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Research Article pubs.acs.org/journal/ascecg

High Modulus, Strength, and Toughness Polyurethane Elastomer Based on Unmodified Lignin Hui Li,†,∥ Jiao-Tong Sun,†,∥ Cun Wang,‡ Songlin Liu,§ Du Yuan,† Xin Zhou,† Jozel Tan,‡ Ludger Stubbs,‡ and Chaobin He*,†,§ †

Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, 117575 Singapore Institute of Chemical and Engineering Sciences, Agency for Science, Technology and Research (A*STAR), 1 Pesek Road, Jurong Island, 627833 Singapore § Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, 138634 Singapore ‡

S Supporting Information *

ABSTRACT: Lignin-based polyurethane elastomers (LPUe) with high stiffness, strength, and toughness were facilely prepared by direct cross-linking of unfunctionized lignin as hard segments and poly(propylene glycol) tolylene 2,4-diisocyanate terminated (PPGTDI) as soft domains. The effects of lignin molecular weight (3600 and 600 g mol−1) and weight fraction (5−40 wt %) on the thermal and mechanical properties of LPUe were studied. With an increase in lignin content, LPUe exhibited improved thermal stability, and the glass transition temperature (Tg) also increased, especially for LPUe derived from lignin with low lignin molecular weight of 600 g mol−1 (600LPUe). Furthermore, LPUe also exhibits excellent mechanical properties. For 600-LPUe with 40 wt % of lignin, the Young’s modulus, tensile strength, and strain at break reach 176.4 MPa, 33.0 MPa, and 1394%, respectively, which could be attributed to better dispersion of low molecular weight lignin in elastomers as evident from DSC, SEM, and TEM studies. Our results demonstrate the potential application of unmodified lignin in developing biobased high-performance PU materials. This is in contrast to many current studies of LPUe systems that need lignin modification to prepare PU materials. KEYWORDS: Unmodified Lignin, Polyurethane elastomer, Mechanical property

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INTRODUCTION

Lignin, the second most abundant biomass after cellulose, is widely available as a byproduct of the pulp and paper industry.11−13 With the advantages of high thermal stability, renewable, and biodegradability,13−15 lignin has been widely investigated to produce biobased PU materials. However, most of them are used as rigid foams and thermoset films since lignin offers mechanical rigidity as hard segments.16−19 As for these products, a high Young’s modulus of 1000−2000 MPa and ultimate strength of 30−90 MPa are obtained, while ultimate strain is less than 20%.20−22 Therefore, additional polyols (such as polyethylene glycol (PEG), poly(propylene glycol) (PPG)) or lignin modification are applied to modify the performance of lignin-based PU and widen their commercial applications, such as PU elastomers (PUe).23,24 Reimann et al. prepared threecomponent LPU films (lignin-PEG-diphenylmethane diisocyanate (MDI)) with a PEG content of 20−70 wt %. The ultimate strain of LPU could be increased to 250%, while Young’s

PU elastomers (PUe) have attracted much attention because of their high ductility, toughness, and good stability.1,2 However, low stiffness and low strength limit their application, which is about 10 MPa for the Young’s modulus and 10−25 MPa for tensile strength.3−6 A general strategy to reinforce PUe is by blending them with other polymers such as polycaprolactone and poly(acrylic acid) or by incorporating inorganic/organic fillers, including graphene, carbon nanotubes (CNTs), et al.7,8 However, due to aggregation of the fillers, the improved modulus and strength at high filler content are also accompanied by reduction of ductility and toughness.4,6 To facilitate a better dispersion, in situ polymerization of monomers that possess reactive functional groups is another approach. Lignin, with abundant phenolic and aliphatic hydroxyl groups that could readily react with isocyanates to form urethane linkage, is a good candidate for preparation of biobased high value-added products,9,10 especially for PU materials. The stiff nature of lignin could endow lignin-based PUe with high modulus and strength. © 2017 American Chemical Society

