Reprocessable Supramolecular Thermoplastic BAB-Type Triblock

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Cite This: ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

Reprocessable Supramolecular Thermoplastic BAB-Type Triblock Copolymer Elastomers with Enhanced Tensile Strength and Toughness via Metal−Ligand Coordination Wentao Wang,† Juan Zhang,† Feng Jiang,‡ Xuehui Wang,*,† and Zhigang Wang*,† †

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CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ‡ School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, Guangdong 510006, P. R. China S Supporting Information *

ABSTRACT: A new strategy was proposed to enhance tensile strength, toughness, and extensibility of BAB-type reprocessable supramolecular triblock copolymer elastomers (STBCPEs) by simply incorporating 1-vinylimidazole (VI) as a minor comonomer to introduce metal−ligand coordinate bonds of minor contents into the soft matrix. Poly(n-butyl acrylate-co-1-vinylimidazole)-b-poly(isobornyl acrylate)-bpoly(n-butyl acrylate-co-1-vinylimidazole) (P(BA-co-VI)-bPIBA-b-P(BA-co-VI)) triblock copolymers with varied molecular masses were synthesized via successive reversible addition−fragmentation chain transfer (RAFT) polymerization. Zinc chloride (ZnCl2) was used to coordinate with the pendent imidazole groups in the soft matrix. These supramolecular STBCPEs exhibited simultaneously improved creepresistance, Young’s modulus, tensile strength, and toughness without sacrificing any extensibility. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) results indicated that these supramolecular elastomers showed typical microphase-separated morphology. The microstructure of elastomeric STBCPEs consisted of microphase-separated glassy PIBA domains, serving as physical cross-linking points for one network, and metal−ligand cross-linked clusters formed by ZnCl2 and imidazole groups, which constrained the soft flexible end P(BA-co-VI) blocks, serving as cross-linking points for the second network. This strategy provides a novel means to design high-performance reprocessable supramolecular elastomers through strategically introducing a network of dynamic metal−ligand interaction into the microphase-separated thermoplastic elastomeric system. KEYWORDS: RAFT polymerization, supramolecular network, noncovalent, glass transition, microphase separation, mechanical property

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INTRODUCTION Supramolecular polymers have been studied intensively with tremendous attention. They possess intrinsic directionality and variability of cohesive strength, which makes them easily tailored as functional materials regarding their chemical, physical, and stimuli-responsive nature.1−3 Supramolecular reversible interactions are introduced to design complex macromolecular structures with particular properties normally unmet with covalent bonds alone. Hydrogen bonding, π−π stacking, host−guest complexation, and metal−ligand coordination are typical employed supramolecular interactions.4−6 Supramolecular networks formed by reversible interactions have relatively weaker binding energies compared to chemically cross-linked covalent networks. Therefore, by certain external stimuli, their easy breakage and reformation enable them to be preferable to make engineered smart materials.7,8 Among the above-mentioned four interactions, metal−ligand © XXXX American Chemical Society

coordination is of particular versatility. The strength of metal− ligand coordinate bonds varies in a wide range from weak to closely covalent with changes of metal ions, ligand types, and counterions.9−11 They dissipate energy with high efficiency due to their reversibility and highly adjustable bond energies. This energy dissipation can be efficiently operated with hundreds or more extension cycles. Thus, to reinforce the soft materials, such as hydrogels and elastomers, introduction of metal−ligand coordinate bonds is favored.9,12,13 As biphasic synthetic polymer materials, the thermoplastic elastomers have been applied extensively in various commercial fields due to their excellent mechanical properties.14−16 A variety of technologically applicable smart materials can be Received: December 28, 2018 Accepted: February 15, 2019

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DOI: 10.1021/acsapm.8b00277 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials Scheme 1. (a) Illustration of Formation of STBCPE, P(BA-co-VI)-b-PIBA-b-P(BA-co-VI)/ZnCl2 and (b) Schematic Illustration of the Microstructure for STBCPEa

a

The microdomains in blue color represent aggregation of hard middle PIBA block, and the middle yellow lines among the microdomains represent the chain entanglements of P(BA-co-VI) chains. The magenta spheres connected by magenta lines represent the metal−ligand coordinate bonds.

hydrogels by Gong and co-workers, which was similar to our thought on architectural design.27,28 Herein, we propose a novel but simple design of reprocessable surpramolecular triblock copolymer elastomers (STBCPEs), consisting of soft−hard−soft BAB-type triblock copolymers and metal−ligand coordination, for which poly(isobornyl acrylate) (PIBA) was designed as the hard middle A block and poly(n-butyl acrylate-co-1-vinylimidazole) (P(BA-coVI)) as the soft two end B blocks. The P(BA-co-VI)-b-PIBA-bP(BA-co-VI) TBCPEs were synthesized by successive reversible addition−fragmentation chain transfer (RAFT) polymerization, employing 1,4-phenylenebis(methylene)bis(2-hydroxyethyl) dicarbonotrithioate (PMHD) as a bifunctional chain transfer agent (CTA). The metal−ligand coordination was subsequently introduced into TBCPEs, which is schematically illustrated in Scheme 1. Such structural design brings about two types of network into the system, with one physical network formed by microphase separation, the self-assembled PIBA hard microdomains serving as the physical cross-linking points, and with another network formed by the metal−ligand coordination, the metal−ligand clusters as the cross-linking points. The metal−ligand coordination can obviously enhance the macroscopic mechanical performance of STBCPEs. Noticeably, this architectural design exhibits high efficiency in improving Young’s modulus, tensile strength, and extensibility for STBCPEs simply by incorporating a functional comonomer with minor mass contents during the polymerization of the BAB-type TBCPEs and metal−ligand coordination. The correlation of bulk mechanical properties to

provided on the basis of microphase separation of triblock copolymer elastomers (TBCPEs). However, the practical applications of TBCPEs are greatly hampered due to their low Young’s modulus and tensile strength.17−19 To cover this shortcoming, an incorporation of various nanofillers has been practically applied for the enhancement of mechanical performance of TBCPEs20,21 but along with several crucial defects, such as high filler loading and difficulty in processing and regulations of filler dispersion or the polymer−filler interface, which restricts the use of nanofillers.22 On the other hand, chemically cross-linked networks can be implemented into TBCPEs to improve the mechanical performance.23,24 For example, an ionic cross-linked network was recently incorporated into TBCPEs as an additional network to enhance the mechanical property.24 The drawback was that once the ionic cross-linked network in TBCPEs was broken, the covalent linkages were not able to reform, leading to reduced extensibility. The ionic cross-linked TBCPEs showed poor reprocessability and extensibility, apparently, because the covalent linkages experienced irreversible rupture. Overall, it remains challenging for designing desirable architectures of TBCPEs for approaching an engineering trade-off among the tensile strength, toughness, and extensibility. Fortunately, the natural materials, such as spider silks, bones, and mussel byssus inspired people to consider the usage of the function of metal−ligand coordination in TBCPEs, which help fabricate supramolecular thermoplastic elastomers with improved strength and extensibility.25,26 A double network mode was proposed to make strong and tough B


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ACS Applied Polymer Materials

Table 1. PIBA and PVI Contents, Molecular Masses, PDI, and Glass Transition Temperature for the BAB-Type STBCPEs sample code STBCPE10.5 STBCPE20.5 STBCPE30.5

PIBA contenta (wt %)

PVI contenta (wt %)

number density of imidazole groupa,b,c

PIBA block Mnb (kg/mol)

total Mnb (kg/mol)

PDIb(Mw/ Mn)

Tg,P(BA-coVI)d (°C)

Tg, PIBAd (°C)

