Design of Novel Self-Healing Thermoplastic Vulcanizates Utilizing

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 40996−41002

Design of Novel Self-Healing Thermoplastic Vulcanizates Utilizing Thermal/Magnetic/Light-Triggered Shape Memory Effects Jiarong Huang,‡,§,† Liming Cao,‡,§,† Daosheng Yuan,*,‡,§ and Yukun Chen*,‡,§ ‡

Lab of Advanced Elastomer, South China University of Technology, 381 Wushan Road, Tianhe District, Guangzhou 510640, China School of Mechanical and Automotive Engineering, South China University of Technology, 381 Wushan Road, Tianhe District, Guangzhou, 510640, China

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ACS Appl. Mater. Interfaces 2018.10:40996-41002. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/17/19. For personal use only.

S Supporting Information *

ABSTRACT: We designed novel self-healing thermoplastic vulcanizates (TPVs), achieving excellent thermal/magnetic/light-triggered shape memory assisted self-healing behavior. Damage on polylactide (PLA)/epoxidized natural rubber (ENR)/Fe3O4 TPVs could be healed via three events synergistically: the shape memory effect of TPVs resulted in the physical contact of damaged surfaces; the desorption−absorption of ENR/Fe3O4-bound rubber promoted interdiffusion of ENR chains, leading to the self-healing of ENR phase; ENR was grafted onto PLA segments to assist PLA rearranging and entangling again to achieve the repair of TPVs. This self-healing TPV is reported for the first time and paves the way to design nextgeneration self-healing materials. KEYWORDS: thermoplastic vulcanizates, self-healing, shape memory effect, multistimuli response, PLA/ENR

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assists in closure of cracks and damage, whereas linear poly(εcaprolactone) chains diffuse and rearrange between the cracks to achieve self-healing with the efficiency of 95%. Such selfhealing behavior is widely reported in the field of plastics,17 elastomers,9,18 coatings,19 and hydrogels.20 However, no related research can be found in the field of thermoplastic vulcanizates (TPVs). As a new type of elastomer, TPVs are widely used in industry because of the combination of the recyclability of thermoplastics and the excellent resilience of conventional rubbers.21−24 In view of better sustainability, it is significant to endow TPVs with self-healing property. Recently, we have designed PLA/NR TPVs with cocontinuous structure,23 endowing TPVs with excellent shape memory effect, which may be used to assist the healing process. As the shape memory assisted self-healing polymers need to be heated above its Tm for a long time to achieve shape recovery and chain diffusion, the risk of unexpectedly adverse effect for circumjacent parts and excessive energy consumption will be raised.25−28 Therefore, different external stimuli have been introduced to achieve local/noncontact healing behavior in recent years.25−28 Corten et al.25 synthesized p-methyl methacrylate/n-butyl acrylate/heptadecafluorodecyl methacrylate/γ-Fe2O3 elastomer and found that γ-Fe2O3 on the damaged region could generate thermal energy because of its magnetocaloric effect, strong chain diffusion promoted the

ith the demand for improving the reliability and durability of materials, self-healing materials that can recover from suffered physical damage on themselves autonomously or with the aid of a stimulus have received numerous attention in recent years.1,2 Self-healing materials exhibit sufficient latent capacity in sensors, smart actuators, artificial skin and so on.3−5 Broadly speaking, two main strategies have been used to incorporate self-healing functionality to polymer systems.6−15 First, microencapsulated healing agents were embedded into the matrix to repair the crack surface via in situ polymerization.6,7 Second, dynamic bonds, including reversible covalent bonds (such as Diels− Alder bonds,8,9 schiff base bonds10) and reversible noncovalent bonds (such as hydrogen bonding,11,12 metal−ligand interaction,13 and ionic bonding14,15), exhibit reversible association−dissociation behavior under external stimulus to realize the damage-repair process. However, closure of the crack is the precondition for the self-healing process,8,16 which can only be realized by manually forcing fractured surfaces back and is difficult to achieve in practical applications. As physical damage is generally generated during the deformation process, shape memory effect has been introduced to assist the healing process, and these materials are called shape memory assisted self-healing materials.1,8,9,17 Shape memory polymers with interpenetrating network has been proved to have the ability of assisting the self-healing process.9,17−20 Rodriguez et al.17 fabricated a self-healing blend with interpenetrating network, in which the shape memory effect of cross-linked poly(ε-caprolactone) network © 2018 American Chemical Society

