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Vanillin-based polyschiff vitrimers: reprocessability and chemical recyclability Hongwei Geng, Yuli Wang, Qingqing Yu, Shaojin Gu, Yingshan Zhou, Weilin Xu, Xi Zhang, and De-zhan Ye ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03925 • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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ACS Sustainable Chemistry & Engineering

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Vanillin-based polyschiff vitrimers: reprocessability and chemical

2

recyclability

3

Hongwei Geng c, Yuli Wang c, Qingqing Yu c, Shaojin Gu c, Yingshan Zhou c, Weilin Xu a, Xi Zhang b, Dezhan Ye a,b,c*

4

a

State Key Laboratory Cultivation Base for New Textile Materials & Advanced Processing Technology , Wuhan

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Textile University, No. 1 Yangguang Avenue, Jiangxia District, Wuhan, Hubei 430200, China.

b

State Key Laboratory of Polymer Material Engineering, Sichuan University, No. 24 South Section 1, Yihuan

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8

Road, Chengdu 610065, China.

c

School of Materials Science and Engineering, No. 1 Yangguang Avenue, Jiangxia District, Wuhan, Hubei

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430200, China.

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*Corresponding authors: [email protected]

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ABSTRACT

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In this study, dynamic imine covalent bonds were introduced into vanillin-based vitrimers networks,

13

endowing thermosets with hot-reprocessing ability and chemical recyclability under acid hydrolysis. First,

14

dialdehyde monomer, which was synthesized from lignin-derived vanillin monomer, was reacted with

15

conventional amine cross-linkers to form dynamic imine bond networks. Even after three hot-processing cycles,

16

the tensile strength and elongation at break of polyschiff vitrimers could be recovered at least up to 71.2% and

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72.8%, respectively, through the imine metathesis reaction. Importantly, the dialdehyde monomers showed

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enhanced recyclability under strong acid solution, and could be reused to regenerate polyschiff vitrimers. These

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characteristics of reprocessablility, recyclablility, and bio-based monomer present a feasible way to satisfy the

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demands of sustainability.

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Keywords: vanillin, bio-based thermoset, dynamic covalent bond, imine, self-healing, reprocessing, chemical

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recycling 1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Introduction

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To address sustainability challenges caused by petrochemical depletion, researchers are exploiting

25

renewable resources to generate bio-based chemicals, materials, and biofuels. Lignin, a byproduct of pulp and

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paper refining, shows great potential to be a desirable alternative to petroleum feedstock if we consider its

27

abundance as a natural resource 1. To take advantage of its renewability and backbone rigidity, lignin-based

28

thermosets, such as epoxy and phenol resin , have been extensively studied.

29

2

3

Two approaches, fractionation and chemical modification (e.g., anhydride reaction and epoxidation), have 4

30

been traditionally adopted for preparing lignin-derived thermosets

31

deconstruction of its aromatic structures is challenging because of its poor reactivity and solubility. Compared

32

with these methods, the preparation of bio-based thermosets from well-characterized lignin-derived monomers

33

is desirable and has received extensive attention. Vanillin, a renewable and aromatic compound from lignin

34

and its derivatives (e.g., eugenol, 4-Hydroxy-3-methoxybenzyl alcohol, and vanillic acid) have been widely

35

reported for producing epoxy and phenolic resin

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reaction to synthesize UV-curable monomers for coating applications

37

most important thermosets, has also been prepared by epoxidation of vanillyl alcohol

38

vanillin, and vanillic acid derivations

39

monomer in the synthesis of high-performance of benzoxazines resin

40

obtained by vanillin .