21.9

2.2

19.0

14.4

65.7

1.4

−37.7

103.5

25.1

2.3

20.7

14.4

57.4

1.4

−35.8

104.3

27.8

2.1

19.6

14.4

51.7

1.3

−33.1

104.5

a

Determined by 1H NMR spectroscopy. bDetermined by GPC. cPer 1000 carbons in the end B blocks. dDetermined by DSC. transmission electron microscope (TEM) at an acceleration voltage of 80 kV. A thermal annealing under vacuum at 150 °C for 48 h was applied to each sample prior to SAXS measurement, TEM observation, and the mechanical property test. The annealing process allowed for sufficient microphase separation.

molecular structures of STBCPEs exhibits the obvious contribution of metal−ligand coordination, paving a road toward de novo design of reprocessable supramolecular elastomers with applicable properties.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION Preparation of Metal−Ligand Cross-Linked STBCPEs. Successive RAFT polymerization was applied to synthesize the BAB-type triblock copolymers behaving as polymeric elastomers (TBCPEs). PMHD was used as the bifunctional chain transfer agent for synthesis of telechelic PIBA. PIBA had the number-average molecular mass of 14.4 kg/mol and polydispersity index (PDI) of 1.1. PIBA with the high glass transition temperature serves as the A block, the hard middle block. On the other hand, P(BA-co-VI) with the much lower glass transition temperature serves as the end B blocks, the soft end blocks. The above-mentioned telechelic PIBA block was used as the macromolecular chain transfer agent to synthesize the three P(BA-co-VI)-b-PIBA-b-P(BA-co-VI) TBCPEs via RAFT polymerization. Figure S1 shows the GPC traces for the synthesized three samples with different total molecular masses. To endow the elastomeric property for the BAB-type TBCPEs, a mass range from 21.9 to 27.8 wt % was controlled for the PIBA content.30 Scheme 1 illustrates that the hard PIBA microdomains appear to be isolated in the soft matrix; however, they are connected through entanglements of the soft P(BA-co-VI) chains.30 In other words, the soft outer B blocks eventually serve as a “bridge” between neighboring hard A microdomains. Therefore, the BAB-type TBCPEs also behave as thermoplastic elastomers, similar to the ABA-type ones. It is well-known that the vinyl and imidazole groups of VI are not conjugated, the propagating vinyl radicals become highly reactive and unstable during polymerization.31,32 Therefore, the low contents of VI were designed to be just around 2.2 wt % for the synthesis of TBCPEs. A pair of free electrons on the nitrogen atom in imidazole occupies one sp2 hybrid orbital without participating in electron delocalization among the aromatic ring, and the imidazole group becomes weakly alkaline, and an electrophilic attack is possible for the formation of the bond interactions. Thus, in the coordination chemistry, the selective ligands can be imidazole or its derivatives.33−35 To introduce a metal−ligand coordinate bonding moiety into TBCPEs, the VI monomer was adopted to copolymerize in very minor mass percentages with n-butyl acrylate. Imidazole groups served as the ligands to form metal− ligand coordinate bonds by simply mixing with metal salts.13,36,37 ZnCl2 was selected to participate in the above synthesized TBCPEs for fabricating cross-linked networks by constructing metal−ligand coordinate bonds between zinc ions and imidazole groups in the soft end B blocks (Scheme 1). In fact, a tetrahedral geometry is favored for the zinc ion to

Materials. 1-Vinylimidazole (VI), n-butyl acrylate (BA), and isobornyl acrylate (IBA) were purchased from TCI. Zinc chloride (ZnCl2), 2,2′-azobis(2-methylpropionitrile) (AIBN), and other reagents were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). The monomers were passed through basic alumina in a short column to remove the inhibitors. AIBN was purified by recrystallization in ethanol before use. The bifunctional chain transfer agent, PMHD, was synthesized as reported.29 All other reagents were used as received. The P(BA-co-VI)-b-PIBA-b-P(BA-coVI) TBCPEs, TBCPE1−3 series, were synthesized by our reported method, and the details can be found in our recent publication.30 Preparation of STBCPEs with ZnCl2. A typical preparation procedure for STBCPEs can be described as follows. TBCPE1 (2.0 g) was dissolved in chloroform (20 mL), and then a desired amount of ZnCl2 was introduced into the above solution. The molar ratio of ZnCl2 to the imidazole groups in TBCPE1 was controlled at 0.5 to obtain STBCPE1-0.5. The mixture was stirred for 8 h and then poured into a Teflon mold. Chloroform was evaporated at room temperature to obtain the film sample. The film sample was further dried under vacuum at 40 °C for 24 h. STBCPE2-0.5 and STBCPE30.5 films were prepared by the same procedure. Characterizations. The chemical structures and molecular masses were determined by nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC) measurements.30 UV−vis spectra were acquired by using an UV-3600 UV−vis−NIR spectrophotometer (Shimadzu Company). Titration was performed by the addition of ZnCl2 methanol solution with a concentration of 0.1 mg/mL into TBCPE1 chloroform solution with a concentration of 1 mg/mL in a quartz cuvette by a step of 50 μL of ZnCl2 methanol solution per titration. The glass transition temperatures (Tg) and viscoelastic properties of these samples were measured by using a differential scanning calorimeter (TA-Q2000 DSC) and stress-controlled rheometer (TA-AR2000EX) with 8 mm parallel plates, respectively. Oscillatory frequency sweeps from 500 to 0.01 rad/s were carried out at a fixed strain amplitude of 1%, which was in the linear viscoelastic regime. Shear creep and creep recovery measurements were carried out by monitoring the changes in strain of the samples over a period of 40 min at a fixed shear stress of 100 Pa and another certain period immediately after the shear stress was removed. Tensile property measurements were performed on a Suns UTM2502 universal testing machine at a constant extension rate of 10 mm/min at room temperature. The detailed testing procedure can be found in our previous reports.21,24 Small-angle X-ray scattering (SAXS) measurements were carried out to acquire the structural information at the nanometer scale using a modified Xeuss system (Xenocs SA, Sassenage, France). The wavelength of the incident X-ray and the sample-to-detector distance were 0.154 nm and 2500 mm, respectively. The collection time for each SAXS pattern was 30 min. The ultrathin sections cut by a Leica EM FC6 ultramicrotome were stained by 1.0 wt % ruthenium tetraoxide (RuO4) solution for 4 h at room temperature before observation with a Hitachi HT7700 C


