Healability Demonstrates Enhanced Shape-Recovery of Graphene

Dec 19, 2017 - (34-38) However, SMPs with an excellent shape-memory effect contain chemical and/or physical cross-linking networks and are usually sti...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 2897−2906

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Healability Demonstrates Enhanced Shape-Recovery of GrapheneOxide-Reinforced Shape-Memory Polymeric Films Zilong Xiang, Ling Zhang, Tao Yuan, Yixuan Li, and Junqi Sun* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, PR China

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S Supporting Information *

ABSTRACT: The fabrication of shape-memory polymers or films that can simultaneously heal the mechanical damage and the fatigued shape-memory function remains challenging. In this study, mechanically robust healable shape-memory polymeric films that can heal the mechanical damage and the fatigued shape-memory function in the presence of water are fabricated by layer-by-layer assembly of branched poly(ethylenimine) (bPEI)-graphene oxide (GO) complexes with poly(acrylic acid) (PAA), followed by the release of the (PAA/ bPEI-GO)*n films from the underlying substrates. The free-standing (PAA/bPEI-GO0.02)*35 films made of bPEI-GO complexes with a mass ratio of 0.02 between GO nanosheets and bPEI are mechanically robust with a Young’s modulus of 19.8 ± 2.1 GPa and a hardness of 0.92 ± 0.15 GPa and exhibit excellent humidity-induced healing and shape-memory functions. Benefiting from the highly efficient healing function, the (PAA/bPEI-GO0.02)*35 films can heal cuts penetrating thorough the entire film and achieve an ∼100% shape-recovery ratio for a long-term shape-memory application. Meanwhile, the shape-memory function of the mechanically damaged (PAA/bPEI-GO0.02)*35 films can be finely restored after being healed in water. The shape-memory functions of the (PAA/bPEI-GO0.02)*35 films and their healing capacity originate from the reversibility of electrostatic and hydrogen-bonding interactions induced by water between PAA and bPEI-GO complexes. KEYWORDS: layer-by-layer assembly, self-healing materials, shape-memory polymers, materials science, nanofiller-reinforcement

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by light, magnetic field, and electricity.12,15,16 The development of new types of SMPs that can be manipulated by convenient stimuli with long-term service life is one of the main pursuits of scientists in this research area. Compared with other stimuli, moisture is only of moderate harshness, while it is also inexpensive and environmentally friendly, making moisture-triggered SMPs suitable for use as smart biomedical devices.14,24−26 During usage, SMPs are unavoidably subjected to mechanical damage such as accidental scratches or impinging, leading to deterioration of shape-memory functions.9,12,27,28 Moreover, shape-recovery ratios of thermoplastic SMPs generally decrease with increasing shape-memory cycles.29−32 This decrease will limit the repeated usage of thermoplastic SMPs in a precise way. The factors causing the decrease of shape-recovery ratios for different types of thermoplastic SMPs over usage are complicated but can be generally ascribed to the incomplete relaxation of polymer chain segments to their original conformational state after multiple cycles of shape-memory performance. Therefore, strategies that can repair the mechanical damage on SMPs and simultaneously restore their original shape-recovery ratios are highly desirable.

INTRODUCTION Shape-memory polymers (SMPs) are smart polymers that can be manipulated to fix a temporary shape and triggered to recover to a memorized permanent shape upon external stimuli1−7 such as heat,6−10 light irradiation,11,12 solvent,13,14 electrical current,15 magnetic fields,16 and so forth. Compared with shape-memory alloys, SMPs have several advantages that include higher deformation strain, lower stiffness, density and manufacturing cost, potential biodegradability and healability, and the capability to be activated by various stimuli.3,4,17−19 Therefore, SMPs have potential application in areas such as biomedical devices, the aerospace industry, textiles, actuators, flexible electronics, and so forth.3−5,20−23 Taking heat-triggered SMP as an example, a shape-memory cycle is typically achieved in three steps that include: (i) elastic deformation of the SMP sample above a specific transition temperature such as the glass transition or crystal melting temperatures, leading to an decrease in conformational entropy of the constituent polymer chains; (ii) shape fixing at lowered temperature to freeze the temporary shape; and (iii) recovery by reheating the SMP sample above the transition temperature to relax the polymer chains to their entropically favored conformational state, thus returning to the memorized permanent shape.3−5,7 To accommodate a broad application of SMPs, heat-generating species have been integrated into heat-triggered SMPs to allow operation of the deformation, shape fixing, and recovery steps © 2017 American Chemical Society

Received: September 26, 2017 Accepted: December 19, 2017 Published: December 19, 2017 2897

