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CNTs nanocomposite elastomers with high stretchability and toughness based on novel dual-dynamic covalent sacrificial system
Composites Part B 177 (2019) 107270
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Degradable, reprocessable, self-healing PDMS/CNTs nanocomposite elastomers with high stretchability and toughness based on novel dual-dynamic covalent sacrificial system Chi Lv, Jinke Wang, Zhongxiao Li, Kaifeng Zhao, Junping Zheng * Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Degradability Reprocessability High stretchability Toughness Self-healing
Synthesizing materials that possesses degradability, reprocessability and self-healability simultaneously without compromising stretchability and toughness is considered a huge challenge. In this paper, a pioneering dualdynamic covalent sacrificial system is exploited for the fabrication of PDMS/CNTs nanocomposite. In the network structure, aromatic disulfide bonds and imine bonds act as sacrificial units and semi-permanent crosslinking points, respectively. The unique design endows the nanocomposite elastomers with high elonga tion at break (up to 1420%), high toughness (up to 5000 kJ m 3) and good tensile strength (up to 1.10 MPa). Moreover, these mechanical properties can be regulated by varying the chemical composition and CNTs content. Additionally, the nanocomposite elastomers exhibit excellent self-healing efficiency (12 h, 95% of toughness) at ambient temperature and the ability to be processed multiple times. More importantly, the nanocomposite elastomers can be degraded controllably by three ways, achieving the complete recycling of CNTs and PDMS. We believe that our design strategy will provide a new way for the development of green stretchable tough materials used in a sustainable way.
1. Introduction Polysiloxane elastomers are fascinating materials due to their impressive intrinsic properties, such as nontoxicity and excellent flexi bility at broad temperature ranges [1]. They have attracted significant attention in the applications of electronic skins, artificial muscles, and sensors recently, especially for stretchable devices [2–4]. However, most neat polysiloxane elastomers exhibit relatively poor stretchability and toughness, which severely limits their practical applications. Besides, they often be damaged in their lifecycle, which causes security risks and huge resources waste. Therefore, it is of significance to design sustain able silicone elastomer with high stretchability and toughness. Recently, materials with self-healability and reprocessability have been attracting much interest because they can greatly reduce the environmental pollution and material consumption. Thermosetting polymers often lack these abilities after curing because the molecular chains are crosslinked by permanent chemical bonds. People have tried to solve the problem by replacing the permanent bonds with dynamic covalent bonds and dynamic noncovalent interactions. The former in volves the hindered urea bond [5], boroxines linkage [6], Diels-Alder
reaction [7], disulfide exchange [8], and imine bond [9], etc, while the latter deals with host-guest interaction [10], ionic interaction [11], hydrogen bond [12,13], and metal-ligand coordination [14], etc. However, the mechanical properties of material are often compromised due to the incorporation of dynamic bonds. A majority of materials with dynamic bonds are not stretchable and not tough because these dynamic bonds are prone to preferential dissociation under load. Therefore, it’s difficult to obtain a stretchable and tough material in the presence of dynamic bonds. Degradability, as another promising property, enables the materials to be completely recycled. However, a majority of self-healing materials don’t possess degradability. For example, per manent covalent bonds are often introduced into some self-healing systems to increase mechanical strength [15,16], which makes the ma terials difficult to degrading. In addition, in order to obtain degradable materials, the crosslinking bonds need to be completely dissociated, but many dynamic bonds don’t have the ability [17–19]. Therefore, it’s a huge challenge to combine so many properties, such as degradability, reprocessability, stretchability, toughness and self-healability in one material. To achieve stretchability and toughness, one strategy is to
* Corresponding author. E-mail address: [email protected] (J. Zheng). https://doi.org/10.1016/j.compositesb.2019.107270 Received 2 April 2019; Received in revised form 20 July 2019; Accepted 9 August 2019 Available online 10 August 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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constructing sacrificial bond system. It is considered to be effective for improving stretchability and mechanical strength [20–24]. The sacrifi cial bonds preferentially rupture during the large deformation, thus resulting energy dissipation. This energy dissipation plays an important role in improving mechanical properties. When the dynamic bond is used as sacrificial unit, the material will be given self-healability. For example, Gong et al. [25] designed a novel tough hydrogel by intro ducing multiple ionic bonds. The mechanical properties can be tuned by varying ionic combinations. Guan et al. [26] reported a self-healing material that combine outstanding mechanical performance and excel lent self-healability by introducing sacrificial hydrogen bond. Guo and Huang et al. [27] developed a high strength and high stretchability polyisoprene rubber. In this rubber network, two sacrificial bonds are employed. Although various sacrificial bond systems have been devel oped, most of these sacrificial bond systems involved in permanent chemical crosslinking, which makes it impossible to reprocess and degrade the crosslinking network. In order to obtain tough materials, another alternative method is to preparing nanocomposite. Various nanofillers, such as nano-silicon di oxide, graphene or carbon nanotubes (CNTs), are used to improve toughness [28]. Although huge progresses have been achieved, the self-healing efficiency is restricted. It has been demonstrated that the introduction of nanofillers reduces chain mobility, which is a key factor in determining self-healing properties [29]. For instance, Zhang et al. [30] reported a GO/polyurethane nanocomposite, the self-healing effi ciency is only around 70% after healing at 150 � C. In addition, these nanofillers are usually modified by permanent covalent bonds, making the prepared nanocomposite difficult to be reprocessed and degraded. Here, we designed a unique sacrificial system with two dynamic covalent bonds: a strong yet semipermanent dynamic covalent bond (imine bond) [31] and a weak sacrificial dynamic covalent bond (aro matic disulfide bond) [32]. Unlike aliphatic disulfide bond and other dynamic bonds, aromatic disulfide bond has weaker bond energy due to the presence of aryl groups. Therfore, it can be reversibly exchanged at ambient temperature, compensating for the self-healability of nano composites. Imine bond, also show excellent dynamic property for reprocessing and self-healing. Particularly, the exchange reaction be tween aldehydes and amines and the pH response ability endows the material with unique degradability [9,33]. Such combination provides us with a viable strategy for reprocessing and degrading sacrificial bond systems. To our knowledge, this is the first degradable and reprocessable sacrificial network. And this dual-dynamic covalent sacrificial system is applied to nanocomposite for the first time. In present work, we prepared degradable, reprocessable, self-healing PDMS/CNTs nanocomposite elastomers with high stretchability and toughness by constructing a novel dual-dynamic covalent sacrificial system. The nanocomposite elastomers exhibit outstanding stretch ability and toughness, and the mechanical performance of the nano composite elastomers can be tuned by varying content of aromatic disulfide bonds and CNTs. In addition, these nanocomposite elastomers also show excellent room-temperature self-healability and excellent reprocessability. More importantly, in the presence of competing alde hyde, stronger nucleophile and acid, the nanocomposite elastomers can be completely degraded, achieving the recycling of CNTs and PDMS. All of these properties can give the nanocomposite elastomer a great po tential as a green material for sustainable use.
makes the prepared nanocomposite easier to be reprocessed and degraded. In consideration of the bifunctional nature of pyrenecarbox aldehyde (PA), PA is chosen as the functional molecule. With simple stirring, PA molecule can interact strongly with CNTs via π-π in teractions. To confirm the attachment of PA onto CNTs surfaces, TGA was applied. The TGA curves of pristine-CNTs, CNTs/PA and PA are shown in the Fig. S1. It can be seen that the pristine-CNTs has hardly any weight loss, while the CNTs/PA with a weight loss of 4 wt %. The FT-IR of CNTs/PA was also performed (Fig. S2). As shown in the infrared spectrum of PA, the peak at 1595 cm 1 is ascribed to the stretching vi bration of benzene ring and the peak at 1680 cm 1 is ascribed to the aldehyde group. These characteristic peaks of PA appear in the infrared spectrum of CNTs/PA and these peaks are shifted to lower wavenumber. The phenomenon may be attributed to π-π interactions and hydrophobic interactions between the walls of the CNTs and the pyrene groups. Additional evidence of the CNT functionalization was provided by Raman spectroscopy. As we all known, the D band reflects disordered graphite structure and defects, while the G band reflects the sp2 hy bridized carbon atoms. ID/IG, the intensity ratio of D to G bands is widely used to evaluate the defects and degree of functionalization. As shown in Fig. S3, the ID/IG are 1.05 and 0.94 for pristine-CNTs and CNTs/PA, respectively. The decrease of ID/IG indicates that the original defects are covered by the PA molecules and the adsorption of PA decreases the disorder of graphene layers. These above investigations verified that the CNTs have been successfully functionalized by PA.
