Research Article www.acsami.org
Dual-Triggered and Thermally Reconfigurable Shape Memory Graphene-Vitrimer Composites Zenghui Yang,†,‡ Qihua Wang,*,† and Tingmei Wang† †
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, P. R. China ‡ University of Chinese Academy of Sciences, Beijing, 100039, P. R. China S Supporting Information *
ABSTRACT: Conventional thermoset shape memory polymers can maintain a stable permanent shape, but the intrinsically chemical cross-linking leads to shape that cannot be altered. In this paper, we prepared shape memory graphene-vitrimer composites whose shape can be randomly changed via dynamic covalent transesterification reaction. Consecutive shape memory cycles indicate stable shape memory with undetected strain shift and constant shape fixity and recovery values (Rf > 99%, Rr > 98%). Quantitative characterization of shape reconfiguration by dynamic mechanical thermal analysis (DMA) shows prime reconfigurable behavior with shape retention ratio of 100%. Thus, the arbitrary 2D or 3D newly permanent shape can be easily obtained from a simple plain sample by facile thermal treatment at 200 °C above transesterification temperature (Tv). Besides, it is found that graphene-vitrimers show a ductile fracture in tensile test with a large breaking strain and classical yield phenomenon because of the well-dispersed graphene sheets in the vitrimer that endow effective stress transfer. As the graphene loading increases from 0% to 1%, the yield strength and breaking stain increase from 12.0 MPa and 6% to 22.9 MPa and 44%, respectively. In addition, graphene also serves as energy convertor to convert near-infrared (NIR) irradiation into thermal energy to induce a helix shape sample that is recovered totally within 80 s sequent NIR irradiation. These dual-triggered and reconfigurable shape memory graphene-vitrimers are expected to significantly simplify processing of complex shape and broaden the applications of shape memory polymers. KEYWORDS: shape memory polymer, vitrimer, reconfiguration, graphene, stimulus
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elastic transitions are in charge of fixing the temporary shapes and driving shape recovery. In recent years, the main points of SMPs are focused on designing new shape memory networks, multifunctional shape memory devices, as well as high temperature shape memory polymers. For instance, Deng et al. and Huang et al. exhibit a kind of fiber-shaped supercapacitor with shape memory ability and well-maintained electrochemical performance.14,15 Kim et al. present a triboelectric nanogenerator that has the capacity to be healed and to recover good performance after degradation of its triboelectric layer with the help of shape memory.11 To meet potential applications of SMPs in a harsh environment, high temperature shape memory polymers based
INTRODUCTION Shape memory polymers (SMPs) are mechanically active materials featuring memory of the programmed temporary shape and later return to their permanent shape upon exposure to external stimulus such as heat,1−3 light,4,5 solvent,6,7 or other means.8 The extensive potential application in aerospace deployable structures9 and biomedical10 and functionally tunable devices11,12 has stimulated intense interest in this area during the past few years. The emergence of triple, multiple, and reversible shape memory further broadens the types of SMPs and at the same time accelerates the development of SMPs. The shape memory behavior of SMPs originated from the networks by chemical (thermoset SMP) or physical cross-links (thermoplastic SMP) and elastic transitions by means of crystallization or vitrification.13 The cross-linked networks are responsible for maintaining the original (permanent) shape after processing, whereas the © XXXX American Chemical Society
Received: June 19, 2016 Accepted: July 27, 2016
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DOI: 10.1021/acsami.6b07403 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces on polyimide and its composites have also been widely revealed.16−22 However, the aforementioned SMPs usually maintain a simple permanent shape that neither can be altered or reprocessed for thermoset SMPs nor can be reprogramming without traversing a fluid state by remolding thermoplastic SMPs.23 Furthermore, previously prepared SMPs are usually limited to simple films, a sample geometry easily obtained by facile casting or molding technique in shape processing. Thus, to prepare an SMP with a complex 3D-structure, a new sample must be processed from a complicated model and this process is both time-consuming and inefficient for large scale applications of SMPs.23 Recently, using dynamic covalent chemistries to design reprocessing SMPs by facile thermomechanical approach has attracted significant attention and