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Determination of Molar Ratio of ZnCl 2 /VI by Rheology. Since the rheological properties of STBCPEs are closely related to the number of metal−ligand coordinate bonds,42,43 the oscillatory rheology was taken to probe the dynamic viscoelastic properties of STBCPE1. Figure 2a shows the changes of the storage modulus, G′, and loss modulus, G″, versus frequency measured at 25 °C for STBCPE1 with selected molar ratios of ZnCl2/VI. The measurements were performed in the linear viscoelasticity regime. It can be seen, that, overall, the moduli increase with an increasing molar ratio of ZnCl2/VI up to 0.4, resulting from the contribution of the formed metal−ligand coordinate bonds.44 When the molar ratio of ZnCl2/VI reaches 0.4 and above, the storage modulus curves show bending; meanwhile, the loss modulus curves show well-defined minima in the high frequency range, with one of them pointed out by a purple arrow shown in Figure 2a, indicating the formation of a transient network.44,45 The complex viscosity also increases with an increasing molar ratio of ZnCl2/VI up to 0.5, and at a certain point, the contribution of the metal−ligand coordinate bonds reaches saturation, as shown in Figure S2. Figure 2b shows the van Gurp−Palmen plot for STBCPE1 with different molar ratios of ZnCl2/VI.46 At the low molar ratio of ZnCl2/VI below 0.3, one minimum around G* = 105 Pa for each curve can be found, possibly due to the slow relaxation of the backbone chain of TBCPE1.47,48 When the molar ratio of ZnCl2/VI is above 0.3, an extra minimum around G* = 7 × 105 Pa for each curve can be clearly seen, due to relatively fast relaxation of the newly formed transient network in STBCPE1.48 The olive curve for the molar ratio of ZnCl2/VI of 0.3 represents a transition between the above two cases. The changes of tan δ with frequency, as shown in Figure 2c,d, reveal similar results, as shown in Figure 2b, with a singular minimum in each curve for a molar ratio less than 0.3 and with double minima in each curve for a molar ratio higher than 0.3. Moreover, the olive curve at the molar ratio of 0.3 represents the transition between the two cases. The results in Figure 2 indicate that when the molar ratios of ZnCl2/VI are above 0.4, the extra relaxation due to the formation of a transient network by metal−ligand coordinate bonds approaches saturation, with the storage modulus, loss modulus, tan δ, and δ angle at the complex modulus of the extra minimum reaching steady values. Therefore, it can be readily confirmed that the molar ratio of ZnCl2/VI can be chosen at the value of 0.5 for fabricating STBCPEs with sufficient network formation by metal−ligand coordinate bonds. Order-to-Disorder Transition and Relaxation Spectra for STBCPEs. For the supramolecular triblock copolymers with metal−ligand coordinate bonds, temperature sweeps during cooling were traced with a constant frequency of 1 rad/s and a strain of 1%, falling in the linear viscoelastic regime.30 Figure 3 shows the temperature dependences of the storage modulus, G′, and loss modulus, G″, for STBCPEs during the cooling process. The G′ and G″ curves for TBCPEs are enclosed for the comparison purpose. For all three pairs, as compared with TBCPEs, notable increases are apparent for both storage and loss moduli of STBCPEs, which is apparently due to the additional cross-linking formed by metal−ligand coordinate bonds in P(BA-co-VI) domains.12,43 The temperatures remarked by orange arrows indicate the points of orderto-disorder transition (TODT). The TODT values are 182, 177, and 173 °C for TBCPE1, TBCPE2, and TBCPE3, respectively. The TODT values are 241, 235, and 233 °C for STBCPE1-0.5,

coordinate with two imidazole groups and two coordinating chloride anions.38−40 The characteristics of these STBCPEs are listed in Table 1. The number densities of the imidazole group are around 20.0 per 1000 carbons in the end B blocks. Determination of Molar Ratio of ZnCl2/VI through Titration. The molar ratio of ZnCl2/VI was determined through titration as traced by an UV−vis−NIR spectrophotometer. The formation of the metal−ligand coordinate bonds for TBCPE1 with the addition of ZnCl2 was confirmed by UV−vis spectra. The traced UV−vis absorption spectra for the TBCPE1 solution after stepwise addition of ZnCl2 are shown in Figure 1a. It can be seen, that the characteristic absorption

Figure 1. (a) UV−vis spectra of TBCPE1 chloroform solution titrated with ZnCl2 methanol solution. (b) Changes of absorption peak intensities at 275 and 282 nm as functions of molar ratio of ZnCl2/VI for TBCPE1 chloroform solution titrated with ZnCl2 methanol solution.

peaks appear at 275 and 282 nm, originating from the ligand (VI)-to-metal (Zn) charge-transfer (LMCT) absorptions.41 The distinct doublets at 275 and 282 nm might indicate the existence of geometrically nonequivalent Zn(II)-VI units. Note, that the broad absorption peak at 303 nm originates from the n-π* transition of imidazole. Figure 1b further shows that the absorption peak intensities at 275 and 282 nm increase about linearly with an increasing molar ratio of ZnCl2/VI prior to the value of 0.5 and approach plateaus beyond that. The molar ratio of ZnCl2/VI at 0.5 indicates the equivalence point12 and, therefore, was selected for the preparation of each STBCPE. D


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Figure 2. (a) Changes of storage modulus, G′, and loss modulus, G″, versus frequency and (b) van Gurp−Palmen plot for STBCPE1 with different molar ratios of ZnCl2/VI. Changes of loss tangent, tan δ, versus frequency with molar ratios of ZnCl2/VI at (c) 0, 0.1, and 0.2 and (d) 0.3, 0.4, 0.5, and 0.6. G′ and G″ curves in (a) are shifted horizontally for providing clear comparison and avoiding overlap.

°C, the relaxation spectra of TBCPEs are quite similar with slightly visible differences (Figure 4a), while the relaxation spectra of TBCPEs become isolated from each other with no overlap at the short relaxation time scales at 25 °C (Figure 4c). This result is apparently related to temperature, recalling that 150 °C is about 46 °C higher than Tg of the PIBA block. The increasing content of the PIBA block also brings about slower relaxation of the molecular chains at the measuring temperatures. At the high measurement temperature of 150 °C, the introduction of metal−ligand coordinate bonds leads to evident relaxation peaks for STBCPEs-0.5, as shown in Figure 4b, and the effect becomes more obvious with PIBA content increasing.51 At 25 °C, the relaxation peaks become more profound due to the introduction of metal−ligand coordinate bonds, and the separation of these peaks due to the different PIBA contents becomes much clearer, as shown in Figure 4d. Overall, the relaxation of the STBCPEs-0.5 series is obviously retarded due to the formation of metal−ligand coordinate bonds in the system. Shear Creep and Creep Recovery for STBCPEs. Shear creep and creep recovery measurements were performed on STBCPEs to disclose the effect of metal−ligand coordinate bonds on elastic deformation and viscous flow in the samples. Figure 5 shows the changes of strain as functions of time during shear creep and creep recovery processes for TBCPEs and STBCPEs-0.5 at 25 °C. The corresponding strain-time curves, as measured at 150 °C for these samples, are shown in Figure S4. A constant stress of 100 Pa was applied for the first period of 40 min up to the instant t1 (Figure 5b) during the creep stage, and then the stress was removed for creep recovery, remarked on in Figures 5a and S4a. Typically from the blue strain-time curve in Figure 5b, the following two parameters, the steady-state compliance, Je0, and Newtonian viscosity, η0, can be obtained; noting, that σ0 = 100 Pa in this measurement.53 It can be clearly seen from Figure 5 that the increase in strain is obviously suppressed when the metal−

STBCPE2-0.5, and STBCPE3-0.5, respectively, indicating obvious enhancements for TODT with incorporation of the metal−ligand coordinate bonds. On the other hand, the G′ and G″ curves for STBCPEs show a much more extended rubbery plateau in the high temperature range, about 60 °C wider than those for TBCPEs, also indicating enhancement of thermal stability upon addition of ZnCl2. The increase in TODT and extension in the rubbery plateau reflect the reduction of chain mobility due to metal−ligand coordinate bonds in the soft domains.49,50 The result in this section clearly demonstrates that the transient network formed by the metal−ligand coordinate bonds influences the viscoelastic property of the associated triblock copolymers at a much greater extent than by just increasing the chain lengths of the blocks.30,50 According to previous studies on transient networks, the distribution of the associating groups along the backbone chains controls the performance of the copolymer systems.43,44,50 In our study, the associating groups of VI among TBCPEs bonding with ZnCl2 in STBCPEs slow the copolymer chain dynamics and produce effective interchain interactions by metal−ligand coordinate bonds. The relaxation dynamics can be represented by the continuous relaxation spectrum, which discloses the molecular motion processes occurring at certain time scales and their dependences on the applied temperature.51 Besides, the molecular structures can be inferred from the relaxation spectrum according to the embedded molecular motion processes.52 Due to incompatibility between P(BA-co-VI) and PIBA blocks, the synthesized BAB-type triblock copolymers are expected to show complicated relaxation behaviors after introducing the metal−ligand coordinate bonds.51 Figure S3 shows the frequency dependences of storage and loss moduli for TBCPEs and the STBCPEs-0.5 series measured at 150 and 25 °C, respectively. Figure 4 shows the extracted continuous relaxation spectra from Figure S3 for TBCPEs and STBCPEs-0.5. At the high measurement temperature of 150 E


