Jianqiao Wu

Xia Yu

Hao Zhang

Junbo Guo

Jun Hu

Min-Hui Li
email: min-hui.li@chimieparistech.psl.eu

Fully Biobased Vitrimers from Glycyrrhizic Acid and Soybean Oil for Self-Healing, Shape Memory, Weldable and Recyclable Materials

Keywords: biobased vitrimers, glycyrrhizic acid, transesterification, shape memory polymers, recyclable adhesives

Vitrimers are covalent polymer networks that can change their topology through thermally activated bond exchange reactions. In this work fully biobased and recyclable vitrimers were developed from epoxidised soybean oil (ESO) and natural glycyrrhizic acid (GL) as it was without additional chemical modification, which avoided the use of nonrenewable petroleum resources and resolved the disposal problems of materials. Due to the unique rigid skeleton of GL, ESO/GL vitrimers showed good thermal stability and mechanical properties. Driven by the transesterification-induced topological network rearrangements, these ESO/GL vitrimers exhibited high performance of welding, repairing and shape memory. They were also recyclable and chemically degradable by ethylene glycol. More importantly, these vitrimers could be used as repairable and recyclable adhesives.

INTRODUCTION

Thermosetting polymers such as Bakelite are permanently cross-linked materials which present excellent thermal stability, dimensional stability, chemical resistance and mechanical properties, and have been widely used in coatings, composites, adhesives and electronic packaging over the past century. [1][2][3][4] But they cannot be reprocessed and reshaped once synthesized. On the other hand, non-cross-linked polymers and those with reversible crosslinks are reprocessable. However, they are soluble and have modest chemical resistance. The concept of vitrimer invented by the group of Leibler [5][6][7] in 2011 has addressed elegantly this dilemma. Vitrimers are covalent polymer networks that can change their topology through thermally activated bond exchange reactions. At high temperatures, vitrimers can flow and behave like viscoelastic liquids. Therefore, they can be reprocessed and recycled as thermoplastics. At low temperatures, exchange reactions are very slow and vitrimers behave like classical thermosets. Having the properties of both thermosets and thermoplastics, vitrimers have immerged as appealing candidates for self-healing materials, [8][9][10] shape memory polymers, [11][12][13] soft actuators, [14][15][16][START_REF] Lu | Tunable photocontrolled motions using stored strain energy in malleable azobenzene liquid crystalline polymer actuators[END_REF] and 3D printing thermosets. [START_REF] Shi | Recyclable 3D printing of vitrimer epoxy[END_REF][START_REF] Zhang | Reprocessable thermosets for sustainable three-dimensional printing[END_REF] At the same time, the interest in polymers from renewable resources has grown incessantly both in academia and in industry. The green chemistry connotations in the conception of polymer materials are opening the way to the sustainable development of new generation of polymers. [START_REF] Gandini | Progress of polymers from renewable resources: Furans, vegetable oils, and polysaccharides[END_REF][START_REF] Zhu | Sustainable polymers from renewable resources[END_REF] For vitrimer preparation, renewable resources from vegetal, animal, or fungal have been used more and more as the building blocks, [START_REF] Altuna | Self-healable polymer networks based on the crosslinking of epoxidised soybean oil by an aqueous citric acid solution[END_REF][START_REF] Liu | Eugenol-derived biobased epoxy: shape memory, repairing, and recyclability[END_REF][START_REF] Zhang | Preparation of a ligninbased vitrimer material and its potential use for recoverable adhesives[END_REF][START_REF] Liu | A selfhealable high glass transition temperature bioepoxy material based on vitrimer chemistry[END_REF][START_REF] Wang | Robust, Fire-safe, monomer-recovery, highly malleable thermosets from renewable bioresources[END_REF][START_REF] Wang | Facile in situ preparation of high-performance epoxy vitrimer from renewable resources and its application in nondestructive recyclable carbon fiber composite[END_REF][START_REF] Ma | Readily recyclable, high-performance thermosetting materials based on a lignin-derived spiro diacetal trigger[END_REF][START_REF] Geng | Vanillin-based polyschiff vitrimers: Reprocessability and chemical recyclability[END_REF][START_REF] Dhers | Fully bio-based polyimine vitrimer derived from fructose[END_REF] which can reduce the consumption of non-renewable resources and may endow materials with biocompatible and biodegradable properties. For example, Zhang and co-workers prepared a series of vitrimer materials from eugenol, [START_REF] Liu | Eugenol-derived biobased epoxy: shape memory, repairing, and recyclability[END_REF] lignin, [START_REF] Zhang | Preparation of a ligninbased vitrimer material and its potential use for recoverable adhesives[END_REF] vanillin, [START_REF] Liu | A selfhealable high glass transition temperature bioepoxy material based on vitrimer chemistry[END_REF] and guaiacol, [START_REF] Liu | A selfhealable high glass transition temperature bioepoxy material based on vitrimer chemistry[END_REF] which were weldable, reprocessable and self-healable. The group of Zhu found that the Schiff base covalent vitrimers prepared from vanillin exhibited the high performance beyond the ones from commercial bisphenol A epoxy resin, [START_REF] Wang | Robust, Fire-safe, monomer-recovery, highly malleable thermosets from renewable bioresources[END_REF][START_REF] Wang | Facile in situ preparation of high-performance epoxy vitrimer from renewable resources and its application in nondestructive recyclable carbon fiber composite[END_REF][START_REF] Ma | Readily recyclable, high-performance thermosetting materials based on a lignin-derived spiro diacetal trigger[END_REF] and could be facilely recycled from carbon fiber reinforced composites. [START_REF] Wang | Facile in situ preparation of high-performance epoxy vitrimer from renewable resources and its application in nondestructive recyclable carbon fiber composite[END_REF][START_REF] Ma | Readily recyclable, high-performance thermosetting materials based on a lignin-derived spiro diacetal trigger[END_REF] However, in these reported works tedious chemical modifications on biological resources were necessary to offer reactive sites for curing. Therefore, we find necessary to develop the fully biobased vitrimers using renewable resources without excessive chemical modification before curing. In this paper, we report on the preparation of fully biobased vitrimers from epoxidised soybean oil (ESO) and natural glycyrrhizic acid (GL) based on the transesterification reactions. [START_REF] Li | Study of the transesterification of a main-chain mesomorphic polyester by small-angle neutron scattering[END_REF] Glycyrrhizic acid (GL) is a natural triterpene saponin found mainly in licorice roots and has been widely applied as food sweeteners and medicinal ingredients. [START_REF] Ploeger | The pharmacokinetics of glycyrrhizic acid evaluated by physiologically based pharmacokinetic modeling[END_REF][START_REF] Kornievskaya | NMR and photo-CIDNP investigations of the glycyrrhizinic acid micelles influence on solubilized molecules[END_REF][START_REF] Tykarska | Supramolecular organization of neutral and ionic forms of pharmaceutically relevant glycyrrhizic acid-amphiphile self-assembly and inclusion of small drug molecules[END_REF][START_REF] Han | Anti-allergic activity of glycyrrhizic acid on IgE-mediated allergic reaction by regulation of allergyrelated immune cells[END_REF] It has a rigid hydrophobic triterpenoid and a hydrophilic diglucuronic unit (Figure 1), which allow GL to be easily incorporated into the biocompatible self-assembled materials. [START_REF] Saha | Fibrillar networks of glycyrrhizic acid for hybrid nanomaterials with catalytic features[END_REF][START_REF] Ma | Multiple water-in-oil-in-water emulsion gels based on selfassembled saponin fibrillar network for photosensitive cargo protection[END_REF][START_REF] Wan | Thermoresponsive structured emulsions based on the fibrillar self-assembly of natural saponin glycyrrhizic acid[END_REF][START_REF] Ma | A natural triterpene saponin-based pickering emulsion[END_REF] GL contains three carboxylic acid groups and can react with epoxy directly as hardener. In addition, the multiple hydroxyl groups in GL are also beneficial to the transesterification reactions of epoxy-acid vitrimer networks. [START_REF] Han | A catalyst-free epoxy vitrimer system based on multifunctional hyperbranched polymer[END_REF] Moreover, the unique rigid skeleton of GL should be able to improve the thermal stability and mechanical properties of materials. On the other hand, epoxidised soybean oil (ESO), as a common plasticizer in the plastics industry, is low-cost, non-toxic and non-volatile substance, which can be crosslinked with amines, [START_REF] Ratna | Mechanical properties and morphology of epoxidized soyabean-oil-modified epoxy resin[END_REF] carboxylic acid, [START_REF] Wang | Flame-retardant pressure-sensitive adhesives derived from epoxidized soybean oil and phosphorus-containing dicarboxylic acids[END_REF] and anhydrides [START_REF] Gerbase | Dynamic mechanical and thermal behavior of epoxy resins based on soybean oil[END_REF] to form tridimensional networks. [START_REF] Qi | Epoxidized soybean oil cured with tannic acid for fully bio-based epoxy resin[END_REF] Taking advantage of the properties of both GL and ESO, we prepared here the fully biobased vitrimers from ESO and GL with different ESO/GL stoichiometric ratios. Both materials come from food grade sources and are used directly without additional chemical modification. The schematic diagram of the curing and network topological rearrangements is shown in Figure . 1.