has the potential to simplify the preparation of complex SMPs’ geometry.23,24 Vitrimers, as organic strong glass-formers, have dynamic covalent bonds and exchangeable property by thermo or light to enable topological network rearrangements of thermoset polymers for self-healing, welding, and reprocessing of thermoset polymers.25−31 But the shape memory reconfigurable properties of vitrimers are rarely systematically demonstrated, and the intrinsic fragility restricts their wide range of application. Graphene, a one-atom-thick planar sheet of carbon atoms densely packed in a honeycomb crystal lattice, has recently emerged as a promising functional filler to improve mechanical properties such as strength, modulus, and toughness.32 Besides, graphene is often used as photothermal transformation material to comvert light energy to thermal energy due to its fast and efficient photothermal transformation performance.33 In this study, we prepared shape memory graphene-vitrimers via reaction of the diglycidyl ether of bisphenol A with sebacic acid, using 1,5,7-triazabicyclo[4.4.0]dec-5-ene as catalyst. Graphene is introduced into the epoxy vitrimer that serves not only as reinforced phase to enhance the physical properties but as energy convertor to convert NIR irradiation into thermal energy to induce shape recovery. The thermal expansion, creep recovery, and thermomechanical properties were characterized by DMA. The interface, thermal, and mechanical properties were assessed by SEM, DMA, DSC, and tensile tests. We also quantitatively and qualitatively investigated reconfigurable behaviors of Graphene-vitrimers in virtue of covalent transesterification reactions. The reconfigurable ability of these materials by facile thermomechanical approach can be used to obtain another newly permanent shape with 2D or 3D geometry. Furthermore, the photothermal transformation efficiency of graphene and NIR (808 nm, 100 mW/cm2) triggered shape recovery of graphene-vitrimers were discussed and demonstrated.
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Figure 1. Precursors structure of vitrimer, synthesis scheme of graphene-vitrimers, and the potential macromolecular structure changes in shape memory and shape reconfiguration process. for 1 h (500 W). The obtained graphene solution (15 mg/mL) was used immediately in the following part. Synthesis of Graphene-Vitrimers. The samples of graphenevitrimers are named as EP-x%, where x represents the weight percentage of graphene in epoxy resin. Taking EP-0.5% as an example, stoichiometric amounts of E51 (5 g, 12.75 mmol) and SA (2.58 g, 12.75 mmol) were mixed with 2.53 mL of graphene dispersion. The mixture was heated to 160 °C until the NMP was totally evaporated under manual stirring. Then, TBD (5 mol % to the COOH) was introduced and stirred manually until homogeneous. The mixture was quickly poured into PTFE mold and left at 180 °C for 12 h. The thickness of the obtained samples is about 1 mm. Scanning Electron Microscopy (SEM). The graphene-vitrimers were cryofractured in liquid nitrogen, and the fracture surfaces were observed on field emission scanning electron microscope (FESEM, JSM-6701F). The fracture surfaces of the samples after the tensile tests were also observed by the FESEM. All the surfaces were sputtered with a thin gold film before observation. Dynamic Mechanical Thermal Analyses (DMA). The thermal mechanical properties of graphene-vitrimers were measured on Netzsch DMA 242C heated from −30 to 100 °C at a rate of 5 °C min−1 and a frequency of 1 Hz. Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetric (DSC). The thermal stability was performed under nitrogen on a Netzsch-STA449F3 simultaneous thermal analysis system using a heating rate of 10 °C min−1 from 25 to 800 °C. DSC was conducted on Netzsch-DSC200F3 using a heating rate of 5 °C min−1 from −15 to 120 °C. Tensile Tests. Tensile tests were conducted on universal testing machine (Shimadzu AG-X, 500N) to study the mechanical properties with a constant crosshead rate of 5 mm min−1 at room temperature. The samples were carefully cut to dog-bone type dimensions according to ISO527-2/1BB. Creep and Recovery Test. The creep-recovery curves were measured using DMA 242C at different temperatures (160, 200 °C). The specimen was first stretched under constant load for 30 min; then, the load was released instantaneously. The creep recovery process lasted for 30 min, and the strain as a function of the time was recorded. The temperature is kept constant in each step. Dilatometry Experiment. Dilatometry experiment was performed on a DMA 242C apparatus in the tension geometry under the controlled force mode, with a rectangular tension dimension of 20.0 mm × 3.0 mm × 1.0 mm. The length was measured when the sample was heated from −30 to 250 °C at a rate of 5 °C min−1.
