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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Photothermal-Induced Self-Healable and Reconfigurable Shape Memory Bio-Based Elastomer with Recyclable Ability Zhanbin Feng,† Jing Hu,† Hongli Zuo,† Nanying Ning,*,†,‡,§ Liqun Zhang,†,‡,§ Bing Yu,*,†,‡,§ and Ming Tian*,†,‡,§ State Key Laboratory of Organic-Inorganic Composites, ‡Beijing Advanced Innovation Center for Soft Matter Science and Engineering and §Key Laboratory of Carbon Fiber Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China

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†

S Supporting Information *

ABSTRACT: Photothermal-induced self-healable and shape memory materials have drawn much attention due to the rapidly growing technical applications and environmental requirements. As epoxy natural rubber (ENR) is a kind of bio-based elastomer with good mechanical properties, weather resistance, and air impermeability, it is of great significance to incorporate ENR with recyclable, photothermal-induced self-healable and shape memory properties. In this study, we report a simple method to cross-link ENR with dodecanedioic acids (DAs) through esterification reaction, and during the cross-linking process, a little aniline trimer (ACAT, a kind of oligoaniline) was added at the same time. Then, the ENR-DA-ACAT vitrimers that were covalently cross-linked with recyclable, selfhealable, and multiple responsive properties were obtained, which also possessed various functions. As a result of the transesterification reactions at elevated temperatures, the ENR-based vitrimers possess the ability to be reprocessed and self-healed, and the mechanical properties could be maintained even after three consecutive breaking/mold pressing cycles. Besides, the vitrimer is also responsive to near-infrared (NIR) light and pH with the introduction of ACAT, and we also find that ACAT can be used as a catalyst to accelerate the transesterification reaction. Moreover, it is demonstrated that the ENRDA-ACAT vitrimer could also be used to construct the reconfigurable shape memory polymer; the shape fixing ratio and shape recovery ratio are both above 95% in the reconfiguration process, and the multistage shape memory performance can also be achieved by NIR irradiation, which will potentially lead to a wide application for ENR in the field of actuators. KEYWORDS: ENR, transesterification reaction, self-healing, recyclable, reconfigurable shape memory, photothermal effect

1. INTRODUCTION Epoxidized natural rubber (ENR) is a kind of unique elastomer obtained from the epoxidation of natural rubber. Because of the existence of polar oxirane groups on the backbone, ENR has been proved to possess a number of interesting properties, such as good mechanical properties, excellent weather/oil resistance, and low air permeability. Just as the other traditional thermoset rubbers, ENR could also be cross-linked through the reaction of double bonds by using sulfur1 or peroxide2 to form covalent chemical cross-links. However, this permanent cross-link makes the ENR difficult to be recycled, leading to a great challenge to dispose the waste rubbers. To make these thermoset rubbers easily reprocessed like thermoplastics, various dynamic covalent bonds were incorporated into the cross-linked polymer networks,3,4 and some reactions that were used such as the Diels−Alder reaction, which could result in reversible chemical cross-linking,5−8 acylhydrazone bonds,9,10 or siloxane equilibration,11 have also been investigated. Vitrimers, which were first reported by Leibler et al. in 2011, are a new type of polymers that are based on exchange reactions.12 The vitrimers could alter their topologies through © XXXX American Chemical Society

intrinsically and thermally triggered exchange reactions, resulting in the thermal plasticity of the network.13 As a result, vitrimer is a kind of covalently cross-linking material that combines the advantage of desired mechanical properties and the ability to be reshaped and recycled after being covalently cross-linked.14 Since then, many other polymer systems have been reported as vitrimer or vitrimer-like materials, such as citric acid-based polyester vitrimer,15 polylactide vitrimer,16 vitrimer based on olefin,17 disulfide bonds,18−21 imine bonds,14,22−25 boronic ester linkages,26,27 and transalkylation exchange reaction of C−N bonds.28 This new kind of polymer material is covalently cross-linked and could be almost recycled, exhibiting great application prospects in various fields such as actuators,29 electrolyte membrane,30 liquid crystalline,31 and shape memory polymer.32 To introduce the concept of vitrimers into the ENR, the transesterification chemistry was also incorporated. For example, Norvez et al. reported that dicarboxylic acids could Received: October 15, 2018 Accepted: December 20, 2018

