PVA47k (Figure S9d−f). Interestingly, the average out-of-plane and in-plane diffusion rates are close to each other (9.41 vs 8.33 μm/min) without BN nanoplatelets. So, the anisotropic diffusion of PVA is directly due to the presence of two-dimensional nanoplatelets. It is quite reasonable that the retardation of out-ofplane diffusion induced by the preferred in-plane alignment of BN nanoplatelets (Figure 2e,f) has been compensated by the enhanced in-plane diffusion. We then tried to establish a theoretical model which can correlate the influence of BN nanoplatelets on polymer diffusion to the self-healing properties of e-LBL films. Here, we consider the diffusion of PVA and TA as a whole to simplify the model and study the diffusion in terms of mass transfer. Importantly, the diffusion of polymers into the solution is considered as a competitive process for self-healing (Figure 7). The flux (J) into and out of the crack area can be expressed as Jin = Ak in(c1 − c 2) = hwk in(c1 − c 2)

(1)

Jout = A′kout(c 2 − c3) = zwkout(c 2 − c3)

(2)

where kin and kout are the mass transfer coefficients into and out of the crack with the cross-sectional areas of A and A′, respectively; h is the thickness of the film, z is the length of the crack, and w is the width of the film; c1, c2, and c3 are polymer concentrations in the film, crack area, and solution, respectively. We also have V 9440

dc 2 = Ak in(c1 − c 2) − A′kout(c 2 − c3) dt

(3)

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(5)

After a time of te, an equilibrium is established

Jin = Jout

(6)

hwk in(c1 − c 2) = zwkout(c 2 − c3)

(7)

So

Combining eqs 5 and 7 gives hk ine

⎛k A⎞ −⎜ in ⎟te ⎝ V ⎠

⎛k A⎞ ⎞ ⎛ −⎜ in ⎟t c1 = zkout⎜⎜1 − e ⎝ V ⎠ e⎟⎟c1 ⎝ ⎠

(8)

Then we have Figure 7. Side view of the self-healing process on an e-LBL film with a thickness of h and a width of w (not shown in this schematic drawing). The white rectangular region denotes the crack with a width of z. c1, c2, and c3 are polymer concentrations in the film, crack area, and solution, respectively. kin and kout are the mass transfer coefficients into and out of the crack area, respectively.

e

=

zkout hk in + zkout

(9)

According to eq 9, te is in reverse proportion to kout. On the other hand, suppose that after a time period (tr), a complete repair can be achieved. According to eq 5, tr is mainly determined by and inreversely proportional to kin. As at the equilibrium, no further accumulation in the crack area can be expected, and to make a full recovery, it is necessary to have tr < te

where V is the volume of the crack part. We use the initial condition (t = 0; c2 = 0) to integrate the mass balance, which gives ⎛ Ak + A k ⎞ ⎞ Ak inc1 + A′koutc3 ⎛ −⎜ in ′ out ⎟t V ⎠⎟ ⎜⎜1 − e ⎝ c2 = ⎟ Ak in + A′kout ⎝ ⎠

⎛k A⎞ −⎜ in ⎟te ⎝ V ⎠

So, in the absence of BN nanoplatelets, the incomplete recovery can be due to the large kout, making te smaller than tr. According to the experiments, the addition of BN nanoplatelets can decrease the average out-of-plane diffusion rate, which means kout is reduced. This will increase te so that a complete recovery is possible (te > tr). The use of PVA with smaller molecular weight alone may not guarantee a complete recovery because it increases kin and kout simultaneously. However, when

(4)

Here, c1 can be regarded as a constant, and c3 can be essentially zero due to the large volume of the solution. Also consider that kout is much smaller than kin due to the interactions between PVA and TA, so we have

Figure 8. Self-healing of free-standing (PVA145k-BN1μm/TA)50 through (a−c) an overlay of the surface near cut or (d−f) direct attachment of cross-sectional area of the cut: (a,d) optical images of (PVA145k-BN1μm/TA)50 after self-healing; (b,e) stress−strain curves of original and selfhealed films, with the insets showing that the ruptures occur at the area other than the healing parts; (c,f) fluorescent optical images showing the diffusion of PVA-c across the attached interface (the top layer of one cut strip was coated with PVA-c before being attached to the other). The red lines in (c,f) indicate the contact area. The time for self-healing is 90 min. 9441

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bonding for effective load transfer and induce an anisotropic diffusion of polymers, inhibiting polymer release into the solution while maintaining the overall diffusion ability. The established theoretical model not only reveals the critical role of BN nanoplatelets for improving the self-healing performance but also can be used to understand the healing mechanism of other e-LBL films. Future work may include the use of other two-dimensional fillers in self-healable e-LBL films to further optimize the performance and diversity the functionalities.

