Lignin-based epoxy composite vitrimers with light-controlled remoldability

Vitrimers open new possibilities in the reprocess of epoxy and other thermosets. However, direct heating is not practical on many occasions, and the waste vitrimers would cause great harm to the environment. In this work, we propose to use kraft lignin (KL) to fabricate vitrimer with reprocessability and environmental friendliness. The lignin-based epoxy vitrimer was fabricated by blending epoxy-modified KL and poly(ethylene glycol) bis(carboxymethyl) ether (PEG-DCM). The obtained lignin-based epoxy vitrimer (EML/PEG-DCM) showed good light-to-heat capability. Under the infrared radiation (808 nm, 1 W cm−2) for only 30 s, the surface temperature of EML/PEG-DCM was over ∼148 °C, and reached the maximum at ∼231 °C for 5 min. This good light-to-heat effect can activate the dynamic 3D cross-linking networks and repair the vitrimer. The energy consumption of the light-controlled remolding process is only one-thousandth of the conventional hot-press. This study not only helps to explore the natural characteristics of lignins, promoting their functional and intelligent utilization, but also provides a new raw material platform for the development of green vitrimer materials.


Introduction
Thermosetting polymers such as cured epoxy resin are permanently cross-linked materials, which present excellent electrical insulation, high adhesion, dimensional stability, and corrosion resistance. In the last few decades, they have been widely used as coatings [1][2][3], adhesives, electronic, and electrical [4][5][6] materials in many areas, including machinery and aviation, chemical industry, construction, and automobile. Nonetheless, due to its insoluble and infusible polymer network, conventional thermoset epoxy resin cannot be melted and re-shaped after it is cured, which usually prevents recycling. The concept of vitrimer, introduced by Leibler's team in 2011, makes it possible to reprocess or recycled epoxy resin and other thermosetting polymers [7][8][9]. Profiting from the existence of exchangeable bonds in the crosslinking network, vitrimers not only have similar properties to traditional thermosetting epoxy resins at low temperature but also can be reprocessed (reshaping, welding, recycling) at high temperature. With both thermosetting and thermoplastic properties, vitrimer has become a popular candidate for a variety of functional materials. For example, Wu et al. prepared fully biobased vitrimers with good thermal stability and mechanical properties that could be used as an adhesive [10] and carbon fiber composites with good recycling property [11]. Gao and co-workers combined hydrogen bonds and exchangeable β-hydroxyl esters into acrylate vitrimers, which demonstrated a new strategy for developing a kind of mechanically robust and reprocessable 3D printing thermosets [12]. Niu et al. presented a self-repairable and visualized interactive human motion detection sensor by integrating the vitrimer elastomer with photonic crystals [13]. Numerous studies to improve the reprocessability of vitrimers have been reported; most of them were synthesized from petroleum-based materials and required hotcompaction processes. With the increased awareness in the end of life recyclability, convenient operation, and energy consumption, the fabrication of a green vitrimer which can be remolded accurately and easily still remains a challenge.
In recent years, the development of polymers from renewable resources has grown incessantly both in academia and in the industry [10,[14][15][16][17][18]. Lignin is the largest renewable source of aromatic building blocks in nature [19][20][21][22] and has significant potential to serve as starting material for the production of bulk or functionalized aromatic compounds to offer suitable alternatives to the universally used, petroleum-derived BTX (benzene, toluene, and xylene) [23][24][25][26][27][28]. There are a large number of aromatic rings and conjugated functional groups inside the molecular structure of lignin, which allow the formation of strong conjugation and π-π molecular interactions among lignin molecules [26,[29][30][31], endowing lignin with unique optical properties including aggregation-induced emission, UV absorbance and great potential for sustainable photothermal conversion [32,33]. Zhang et al. obtained a ligninbased photoresponsive actuator that can achieve up to 18% light-driven contraction under loading within 3 s and was successfully applied to power a thermoelectric generator [25]. Inspired by the interaction of the conjugated structure in melanin, Chen et al. used lignin nanoparticles to create a solar-powered thermoelectric generator that was able to drive a motor [26]. Mika et al. report for the first time a onepot catalyst-free preparation of lignin-based vitrimers, and the mechanical properties of the vitrimers can be widely tuned in a facile way [34]. Xu et al. developed an information encryption device using shape memory cellulose acetate as a matrix, and lignin as a photothermal agent [27].
