High performance thermoplastic elastomers with biomass lignin as

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High performance thermoplastic elastomers with biomass lignin as plastic phase Jinhao Huang, Weifeng Liu, and Xueqing Qiu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04936 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 17, 2019

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ACS Sustainable Chemistry & Engineering

High Performance Thermoplastic Elastomers with Biomass Lignin as Plastic Phase Jinhao Huang, 1,† Weifeng Liu, 1,†, * and Xueqing Qiu †, ‡, * †School of Chemistry and Chemical Engineering, South China University of Technology, Wushan Road 381, Guangzhou, Guangdong, 510640, China ‡State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Wushan Road 381, Guangzhou, Guangdong, 510640, China 1

These authors contributed equally.

*Corresponding Authors E-mail: [email protected] & [email protected]

ABSTRACT: For the first time, the coordination-based energy sacrificial bonds have been constructed in the interface between lignin and polyolefin elastomer for preparing a new class of high performance thermoplastic elastomers (TPEs) with biomass lignin as the hard plastic phase. The coordination bonds not only promoted the dispersion of lignin, but also improved the interfacial interactions between lignin and polyolefin elastomer matrix, and also facilitated the orientation of chain segments during stretching. The synergistic coordination effect of lignin promoted higher energy dissipation, leading to simultaneously enhanced strength and toughness of lignin-based TPEs even with the lignin loading as high as 30 wt%. The lignin-based TPEs also exhibited excellent shape memory performance. Our strategy offers a promising methodology for the facile production of high performance but cost-effective TPE materials using bio-renewable resources as plastic phase.

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KEYWORDS: thermoplastic elastomer, lignin, polyolefin, coordination bonds

INTRODUCTION Thermoplastic elastomer (TPE) is a type of sustainable polymeric materials that have been widely used in many applications as a substitution of traditional rubbers due to their excellent recyclability and energy-saving processability1. TPE typically consist of hard plastic domains of less than 200 nm homogeneously dispersed in the soft rubber matrix, forming thermal reversible physical crosslink network2. The plastic domains are primarily made of hard segments with crystallization or high glass transition temperature, such as the well-known olefin multi-block copolymer (OBC)3 and styrene-butadiene-styrene triblock copolymer (SBS)4. Most commercial TPE are highly dependent on petroleum-derived raw chemicals and the synthesis technologies are sophisticated. High performance but cost-effective TPE made of bio-renewable resources are intensely demanded. Lignin, an important component of woody plants, is the second largest biomass resource in nature. Up to fifty million tons of technical lignin is produced annually as a major industrial by-product of pulping and biorefinery plants5. Nonetheless, the vast majority of lignin is combusted for cheap fuel, causing serious waste of resources. The biomass industries are vitally interested in exploring high value-added utilizations for lignin. Being a type of non-toxic aromatic biopolymer with relatively high glass transition temperature (114~150 °C)6 and excellent anti-UV radiation performance7, lignin is found useful in many potential applications such as storage energy devices8,


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9,

sunscreen cream10, carbon fibers11, bio-derived synthetic polymers and composites5,

12, 13, 14, 15,

hybrid fillers with carbon black in rubbers etc16, 17. As lignin is a cheap,

readily available and environmentally friendly biopolymer, the polymer scientists are particularly interested in applying lignin to thermoplastics and thermoplastic elastomer composites for the substitution of conventional polystyrene or polyolefin plastic phase. However, since the first attempt of Naskar et al.18 who made renewable thermoplastics by introducing lignin into nitrile rubber (NBR) without chemical crosslinking, there are few reports on the novel lignin-based TPE. Although lignin/NBR composite thermoplastics were obtained with good tensile strength, the anti-aging and weatherability of the composites suffered from the unsaturated double bonds in NBR chains. They also derived multiphase lignin/NBR composite elastomers and thermoplastics via reactive mixing with carbon black as a reinforcing agent, dicumyl peroxide and boric acid as cross-linkers, and polyethylene oxide as the compatilizer19. In this paper, we present a new class of lignin-based TPE using saturated polyolefin elastomer (POE) as the rubber matrix. POE is a type of non-polar ethylene/α-olefin random copolymer with good elasticity and good thermostability but low melting temperature and weak mechanical strength15,

16.

