Triblock Copolymer Elastomers with Enhanced Mechanical Properties

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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 Feng Jiang, Chu Fang, Juan Zhang, Wentao Wang, and Zhigang Wang* CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *

ABSTRACT: ABA triblock copolymer elastomers (TBCPEs) were first synthesized via reversible addition−fragmentation chain transfer (RAFT) polymerization, for which poly(isobornyl acrylate) (PIBA) was chosen as the dispersed hard microdomains, whereas poly(n-butyl acrylate)-co-poly(1vinylimidazole) (P(BA-co-VI)) was chosen as the rubbery matrix. Two much distinct glass transition temperatures were found, corresponding to the soft matrix and hard microdomains, respectively. Although the mass percentages of the incorporated third comonomer, 1-vinylimidazole, were just about 2.0%, the imidazole groups on the TBCPE chains could be ionically cross-linked by 1,6-dibromohexane to bring these TBCPEs into much strong ones, half-cross-linked TBCPEs-HC and full-cross-linked TBCPEs-FC. It is interesting to demonstrate that the ultimate tensile strength for TBCPEs-FC could be increased up to 10 times that for TBCPEs, and the elastic recovery could also be improved to above 90%, while the elongation at break just showed modest decreases. Transmission electron microscope (TEM) and small-angle X-ray scattering (SAXS) measurements disclosed that TBCPEs, TBCPEs-HC, and TBCPEs-FC all had typical microphase-separated morphology, with the interdomain distance tunable by the molecular mass of TBCPEs. In-situ SAXS measurements revealed that the hard microdomains in TBCPEs-HC could be orientated along the tensile direction during stretching and be recovered to the original state after stress release.

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INTRODUCTION As one of the most used materials in our daily life, elastomers can be reversibly extended up to several times their original length and have received tremendous attention due to their unique mechanical features and wide applications in automotive, footwear, adhesives, electronics, and tissue engineering.1−4 In general, elastomers have microphaseseparated structures, in which the hard microdomains with a high glass transition temperature (Tg) act as physical cross-links to provide sufficient tensile strength, while the rubbery domains with a low Tg serve as elastic strands to bridge the dispersed hard microdomains in the matrix.5,6 Reversible addition− fragmentation chain transfer (RAFT) polymerization is robust and convenient to produce different kinds of well-defined ABA triblock copolymers containing (meth)acrylate and styrene type monomers with a suitable bifunctional chain transfer agent (CTA).7 The macroscopic performance of elastomers is strongly related to the composition, microstructure, and network architecture.8 Applications of most unfilled elastomers are limited due to their poor mechanical strength. Therefore, incorporation of natural or synthetic nano-objects into © XXXX American Chemical Society

elastomeric matrixes is an effective strategy to enhance the mechanical properties of the resulted composites. Recently, our group found that the mechanical properties of elastomers could be dramatically improved by introducing semirigid cellulose chains or stiff nanofillers via grafting methods.9−13 Another strategy to construct elastomeric materials with superior mechanical properties is to introduce additional networks into the as-prepared triblock copolymers, in which the networks can be formed by noncovalent or covalent interactions. Noncovalent interactions, such as metal−ligand coordination and hydrogen bonding, have been applied to fabricate elastomeric materials with well-defined architecture and fascinating properties by Guan and co-workers.14−16 Covalent interactions formed by various cross-linkers also can significantly improve the mechanical performance of elastomers to meet certain application demands.17−19 Poly(n-butyl acrylate) is an excellent rubbery matrix with a low Tg and has been used to prepare various elastomeric Received: July 3, 2017 Revised: July 30, 2017

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DOI: 10.1021/acs.macromol.7b01414 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Illustration of Synthesis of (a) PIBA-b-P(BA-co-VI)-b-PIBA TBCPE and (b) Ionically Cross-Linked TBCPE

incorporated 1,6-dibromohexane in TBCPEs. We emphasize that the mass percentages of the incorporated third comonomer, 1-vinylimidazole, with imidazole groups in TBCPEs were just 2.0%.

