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Design of multi-stimuli responsive shape memory biobased PLA/ENR/Fe3O4 TPVs with balanced stiffnesstoughness based on selective distribution of Fe3O4 Jiarong Huang, Liming Cao, Daosheng Yuan, and Yukun Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05025 • Publication Date (Web): 14 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

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ACS Sustainable Chemistry & Engineering

Design of multi-stimuli responsive shape memory biobased PLA/ENR/Fe3O4 TPVs with balanced stiffness-toughness based on selective distribution of Fe3O4 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 Corresponding Author: Yukun Chen [email protected]; [email protected] Abstract: In this work, multi-stimuli responsive shape memory PLA/epoxidized natural rubber (ENR)/ferriferrous oxide (Fe3O4) thermoplastic vulcanizates (TPVs) with balanced stiffnesstoughness were designed via dynamic vulcanization. Regulated by thermodynamic factors and kinetic factors, Fe3O4 was selectively distributed in continuous ENR phase or at PLA/ENR interface, which played a crucial role in reinforcing rubber and interfacial compatibilization. Therefore, excellent multi-stimuli responsive shape memory behavior and significantly improved impact strength were achieved without decreasing its tensile strength. With 30 phr Fe3O4, impact strength reached to 90.08 kJ/m2 (without fracture), which was 31 times of neat PLA. Meanwhile, the TPVs could recover its original shape in merely several seconds along with the shaperecovery ratio of 96.63% in thermal field. Additionally, Fe3O4 also endowed the TPVs with magnetic/light induced shape memory effect in an alternating magnetic field and under the nearinfrared light (808 nm) irradiation, which exhibited great potential in intelligent biomedical areas. Keywords: Thermoplastic vulcanizates; shape memory effect; multi-stimuli response; balanced stiffness-toughness; co-continuous structure; selective distribution Introduction

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Shape memory polymers (SMPs) are stimuli-responsive materials, which can recover their permanent shape from large deformation by exposing to various external stimuli (temperature, light, pH, moisture, electric and magnetic field, etc.).1-6 SMPs show advantages of low response temperature, fast response and wide response strain, which endue them with great potential to be applied in aerospace industries, intelligent biomedical devices, sensors, and so on.7-11 As a biobased polymer, PLA exhibits sufficient latent capacity in fabricating heat-triggered SMPs due to its high strength and intrinsic shape memory effect.12-14 However, the inherent brittleness, single stimulus and poor shape memory effect restrict the wide applications of PLA-based composites seriously.15-17 To achieve precise control of the shape recovery process, various strategies have been developed to improve the shape recovery ability of PLA. A series of PLA-based copolymers 18-21 have been synthesized to obtain shape memory property with high shape fixity ratio (Rf) and shape recovery ratio (Rf, over 90%). For example, Odent et al.

22

synthesized ionic hybrids

consisting of imidazolium-terminated PLA, poly[ε-caprolactone-co-D,L-lactide] (P[CL-co-LA]) and surface-modified silica nanoparticles, which showed Rf of ~100% and Rr of 79%. They ascribed the excellent shape memory behavior to the ionic interactions between the matrix and silica nanoparticles. Compared with the synthesized copolymers, the incorporation of elastomers is a feasible method to prepare PLA-based SMPs.14,17,23-26 Many researchers

24-26

fabricated

PLA/TPU composites with good shape memory behavior by melt blending. Recently, we also 14,17,27

designed PLA/NR (ENR) thermoplastic vulcanizates (TPVs) with co-continuous structure

via dynamic vulcanization, and found that such novel co-continuous structure could offer strong resilience for the shape recovery process. Therefore, the resultant TPVs showed excellent Rf (~100%) and Rr (＞95%) at high deformation. Meanwhile, as a new type of elastomer, the TPVs


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combine the excellent resilience of conventional rubbers and the easy recyclability of thermoplastics, effectively reducing the waste of resources and attenuating environmental pollution caused by the use of traditional non-reusable vulcanized rubber, which can promote sustainable development of resources. 14,17,24-27 Although the incorporation of elastomers could improve the shape memory performance of PLA-based SMPs, a higher content of elastomer is required to achieve better shape memory effect (SME), which will lead to the imbalance between stiffness and toughness.17,28,29 Additionally, the applications of such direct heat-triggered SMPs are restricted because it is difficult to direct heat the SMPs to activate its SME in some temperature sensitive conditions.30 Therefore, much attention has been focused on developing SMPs that can be indirectly heated by different stimuli. Especially, the magnetic-sensitive SMPs 31-35 and light-induced SMPs 36-38 have shown great potential to be applied in some shielded environments and noncontact systems because of the advantages of noncontact and precise control. Zheng et al.

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synthesized

poly(D,L-lactide)/Fe3O4 nanocomposites and observed the remotely actuated SME in an alternating magnetic field (f =20 kHz). When the weight ratio of PDLLA/Fe3O4 was 1:1, the composite exhibited excellent Rr (93%) with recovery time of 35 s. Wei et al.