2. Results and discussion
Firstly, to verify the sacrificial nature of the dual-dynamic covalent sacrificial system, various neat PDMS elastomers those without adding CNTs were prepared. (Molar ratios: PDMS-1S: H2N-PDMS-NH2/TFB/ APDS ¼ 2: 2: 1; PDMS-2S: H2N-PDMS-NH2/TFB/APDS ¼ 1: 2: 2) As a comparison, we replaced the disulfide bond with a methylene bridge and synthesized control samples. (Molar ratios: PDMS-1S-ctrl: H2NPDMS-NH2/TFB/DADPM ¼ 2: 2: 1; PDMS-2S-ctrl: H2N-PDMS-NH2/ TFB/DADPM ¼ 1: 2: 2). The stress-strain curves are shown in Fig. 2a. Despite the decrease in tensile strength, PDMS-2S and PDMS-1S all
2.2. Preparation of PDMS/CNTs nanocomposite elastomers The typical procedure for preparation of PDMS/CNTs nano composite elastomers is presented in Fig. 1a. H2N-PDMS-NH2, 4-amino phenyl disulfide (APDS), 1, 3, 5-triformylbenzene (TFB), and CNTs/PA were added into a reaction bottle to form uniform slurry by a one-pot reaction process. After the solvent evaporated, the nanocomposite elastomers were fabricated by hot press molding. In this reaction, TFB acts as the crosslinker and the crosslinking is formed by condensation reaction of amino group and aldehyde group. The newly formed imine bond exists in the cross-linking point and is mainly responsible for pri mary crosslinking. Unlike the imine bond, the aromatic disulfide bond is introduced directly through the APDS during the crosslinking process. It should be pointed out that since the aldehyde group content on CNTs/ PA is much lower than TFB, we believe that the nanocomposite elas tomer is still mainly chemically crosslinked by TFB. The formation of imine crosslinked network was verified by FT-IR spectrum. As seen in Fig. 1b, the peak at 1695 cm 1 is ascribed to the stretching vibration of aldehyde group in TFB linker, but this characteristic peak is barely detectable in the infrared spectrum of nanocomposite elastomer. Meanwhile, a new peak appears at 1648 cm 1. This peak is assigned to the newly formed imine bond. The dispersion of CNTs was studied by XRD. XRD patterns (Fig. S4) reveal a peak of CNTs/PA at 2θ position of 26� , while the PDMS/CNTs nanocomposite elastomer shows a XRD pattern without the sharp peak, indicating that the CNTs are well dispersed [34]. The morphology of fracture surface was investigated by SEM (Fig. S5). This further proved that the CNTs are uniformly dispersed. 2.3. Dynamic features of PDMS/CNTs nanocomposites elastomers
2.1. Characterization of the aldehyde-functionalized carbon nanotubes (CNTs/PA) In this work, the CNTs are functionalized by aldehyde group. These aldehyde groups can effectively react with amino groups to form imine bond. The presence of imine bonds on the surface of the CNTs not only increases the compatibility of the polymer matrix and the CNTs, but also 2
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Fig. 1. (a) Illustration for preparation of the PDMS/CNTs nanocomposite elastomers. (b) FT-IR spectra of a) TFB and b) PDMS/CNTs nanocomposite elastomer.
to 103.9 kJ m 3, the energy dissipation ratio increase from 27.7% to 54.8% (Fig. 3d). The result shows that the PDMS/CNTsx-1S elastomers exhibit lower energy dissipation but higher energy dissipation ratio under the same CNTs content as PDMS/CNTsx-2S. To illustrate the effect of aromatic disulfide bond content on energy dissipation, we prepared two other control samples PDMS/CNTs1.0-0.5S and PDMS/CNTs1.0-1.5S. The cyclic tensile tests were conducted. (Aromatic disulfide bond con tent: 0.19 mmol/g for PDMS/CNTs1.0-2S; 0.09 mmol/g for PDMS/CNTs1.0-1.5S; 0.05 mmol/g for PDMS/CNTs1.0-1S; 0.02 mmol/g for PDMS/CNTs1.0-0.5S) The dissipation energies of PDMS/CNTs1.0-0.5S and PDMS/CNTs1.0-1.5S are 37.8 and 68.5 kJ m 3, respectively. Under the same strain, the dissipation energies of PDMS/CNTs1.0-1S and PDMS/CNTs1.0-2S are 42.3 and 94.7 kJ m 3, respectively (Fig. 3e and f). With the increase of aromatic disulfide bonds, the dissipation energy shows an increasing trend. The results indicate that the content of aro matic disulfide bonds indeed play a key role in dissipating energy. The more the aromatic disulfide bonds, the more the energy dissipation. Taking PDMS/CNTs1.0-2S as an example, its cyclic tensile curves at different strains are shown in Fig. 4a, exhibiting a strain dependence. Pronounced hysteresis loops can be observed during the cycles and the loops become larger as the strain increases, indicating that the elastomer can effectively dissipate energy during loading. The quantified results are shown in Fig. 4b. As shown, the elastomer can dissipate energy of 9.7 kJ m 3 at strain of 100%. When the strain increases to 600%, the elastomer can dissipate energy of 537.3 kJ m 3. Meanwhile, the energy dissipation ratio increase from 18.9% to 38.8%. With increasing strain, the energy dissipation ratio exhibits a monotonic increasing trend, indicating increasing hysteresis. The above phenomenon is caused by the sacrifice of the aromatic disulfide bond. With small strains applied, weaker aromatic disulfide bonds are fracture to dissipate energy firstly. As the strain increases, the stronger imine bonds begin to break and finally the energy dissipation is maximized. This explains why the en ergy dissipation is greater under large strains during the cyclic tests. The ability to undergo repeated association of dynamic bonds also endows the nanocomposite elastomer with excellent self-recovery abil ity. To evaluate the self-recovery ability, cyclic tensile tests at a strain of 300% were carried out. After resting at ambient temperature for different times, the samples were re-stretched to 300%. As shown in Fig. 4c and d, for the first cycle, a large hysteresis loop with the dissi pated energy of 94.8 kJ m 3 is observed. Without resting time, the sec ond loading-unloading cycle shows a smaller hysteresis loop (39.9 kJ m 3). After resting for a period of time, the cycle gradually recover to the first cycle. The recovery ratio is applied to evaluate re covery efficiency. It is noticeable that the recovery ratio reaches 81.6% after 1 h and the areas of hysteresis loops almost fully recover after 6 h with the recovery ratio of 96.9%. The time-dependent on the recovery ratio indicate that the recovery process involves competition between
showed higher elongation at break compared to the control samples. The results indicate that aromatic disulfide bonds can significantly affect the mechanical properties, especially for stretchability. To further illustrate the sacrificial nature, we performed tensile tests on PDMS-2S and PDMS2S-ctrl at different stretching speeds. Obviously, the mechanical prop erties of PDMS-2S shows a strong stretching speed dependence (Fig. 2b). As the stretching speed increases, less time is allowed for the reformation of the broken aromatic disulfide bonds, which reduces the fracture tolerance. Such phenomenon is typical of elastic polymers with dynamic weak interactions. However, there is no significant difference in stress-strain curves for PDMS-2S-ctrl, indicating the poor dynamic nature (Fig. 2c). We also performed cyclic tensile tests using PDMS-2S and PDMS-2S-ctrl as examples. The hysteresis behaviour of elastomers during the loading cycle reflects the energy dissipation. As seen from Fig. 2d, PDMS-2S shows pronounced hysteresis during the loadingunloading cycle, indicating the energy dissipation during loading. Meanwhile, hysteresis can be negligible for PDMS-2S-ctrl (Fig. 2e). All of the above experiments show that aromatic disulfide bonds have sacri ficial effect in our elastomers. After forming nanocomposite elastomers, the dynamic features of the nanocomposite elastomers were also investigated by cyclic tensile tests. Fig. 3a shows the cyclic curves of PDMS/CNTsx-2S with different CNTs content at the same strain of 300%. Notably, with the increase of CNTs content, the elastomers exhibit improved tensile strength and hysteresis. Pronounced hysteresis loops can be observed during the cycles and the hysteresis loop becomes lagrer as the CNTs content increases, indicating that the addition of CNTs may have an impact on energy dissipation. This phenomenon may attribute to stress transfer effect of the CNTs [35, 36]. During stretching, the CNTs that uniformly dispersed in the elas tomer dissipate energy by transferring stress. More CNTs result in more entanglement and interaction between CNTs and PDMS chains, thus dissipate more energy. Fig. 3b shows the quantified results. As the CNT content increases from 0.5% to 2.0%, the elastomer can dissipate more energy and higher energy dissipation ratio is achieved. When CNTs content is 0.5 wt %, it can be seen that the elastomers could dissipate energies effectively as much as 36.8 kJ m 3 with a energy dissipation ratio of 23.8%. When CNTs content increases to 1.5 wt %, the dissipate energy and energy dissipation ratio increase to 208.2 kJ m 3 and 42.7%, respectively. Further increase the CNTs content to 2.0 wt %, the dissi pate energy increase to 237.2 kJ m 3, while the energy dissipation ratio decrease to 42.3%. The slight decrease of the energy dissipation ratio may attribute to the aggregation of the increasing CNTs. The morphology of fracture surface of PDMS/CNTs2.0-2S was investigated to verify our explanation (Fig. S6). Clearly, the aggregation of CNTs was observed in the SEM image of PDMS/CNTs2.0-2S. The cyclic tensile tests of PDMS/CNTsx-1S elastomers are also conducted (Fig. 3c). With the increase of CNTs content, the dissipate energy increase from 24.6 kJ m 3 3
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Fig. 2. (a) Stress-strain curves of PDMS-2S, PDMS-1S, PDMS-2S-ctrl and PDMS-1S-ctrl, respectively. (b) Stress-strain curves of PDMS-2S at different stretching speeds. (c) Stress-strain curves of PDMS-2S-ctrl at different stretching speeds. (d) Consecutive loading-unloading curves of PDMS-2S. (e) Consecutive loadingunloading curves of PDMS-2S-ctrl.