Received: May 12, 2017 Revised: June 30, 2017 Published: July 18, 2017 7942

DOI: 10.1021/acssuschemeng.7b01481 ACS Sustainable Chem. Eng. 2017, 5, 7942−7949

Research Article

ACS Sustainable Chemistry & Engineering modulus and tensile strength remain high.25 Avérous et al. prepared lignin−fatty acid-based polyol which was then reacted with isocyanate prepolymers.26 However, the additional polyol or lignin modification will complicate the synthesis procedure and increase production cost and environmental impact. So far, there is no report of direct reaction using unmodified lignin and diisocyanate for LPUe. Recently, thermoplastic lignin (precross-linked)-based PU was developed from the reaction of lignin and oligomeric polybutadiene diisocyante with high lignin content of 65−80 wt %.27 On the other hand, soft diisocyanate such as 1,2-bis(isocyanate)ethoxyethane (TEGDI) could be used as soft segments for the preparation of PU elastomers, whose backbone is flexible ether chains.28,29 This suggests a promising approach to prepare lignin-based LPUe by reacting lignin with suitable soft elastomeric diisocyanates. In this work, LPUe was facilely prepared by direct reaction of unmodified lignin and poly(propylene glycol)tolylene 2,4diisocyanate terminated (PPGTDI), in which lignin serves as a biomass polyol provider to replace petroleum-based polyol. With respect to stiff lignin and elastomeric PPGTDI, excellent mechanical properties with high modulus, strength, and ductility were expected. In this study, two types of lignin with different molecular weights (3600 and 600 g mol−1) were used, and the properties of LPUe were tuned by varying lignin content from 5 to 40 wt %. The morphological, thermal, and mechanical properties of the obtained LPUe were thoroughly characterized using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and mechanical tests. At low lignin content, LPUe with high ductility of 2000% was obtained. With increasing lignin content, Young’s modulus and tensile strength increased significantly, while ductility remained relatively high. These results clearly demonstrated the potential application of biomass lignin in the preparation of high-performance renewable PU elastomers.

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Table 1. NCO/OH Ratios and Results from TGA, DTG Analysis of Lignin, PPGTDI, and LPUe at Different Lignin Contents Materials

Ratios of NCO/OH

T5%/ °Ca

T10%/ °Ca

T50%/ °Ca

R800/ %b

TDTGmax/ °Cc

PPGTDT 3600-Lignin 5 wt % LPUe 13 wt %LPUe 20 wt % LPUe 30 wt % LPUe 40 wt % LPUe 600-Lignin 5 wt % LPUe 13 wt %LPUe 20 wt % LPUe 30 wt % LPUe 40 wt % LPUe

− − 2.92 1.03 0.61 0.36 0.23 − 1.78 0.63 0.37 0.22 0.14

280 204 289 289 289 287 279 237 291 292 283 279 263

302 273 319 319 321 318 315 305 324 327 320 322 309

351 507 380 383 387 391 393 382 390 393 396 399

0.17 37.8 2.6 3.6 7.4 13.6 17.3 56.7 3.4 9.8 13.4 17.3 24.1

375 380 383 386 388 393 393 337 384 390 393 395 395

a

T5%, T10%, and T50% represent thermal degradation temperatures at 5%, 10%, and 50% mass loss. bR800 represents final amount of char residue at 800 °C. cTDTGmax is the temperature at maximum mass loss derivative. an air-circulated oven for 2 h at 120 °C. Then, the film was stored in a desiccator before testing. The thickness of the films was about 0.5 mm. Characterization. Attenuated total reflection Fourier transform infrared spectra (ATR-FT-IR) were obtained using a Shimadzu IR Tracer-100 spectrometer with the wavenumber range of 600−4000 cm−1. DSC was performed on a TA Instruments DSC Q100 under N2 at a heating rate of 10 °C min−1 to determine the glass transition temperature (Tg); Tg was calculated from the second scan for all samples. The thermal stability of the LPUe film was tested by thermal gravimetric analysis (TGA) on a TA Instruments SDT Q600 under N2. The samples were heated from the room temperature to 800 °C at a heating rate of 10 °C min−1. Tensile tests were performed using an Instron Universal Tester 5569 at a crosshead speed of 50 mm min−1. Dumbell-shaped tensile specimens were cut using a die with a gauge length of 10 mm and width of 3.18 mm. At least five specimens were measured for the tensile test. The fracture surfaces after tensile tests were observed using ZEISS field-emission SEM. The fracture surfaces were coated with a thin layer of gold before examination by sputtering. Transmission electron microscopy (TEM) was performed with a JEOL 2010F system.