materials, such as poly(methyl methacrylate)-b-poly(n-butyl acrylate)-b-poly(methyl methacrylate) (PMMA-b-PBA-bPMMA), polystyrene-b-poly(n-butyl acrylate)-b-polystyrene (PS-b-PBA-b-PS), and star poly(n-butyl acrylate)-b-polyacrylonitrile (PBA-b-PAN) copolymer elastomers. 20−22 Poly(isobornyl acrylate) (PIBA) is a rigid matrix with hardness of 19.6 kg/mm2 and a high Tg, which is comparable to that of PMMA and PS.23 To the best of our knowledge, only limited research has been reported on the elastomers taking PIBA as the hard microdomains.24 Poly(ionic liquid)s can be prepared either by direct polymerization of ionic liquid monomers or by polymerization of imidazolium-type ionic liquid monomers at first and then by quaternization and anion exchange reactions. The latter one is versatile and can be performed to obtain functional polymers containing imidazole groups with a postpolymerization modification.25−27 Herein, we consider that we can first synthesize ABA triblock copolymer elastomers (TBCPEs) via RAFT polymerization, in which we intend to incorporate a third comonomer with imidazole groups in the TBCPE chains, and then we employ a bromine-containing cross-linker to ionically cross-link the TBCPEs to obtain strong ABA triblock copolymer elastomers. As illustrated in Scheme 1, PIBA was chosen as the dispersed hard microdomains, while poly(n-butyl acrylate)-co-poly(1-vinylimidazole) (P(BA-co-VI)) was chosen as the rubbery matrix, in which imidazole groups could be ionically cross-linked by 1,6-dibromohexane. Basically, S,S′-bis(α,α′-dimethyl-α″-acetic acid)trithiocarbonate (BDAT) was used as a bifunctional chain transfer agent to synthesize PIBA-based macromolecular chain transfer agent (PIBA-CTA). Then, PIBA-b-P(BA-co-VI)-b-PIBA triblock copolymer elastomer was synthesized with PIBA-CTA via RAFT polymerization. A subsequent quaternization cross-linking was performed. In this design, the PIBA hard microdomains can act as physical cross-links to provide the base for the first physical network, and the ionic cross-links can further act as the cross-linking points for the second network to enhance the macroscopic performance of TBCPEs. Thus, ionically crosslinked TBCPEs can be synthesized with enhanced mechanical properties by manipulating the composition and the amount of

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EXPERIMENTAL SECTION

Materials. Isobornyl acrylate (IBA), n-butyl acrylate (BA), and 1vinylimidazole (VI) were purchased from TCI and were purified by passing through the column of basic alumina. The bifunctional chain transfer agent, S,S′-bis(α,α′-dimethyl-α″-acetic acid)trithiocarbonate (BDAT), was prepared according to the reported method.28 1,6Dibromohexane, 2,2′-azobis(2-methylpropionitrile) (AIBN), and other reagents were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). AIBN was recrystallized in ethanol prior to use and the others were used as received. Synthesis of PIBA-Based Macromolecular Chain Transfer Agent (PIBA-CTA). PIBA-CTA was synthesized via RAFT polymerization with S,S′-bis(α,α′-dimethyl-α″-acetic acid)trithiocarbonate (BDAT) as the bifunctional chain transfer agent. A typical polymerization procedure was described below. BDAT (70.5 mg, 0.25 mmol), AIBN (8.2 mg, 0.05 mmol), and IBA (15.62 g, 75 mmol) were dissolved in 1,4-dioxane (15 mL) in a flask. The above solution was degassed by three freeze−pump−thaw cycles, and then the flask was sealed and immersed into an oil bath set at 70 °C. After a reaction period of 24 h with stirring, the polymerization was stopped, and the mixture was precipitated into methanol. The obtained product PIBACTA1 was dried at 40 °C under vacuum overnight. PIBA-CTA1, PIBA-CTA2, and PIBA-CTA3 with number-average molecular masses, Mn, of 48, 23, and 12 kg/mol and polydispersity indexes, DPI, of 1.1, 1.2, and 1.1, respectively, were prepared via the above RAFT polymerization procedure. Synthesis of PIBA-b-P(BA-co-VI)-b-PIBA Triblock Copolymer Elastomer (TBCPE). A typical procedure for synthesis of PIBA-bP(BA-co-VI)-b-PIBA triblock copolymer elastomer was described below. PIBA-CTA1 (2.4 g, 0.05 mmol), AIBN (1.6 mg, 0.01 mmol), BA (6.91g, 54 mmol), VI (0.56 g, 6 mmol), and 1,4-dioxane (12 mL) were introduced into a flask. After PIBA-CTA1 was dissolved, the flask was degassed by three freeze−pump−thaw cycles and then sealed. The polymerization was conducted at 70 °C with stirring. After a suitable reaction time, the polymerization was stopped by opening the flask, and the product was precipitated in water/methanol (2/8 v/v) mixture. The obtained product, TBCPE1, was dried at 40 °C under vacuum overnight. TBCPE2 and TBCPE3 were prepared according to B