the reversible aromatic disulfide bonds and the elastic contraction of the primary network.
chemical composition and CNTs content, materials with various me chanical properties can be designed. Mechanical properties of the asprepared PDMS/CNTs nanocomposite elastomers were measured by stress-strain experiments. The stress-strain curves of PDMS/CNTsx-2S are shown in Fig. 5a. Without CNTs, the elastomer shows a tensile strength only 0.15 MPa (see Fig. 2a, PDMS-2S). After the addition of CNTs of 0.5 wt %, the mechanical properties of PDMS/CNTs0.5-2S improve remarkably. In particular, the elongation at break increase to 980%. Notably, when the CNTs content increase to 2.0 wt %, PDMS/
2.4. Mechanical properties of PDMS/CNTs nanocomposite elastomers With the architecture design of dual-dynamic covalent sacrificial system into the PDMS crosslinking network, the obtained PDMS/CNTs nanocomposite elastomers exhibit a huge increase in tensile strength and elongation at break after the addition of CNTs. By varying the 4
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Fig. 3. (a) Loading-unloading curves of PDMS/CNTsx-2S with different CNTs content and (b) corresponding toughness and energy dissipation ratio. (c) Loadingunloading curves of PDMS/CNTsx-1S with different CNTs content and (d) corresponding toughness and energy dissipation ratio. (e) Loading-unloading curves of elastomers with different aromatic disulfide bond content and (f) corresponding toughness.
CNTs2.0-2S shows a tensile strength of 1.10 MPa, almost 8 times of that of PDMS-2S. However, it should be noted that the improvement in the mechanical properties is due to the synergistic effect of the disulfide bond and the CNTs. To verify this, we synthesized a control sample PDMS/CNTs1.0-ctrl that does not contain disulfide bonds. Obviously, PDMS/CNTs1.0-ctrl only shows the elongation at break of 570% (Fig. S7), while PDMS/CNTs1.0-2S shows a high elongation at break of 920%. The results show that the aromatic disulfide bond is a guarantee for the high elongation at break of the nanocomposite and the CNTs mainly affects the tensile strength. The stress-strain experiments of
PDMS/CNTsx-1S were also conducted. Fig. 5b shows the stress-strain curves of PDMS/CNTsx-1S. Under the same CNTs content as PDMS/ CNTsx-2S, the elongation at break of PDMS/CNTsx-1S elastomers are 1030%, 1090%, 1200%, and 1420%, respectively. The tensile strength ranges from 0.25 to 0.32 MPa. Fig. 5c shows the toughness of PDMS/CNTs nanocomposite elasto mers. Without addition of CNTs, the toughnesses of PDMS-2S and PDMS1S are 710 and 430 kJ m 3, respectively (Calculated from Fig. 2a). It’s attributed to the different content of aromatic disulfide bond. After the addition of CNTs, the toughness increases as the CNTs increases. Noted 5
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Fig. 4. (a) Loading-unloading curves of PDMS/CNTs1.0-2S under different strains (100%, 200%, 300%, 400% and 600%) and (b) corresponding toughness and energy dissipation ratio. (c) Loading-unloading curves of PDMS/CNTs1.0-2S with different resting times. (d) Time-dependent recovery ratio and toughness.
Fig. 5. (a) Stress-strain curves of PDMS/CNTsx-2S elastomers with different CNTs contents. (b) Stress-strain curves of PDMS/CNTsx-1S elastomers with different CNTs contents. (c) Toughness of PDMS/CNTsx-2S and PDMS/CNTsx-1S elastomers.
that at CNTs content of 2.0 wt %, the toughness of PDMS/CNTs2.0-2S reaches the maximum of 5000 kJ m 3. As a visual demonstration, the PDMS/CNTs nanocomposite elastomers can withstand various de formations, like stretching, twisting and knotting (Fig. 6a–d). A PDMS/ CNTs1.0-2S sheet with wide of 5 mm and thick of 1 mm can hold a weight of 500 g (Fig. 6e). Notably, the elastomer can be stretched into a semi transparent membrane which can tolerate a certain strain (Fig. 6f).
2.5. Self-healability of PDMS/CNTs nanocomposite elastomers To demonstrate the self-healability of PDMS/CNTs nanocomposite elastomers, a visual macroscopic self-healing test was performed. As seen in Fig. 7a, b, a PDMS/CNTs1.0-2S sheet was cut into halves. Then, the two fracture faces are connected together. The healed elastomer could withstand stretching by hand to a large extend (Fig. 7c). To further evaluate the self-healability of the nanocomposite elastomer quantita tively, tensile tests were conducted on the healed and original elasto mers. The stress-strain curves of the healed PDMS/CNTs1.0-2S with 6
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Due to the multi-stimulus-responsive characteristic of imine bond, the crosslinked PDMS/CNTs nanocomposite elastomers can be decros slinded by relying on competing reaction between various aldehydes and amines and the pH response ability. In addition, the CNTs can be completely recycled. As seen in Fig. 8d and e, the PDMS/CNTs1.0-2S elastomer sample was immersed into dichloromethane with excess tri fluoroacetic acid (TFA), O-ethylhydroxylamine (EHA), and benzalde hyde (BA). Due to the protonation of the amino group, the solution turned orange immediately after the addition of TFA. Within 3 min, the nanocomposite elastomer was completely degraded. Similarly, addition of BA (competing aldehyde) and EHA (stronger nucleophile) also shows quick degradation within 24 h. In these cases, BA and EHA undergoes an exchange reaction with imine bonds, respectively. The competing aldehyde and stronger nucleophile can substitute the original aldehyde and amine. The degradation ratio of the nanocomposite elastomer is defined by the mass of residue (mr) and original elastomer (mo), the � � mr equation is showed as follow: 1 m *100%. The calculated values are o 98.4% (TFA), 97.5% (EHA) and 97.3% (BA), respectively, indicating the excellent degradability. Fig. 8f shows the TGA curves of the nano composite elastomer and the residues. It can be seen that the nano composite elastomer with a large weight loss of 90.3%, while the residues only with the weight loss of 16.1% (TFA), 19.7% (EHA) and 26.4% (BA), respectively. The result indicates that CNTs are the main composition of residue. The CNTs content in the residues also can be calculated by the TGA weight loss (mloss) and the original CNTs content in nanocomposite elastomer, the equation is showed as follow: 100% � � mloss 90:3% *8:7% þ mloss . The calculated CNTs content are 82.3% (TFA),
Fig. 6. Digital images of the PDMS/CNTs nanocomposite elastomer under different conditions demonstrating their excellent mechanical deformation and processability. The elastomer showing its ability to withstand (a) knotting, (b) twisting, (c–d) stretching, (e) loading, and (f) the malleability.
different healing times are shown in Fig. 7d. As the healing time in creases, both the tensile strength and elongation at break are gradually recovered. Self-healing efficiency was calculated through the toughness. After 12 h healing, up to 95% of the self-healing efficiency can be ob tained (Fig. 7e). Furthermore, the self-healability of PDMS/CNTsx-2S and PDMS/CNTsx-1S elastomers with different CNTs content is also investigated (Fig. 7f and g). The self-healing time is 12 h for all elasto mers. As the CNTs content increases, the self-healing efficiency shows an overall decrease trend for both the PDMS/CNTsx-2S and PDMS/CNTsx1S elastomers. However, the lowest self-healing efficiencies are still up to 90.3% for PDMS/CNTs2.0-2S and 74.7% for PDMS/CNTs2.0-1S (Fig. 7h). The difference of self-healing efficiencies may be due to the fact that aromatic disulfide bonds are the most important factor for selfhealability. It can be substantiated by comparing the self-healability of PDMS-2S and PDMS-2S-ctrl. PDMS-2S exhibits fast healing time of 4 h with self-healing efficiency of 95% (Fig. S8). For PDMS-2S-ctrl, only 55% of the self-healing efficiency obtained (Fig. S9) even after a healing time of 24 h. The results indicate that the self-healability is mainly stems from the aromatic disulfide bonds. Meanwhile, the imine bonds play a synergistic role in self-healing during this period. In order to further investigate the effect of aromatic disulfide bond content, we also compared the self-healing efficiency of elastomers with different aro matic disulfide bond content. As shown in Fig. 7i, with the increase of aromatic disulfide bonds, the self-healing efficiency shows an increasing trend. This indicates that the more aromatic disulfide bonds, the easier it is to achieve high self-healing efficiency.
