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Jan 18, 2018 - A Fascinating Metallo-Supramolecular Polymer Network with. Thermal/Magnetic/Light-Responsive Shape-Memory Effects. Anchored by Fe3O4 ...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

A Fascinating Metallo-Supramolecular Polymer Network with Thermal/Magnetic/Light-Responsive Shape-Memory Effects Anchored by Fe3O4 Nanoparticles Lan Du,† Zhi-Yuan Xu,† Cheng-Jie Fan,† Gang Xiang,‡ Ke-Ke Yang,*,† and Yu-Zhong Wang† †

Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MOE), College of Chemistry, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, and ‡College of Physics, Sichuan University, Chengdu 610064, China S Supporting Information *

ABSTRACT: Two-way shape-memory polymers (2W-SMPs) show great potential in actuating applications such as robotics due to their reversibility; indeed, multiresponsive 2W-SMPs are more expected. Inspired by the fascinating adhesion effects of mussels, we herein describe a metallo-supramolecular poly(ε-caprolactone) (PCL)-based network constructed around catechol chemistry, leading to excellent thermal/ magnetic/light-responsive two-way shape-memory effects (2W-SME) as well as self-healing capacity. These hybrid networks get readily self-assembled upon metal coordination interaction between superparamagnetic iron oxide nanoparticles (Fe3O4 NPs) and catechol-telechelic PCL. The incorporation of Fe3O4 NPs may act as the strong netpoints which allow the networks with excellent thermal-responsive one-way (1W) and 2W-SME, due to the ability of the semicrystalline PCL segments to present both crystallization-induced elongation (CIE) and melting-induced contraction (MIC) under constant stress. As a multifunctional medium, moreover, it also endows the hybrids with the magnetic-responsive 2W-SME in an alternating magnetic field and the light-induced SME triggered by the near-infrared light. Thanks to the dynamic nature of this metal coordination interaction inspired by mussel, the target hybrid networks also showed good self-healing capability. A model of magnetic-triggered actuator was well-established, which allows to the material with interesting applications such as remotecontrol or intelligent magnetic-sensitive devices.

1. INTRODUCTION Shape-memory polymers (SMPs), as an emerging class of smart materials, have the ability to respond to various external stimuli (such as heat, light, electricity, and so on) by changing their shapes.1−6 This unique feature endows SMPs to a variety of applications, ranging from actuator and sensors to biomedical devices and space deployable materials.7−11 Generally, a shapememory cycle including programing and recovery process is needed to get the so-called one-way SMP (1W-SMP). This irreversible behavior of 1W-SMP greatly limits its real application in remote-control devices or shielding systems. Recently, the reversible shape-memory effect or so-called twoway shape-memory effect (2W-SME) has been found in liquidcrystalline elastomers (LCE) and semicrystalline polymer networks (SCN) successively.12−19 Compared to 1W-SMP, the 2W-SMP does not need to be reprogrammed under successive cycles, which makes them good candidates in the field of sensors, actuators, and artificial muscles.20 Regarding the working mechanism, 2W-SME can be realized by the reversible anisotropic−isotropic transition in the case of LCEs and crystallization-induced elongation (CIE) and melting-induced contraction (MIC) under constant stress in the case of SCNs. Commonly, SCNs are much easier to tailor © XXXX American Chemical Society

and synthesize than LCEs. Up to now, various 2W-SMPs based on SCNs have been developed, wherein the semicrystalline segments such as poly(cyclooctene) (PCO),13 poly(ethyleneco-vinyl acetate) (EVA),14 poly(ε-caprolactone) (PCL),15 poly(ω-pentadecalactone) (PPD),16 poly(1,4-butylene adipate) (PBA),18 and poly(octylene adipate) (POA)19 have been employed as a matrix in 2W-SMP systems. Because of the origin of CIE and MIC, such SCNs can only exhibit thermalresponsive 2W-SME. Indeed, realizing multiresponsive 2WSME in a simple system is still a major challenge for most researchers. In addition, the network with well-defined architecture is also a key point to achieve good performances in 2W-SME. As a mature approach, the chemical cross-linking is widely employed to create these SCNs.21 Nevertheless, this irreversible feature will restrict its implementation on e.g. melt-processing. Recently, dynamic networks have been highly considered in the area of SMPs owing to their unique reversibility. Up to now, a variety of reversible covalent or noncovalent Received: December 12, 2017 Revised: January 18, 2018

