One-pot synthesis of lignin thermosets exhibiting widely tunable

Jul 8, 2019 - One-pot synthesis of lignin thermosets exhibiting widely tunable mechanical properties and shape memory behaviour ...
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Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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One-Pot Synthesis of Lignin Thermosets Exhibiting Widely Tunable Mechanical Properties and Shape Memory Behavior Yunsheng Xu, Karin Odelius, and Minna Hakkarainen* Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden

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S Supporting Information *

ABSTRACT: A series of kraft lignin based thermosets were successfully synthesized by a one-pot heat curing method composed of lignin, PEG400, and citric acid through esterification reactions with water as the only produced byproduct. The polyester thermosets were prepared by varying the ratio of lignin and PEG400 in combination with citric acid as the cross-linker. Lignin and PEG400 were chosen as the rigid and soft segments, respectively, to tailor the thermal mechanical properties of the thermosets. An increase of lignin content from 20 to 40 wt % facilitated an increase in the cross-linking density and aromatic content. This was reflected in the storage modulus at 25 °C, which increased from 5.7 to 2000 MPa, and the glass transition temperature, which increased from −0.3 to 102 °C. At the same time, the tensile strength changed from 1.2 to 34.3 MPa. The mechanical properties were, thus, tunable from flexible to rigid, demonstrating a significantly high storage modulus and tensile strength for a biobased thermoset. Furthermore, a superb thermally stimulated shape memory property was illustrated. This is promising for the use of commercial kraft lignin as a building block for versatile applications. KEYWORDS: Lignin, Polyester thermoset, One-pot synthesis, Shape memory

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INTRODUCTION Lignin is the most abundant natural aromatic resource, and it is today mainly produced by the pulp and paper industry as a byproduct. At the moment, most of these products are incinerated for energy, and a minority (below 2%) is used for other low-value products such as dispersants, adhesives, and surfactants.1 Due to its aromatic backbone structure and abundancy, lignin is of great interest for utilization as a replacement for petroleum-based chemicals for synthesis of aromatic thermoplastics and thermosets. To achieve this goal, the current research work is mainly divided into two paths: The first utilizes lignin model compounds including vanillin,2−5 2-methoxy-4-propylphenol,6,7 and eugenol8−11 etc., which could be depolymerized from lignin as precursors and utilized for the synthesis of different types of polymeric materials. However, the bottleneck of this route is the shortage of commercial processes for green conversion of lignin into most of these model compounds.12 The second route is to use technical lignin from the industry, directly or after fractionation, as a rigid building block to prepare polymeric materials. Due to the inherent presence of multiple hydroxyl groups (both aliphatic and phenolic) in lignin, this route commonly leads to cross-linked materials, e.g. epoxy,13−16 phenolic,17−20 and polyester21−23 thermosets. However, lignin is also wellknown for its heterogeneous bulk structure and its relatively low reactivity and poor compatibility, which limits its use as a resin component. To overcome this issue, chemical mod© XXXX American Chemical Society

ification and solvent fractionation has commonly been utilized to increase the reactivity of the functional groups or to reduce lignin’s molecular weight and dispersity.23−29 These methods are valuable when well-defined narrow dispersity materials are required. However, many of these routes require multistep procedures, which can hamper commercial applications of the lignin based products. Synthesis of lignin based materials, through reaction of lignin and dicarboxylic acid chlorides, has been reported.30−32 However, in addition to the use of dicarboxylic acid chlorides, large amounts of solvents were needed to fractionate lignin, during synthesis and the following Soxhlet extraction purification of the synthesis products to remove unreacted lignin and formed byproducts such as HCl. Potentially interesting materials were synthesized, but no films were prepared and no mechanical testing was performed, so the material properties are not possible to assess. More recently, esterification of lignin was successfully demonstrated by reaction with poly(ester-amide) or poly(ester-amine) to form hyperbranched poly(ester-amine-amide) or with adipic acid, glycerol, and amines to first form hyperbranched prepolymers, which were further reacted with lignin.21,22 Due to the presence of both soft and rigid segments in the cross-linked Received: May 23, 2019 Revised: July 4, 2019 Published: July 9, 2019 A