As lignin has high Tg and good

stiffness, ideally we can combine the advantages of lignin and POE to make good TPEs. Unfortunately, this idea is constrained by huge challenges attributing to the weak interfacial interactions between lignin and non-polar polyolefin and easy



aggregation of lignin in polyolefin matrix22. Many efforts have been devoted to conquer these obstacles, such as the chemical modification on lignin by esterification23, alkylation24, 25, silylation26, and directly blending by solid state shear pulverization27 etc. Although these methods are capable of improving the performance of lignin/polyolefin composites in limited degree at low lignin loadings (< 10 wt%), there are still drawbacks such as uncompetitive cost and particularly their inability to effectively improve the compatibility and dispersibility of lignin in polyolefins. Recently, the energy sacrificial bonds have been demonstrated as a key mechanism in determining the outstanding properties of some strong and tough biomaterials such as the spider silk and mussel byssus etc28, 29. The energy sacrificial bonds commonly exist in the hierarchical structures of biomaterials, generally including the hydrogen bonds, ionic bonds and metal coordination interactions. The sacrificial bonds in biomaterials can dissipate energy on molecular scale through dynamic rupture and re-form upon stretching, endowing protein materials with super strong and tough properties and even self-healing capacity30. When the energy sacrificial mechanism is adopted in hydrogels and rubber elastomers, it can not only dissipate energy, but also eliminate the stress concentration and promote the orientation of chain segments, thus significantly improving the strength and toughness of materials31-36. As an example, very recently, Filippidi et al.37 reported that by introducing reversible Fe3+-catechol units into a lightly crosslinked elastic epoxy network, the elastic modulus and tensile strength of elastomer was enhanced by 770 times and 58 times, respectively, along with a 92-fold increase in the fracture toughness in comparison with its iron-free


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precursor. Until now, energy sacrificial mechanism has achieved success in reinforcing the chemically crosslinked rubber elastomers. However, it has not been applied in environmentally friendly thermoplastic elastomers. Inspired by the energy sacrificial mechanism from biomaterials, we introduced Zn-based coordination bonds into the interface between lignin nanoparticles and elastomer matrix for preparing a novel type of high performance lignin-based TPEs. The interfacial interaction between lignin and non-polar polyolefin elastomer was significantly enhanced by the coordination bonds, which promoted the dispersion of lignin in the polyolefin matrix. Importantly, we demonstrated the synergistic coordination effect of lignin endowed the lignin-based TPE with simultaneously enhanced strength and toughness. EXPERIMENTAL SECTION Materials. Enzymatic hydrolysis lignin (EHL) (purity ~85%) was provided by Shandong Longli Bio-Technology (China) and used without further purification or chemical modification. The weight average molecular weight of EHL is 5000 Da with the PDI of 4.0. The phenolic hydroxyl group content was about 1.55 mmol·mg−1. Maleic anhydride grafted POE (POE-MA 9702) with the grafting ratio of 1.0 mol% was supplied by Kingfa Sci. & Tech. Co., Ltd. 3-Amino-1H-1,2,4-triazole (ATA, 96 wt%) was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. ZnCl2 (AR grade) was purchased from Damao Chemical Reagent Factory (Tianjin). All chemicals and materials were used as received. Sample preparation. EHL and POE-MA were dried in vacuum oven at 50 °C for 24



hours prior to use. POE-MA and ATA were first mixed in an internal mixer at 50 rpm and 100 °C for 15 min to incorporate ATA onto the main chain of POE-MA. Then pre-weighted EHL and ZnCl2 were added and blended for another 15 min to prepare lignin/POE composites. The samples were finally pressed into flat sheets with 1 mm thickness at 200 °C. The prepared samples were named as PMxLyAmZn, with the numbers x, y, m and n representing the corresponding loadings of POE-MA, lignin, ATA and ZnCl2 with a unit of g, respectively. PM is an abbreviation of POE-MA. Characterizations. Fourier transform infrared (FTIR) spectra were collected on a Bruker Vertex 70 FTIR spectrometer (Germany) using attenuated total reflection (ATR) mode in order to substantiate the interactions between POE-MA and ATA, the Zn2+ and triazole units. The tensile stress-strain curves were determined on CMT 4204 electronic universal testing machine (MTS Systems Co. Ltd, China) according to the standard GB/T1040-2006 under a stretching speed of 200 mm/min at room temperature. The fracture energy (W) is defined as the integral area of the stress-strain curves. The stress-relaxation testing was also performed on CMT 4204. The samples were elongated to 100% strain at the speed of 100 mm/min. The variation of stress was recorded for 3600 s under the constant strain at 25 °C. AFM force measurements were performed by a Park XE-100 atomic force microscope (Korea) in deionized water at 25 °C. The instrument parameters such as the spring constant and deflection sensitivity of each cantilever were all determined before measurements according to our previous reported methods38. Silicon nitride