A

DOI: 10.1021/acsami.8b18002 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

were all purchased from Sigma-Aldrich. Aniline (AN, 99%) was purchased from Adamas. p-Phenylenediamine (98%) was purchased from TCI. Ammonium persulfate (98%) was purchased from AlfaAesar. 2.2. Synthesis of Aniline Trimer (ACAT, an Oligoaniline). ACAT was synthesized based on the previously described method,39 and the detailed procedures are shown in the Supporting Information. The Fourier transform infrared (FT-IR) spectra and 1H NMR test were used to characterize the structure of ACAT (Figure S1). 2.3. Preparation of the Cross-Linked ENR-DA-ACAT Samples. Different amounts of Zn(Ac)2, DMI, DA, and ACAT were successively mixed with ENR in proportion by using the Haake rheomixer for about 30 min (Table S1). After being mixed, the uncured samples were compressed at 180 °C, and the optimal curing time was determined by the vulcameter (Figure S2). The amounts of DA were 10, 20, and 30 mol % with respect to the epoxy groups. To accelerate the cross-linking reaction, DMI was also added; the amounts of DMI and Zn(Ac)2 were 10 and 20 mol % with respect to the carboxyl groups, respectively. Zn(Ac)2 was added as the transesterification catalyst.34,40 In addition, if ACAT was added as a tetrafunctional cross-linker for epoxy groups, which can also act as the catalyst for transesterification,38 and the amount of ACAT added was only 10 mol % relative to the carboxyl groups, then the topological rearrangement of the formed network was not that difficult. The resulting cross-linked ENR elastomers were named as ENR-DAxACAT, where x represents the mol % relative to the epoxy groups. For comparison, the sample without DA and the sample without ACAT were also prepared, which were named as ENR-ACAT and ENR-DA, respectively. 2.4. Characterizations. The 1H NMR spectra were carried out on a 400 MHz instrument of Bruker AV400 produced in Germany; d6-dimethyl sulfoxide (DMSO) was used as the solvent and tetramethylsilane was used as an internal standard. Fourier transform infrared (FT-IR) spectra were carried out on a Bruker TENSOR 27 Fourier transformation infrared spectrometer of Germany from 4000 to 500 cm−1. An average of about 32 scans with a resolution of 2 cm−1 was recorded for each measurement. Differential scanning calorimetry (DSC) analysis and thermal gravimetric analysis (TGA) were carried out in a STARe system (Mettler-Toledo Inc.) at a heating rate of 10 K/min. DSC analysis was measured from 143 to 473 K, and the TGA was measured from 303 to 1273 K. The above tests were carried out under a nitrogen atmosphere with the nitrogen flow rate of 50 mL/min, and the mass of the desired sample was about 10 mg. Curing characteristics (Figure S2) were determined at 180 °C using a P355C2 vulcameter (Beijing Ruida Yu Chen Instrument Co., Ltd). The mechanical properties of the cross-linked samples were tested using a USA Instron 5567 instrument. The experiment was carried out at room temperature with a tensile rate of 100 mm/min. The sample for test was a dumbbell type (length × width × thickness: 50 × 4 × 1 mm3). Each sample was tested five times under the same condition, and the results are shown in Figure S13. For the cyclic tensile test, the stretching and recovery rates were both set at 100 mm/min, and the results are shown in Figure S14. A rectangular sample (15 mm × 5 mm × 1 mm) was subjected to stress−relaxation experiments, dynamic mechanical analysis (DMA), and dilatometry experiments by using a TA-Q800 DMA device. For stress−relaxation experiments, the sample was first subjected to a preload force of 1 × 10−3 N. After the temperature reached equilibrium at 80 °C, it was kept for 5 min. The sample was then stretched to a strain of 5%; the changes in stress of the sample throughout the experiment were measured. For the DMA test, the dynamic strain was set to 0.5% under a tensile mode, and the frequency and heating rate were 10 Hz and 3 °C/min, respectively. The samples were subjected to temperature scanning from −80 to 100 °C. For dilatometry tests, a stress of 4 kPa was applied throughout the experiment, and the changes of the sample length from −80 to 250 °C were measured at a heating rate of 3 °C/min.