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increases the η0 values for STBCPEs as compared with TBCPEs. At 150 °C, the changes of Newtonian viscosity, η0, with increasing PIBA content are a little complicated (Figure 6d), in the sense that the Newtonian viscosity does not increase with increasing PIBA content, unlike that in Figure 6b, which might be explained as follows. At 150 °C, for TBCPEs, the Newtonian viscosity values might be affected more by the total molecular masses of TBCPEs (decreasing from TBCPE1 to TBCPE3 in Figure 6d) rather than their PIBA contents because PIBA blocks are now sitting above the Tg of PIBA (Table 1). For STBCPEs, the Newtonian viscosity values do not change obviously at 150 °C, since the viscosity values are significantly enhanced by the introduction of metal−ligand coordinate bonds into STBCPEs, which also removes the effect of the total molecular mass of TBCPEs. Overall, we can conclude, that the introduction of metal−ligand coordinate bonds into TBCPEs can obviously increase the Newtonian viscosity, η0, reducing the viscous flow, and decrease the steady-state compliance, Je0, reducing the elastic deformation. Thermal Properties and Microphase-Separated Morphology for STBCPEs. Due to incompatibility between PBA and PIBA blocks, microphase-separated structures are expected for TBCPEs, which brings about two well-separated Tg values.24,30 The introduction of metal−ligand coordinate bonds in STBCPEs might enhance the separation of the two Tg values. The Tg values of TBCPEs and STBCPEs were determined by using a differential scanning calorimeter (DSC). Figure S5 shows the heat flow curves and derivative heat flow curves for TBCPEs and STBCPEs-0.5. A heating rate of 10 °C/min was applied. Tg values can be well determined from the temperature derivative heat flow curves. Two separate Tg values are found for TBCPEs and STBCPEs, with low Tg corresponding to the glass transition for P(BA-co-VI) soft matrix and high Tg to that for the PIBA hard microdomains, confirming the existence of the microphase-separated structures.30 Figure S5a shows that TBCPE1 has two Tg’s at −39.3 and 103.4 °C, respectively, while STBCPE1−0.5 has two Tg’s at −37.7 and 103.5 °C, respectively. The Tg value for the P(BA-co-VI) soft matrix shows a certain increase in STBCPE1-0.5, indicating that the chain mobility of P(BA-coVI) is more or less restricted by the incorporated metal−ligand coordinate bonds, which is in accordance with the rheological result. Furthermore, the Tg values for the P(BA-co-VI) soft matrix increase from −37.7 °C for TBCPE2 to −35.8 °C for STBCPE2-0.5 and from −36.8 °C for TBCPE3 to −33.1 °C for STBCPE3-0.5. The Tg values of the PIBA hard microdomains are just slightly increased (less than 1 °C) by the introduction of metal−ligand coordinate bonds. The two separate Tg values ensure the necessary elastomeric properties for STBCPEs. The polar metal−ligand coordinate bonds, as illustrated in Scheme 1, might restrict the potential microphase separation of STBCPEs-0.5, even though the amounts of the metal−ligand coordinate bonds are just minor. In order to get insight into the effect of metal−ligand coordinate bonds on the resulted microphase-separated morphology, a small-angle X-ray scattering (SAXS) technique was applied to measure the long period of microdomains and the density contrast of phase domains for TBCPEs and STBCPEs-0.5.30 Figure 7 shows the SAXS intensity profiles (I(q) versus q) for TBCPEs and STBCPEs0.5. It can be seen, that all the samples show the first-order scattering peak, which indicates the existence of microphaseseparated structures. The SAXS peak intensities for TBCPEs

Figure 3. Temperature dependences of storage modulus, G′, and loss modulus, G″, for (a) TBCPE1 and STBCPE1-0.5, (b) TBCPE2 and STBCPE2-0.5, and (c) TBCPE3 and STBCPE3-0.5 during the cooling process with a cooling rate of 1 °C/min.

ligand coordinate bonds are introduced in each one of the elastomers, and the creep recovery seems to be suppressed also when the metal−ligand coordinate bonds play a role. Figure 6 shows the changes of steady-state compliance, Je0, and Newtonian viscosity, η0, with increasing PIBA content for TBCPEs and TBCPEs-0.5. At the two measurement temperatures, steady-state compliance, Je0, decreases with increasing PIBA content for both elastomer series (Figure 6a,c), indicating that PIBA blocks serving as hard domains in the elastomeric systems function as physical cross-linking points, which reduce the elastic deformation. On the other hand, the introduction of metal−ligand coordinate bonds into the elastomers (STBCPEs) significantly reduces the Je0 values. At 25 °C, the Newtonian viscosity, η0, increases with increasing PIBA content for both elastomer series (Figure 6b), demonstrating that the PIBA physical cross-linked network reduces the viscous flow, and the additional cross-linked network formed by metal−ligand coordinate bonds further F


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Figure 4. Continuous relaxation spectra for TBCPEs at (a) 150 and (c) 25 °C and STBCPEs at (b) 150 and (d) 25 °C.

micrograph.54,55 For STBCPE1-0.5, some dark dots can be found, which are relatively homogeneously dispersed throughout the sample. The dark dots are considered to be the metal− ligand clusters located in-between the PIBA microdomains. The PIBA microdomains exhibit a relatively bright color. This result is reasonable if considering that the metal−ligand coordinate bonds are basically staying together with the P(BAco-VI) matrix. The metal−ligand clusters can act as extra crosslinkers in STBCPEs-0.5, which are anticipated to lead to distinct mechanical performance for STBCPEs. This will be presented in the next section. A number of systems containing transient network cross-links were studied previously, including some block copolymers containing terpyridine groups at the chain ends.56−59 For this type of copolymer materials, their performance as thermoplastic elastomers is attributed to the presence of aggregates of metal−ligand clusters. Improvements of Mechanical Properties for STBCPEs. Monotonic and step-cyclic tensile tests were carried out to obtain the Young’s modulus, ultimate tensile strength, elongation at break, and elastic recovery for STBCPEs0.5.60−62 The typical tensile deformation curves for TBCPEs and STBCPEs-0.5 are shown in Figure 8a. It can be found, that TBCPEs and STBCPEs-0.5 show typical elastomeric behavior, and the observed mechanical performance is related to the total molecular mass and the incorporation of the metal− ligand coordinate bonds. In order to clarify the relationships among the mechanical property, molecular mass, and metal− ligand coordinate bond, the changes of the ultimate tensile strength and elongation at break versus total molecular mass are shown in Figure 8b,c, respectively. The ultimate tensile strength decreases and elongation at break increases with increasing molecular mass for TBCPEs and STBCPEs-0.5. Figure S7 shows the variations of the Young’s modulus, ultimate tensile strength, and elongation at break versus PIBA content for TBCPEs and STBCPEs-0.5. The values of Young’s modulus and ultimate tensile strength demonstrated a change from a soft elastomer (TBCPEs) to a relatively strong elastomer (STBCPEs-0.5) (Figures 8b and S7a,b), indicating