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(Si3N4) probes were coated with EHL following the procedures described in our previous work38. The limited force was set 20.0 nN, the interaction time was fixed at 500 ms and the force distance was set 1.0 μm. To ensure the reliability of data, each measurement was repeated 150 times using three different tips. The scanning electron microscopy (SEM) was conducted on a Hitachi UHR FE-SEM SU8220 instrument (Japan) to study the fracture surface morphology. The samples were sputtered with a thin gold film. The Shore A hardness was measured by a LX-A durometer (Kunshan Creator Testing Instrument Co., Ltd., China) according to GB/T531-1999. The loading-unloading hysteresis tensile tests were performed on CMT 4204 with a loading and unloading speed of 100 mm/min. To obtain the hysteresis curves at the fixed strain of 300%, the samples were elongated to 300% strain and then stayed for a respective period of time at 25 °C (0, 60, 120, 300, 600, 1800, 3600 and 7200s) followed by subsequent tensile cycles. After that the samples were warmed for 3 min at 60 °C for quick recovery and cooled to room temperature, and followed by the last hysteresis cycle. The dissipation energy of the hysteresis cycle was calculated as the area integral of the loading-unloading loop, W1 is the dissipation energy of the first hysteresis loop and W2 is for the second loop. The hysteresis difference ΔW is defined as W1 - W2. For the hysteresis curves at various strains, the samples were elongated to the targeted strain from 25% to 650%, unloaded to stress zero and followed by subsequent tensile cycle. The hysteresis ratio was calculated as the value of hysteresis loss divided by the stretching energy applied in each cycle.



X-ray diffraction (XRD) analysis was carried out by a Bruker D8 Advance X-ray diffractometer using Cu radiation with Kα wavelength of 0.1541 nm. The samples with strain of 0% and 400% were scanned at room temperature with a scanning increment of 0.02º. Small-angle X-ray scattering (SAXS) experiments were conducted by a Xenocs Xeuss 2.0 instrument (France). The X-ray source was Excillum MetalJet-D2 with the wavelength of 1.34 Å and the detector was Dectris Pilatus 3R1M with the sample-to-detector distance of 2521.81 mm. The exposure time was set 300 s for each scan. RESULTS AND DISCUSSION Preparation of lignin-based TPE Ethylene/1-octene random copolymer elastomer POE was chose as the rubber phase due to its saturated chain structure which endows good weatherability. However, as the non-polar POE is incompatible with the polar lignin particles, directly blending POE with lignin resulted in no reinforcement effect (Figure S2). Our method for preparing high-performance lignin-based TPE composites originates from the energy sacrificial mechanism found in biological materials. To construct energy sacrificial bonds in the lignin/POE composite system, two critical issues need to be solved: (a) modification of POE with functional groups to provide ligands for ion complexation, (b) design of interfacial bonds between lignin and elastomer matrix to form strong interactions. For easier functionalization, the maleic anhydride modified POE (POE-MA) with a grafting ratio of 1.0 mol% was chose as the rubber matrix. We developed a two-step compounding process in one internal mixer to incorporate


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Zn-based coordination bonds into the lignin-based TPE system, as shown in Figure 1. POE-MA was first reacted with 3-amino-1,2,4-triazole (ATA) to produce the ATA-modified POE-MA with functional amide triazole-carboxylic acid groups. Lignin was then added together with ZnCl2 for further blending. Zn-based coordination bonds could form between the triazole groups grafted on the POE backbones and the phenoxy and carboxylate groups in lignin molecules (confirmed by FTIR spectra in Figure S3). The amide triazole-carboxylic acid groups were also able to form hydrogen bonding interactions with the polar groups in lignin.

Figure 1. Illustration for the preparation of ATA-modified POE-MA and lignin-based TPE with coordination sacrificial bonds.



Figure 2. (a) Uniaxial tensile curves of lignin-based TPE in comparison with PM matrix. (b) SEM images of the fracture surface of lignin-based TPE composites after tensile break with the scale bar of 2 μm, sample 1 represents for PM32L8, 2 for PM32L8A1Z1. (c) Comparison of interaction forces between lignin and PM matrix with and without sacrificial bonding agents. (d) Stress relaxation curves of PM and its blends under the strain of 100%.