be used as the cross-linking agent to covalently cross-link ENR through the esterification reaction.33 The cross-linking rate was greatly accelerated by adding 1,2-dimethylimidazole (DMI) as an accelerator. Besides, Norvez et al. found that zinc acetate (Zn(Ac)2) could also be used as the transesterification catalyst and the stress of the resulting cross-linked ENR could be relaxed at elevated temperatures, exhibiting vitrimer-like behaviors.34 More recently, Guo et al.35 reported a simple method for the preparation of covalently cross-linked ENRbased vitrimers with recyclable and responsive properties. A number of exchangeable bonds were incorporated into the rubber-carbon nanodot (CD) interphase by using CD as the high functional cross-linker. The cross-linked ENR-CD vitrimer can alter its topology via transesterification reactions in the rubber−CD interphase, endowing the rubbers the ability to be reprocessed and reshaped. At the same time, photothermal-induced smart polymer is a kind of polymer that could change dimensions, shapes, and chemical or physical behaviors depending on the heat generated by photoirradiation. To make the vitrimer possess photothermal effect, various photothermal moieties have been incorporated into vitrimers to endow functional properties.31,36,37 For example, a well-defined molecular oligoaniline has also been used to prepare functionalized vitrimer, and this molecule is well soluble in common solvents and possesses excellent electroactivity.38 Wei et al. prepared covalently crosslinked epoxy-based vitrimers that were responsive to six different stimuli and exhibited photothermal effect by incorporating a small quantity of aniline trimer (ACAT).38 Although the ACAT has been used to construct epoxy resinbased vitrimers, there is no report about the elastomer−ACAT vitrimer composites enabled by transesterification reaction. Meanwhile, due to the near-infrared (NIR) light absorption of ACAT, the photothermal effect of the elastomer−ACAT vitrimer may exist, which may be used for the photohealing or photoinduced shape memory of the elastomer. In this study, we report a simple method for the preparation of covalently cross-linked ENR-based vitrimers with recyclable, self-healable, and multiple responsive properties. ENR is covalently cross-linked by dodecanedioic acids (DAs) via transesterification reaction, and the various functions such as pH and light response are realized by introducing a small quantity of ACAT. The obtained ENR-DA-ACAT vitrimers can change their topology through transesterification reactions at high temperature, endowing the vitrimers with the ability to be self-healed and reshaped. The mechanical properties of ENR-DA-ACAT vitrimers could be maintained even after three consecutive cut/mold pressing cycles. It was also found that ACAT could catalyze the transesterification reaction, which may accelerate the self-healing and shape memory process. Moreover, another important property of ENR-DAACAT vitrimer is the reconfigurable shape memory behavior, and the addition of ACAT makes the vitrimer also exhibit light-induced multistage shape memory behaviors, which will potentially lead to a wider application for ENR in the field of actuators.

2. EXPERIMENTAL SECTION 2.1. Materials. Epoxidized natural rubber (ENR) with an epoxidation degree of 25% was purchased from the Agricultural Products Processing Research Institute of Chinese Academy of Tropical Agricultural Science, China. Dodecanedioic acid (DA, 99%), zinc acetate (Zn(Ac)2, 98%), and 1,2-dimethylimidazole (DMI, 99%) B


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Scheme 1. (a) Synthetic Protocol of the Cross-Linked ENR-DA-ACAT Network; (b, c) An Illustration of Reversible Transesterification

TA DMA Q800 was also used to carry out the quantitative shape memory measurements. After reaching equilibrium at 80 °C for 3 min, the sample was stretched to 0.5 N at a rate of 0.2 N/min. Then, the sample was rapidly cooled to about −60 °C and the temperature was kept for 5 min to fix the temporary shape. The force applied to the sample was then removed at a rate of 0.2 N/min until a preload of 10−3 N was reached. After equilibrium for about 5 min, the temperature was increased to 80 °C and kept for 5 min to recover the permanent shape. The reprogramming of the permanent shape could be achieved by stretching the sample at 200 °C for about 45 min at a constant force of 0.2 N and then recovering for 10 min. The new shape memory cycle was also performed by using the above condition. The fixing ratio (Rf) and recovering ratio (Rr) were used to reflect the shape memory behavior quantitatively. The Rf and Rr were calculated by eqs 1 and 2, respectively.41 R f = 100% ×

ε εload

ε − εrec R r = 100% × ε

infrared (NIR) light for about 15 min. The self-healing efficiency was calculated by eq 342 HE =

E healed × 100% Eoriginal

(3)

where Eoriginal and Ehealed correspond to the tensile energy (E) for original and healed samples, respectively; the tensile energy (E) was calculated based on the area that was under the stress−strain curve. Besides, Eoriginal represents the tensile energy (E) of the original sample; Ehealed represents the tensile energy (E) of the sample that was self-healed. Recycling experiment was carried out in the compression molding machine. The samples for the tensile test were cut into some small pieces, followed by placing into a mold (50 × 50 × 1 mm3) and reprocessing at 200 °C under the pressure of 15 MPa for about 20 min. The recycled sample could be obtained after cooling back to room temperature. The recyclability efficiency was calculated by eq 4