are much stronger than those for the corresponding STBCPEs0.5. The SAXS peak intensity increases, and peak width becomes narrower from TBCPE1 to TBCPE3, indicating gradual enhancement of the ordering of microphase separation, possibly due to gradual reduction of the total molecular mass; whereas, the SAXS peak intensity decreases from STBCPE10.5 to STBCPE3-0.5, indicating a gradual reduction of the ordering of microphase separation. In other words, gradual enhancement in the restriction of microphase separation is also possibly due to gradual reduction of the total molecular mass. It is noted, that there is an obvious intensity upturn at the lower q values for STBCPEs-0.5, apparently coming from the electron density contrast enhancement due to the introduction of the zinc ions.54 From the SAXS peak position, the interdomain distance, d, can be calculated by using d = 2π/ qmax. Figure S6 shows the changes of interdomain distance, d, with the total number-average molecular mass for TBCPEs and STBCPEs-0.5. The interdomain distance increases with increasing total molecular mass, consistent with the previous result.30 The introduction of metal−ligand coordinate bonds in the elastomers only slightly reduces the interdomain distance; however, it seriously disturbs the formation of microphase separation due to the formation of extra cross-links in the system. The microphase-separated morphologies for STBCPEs were further examined by using TEM. The typical microphaseseparated morphology for TBCPE1 and STBCPE1-0.5 can be clearly seen in Figure 7. However, the exact microphaseseparated structure cannot be determined from TEM observation. It cannot be determined by the SAXS profiles, as shown in Figure 7a, due to absence of the high-order SAXS peaks. Figure 7e shows that with introduction of metal−ligand coordinate bonds into the elastomer, the microphase-separated structure in STBCPE1-0.5 looks blurred as compared with that for TBCPE1, consistent with the SAXS result shown in Figure 7a. The metal−ligand coordinate bonds might form certain clusters in the sample. The higher mass density of metal− ligand clusters makes them be recognized in the TEM G


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elongation at break values increase to 1074% for STBCPE10.5, to 818% for STBCPE2-0.5, and to 717% for STBCPE30.5. This phenomenon is in agreement with the result reported by Craig et al. on the metal−ligand cross-linked gels.63 They also found that the tensile strength and extensibility could be significantly enhanced by the transient metal−ligand crosslinks. In general, an improvement of tensile strength conflicts with the desire for improvement of extensibility because a flexible network usually can sustain large deformation but results in weak tensile strength.64 On the contrary, a rigid or restricted network brings about an enhanced tensile strength but fails with a poor extension.65 For example, the Young’s modulus and tensile strength could be improved notably in the filled composites; however, ductility was not improved due to restricted chain mobility imposed by added fillers. In this work, the metal−ligand coordinate bonds might form the aggregated clusters, which can act as reinforcing fillers in the copolymer matrix. Therefore, the elastomer becomes similar to the composite elastomer, leading to obviously improved tensile strength. On the other hand, the metal−ligand coordinate bonds can be broken or dissociated by elongation; therefore, the soft end B blocks can be extended with a reduced degree of freedom for chain conformation, and then the reestablished metal−ligand coordinate bonds can allow for higher extensibility for the elastomers.13,27,28,36 The above two aspects infer that the introduction of metal−ligand coordinate bonds into TBCPEs may provide an opportunity to balance the mechanical performance among the Young’s modulus, tensile strength, and extensibility. To evaluate the elastic recovery performance, step-cyclic tensile tests were performed for TBCPEs and STBCPEs-0.5. The nominal stress−strain curves during cyclic tensile deformation were recorded with maximum strain values increasing sequentially to 50, 100, 150, 200, and so on up to 400%. Figure 8d,e shows the step-cyclic tensile deformation curves for TBCPE1 and STBCPE1-0.5, and Figure S8 shows the step-cyclic tensile deformation curves for TBCPE2, STBCPE2-0.5, TBCPE3, and STBCPE3-0.5. As compared to TBCPEs, a larger hysteresis is observed for the loading− unloading cycle for STBCPEs-0.5, indicating the presence of a large amount of energy dissipation through rupture of the metal−ligand coordinate bonds. A similar phenomenon has been observed for other polymers containing the metal−ligand coordinate bonds, which is attributed to the above-mentioned energy dissipation on the basis of plastic deformation, which cannot relax to the equilibrium on the time scale of the individual cycle.12,36 The elastic recovery values can be obtained from the nominal stress−strain curves as ER% = 100% × (εmax − ε(0,εmax))/εmax, where εmax and ε(0,εmax) are the maximum strain and the strain at zero stress in one cycle, respectively. Figure 8f shows the changes of elastic recovery with maximum strain in each cycle for TBCPEs and STBCPEs-0.5 during stepcyclic tensile deformation. The ER values increase with the maximum strain in each cycle and reach plateaus at high maximum strains, demonstrating the typical elastomeric behavior. Note, that the dash line indicates the ER value of 90% in the plot. The ER values for STBCPEs-0.5 are lower than those for TBCPEs, indicating the existence of more energy dissipation during step-cyclic tensile deformation for the former ones than for the latter ones.

Figure 5. Changes of strain as functions of time during shear creep and creep recovery processes for (a) TBCPE1 and STBCPE1-0.5, (b) TBCPE2 and STBCPE2-0.5, and (c) TBCPE3 and STBCPE3-0.5. Constant stress of 100 Pa was applied for the first period of 40 min, and then the stress was removed. The measuring temperature was 25 °C.

an efficient enhancement on mechanical properties by introduction of metal−ligand coordinate bonds.13,27,28,36 Interestingly, the ultimate tensile strength and elongation at break for STBCPEs-0.5 simultaneously increase up to above 3.2 and 1.3 times those for corresponding TBCPEs (Figure S7b,c). The ultimate tensile strength values are 1.47, 1.50, and 1.89 MPa, respectively, while elongation at break values are 778, 639, and 434%, respectively, for TBCPE1, TBCPE2, and TBCPE3. While as compared with TBCPEs, STBCPEs-0.5 samples show superior tensile strength and elongation at break due to the contribution of metal−ligand coordinate bonds. The evidence is that the ultimate tensile strength values increase to 4.71 MPa for STBCPE1-0.5, to 5.29 MPa for STBCPE2-0.5, and to 6.21 MPa for STBCPE3-0.5, while the H


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Figure 6. Changes of steady-state compliance, Je0, and Newtonian viscosity, η0, as functions of PIBA content measured at 25 (a, b) and 150 °C (c, d) for TBCPEs and STBCPEs-0.5. The open symbols represent the data points for TBCPEs, and the filled symbols represent the data points for STBCPEs-0.5.

Figure 7. SAXS profiles (I(q) vs q curves) for (a) TBCPE1 and STBCPE1-0.5, (b) TBCPE2 and STBCPE2-0.5, and (c) TBCPE3 and STBCPE30.5, and typical TEM micrographs for (d) TBCPE1 and (e) STBCPE1-0.5.

meric matrix, while restricting the polymer chain elongation.21,61,62,64−66 The additional chemically cross-linked network also brings about the reduced extensibility.23,24 In this work, a minor amount of VI commoner was copolymerized in the soft end B blocks of the BAB-type TBCPEs, with VI groups serving as ligands for the formation of metal−ligand coordinate bonds when ZnCl2 was added in to obtain STBCPEs-0.5. In

It is quite challenging, indeed, to simultaneously combine the tensile strength and extensibility into TBCPEs. The contradictory is represented in that the introduction of nanoparticles through either compounding or grafting leads to enhancement of the mechanical tensile strength but also leads to the reduced extensibility, because the necessary strong particle−polymer interfacial interaction rigidifies the elastoI


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Figure 8. Tensile deformation curves (a), ultimate tensile strength (b), and elongation at break (c) versus Mn for TBCPEs and STBCPEs-0.5. Stepcyclic tensile deformation curves for TBCPE1 (d) and STBCPE1-0.5 (e). (f) Elastic recovery versus maximum strain of each cycle for TBCPEs and STBCPEs-0.5.

minor contents; thus, the STBCPEs can be reprocessed easily by either solvent dissolution or heat fusion. Moreover, the mechanical properties of the reprocessed products still remain about the same.

