Eugenol-Derived Biobased Epoxy: Shape Memory, Repairing, and

Oct 16, 2017 - All vitrimers exhibit excellent shape changing, crack healing, and shape memory properties. Although vitrimers with 1:0.75 and 1:1 rati...
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Eugenol-Derived Biobased Epoxy: Shape Memory, Repairing, and Recyclability Tuan Liu,† Cheng Hao,† Liwei Wang,‡ Yuzhan Li,‡ Wangcheng Liu,† Junna Xin,† and Jinwen Zhang*,† †

School of Mechanical and Materials Engineering, Composite Materials and Engineering Center, and ‡School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington 99164, United States S Supporting Information *

ABSTRACT: Conventional epoxy polymers are constructed by petro-based resources that are toxic and nonrenewable, and their permanent cross-links make them difficult to be reprocessed, reshaped, and recycled. In this study, a unique eugenol-derived epoxy (Eu-EP) is synthesized, and then vitrimeric materials are prepared by reacting Eu-EP with succinic anhydride (SA) at various ratios (1:0.5, 1:0.75, and 1:1) in the presence of zinccontaining catalysts. All vitrimers exhibit excellent shape changing, crack healing, and shape memory properties. Although vitrimers with 1:0.75 and 1:1 ratios cannot be physically reprocessed, they can be well reprocessed by the chemical method of being simply decomposed in a benign ethanol solution without loading additional catalyst. The collected decomposed polymers can form vitrimers again after exposure at 190 °C for 3 h. This work combines the concepts of vitrimer preparation, chemical recycling, and biobased polymer together, which would bring a feasible way to satisfy the demands of sustainability.

1. INTRODUCTION Conventional thermosetting polymers have excellent dimensional stability, chemical resistance, and thermal stability, but they cannot be reprocessed like the thermoplastics by remelting due to the permanent cross-linked structure. There is a growing interest in recycling thermosetting polymers, but the highly cross-linked structures of thermosets make recycling and reuse of the materials a great challenge.1−3 To address this dilemma, some reversible thermosetting polymers have been introduced in recent years,4−6 in which the covalent linkages can break and re-form under external stimulus like heat or UV light.7−9 A representative example is based on the reversible Diels−Alder (D−A) reaction. For example, the D−A adduct of maleimide and furan at over 120 °C undergoes a retro-D−A reaction, and the cross-linked polymer dissociates into liquid.10,11 At low temperature (∼60 °C), the D−A addition reoccurs to form the cross-linked polymers. However, such a solid−liquid transformation at an elevated high temperature leads to a sudden drop of viscosity, which causes loss of the dimensional stability of the bulk materials.12 In 2011, Leibler’s group introduced the concept of vitrimer, which relies on dynamic bond exchange mechanism.13 When subjected to heat, the reversible chemical bonds exchange in a dynamic manner, and the cross-linked structures remain integrally, which allows the polymer to be reprocessed and repaired like thermoplastics.14−16 One of the most representative vitrimers is prepared from the epoxy−anhydride reaction system.17 In cross-linked polymer network, the hydroxyl groups resulting from the ring-open reaction of epoxies can induce the © XXXX American Chemical Society

dynamic transesterification with the ester linkages at high temperatures (>150 °C), which imparts multifunctional properties such as shape changing, repairing, and reprocessing to the materials. In order to reserve sufficient free hydroxyl groups for transesterification, the ratio of epoxy to anhydride in vitrimer is often kept at ∼1:0.5. Reaction of epoxy and anhydride at this stoichiometric ratio tends to yield insufficiently cross-linked polymer network compared with that of the conventional thermosets. In industrial practice, epoxy and anhydride are often formulated by stoichiometric ratios of 1:0.7 to 1:1 (especially from 1:0.8 to 1:0.9) for better completion of curing. However, under such stoichiometric ratios, the cross-linked polymer networks cannot be reprocessed or physically recycled due to an insufficient amount free hydroxyl groups available for transesterification. For this reason, the repairability and recyclability of the epoxies formulated in a conventional way have received little attention. In this study, cross-linked polymer networks were prepared by reacting epoxy and anhydride at different stoichiometric ratios of 1:0.5, 1:0.75, and 1:1 under the catalysis of zinc acetylacetonate hydrate (Zn(acac)2). It was similar to those reported elsewhere;18,19 in this study the vitrimer prepared with a 1:0.5 stoichiometric ratio exhibited fast transesterification at an elevated temperature, excellent crack healing, shape changing, and reprocessing (or physical recycling) properties. Received: September 1, 2017 Revised: September 29, 2017