13

8-11

. However, using lignin without

5-7

,

. For example, eugenol has been modified by thiolene 12

. Interestingly, epoxy resin, one of the 10

, dihydroeugenol 9,

. Recently, vanillin and guaiacol have been utilized as bio-based aromatic 14

. Moreover, phenolic resin also can be

3

41

Traditionally, thermosets have permanent covalent bond networks that preclude the reprocessing of cured

42

polymers. Designing cross-linked networks that can be reprocessed, self-healed, and reshaped is still a challenge

43

to enable available manufacturing techniques for thermoset materials. One approach to solving these limitations

44

is to introduce the dynamic covalent into thermoset structures . For example, Leibler et al. prepared a malleable

15

2


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polyester thermoset based on transesterification reactions at high temperatures with metal catalysts

. Jianjun

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Cheng et al. designed hindered urea bonds and used them to make polyureas and poly(urethane-urea)s

47

cross-linked networks that can self-heal at room temperature . Other reversible reactions, such as retro-Diels–

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Alder , disulfide metathesis , boronic ester exchange , oxime−carbamate , and transcarbamoylation

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also been adopted in the preparation of reprocessable and self-healing cross-linked polymeric materials.

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Considering the superior renewability of lignin in natural plants, it is desirable to prepare lignin-based thermosets

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with intrinsically reprocessable, recyclable, and self-healable ability if one is concerned about the impact of the

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depletion of petrochemical resources. However, few studies have been performed in this emerging field so far.

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Very recently, Jinwen Zhang reported a unique eugenol-derived epoxy that was cured with succinic anhydride .

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Under the catalysis of zinc acetylacetonate hydrate, this cross-linked polymer exhibited excellent shape-changing

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and crack-healing ability. They also prepared lignin-based vitrimers from ozone-treated Kraft lignin and sebacic

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acid epoxy, showing excellent repair properties at elevated temperatures .

17

18

19

20

21

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have

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24

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The imine bond is a classic dynamic covalent bond, which is formed by the condensation reaction between

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aldehydes and amines. Three equilibrium processes are involved in the imine reactions, including imine

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condensation/hydrolysis, imine exchange, and imine metathesis

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widespread use as self-healing hydrogel

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resins. Recently, imine chemistry has been designed to prepare ultra-thin solid-state Li-Ion electrolyte membranes

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and repairable woven carbon fiber composites, in which the dialdehyde is terephthaldehyde

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exchange reactions of imine, the reported repairable polyimine could be dissolved/recycled by using amine

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monomer. Benezene-1,3, 5-tricarbaldehyde also has been used as a functional cross-linker to form dynamic imine

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bonds in self-healing and reprocessable poly(dimethyl siloxane) elastomer

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regard to vanillin-derived Schiff base materials, their utilizations are usually limited to the fields of water

26-28

25

and drug-release carriers

. Imine chemistry, traditionally finding 29,30

3


has been under-used for thermoset

33

31,32

. Owing to the

and polybutadiene rubber

34

. With


67

treatment because of their excellent chelation with metal ions

35,36

.

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In this study, cross-linked polyimine/polyschiff vitrimers from a lignin-based monomer, vanillin, were

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prepared by employing imine chemistry. These thermosets could be reprocessed and self-repaired under elevated

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temperatures, because of the thermal reversibility of imine bonds. Interestingly, the chemical recycling of

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dialdehyde monomer was demonstrated under acid solution at moderate temperature (50 °C). Moreover, the

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recycled dialdehyde monomer can be reused in the synthesis of polyschiff vitrimers. This work may provide some

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new insights in the design of recyclable lignin-based vitrimers with reprocessable and self-healing properties.

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Experimental

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Materials

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Vanillin (≥ 99%), tris(2-aminoethyl)amine (≥ 96%), diethylenetriamine (≥ 99%) and 1,4-dibromo butane (≥

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98%) were purchased from Aladdin company. All other analytical reagents were commercial chemicals and used

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without further purification.