The typical mechanical performances of the prepared lignin-based TPEs are shown in Figure 2a. Compared with the pristine PM sample, directly blending with 20 wt% of lignin (PM32L8) resulted in a slight increase in the stress, attributing to the hydrogen bonding interactions between the maleic anhydride groups in PM and the polar groups in lignin. Nevertheless, the sample PM32L8 failed at smaller tensile strength, due to the serious aggregation of lignin particles in PM matrix (Figure 2b).


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After introducing ATA and Zn2+ into the lignin/PM composite system, the toughness was significantly increased by nearly 100% from 34.6 J/cm3 for PM to 68.5 J/cm3 for PM32L8A1Z1. The stress at break of PM32L8A1Z1 was also increased by 35% compared with neat PM. For comparison, neat PM was first grafted with ATA and then blended with ZnCl2 in the absence of lignin to produce the sample PM32A1Z1. No significant increment in the tensile strength and toughness was observed for PM32A1Z1, suggesting that lignin was essential for the strength and toughness improvement. This is because lignin is rich in oxygen-containing polar groups, such as the phenolic hydroxyl groups, aliphatic hydroxyl groups, carbonyl groups etc., which could function as natural ligands for the coordination complexation. Therefore, lignin played a synergistic coordination enhancement effect in the lignin-containing sample PM32L8A1Z1, forming stronger coordination dynamic network than that without lignin (PM32A1Z1). Both the strength and toughness of PM32L8A1Z1 were dramatically increased in relative to the PM matrix, PM32L8 and PM32A1Z1. Before this work, lignin was usually reported to deteriorate the toughness of polyolefin materials when the loading content was above 10 wt%27. Our work is the first demonstration that the strength and toughness of lignin/polyolefin composites can be enhanced simultaneously. The as-designed lignin-based TPE is strong, tough, and ductile, benefiting from the significant synergistic coordination enhancement effect of lignin. SEM images of the fracture surface clearly provide visual evidence for the dispersion of lignin particles in the PM matrix, as shown in Figure 2b. For the directly



blending product PM32L8, serious particle agglomeration was observed with an average particle size larger than 2000 nm, the interface between the agglomerated lignin particles and polymer matrix was obviously exfoliated. After introducing the coordination bonds, a uniformly granular phase-separated structure was observed for PM32L8A1Z1 where the lignin particle size was reduced to 200~400 nm, and there was no obvious interface exfoliation between lignin particles and polymer matrix, suggesting that the coordination bonds could not only promote the dispersion of lignin, but also improve the interfacial interactions between lignin and PM matrix. To further quantitatively measure the interfacial interactions between lignin and PM, the interfacial force measurements were conducted by the atomic force microscopy (AFM)38. As revealed in Figure 2c, the average interaction force between lignin and PM matrix was increased by more than 86% after introducing Zn-based bonds, from 228±113 mN/m for PM32L8 to 425±105 mN/m for PM32L8A1Z1. The AFM force measurements clearly demonstrated that the coordination bonds significantly improved the interfacial interactions between lignin and polymer matrix, which promoted the stress transfer from matrix to lignin particles, leading to better shearing on lignin particles during mixing, and thus resulting in smaller particle size and better dispersion of lignin in polymer matrix. Stress relaxation analysis was employed to further illustrate the Zn-based interactions between lignin and PM, as shown in Figure 2d. Compared with the PM matrix, PM32L8A1Z1 and PM32L8 released the force faster, because the polymer chains could slip from the surface of lignin particles under stretching, leading to the


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faster stress relaxation. PM32L8 released the force fastest, because of the weak interfacial interaction between lignin and PM. Introducing the Zn-based coordination bonds between lignin and PM led to a slower relaxation process of PM32L8A1Z1 than PM32L8, indicating the stronger restriction effect of coordination bonds on the relaxation of PM chains. Effects of sacrificial bonds on the mechanical performance Typical tensile stress-strain curves of lignin-based TPE with various contents of ATA and ZnCl2 are shown in Figure 3. The increased strength and toughness of PM32L8A0Z1 compared with PM32L8 indicated that the Zn-based bonds could form between lignin and PM even in the absence of the modifier ATA (Figure 3a). After introducing a small dosage of ATA (0.5 g or 1.2 wt%), a further significant increase in the strength and toughness was achieved for PM32L8A0.5Z1, indicating that the triazole groups introduced by ATA promoted the complexation of Zn2+ and formed stronger coordination sacrificial network39. Interestingly, further increasing the content of ATA from 0.5 to 2 g led only slight improvements in the tensile performance (Figure 3a and Figure 3c).



