Environmentally Friendly Method To Prepare Thermo-Reversible, Self

Jan 30, 2019 - Thermo-reversible elastomers (TRE) can be repeatedly processed and, thus, can reduce the dependence of the petroleum for synthetic rubb...
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Environmentally Friendly Method To Prepare Thermo-Reversible, Self-Healable Biobased Elastomers by One-Step Melt Processing Zhanbin Feng,† Jing Hu,† Bing Yu,†,§ Hongchi Tian,‡ Hongli Zuo,† Nanying Ning,*,†,§ Ming Tian,*,†,§ and Liqun Zhang†,§ †

ACS Appl. Polym. Mater. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/31/19. For personal use only.

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡ Dawn Polymer Material Co., LTD, Shandong 265700, China § Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: Thermo-reversible elastomers (TRE) can be repeatedly processed and, thus, can reduce the dependence of the petroleum for synthetic rubber. The preparation of TRE is generally carried out in solution, and it requires a long reaction time. These disadvantages limit the large scale preparation of TRE. In the present work, we chose the epoxidized natural rubber (ENR) as the matrix and designed a new method to prepare thermo-reversible, self-healable ENR by a catalyzed ring-opening (RO) reaction of ENR followed by a Diels−Alder (DA) reaction through one-step melt blending for the first time. Furfurylamine (FA) was first grafted onto ENR (ENR−FA) by an RO reaction under the presence of catalyzer, and then bismaleimide (BMI) was added to induce the cross-linking of ENR−FA via DA reaction, which can be easily realized during melt blending. As the DA reaction is reversible at different temperatures, the mechanical properties of thermoreversible ENR (TRENR) could be tailored by the ratio of FA to BMI. The TRENR is recyclable twice by compression molding, while retaining 90% of its mechanical properties after the first recycling. In addition, the as-prepared TRENR exhibits self-healing ability. The preparation of TRENR was carried out in a Haake rheomixer batch mixer with a short reaction time, and it requires no solvent and does not produce a stimulating smell. This new technique is promising in industry for rubber recycling and large-scale preparation of TRE in an environmentally friendly method. KEYWORDS: thermo-reversible elastomers, Diels−Alder reaction, epoxidized natural rubber, one-step melt processing, rubber recycling

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INTRODUCTION In recent years, rubbers have been used in a wide range of applications, such as tires, sealing devices, building materials, etc. It is well-known that traditional rubbers commonly use either sulfur or peroxide to form irreversible chemical crosslinks, which make it difficult to be recycled and consequently causes large amounts of polymer wastes and some serious environmental issues.1 It is of great significance to prepare the thermo-reversible rubber that can be covalently cross-linked at room temperature and can be de-cross-linked at elevated temperatures, and then this kind of rubber can be repeatedly reprocessed and reused just like thermoplastics. In this way, the amount of synthetic rubbers used in the industrial production and daily life can be greatly reduced, resulting in a smaller consumption of the petroleum and fewer productions of solid wastes. In the past few years, lots of polymer materials have been successfully fabricated through the reversible noncovalent and covalent bonds method, which possessed the self-healing and © XXXX American Chemical Society

remolding ability. For example, self-healable and recyclable polymers including rubbers could be constructed via ionic bonds,2−6 hydrogen bonding interaction,7−9 π−π stacking interactions,10 and could also be fabricated through dynamic covalent bonds, such as a schiff base,11−13 acylhydrazone bonds,14−16 disulfide bonds,17−19 alkoxyamine bonds,20,21 and siloxane equilibration.22 Comparatively, it is more desirable to take advantage of reversible covalent bonds to fabricate the self-healing polymers as heating, or solvent can make the noncovalent interactions unstable. Among the covalent bonds method, the Diels−Alder (DA) reaction is a representative thermo-reversible reaction with the advantages of relatively fast kinetics and mild reaction conditions and, thus, has been widely used in the past decade,23−31 especially in the preparation of a broad range of Received: October 22, 2018 Accepted: December 27, 2018

A

DOI: 10.1021/acsapm.8b00040 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials

FA) at about 120 °C via DA reaction. The TRENR with comparable crosslink density to traditional cross-linked rubber was achieved by optimizing the ratio of FA and BMI. The thermal reversibility of TRENR was proven by differential scanning calorimeter (DSC), dynamic mechanical thermal analysis (DMTA), and dissolution experiments, which indicated that the de-cross-linking reaction can occur at about 150 °C, and the DA cross-linking network can be reformed at about 125 °C. The TRENR is recyclable twice by compression molding, while retaining 90% of its mechanical properties after the first recycling. In addition, the as-prepared TRENR exhibits a self-healing ability. This method was carried out in a Haake rheomixer batch mixer at a short reaction time by one-step melt blending, which requires no solvent and does not produce a pungent smell; this new technique can be potentially used in large-scale industrial production of TRE and the rubber recycling in an environmentally friendly way.

polymers.32−35 In recent years, the preparation of thermoreversible cross-linked resins by using a DA reaction has attracted much attention.36,37 For example, new kinds of covalently cross-linked epoxy38−41 and polyurethane42−47with recyclable properties have been prepared based on a DA reaction.48−51 The DA reaction between a conjugated diene and a dienophile can undergo from room temperature to about 80 °C, resulting in a cyclohexene derivative.52 The reverse reaction of the cross-linked DA adducts can typically occur at temperatures above 100 °C to return to its original state, named as the retro-Diels−Alder (rDA) reaction.32 Unlike the materials which possess diene/dienophile structure or can be easily functionalized with diene/dienophile groups, and the DA reaction can be easily designed between these structures. For most of the typical rubbers, it is difficult to achieve the thermoreversible cross-linking network as there are few active bonding sites that could be attached or modified. Only double bonds in elastomers could be used to achieve functionalization.53 As a result, the modification of these elastomers is necessary to acquire some active sites. For example, Jiao et.al54 used the derivatives of dicyclopentadiene (DCPD) to react with chlorohydrine rubber (CHR) to achieve a thermo-reversible elastomer (TRE) in the solution, which can avoid the complex synthesis process. In addition, the cross-linking and de-crosslinking of DCPD could produce some side reactions. In recent years, Shi et.al grafted the furfuryl mercaptan (Fu) onto polybutadiene (PB) chains, which acted as diene through the thiol−ene click reaction, and then the DA reaction between the Fu-grafted PB and bismaleimide (BMI), which acted as both the dienophile and crosslinker, was achieved at about 100 °C. The synthesized DA-cross linked PB can be recycled through remolding several times.55−57 Meanwhile, Picchioni et.al used furfurylamine (FA) instead of Fu as diene to achieve the DA reaction, avoiding an uncomfortable smell.58,59 However, to the best of our knowledge, direct grafting of FA on the natural rubber in a melt state was less reported. Epoxidized natural rubber (ENR), which is a biobased rubber with good mechanical properties, excellent weather resistance, and good air impermeability, is obtained by epoxidation of natural rubber (NR). According to Norvez’s report,60 ENR can be directly covalently cross-linked by dicarboxylic acids via the ring-opening (RO) reaction between the carboxylic acid groups and the epoxy groups. Zinc acetate (Zn(Ac)2), which acted as a transesterification catalyst, was added into the dodecanedioic acid-cross-linked ENR, and the covalently cross-linked ENR with recyclable properties could achieve stress relaxation at elevated temperatures. Norvez et al.61 also used dithiodibutyric acid as a crosslinker to synthesize chemically cross-linked ENR via thermo-activated disulfide rearrangements. However, up to now, there are no reports about thermo-reversible ENR (TRENR) based on DA reaction. Besides, the preparation of TRE based on the DA reaction is generally carried out in solution, which requires a large amount of solvent and a long reaction time. These disadvantages limit the large scale preparation of TRE. In this study, we report an environmentally friendly route to synthesize TRENR based on a catalyzed RO reaction and DA reaction by one-step melt blending for the first time. We chose ENR as the matrix. Furfurylamine (FA) was first grafted onto the ENR through RO reaction under the catalysis of Ytterbium(III) trifluoromethanesulfonate hydrate (Yb(OTf)3, named as Yb) at about 110 °C for 10 min, and then BMI was added to induce the cross-linking of FA-grafted ENR (ENR−