During the curing process the carboxyl groups in GL attacked the epoxy on ESO to form ester bonds with the generation of additional hydroxyl groups, and the hydroxyl groups in GL can also react with epoxy to afford ether bonds, etc. (Scheme S1), resulting in the formation of cross-linked networks. In the presence of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) catalysis, transesterification reactions took place at elevated temperature to induce the topological rearrangements of networks. The ESO/GL networks were studied in detail by stress relaxation tests, creep-recovery test, thermodilatometry tests, etc., and were evidenced to be weldable, repairable, reprocessable and recyclable vitrimers. Moreover, we used ESO/GL vitrimers to produce a repairable and recyclable adhesive. This fully biobased materials containing 100% renewable carbon content will enrich the species of biobased vitrimers and show a potential for biomedical applications.

Preparation of ESO/GL networks.

As shown in Figure S2, TBD (5% molar ratio to carboxylic acid) was first added into ESO using a beaker at 100°C, and then the solution was heated to 120°C.

Upon addition of GL, the mixture was poured into a mould with a dimension of 100 mm × 80 mm × 1 mm. Finally, the mould was put into a press vulcanizer and cured at 160°C for 1 h, 180°C for 2 h, and 200°C for 2 h at a pressure of 200 kg/cm 2 .

Gel content test.

The dry sample (~30 mg, m0) was immersed in 5 mL acetonitrile for 20 h. After 20 h, the samples were dried in a vacuum oven at 60°C for 10 h and then weighted (m1). The gel content was calculated according to the following equation,

Gel contents (%) = 𝑚𝑚 1 𝑚𝑚 0 × 100% (1)
Swelling test. The swelling ratio was measured by immerging the sample (~30 mg, m0) in acetonitrile at room temperature. When the absorption reached equilibrium, the sample was taken out of the solution. The excessive surface solvent was removed with a filter paper, and the swollen sample was weighted (m1). The swelling ratio was calculated according to the following equation,

Swelling ratio (%) = 𝑚𝑚 1 -𝑚𝑚 0 𝑚𝑚 0 × 100% (2)
Dynamic mechanical analysis. Dynamic mechanical properties were measured using a dynamic mechanical analyzer (DMA, TA-Q800) in tension mode. The sample with a dimension of 30 mm × 5 mm × 1 mm was scanned from -50 to 200°C at a heating rate of 5°C/min. The amplitude was set at 5 µm and the frequency was 1 Hz.

Stress relaxation test.

Stress relaxation was measured using DMA (TA-Q800). The sample dimension was 30 mm × 5 mm × 1 mm. The sample was heated to appointed temperature and soaked for 15 min with a static force (0.01 N). After that, an instantaneous strain of 5% was forced to the material. The stress and modulus was detected for several hours until it achieved the equilibrium.

Creep-recovery test. Creep-recovery behavior was measured using DMA (TA-Q800). The sample dimension was 30 mm × 5 mm × 1 mm. Initially the sample was heated to 200°C and soaked for 15 min, and then a constant force was added to stretch material (stress constant at 8 kPa). After 1 h, it was revoked and the material recovered for another 1 h.

Thermodilatometry test. Thermodilatometry was measured using DMA (TA-Q800) in controlled force tension mode. The sample dimension was 30 mm × 5 mm × 1 mm. A constant force was added to stretch material which made the stress constant at 2 kPa. Sample was heated to 250°C as a heating rate of 5°C/min.

Welding test. Welding property was studied using an oven at 200°C. Three pieces were assembled into an "H" pattern without any pressure at 200°C for 15 min.

Repairing test. Repairing property was studied using an optical microscope (UPT200I). The sample was cracked using a knife and repaired at an oven at 200°C. A glass slide was added on the sample to offer a small pressure.

Shape memory test. Shape memory property was characterized using DMA (TA-Q800) in the tension mode. The sample dimension was 30 mm × 5 mm × 1 mm. Initially, the sample was equilibrated at 60°C (above Tg) for 40 min. Subsequently, a constant force of 0.03 N was applied to the sample and the temperature was decreased to 0°C (below Tg) at a cooling rate of 5°C/min.

When the temperature reached 0°C, the constant force was removed, and the sample was heated back to 60°C at a heating rate of 5°C/min. The above steps were repeated for three times and the displacements were recorded during the test.

Physical recycle. Physical recycle properties were studied using DMA (TA-Q800). The sample pieces were gathered and put into a mould on a press vulcanizer at 200°C for 135 min at 200 kg/cm 2 pressure to form new samples. The sample dimension was 30 mm × 5 mm × 1 mm. All the samples were cooled to -40°C for soaking 15 min and then tested using DMA stress-strain mode.

Lap-shear test. Lap-shear measurements were carried out on testing machine (UTM5205XHD) according to ISO 4587: 2003 standard. The aluminum sheets were 100 mm × 25 mm × 1 mm, and cleaned by soap water, Di-water and isopropanol before use. Initially, uncured ESO/GL (R = 0.5, TBD 5 mol%) mixture was loaded between two aluminum sheets, and the overlap area was 12.5 mm × 25 mm with a thickness of the interval 0.2 mm. After that, the curing was performed on a press vulcanizer with the same condition as ESO/GL networks. For repairing process (Mode I), the failed two halves of adhesive were re-bonded together through hot press at 200°C with 200 kg/cm 2 pressure for 135 min. For recycled process (Mode II), the broken samples and redundant cured samples were collected and grinded into powders. After that, ~70 mg of them was loaded on aluminum, followed by hot press at 200°C for 135 min at 200 kg/cm 2 pressure. At least five repeats were performed for each sample.

Chemical degradable test. Chemical degradable property was investigated by using ethylene glycol (EG, 5 mL) in vials at 180°C. The sample was cut to 20 ~30 mg pieces (m0), and then immersed in EG at 180°C. The sample was taken out of the vial at 1 h, 2 h, till to 14 h, and dried at 180°C and weighted (m1). The relative weight was calculated according to the following equation and at least three samples were studied to get an average value.

Relative weight (%) =

𝑚𝑚 1 𝑚𝑚 0 × 100% (3)

RESULTS AND DISCUSSION

Curing and structure characterization of ESO/GL networks. The curing reactions of ESO and GL were performed in the presence of TBD (5% molar ratio to carboxylic acid) which served as a catalyst for the transesterification reactions. Due to the poor solubility of GL in ESO, the stoichiometric ratios (R = carboxylic acid/epoxy) were set up as 0.4, 0.5 and 0.6. In this case, the excess epoxy groups would react with hydroxyl groups to offer a fraction of partial permanent linkages. Meanwhile, a control system was designed by using glycyrrhetinic acid (GA) to react with ESO for verifying the curing reactivity of all three carboxyl groups of GL (Figure S3). As can be seen from the structures of GA and GL in Figure S3a, all three carboxyl groups of GL are alicyclic with the similar chemical environment, and their pKa values are 3.98, 4.62 and 5.17 for carboxyl group I, II, and III, respectively, [START_REF] Zeng | Determination of the polyacid dissociation constants of glycyrrhizic acid[END_REF] while only carboxyl group III exists in GA. That is to say, carboxyl group III is less reactive than carboxyl group I and II. The control results showed that carboxyl group III can easily attack epoxy group to form polymer networks supported by the exothermic peak at 210°C (Figure S3b) and the obvious glass transition around 53°C after curing (Figure S3c). Because there were excess epoxy groups in ESO/GL systems, it was assumed that all three carboxyl groups of GL could participate in the curing reactions.

In order to establish the curing process, differential scanning calorimeter (DSC) was used to determine the phase transition and the reaction peaks in the range from 50 to 250°C with a heating rate 5°C/min. As shown in Figure 2a, there was a broad endothermic peak at 195°C for the melting point of GL, while no peak for liquid ESO. When heating the mixture of GL and ESO (R = 0.5) without catalyst, a broad exothermic peak at 180°C was observed clearly with enthalpy of 165 J/g, indicating the reaction between carboxylic acid and epoxy groups and the formation of crosslinked networks. As for the mixture of ESO/GL (R = 0.5, TBD 5 mol%), a broad exothermic peak at a little higher temperature (200°C) was detected as shown in Figure 2a, indicating again the curing reaction and the formation of crosslinked networks. To ensure no degradation of catalyst during the curing reactions at such high temperature, the thermal stability of TBD and ESO/GL/TBD mixture were studied using thermogravimetric analyzer (TGA) in Figure S4. Generally, free TBD monomer at high temperature (above 150°C) can cause side reactions and degradation (Figure S4a), whereas the mixture of ESO/GL/TBD (R = 0.5, TBD 5 mol%, 0.945 wt%) showed well thermal stability with a 0.945% degradation at 215°C (Figure S4b), revealing that TBD did not decompose as the pure one. This phenomenon also existed even for higher TBD content (R = 0.5, TBD content: 50 mol%, 8.6 wt%), where the 8.6 wt% loss was achieved at 269°C (Figure S4c).