78.4% (EHA) and 71.1% (BA), respectively. The above results demon strate that the degradation process is complete and efficient. The excellent degradability provides a potential resources-recycling solution with attractive application prospect. Here, we compare the previously reported PDMS-based elastomers with our elastomer (Table S1). As an elastomer with so many properties, our elastomer shows prominent overall performance. 3. Conclusions
In summary, a pioneering dual-dynamic covalent sacrificial system is exploited for the fabrication of degradable, reprocessable, self-healing PDMS/CNTs nanocomposite elastomers with high stretchability and toughness. The cross-linking network is constructed through the for mation of imine, meanwhile the aromatic disulfide bonds are incorpo rated at the same time. The as-prepared nanocomposite elastomers exhibit tunable elongation at break (870–1420%) and tensile strength (0.25–1.10 MPa) by adjusting the content of aromatic disulfide bonds and CNTs. At ambient temperature, the toughness of the damaged elastomers can completely and rapidly recover after 12 h healing. The self-healing efficiency is up to 95%. Moreover, the elastomers can be reprocessed multiple times and the mechanical properties still have not dropped significantly. It is important that the elastomers can be con trollably degraded by adding trifluoroacetic acid, O-ethylhydroxylamine and benzaldehyde. All the aforementioned properties make the nano composite elastomers attractive for applications in various fields such as stretchable, flexible sensors, electronic skins, and artificial muscles.
2.6. Reprocessability and degradability of PDMS/CNTs nanocomposite elastomers
4. Experimental section
The dynamic nature of the dual-dynamic covalent system allows the PDMS/CNTs nanocomposite elastomers to be reprocessed and recycled. To examine the reprocessability, the nanocomposite elastomers were cut into pieces (Fig. 8a), then the pieces were pressed at 120 � C for 30 min. The reprocessed nanocomposite elastomer sheet looks integrated (Fig. 8b). Fig. 8c shows the stress-strain curves of the reprocessed nanocomposite elastomer. It is worth mentioning that even after 4 cycles of cutting/recycling process, the elastomer can still maintain its original mechanical strength, showing outstanding reprossability.
4.1. Materials and characterization Bis (3-aminopropyl)-terminated poly(dimethylsiloxane) (H2NPDMS-NH2, Mn ¼ 10000 g mol 1) was provided by Heowns. 1, 3, 5-tri formylbenzene (TFB) was purchased from Sanbang Chemical. 4, 40 diaminodiphenylmethane (DADPM) was provided by J&K Chemical. 4aminophenyl disulfide (APDS) was obtained from Adamas. 7
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Fig. 7. (a) Digital images of the original PDMS/CNTs1.0-2S sheet and (b) the PDMS/CNTs1.0-2S sheet cut in half. (c) The stretched self-healed PDMS/CNTs1.0-2S sheet. (d) Stress-strain curves of self-healed PDMS/CNTs1.0-2S with different healing time and (e) the self-healing efficiencies with different healing time. (f) Stressstrain curves of PDMS/CNTsx-2S elastomers before (solid lines) and after (dashed lines) healing 12 h. (g) Stress-strain curves of PDMS/CNTsx-1S elastomers before (solid lines) and after (dashed lines) healing 12 h. (h) The self-healing efficiencies of PDMS/CNTsx-2S and PDMS/CNTsx-1S elastomers with healing time of 12 h. (i) Comparison of self-healing efficiency of elastomers with different aromatic disulfide bond contents.
Benzaldehyde (BA) and trifluoroacetic acid (TFA) was obtained from Heowns and Adamas, respectively. According to previously reported paper [37], O-ethylhydroxylamine (EHA) was synthesized. Graphitized CNTs (TNSM3, length ¼ 0.5–2 μm, OD ¼ 10–20 nm, 98%) were provided by Chengdu organic chemicals Co., Ltd. 1-pyrenecarboxaldehyde (PA) was purchased from Shanghai D&B chemical. Fourier-transform infrared (FT-IR) spectra were obtained with range from 600 to 4000 cm 1. (Nicolet 6700, USA). The spectra were collected with a spectral resolution of 4 cm 1 by Attenuated total reflectance (ATR) method. X-ray diffraction (XRD) was conducted on a MiniFlex 600 diffractometer (Rigaku, Japan), the peaks were recorded from 15o70� . Thermal gravimetric analysis (TGA) was carried out on Netzsch STA449F3 thermal analyser. (temperature range: 40–700 � C; heating rate: 10 � C min 1, argon atmosphere) The fracture surface were observed on a Hitachi s4800 scanning electron microscope (SEM) Mechanical tests: The loading-unloading cycles and stress-strain curves were obtained at room temperature by SANS CMT4203 testing machine. The size of as-prepared nanocomposite elastomer samples is 1 mm � 12 mm � 2 mm. Stretching rate of 50 mm min 1 was adopted. In loading-unloading tests, various strains (100%, 200%, 300%, 400% and 600%) were conducted on samples. When the predetermined strain is 300%, the elastomers were relaxed for a waiting time (0–6 h) prior to the next stretching process. Fracture energy (W, KJ m 3) is used to
characterize fracture toughness. It is defined as the area below the stressR strain curve until fracture and calculated as follow:W ¼ σdε. The re covery ratio was defined by a ratio of energy dissipation after different resting time to the first cycle. The dissipated energy for each cycle, ΔU, was defined as the area of the hysteresis loop, which is calculated as R R follow: ΔU ¼ σ dε σ dε. The energy dissipation ratio (δ) is loading
unloading
R σmax calculated as below: δ ¼ ΔU σdε, where U is the elastic energy U,U ¼ 0 when it is loaded elastically to the stress σmax . Self-healing tests: The as-prepared nanocomposite elastomers were cut on the edge partially in the width direction with 1 mm. Subse quently, the cut parts were put together. Then, the connected elastomers were placed in a tetrafluoroethylene dish at room temperature. After a certain time, the healed elastomers were tested. The self-healing effi ciency was obtained by calculating the raito of toughness of the healed elastomers and the original elastomer. 4.2. Preparation of aldehyde-functionalized carbon nanotubes (CNTs/ PA) The functionalization of CNTs with PA was conducted according to a similar method as previously published [38]. Briefly, the pristine CNTs (200 mg) were dispersed in N, N-dimethylformamide (60 mL) 8
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Fig. 8. (a) Fragments of PDMS/CNTs1.0-2S and (b) the reprocessed sheet. (c) Stress-strain curves of PDMS/CNTs1.0-2S with different reprocessing times. (d) De gradability test of PDMS/CNTs1.0-2S before treatment. (e) Degradability test of PDMS/CNTs1.0-2S after treatment. (f) TGA curves of PDMS/CNTs1.0-2S and the residues in the presence of TFA, EHA and BA.
containing PA (50 mg) by ultrasonic for 2 h. Next, the solution was stirred overnight. Finally, the mixed solution was centrifuged and washed with ethanol. The functionalized carbon nanotubes were dried at 30 � C for 24 h.
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4.3. Preparation of PDMS/CNTs nanocomposite elastomers PDMS/CNTs nanocomposite elastomers were prepared by solution mixing CNTs/PA during the crosslinking process of H2N-PDMS-NH2, APDS, and TFB. Firstly, H2N-PDMS-NH2,TFB and APDS was dissolved in CH2Cl2 with different molar ratios. Secondly, the CNTs/PA dispersion was prepared by sonication of CNTs/PA in CH2Cl2. Then, the dispersion was poured into the mixture solution. After being stirred 24 h at 30 � C under N2 atmosphere, the mixture solution was poured into a mold and dried in vacuum for 24 h, the PDMS/CNTs nanocomposite elastomers (Molar ratios: PDMS/CNTsx-2S: H2N-PDMS-NH2/TFB/APDS ¼ 1: 2: 2; PDMS/CNTsx-1.5S: H2N-PDMS-NH2/TFB/APDS ¼ 1.5: 2: 1.5; PDMS/ CNTsx-1S: H2N-PDMS-NH2/TFB/APDS ¼ 2: 2: 1; PDMS/CNTsx-0.5S: H2N-PDMS-NH2/TFB/APDS ¼ 2.5: 2: 0.5; x represents the CNTs content, wt %) were obtained. These elastomers are further molded by hot pressing at 120 � C. Acknowledgements We are grateful to the National Natural Science Foundation of China (No. 51473114). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107270.