(1)

efficiency = (2)

Erecyclable Eoriginal

× 100% (4)

where Eoriginal and Erecyclable correspond to the tensile energy (E) for original and recyclable samples, respectively; the tensile energy (E) was calculated based on the area under the stress−strain curve.

where εload is the strain under load upon the cooling process, ε corresponds to the strain when the load is removed before heating, and εrec represents the strain when the heating process is accomplished under a stress-free condition. The light-responsive performance was carried out in the atmosphere. A laser with the wavelength of 808 nm was selected as the light source, and the distance between the samples and the light source was 10 cm. The energy density irradiated to the sample was 3 W/cm2. The self-healing property of the crack in the films was observed by using the scanning electron microscope (SEM, S-4800) at accelerated electron energy of 5.0 kV. The sample for tensile test was first cut off, and then the separated film surface was recontacted in the hot stage at 200 °C for about 15 and 30 min, separately, or irradiated by near-

3. RESULTS AND DISCUSSION 3.1. Covalent Cross-Linking of ENR with DA and ACAT. The vitrimer was prepared via the ring-opening reaction, which occurred between carboxyl groups of DA and epoxy groups of ENR (Scheme 1). During the reaction process, the carboxyl groups or the epoxy groups in the polymer system can further react with the hydroxyl groups generated, following which the polymer network is formed. At the same time, in this study, ACAT was also added, which could be also regarded as a cross-linker with four C


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Figure 1. (a) FT-IR spectra of ACAT, uncured ENR-DA-ACAT, and cured ENR-DA-ACAT. (b) Variations of hydroxyl groups (∼3530 cm−1) and (c) carboxyl groups (∼1820−1560 cm−1) as well as epoxy groups (∼960−760 cm−1) of FT-IR spectra in the curing process of ENR-DA-ACAT at 180 °C.

process, carboxyl and epoxy groups were consumed and hydroxyl groups were generated.43 In addition, to monitor the cross-linking process of the ENR-DA-ACAT, the variations of the epoxy groups and the ester groups during in situ cross-linking were observed by using in situ FT-IR at 180 °C. As the cross-linking reaction proceeds, absorption peak intensity of the epoxy group at 877 cm−1 decreased gradually, whereas the absorption peak intensity of the ester group at 1722 cm −1 gradually increased. The peak intensities for the epoxy group and the ester group remained nearly constant after 40 min, indicating that the reaction was completed in about 40 min (Figure 1b,c). The actual crosslinking time of ENR-DA-ACAT could be determined by monitoring the torque of the sample in the vulcanizer (Figure S2). A sharp increase of the torque was observed at ∼5 min, which indicated that the cross-linking reaction for the ENRDA-ACAT started; finally, we observed that the torque remained almost unchanged at about 40 min, indicating that the cross-linking reaction had been completed. The occurrence of the cross-linking was also verified by solubility testing. As shown in Figure S3, the ENR-DA20-ACAT vitrimers were not soluble in DMSO even after immersing in the solvent for 6 h at a high temperature, and this behavior was similar to the vitrimers based on dynamic covalent bonds reported in other references.23,44 In our opinion, this superior property can be explained by the characteristic of transesterification reaction. As a kind of exchange bonds, the bond-forming and bondbreaking of the ester bond occur at the same condition and rate; thus, cross-linking density does not alter during the

functionalities. During the curing process, a ring-opening reaction occurred between the amino groups and the epoxy groups. Three kinds of samples with various cross-linkdensities were synthesized through the different addition amounts of DA in the preparation process (named as ENRDA10-ACAT, ENR-DA20-ACAT, and ENR-DA30-ACAT). According to the gel fraction (Table S2) and mechanical properties (Figures S9 and S10), the sample of ENR-DA20ACAT has suitable mechanical strength (about 1.5 MPa), elongation at break (about 180%), and a suitable modulus (about 2 MPa). Therefore, the sample of ENR-DA20-ACAT was chosen to carry out the further study. The occurrence of the chemical reaction was further verified with FT-IR. The FT-IR spectrum of the cured ENR-DAACAT is shown in Figure 1a, which indicates that the absorptions of carboxyl groups at 3430 cm−1 disappeared, whereas the absorption for epoxy group at 877 cm−1 decreased in the cured ENR-DA-ACAT compared with the uncured sample. Meanwhile, the N−H stretching vibration absorption in terminal amino groups (3310 and 3210 cm−1) of ACAT disappeared, whereas the absorptions at ∼3530 and 1722 cm−1 appeared in the spectrum of cured ENR-DA-ACAT, which were attributed to the O−H stretching vibrations in hydroxyl groups and CO stretching vibrations in ester groups, respectively (Figure 1a). These results confirmed the occurrence of the cross-linking reaction between carboxyl/ amino groups and epoxy groups, and the ACAT was fully integrated into the vitrimer network. During the cross-linking D