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DOI: 10.1021/acs.macromol.7b01889 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. (a) Synthetic Route of Eugenol Epoxy (Eu-EP); (b) Curing Reaction of Eu-EP and SA and Illustration of HeatInduced Transesterification Reactions (TERs) in the Presence of Zinc Catalyst

filtration. Figure 1a shows the FTIR spectra of Eu-DI. After reaction, the absorption band of eugenol hydroxyl groups at ∼3500 cm−1 totally disappeared, while the absorption band of double bond at 1638 cm−1 remained intact. In addition, the structure of Eu-DI was also confirmed by the 1H NMR (Figure 1b) and 13C NMR (Figure 1c) spectra. These results demonstrate the successful synthesis of Eu-DI. In the second step, Eu-EP was prepared by the epoxidation of Eu-DI. The reaction was performed at room temperature in CH2Cl2 using m-chloroperoxybenzoic acid (mCPBA) as the epoxidation reagent. After reaction, the 1H NMR signals of the double bonds at around 5.1 and 5.9 ppm disappeared (Figure 1b), whereas new signals appeared at the lower field, 2.5−3.2 ppm, due to the formation of epoxypropyl groups. In the FTIR spectrum (Figure 1a), the absorption of double bonds at 1638 cm−1 disappeared, and a new absorption peak at 829 cm−1 which belongs to epoxy groups was noted after epoxidation. These evidences indicate the successful epoxidation of the double bonds to afford Eu-EP. Moreover, the epoxide equivalent weight (EEW) of Eu-EP was found to be 205.1 g equiv−1, which was closed to the theoretical value (207.0 g equiv−1). The slight deviation of EEW could be related to the ring-opening side reaction of epoxy which resulted in the formation of some hydroxyl groups (Figure 1a). 2.2. Curing and Structure Characterization of Eu-EP/ SA Polymer Networks. Eu-EP and succinic anhydride (SA) at different stoichiometric ratios (R = epoxy/anhydride) were reacted. Zinc acetylacetonate (Zn(acac)2) was selected to catalyze the curing reaction and also served as the catalyst for subsequent transesterification in the formed material at elevated temperatures. Table 1 shows the summary of the formulations of the vitrimer samples. For all formulations, Eu-EP accounts for over 67 wt %, suggesting high biomass contents in the products. The possible reaction mechanism between Eu-EP and SA is shown in Scheme S1. At the beginning, the anhydride ring is opened and activated by Zn(acac)2, and the resulting carboxylate anions react with epoxy groups to afford ester bonds and release oxygen anions which will further react with anhydride or epoxy; the hydroxyl anions could also attract active hydrogen to form hydroxyl groups.23−25 Because acetylacetone generated from Zn(acac)2 has a low boiling point of ∼140 °C which is much lower than the postcuring