EXPERIMENTAL SECTION
Materials. Epoxy resin, DGEBA E51 (CYD 128), with an epoxy equivalent weight of 184−194 g (equiv)−1, was purchased from Yueyang Baling Petrochemical Industry Co. Ltd., China. Curing agent sebacic acid (SA) was purchased from the Sinopharm Chemical Reagent Co., Ltd., China. Catalyst 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) was purchased from J&K Chemical Ltd. The chemical structures of these three reactants are shown in Figure 1. Graphene (SE1430) was supplied by the Sixth Element (Changzhou) Materials Technology Co., Ltd. N-Methyl-2-pyrrolidone (NMP, >99.0%) was obtained from Shanghai Kefeng Industry & Commerce Co., Ltd. The materials were used as received without further purification. Sample Preparation. Dispersion of Graphene. An amount of 300 mg graphene was suspended in 20 mL of NMP and ultrasonicated B
DOI: 10.1021/acsami.6b07403 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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at 1735 cm−1) appears. Therefore, it is illustrated that the precursors completely reacted with each other and chemical crosslinked networks were formed, and the photographic images are shown in Figure 2a. The well-established chemical reaction between epoxy and carboxyl acid is a complicated process. Once hydroxyl groups are generated by the reaction of epoxy and acid, they continue to react with either epoxy or acid, leading to the branching of polymer chains and the formation of a cross-linked network.31,34 Stoichiometric amounts of epoxy and carboxylic acid were used in order to the obtained network containing both hydroxyl and ester groups to conduct transesterification exchange reaction. It is the transesterification reaction that plays an important role to shape reconfiguration properties in thermoset SMPs. Figure 1 illustrates the potential macromolecular structure changes during shape memory cycle and shape reconfiguration process. In our expectations, the networks could not only exhibit shape memory by virtue of activated molecular chains mobility around glass transition temperature (Tg) with the classical transition between glass state and rubbery state but possess reconfigurable property due to faster dynamic exchangeable of covalent bonds above transesterification temperature (Tv). In other words, any deformed shape around Tg can be recovered to its original shape through entropy elasticity, but the reprogrammed shapes retained above Tv are nonrecoverable because the activated covalent bonds take place in the exchange reaction and the network topographic is changed. To determine the dispersion of graphene in vitrimer matrix, the cryofractured surfaces of graphene-vitrimers containing 0.1 and 1.0 wt % of fillers were examined by SEM, and images are shown in Figure 2. As shown in SEM images of the graphenevitrimers, most of the graphene sheets are fully exfoliated and clearly well-dispersed in the EP matrix with its wrinkled feature and no obvious aggregation exists even further increasing filler content (Figure S2). The image also reveals that the graphene nanosheets are well-wrapped in or covered with a thick EP layer. The linear viscoelastic properties of the graphene-vitrimers were characterized using DMA. The tensile storage modulus for these shape memory graphene-vitrimers are revealed in Figure 3a and Table 1. The results show that all samples exhibit similar temperature-dependent viscoelastic properties with high storage modulus (E′) at glass state and low E′ at rubbery state and importantly a sharp drops of magnitude around their respective glass transitions region, indicating their potential shape memory behaviors. Like classical thermoset SMPs, EP exhibits major molecular chain mobility related to Tg and one rubbery plateau with a modulus of about 4.1 MPa. The storage modulus in glass state is about 1.8 GPa. The incorporation of graphene induces a small increase of tensile storage modulus. Figure 3b indicates that