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Macromolecules the above procedure with PIBA-CTA2 and PIBA-CTA3 as the macromolecular chain transfer agents, respectively. Preparation of Ionically Cross-Linked Triblock Copolymer Elastomers. Ionically cross-linked triblock copolymer elastomers were prepared with TBCPEs via quaternization. As an example, TBCPE1 of 1.5 g was dissolved in tetrahydrofuran of 20 mL, and then 1,6-dibromohexane of a desired amount was added. Note that the molar ratios of 1,6-dibromohexane to the imidazole groups in TBCPE1 were chosen to be 0.25 or 0.50 to obtain TBCPE1-HC and TBCPE1-FC, respectively. After being stirred for 2 h, the above solution was poured into a Teflon mold. After removal of tetrahydrofuran at room temperature the film sample was taken out from the Teflon mold. The film sample was further dried under vacuum at 50 °C for 12 h. TBCPE2-HC, TBCPE2-FC, TBCPE3-HC, and TBCPE3-FC film samples were prepared by using the same method. Characterizations. A Bruker AVANCE 400 nuclear magnetic resonance (NMR) spectrometer was used to characterize the chemical structures of the synthesized samples. A Waters 150C gel permeation chromatography (GPC) apparatus equipped with three columns (500, 103, and 104 Å) and a 2414 refractive index detector was applied to measure the number-average molecular masses, Mn, and polydispersity indexes, PDI, of the samples using THF as an eluent and polystyrenes as standards. A TA Q2000 differential scanning calorimeter (DSC) was used to determine the glass transition temperatures, Tg, of the samples under a nitrogen atmosphere. Thermogravimetric analysis (TGA) measurements were performed from 40 to 600 °C on a TA Q5000IR thermogravimetric analyzer with a heating rate of 10 °C/ min. Small-amplitude oscillatory frequency sweeps for the samples were carried out from 500 to 0.05 rad/s on an AR2000EX rheometer (TA Instruments) with a parallel-plate geometry (diameter of 25 mm) at 50 °C under a nitrogen atmosphere. The mechanical tensile property tests were carried out on a Suns UTM2502 universal testing machine at room temperature with the crosshead speed of 10 mm/ min. To observe the microphase separation morphology of the samples, a Leica EM FC6 ultramicrotome was used to obtain the ultrathin sections at −90 °C. The ultrathin sections were stained by using ruthenium tetraoxide, RuO4, with a concentration of 1.0 wt % at room temperature for 4 h. Transmission electron micrographs for the above stained ultrathin sections were taken with a JEOL JEM-2100 transmission electron microscope (TEM) at an acceleration voltage of 200 kV. Small-angle X-ray scattering (SAXS) measurements were conducted on a modified Xeuss system from Xenocs SA (Sassenage, France). The sample-to-detector distance was 2500 mm, and the wavelength of the X-ray radiation was 0.154 nm. After the film sample was stretched to a predetermined strain with a Linkam TST350 tensile stage, SAXS patterns were collected with an exposure time of 900 s for each pattern. The SAXS data were corrected for the background scattering.