DOI: 10.1021/acssuschemeng.9b02921 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Characterization. Size Exclusion Chromatography (SEC). The weight-average molecular weight (Mw) and dispersity (Đ) of the lignoboost were determined by a SEC 1260 infinity (Polymer standard service, Germany) equipped with a PSS precolumn, PSS column 100 Å, and PSS GRAM 10000 Å analytical column. The samples were prepared by dissolving 5 mg/mL of lignin in a DMSO + 0.5 wt % LiBr solution. Molecular weight was calculated based on the retention time of pullulan standards. Phosphorus Nuclear Magnetic Resonance (31P NMR). The different types of hydroxyl and carboxylic groups of lignoboost were quantitatively determined by 31P NMR. The samples were prepared according to a previously reported procedure.33 The integration of the peak at 151.1−151.5 ppm was used as an internal standard, while the integration of 150−144.5 ppm was assigned to aliphatic OH groups, 144.5−136.5 ppm to phenolic OH groups, and 136−133 ppm to COOH groups, respectively. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR FTIR). To confirm that cross-linking had occurred, the lignin based thermosets were investigated by Fourier transform infrared spectroscopy. The measurements were performed on a PerkinElmer Spectrum400 FTIR spectrometer (Norwalk, CT). The instrument was equipped with an attenuated total reflectance (ATR) accessory (golden gate) from Graseby Specac (Kent, United Kingdom). All the spectra were recorded using 16 scans at a resolution of 4 cm−1. Fourier Transform Infrared Spectroscopy Imaging. FTIR spectra and single-peak absorbance images of the thermosets were recorded using a PerkinElmer Spotlight 400 system equipped with an optical microscope (Bucks, U.K.). Images of the ester bond and aromatic double bond absorbance intensity at 1730 and 1590 cm−1 were used to evaluate the amount and distribution of ester bonds and lignin. Differential Scanning Calorimetry (DSC). DSC analysis of the cured thermosets was performed on a Mettler-Toledo DSC 820 to determine the glass transition temperatures (Tg), taken as the midpoint of the transition, of the cured thermosets. Samples (3−5 mg) were weighed and sealed into 40 mL alumina crucibles. All the samples were first cooled to −80 °C and kept at that temperature for 2 min, after which they were heated to 150 °C at a heating rate of 10 °C/min. The entire DSC test was performed under a nitrogen atmosphere using a flow rate of 50 mL/min. Gel Content. Rectangular-shaped specimens of the thermosets (∼100 mg, 50 mm × 20 mm × 0.1 mm) were extracted with 1,4dioxane (∼20 mL) for 48 h. The insoluble fraction was then dried in a vacuum oven at 30 °C until a constant weight was reached. The gel content of the thermosets was calculated by the equation:

system, the achieved lignin copolymer exhibited shape memory property (SMP). To fabricate commercially viable lignin based thermosets, it is an advantage if the lignin comes directly from the industry with little or no processing and without chemical modification. Further to reach as high biobased content as possible, the other building blocks should also be derivable from biomass. Therefore, in this work, we utilized a commercialized kraft lignin (lignoboost) in combination with citric acid and PEG400 to prepare a polyester thermoset that could potentially be produced fully from biobased resources. PEG400 was expected to function as a reactive diluent and soft segment in the thermosets aiming to increase the dispersion of the components, facilitate the cross-linking reaction, and balance and tune the final mechanical properties of the materials. As a soft segment PEG could also promote the formation of a material with shape-memory properties. Finally, it was anticipated that this system could combine synthesis and curing to thermoset films in one-pot by simple introduction of heat and with water as the produced byproduct.