EXPERIMENTAL SECTION

Materials. Lignin (Mn = 3600, Mw/Mn = 1.65, hydroxyl group = 5.6 mmol g−1) was purchased from Sigma-Aldrich and was used after acid washing according to the literature.14 The partially depolymerized lignin (Mn = 600, Mw/Mn = 1.71, hydroxyl group = 9.2 mmol g−1) was supplied by the Institute of Chemical and Engineering Sciences, Singapore. It was obtained after depolymerization at 280 °C for 15 min in sodium hydroxide aqueous solution and then precipitated in HCl solution. The molecular weights are obtained by the gel permeation chromatography (GPC) measurements. The concentrations of hydroxyl groups were determined by the 1H NMR method.17 PPGTDI (Mn = 2300) and tetrahydrofuran (THF) were supplied by Sigma-Aldrich without further purification. Preparation of Lignin-Based Polyurethane Elastomer (LPUe). Two series of LPUe derived from lignin with different molecular weights (3600 and 600 g mol−1) were prepared and denoted as 3600-LPUe and 600-LPUe, respectively. Lignin with varying weight ratios of 5, 13, 20, 30, and 40 wt % were reacted with PPGTDI to produce the above two series of LPUe. With a higher lignin content of 50 wt %, rigid and brittle 600-LPUe was obtained in our work (Figure S1). The NCO/OH ratios were calculated according to the isocyanate group from PPGTDI and total hydroxyl content (both phenolic and aliphatic group) from lignin. The composition and corresponding NCO/OH ratios are given in Table 1. Typically, 5 wt % LPUe was synthesized as follows: 0.3 g of lignin was dissolved in 30 mL of THF under magnetic stirring. Subsequently, 5.7 g of PPGTDI was added under N2 flow. After stirring for 10 h at room temperature, the reaction solution was poured into a PTFE plate, and standing in the fume hood allowed solvent evaporation. Finally, the film was cured in

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RESULTS AND DISCUSSION Chemical Characterization of LPUe. LPUe copolymers were synthesized by directly cross-linking between PPGTDI and lignin with molecular weights of 3600 and 600 g mol−1. The reaction between lignin and PPGTDI is confirmed by FTIR. The FTIR spectrum of lignin, PPGTDI, and 40 wt % LPUe are presented in Figure 1, while LPUe with other lignin contents are shown in Figure S2. As shown in Figure 1(a), a broad peak around 3395 cm−1 in the FTIR spectrum of lignin, which is assigned to a hydroxyl group (−OH), greatly decreases upon reaction with PPGTDI. Isocyanate group (−NCO) absorption at 2270 cm−1 in the FTIR spectrum of PPGTDI is barely seen in LPUe products, indicating the consumption of isocyanate groups.27 Concurrently, a peak at a lower frequency (3332 cm−1) and a sharp peak at 1728 cm−1 are observed in the LPUe spectra, which are ascribed to N−H stretching vibrations and C = O stretching vibrations, respectively.16 These results indicated the successful formation of the urethane group upon cross-linking of the hydroxyl group from lignin and isocyante group of PPGTDI. Furthermore, a strong peak at 1088 cm−1 7943


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Figure 1. FTIR spectra of LPUe with different lignin molecular weights: (a) 3600-LPUe, (b) 600-LPUe, absorption spectra of urethane C=O groups from 3600-LPUe (c), and 600-LPUe (d), absorption spectra of C−O−C ether groups from 3600-LPUe (e), and 600-LPUe (f). Arrows indicate LPUe with increasing lignin content.