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also fabricated

ultraviolet (UV)-cured PLA/Fe3O4 nanocomposites via 3D printer, and the composite recovered to its original shape with an alternating magnetic field (f =20 kHz) for 23s. Hua et al.

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constructed photothermal-responsive PLA/multi-walled carbon nanotubes shape changing composites with fused deposition modeling printing technology, and found that composites could change its shape under near-infrared (NIR) irradiation within several seconds. However, a relatively high content of magnetocaloric/photothermal fillers is required to achieve stronger magnetocaloric effect and photothermal effect, which will deteriorate the


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mechanical properties and deteriorate the reliability and durability.31, 33-35 For example, Wei et al. 33

found that the extensibility of the UV-cured PLA/Fe3O4 (80/20 wt%) nanocomposites

decreased 170% compared with the neat PLA. Therefore, how to improve the toughness of PLAbased magnetic/light-induced SMPs with strong magnetic/light sensitivity is a concern should be addressed. As the introduction of elastomers is one of the most effective methods to toughen PLA, it will be very interesting to incorporate both elastomers and magnetocaloric/photothermal media into PLA to obtain super-toughness and excellent multi-stimuli responsive SME simultaneously. Here, we present multi-stimuli responsive shape memory TPVs with balanced stiffnesstoughness via dynamic vulcanization. PLA/ENR/Fe3O4 TPVs with co-continuous morphology were prepared, in which PLA was used as the switching phase and cross-linked continuous ENR could provide strong resilience to achieve shape recovery. The dispersion of Fe3O4 in two phases was regulated by thermodynamic factors and kinetic factors in order to achieve rubber reinforcement and interfacial compatibilization. The PLA/ENR/Fe3O4 TPVs showed excellent shape memory behavior triggered by thermal field, alternating magnetic field and NIR irradiation. Meanwhile, the TPVs showed super-toughness without sacrificing the stiffness. The synergistically toughen mechanism of rigid nanoparticles and rubber was discussed via a series of testing methods. Such multi-stimuli responsive shape memory materials demonstrate great potential in the intelligent biomedical areas. Experimental section Materials. Polylactide (PLA REVODE101, PLLA), MI = 2-10 g/10 min (190 °C, 2.16 kg), ρ = 1.25 g/cm3, average molecular weight (Mw) ≈ 150000 g/mol, was purchased from Zhejiang Haizheng Biomaterials Co., Ltd. Epoxidized natural rubber (ENR, containing 50 mol % of epoxy


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groups) was purchased from Chinese Academy of Tropical Agricultural Sciences. Fe3O4 (ST-O013-1, density = 5.7 g/cm3, specific surface area = 35 m2/g) with particle size no more than 200 nm, was purchased from Shanghai ST-NANO Science Technology Co., Ltd.. Dicumyl peroxide (DCP) was purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Dichloromethane (DCM), analytical reagent, was purchased from Shanghai Richjiont Chemical Reagents Co., Ltd. (China). Irganox 1010, antioxidant, industrial grade, was obtained from the open market. Sample preparation. PLA pellets and Fe3O4 powder were dried at 60 °C for 10 h in a vacuum oven. In this work, PLA/ENR weight ratio was fixed at 70/30 and the loading amount of Fe3O4 was 0, 10, 30, 50 and 70 phr (parts per 100 parts of (PLA + ENR)), the corresponding volume fraction was 0, 2, 5.7, 9.1 and 12.3 vol%, respectively. And the corresponding mass fraction was 0, 9.1, 23.1, 33.3 and 41.2 wt%, respectively. For easy reading, here we used the mass fraction instead of volume fraction. ENR/Fe3O4 compounds were prepared on a two-roll mill first. PLA/ENR/Fe3O4 ternary TPVs were prepared in a Haake Rheocord 90 at 150 °C with a rotor speed of 60 rpm. Firstly, PLA and Irganox 1010 (0.2wt% of (PLA + ENR)) were added into the cave for melting. Then the prepared ENR/ Fe3O4 compound was added and mixed for 3 min. Followed by the addition of DCP (1.5 wt % of ENR) to initiate the crosslinking of ENR for about 4 min. After that, the composites were cooled down to room temperature, and the lumpshaped samples were chopped into small granules. The standard specimens were prepared by injection molding machine (TTI-160F, Welltec Machinery & Equipment Co. Ltd., China) at 170180 °C with injection pressure of 45 MPa. For convenience, the composites are coded according to the PLA/ENR/Fe3O4 ratio. Fox example, P7E3F3 represents PLA/ENR weight ratio was 70/30, and the loading amount of Fe3O4 was 30 phr. For comparison, PLA/Fe3O4 composites were also prepared with the same process.