Although the vitrimers with epoxy/anhydride ratios of 1:0.75 and 1:1 displayed much slower transesterification rates than the vitrimer with an epoxy/anhydride ratio of 1:0.5 and could not be physically reprocessed, they exhibited considerable crack healing and shape changing properties. Nevertheless, these two compositions were easily decomposed by ethanol at mild temperature (150 °C). The thermal degradation could be another reason for the drop of Tg. 2.7. Shape Memory Properties of Eu-EP/SAs. Figure 6a shows the deformation−time curves of Eu-EP/SAs. For better comparison, all curves were normalized based on the instantaneous deformation at the first cycle. It can be observed that deformation, shape fixity, and shape recovery remained stable over three cycles, indicating their excellent shape memory properties. Figure 6b and Video S1 provide a visualized version for the shape memory. At the beginning, the samples with permanent shape of “W−S−U” were deformed at above Tg. Upon cooling below Tg, the temporary shape of the films was fixed. Subsequently, the film samples were moved to a heating plate with a temperature of 80 °C (above Tg) to allow the recovery of the shape to “W−S−U”. It has been reported that for polymer materials that exhibit thermal responsive shape memory properties the polymer should possess chemical or physical cross-linking networks and a suitable ratio of stiff and flexible segments.35−37 Obviously, all Eu-EP/SAs with different ratios are cross-linked polymers and are constructed by flexible alkyl chains and stiff benzene groups. Shape fixity (Rf) and shape recovery (Rf) were calculated according to the following equations:

reprocessing, both modulus and strength decreased. In addition, the temperature at 5 wt % loss showed a decrease of about 16 °C. Therefore, the thermal degradation during the high temperature processing could be a reason for the decrease of mechanical properties. Although physical recycling is a simple means for reuse of the material, it has some limitations. First, this method only works for vitrimers that can undergo fast TER, in this study it is the Eu-EP/SA sample with R = 1:0.5. Second, epoxies are often used as matrix resins for composite materials and encapsulating materials for electronic products, mechanical recycling and of such waste products, and reuse of the recylates are great challenges. Chemical degradation of thermosetting polymers and reuse of the decomposed polymers in new thermosetting materials may provide a sound solution to these limitations of the physical recycling method.31−34 In this work, we found that decomposition of the Eu-EP/SA vitrimer was easily achieved by reacting with ethanol. Because the prepared Eu-EP/SA vitrimers inherently contain Zn2+ ligand complexes, no new catalyst is needed for this process. The mechanism for the degradation reaction is shown in Scheme 2. The hydroxyl groups of ethanol can initiate the TERs with the ester bonds of Eu-EP/SAs in the presence of Zn(acac)2. This reaction results in a dissolution of the cross-linked polymers. The degradation temperature was selected as 160 °C, which is above the onset temperature for the TERs (Figures 3b,c). Figure 5c shows the effect of reaction time on degradation degree of Eu-EP/SA with R = 1:0.5. The sample was completely decomposed within 7 h, and the decomposed polymer was soluble in ethanol (Figure 5b). The same experiments were also performed to decompose Eu-EP/SAs with R = 1:0.75 and 1:1. Similarly, both of them were completely decomposed at 160 °C within 7 h. Although they did not have sufficient hydroxyl or carboxyl groups in their structures, the ethanol solution provided extra hydroxyl groups to induce the TERs and resulted in complete dissolution of polymers. After reaction, ethanol was removed from the reaction solution, and the decomposed matrix polymer (DMP) with catalyst was collected and appeared as a highly viscous liquid. The 1H NMR spectrum of the DMP is shown in Figure S10. Obviously, the ethyl group (CH3CH2−), which was derived from ethanol, was noted, demonstrating the occurrence of TERs between ethanol and cross-linked polymer. When the DMP was heated at 190 °C, transesterification took place in the DMP, and ethanol was reduced and evaporated. The regenerated Eu-EP/SAs were insoluble in any solvent at room temperature, indicating the formation of cross-linked

Rf =

εd × 100% εload

(1)

Rr =

εd − εrec × 100% εd

(2)

where εload and εd are the instantaneous deformation upon loading and the fixed deformation after cooling and load removal, and εrec is the deformation after revovery. As shown in Table 2, all samples showed a shape recovery of almost 100%. The shape fixity for Eu-EP/SA with R = 1:0.5 was 88.8%, while the other Eu-EP/SAs with R = 1:0.75 and 1:1 were around 92%. Xie et al.35 reported that the difference in shape fixities could be correlated to the difference in glassy modulus (Gg′) and rubbery modulus (Gr′) for each individual sample. In this G