Figure 2. TGA curves of lignin, PPGTDI, and LPUe at varying lignin weight ratios of 5, 13, 20, 30, and 40 wt %. (a) 3600-LPUe and (b) 600-LPUe. Arrows indicate LPUe with increasing lignin content.

peak increases, indicating improved hydrogen bonding from urethane C=O groups. While for 40 wt % LPUe, the relative intensity of this peak is even higher than the peak at 1728 cm−1 (free carbonyl groups), indicating strong hydrogen bonds are formed between C=O groups and proton donator groups. This is mainly ascribing to the increased O−H groups from lignin, which is in excess relative to isocyanate groups in PPGTDI (NCO/OH is 0.14 as shown in Table 1). Furthermore, a wavenumber shift of C−O−C ether groups (from 1088 to 1078

appears in LPUe ascribing to the vibration of C−O deformation from polyether segments of PPGTDI.30,31 All these results further confirmed that lignin was covalently bonded with PPGTDI to yield urethane linkage of LPU. In addition, a shoulder peak around 1705 cm−1 is observed as shown in Figure 1(d), which is assigned to the vibration of hydrogen-bonded carbonyl groups from urethane C = O groups with both N−H groups and O−H groups.40,41 With increasing lignin content for 600-LPUe, the intensity of this 7944


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Figure 3. DSC curves of LPUe at various lignin contents of 5, 20, and 40 wt %: (a) 3600-LPUe and (b) 600-LPUe.

Figure 4. SEM fracture surfaces of LPUe after tensile test: 3600-LPUe (top images: a−d) and 600-LPUe (bottom images: e−h) at varying lignin content of 5 (a,e), 20 (b,f), and 40 wt % (c,g). (d,h) TEM images of 40 wt % 3600-LPUe and 600-LPUe, respectively.

cm−1) is observed with high lignin contents (Figure 1(f)), suggesting strong hydrogen bonding interaction between C− O−C ether groups and N−H/O−H groups.32,33 With low lignin content, hydrogen bonding is mainly formed between a carbonyl group/C−O−C group and N−H groups from adjacent urethane chains. With increasing lignin contents, O− H groups increase, which could serve as proton donators for hydrogen bonding. Therefore, intermolecular interactions in LPUe at high lignin content are enhanced, which give rise to improved thermal and mechanical properties.34 The absorption spectra of urethane C=O groups and C−O−C ether groups for 3600-LPUe are presented for comparison (Figure 1(c,e)). However, due to the steric hindrance and poor dispersion of lignin with high molecular weight, hydrogen bonding was not prominent for all 3600-LPUe samples. Thermal Stability of LPUe. The thermal stability of these LPUe was investigated by TGA measurements under N2 atmosphere. The TGA curves of the two series of LPUe are presented in Figure 2, and the degradation temperatures at various mass losses and the amounts of char resident are summarized in Table 1. Lignin with different molecular weight and PPGTDI were included for comparison. From the TGA curves, the initial thermal degradation temperature of lignin is around 200 °C, which is associated with a dehydration reaction of hydroxyl groups and heterolysis of ether linkages.16,22 While for PPGTDI, the degradation starts at 280 °C. Upon crosslinking, all the LPUe products exhibited improved thermal stability, irrespective of lignin molecular weight. Higher

decomposition temperatures are observed compared with raw materials of lignin and PPGTDI. The temperature at the maximum decomposition, which is associated with the degradation of the urethane structure,16 was increased after cross-linking, further indicating that the thermal stability of LPUe was improved.19 With respect to the effect of lignin content (Table 1), the initial decomposition temperature (T5%) was essentially constant but gradually decreased with increasing lignin contents for these two series of LPUe. This could be caused by the low cross-linking of urethane structures and the excess of hydroxyl group at high lignin content. NCO/OH ratios are just 0.23 and 0.14 for 40 wt % 3600-LPUe and 600LPUe, suggesting that the cross-linking density is relative low in these LPUe products. Furthermore, the decomposition temperature of excess hydroxyl groups is relatively lower than the urethane group.16 Despite the decreased initial decomposition temperature of LPUe with high lignin content, its thermal stability was still enhanced compared with lignin and PPGTDI. Finally, the amount of char residues at 800 °C is summarized in Table 1, and it increased with increasing lignin content. This is attributed to the higher thermal resistance of lignin, which possesses a large number of aromatic groups.22 Thermal Transition of LPUe. The Tg values of LPUe materials at varying lignin contents were investigated by DSC analysis. As shown in Figure 3, Tg increases with increasing lignin contents for both 3600-LPUe and 600-LPUe materials. As for 3600-LPUe, Tg increases from −47 to −36 °C, while a progressive increase in Tg from −44 to 8 °C is observed for 7945