Table 1. Molecular Masses and Polydispersity Indexes and PIBA and PVI Contents for Synthesized TBCPEs

a

sample code

Mna (kg/mol)

PDIa

PIBA contentb (wt %)

PVI contentb (wt %)

TBCPE1 TBCPE2 TBCPE3

136 66 34

1.6 1.4 1.3

32.4 32.0 31.8

2.0 1.9 1.9

Determined by GPC. bDetermined by 1H NMR spectroscopy.

the mass percentage of PVI component for TBCPE1 is just 2.0%, and the mass percentage of PVI component for TBCPE2 and TBCPE3 is just 1.9%. Two types of ionically cross-linked TBCPEs, half-cross-linked TBCPEs-HC and full-cross-linked TBCPEs-FC, were subsequently prepared with the PIBA-bP(BA-co-VI)-b-PIBA TBCPEs as precursors, for which the imidazole groups were half cross-linked or fully cross-linked through fixing the molar ratios of 1,6-dibromohexane/imidazole groups at 0.25 or 0.50, respectively. When subjected to a small-amplitude sinusoidal deformation, polymer materials can give a complex response, which is sensitive to the existence of inherent network structures.29 Therefore, rheological measurement was conducted to investigate the viscoelastic properties of TBCPEs and their ionically cross-linked counterparts. As an example, the changes of storage modulus, G′, and loss modulus, G″, with angular frequency, ω, for TBCPE1, TBCPE1-HC, and TBCPE1-FC as measured at 50 °C are shown in Figures 1a and 1b, respectively. For TBCPE1, it can be found that the values of G′ are higher than that of G″ from 500 to 0.05 rad/s, and G′ reaches plateau

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RESULTS AND DISCUSSION Syntheses of PIBA-b-P(BA-co-VI)-b-PIBA Triblock Copolymer Elastomers (TBCPEs) and Ionically Cross-Linked TBCPEs. To demonstrate our concept, PIBA-b-P(BA-co-VI)-bPIBA TBCPEs with varied molecular masses and polydispersity indexes were first synthesized via RAFT polymerization. The chemical structures of synthesized TBCPEs were characterized via nuclear magnetic resonance (NMR) spectrometer. Figure S1 shows the 1H NMR spectrum for TBCPE1 as an example, in which the signals appeared at 6.8 and 7.1 ppm can be ascribed to protons of the imidazole ring, and the signals appeared at 4.0 and 4.6 ppm can be ascribed to protons of −OCH2 in PBA and −OCH in PIBA repeat units, respectively. Therefore, the contents of PVI and PIBA in TBCPEs can be obtained from their 1H NMR spectra. The molecular masses and polydispersity indexes as determined from GPC measurements and PIBA and PVI contents as determined from 1H NMR spectra for the synthesized TBCPEs are listed in Table 1. It can be seen that

Figure 1. Changes of (a) storage modulus, G′, and (b) loss modulus, G″, with angular frequency, ω, for TBCPE1, TBCPE1-HC, and TBCPE1-FC measured at 50 °C. C