A

DOI: 10.1021/acs.macromol.7b02641 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Preparation Route of the PCL-DA (a) and the PCL-DA-Fe3O4 Networks (b)

interactions, such as D−A bonding,22 hydrogen bonding,23 π−π stacking,24 host−guest recognition,25 and metal−ligand coordination,26 have been utilized to construct shape-memory dynamic networks. Commonly, the noncovalent interaction can be readily assembled under mild conditions, and it has been widely used to develop self-healing materials.27−29 However, networks constructed upon weak noncovalent interactions can hardly show ideal SME. In the present work, we aim to develop a dynamic SCN in order to achieve desirable multiresponsive 2W-SME as well as self-healing capacity. To meet this goal, a stable metallosupramolecular interaction inspired by mussels was considered.30 Indeed, mussels can adhere their byssal threads tightly to all types of surfaces in a wet environment due to it secretion of an adhesive protein.31 An unusual structural component found in these proteins is 3,4-dihydroxyphenylalanine (DOPA).32,33 Catechol groups in DOPA can form stable coordination bonds with the metal atoms such as Fe at the interface.34 To construct SCN with 2W-SME, poly(εcaprolactone) (PCL) was herein chosen as the building block in virtue of its desirable crystalline property as well as its low toxicity, biodegradability, and biocompatibility.16,21 The target PCL-based dynamic networks were straightforwardly fabricated by mixing iron oxide nanoparticles (Fe3O4 NPs) with catecholtelechelic PCL (PCL-DA), leading to metal coordination interaction between the Fe3O4 NPs and catechol groups. The semicrystalline PCL segment can endow the network thermal-

responsive 1W and 2W-SME, while incorporating Fe3O4 NP also endues the network to magnetic-responsive 2W-SME in an alternating magnetic field due to the magnetic hyperthermia35 and light-responsive SME triggered by near-infrared (NIR) light due to the photothermal effect.36 Utilizing the dynamic nature of this metal coordination interaction inspired by mussels enabled us to address self-healing capability of these hybrid networks.

2. EXPERIMENTAL SECTION Materials. ε-Caprolactone (ε-CL) and stannous octoate (Sn(Oct)2) were purchased from Sigma-Aldrich. 1,6-Hexamethylene diisocyanate (HDI) (AR grade) from TCI was used without further purification. Dopamine hydrochloride (DA-HCl), sodium oleate, ferrous chloride tetrahydrate (FeCl2·4H2O), ferric chloride hexahydrate (FeCl3·6H2O), and dibutyltin dilaurate (DBTDL) were purchased from J&K Scientific Ltd. 1,4-Butanediol (BDO) and N,Ndimethylformamide (DMF) were purchased from Kelong Reagent Corp. (Chengdu, China). Before use, ε-CL, BDO, and DMF were dried over CaH2 and distilled under reduced pressure. Triethylamine from Kelong Reagent Corp. (Chengdu, China) was refluxed by 4toluenesulfonyl chloride for 3.5 h, dried over CaH2, and distilled. All other reagents and solvents of A.R. grade were supplied by Kelong Reagent Corp. (Chengdu, China) and were used as received without further purification. Synthesis of PCL-Diols. PCL-diols was prepared by typical bulk ring-opening polymerization (ROP) of ε-CL using BDO as an initiator and Sn(Oct)2 as catalyst according to our previous report.37 As shown in Scheme 1, preweighed ε-CL, BDO, and Sn(Oct)2 were quickly B

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Macromolecules added into a 50 mL round flask under nitrogen, which had been under vacuum for 3 h to exhaust residual water. After that, the reaction system was polymerized at 120 °C for 24 h. The obtained product was dissolved in chloroform, precipitated in methanol, and dried under vacuum at room temperature. The molecular weight of PCL-diols was controlled by adjusting the feed molar ratio of ε-CL and BDO, and the detailed information is given in Table 1.

room temperature in chloroform (CDCl3) and DMSO-d6, using tetramethylsilane (TMS) as an internal reference. Gel Permeation Chromatography (GPC). GPC was performed on HLC-8320GPC (TOSOH Corporation) apparatus equipped with a refractive index detector, using poly(methyl methacrylate) (PMMA) standards to obtain a calibration curve. The elution solvent used was N,N-dimethylformamide (DMF) at a flow rate of 0.4 mL/min at 40 °C. Ultraviolet−Visible (UV−Vis) Spectrophotometer. The UV spectra of the each sample were recorded on a Varian Cary 50 spectrophotometer using DMF as solvent at room temperature in the wavelength range 200−500 nm. Swelling Test of the PCL-DA-Fe3O4 Networks. The samples were cut into small slices, swelled with chloroform for 24 h, and then extracted. The mass of the original sample (m0), the swelled extracted sample (m1), and the dried extracted sample (m2) were recorded. The gel content (G (%)) and the degree of swelling (S (%)) were calculated by the following formulas:

Table 1. Molecular Weight of the PCL-Based Samples samples

Mn,NMRa (103 g/mol)

PCL2.8K-diols PCL4.9K-diols PCL6.1K-diols PCL2.8K-DA PCL4.9K-DA PCL6.1K-DA

2.76 4.87 6.09 3.45 5.55 6.76

Mn,UV−visb (103 g/mol)

Mn,GPCc (103 g/mol)

PDIc

3.66 5.14 6.42

5.17 8.50 14.5 6.93 8.93 14.7

1.48 1.83 1.21 2.73 2.52 2.24

a

Calculated from H NMR spectra. bCalculated from UV−vis spectra. c Determined by GPC.