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EXPERIMENTAL SECTION

Materials. Softwood kraft lignin was obtained from the Lignoboost process (LB) and conditioned in a fume hood overnight before use, citric acid (CA, ≥99.5%, Fluka), poly(ethylene glycol) 400 (PEG400, Mn = 400 g/mol, Sigma-Aldrich), 4-(dimethylamino)pyridine (DMAP, ≥99.0%, Fluka), and 1,4-dioxane (99.9%, VWR) were all used as received. Synthesis of Lignin-Based Thermoset Resins. To prepare lignin thermosets with different compositions, the catalyst DMAP (1 wt % of sum of reactants) and different molar ratios of reactants (LB, CA, and PEG, Table 1) were added into a glass flask together with a

Table 1. Feed Ratios of the Synthesized Thermosets and Their Gel Content Values Sample

LB (wt %)

PEG (wt %)

CA (wt %)

OH/COOH (mol %)a

Gel content (%)

X-0LB X-20LB X-30LB X-35LB X-40LB

0 20 30 35 40

60 40 30 25 20

40 40 40 40 40

81.4 88.3 91.6 93.2 94.2

47.6 90.1 89.2 99.9 99.9

Gel content = mae mbe × 100%

a

OH/COOH was measured from the equation: MolOH/COOH = (COH*LBwt % + 2*PEGwt %/400 + CAwt %/192)/(CCOOH*LBwt % + 3*CAwt %/192), where is COH and CCOOH is the hydroxyl and carboxylic acid concentration of lignoboost calculated from 31P NMR results (Table S2).

(1)

mbe and mae represent the mass before and after the extraction, respectively. Dynamic Mechanical Thermal Analysis (DMTA). The thermomechanical properties of the lignin-based thermosets were measured using a TA Instrument Q800 in tension clamp mode. All the samples were cut into nominal dimensions of 20 mm × 5 mm × 0.1 mm and fastened on both sides of the clamp to ensure a uniform crosssectional area. The samples were tested from −50 to 150 °C at 1 Hz with a deflection of 0.1% strain and at a 3 °C/min heating rate. Thermal Gravimetric Analysis (TGA). The thermal stability of the thermosets was evaluated by TGA using a TGA/SDTA 851e (Mettler-Toledo, USA). Approximately 10 mg of each sample was weighed into 70 μL alumina crucibles. The experiments were performed under a nitrogen flow of 50 mL/min with a temperature range from 25−650 °C at a heating rate of 10 °C/min. Tensile Properties. The tensile properties of the lignin based thermoset samples were determined by Instron5944 Universal Testing Machine. Before testing, the samples had the approximate dimensions of 40 mm × 5 mm × 0.1 mm. The Young’s modulus, elongation at break, and tension strength were all calculated according to the standard of ASTM D882-18.

minimum amount of 1,4-dioxane (the concentration of all reactants was 1.6 mol/L) required to achieve a homogeneous resin. As an example in the case of X-30LB, LB (1.2 g, 1.2 mmol), CA (1.6 g, 8.3 mmol), PEG (1.2 g, 3 mmol), and 40 mg of DMAP were mixed with 8 mL of dioxane. After 1 h of stirring, the resulting mixtures were cast into round aluminum molds (diameter = 70 mm). After leaving the molds in the fume hood for 24 h to evaporate 1,4-dioxane, the molds were transferred into a heating oven. The curing reaction was performed at 110 °C for 48 h. After being cooled down to room temperature, the samples were removed from the molds by soaking in 5 mol/L hydrochloric acid solution followed by washing using distilled water to remove the remaining acid. Finally, the samples were transferred into a vacuum oven at 20 °C for 3 days to remove unbound water. The cured thermosets were denoted according to the feed ratio of lignin (X-30LB represents a cross-linked resin containing 30 wt % ratio of lignin). B

DOI: 10.1021/acssuschemeng.9b02921 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematics over the transformation of lignin (illustrated by a simplified model structure), PEG400, and citric acid into a thermoset material.