Based on the green character and photothermal conversion capability of lignin, here, we synthesized a lignin-based epoxy vitrimer and proposed a light-to-heat approach for light-controlled remolding of the vitrimer. Lignin has abundant functional groups, such as carboxylic, methoxy, aliphatic and phenolic hydroxyls, and carbonyl groups [35], which give it great potential for chemical modification [36]. In this work, lignin was modified by epoxy (epoxy modified lignin, EML) and mixed with poly(ethylene glycol) bis(carboxymethyl) ether (PEG-DCM). The carboxylic acid groups in PEG-DCM reacted with epoxy directly to form tridimensional networks of lignin-based epoxy vitrimer (EML/PEG-DCM). Under the photothermal conversion effect of lignin, the dynamic networks can be activated under an 808-nm infrared laser without the addition of common expensive photothermal materials. During the curing process, the carboxyl groups in PEG-DCM attacked the epoxy on EML to form ester bonds with the generation of additional hydroxyl groups, resulting in the formation of crosslinked networks. In the presence of zinc acetate catalysis, transesterification reactions took place at elevated temperatures to induce the topological rearrangements of networks. The chemical structures and mechanical properties of the resulting lignin-based epoxy vitrimers were systematically analyzed using Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, tensile strength, and elongation at break. Moreover, we also achieved lightcontrolled remold processes of the workpieces and evaluated their energy consumption and mechanical properties.

Preparation of lignin-based epoxy vitrimers
The lignin-based epoxy vitrimers were prepared by reaction of EML and PEG-DCM under the catalysis of zinc acetate. Firstly, EML, zinc acetate, and PEG-DCM were evenly mixed at room temperature to get a brown sticky mixture. Then, the mixture was poured into a standard dumbbell Teflon mold and cured in a convection oven at 120 °C (or 160 °C) for 4 h (or 6 h). For all samples, the molar ratio of epoxy/COOH was set as 1/1.

Characterization
Fourier transform infrared spectroscopy (FTIR) analysis was measured on an FTIR-650 spectrometer. The sample was scanned 32 times from 4000 cm −1 to 400 cm −1 with a resolution of 1.5 cm −1 .
The epoxy value (mol /100 g) was determined by the acid-acetone titration method. The HCl-acetone solution was obtained by mixing hydrochloric acid and acetone at a volume ratio of 1:40 in a glass vial at room temperature. 1.0 g (accurate to 0.0002 g) of EML sample was accurately weighed and added in a 250 mL conical flask with 25 mL HCl-acetone solution added and reacted with the sample in the dark at room temperature for 1 h. Then, the solution was titrated with 0.1 mol/L NaOH standard solution. Until the pH value of the system reached and kept stable at 7.0, the consumption volume of the NaOH standard solution was recorded as V 1 . At the same time, two blank titrations were carried out according to the above conditions, and the consumption volume of the NaOH standard solution was recorded as V 0 . The epoxy value (EV) can be calculated according to Eq. (1): where V 0 and V 1 represent the consumptions of NaOH standard solution in the blank and the experimental part, respectively. W represents the weight of the sample.
Thermogravimetric analysis was measured on an STA 7500 thermogravimetric (TG) analyzer (TA instruments, America). Samples (about 3 ~ 5 mg) were heated from room temperature to 800 °C at the heating rate of 10 °C/min in a nitrogen atmosphere (100 mL/min).
Proton nuclear magnetic resonance ( 1 H-NMR) spectra were recorded on a Bruker Avance III 400 spectrometer (Bruker, Germany) at room temperature for 8 scans. For preparing the NMR sample, 5 mg of lignin samples were dissolved in 0.5 mL of DMSO-d 6 .
Mechanical properties were tested at room temperature with an LD23.503 testing machine (Lishi Instruments Co. Ltd, Shanghai), and all measurements were made at an extension rate of 5 mm/min. All samples are molded in standard dumbbell molds with a length of 35 mm, a narrow section width of 2 mm, and a thickness of 1 mm. At least three replicate experiments were performed.