Figure 3. Stress-strain curves of lignin-based TPE composites with various ATA contents (a) and Zn2+ contents (b). Fracture energy of lignin-based TPE composites with various ATA contents (c) and Zn2+ contents (d).

When ATA was added without ZnCl2, the tensile strength and ductility of PM32L8A1Z0 increased obviously in relative to PM32L8, as shown in Figure 3b. This confirmed that the amide triazole-carboxylic acid groups, the reaction products of maleic anhydride in PM and ATA, could generate stronger hydrogen bonding interactions with the polar groups in lignin than that between PM and lignin. Similarly, addition of a small weight ratio of ZnCl2 (0.5 g or 1.2 wt% for PM32L8A1Z0.5) into the TPE led to a significant increase in the strength and toughness, but further increasing the ZnCl2 content from 0.5 to 2 g did not improve the tensile performance obviously (Figure 3b and Figure 3d). The results shown in


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Figure 3 implied that, to construct the coordination sacrificial network, a minimum dosage of ATA and zinc ion (1.2 wt% in this work) was requisite, once the coordination dynamic network was established in the composite system, the tensile strength and strain at failure were not sensitive to the loading amount of ATA and zinc ion studied in this work. As lignin provided ligand groups for the coordination bonds and was essential for the strength improvement, the effect of lignin content on the mechanical performance of lignin-based TPE was then examined. As shown in Figure 4a and Table S1, the elastic modulus of the lignin-based TPE increased with the lignin loading, on the contrary, the tensile strength and elongation at break decreased, due to the agglomeration of lignin particles at high lignin contents (Figure 4b). The average lignin particle size increased from less than 200 nm without obvious agglomerates for PM36L4A1Z1 to 400-600 nm for PM24L16A1Z1. Nevertheless, if there were no Zn-based coordination bonds, lignin agglomerates larger than 2000 nm would form in the composite (see Figure 2b). The TPE containing 10 wt% of lignin (PM36L4A1Z1) achieved the maximum tensile strength of 28 MPa as well as the same high ductility as PM, along with the fracture energy reaching 72.3 J/cm3, which was 2.1 times of the PM matrix. As the lignin content increased to 30 wt%, the elongation at break for PM28L12A1Z1 still exceeded 500%, and the tensile strength was still higher than the PM matrix, leading to the fracture energy much higher than the PM matrix and PM32L8 without coordination bonds (Table S1). Even when the lignin content reached 40 wt%, the composite PM24L16A1Z1 still exhibited comparable tensile



strength to the PM matrix, and the elastic modulus (5% strain) and the stress at 200% strain of the former were about 1.0 and 3.0 times higher, respectively. Although the lignin-polyolefin incompatibility remains an issue at high lignin loading, through the construction of interfacial interactions (including coordination bonds and hydrogen bonds), the mechanical performance of lignin/polyolefin composites can be significantly improved in comparison with those without good interfacial interactions. The mechanical performance of the prepared lignin-based TPE was comparable to most of commercial TPE products, such as the SBS, OBC, TPU, polyolefin blends-based TPO and TPV. Different from the commercial TPE products in which the plastic phase is built from petroleum-derived hard blocks, our lignin-based TPE composites adopt the natural biomass lignin as the plastic phase.

Figure 4. (a) Tensile curves of lignin-based TPE composites with various lignin contents. (b) SEM images taken from the cross-sections of composites after tensile break with the scale bar of 2 μm. Sample 1 represents for PM36L4A1Z1, 2 for PM32L8A1Z1, 3 for PM28L12A1Z1, and 4 for PM24L16A1Z1.

Energy dissipation of the lignin-based TPE To study the viscoelastic energy dissipation of the lignin-based TPE, hysteresis