To avoid buckling, a weak force of 0.1 N was applied throughout the experiment. Quantitative Shape Memory and Shape Reconfiguration Tests. The quantitative shape memory behavior of the graphenevitrimer materials was evaluated by DMA using a tensile film fixture and the controlled force mode. The sample was first heated to 70 °C above switching temperature of the switching domains with 10 °C min−1; then the sample was stretched under a load of 0.8 N for 5 min and afterward cooled at 5 °C min−1 to 10 °C under the load, upon which the sample length was εload. After subsequent unloading, the length of the sample in the temporary shape εunload was obtained. The recovery process was then triggered by heating the sample back to 70 °C with a heating rate of 5 °C min−1, upon which the sample length decreased to εrec. The shape fixity ratio (Rf) and shape recovery ratio (Rr) were determined using the eqs 1 and 2:
Rf =
εunload × 100% εload
(1)
Rr =
εunload − εrec × 100% εunload
(2)
Reconfiguration testing was conducted by DMA under control force mode. The first shape memory cycle was performed as described above. After shape recovery at 70 °C, the temperature increased to 200 °C to reconfigure. The sample was stretched under a load of 0.8 N for 5 min, then the force was removed and the sample kept for 5 min at 200 °C. After stress relaxation, the temperature was decreased to 70 °C and the new shape was obtained, and then another shape memory cycle was conducted. Consecutive shape memory and reconfiguration were measured. Qualitative Shape Memory and Shape Reconfiguration Tests. Shape Memory Test. A permanent shape was deformed to a temporary shape above Tg and then cooled to room temperature to fix the temporary shape. The fixed shape was then placed back in heating stage to recover. Shape Reconf iguration Testing. The various 2D and 3D structures were manually folded from flat samples. The folded objects were placed into a 200 °C oven and annealed for about 30 min. A new shape memory cycle was conducted again using same methods described above with a different temporary shape.
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RESULTS AND DISCUSSION Synthesis and Characterization of Reconfigurable Shape Memory Graphene-Vitrimers. We prepared shape memory graphene-vitrimers via reaction of the diglycidyl ether of bisphenol A with sebacic acid in the presence of TBD as a catalyst for transesterification and graphene as functional filler (Figure 1). FTIR test was conducted for precursors (E51, SA) and the resulting shape memory vitrimers. The results are show in Figure S1 in Supporting Information. The characteristic peaks of epoxy group (νCOC at 910 cm−1) and carboxylic acid (νCOacid at 1706 cm−1) totally disappear, and a new ester peak (νCOester
Figure 2. SEM images of cryofractured surfaces for (a) EP-0.1% and (b) EP-1%. C
DOI: 10.1021/acsami.6b07403 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 3. (a) Storage modulus (E′) and (b) loss factor (tan δ) vs temperature curves, (c) differential scanning calorimeter (DSC) curves, and (d) thermogravimetric analysis (TGA) of the graphene-vitrimers.
Table 1. Physical Properties of Shape Memory Graphene-Vitrimers sample
E′ga (MPa)
E′rb (MPa)
TgDMA (°C)
TgDSC (°C)
TvDMA (°C)
T5%c (°C)
EYoungd (MPa)
σYielde (MPa)
EP EP-0.1% EP-0.5% EP-1% EP-3%
1885 ± 28 1759 ± 20 2006 ± 12 2160 ± 18 2141 ± 32
4.1 ± 0.9 4.6 ± 0.4 3.8 ± 1.0 4.4 ± 1.2 4.9 ± 0.5
42.9 43.8 43.6 48.3 45.4
39.2 39.1 40.7 39.0 40.8
170 155 150 136 116
345 344 332 348 355
565.9 ± 10.1 767.3 ± 8.5 797.5 ± 7.9 1232 ± 23.5 1022 ± 14.3
12.0 ± 0.8 13.4 ± 1.2 14.4 ± 1.0 22.9 ± 1.7 17.4 ± 0.6
a Storage modulus at glass state. bStorage modulus at rubbery state. cTemperature at which 5% weight loss was recorded by TGA. dYoung modulus in tensile test at room temperature. eYield strength in tensile test at room temperature.