DOI: 10.1021/acsami.7b14588 ACS Appl. Mater. Interfaces 2018, 10, 2897−2906

ACS Applied Materials & Interfaces

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Increasing netpoints in SMPs such as chemical or physical cross-linking can significantly improve the durability, shapememory behavior, and shape-recovery ratios of the resultant SMPs.15,19,33 However, this strategy cannot fully address the problems of mechanical damage and decrease in shape-recovery ratios of SMPs. Artificial self-healing/healable materials can repair mechanical damage autonomically or under the assistance of external stimulus.34−40 In the past decade, various self-healing/healable polymer composite materials have been fabricated, aiming for artificial materials with enhanced durability and improved reliability.34−40 Among them, intrinsic self-healing/healable materials that can heal mechanical damage through the reversibility of noncovalent interactions and dynamic covalent bonds have attracted much attention from scientists. Intrinsic self-healing/healable materials can achieve multiple rounds of damage healing in a given region, and their fabrication does not involve the complicated steps of incorporating externally added healing agents.34−38 Endowing SMPs with self-healing ability provides a practically applicable way to repair mechanical damage to extend their service life. The fabrication of SMPs that can heal damage through the reversibility of noncovalent interactions and dynamic bonds is challenging because the contradiction between SMPs and selfhealing materials needs to be compromised.12,41 Self-healing/ healable polymers need to have high polymer chain mobility to heal damage.34−38 However, SMPs with an excellent shapememory effect contain chemical and/or physical cross-linking networks and are usually stiff with a high modulus, which restricts the mobility of polymer chains and makes the healing process difficult.3−5 Several groups have reported the fabrication of healable SMPs.27,28,41−48 However, healable SMPs that are highly mechanically robust and can perform shaperecovery and damage healing under the same stimulus are rarely reported. Moreover, despite the fact that mechanical damage on SMPs can be healed, the self-healing function of SMPs has not been exploited to completely restore their permanent shape to improve the shape-recovery ratio after multiple cycles of shapememory performance. The layer-by-layer (LbL) assembly, which involves alternate deposition of species with complementary interactions, provides a facile method for the fabrication of polymeric composite films with well-controlled composition and structures.49−52 The electrostatic and hydrogen-bonding interactions in LbL-assembled polyelectrolyte films are dynamic in nature and can be partially broken in the presence of water to enhance the chain mobility of polyelectrolytes.37 By making use of this property, our group and others have demonstrated that LbL-assembled polyelectrolyte films and bulk polyelectrolyte complexes are promising healable materials that can accomplish healing with the assistance of water.37,53−56 We believe that the reversibility of the electrostatic and hydrogen bonding interactions in LbL-assembled polyelectrolyte films can be exploited for the fabrication of shape-memory films. In this work, for the first time, we report the fabrication of healable SMP films by LbL assembly of branched poly(ethylenimine) (bPEI)-graphene oxide (GO) complexes with poly(acrylic acid) (PAA). The free-standing PAA/bPEI-GO composite films are mechanically robust and can achieve excellent shape-memory and damage healing properties under the stimulus of humidity. Most importantly, we demonstrate that the films can significantly enhance their shape-recovery behavior through the healing step to reach an ∼100% shape-recovery ratio during long-term usage.