Figure 2. (a) FTIR spectra of the uncured 30LB and the corresponding cured X-30LB. (b) FTIR spectra of all the cured thermosets with different lignin contents. Shape Memory Behavior Test. The shape memory property of one of the lignin thermosets was characterized using a TA Instrument Q800 in the tension clamp mode. The dimension of the sample was 20 mm × 5 mm × 0.1 mm. First, the sample was equilibrated at 80 °C (above Tg) for 10 min. Subsequently, a 0.4 N of static force was applied to the sample with the ramp of 0.4 N/min. The sample was stretched to a temporary shape, and the temperature was quickly lowered to 20 °C and kept at 20 °C isothermally for 10 min. The constant force was removed and the sample was kept for an additional 10 min at 20 °C, after which the sample was heated back to 80 °C and kept isothermally at 80 °C for 10 min. The above steps were repeated three times, and the strain changes were recorded during the test.

groups are expected to provide enough cross-linking sites for the formation of thermosets, while varying the amount of lignin with plenty of aromatic groups and PEG with a flexible ether chain could provide tunable stiffness and toughness to the thermosets. Furthermore, lignoboost as kraft lignin could be dissolved in PEG400, which makes the reaction mixture more homogeneous, and it is also anticipated to increase the reaction rate. Previously lignin based polyester thermosets were prepared from a mixture of dioxane fractionated kraft lignin and PEG, which were cured with dicarboxylic chloride and then purified by Soxhlet extraction with two solvents.30−32 In comparison, the system here is fabricated and cured to good quality free-standing thermoset films in one-pot. In addition citric acid was used as a green cross-linker to replace dicarboxylic chloride, the esterification reaction took place under milder conditions, and the final products have a higher biobased content. Confirmation of the Curing Reaction by FTIR. The reaction for the formation of the thermosets starting with lignin, PEG, and citric acid was a simple one-step heat curing reaction with water as the only produced byproduct during the process (Figure 1). In order to verify the esterification reaction during the curing process, ATR FTIR was utilized to compare the changes in functional groups before and after the reaction. As an example the spectra of the resin 30LB before and after curing are shown in Figure 2a. The carbonyl peak shift from

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RESULTS AND DISCUSSION A series of lignin-based polyester thermosets with tunable properties was designed and synthesized by a one-pot reaction, which occurred simply due to heating the curing mixture of lignin, citric, acid and PEG400. The influence of the lignin content, as the rigid constituent, on the thermoset’s molecule structure and properties, including gel content and thermomechanical and tensile properties, was further investigated by multiple techniques. Polyester Thermoset Synthesis. Lignin, PEG400, and citric acid, all derivable from renewable resources, were chosen as building blocks for the fabrication of biobased polyester thermosets. The rationale behind the choice was the following: lignin and citric acid with multiple hydroxyl and carboxyl C

DOI: 10.1021/acssuschemeng.9b02921 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering 1714 to 1730 cm−1 was clearly observed after the curing process, which was further confirmed by real-time FTIR experiments (Figure S3). This indicates that the carboxylic carbonyl group was transformed into an ester carbonyl group, indicating a successful reaction. Furthermore, the intensity of the hydroxyl group absorbance at 3430 cm−1 decreased and a new ester −C−O band at 1248 cm−1 was formed after the curing process. Furthermore, all lignin thermosets had high gel content above 80%, indicating a well cross-linked material (Table 1). All of these changes strongly support that the esterification condensation reaction was successfully accomplished during the curing process. Comparison of the chemical structures of the cured thermosets with different lignin contents through their FTIR spectrum (Figure 2b) clearly illustrates that increasing the lignin content increases the intensity of the aromatic −CC bond (at 1510 cm−1), which supports that more aromatic rings were incorporated into the thermoset. In addition, a larger amount of ester −C−O bonds (at 1250 cm−1) was formed when the lignin content in the thermosets increased. This could be explained by lignin having a larger number of functional groups as compared to PEG400. Based on SEC and 31 P NMR results, the used lignoboost has 7.6 hydroxyl groups and 0.6 carboxylic groups per molecule, while PEG400 only has two hydroxyl groups per molecule (see Supporting Information, Figure S2), as a result, increasing the lignin content should provide more cross-linking sites and formation of more ester bonds in the thermosets. FTIR Imaging of the Functional Group Distribution. FTIR imaging with high spatial resolution and sensitive structural identification was utilized to gain a better understanding of the formation and distribution of the functional groups on the surface of the thermosets with a spatial view. Two specific wavenumbers, 1510 and 1250 cm−1, representing aromatic double bond and ester bonds were chosen, and their absorbance intensity was imaged on the surface of the thermosets (Figure 3a and 3b). The absorbance intensity at