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Figure 5. Tensile stress−strain curves of 3600-LPUe (a) and 600-LPUe (b) at varying lignin contents. Young’s modulus (c), tensile strength (d), elongation at break (e), and toughness (calculated as the area under stress−strain curves) (f) of the two series of LPUe at different lignin contents.

Table 2. Mechanical Properties of 3600-LPUe and 600-LPUe at Different Lignin Contents Lignin content 3600-LPUe 5 wt % 13 wt % 20 wt % 30 wt % 40 wt % 600-LPUe 5 wt % 13 wt % 20 wt % 30 wt % 40 wt %

Young’s modulus/MPa

Tensile strength/MPa

Elongation/%

Toughness/MJm−3

1.14 1.20 1.43 1.67 3.92

± ± ± ± ±

0.04 0.06 0.05 0.06 0.60

3.34 5.12 4.10 3.90 4.44

± ± ± ± ±

0.29 0.44 0.35 0.51 0.25

1969 1839 1960 2113 1646

± ± ± ± ±

170 78 109 255 108

30.1 37.8 38.1 42.9 45.2

± ± ± ± ±

4.1 4.0 3.7 7.5 4.9

0.90 0.97 1.31 4.97 176.4

± ± ± ± ±

0.04 0.04 0.04 0.50 9.88

2.94 4.50 5.72 12.2 33.0

± ± ± ± ±

0.43 0.73 0.70 1.84 1.05

1781 2079 2289 2149 1394

± ± ± ± ±

131 194 198 137 99

22.9 37.1 52.6 98.5 264.7

± ± ± ± ±

4.0 6.0 7.8 15.3 24.8

interactions in 600-LPUe, contributing to the increased Tg in 600-LPUe with high lignin content. Morphology of LPUe. The cross-section morphology of LPUe after tensile fracture was studied using SEM. Two series of SEM micrographs of LPUe with lignin contents of 5, 20, and 40 wt % are presented. As shown in Figure 4(e−g), smooth and homogeneous morphology without obvious aggregates is observed for 600-LPUe samples at varying lignin contents, implying that lignin is well incorporated in PU systems. No obvious clusters were exhibited for 600-LPUe with lignin content of 40 wt % in TEM images, consistent with SEM

600-LPUe. This could be ascribed to the contribution of the lignin hard segment in the PU network.25,35 Besides, a broadening of the glass transition is observed with high lignin content for 600-LPUe, indicating well miscibility of lignin with PPGTDI.36 With the advantage of low molecular weight, high solubility, and high reactivity, 600-lignin could be well reacted with PPGTDI and form a branched network via urethane groups. With the introduction of lignin rigid structure in PU back bones, the chains mobility of LPUe decreases and thereby Tg increases with increasing lignin content. On the other hand, strong hydrogen bonding could enhance the intermolecular 7946


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from the cross-linking of low molecular weight of lignin and PPGTDI. With high solubility, reactivity, and hydroxyl group content (9.2 mmol g−1), 600-lignin is able to well react with PPGTDI and incorporate into PU systems to facilitate enhancing chain stiffness. DSC, SEM, and TEM results also indicate well miscibility of 600-lignin and PPGTDI with varying lignin contents. For LPUe with a low lignin content, the Young’s modulus and tensile strength remain relatively low due to the existence of a great deal of elastomeric segments in LPUe. With increasing lignin content, NCO/OH ratios decrease to 0.14 for 40 wt % 600-LPUe, suggesting that the covalent cross-linking density via urethane groups is relative low. The swelling test also indicated that the cross-linking density decreased with increasing lignin content (Figure S5). Therefore, slightly branched polymer networks with high lignin contents were constructed. A high content of hard segment lignin was incorporated into PU systems to improve the stiffness of the PU network. Besides, as shown in FTIR results, strong hydrogen bonding was formed for lignin content of 40 wt %, which improved intermolecular interactions and enhanced physical cross-linking of urethane and lignin (Figure 6). Consequently, tough PU elastomers were prepared with a