According to the above TGA results and other reported of TBD, [START_REF] Binkowska | Structural and spectroscopic studies of the 1:1 complex of 4-nitrophenyl[bis(ethylsulfonyl)]methane-1,5,7-triazabicyclo[4.4.0]dec-5-ene[END_REF][START_REF] Ng | Structural and spectroscopic studies of the 1:2 complex of 1,5,7-triazabicyclo[4.4.0]dec-5-ene-4-nitrophenyl[END_REF][START_REF] Huczyński | Spectroscopic studies of the 1:1 complexes of 4-nitrophenyl[bis(ethylsulfonyl)]methane and phenyl[bis(ethylsulfonyl)]methane with 7-methyl-1,5,7-triazabicyclo[END_REF] it was assumed that TBD may form a transition structure with carboxyl group of GL as shown in Scheme S2, thus enhancing the thermo stability of TBD in comparison with free TBD monomer. However, this transition structure may hinder the curing reactions, which consequently caused the temperature of exothermic peak of of ESO/GL with TBD a little higher than mere ESO/GL. As can be seen from the DSC results (Figure 2a), the exothermic peak of curing reaction of ESO/GL ranged from 170 to 230°C. In this case, in order to make the curing process cover this temperature range and avoid the degradation of GL, 160°C, 180°C, and 200°C were set up as the pre-curing, curing, and post-curing temperatures, respectively. The pre-curing temperature (160°C) was used to make the initial crosslinking slow and prevent flash polymerization, while the post-curing temperature (200°C) was used to guarantee the complete reaction and to remove internal stress of materials. Meanwhile, pressure was also necessary for the curing process [START_REF] Zhang | Preparation of a ligninbased vitrimer material and its potential use for recoverable adhesives[END_REF][START_REF] Dhers | Fully bio-based polyimine vitrimer derived from fructose[END_REF] because it not only made resin flow but also exhausted voids to help vitrimer preparation. Given above, the curing process was set as "160°C /1 h + 180°C /2 h + 200°C /2 h" at a pressure of 200 kg/cm 2 for ESO/GL with different stoichiometric ratios and 5 mol% of TBD.

The curing reaction between ESO and GL in ESO/GL mixture was further confirmed by FTIR, where peaks of epoxy at 843 and 823 cm -1 disappeared after curing, revealing the complete consumption of epoxy groups (Figure 2b). [START_REF] Altuna | Self-healable polymer networks based on the crosslinking of epoxidised soybean oil by an aqueous citric acid solution[END_REF] Additionally, the signals at 3430, 2925, 1740, 1730, and 1655 cm -1 were assigned to hydroxyl, methylene, ester, carboxyl and ketone groups from backbones of ESO and GL, respectively (Figure S5). Notably, the materials were black after curing which may due to the oxidation of trace diglucuronic units on GL. For confirming this assumption, a control experiment was performed by using carbenoxolone (CBX, Figure S6a) to react with ESO under the similar curing condition as ESO/GL. As shown in Figure S6b, the cured sample of ESO/CBX was transparent, implying that the black color of ESO/GL vitrimer was attributed to the oxidation reaction of trace diglucuronic units. The formulations and thermal properties of ESO/GL vitrimers were summarized in Table 1. Gel content and swelling ratio were investigated for proving the effective crosslinking. All networks exhibited similar gel contents in the range from 89 to 91% and had low swelling ratios at room temperature from 14 to 17%, similar to those of thermosets with high crosslinking density.

Moreover, the swelling ratio decreased as expected when the loading of GL increased, i.e., when the concentration of carboxyl and hydroxy groups increased. a R represents the stoichiometric ratio of carboxylic acid/epoxy. b The swelling ratio and gel content of ESO/GL networks were measured using acetonitrile as solvent. c The temperature at 5 wt% loss (Figure S7). d Tg values determined from DSC (Figure 2c). e The peak temperature from tan δ curve (Figure 2d).

To test the thermal stability of ESO/GL networks, TGA under nitrogen atmosphere was used.

As shown in Figure S7 and Table 1, the temperature at 5% of weight loss (Td5) for each material was above 310°C, revealing a good thermal stability of ESO/GL networks that did not decompose under 300°C. Glass transition temperature (Tg) is a parameter acting as the upper limit temperature for the use as thermosets. The Tg of ESO/GL networks measured by DSC were shown in Figure 2c, the middle point of the steps at 8, 13, and 18°C being assigned as Tg for ESO/GL networks with R = 0.4, 0.5 and 0.6, respectively. The gradient increase of Tg with R value was mainly due to the rigid skeleton of GL in the networks. On one hand, the higher content of GL offered more reactive sites (carboxyl and hydroxyl groups) for curing reaction and caused an increase in crosslinking density as shown in the Table 1. On the other hand, the skeleton of GL made polymer chains more rigid, and could also increase the value of Tg. Indeed, the viscoelastic properties of ESO/GL networks (R = 0.4, 0.5, 0.6) were investigated using dynamic thermomechanical analysis (DMA). As shown in Figure . 2d, a clear transition from glass plateau to rubbery plateau was observed for each sample, and the storage modulus at glass plateau was 2200 MPa for all three samples. Compared with the system based on ESO and citric acid with the storage modulus ranging from 600 to 400 MPa, [START_REF] Altuna | Self-healable polymer networks based on the crosslinking of epoxidised soybean oil by an aqueous citric acid solution[END_REF] the higher storage modulus of our system was mainly due to the stiff structure of GL. The peaks of tan δ at 39, 49, 64°C were assigned as Tg, and their variation tendency was consistent with the one observed by DSC, which further indicated that the rigid structure of GL and the high crosslinking density led to the increase of Tg. Such mild temperatures of Tg are favorable for obtaining shape memory properties around room temperature.

Characterization of topological network rearrangements. The mechanism of transesterification reactions (TERs) was shown in Figure 1, where hydroxyl groups reacted with ester bonds to generate other hydroxyl groups and ester bonds, consequently resulting in the topological rearrangements of crosslinked networks. Since the breakage of old bonds and the formation of new bonds happened simultaneously, the rearrangements did not influence the crosslinking density of materials. 5,8 The evidence of this thermally-activated TERs could be revealed by thermal stress relaxation experiment, which examined the stress as a function of an external applied strain to the materials. Figure 3a Analyzing in detail the relaxation process, we found the system relaxation followed a Maxwell model fitted by an Arrhenius-type equation:

𝑙𝑙𝑙𝑙𝜏𝜏 * = 𝐸𝐸 𝑎𝑎 𝑅𝑅𝑅𝑅 -𝑙𝑙𝑙𝑙𝑙𝑙 (4)
where τ* is the relaxation time defined as the time needed to achieve a particular G/G0 value (1/e) at a given temperature, Ea is the activation energy of the bond exchange, T is the experimental temperature, and R is the universal gas constant. As shown in Figure 3b, the perfect fitting was obtained with Ea around 112 kJ mol -1 , which was in good agreement with the values reported by Leibler 5 and other researchers [START_REF] Shi | Recyclable 3D printing of vitrimer epoxy[END_REF][START_REF] Altuna | Self-healable polymer networks based on the crosslinking of epoxidised soybean oil by an aqueous citric acid solution[END_REF][START_REF] Yu | Influence of stoichiometry on the glass transition and bond exchange reactions in epoxy thermoset polymers[END_REF]50 (69~150 kJ mol -1 ). In addition, using Maxwell equation 50,[START_REF] Chen | Durable liquid-crystalline vitrimer actuators[END_REF] a topology freezing transition temperature (vitrification temperature, Tv) of ESO/GL networks (R = 0.5, TBD 5 mol%) was determined as 69°C (Figure S8).

Besides, the stress relaxation curves of ESO/GL networks with different stoichiometric ratios at 200°C were measured as shown in Figure 3c. Interestingly, the relaxation time of ESO/GL networks with R = 0.6 as well as with R = 0.4 was slightly higher than the one with R = 0.5.

Normally, a lower R = 0.4 corresponded to a higher content of ESO and more flexible chains within networks, facilitating the TERs. However, with R = 0.4, the amount of reactive hydroxyl groups from GL decreased simultaneously. Consequently, the relaxation time of ESO/GL networks with different R = 0.4, 0.5 and 0.6 was in the same order with only slight difference. As a matter of fact, the relaxation time was the result of the synergistic effect between the flexible chains and the hydroxyl groups. [START_REF] Han | A catalyst-free epoxy vitrimer system based on multifunctional hyperbranched polymer[END_REF][START_REF] Snyder | Reprocessable acid-degradable polycarbonate vitrimers[END_REF] In the following study, we continued to select ESO/GL networks with R = 0.5 as the example.

Creep-recovery behavior and thermodilatometry of ESO/GL networks (R = 0.5, TBD 5 mol%)

were also investigated for verifying further TERs. As shown in the Figure 3d, the material flowed after an initial elastic response at 200°C, due to the TERs. The strain increased to 10.5% until the stress was revoked after 60 min. Then, the strain decreased slightly and retained 7.1% of elongation in the remaining time. Furthermore, its malleability temperature (Tmall) was measured using DMA with controlled force model as shown in Figure S9. A notable increase in the expansion coefficient appeared at the temperature above 138°C, defined as Tmall. 14,[START_REF] Chen | Rapid stress relaxation and moderate temperature of malleability enabled by the synergy of disulfide metathesis and carboxylate transesterification in epoxy vitrimers[END_REF] After this temperature, the strain increased drastically with temperature because of the accelerated transesterification rate, and the material became malleable driven by the topological network rearrangements.

Welding, repairing, and shape changing properties. The TERs-induced stress relaxation imparted multi-functionalities to materials such as welding, repairing, and shape changing. To study welding properties three pieces of ESO/GL (R = 0.5, TBD 5 mol%) materials were selected to make up an "H" pattern as shown in Figure 4a, and the welding was proceeded at 200°C for only 15 min without pressure. After the network rearrangements, an integral material was produced and could bear nearly ~100 g weight which was 300 times heavier than itself (Figure 4b), highlighting the good weldability.

