Article pubs.acs.org/Macromolecules
Mechanoresponsive Healable Metallosupramolecular Polymers Guangning Hong, Huan Zhang, Yangju Lin, Yinjun Chen, Yuanze Xu, Wengui Weng,* and Haiping Xia* Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, P. R. China S Supporting Information *
ABSTRACT: The development of polymers that possess superb mechanical properties and at the same time are capable of sensing damage and self-healing is presented. Coppercatalyzed azide−alkyne cycloaddition (CuAAC) based tridentate ligand 2,6-bis(1,2,3-triazol-4-yl)pyridine (BTP) and covalent mechanophore spiropyran (SP) units are incorporated into the polymer backbone to prepare ligand macromolecule. Upon coordinating with transition or lanthanide metal salts, metallosupramolecular films with phased-separated soft/hard morphology are spontaneously formed. The resulting materials show a rare combination of strong, tough, and elastic mechanical properties and are able to sense damage by changing optical properties. The Zn2+-containing material can self-heal in the presence of solvent and fully restore its mechanical properties. The underlying structure−property relationship is unveiled. In particular, the interplay between the covalent SP mechanophore and the noncovalent metal−ligand interactions and their hard phase is demonstrated.
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INTRODUCTION Inspired by nature’s intriguing way and extraordinary biologically produced substances, a great number of approaches have been explored to develop biomimetic processes and materials.1−3 One subarea of this biomimetics is the study of healable polymeric materials that can undergo healing either in autonomous ways or via induced mechanisms. A variety of approaches, such as the uses of microencapsulation of reactive species,4,5 dynamic covalent chemistry, and noncovalent interactions,6−13 have emerged to access such healable materials. Of particular interest to us is the use of noncovalent interactions in building healable materials that can respond to variant external stimuli.12,14−19 There are a wide range of noncovalent interactions available, allowing the design of many innovative and elegant supramolecular healable materials whose healing could be induced by stimuli such as light, heat, solvent, and pressure. The use of noncovalent interactions also provides opportunity of imparting other biomimetic functionalities into the materials. For instance, inspired by titin, Guan and co-workers used 2-ureido-4[1H]-pyrimidone (UPy) motif based doubleclosed-loop module to build biomimetic modular polymers that exhibit a rare combination of self-healing, shape memory, and superior mechanical properties, that is, high tensile modulus, high toughness, and resilience.20−23 The UPy based hydrogenbonding interaction not only elicits healing and shape memory but also provides sacrificial weak bonds to dissipate strain energy and prevents the materials from premature fracture. By using micellar polymerization technique in the presence of electrolyte, Tuncaboylu et al. showed that n-alkyl methacrylates with alkyl chain lengths of 18 and 22 carbon atoms could be © XXXX American Chemical Society
copolymerized with acrylamide to obtain self-healing and tough supramolecular hydrogels based on strong hydrophobic associations.24 Given that mechanical loading is inevitable for many selfhealing materials, it would be of great importance to have the abilities to sense stress and provide visible warning signals before the materials are deformed to failure. Many polymers capable of providing visible detections of stress by changing their optical properties, such as absorption, fluorescence, and luminescence, have been successfully made by using mechanoresponsive motifs (mechanophores).6,25,26 The principles behind these systems include changes in the intermolecular interactions between blended dye molecules,27−29 variations in the molecular conformation of dye motifs covalently incorporated into polymers,30 and chemical reactions in mechanophores integrated into the polymer backbones.31−35 Mechanically induced chemical changes in mechanophores have recently attracted considerable interests, since this mechanism is fundamentally similar to mechanical responses found in nature.36−47 We recently developed mechanoresponsive triblock copolymers via covalently embedding SP mechanophore in the center of the soft block.35 Unique hard block fraction dependent mechanoresponsive properties have been found. However, no self-healing design has been introduced into the block copolymers. Urban and coworkers recently developed healable copolymer films that exhibit color changes in response to mechanical scratch. The Received: August 22, 2013 Revised: September 28, 2013