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tensile tests were first conducted at a fixed strain of 300%. Evident hysteresis behavior was observed for all composites (Figure S6). Taking sample PM32L8A1Z1 as an example (Figure 5a), a significantly greater hysteresis loss than PM matrix (Figure S6a) was observed in the first tensile loop. The sample did not recover to its original length and left a notable residual strain of 70% after the first tensile cycle. Subsequent tensile cycles after relaxing for various times at room temperature gave similar hysteresis loss. The residual strain totally recovered after heating at 60 °C for 3 min. During heating, most of the temporarily re-constructed hydrogen bonds and coordination bonds broke, the elastic physical crosslinking network contracted to its high entropy state, contributing to the recovery of residual strain. Upon cooling, the coordination bonds and hydrogen interactions healed to their original configurations, leading to a mostly recovered stress-strain profile and hysteresis behavior (Figure 5a). The dependences of the hysteresis ratio W2/W1 and the hysteresis difference ΔW on the ZnCl2/ATA mass ratios are presented in Figure 5c, where W1 is the dissipation energy of the first hysteresis loop, and W2 is the dissipation energy of the second hysteresis loop. With the increasing ZnCl2 content, the value of W2/W1 decreased slightly, suggesting higher dissipation energy at the increased amount of coordination bonds. The value of ΔW first increased then decreased (Figure 5c), with the maximum ΔW value achieved at the ZnCl2/ATA mass ratio of 1.0 (sample PM32L8A1Z1), which was an approximately 1.3-fold increase in comparison with that of PM32L8, intuitively revealing higher energy dissipation induced by the dynamic fracture of Zn-based coordination bonds. As shown in Figure 5d, higher lignin content resulted in



larger energy dissipation due to the breakage of more interfacial interactions between lignin and elastomer matrix, confirming the synergistic coordination effect of lignin.


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Figure 5. (a) Hysteresis curves of PM32L8A1Z1 at the fixed strain of 300%. (b) Hysteresis curves of sample PM32L8A1Z1under different strains. (c) Variation of W2/W1 and ΔW for PM32L8A1Z-x with ZnCl2/ATA mass ratios. (d) Variation of W2/W1 and ΔW for PMxLyA1Z1 with lignin loadings. (e) Hysteresis loss of each cycle at different strains. (f) Hysteresis ratio of each cycle at different strains. The hysteresis ratio was calculated as the value of hysteresis loss divided by the stretching energy applied in each cycle.

The hysteresis tensile tests under various strains were also performed to distinguish the toughness differences. The hysteresis curves, hysteresis losses and hysteresis ratios at various strains from 25% to 650% are presented in Figure 5. The hysteresis curves of other samples can be found in Figure S7. As shown in Figure 5b, it is evident that the lignin-based TPE with Zn-based bonds showed higher hysteresis loss than that without coordination bonds. With an increasing applied strain, the residual strain increased gradually, indicating changes of microstructure during stretching. The



microstructure changes may include breakage of interactions between the elastomer and lignin, rupture of the dynamic sacrificial network, disentanglement of elastomer chains and slipping of elastomer chains along the lignin surface40. Figure 5e visually compares the differences of hysteresis loss. The differences were more obvious especially when the strain was larger than 250%. By comparing with the samples PM32L8, PM32L8A1 and PM32L8Z1, the hysteresis loss of PM32L8A1Z1 was the highest (Figure 5e left), further verifying that adding ATA together with ZnCl2 could construct stronger dynamic network and could increase the interfacial interactions. Increasing the lignin content led to higher hysteresis loss (Figure 5e right), consistent with the results revealed in Figure 5e, further verifying the synergistic coordination effect of lignin. Interestingly, a closer observation reveals that the hysteresis loss of all composites increased linearly at the strains larger than 250% (Figure 5e). The hysteresis ratio, calculated as the value of hysteresis loss divided by the stretching energy applied in each cycle, increased with the strain when the applied strain was less than 250%, but at strains larger than 250%, the hysteresis ratio held nearly constant, as shown in Figure 5f. This implied that after the adjustment in the early stage deformation, the chain conformation and the phase microstructure inside the composites were stably oriented, thus exhibiting a constant ratio of energy dissipation to energy storage at large strains. This characteristic coincided with the elastomeric performance demonstrated by the hysteresis curves in Figure 5a. Deformation mechanism of the lignin-based TPE


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The effect of coordination bonds on the chain alignment was first studied by the Wide-angle X-ray diffractograms (XRD). As depicted in Figure 6a, PM and its lignin blends all exhibited broad amorphous patterns without obvious highly-ordered crystalline phase at 0% strain. The weak reflections of fringed micellar crystals in the PM matrix were overlapped in the amorphous pattern. However, when stretched to 400% strain, two sharp diffraction peaks at 2θ of 20.5° and 22.8° were observed in PM32L8A1Z1, which were assigned to the (110) and (200) lattice planes of the polyethylene orthorhombic crystal cell41, respectively. But in the samples of PM and PM32L8 without coordination bonds, no obvious crystalline reflections were observed even under 400% strain. The crystalline reflections were not found in the sample PM32L8A1 either (Figure S8), verifying that the weak hydrogen bonding did not induce the chain orientation during stretching. These results indicated that the coordination bonds in the lignin-based TPE composites facilitated the orientation of chain segments during stretching, promoted the strain-induced-crystallization, thus causing self-reinforcing42, 43. To further explore the deformation mechanism of the lignin-based TPE, small-angle X-ray scattering (SAXS) analysis were implemented. The scattering patterns shown in Figure S9 provided evident difference in the deformation of lignin-based TPE in comparison with the PM matrix. Before tension, no scattering circle was observed for PM (Figure S9A1), whereas it was determined in the lignin-blended sample PM32L8 (Figure S9B1), and the scattering circle became larger after introducing the coordination bonds (Figure S9C1), suggesting more