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EXPERIMENTAL SECTION

Materials. ENR (Mw ∼ 250 000, the epoxy degree is 40%) was purchased from Zhanjiang Tropical Academy (China). BMI (98%) and FA (99%) were purchased from Alfa-Aesar Chemical Co., Ltd. (China). Zinc tetrafluoroborate hydrate (Zn(BF4)2, 99%), magnesium perchlorate (Mg(ClO4)2, 99%), and Yb(OTf)3 (99%) were purchased from J&K Scientific Ltd. (China). Toluene (99%) and cyclohexane (99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All the reagents were used as received. Catalyzed Chemical Modification of ENR with FA (ENR−FA). To obtain ENR−FA with a good grafting ratio, we chose Yb(OTf)3 (named as Yb) as the catalyst and optimized its dosage, and then we optimized the molar ratio between FA and the epoxy groups of ENR. The chemical modification of ENR with different molar ratios of FA to epoxy groups of ENR (1:0.25, 1:0.125, 1:0.1, named as ENR− 0.25FA, ENR−0.125FA, and ENR−0.1FA) was achieved in a Haake rheomixer by using Yb as the catalyst. Take ENR−0.125FA (0.2%Yb) (0.2% represents the molar ratio between catalyst and epoxy groups of ENR) for example, the preparation procedure of it was as follows. First, 40 g of ENR was added to the Haake rheomixer to be plasticized at 80 °C for 3 min. Then, 0.23 g (the molar ratio between Yb and epoxy groups of ENR is 0.2%) of Yb(OTf)3 was added slowly for 3 min to be well-mixed. Finally, 2.1 mL (2.2 g) of FA was added slowly, and the mixture was allowed to react at 110 °C for about 10 min. Then, the modified ENRs with different molar ratios of FA were obtained. The recipes for different molar ratios between FA and ENR based on Yb(OTf)3 are shown in Table 1.

Table 1. Recipes for Different Samples of ENR−xFA

ENR−0.25FA ENR−0.125FA ENR−0.1FA

weight of ENR (g)

weight of FA (g)

weight of Yb(OTf)3 (g)

40 40 40

4.4 2.2 1.7

0.23 0.23 0.23

Diels−Alder Crosslink of ENR. BMI was added into the ENR− FA melt in the Haake rheomixer batch mixer. Typically, 40.0 g of ENR−FA (23.8 mmol furan) and 4.4 g of BMI (11.9 mmol) were added to the Haake rheomixer with the temperature set to 80 °C. Here, it should be noted, that the key factor in this experiment is temperature, and the temperature will go up when rubber suffers from the strong shearing force in the Haake rheomixer; so, repeated experiments were carried out to determine the temperature that should be set at the beginning, and the temperature was detected by the temperature sensor in the Haake rheomixer. Finally, it was found, that the appropriate temperature was 80 °C. After the addition of BMI, the temperature increased to about 120 °C to achieve a DA reaction, the mixture was allowed to react for 10 min, and then the B

DOI: 10.1021/acsapm.8b00040 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials product was withdrawn from the Haake rheomixer. The recipes of different proportions of FA and BMI are shown in Table 2. The

Healing Efficiency =

weight of ENR (g)

weight of FA (g)

weight of BMI (g)

ENR−0.25FA ENR−0.125FA ENR−0.1FA

40 40 40

4.4 2.2 1.7

4.4 4.4 4.4

product was molded at about 160 °C under the pressure of 20 MPa for 20 min. “Reprocessed” samples were prepared according to the following procedure. The samples were cut into small pieces and then were hot pressed under the same condition. Characterizations and Measurements. The grafting ratios of FA were measured by Fourier transform infrared (FT-IR) and 1H NMR spectra. The grafting ratio was calculated according to eq 1 Grafting ratio =

s

(4) where V2 is the volume fraction of rubber in the swollen sample, χ is the interaction parameter between ENR and toluene, ρ is the density of ENR, and ρs is the density of toluene.