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[15] Zuo Y, Gou Z, Zhang C, Feng S. Polysiloxane-based autonomic self-healing elastomers obtained through dynamic boronic ester bonds prepared by thiolEne“Click” chemistry. Macromol Rapid Commun 2016;37(13):1052–9. [16] Wu J, Cai L, Weitz DA. Tough self-healing elastomers by molecular enforced integration of covalent and reversible networks. Adv Mater 2017;29(38):1702616. [17] Nishimura Y, Chung J, Muradyan H, Guan Z. Silyl ether as A robust and thermally stable dynamic covalent motif for malleable polymer design. J Am Chem Soc 2017; 139(42):14881–4. [18] Lu L, Pan J, Li G. Recyclable high-performance epoxy based on transesterification reaction. J Mater Chem A 2017;5(40):21505–13. [19] Lu Y, Tournilhac F, Leibler L, Guan Z. Making insoluble polymer networks malleable via olefin metathesis. J Am Chem Soc 2012;134(20):8424–7. [20] Zhou X, Guo B, Zhang L, Hu G. Progress in bio-inspired sacrificial bonds in artificial polymeric materials. Chem Soc Rev 2017;46(20):6301–29. [21] Lu H, Shi X, Yu K, Fu YQ. A strategy for modelling mechanochemically induced unzipping and scission of chemical bonds in double-network polymer composite. Compos B Eng 2019;165:456–66. [22] Wu S, Qiu M, Tang Z, Liu J, Guo B. Carbon nanodots as high-functionality crosslinkers for bioinspired engineering of multiple sacrificial units toward strong yet tough elastomers. Macromolecules 2017;50(8):3244–53. [23] Shao C, Chang H, Wang M, Xu F, Yang J. High-strength, tough, and self-healing nanocomposite physical hydrogels based on the synergistic effects of dynamic hydrogen bond and dual coordination bonds. ACS Appl Mater Interfaces 2017;9 (34):28305–18. [24] Zhang X, Tang Z, Guo B, Zhang L. Enabling design of advanced elastomer with bioinspired metal-oxygen coordination. ACS Appl Mater Interfaces 2016;8(47): 32520–7. [25] Sun T, Kurokawa T, Kuroda S, Ihsan AB, Akasaki T, Sato K, Haque MA, Nakajima T, Gong J. Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity. Nat Mater 2013;12(10):932–7. [26] Neal JA, Mozhdehi D, Guan Z. Enhancing mechanical performance of A covalent self-healing material by sacrificial noncovalent bonds. J Am Chem Soc 2015;137 (14):4846–50.
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Degradable, reprocessable, self-healing PDMS/CNTs nanocomposite elastomers with high stretchability and toughness based on novel dual-dynamic covalent sacrificial system Chi Lv, Jinke Wang, Zhongxiao Li, Kaifeng Zhao, Junping Zheng * Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Degradability Reprocessability High stretchability Toughness Self-healing
Synthesizing materials that possesses degradability, reprocessability and self-healability simultaneously without compromising stretchability and toughness is considered a huge challenge. In this paper, a pioneering dualdynamic covalent sacrificial system is exploited for the fabrication of PDMS/CNTs nanocomposite. In the network structure, aromatic disulfide bonds and imine bonds act as sacrificial units and semi-permanent crosslinking points, respectively. The unique design endows the nanocomposite elastomers with high elonga tion at break (up to 1420%), high toughness (up to 5000 kJ m 3) and good tensile strength (up to 1.10 MPa). Moreover, these mechanical properties can be regulated by varying the chemical composition and CNTs content. Additionally, the nanocomposite elastomers exhibit excellent self-healing efficiency (12 h, 95% of toughness) at ambient temperature and the ability to be processed multiple times. More importantly, the nanocomposite elastomers can be degraded controllably by three ways, achieving the complete recycling of CNTs and PDMS. We believe that our design strategy will provide a new way for the development of green stretchable tough materials used in a sustainable way.
1. Introduction Polysiloxane elastomers are fascinating materials due to their impressive intrinsic properties, such as nontoxicity and excellent flexi bility at broad temperature ranges [1]. They have attracted significant attention in the applications of electronic skins, artificial muscles, and sensors recently, especially for stretchable devices [2–4]. However, most neat polysiloxane elastomers exhibit relatively poor stretchability and toughness, which severely limits their practical applications. Besides, they often be damaged in their lifecycle, which causes security risks and huge resources waste. Therefore, it is of significance to design sustain able silicone elastomer with high stretchability and toughness. Recently, materials with self-healability and reprocessability have been attracting much interest because they can greatly reduce the environmental pollution and material consumption. Thermosetting polymers often lack these abilities after curing because the molecular chains are crosslinked by permanent chemical bonds. People have tried to solve the problem by replacing the permanent bonds with dynamic covalent bonds and dynamic noncovalent interactions. The former in volves the hindered urea bond [5], boroxines linkage [6], Diels-Alder
reaction [7], disulfide exchange [8], and imine bond [9], etc, while the latter deals with host-guest interaction [10], ionic interaction [11], hydrogen bond [12,13], and metal-ligand coordination [14], etc. However, the mechanical properties of material are often compromised due to the incorporation of dynamic bonds. A majority of materials with dynamic bonds are not stretchable and not tough because these dynamic bonds are prone to preferential dissociation under load. Therefore, it’s difficult to obtain a stretchable and tough material in the presence of dynamic bonds. Degradability, as another promising property, enables the materials to be completely recycled. However, a majority of self-healing materials don’t possess degradability. For example, per manent covalent bonds are often introduced into some self-healing systems to increase mechanical strength [15,16], which makes the ma terials difficult to degrading. In addition, in order to obtain degradable materials, the crosslinking bonds need to be completely dissociated, but many dynamic bonds don’t have the ability [17–19]. Therefore, it’s a huge challenge to combine so many properties, such as degradability, reprocessability, stretchability, toughness and self-healability in one material. To achieve stretchability and toughness, one strategy is to
* Corresponding author. E-mail address: [email protected] (J. Zheng). https://doi.org/10.1016/j.compositesb.2019.107270 Received 2 April 2019; Received in revised form 20 July 2019; Accepted 9 August 2019 Available online 10 August 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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constructing sacrificial bond system. It is considered to be effective for improving stretchability and mechanical strength [20–24]. The sacrifi cial bonds preferentially rupture during the large deformation, thus resulting energy dissipation. This energy dissipation plays an important role in improving mechanical properties. When the dynamic bond is used as sacrificial unit, the material will be given self-healability. For example, Gong et al. [25] designed a novel tough hydrogel by intro ducing multiple ionic bonds. The mechanical properties can be tuned by varying ionic combinations. Guan et al. [26] reported a self-healing material that combine outstanding mechanical performance and excel lent self-healability by introducing sacrificial hydrogen bond. Guo and Huang et al. [27] developed a high strength and high stretchability polyisoprene rubber. In this rubber network, two sacrificial bonds are employed. Although various sacrificial bond systems have been devel oped, most of these sacrificial bond systems involved in permanent chemical crosslinking, which makes it impossible to reprocess and degrade the crosslinking network. In order to obtain tough materials, another alternative method is to preparing nanocomposite. Various nanofillers, such as nano-silicon di oxide, graphene or carbon nanotubes (CNTs), are used to improve toughness [28]. Although huge progresses have been achieved, the self-healing efficiency is restricted. It has been demonstrated that the introduction of nanofillers reduces chain mobility, which is a key factor in determining self-healing properties [29]. For instance, Zhang et al. [30] reported a GO/polyurethane nanocomposite, the self-healing effi ciency is only around 70% after healing at 150 � C. In addition, these nanofillers are usually modified by permanent covalent bonds, making the prepared nanocomposite difficult to be reprocessed and degraded. Here, we designed a unique sacrificial system with two dynamic covalent bonds: a strong yet semipermanent dynamic covalent bond (imine bond) [31] and a weak sacrificial dynamic covalent bond (aro matic disulfide bond) [32]. Unlike aliphatic disulfide bond and other dynamic bonds, aromatic disulfide bond has weaker bond energy due to the presence of aryl groups. Therfore, it can be reversibly exchanged at ambient temperature, compensating for the self-healability of nano composites. Imine bond, also show excellent dynamic property for reprocessing and self-healing. Particularly, the exchange reaction be tween aldehydes and amines and the pH response ability endows the material with unique degradability [9,33]. Such combination provides us with a viable strategy for reprocessing and degrading sacrificial bond systems. To our knowledge, this is the first degradable and reprocessable sacrificial network. And this dual-dynamic covalent sacrificial system is applied to nanocomposite for the first time. In present work, we prepared degradable, reprocessable, self-healing PDMS/CNTs nanocomposite elastomers with high stretchability and toughness by constructing a novel dual-dynamic covalent sacrificial system. The nanocomposite elastomers exhibit outstanding stretch ability and toughness, and the mechanical performance of the nano composite elastomers can be tuned by varying content of aromatic disulfide bonds and CNTs. In addition, these nanocomposite elastomers also show excellent room-temperature self-healability and excellent reprocessability. More importantly, in the presence of competing alde hyde, stronger nucleophile and acid, the nanocomposite elastomers can be completely degraded, achieving the recycling of CNTs and PDMS. All of these properties can give the nanocomposite elastomer a great po tential as a green material for sustainable use.
makes the prepared nanocomposite easier to be reprocessed and degraded. In consideration of the bifunctional nature of pyrenecarbox aldehyde (PA), PA is chosen as the functional molecule. With simple stirring, PA molecule can interact strongly with CNTs via π-π in teractions. To confirm the attachment of PA onto CNTs surfaces, TGA was applied. The TGA curves of pristine-CNTs, CNTs/PA and PA are shown in the Fig. S1. It can be seen that the pristine-CNTs has hardly any weight loss, while the CNTs/PA with a weight loss of 4 wt %. The FT-IR of CNTs/PA was also performed (Fig. S2). As shown in the infrared spectrum of PA, the peak at 1595 cm 1 is ascribed to the stretching vi bration of benzene ring and the peak at 1680 cm 1 is ascribed to the aldehyde group. These characteristic peaks of PA appear in the infrared spectrum of CNTs/PA and these peaks are shifted to lower wavenumber. The phenomenon may be attributed to π-π interactions and hydrophobic interactions between the walls of the CNTs and the pyrene groups. Additional evidence of the CNT functionalization was provided by Raman spectroscopy. As we all known, the D band reflects disordered graphite structure and defects, while the G band reflects the sp2 hy bridized carbon atoms. ID/IG, the intensity ratio of D to G bands is widely used to evaluate the defects and degree of functionalization. As shown in Fig. S3, the ID/IG are 1.05 and 0.94 for pristine-CNTs and CNTs/PA, respectively. The decrease of ID/IG indicates that the original defects are covered by the PA molecules and the adsorption of PA decreases the disorder of graphene layers. These above investigations verified that the CNTs have been successfully functionalized by PA.