images. With a high content of hydroxyl groups and high solubility, lignin is well reacted with PPGTDI via urethane groups to form a continuous network. Furthermore, strong hydrogen bonding enhances intermolecular interactions of lignin and PPGTDI. Thus, homogeneous morphology with well dispersion of lignin in the PU matrix is obtained, which is beneficial for the mechanical properties of LPUe, while microaggregates are dispersed among 3600-LPUe samples (Figure 4(a−d)). Furthermore, the aggregates of 3600-LPUe could also be observed by the naked eye, and the photographs are shown in Figure S3. These aggregates probably result from the poor solubility of lignin with high molecular weight. They could act as the origin of cracks in this LPUe system and have adverse effects on the mechanical properties.37 Mechanical Property of LPUe. The mechanical property of LPUs was studied by tensile test. The stress−strain curves, calculated Young’s modulus, tensile strength, ultimate elongation, and toughness are presented in Figure 5 and Table 2. As shown in Figure 5(a) and (b), 600-LPUe exhibited superior mechanical properties to 3600-LPUe, especially at a lignin content of 40 wt %. Figure 5(c) demonstrates that Young’s modulus increases as the amount of lignin increases in both of the two series of LPUe. Lignin itself possesses a high modulus of 2.31−4.65 GPa, expecting increases in chain stiffness as more aromatic rings are incorporated into the PU structure.27,38 For lower lignin contents (5−20 wt %), the Young’s modulus remained very low due to the existence of a great deal of elastomeric segments in LPUe. With increasing lignin content, maximum Young’s modulus values of 3.92 ± 0.60 and 176.4 ± 9.88 MPa were obtained for 3600-LPUe and 600-LPUe at a lignin content of 40 wt %, respectively. For the tensile strength, different results are shown in Figure 5(d) for LPUe derived from the high and low molecular weight of lignin. For the 3600-LPUe series, the tensile stress achieved a maximum value (5.12 ± 0.44 MPa) at lignin content of 13 wt % and then decreased at higher lignin weight ratios. The subsequent decrease could be attributed to the poor solubility of lignin aggregates, which act as the origin of cracking in the PU systems.24,37 While for the 600-LPUe series, the tensile strength increased with increasing lignin contents. With lignin content of 40 wt %, the maximum tensile strength of 33.0 ± 1.05 MPa was obtained. Figure 5(e) shows that more than 2000% of ultimate strain could be obtained for lignin content less than 30 wt % since a great deal of elastomeric PPGTDI is introduced into LPUe systems. With increasing lignin content to 40 wt %, the ultimate strain decreases. However, 1394 ± 99% and 1646 ± 108% could still be achieved for 600-LPUe and 3600-LPUe, respectively, which is relatively higher than most PU elastomers reported in the literatures.6,39,40 Besides, this material exhibits good stress−strain recovery after stretching to 1000% (Figure S4). It should be noted that Young’s modulus and tensile strength increased by a factor of 196 and 11 from a lignin content of 5 to 40 wt %, while elongation dropped slower, reduced by 22% for 600-LPUe. The modulus and strength were increased at a rather faster rate than ductility decreased with increasing lignin content. Thus, a high toughness of 264.7 MJ m−3 was obtained for 40 wt % 600-LPUe (Figure 5(f)), higher than most PU elastomers in the literatures (10−150 MJ m−3).4,6,40,41 Due to low stiffness and strength, 3600-LPUe exhibited a low toughness of 45.2 MJ m−3. All these results indicate that high modulus and strength, along with high ductility and toughness, of LPUe was obtained