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RESULTS AND DISCUSSION The reaction principle of the grafting of FA on ENR and the DA-cross-linking of ENR is shown in Scheme 1. The furan

integral(δ = 3.67) integral(δ = 3.67) + integral(δ ∼ 2.7 − 2.9)

(1)

where integral (δ=3.67) represented the peak area of δ = 3.67; integral represented the peak area between δ ∼ 2.7−2.9. The H NMR spectra were performed by using an AV600 (Bruker Inc., Germany) 600 MHz instrument, and CDCl3 was used as the solvent. Tetramethylsilane (TMS) was used as an internal standard, and 7−8 mg of sample was needed. FT-IR spectra were carried out on a Bruker TENSOR 27 Fourier transformation infrared spectrometer of Germany from 4000 to 500 cm−1. All the samples were dissolved in THF and then coagulated by alcohol. The thermo-reversible properties of cross-linked ENR were measured by DSC analysis, dynamic mechanical thermal analysis (DMTA), and dissolution experiments. DSC analysis was carried out with a STARe system (Mettler-Toledo Inc.), and the heating rate was set at 10 K/min, measured from 303 to 473 K; about 7 mg of samples was needed for the tests. The specimens (30 mm × 4 mm × 2 mm) were used to carry out the dynamic mechanial analysis (DMA) by using a TA Q800 DMA device. The temperature was set from 293 to 473 K under the frequency of 1.0 Hz and the strain of 0.3%, and the heating rate was set at 3 K/min. The dissolution experiments of the DA-cross-linked ENR were carried out in DMF at room temperature for 72 h and at 170 °C for about 2 h in a three-neck flask, and then, the solution was cooled to about 60 °C. The sample used in the dissolution experiment was cut into pieces and swollen in toluene for 48 h in advance to make sure that the unreacted BMI had been removed completely, and then the sample was dried in vacuum for 24 h. The recycling experiment was carried out in the compression molding machine. The samples for the tensile test were cut into some small pieces, followed by placing them into the mold to be reprocessed at 160 °C under the pressure of 15 MPa for about 15 min. The recycled sample was obtained after cooling back down to the room temperature. The thermo-reversible efficiency was calculated by eq 2

Scheme 1. Reaction Principle of the Grafting of FA on ENR and Diels-Alder Crosslink of ENR

(δ∼2.7−2.9) 1

Efficiency =

Stress (reprocessed) Stress (DA‐cross‐linked)

(3)

The stress−strain curves of DA-cross-linked samples were obtained by using an Instron 5567 tensile apparatus at room temperature, and the stretching speed was set at 100 mm/min. The samples for the tensile test were a dumbbell type (length × width × thickness, 110 × 5 × 2 mm3). The crosslink density of cross-linked ENR was determined by the equilibrium swelling method. The detailed procedure is shown in the Supporting Information. The crosslink density for the cross-linked samples was calculated based on the Flory eq 4 É ÅÄ 2Ñ W3/ρ 1 ÅÅ ln(1 − V2) + V2 + χV2 ÑÑÑÑ Vr = − ÅÅÅÅ ÑÑ, where V2 = W W3 − W ÑÑ 3 V ÅÅÅÇ V21/3 + ρ 1 ÑÖ ρ

Table 2. Recipes for Different Samples of ENR−xFA− 0.125BMI sample names

Stress (self‐healing) Stress (DA‐cross‐linked)

group of FA was first grafted onto ENR chains via a catalyzed RO reaction. The prepared furan-modified ENR (ENR−FA) was then cross-linked with BMI through a DA reaction in a Haake rheomixer. Catalyzed Chemical Modification of ENR with FA (ENR−FA) via RO Reaction. ENR−FA was prepared by an RO reaction, and Yb was used as a catalyst for this RO reaction. According to previous studies,62 the RO reaction could occur between epoxy resin and amine derivatives without catalysts at about 100 °C. However, the epoxy groups on the main chain of ENR have low activity, and the RO reaction cannot happen without catalyst according to our experiments (see Figure S3); therefore, the catalyst is essential for an RO reaction between ENR and FA. It should be noted, that the boiling point of FA is about 146 °C, and the temperature of an RO reaction is about 110 °C, so the evaporation of FA can be avoided in a closed state. The dosage of catalyst was first optimized according to the Haake rheomixer results (see Figure S3), as analyzed in the Supporting Information. The recipes and optimal dosage of the catalyst are shown in Tables S1. The optimal dosage of Yb was 0.2% molar ratio of ENR.