Light-healing properties were performed on an LD23.503 testing machine (Lishi Instruments Co. Ltd, Shanghai). The specimens were completely cut perpendicular to the tensile direction and then jointed the fracture surfaces. The healing was performed under 808 nm, 1W/cm −2 infrared laser irradiation for 20 min, and then the test was performed. The tensile rate at room temperature is 5 mm/min. Photothermal characterizations were tested at room temperature by a HIKMICRO H21PRO thermal imaging camera (Hangzhou Hikvision Digital Technology Co. Ltd, Hangzhou) and an 808-nm infrared laser with a power density of 1.00 W/cm −2 .
The photothermal conversion efficiency (η) can be calculated according to Eq. (2) [37]: where Q is the heat generation rate of the sample under irradiation and P is the power of the light. The calculation of Q is critical as P is constant at a given light power. At a specific point of the heating process, Eq. (3) is satisfied: where h represents the heat transfer coefficient and A is the area of heat transfer. ΔT represents the difference between the temperature of our E-P600 and the ambient temperature at a certain time t. Cp is the specific heat of the sample. In this system, the Cp differs as the temperature change, which could be determined by the method of sapphire with DSC is defined as dt/dT, which can be solved through the differential operation of the inverse function of the temperature-rising curve. g(Cp) is defined as Eq. (4): According to the Q, the photothermal efficiency of E-P600 is 86.20% under light radiation at 1 W. (Due to the small sample area, the calculation here ignores the heat loss of heat transfer, and the photothermal efficiency obtained is smaller than the actual one).
The calculation formula of energy utilization coefficient (Eq. 5) is defined as healing efficiency to energy used.
where J is the energy utilization coefficient, H is the healing efficiency, and J is the energy expended to repair the sample.
Differential scanning calorimetry (NETZSCH DSC214) was utilized to determine the specific heat of E-P600. Heating cycles between 0 and 200 ℃ were recorded with a heating speed of 15 ℃/min (Fig. S1).
The angle of contact was were tested at room temperature by a Contact Angle tester (FIBRO System AB, SWEDEN).

Results and discussion
In this work, based on the photothermal effect of lignin [32], a new kind of lignin-based epoxy vitrimers was synthesized to achieve light-controlled remoldability. The synthetic routes, curing, and network topological rearrangements to the lignin-based epoxy vitrimers are shown in Fig. 1. First, epoxy-modified lignin was produced by KL reacted with epichlorohydrin. Excess epichlorohydrin was used as a solvent to reduce the viscosity and hydrolysable chlorine content in epoxy prepolymers. The epoxide equivalent per weight of EML was measured by the titration method to be 0.55 mol/100 g. Subsequently, EML and PEG-DCM were blended homogeneously and cured at 120 °C for 4 h (or 160 °C for 6 h), the carboxylic acid opened epoxies while hydroxyls were generated at the same time. According to the different molecular weights of PEG-DCM, the corresponding samples of the synthesized lignin-based epoxy vitrimers were labeled as E-P600 and E-P2000.
FT-IR, TG, and 1 H NMR tests were measured to verify the successful epoxy modification of KL. Figure 2 shows the FT-IR spectra of KL, EML, and lignin-based epoxy vitrimer (E-P600), indicating that compared with KL, the benzene ring skeleton and the basic structure of EML have not been destroyed. To make a better comparison, the spectra were normalized based on the internal standard peak at 1512 cm −1  Figure 2b shows the TGA and DTG plots of KL and EML. The main weight loss of KL at 250 °C and 365 °C is attributed to the partial degradation of lignin and the removal of the methoxyl group from benzene ring, respectively [39]. For EML, the first peak temperature of DTG appears at 325 °C, which is mainly due to the decomposition of epoxy groups [40]. The second peak temperature of DTG at 385 °C is ascribed to the breakdown of the backbone of KL (Fig. S2). The 1 H-NMR spectrum of KL and EML are shown in Fig. S2. As shown in Fig. S2, protons associated with aromatic protons are observed between 6.5 and 7.5 ppm. The proton signal at around 3.75 ppm was assigned to the methoxy group of lignin [41]. The proton signals at 4.25 ppm (peak a), 2.83 ppm (peak b), and 2.67 / 3.17 ppm (peak c) are chemical shifts of protons on the epoxy group of EML [42]. These results confirmed the successful epoxy modification of lignin.