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Synthesis of dialdehyde monomer from vanillin (DAV)

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Vanillin (12 mmol, 2.4 eq), anhydrous K2CO3 (12 mmol, 2.4 eq), and CH3CN (30 mL) were added to a 150 mL

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three-necked bottle under magnetic stirring. Then, the mixture was degassed with nitrogen for at least 30 min,

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and the reaction was started by adding 5 mmol (1.0 eq) 1,4-dibromo butane. The reaction was conducted for 24 h

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under reflux. Finally, the reaction mixture was poured into a large volume of water to remove salt. After filtration,

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the precipitation was dissolved into CH2Cl2 and was washed three times with NaOH (1 M) and distilled water. The

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organic phase was dried over anhydrous MgSO4 before the solvent was condensed by a rotary evaporator in

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vacuum. After being vacuum dried in an oven overnight, DAV with a yield of 86% was obtained as a white powder.

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1

H-NMR (400 MHz, CDCl3) δ (ppm): 9.76 (s, 2H, Ar-CHO), 7.36 (d, 2H, Ar-H), 7.34 (s, 2H, Ar-H), 6.90 (d, 2H, 4


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Ar-H), 4.13 (t, 4H, O-CH2-), 3.81 (s, 6H, OCH3), 2.10-1.99 (m, 4H, O-CH2-CH2-). C-NMR (400 MHz, CDCl3) δ (ppm):

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190.88, 153.92, 149.77, 130.00, 126.69, 111.37, 109.20, 68.64, 55.94, and 25.75.

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Polyimine cross-linked film preparation

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The cross-linked films were prepared by dissolving DAV in an appropriate amount of CH2Cl2 solvent; then, a

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precise amount of diethylenetriamine and tris(2-aminoethyl)amine were added into the solution under stirring

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conditions at 50 °C for 10 min. Immediately, the mixture was poured into a square PTFE tray, and a glass plate was

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used to cover it, which was then left at room temperature overnight. The obtained defect-free film was further

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cured in a drying oven at 50 °C for at least 24 h. Aimed at obtaining polyschiff vitrimers with different crosslinking

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degrees, the molar ratio of tris(2-aminoethyl)amine crosslinker to diethylenetriamine monomer was adjusted.

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The details for preparing the cross-linked polyimine films are recorded in Table 1.

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Table 1. Predetermined content of DAV and amine monomers used for preparing cross-linked polyschiff films

DAV

diethylenetriamine

tris(2-aminoethyl)amine

Sample 1

6mmol, 2.1504 g

1.8 mmol, 0.1974 ml

2.8 mmol, 0.437 ml

Sample 2

6 mmol, 2.1504 g

3.0 mmol, 0.329 ml

2 mmol, 0.3122 ml

Sample 3

6 mmol, 2.1504 g

--

4 mmol, 0.6244 ml

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100

Characterization

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Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 560 FTIR spectrometer. The powder

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samples were characterized using the KBr pellet technique. With respect to the polyschiff films, they were directly

103

measured by the ATR model. Each spectrum was recorded with 32 scans in the frequency range of 4000–400 cm

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with a resolution of 2 cm−1.

5


−1


105

106

Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker 400 (400 MHz) spectrometer, using

CDCl3 as a solvent.

107

The thermal properties of polyimine film were performed on a differential scanning calorimeter (DSC)

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(NETZSCH, DSC 214). The sample was dried in a vacuum oven at 100 °C for 3 h before conducting experiments.

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First, the sample was heated from room temperature to 105 °C and kept at equilibrium in this state for 3 min to

110

remove the water. Then, the temperature was immediately dropped to −40 °C before being gradually increased to

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200 °C at 10 °C/min. All the abovementioned temperature controls were performed under nitrogen atmosphere

112

(60 mL/min flow rate).

113

The thermal stability of the films was measured by thermal gravimetric analysis (TGA; TA instrument, model

114

Q-500) at a heating rate of 10 °C/min from room temperature to 600 °C under nitrogen atmosphere (50 mL/min

115

flow rate).