(2)

The self-healing behavior of the scratch in the samples was observed by a scanning electron microscope (SEM, S-4800) at an accelerated electron energy of 5.0 kV. The cross-linked film was first cut off, and then, the separated film surface was recontacted in the hot stage at 150 °C for about 0, 10, and 20 min, respectively. The selfhealing efficiency was calculated according to the stress−strain curves before and after healing by eq 3 C

DOI: 10.1021/acsapm.8b00040 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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Figure 1. (A) The ring-opening reaction between ENR and FA and their corresponding molecular structure. (B) 1H NMR spectra of ENR and ENR−FA with Yb. Peask a−h represent the corresponding peaks in the molecular structure of ENR, and peaks 1−5 represent the corresponding peaks in the molecular structure of FA and ENR−FA in Figure 1A.

Figure 2. Haake rheomixer curves of (A) ENR−0.125FA−0.125BMI and (B) ENR−0.125BMI.

The structure of ENR−FA was evidenced by using 1H NMR, and the results are shown in Figure 1B. Compared with the 1H NMR spectrum of unmodified ENR, new characteristic peaks at δ = 6.37, 6.31, and 7.54 ppm on the spectrum of ENR−FA were assigned to the protons 1, 2, and 3 of the furan ring, respectively (Figure 1B). The peak at δ = 3.67 was attributed to the proton 4 of FA in Figure 1A. This result agrees well with that reported in a previous study.49 Besides, the 1H NMR spectrum of FA in CDCl3 and DMSO are shown in Figure S1. Five different characteristic peaks of FA are observed, and the area ratio of five peaks is about 1:1:1:2:2, indicating that the new peak at δ = ∼1.55 ppm (proton 5) of ENR−FA in Figure 1 was assigned to the proton of an amine group connecting to the furan ring. After being grafted onto ENR, the NH2 of FA had transformed into NH due to the RO reaction, and the corresponding characteristic peak may be shifted to δ = ∼1.45 ppm as a result of the electron-donating effect of the methylene groups in polymer chains. Meanwhile, after the functionalization, the area of the peak located between 2.70 and 2.90 ppm (proton a) decreases after the RO reaction, which represents the proton connected to the carbon atom of epoxy groups on the main chain. These 1H NMR

results indicate that FA has been grafted onto the main chains of ENR successfully. The grafting ratio of FA was calculated based on the two peak (δ) areas at 3.67 (peak 4) and between 2.7 and 2.9 (peak a). The grafting ratios were calculated according to 1H NMR spectra of ENR−FA (Figure S2A) based on eq 1, and the results are shown in Figure S2B. The grafting ratio of the furfuryl groups increased with the increase of FA in feed, as shown in the figure. Diels−Alder Crosslink of ENR−FA and BMI. As the unmodified ENR can be directly cross-linked by BMI at about 160−170 °C (see Figure S4) and form irreversible crosslinking,63 here, we carried out the DA cross-linking reaction of ENR−FA−BMI (DA cross-linking of ENR−FA and BMI) at lower temperatures (about 115−127 °C). The ENR− 0.125FA−0.125BMI sample was studied with the Haake rheomixer as an example. The torque and temperature as a function of time are shown in Figure 2A. The temperature decreases first with the addition of Yb and FA (room temperature) and then increases due to the strong shearing force, which is detected by the temperature sensor in the Haake rheomixer. The torque slightly increases with the addition of Yb because it is in powder form, whereas it D

DOI: 10.1021/acsapm.8b00040 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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linking (rDA) reaction. After being cooled down to room temperature, some precipitates appeared at the bottom of the bottle, and then these precipitates were dissolved again when heating at 170 °C, demonstrating that the cross-linked ENR has a thermal reversible dynamic-cross-linked network. It should be noted, that before the dissolution experiment, the sample was cut into pieces and swollen in toluene for 48 h to make sure that the unreacted BMI has been removed completely. Therefore, the rDA (de-cross-linking) reaction becomes preponderant at 170 °C, and few permanent crosslinks can be formed. In order to further verify the temperature range for the DA and rDA reaction (cross-linking and de-cross-linking reaction), a DSC test was used to verify the thermo-reversibility of the TRENR sample. As shown in Figure 4B, during the heating process, a single endothermic peak at about 150 °C is observed, representing the occurrence of the rDA reaction and the rupture of the DA crosslink network, which is in agreement with the DMTA results as shown in Figure 4D. In addition, during the cooling process, a single exothermic peak occurs at about 130 °C, indicating the occurrence of a re-cross-linking (DA) reaction, which is in agreement with the results from the temperature curve of the Haake rheomixer in Figure 2B. The tensile storage moduli (E′) of ENR, ENR−FA, and ENR−FA−BMI as a function of temperature are shown inFigure 4C. All heating curves exhibit a decrease in E′ with the increasing of temperature, especially for non-cross-linked ENR and ENR−0.125FA due to the absence of crosslinks. The large decrease in E′ makes it is impossible to carry out any further measurements on ENR and ENR−0.125FA at a temperature above 80 °C. However, a stable modulus plateau with a slight decrease in temperature can be observed in the heating curve of ENR−0.125FA−0.125BMI, which verifies the cross-linking state of the sample and the de-cross-linking process at a high temperature. The rDA cross-linking reaction and re-cross-linking reaction are further studied via a heating and cooling cycle of the DMTA curve of ENR−FA−BMI, and the results are shown in Figure 4D. We observed that the original value of E′ for DA-cross-linked ENR can almost be recovered after cooling. The result indicates that there is no (or very little) degradation or side reactions occur during the heating or cooling cycle at 20−190 °C. Likewise, precise temperatures for the rDA cross-linking and re-cross-linking reaction in ENR−FA−BMI are also measured through the heating and cooling cycle. The E′ of the ENR−0.125FA− 0.125BMI remains unchanged below 100 °C, whereas it has a sharp decrease above 150 °C. This is attributed to the fact that the cross-linked chemical bonds rupture when the temperature is above 150 °C. It is at about 150 °C that the rDA crosslinking reaction happens. In the cooling cycle, especially at about 125 °C, the ruptured chemical bonds are rebuilt, leading to the increase in modulus. Mechanical Properties, Self-Healing Behavior, and Recyclability of TRENR. The stress−strain curves of all crosslinked samples are shown in Figure 5A. The samples of ENR− FA−BMI with different molar ratios of FA and ENR (ENR− 0.1FA−0.125BMI, ENR−0.125FA−0.125BMI, and ENR− 0.25FA−0.125BMI) werecross-linked via DA reaction (thermo-reversible) at 115−127 °C. To compare the mechanical properties, the sample of ENR−0.125BMI was also directly cross-linked (irreversible) by BMI at high temperature (170 °C). As shown in Figure 5A, with the increase of the molar ratio of FA and ENR, the tensile strength and elongation at the

decreases with the addition of FA because FA acts as a plasticizer. It is noted, that the torque has a sharp decrease once Yb or FA is added, whereas it has a sharp increase once the feeding process is finished. These phenomena are attributed to the opening and closing of the ram of the Haake rheomixer, respectively. After the addition of BMI, the temperature decreases first and then increases to about 115 °C due to the continuous shearing force. The torque of ENR− 0.125FA−0.125BMI decreases first and then starts to increase at about 115 °C, indicating the occurrence of a cross-linking reaction. Both the torque and temperature of ENR−0.125FA− 0.125BMI increase continuously with the increasing of the cross-linking time, and then they remained constant 3 min later, indicating that the DA cross-linking reaction between ENR−FA and BMI has been finished within 3 min. For comparison, BMI was also blended with ENR. Since FA was absent, no DA reaction occurred. The torque and temperature as a function of time of ENR−0.125BMI are shown in Figure 2B. The torque and temperature possess a similar change obviously after the addition of Yb. The significant difference is that the torque decreases when BMI is added into the Haake rheomixer, and it continuously decreases while the temperature increases to 129 °C, indicating that the cross-linking reaction of unmodified ENR does not occur in the presence of BMI at this range of temperature. In order to further verify the occurrence of the DA reaction, the FT-IR spectra were also carried out. As shown in Figure 3,

Figure 3. FT-IR spectra of ENR−FA and ENR−FA−BMI.

after being cross-linked, two characteristic peaks at 1714 and 1188 cm−1 appeared in the spectrum of ENR−FA−BMI, which represented the CO stretching vibration of the BMI and the DA ring formed by the DA reaction, respectively.58 This result also indicates the occurrence of the DA reaction by one-step melt blending. Thermo-Reversibility of TRENR. Since the reversibility of the DA reaction at different temperatures existed, the obtained TRENR was also thought to possess thermo-reversible property, so the dissolution experiment was used to verify the thermo-reversibility of TRENR under different temperatures. The dissolving behavior of the DA-cross-linked sample (taking the sample ENR−0.125FA−0.125BMI as an example) in DMF is shown in Figure 4A. As shown in the photograph, DA-cross-linked ENR can only swell at 30 °C in DMF. However, when the temperature increases up to 170 °C, the cross-linked ENR is gradually dissolved in DMF to form solution after 1 h, indicating the occurrence of a de-crossE

DOI: 10.1021/acsapm.8b00040 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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Figure 4. (A) The dissolving of DA-cross-linked ENR, de-cross-linked ENR, and re-cross-linked ENR in DMF. (B) DSC curve for re-DA and rDA reaction of cross-linked ENR. (C) Storage modulus of ENR, ENR−0.125FA, and ENR−0.125FA−0.125BMI. (D) Storage modulus and tan δ versus temperature (30−190 °C) of ENR−0.125FA−0.125BMI.

Figure 5. (A) Stress−strain curves of ENR−0.125BMI directly cross-linked by BMI at 170 °C, and the samples of ENR−0.1FA−0.125BMI, ENR− 0.125FA−0.125BMI, and ENR−0.25FA−0.125BMI cross-linked via DA reaction (thermo-reversible) at 127 °C. (B) Recycling study. The heat press process of polymer pieces into solid film, taking the sample ENR−0.125FA−0.125BMI as an example. (C) Stress−strain curves of ENR− 0.125FA−0.125BMI, and the same sample recycled for one time or two times.

break of TRENR increases. For example, with the molar ratio varying from 0.1 to 0.25, the elongation at break increases from about 120−320%, while the tensile strength increases from 5.7 to 8.7 MPa. This is mainly attributed to the corresponding changes of crosslink density (Table S3) caused by the different grafting ratios of FA. The modulus of TRENR decreases with the increasing dosage of FA, which is attributed to the

introduction of FA via RO reaction. FA can also be acted as plasticizer and can decrease the modulus of TRENR.64 Besides, the Tg of ENR−FA is lower than that of ENR from the DSC curves (Figure S7), indicating that FA behaves as a plasticizer. As a result, the modulus of ENR−FA−BMI is lower than that of ENR−BMI, with a similar BMI content as the presence of plasticizer FA. F

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Figure 6. SEM images of ENR−0.125FA−0.125BMI after being self-healed for 0 (A), 10 (B), and 20 min (C) at 150 °C.

In addition, due to the reversible properties of the DA reaction at different temperatures, the cross-linked ENR−FA− BMI could also exhibit recyclable properties. Taking ENR− 0.125FA−0.125BMI as an example, the sample was recycled through hot pressing, as shown in Figure 5B, and the corresponding stress−strain curve is shown in Figure 5C. It demonstrates that the remolded sample still exhibits good mechanical properties. The retention rate of tensile strength is about 90% after the first recycling, and it is still above 80% even after the second recycling, indicating that DA-cross-linked ENRs possess good thermo-reversibility. Despite the decrease in elongation at break after recycling, the elongation at the break of the sample still reaches 200% after two cycles. On the basis of the mechanism of thermal reversibility, the reason for the decrease in tensile strength is that the cross-linking density of the reprocessed ENR−0.125FA−0.125BMI is slightly decreased (Table S2). In our future work, a flexible dienophile, such as bismaleimidohexane, will be synthesized to improve the thermo-reversible efficiency andrecyclability. Meanwhile, the glass transition temperature of the samples does not change obviously during the recycling process as well as the decomposition temperature, which are shown in Table S2 and Figure S5, respectively. Therefore, the recycled TRENR exhibits neither obvious mechanical strength loss nor thermal degradation through two times of recycling. To confirm this assumption, FT-IR was also performed to monitor the structure changes of the recycled materials. As shown in the ATR−FT-IR spectra of the recycled samples (Figure S6), the absorption at 1188 cm−1 (DA ring stretch vibration) does not change after recycling, exhibiting that no degradation occurs through the rDA reaction, and the cross-linking density of the recycled samples do not change obviously compared to the original one. Overall, this DA-cross-linked ENR networks exhibit good recyclability. Due to the existence of the reversible dynamic covalent bonds in the ENR−FA−BMI, the cross-linked ENR potentially possess the characteristic of self-healing. In order to investigate the self-healing behavior, a small crack was cut on the surface of the sample by a blade. The sample with a scratch was then put onto the heating stage at 150 °C for different times, and then the healing of the crack was observed via SEM. As shown in the SEM images (Figure 6A−C), the crack on the film of the ENR−0.125FA−0.125BMI sample becomes gradually smaller and finally almost disappears when equilibrating at 150 °C; this result indicated that the cross-linked samples could be healed at about 150 °C. Moreover, as the cross-linked ENR networks do not melt during the healing process, the reversible DA reaction between ENR−FA and BMI leads to the self-healing property. The self-healing efficiency could be quantitatively characterized according to eq 3, and the efficiency is about 87% (Figure S8), indicating that the ENR−FA−BMI possessed considerable self-healing behavior.

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CONCLUSION

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ASSOCIATED CONTENT

TRENR with good thermo-reversibility and self-healable behavior has been successfully prepared by using a simple one-step melt blending approach based on a catalyzed RO and DA reaction for the first time. ENR was first grafted with FA, which acted as dienes, via a catalyzed RO reaction in a Haake rheomixer, and then the as-prepared ENR−FA was crosslinked by BMI through DA reaction. TRENR with comparable crosslink density to traditional cross-linked rubber was achieved by optimizing the ratio of FA and BMI. The recross-linking temperature and de-cross-linking temperature of TRENR were verified at about 130 and 150 °C, respectively, as demonstrated by DSC and DMTA tests. The TRENR is recyclable twice by compression molding, while retaining 90% of its mechanical properties after the first recycling. In addition, the as-prepared TRENR exhibits a self-healing ability. Compared with the DA reaction in solution reported in previous studies, our method was carried out in a Haake rheomixer batch mixer at a much shorter reaction time by onestep melt blending. In addition, this method requires no solvent and does not produce a stimulating smell, and thus, it is promising in industry for rubber recycling and large-scale preparation of TRE in an environmentally friendly method.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.8b00040. Description of the swelling method, the confirmation of optimal dosage of the catalyst, the FA-modified ENR, and the curing curves of ENR−0.125BMI; recipe for different samples of ENR−xYb; summary of the crosslink density, glass transition temperature, tensile strength, and elongation at the break; crosslink density of the DA-cross-linked ENR; 1H-NMR spectra; grafting ratio; Haake curves; curing curves; TGA curves; FT-IR spectra; DSC curves; and stress−strain curves (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ming Tian: 0000-0002-4820-7372 Liqun Zhang: 0000-0002-8106-4721 Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acsapm.8b00040 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS We would like to express our sincere thanks to National Natural Science Foundation of China (Grants 51525301 and 51521062) for financial supports.

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