The optical image of the contact angle test is shown in Fig. 3a. The contact angles of E-P600 and E-P2000 with water are 45.5° and 28.5°, respectively, indicating that the lignin-based epoxy vitrimers are hydrophilic materials. The hydrophilicity of the lignin-based epoxy vitrimers can be inferred from PEG-DCM in the raw material. The contact angle of E-P2000 is smaller than that of E-P600, which can be inferred from that E-P2000 has a higher content of PEG-DCM and a lower crosslinking density. Swelling tests of the  samples were carried out, as Fig. 3c, to demonstrate their hydrophilic differences. As shown in the figure, the initial sizes of samples E-P600 and E-P2000 are both 1 cm × 1 cm. After 2 h, the sizes of E-P600 and E-P2000 increased to 1.1 cm × 1.1 cm and 1.5 cm × 1.5 cm, respectively, and 1.1 cm × 1.1 cm and 1.6 cm × 1.6 cm, respectively, after 24 h. Their weight changes are shown in Table 1, which can calculate that the swelling rates of E-P600 and E-P2000 in water are 179.29% and 414.61%, respectively. In addition, the Shore hardness tester is also used to test the relative hardness of the sample (Fig. 3b). The Shore hardness of the samples E-P600 and E-P2000 are 51 HA and 90 HA, respectively.
The photothermal tests were carried out to evaluate the photothermal effects and stabilities of lignin-based epoxy vitrimers (Fig. 4). The rapid photothermal conversion of lignin-based epoxy vitrimers was detected by an infrared camera under infrared laser irradiation. However, the pure PEG-DCM exhibited near-infrared inertness and little change in its surface temperature, indicating that the conversion of infrared light energy to heat was caused by lignin. The heating and cooling curves of lignin-based epoxy vitrimers and PEG-DCM triggered by an infrared laser with a power density of 1.00 W/cm 2 are shown in Fig. 4b. All samples were heated for 5 min under an infrared laser and then cooled for 7 min with the laser turned off. The surface temperature of E-P600 and E-P2000 rose rapidly after the infrared laser was turned on, and slowed down and leveled off after 1.5 min, with the highest temperatures reaching 231 °C and 170 °C, respectively, compared with no significant changes in those of the pure PEG-DCM control group. It was calculated that the photothermal efficiency of E-P600 is 86.20% under light radiation at 1 W. The photothermal phenomenon of lignin-based epoxy vitrimers can be attributed to a large number of aromatic rings and conjugated functional groups in the molecular structure of EML, which allow the formation of strong conjugation and π-π molecular interaction between EML molecules [26,29]. The conjugated structure of EML effectively promotes electron transitions from low-energy orbitals to high-energy states [32], and the visible and near-infrared light energy absorbed by EML is mainly released in the form of non-radiative transitions. Compared with the E-P2000, the E-P600 has a faster rate of light-to-heat conversion and a higher maximum temperature. This phenomenon could be due to the stronger hydrogen bond interaction between EML molecules in E-P600 and the weaker conjugation effect of benzene rings by the longer molecular chain of E-P2000. The strong molecular interaction promotes the π-π aggregation of EML, thus promoting the photothermal transformation [43]. Moreover, E-P600 showed excellent photothermal stability during five cycles of light-to-heat and cooling Fig. 3 Optical images of E-P600 and E-P2000. a Contact angle test, b shore hardness, and c changes in sizes after swelling  (Fig. 4c), suggesting its good practical application potential as a photothermal functional material. As with traditional vitrimers, lignin-based epoxy vitrimers can be repaired under a thermal stimulus, such as hot pressing, to activate the dynamic covalent bonds in the cross-linked network. However, the processes of the hot press were tedious and energy-consuming. Figure 5 shows that the remolding of E-P600 by an 11 kW hot press at 150 °C needed 2 h and consumed 22 kW·h of electric energy, which is far more than the heat required for the remolding. Based on the good effect and stability of photothermal conversion of lignin-based epoxy vitrimers, here, the in situ light-controlled remolding method was used to improve the convenience and reduce energy consumption (Fig. 4). Compared with the general hot-pressing, the in situ light-controlled method can remold the material without disassembling the damaged parts. The whole procedure only takes 0.02 kW·h of electric energy and 20 min.