a unique loss factor (tan δ) peak associated with the glass transition temperature is observed for all samples. The Tg of pure EP is 42.9 °C, yet the Tg of graphene-vitrimers was little fluctuated by incorporating the filler of graphene. From the middle point of the endothermic step during the second scan of DSC measurements, the Tg values of graphene-vitrimers are around 39 °C and keep basically identical, which is different from the data of DMA but shows similar trend as DMA with the increase of graphene. Figure 3d shows the weight loss during the heating of graphene-vitrimer nanocomposites under inertia nitrogen atmosphere. It can be seen that the starting decomposition (∼5 wt % weight loss) of EP is at ∼345 °C, whereas that of EP-3% is at ∼355 °C, indicating the slight increase of decomposition temperature when graphene is added. The fastest weight loss happens at almost the same temperature of ∼430 °C for all samples. Thus, the shape memory graphene-vitrimers exhibit good thermal stability. The typical stress−strain curves at room temperature are shown in Figure 4a, and the data of Young’s modulus (EYoung) and yield strength (σYield) are summarized in Table 1. The addition of graphene significantly enhances the mechanical properties of the vitrimer. EYoung shows an obvious enhancement with an increment from 565 to 1232 MPa with the graphene loading
increase from 0 to 1 wt %. Further increasing of graphene has side effect on EYoung. The incorporation of graphene also has a more profound effect on the tensile strength, interestingly, simultaneously on breaking strain. The same as traditional epoxy, the pure vitrimer exhibits brittle fracture with small breaking strain of 6% due to intrinsic brittleness and vulnerability to cracks. But introducing of graphene changes the fracture mode and enhances toughness of graphene-vitrimers along with apparent yield phenomenon and large breaking strain. Combined with the SEM results in Figure 4, the pure EP displays a smooth fracture surface and almost no events are observed during the crack propagation because of the lack of energy-absorbing events. Nevertheless, graphene-vitrimers show rough surface as an indication of ductile fracture and more resistance to microcrack propagation. As the graphene loading increase from 0 to 1%, the yield strength and breaking stain increase from 12.0 MPa and 6% to 22.9 MPa and 44%, respectively. Compared with neat EP, the graphene-vitrimer composites with only 1.0 wt % graphene exhibit a dramatic increase in yield strength (approximately 1.9-fold) and breaking strain (approximately 7.3-fold), revealing that graphene possesses a high reinforcing efficiency for promoting the load transfer from the polymer to graphene and endowing a ductile fracture with large strain.32 This high efficiency D
DOI: 10.1021/acsami.6b07403 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. (a) Typical stress−strain curves of the graphene-vitrimer. SEM images of the fracture surfaces of tensile test specimens for (b) neat EP, (c, d) EP-0.5%, and (e, f) EP-1%.