a complete recovery is possible with BN nanoplatelets, PVA47k can favor a quicker recovery due to the higher mobility (larger kin). The self-healing rate is similar for films with different sized BN nanoplatelets because the overall diffusion ability (kin) is not significantly affected by them. This model can also explain other previous observations:46 for example, according to eq 9, increasing the length of cut (z) also decreases te, making a complete self-healing hard to achieve; the increase of film thickness h will increase te, allowing for better healing performance; in addition, the reason that an e-LBL system such as PEI/PAA can self-heal without the addition of nanoplatelets is due to their smaller kout due to the stronger electrostatic interactions between PEI and PAA than hydrogen bonding. Self-Healing Properties as Free-Standing Films. The investigation of self-healing properties of free-standing e-LBL films is very limited compared with surface coatings.90 One possible reason is that some e-LBL films are difficult to be freestanding due to their brittle nature.57 Another reason is that the lack of substrate may bring an additional solid−liquid interface, which may alter the diffusion kinetics. We tested the possibility of our free-standing films to self-heal through two different ways: the two cut rectangular strips were attached to each other either by stacking the film surface near the cut or by connecting the fracture surface directly. A certain amount of water was then added onto the contact region to initiate the self-healing process. By attaching the strips through the surface near the cut, all three films, that is, (PVA145k/TA)50, (PVA145k-BN0.5μm/ TA)50, and (PVA145k-BN1μm/TA)50, can effectively self-heal within 90 min (Figure 8a,b and Figure S11). Importantly, the failures of the self-healed films during the tensile test typically occurred at the area other than the healing parts (Figure 8b and Figure S11c,f). The mechanical properties of these films were found to be completely comparable with that of the original films (Table S2). The diffusion of PVA145k across the interface can be visualized by coating the top layer of one cut strip with PVA-c before attaching to the other unlabeled for self-healing (Figure 8c). It is, however, more challenging to heal the films by attaching the cross section of the cuts for a complete recovery to the initial form. This is due to not only the much smaller contact area but also the lack of substrate support, which exposes another surface for polymer release. For (PVA145k/TA)50 and (PVA145k-BN0.5μm/TA)50, after water-induced self-healing for 90 min, the tensile failure usually occurred at the healed parts where defects may exist due to the incomplete recovery (Figure S12). Accordingly, the mechanical properties after self-healing decreased compared with those of initial films (Figure S12c,f and Table S2). Intriguingly, a full recovery can be achieved for (PVA145k-BN1μm/TA)50, which has essentially the same modulus and ultimate strength with the film before damage (Figure 8d,e and Table S2). Here, the lack of support may necessitate the use of BN1μm to more effectively inhibit polymer release into the solution so that a defect-free interface can be obtained through interfacial diffusion (Figure 8f).

EXPERIMENTAL SECTION Materials. BN nanoplatelets (sizes 0.5 and 1 μm) were provided by Shanghai ST-NANO Science & Technology Co., Ltd., China. Poly(vinyl alcohol) (Mw = 47 000 and 145 000 g/mol) and tannic acid (Mw = 1701 g/mol) were obtained from Aladdin. Hydrochloric acid (HCl), 98% sulfuric acid (H2SO4), and 30% hydrogen peroxide (H2O2) solution were purchased from Tianjin Fu Yu Fan Chemical Co., Ltd. Deionized (DI) water (18 MΩ cm−1) was obtained from the Milli-Q system and used in all experiments. The substrates applied in LBL assembly were microscope glass slides. Preparation of PVA, PVA-BN, and TA Solutions. PVA (1.2 g) was added in 200 mL of DI water and heated at 80 °C for 1 h to make 6 mg/mL solutions. TA (1.2 g) was dissolved in 200 mL of DI water to make 6 mg/mL solutions. For the preparation of PVA-BN dispersions, BN nanoplatelets (0.4 g) were added to PVA solutions and the mixture was stirred for 30 min followed by ultrasonication (KQ-500DE, 400 W) for 2 h. The dispersion was then centrifuged at 3000 rpm for 10 min, and the supernatant was collected. The concentration of BN nanoplatelets can be determined based on the TGA result of the dried powders from a certain amount of PVA-BN dispersion, which can give mass percentages of PVA and BN. The pH of all solutions was adjusted to 2 using 2.4 M HCl solution. LBL Assembly. The microscope glass slides were first sonicated in ethanol for 20 min, followed by being rinsed with DI water and immersed in piranha solution (3:1 ratio of 98% H2SO4 and 30% H2O2) for 1 h. Then, the slides were thoroughly rinsed with large amounts of DI water and dried at room temperature. For the preparation of multilayers, glass slides were immersed alternatively in PVA (or PVA-BN) solutions and TA solutions for 5 min with intermediate DI water washing and air drying. Typically, 50 bilayers were achieved for different combinations. Fluorescent Imaging. PVA-coated carbon quantum dots (PVA-c) were used as the top layer during LBL assembly to cover the whole surface for out-of-plane diffusion tracking. For in-plane diffusion tracking, a partial surface of the film was coated by PVA-c and the whole film was immersed in water for 14 h. To observe interfacial diffusion between free-standing films, the top layer of one cut strip was coated with PVA-c before being attached to the other unlabeled one. PVA-c was made as follows: 30 mL of 6 mg/mL PVA aqueous solutions was transferred into a poly(tetrafluoroethylene)-lined stainless steel autoclave (40 mL). The system was heated at 180 °C for 8 h and then cooled to room temperature. Self-Healing of Surface Coatings and Free-Standing Films. To study the self-healing properties of surface coatings, a 50 μm wide cut was made through the films by a No. 23 surgical scalpel blade. The damaged surface coatings were then immersed in water for different lengths of time. For self-healing of free-standing films, two cut rectangular strips were attached to each other by an overlay of the film surface near the cut or a direct connection of the cut cross sections on a Petri dish. Small amounts of water were then added to the contact area for 90 min. Instruments and Testing. Atomic force microscopy (AFM) experiments were performed in a tapping mode using a Dimension Icon atomic force microscope system. The sample for AFM testing was prepared by drying a droplet of PVA-BN dispersion on silicon wafer. X-ray photoelectron spectroscopy was performed using PHI ESCA 5700 with Al Kα (1486.6 eV). The growth of the first 10 bilayers was monitored using QCM200 (Stanford Research Systems, Inc.). Before