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Figure 2. (a) Stress−relaxation curves after normalization at different temperatures. (b) Linear fitting based on the Arrhenius equation according to the relaxation time determined by the experiments.

well confirmed by dilatometry experiments.12,35 In Figure 3, the slope of the curve for the ENR-DA20-ACAT was nearly

exchange reactions, resulting in the ability of maintaining its topology in solvent at elevated temperatures.45,46 3.2. Stress−Relaxation Experiments. To use or reprocess the obtained vitrimers at different temperatures, the flow properties of the cross-linked ENR networks at different temperatures were studied by stress−relaxation experiments. The changes of relaxation modulus at a strain of 5% were measured after different times of heating. In Figure 2a, the sample of ENR-DA20-ACAT exhibited significant stress relaxation at above 190 °C over time according to stress−relaxation experiments, further indicating that this covalent cross-linked network possessed dynamic property at elevated temperatures. However, the relaxation was not complete even at about 210 °C; this might be due to the formation of irreversible ether bond cross-linking by DMIcatalyzed epoxy homopolymerization.35 According to the relaxation modulus after normalization, the relaxation time was also determined at 37% (1/e) of σ/σ0, and the correlation between the relaxation time and the temperature satisfied the Arrhenius law.47 With the increase of temperature, the relaxation time decreased from 4161 s at 160 °C to 790 s at 210 °C. As shown in Figure 2b, the relationship between relaxation time and temperature satisfied the Arrhenius law, and we could calculate the activation energy of ENR-DA-ACAT vitrimer based on the slope of the line (Ea ∼ 58.5 kJ/mol). Although Leibler et al.40 also reported a vitrimer based on transesterification reaction (Ea ∼ 80−90 kJ/ mol), the Ea of ENR-DA-ACAT was much lower. A controlled stress−relaxation experiment of ENR-DA20 and ENR-DA20ACAT was carried out, which is shown in Figure S4. The relaxation time τ of ENR-DA20-ACAT was much shorter than that of ENR-DA20. Moreover, it is reported that the transesterification could be catalyzed by tertiary amine moieties, which could lead to lower activation energy.38,48 So it may be due to the catalysis effect of tertiary amine moieties formed by ACAT that reduced the Ea of the ENR-DA-ACAT vitrimer. The glass transition temperatures (Tg) of the samples were determined by DMA, and the results are shown in Table S2. As a kind of rubber, the Tg values of the ENR-DA-ACAT samples were between −21.6 and −6.3 °C and increased with the crosslinking density. In addition, vitrimers possessed another topology freezing transition, and the temperature (Tv) corresponded to the temperature when the vitrimer transited from the solid state to the liquid state due to the exchange reactions in the polymeric network.12 The existence of Tv was

Figure 3. Dilatometry experiments for ENR-DA-ACAT under the heating rate of 3 °C/min.

unchanged from 70 to 180 °C, which could be regarded as a permanently cross-linked network at this range of temperature. However, with the occurrence of transesterification reactions, the slope of the curve increased sharply when the temperature increased to above 180 °C. Moreover, an increase of the slope was also observed in the strain−temperature curve near 50 °C, which was derived from the glass transition. These two transition points can be used as the temperature for the following shape memory experiments. 3.3. Self-Healing and Shape Memory Property of ENR-DA-ACAT Vitrimer. 3.3.1. Self-Healing Performance of ENR-DA-ACAT Vitrimer. Because of the existence of the transesterification reaction in the vitrimers, our ENR-based vitrimer may potentially possess the characteristic of selfhealing. To investigate this self-healing process, a small scratch was made on the surface of the cross-linked ENR-DA-ACAT sample by a blade. The ENR-DA-ACAT sample with the scratch was then set onto the heating stage whose temperature was about 200 °C, and the healing process of the crack was observed through scanning electron microscopy (SEM). As shown in the SEM images (Figure 4a−c), the crack on the sample of ENR-DA20-ACAT became gradually smaller and finally almost disappeared when equilibrating at 200 °C on the heating stage for about 30 min, indicating that the self-healing process could be achieved by direct heating at about 200 °C for the cross-linked ENR-DA-ACAT sample. Moreover, the E


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Figure 4. (a−c) SEM images of ENR-DA-ACAT vitrimer after being self-healed for 0, 15, and 30 min at 200 °C; (d) sample of ENR-DA-ACAT vitrimer was cut into two halves; (e) the two halves were self-healed completely after heating at 200 °C; (f) stress−strain curves of ENR-DA-ACAT vitrimer sample after and before healing.