Synthesis of Catechol Group Terminated PCL (PCL-DA). The synthetic route of PCL-DA is also shown in Scheme 1a. The reaction was performed in a round flask by two steps. In the first step, quantitative PCL-diols were added and dried at 75 °C for 3 h under vacuum. It was then purged three times with dry nitrogen and kept in a nitrogen flow. The DMF was injected into the reactor to dissolve the PCL-diols with magnetic stirring. After that, a predetermined amount of HDI and five drops of DBTDL were injected into flask with reaction time for 3.5 h at 75 °C under persistent stirring to prepare the prepolymer (NCO-PCL-NCO). In the second step, dopamine hydrochloride in DMF was added, and triethylamine was injected dropwise into the reaction system to neutralize the hydrochloric acid. This end-capping progress was carried out for another 12 h at 40 °C under nitrogen. After purification by separation and precipitation in methanol, the final product was dried under vacuum at room temperature to obtain the PCL-DA. The molecular weight of PCL-DA is also shown in Table 1. Preparation of Fe3O4 Nanoparticles (Fe3O4 NPs). Fe3O4 NPs with the coating agents sodium oleate were synthesized according to previously reported literature by the controlled chemical coprecipitation method.35 First, 5.74 g of sodium oleate, 3.98 g (0.02 mol) of FeCl2·4H2O, and 10.81 g (0.04 mol) of FeCl3·6H2O were dissolved in 100 mL of distilled water in a three-neck flask. 0.5 g of sodium oleate in 10 mL of ammonium hydroxide was added into the flask dropwise with 1000 rpm stirring under the protection of dry nitrogen at the 50 °C. After 1 h, the precipitated Fe3O4 NPs were washed by five cycles of centrifugation and redispersion in ethanol. Then, the precipitated Fe3O4 NPs were redispersed in the sodium oleate solution (5%, w/w) under the conditions of ultrasonic agitation for 0.5 h and strong stirring for another extra 0.5 h. The products were also washed by five cycles of centrifugation and redispersion in ethanol. Finally, the products were dried under vacuum at room temperature to obtain the Fe3O4 NPs. The detailed characterization of Fe3O4 nanoparticles was recorded (Methods S1, Supporting Information). Preparation of PCL-DA-Fe3O4 Networks. The desired amount of Fe3O4 NPs and PCL-DA were mixed in chloroform. The dispersion solution was treated with strong ultrasonication for 1.5 h. Then, the mixture was poured into a horizontal Teflon dish at room temperature for 2 days to exhaust solvent, transferred into a drying oven at 40 °C for 1 day, and under vacuum for another 2 days (shown in Scheme 1b). Fourier Transform-Infrared Spectrometry (FT-IR). FT-IR spectral analysis was carried out in a range of wavenumbers from 4000 to 500 cm−1 with a Fourier transform infrared spectrometer (Nicolet 6700, USA). Nuclear Magnetic Resonance (NMR). The 1H NMR spectra were obtained at 400 MHz with Bruker400 (Bruker, Switzerland) at

G (%) =

m2 × 100% m0

(2)

S (%) =

m1 × 100% m2

(3)

The cross-linking density (v) was calculated by the Flory−Rehner equation:38

v=

− [ln(1 − v2) + v2 + χv 2]

(

Vs v21/3 −

v2 2

)

(4)

where χ is the polymer−solvent interaction parameter, which is related to the solubility patameters of the solvent (δsolvent) and the polymer (δpolymer): χ = 0.34 +

Vs (δpolymer − δsolvent)2 RT

(5)

where Vs is the molar volume of the solvent, R is the gas constant, and T is the absolute temperature. All of these data were measured three times and averaged. Differential Scanning Calorimetry (DSC). DSC was performed with a DSC-Q200 (TA Instruments, USA). Samples were heated to 100 °C for 3 min to eliminate the previous thermal history and then cooled down at 10 °C/min to −50 °C. And then, the samples were heated again at the same rate up to 100 °C. Dynamic Mechanical Analysis (DMA). The thermomechanical properties of the samples were tested by using a dynamic mechanical analyzer DMA Q800 (TA Instruments, USA) with a heating rate of 3 °C/min from −80 to 90 °C at the amplitude of 0.2% and a frequency of 1 Hz. Transmission Electron Microscopy (TEM). TEM images were acquired with a Tecnai G2 F20S-TWIN transmission electron microscope (FEI Co., USA) at an acceleration voltage of 200 kV. The thin slice of sample was microtomed at subambient temperature. Tensile Stress Test. The PCL-DA-Fe3O4 networks films with a dumbbell shape were used for the tensile stress test with using a SANS CMT4104 (SANS Group, China) at a strain rate of 50 mm/min at room temperature. Each sample was tested at least three times, and the results reported were average values of each group. Shape-Memory Effect. 1W and 2W-SME of PCL-DA-Fe3O4 networks were conducted by cyclic thermalmechanical tested with a DMA Q800 (TA Instruments, USA) in a force-controlled mode. The detailed procedures and evaluation formulas were addressed in the Supporting Information Methods S3. Self-Healing Effect. The efficiency of self-healing was calculated from the tensile strength before and after self-healing test by an Instron Universal Testing Machine (SANS CMT4104, SANS Group, China). In this work, five strips of PCL-DA-Fe3O4 networks with a dumbbell shape were prepared, four of them were cut with a razor blade, and the width of cut marks was about 0.1 mm. Then the C