Figure 6. Schematics of structure of LPUe with increasing lignin content for 600-LPUe.

high content of lignin hard segments, strong hydrogen bonding, and elastomeric polyether segments. As a result, excellent mechanical properties of 176.4 ± 9.88 MPa for Young’s modulus, 33.0 ± 1.05 MPa for tensile strength, 1394 ± 99% for ultimate elongation, and 264.7 ± 24.8 MJ m−3 for toughness was obtained for 600-LPUe with a lignin content of 40 wt %. The mechanical properties are superior to most reported PUe, even higher than PUe optimized with various fillers, which exhibit a Young’s modulus of 20−50 MPa, tensile strength of 10−30 MPa, and elongation of 200−800% (Figure 7).3,5,42−47 With the advantage of renewable, abundant, and environmental friendliness, LPUe exhibits promising application in the PU industry.

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CONCLUSION A high modulus, strong, and tough lignin-based PU elastomer (LPUe) was synthesized by the direct reaction of unmodified lignin and PPGTDI for the first time. Two series of LPUe derived from low (600 g mol−1) and high (3600 g mol−1) molecular weights of lignin were studied. FTIR confirmed the formation of urethane groups by the reaction of hydroxyl groups of lignin and isocyanate groups from PPGTDI. Upon cross-liking, the thermal stability of LPUe was improved compared with raw materials. With increasing lignin content, Tg increased as more hard segments were incorporated into the LPUe systems. Due to the higher content of hydroxyl groups, higher solubility, and higher reactivity, 600-lignin was better dispersed into the PU matrix, and 600-LPUe exhibited superior mechanical properties compared with 3600-LPUe. For 600LPUe, with increasing lignin content, the modulus and strength 7947