STBCPEs-0.5, the microphase-separated domains and the metal−ligand cross-linked clusters in the soft B end block matrix intertwined, behaving as a double-network system.27,28 For such a system, the role of sacrificial units was taken by the metal−ligand coordinate bonds. The energy could be dissipated substantially through the preferential detaching/ rupturing of the metal−ligand coordinate bonds.12,36 During the tensile deformation process, the metal−ligand coordinate bonds were disabled first due to their dissociation energy being relatively lower than that for the covalent bonds, and by such a way, the stretched chains could be relaxed, and mechanical energy could be dissipated on the basis of the reversible crosslinking mechanism. In STBCPEs-0.5, the architectural design was that the sacrificial units were introduced into the soft B end block matrix, and such obtained STBCPEs-0.5 exhibit dramatic increases in both tensile strength and elongation at break without losing any extensibility. Our results also demonstrate that, by introducing the metal−ligand coordinate bonds, the mechanical properties of STBCPEs can be improved and adjusted according to certain application purposes. Therefore, the strategy applied in this work confirms the natural manner to be borrowed for design of functional materials through benefits of sacrificial metal−ligand coordinate bonds and double network mechanisms. It is further noted, that the term “reprocessable” implies that the metal− ligand interactions are based on the noncovalent bonds with

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CONCLUSIONS In this study, the metal−ligand interaction was intended to be incorporate into the BAB-type triblock copolymers to produce reprocessable supramacromolecular elastomers (STBCPEs), in which the metal−ligand coordinate bonds formed the additional noncovalent network, serving to enhance the mechanical performance in several aspects, the improvements in the Young’s modulus, tensile strength, toughness, and extensibility. A low mass percentage (2.2 wt %) of VI comonomer was copolymerized into the soft B end blocks of the BAB-type triblock copolymers in the initial design by RAFT copolymerization. Subsequently, the metal−ligand coordination bonding of VI rings in the B blocks with ZnCl2 was accomplished in solutions to produce STBCPEs. Titration as traced by UV−vis-NIR spectrophotometer and rheological measurements disclosed an appropriate molar ratio of ZnCl2/ VI at 0.5. SAXS and TEM measurements revealed the microphase-separated morphology of the triblock copolymers associated with the metal−ligand clusters in STBCPEs-0.5. The dynamic nature of metal−ligand coordinate bonds served as the sacrificial noncovelent units, which dissipated energy J


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Experimental Behavior with Transient Network Theory. J. Rheol. 1993, 37, 695−726. (9) Holten-Andersen, N.; Harrington, M. J.; Birkedal, H.; Lee, B. P.; Messersmith, P. B.; Lee, K. Y. C.; Waite, J. H. pH-Induced MetalLigand Cross-Links Inspired by Mussel Yield Self-Healing Polymer Networks with Near-Covalent Elastic Moduli. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 2651−2655. (10) Wei, P. F.; Yan, X. Z.; Huang, F. H. Supramolecular Polymers Constructed by Orthogonal Self-assembly Based on Host-Guest and Metal-Ligand Interactions. Chem. Soc. Rev. 2015, 44, 815−832. (11) He, C. B.; Liu, D. M.; Lin, W. B. Nanomedicine Applications of Hybrid Nanomaterials Built from Metal-Ligand Coordination Bonds: Nanoscale Metal-Organic Frameworks and Nanoscale Coordination Polymers. Chem. Rev. 2015, 115, 11079−11108. (12) Tang, Z. H.; Huang, J.; Guo, B. C.; Zhang, L. Q.; Liu, F. Bioinspired Engineering of Sacrificial Metal-Ligand Bonds into Elastomers with Supramechanical Performance and Adaptive Recovery. Macromolecules 2016, 49, 1781−1789. (13) Mozhdehi, D.; Ayala, S.; Cromwell, O. R.; Guan, Z. B. SelfHealing Multiphase Polymers via Dynamic Metal-Ligand Interactions. J. Am. Chem. Soc. 2014, 136, 16128−16131. (14) Huang, Y. F.; Chang, R. X.; Han, L. L.; Shan, G. R.; Bao, Y. Z.; Pan, P. J. ABA-Type Thermoplastic Elastomers Composed of Poly(εcaprolactone-co-δ-valerolactone) Soft Midblock and Polymorphic Poly(lactic acid) Hard End Blocks. ACS Sustainable Chem. Eng. 2016, 4, 121−128. (15) Nagapudi, K.; Brinkman, W. T.; Leisen, J.; Thomas, B. S.; Wright, E. R.; Haller, C.; Wu, X. Y.; Apkarian, R. P.; Conticello, V. P.; Chaikof, E. L. Protein-Based Thermoplastic Elastomers. Macromolecules 2005, 38, 345−354. (16) Wanamaker, C. L.; O’Leary, L. E.; Lynd, N. A.; Hillmyer, M. A.; Tolman, W. B. Renewable-Resource Thermoplastic Elastomers Based on Polylactide and Polymenthide. Biomacromolecules 2007, 8, 3634− 3640. (17) Guo, B. C.; Tang, Z. H.; Zhang, L. Q. Transport Performance in Novel Elastomer Nanocomposites: Mechanism, Design and Control. Prog. Polym. Sci. 2016, 61, 29−66. (18) Jochum, F. D.; Theato, P. Temperature-and Light-Responsive Smart Polymer Materials. Chem. Soc. Rev. 2013, 42, 7468−7483. (19) Chan, B. Q. Y.; Low, Z. W. K.; Heng, S. J. W.; Chan, S. Y.; Owh, C.; Loh, X. J. Recent Advances in Shape Memory Soft Materials for Biomedical Applications. ACS Appl. Mater. Interfaces 2016, 8, 10070−10087. (20) Helal, E.; Demarquette, N. R.; Amurin, L.; David, E.; Carastan, D.; Fréchette, M. Styrenic Block Copolymer-Based Nanocomposites: Implications of Nanostructuration and Nanofiller Tailored Dispersion on the Dielectric Properties. Polymer 2015, 64, 139−152. (21) Fang, C.; Zhang, Y. Q.; Wang, W. T.; Wang, Z. K.; Jiang, F.; Wang, Z. G. Fabrication of Copolymer-Grafted Multiwalled Carbon Nanotube Composite Thermoplastic Elastomers Filled with Unmodified MWCNTs as Additional Nanofillers to Significantly Improve Both Electrical Conductivity and Mechanical Properties. Ind. Eng. Chem. Res. 2015, 54, 12597−12606. (22) Creton, C. 50th Anniversary Perspective: Networks and Gels: Soft but Dynamic and Tough. Macromolecules 2017, 50, 8297−8316. (23) Voorhaar, L.; Hoogenboom, R. Supramolecular Polymer Networks: Hydrogels and Bulk Materials. Chem. Soc. Rev. 2016, 45, 4013−4031. (24) Jiang, F.; Fang, C.; Zhang, J.; Wang, W. T.; Wang, Z. G. Triblock Copolymer Elastomers with Enhanced Mechanical Properties Synthesized by RAFT Polymerization and Subsequent Quaternization Through Incorporation of a Comonomer with Imidazole Groups of about 2.0 Mass Percentage. Macromolecules 2017, 50, 6218−6226. (25) Schmitt, C. N.; Politi, Y.; Reinecke, A.; Harrington, M. J. Role of Sacrificial Protein-Metal Bond Exchange in Mussel Byssal Thread Self-healing. Biomacromolecules 2015, 16, 2852−2861.

when being deformed and ruptured, and then they recovered through reformation of the bonds when being relaxed. In such an elegant way, the STBCPEs-0.5 showed obviously improved extensibility, creep resistance, and toughness besides the improved Young’s modulus and tensile strength. It was obvious that stretchable synthetic polymeric elastomers could borrow such an architectural design for pursuing simultaneously enhanced tensile strength, elongation at break, and toughness without losing any extensibility.

