Letter Cite This: ACS Macro Lett. 2019, 8, 506−511
pubs.acs.org/macroletters
Unexpected Healability of an ortho-Blocked Polybenzoxazine Resin Lei Zhang,†,‡ Zhongxiang Zhao,†,‡ Zenghui Dai,† Linghui Xu,† Feiya Fu,† Takeshi Endo,§ and Xiangdong Liu*,† †
College of Materials and Textile, Zhejiang Sci-Tech University, Xiasha Higher Education Zone, Hangzhou 310018, China Molecular Engineering Institute, Kinki University, Kayanomori, Iizuka 820-8555, Japan
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S Supporting Information *
ABSTRACT: Ring-opening polymerization of bifunctional benzoxazine has long been thought to produce a permanent network structure without reprocessing ability. Here, we demonstrate that surprising healability can be achieved by a controlled polymerization of an ortho-blocked bifunctional benzoxazine poly(oC-hda). The cured resin possesses a crosslinked structure, but can be deformed, remolded from crushed pieces or healed from mechanical damage. Based on a series of intensive experiments, we show that the healability can be explained by a dynamic bonding exchange mechanism between the phenoxy structures existing during the curing process. Moreover, we verify the possibility to heal the fatigue damaged poly(oC-hda) based composite to extend its service life. Our study provides another dynamic covalent bond to synthesize healable polymers, offering a broad platform for combining healability and desired thermosetting features together.
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Polybenzoxazine, derived from 1,3-benzoxazine through ring-opening polymerization, has gained intensive focus recently due to its synthetic versatility together with attractive thermal and mechanical properties, as a combination of phenolic resin and epoxy resin.25,26 On the contrary, comparing to the enormous published polybenzoxazine species, polybenzoxazine possessing healable, malleable, and recyclable abilities is relatively scarce. Liu et al.27 and Krajnc et al.28 prepared benzoxazine precursors having potential healing ability based on Diels−Alder reaction. Yagci et al. are the most active in developing healable polybenzoxazines through analogous Friedel−Crafts reaction,29 hydrogen bonding,30 coumarine cyclization,31 ketene chemistry,32 and coordination.33 We have reported a healable polybenzoxazine/ anhydride system based on transesterification.34 Up to date, healable polybenzoxazines are mostly realized by special structure design, such as introducing functional groups. Conventional polybenzoxazine has long been considered as a nonhealable material, since the Mannich bridge structure is mostly irreversible. However, Mannich bridge is not the unique linkage produced during ring-opening polymerization, N− CH2−X (X = O, N, S) and other structures can be generated as well,35−38 while their possibility for healing has long been ignored.
ncorporating dynamic covalent bonds into polymers to achieve healability is one of the expanding interests
Scheme 1. Structures of the Investigated Benzoxazine Monomers
currently in the field of polymer chemistry.1−5 Dynamic covalent bonds allow (1) dissociation and association of polymers or (2) bond exchange between polymer chains. Thus, thermosets containing dynamic covalent bonds become healable, malleable or recyclable, which are unique from conventional thermosets. Till now, various reversible reactions have been applied therein, which include but are not limited to (1) Diels−Alder reaction,6−8 (2) olefin metathesis,9−11 (3) urea exchange,12−14 (4) transesterification,15−17 (5) disulfide exchange,18−20 and (6) boron-diol reactions.21−24 © XXXX American Chemical Society
Received: January 28, 2019 Accepted: February 27, 2019
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DOI: 10.1021/acsmacrolett.9b00083 ACS Macro Lett. 2019, 8, 506−511
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ACS Macro Letters
Figure 1. (a) Compress deformation, (b) bending and recovery, (c) stamping, (d) recycling, (e) swelling, and (f) dynamic mechanical analysis of poly(oC-hda).