Figure 5. (a) Dilatometry experiments of graphene-vitrimers and (b) creep and recovery curves for EP, EP-0.1%, and EP-1% at different temperatures.
strain, that are very important for graphene-vitrimers to exhibit larger deformation and shape reconfiguration. Dilatometry experiments confirm the existence of the two transitions of vitrimer that are responsible for elastic switch and topology rearrangements, respectively. Figure 5a shows that there are two phase transitions in graphene-vitrimers, named “amorphous I to II” and “amorphous II to III”. The transition of phase I to
reinforcement might be attributable to the well-dispersed graphene sheets in the EP vitrimer matrix. The magnified photographs shown in Figure 4d,f further confirm the function of graphene in vitrimer and the features of ductile fracture. The red arrows indicate stress concentrated area that crack in the last. In summary, the preferable dispersion and lubrication of graphene nanosheets render better mechanical properties, especially the large breaking E
DOI: 10.1021/acsami.6b07403 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces phase II at a relatively low temperature similar to that in a conventional cross-linked polymer networks is attributed to the transition between glass state and rubbery state. This transition temperature is classical glass transition temperature. The transition of phase II to phase III is related to covalent bonds exchange reaction, and the responsive temperature reflects freezing topology transition but is not a viscous-to-elastic gel transition. Above Tv, the transesterification reaction is accelerated and the network starts to flow like a viscoelastic fluid while keeping its integrity and is insoluble, because the number of bonds remains constant; namely, transesterfications are equilibrium dynamic reactions, where an ester and an alcohol are transformed into another ester and another alcohol via transesterification.31 Herein, the unique property of vitrimer will provide a new approach for easily reconfiguring arbitrary shape from processed permanent shape. It is shown that the Tg change is inconspicuous for all samples, while the Tv decreases with the content of graphene increase. The Tv of neat epoxy is ∼170 °C, yet that of EP-3% is about ∼116 °C. Creep and recovery experiments allow for determining the efficiency of the transesterification reaction at a particular temperature and optimum operation temperature of shape reconfiguration. Figure 5b depicts the elongational creep experiments obtained for EP, EP-0.1%, and EP-1% under temperatures of 160 and 200 °C. It is shown that the creep and recovery curve of EP is very different at different temperatures. For instance, after the instantaneous stress release and strain start to recover, the EP tested at 160 °C displays a distinct creep recover and the value of shape retention is 95.4% after 30 min (Figure S3), whereas that at 200 °C keeps a shape retention of 100% with stretched strain of 10.1%. The shape retention ratio gradually increases under creep test at 160 °C with increase of graphene content. The shape retention ratios of all samples at 200 °C are nearly 100%. These differences of creep recover behavior are closely associated with the efficiency of covalent bonds exchange reaction under different temperatures. At relative low temperature, the transesterfication reaction ratio is slow, leading to more time needed to completely relax loaded stress, that is to say that the loaded stress is not totally relaxed within the given time. The residual stress drives the strain recovery in creep recovery step, leading to lower shape retention ratio. In contrast, at higher temperature above Tv, the transesterification reaction ratio is faster, indicating the loaded stress is rapidly relaxed and higher shape retention is obtained with no creep recovery occurring. In addition, the complete stress relaxation temperatures descend with the component of graphene increase. Therefore, in the following shape reconfiguration tests, we set a temperature of 200 °C as the operation temperature. Shape Memory and Reconfigurable Properties. To investigate the shape memory cycling stability of graphenevitrimers, EP-1% was subjected to consecutive shape memory cycles with the strain above 31% and recovery temperature of 70 °C. As shown in Figure 6a, the cycle to cycle curves show stable shape memory property with almost identical strain. Except for the first shape memory cycle, the shape fixity (Rf) and shape recovery values (Rr) are above 98% and 99%, respectively. Consecutive thermomechanical cycling indicates very little degeneration of shape memory with undetected strain shift and constant shape fixity and recovery after five shape memory cycles (Figure 6a, Figure S4 in Supporting Information). As previously shown, the graphene-vitrimers possess enough toughness to conduct high strain shape memory. Figure 6b shows high strain shape memory effect, where a strip sample is stretched at 70 °C
Figure 6. (a) Consecutive shape memory cycles of EP-1%. (b) Demonstration of the high strain shape memory performance of EP-1%.