Figure 5. Shape recovery process and reconfiguration of ENR-DA20-ACAT vitrimer. Experiment conditions: (a → b) heated at 80 °C for about 1 min and then cooled at 0 °C; (b → c) heated at 80 °C; (c → d) heated at about 200 °C for about 10 min and then cooled at 0 °C; (d → e) heated at 80 °C for about 1 min and then cooled at 0 °C; (e → f) heated at 80 °C.

self-healing characteristic could be only attributed to the transesterification reaction in the vitrimers as no signal of melting was observed for the cross-linked ENR networks during the healing process. At an elevated temperature, the crack became smaller as a result of the compression formed at

the back of the interface; then, the crack could be healed after the formation of new networks due to the reconnecting of liberated polymer chains. This self-healing behavior could not only occur at the surface of the samples, but also occur at the sectional surface of the dumbbell specimens (Figure 4d,e), and F


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Figure 6. (a) Reversible doping/undoping of ENR-DA-ACAT vitrimer. (b) Extension−contraction behaviors of an ACAT−vitrimer film under the stimulus of pH (acid solution was 1 M PTSA/tetrahydrofuran (THF) solution and base solution was 1 M TEA/THF solution).

Figure 7. (a) Temperature curves of ENR-DA-ACAT vitrimer and the sample without ACAT (ENR-DA vitrimer) dependent on NIR light irradiation time. (b) Temperature of ENR-DA-ACAT vitrimer when irradiated for 900 s. NIR-light-triggered healing of ENR-DA-ACAT vitrimer with crack healed by using local irradiation for (c) 0 min and (d) 15 min.

above the Tv measured in the dilatometry experiments. The shape recovery process and reconfiguration of ENR-DA20ACAT vitrimer are illustrated in Figure 5 as an example. A typical thermally triggered shape transition of ENR-DA20ACAT vitrimer between permanent rodlike shape and temporary “Z” shape or “knot” shape is shown in Figure 5b1,b2, respectively, and the different temporary shapes could almost return to their original states in the following shape recovery step (Figure 5c), demonstrating that the ENR-DA20ACAT vitrimer could exhibit excellent basic one-way shape memory behavior. Meanwhile, the shape of the vitrimer sample could also be reconfigured at about 200 °C, when transesterification reaction occurred in the polymer network. Figure 5d−f demonstrates the ability of ENR-based vitrimer to reconfigure its shape. This rodlike permanent shape could be reprocessed to a fan shape as the second permanent shape; then, a temporary “M” shape can be fixed elastically. The temporary M shape can fully recover to the fan-shaped

the self-healing efficiency could be calculated based on the tensile energy (E) ratio of the ENR-DA20-ACAT sample after and before healing. The stress−strain curves for ENR-DA20ACAT samples after and before healing are shown in Figure 4f. The self-healing efficiency was about 80% according to the tensile energy (E) ratio, indicating that the ENR-based vitrimer exhibited the self-healing ability. 3.3.2. Shape Memory and Reconfigurable Performance of ENR-DA-ACAT Vitrimer. Shape memory polymers are a kind of smart materials that could memorize their temporary shapes and recover the permanent shapes under external stimulus (such as heat, light, pH, etc.). According to the dilatometry experiments in Figure 3, shape memory behaviors could be designed based on our ENR-DA-ACAT samples. In the shape memory experiment, the thermal-induced temperature was about 80 °C, which was above the first transition temperature in Figure 3, and the shape was fixed at about 0 °C; the shape reconfiguration temperature was about 200 °C, which was G


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ACS Applied Materials & Interfaces Scheme 2. Mechanism of NIR-Induced Self-Healing Behavior