Figure 2. (a) Sample features for shear-damage and healing test and poly(oC-hda) healed under various temperatures and durations; (b) crackhealing behavior of poly(oC-hda).
the influence of incomplete curing. The results prove that the curing condition (210 °C/4 h) is sufficient to enable complete ring-opening of benzoxazine, as evidenced by the disappearance of the FT-IR peak at 923 cm−1 and the curing peak vanishing in DSC (Figure S2). Besides, poly(oC-hda) can be bent at 180 °C and fixed after cooling. The fixed feature is able to recover back to its original shape once the sample is heated again (Figure 1b). Stamping results further confirm the malleability of poly(oC-hda) (Figure 1c, coin diameter is 19 mm). An embossment appears on the surface of poly(oC-hda), which is generated by placing a 250 g weight on the top of the coin and heating at 180 °C for 15 min. Moreover, unlike conventional thermosets or other polybenzoxazines, poly(oChda) is recyclable even after crushing into pieces. We have successfully remolded poly(oC-hda) fragments into the desired shape after compressing at 180 °C/15 MPa/2 h. A higher pressure is selected here in order to reduce defects in the sample. The recycled sample has a tensile strength of 48.3 ± 2.1 MPa, with a high mechanical healing efficiency larger than 90% (the tensile strength of original sample is 52.6 ± 4.5 MPa; Figure 1d). It is worth mentioning that poly(oC-hda)
During the past, we have systematically studied the reversibility of N−CH2−X bonds based on benzoxazine/thiol and benzoxazine/amine systems.39,40 Comparing with Mannich bridge, N−CH2−X bonds are more reactive, so that the reversible reaction can take place at a lower temperature. Besides, as the reversibility of N−CH2−X bonds are similar as other reported dynamic covalent bonds, it is anticipated that polybenzoxazines constructed by N−CH2−X structures can be healable and recyclable. To verify this hypothesis, we investigate several benzoxazines with different blocking positions (Scheme 1, synthesis details can be found in the SI), together with a commercialized benzoxazine BF-a. After curing at 210 °C/4 h, the polybenzoxazines are compressed at a condition of 180 °C/5 MPa/20 min. Poly(oC-hda) and poly(pC-hda) deform into sheets (Figures 1a and S1), while the other polymerized benzoxazines (mC-hda, P-hda, BF-a) become fragments after compression (Figure S1). Since the deformation and healing behaviors of the two polybenzoxazines are quite similar during our preliminary experiments, we then select poly(oC-hda) for further investigation. FT-IR and DSC were applied to exclude 507
DOI: 10.1021/acsmacrolett.9b00083 ACS Macro Lett. 2019, 8, 506−511
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ACS Macro Letters
Figure 3. (a) TGA-GC-MS result and proposed structures of poly(oC-hda), (b) 1H NMR of poly(oC-ha), (c) solubility change of poly(oC-hda) before and after exchange reaction, and (d) proposed phenoxy exchange mechanism.
evaluated under a shear mode by placing poly(oC-hda) with a thickness of 0.2 mm in between two aluminum plates (Figure 2a, details can be found in the SI). Healing efficiency is obtained by comparing the strengths of the healed samples with the original samples. The original sample shows an adhesion force of 1436 N. The damaged samples are compressed together and healed under various conditions to investigate the influences of temperature and healing duration. We found that a healing temperature below 160 °C is inadaptable to achieve high healing efficiency. On the other hand, healing at 180 °C provides the best adhesion force of 1427 N, which corresponds to an almost complete recovery. A further increase in healing temperature up to 200 °C leads to reduction in adhesion force. We also characterize the adhesion
dramatically loses its recyclability if a higher curing temperature (240 °C) is selected. Poly(oC-hda) can be swollen in DMF at 70 °C without dissolution (Figure 1e). Dynamic mechanical analysis also shows poly(oC-hda) has a Tg at 123 °C and a rubbery plateau with a constant value near 7 MPa (Figure 1f), which are consistent with a previous study.41 These results confirm that poly(oC-hda) is a cross-linked structure. Therefore, the deformation and recycling abilities of cross-linked poly(oC-hda) are resulted from reasons other than thermoplasticity, like linear polymers. The bonds within the network are likely to be dynamic to enable macroscale flow so that deformation, stamping, and recycling are allowed. Mechanical damage recovery is selected to show the healing performance of poly(oC-hda). Herein, adhesion force is 508