Research Article

RESULTS AND DISCUSSION

LbL Assembly of PAA/bPEI-GO Films. Because of their extremely high modulus, GO nanosheets are frequently employed as nanofillers to reinforce polymer composites. GO nanosheets contain hydroxyl, epoxide, carbonyl, and carboxyl groups on their basal planes and sheet edges. Therefore, GO nanosheets can have electrostatic and hydrogen bonding interactions and covalent bonds with bPEI to produce bPEI-GO complexes in aqueous solutions (Figure S1). bPEI-GO complexes were prepared by keeping the bPEI concentration in the complex solutions at 2.0 mg/mL while varying the concentrations of GO nanosheets in the complex solutions to be 0, 0.02, 0.04, 0.08, and 0.20 mg/mL, respectively. For simplicity, they are denoted as bPEI-GOm with m referring to the feed mass ratio of GO nanosheets to bPEI in the complex solutions, which is 0, 0.01, 0.02, 0.04, and 0.10, respectively. Atomic force microscopy (AFM) was used to characterize the formation of bPEI-GO0.02 complexes. With bPEI-GO0.02 complexes as a typical example, GO nanosheets have a flat paper-like structure with their average sheet thickness being ∼1.0 nm (Figure S2a). After complexing with bPEI, the surface of the bPEI-GO0.02 nanosheets becomes rougher, with their average sheet thickness increasing to ∼2.3 nm (Figure S2b). This result demonstrates the successful preparation of bPEI-GO complexes. As illustrated in Scheme 1a, positively charged bPEI-GOm complexes can be LbL-assembled with PAA for the fabrication of (PAA/bPEI-GOm)*n films (where n represents the number of film deposition cycles, and a half number means that PAA is the outmost layer of the films) through electrostatic and hydrogen-bonding interactions between bPEI and PAA. The thicknesses of the (PAA/bPEI-GOm)*n films with different numbers of deposition cycles were measured with a surface stylus profilometer. As shown in Figure 1a, the (PAA/ bPEI-GO0.02)*n films exhibit a typical exponential growth in the initial 8 deposition cycles and thereafter a rapid linear growth with increments of ∼885 nm per deposition cycle. After 35 deposition cycles, the average thickness of the (PAA/bPEI-GO0.02)*35 films rapidly reaches ∼29.4 μm. The cross-sectional SEM image of the (PAA/bPEI-GO0.02)*35 film indicates that the film has a rough surface because of the aggregation of bPEI-GO0.02 complexes with PAA (inset in Figure 1a). Previous studies showed that the rapid growth of exponential LbL-assembled polyelectrolyte films is mainly ascribed to the “in-and-out” diffusion of polyelectrolytes during the film deposition process.37,57−59 To elucidate the exponential growth of the (PAA/bPEI-GO0.02)*n films, PAA grafted with lucifer yellow cadaverine (denoted as PAA-LYC) and bPEI-GO0.02 complexes with bPEI being labeled with fluorescein isothiocyanate (denoted as bPEI-FITC-GO0.02) were deposited as the outmost layer of the (PAA/bPEI-GO0.02)*35 and (PAA/ bPEI-GO0.02)*35.5 films, respectively. A confocal laser scanning microscope (CLSM) was used to observe the diffusion of the polyelectrolytes along the film deposition direction. It can be clearly seen that PAA-LYC can diffuse throughout the entire (PAA/bPEI-GO0.02)*35/PAA-LYC film, disclosing the high diffusion ability of PAA-LYC within the films (Figure 1b). By contrast, bPEI-FITC can only diffuse to a limited depth of ∼5 μm within the (PAA/bPEI-GO0.02)*35.5/bPEI-FITCGO0.02 film (Figure 1c). The strong interactions between bPEI-FITC and GO nanosheets restrict the diffusion of bPEIFITC within the film. When the (PAA/bPEI-GO0.02)*n films were immersed into PAA solution, an excess amount of PAA 2898

DOI: 10.1021/acsami.7b14588 ACS Appl. Mater. Interfaces 2018, 10, 2897−2906

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Scheme 1. (a) Fabrication Process of the (PAA/bPEI-GOm)*n Films and† (b) Schematic Illustration of the Shape-Memory and Healing Process of the (PAA/bPEI-GO0.02)*n Films‡

†

For clarity, the interactions between GO and polymer matrices are not presented in the scheme. ‡The dash lines mark the original length of the (PAA/bPEI-GO0.02)*n films. Lori represents the original length of the (PAA/bPEI-GO0.02)*n film. LT(1), LP(1), and LP(N) represent the length of the film in temporary shape (LT(1)) and permanent shape (LP(1) and LP(N)), in which (1) and (N) means in the 1st and Nth cycles of shapememory performances.

Figure 1. (a) Thickness of the (PAA/bPEI-GO0.02)*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-GO0.02)*35 film. (b) CLSM image of a (PAA/bPEI-GO0.02)*35 film with an outmost layer of PAA-LYC. (c) CLSM image of a (PAA/bPEI-GO0.02)*35.5 film with an outmost layer of bPEI-FITC-GO0.02 complexes. (d) Thickness of the (PAA/bPEI-GOm)*35 film as a function of feed mass ratio of GO to bPEI in bPEI-GOm complexes. (e) TGA curves of GO nanosheets and the (PAA/bPEI-GOm)*35 films with m being 0, 0.02, and 0.10, respectively.

can diffuse into the films. Next, when the (PAA/bPEIGO0.02)*n/PAA films were transferred into the solution of bPEI-GO0.02 complexes, PAA deposited in the previous step can diffuse out to the film/solution interface, enabling a large amount of bPEI-GOm complexes to be deposited. The “in-andout” diffusion of PAA leads to the deposition of more amounts of PAA and bPEI-GO0.02 complexes in the current layer than in the previous layer. In this way, the (PAA/bPEI-GO0.02)*n films undergo an exponential growth process until the amount of the in-diffused PAA reaches constant. The other (PAA/bPEI-GOm)*n films with m being 0, 0.01, 0.04, and 0.1 also have a similar exponential growth behavior as that of the (PAA/bPEI-GO0.02)*n films. However, as shown in Figure 1d,

the thickness of the (PAA/bPEI-GOm)*35 films decrease with increasing GO contents in bPEI-GOm complexes. GO nanosheets act as barriers to inhibit the diffusion of PAA. Therefore, the (PAA/bPEI-GOm)*35 films with a higher loading contents of GO nanosheets have a lower growth speed. Thermal gravimetric analysis (TGA) was used to measure the mass ratio of GO nanosheets in (PAA/bPEI-GOm)*35 films (Figures 1e and S3). According to the mass loss of GO nanosheets as well as that of the (PAA/bPEI)*35 and (PAA/bPEI-GOm)*35 films at 800 °C, the mass ratios of GO nanosheets in (PAA/ bPEI-GOm)*35 films are calculated to be ca. 0.65, 1.6, 3.0, and 6.8 wt %, respectively, for m being 0.01, 0.02, 0.04 and 0.1, respectively. In addition, the (PAA/bPEI-GOm)*35 films 2899

DOI: 10.1021/acsami.7b14588 ACS Appl. Mater. Interfaces 2018, 10, 2897−2906

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Figure 2. (a) Young’s modulus and hardness of the (PAA/bPEI)*35 and (PAA/bPEI-GO0.02)*35 films as a function of penetration depth. (b) Young’s modulus and hardness of the (PAA/bPEI-GOm)*35 films as a function of feed mass ratio of GO to bPEI in the bPEI-GOm complexes.

(PAA/bPEI-GO0.02)*35 strip has a permanent shape of 10 × 3 mm (Figure 3a(i). Under ∼80% relative humidity (RH), the (PAA/bPEI-GO0.02)*35 film can be stretched to ∼200% of its original length to obtain a temporary shape (Figure 3a(ii). This temporary shape is then fixed by drying the film with nitrogen flow (∼5% RH) for several seconds (Figure 3a(iii). Switching the RH to ∼80%, the stretched (PAA/bPEI-GO0.02)*35 film, which is in a temporary shape, rapidly returns to its permanent shape with a 100% recovery ratio within 4 s (Figure 3a(iv−vi)). Moreover, the free-standing (PAA/bPEI-GO0.02)*35 film was cut into a permanent shape of the letter “C” (Figure S5a). The film was stretched to a temporary shape of a rectangular strip (10 × 3 mm) under an ∼80% RH environment at room temperature, and then the temporary shape was fixed by decreasing the environmental RH to ∼5% (Figure S5b). When the RH returns to ∼80%, the stretched (PAA/bPEI-GO0.02)*35 strip rapidly and completely returns to the original “C” shape within 4 s (Figure S5c−f). In addition, the (PAA/bPEIGO0.02)*35 film can also undergo a bending/unbending shapememory process under the stimulus of moisture (Figure S6). These results demonstrate that the (PAA/bPEI-GO0.02)*35 films show satisfactory shape-memory properties upon a change in environmental humidity. A proposed mechanism of humidity-induced shape-memory behavior for the (PAA/ bPEI-GO0.02)*35 films is illustrated in Figure 3b. When the film is exposed to an environment with high RH, water molecules are absorbed by the film because of the highly hydrophilic nature of PAA and bPEI-GO components. The electrostatic and hydrogen bonding interactions between PAA and bPEI are largely weakened in the presence of water, decreasing the physical cross-linking density of the film and enhancing the chain mobility of the PAA and bPEI polyelectrolytes.14,37,54−56 Therefore, the (PAA/bPEI-GO0.02)*35 film becomes soft and highly stretchable and can be stretched to a temporary shape in a humid environment. The temporary shape of the (PAA/bPEI-GO0.02)*35 film can be fixed in a dry environment because the electrostatic and hydrogen-bonding interactions between PAA and bPEI-GO0.02 complexes are reconstructed after water desorption, decreasing the chain mobility of polyelectrolytes within the film. Consequently, the polyelectrolytes in the temporary shape of the film are locked in their extended conformations. Restoration of the environmental humidity to 80% RH leads to partial breakage of the electrostatic and hydrogen-bonding interactions between PAA and bPEI-GO in their temporary shape. Therefore, restoration of the film from its temporary shape to permanent shape is realized by the conformation transformation of the polymer chains from a highly extended state to the entropically favored coiled state. Nanoindentation measurements indicate that the modulus of the (PAA/bPEI-GO0.02)*35 SMP films in