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Characterizations. The contact angles of ENR and Fe3O4 were measured by a sessile-drop method on an apparatus model OCA 15 PLUS, DATAPHYSICS. The volume of each droplet (water or diiodomethane) was 2 μL. Contact angles measurement was carried out at least 10 times at different positions. The final contact angle was the average value of 10 results. And the contact angles of PLA were cited from our previous work,39 which was measured on the same machine. The localization of Fe3O4 in the PLA/ENR blends were observed on a JEM-100CX II transmission electron microscope (JEOL, Japan) at an accelerating voltage of 100 kV. The samples with about 100 nm in thickness were prepared by thin sections of the composite sheets with an ultramicrotome Leica EMUC6 at -100 °C. Then the samples were soaked in osmium tetroxide with concentration of 1% for 4 h. The phase morphology of cryogenically fractured samples, dichloromethane (DCM)-etched samples and impact fractured samples were observed by using Merlin field emission scanning electron microscopy (FE-SEM, Carl Zeiss). Some cryogenically fractured samples were etched by DCM to remove the PLA phase , then all of the samples were dried under vacuum at 60 °C for 1h and coated with a gold layer. Energy dispersive X-ray spectra (EDS) were measured by INCA 350 spectrometer (Oxford, England) to observe the localization of Fe3O4 in the PLA/ENR blends. The tensile tests of PLA/ENR/Fe3O4 TPVs and PLA/Fe3O4 composites were carried out on a universal testing instrument (Tensile mold, Shimadzu AG-1, 10kN, Japan) with a strain rate of 50 mm/min according to ISO 527-2. The notched Izod impact toughness of PLA/ENR/Fe3O4 TPVs were tested on an impact testing machine (ZWICK5331, Zwick/Roell, Germany) and the sample bars were 2 mm in depth and 45° angle according to ISO 180. All the tests were conducted under ambient conditions.


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The tensile tests of ENR/Fe3O4 composites at room temperature were carried out on a Computerized Tensile Strength Tester (UT-2080, U-CAN Dynatex Inc, Taiwan) under a tensile mode with a strain rate of 500 mm/min according to ISO 37-2005. Thermo mechanical properties of the PLA/ENR/Fe3O4 TPVs were obtained by a dynamic mechanical analyzer (METTLER DMA 1) using a tensile mode in the solid state. The tests were performed from -50-100 °C at a heating rate of 2 °C/min and an oscillation frequency of 1 Hz. Bound rubber content was conducted to evaluate the physical entanglements between Fe3O4 and ENR. In order to eliminate the influence of the inherent gel of ENR, the Neat ENR and ENR/Fe3O4 compounds were masticated on a two-roll mill (equipped with cooling device) with same condition. Then 0.5g (W1) ENR/Fe3O4 compound was packed in filter paper (W2), and then it was immersed in toluene at room temperature for 72 h to remove the movable molecule chains, the residue and filter paper was dried in oven to a constant weight (W3), bound rubber content of samples were calculated by the Equation 1 followed. 40,41 Bound rubber content(wt%) =

W3 - W2 - W1 × F W1 × R

(1)

Where F and R are the mass fraction of Fe3O4 and ENR in ENR/Fe3O4 blend, respectively. The shape memory properties of PLA/ENR/Fe3O4 TPVs were measured by using a TA DMA Q800 instrument. The stress-controlled cyclic thermal mechanical tests contained four stages: (1) The samples of 8.0 mm ×6.0 mm ×1.0 mm (length × width × thickness) were heated to 70 °C and then stretched from its original shape to 50% strain. (2) The deformed samples were then cooled down to 25 °C with the stress kept constant, and the sample’s length was recorded as εF. (3) After the applied force was removed and the sample was held at 25 °C for 10 min to ensure the full shape fixing, then the sample strain was recorded as εU. (4) Lastly, the samples were reheated to 70 °C and maintained for 10 min to recover any possible residual strain.


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The final permanent strain was recorded as εP. Two cycles were performed in the same way for each sample. The heating/cooling rate was 3 °C/min, and the total time used for shape memory cycles was 100 min. The shape fixity ratio (Rf) and shape recovery ratio (Rr) of PLA/ENR/Fe3O4 TPVs in a given cycle N were calculated according to the following Equation 2 and 3: εU(N) ― εP(N ― 1)

Rf = εF(N) ― εP(N ― 1) × 100% εF(N) - εP(N)

(2) (3)

Rr = εF(N) - εP(N - 1) × 100%

The magnetic-induced shape memory properties of PLA/ENR/Fe3O4 TPVs were investigated by fold-deploy shape memory tests

25,28

in an alternating magnetic field with a

frequency of f = 45 kHz and a magnetic field strength of H = 29.7 kAm-1. (SP-15A, Inductive Heating Machine, Shenzhen, China). Rectangular specimens (30 mm×10 mm×1.0 mm) were folded after heating to 70 °C for 10 min, and then cooled down to 25 °C for another 10 min. The folded specimens were placed in the alternating magnetic field to recover their original shape. In order to evaluate the light-induced shape memory effect of PLA/ENR/Fe3O4 TPVs, the “V” shape specimen was exposed to near-infrared (NIR) laser (808 nm, 1W) from 20 cm. The magnetic/light-induced shape recover process of all samples were repeated for three times and recorded by a digital camera to record the final angle (𝜃𝑟) after each shape recovery process. The shape recovery ratio (Rr induced) was calculated using the following Equation 4: Rr

θr

induced

= 180° × 100%

(4)