DOI: 10.1021/acsmacrolett.9b00083 ACS Macro Lett. 2019, 8, 506−511
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ACS Macro Letters
Meanwhile, peaks assigned to the Mannich bridge become higher. This is due to the conversion of phenoxy structure to phenolic structures, as mentioned in the literature.36,40 In addition, since the reversion tendency of the Mannich bridge in benzoxazine is weak, the results also explain the reason that poly(oC-hda) loses its recyclability after curing at 240 °C. The above results confirm the substantial amount of phenoxy structures in poly(oC-hda). However, these structures have to be dynamic so that healing and recycling are permitted. To demonstrate the possible exchange between phenoxy structures, we place a cross-linked poly(oC-hda) in an excess amount of monofunctional oC-ha monomer (1:20, m/m) and heat at 180 °C (Figure 3c). As linear poly(oC-ha) forms, the sample color becomes dark red. After removing from the oven, a viscous fluid is obtained, which can be completely dissolved in DMF. The result proves the existence of covalent bonding exchange, which makes the cross-linked structure of poly(oChda) collapse into linear and branched segments. The chemical structure of the reacted mixtures was proved by NMR (Figure S13) and GPC (Figure S14). Otherwise, cross-linked poly(oChda) would remain after treatment and the final product cannot be thoroughly dissolved. Phenoxy structure is a kind of N−CH2−X structures, which are active at elevated temperatures. For oC-hda, only the oxygen of oxazine ring and the para-position of aryl ring are reactive sites during ring-opening (Figure 3a). As aryl ring attacking generates stable Mannich bridge, both the reversible nature of N−CH2−X species39,40 and experimental results in this study imply that phenoxy in poly(oC-hda) is dynamic (Figure 3c). With imine as intermediate, exchange between N−CH2−X bonds becomes highly possible (Figure 3d), which enables rearrangement of the network so that macroscopic changes, such as deformation, healing, and recycling, of poly(oC-hda) are allowed. However, once the phenoxy structures convert to an irreversible Mannich bridge, the network becomes fixed and the functional abilities of poly(oChda) no longer exist. On the other hand, poly(P-hda), poly(mC-hda), and poly(BF-a) have more reactive sites on the aryl ring. Multiple reactive sites on the same aryl ring likely make the ring become a cross-linking point, which is adverse to the macroscopic flowability of the material. According to Nielsen’s equation,43 the cross-linking density of poly(P-hda), poly(mC-hda), and poly(BF-a) are 3.09, 2.43, and 2.16 mol· dm−2, which are higher than 1.49 mol·dm−2 of poly(oC-hda). Therefore, the balance of cross-linking density and phenoxy exchange largely influences the malleable and healable window. The attractive healability of poly(oC-hda) is then applied in composite to elongate its service life. This is the first reported prototype to recover the fatigue damage of fiber-reinforced composite based on healable polymers. In this study, cotton fiber-reinforced poly(oC-hda) based composite is fabricated through an environmentally friendly slurry method42 (details can be found in SI), and its fatigue-healing behavior is evaluated under displacement-control mode (Figure 4). The initial resistance force is 3 N and gradually decreases with the increase of fatigue cycles. After every 2000 cycles, the composite is healed at 180 °C/4 h and the resistance force of the composite recovers back to 3 N. Remarkably, fatigue is carried out up to 8000 cycles and the healed composite does not show apparent change in mechanical performance. In summary, our work offers a new dynamic covalent bond based on the study of an ortho-blocked polybenzoxazine. By controlling the curing temperature, a large amount of phenoxy
Figure 4. Fatigue-healing performances of cotton fiber reinforced poly(oC-hda) for 8000 cycles.