For repairing properties, a piece of sample was scratched to have a crack on the surface, and then was heated at 200°C in an oven. As shown in Figure 4c, the width of the crack decreased from 100 to 19 μm within 5 min and eventually disappeared after 60 min, exhibiting the repairing ability.

The similar repairing results were also obtained for R = 0.4 and 0.6, which indicated the quick rate of TERs in ESO/GL networks (Figure S10). Note that for the ESO/GL networks (R = 0.5, TBD 5 mol%), there were four characteristic temperatures: Tg(DMA) = 49°C, Tv = 69°C, Tmall = 138°C and Td5 = 328°C. Figure 5a showed its deformation-time curves for temperature range from 0 to 60°C to evaluate the consecutive shape memory around the glass transition (Tg), where all curves were normalized according to the maximum deformation at the first cycle for a better comparison. The shape fixity (Rf) and shape recovery (Rr) were defined as the following equations:

𝑅𝑅 𝑓𝑓 = 𝜀𝜀 𝑑𝑑 𝜀𝜀 𝑙𝑙𝑙𝑙𝑎𝑎𝑑𝑑 × 100% (5)
𝑅𝑅 𝑟𝑟 = 𝜀𝜀 𝑑𝑑 -𝜀𝜀 𝑟𝑟𝑟𝑟𝑟𝑟 𝜀𝜀 𝑑𝑑 × 100% (6)
where εload represents the maximum deformation under load, εd is the fixed deformation after cooling and load removal, and εrec is the recovered deformation. The calculated Rf and Rr values for each cycle were listed in Table 2. The value of Rf was approximately 92% for all three cycles, and such good dimensional stability was attributed to the rigid backbone of GL within the networks.

In addition, the value of Rr was higher than 94% in each cycle which revealed that ESO/GL networks (R = 0.5, TBD 5 mol%) had a great shape memory property. Figure 5b and Movie S1 demonstrated the direct visualization of the vitrimer reprocessing and shape memory. Initially, the flat ribbon of ESO/GL networks (R = 0.5, TBD 5 mol%) was heated above Tg for 10 s to deform a twisted ribbon fixed by using a tweezer (Process I). Subsequently, the twisted ribbon was heated above Tmall for 10 min using a blower, which made stresses relax due to the topological network rearrangements. After cooling to room temperature the permanent twisted ribbon was obtained (Process II). This permanent twisted ribbon shape could not recover to the original flat shape because of the "plasticity deformation" by TERs. 11,12,[START_REF] Zou | Dynamic covalent polymer networks: from old chemistry to modern day innovations[END_REF] Furthermore, it could be fixed to another "temporary form" and recovered using heating-cooling process at a temperature above Tg. As shown in Figure 5b (Process III), the twisted ribbon was heated above Tg for 10 s and deformed to a temporary form "J". Finally, the "J" recovered to the twisted ribbon again after heating above Tg for 10 s (Process IV). In this cycle (Process III and IV), the external stress was applied when the mobility of chain segments was activated above Tg to get a macroscopic deformation. Such a deformation can be temporarily fixed by decreasing the chain mobility through cooling, while re-activating the chain segments by heating led to a shape recovery driven by the stored entropic energy. Obviously, the permanent deformation and shape memory properties allow the design of 3D structures from our biobased ESO/GL vitrimers. Furthermore, as Tg of ESO/GL networks (R = 0.5, TBD 5 mol%) was close to apparent temperature of human beings, we tried to achieve the shape changing and recovery of ESO/GL vitrimer on hands instead of using heating stage or oven. As shown in Movie S2, the shape of flat film was changed by apparent temperature and fixed easily at room temperature. Also, it just took 100 s to recover the original flat shape upon heating by hands (Movie S3). Since the shape memory property of ESO/GL vitrimers (R = 0.5, TBD 5 mol%) only depended on apparent temperature of hands, we called it "apparent temperature shape memory, ATSM", which may have the potential application in wearable soft devices.

Recycling and lap-shear properties. After confirming the malleability of the TERs-based ESO/GL networks (R = 0.5, TBD 5 mol%), we evaluated their recyclability. The physical recycle process was shown in Figure 6a. The original samples were initially broken into pieces, and then were re-filled into a mound. After hot press at 200°C, these pieces were fully recycled to form integral samples. Figure 6b showed the representative stress-strain curves before and after recycling. The tensile strength reduced from 3.1 to 1.4 MPa with a recycle efficiency of ~45%, and the decrease in tensile strength was mainly due to: 1) the breakage of partial permanent linkages between hydroxyl groups with ESO, 2) the oxidation of trace diglucuronic units on GL, and 3) the interfacial defects in recycled vitrimers (Figure S11). Conversely, the tensile modulus of recycled material maintained 2366 MPa in comparison with 2483 MPa of the original one. The high recycle efficiency in tensile modulus (~95%) indicated that the holistic chemical structure of networks hardly changed after the recycle process, with keeping the similar molecular weight and rigidness of vitrimer networks. Having the repairing and recycling properties of ESO/GL networks (R = 0.5, TBD 5 mol%) in hands, its application as adhesive was investigated using lap-shear test. Adhesion samples were prepared by bonding two aluminum sheets with uncured ESO/GL (R = 0.5, TBD 5 mol%) as

shown in Figure 6c. The bonding area was 12.5 mm × 25 mm and the thickness of the interval was 0.2 mm. After curing, two aluminum sheets were bonded together successfully (original adhesive)

and the lap-shear strength was measured to 3.42 MPa (Figure 6d). Unlike traditional epoxy adhesives the network of ESO/GL adhesive can be topologically rearranged due to the TERs.

Therefore, the failed two halves of adhesive were facilely re-bonded together using hot press at 200°C (Mode I, repaired adhesive, Figure 6c) with a lap-shear strength 2.89 MPa (Figure 6d).

Notably, after the tests the adhesive can be found on both halves of aluminum sheets for original and repaired ones (Figure S12), which meant that the lap-shear damage was caused by cohesive failure, not adhesive failure between aluminum and adhesive interfaces. In addition, the redundant cured ESO/GL (R = 0.5, TBD 5 mol%) grinded into powder could also be employed as adhesive (Mode II, recycled adhesive, Figure 6c) with a lap-shear strength 2.49 MPa (Figure 6d). However, for the recycled adhesive sample the lap-shear damage was mainly caused by adhesive failure (Figure S12), probably because the cured ESO/GL powder could not contact with aluminum surface as well as the uncured one. It was worth noting that the biobased ESO/GL adhesive had comparable properties with those of biobased adhesives and of some commercial adhesives (Table S1). Moreover, this is the first report on the recyclability of biobased adhesives. 10,[START_REF] Zhang | Preparation of a ligninbased vitrimer material and its potential use for recoverable adhesives[END_REF] Chemical degradability and cell viability. In practice, materials need to be degraded when they could not maintain their useful properties. The chemical degradability of ESO/GL networks (R = 0.5, TBD 5 mol%) was investigated at high temperature (180°C) by using ethylene glycol (EG) as the reactant and solvent. As shown in Figure 7a, the square sample became smaller and irregular in shape, and the EG solution changed from transparent to brown after 14 h. The relative weight as a function of degradation time was shown in Figure 7b, where the weight decreased rapidly to 30% within the first 6 h, then decreased slowly to 7% until 12 h, and finally stabilized until 14 h.

Apparently, the hydroxyl groups of EG participated in TERs of ESO/GL networks at 180°C, thus causing the network degradation as shown in Figure 7c. [START_REF] Yu | Carbon fiber reinforced thermoset composite with near 100% recyclability[END_REF][START_REF] Kuang | Dissolution of epoxy thermosets via mild alcoholysis: the mechanism and kinetics study[END_REF] Notably, the residual of sample (7%)

was mainly due to partial permanent linkages offered by hydroxyl-epoxy reactions during curing process.

The biocompatibility of ESO/GL vitrimer (R = 0.5, TBD 5 mol%) was evaluated by the cell culture experiment of fibroblasts NIH-3T3 in the presence of the vitrimer. As shown in Figure 7d, dead cells were stained in red and live cells in green. Cell images showed that the majority of fibroblasts NIH-3T3 still survived in the presence of vitrimer for 24 h. Due to its outstanding biocompatibility and natural origin, the ESO/GL vitrimer may find potential application in biorelated fields. MPa were detected for original, repaired and recycled samples of ESO/GL, demonstrating that this vitrimer can be applied as repairable and recyclable adhesives. Furthermore, the vitrimer could be degraded effectively by ethylene glycol, and exhibited excellent biocompatibility. Our fully biobased multifunctional vitrimers enrich the library of biobased vitrimers and thermosets.

ASSOCIATED CONTENT Supporting Information.

The following files are available free of charge.

Notes

The authors declare no competing financial interest.

For Table of Contents Use Only

Fully biobased vitrimers have been prepared by using natural glycyrrhizic acid and epoxidised soybean oil as raw materials without any chemical modification.

For R = 0.4, the width of the crack decreased from 77 to 17 μm within 5 min, and for R = 0.6 the width nearly disappeared after 5 min, exhibiting the great repairing ability.

S12

Apparent temperature shape memory of ESO/GL Apparent temperature shape memory of ESO/GL networks (R = 0.5, TBD 5 mol%) was shown in Movie S2 and Movie S3. In Movie S2, a flat sample was heated using hands and fixed to an "S" and a "chair", respectively. Finally, it was recovered to the original shape. In Movie S3, a bent "chair" was set on the hands, and gradually became to a flat sample with an angle approach 180° within 100 s upon heating by hands.