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Figure 1. (a) Multiple BTP ligand units (blue) and covalent mechanophore molecules SP (orange) are incorporated into the polymer backbone to afford ligand macromolecule 2. (b) The metal−ligand complexes phase separate from the linker chains to form “hard phase” domains in the 3D network (only the case with Zn2+ ions is shown here), leaving the mechanophores SP in the “soft phase”. dialysis over chloroform afforded clean macromolecule 2 (1.2 g, 90% yield, Figures S4 and S5). Preparation of Metallosupramolecular Films. Macromolecule 2 was dissolved in chloroform in homemade Teflon casters. Zn(OTf)2 (0.05 mol L−1) or Eu(OTf)3 (0.015 mol L−1) in acetonitrile was poured into the polymer solutions under stirring. The mixtures were then centrifugated to remove air bubbles and placed under room temperature for 24 h for slow drying. Traces of remaining solvent were eliminated by vacuum-drying for another 24 h. Clean and dry films of about 0.5 mm thick were peeled off from the casters for subsequent tests. Tensile Test. Films were cut into strips of about 5 mm in width, and tensile tests were carried out on an Instron 3343 machine with an initial strain rate of 0.05 s−1. Specimens were exposed to intense white light for about 10 min to drive the mechanophore to the colorless closed form prior to any tensile test. A SONY camera (HDR-CX210) was utilized to record the color change of the samples with a frame rate of 25 fps. Self-Healing by Solvent. Strips of about 0.5 mm thick were cut by a razor blade to have a 0.2 mm deep notch. Solvent (chloroform or toluene) was dropped to cover the notch every hour, and the images of the wound were taken by a VHX-600 digital microscope (Keyence, UK). Films with various healing times were then dried in vacuum for 24 h to get rid of the solvent. The healed films were then stretched following the same procedure to obtain the stress−strain curves. SAXS Measurements. All scattering tests were performed on an Anton-Paar SAXSess mc2 platform using slit collimation (slit dimensions: 20 × 0.33 mm2). The wavelength of the X-ray was 0.1542 nm, and the camera length was calibrated to be 260 mm using silver behenate standard. The scattering vector is defined as q = 4π/λ sin θ, where λ is the wavelength and 2θ is the scattering angle. For temperature-dependent SAXS, films were simply elevated to wanted temperature and allowed for a 5 min waiting to ensure thermal equilibrium before exposure. Strain-dependent SAXS tests were also
color changes are caused by the ring-opening of spironapthoxazine side groups to form merocyanine (MC) and the healing is induced by the dissociation of intermolecular hydrogen bonding between MC pairs, which results in collapse of copolymer backbone and consequent pulling of neighboring chains to fill the removed mass and repair the scratch.48 This mechanism is essentially different from those of other healable supramolecular polymers. In this study, we show, for the first time to the best of our knowledge, metallosupramolecular polymers that are able to exhibit a rare combination of selfhealing, stress sensing, and superior mechanical properties.
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EXPERIMENTAL SECTION
Synthesis of Ligand Macromolecules 2. The source of all reagents, chemicals, materials, solvents, and the intermediate substances evolved in the synthesis of SP diol are listed and described in the Supporting Information (Figures S1 and S2). The synthesis of dihydroxylated BTP ligand can be found elsewhere.49 Macromolecule 2 was prepared using the polyurethane reaction method shown in Figure S3. SP diol (5.0 mg, 0.013 mmol, 1.0 equiv), HDI (21 μL, 0.13 mmol, 10 equiv), and dibutyltin dilaurate (DBTDL) (2.0 μL, 0.003 mmol, 0.23 equiv) were dissolved in anhydrous N,N-dimethylformamide (DMF, 0.5 mL) and stirred for 1 h. The reaction mixture was then filtered with a syringe filter (0.45 μm) and added dropwise into a solution of PTHF (Mn = 2000 g mol−1, 1.0 g, 0.5 mmol, 38.46 equiv) in anhydrous DMF (1.0 mL). The mixture was heated and stirred at 30 °C for 1 h. HDI (142 μL, 0.883 mmol, 67.92 equiv), dihydroxylated BTP ligand (164.58 mg, 0.5 mmol, 38.46 equiv), and DBTDL (18 μL, 0.027 mmol, 2.07 equiv) were dissolved DMF (1.0 mL) and then added into the mixture. The solution was stirred at 30 °C for 2 days, resulting in a viscous solution. Precipitation from methanol and further B
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conducted on 100:0 and 0:100 films. Two black markers were made in the gauge region to ensure better control of local strain. The samples were slowly stretched until the distance between the two markers reached certain values and then tightened into the special sample clamps for SAXS tests. Cares were taken to avoid any retraction during clamping. A typical exposure time was 30 min, and the 1D scattering intensities were desmeared by the commercial software SAXSqudrant. Because of the slit collimation, the obtained 1D scattering curves are the slice along the tensile direction.