2. Results and discussion
Firstly, to verify the sacrificial nature of the dual-dynamic covalent sacrificial system, various neat PDMS elastomers those without adding CNTs were prepared. (Molar ratios: PDMS-1S: H2N-PDMS-NH2/TFB/ APDS ¼ 2: 2: 1; PDMS-2S: H2N-PDMS-NH2/TFB/APDS ¼ 1: 2: 2) As a comparison, we replaced the disulfide bond with a methylene bridge and synthesized control samples. (Molar ratios: PDMS-1S-ctrl: H2NPDMS-NH2/TFB/DADPM ¼ 2: 2: 1; PDMS-2S-ctrl: H2N-PDMS-NH2/ TFB/DADPM ¼ 1: 2: 2). The stress-strain curves are shown in Fig. 2a. Despite the decrease in tensile strength, PDMS-2S and PDMS-1S all
2.2. Preparation of PDMS/CNTs nanocomposite elastomers The typical procedure for preparation of PDMS/CNTs nano composite elastomers is presented in Fig. 1a. H2N-PDMS-NH2, 4-amino phenyl disulfide (APDS), 1, 3, 5-triformylbenzene (TFB), and CNTs/PA were added into a reaction bottle to form uniform slurry by a one-pot reaction process. After the solvent evaporated, the nanocomposite elastomers were fabricated by hot press molding. In this reaction, TFB acts as the crosslinker and the crosslinking is formed by condensation reaction of amino group and aldehyde group. The newly formed imine bond exists in the cross-linking point and is mainly responsible for pri mary crosslinking. Unlike the imine bond, the aromatic disulfide bond is introduced directly through the APDS during the crosslinking process. It should be pointed out that since the aldehyde group content on CNTs/ PA is much lower than TFB, we believe that the nanocomposite elas tomer is still mainly chemically crosslinked by TFB. The formation of imine crosslinked network was verified by FT-IR spectrum. As seen in Fig. 1b, the peak at 1695 cm 1 is ascribed to the stretching vibration of aldehyde group in TFB linker, but this characteristic peak is barely detectable in the infrared spectrum of nanocomposite elastomer. Meanwhile, a new peak appears at 1648 cm 1. This peak is assigned to the newly formed imine bond. The dispersion of CNTs was studied by XRD. XRD patterns (Fig. S4) reveal a peak of CNTs/PA at 2θ position of 26� , while the PDMS/CNTs nanocomposite elastomer shows a XRD pattern without the sharp peak, indicating that the CNTs are well dispersed [34]. The morphology of fracture surface was investigated by SEM (Fig. S5). This further proved that the CNTs are uniformly dispersed. 2.3. Dynamic features of PDMS/CNTs nanocomposites elastomers
2.1. Characterization of the aldehyde-functionalized carbon nanotubes (CNTs/PA) In this work, the CNTs are functionalized by aldehyde group. These aldehyde groups can effectively react with amino groups to form imine bond. The presence of imine bonds on the surface of the CNTs not only increases the compatibility of the polymer matrix and the CNTs, but also 2
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Fig. 1. (a) Illustration for preparation of the PDMS/CNTs nanocomposite elastomers. (b) FT-IR spectra of a) TFB and b) PDMS/CNTs nanocomposite elastomer.
to 103.9 kJ m 3, the energy dissipation ratio increase from 27.7% to 54.8% (Fig. 3d). The result shows that the PDMS/CNTsx-1S elastomers exhibit lower energy dissipation but higher energy dissipation ratio under the same CNTs content as PDMS/CNTsx-2S. To illustrate the effect of aromatic disulfide bond content on energy dissipation, we prepared two other control samples PDMS/CNTs1.0-0.5S and PDMS/CNTs1.0-1.5S. The cyclic tensile tests were conducted. (Aromatic disulfide bond con tent: 0.19 mmol/g for PDMS/CNTs1.0-2S; 0.09 mmol/g for PDMS/CNTs1.0-1.5S; 0.05 mmol/g for PDMS/CNTs1.0-1S; 0.02 mmol/g for PDMS/CNTs1.0-0.5S) The dissipation energies of PDMS/CNTs1.0-0.5S and PDMS/CNTs1.0-1.5S are 37.8 and 68.5 kJ m 3, respectively. Under the same strain, the dissipation energies of PDMS/CNTs1.0-1S and PDMS/CNTs1.0-2S are 42.3 and 94.7 kJ m 3, respectively (Fig. 3e and f). With the increase of aromatic disulfide bonds, the dissipation energy shows an increasing trend. The results indicate that the content of aro matic disulfide bonds indeed play a key role in dissipating energy. The more the aromatic disulfide bonds, the more the energy dissipation. Taking PDMS/CNTs1.0-2S as an example, its cyclic tensile curves at different strains are shown in Fig. 4a, exhibiting a strain dependence. Pronounced hysteresis loops can be observed during the cycles and the loops become larger as the strain increases, indicating that the elastomer can effectively dissipate energy during loading. The quantified results are shown in Fig. 4b. As shown, the elastomer can dissipate energy of 9.7 kJ m 3 at strain of 100%. When the strain increases to 600%, the elastomer can dissipate energy of 537.3 kJ m 3. Meanwhile, the energy dissipation ratio increase from 18.9% to 38.8%. With increasing strain, the energy dissipation ratio exhibits a monotonic increasing trend, indicating increasing hysteresis. The above phenomenon is caused by the sacrifice of the aromatic disulfide bond. With small strains applied, weaker aromatic disulfide bonds are fracture to dissipate energy firstly. As the strain increases, the stronger imine bonds begin to break and finally the energy dissipation is maximized. This explains why the en ergy dissipation is greater under large strains during the cyclic tests. The ability to undergo repeated association of dynamic bonds also endows the nanocomposite elastomer with excellent self-recovery abil ity. To evaluate the self-recovery ability, cyclic tensile tests at a strain of 300% were carried out. After resting at ambient temperature for different times, the samples were re-stretched to 300%. As shown in Fig. 4c and d, for the first cycle, a large hysteresis loop with the dissi pated energy of 94.8 kJ m 3 is observed. Without resting time, the sec ond loading-unloading cycle shows a smaller hysteresis loop (39.9 kJ m 3). After resting for a period of time, the cycle gradually recover to the first cycle. The recovery ratio is applied to evaluate re covery efficiency. It is noticeable that the recovery ratio reaches 81.6% after 1 h and the areas of hysteresis loops almost fully recover after 6 h with the recovery ratio of 96.9%. The time-dependent on the recovery ratio indicate that the recovery process involves competition between
showed higher elongation at break compared to the control samples. The results indicate that aromatic disulfide bonds can significantly affect the mechanical properties, especially for stretchability. To further illustrate the sacrificial nature, we performed tensile tests on PDMS-2S and PDMS2S-ctrl at different stretching speeds. Obviously, the mechanical prop erties of PDMS-2S shows a strong stretching speed dependence (Fig. 2b). As the stretching speed increases, less time is allowed for the reformation of the broken aromatic disulfide bonds, which reduces the fracture tolerance. Such phenomenon is typical of elastic polymers with dynamic weak interactions. However, there is no significant difference in stress-strain curves for PDMS-2S-ctrl, indicating the poor dynamic nature (Fig. 2c). We also performed cyclic tensile tests using PDMS-2S and PDMS-2S-ctrl as examples. The hysteresis behaviour of elastomers during the loading cycle reflects the energy dissipation. As seen from Fig. 2d, PDMS-2S shows pronounced hysteresis during the loadingunloading cycle, indicating the energy dissipation during loading. Meanwhile, hysteresis can be negligible for PDMS-2S-ctrl (Fig. 2e). All of the above experiments show that aromatic disulfide bonds have sacri ficial effect in our elastomers. After forming nanocomposite elastomers, the dynamic features of the nanocomposite elastomers were also investigated by cyclic tensile tests. Fig. 3a shows the cyclic curves of PDMS/CNTsx-2S with different CNTs content at the same strain of 300%. Notably, with the increase of CNTs content, the elastomers exhibit improved tensile strength and hysteresis. Pronounced hysteresis loops can be observed during the cycles and the hysteresis loop becomes lagrer as the CNTs content increases, indicating that the addition of CNTs may have an impact on energy dissipation. This phenomenon may attribute to stress transfer effect of the CNTs [35, 36]. During stretching, the CNTs that uniformly dispersed in the elas tomer dissipate energy by transferring stress. More CNTs result in more entanglement and interaction between CNTs and PDMS chains, thus dissipate more energy. Fig. 3b shows the quantified results. As the CNT content increases from 0.5% to 2.0%, the elastomer can dissipate more energy and higher energy dissipation ratio is achieved. When CNTs content is 0.5 wt %, it can be seen that the elastomers could dissipate energies effectively as much as 36.8 kJ m 3 with a energy dissipation ratio of 23.8%. When CNTs content increases to 1.5 wt %, the dissipate energy and energy dissipation ratio increase to 208.2 kJ m 3 and 42.7%, respectively. Further increase the CNTs content to 2.0 wt %, the dissi pate energy increase to 237.2 kJ m 3, while the energy dissipation ratio decrease to 42.3%. The slight decrease of the energy dissipation ratio may attribute to the aggregation of the increasing CNTs. The morphology of fracture surface of PDMS/CNTs2.0-2S was investigated to verify our explanation (Fig. S6). Clearly, the aggregation of CNTs was observed in the SEM image of PDMS/CNTs2.0-2S. The cyclic tensile tests of PDMS/CNTsx-1S elastomers are also conducted (Fig. 3c). With the increase of CNTs content, the dissipate energy increase from 24.6 kJ m 3 3
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Fig. 2. (a) Stress-strain curves of PDMS-2S, PDMS-1S, PDMS-2S-ctrl and PDMS-1S-ctrl, respectively. (b) Stress-strain curves of PDMS-2S at different stretching speeds. (c) Stress-strain curves of PDMS-2S-ctrl at different stretching speeds. (d) Consecutive loading-unloading curves of PDMS-2S. (e) Consecutive loadingunloading curves of PDMS-2S-ctrl.