Tensile tests for original and recycled samples

Samples after tensile tests were shown in Figure S6. It can be observed clearly that ESO/GL networks was broken neatly, while for recycled sample it was broken like zigzag. It was assumed that due to the incompleted TERs at interfaces between joined pieces, the cracks would develop along the weak interfacs, thus resulting in the decrease in tensile strength of recycled samples.

Figure 1 .

 1 Figure 1. Schematic illustration of chemical structures of ESO and GL, curing reaction and

Figure 2 .

 2 Figure 2. (a) DSC curves of ESO, GL, ESO/GL without catalyst (R = 0.5) and ESO/GL (R = 0.5,

  showed the evolution of the relaxation modulus of ESO/GL networks (R = 0.5, TBD 5 mol%) at 140, 160, 180 and 200°C, respectively, and the relaxation time (τ*) was determined as the material relaxed to 1/e of the initial modulus (G/G0 = 1/e). The τ* was 28117 s (7.81 h) at 140°C, 4118 s (1.14 h) at 160°C, 1351 s (23 min) at 180°C, and 425 s (7 min) at 200°C, and the full stress relaxation was achieved after 9500 s (2.64 h) at 180°C, but only after 3200 s (53 min) at 200°C. The drop in relaxation time was explained by the acceleration of transesterification at high temperature. The suitable stress relaxation time of ESO/GL (R = 0.5) benefitting from the flexible chains of ESO and the large amount of hydroxyl groups on GL ensured the material welding, repairing, reprocessing and recycling.

Figure 3 .

 3 Figure 3. (a) Stress relaxation of ESO/GL networks (R = 0.5, TBD 5 mol%) at different

Figure 4 .

 4 Figure 4. (a) and (b) Welding test of ESO/GL networks (R = 0.5, TBD 5 mol%). (c) Optical

Figure 5 .

 5 Figure 5. (a) Consecutive shape memory cycles for ESO/GL networks (R = 0.5, TBD 5 mol%)

Figure 6 .

 6 Figure 6. (a) Photos showing the recycling process of ESO/GL networks (R = 0.5, TBD 5 mol%);

Figure 7 .

 7 Figure 7. (a) Photo showing the chemical degradation experiment. (b) The weight loss of ESO/GL

Figure S10 .

 S10 Figure S10. Optical microscope images of ESO/GL networks (R = 0.4, 0.6; TBD 5 mol%) during

Figure S11 .

 S11 Figure S11. Photos of tensile tests for original (a) and recycled (b) samples.

Table 1

 1 Compositions and thermal properties of ESO/GL networks containing 5 mol% of TBD.

	R a	Gel content	Swelling ratio	Td5	Tg(DSC)	Tg(DMA)
		(%) b	(%) b	(°C) c	(°C) d	(°C) e
	0.4	89.0 ± 1.0	17.0 ± 0.8	315	8	39
	0.5	90.1 ± 0.3	16.1 ± 0.8	328	13	49
	0.6	90.9 ± 0.4	14.1 ± 0.3	314	18	64

Table 2 .

 2 Values of shape fixity (Rf) and shape recovery (Rr) for ESO/GL networks (R = 0.5, TBD 5 mol%).

		Rf (%)			Rr (%)
	cycle 1	cycle 2	cycle 3	cycle 1	cycle 2	cycle 3
	92.9	92.7	91.8	94.8	94.1	95.4

  Characterization; reaction mechanism of ESO with GL; 1 H NMR spectrum of epoxidised soybean oil; preparation of ESO/GL networks; curing reactivity study using a control system ESO/GA; thermal stability of TBD and ESO/GL/TBD mixture before the curing reactions; the possible transition structure of TBD with carboxyl groups of GL; FTIR spectra analysis for curing reactions; thermal stability of ESO/GL networks; chemical structure of carbenoxolone (CBX) and photo of cured ESO/CBX material; topology freezing transition temperature of ESO/GL networks; thermodilatometry testing of ESO/GL networks; repairing tests of ESO/GL networks; apparent temperature shape memory of ESO/GL; tensile tests for original and recycled samples; photos of samples after lap-shear tests and the comparison between this work and other adhesives; Movie S1; Movie S2; Movie S3.

Table S1 .

 S1 Lap-shear properties of this work and other reports.

	Lap-shear strength (aluminum)	Biobased	Can be repaired	Can be recycled	Reference
	0.6 MPa	√	×	×	Green Chem., 2013, 15, 3360
	6.5 MPa	√	√	×	Green Chem., 2018, 20, 2995
	24 MPa	×	√	×	J. Mater. Chem. A, 2017, 5, 21169
	4-6 MPa	×	×	×	Int. J. Adhes. Adhes., 2006, 29, 36
	13-19 MPa

3.42 MPa√ √ √ This work

ACKNOWLEDGMENT

This work is supported by NSFC (no. 21604085) and National Key R&D Program of China (no. 2017YFD0200302).

Supporting Information

Fully Biobased Vitrimers from Glycyrrhizic Acid and Soybean Oil for Self-Healing, Shape Memory, Weldable and Recyclable Materials Jianqiao Wu, † Xia Yu, † Hao Zhang, † Junbo Guo, †

*Corresponding Authors

Email: jhu@mail.buct.edu.cn (Jun Hu); min-hui.li@chimieparistech.psl.eu (Min-Hui Li).

The supporting information includes 13 pages, 12 figures, 1 table and 2 schemes.

Characterization

Thermal stability test: Thermal stability was measured using a thermogravimetric analyzer (TGA, TA-Q50). Samples (~5 mg) were loaded into alumina crucibles and scanned from room temperature to 700°C at a heating rate of 10°C /min under nitrogen atmosphere.

Differential scanning calorimeter (DSC) test: Differential scanning calorimeter (DSC, TA-Q20) was used to investigate the curing behavior and determine the Tg of vitrimers. The samples (~5 mg) were sealed in aluminum crucibles, and scanned at a heating rate of 5°C /min for curing samples and raw materials, while at a heating rate of 20°C /min for cured vitrimers to determine their Tg.

Fourier transform infrared spectroscopy (FTIR): Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a spectrophotometer (Nicolet iS5). The sample was grinded with potassium bromide (KBr, 100 mg) in a mortar, and the mixture was compressed into a disk using a mould. All samples were scanned from 4000 to 400 cm -1 .

Cell viability test:

The samples with 15 mg each piece were used for cell viability study.

Before use, the samples were sterilized by immersion in sterile PBS for 30 min, followed by UV irradiation for 30 min. Fibroblasts NIH-3T3 were grown in Dulbecco's Modified EaGle's Medium (DMEM) supplemented with 10% fetal bovine serum (FBS) in an atmosphere of 5% CO2 at 37°C. The cells were seeded in 48-well culture plate with a density of 2 × 10 4 cells per well. After overnight incubation, the medium was replaced by fresh DMEM containing 10% FBS and the sterile samples. After co-culturing for 24 h at 37°C, 10% of CellTiter-Blue reagent was added to each well and incubation continued for a further 3 h to perform the cell viability assay. The fluorescence intensity was measured at 560/590 nm (Ex/Em) by using a BioTek Synergy H1 microplate reader.


Dr Bin Ni

Dr Mengxue X Zhang

Prof Cedric M Guyon

Dr Patrick Keller
email: patrick.keller@curie.fr

Dr Michael H Tatoulian
email: michael.tatoulian@chimieparistech.psl.eu

Min-Hui Li
email: min-hui.li@chimieparistech.psl.eu

Li

Plasma-induced polymerizations: a new synthetic entry in liquid crystal elastomer actuators

Keywords: Liquid crystal elastomer, Plasma, Magnetic field, Thermomechanical, Soft robots

The research on soft actuators including liquid crystal elastomers (LCEs) becomes more and more appealing at a time when the expansion of artificial systems is blooming. Among the various LCEs actuators, the bending deformation is often in the origin of many actuation modes.

Here a new strategy with plasma technology was developed to prepare single-layer main-chain LCEs with thermally actuated bending and contraction deformations. Two distinct reactions, plasma polymerization and plasma-induced photopolymerization, were used to polymerize in one step the nematic monomer mixture aligned by magnetic field. The plasma polymerization formed crosslinked but disoriented structures at the surface of the LCE film, while the plasmainduced photopolymerization produced aligned LCE structure in the bulk. The actuation behaviors (bending and/or contraction) of LCE films could be adjusted by plasma power, reaction time, and sample thickness. Soft robots like crawling walker and flower mimic were built by LCE films with bending actuation.

Introduction

At a time when the expansion of artificial systems through our society is rapidly blooming, the research on soft actuators becomes more and more appealing and necessary. [1,2] Liquid crystal elastomers (LCEs), as a type of stimuli-responsive materials, [3][4][5][6][7] exhibit outstanding reversible shape-memory properties when exposed to external stimulations. [8][9][10][11][12][13] Since P. G. De Gennes [14] predicted the possibility of making LCE actuators in 1975, the continuous research and developments on LCEs attracted considerable attention and found potential applications as artificial muscles, [15][16][17] smart sensors, [18][19][20] motors [21] and actuators. [22][23][24] Finkelmann [25] pioneered the two-step mechanical stretch and hydrosilylation method to obtain aligned LCEs with reversible contraction and extension deformation. On the other hand, radiation-induced polymerizations such as photopolymerization [26,27] are the most popular methods. Being combined with different alignment procedures like magnetic alignment, [8,28] surface patterning technology, [29,30] microfluidic alignment [12] and microchannel alignment, [31,32] photopolymerization has contributed widely to the development of the LCEs, especially those with sophisticated three-dimensional (3D) shape deformations. More recently, another radiation-induced polymerization, electron-beam polymerization [33,34] has also been used to prepare LCEs.

Among the various LCEs actuators, the bending deformation is often in the origin of many actuation modes. A common strategy to achieve bending deformation used bilayer or multilayer heterogeneous materials, [9,33,35,36] similar to the bimorph concept. However, this strategy might suffer from delamination or interfacial sliding during repeating actuations. Thus, it would be desirable to have access to the bending deformation with homogeneous single-layer materials.

For this purpose, spatially modulated mesogens' orientations, such as twist, [37] splay, [38] and azimuthal or radial alignments, [29] were designed in single-layer using surface anchoring technologies for alignment. These single-layer LCE actuators were finally obtained by photopolymerization, which exhibited bending or others sophisticated deformations. However, surface anchoring technologies have limitation in sample thickness, and the reported LCE films have thickness < 200 m. On the other hand, sequential or gradient crosslinking, [22], [39,40] 3D printing, [41] and photolithography [42,43] were also employed to make complex LCEs with 3D deformations. In particular, 3D printing as an emerging technology to prepare LCEs allows to prepare sophisticated LCE structures. [44] Nevertheless, the alignment direction is restricted, which depends on the extrusion direction of the LC mixture from the printer. Here, by combining plasma-induced polymerizations and magnetic field alignment we report on a new and simple strategy to achieve single-layer LCE bending actuation without the problem of delamination and without the need to use complex techniques of surface alignment. It is also a flexible strategy to obtain various deformation behaviors (bending and/or contraction) by adjusting the thickness of the LCE films or plasma powers. As examples of potential application, thermally actuated soft crawling walker and flower mimics were made by this LCE.

Plasma, which consists of a mixture of ions, electrons, radicals, excited species, photons, has attracted a considerable attention for surface cleaning, [45] catalysis [46] and polymerization. [START_REF] Yasuda | Plasma polymerization[END_REF] Low pressure plasma polymerization is considered as a common method to prepare polymer thin films with excellent mechanical properties and chemical stabilities. [START_REF] Thiry | [END_REF] Gas is excited under vacuum conditions by supplying electrical energy and results in activated species, which initiate the surface polymerization to obtain thin cross-linked polymer films. Moreover, with the increase of electrical energy supplied, an increased electron density enhances the probability of molecules undergoing multiple electron impacts, [49] which allows the preparation of polymer thin film with high crosslinking density.

Meanwhile, for people familiar with the plasma technology, a striking fact, which has been frequently overlooked, is that plasma generates lights of different wavelengths, [50] which could be used to promote the photopolymerization of monomers. For example, Wertheimer et al [51] reported the use of vacuum ultraviolet light (VUV,  = 200 nm) to carry out the photopolymerization of amine-rich thin films from ammonia-hydrocarbon plasma gas mixtures.

Nevertheless, the use of longer wavelengths ( = 300 -450 nm) in the plasma for photopolymerization has not been reported. In this work, we combined the plasma-induced high crosslinking reactions on the surface and the plasma-induced photopolymerization ( = 300 -450 nm) in the bulk of LC monomers mixture to produce single-layer LCEs with bending deformation. Indeed, the plasma-induced high crosslinking reactions randomized the orientation of LC monomers aligned beforehand by magnetic field and resulted in a crosslinked surface without preferential alignment. Meanwhile the UV and visible light ( = 300 -450 nm) generated by plasma penetrated the sample of LC monomers and polymerized them in the presence of photoinitiator to achieve aligned monodomain LC network. Consequently, the whole LCE sample presented a bending behavior under thermal stimulation. Moreover, by adjusting the supplied plasma power energies, the reaction times, and the sample thickness, LCE films with bending-contraction and only contraction behaviors were obtained. Owing to the simultaneous bending and contraction deformation in these LCE films, soft crawling walker and flower mimics were prepared. Finally, when LC monomers mixture was filled in a closed cell, the plasma-induced reactions could be suppressed and only plasma-induced photopolymerization operated to produce macro/micro-scale samples with contraction behavior.

In brief, plasma-induced polymerizations provide a new and simple synthetic entry to LCE actuators with different deformation behaviors and various shapes and sizes.

Results and discussion

Synthesis of the LC monomer and preparation of LCEs

The LC monomer C4 in Scheme 1 was synthesized according to Scheme S1. The details of syntheses and characterizations were described in the Supporting information.

The phase behavior of the LC monomer was studied by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The LC monomer C4 exhibited two endothermic transitions at 68 and 128 °C upon heating according to DSC measurement (Figure S3a). The observation by POM allowed to attribute these two transitions to crystal-to-nematic and nematic-to-isotropic phase transitions, respectively (Figure S3b). Upon cooling process, the LC monomer C4 exhibited a broad nematic phase from 126 to 20 °C. For plasma-induced polymerizations, the LC monomers mixture was composed of LC monomer C4, chain-extender (2,2′-(ethylenedioxy)diethanethiol) (EDDET), LC crosslinker (RM82) and, when necessary, the photo-initiator 2-benzyl-2-(dimethylamino)-4'-morpholino butyrophenone (Iragure 369) (Scheme 1). The molar ratios of LC monomer, chain-extender, and LC crosslinker were kept at C4 : EDDET : RM82 = 1: 1: 0.4 in all the experiments. The concentration of the photoinitiator, when present, was kept at 2 wt%. The phase transition of the LC monomers mixture (noted as LC mixture hereafter for simplicity) was checked beforehand to ensure that the mixture stayed in the nematic phase during the plasma-induced polymerizations at room temperature (RT). As shown in the DSC (Figure S4a), a phase transition appeared at 80°C during the cooling process, which was ascribed to the isotropic-nematic phase transition by POM. Interestingly, no recrystallization was observed upon cooling until RT. Moreover, recrystallization did not occur in the LC mixture left at RT for at least 4 days (see Figure S5 for DSC at different days after mixture preparation). Therefore, once the LC mixture was aligned beforehand, the LC alignment could be maintained during the vacuum process and the plasma process, which were performed at RT without intentional heating in the time scale of several hours. Besides, considering that the continued plasma treatment might increase the environment temperature, the temperature of the plasma chamber was monitored as a function of time under Argon plasma powers of 150 W and 200 W, as shown in Figure S4b. Under 200 W and during 120 min, which were the highest power and longest duration used in this research, the environmental temperature of the plasma could reach to 70 o C that was still lower than the nematic-isotropic phase transition of LC mixture. Therefore, the aligned nematic phase of LC mixture was maintained during the used plasma process. This is a key point that enables to prepare aligned LCEs by plasma technique.

LCEs prepared by plasma polymerization without photoinitiator

Since the electrons, ions, and photons generated by plasma could initiate the polymerization, the plasma polymerization of the LC mixture was firstly investigated in the absence of photoinitiator (Figure 1a). The LC mixture was aligned in magnetic field as described in Experimental section below (see also Scheme S2a and 3a in Supporting Information). The aligned monomer mixture appeared almost transparent as observed by the naked eye (Figure 1b). Meanwhile, it exhibited transmittance variations observed by POM when changing the angle  between its alignment direction and the POM polarizer direction from 0 o to 45 o (Figure S6a). Both observations indicated that the nematic phase of the LC mixture was well aligned.

Then, a series of samples of the same aligned LC mixture were treated by plasmas of various power energies, i.e., 50 W, 100W, 150W and 200 W, using Argon gas and during 30 min. A white layer appeared on the surface of the film samples after the plasma polymerization (Figure 1c-d). POM observations showed that there were not clear light transmittance variations with the angle  changing from 0 o to 45 o (Figure S6b-e). Therefore, the LC alignment of the sample at the surface has been disrupted during the plasma process. Other plasma parameters, such as pulsed plasma instead of continuous plasma, different gas (N2 and O2) and different reaction time (120 min), were also used to polymerize the LC mixture. All samples showed similar results as observed by POM (Figure S7). Hence, the LC mixture without photoinitiator could not produce aligned LCEs by the plasma polymerization.

When immersing the above plasma-exposed films in dichloromethane, we noticed that a fraction of the material dissolved, but the upper white layer was insoluble. Thus, the plasma polymerization generated a crosslinked, but disorganized (non-aligned) layer at the sample surface, leaving unreacted the main body of the LC mixture. The gel content, i.e., the weight ratio of crosslinked part versus the uncrosslinked part after plasma treatment was measured by gravimetry (see Experimental). The thickness of crosslinked part was then evaluated as 2-10 m, which increased with the increase of plasma power energy, reaching ~ 10 m for a power of 200 W (Figure 1g).

LCEs prepared in the presence of photoinitiator by plasma-induced polymerizations

As described above, plasma polymerization in the absence of photoinitiator could only polymerize a thin layer (2 -10 m) of monomer on the top of the sample. Then, lights of different wavelengths generated during the plasma process were examined, and the plasmainduced photopolymerization was attempted to polymerize/crosslink the LC mixture with photoinitiator added.

The spectra of the Argon plasma-generated lights at different plasma power energies are shown in Figure 2. Strong emissions in the near infrared (NIR) from 700 -850 nm were detected, which could explain the moderate temperature elevation of plasma chamber described in the end of part 2.1. Emission peaks could also be observed between 300 nm to 450 nm (Figure 2a), which fall in the absorbance region of the photoinitiator Iragure 369. Moreover, the intensity of those peaks increased with the increase of plasma power (Figure 2b). Thus, the UV-Visible lights produced by the plasma could be used to initiate the photopolymerization of the LC mixtures in the presence of photoinitiator.

The polymerization of the aligned LC mixtures containing Iragure 369 was then carried out using plasma of different powers. The reaction time of 30 min was first used and the sample thickness was around 200 m. After polymerization, the crosslinked gel contents of the final samples were estimated by gravimetry. The gel content was 15% and 56% for samples prepared with 50 W and 100 W plasma, respectively. That means 85% and 42% of starting materials were not crosslinked into LCEs (Table S1). In contrast, when the plasma power increased to 150 W and 200 W, the gel content of the final sample were 69%, 71%, respectively, which was similar to gel contents of the LCEs prepared by UV polymerization for 30 min (73%). [52] The relatively low gel content of this system was caused by the stoichiometric excess of allyl groups from both C4 and RM82 in the formulations (C4 : EDDET : RM82 = 1: 1: 0.4). RM82 participated in the elastomer as monomer and crosslinker, which might lead to unreacted allyl groups. The same level of gel contents were obtained by other groups [53], [54] in similar LCEs prepared by photopolymerization, which exhibited stable reversible deformation upon stimulation. Thus, the plasma power of 150W and 200W and the reaction time of 30 min were sufficient here to conduct the plasma-induced photopolymerization of LC mixtures.

The deformation properties of LCE films with 150W and 200W plasma power were then investigated. Interestingly, the LCE film produced by 150W plasma exhibited a reversible bending behavior upon heating and cooling (Figure 3a). In fact, a crosslinked surface part (face to plasma) without alignment was made by the normal plasma reactions, while an aligned LCE bulk part was formed by the photopolymerization. Therefore, in this single-layer LCE, the bulk part contracted along the direction of alignment, while the surface part did not contract.

Consequently, the bending behavior was observed. Because the contraction part is much thicker (~ 190 m) than the surface part (≤10 m), a shortening of the whole length was also obtained upon heating (Figure 3b, 150W, 130 °C). Intriguingly, the LCE film produced by 200W plasma showed only a reversible contraction behavior (Figure 3b). It was hypothesized that the photopolymerization might be faster than the plasma polymerization under 200W plasma power.

Thus, the whole sample was an aligned LCE obtained by photopolymerization. Indeed, POM image of LCE films prepared by 200 W plasma power confirmed the alignment of the whole sample (Figure S8). Moreover, a LCE film prepared by classical photopolymerization under 365 nm UV light was subsequently submitted to 200W Argon plasma treatment for 30 min.

The final sample showed almost the same contraction behavior as before plasma treatment, indicating 200W plasma could hardly destroy the LCE surface alignment (Figure S9).

The evolutions of the bending deformation characterized by the curvature 1/R and of the contraction by length ratio L/Liso of the LCE films as a function of temperature are shown in Figures 3c and3d. For the LCE film made with 150W plasma, upon heating the curvature remained near 0 when T < 70 ℃, and increased rapidly for T increasing from 80 to 120 ℃.

Upon cooling, the variation of the curvature was reversible with a slight hysteresis. On the other hand, the total length of the bent film at 130 ℃ was estimated as 4.3 mm, relative to its initial length of 6 mm at RT (a 28% of shortening took place, see Figure 3a). As for the LCE film made with 200W plasma, in the whole temperature range from RT to 130 o C the LCE film shrank to half of its initial length at RT (LRT/Liso = 2). Abrupt contraction took place upon heating in the temperature range 100-120 o C, around the nematic-isotropic transition temperature (TNI) of the LCE (Figure S10). The contraction and extension of LCE film were also reversible with a slight hysteresis. Figure 3e is the schematic illustration of the actuation of both LCE films.

Further, the reaction times were increased to 120 min to carry out the polymerization of the aligned LC mixtures containing Iragure 369 with plasma of different powers. Gel contents of the final sample increased: with 50W, 100W, 150W and 200W plasma, gel contents became 55%, 70%, 71% and 73%, respectively (instead of 15%, 56%, 69%, 71% with reaction time of 30 min). The LCE films made with 100W and 150W plasma exhibited thermo-responsive reversible bending-contraction behaviors (Figure S11a andb Finally, the effect of the sample thickness on the deformation of LCEs prepared by plasmainduced polymerizations was studied. LCE films of thickness of 120, 190, 300, 380 and 450 m were prepared with the plasma power of 150W and the reaction time of 120 min. As shown in Figure S13a-c, LCE films exhibited thermally actuated bending-contraction behaviors with the thickness of 120, 190 and 300 μm. However, with the thickness of 380 and 450 μm, LCE films showed only reversible contraction and extension (Figure S13d-e). Such sole contraction phenomena can be explained by the fact that the very thin crosslinked layer (~10 μm) generated by plasma reactions could not resist the contraction force of the thick part of LCEs (> 300 μm) upon heating process.

LCEs prepared by plasma-induced photopolymerization without plasma contact

As discussed above, the UV-Visible light generated by plasma did polymerize efficiently the LC mixture to prepare aligned LCEs. We then wondered if the UV-Visible light would still work when the plasma flux was not in direct contact with the monomer sample. In fact, the plasma needs to be in "touch" with the surface of the LC monomers mixture to induce any plasma reactions. To prevent the direct action of the plasma, a closed PDMS mold (Scheme S2b) was used to pack the LC mixture with a thickness of 160 m. After exposing to a 150 W plasma during 120 min, the LCE film was removed from the PDMS mold. The LCE film contracted to half of its initial length at RT (LRT/Liso = 2) when heated to isotropic phase as shown in Figure 4b. Therefore, it was confirmed definitively that the light produced by the plasma was able to induce the photopolymerization of the LC monomers.

By making use of this close system, LCE micropillars of 100 m height and 20 m diameter (H100µm x Φ20µm) as shown in Figure 4c were also prepared by plasma-induced photopolymerization with the same alignment setup developed previously [28,55] (see also Experimental section and Scheme S2c and S3c). The aligned LC mixture in the closed PDMS micropillar mold was put in the plasma chamber and polymerized under 150W plasma during 120 min. The obtained micropillars (H100µm x Φ20µm) cut from the sample exhibited typical reversible contraction-extension with temperature changes. Thermal stability was tested by 10 heating-cooling cycles (Figure S14), showing no change in contraction ratio.

Soft crawling walker and flower mimics

Taking advantage of the simultaneous bending and contraction deformation of LCEs described above, we have made a soft crawling that mimicked the motion of caterpillar. As shown in Figure 5a, a LCE film with thermally activated bending-contraction behavior (length 4.5 mm, width 1.5 mm; thickness: ~300 μm) was placed on a surface with micro-ratchets roughness.

The LCE film was prepared with 150 W Argon plasma during 120 min. Stimulated by heatingcooling cycles, the LCE film moved along the surface as shown in Figure 5b. The film bended and contracted on heating, while on cooling the position return of the left side of the LCE due to the extension was blocked by the steep side of one of the ratchet teeth (Figure S15b).

Consequently, the bending film extended towards the smooth side of the teeth and a forward motion in right direction was obtained along the surface. As shown in Figure 5c, after five successive heating and cooling cycles, the LCE film moved forward about 7 mm. On average, LCE film moved 30 % of its original length in each heating-cooling cycle.

In addition, a flower mimic was also prepared with a cross-shaped LCE sample as shown in Figure 5d and 5e. The sample was prepared with homeotropic alignment (see Experimental and Scheme S2d and S3d) and polymerized with 150W Argon plasma during 120 min. The cross-shaped LCE was composed of a crosslinked surface without alignment and a homeotropically aligned bulk (Figure 5d). Upon heating, the bulk part of cross-shaped LCE expanded because of the contraction along the thickness direction, while the surface part maintained the same size. Consequently, the sample curved up like the closure of a four-petal flower (Figure 5e). Upon cooling, the flower mimicking cross-shaped LCE re-opened.

Conclusions

Main-chain nematic liquid crystal elastomers (LCEs) were successfully prepared by plasmainduced polymerizations. Two distinct reactions were identified, plasma polymerization and plasma-induced photopolymerization (in the presence of photoinitiator). The plasma polymerization formed crosslinked but disoriented structures at the surface of the film sample, while the plasma-induced photopolymerization produced aligned LCE structure from the remaining monomers in the bulk. The actuation behaviors of these single-layer LCE films could be adjusted by the plasma power, the reaction time, and the sample thickness. Typically, with plasma power of 150 W, reaction time of 30 min or 120 min, and sample thickness smaller than 300 m, thermally activated simultaneous bending and contraction could be obtained in these single-layer LCEs. The bending deformation was caused by the contraction of the bulk part along the direction of alignment and the non-contraction of the surface part face to the plasma flux. On the other hand, only contraction behavior was observed when LCE sample had a thickness superior to 300 m or was polymerized with high plasma power (200W). Moreover, it was demonstrated that the light generated by the plasma could photopolymerize alone the LC mixtures packed in the closed cells, where the plasma flux was not in contact with monomers, to obtain macroscopic LCE films and microscopic micropillars with reversible contractionextension behavior. Finally, the LCE films with simultaneous bending and contraction actuation were used to construct a soft crawling walker that could move along the surface with microratchet roughness upon heating-cooling cycles and a flower mimic that could close and open with temperature change.

Plasma technology is enriching the preparation methods available for LCEs and shows advantages in the preparation of single-layer LCEs with bending and contraction actuation.

Experimental section

Materials and characterization

Allyl alcohol (98%), potassium carbonate (K2CO3), sodium hydroxide (NaOH) were purchased from Alfa-Aesar. Ethyl 4-hydroxybenzoate (98%), sodium hydride (90%), N,N'dicyclohexylcarbodiimide (99%), 4-pyrrolidinopyridine( 98%), methylhydroquinone (99%), Polarized Optical microscopy (POM) observations were carried out on a Motic BA310POL equipped with a Motic CCD camera and a Mettler FP 80 hot stage. The emission spectra of the plasma were measured with a detector SpectraPro 2750. The low-pressure plasma polymerization was done using the Flarion Series FLR300 (13.56MHz-300 W) apparatus provided by Plasmionique (Québec, Canada) [56][57][58] equipped with a two-stage rotary vacuum pump. The images were captured by the camera of a smartphone fixed on a tripod.

Alignment method for the LC monomer mixtures

The different molds and alignment processes are shown in Scheme S2 and S3.

For planar alignment of film samples, the LC mixture, containing the LC monomer C4, the chain-extender EDDET, the LC crosslinker RM82 and, if needed, the photo-initiator Iragure 369, was added into the rectangular hole of a PDMS mold placed on a glass slide, the common size of the rectangular hole being ~ 6mm x 2 mm x ~200µm (length x width x thickness) (Scheme S2a). There were two kinds of molds: closed mold and open mold. To fill uniformly the mold, the LC mixture was heated up to its isotropic phase (110 °C) and added into the mold progressively. Then, the LC mixture-filled mold was placed at the center of a holder equipped with two permanent magnets facing each-other (1 T NdFeB rare earth magnet) (Scheme S3a and S3b). A slow cooling (1 o C/ min) from isotropic phase to room temperature in the presence of the magnetic field resulted in the planar alignment of the nematic phase in the LC mixture.

After alignment the LC mixture-filled mold was moved to the plasma chamber.

For micropillars preparation, the LC mixture was deposited on a glass coverslip placed atop a permanent magnet (~1 T NdFeB rare earth magnet) in a heating stage (Scheme S3c). The mixture was first heated up to its isotropic phase (110 o C). A soft PDMS mold with cylindrical holes (H100µm x Φ20µm) was gently pressed onto the melted mixture and then cooled down to room temperature at a slow rate (1 o C/ min). During the cooling process, the magnetic field aligned the nematic phase with the nematic director along the long axis of the cylinder holes.

For the cross-shaped LCE, LC mixture was filled into a cross-shaped hole with four arms, the size of each arm being ~ 6 mm x ~ 2 mm x ~200µm (length x width x thickness). The alignment setup was shown in the Scheme S3d where a homeotropic alignment was produced. The same alignment process was carried out as that described for planar alignment described above.

Low-pressure plasma polymerization process

The aligned LC mixture with its mold was put inside the low-pressure plasma device. Turbomolecular pump backed by a two-stage rotary vane pump was used to evacuate the air from the chamber to a base pressure of <10 -6 mbar. Then, Argon gas was introduced into the chamber until the chamber pressure reached 10 -1 mbar. At last, the plasma power energy and the reaction time were adjusted according to the conditions chosen. After polymerization, the LCE film was recovered by removing it from the mold.

Gel content measurement by gravimetry

After polymerization, the crosslinked gel contents of the final samples were estimated by gravimetry as follows. The polymerized sample was immersed in dichloromethane for 30 min to solubilize the unreacted monomers and un-crosslinked polymers, and the residues were dried overnight in vacuum at 70 °C. The weight of the dried residue (mr) over that of the initial sample (m0) gave the gel content (mr/m0).

  ), while the LCE film made with 200W plasma exhibited again thermo-responsive reversible contraction (Figures S11c). These results indicated that a low plasma power could also produce crosslinked LCE films with reaction time extension. Therefore, the deformation behaviors of the LCE films (bending or contraction) could be controlled by adjusting both the plasma power and the reaction time. The bending deformation (1/R) and the contraction/extension (L/Liso) of the LCE film obtained under 150W and 200W plasma powers and with two different reaction times ("plasma 30 min" and "plasma 120 min") are summarized together in Figure S12. With longer reaction time, the deformation extent of the final sample increased.

(

  ethylenedioxy) diethanethiol (EDDET, 95%) and Irgacure 369 (2-benzyl-2-(dimethylamino)-4′-morpholinobutyrophenone) (97%) were purchased from Sigma-Adrich. All these compounds were used as received without purification. 2-Methyl-1,4-phenylene bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate) (RM82, 95%) was purchased from Hunan Huateng Pharmaceutical Co., Ltd in China and recrystallized in ethanol before using. Solvents like tetrahydrofuran (THF), dichloromethane (DCM), petroleum ether (PE, 40-60℃) and acetone with analytical pure grade were obtained from VWR and used without purification. 1 H NMR and 13 C NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer at 297 K. Deuterated chloroform (CDCl3) was used as the solvent and TMS (tetramethylsilane) as the internal reference. Calorimetric measurements of LC monomers and elastomers were performed using a PerkinElmer DSC7 instrument. LC monomer or monomer mixture were scanned over a temperature range between -40 °C and 120 °C at a rate of 5 °C/min. LCE samples were scanned over a temperature range between -40 °C and 150 °C at 10 °C/min.

Scheme 1 .

 1 Scheme 1. Molecular structures of the components in the LC monomers mixture.

Figure 1 .

 1 Figure 1. (a) Schematic of "open" mold for LCE preparation via plasma polymerization. (b-f) Photographs of the sample surfaces before (b) and after Argon plasma polymerization with plasma power of 50 W (c), 100 W (d), 150 W (e) and 200 W (f), respectively. (g) The thickness of polymerized LCE upper layer under different plasma energies. Argon flow rate was fixed at 50 mL/min, the reaction time as 30 min. The double arrow represents the orientation direction of nematic director in LC mixture.

Figure 2 .

 2 Figure 2. (a) Light emission spectra of the Argon plasma at 200 W. (b) Light emission spectra of the Argon plasma of different powers in UV-Visible region.

Figure 3 .

 3 Figure 3. (a) and (b) The deformation behaviors of the LCE films fabricated by plasma-induced polymerizations with 150W and 200W plasma powers during 30 min and in the presence of photoinitiator. The surface part face to plasma of the sample was put down on the hot stage (gray blocks in the photos). (c) The curvature (1/R) of the LCE film polymerized by 150W plasma as a function of temperature upon heating and cooling. R is the radius of the curved film. (d) The contraction and extension of the LCE film polymerized by 200W plasma upon heating and cooling, represented by L/Liso as a function of temperature. L and Liso are the lengths of the LCE film along the netmatic alignment at a given temperature T and in its isotropic state, respectively. (e) Schematic illustration of the bending and contraction behavior of the LCE films.

Figure 4 .

 4 Figure 4. Thermo-responsive deformation of the LCE polymerized in the closed PDMS mold only by the light produced from the 150 W plasma for 120 min. (a) Macroscopic LCE film sample: preparation (left) and their size upon heating and cooling at 25°C, 130°C at 25°C (right). (b) Micro-pillars sample: preparation (left) and the sizes of the cut pillars upon heating and cooling at 25°C, 130°C at 25°C (right). The alignment direction of the nematic phase is indicated by black double arrow in (a) and by write double arrow in (b).

Figure 5 .

 5 Figure 5. (a) Schematic presentation of the LCE film on the ratchet surface. LCE was prepared by plasma-induced polymerizations with non-aligned surface part (brown) and planarly aligned bulk part (blue). (b) Locomotion illustration of LCE film with bending-contraction behaviors on a surface with micro-ratchet roughness. (c) Photographs showing the displacements of the LCE film on the ratchet surface made of PDMS upon several heating-cooling (H-C) cycles. The rachet surface was put on a glass slide, under which a grid paper was placed for position measurement. Both ratchet surface and glass slide are transparent and not visible in the picture. (d) Schematic presentation of the cross-shaped LCE prepared by plasma-induced polymerizations with non-aligned surface part (brown) and homeotropically aligned bulk part (blue). The size of each of four petals is 6 mm x 1.5 mm x ~200µm (length x width x thickness). (e) Cross-shaped LCE exhibits flower-like opening and closing upon thermal stimulation.

Acknowledgements

This work has received the support from the French National Research Agency (ANR-16-CE29-0028) and from the "Institut Pierre-Gilles de Gennes" (IPGG, laboratoire d'excellence, "Investissements d'avenir" programs ANR-10-IDEX-0001-02 PSL, ANR10-LABX-31 and ANR-10-EQPX-34). Bin NI gratefully acknowledges the China Scholarship Council for funding his Ph.D. scholarship. We would like to thank Prof. Axel Bugin from Sorbonne University and Institut Curie for fruitful discussions. Mr. Rémy Fert and Mr. Eric Nicolau in the mechanical workshop of UMR168 CNRS in Institut Curie are acknowledged for building the setup used in the alignment by two magnets.

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Conflicts of interest

There are no conflicts to declare.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.