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Results and discussion The selective distribution of Fe3O4. The distribution of fillers will have significant influence on microstructure, mechanical properties and shape memory behavior of the TPVs. Therefore, the potential distribution of Fe3O4 was predicted from the thermodynamic view firstly. The surface energy of PLA, ENR and Fe3O4 were measured at room temperature (Table S1), the interfacial intension (𝛾𝑎𝑏) between different pairs were calculated based on Equation S1 and the results are shown in Figure 1. The calculated wetting coefficient ωa of Fe3O4 with PLA/ENR system is ωa = 2.47, indicating that Fe3O4 tends to distribute in ENR phase from thermodynamic view.24 Meanwhile, from the kinetics view, Fe3O4 was firstly mixed with ENR via a two-roll mill, the strong physical entanglements between rubber chains and fillers also restricted the migration of fillers. In other words, from both thermodynamics and kinetics view, Fe3O4 tends to distribute in ENR phase.

Figure 1. Interfacial tension between different pairs. To further observe the final location of Fe3O4, TEM was employed (Figure 2a). It can be clearly seen that PLA/ENR/Fe3O4 TPVs exhibit special co-continuous phase morphology, which is similar to PLA/ENR blend in our previous research.17 Considering the relatively low depth of field of the TEM, here TEM was just employed to observe the location of Fe3O4,24,41 The white


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phase is considered as PLA phase, while the gray phase in which most of Fe3O4 localized is considered as ENR phase. And some agglomerates of Fe3O4 (around 200 nm) can be seen from TEM, which is due to its magnetic attractive forces, inherently large surface energies and high dosage.31,33-35 This observation is in accordance with the thermodynamic prediction. However, part of Fe3O4 is observed to be located at PLA/ENR interface in P7E3F3, indicating that Fe3O4 has a tendency to migrate into PLA phase when its content was higher. In order to further study this migration tendency, Energy Dispersive Spectrum (EDS) was conducted to confirm the elements composition. At least 10 micro-domains on the ENR phase were analyzed and the results are shown in Figure S1. It is conspicuously seen that the weight percentage of iron element in P7E3F1 from EDS data is very close to the assumed value, while it is increasingly far from the assumed value with increasing Fe3O4 content. According to the minimization of dissipative energy theory,41,42 the strong shear force and viscosity difference between PLA and ENR during dynamic vulcanization result in the inevitable migration tendency of Fe3O4. In addition, as previously reported, a higher epoxidation level of ENR would increase the concentration of ring-opened byproducts (i.e., -OH, C=O, or cyclic ethers, etc.) in ENR.43,44 Therefore, the interaction between Fe3O4 and the oxygenous groups of ENR could result in the strong entanglements between Fe3O4 and ENR chains. The entanglements will drive some of the ENR chains into the PLA phase to compatibilize the TPVs.


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Figure 2. TEM images of P7E3F1 and P7E3F3 (a), SEM images of PLA/ENR/Fe3O4 TPVs (b). SEM was also employed to investigate the morphology of PLA/ENR/Fe3O4 TPVs and the results are shown in Figure 2b. It is seen that obvious interface of PLA and ENR is observed in P7E3F0, showing the poor compatibility between them. Surprisingly, with the incorporation of Fe3O4, a rather fuzzy PLA/ENR interface is observed, indicating the improved compatibility between them. This is due to the excellent compatibilization effect of Fe3O4 migrated to the interface during processing. Notably, with increasing Fe3O4 content, the temperature difference (△Tg) of Tg of PLA and ENR decrease gradually (Figure 3b and Table S3), which also disclosed the interfacial compatibilization effect of Fe3O4 in PLA/ENR system. When Fe3O4 content increased to 70 phr, it is difficult to distinguish PLA phase and ENR phase and Fe3O4 is uniformly dispersed in the TPVs, which also indicates the inevitably migration of the excess Fe3O4. In the etched image of Figure 2b, the interpenetrating pores are the PLA phase selectively etched by dichloromethane (DCM) and the continuous network is the cross-linked ENR phase. This morphology indicated that the incorporation of Fe3O4 did not change the co-continuous phase structure, which was confirmed to play an important role in improving toughness and shape memory properties of PLA/ENR TPVs.14 When Fe3O4 content was no more than 50 phr, the ENR network was very impeccable with uniform pores, and the thickness of adjacent pores diminished rapidly with increasing Fe3O4 content. However, the continuous rubber phase was partially destructed as observed in P7E3F7, in which the bigger pore diameter was ~15 μm, while the smaller pores showed diameter of ~0.5 μm, and the average thickness of adjacent pores was less than 1 μm. This change is ascribed to that the volume fraction of ENR increases remarkably with the incorporation of Fe3O4, which improved the shear force of ENR during dynamic vulcanization. The integrity of the co-continuous phase structure is also confirmed by


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the swelling experiments (Figure S2). Neat PLA film dissolved completely in DCM within 30 min at ambient temperature, while PLA/ENR/Fe3O4 TPVs only crimped within 30 min without the support of PLA phase, indicating the continuous structure of ENR. Because if the rubber phase is dispersed as particles in plastic matrix, when the plastic is dissolved, the rubber particles would disperse in the solvent rather than just crimp without the support of continuous plastic. However, many small peeling rubber pieces can be observed in P7E3F7, which indicates the partial destruction of P7E3F7.