Figure 7. (a) Quantitative shape memory and shape reconfiguration cycles for EP-1%. (b) Thermally induced transesterification reactions during consecutive shape reconfigurable process in graphene-vitrimers.
and fixed at room temperature to a temporary shape that is about 250% times as long as the original shape. Importantly, this high stretched temporary shape can be fully recovered to original shape after reheating. These results suggest that excellent shape memory could be obtained around the temperature of Tg and the F
DOI: 10.1021/acsami.6b07403 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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temperature below Tv. To further illustrate the cycling stability of shape memory after shape reconfiguration, multiple consecutive shape memory and reconfiguration in the alternative thermomechanical cycles were conducted and shown in Figure 7a. The results indicate a shape resetting process is conducted to generate a newly reconfigurable shape after each shape memory cycle. Within each cycle, the excellent shape memory is obtained with Rf and Rr above 98%, followed by a reconfigurable shape process with the shape retention value approaching 100%. Unlike conventional systems, this original shape of thermoset can be reprogrammed by rearrangement of exchangeable covalent bands under reconfigurable process so that the following shape memory cycle starts from a newly permanent shape. Of note is that the shape memory performance is not deteriorated after shape reconfiguration, indicating high performance shape memory and reconfiguration can be obtained in this system by facile thermomechanical mode. For a vivid description of the shape memory and reconfiguration, samples of the EP-1% are used to conduct a macroscopical
plasticity creep is negligible even at high stretched temporary shape. To demonstrate shape reconfigurable property, quantitative experiment was first conducted. Sample of EP-1% combined shape memory with reconfiguration was used to suffer thermal mechanical cycles. Figure S5 illustrates a reprogrammed cycle after shape memory, in which the deformation strain of about 8% is fully and permanently retained (shape retention approach 100%) because of the completely stress relaxation. The dynamic chemical reaction mechanism shown in Figure 7b can provide in-depth explanation for shape reconfiguration. When heating increases the temperature to above Tv, the transesterfication is activated, and an ester group and a hydroxyl group react with each other with the assistance of catalyst, producing a new ester group and a new hydroxyl group which will continue to react with other hydroxyl or ester groups to produce hydroxyl and ester groups until a dynamic equilibrium of bond exchange reactions is formed and the loaded stress is totally relaxed. Therefore, a new geometrical configuration is maintained when cooling the
Figure 8. (a) Images of qualitative study demonstrating shape memory of strip sample. (b) Reconfiguration behavior of strip sample into ringlike shape and demonstration of shape memory cycle. (c) Reconfiguration behavior of ringlike sample into “M” shape and emonstration of shape memory cycle. (d) Schematic of shape reconfiguration and shape memory cycle from 2D to 3D geometry. (e) Reconfiguration behavior of plain shape into 3D origami structure and demonstration of the shape memory effect: i represents shape fix process above glass transition temperature, and ii represents the shape recovery process. G
DOI: 10.1021/acsami.6b07403 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 9. (a) Temperature elevation of different samples as a function of irradiation time. (b) Shape recovery of folded strip sample with the increase of irradiation time by local NIR irradiation. (c) Step by step shape recovery of helix shape by sequence NIR irradiation.