DOI: 10.1021/acs.macromol.7b01889 Macromolecules XXXX, XXX, XXX−XXX

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used as received. All solvents (GR grade) were used without further purification. 4.2. Synthesis of Eugenol Diene (Eu-DI) Compound. Under magnetic stirring, eugenol (32.8 g, 1 equiv), ethanol (200 mL), K2CO3 (27.6 g, 1 equiv), and KI (1.28 g, 0.04 equiv) were charged into a 500 mL three-necked flask and heated to 70 °C. After the color of the solution turned to dark green, 1,4-dibromobutane (17.28 g, 0.4 equiv) in 20 mL of ethanol was added dropwise into the mixture through a constant pressure funnel. The mixture was allowed to react for 12 h. After the reaction mixture was cooled to room temperature and then remained still for 12 h, a solid crude product was collected by filtration. The crude product was washed with hot water, recrystallized in ethanol, and dried in a vacuum oven at 60 °C. The product was a white powder with a yield of 93%. 1H NMR (400 MHz, CDCl3, δ): 1.96−2.06 (m, 4H, OCH2(CH2)2CH2O), 3.28−3.34 (d, 4H, CH2 CHCH2−), 3.82−3.84 (s, 6H, ArOCH3), 4.03−4.10 (t, 4H, ArOCH2−), 5.02−5.10 (m, 4H, CH2CH−), 5.88−6.00 (m, 2H, CH2CH−), 6.66−6.71 (d, 4H, Ar−H), 6.79−6.83 (d, 2H, Ar−H). 13 C NMR (400 MHz, CDCl3, δ): 25.99, 39.79, 55.88, 68.77, 112.32, 113.27, 115.53, 120.40, 132.77, 137.68, 146.77, 149.36. 4.3. Synthesis of Eugenol-Derived Epoxy (Eu-EP). Eu-DI (10 g, 1 equiv) was dissolved in 100 mL of CH2Cl2 and then charged into a 250 mL three-necked flask. Under magnetic stirring, mCPBA (3 equiv) was slowly added into the solution, and the reaction was continued at room temperature for 3 days. After the solid was removed by filtration, the mixture was washed twice with saturated Na2SO3 aqueous solution. The organic phase was concentrated using a rotary evaporator. The collected crude product was recrystallized in ethyl acetate and dried in a vacuum oven at 60 °C. The final product was a light yellow powder with a yield of 75%. 1H NMR (400 MHz, CDCl3, δ): 1.96−2.05 (m, 4H, OCH2(CH2)2CH2O), 2.50−2.55 (dd, 2H, −CH−CH2−O), 2.73−2.87 (m, 6H, Ar−CH2− and −CH−CH2−O), 3.07−3.16 (m, 2H, −CH−CH2−O), 3.83−3.86 (s, 6H, −O−CH3), 4.03−4.10 (t, 4H, OCH2(CH2)2CH2O), 6.72−6.85 (dd, 6H, Ar−H). 13 C NMR (400 MHz, CDCl3, δ): 25.98, 38.29, 46.79, 52.58, 55.94, 68.72, 112.73, 113.22, 120.93, 129.88, 147.24, 149.38. 4.4. Preparation of Eu-EP/SA Polymer Networks. Eu-EP/SA vitrimer materials were prepared through the curing reaction between Eu-EP and succinic SA. Zn(acac)2 was used as catalyst and added at an amount of 10 mol % of SA. After these three compounds were mixed using a mortar, the powder mixture was added into a Teflon mold with a dimension of 5 cm × 5 cm × 0.25 cm. The mold was placed in a convention oven at 120 °C. After the powder mixture was totally melted, the Teflon mold was transferred to a hot-press machine. The mixture was cured at 150 °C for 1 h and postcured at 190 °C for 2 h. After curing, the samples were cooled to room temperature in air. 4.5. Characterizations. Nuclear magnetic resonance (NMR) spectra were recorded on a fully automated Varian 400-MR spectrometer (400 MHz). CDCl3 was used as deuterated solvent to dissolve the samples. Chemical shifts of 1H and 13C NMR peaks were reported in ppm. Fourier transform infrared spectroscopy (FTIR) spectra were collected on a NICOLET iS50 FTIR spectrophotometer. The sample for FTIR experiment was prepared by grinding the sample (1 mg) and potassium bromide (KBr, 100 mg) powder in a mortar and then compressed the mixture into a disk using a mold. The sample was scanned from 4000 to 400 cm−1 with a resolution of 4.0 cm−1 for 64 times. The epoxy equivalent weight (EEW) of Eu-EP was determined by titration using the HCl−acetone method. Differential scanning calorimeter (DSC1, Mettler-Toledo, Switzerland) was used to test the Tg values and the curing behavior. The samples (∼5 mg) were scanned from 25 to 150 or 200 °C at a heating rate of 10 K min−1 under a nitrogen atmosphere. Thermal stability was measured using a TGA/DSC1 thermogravimetric analyzer (TGA), and the samples (∼10 mg) were scanned from 50 to 800 °C at a heating rate of 10 K min−1 under a nitrogen atmosphere. Dynamic mechanical properties were characterized using a dynamic mechanical analyzer (DMA) (Q800, Thermal Instrument) in the single cantilever mode. The dimension of the sample was 35.0 mm × 12.8 mm × 3 mm. The frequency was set at 1 Hz, and the oscillating amplitude was set at 15 μm. The samples were scanned from −10 to 150 °C at a