Received: October 18, 2018 Accepted: November 20, 2018 Published: November 20, 2018 40996

DOI: 10.1021/acsami.8b18212 ACS Appl. Mater. Interfaces 2018, 10, 40996−41002

Letter

ACS Applied Materials & Interfaces

Figure 1. (a) DMA curve of P7E3F1, photographs of shape-memory behavior of P7E3F1: (b) folded shapes in thermostat at 70 °C, (c) spiral shapes in an alternating magnetic field (AFM, f = 45 kHz), (d) folded shapes under near-infrared (NIR) light, (e) photographs of P7E3F0 exposed to AFM and NIR light.

healing TPVs for the first time in the field of dynamic vulcanization, and it may expand the research scope of selfhealing materials and improve the reliability and durability of the TPVs. The special cocontinuous structure (Figure S2) could endow PLA/ENR/Fe3O4 TPVs with outstanding shape memory property.23 The shape memory property of P7E3F1 was quantitatively evaluated by stress-controlled dynamic mechanical analysis (DMA) at 70 °C (above Tg of PLA, Figure S3). As shown in Figure 1a, most of the fixed strain was recovered, indicating the excellent shape memory behavior of PLA/ENR/Fe3O4 TPVs with shape fixity ratio (Rf) of ∼100% and shape recovery ratio (Rr) of ∼98% (Table S4). The folded specimen was heated to 70 °C to visually observe its shape memory behavior (Figure 1b) and found that PLA/ENR/ Fe3O4 TPVs could recover to its permanent shape completely within 50 s, which was consistent with the DMA results. As a ferromagnetics material, Fe3O4 could endow PLA/ ENR/Fe3O4 TPV with excellent magnetically sensitive shape

repair of the materials. Yang et al.8 synthesized polyurethane/ polydopamine composites and found that the photothermal effect of polydopamine endowed the composites with rapid light-triggered self-healing properties with the healed strength of 22 MPa. The near-infrared light and alternating magnetic field could locally heat the damaged area to achieve repair without disrupting the “healthy” regions, and save considerable amount of energy, promoting sustainable development of resources. Therefore, to repair the damage generated during the deformation process, we presented a shape memory assisted self-healing PLA/ENR/Fe3O4 TPV via dynamic vulcanization. The novel cocontinuous structure could endow TPVs with excellent shape memory effect. Fe3O4 particles were regulated to distribute in ENR phase to form bound rubber, which could desorb to achieve the self-healing ability. Meanwhile, the excellent photothermal/magnetocaloric effect endowed TPVs with remote/noncontact heating manner. We presented a universal approach to prepare shape memory assisted self40997

DOI: 10.1021/acsami.8b18212 ACS Appl. Mater. Interfaces 2018, 10, 40996−41002

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ACS Applied Materials & Interfaces

Figure 2. (a) Stress−strain curves of original, damaged and healed P7E3F1, (b) the self-healing efficiencies of PLA/ENR/Fe3O4 TPVs, (c) optical micrographs of PLA/ENR/Fe3O4 TPV before healing, healed with shape memory effect (SME) assist and without SME assist, (d) schematic illustration of the damaged process, (e) self-healing efficiencies of PLA/ENR/Fe3O4 TPVs with SME and without SME. Sample compositions are the same as in Table S1.

memory effect because of its magnetocaloric effect in AMF.25 P7E3F1 was put into an AMF (Figure 1c, Movie S1) and was heated to 93 °C within several seconds (Figure S4), the spiral specimen recovered to its initial shape completely within 6 s. Additionally, the photothermal effect of Fe3O4 made it possible to achieve shape memory effect triggered by NIR light.27 As the surface temperature rapidly increased to 86 °C under the irradiation of NIR light, the TPVs could also achieve its shape memory behavior rapidly with higher shape recovery ratio (Movie S2, Table S4). As a comparison, Figure 1e confirmed that P7E3F0 could not be heated by AMF/NIR and no shape recovery was observed. Shape memory polymers with interpenetrating network are able to achieve self-healing performance assisted by its SME.17−20 Moreover, ENR with high epoxidation level also exhibits self-healing ability because of its interdiffusion and self-adhesion.12,29 Therefore, PLA/ENR/Fe3O4 TPVs with cocontinuous structure are also supposed to exhibit shape memory assisted self-healing ability. To achieve perfect shape recovery (Figure S3), the specimens were stretched with 50% strain at 70 °C and then damaged by a homemade device (Figure S1) to ensure the identical depth and width (50% width crack) according to Figure 2d.8 Figure 2a shows the stress−strain curves of P7E3F1 in original state, just damaged, and healed at different conditions. The tensile strength and strain at break of original P7E3F1 were 27.61 MPa and 56.50%