Figure 3 ． Storage modulus (E′) (a) and loss factor (Tan δ) (b) of PLA/ENR/Fe3O4 TPVs, Stress-strain curves of ENR/Fe3O4 composites (c), bound rubber content of ENR/Fe3O4


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compounds with various Fe3O4 content according to EDS result (d). Attention: E30F5 represents 100 phr ENR and 16.65 phr Fe3O4. In this co-continuous PLA/ENR system, both the morphology and rubber strength play a momentous role in its mechanical properties and shape memory behavior.14,41 Therefore, the reinforcing effect of Fe3O4 on ENR is also studied, as shown in Figure 3c. It should be noted that Fe3O4 content in the ENR/Fe3O4 composites is calculated according to the corresponding content of Fe3O4 in PLA/ENR/Fe3O4 TPVs based on EDS results. It is clearly seen that Fe3O4 shows excellent reinforcing effect for ENR. For example, the tensile strength is 8.65 MPa of E30F22.8 (corresponding to P7E3F3), which is 6 times of neat ENR. The inorganic fillers reinforce the rubber by generating physical entanglement with the rubber.45,46 The investigation of bound rubber content is an effective method to figure out the physical entanglements between rubber and fillers.40,41,46 As shown in Figure 3d, with increasing Fe3O4 content, the bound rubber content of ENR/Fe3O4 compounds firstly increases and then decreases, indicating that there are strong physical entanglements between Fe3O4 particles and ENR chains, while the slight decline is due to the aggregation of excess Fe3O4. With increasing Fe3O4 content, the Tg of ENR turned to higher temperature (Figure 3b, from -18.10 °C to -12.90 °C), which also disclosed that strong physical entanglements between Fe3O4 and ENR. Therefore, the selective distribution of Fe3O4 altered the structural integrity and strength of continuous ENR networks, which would play a momentous role in its mechanical properties and shape memory behavior. Balanced stiffness-toughness. As a key factor to expand the applications of PLA-based SMPs, the toughness and stiffness of the TPVs were investigated. As shown in Figure 4a, the notched impact strength improved significantly with the incorporation of Fe3O4. For example, it increased to 81.19 kJ/m2 with only 10 phr Fe3O4, which was nearly 5 times of the PLA/ENR blend and 30


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times of neat PLA. Notably, each specimen of PLA/ENR/Fe3O4 TPVs was not fractured completely, and the corrected impact strength for completely fractured specimens were calculated approximately though dividing the fractured length by the whole length and the results were shown in Figure 4a. P7E3F3 shows the highest notched impact strength of 90.08 kJ/m2, while its corrected impact strength even reached to 146.29 kJ/m2, which was 8 times of the PLA/ENR blend and 53 times of neat PLA. The super-toughness of PLA/ENR/Fe3O4 TPVs was also confirmed by the great improvement in tensile toughness (Figure S3). In the field of rubber toughened PLA, a higher content elastomer was required to endow PLA with better toughness, which resulted in a sharp decline in tensile strength.

24,27-29,47

However, there was no obvious

decrease in tensile strength (Figure 4b) and Young's modulus (Figure S3), indicating this is an effective method to prepare rubber/plastic blends with balanced stiffness and toughness.48,49 For comparison, PLA/Fe3O4 composites were also prepared in a similar way, and the corresponding mechanical properties are shown in Figure S4. Nearly no improvement in impact toughness was observed in PLA/Fe3O4 composites, while the tensile strength reduced obviously as the content of Fe3O4 increased. The above results indicate that neither ENR nor Fe3O4 can effectively toughen PLA, but the combination of ENR and Fe3O4 achieves a synergistic effect in toughening PLA without sacrificing its stiffness. The mechanism of the stiffness-toughness balance is worth to ascertain. Meanwhile, PLA/ENR/Fe3O4 TPVs exhibit excellent recycling ability with the tensile strength retention rate of 95.61% and impact strength retention rate of 102.83% after third reprocessing (Figure S5), effectively reducing the waste of resources and attenuating environmental pollution caused by the use of traditional non-reusable vulcanized rubber, which can promote sustainable development of resources.


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Figure 4. Notched impact strength (a) and stress-strain curves (b) of PLA/ENR/Fe3O4 TPVs. Toughening mechanism. Digital izod impact test is an effective method to describe the fracture process by recording the process of change in force during fracture.39,41 It can be seen from Figure 5a that the fracture process of PLA/ENR/Fe3O4 TPVs could be divided into two stages: the crack initiation stage and the crack propagation stage. Compared with the rapid decline of P7E3F0 at the crack propagation stage, the curves of the PLA/ENR/Fe3O4 TPVs declined slowly, indicating that much energy was absorbed during the crack propagation stage. The absorbing energy of crack initiation (Wi) and propagation (Wp) and the toughness index (the rate of Wp/Wi) were calculated and shown in Figure 5b. It is clearly seen that Wi showed little increase with the increase of Fe3O4, which was due to the little crack in PLA caused by the migrated Fe3O4. On the contrary, Wp increased significantly with increasing Fe3O4 content, which was due to that the deformation and cavitation of rubber phase initiated the shear yielding of TPVs with some degree of multiple crazing at the crack propagation stage. When the content of Fe3O4 was 30phr, the TPV had the most impeccable network and better enhanced ENR phase, thus it had the highest Wp and toughness index.