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(2) Benli, S.; Yilmazer, U.; Pekel, F.; Oezkar, S. Effect of fillers on thermal and mechanical properties of polyurethane elastomer. J. Appl. Polym. Sci. 1998, 68, 1057−1065. (3) Phua, S. L.; Yang, L.; Toh, C. L.; Huang, S.; Tsakadze, Z.; Lau, S. K.; Mai, Y.-W.; Lu, X. Reinforcement of polyether polyurethane with dopamine-modified clay: the role of interfacial hydrogen bonding. ACS Appl. Mater. Interfaces 2012, 4, 4571−4578. (4) Khan, U.; May, P.; O’Neill, A.; Coleman, J. N. Development of stiff, strong, yet tough composites by the addition of solvent exfoliated graphene to polyurethane. Carbon 2010, 48, 4035−4041. (5) Dai, X.; Xu, J.; Guo, X.; Lu, Y.; Shen, D.; Zhao, N.; Luo, X.; Zhang, X. Study on structure and orientation action of polyurethane nanocomposites. Macromolecules 2004, 37, 5615−5623. (6) Pei, A.; Malho, J.-M.; Ruokolainen, J.; Zhou, Q.; Berglund, L. A. Strong nanocomposite reinforcement effects in polyurethane elastomer with low volume fraction of cellulose nanocrystals. Macromolecules 2011, 44, 4422−4427. (7) Podsiadlo, P.; Arruda, E. M.; Kheng, E.; Waas, A. M.; Lee, J.; Critchley, K.; Qin, M.; Chuang, E.; Kaushik, A. K.; Kim, H.-S.; et al. LBL assembled laminates with hierarchical organization from nano-to microscale: high-toughness nanomaterials and deformation imaging. ACS Nano 2009, 3, 1564−1572. (8) Zhou, Y.; Xiu, H.; Dai, J.; Bai, H.; Zhang, Q.; Fu, Q. Largely reinforced polyurethane via simultaneous incorporation of poly(lactic acid) and multiwalled carbon nanotubes. RSC Adv. 2015, 5, 30912− 30919. (9) Sun, J.; Wang, C.; Yeo, J. C. C.; Yuan, D.; Li, H.; Stubbs, L. P.; He, C. Lignin Epoxy Composites: Preparation, Morphology, and Mechanical Properties. Macromol. Mater. Eng. 2016, 301, 328−336. (10) Kai, D.; Tan, M. J.; Chee, P. L.; Chua, Y. K.; Yap, Y. L.; Loh, X. J. Towards lignin-based functional materials in a sustainable world. Green Chem. 2016, 18, 1175−1200. (11) Upton, B. M.; Kasko, A. M. Strategies for the Conversion of Lignin to High-Value Polymeric Materials: Review and Perspective. Chem. Rev. 2016, 116, 2275−2306. (12) Li, H.; Yuan, D.; Tang, C.; Wang, S.; Sun, J.; Li, Z.; Tang, T.; Wang, F.; Gong, H.; He, C. Lignin-derived interconnected hierarchical porous carbon monolith with large areal/volumetric capacitances for supercapacitor. Carbon 2016, 100, 151−157. (13) Sen, S.; Patil, S.; Argyropoulos, D. S. Thermal properties of lignin in copolymers, blends, and composites: a review. Green Chem. 2015, 17, 4862−4887. (14) Liu, W.; Zhou, R.; Goh, H. L. S.; Huang, S.; Lu, X. From waste to functional additive: Toughening epoxy resin with lignin. ACS Appl. Mater. Interfaces 2014, 6, 5810−5817. (15) Calvo-Flores, F. G.; Dobado, J. A. Lignin as Renewable Raw Material. ChemSusChem 2010, 3, 1227−1235. (16) Griffini, G.; Passoni, V.; Suriano, R.; Levi, M.; Turri, S. Polyurethane Coatings Based on Chemically Unmodified Fractionated Lignin. ACS Sustainable Chem. Eng. 2015, 3, 1145−1154. (17) Chung, H.; Washburn, N. R. Improved lignin polyurethane properties with lewis acid treatment. ACS Appl. Mater. Interfaces 2012, 4, 2840−2846. (18) Yang, Y.; Deng, Y.; Tong, Z.; Wang, C. Renewable lignin-based xerogels with self-cleaning properties and superhydrophobicity. ACS Sustainable Chem. Eng. 2014, 2, 1729−1733. (19) Xue, B.; Wen, J.; Sun, R. Lignin-based rigid polyurethane foam reinforced with pulp fiber: synthesis and characterization. ACS Sustainable Chem. Eng. 2014, 2, 1474−1480. (20) Saraf, V. P.; Glasser, W. G. Engineering plastics from lignin. III. Structure property relationships in solution cast polyurethane films. J. Appl. Polym. Sci. 1984, 29, 1831−1841. (21) Saraf, V. P.; Glasser, W. G.; Wilkes, G. L.; McGrath, J. E. Engineering plastics from lignin. VI. Structure−property relationships of PEG-containing polyurethane networks. J. Appl. Polym. Sci. 1985, 30, 2207−2224. (22) Jia, Z.; Lu, C.; Zhou, P.; Wang, L. Preparation and characterization of high boiling solvent lignin-based polyurethane

Figure 7. Parameters of PU elastomers summarized from the literatures (blue histogram); our work is included and noted with a red histogram.

greatly increased, while ductility remained relatively high. Thus, a high toughness of 264.7 MJ m−3 was achieved with a lignin content of 40 wt %, which is superior to most of the reported PUe. This result highlights the potential application of lignin in the preparation of high-performance biobased PU products.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01481. Mechanical property of 50 wt % 600-LPUe, FTIR spectra of LPUe with different lignin contents, photograph of LPUe samples, photograph of 40 wt % 600-LPUe before and after stretch, photograph of 600-LPUe samples after swelling test. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Chaobin He. Fax: 65 6776 3604. Tel: 65 6601 1427. E-mails: [email protected], [email protected]. ORCID

Xin Zhou: 0000-0002-2843-3065 Chaobin He: 0000-0001-8200-8337 Author Contributions ∥

Hui Li and Jiaotong Sun contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Science and Engineering Research Council (Grant No. R-284-000-112-305) of the Agency for Science, Technology and Research of Singapore.

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REFERENCES

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