higher lignin content thermoset except for X-40LB, the increase of content could increase the amount of higher ester bonds formed in the thermosets. Hence, the increase of lignin content promoted a higher amount of rigid lignin backbone and higher cross-linking density in the formed thermosets. Thermomechanical Properties of the Thermosets. The thermomechanical properties of the lignin thermosets were studied by DMTA. The changes in storage modulus (G) and loss factor (tan δ) as a function of temperature are presented in Figure 4a and b. X-0LB could not be tested through DMTA, as it did not form a uniform and intact film. In the thermosets, the α relaxation temperature (Tα) and modulus are usually related to both the rigidity of the backbones and the cross-linking density. As we expected, an increase in the lignin feed ratio (from 20 to 40%) pronouncedly increased the storage modulus at 25 °C (from 5.7 to 2,000 MPa). Similarly, the Tα of the thermosets was deduced to the feed ratio of lignin and an increase in the feed ratio of lignin significantly increased the Tα of the thermosets. This trend is in agreement with the Tα values obtained from DSC (see Supporting Information Figure S4 for the DSC curves). Compared to the other thermosets, X-30LB shows a broad Tα transition (from 20 to 60 °C). This is possible due to the equal feed ratio of lignin and PEG, which seemed to result in a more heterogeneous material. Tensile Properties. The tensile properties of the thermosets with different lignin contents were all tested under the same conditions and before testing conditioned at 22 °C and 40% relative humidity for 72 h. Their tensile stress− strain behavior and the actual values are shown in Figure 5 and Table 2. Interestingly, depending on the composition, the thermosets exhibited three typical stress−strain behaviors commonly observed for different types of polymers: X-20LB and X-30LB were very flexible with 90−110% elongation at break; X-35LB showed a yielding and strain hardening behavior typical for hard and tough polymers; X-40LB with only a 6% elongation at break exhibited no yielding and behaved like a rigid plastic. It is noticed that the X-40LB thermoset had a significantly higher tensile strength (34.3 MPa) compared to other kraft lignin derived thermosets presented in literature, such as epoxy (5 MPa)27 and polyester (12.9 MPa)34 thermosets which were prepared from heat curing of ozone treated Kraft lignin and sebacic acid epoxy. Furthermore, the tensile strength was also higher than for other polyester thermosets based on citric acid and PEG (8 MPa).35 Generally, an increase of lignin content, thus, had a positive effect on the tensile strength and negative effect on the elongation at break. This could be explained by the increasing of Tg of thermosets with higher lignin contents. Based on these result, we could conclude that the mechanical properties of these lignin thermoset are easily tunable by simply varying the lignin feed ratio. Thermal Stability. The thermal stability of the thermosets was investigated by TGA, and all the TGA curves show three major weight loss stages (Figure 6). The first stage appears at 100 °C, which is probably due to the evaporation of water.23 This water is proposed to originate mainly from water absorption during conditioning. This is supported by the percentage of weight increase for the samples during the conditioning, which was very close to the weight loss percentage around 100 °C during TGA measurements. For example, the X-30LB increased 1.4% of weight during the conditioning, while it lost 1.5% of weight at 100 °C in the TGA

Figure 3. FTIR imaging absorbance intensity patterns of the thermoset surfaces with different lignin contents at 1510 cm−1 (a) and 1250 cm−1 which represent the aromatic −CC and ester −C− O bonds, respectively (b).