Table 2. Values of Shape Fixity (Rf) and Shape Recovery (Rr) for the Cured Eu-EP/SAs Rf (%)

Rr (%)

R

cycle 1

cycle 2

cycle 3

cycle 1

cycle 2

cycle 3

1:0.5 1:0.75 1:1

88.8 91.9 92.4

88.8 91.8 92.6

88.8 91.8 92.5

100.0 100.0 99.9

99.9 100.0 100.0

100.0 100.0 99.9

work, the differences of Gg′ and Gr′ for Eu-EP/SAs with R = 1:0.75 and 1:1 were wider than that for Eu-EP/SA with R = 1:0.5, which may be responsible for their higher shape fixity (Figure 2d). As shown in Figure S2, all the cured Eu-EP/SA samples show a broad tan δ peak typically ranging from 35 to 75 °C. In most cases, the broad Tg could have a dual effect on the shape memory property. On one hand, the broad Tg allows programing of multistage shape memory and enable high shape recovery stress.38 On the other hand, the broad Tg could result in incomplete shape recovery. In this work, in order to achieve complete shape recovery, samples need to be treated at temperature above 75 °C. Table 3. Function Assessment of the Cured Eu-EP/SAs with Different Stoichiometric Ratios R

crack healing

shape changing

physical recycling

chemical recycling

shape memory

1:0.5 1:0.75 1:1

√ √ √

√ √ √

√ × ×

√ √ √

√ √ √

3. CONCLUSIONS In conclusion, biobased eugenol epoxy (Eu-EP) was synthesized and cured with succinic anhydride (SA). Different epoxy− anhydride stoichiometric ratios (R) of 1:0.5, 1:0.75, and 1:1 were selected. The material with Eu-EP/SA in a 1:0.5 ratio exhibited a high content of hydroxyl groups and was able to easily undergo transesterification exchange reactions (TERs) at temperatures above 150 °C. This dynamic reaction imparts fast stress relaxation of the cross-linked polymer network and permits shape changing, crack healing, and physical recycling of the samples. In contrast, due to the lack of hydroxyl groups, EuEP/SAs with 1:0.75 and 1:1 stoichiometric ratios showed slower stress relaxation rates, but they could still afford shape changing and crack healing properties. It was also found that all Eu-EP/SAs could be decomposed in the environmentally benign ethanol at 160 °C. The decomposed polymers can be reconverted into new thermosetting polymers after exposure at 190 °C for 3 h. Furthermore, all Eu-EP/SAs exhibited excellent shape memory properties. This work integrates the concepts of vitrimer, chemical recycling, and biobased polymer together, and the findings may set up an important framework for future design of sustainable polymer materials. 4. EXPERIMENTAL SECTION 4.1. Materials. Eugenol (Aldrich, ≥98%), zinc acetylacetonate hydrate (Zn(acac)2, Aldrich, 99.995%), 1,4-dibromobutane (Acros Organics, 99%), succinic anhydride (SA, Acros Organics, 99%), 3chloroperbenzoic acid (mCPBA, Acros Organics, 70−75 wt %), potassium carbonate (K2CO3, J.T. Baker, ≥99%), potassium iodide (KI, J.T. Baker, ≥99%), magnesium sulfate (MgSO4, Fisher Chemical, ≥99%), and sodium sulfite (Na2SO3, Fisher Chemical, 98.6%) were H