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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/acsapm.8b00277. The GPC traces, additional rheological data, DSC heat flow curves, changes of interdomain distance with total number-average molecular mass, the relationship between mechanical properties and PIBA content, and additional step-cyclic stress−strain curves (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 0551-63607703; Fax: +86 0551-63607703; E-mail: [email protected]. *E-mail: [email protected]. ORCID

Feng Jiang: 0000-0001-5992-0520 Zhigang Wang: 0000-0002-6090-3274 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Natural Science Foundation of China (Grant 51473155 and 51603199) is greatly appreciated for the funding support. We also appreciate the Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences.

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REFERENCES

(1) Aida, T.; Meijer, E.; Stupp, S. Functional Supramolecular Polymers. Science 2012, 335, 813−817. (2) Beck, J. B.; Rowan, S. J. Multistimuli, Multiresponsive MetalloSupramolecular Polymers. J. Am. Chem. Soc. 2003, 125, 13922− 13923. (3) Burnworth, M.; Tang, L. M.; Kumpfer, J. R.; Duncan, A. J.; Beyer, F. L.; Fiore, G. L.; Rowan, S. J.; Weder, C. Optically Healable Supramolecular Polymers. Nature 2011, 472, 334. (4) Zhang, Z. B.; Luo, Y.; Chen, J. Z.; Dong, S. Y.; Yu, Y. H.; Ma, Z.; Huang, F. H. Formation of Linear Supramolecular Polymers That Is Driven by C-H···π Interactions in Solution and in the Solid State. Angew. Chem. 2011, 123, 1433−1437. (5) Folmer, B. J.; Sijbesma, R.; Versteegen, R.; Van der Rijt, J.; Meijer, E. Supramolecular Polymer Materials: Chain Extension of Telechelic Polymers Using a Reactive Hydrogen-Bonding Synthon. Adv. Mater. 2000, 12, 874−878. (6) Lehn, J. M. Dynamers: Dynamic Molecular and Supramolecular Polymers. Prog. Polym. Sci. 2005, 30, 814−831. (7) Tanaka, F.; Edwards, S. Viscoelastic Properties of Physically Crosslinked Networks. Transient Network Theory. Macromolecules 1992, 25, 1516−1523. (8) Annable, T.; Buscall, R.; Ettelaie, R.; Whittlestone, D. The Rheology of Solutions of Associating Polymers: Comparison of K


Article


(45) Tang, M.; Wang, W. T.; Xu, D. H.; Wang, Z. G. Synthesis of Structure-Controlled Polyborosiloxanes and Investigation on Their Viscoelastic Response to Molecular Mass of Polydimethylsiloxane Triggered by Both Chemical and Physical Interactions. Ind. Eng. Chem. Res. 2016, 55, 12582−12589. (46) Van Gurp, M.; Palmen, J. Time-Temperature Superposition for Polymeric Blends. Rheol. Bull. 1998, 67, 5−8. (47) Trinkle, S.; Friedrich, C. Van Gurp-Palmen-Plot: A Way to Characterize Polydispersity of Linear Polymers. Rheol. Acta 2001, 40, 322−328. (48) Dalsin, S. J.; Hillmyer, M. A.; Bates, F. S. Linear Rheology of Polyolefin-Based Bottlebrush Polymers. Macromolecules 2015, 48, 4680−4691. (49) Liu, X.; Zhao, R. Y.; Zhao, T. P.; Liu, C. Y.; Yang, S.; Chen, E. Q. An ABA Triblock Containing a Central Soft Block of Poly[2,5di(n-hexogycarbonyl) styrene] and Outer Hard Block of poly(4vinylpyridine): Synthesis, Phase Behavior and Mechanical Enhancement. RSC Adv. 2014, 4, 18431−18441. (50) Osterwinter, C.; Schubert, C.; Tonhauser, C.; Wilms, D.; Frey, H.; Friedrich, C. Rheological Consequences of Hydrogen Bonding: Linear Viscoelastic Response of Linear Polyglycerol and Its Permethylated Analogues as a General Model for Hydroxyl-Functional Polymers. Macromolecules 2015, 48, 119−130. (51) Grindy, S. C.; Learsch, R.; Mozhdehi, D.; Cheng, J.; Barrett, D. G.; Guan, Z. B.; Messersmith, P. B.; Holten-Andersen, N. Control of Hierarchical Polymer Mechanics with Bioinspired Metal-Coordination Dynamics. Nat. Mater. 2015, 14, 1210. (52) Honerkamp, J.; Weese, J. Determination of the Relaxation Spectrum by a Regularization Method. Macromolecules 1989, 22, 4372−4377. (53) Ferry, J. D.; Ferry, J. D. Viscoelastic Properties of Polymers. John Wiley & Sons, 1980. (54) Lee, S. W.; Kumpfer, J. R.; Lin, P. A.; Li, G. D.; Gao, X. P.; Rowan, S. J.; Sankaran, R. M. In Situ Formation of Metal Nanoparticle Composites via ″Soft″ Plasma Electrochemical Reduction of Metallosupramolecular Polymer Films. Macromolecules 2012, 45, 8201− 8210. (55) Song, B.; Wu, G. L.; Wang, Z. Q.; Zhang, X.; Smet, M.; Dehaen, W. Metal- Ligand Coordination-Induced Self-Assembly of Bolaamphiphiles Bearing Bipyrimidine. Langmuir 2009, 25, 13306− 13310. (56) Shi, Y.; Wang, M.; Ma, C. B.; Wang, Y. Q.; Li, X. P.; Yu, G. H. A Conductive Self-Healing Hybrid Gel Enabled by Metal-Ligand Supramolecule and Nanostructured Conductive Polymer. Nano Lett. 2015, 15, 6276−6281. (57) Li, H. X.; Wei, W.; Xiong, H. M. An Asymmetric A-B-A’ Metallo-Supramolecular Triblock Copolymer Linked by Ni2+-bisTerpyridine Complexes at One Junction†. Soft Matter 2016, 12, 1411−1418. (58) Yuan, J. C.; Fang, X. L.; Zhang, L. X.; Hong, G. N.; Lin, Y. J.; Zheng, Q. F.; Xu, Y. Z.; Ruan, Y. H.; Weng, W. G.; Xia, H. P.; Chen, G. Multi-Responsive Self-Healing Metallo-Supramolecular Gels Based on ″Click″ Ligand. J. Mater. Chem. 2012, 22, 11515−11522. (59) Bode, S.; Zedler, L.; Schacher, F. H.; Dietzek, B.; Schmitt, M.; Popp, J.; Hager, M. D.; Schubert, U. S. Self-Healing Polymer Coatings Based on Crosslinked Metallosupramolecular Copolymers. Adv. Mater. 2013, 25, 1634−1638. (60) Wang, Z. G.; Niu, Y. H.; Fredrickson, G. H.; Kramer, E. J.; Shin, Y. W.; Shimizu, F.; Zuo, F.; Rong, L. X.; Hsiao, B. S.; Coates, G. W. Step-Cycle Mechanical Processing of Gels of sPP-b-EPR-b-sPP Triblock Copolymer in Mineral Oil. Macromolecules 2010, 43, 6782− 6788. (61) Wang, Z. K.; Jiang, F.; Zhang, Y. Q.; You, Y. Z.; Wang, Z. G.; Guan, Z. B. Bioinspired Design of Nanostructured Elastomers with Cross-linked Soft Matrix Grafting on the Oriented Rigid Nanofibers to Mimic Mechanical Properties of Human Skin. ACS Nano 2015, 9, 271−278.