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Tg’s for PIBA hard microdomains in TBCPE2 and TBCPE3 series are around 106.8 and 105.7 °C, respectively. The Tg’s for P(BA-co-VI) soft matrix increase from −38.9 °C (TBCPE2) to −38.1 °C for TBCPE2-HC and to −37.1 °C for TBCPE2-FC. For TBCPE3 series, the Tg’s for P(BA-co-VI) soft matrix increase from −40.1 °C (TBCPE3) to −39.4 °C for TBCPE3HC and to −38.5 °C for TBCPE3-FC. The Tg’s for P(BA-coVI) soft matrix and PIBA hard microdomains apparently increase with increasing molecular mass of TBCPEs. The existence of the two much obviously distinct Tg values emits the necessity of elastomeric property for these TBCPEs. Mechanical Properties of TBCPEs. The mechanical properties of TBCPEs and their ionically cross-linked counterparts were measured by using monotonic and step-cyclic tensile test modes, respectively. Table S2 lists the values for ultimate tensile strength, elongation at break, and average elastic recovery (ER) for TBCPEs. Figure 3a shows the typical monotonic nominal stress−strain curves for TBCPE1, TBCPE1-HC, and TBCPE1-FC. Note that all these elastomers can be stretched up to a strain more than 450%, illustrating their sufficient elastomeric performance. The ultimate tensile strength and elongation at break for TBCPE1 are 2.8 MPa and 804%, respectively. When cross-linked with different amounts of 1,6-dibromohexane the ultimate tensile strengths can be increased to 8.0 MPa for TBCPE1-HC and 11.7 MPa for TBCPE1-FC, while the elongation at break drops to 678% for TBCPE1-HC and 501% for TBCPE1-FC, demonstrating that the incorporation of ionic cross-links can obviously enhance the tensile strength of TBCPEs. Figures S4a and S5a show the monotonic nominal stress−strain curves for TBCPE2 and TBCPE3 and their ionically cross-linked counterparts, respectively. The similar changing trends can be found for TBCPE2 and TBCPE3 series. As a typical commercial elastomer, polystyrene-b-polybutadiene-b-polystyrene (SBS) triblock copolymer is widely used in our daily life.30 SBS with weight-averaged molecular mass of 115 kg/mol and PS content of 39.5 wt % exhibits an ultimate tensile strength of 10.5 MPa,31 comparable to that of the prepared ionically cross-linked TBCPEs in this work. Step-cyclic tensile tests were further applied to measure the elastic recovery of these elastomers. Figures 3b,c,d show representative step-cyclic nominal stress−strain curves for TBCPE1, TBCPE1-HC, and TBCPE1-FC, and Figures S4 and S5 show the step-cyclic nominal stress−strain curves for TBCPE2 and TBCPE3 and their ionically cross-linked counterparts, respectively. Evident stress softening and hysteresis can be observed for TBCPEs because less stress is needed to sustain the samples at the same strain in a subsequent cycle and the residual strain at zero stress increases as the maximum strain in each cycle increases. This interesting behavior is called the Mullins effect, which is caused by the permanent irreversible plastic deformation during the tensile deformation process.32 For ionically cross-linked TBCPEs, the soft P(BA-co-VI) chains are cross-linked by ionic groups; therefore, most of the deformed polymer chains can recover to their initial state during the unloading process. In order to examine the influence of ionic cross-links on the elastomeric performance, the elastic recovery (ER) values during the loading−unloading process were calculated from the step-cyclic nominal stress−strain curves. Obviously, Figure 3e as well as Figures S4e and S5e show that the ionically cross-linked TBCPEs possess much higher elastic recovery values than TBCPEs. For instance, the ER values for TBCPE1 in each cycle

values at low frequencies due to the presence of hard PIBA microdomains serving as physical cross-links in the elastomeric matrix. A similar phenomenon can be found for TBCPE1-HC and TBCPE1-FC. Moreover, the storage and loss moduli for TBCPE1-HC and TBCPE1-FC are much higher than that for TBCPE1, indicating that TBCPEs can be successfully crosslinked by the reaction between imidazole and bromine groups. TBCPE2 and TBCPE3 series show similar rheological changing trends (curves not shown) as TBCPE1 series, except for the gradually decreasing storage and loss moduli with decreasing molecular mass. Thermal Properties. Thermogravimetric analysis (TGA) measurements were performed to examine thermal stability of TBCPEs. Figure S2 shows the mass loss and their derivative mass loss curves for TBCPE1, TBCPE1-HC, and TBCPE1-FC. It can be found that TBCPE1, TBCPE1-HC, and TBCPE1-FC obviously have quite similar thermal decomposition behaviors with two obvious thermal decomposition stages, demonstrating that the incorporation of ionic cross-links does not reduce the thermal stability of TBCPEs, apparently because of the much low mass percentage of incorporated PVI component, which is essential for practical applications. The major thermal decomposition peak temperatures for TBCPEs are summarized in Table S1. Differential scanning calorimeter (DSC) measurements were carried out to determine Tg’s of TBCPEs and ionically crosslinked TBCPEs. Figure 2 shows the DSC heat flow curves for

Figure 2. Heat flow curves from DSC measurements under a nitrogen atmosphere with a heating rate of 10 °C/min for TBCPE1, TBCPE1HC, and TBCPE1-FC.