the reversible aromatic disulfide bonds and the elastic contraction of the primary network.
chemical composition and CNTs content, materials with various me chanical properties can be designed. Mechanical properties of the asprepared PDMS/CNTs nanocomposite elastomers were measured by stress-strain experiments. The stress-strain curves of PDMS/CNTsx-2S are shown in Fig. 5a. Without CNTs, the elastomer shows a tensile strength only 0.15 MPa (see Fig. 2a, PDMS-2S). After the addition of CNTs of 0.5 wt %, the mechanical properties of PDMS/CNTs0.5-2S improve remarkably. In particular, the elongation at break increase to 980%. Notably, when the CNTs content increase to 2.0 wt %, PDMS/
2.4. Mechanical properties of PDMS/CNTs nanocomposite elastomers With the architecture design of dual-dynamic covalent sacrificial system into the PDMS crosslinking network, the obtained PDMS/CNTs nanocomposite elastomers exhibit a huge increase in tensile strength and elongation at break after the addition of CNTs. By varying the 4
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Fig. 3. (a) Loading-unloading curves of PDMS/CNTsx-2S with different CNTs content and (b) corresponding toughness and energy dissipation ratio. (c) Loadingunloading curves of PDMS/CNTsx-1S with different CNTs content and (d) corresponding toughness and energy dissipation ratio. (e) Loading-unloading curves of elastomers with different aromatic disulfide bond content and (f) corresponding toughness.
CNTs2.0-2S shows a tensile strength of 1.10 MPa, almost 8 times of that of PDMS-2S. However, it should be noted that the improvement in the mechanical properties is due to the synergistic effect of the disulfide bond and the CNTs. To verify this, we synthesized a control sample PDMS/CNTs1.0-ctrl that does not contain disulfide bonds. Obviously, PDMS/CNTs1.0-ctrl only shows the elongation at break of 570% (Fig. S7), while PDMS/CNTs1.0-2S shows a high elongation at break of 920%. The results show that the aromatic disulfide bond is a guarantee for the high elongation at break of the nanocomposite and the CNTs mainly affects the tensile strength. The stress-strain experiments of
PDMS/CNTsx-1S were also conducted. Fig. 5b shows the stress-strain curves of PDMS/CNTsx-1S. Under the same CNTs content as PDMS/ CNTsx-2S, the elongation at break of PDMS/CNTsx-1S elastomers are 1030%, 1090%, 1200%, and 1420%, respectively. The tensile strength ranges from 0.25 to 0.32 MPa. Fig. 5c shows the toughness of PDMS/CNTs nanocomposite elasto mers. Without addition of CNTs, the toughnesses of PDMS-2S and PDMS1S are 710 and 430 kJ m 3, respectively (Calculated from Fig. 2a). It’s attributed to the different content of aromatic disulfide bond. After the addition of CNTs, the toughness increases as the CNTs increases. Noted 5
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Fig. 4. (a) Loading-unloading curves of PDMS/CNTs1.0-2S under different strains (100%, 200%, 300%, 400% and 600%) and (b) corresponding toughness and energy dissipation ratio. (c) Loading-unloading curves of PDMS/CNTs1.0-2S with different resting times. (d) Time-dependent recovery ratio and toughness.
Fig. 5. (a) Stress-strain curves of PDMS/CNTsx-2S elastomers with different CNTs contents. (b) Stress-strain curves of PDMS/CNTsx-1S elastomers with different CNTs contents. (c) Toughness of PDMS/CNTsx-2S and PDMS/CNTsx-1S elastomers.
that at CNTs content of 2.0 wt %, the toughness of PDMS/CNTs2.0-2S reaches the maximum of 5000 kJ m 3. As a visual demonstration, the PDMS/CNTs nanocomposite elastomers can withstand various de formations, like stretching, twisting and knotting (Fig. 6a–d). A PDMS/ CNTs1.0-2S sheet with wide of 5 mm and thick of 1 mm can hold a weight of 500 g (Fig. 6e). Notably, the elastomer can be stretched into a semi transparent membrane which can tolerate a certain strain (Fig. 6f).
2.5. Self-healability of PDMS/CNTs nanocomposite elastomers To demonstrate the self-healability of PDMS/CNTs nanocomposite elastomers, a visual macroscopic self-healing test was performed. As seen in Fig. 7a, b, a PDMS/CNTs1.0-2S sheet was cut into halves. Then, the two fracture faces are connected together. The healed elastomer could withstand stretching by hand to a large extend (Fig. 7c). To further evaluate the self-healability of the nanocomposite elastomer quantita tively, tensile tests were conducted on the healed and original elasto mers. The stress-strain curves of the healed PDMS/CNTs1.0-2S with 6
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Due to the multi-stimulus-responsive characteristic of imine bond, the crosslinked PDMS/CNTs nanocomposite elastomers can be decros slinded by relying on competing reaction between various aldehydes and amines and the pH response ability. In addition, the CNTs can be completely recycled. As seen in Fig. 8d and e, the PDMS/CNTs1.0-2S elastomer sample was immersed into dichloromethane with excess tri fluoroacetic acid (TFA), O-ethylhydroxylamine (EHA), and benzalde hyde (BA). Due to the protonation of the amino group, the solution turned orange immediately after the addition of TFA. Within 3 min, the nanocomposite elastomer was completely degraded. Similarly, addition of BA (competing aldehyde) and EHA (stronger nucleophile) also shows quick degradation within 24 h. In these cases, BA and EHA undergoes an exchange reaction with imine bonds, respectively. The competing aldehyde and stronger nucleophile can substitute the original aldehyde and amine. The degradation ratio of the nanocomposite elastomer is defined by the mass of residue (mr) and original elastomer (mo), the � � mr equation is showed as follow: 1 m *100%. The calculated values are o 98.4% (TFA), 97.5% (EHA) and 97.3% (BA), respectively, indicating the excellent degradability. Fig. 8f shows the TGA curves of the nano composite elastomer and the residues. It can be seen that the nano composite elastomer with a large weight loss of 90.3%, while the residues only with the weight loss of 16.1% (TFA), 19.7% (EHA) and 26.4% (BA), respectively. The result indicates that CNTs are the main composition of residue. The CNTs content in the residues also can be calculated by the TGA weight loss (mloss) and the original CNTs content in nanocomposite elastomer, the equation is showed as follow: 100% � � mloss 90:3% *8:7% þ mloss . The calculated CNTs content are 82.3% (TFA),
Fig. 6. Digital images of the PDMS/CNTs nanocomposite elastomer under different conditions demonstrating their excellent mechanical deformation and processability. The elastomer showing its ability to withstand (a) knotting, (b) twisting, (c–d) stretching, (e) loading, and (f) the malleability.
different healing times are shown in Fig. 7d. As the healing time in creases, both the tensile strength and elongation at break are gradually recovered. Self-healing efficiency was calculated through the toughness. After 12 h healing, up to 95% of the self-healing efficiency can be ob tained (Fig. 7e). Furthermore, the self-healability of PDMS/CNTsx-2S and PDMS/CNTsx-1S elastomers with different CNTs content is also investigated (Fig. 7f and g). The self-healing time is 12 h for all elasto mers. As the CNTs content increases, the self-healing efficiency shows an overall decrease trend for both the PDMS/CNTsx-2S and PDMS/CNTsx1S elastomers. However, the lowest self-healing efficiencies are still up to 90.3% for PDMS/CNTs2.0-2S and 74.7% for PDMS/CNTs2.0-1S (Fig. 7h). The difference of self-healing efficiencies may be due to the fact that aromatic disulfide bonds are the most important factor for selfhealability. It can be substantiated by comparing the self-healability of PDMS-2S and PDMS-2S-ctrl. PDMS-2S exhibits fast healing time of 4 h with self-healing efficiency of 95% (Fig. S8). For PDMS-2S-ctrl, only 55% of the self-healing efficiency obtained (Fig. S9) even after a healing time of 24 h. The results indicate that the self-healability is mainly stems from the aromatic disulfide bonds. Meanwhile, the imine bonds play a synergistic role in self-healing during this period. In order to further investigate the effect of aromatic disulfide bond content, we also compared the self-healing efficiency of elastomers with different aro matic disulfide bond content. As shown in Fig. 7i, with the increase of aromatic disulfide bonds, the self-healing efficiency shows an increasing trend. This indicates that the more aromatic disulfide bonds, the easier it is to achieve high self-healing efficiency.