1510 cm−1 increased with increasing lignin feed ratio, which is expected and consistent with the earlier presented FTIR result. Additionally, a more homogeneous dispersion of lignin was observed for the X-20LB compared with X-30LB. This may be explained by the higher content of PEG400, which acted as reactive diluent in the system, and could facilitate dispersion and mobility of lignin in the uncured resins. As for the ester bonds, although the ester distribution was not uniform in the D

DOI: 10.1021/acssuschemeng.9b02921 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 4. Storage modulus (a) and tan delta (b) versus temperature for the thermosets with different lignin contents.

thermosets, respectively, which correspond to breaking of αand β-aryl-alkyl-ether linkages in lignin37 and decarboxylation of any partly reacted citric acid.38 The last and most pronounced stage commences at around 250 and 350 °C for X-0LB and other lignin thermosets, respectively, which should correspond to thermal decomposition of the thermoset. Further, the statistic heat-resistant index (Ts) was used to specify the thermal stability data of the thermosets. Based on the 5% and 30% weight loss temperatures, the Ts is calculated by the equation:39,40 Ts = 0.49 × [T5 + 0.6(T30 − T5)]

Table 3 shows that the addition of lignin greatly improved the thermal stability of the thermoset, which could be explained by

Figure 5. Representative examples of the tensile testing curves for the lignin based thermosets with different compositions showing the tensile stress versus strain.

Table 3. Thermal Stability Data for the Cured Thermosets Derived from TGA Analysis

Table 2. Dynamic Mechanical and Tensile Strength Properties of the Different Lignin Thermosetsa Sample

G25.0 (MPa)

Tαa (°C)

X-20LB X-30LB X-35LB X-40LB

5.7 50 630 2000

−0.3 21.6 73.0 102.0

Tαb (°C) −7.9 3.2 25.6 44.5

± ± ± ±

0.4 0.9 1.7 3.3

Tensile strength (MPa)

Elongation at break (%)

± ± ± ±

113 ± 1 101 ± 14 53 ± 8 6±2

1.2 3.4 14.2 34.3

0.1 0.9 3.5 6.2

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Sample

T5 (°C)

T30 (°C)

Tmax (°C)

Ts (°C)

X-0LB X-20LB X-30LB X-35LB X-40LB

97 213 210 212 212

212 358 359 363 362

287 395 394 395 395

83 150 150 151 151

the fact that higher lignin content could increase the crosslinking density. Furthermore, the thermogravimetry of neat lignin generally leads to high char content (42 wt %),17,41 which contributes to higher char residue at 600 °C when the lignin content increases. Shape Memory Behavior. Polymers containing both soft segments controlling a temporary shape and cross-linking sites are expected to have thermally induced shape-memory behavior due to entropic elasticity.42 Here, we selected X35LB with a Tg higher than room temperature as a potential candidate for exhibiting shape memory behavior and investigated this through two experiments. Figure 7a illustrates the shape memory behavior of X-35LB: the sample was first kept with a curled shape at 80 °C for 2 h to make it a permanent shape, and then it was stretched out to a flattened shape and immediately transferred to room temperature. The sample kept the flattened shape for the tested time (1 h) which means a transient shape was formed. Finally, the sample was heated to 80 °C again and its original curled shape was quickly, within 1 min, recovered and became the new permanent shape. Furthermore, stress-controlled cyclic thermomechanical testing was carried out to quantify the shape memory property (Figure 7b). The shape memory fixity rate (εf) and recovery rate (εr) were calculated according to the following equations:

a

Tga values were measured from the peak of tan delta from the DTMA analysis, and Tgb was obtained from DSC analysis.

Figure 6. TGA curves of the thermosets with different lignin contents.

test. X-0LB shows the highest amount of water evaporation, which indicates higher moisture absorption. This could be explained by its high PEG content, which contributes to a larger water absorption ability.36 The second stage commences at around 150 and 250 °C for X-0LB and the other lignin E

DOI: 10.1021/acssuschemeng.9b02921 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 7. (a) Photographs showing the shape memory behavior of X-35LB. (b) Stress-controlled cyclic thermal mechanical testing of X-35LB.