(Figure S5), and then decreased to 20.33 MPa and 4.15% after damage. Surprisingly, P7E3F1 showed decent healing effect at different conditions. After healing at 70 °C for 3 h, its tensile strength and strain at break reached to 25.31 MPa and 39.09%, and the self-healing efficiency was 69.02%. The self-healing efficiencies of PLA/ENR/Fe3O4 TPVs with different plastic/ rubber ratio were calculated according to eq S48,18,26 and are shown in Figure 2b. With increasing rubber content, the selfhealing efficiencies of tensile strength and tensile toughness were both first increased and then decreased. Such variation tendency was determined by SME, interdiffusion of ENR and PLA/ENR interface collectively, which would be discussed in the next section. Notably, it could be seen in Figure 2a that PLA/ENR/Fe3O4 TPVs also showed self-healing ability triggered by AMF and NIR light. Indirectly heated by AMF and NIR light for only 10 min, the tensile strength reached to 24.51 and 23.89 MPa, respectively. Such remote/noncontact stimuli induced self-healing TPVs could be healed with less energy and time and avoid degradation of the “healthy” regions,25−28 which exhibited great potential in intelligent biomedical areas. An obvious crack could be observed on the surface of damaged specimen (Figure 2c). After heating at 70 °C for 3 h, the damaged surfaces of the prestretched specimens were forced to achieve physical contact, resulting in the disappearance of the crack. However, the local cracks could still be 40998

DOI: 10.1021/acsami.8b18212 ACS Appl. Mater. Interfaces 2018, 10, 40996−41002

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ACS Applied Materials & Interfaces

Figure 3. (a) Photographs of the self-healing behavior of E30F10−0.5DCP with different shapes after healing, (b) stress−strain curves of original and healed ENR/Fe3O4 vulcanizates with different DCP content, (c) photographs of ENR/Fe3O4 compounds swelling in toluene for 3d, (d) bound rubber content of ENR/Fe3O4 compounds, (e) apparent cross-link density of PLA/ENR/Fe3O4 TPVs at different temperature, (f) self-healing efficiency of vulcanizates and corresponding TPVs. Sample compositions are the same as in Tables S2 and S3.

results confirmed that SME of PLA/ENR/Fe3O4 TPVs was one of the key factors for its self-healing process.8,9,17 When TPVs were heated above Tg, the shape recovery of the prestretched specimens drove the release of stored strain energy in the damaged zone, resulting in the closure of the crack. However, P9E1F1 and P8E2F1 exhibited relatively weak shape memory effect with shape-recovery ratio of 79.82 and 84.46%, the insufficient surface contact of the damaged region restricted the subsequent healing of the TPVs. Thus, P9E1F1 and P8E2F1 showed inferior self-healing efficiencies, and the self-healing efficiency of the TPVs gradually increased with the

observed obviously in the unstretched specimens, because the stored strain energy in the plastic zone is insufficient to achieve surface contact perfectly,8,19 confirming that physical contact of the damaged surfaces was the precondition for the self-healing process. The prestretched and unstretched specimens were prepared according to Figure 2d to investigate the role of SME in the self-healing performance. It could be clearly seen that the healing efficiency of the prestretched specimens were higher than the unstretched specimens, for example, the healing efficiency of the stretched P7E3F1 was 69.02%, whereas the unstretched P7E3F1 was only 54.27%. These 40999

DOI: 10.1021/acsami.8b18212 ACS Appl. Mater. Interfaces 2018, 10, 40996−41002

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ACS Applied Materials & Interfaces

Figure 4. (a) FTIR of dichloromethane-extracted PLA/ENR/Fe3O4 TPVs, (b) schematic diagram of grafting reaction, (c) schematic mechanism for the self-healing process of PLA/ENR/Fe3O4 TPVs.