become rougher with the increase of GO contents within the bPEI-GOm complexes (Figure S4). Mechanical Properties of (PAA/bPEI-GOm)*35 Films. Nanoindentation was utilized to investigate the mechanical properties of the (PAA/bPEI-GOm)*35 films with different GO contents. Nanoindentation measurements were performed in air with ∼25% relative humidity (RH) at 25 °C by the “G-Series Continuous Stiffness Measurement (CSM) Standard Hardness, Modulus, and Tip Cal” method.37,56 Figure 2a shows the typical Young’s modulus and hardness of the (PAA/bPEIGO0.02)*35 and the (PAA/bPEI)*35 films as a function of indentation depth. The Young’s modulus and hardness of the films in the plateau region of 300 to 400 nm are used to measure the “real” Young’s modulus and hardness of the films to eliminate the influences from the film surface and the underlying substrates. The Young’s modulus and hardness of the (PAA/bPEI-GOm)*35 films are plotted as a function of GO loading ratios in the corresponding films (Figure 2b). The (PAA/bPEI)*35 film has a Young’s modulus and hardness of 9.8 ± 1.7 GPa and 0.41 ± 0.1 GPa, respectively. The Young’s modulus and hardness of the (PAA/bPEI-GOm)*35 films increase with increasing GO contents within the films and achieve a highest Young’s modulus of 19.8 ± 2.1 GPa and hardness of 0.92 ± 0.15 GPa for the (PAA/bPEI-GO0.02)*35 films. Further increasing GO ratios in bPEI-GOm complexes leads to the decrease in the Young’s modulus and hardness of the (PAA/bPEI-GOm)*35 films. Compared with the (PAA/bPEI)*35 films, the much enhanced mechanical properties of the (PAA/bPEI-GO0.02)*35 films arise from the homogeneous dispersion of GO nanosheets within the films and the strong interactions between bPEI and GO nanosheets within (PAA/bPEI-GO0.02)*35 matrix films. The well-dispersed GO nanosheets can effectively transfer stress from polymers to nanofillers to enhance mechanical properties of the (PAA/ bPEI-GO0.02)*35. The decrease of the Young’s modulus and the hardness of the (PAA/bPEI-GOm)*35 films with m being higher than 0.02 is due to the aggregation of GO nanosheets in the corresponding films, which are disclosed by the crosssectional SEM images of the films (Figure S4). Shape-Memory Performance of (PAA/bPEI-GO0.02)*35 Films and Their Healing Behavior. The remarkable mechanical robustness of the (PAA/bPEI-GO0.02)*35 films guarantee them to be sufficiently stable and durable for use as SMPs. Free-standing (PAA/bPEI-GO0.02)*35 films were released from substrates by dissolving the predeposited poly(4vinylphenol) (P4VP) sacrificial layer sandwiched between the underlying substrates and the targeted films. Figure 3a and the movie in the Supporting Information demonstrate the moisture-induced shape-memory behavior of the (PAA/bPEIGO0.02)*35 films at a room temperature of 25 °C. The rectangular 2900

DOI: 10.1021/acsami.7b14588 ACS Appl. Mater. Interfaces 2018, 10, 2897−2906

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Figure 3. (a) Shape-memory process of the (PAA/bPEI-GO0.02)*35 film. The small piece of paper stuck on the right side of the film was used to stretch the film during the shape-memory process. (b) Schematic illustration of the shape-memory mechanism of the (PAA/bPEI-GO0.02)*35 film.

Figure 4. (a) Dependence of total recovery ratio (Rr,t) of the (PAA/bPEI-GO0.02)*35 films as a function of the number of shape-memory cycles. After the 51th shape-memory cycle, the films are healed in water, and the total recovery ratio returns to 100%. (b) The lengths of the (PAA/bPEIGO0.02)*35 strips (original dimensions: 15 × 3 mm) during 500 shape-memory cycles with the sample strips being immersed in water for 30 min after every 10 shape-memory cycles. In each shape-memory cycle, the strip was stretched to ∼200% of its original length under 80% RH, and the length was fixed by decreasing the RH to 5%; after that, films are exposed in 80% RH to make the films recover to the permanent shape.

water and under an ∼20% RH environment are ∼140.1 MPa and ∼20 GPa, respectively.34,48 The significant difference in film modulus is due to the reversible breakup and formation of the interactions between PAA and bPEI-GO complexes in water and under dry conditions. This result supports the mechanism proposed for the shape-memory behavior of the (PAA/bPEI-GO0.02)*35 films. More importantly, the reversible interactions between PAA and bPEI-GO enables the (PAA/bPEI-GO0.02)*35 films to repeatedly undergo stretching/recovering shape-memory performance. The shape-recovery ratio is an important parameter to evaluate the performance of SMPs for repeated application. The (PAA/bPEI-GO0.02)*35 films were cut into rectangular strips with dimensions of 15 × 3 mm to undergo successive stretching-recovery shape memories by periodically alternating the environmental RH between 5 and 80%. After each shapememory cycle, the length of the (PAA/bPEI-GO0.02)*35 film is measured, and the total recovery ratio Rr,t is calculated by the following equation

R r,t(N ) =

L T (N ) − L P (N ) L T(N ) − Lori

(1)

where N refers to the number of shape-memory cycles, Lori is the original length of the film, and LT(N) and LP(N) represent the temporary and permanent lengths of the film for Nth cycles of shape-memory performance, respectively.29,32,60 Dependence of the Rr,t of the (PAA/bPEI-GO0.02)*35 films as a function of the number of shape-memory cycles is plotted in Figure 4a. An ∼100% recovery ratio is obtained for the (PAA/ bPEI-GO0.02)*35 films in the initial 12 cycles of shape-memory. Starting from the 15th shape-memory cycle, the Rr,t of the samples descends with the increment of the number of shapememory cycles. The Rr,t declines to ∼90% of its original value on the 45th shape-memory cycle, and thereafter, the Rr,t decreases slowly. The incomplete recovery of the (PAA/ bPEI-GO0.02)*35 films to their original permanent shape, which generally leads to negative effects on the SMP films, is mainly caused by the slipping and disentangling of the polymer chains 2901

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Figure 5. (a,b) Photographs (i) and SEM images (ii) of the (PAA/bPEI-GO0.02)*35 film with an ∼75 μm cut before (a) and after (b) being healed in water. (c) Stress−strain curves of the (PAA/bPEI-GO0.02)*35 films after being healed in water for different times. (d) Storage modulus of the (PAA/bPEI-GO0.02)*35 and (PAA/bPEI)*35 films in water as a function of frequency.

during film stretching. However, as shown in Figure 4a, the shape-recovery ratio of the (PAA/bPEI-GO0.02)*35 films returns to ∼100% after being immersed in water for 30 min, meaning that the shape-memory function can be healed in water. Moreover, the (PAA/bPEI-GO0.02)*35 films keep an ∼100% shape-recovery ratio over 500 shape-memory cycles, with the films being healed in water for 30 min after every 10 shape-memory cycles (Figure 4b), demonstrating a long-lasting and reliable shape-memory function of the films without fatigue, with the assistance of a healing step. The healing of the fatigued shape-memory function of the (PAA/bPEI-GO0.02)*35 films is illustrated in Scheme 1b(1−4). When the (PAA/bPEIGO0.02)*35 films are immersed in water, the interactions between PAA and bPEI-GO0.02 complexes can be largely weakened or broken, leading to polyelectrolytes of higher mobility than in films exposed to a highly humid environment. The polyelectrolytes with extremely high mobility significantly facilitate the conformation transformation of polyelectrolyte chains from a stretched high-energy state to an entropically favored coiled state. Therefore, the complete recovery of the SMP films to the original permanent shape is realized. Because the restoration of the (PAA/bPEI-GO0.02)*35 films from a temporary shape to a permanent shape occurs in a humid environment, the healing step also takes place during this restoring process. Therefore, a high shape-recovery ratio for the (PAA/bPEI-GO0.02)*35 SMP films is guaranteed even without the healing step in water. It should be mentioned that the shape fixing ratio of the (PAA/bPEI-GO0.02)*35 films can always maintain 100%. This is because the (PAA/bPEI-GO0.02)*35 films saturated in an environment of 5% RH have a Tg of ∼116 °C. The mobility of the bPEI and PAA chains in the (PAA/bPEI-GO0.02)*35 films are greatly suppressed at room temperature under 5% RH, which ensures a 100% fixing ratio of the (PAA/bPEI-GO0.02)*35 films. Healing Mechanical Damage on (PAA/bPEI-GO0.02)*35 Films. Besides the healing of the shape-memory function, the

(PAA/bPEI-GO0.02)*35 films can also heal damage such as cuts and scratches that can severely destroy the shape-memory function (Scheme 1b(1,5,6)). As shown in Figure 5a, a cut with dimensions of ∼75 μm wide and ∼1.5 mm long was made on the middle of a (PAA/bPEI-GO0.02)*35 SMP film (15 × 3 mm) with a blade. The cut penetrating the entire film is easy to be observed when the damaged film is attached on a glass tube. The cut is completely healed after immersing the film in water for 12 h (Figures 5b and S7). As demonstrated above, the associations between PAA and bPEI-GO are weakened and partially broken when the PAA/bPEI-GO0.02 films are in water, which enables the polyelectrolytes to diffuse to the damaged region. The interactions between polyelectrolytes can be reformed upon drying the films. In this way, the healing of the cuts in the films is accomplished. The stress−strain curves of the cut (PAA/bPEI-GO0.02)*35 films that are healed in water with different times were measured in an environment with ∼40% RH at 25 °C. Calculated form the stress−strain curves in Figure 5c, the healing efficiency of the films in term of the restored stress reaches ∼50 and ∼98% after healing for 6 and 12 h, respectively. Further extending the healing time to 18 h does not increase the healing efficiency of the films, meaning that 12 h is sufficiently long to heal the cut. Our previous study showed that it takes several to several tens of minutes to heal cuts in PAA/PEI films.37 However, the healing of (PAA/bPEIGO0.02)*35 films takes a much longer time of 12 h. As indicated in Figure 5d, the storage modulus of the (PAA/bPEIGO0.02)*35 and (PAA/bPEI)*35 films in water at a frequency of 1 Hz is ∼140.1 and ∼20.3 MPa, respectively. The higher storage modulus of the (PAA/bPEI-GO0.02)*35 films than the (PAA/bPEI)*35 films indicates that the incorporation of GO nanosheets suppresses the mobility of PAA and bPEI in (PAA/bPEI-GO0.02)*35 films, making the healing process difficult to take place. As intrinsic healable film materials, the (PAA/bPEI- GO0.02)*35 SMP films can repeatedly repair deep cuts at the same region (Figure S8). The healing efficiency of 2902

DOI: 10.1021/acsami.7b14588 ACS Appl. Mater. Interfaces 2018, 10, 2897−2906

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ACS Applied Materials & Interfaces

Figure 6. Shape-memory process of the cut (PAA/bPEI-GO0.02)*35 films after being healed in water. (a) The original shape of the healed (PAA/bPEI-GO0.02)*35 film. (b) The healed (PAA/bPEI-GO0.02)*35 film in its temporary shape. (c) Shape recovering process of the healed (PAA/bPEI-GO0.02)*35 film under ∼80% RH. (d) The permanent shape of the healed (PAA/bPEI-GO0.02)*35 film. The small piece of paper stuck on the right side of the film was used to stretch the film during the shape-memory process.

the film can still reach ∼85% of the original film after five cycles of damage/healing processes (Figure S9). The cut (PAA/bPEI-GO0.02)*35 films after healing can still exhibit excellent shape-memory performance. As indicated in Figure 6, the healed (PAA/bPEI-GO0.02)*35 film is stretched to 200% of its original length under an ∼80% RH, and this temporary shape is then fixed under an ∼5% RH. The stretched (PAA/bPEI-GO0.02)*35 film restores to its original length when the environmental humidity increases to ∼80% RH. More importantly, the healed (PAA/bPEI-GO0.02)*35 films can undergo shape-memory for several hundreds of cycles without decreasing their shape-recovery ratios when a healing process is conducted after each 10 cycles of shape-memory performance. This result demonstrates that healability not only repairs mechanical damage but also restores the deteriorated shapememory function of the (PAA/bPEI-GO0.02)*35 films.

healing of the mechanical damage and the fatigued shapememory function of the (PAA/bPEI-GO0.02)*35 films can take place during the shape-recovery process, which is favorable to keep a high shape-recovery ratio for the SMP films and makes the healing step convenient to accomplish. Healable shapememory films of well-designed shapes are expected to be fabricated by the LbL assembly technique because of its capability to deposit polymeric films on substrates of complicated morphologies. As far as we know, this is the first fabrication of shape-memory films that can simultaneously heal mechanical damage and fatigued shape-memory function. This study paves a new avenue for the fabrication of healable shape-memory polymers and films with extended service life and enhanced shape-memory function.

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EXPERIMENTAL SECTION

Materials. PAA (Mw ≈ 450 000 g mol−1), bPEI (Mw ≈ 750 000 g mol−1), PDDA (Mw ≈ 100 000−200 000 g mol−1), P4VP (Mw ≈ 8000 g mol−1), and FITC were obtained from Sigma-Aldrich. LYC was purchased from Biotium. 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.61 PAA-LYC and bPEI-FITC were prepared following previously reported procedures.37,56 Preparation of Aqueous Dispersions of bPEI-GOm Complexes. With bPEI-GO0.02 complexes as a typical example, the complexes were prepared by dropwise addition of a 50 mL GO dispersion (0.08 mg mL−1) into a 50 mL bPEI (4.0 mg mL−1) solution under continuous stirring. After stirring at room temperature for 24 h, the pH of the aqueous dispersion was adjusted to 9.5. The final concentrations of GO and bPEI in the dispersion were 0.04 and 2 mg mL−1, respectively. The bPEI-GOm complexes with different mass ratios of bPEI to GO were prepared in the same way as that of the bPEI-GO0.02 complexes. Fabrication of PAA/bPEI-GOm Films. Silicon and glass substrates were cleaned by immersing the substrates in heated piranha solution (30% H2O2/98% H2SO4 = 1/3 v/v) for 1 h. Caution: Piranha solution reacts violently with organic materials and should be handled carefully! The cleaned substrates were immersed in an aqueous PDDA solution (1 mg mL−1) for 20 min to obtain a positively charged surface. The PDDA-modified substrate was alternately dipped into aqueous