The mechanical properties of vitrimers predominantly determine their suitability for various applications [34]. All lignin-based epoxy vitrimer samples were tested by the uniaxial tensile. The stress-strain curves of the resultant lignin-based epoxy vitrimers with different molecular weights of PEG-DCM contents are shown in Fig. 6a. The introduction of flexible PEG segments rendered the lignin-based epoxy vitrimers with tunable mechanical properties by simply adjusting the molecular weight of PEG-DCM in the reaction mixture [44]. When the molecular weight of PEG-DCM rose from 600 to 2000, the tensile strength increased from 1.45 MPa to 4.52 MPa, and the elongation at break decreased from 22.1 to 7.4%. Due to the entanglement between the longer molecular chains of PEG-DCM2000, the tensile strength of lignin-based epoxy vitrimers increased with the increasing molecular weight of PEG-DCM. At the same time, the decrease in the tensile strength of the lignin-based epoxy vitrimers was attributed to the increase in the glass transition temperature of the E-P2000 (Fig. S3).
According to the in situ light-controlled remolding method, the samples were irradiated for 20 min under an 808-nm infrared laser with a power of 1 W/cm 2 . The stress-strain curves of the reprocessed lignin-based epoxy vitrimers are shown in Fig. 6a. Healing efficiency (%) is defined as the ratio of the stress of healed and virgin vitrimers. The results show that the efficient dynamic transesterification reaction in the cross-linking network structure under external thermal stimulation endows the E-P600 with a healing efficiency of 46.2%. However, with the increase of PEG-DCM molecular weight, the healing efficiency of lignin-based epoxy vitrimers decreased, as the healing efficiency of E-P2000 was only 26.3%. This might be due to the relatively low efficiency of ester exchange reaction within a limited time, and the longer chains of PEG-DCM which impeded the movement of polymer chains could also weaken the rearrangement efficiency of chemical cross-linking networks [45]. The stress-strain curves of the repaired E-P600 sample by the hot-pressing method is shown in Fig. S4. Its tensile strength reached 1.22 MPa, and its healing efficiency achieved 84.1%. Compared with the general hot-pressing repair method, the in situ light-control repair method achieved 55% of its effect with only 0.09% energy consumption, whose energy utilization coefficient increased nearly 600 times.
The TGA/DTA results in Fig. 6b show that the ligninbased epoxy vitrimers has good thermal stabilities. The initial decomposition temperature at 5% weight loss and the maximum decomposition rate temperature of E-P600 reached 280 °C and 374 °C, and E-P2000 reached 315 °C and 392 °C, respectively. Compared with that before curing, the thermal stability of EML cured with PEG-DCM has been greatly improved, which also meant that EML and PEG-DCM react adequately.
During the experiment, we noted that the mixture of uncured EML and PEG-DCM had a high viscosity. The E-P600 was used as an adhesive. As shown in Fig. S5, the E-P600 adhesive can withstand direct pull force exceeding 9.0 N. The adhesive mechanism of lignin was that there were phenolic hydroxyl groups, methoxy groups, and free C5 existing on the benzene ring, which could be further cross-linked [46]. Similar to the mechanism, the experiment was carried out under acidic conditions, and the hydrogen atom in the ortho position of phenol was more active [47]. Meanwhile, the adhesiveness of the mixture was enhanced by the introduction of epoxy groups and hydroxyl groups formed during the curing process.

Conclusion
In this work, we have reported unprecedented but low-cost lignin-based epoxy vitrimers, with good photothermal conversion properties and light-controlled remoldability. The lignin-based epoxy vitrimer was fabricated by blending epoxy modified lignin (EML) and poly(ethylene glycol) bis(carboxymethyl) ether (PEG-DCM). The obtained ligninbased epoxy vitrimers showed good light-to-heat capability, which was attributed to the fact that the conjugated structures of lignin could effectively reduce the energy required for the electronic transition from low-energy orbitals to high-energy states and then release the energy mainly in the form of non-radiative transitions. Applying this property, the lignin-based epoxy vitrimers can be remolded by an in situ light-controlled method. Under the infrared radiation (808 nm, 1 W cm −2 ) for only 30 s, the surface temperature of EML/PEG-DCM was over ∼148 °C and reached the maximum at ∼231 °C for 5 min. More importantly, the whole procedure of light-controlled remolding only took 0.02 kW·h of electric energy and 20 min. We believe that this work provides a new idea for the application of lignin in photothermal materials and the research of low-power light-controlled remolding materials.

Conflict of interest
The authors declare no competing interests.