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RESULTS AND DISCUSSION Material Synthesis. As illustrated in Figure 1, a tridentate ligand BTP synthesized via CuAAC click chemistry49 and a covalent mechanophore SP are employed to construct the target ligand macromolecule 2. Multiple BTP units and SP motifs were incorporated into the polymer backbone by reacting with dihydroxyl-terminated poly(tetrahydrofuran) (PTHF) and hexamethylene diisocyanate (HDI) via a polyurethane reaction method (Figure S3). GPC measurement (Figure S4) gave the number-averaged molecular weight Mn = 32 000 g mol−1 and the polydispersity index (PDI) of 1.56. Estimated from the feed ratio, 0.4 SP motifs and 15 BTP units on average were incorporated into a single polymer chain. The BTP ligand is able to coordinate well with Zn2+ or Eu3+ ions to form 1:2 (metal to ligand, Zn(BTP)2) (Figure 1b) and 1:3 (metal to ligand, Eu(BTP)3) complexes, respectively.50,51 Therefore, upon addition of Zn2+ or Eu3+ into solution of 2, multiresponsive and self-healing supramolecular gels can be readily formed, similar to our previous report.49 Elastic films can be readily harvested by casting the gels in Teflon caster and subsequent drying. Samples were denoted as m:n, where m and n denote the theoretical molar percentage of the BTP ligands that bind to Zn2+ and Eu3+, respectively. Two typical samples, i.e., 100:0 (no Eu3+ and all the BTP ligands bind to Zn2+) and 0:100 (no Zn2+ and all the BTP ligands bind to Eu3+), were investigated. The control sample (0:0) denotes films solely made of the ligand macromolecules 2. The freshly prepared films of 100:0 and 0:100 are light brown red, while the 0:0 film is light blue (Figure S6). After being irradiated with intense white light for 5 min, the colors can be significantly reduced. Upon removal of the white light, the colors returned after about 2 h of exposure to fluorescent room light. Since coordination with metal ions has been found for the MC form of SP,52,53 we speculated that the observed colors of the supramolecular films should involve an interplay between the metal ions and the ring opened MC form of SP. The stabilization by metal−MC interaction may facilitate the thermal isomerization of SP to MC at room temperature.54,55 Similar stabilization effects can be achieved by hydrogen bonding with MC56−58 or by disrupting the solvent ordering around MC.59 Indeed, when much stronger coordinating ligand N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA) was added into the solution of 2 and Eu(OTf)3, the resulting solution exhibited light induced color changing properties similar to that of the solution of 2 alone, owing to the strong competitive binding from PMDETA (Supporting Information). Microscopic Structure. To elucidate the structure− property relationship, our efforts were first placed to investigate the aggregation behavior at nanometer length scale via SAXS technique. In a first set of experiments, variable temperature SAXS measurements were performed on 100:0 and 0:100 films, and the scattering curves are illustrated in Figures 2a and 2b, respectively. Generally, the scattering intensity reflects the
Figure 2. SAXS results of (a) 100:0 and (b) 0:100 films at elevated temperatures indicated in the legends. The black dashed lines are the scattering curves of the control sample (0:0) at room temperature for reference. The black arrows indicate the position of the primary Bragg peak. “Cool” represents the scattering curves from samples that were cooled to room temperature after heating to 150 °C.
electron density fluctuations inside the system. No reflection peak is recognized (black dashed lines) in the 0:0 sample. The expected hard/soft morphology between the metal:BTP complexes (hard phase) and the linker segments (soft phase) in metal ion containing films is verified by the presence of the primary Bragg peak (black arrows in Figure 2), denoted as q1 peak and q2 peak for 100:0 and 0:100, respectively. The Bragg peak is originated from the interference between hard domains and its position gives an appraisal of the average domain spacing. The q values of q1 and q2 peaks are 0.475 nm−1 and 0.488 nm−1, corresponding to domain distances (d = 2π/q) of 13.2 and 12.8 nm for 100:0 and 0:100, respectively. This phaseseparated morphology was confirmed by TEM measurements (Figure S8). When the environment temperature was hoisted up to 150 °C, the shapes of the scattering curves are preserved. This indicates that the hard domains are quite thermally stable for both 100:0 and 0:100 films, which should be ascribed to the thermal stability of metal:BTP complexes and the strong immiscibility between the complexes and the linker segments. Macroscopically, the films did not melt or flow at 150 °C, consistent with the results from the variable temperature SAXS. Mechanical and Mechanoresponsive Properties. A set of tensile experiments were then performed to study the degree of mechanical reinforcement endowed by the metallosupramolecular interactions. Films were treated by intense white light for 10 min prior to all tensile tests to ensure most SP was in the closed form. In fact, similar white light treatments have been adopted by Scotts and Braun.32,60,61 Figure 3 illustrates the representative stress−strain curves of 100:0, 0:100, and the control 0:0 films. Both Zn2+-containing films (100:0) and Eu3+C
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Figure 3. Stress−strain responses of 100:0 film, 0:100 film, and the control sample (0:0).
containing films (0:100) take on sigmoid shaped stress−strain responses that are often observed in stretching biomacromolecules such as collagen62,63 and dragline spider silk.64 The typical curve is composed of an initial low-stress region owing to the uncoiling and straightening of the soft linker strands, an intermediate region right after the pseudo yield point where the stress response comes from the plastic deformation and increases slackly with strain, and a final stress stiffening region where the slope of the curves significantly increases and the stress rapidly upturns. The tensile properties, i.e., Young’s modulus, ultimate strength, strain at break, and material toughness, are summarized in Table 1. As can be seen, the
Figure 4. Mechanically induced color variation for control sample (0:0) and the metallosupramolecular films (100:0 and 0:100) at variable macroscopic strains indicated in numbers. The last column represents the images from fractured and retracted specimens.
and 600% for the 0:0, 100:0, and 0:100 films, respectively. The strong correlation between εonset and εip indicates that sufficient chain alignment along the tensile direction is essential to transfer the load to the mechanophores thus to activate the mechanochemical reaction.65 The windows between εonset and the strain at break for the two metal ion containing samples are wider than that of the control. Therefore, the use of metallosupramolecular interactions provides better opportunity for earlier assessment, modification, and improvement before catastrophic failure occurs. Comparing the colors of the retracted broken samples with those of the unstretched ones without intense white light irradiation (Figure S6), we find that the two sets of colors are essentially the same except that stretching leads to darker colors. Moreover, it is impressive that for 100:0 and 0:100 samples the colors at high strains before fracture are only slightly different from those of the control but are very different right after fracture and retraction (vide inf ra). In order to demonstrate the unique color changing properties in 100:0 and 0:100, a new film was prepared by physically blending the SP diol, Zn2+ salt, and the solution of the BTP containing ligand macromolecule we previously reported.49 This composite film exhibits similar light-induced color changes to those of the metallosupramolecular samples studied here. However, no mechanochromism was observed in this material when subjected to tensile testing (Figure S9). This result implies that the observed color changing behaviors of the metal containing supramolecular films during tensile testing are dominated by the mechanical activation on covalently embedded SP mechanophore. To provide more insights into the mechanical and mechanochromic properties of the metallosupramolecular films, strain-dependent SAXS experiments were performed. In spite of the inertia to heat, strain-dependent SAXS experiments unveil that the hard domains are prone to mechanical force. The representative scattering curves under various strains for a 100:0 film are illustrated in Figure 5a (also see Figure S10a). The evolution of the microscopic structures can also be divided into three stages. At the initial stage before the pseudo yield point, the position of the q1 peak consecutively moves toward lower q as shown by the solid arrows, which relates to the
Table 1. Mechanical Characteristics of 0:0, 100:0, and 0:100 sample
Young’s modulusa [MPa]
strain at break [%]
ultimate strength [MPa]
material toughnessb [MPa]
0:0 100:0 0:100
3.5 ± 0.5 3.5 ± 0.4 4.1 ± 0.4
720 ± 21 1450 ± 40 1053 ± 46
4.9 ± 0.4 9.5 ± 0.4 15.2 ± 0.4
19.6 ± 2.0 50.5 ± 2.6 59.1 ± 1.6
a
Calculated from small strain (