force after curing for various times and figure out that 4 h is the minimum duration to achieve sufficient healing at 180 °C. This is consistent with the crack-healing behaviors of the fractured poly(oC-hda) sample, of which the crack completely disappears after heating at 180 °C for the same duration (Figure 2b). It is well-known that phenol and Mannich bridge structures in cross-linked polybenzoxazine are generated from ringopening polymerization.25 However, depending on the polymerization conditions, the obtained network can be phenolic structures, phenoxy structures, or both.35,37,38 The secret that lies behind the healing and recycling ability of crosslinked poly(oC-hda) is probably related to its structure. To verify this, the cross-linked poly(oC-hda) is analyzed through TGA-GC-MS (Figure 3a). Theoretically, the para-position of the aryl ring and the oxygen on the oxazine ring of poly(oChda) are electron-rich, thus, both of them can be attacked by imine. Attacking to the oxygen generates phenoxy structures, while attacking to the aryl ring forms phenolic structure.36,38 Various fragments can be detected (D1−D6), according to the GC and MS spectra (Figures S3−S11). Among them, D2, a phenoxy structure, is found with the largest amount. The fragments D2, D3, and D4 prove the oxygen-attacking and fragment D3 evidences aryl-attacking. Hence, oxygen-attacking and aryl-attacking can be carried out simultaneously. Besides, oxygen-attacking seems to be preferred, since a higher amount of phenoxy structures is found (Figure 3a). For a further illustration, a monofunctional benzoxazine oC-ha, which has a similar structure of oC-hda is prepared by o-cresol and nhexylamine to avoid cross-linking after polymerization. According to 1H NMR results, slight ring-opening is observed as tiny resonance peaks appear at 4.77 and 3.85 ppm after curing at 180 °C/1 h, which are corresponding to the protons on the phenoxy structures (Figure 3b). Raising the curing temperature up to 210 °C (the same as the curing temperature of poly(oC-hda)) facilitates phenoxy generation, as these two peaks increase significantly. In addition, other than phenoxy structures, phenolic structures are created at 210 °C as well. The smaller resonances at around 3.6 ppm are corresponding to the protons of the Mannich bridge, benzylamine, ditan, and so on.36,38,40 The result again proves that the two reaction paths are parallel. However, the phenoxy peaks disappear after further increasing the curing temperature up to 240 °C/1 h. 509
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structures are produced. The exchange between phenoxy structures is the intrinsic reason that enables healing and recycling of the polybenzoxazine. Moreover, the healability can be inherited by the corresponding composite to recover from fatigue damage. This is the first prototype to show fatiguehealing based on composite matrix. It should be emphasized that since benzoxazine has extreme flexibility in structure design, while phenoxy structure can be obtained controllably, this work is not limited to development of healable polybenzoxazine. In fact, by introducing benzoxazine into other polymer systems, this work introduces a general compatible platform to produce healable polymers, which has superior broad potential applications.
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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/acsmacrolett.9b00083.
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Experimental details, compress deformation, DSC, DMA, and FT-IR, 1H, and 13C NMR spectra used for the study are given (PDF).
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Takeshi Endo: 0000-0001-6903-8048 Xiangdong Liu: 0000-0001-5262-0031 Author Contributions ‡
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51573167 and 51873195) and Zhejiang Provincial Natural Science Foundation of China under Grant No. LQ18E030004.
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ABBREVIATIONS P-hda, 1,6-bis(3,4-dihydro-2H-1,3-benzoxazin-3-yl) hexane; oC-hda, 1,6-bis(8-methyl-3,4-dihydro-2H-1,3-benzoxazin-3-yl) hexane; mC-hda, 1,6-bis(7-methyl-3,4-dihydro-2H-1,3-benzoxazin-3-yl) hexane; pC-hda, 1,6-bis(6-methyl-3,4-dihydro-2H1,3-benzoxazin-3-yl) hexane; oC-ha, 8-methyl-3,4-dihydro-2H1,3-benzoxazin-3-yl hexane.
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