nanoscale phases in PM32L8A1Z1. With the elongation ratio increased, a weak shuttle-like scattering pattern was observed for PM (Figure S9A3). Interestingly, the scattering circles for PM32L8 and PM32L8A1Z1 turned rhombus as the strain increased (Figure S9B2 & S9B3, Figure S9C2 & S9C3), implying different nanoscale deformation process from the PM matrix.

Figure 6. (a) XRD spectra of PM and its blends at 0% and 400% strains, respectively. (b) Normalized 1D correlation function curves for PM32L8A1Z1 at 600% strain. (c) PM32L8 at various strains, and (d) PM32L8A1Z1at various strains.


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Figure 7. Schematic illustration of the deformation mechanism of lignin-based TPE.

The SAXS profiles under various strains are provided in the supporting information in Figure S10. The lignin-based TPE composites exhibited fast decay in the scattering intensity, indicating that the size of lignin domain in the matrix were out of the detection range (1-160 nm) of our SAXS instrument, consistent with the SEM results shown in Figure 2b. The Guinier radius of the inhomogeneity (Rg) fitted from Guinier’s law when q→0 gives an evident judge for the spatial extension of particles44. For both PM32L8 and PM32L8A1Z1, Rg showed little change after stretching (Figure S11), suggesting that the lignin particles in the PM matrix did not deform under loading, as they are harder than the PM matrix. The normalized 1D correlation function curves of the SAXS profiles at various strains are shown in Figure 6c & 6d, which were extracted through inverse Fourier transformation of the Lorenz-corrected SAXS profiles45, 46. The structure parameters summarized in Table S2 were yielded from the analysis of the 1D correlation



functions (Figure 6b), including the long period length (L), the transition layer length (dtr), the length of the crystalline layer plus transition layer (lc). The core-crystalline layer length (d0 = lc − dtr), the amorphous layer length (la = L − lc), and the linear crystallinity (Φl = lc/L) were then calculated accordingly. These structure parameters reflected the evolution of elastomer matrix during stretching. As shown in Table S2, for both PM32L8 and PM32L8A1Z1, the long period L maintained constant as the strain increased, the values of lc and Φl increased, but la decreased, verifying that the amorphous elastomer chain segments were oriented after stretching. For PM32L8 without coordination bonds, the transition layer length dtr increased, while the core-crystalline layer length d0 decreased as the strain increased, suggesting that the fringed micellar crystals in polyethylene elastomer matrix were fragmentized or detached according to the slip-link deformation theory of POE47. This also implied that the deformation process of the polyethylene elastomer matrix was not interfered by lignin due to the poor interfacial interactions in the directly blending sample PM32L8. By contrast, for PM32L8A1Z1 with coordination bonds, the value of dtr decreased and d0 increased as the strain enlarged. This interesting variation implied that new thicker micellar crystals were formed upon stretching. The reason could be that, under the restriction effect of coordination bonds, the newly oriented crystallizable chain segments and the partially oriented chain segments in the transition layer were able to form new micellar crystals nearby the previously existing micellar crystals, which increased the average thickness of the core crystalline layer d0. This finding matched well with the XRD analysis results, explaining the


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strain-induced-crystallization and the reinforcement effect of the lignin-containing TPE with Zn-based coordination bonds. The above evidences proved that the prepared lignin-based TPE mainly consisted of a dual-crosslinking network structure including a dynamic coordination sacrificial network and a physical network endowed by polyethylene fringed micelles. The deformation mechanism of the dual-network lignin-based TPE is proposed in Figure 7. The tensile stress can transfer from the PM matrix to lignin particles through strong interfacial interactions including the H-bonds and Zn-based coordination bonds. At small strains, the hydrogen bonds break first under stretching, accompanied with the rearrangement of elastomer chain segments along the stretching orientation. Upon further stretching, the Zn-based bonds dynamically fracture and the elastomer-lignin interactions break, the synergistic coordination effect of lignin promotes the efficient energy dissipation, leading to the release of excess stress, which prevents the stress concentration and hinders the initiation of micro-cracks. Meanwhile, at large strains, with the restriction of coordination bonds, the oriented elastomer chain segments are able to form new micellar crystals during stretching, causing self-reinforcing. Therefore, a dramatic improvement in strength and toughness is achieved. Shape memory performance Recently, Naskar et al. reported excellent shape-memory composites with switchable electrical conductivity by incorporating lignin into NBR, the chemical crosslinks and hydrogen bonds between lignin and rubber matrix obviously enhanced the shape memory performance of the composites48. As the coordination bonds can