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Figure 5. Displacement-force curves of PLA/ENR/Fe3O4 TPVs (a), the absorbing energy and the toughness index of PLA/ENR/Fe3O4 TPVs (b). The Digital izod impact tests indicated that the energy dissipation mainly focused on the crack propagation stage within the ENR phase and the interface. The toughening mechanism was shown in Figure 6a. (1) The compatibilization between PLA and ENR could facilitate the stress transfer from rigid PLA to ductile ENR phase. (2) Compared with the instant cavitation of the dispersed rubber particles, the continuous ENR network could act as a path for the propagation of the plastic deformation,24 leading to the stretch of ENR network and dissipation of energy effectively. (3) The stress transfer within the ENR phase would be terminated when the ENR network was destroyed, thus the enhanced ENR network could effectively delay this process and dissipate more energy. For P7E3F0, although it had co-continuous phase structure, the neat ENR was too weak to sustain the impact stress, resulting in the quick fracture and “pulling out” of the ENR network (Figure S6). Thus P7E3F0 had the lowest impact strength. However, the continuous ENR phase in PLA/ENR/Fe3O4 TPVs was effectively enhanced by Fe3O4, achieving the super-toughness of the TPVs. (4) The structural integrity also played an important role in toughening PLA via bearing stronger external force and transferring force more effectively. Although the reinforcement effect was slightly lower than that of P7E3F7, P7E3F3 had the


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impeccable ENR network to bear stronger impact stress and was able to be stretched to a larger strain along the force direction (Figure 6b). While the imperfect continuous ENR network of P7E3F7 was too thin to sustain the impact force, the ENR phase was destroyed with slight stretch, causing the slight decline of toughness. (5) Since the Fe3O4 had adverse effect for the tensile strength of PLA, the selective distribution of Fe3O4 in ENR phase weakened the adverse effect of Fe3O4 on tensile strength of PLA.

Figure 6. Toughening mechanism (a), SEM images of impact fractured surfaces of P7E3F3 and P7E3F7 (b). Multi-stimuli responsive shape memory effect. The thermo-mechanical analysis was conducted to evaluate the shape memory behavior of PLA/ENR/Fe3O4 TPVs. As shown in Figure 3a, the E′ decreases rapidly when the temperature is above Tg of PLA, which is originated from the movement of the amorphous PLA chains. Such large decrease of E′ at transition


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temperature (Ttrans) provides the capability to act as a shape memory material, thus Ttrans is set at 70 °C, which is about 10 °C above Tg of PLA. Therefore, firstly, the snapshots of the shaperecovery process at 70 °C are presented in Figure 7a (Movie S1). All of the TPVs recovered quickly at the first 5s and then recovery speed decreased in the following time, which is ascribed to that the energy stored in the stretched ENR chains releases fast in the initial stage and then the recovery driving force dwindles subsequently. With increasing Fe3O4 content, the recovery speed and recovery rate of the TPVs are both firstly increased and then decreased, and P7E3F3 shows the highest recovery speed and shape recovery ratio.

Figure 7. Digital photographs of the shape recovery sequence (spiral shapes) of PLA/ENR/Fe3O4 TPVs in hot water at 70 °C (a), Stress-controlled cyclic thermal mechanical testing (b), shape fixity ratio (Rf) and shape recovery ratio (Rr) for PLA/ENR/Fe3O4 TPVs (c). The shape memory performance of PLA/ENR/Fe3O4 TPVs with various Fe3O4 content were further quantitatively measured by DMA at 70 °C and the results are shown in Figure 7b.


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The calculated Rf and Rr are shown in Figure 7c and Table 1. As the continuous PLA phase is in glassy state at 25 °C, most of the elastic retraction of the elongated ENR chains during deformation can be restricted absolutely. It is clearly seen that all the TPVs exhibit a high Rf nearly 100% in two cycles, indicating that the incorporation of Fe3O4 had little effect on the Rf of PLA-based SMPs. With Fe3O4 content increasing, the Rr of PLA/ENR/Fe3O4 TPVs first increased and then decreased, in which P7E3F3 exhibited the highest Rr in two cycles (96.63% and 93.47%), because of the structural integrity and higher mechanical strength of ENR phase, which will be discussed in the next section. In addition, Rr slightly decreased in the second cycle because of the partial destruction of the ENR phase at high deformation. Table 1. The shape fixity ratio (Rf) and shape recovery ratio (Rr) of PLA/ENR/Fe3O4 TPVs under different conditions Thermal Field Sample