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Figure 1. (a) 1H NMR spectrum of PCL2.8K-diols in CDCl3. (b) 1H NMR spectrum of PCL2.8K-DA in DMSO-d6. (c) FTIR spectra of HO-PCL2.8KOH, NCO-PCL2.8K-NCO, and PCL2.8K-DA. (d) UV−vis spectra of PCL-DA and PCL-diols at 0.4 mg/mL in DMF. (e, f) TEM micrographs for PCL2.8K-DA-15%Fe3O4 networks. samples were placed into the oven at 100 °C for 0, 1, 5, and 10 h. After that, all five strips were tested by Instron Universal Testing Machine at a strain rate of 50 mm/min at room temperature.

preprepared by 1,4-butanediol (BDO)-initiated ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). In order to explore the effect of the architecture of networks on their performance, the molecular weight of precursors was adjusted by varying the mass feed ratio of ε-CL to BDO, and the molecular features of resulting PCL were determined by 1H NMR, UV−vis spectra, and GPC (Table 1). The detailed calculation method was recorded in the Supporting Information (Methods S2). The structures of PCL-diols and PCL-DA were confirmed by FTIR, 1H NMR, and UV−vis spectroscopy. Taking PCL2.8K-diols as an example, Figure 1a illustrates its

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of the PCL-DA. In our present work, the target PCL-DA-Fe3O4 networks were constructed by reversible metallo-supramolecular interaction between PCL-DA and Fe3O4 NPs. Herein, the precursor PCLDA was obtained by terminating catechol groups using diisocyanate coupling agent to PCL-diols, which was D

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increasing molecular weight of PCL-DA precursor exhibited lower G% the corresponding networks, and an evident increase in S% and a decrease in v value could be found. The morphology of the hybrid networks was clarified with the help of TEM analyses. From TEM images (Figure 1e,f), it is evident that the Fe3O4 NP was finely dispersed within matrix, and no obvious aggregate could be found. 3.3. Thermal Properties of the PCL-Based Samples. The thermal properties of the PCL-diol and PCL-DA-Fe3O4 networks were investigated by DSC and DMA. Figure 2 displays the DSC curves of the PCL2.8K-based samples in the cooling scan (a) and the following heating scan (b). The relevant thermal parameters and their corresponding values of all the samples are summarized in Table 3. For the samples derived from the same PCL precursor, both the crystallization temperature (Tc), the melting temperature (Tm), and their relevant enthalpy ΔHc and ΔHm were reduced with the Fe 3O 4 NP content. This indicates that the crystallization ability of samples was impeded due to the embedment of Fe3O4 NP in the matrix. On one hand, the network was formed by metal−ligand interaction between Fe3O4 NP and the terminated catechol groups in PCL, which restricts the motion of the PCL chain segments. On the other hand, the presence of Fe3O4 NP also acts as a hurdle during the compacting of polymer chains. When the same content of Fe3O4 NP was fixed and in logical way, the samples derived from higher molecular weight precursor presented better crystallization ability with higher Tm and ΔHm. Figure S2 presents the storage modulus of the PCL-based networks as a function of temperature. A distinct decrease of storage modulus was found when the temperature was above the Tm of PCL segment. A plateau was then observed at high temperature which proves that the cross-linked network was formed. 3.4. Mechanical Properties. The tensile test of the PCLDA-Fe3O4 networks was conducted, and the results are summarized in Table 4. Compared to the poor mechanical performance of PCL-DA precursors which cannot be molded, those of hybrid networks were remarkably enhanced after the incorporation of Fe3O4 NP as cross-linking nodes. Actually, the samples with 5% Fe3O4 NP already shows desirable mechanical performances because the network architecture has been basically formed. Further increasing Fe3O4 NP will not benefit the mechanical performances, on the contrary, the excess of nanoparticles restricts the motion of the PCL chain segments. In fact, the mechanical performance is just one of properties we are concerned with; the magnetic responsibility and light responsibility of the material are also very important. Then, we still prepared the samples with higher Fe3O4 NP content in order to systematically explore multiresponsive SME. 3.5. Shape-Memory Effects. From the view of the architecture design and the working mechanism, a schematic sketch of 1W and 2W-SME in the present system is depicted on Figure 3a, and the quantitative evaluation of 1W and 2W-SME for the PCL-DA-Fe3O4 networks was conducted via cyclic thermomechanical analysis by DMA. As an example, Figure 3e,f describes the cyclic 1W-SME and 2W-SME of PCL2.8K-DA-15% Fe3O4, respectively. For 1W-SME, the shape fixity ratio (Rf) and recovery ratio (Rr) of all the PCL-DA-Fe3O4 networks are quantified according to eqs 4 and 5 (Methods S4, Supporting Information), and the results are summarized in Table 5. In our present hybrid networks, the crystalline PCL segment acts as switching domains, and the cross-links formed by