experiment. Figure 8a shows the shape fixity and shape recovery course of a strip sample (22 mm × 3 mm × 1 mm) (sample a1). The a1 is fixed to temporary “S” shape (a2) by shape fixity strategy (Experimental Section) and then recovers to its strip shape (a3) with almost no difference with a1 when heated to 70 °C (Video S1), representing excellent shape memory properties. Subsequently, the strip sample is bent and reconfigured into a newly permanent ringlike shape (b1) by annealing at 200 °C for 30 min. The regenerative ringlike shape can be deformed into a temporary helix shape (b2) at 70 °C that can fully recover to permanent ringlike shape in the following shape recovery step (Video S2). Furthermore, the ringlike shape can be further reconfigured to yield another permanent “M” shape (c1), which also can be deformed into a temporary ringlike shape (c2) that also can recover by thermal stimulus at 70 °C. Above all manipulation of arbitrary transformation between permanent shape and temporary shape is complex for conventional processing techniques. The covalent bonds’ exchange nature of the reconfiguration in this work provides an additional unique freedom for arbitrary shape manipulation range from two-dimensional (2D) to 2D or from 2D to 3D shapes. Figure 8d,e demonstrates the shape change from 2D to 3D structure by shape reconfiguration. The plain shape is reconfigured into an open box via annealing at 200 °C for 30 min. The newly permanent open box can also undergo shape memory cycles. First, the open box (e1) is heated to 70 °C and manually converted to the temporary plain form (e2) by shape fix strategy, and then the temporary plain shape is recovered to the reconfigured permanent open box shape (e3) near totally by thermal triggering. NIR-Triggered Shape Recovery. With the incorporation of graphene into the reconfigurable shape memory vitrimer, we expect to fulfill noncontact triggered shape recovery by NIR irradiation with graphene serving as energy convertor to convert NIR irradiation to thermal energy. The temperature change of graphene-vitrimers under NIR irradiation (100 mW/cm2) was observed and shown in Figure 9a. The temperature increases quickly for graphene-vitrimers, and the increasing rate becomes faster and the temperature reaches higher with the graphene content increasing. The temperature of EP-1% reaches 48 °C within 40 s, whereas for the vitrimer without graphene there is no obvious increase after 50 s of NIR irradiation. This fact confirms the high efficiency of the NIR energy absorption and transformation
of graphene in virtimer, which is expected in terms to the faster shape recovery of graphene-vitrimers under NIR irradiation. Figure 9b illustrates the shape recovery of folded strip sample under local NIR irradiation. The folded shape totally recovered to its original dumbbell shape within 20 s of NIR irradiation. A helix temporary shape obtained through thermal fixed can also recover to its original shape step by step using NIR in sequent irradiation. It is clear that the recovery begins after 7 s of irradiation, and the permanent shape is recovered within 80 s (Video S3). In addition, local trigger by NIR could selectively drive shape recovery of the temporary strip shape to its permanent shape c1 by three steps (Figure S6).
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CONCLUSIONS In summary, dual-triggered shape recovery and shape reconfigurable shape memory composites based on graphene-vitrimers are prepared with dynamic transesterification reaction. The incorporation of graphene into vitrimer significantly improves mechanical properties, especially enhancing toughness with a classical ductile fracture. Reconfiguring measurements show that the established dynamic covalent bonds can undergo shape reconfiguration above Tv with the shape retention ratio approaching 100%, indicating that the arbitrary newly permanent shape can be obtained from 2D to 2D or from 2D to 3D by the facile thermal mechanical mode. Moreover, the arbitrary shapes also show excellent shape memory effect. We also have demonstrated that the graphene-vitrimers exhibit shape recovery behavior by NIR trigger by virtue of high-efficiency photothermal conversion of graphene. This obtained dual-triggered and reconfigurable shape memory graphene-vitrimers will significantly simplify processing of complexly permanent shape and broaden the applications of shape memory polymers.
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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.6b07403. FTIR spectra; SEM image; data on shape creep, recovery, shape retension ratio, shape fixity, shape memory cycle; NIR triggered recovery of permanent shape c (PDF) H
DOI: 10.1021/acsami.6b07403 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Video S1 of thermally induced shape recovery of permanent shape a (AVI) Video S2 of thermally induced shape recovery of reconfigured permanent shape b (AVI) Video S3 of mobile NIR induced shape subsequently recovery of a helix shape (AVI)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We greatly appreciate the financial support from the National Basic Research Program of China (973 Program, Grant 2015CB057502). The National Defense Innovation Fund of Chinese Academy of Sciences (Grant CXJJ-14-M43) and the National Natural Science Foundation of China (Grant 51305431) are duly acknowledged.
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DOI: 10.1021/acsami.6b07403 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