Rf (%)

Rr (%)

AMF

NIR

Rr induced (%)

Rr induced (%)

1st

2nd

1st

2nd

P7E3F0

99.52

99.23

91.22

84.76

0

0

P7E3F1

99.54

99.51

93.89

89.73

94.72

93.42

P7E3F3

99.31

99.25

96.63

93.47

97.72

96.05

P7E3F5

99.76

99.58

94.11

90.39

96.11

94.36

P7E3F7

99.53

99.52

90.25

83.91

90.32

91.28

Fe3O4 is ferromagnetics,32,33,35 which could endow the PLA/ENR/Fe3O4 TPVs with excellent soft magnetic properties. As shown in Figure S7, the saturation magnetization (Ms) increased from 16.72 emu/g for P7E3F3 to 25.13 emu/g and 30.13 emu/g for P7E3F5 and P7E3F7, respectively. Meanwhile, the hysteresis loss of Fe3O4 particles

33

in an alternating

magnetic field can generate heat, which could endow the TPVs with magnetic-induced shape memory behavior. Therefore, in order to evaluate the effects of Fe3O4 on the magnetic-induced


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shape memory property, PLA/ENR/Fe3O4 TPVs were put into an alternating magnetic field with frequency of f = 45 kHz and a magnetic field strength of H = 29.7 kAm-1 (Movie S2) and the temperature curves of the TPVs surface are shown in Figure 8a. With the incorporation of Fe3O4, the temperature of the TPVs increased quickly and eventually reached an equilibrium value within several seconds. As shown in Figure 8b, P7E3F0 was unable to change its shape in the alternating magnetic field, while other samples with different contents of Fe3O4 recovered to their permanent shape completely within several seconds. In order to evaluate the effect of Fe3O4 dosage on shape memory effect of TPVs, the beginning time of shape recovery (response time), and the total time for shape recovery (recovery time) were recorded according to the experimental video, while the shape recovery ratios (Rr induced) was calculated with the final angle (𝜃𝑟) after each shape recovery process. As shown in Figure 8c, with Fe3O4 content increasing, the response time and the recovery time decreased rapidly, because the excellent magnetocaloric effect of Fe3O4 can indirectly heat matrix fast to the Tg of PLA to begin and finish the shape recovery. Notably, the recovery time of P7E3F7 increased finally, which was due to its structural integrity and would be discussed in the next section. Meanwhile, all the PLA/ENR/Fe3O4 TPVs exhibited the Rr induced over 90% in the alternating magnetic field (Table 1). For example, the Rr induced

of P7E3F1, P7E3FF3, P7E3F5 and P7E3F7 were 94.72%, 97.72%, 96.11% and 90.32%,

respectively, which was in accordance with the DMA results.


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Figure 8. Temperature change of different samples in the alternating magnetic fields, the inset images are the surface temperature of P7E3F1 at H=29.7 kAm-1 recorded by IR thermography (a), digital photographs of the TPVs with folded shapes in an alternating magnetic field (b), the response time, recovery time and shape recovery ratio for the TPVs in the alternating magnetic field (c), digital photographs of the shape recovery sequence (“V” shape) of P7E3F3 under nearinfrared irradiation (d). Additionally, the photothermal effect of Fe3O4 made it possible to achieve shape memory effect triggered by NIR irradiation (Movie S3). 36-38 Digital photographs in Figure 8d showed the shape recovery behavior of P7E3F3 under NIR irradiation. As the surface of the sample was rapidly heated above the Tg under the NIR irradiation (Figure 8a), the restriction of PLA phase was released and the elongated continuous ENR phase drove the process of shape recovery. As the safe and remote/noncontact stimuli, the alternating magnetic field and NIR irradiation could


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heat the materials locally, achieving remotely actuated shape memory performance of PLA/ENR/Fe3O4 TPVs rapidly. The near-infrared (NIR) light and alternating magnetic field were the remote/noncontact heating manners, which could localized heat the target area to achieve shape recovery, reducing the risk of unexpectedly adverse effects (such as degradation, failure of dimensional stability, etc.) on non-functional areas.34-38 In addition, such localized heating manner could also save considerable amount of energy, which could promote sustainable development of resources.37 Shape memory mechanism. We propose a schematic to make clear the shape memory mechanism of PLA-based TPVs in Figure 9. When the TPVs are heated to 70 °C (>Tg of PLA), the TPVs can be deformed to different shapes easily because PLA is in rubbery state. Then, the amorphous PLA chain segments are frozen at 25 °C. Since the E′ of the PLA phase in frozen state is far beyond the resilience force of the stretched ENR phase, thus the continuous PLA phase can act as a heat-control switch to hold the temporary shape perfectly with Rf of ~100%. Finally, when the TPVs are reheated above Ttrans through thermal/magnetic/light manner, the PLA chain segments are unfrozen and the ENR phases become “unlocked”, thus the continuous ENR phase can provide strong resilience force to drive the TPVs back to its original shape without much hindrance of the PLA phase,12,50,51 resulting in a perfect shape memory performance. Notably, the ENR phase is cross-linked and consecutive in PLA, which provides a strong recovery driving force and is quite different from the conventional heat-triggered shape memory polymers. The incorporation of Fe3O4 particles can act as strong net-points, which promotes the formation of the crosslink network and plays an important role in reinforcing rubber.45,46,52,53 The improved mechanical properties of rubber phase, together with the


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impeccable continuous network can offer stronger resilience for the shape recovery. Therefore, PLA/ENR/Fe3O4 TPVs achieve excellent shape memory property.