CONCLUSIONS We demonstrated that incorporating BN nanoplatelets into eLBL films (PVA/TA) consisting of a hydrogen bonding network resulted in simultaneous improvements of both stiffness/strength and self-healing ability. The dual enhancements can be attributed to the two-dimensional structure of BN nanoplatelets, which can strengthen the interface by covalent 9442

DOI: 10.1021/acsnano.6b04482 ACS Nano 2016, 10, 9434−9445

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ACS Nano LBL assembly on quartz crystals, they were treated using piranha solution and washed with DI water. The cross sections of different films as well as their surface morphologies were observed by SEM of a Hitachi S4800 apparatus. IR spectra were obtained using a PerkinElmer Spectrum Frontier optical spectrometer. The mechanical properties of surface coatings were characterized by a Nanoinstruments Nanoindenter II model provided by MTS Nanoinstruments Inc., Oak Ridge, TN. A Berkovich-shaped indenter was used with a penetration depth of 1 μm. The hardness and modulus were calculated and recorded from five different points. The mechanical properties of free-standing films were evaluated by a room temperature uniaxial tensile test at a force rate of 0.1 N/min using the inorganic/film mode on a TA Q400EM machine. TGA results were obtained on a thermogravimetric analyzer (TA Q500) under an atmosphere of oxygen with a heating rate of 10 °C/min. The optical and fluorescent images with a UV light excitation were obtained using an inverted microscope (IX-81, Olympus, Japan). UV−vis spectra were collected on a TU-1810 spectrophotometer. The rheological behaviors of different solutions were investigated by a MCR 300 (Paar Physica) rheometer using a 25 mm parallel-plate geometry at 25 °C. Dynamic frequency sweep experiments were measured from 1 to 100 rad/s at a fixed oscillatory strain of 0.2%.

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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.6b04482. Statistical analysis of the dimensions of BN nanoplatelets by DLS, TEM, and AFM; IR spectra of different BN nanoplatelet dispersions and PVA; TGA results of PVA145k-BN1μm and different films; additional optical, SEM, and fluorescent optical images of different films during self-healing; additional cross-sectional SEM images showing the thicknesses of different films; additional UV−vis spectra of TA diffusing into water; additional self-healing results of free-standing films; summaries of mechanical properties based on nanoindentation and tensile test (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS M.Y. thanks the National Natural Science Foundation of China (Grant Nos. 21303032 and 21571041) for financial support. Y.H. thanks the HIT 100-talent program (Grant No. AUGA5710006813) and Fundamental Research Funds for the Central Universities (Grant No. HIT. IBRSEM. A. 201405) for financial support. REFERENCES (1) Wang, C.; Wu, H.; Chen, Z.; McDowell, M. T.; Cui, Y.; Bao, Z. A. Self-Healing Chemistry Enables the Stable Operation of Silicon Microparticle Anodes for High-Energy Lithium-Ion Batteries. Nat. Chem. 2013, 5, 1042−1048. (2) Cordier, P.; Tournilhac, F.; Soulie-Ziakovic, C.; Leibler, L. SelfHealing and Thermoreversible Rubber from Supramolecular Assembly. Nature 2008, 451, 977−980. (3) White, S. R.; Moore, J. S.; Sottos, N. R.; Krull, B. P.; Santa Cruz, W. A.; Gergely, R. C. R. Restoration of Large Damage Volumes in Polymers. Science 2014, 344, 620−623. 9443

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DOI: 10.1021/acsnano.6b04482 ACS Nano 2016, 10, 9434−9445

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DOI: 10.1021/acsnano.6b04482 ACS Nano 2016, 10, 9434−9445