116

117

Dynamic mechanical analysis (DMA) was performed on a Q 800 instrument using single-beam mode at 1 Hz

with a heating rate of 3 °C/min in air atmosphere.

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The mechanical properties of the polyschiff cross-linked films were measured on a 5567 Universal Testing

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machine with a 1 kN sensor. The gauge length was 50 mm, and the test was performed at a 20 mm/min

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cross-head speed. The average thickness values of the films with 1 cm width were calculated with a micrometer

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over five measurements. At least eight species were tested for each sample.

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Optical microscopy images, which were used to monitor crack healing, were obtained with an Olympus BH-2

equipped with a Canon EOS 1100D, using 2× and 5× objectives.

124

The self-healing ability of dumbbell-shaped samples was tested by cutting the center of each sample using

125

scissors, after which the cut pieces were gently pressed by a 200 g weight at 150 °C for 1 h. After cooling, the

126

pieces were subjected to strain-stress experiments to test the recovery of mechanical properties.

6


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A reprocessing test was performed on a hot-pressing machine. The films were cut into small pieces with

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scissors, placed into a round steel mold (10 cm diameter and 0.5 mm thickness), and reprocessed at 150 °C for 10

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min at 10 MPa. After cooling to room temperature, the recycled films were obtained and stress−strain tests were

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performed as previously described.

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Chemical recycling of cross-linked films was performed by immersing the film in HCl solution (pH = 2, 4, 6);

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then, the insoluble residuals were collected, vacuum-dried, and characterized by FTIR and 1H-NMR methods. The

133

weighted residuals were used as recycled DAV monomer to prepare cross-linked polyimine films as previously

134

described.

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Results and discussion

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Synthesis and characterization of polyschiff films

137

The preparation of cross-linked polyimine vitrimers is schematically illustrated in Figure 1. First, dialdehyde

138

was synthesized from vanillin and 1,4-dibromo butane; then, it was condensed with amine monomers to obtain a

139

light yellow film.

140 141

Figure 1. Synthetic route of polyschiff thermosets from vanillin

142

143

Dialdehyde monomer was successfully prepared as shown in the FTIR spectra (Supporting Information Figure

144

S1). The absorption peak around 3165.1 cm , corresponding to the vibration of the vanillin–OH group,

−1

7



145

disappeared after the reaction with 1,4-dibromo butane. The new vibration of methyl and methylene groups near

146

2974.7, 2944.7, and 2858.5 cm also demonstrated the successful synthesis of DAV. Furthermore, the absorption

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peak of aromatic aldehyde, centered near 1670 cm , was also observed. As shown in the NMR spectra of Figure

148

S2, the absorption signal for the methylene proton, which is adjacent to the phenolic ether (Ph-O-CH2-) group,

149

was detected at 4.13 and 68.64 ppm, indicating that the dialdehyde monomer was successfully synthesized from

150

vanillin. Moreover, the absorption peak at around 9.76 and 190.88 ppm is confirmed as the aldehyde group.

−1

−1

151

Because of the fast imine condensation reaction, cross-linked polyimine films were easily prepared through

152

the reaction between the dialdehyde (DAV), diethylenetriamine, and tris(2-aminoethyl)amine) monomer even at

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room temperature. The mixed solution became light yellow from colorless in only 10 min at 50 °C, suggesting the

154

beginning of the imine reaction. FTIR results were used to prove the formation of Schiff structures in the film

155

(Supporting Information Figure S3). A new emerging peak near 1640 cm and a disappearing absorption at 1678

156

cm , correlating with imine and aromatic aldehyde, respectively, strongly suggests the formation of imine

157

bonding networks . A gel fraction experiment was conducted to check the imine networks. After being refluxed

158

with CH2Cl2 at 50 °C for 12 h, the bulk film of sample 3 still retained its original shape without breaking into small

159

particles, with approximately 96% residual weight percentage.

−1

−1

32

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The thermal properties of polyschiff films were studied here in order to confirm the thermal reprocessing

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and self-healing conditions. The results shown in Figure S4 indicate that the glass transition temperature (Tg) of

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polyimine films increased with the cross-linking degree, which rose from 48 °C to 64 °C. Furthermore, their

163

thermal stabilities were characterized by the TGA method as illustrated in Figure S5. Clearly, all films are thermally

164

stable up to a temperature of at least 300 °C (onset decomposition temperature, To). The maximum

165

decomposition temperature (Tp) ranged from 349 to 368 °C, which is obviously associated with the cross-linking

166

degree. Finally, the residual weight percentages of polyimine films are still almost 36% up to 600 °C. All these

8


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results demonstrate that polyschiff networks are thermally stable under conventional modeling approaches. The

168

detailed results of thermal properties of polyschiff vitrimers are recorded in Table 2.

169

170

Table 2. Summary of the thermal properties of polyschiff films

a

Toa (oC)

Tpb (oC)

Residual weight (wt %) at 600 oC

Tg (oC) DSC

νec (mol/m 3)

Pristine Sample 1

315

355

36.1

60

311

Pristine Sample 2

301

349

37.4

48

--

Pristine Sample 3

335

368

35.2

64

1120

Initial decomposition temperature; b maximum decomposition temperature; c calculated from rubbery modulus measured by DMA experiments

171

172

Figure 2 illustrates the dynamic mechanical properties of polyschiff thermosets. The storage modulus of

173

pristine 1 (E′ = 1713 MPa) is lower than that of pristine 3 (E′ = 2341 MPa) at 25 °C, which is obviously attributed to

174

the higher crosslinking degree in sample 3. The tanδ peak is often used to determine the Tg. It was noticed that

175

the Tg of crude samples 3 and 1 are 76 and 75 °C, respectively, as shown in Figure 2b. A rubbery plateau can be

176

observed above the Tg temperature for both, indicating the presence of cross-linking networks in this system. The

177

cross-linking density (νe) of virginal samples 3 and 1 are 1120 and 311 mol/m , respectively, according to the

178

previously reported method 37.

3

179 180

Figure 2. (a) Storage Modulus and (b) tanδ of polyschiff thermosets from vanillin

181

182

The mechanical properties of pristine polyimine vitrimers were tested as illustrated in Figure S6. Overall, the

183

tensile strength and elongation at break of polyschiff vitrimers ranged from 47.43 ± 2.46 MPa to 57.1 ± 6.01 MPa, 9



Page 10 of 21

184

and 16.34 ± 2.86% to 13.01 ± 1.51%, respectively, obviously showing no large differences. The polyschiff vitrimers

185

in this work are much tougher than those of recently reported polyimine vitrimers, whose stress and elongation

186

at break are approximately 40 MPa and 5%, respectively. In contrast with the reported rigidity of

187

terephthalaldehyde, the aromatic aldehyde group is adjacent to the butyl group, resulting in the flexible structure

188

of DAV 31,32.

189

Dynamic properties

190

Self-healing ability

191

Traditional thermosets do not reform their bulk integrity once they are damaged or scratched; this problem 38

39,40

192

can be resolved by introducing dynamic bonds into networks, such as Diels-alder adduct , disulfide linkage

193

and dioxaborolane

194

condensation, exchange, and metathesis

195

thermosets. With the expected behaviors of dynamic imine bonds, the self-healing properties of polyschiff

196

vitrimers were first evaluated by a scratch recovery test. The scratched film was subjected to a heating stage at

197

180 °C and monitored using optical microscopy, as shown in Figure 3. Obviously, the scratch immediately became

198

shallower in only 15 min, and it finally disappeared after 2 h, indicating its self-healing ability at the evaluated

199

temperature.

200

41

,

. With regard to the imine bond, three types of reverse imine reactions, including imine 25

, are obviously desirable for designing the dynamic properties of

It is widely acknowledged that covalent adaptable networks are classified into two groups, dissociative and 42

201

associative, depending on their exchange mechanism

202

excluded, mainly because of the trace amount of water under the high self-healing temperature. This explanation

203

could be proved by the absence of aromatic aldehyde absorption in the FTIR spectra of the first recycled sample 1,

204

as shown in Figure S7. With the equivalent molar ratio of aldehyde and amine in the vitrimers preparation, the

205

self-healing mechanism polyschiff vitrimers does not belong to the imine exchange reaction (residual amine 10

. In this work, the breakage of imine bonds could be


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34

206

group exchanged with imine)

207

metathesis, as shown in Figure 4.

. Finally, we deduced that its dynamic properties are controlled by imine

208 209

Figure 3. Optical microscopy images of scratched pristine sample 1 before and after healing at 180 °C for different times

210

211 212

Figure 4. Self-healing ability of crosslinking polyschiff vitrimers endowed by dynamic imine metathesis reaction

213

214

To visualize the self-healing properties of polyimine vitrimers, the dog-bone shape of sample 1 was cut with

215

scissors, and the cut pieces were realigned with an overlap of 2 mm × 3 mm with a 250 g weight at 150 °C for 1 h.

216

As shown in Figure 5a, the cut edges were melted together. The Young’s modulus, stress at break, and elongation

217

at break of the original sample 1 are 724.38 ± 43.65 MPa, 51.12 ± 3.99 MPa, and 13.01 ± 1.51%, respectively. A

218

healing time of 1 h results in 74.5% and 118.5% recovery of tensile strength and elongation at break, respectively

219

(Figure 5b).

11



220 221

Figure 5. (a) Photo of cut sample 1 before and after healing for 1 h at 150 °C under 200 g weight in oven; (b) stress−strain curves of the thermally healed

222

sample

223

224

Mechanical properties recovery

225

Once the self-healing ability of polyschiff vitrimers was confirmed, their reprocessability was studied. First,

226

the polyschiff vitrimers were cut into small pieces and then hot-pressed at 150 °C for 10 min to obtain the bulk

227

materials. These procedures were repeated three times to obtain the dog-bone-shaped samples for tensile-strain

228

experiments.

229 230

Figure 6. (a) Reprocessing experiments of polyimine vitrimers in a hot press; (b) bar chart of Young’s modulus, (c) tensile strength at break, and (d) fractured

231

elongation of the original, first, second, and third cycles of vitrimers obtained using hot processing at 150 °C for 10 min

232 12


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233

The tensile-strain tests (Figure 6) demonstrated that the stress and elongation at break for samples 3 and 1

234

still kept at the same level of original, and sometimes even slightly higher after three recycles. However, these

235

properties of sample 2 become inferior with increasing hot processing cycles, perhaps because of its faster

236

consumption of dynamic bonds. Furthermore, it was found that the tensile strength and elongation at break is

237

proportional to the crosslinking degree under the same hot-processing batch. Meanwhile, the Young’s modulus

238

shows an increasing trend with the declining cross-linking degree. These results demonstrate that the mechanical

239

properties of polyschiff films are better with the higher cross-linking points in this work. Finally, though sample 2

240

shows the lowest tensile strength and elongation at break after three hot-processing cycles, it still exhibits up to

241

71.2 % and 72.8 % recovery.

242

Chemical recycling of DAV monomer

243

In taking advantage of the unstable Schiff structure under acid conditions

43

, it is interesting to test the

244

chemical recycling of DAV monomer and its reuse in the formation of the cross-linked polyschiff film. Here, the

245

samples were immersed in an acid solution (pH =2, 4, 6) at 50 C for 24 h under continuous stirring. Obviously, the

246

bulk materials break into small particles at a pH of 2, leading to the milky mixtures after acid hydrolysis (Figure 7a).

247

In order to quantitatively evaluate the influence of pH on this chemical recycling process, the residual weight

248

percentages of vitrimers were compared with the theoretical content of DAV in vitrimers, as illustrated in Figure

249

7b. The weight percentage of insoluble substances at a pH of 2 is slightly lower than theoretical content of DAV,

250

indicating complete decomposition. However, the opposite trend was observed at higher pH values. Thus, it could

251

be concluded that a stronger acid condition is beneficial to chemical recycling. Furthermore, under the same

252

conditions, the chemical degradation of sample 3 is much slower in contrast with that of the others, which is

253

clearly associated with the cross-linking degree. These results are attributed to the nature of the dynamic imine

254

reaction, in which reversible equilibrium could be accelerated under acid water solution, thus resulting in the 13

°



255

breakage of imine bonds and the new formation of aldehyde structures.

256

In order to test the film reforming ability, the white powder was recovered by filtration and its structure was

257

characterized by the FTIR and H-NMR methods, as shown in Figure 8. Compared with the original vanillin-based

258

dialdehyde, the H-NMR spectrum of the recycled dialdehyde does not show any differences in the peak positions

259

and area integral of hydrogen protons. The H-NMR signal of the aromatic aldehyde near 9.85 appeared after acid

260

hydrolysis, strongly indicating the rupture of imine bonds. After the recycled dialdehyde structure was confirmed,

261

it was reused in the polyschiff vitrimer preparation with amine monomers, as shown in Figure 7a. As expected, a

262

flawless film was also obtained, showing the chemical recyclability of DAV monomer in polyschiff vitrimers.

1

1

1

263 264

Figure 7. (a) Chemical recycling of cross-linked polyimine films (conditions: 50 °C, pH =2, 24 h); (b) weight percentages of residuals after acid hydrolysis at

265

50 °C for 24 h

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Figure 8. (a) FTIR and (b) 1H-NMR spectra of recycled DAV from acid hydrolysis (conditions: pH = 2, 50 °C, 24 h)

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Conclusions

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Bio-based polyschiff vitrimers were synthesized from vanillin and a commercial amine cross-linker. With the

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introduction of dynamic imine covalent bonds into these crosslinked networks, the obtained bio-based vitrimers

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show self-healing ability, hot-reprocessing properties, and chemical recyclability under acid hydrolysis. Even after

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three hot-processing cycles, the tensile strength and elongation at break remained at the same level as those of

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the original, sometimes even slightly higher. Importantly, dialdehyde monomers could be recycled from polyschiff

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vitrimers under acid solution at 50 °C, for which structure is the same as that of the synthesized dialdehyde

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monomer and could be used to regenerate polyschiff vitrimers. This work offers a significant opportunity for

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designing self-healing and chemically recyclable polymer materials from lignin-based monomers.

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Supporting Information

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Figure S1. FTIR spectra of vanillin and DAV; Figure S2. NMR spectra of DAV monomer; Figure S3. FTIR spectra

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of pristine polyschiff vitrimers; Figure S4. DSC results of pristine polyschiff vitrimers; Figure S5. TG results of

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pristine polyschiff vitrimers; Figure S6. Strain-stress plot of pristine polyschiff vitrimers; Figure S7. FTIR spectra of

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fractured sample 1 before and after 200 g weight hot-pressing; Table S1. Mechanical properties of pristine and

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recycled polyschiff vitrimers using hot-processing.

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Conflicts of interest

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There are no conflicts to declare.

Acknowledgements

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This work was supported by the foundation of Science Research Program from the Hubei Provincial

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Department of Education (Grant No.Q20171602) and the Opening Project of State Key Laboratory of Polymer

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Materials Engineering (Sichuan University) (Grant No. sklpme2017-4-14).

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TOC:

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Thermally reprocessable and acid-degradable vanillin-derived vitrimers were prepared, showing a total chemical

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recyclability of bio-resource based materials.

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Vanillin-Based Polyschiff Vitrimers ... - ACS Publications

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