ENR-DA-ACAT vitrimer could expand in acid p-toluenesulfonic acid (PTSA) and contract in base (triethylamine, TEA), and this process was reversible for several times, exhibiting a pH-induced shape memory behavior. Besides, the ENR-DA could not possess the same response behavior as the ENR-DAACAT (Figure S7), so ACAT was essential for the pHresponsive performance of ENR-DA-ACAT vitrimer. 3.4.2. Light Responses Triggered by the Photothermal Effect of ACAT. Because of the existence of the large conjugation groups, polyaniline moieties were also reported to possess a photothermal effect.52 The photothermal effect of the ACAT in our ENR vitrimers was also investigated, and an infrared thermal imager was used to monitor the temperatures of ENR-DA and ENR-DA-ACAT vitrimers after the samples were irradiated under the near-infrared (NIR) light at a wavelength of 808 nm for ∼900 s. As shown in Figure 7a, the ENR-DA-ACAT vitrimer possesses a much higher temperature than that of ENR-DA. For example, after the NIR irradiation for 900 s, the temperature of the ENR-DA-ACAT vitrimer sample increased to ∼240 °C, whereas that of ENR-DA was only about 60 °C, indicating obvious thermal effects induced by NIR irradiation (Figure 7b). This photothermal effect could be utilized to realize the photoinduced self-healing of the ENRDA-ACAT samples, and the mechanism is shown in Scheme 2. Similar to the self-healing triggered by heating, a small scratch was made on the surface of the cross-linked ENR-DA-ACAT sample by a blade, and then the ENR-DA-ACAT sample with the scratch was irradiated under NIR; the healing process of the crack was observed via SEM. As shown in the SEM images (Figure 7c,d), the crack on the film of ENR-DA20-ACAT sample healed completely via NIR light for about 15 min. It was noted that only the crack part was directly irradiated by the NIR to achieve self-healing, whereas the other part of the

permanent shape during the recovery process, exhibiting that this kind of vitrimer possessed good shape memory and reconfigurable performance. The quantitative shape memory cycle of ENR-DA-ACAT sample was also carried out, and the results are shown in Figure S5. The permanent shape of ENR-DA20-ACAT vitrimer was constructed based on dynamic transesterification cross-links, serving to rememorize the permanent shape; both the shape fixing ratio (Rf) and the shape recovery ratio (Rr) are above 95% for the reconfiguration process. The original shape of ENR-DA-ACAT vitrimer could be reprogrammed by transesterification reaction. As a result, besides a single shape memory evaluation, a cycling shape memory experiment of ENR-DA-ACAT was also carried out. As shown in Figure S6, both of the Rf and Rr were above 90% throughout the three cycling experiments, indicating the ENR-DA-ACAT vitrimers possessed excellent and repeatable shape memory performances. 3.4. Multifunctionality of ENR-DA-ACAT Vitrimer. 3.4.1. pH-Responsive Performance of ENR-DA-ACAT Vitrimer. As a kind of pH-responsive moieties, polyaniline and its oligomers exhibit the characteristic of doping in acid49 and undoping in base,50 resulting in the variation of its polarity (Figure 6a). When these moieties were introduced into the polymer matrix, a kind of pH-induced responsive performance was possibly realized, whose mechanism was a little different from the heat-induced shape memory behavior based on Tg or Tv.51 In this research, the aniline trimer (ACAT) was introduced into the network of ENR vitrimer. According to the previous research reported by Wei et al.,38 doping of the ACAT could cause the extension of ACAT vitrimer, whereas undoping of the ACAT could cause the contraction of the ACAT vitrimer. As shown in Figure 6b, the rectangular film of H


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Figure 8. Multistage shape recovery of the sample constructed by NIR irradiation: (a) original shape, (b) temporary shape after being programmed, (c) local shape recovery after being NIR irradiated, and (d) permanent shape after undergoing another local NIR irradiation.

Scheme 3. Mechanism of NIR-Induced Multistage Shape Recovery Performancea

a

(a) Original shape, (b) temporary shape after being programmed, (c, d) local shape recovery after being NIR irradiated, and (e) permanent shape after undergoing another local NIR irradiation.

Figure 9. (a) Recycling study: process of heat pressing ENR-DA-ACAT vitrimer pieces into a solid film, (b) stress−strain curves obtained from the experiment performed in (a).

sample was maintained at room temperature, which reduced the possibility of thermal oxygen aging caused by heating compared with thermal self-healing using a hot stage at about 200 °C. Meanwhile, the ENR-DA could not be healed even when irradiated by NIR for the same time (Figure S8c,d) as it could not reach the temperature that activated the transesterification reaction. The ENR-ACAT could also not be healed (Figure S8a,b) because of the absence of exchange bonds although the temperature could reach about 200 °C (Figure S9) after being irradiated for 15 min. So the simultaneous presence of DA and ACAT in ENR-DA-ACAT vitrimer was required to achieve NIR-induced self-healing. The photothermal effect was also used to achieve the multistage shape memory performance as shown in Figure 8,

and the mechanism is shown in Scheme 3. The ENR-DAACAT vitrimer sample was first programmed into a spiral shape at 80 °C (Figure 8b); as thermal effect could only be generated at the NIR irradiation part, gradual NIR irradiation of different parts of the sample could lead to sequential shape recovery (Figure 8c,d). It would potentially lead to wider application of shape memory performance for ENR-based vitrimers via NIR irradiation. 3.4.3. Recyclability of ENR-DA-ACAT Vitrimer. To investigate this recyclability of the ENR-DA-ACAT vitrimer, the ENR-DA-ACAT vitrimer samples were cut into small pieces and then recycled through thermal pressing under 15 MPa pressure at 200 °C for 20 min. As shown in Figure 9a, a defectfree polymer film could be obtained even after three times of I


Research Article

ACS Applied Materials & Interfaces recycling process. Then, we investigated the mechanical properties and thermal properties of the recycled materials. Taking ENR-DA20-ACAT as the example, the stress−strain curves of the original and the recycled ENR-DA-ACAT vitrimer are shown in Figure 9b; the mechanical properties could be maintained (about 88% after the third recycling) for the recycled samples. Meanwhile, the glass transition temperatures of the samples did not change obviously during the recycling process as well as the decomposition temperature, which are shown in the DSC and TGA curves, respectively (Figures S10 and S11), indicating neither mechanical strength loss nor thermal degradation through three times of recycling of the ENR-DA-ACAT vitrimer. To confirm this assumption, FT-IR was also performed to monitor the structure changes of the recycled materials. As shown in the ATR-FT-IR spectra of the recycled samples (Figure S12), the absorption at 1722 cm−1 (ester stretch vibration) did not change after three times of recycling, exhibiting that no degradation occurred through the transesterification reactions and the cross-linking density of the recycled samples did not change compared to the original one. Overall, this ENR-DA-ACAT vitrimer exhibited considerable recyclability, indicating a considerable stability of the network during reprocessing and high-temperature experiments.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N.N.). *E-mail: [email protected] (B.Y.). *E-mail: [email protected]. Tel: +86 10 64456158. Fax: +86 10 64433964 (M.T.). ORCID

Liqun Zhang: 0000-0002-8106-4721 Ming Tian: 0000-0002-4820-7372 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to express our sincere thanks to National Natural Science Foundation of China (Grant nos. 51525301 and 51521062) for financial supports.

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4. CONCLUSIONS In this study, we report a simple method for the synthesis of covalently cross-linked ENR-based vitrimers with recyclable, self-healable, and macroscopically responsive properties. ENR is covalently cross-linked by using dodecanedioic acids (DAs) as cross-linker via transesterification reaction, and various functions are achieved by adding a small quantity of ACAT into the vitrimer. These ENR-DA-ACAT vitrimers can change their topology networks and relax stress via transesterification reactions at high temperatures. Besides, the ENR-based vitrimers have the ability to be reprocessed and self-healed, and the mechanical properties could be maintained even after three consecutive cut/mold pressing cycles. Especially, the vitrimer is also responsive to light and pH due to the addition of ACAT, resulting in a photoinduced self-healing behavior. We also find that ACAT could catalyze the transesterification reaction, which may accelerate the self-healing process. Moreover, we demonstrate that ENR-DA-ACAT vitrimer can be used as reconfigurable shape memory materials with the shape fixing ratio and shape recovery ratio above 95% in the reconfiguration process, and the addition of ACAT makes the vitrimer also exhibit light-induced multistage shape memory behaviors, which will potentially lead to a wider application for ENR in the field of actuators.

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ACAT and ENR-DA after being self-healed for 0 and 15 min irradiated by NIR; normalized stress−relaxation curves at 180 °C for ENR-DA20 and ENR-DA20-ACAT (PDF)

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.8b18002. Synthesis and characterization of ACAT; different samples of the ENR-DA-ACAT vitrimer; summary of the physical properties of ENR-based vitrimer; crosslinking kinetics of ENR-DA-ACAT; quantitative shape memory and shape reconfiguration cycles; DSC curves, TGA curves, FT-IR spectra, and mechanical properties of ENR-DA20-ACAT vitrimer; images of ENR-DA that was placed in THF, acid and base; SEM images of ENRJ


Research Article

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