TBCPE1, TBCPE1-HC, and TBCPE1-FC under a nitrogen atmosphere with a heating rate of 10 °C/min. Two different Tg’s can be found for these elastomers, corresponding to P(BAco-VI) soft matrix (low Tg) and PIBA hard microdomains (high Tg). TBCPE1 shows two Tg’s around −37.4 and 107.5 °C, respectively. Higher Tg keeps around 107.5 °C for TBCPE1HC and TBCPE1-FC, indicating that the Tg of PIBA hard microdomains is independent of ionic cross-links. However, the lower Tg for P(BA-co-VI) soft matrix increases to −36.2 °C for TBCPE1-HC and to −35.3 °C for TBCPE1-FC, indicating that the soft polymer chains become slightly restricted by the incorporated ionic cross-links, implying successful cross-linking reactions, in accordance with the rheological test results. The DSC heat flow curves for TBCPE2, TBCPE3, and their ionically cross-linked counterparts are shown in Figure S3. The D

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Figure 3. (a) Monotonic nominal stress−strain curves for TBCPE1, TBCPE1-HC, and TBCPE1-FC. Step-cyclic nominal stress−strain curves for (b) TBCPE1, (c) TBCPE1-HC, and (d) TBCPE1-FC. Changes of (e) elastic recovery and (f) hysteresis loss with maximum strain in each cycle during the step-cyclic tensile deformation for TBCPE1, TBCPE1-HC, and TBCPE1-FC.

are much lower than 90% during the step-cyclic tensile deformation. However, after several cycles in the loading− unloading process, the ER values for TBCPE1-HC and TBCPE1-FC become higher than 90%. The average elastic recovery for TBCPE1 is 72%, which is much lower than that for TBCPE1-HC (90%) and TBCPE1-FC (94%), indicating the excellent elasticity for ionically cross-linked elastomers. The elasticity of elastomers can be explained as the ability to recover their conformations after removal of externally applied forces. In our case, the elasticity of TBCPEs can be attributed to the physical cross-links provided by PIBA hard microdomains. While for ionically cross-linked TBCPEs, besides the physical cross-links, the ionic cross-links can serve as additional crosslinks to prevent the polymer chains from flowing under an external stress and thus to significantly enhance elasticity. In order to clarify clearly the role of ionic cross-links, hysteresis loss defined as the amount of energy dissipated during a cyclic tensile deformation was calculated from the integrated area of the hysteresis loop under the loading and subsequent unloading curve. The changes of hysteresis loss for TBCPE1, TBCPE1HC, and TBCPE1-FC during the step-cyclic tensile deformation are depicted in Figure 3f. The hysteresis loss of TBCPE1 increases linearly as strain increases, and the values of hysteresis

loss are low during the whole loading−unloading cycles. However, the hysteresis loss values for TBCPE1-HC and TBCPE1-FC increase exponentially with increasing strain, and the hysteresis loss values are much higher than that for TBCPE1, indicating that the incorporation of ionic cross-links can dramatically increase the energy dissipation of elastomers. The changes of hysteresis loss for TBCPE2 and TBCPE3 and their ionically cross-linked elastomers are shown in Figures S4f and S5f, respectively. It can be seen that the hysteresis loss values for TBCPE2 and TBCPE3 are much lower than their ionically cross-linked counterparts, especially at higher strains, with similar changing trends to that for TBCPE1, TBCPE1HC, and TBCPE1-FC. It can be considered that at low strains the deformation of ionically cross-linked TBCPEs is ascribed to the tension of soft P(BA-co-VI) chains, and the hysteresis loss values are low. When the strain increases, higher stress is needed to stretch the elastomers due to the presence of additional ionically cross-linked network, and the introduced ionic cross-links can transfer the applied stress into the hard microdomains more effectively; thus, much more energy is dissipated, which makes ionically cross-linked TBCPEs much tougher than TBCPEs. Further deformation leads to occurrence of chain pullout, breakage of ionic cross-links, and E

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FC, increases linearly with increasing molecular mass. However, as seen from Figure 4b the elongation at break for these elastomers only shows slight changes as the molecular mass increases. The enhancement in ionic cross-linking density due to full ionic cross-linking for TBCPEs-FC provides its highest slope for the ultimate tensile strength increase with increasing molecular mass. The molecular mass does not show an obvious effect on the elongation at break for each type of TBCPEs, indicating the molecular masses of these synthesized elastomers are already above the critical entanglement molecular mass for sufficient physical performances. Transmission electron microscope (TEM) and small-angle X-ray scattering (SAXS) techniques were applied to examine the possible influence of molecular mass on the microphaseseparated morphology of TBCPEs and their ionically crosslinked counterparts. The typical TEM micrographs for TBCPE1, TBCPE1-HC, and TBCPE1-FC are shown in Figure 5, indicating the typical microphase-separated structure, which

deformation of hard microdomains, which are expected to contribute to hysteresis.33,34 When elastomeric materials are fixed at a constant strain, the stress will decrease with time gradually, which is known as the stress relaxation.35 Stress relaxation behavior is important for elastomers in engineering applications. Therefore, stress relaxation measurements were conducted to characterize the time-dependent relaxation of ionically cross-linked TBCPEs. Figure S6 shows the typical stress relaxation curves for TBCPE1, TBCPE1-HC, and TBCPE1-FC at a constant strain of 400%. TBCPE1 shows a total stress relaxation of 79.0% as the nominal stress decreases from 1.62 to 0.34 MPa within 10 h due to the presence of a large amount of plastic deformation. However, the total stress relaxation decreases to 53.4% and 36.0% for TBCPE1-HC and TBCPE1-FC, respectively, suggesting that incorporation of ionic cross-links can reduce the stress relaxation and enhance elasticity, which is in accordance with the step-cyclic tensile deformation results. Molecular Mass Dependence of Mechanical Properties and Microphase Morphology for TBCPEs and Ionically Cross-Linked TBCPEs. Table 1 indicates that the synthesized TBCPEs have close PIBA and PVI contents. In order to reveal the relationship between the mechanical property and molecular mass of TBCPEs and ionically crosslinked TBCPEs, changes of ultimate tensile strength and elongation at break with number-average molecular mass for these elastomers are shown in Figure 4. It can be found from Figure 4a that the ultimate tensile strength for TBCPEs and ionically cross-linked counterparts, TBCPEs-HC and TBCPEs-

Figure 5. Typical TEM micrographs for (a) TBCPE1, (b) TBCPE1HC, and (c) TBCPE1-FC. (d) Change of interdomain distance, d, with number-average molecular mass for the synthesized TBCPEs.

is in accordance with the DSC result. Figure S7 displays the SAXS intensity profiles for TBCPEs and their ionically crosslinked counterparts. It can be seen from Figure S7a that TBCPE1, TBCPE1-HC, and TBCPE1-FC display a maximum scattering peak locating at qmax of 0.092 nm−1, and an interdomain distance of d = 68 nm can be calculated by d = 2π/qmax. Overall, the TEM and SAXS results indicate that TBCPE1, TBCPE1-HC, and TBCPE1-FC possess the same microstructures and the same interdomain distance, which are independent of incorporated amounts of ionic cross-links, possibly due to the much low PVI mass percentage of 2.0% in the sample. Similar to the TBCPE1 series, the SAXS intensity profiles as shown in Figures S7b and S7c for the TBCPE2 and TBCPE3 series also indicate the microphase-separated morphology. Interestingly, the maximum scattering peak, qmax, shifts to 0.152 nm−1 for TBCPE2, TBCPE2-HC, and TBCPE2FC, pointing at an interdomain distance of d = 41 nm. For TBCPE3, TBCPE3-HC, and TBCPE3-FC, the maximum peak qmax shifts to 0.237 nm−1, indicating an interdomain distance of d = 26 nm. In general, block copolymers can form microphaseseparated morphology due to the presence of incompatible components and the chemical bonding between blocks, and the microstructures of block copolymers are related to the

Figure 4. Changes of ultimate tensile strength (a) and elongation at break (b) with number-average molecular mass for TBCPEs, TBCPEsHC, and TBCPEs-FC. F

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oriented PIBA hard microdomains recover to the original state after release of the externally applied stress.

composition, interaction parameter, and molecular mass of the blocks.36 In order to clarify the relationship between interdomain distance and molecular mass of TBCPEs, the change of interdomain distance, d, with molecular mass for the synthesized TBCPEs is plotted in Figure 5d. Obviously, the interdomain distance increases linearly with increasing molecular mass of TBCPEs. Similar results have been reported by Skoulios and co-workers, who found that the microdomain spacing of diblock or triblock copolymers increases monotonically with molecular mass.37 In summary, the mechanical property and interdomain distance of ionically cross-linked TBCPEs can be tuned by adjusting the molecular mass of TBCPEs. Structural Evolutions of Ionically Cross-Linked TBCPEs during Tensile Deformation. To well understand the morphological evolution for TBCPEs during tensile deformation, in situ SAXS measurements were conducted. Figure 6a shows the photographs for TBCPE1-HC taken at

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CONCLUSIONS In summary, here we report our effort to synthesize TBCPEs (PIBA-b-P(BA-co-VI)-b-PIBA triblock copolymer elastomers) by RAFT polymerization, during which imidazole groups in the third comonomer, 1-vinylimidazole, were incorporated in TBCPEs. Even with much low mass percentage of 2.0% for poly(1-vinylimidazole) component, the imidazole groups on TBCPE chains could be ionically cross-linked by 1,6dibromohexane, which could strongly enhance the mechanical performance of TBCPEs. The ionically cross-linked TBCPEs showed similar thermal decomposition behaviors as TBCPEs, which meant that an incorporation of ionic cross-links did not change thermal stability of TBCPEs. P(BA-co-VI) soft matrix and PIBA hard microdomains in TBCPEs and ionically crosslinked TBCPEs showed much different low and high Tg values, and the low Tg values for P(BA-co-VI) soft matrix were slightly affected by the introduced ionic cross-links. The mechanical properties of TBCPEs and ionically cross-linked TBCPEs could be tuned by controlling of molecular mass and ionic crosslinking degree. The ultimate tensile strength and elastic recovery were significantly improved for ionically cross-linked TBCPEs as compared with TBCPEs due to coexistence of a physical network and an ionically cross-linked network in the elastomeric matrix. TBCPEs and ionically cross-linked TBCPEs all showed typical microphase-separated morphology as observed from transmission electron microscope (TEM) and small-angle X-ray scattering (SAXS) measurements. In-situ SAXS measurements demonstrated that the PIBA hard microdomains could be orientated along the tensile direction during stretching and then be recovered to the original state after release of stress. The synthesis approach employed in the report is versatile and can be conveniently utilized to prepare novel ionically cross-linked elastomers with tunable mechanical performance through copolymerization of certain comonomers with very minor mass percentages.

Figure 6. (a) Photographs taken at different strains and (b) a typical nominal stress−strain curve for TBCPE1-HC during a loading and unloading cycle. Selected 2D-SAXS patterns for TBCPE1-HC during (c) the loading and (d) unloading cycle. The stretching direction is horizontal.

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ASSOCIATED CONTENT

S Supporting Information *

different strains during a loading and unloading cycle, and the typical nominal stress−strain curve during this cycle is displayed in Figure 6b, in which the transparent sample was stretched gradually from 0% to 400% strain and then retracted back to zero stress with a residual strain of 65%. The selected 2D-SAXS patterns for TBCPE1-HC during the loading and unloading cycle are exhibited in Figures 6c and 6d, respectively. Before stretching (0% strain), the 2D-SAXS pattern presents a circular scattering halo, demonstrating a typical scattering feature for the microphase-separated structure. Moreover, the isotropic scattering pattern indicates that the PIBA microdomains are randomly distributed in TBCPE1-HC. When stretched to 100% strain, an H-shaped SAXS pattern can be found in Figure 6c, illustrating that the PIBA hard microdomains are forced to be stacked and tilted along the stretching direction.38 Further deformation leads to the formation of a two-lobe pattern, implying orientation of the stacked PIBA hard microdomains along the stretching direction. During the unloading process, the SAXS patterns change from the twolobe pattern to H-shaped pattern, as shown in Figure 6d, and then to the initial circular scattering halo as the strain decreases from 400% to 65%, which demonstrates that most of the

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01414. Figures S1−S7; Tables S1 and S2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel +86 0551-63607703; Fax +86 0551-63607703; e-mail [email protected] (Z.G.W.). ORCID

Zhigang Wang: 0000-0002-6090-3274 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (Grants 51473155 and 51603199). This work is also supported by the China Postdoctoral Science Foundation (Grant 2015M582013). Prof. Yongfeng Men at the Changchun Institute of applied G

DOI: 10.1021/acs.macromol.7b01414 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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Chemistry, Chinese Academy of Sciences, is acknowledged for providing use of the SAXS facility.

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DOI: 10.1021/acs.macromol.7b01414 Macromolecules XXXX, XXX, XXX−XXX