Figure 9. 3D illustration of the shape memory mechanism under different conditions It is worth noting that the Rr of P7E3F3 in the thermal field and the alternating magnetic field are both higher. As mentioned above, the integrity of continuous rubber network and mechanical properties of rubber phase have significant influence on the shape memory behavior. P7E3F3 exhibits the impeccable structure of co-continuous phase and relative higher mechanical strength among the TPVs, which were beneficial for shape recovery. In addition, the inherent shape memory effect of PLA also promotes shape recovery of the TPVs (Figure S8). However, as shown in Figure S8, the neat PLA cannot recover to its original shape after 100 s, which is due to the relatively low crystallinity of this kind of PLA,14 Meanwhile, Figure S8 also declares that the inherent shape memory effect of PLA is too weak to achieve the complete recovery of the TPVs (around 15 s, Figure 7a). Meanwhile, the Rr of the TPVs decreased slightly when Fe3O4 exceeded 30 phr because of the partial destruction of co-continuous morphology (Figure 2b and Figure S2) Notably, the recovery time of the TPVs in the alternating magnetic field and under NIR irradiation was lower than in the thermal field (Figure 7a, Figure 8). The heat of the former comes from the Fe3O4 particles inside the TPVs, in which the Fe3O4 particles generate heat


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immediately and transferred to ENR and PLA rapidly (Figure 9). While for thermal field, the heat comes from the environment and could only be transferred from the surface to the inside of the TPVs slowly. Therefore, the TPVs in the alternating magnetic field and under NIR irradiation could be heated over transition temperature and then recover to their permanent shape more quickly. Such multi-stimuli responsive shape memory TPVs with balanced stiffness-toughness and recycling ability exhibit massive potential in biomimetic field and aerospace industries. Conclusion In conclusion, we successfully designed and prepared multi-stimuli responsive shape memory PLA/ENR/Fe3O4 TPVs with balanced stiffness-toughness via dynamic vulcanization. The distribution of Fe3O4 was regulated by thermodynamic factors and kinetic factors to reinforce ENR and achieve interfacial compatibilization. The TPVs showed excellent shape memory properties in thermal field with the Rf close to 100% and Rr beyond 96%. Additionally, the incorporation of Fe3O4 also endowed the TPVs with magnetic/light triggered SME in an alternating magnetic field and under the near-infrared light (808 nm) irradiation, in which the TPVs could recover to their original shape in merely several seconds along with the Rr of 97.72%. When the content of Fe3O4 was 30 phr, the impact strength of the TPV reached to 90.08 kJ/m2 (without fracture), which was 5 times of blank sample and 31 times of neat PLA. Meanwhile the tensile strength of the TPV was 29.62 MPa, which was 97% of blank sample, demonstrating that ENR and Fe3O4 could synergistically toughen PLA without sacrificing its stiffness. Such multi-stimuli responsive shape memory TPVs with balanced stiffness-toughness achieved substantial and critical potential in intelligent biomedical areas. Supporting Information The Supporting Information is available free of charge on the ACS Publications website.


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Contact angle test (Table S1, Table S2), Energy Dispersive Spectrum (Figure S1), Glas transition temperature (Table S3), Swelling experiments (Figure S2), Tensile toughness and Young's modulus of PLA/ENR/Fe3O4 composites (Figure S3), Mechanical properties of PLA/Fe3O4 composites (Figure S4), Recycling ability of PLA/ENR/Fe3O4 TPVs (Figure S5), Morphology of impact fractured surfaces (Figure S6), Soft magnetic properties (Figure S7), Shape recovery of the neat PLA (Figure S8). (PDF) Movie S1. Video of the qualitative shape-memory test in hot water at 70 °C. (AVI) Movie S2. Video of the shape-memory test in an alternating magnetic field. (AVI) Movie S3. Video of the shape-memory test under near-infrared (NIR) irradiation. (AVI) Author Information Corresponding Author *Yukun Chen [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgment This work was supported by the Program of Guangzhou Science Technology and Innovation Commission [Grant No. 201607010103], Program of Guangdong Provincial Department of Science and Technology [Grant No. 2016A010103004], Project funded by China Postdoctoral Science Foundation [Grant No. 2017M622683], the Fundamental Research Funds for the Central Universities and the National Natural Science Foundation of China [Grant No. 21704028].


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For Table of Contents Use Only

The multi-stimuli responsive shape memory PLA/ENR/Fe3O4 TPVs with balanced stiffnesstoughness and recycling ability, which are renewable and sustainable.


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TPVs with B - ACS Publications - American Chemical Society

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