was too fragile to keep its shape when taking out of toluene, whereas E30F10 showed a reinforced shape-keeping (Figure 3c) because of the absorption and interlacements between the filler and the matrix,30 leading to the increase in the bound rubber content (Figure 3d). And the decline in bound rubber content at high temperature was due to desorption of loosely bound rubber,24 ensuring the sufficient chain mobility for the diffusion of ENR chains. However, the desorbed ENR chains would absorb onto the Fe3O4 surface and entangle again to form bound rubber again (Figure S10), thus the desorption− absorption of ENR/Fe3O4 bound rubber endow ENR phase with excellent self-healing performance. As shown in Figure 3e, although no DCP was used to cure ENR, E30F10−0DCP still exhibited a certain cross-link density, indicating that Fe3O4 can act as physical cross-link points to improve the cross-link density of ENR.24,30 And desorption of loosely bound rubber could also decrease the cross-link density at high temperature, realizing the interdiffusion and healing of ENR chains. Meanwhile, it could be seen from the comparison of P7E3F0 and P7E3F1 (dotted box in Figure 3e) that partial of DCP could be adsorbed and shielded by Fe3O4, reducing the content of covalent cross-link density.30 The incorporation of Fe3O4 could not only form bound rubber but also decline covalent cross-link density, thus PLA/ENR/Fe3O4 TPVs showed better self-healing efficiency than PLA/ENR TPVs (Figure S6). ENR/Fe3O4 vulcanizates with low cross-link density exhibited strong peel strength (Figure S11), which was ascribed to interdiffusion of ENR chains29 and could result in the excellent self-healing efficiency. Therefore, the healing of ENR continuous phase could achieve the healing of PLA/ENR/Fe3O4 TPVs. Especially when the healing efficiency of E30F10−0.5DCP was 85.56%, the corresponding P7E3F1−0.5DCP achieved a high efficiency of 82.61%, which was very incredible for TPVs. Confusingly, although E30F10−0DCP showed higher selfhealing efficiency than E30F10−0.5DCP, the efficiency of P7E3F1−0DCP was only 74.29% (Figure 3f), which was lower than P7E3F1−0.5DCP. To understand the reason for this

increase of rubber content. Similarly, the shape memory effect and segment mobility were too weak to achieve the healing of neat PLA (Figure S6). Meanwhile, the low self-healing efficiencies of P6E4F1 and P5E5F1 were ascribed to their high cross-link density (Figure S7), which will be discussed in the next section. The healing of continuous ENR phase was surmised as another key factor to achieve self-healing behavior of TPVs. As the cross-link density of rubber played a vital role in its selfhealing effect,29 we chose E30F10 (corresponding to P7E3F1) with different DCP content (0, 0.5, 1, 1.5 phr) to evaluate the self-healing performance of ENR/Fe3O4 vulcanizates. The specimens were cut into two sections and put close to each other to achieve spatial contact, and then heated at 70 °C for 3 h. The healed sample did not fracture at the joint position even under strong twisting, bending or stretching (Figure 3a). With increasing DCP content, the pure ENR showed a declining healing result (Figure S8) due to its high cross-link density.29 However, the incorporation of Fe3O4 endowed ENR with better self-healing performance (Figure 3a, Figure S9). Figure 3b showed that E30F10 with lower DCP content exhibited remarkable healing effect in tensile strength. Higher DCP content could endow vulcanizates with higher tensile strength, but sacrifice its self-healing capacity. When the DCP content increased from 0 phr to 1.5 phr, the healing efficiency of the ENR/Fe3O4 vulcanizates were 90.37, 85.56, 79.57, and 58.53%, respectively. Correspondingly, the healing efficiency of the P7E3F1 with different DCP content were 74.29, 82.61, 73.55, and 69.02%, respectively (Figure 3f). The neat ENR and ENR/Fe 3 O 4 compounds were masticated with same degree on a two-roll mill, and then immersed in toluene to evaluate the effect of Fe3O4 in the selfhealing behavior. As shown in Figure 3c, neat ENR was first swollen and then almost dissolved within 3d. However, E30F10 compound was just swollen after immersing in toluene for 3d, which suggested that Fe3O4 particles can act as strong net-points to absorb ENR chains and form bound rubber to resist the dissolution of ENR.15 Meanwhile, the gel of E50F10 41000