dynamically break and reform, principally they can be used as anchors to lock shapes in shape memory polymers (SMP)49. To quantitatively examine the shape memory performance of the lignin-based TPE, the stress-controlled program cycling was performed on the sample of PM32L8A1Z1, as depicted in Figure 8a. According to the DSC melting curves (Figure S12), the PM matrix totally melted at about 75 °C. But PM32L8A1Z1 kept solid state even at 100 °C because of the coordination-based physical crosslinking network still existing at temperatures above the melting point of the PM matrix (Figure S13 & Figure S14). Therefore, the programming temperature Tp was set at 80 °C and the fixing temperature Tlow at 0 °C. The lignin-based TPE (PM32L8A1Z1) showed excellent shape memory performance. The shape fixing ratio Rf reached 94% and the recovery ratio was about 82%. In thermoplastic elastomers, the shape fixing ratio and the recovery ratio are determined by the balance of the temporarily formed physical crosslinking and elastic contraction50. As PM32L8A1Z1 contained both Zn-based bonds and fringed micelles or bundled crystals to fix the temporary shape after cooling from the programming temperature, the recovery ratio was thus smaller than the fixing ratio due to the retardation effect of coordination bonds on the elastic contraction. The visual demonstration of the shape memory effect was shown in Figure 8b. The sample PM32L8A1Z1 was programmed from a flat permanent shape to a temporary spiral shape at 80 °C, and was subsequently quenched to 5 °C quickly, at which the temporary spiral shape was fixed after removing the external force. This spiral shape was stable at room temperature. When heated at 80 °C, the spiral shape recovered to


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its original flat shape in seconds. Excellent shape memory performance was clearly demonstrated for the lignin-based TPE. (Additional video for shape memory effect of the lignin-based TPE can be found in supporting information.)

Figure 8. (a) Stress-controlled program for the shape memory performance of PM32L8A1Z1: Tp = 80 °C, Tlow = 0 °C, Rf = 94%, Rr = 82.4%. (b) Photo illustration of the shape memory performance of PM32L8A1Z1.

CONCLUSION For the first time, a new type of high performance lignin-based thermoplastic elastomers has been prepared by adopting biomass lignin as the hard plastic phase and polyolefin elastomer (POE) as the rubber matrix. The coordination sacrificial bonds were constructed in the interface between lignin nano-particles and elastomer matrix, which not only promoted lignin’s dispersion with granular particle size around 200 nm, but also improved the interfacial interactions between lignin and polyolefin elastomer matrix, and also facilitated the orientation of chain segments during stretching. The synergistic coordination effect of lignin promoted higher energy dissipation before failure, leading to simultaneously enhanced strength and toughness of lignin-based TPEs with the loading content of lignin as high as 30 wt%. The designed lignin-based TPEs were strong, tough and ductile, comparable to most



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commercial TPE products. They also exhibited excellent shape memory performance. The enzymatic hydrolysis lignin was used directly as provided by biomass industries without any further purification or chemical modification. Our strategy presented in this work is facile and feasible for production. We envision that the design concept can offer a promising methodology for the preparation of high performance but cost-effective TPE products using bio-renewable raw materials as plastic phase. ASSOCIATED CONTENT Supporting Information Supporting Information is available free of charge on the ACS Publications website. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors gratefully thank the National Natural Science Foundation of China (21706082), the Science and Technology Program of Guangzhou (201707020025, 201804010140), Guangdong Province Science Foundation (2017B090903003), and the China Postdoctoral Science Foundation (2017M622693) for the financial support.

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Table of Contents (TOC) graphic

Synopsis Using sustainable lignin as plastic phase to replace traditional petroleum-derived plastic phase, a new type of green and sustainable lignin-based thermoplastic elastomer composite was prepared.


High performance thermoplastic elastomers with biomass lignin as

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