CONCLUSIONS In summary, by LbL assembly of PAA and bPEI-GO complexes, we have fabricated GO-reinforced shape-memory polymeric films that can efficiently heal the mechanical damage and the fatigued shape-memory function. The healable (PAA/ bPEI-GO0.02)*35 SMP films with well-dispersed GO nanosheets are mechanically robust with a Young’s modulus of 19.8 ± 2.1 GPa and a hardness of 0.92 ± 0.15 GPa and exhibit excellent humidity-induced shape-memory performance. Because of the reversible electrostatic and hydrogen-bonding interactions between PAA and bPEI-GO0.02 complexes, the (PAA/bPEI-GO0.02)*35 films can undergo multiple times of stretching/contracting shape-memory performance and can heal cuts penetrating through the entire film in the presence of water. Meanwhile, the healing of the fatigued shape-memory function is achieved because the healing step enables the restoration of the original conformations of PAA and bPEI polyelectrolytes after multiple times of shape-memory performance. The integration of healing function into (PAA/bPEIGO0.02)*35 SMP films significantly enhances their durability and enables their repeated usage in a precise way. More importantly, 2903

DOI: 10.1021/acsami.7b14588 ACS Appl. Mater. Interfaces 2018, 10, 2897−2906

Research Article

ACS Applied Materials & Interfaces PAA solution (2 mg mL−1, pH 3.5) and an aqueous dispersion of bPEI-GOm complexes (pH 9.5) for 15 min each with an intermediate water wash in three water baths for 2, 1, and 1 min each and N2 drying. The deposition of PAA and bPEI-GOm was repeated until the desired number of deposition cycles was obtained. The PAA/bPEI films were fabricated in a similar way as those of PAA/bPEI-GOm films by replacing bPEI-GO complexes with bPEI. Fabrication of PAA/bPEI-GOm Free-Standing Films. An ethanol solution of P4VP (20 mg mL−1) was spin-coated on a clean silicon or quartz substrate at 3000 rpm for 60 s to produce a P4VP sacrificial layer. The substrate covered with a P4VP layer was treated with oxygen plasma (20 W) for 3 min to produce negatively charged oxygen-containing groups on the surface of P4VP layer. The resulting substrate was immersed in an aqueous PDDA solution (1 mg mL−1) for 20 min to obtain a positively charged surface. Then, (PAA/bPEIGOm)*n films were fabricated on the P4VP/PDDA-modified substrates. Finally, defect-free free-standing (PAA/bPEI-GOm)*n films were obtained by dipping the films in ethanol for 5 min to dissolve the P4VP sacrificial layer. Instruments and Characterizations. AFM images were taken with a commercial instrument, Veeco Company Nanoscope IV, in tapping mode. TGA measurements were performed on a Q500 thermogravimeter (TA Instruments, U.S.A.) with a heating rate of 10 °C min−1 under an N2 atmosphere. The film thickness was measured by a Dektak 150 surface stylus profilometer (Vecco) using a 5 μm stylus tip with a 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). The Young’s modulus, storage modulus, and hardness of the films attached on silicon substrates were measured with an Agilent Nano Indenter G200 with an XP-style actuator and the continuous stiffness measurement (CSM) method. The details of the nanoindentation measurements are available in our previous publications.37,56 Stress−strain curves of the free-standing films were measured with a 410R250 Tension Instrument (TestResources Inc., U.S.A.) at 25 °C and 40% RH. The sample size was 5 × 20 mm, and the tension rate was 10 mm min−1. In each shape-memory cycle, the lengths of the (PAA/bPEI-GO0.02)*35 SMP film in its temporary and permanent shapes were measured by a vernier caliper.

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Funding

This work was supported by the National Natural Science Foundation of China (NSFC grants 21225419) and the National Basic Research Program (2013CB834503). Notes

The authors declare no competing financial interest.

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REFERENCES

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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/acsami.7b14588. Shape-memory processes of the (PAA/bPEI-GO0.02)*35 film (MPG) Synthetic route of the bPEI-GOm complexes, AFM images of GO and bPEI-GO0.02 complexes, cross-sectional SEM images of the (PAA/bPEI-GOm)*35, TGA curves of the (PAA/bPEI-GOm)*35 films with m being 0.01 and 0.04, shape-memory processes of the (PAA/bPEI-GO0.02)*35 film with a permanent “C” shape, bending/unbending shape-memory process of the (PAA/bPEI-GO0.02)*35 films, optical microscope images of healing process of the (PAA/bPEI-GO0.02) film (PDF)

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ABBREVIATIONS PAA, poly(acrylic acid) bPEI, poly(ethylene oxide) PDDA, poly(dimethyl diallylammonium chloride) P4VP, poly(4-vinylphenol) GO, graphene oxide FITC, fluorescein isothiocyanate LYC, lucifer yellow cadaverine TGA, thermal gravimetric analysis RH, relative humidity AFM, atomic force microscopy CLSM, confocal laser scanning microscopy SEM, scanning electron microscopy LbL, layer-by-layer

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Junqi Sun: 0000-0002-7284-9826 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 2904

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