DOI: 10.1021/acsami.8b18212 ACS Appl. Mater. Interfaces 2018, 10, 40996−41002

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ACS Applied Materials & Interfaces contradiction, we conducted FT-IR spectra of the residues from dichloromethane-extracted P7E3F1−0DCP and P7E3F1−0.5DCP and are shown in Figure 4a. The absorption peak at 1750 cm−1 was attributed to the stretching vibration of carbonyl groups of PLA and 870 cm−1 was the characteristic absorption peak of epoxy group, indicating that the PLA chains have been grafted on to the ENR chains induced by DCP (Figure 4b).23 However, the FT-IR spectrum of P7E3F1− 0DCP was the same as ENR, declaring no grafting reaction in this system. Meanwhile, the migrated Fe3O4 could drag some ENR chains into PLA phase, achieving physical interlacement and interlock between PLA and ENR. Such grafting reaction and interlacement could work together to drive the rearrangement of PLA at PLA/ENR interface during the healing of ENR phase. P7E3F1−0.5DCP showed better self-healing efficiency than P7E3F1−0DCP because of the grafting reaction between PLA and ENR, while further increasing the DCP content would increase the cross-link density and hinder the chain mobility. Notably, although P7E3F1−0DCP showed weaker healing efficiency than E30F10−0DCP, the incorporation of PLA could equip the PLA/ENR system with higher mechanical strength than neat ENR, which was significant for healable elastomers.17−20 The mechanism of the self-healing behavior was shown in Figure 4c: (1) when the TPVs were heated over Tg, the deformation caused by the damage would recover to its initial shape driving by its SME, resulting in the physical contact of damaged surfaces. (2) High temperature promoted interdiffusion of ENR chains, leading to the self-adhesion of ENR phase. And then the desorption-absorption effect of the bound rubber promoted the self-healing of ENR phase. (3) PLA chains had been grafted onto the ENR chains induced by DCP and entangled with ENR chains driving by the migrated Fe3O4, thus PLA segments at the joint surfaces could rearrange and entangle again with the help of ENR chains, ultimately resulting in complete entanglement of the chains at the interface and healing the damaged region. To conclude, this communication designed a novel shape memory assisted self-healing TPVs. The selective distribution of Fe3O4 in cocontinuous structure endowed TPVs with excellent thermal/magnetic/light-induced self-healing behavior. We proposed a synergistic self-healing mechanism: the shape recovery of TPVs resulted in the physical contact of damaged surfaces. The desorption−absorption of ENR/Fe3O4 bound rubber promoted the self-healing of ENR phase, while the grafting reaction between PLA and ENR could assist PLA segments to rearrange and entangle again to achieve the selfhealing of TPVs. This multistimuli responsive shape memory assisted self-healing TPV was reported for the first time in the field of dynamic vulcanization and may find applications in intelligent medical devices.

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PLA and PLA/ENR TPV, apparent cross-link density, self-healing behavior of ENR and E30F10−1.5 DCP, bound rubber content, force−displacement curves (PDF) Movie S1, video of the shape-memory test in an alternating magnetic field (AVI) Movie S2, video of the shape-memory test under nearinfrared (NIR) irradiation (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.C.). *E-mail: [email protected] (D.Y.). ORCID

Yukun Chen: 0000-0001-5523-3942 Author Contributions †

J.H. and L.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Program of Guangzhou Science Technology and Innovation Commission [Grant 201607010103], Program of Guangdong Provincial Department of Science and Technology [Grant 2016A010103004], Project funded by China Postdoctoral Science Foundation [Grant 2017M622683], the Fundamental Research Funds for the Central Universities, and the National Natural Science Foundation of China [Grant 21704028].

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.8b18212. Experimental details, sample compositions, homemade device for making damage, morphology of the PLA/ ENR/Fe3O4 TPVs, DMA analysis, shape memory properties, surface temperature, stress−strain curves of PLA/ENR/Fe3O4 TPVs, self-healing efficiency of neat 41001

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DOI: 10.1021/acsami.8b18212 ACS Appl. Mater. Interfaces 2018, 10, 40996−41002