78.4% (EHA) and 71.1% (BA), respectively. The above results demon strate that the degradation process is complete and efficient. The excellent degradability provides a potential resources-recycling solution with attractive application prospect. Here, we compare the previously reported PDMS-based elastomers with our elastomer (Table S1). As an elastomer with so many properties, our elastomer shows prominent overall performance. 3. Conclusions
In summary, a pioneering dual-dynamic covalent sacrificial system is exploited for the fabrication of degradable, reprocessable, self-healing PDMS/CNTs nanocomposite elastomers with high stretchability and toughness. The cross-linking network is constructed through the for mation of imine, meanwhile the aromatic disulfide bonds are incorpo rated at the same time. The as-prepared nanocomposite elastomers exhibit tunable elongation at break (870–1420%) and tensile strength (0.25–1.10 MPa) by adjusting the content of aromatic disulfide bonds and CNTs. At ambient temperature, the toughness of the damaged elastomers can completely and rapidly recover after 12 h healing. The self-healing efficiency is up to 95%. Moreover, the elastomers can be reprocessed multiple times and the mechanical properties still have not dropped significantly. It is important that the elastomers can be con trollably degraded by adding trifluoroacetic acid, O-ethylhydroxylamine and benzaldehyde. All the aforementioned properties make the nano composite elastomers attractive for applications in various fields such as stretchable, flexible sensors, electronic skins, and artificial muscles.
2.6. Reprocessability and degradability of PDMS/CNTs nanocomposite elastomers
4. Experimental section
The dynamic nature of the dual-dynamic covalent system allows the PDMS/CNTs nanocomposite elastomers to be reprocessed and recycled. To examine the reprocessability, the nanocomposite elastomers were cut into pieces (Fig. 8a), then the pieces were pressed at 120 � C for 30 min. The reprocessed nanocomposite elastomer sheet looks integrated (Fig. 8b). Fig. 8c shows the stress-strain curves of the reprocessed nanocomposite elastomer. It is worth mentioning that even after 4 cycles of cutting/recycling process, the elastomer can still maintain its original mechanical strength, showing outstanding reprossability.
4.1. Materials and characterization Bis (3-aminopropyl)-terminated poly(dimethylsiloxane) (H2NPDMS-NH2, Mn ¼ 10000 g mol 1) was provided by Heowns. 1, 3, 5-tri formylbenzene (TFB) was purchased from Sanbang Chemical. 4, 40 diaminodiphenylmethane (DADPM) was provided by J&K Chemical. 4aminophenyl disulfide (APDS) was obtained from Adamas. 7
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Fig. 7. (a) Digital images of the original PDMS/CNTs1.0-2S sheet and (b) the PDMS/CNTs1.0-2S sheet cut in half. (c) The stretched self-healed PDMS/CNTs1.0-2S sheet. (d) Stress-strain curves of self-healed PDMS/CNTs1.0-2S with different healing time and (e) the self-healing efficiencies with different healing time. (f) Stressstrain curves of PDMS/CNTsx-2S elastomers before (solid lines) and after (dashed lines) healing 12 h. (g) Stress-strain curves of PDMS/CNTsx-1S elastomers before (solid lines) and after (dashed lines) healing 12 h. (h) The self-healing efficiencies of PDMS/CNTsx-2S and PDMS/CNTsx-1S elastomers with healing time of 12 h. (i) Comparison of self-healing efficiency of elastomers with different aromatic disulfide bond contents.
Benzaldehyde (BA) and trifluoroacetic acid (TFA) was obtained from Heowns and Adamas, respectively. According to previously reported paper [37], O-ethylhydroxylamine (EHA) was synthesized. Graphitized CNTs (TNSM3, length ¼ 0.5–2 μm, OD ¼ 10–20 nm, 98%) were provided by Chengdu organic chemicals Co., Ltd. 1-pyrenecarboxaldehyde (PA) was purchased from Shanghai D&B chemical. Fourier-transform infrared (FT-IR) spectra were obtained with range from 600 to 4000 cm 1. (Nicolet 6700, USA). The spectra were collected with a spectral resolution of 4 cm 1 by Attenuated total reflectance (ATR) method. X-ray diffraction (XRD) was conducted on a MiniFlex 600 diffractometer (Rigaku, Japan), the peaks were recorded from 15o70� . Thermal gravimetric analysis (TGA) was carried out on Netzsch STA449F3 thermal analyser. (temperature range: 40–700 � C; heating rate: 10 � C min 1, argon atmosphere) The fracture surface were observed on a Hitachi s4800 scanning electron microscope (SEM) Mechanical tests: The loading-unloading cycles and stress-strain curves were obtained at room temperature by SANS CMT4203 testing machine. The size of as-prepared nanocomposite elastomer samples is 1 mm � 12 mm � 2 mm. Stretching rate of 50 mm min 1 was adopted. In loading-unloading tests, various strains (100%, 200%, 300%, 400% and 600%) were conducted on samples. When the predetermined strain is 300%, the elastomers were relaxed for a waiting time (0–6 h) prior to the next stretching process. Fracture energy (W, KJ m 3) is used to
characterize fracture toughness. It is defined as the area below the stressR strain curve until fracture and calculated as follow:W ¼ σdε. The re covery ratio was defined by a ratio of energy dissipation after different resting time to the first cycle. The dissipated energy for each cycle, ΔU, was defined as the area of the hysteresis loop, which is calculated as R R follow: ΔU ¼ σ dε σ dε. The energy dissipation ratio (δ) is loading
unloading
R σmax calculated as below: δ ¼ ΔU σdε, where U is the elastic energy U,U ¼ 0 when it is loaded elastically to the stress σmax . Self-healing tests: The as-prepared nanocomposite elastomers were cut on the edge partially in the width direction with 1 mm. Subse quently, the cut parts were put together. Then, the connected elastomers were placed in a tetrafluoroethylene dish at room temperature. After a certain time, the healed elastomers were tested. The self-healing effi ciency was obtained by calculating the raito of toughness of the healed elastomers and the original elastomer. 4.2. Preparation of aldehyde-functionalized carbon nanotubes (CNTs/ PA) The functionalization of CNTs with PA was conducted according to a similar method as previously published [38]. Briefly, the pristine CNTs (200 mg) were dispersed in N, N-dimethylformamide (60 mL) 8
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Fig. 8. (a) Fragments of PDMS/CNTs1.0-2S and (b) the reprocessed sheet. (c) Stress-strain curves of PDMS/CNTs1.0-2S with different reprocessing times. (d) De gradability test of PDMS/CNTs1.0-2S before treatment. (e) Degradability test of PDMS/CNTs1.0-2S after treatment. (f) TGA curves of PDMS/CNTs1.0-2S and the residues in the presence of TFA, EHA and BA.
containing PA (50 mg) by ultrasonic for 2 h. Next, the solution was stirred overnight. Finally, the mixed solution was centrifuged and washed with ethanol. The functionalized carbon nanotubes were dried at 30 � C for 24 h.
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4.3. Preparation of PDMS/CNTs nanocomposite elastomers PDMS/CNTs nanocomposite elastomers were prepared by solution mixing CNTs/PA during the crosslinking process of H2N-PDMS-NH2, APDS, and TFB. Firstly, H2N-PDMS-NH2,TFB and APDS was dissolved in CH2Cl2 with different molar ratios. Secondly, the CNTs/PA dispersion was prepared by sonication of CNTs/PA in CH2Cl2. Then, the dispersion was poured into the mixture solution. After being stirred 24 h at 30 � C under N2 atmosphere, the mixture solution was poured into a mold and dried in vacuum for 24 h, the PDMS/CNTs nanocomposite elastomers (Molar ratios: PDMS/CNTsx-2S: H2N-PDMS-NH2/TFB/APDS ¼ 1: 2: 2; PDMS/CNTsx-1.5S: H2N-PDMS-NH2/TFB/APDS ¼ 1.5: 2: 1.5; PDMS/ CNTsx-1S: H2N-PDMS-NH2/TFB/APDS ¼ 2: 2: 1; PDMS/CNTsx-0.5S: H2N-PDMS-NH2/TFB/APDS ¼ 2.5: 2: 0.5; x represents the CNTs content, wt %) were obtained. These elastomers are further molded by hot pressing at 120 � C. Acknowledgements We are grateful to the National Natural Science Foundation of China (No. 51473114). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107270.
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