(26) Degtyar, E.; Harrington, M. J.; Politi, Y.; Fratzl, P. The Mechanical Role of Metal Ions in Biogenic Protein-Based Materials. Angew. Chem., Int. Ed. 2014, 53, 12026−12044. (27) Gong, J. P. Why are Double Network Hydrogels so Tough? Soft Matter 2010, 6, 2583−2590. (28) Zhang, H. J.; Sun, T. L.; Zhang, A. K.; Ikura, Y.; Nakajima, T.; Nonoyama, T.; Kurokawa, T.; Ito, O.; Ishitobi, H.; Gong, J. P. Tough Physical Double-Network Hydrogels Based on Amphiphilic Triblock Copolymers. Adv. Mater. 2016, 28, 4884−4890. (29) Cong, H. L.; Zheng, S. X. Poly(N-isopropylacrylamide)-blockpoly(acrylic acid) Hydrogels: Synthesis and Rapid Thermoresponsive Properties. Colloid Polym. Sci. 2014, 292, 2633−2645. (30) Wang, W. T.; Wang, X. H.; Jiang, F.; Wang, Z. G. Synthesis, Order-to-Disorder Transition, Microphase Morphology and Mechanical Properties of BAB Triblock Copolymer Elastomers with Hard Middle Block and Soft Outer Blocks. Polym. Chem. 2018, 9, 3067− 3079. (31) Ge, Z. S.; Xie, D.; Chen, D. Y.; Jiang, X. Z.; Zhang, Y. F.; Liu, H. W.; Liu, S. Y. Stimuli-Responsive Double Hydrophilic Block Copolymer Micelles with Switchable Catalytic Activity. Macromolecules 2007, 40, 3538−3546. (32) Allen, M. H., Jr; Hemp, S. T.; Smith, A. E.; Long, T. E. Controlled Radical Polymerization of 4-Vinylimidazole. Macromolecules 2012, 45, 3669−3676. (33) Cai, S. L.; He, Z. H.; Wu, W. H.; Liu, F. X.; Huang, X. L.; Zheng, S. R.; Fan, J.; Zhang, W. G. A Series of Alkaline Earth Metal Coordination Polymers Constructed from Two Newly Designed Imidazole-Based Dicarboxylate Ligands Containing Pyridinylmethyl Groups. CrystEngComm 2017, 19, 3003−3016. (34) Lu, W.; Le, X. X.; Zhang, J. W.; Huang, Y. J.; Chen, T. Supramolecular Shape Memory Hydrogels: a New Bridge Between Stimuli-Responsive Polymers and Supramolecular Chemistry. Chem. Soc. Rev. 2017, 46, 1284−1294. (35) Sanoja, G. E.; Schauser, N. S.; Bartels, J. M.; Evans, C. M.; Helgeson, M. E.; Seshadri, R.; Segalman, R. A. Ion Transport in Dynamic Polymer Networks Based on Metal-Ligand Coordination: Effect of Cross-Linker Concentration. Macromolecules 2018, 51, 2017−2026. (36) Neal, J. A.; Mozhdehi, D.; Guan, Z. B. Enhancing Mechanical Performance of a Covalent Self-Healing Material by Sacrificial Noncovalent Bonds. J. Am. Chem. Soc. 2015, 137, 4846−4850. (37) Chen, Y. L.; Guan, Z. B. Multivalent Hydrogen Bonding Block Copolymers Self-Assemble into Strong and Tough Self-Healing Materials. Chem. Commun. 2014, 50, 10868−10870. (38) Matthews, C. J.; Clegg, W.; Heath, S. L.; Martin, N. C.; Hill, M. S.; Lockhart, J. C. Coordination of Zn(II), Cd(II), Hg(II), and Ag(I) by Bis(benzimidazole) Ligands. Inorg. Chem. 1998, 37, 199−207. (39) Srivastava, A.; Holten-Andersen, N.; Stucky, G. D.; Waite, J. H. Ragworm Jaw-Inspired Metal Ion Cross-Linking for Improved Mechanical Properties of Polymer Blends. Biomacromolecules 2008, 9, 2873−2880. (40) Ismael, A.; Cristiano, M. L. S.; Fausto, R.; Gómez-Zavaglia, A. Tautomer Selective Photochemistry in 1-(tetrazol-5-yl) Ethanol. J. Phys. Chem. A 2010, 114, 13076−13085. (41) Fawcett, T. G.; Bernarducci, E. E.; Krogh-Jespersen, K.; Schugar, H. J. Charge-Transfer Absorptions of Copper(II)-Imidazole and Copper(II)-Imidazolate Chromophores. J. Am. Chem. Soc. 1980, 102, 2598−2604. (42) De Lucca Freitas, L.; Jacobi, M. M.; Gonçalves, G.; Stadler, R. Microphase Separation Induced by Hydrogen Bonding in a Poly(1,4butadiene)-block-Poly(1,4-isoprene) Diblock Copolymer an Example of Supramolecular Organization via Tandem Interactions. Macromolecules 1998, 31, 3379−3382. (43) Feldman, K. E.; Kade, M. J.; Meijer, E.; Hawker, C. J.; Kramer, E. J. Model Transient Networks from Strongly Hydrogen-Bonded Polymers. Macromolecules 2009, 42, 9072−9081. (44) Lewis, C. L.; Stewart, K.; Anthamatten, M. The Influence of Hydrogen Bonding Side-Groups on Viscoelastic Behavior of Linear and Network Polymers. Macromolecules 2014, 47, 729−740. L


Article

ACS Applied Polymer Materials (62) Jiang, F.; Wang, Z. K.; Qiao, Y. L.; Wang, Z. G.; Tang, C. B. A Novel Architecture Toward Third-Generation Thermoplastic Elastomers by a Grafting Strategy. Macromolecules 2013, 46, 4772−4780. (63) Kean, Z. S.; Hawk, J. L.; Lin, S. T.; Zhao, X. H.; Sijbesma, R. P.; Craig, S. L. Increasing the Maximum Achievable Strain of a Covalent Polymer Gel Through the Addition of Mechanically Invisible CrossLinks. Adv. Mater. 2014, 26, 6013−6018. (64) Wang, Z. K.; Yuan, L.; Jiang, F.; Zhang, Y. Q.; Wang, Z. G.; Tang, C. B. Bioinspired High Resilient Elastomers to Mimic Resilin. ACS Macro Lett. 2016, 5, 220−223. (65) Jiang, F.; Zhang, Y. Q.; Wang, Z. K.; Wang, W. T.; Xu, Z. H.; Wang, Z. G. Combination of Magnetic and Enhanced Mechanical Properties for Copolymer-Grafted Magnetite Composite Thermoplastic Elastomers. ACS Appl. Mater. Interfaces 2015, 7, 10563−10575. (66) Jiang, F.; Zhang, Y. Q.; Wang, Z. K.; Fang, H. G.; Ding, Y. S.; Xu, H. X.; Wang, Z. G. Synthesis and Characterization of Nanostructured Copolymer-Grafted Multiwalled Carbon Nanotube Composite Thermoplastic Elastomers Toward Unique Morphology and Strongly enhanced mechanical properties. Ind. Eng. Chem. Res. 2014, 53, 20154−20167.

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