DOI: 10.1021/acs.macromol.7b01889 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules heating rate of 5 K min−1. The dimension change as a function of temperature was characterized on a thermal mechanical analyzer (TAQ400) from 50 to 230 °C at 3 K min−1 under ambient atmosphere. An oscillating force ramp method (modulate force 0.0200 N at 0.50 Hz) was used for the test. Isothermal stress relaxation was determined on an ARES G2 TA Instruments using a 25 mm parallel plate. The measurements were carried out at different temperatures in the range from 120 to 200 °C. The sample with uniform thickness was heated to the test temperature. After a 10 min temperature equilibration, a constant normal force of 2 N was applied to obtain a good contact of sample with the parallel plate. During the test, a 1.5% strain was applied and maintained, and the relaxation modulus as a function of time was recorded. Shape memory properties were evaluated on a DMA Q800 instrument in the single cantilever mode. The dimension of the sample was 35.0 mm × 12.8 mm × 0.25 mm. The sample was equilibrated at 70 °C (above Tg) for 10 min. An external constant force of 0.1 N was applied to induce a shape changing of the sample, and the temperature was decreased to 15 °C (below Tg) at a cooling rate of 5 K min−1. As soon as the film reached to 15 °C, the external constant force was canceled, and the film was heated to 70 °C again at a heating rate of 5 K min−1. During the second heating process, the shape of the film recovered. These above steps were repeated for three times, and the position changes were recorded during the test. Phycial Recycling. The sample was chopped into small pieces and hot pressed at 200 °C for 1 h. After being released from the hot press, the sample was cooled to room temperature in air. Chemical Recycling. The degradation reaction was carried out using a 100 mL pressure reactor (Series 4842, Parr Instrument Company). 1 g of sample and 20 mL of ethanol were charged to the reactor. After the temperature was raised to 160 °C, the reaction was continued for 5 h. After reaction, the reactor vessel was placed in a water bath and cooled to room temperature prior to opening. The reaction mixture was filtered. The insoluble part was the undecomposed resin. The filtrate was concentrated using a rotary evaporator and then dried further in a vacuum oven at 60 °C for 12 h. The decomposed polymer appeared as a high viscosity liquid. The degradation degree (Dd) was calculated by the following equation:

⎛ W⎞ Dd% = ⎜1 − 2 ⎟ × 100% W1 ⎠ ⎝

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(3)

where W1 is the initial weight of polymer before degradation and W2 is the weight of insoluble polymer after degradation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01889. Scheme S1 and Figures S1−S11 (PDF) Video 1 (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.Z.). ORCID

Jinwen Zhang: 0000-0001-8828-114X Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.macromol.7b01889 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.7b01889 Macromolecules XXXX, XXX, XXX−XXX