FTIR spectra before and after functionalization. For the intermediate compound (OCN-PCL2.8K-NCO), the appearance of the 2270 cm−1 band indicated that the −NCO was introduced in the PCL polymer chains. After reacting with dopamine hydrochloride, this peak disappeared in the resultant sample PCL2.8K-DA, but leading to new peaks at 3390 and 1640 cm−1 as associated with the stretching vibration of the hydroxyl H−O in (HO)2−Ar− and the stretching vibration of carbonyl CO in −NH−CO−NH−, respectively. This reveals that the catechol groups were successfully decorated at the extremities of PCL chains. Figure 1b,c shows the 1H NMR spectra of PCL2.8K-diols and PCL2.8K-DA. For PCL2.8K-diols, the characteristic peaks of four different methylenes within repeating units appear at 1.38 ppm (δHe), 1.65 ppm (δHd), 2.31 ppm (δHc), and 4.06 ppm (δHa). The peak at 3.66 ppm (δHb) is ascribed to −CH2−CH2−OH in the chain end of PCL2.8K-diols. As compared to PCL2.8Kdiols, the chemical shifts for PCL2.8K-DA at 8.5−9.0 (δHa), 6.8−7.1 (δHb), 6.3−6.7 (δHc), and 5.5−6.0 (δHd) ppm correspond to protons on −Ar(−OH)2, −O−CO−NH− CH2−, Ar of catechol group, and the secondary amino group in catechol reacted with the NCO− group, respectively. In the UV−vis spectra (as shown in Figure 1d), there is an absorbance peak related to the catechol group at 283 nm in PCL-DA, not present in the case of PCL-diols. This again indicates that we successfully introduced catechol group into polymer chains. Moreover, the content of catechol in the chain could be tuned up by changing the PCL chain length. According to all results above, we can conclude that the catechol-telechelic PCL precursors with different molecular weight were well-prepared. 3.2. Characterization of the PCL-DA-Fe3O4 Networks. Fe3O4 NP was prepared according to the literature35 and wellcharacterized by XRD, VSM, DLS, and SEM (all details are reported in the Supporting Information). The mean particle diameter (Dc) of Fe3O4 NP was about 20 nm in a narrow distribution determined by DLS. The PCL-DA-Fe3O4 networks were straightforwardly fabricated by mixing the PCL-DA precursor with Fe3O4 NP in chloroform. For a systematic investigation, both the molecular weight of PCL-DA precursor and the content of Fe3O4 NP were adjusted, while the gel fraction (G%), swelling ratio (S%), and cross-linking density (v) of networks obtained from swelling test are reported in Table 2. By fixing the molecular weight of PCL-DA precursor, the network containing more nanoparticles exhibited higher G % but lower S%. Increasing the Fe3O4 NP content leads to higher cross-linking density due to the strong binding of catechol group to iron oxide. As a consequence, it resulted higher G% but lower S%. When Fe3O4 NP content was fixed, Table 2. Results of Swelling Test for PCL-DA-Fe3O4 Networks samples PCL2.8K-DA-5% Fe3O4 PCL2.8K-DA-15% Fe3O4 PCL2.8K-DA-30% Fe3O4 PCL4.9K-DA-5% Fe3O4 PCL4.9K -DA-15% Fe3O4 PCL4.9K -DA-30% Fe3O4 PCL6.1K -DA-5% Fe3O4 PCL6.1K-DA-15% Fe3O4 PCL6.1K-DA-30% Fe3O4

G (%) 83.19 87.66 92.80 80.73 86.38 89.28 77.18 83.82 86.46

± ± ± ± ± ± ± ± ±

0.71 0.62 0.18 1.23 0.29 0.40 0.57 0.37 0.61

S (%) 3104 1575 1441 4599 2219 1518 5287 2915 2032

± ± ± ± ± ± ± ± ±

33 42 34 65 24 23 201 62 30

v × 105 (mol/cm3) 1.07 3.36 3.91 0.58 1.89 3.58 0.45 1.19 2.19

± ± ± ± ± ± ± ± ±

0.03 0.20 0.21 0.02 0.03 0.09 0.04 0.04 0.06 E

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Figure 2. DSC curves of PCL2.8K-based samples: cooling run (a) and subsequent heating run (b) at rate of 10 °C/min after eliminate the previous thermal history.

The thermally induced 1W-SME of the PCL2.8K-DA-15% Fe3O4 network was also recorded by a digital photograph (Figure 3b). The sample was programmed as follows: at the beginning, a straight sample was heated to Thigh = 80 °C, which is above the Tm of PCL segment; subsequently, the sample was deformed to spiral and cooled to Tlow = −20 °C at which the temporary shape can be fixed. Finally, the sample was heated to Thigh = 80 °C, which enabled the sample to recover its original straight shape. In other terms, this sample displayed a good 1W-SME under the thermal conditions. From Figure 3e, the obvious CIE occurred. This phenomenon allowed us to expect 2W-SME in this material. Then, the thermally induced 2W-SME of the PCL-DA-Fe3O4 networks were performed under a constant stress; the actuation magnitude (Ract(σ)) and recovery magnitude (Rrec(σ)) were calculated from cyclic testing. Taking PCL2.8K-DA-15% Fe3O4 as an example, different stresses were applied (see details in Supporting Information Methods S5 and Figure 6S). Clearly, Ract(σ) increased gradually with the applied stress, but the Rrec(σ) decreased. When 0.48 MPa stress was applied, comparatively, the sample showed desirable reversibility in fairly strain range with the average Rrec,2−6 of 97.20% and Ract,2−6 of 26.05% (cyclic 2W-SME of PCL2.8K-DA-15% Fe3O4 is shown in Figure 3c). Then, 0.48 MPa stress was chosen to be applied to the other samples (the results are summarized in Table 5). It reveals that both the molecular weight of PCL precursors and the Fe3O4 NP loading content affect the 2W-SME evidently. It can be stated out that Ract(σ) decreased with the increase of the Fe3O4 NP as well as the decrease of the molecular weight of PCL precursors. This is due to the increase of cross-linking density in hybrid network, retarding the crystallization of polymer chains. In this respect, PCL6.1K-DA-5% Fe3O4 with lowest Fe3O4 NP content but highest PCL molecular weight exhibits the largest Ract(σ) of about 34.28%. For the Rrec(σ), an opposite trend with Ract(σ) was however found. In general, the hybrid networks show good reversibility with Ract(σ) more than 90% except for PCL2.8K-DA-30% Fe3O4. This discrepancy may be ascribed to the excess of Fe3O4 NP. In fact, the strong interaction between Fe3O4 NP and catechol groups ensures the network to present a tough architecture, which may explain the good reversibility in 2W-SME. The 2W-SME was also recorded by a digital photograph (Figure 3c). As we know, Fe3O4 NP presents perfect superparamagnetism, expecting a magnetic-responsive 2W-SME for these

Table 3. DSC Data of PCL-Diols, PCL-DA, and the Hybrid Networks sample

Tca (°C)

ΔHca (J/g)

Tmb (°C)

ΔHmb (J/g)

Xcc (%)

PCL2.8K-diols PCL2.8K-DA PCL2.8K-DA-5% Fe3O4 PCL2.8K-DA-15% Fe3O4 PCL2.8K-DA-30% Fe3O4 PCL4.9K-diols PCL4.9K-DA PCL4.9K-DA-5% Fe3O4 PCL4.9K-DA-15% Fe3O4 PCL4.9K-DA-30% Fe3O4 PCL6.1K-diols PCL6.1K-DA PCL6.1K-DA-5% Fe3O4 PCL6.1K-DA-15% Fe3O4 PCL6.1K-DA-30% Fe3O4

28.02 4.98 0.06 −0.92 −8.84 29.76 17.26 12.67 11.21 9.84 30.58 17.90 16.31 14.96 12.36

93.48 52.19 33.79 29.87 27.88 80.06 60.16 38.20 35.30 30.09 76.75 58.97 41.97 37.66 32.37

50.09 48.05 42.84 40.31 36.97 53.13 53.09 50.12 49.19 48.45 54.63 53.82 51.24 49.83 46.97

95.56 65.94 37.65 32.05 28.25 80.14 64.53 39.77 35.92 30.24 76.66 62.75 41.96 37.88 32.28

69.25 47.78 28.65 26.71 26.61 58.07 46.76 30.26 29.93 28.49 55.55 45.47 31.93 31.57 30.41

a

Obtained from the cooling scan. bObtained from the heating scan. The mass crystallinity Xc (%) = ΔHm × CPCL ÷ ΔH0m; the enthalpy of melting ΔH0m of PCL (100%) is 138 J/g.

c

Table 4. Tensile Properties of the PCL-DA-Fe3O4 Networks samples PCL2.8K-DA-5% Fe3O4 PCL2.8K-DA-15% Fe3O4 PCL2.8K-DA-30% Fe3O4 PCL4.9K-DA-5% Fe3O4 PCL4.9K-DA-15% Fe3O4 PCL4.9K-DA-30% Fe3O4 PCL6.1K-DA-5% Fe3O4 PCL6.1K-DA-15% Fe3O4 PCL6.1K-DA-30% Fe3O4

strain at break (ε, %) 945 866 838 862 845 844 770 712 557

± ± ± ± ± ± ± ± ±

26 25 37 13 16 24 60 46 46

tensile strength at break (σb, MPa) 24.30 26.89 22.21 25.26 24.69 23.12 20.90 20.04 18.41

± ± ± ± ± ± ± ± ±

1.00 0.34 0.66 0.93 0.31 0.60 1.24 0.75 0.65

metal−ligand coordination between Fe3O4 NPs and catechol groups in PCL chains act as netpoints. As mentioned above, the PCL segment in all samples exhibited desirable crystallinity, which allowed to the samples by exhibiting excellent fixity ratio with the Rf close to 100%. Thanks to the fact that the netpoints were constructed upon strong interactions between Fe3O4 NPs and catechol groups, the outstanding recovery property was also achieved, and the Rr of the most samples is beyond 95%. F

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Figure 3. (a) Schematic mechanism for one-way and two-way SME of PCL-DA-Fe3O4 hybrid networks. (b) Digital photographs showing the oneway SME of PCL2.8K-DA-15% Fe3O4 network. (c) Digital photographs shows the two-way SME PCL2.8K-DA-15% Fe3O4 network with cooling and heating process. (d) Digital photographs of two-way SME of PCL2.8K-DA-15% Fe3O4 in an alternating magnetic field with a frequency of 300 kH and a field strength of 4.5 kA/m. Cyclic one-way (e) and two-way (f) SME of PCL.2.8K-DA-15% Fe3O4 recorded by DMA.

that of the networks gradually reduced from 100 to 50 °C while the Fe3O4 NPs concentration decreased to 5% from 30%. According to Tm of PCL segment, PCL2.8K-DA-15% Fe3O4 network was chosen to evaluate the magnetic-responsive 2WSME. Exhilaratingly, it was demonstrated in the case of by applying an alternating magnetic field with a frequency of 300 kHz (Figure 3d). A detailed process was recorded (Methods S6, Supporting Information).

hybrids networks because of the magnetocaloric effect. In order to explore the magnetic-responsive shape-memory behavior of the samples in an alternating magnetic field, we determined the temperature curves of the Fe3O4 NPs and the networks with different Fe3O4 NPs contents in an alternating magnetic field with a frequency of 300 kHz and a field strength of 4.5 kA/m (as shown in Figure S4). It indicates that the temperature rises with time and eventually reaches an equilibrium value. The equilibrium temperature of Fe3O4 NPs is about 150 °C, and G

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Macromolecules Table 5. 1W and 2W-SME of PCL-DA-Fe3O4 Networks 1W-SME samples PCL2.8K-DA-5% Fe3O4 PCL2.8K-DA-15% Fe3O4 PCL2.8K-DA-30% Fe3O4 PCL4.9K-DA-5% Fe3O4 PCL4.9K-DA-15% Fe3O4 PCL4.9K-DA-30% Fe3O4 PCL6.1K-DA-5% Fe3O4 PCL6.1K-DA-15% Fe3O4 PCL6.1K-DA-30% Fe3O4

2W-SME

Rf (%) 99.74 98.57 99.29 99.71 99.47 99.15 99.67 99.35 99.01

± ± ± ± ± ± ± ± ±

0.05 0.01 0.07 0.01 0.03 0.05 0.02 0.03 0.01

Rr (%) 95.07 97.99 95.97 93.29 96.83 96.50 94.92 95.84 96.43

± ± ± ± ± ± ± ± ±

3.19 1.09 1.88 2.84 1.90 1.88 2.96 1.90 1.75

Rrec (%) 93.44 97.20 86.20 89.44 90.33 92.42 91.76 92.16 93.72

± ± ± ± ± ± ± ± ±

2.59 0.44 4.74 2.35 2.22 2.87 1.13 1.22 0.77

Ract (%) 32.23 26.05 20.47 33.19 30.08 23.32 34.28 32.43 24.79

± ± ± ± ± ± ± ± ±

1.71 0.26 0.14 1.47 1.34 0.24 1.56 1.10 0.57

Figure 4. (a) Digital photographs showing the one-way SME of PCL2.8K-DA-15% Fe3O4 network in near-infrared light. (b) Digital photographs of two-way SME of PCL2.8K-DA-15% Fe3O4 in near-infrared light.

As a filler, Fe3O4 NPs may not only provide the magnetic hyperthermia, but they also can serve as nanoscale heat sources through efficiently absorbing near-infrared (NIR) light and transforming it into thermal energy. Thus, we evaluate the equilibrium temperature of the networks with different Fe3O4 NPs contents under the irradiation of NIR light, and the effect of distances between the sample and light source was also investigated (as shown in Figure S5). Based on the results, the PCL2.8K-DA-15% Fe3O4 network with a distance of 20 cm was confirmed for the final SME testing; as expected, both NIR light-responsive 1W and 2W SME were identified in this network in virtue of the photothermal effect of Fe3O4 NPs (Figure 4, Movie S1, and Movie S2). Figure 4a shows the directional recovery process of the cube when the NIR laser is

switched on. Figure 4b records light-responsive 2W-SME of this hybrid network under a constant stress. A detailed process is addressed in the Supporting Information (Methods S7). 3.6. Self-Healing Behavior. Indeed, the PCL-DA-Fe3O4 network constructed by reversible metal coordination bonds with catechol group and Fe3O4 NPs possesses a dynamic feature. We assumed that it could exhibit intrinsic self-healing ability. Herein, the PCL4.9K-DA-15% Fe3O4 network, the sample with best multiresponsive shape-memory performance, was chosen for this evaluation. Actually, the mobility of the polymer segments bound with catechol groups can also play an important role on their healing ability. Hence, the damaged samples were healed at 100 °C to ensure PCL chains with good H

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Figure 5. (a) Typical engineering stress−strain curve for PCL4.9K-DA-15%Fe3O4 networks in original state, just damaged, and healed at 100 °C with different times. (b) Schematic mechanism for the self-healing process of PCL-DA-Fe3O4 hybrid networks. Optical micrographs of the PCL4.9K-DA15% Fe3O4 dynamic network before healing (c), healed at 100 °C for 1 h (d), and healed at 100 °C for 5 h (e).