εf (N ) =

εr (N ) =

εu(N ) εd(N )

of the thermosets was, thus, tunable from flexible to rigid by varying the lignin content. The X-40LB with the highest lignin content also exhibited the highest tensile strength (34.3 MPa), higher than what is found for most literature reported lignin based thermosets. X-35LB illustrated a superb shape memory property with shape fixity ratio of 95% and shape recovery ratio of 99%. In conclusion, an easily accessible and green strategy to develop lignin based thermosets with tailorable properties was presented.

(3)

εd(N ) − εp(N ) εd(N ) − εp(N − 1)

(4)

where εf(N) is the shape fixity ratio at the N cycle and εr(N) is the shape recovery ratio at the Nth cycle. εd is the maximum tensile strain with load, εu is the tensile strain after cooling and uploading, and εp is the recovered strain. The calculated εf and εr at different circles were summarized in Table S3. The lignin thermoset showed a constant shape fixity ratio of around 95% during the three performed cycles, while the shape recovery ratio was close to 99% with the exception of the first circle, when it was 85%. The reason why the first circle has a lower recovery ratio can be due to the inherent internal stress existing in the sample or insufficient activation for the soft chains to move in the first circle. Compared to other literature that reported lignin based thermosets such as ozone treated kraft lignin cured with sebacic acid epoxy (εf was 92% and εr was 97%),34 lignin and glycerol-adipic acid based hyperbranched prepolymer epoxy (εf was 91%, εr was 98%),22 our lignin based thermoset exhibited higher εf and εr. Furthermore, comparing our lignin based thermoset to a representative petroleum based thermoset constituted of bisphenol A and epoxy (εf was 97− 99%, εr was 98−99%), the values of εf and εr were similar.43−45 From these results, we can conclude that the synthesized lignin based thermoset, X-35LB, had superb shape memory performance, which could be repeated in several circles. th

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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.9b02921. SEC and 31P NMR spectrum of lignoboost, real time FTIR spectrum of 30LB after different curing time, DSC curves of the thermosets with different lignin contents, TGA curves of the citric acid, PEG400, and lignoboost, and summary of shape fixity and shape recovery for the X-35LB. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Karin Odelius: 0000-0002-5850-8873 Minna Hakkarainen: 0000-0002-7790-8987

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Notes

The authors declare no competing financial interest.

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CONCLUSIONS A series of lignin based thermosets with tunable mechanical properties were successfully synthesized by a green one-pot heat curing of lignin, PEG400, and citric acid through an esterification reaction and water as the only produced byproduct. The esterification reaction was verified by comparison of the FTIR spectrum before and after curing. The increased content of aromatic rings with increasing lignin content was confirmed by IR imaging, which also revealed increasing cross-linking density with increasing lignin content. The thermal mechanical properties of the thermosets was shown as an increase of storage modulus at 25 °C from 5.7 MPa to the significantly higher value of 2 GPa when the lignin feed ratio increased from 20 to 40%. The mechanical behavior

ACKNOWLEDGMENTS The authors gratefully appreciate the support from China Scholarship Council (CSC). REFERENCES

(1) Laurichesse, S.; Avérous, L. Chemical modification of lignins: Towards biobased polymers. Prog. Polym. Sci. 2014, 39 (7), 1266− 1290. (2) Fache, M.; Boutevin, B.; Caillol, S. Vanillin Production from Lignin and Its Use as a Renewable Chemical. ACS Sustainable Chem. Eng. 2016, 4 (1), 35−46. (3) Gang, H.; Lee, D.; Choi, K. Y.; Kim, H. N.; Ryu, H.; Lee, D. S.; Kim, B. G. Development of High Performance Polyurethane

F

DOI: 10.1021/acssuschemeng.9b02921 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssuschemeng.9b02921 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssuschemeng.9b02921 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX