Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Self-Healing Alkyl Acrylate-Based Supramolecular Elastomers CrossLinked via Host−Guest Interactions Suguru Nomimura,† Motofumi Osaki,† Junsu Park,† Ryohei Ikura,† Yoshinori Takashima,*,†,‡ Hiroyasu Yamaguchi,† and Akira Harada*,§ Department of Macromolecular Science, Graduate School of Science, and ‡Institute for Advanced Co-Creation Studies, Osaka University, Toyonaka, Osaka 560-0043, Japan § The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan Macromolecules Downloaded from pubs.acs.org by UNIV AUTONOMA DE COAHUILA on 03/21/19. For personal use only.
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S Supporting Information *
ABSTRACT: We prepared acrylamide monomers with permethylated cyclodextrins (PM-CDAAmMe) or peracetylated cyclodextrins (PAc-CDAAmMe). PM-CDAAmMe and PAc-CDAAmMe are soluble in various hydrophobic liquid acrylate monomers, and they can form inclusion complexes with guest monomers such as adamantane or fluoroalkyl groups tethered to a vinyl residue. The bulk polymerization of the liquid acrylate monomers with the PMCDAAmMe or PAc-CDAAmMe monomers and the guest monomers gave highly flexible and tough elastomers. Tensile tests on the obtained supramolecular elastomers showed fracture strains of over 800% and fracture energies that were 12 times larger than those of covalently cross-linked conventional elastomers, indicating that the host−guest cross-linking made the supramolecular elastomers quite tough. During the deformation process, the applied stress is dispersed into the supramolecular elastomers by dissociation and recombination of the reversible host−guest complex. Moreover, these host−guest complexes also allow the adhesion of fractured pieces of the supramolecular elastomers without adhesives. The mechanical strength of the fractured elastomer was restored to ∼99% of its initial strength within 4 h. The self-healing properties can be attributed to the reversible cross-linking by the host−guest interactions.
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self-healing abilities,34−44 which allow the materials to maintain their original manufactured form after sustaining external damage. Host−guest interactions are one of noncovalent interactions that enable easy introduction of various functions into materials45−47 using macrocyclic molecules such as crown ethers,48 calixarenes,49 cucurbiturils,50,51 pillararenes,52 cyclodextrins (CDs),53−55 and functional guest molecules. We investigated host−guest interactions between CDs and hydrophobic guest molecules as supramolecular reversible bonds and found a variety of supramolecular polymeric materials, such as macroscopic self-assemblies,56 self-healing materials,57,58 artificial muscles,59−61 and shape memory materials.62 Some polymer networks cross-linked by host− guest supramolecular bonds can serve as stress dispersing systems.63,64 However, these stress dispersion systems must be prepared in aqueous media and are not applicable for generalpurpose in which hydrophobic bulk polymers are employed. Here, we prepared tough and flexible bulk polymeric materials by using various hydrophobic acrylates modified with CD hosts or adamantane guests. The obtained polymers have host and guest moieties as their side chains to form elastomers with high flexibility and toughness. Furthermore, the supramolecular elastomers showed self-healing abilities, which restores them quantitatively within hours. These properties
INTRODUCTION Tough and flexible polymeric materials are used in various manufacturing industries and healthcare applications. In particular, in the automotive industry, lighter weight polymeric materials are now being replaced from conventional heavyweight metal materials in the frames, functional components, and interiors and exteriors of vehicles to improve their fuel economy. Although some polymeric materials are stretchable and show fracture energy values as high as metals and woods, they easily yield to show irreversible deformation with small amount of external stress. To enhance their mechanical strength, materials scientists have improved polymeric materials by physical or chemical modifications, such as cross-linking polymer chains, blending various polymers, or adding fillers to the materials.1−3 In the field of polymer science, some researchers have attempted to prepare tough polymeric materials by introducing new kinds of cross-linking to improve stress distribution. For example, multiple network gels,4−7 nanocomposite gels,8 slide-ring gels,9−11 and ideal polymer networks based on four-arm macromolecules12,13 have been developed. In recent decades, supramolecular scientists have investigated molecular self-assemblies by utilizing reversible noncovalent bonds for developing new materials.14−19 Introduction of the reversible bonds (i.e., hydrogen bonds,20 π−π stacking,21−23 ions,24 hydrophobic interactions,25 and metalcoordination bonds26,27) has achieved tough materials28−33 or © XXXX American Chemical Society
Received: March 8, 2019
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DOI: 10.1021/acs.macromol.9b00471 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. Methods of synthesizing host−guest materials.
because of the high number of hydroxyl groups on the CD residue. Thus, we prepared permethyl γCD monomers (PMγCDAAmMe) and peracetyl CD monomers (PAcγCDAAmMe and PAcβCDAAmMe) (Figure 2a). These mono-
will contribute to resolving the existing challenges in the materials industries.
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RESULTS AND DISCUSSION Design Concept of Polymeric Material Cross-Linked by Host−Guest Interactions. Figure 1 shows two potential preparative methods for the synthesis of the polymeric material cross-linked by host−guest interactions. The host and guest molecules are attached to a polymerizable functional group, such as a vinyl group. One of the preparation methods is mixing the host polymer and guest polymer (method A). The host polymer and the guest polymer are individually prepared via polymerizations of the host and guest monomers, respectively. Then, the host and guest polymers are crosslinked by host−guest inclusion complexation at their side chains with mixing them in a solution to obtain the target polymeric material.65 The other method is the polymerization of host−guest inclusion complexes (method B). First, the host monomer forms an inclusion complex with the guest monomer in a solution of the monomers. Then, the host−guest monomer complex is polymerized to form the polymers cross-linked through the host−guest complexes. Method A requires both the host polymer and the guest polymer should be dissolved in the same solution or molten to be kneaded in the bulk mixture. On the other hand, in method B, only the inclusion complex monomer needs to be dissolved in the solvent. Moreover, method B requires only two steps (complexation and polymerization). Notably, the complexation ratio of the host−guest complex in the product of method B should be larger than that obtained from method A, as the host−guest inclusion complex was formed prior to polymerization in method B. Therefore, we used method B in this report. Preparation of Poly(alkyl acrylate)s Cross-Linked by Host−Guest Interactions. To obtain the hydrophobic bulk elastomer based on poly(alkyl acrylate)s with the CD host/ guest system, the inclusion complex of the CD monomer with the guest monomer should be dissolved in a liquid alkyl acrylate as solvent. Then, copolymerization of the bulk mixture of these monomers should be conducted. However, the CD monomers are hydrophilic and insoluble in alkyl acrylates
Figure 2. Chemical structures of the host monomers (PMγCDAAmMe, PAcγCDAAmMe, and PAcβCDAAmMe) (a), the guest monomers, ethyladamantyl acrylate (AdEtA) and fluorooctyl acrylate (H2F6) (b), alkyl acrylates as main-chain monomers, ethyl acrylate (EA) and butyl acrylate (BA) (c), and the chemical cross-linking reagent, butane diacrylate (BDA) (d).
mers were prepared from 6-acrylamido methyl ether-γCD (γCDAAmMe) by a Williamson ether synthesis or acetylation using acetic anhydride as described in the Supporting Information. The obtained monomers were characterized by 1 H and 13C NMR spectroscopies and MALDI-TOF mass spectrometry (Figures S4−S9). PMγCDAAmMe dissolves in water, acetone, and ethyl acetate. PAcγCDAAmMe and PAcβCDAAmMe dissolve in acetone and ethyl acetate. We also employed two guest monomers: 2-ethyladamantyl acrylate B
DOI: 10.1021/acs.macromol.9b00471 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules (AdEtA) and fluorooctyl acrylate (H2F6) (Figure 2b). PMγCDAAmMe, PAcγCDAAmMe, PAcβCDAAmMe, AdEtA, and H2F6 are highly soluble in ethyl acrylate (EA) and butyl acrylate (BA) (Figure 2c). Scheme 1 shows the bulk copolymerization of EA with PMγCDAAmMe and AdEtA to obtain poly(EA)-tethered CD
mg (0.020 mmol) of 1-hydroxycyclohexylphenyl ketone (Ciba IRGACURE184) was added to the monomers as a photoinitiator. Free radical bulk copolymerization of the monomers was performed by UV irradiation with a high-pressure Hg lamp (λ = 253 and 365 nm), and the bulk mixture of the monomers became a transparent elastic solid within 30 min. The product was dried at 80 °C in vacuo to remove the small amount of residual EA and afford pEA-PMγCD-Ad(1, 1). The other elastomers used in this study, including control samples, were prepared in the same manner (Figure 3). FT-IR (Figure S10) and 1H FG/MAS NMR (Figure S11) spectroscopies were performed to characterize the elastomer. Mechanical Properties of pEA-PMγCD-Ad(x, y) vs Reference Elastomers. The mechanical properties of pEAPMγCD-Ad(x, y) were investigated by tensile tests. Figure 4a shows the stress−strain curves of pEA-PMγCD-Ad(0.5, 0.5) and the control samples. The homopolymer poly(EA) (pEA; with no cross-linking) was easily fractured with showing plastic deformation. On the other hand, the host−guest elastomer pEA-PMγCD-Ad(0.5, 0.5) showed a high fracture stress of 440 kPa and a strain of 900%, indicating that the host and guest moieties play important roles in enhancing the toughness and flexibility. Interestingly, the covalently cross-linked elastomer pEA-BDA(0.5), which was prepared from the copolymerization of EA and butane diacrylate (BDA) (Figure 2d), immediately ruptured with a low strain of 100%. The fracture energy (corresponding to the integral of the stress−strain curve) of pEA-PMγCD-Ad(0.5, 0.5) (51.7 kJ/m2) was 12 times larger than that of pEA-BDA(0.5) (4.4 kJ/m2), indicating that the introduction of the host and guest moieties
Scheme 1. Preparation of Supramolecular Elastomer pEAPMγCD-Ad(x, y)
hosts and Ad guests, abbreviated pEA-PMγCD-Ad(x, y), in which x and y are the mol % contents of the host and guest units in the copolymer, respectively. PMγCDAAmMe (170 mg, 0.10 mmol) and AdEtA (23.0 mg, 0.10 mmol) were dissolved in EA (981 mg, 9.80 mmol). The homogeneous mixture of the monomers was sonicated for 30 min. Then, 4.1
Figure 3. Chemical structures of polymers having host/guest moieties. The host−guest polymers (a), covalently cross-linked polymers (b), and homopolymers with no cross-linking (c). C
DOI: 10.1021/acs.macromol.9b00471 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. (a) Stress−strain curves of pEA-PMγCD-Ad(0.5,0.5) and the controls. (b) Stress−strain curves of pEA-PMγCD-Ad(x, y) with various (x, y) values. (c) Swelling ratios of pEA and pEA-PMγCD-Ad(x, y)s using acetone as solvent. (d) Stress−strain curves of pEA-PMγCD-Ad(0.5, 0.5) with/without adamantane in the preparations.
Dependence of the Swelling Ratios on the Host and Guest Contents in pEA-PMγCD-Ad(x, y). Swelling tests on the elastomers were performed using acetone as the solvent (Figure 4c). pEA-PMγCD-Ad(0.5, 0.5) and -(1, 1) were swollen in acetone to give organogels with swelling ratios of 1210% and 1810%, respectively. On the other hand, pEAPMγCD-Ad(0.1, 0.1) and -(0.25, 0.25) dissolved in acetone. These results agree with the results that pEA-PMγCD-Ad(0.5, 0.5) and -(1, 1) showed high fracture stresses and that pEAPMγCD-Ad(0.1, 0.1) and -(0.25, 0.25) were irreversibly deformed by the lower strain. In pEA-PMγCD-Ad(0.5, 0.5) and -(1, 1), polymer networks cross-linked by the host/guest interactions are supposed to percolate through the materials. Mechanical Properties of pEA-PMγCD-Ad(x, y) with Competitive Guest Molecules. Competitive guest testing was performed by using adamantane (Ad) molecules without the polymerizable group as competitive guest molecules (Figure 4d). Here, Ad (0.20 mmol, saturation amount in EA) was added to the mixture of pEA-PMγCD-Ad(0.5, 0.5) monomers to be solubilized in the bulk monomers, and the radical copolymerization of the mixture was initiated to give a transparent solid. Figure 4d shows the stress−strain curve of
resulted in a higher toughness than what is seen with covalent cross-linking. Figure 4b shows the effect of the cross-linking density on the tensile strength. The stress−strain curves of the host−guest elastomers pEA-PMγCD-Ad(0.1, 0.1), -(0.25, 0.25), -(0.5, 0.5), and -(1, 1) showed that the maximum stress increased with increasing content of host and guest residues. Although pEA-PMγCD-Ad(0.1, 0.1) and -(0.25, 0.25) showed irreversible deformations to yield within a strain of 200%, pEAPMγCD-Ad(0.5, 0.5) and -(1, 1) showed high fracture stresses. The elastomers showed increased toughness as the concentration of host/guest residues increased, indicating that the host−guest interactions act as cross-linking points in the polymer network. Figure S12 shows the stress−strain curves of the elastomers obtained by methods A and B. The supramolecular elastomer prepared by the method B shows higher fracture stress, fracture strain, and fracture energy than the corresponding mixture of the host polymer and guest polymer prepared by method A. These results indicate that the material design concept (method B in Figure 1) is suitable for preparing the tough polymeric materials. D
DOI: 10.1021/acs.macromol.9b00471 Macromolecules XXXX, XXX, XXX−XXX
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Figure 5. (a) Stress−strain curves of pEA-PMγCD-Ad(1, 1), pEA-PAcγCD-Ad(1, 1), and pEA-PAcβCD-Ad(1, 1). (b) Stress−strain curves of pEAPMγCD-Ad(1, 1) and pEA-PMγCD-H2F6(1, 1). (c) Stress−strain curves of pBA-PMγCD-Ad(0.5, 0.5) and controls.
the permethyl γCD moieties and the Ad moieties of the side chains. Dependence of the Mechanical Properties on the Cavity Size of the CDs. Through careful molecular design and preparation, other host, guest, and main chain monomers can also be used to give supramolecular elastomers cross-linked by host−guest inclusion complexes. We employed peracetyl CD host monomers such as PAcγCDAAmMe and PAcβC-
the resulting material. The fracture stress was found to be lower than that of the elastomer of pEA-PMγCD-Ad(0.5, 0.5) prepared in the absence of the Ad competitive guest. The Ad competitive guest is supposed to prevent inclusion complex formation between PMγCDAAmMe and AdEtA during polymerization. This result indicates that the poly(EA) chains in the supramolecular elastomer pEA-PMγCD-Ad(0.5, 0.5) are cross-linked by the host−guest inclusion complexes involving E
DOI: 10.1021/acs.macromol.9b00471 Macromolecules XXXX, XXX, XXX−XXX
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Figure 6. Photographs demonstrating the stab-resistant properties of pEA-BDA(1) (a) and pEA-PAcγCD-Ad(1, 1) (b) using a cutter blade.
Figure 7. (a) Photographs from the self-healing experiments. Dumbbell-shaped pieces of pEA-BDA(1) and pEA-PAcγCD-Ad(1, 1) were cut into two pieces under a powerful force, and then two cut pieces were reattached. Soon after the attachment, the two pieces adhered to each other and could be lifted up against their own weight. After standing for 24 h, the two pieces adhered strongly enough to be pulled from opposite sides without breakage. (b) The stress−strain curves of the pEA-PAcγCD-Ad(1, 1) readhered at room temperature or at 80 °C. (c) The healing ratio (E/ E0) of pEA-PAcγCD-Ad(1, 1) at room temperature and at 80 °C after 4 h (E0: fracture energy of the original elastomer; E: fracture energy of the readhered elastomer). (d) Time course of the healing ratios of pEA-PAcγCD-Ad(1, 1) at 80 °C. The solid line shows the result of the nonlinear least-squares curve fitting using a model, E/E0 = 1 − exp(−t/τ) (t: time after the readhesion; τ: lifetime of the damage (fitting parameter)). From these results, τ was calculated to be 1.54 h.
Ad(1, 1) and pEA-PAcβCD-Ad(1, 1) show high flexibility and toughness, indicating that other CD host monomers are also available for the preparation of supramolecular elastomers. The fracture stress of pEA-PAcγCD-Ad(1, 1) (740 kPa) is higher than that of pEA-PMγCD-Ad(1, 1) (540 kPa). Interestingly,
DAAmMe (Figure 2a). The bulk copolymerization of these host monomers with AdEtA and EA gave the supramolecular elastomers pEA-PAcγCD-Ad(1, 1) and pEA-PAcβCD-Ad(1, 1), respectively (Figure 3a). The tensile tests of these elastomers (Figure 5a) revealed that both pEA-PAcγCDF
DOI: 10.1021/acs.macromol.9b00471 Macromolecules XXXX, XXX, XXX−XXX
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Figure 8. (a) Photographs from the recycling experiments. Sheet-shaped supramolecular elastomer pEA-PAcγCD-Ad(1, 1) (i), the supramolecular elastomer’s organogel swollen in toluene (ii), the fluid containing microparticles of the organogel, which was milled into a microgels of the supramolecular elastomer (iii), and the supramolecular elastomer recycled from the fluid, which was casted and dried at 100 °C (iv). (b) Stress− strain curves of the recycled pEA-PAcγCD-Ad(1, 1).
the energy dissipation mechanism inside the host−guest elastomer gives both reversible elastic deformation and high toughness. Self-Healing Properties of the Supramolecular Elastomers. As host−guest complexation is a reversible process, fractured pieces of the supramolecular elastomer should readhere. That is, the supramolecular elastomers might have self-healing properties. pEA-PAcγCD-Ad(1, 1) was molded into a dumbbell shape by the copolymerization of PAcγCDAAmMe, AdEtA, and EA in a poly(tetrafluoroethylene) (PTFE) template. The obtained dumbbell-shaped pEA-PAcγCD-Ad(1, 1) was divided at its center with a knife blade. Then, the pieces of the supramolecular elastomer were rejoined and allowed to set for 24 h at room temperature. The divided supramolecular elastomer was found to adhere so tightly that one of the pieces lifted by tweezers was able to hold the weight of the other piece (Figure 7a). Other supramolecular elastomers, such as pEA-PMγCD-Ad(1, 1) and pEAPAcβCD-Ad(1, 1), also showed self-healing behaviors (Figure S13). On the other hand, the covalently cross-linked material pEA-BDA(1) could not readhere. These results indicate that the self-healing ability of these supramolecular elastomers is due to the reversible host−guest cross-linking. Interestingly, the self-healing phenomena were observed only between newly sliced surfaces of the elastomers (Movie S3). The original surfaces generated by the PTFE template did not show adhesion at all. Even the newly sliced surface did not adhere to the original molded surface. These results indicate that the host−guest cross-linking points are dissociated in the rupturing process and that the uncomplexed host and guest moieties might only be located on the newly sliced surface to allow readhesion. Almost all of the host and guest moieties can form inclusion complexes in the molded elastomer. This selfhealing ability, which selectively restores the damaged portion of the material, would broaden the applicability of the elastomer. Figures 7b and 7c show the results of self-healing tests with pEA-PAcγCD-Ad(1, 1) subjected to tensile tests at room temperature and 80 °C. Here, the healing ratio (E/E0) is defined as the fracture energy value of the readhered material compared to that of the original material before being cut by a knife blade (E0: fracture energy of the original elastomer; E: fracture energy of the readhered elastomer). The readhered pEA-PAcγCD-Ad(1, 1) elastomer showed a healing ratio of
pEA-PAcβCD-Ad(1, 1) was found to show a fracture stress (1090 kPa) twice as high as that of pEA-PMγCD-Ad(1, 1); however, both elastomers showed similar stress at a lower strain (50 kJ/ m2) is as high as those of tough conventional materials, including polymers, metals, and composite materials. In the supramolecular elastomers, the cross-linking points might disperse stress during deformation. Furthermore, these host− guest elastomers were found to show self-healing behavior. pEA-PAcγCD-Ad(1, 1) quantitatively repaired its physical properties within 4 h at 80 °C. As described above, tough and flexible elastomers with self-healing abilities were successfully prepared using host−guest supramolecular interactions. We
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