Figure 6. A model of magnetic-triggered actuator from two-way shape-memory PCL-DA-Fe3O4 hybrid networks to realize on−off control in a circuit.

mobility, which increased the probability of the dangling catechol group bounded with the Fe3O4 NPs. The tensile tests were carried out to evaluate their selfhealing ability. For sake of comparison, we herein choose five strips of PCL4.9K-DA-15% Fe3O4 network for testing, i.e., a nondamaged strip, a damaged strip without any treatment, and three damaged strips healed for 1, 5, and 10 h. Figure 5a displays the typical engineering stress−strain curves of these five strips. The efficiency of self-healing was calculated from the tensile strength by the following formula:

η=

σh × 100% σo

(1)

Here, σo is the tensile strength of the original strip, and σh refers to the tensile strength of the healed sample. As compared to the original strip, the notched specimen after damage failed immediately at the fracture and showed very poor tensile property. After healing, the mechanical property was obviously enhanced as compared to the notched specimen. Increasing the healing time increases the healing efficiency by reaching 63% after 1 h and about 91% after 10 h. I

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established, which promises the material to have great prospective in different applications such as remote-control or intelligent magnetic-sensitive devices.

In order to reveal the essential reason which answers for this desirable healing capacity of the composites, a control experiment was also conducted on a covalently cross-linked PCL network under the same healing condition (see details in Figure S7). The healing efficiency of the pure chemically crosslinked PCL is about 46%, which is much lower than the healing efficiency of the hybrid composites. Thereby, we can believe that the dynamic interaction between Fe3O4 NPs and the catechol group plays a critical role during the healing process of the composites at 100 °C. Furthermore, in order to inspect the influence of this dynamic interaction on the shape-memory performance, we also evaluated the self-healing behavior at 80 °C, at which temperature the shape-memory properties were tested (see details in Figure S8). Indeed, the healing efficiency of the hybrid composites at 80 °C is only about 57%, which is much lower than what obtained at 100 °C (91%). It suggests that the netpoints based on the metal coordination interaction are very stable at 80 °C which enable the material a good shapememory performance. The inherent self-healing process of PCL4.9K-DA-15% Fe3O4 dynamic network was also recorded by an optical microscope. First, the sample was cut with a razor blade and a crack was observed (Figure 5c). Then, the sample was put into an oven at 100 °C for 1 h (Figure 5d) and 5 h (Figure 5e). Evidently, the crack almost disappeared after 5 h. The detailed process and mechanism are shown in Figure 4b. 3.7. A Model of Magnetic-Responsive Actuator. On the basis of the excellent 2W-SME of PCL-DA-Fe3O4 hybrid networks, we further designed a model of magnetic-triggered actuator, as obtained under high-frequency alternating magnetic field. We inserted the PCL-DA-Fe3O4 hybrid networks into a magnetic circuit. The elongation or shrinkage of the materials was respectively obtained after switching the alternating magnetic field or not, as highlighted by the lamp appearing bright or dark (Figure 6 and Movie S3). This reversible behavior as obtained after applying alternating magnetic field on/off was demonstrated, so that these PCL-DAFe3O4 hybrid networks present a great potential as intelligent actuators as well as to protect some sensitive equipment, which could be interfered with or damaged by the magnetic field.

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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/acs.macromol.7b02641. Figures S1−S7 (PDF) Movie S1 (AVI) Movie S2 (AVI) Movie S3 (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.-K.Y.). ORCID

Ke-Ke Yang: 0000-0002-7019-6059 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We greatly appreciate Prof. Jean-Marie Raquez at the Universite de Mons for his valuable comments on this work. This work was supported financially by the National Natural Science Foundation of China (51473096, 51721091) and the International S&T Cooperation Project of Sichuan Province (2017HH0034).

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REFERENCES

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4. CONCLUSION In summary, a series of PCL-DA-Fe3O4 metallo-supramolecular networks were successfully prepared upon the strong bonding of catechol groups present at the end of PCL chains to iron oxide as compared by the interfacial adhesion chemistry of mussel threads. The molecular weight of PCL precursors and the content of nanoparticles were varied to tune the architecture, thermal behaviors, and SME of PCL-DA-Fe3O4 hybrid networks. All the networks display good mechanical performances compared to the poor mechanical performance of PCL-DA because of the formation of the cross-linking network. The thermal-switched 1W and 2W-SME were evaluated by DMA, a desirable 1W-SME was obtained with the Rf close to 100% and Rr beyond 95%, and the good reversibility was demonstrated in the 2W-SME evaluation. In addition, the magnetic-triggered 2W-SME and near-infrared light-triggered SME were also realized with the incorporation of Fe3O4 NPs into the matrices. At the same time, the dynamic feature of the metal coordination endowed the hybrid networks with good self-healing capability; 91% recovery of σ was achieved in a typical sample PCL4.9K-DA-15% Fe3O4 after 10 h healing. Finally, a model of magnetic-triggered actuator was successfully J

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