Improvement of Mechanical and Self-Healing Properties for Polymethacrylate Derivatives Containing Maleimide Modiﬁed Graphene Oxide

polymers

Article Improvement of Mechanical and Self-Healing Properties for Polymethacrylate Derivatives Containing Maleimide Modiﬁed Graphene Oxide

Won-Ji Lee and Sang-Ho Cha *

Department of Chemical Engineering Kyonggi University, 154-42, Gwanggyosan-ro, Yeongtong-gu, Suwon, Gyeonggi 16227, Korea; [email protected] * Correspondence: [email protected]

Received: 31 January 2020; Accepted: 3 March 2020; Published: 6 March 2020

Abstract: In this paper, a self-healable nanocomposite based on the Diels-Alder reaction is developed. A graphene-based nanofiller is introduced to improve the self-healing efficiency, as well as the mechanical properties of the nanocomposite. Graphene oxide (GO) is modified with maleimide functional groups, and the maleimide-modified GO (mGO) enhanced the compatibility of the polymer matrix and nanofiller. The tensile strength of the nanocomposite containing 0.030 wt% mGO is improved by 172%, compared to that of a polymer film incorporating both furan-functionalized polymer and bismaleimide without any nanofiller. Moreover, maleimide groups of the surface on mGO participate in the Diels-Alder reaction, which improves the self-healing efficiency. The mechanical and self-healing properties are significantly improved by using a small amount of mGO.

Keywords: self-healing; modiﬁed GO; nanocomposite; Diels-Alder reaction

1. Introduction Polymers have been used in various applications, such as biomedical devices, transportation, artificial electron skin and coating [1–4]. However, polymers are vulnerable to damage and loss of durability to external stimuli; thus, polymer composites have been widely studied as a solution to these problems [5]. They have been used in a variety of fields, owing to their superior properties such as good abrasion resistance, corrosion resistance, improved mechanical properties and a light weight. Graphene-based materials have outstanding properties such as high conductivity, thermal stability, good mechanical properties and compatibility with polymers [6–8]. The durability of polymer composites depends on the integrity of the polymer matrix and reinforcing material such as nanofiller [9]. Poor compatibility between the polymer matrix and the filler causes micro cracks. Thus, the durability and interfacial adhesion are improved by the surface treatment of the reinforcing material [10]. The fillers used in polymer composites are modified by various methods [11] to reduce aggregation, which degrade the mechanical properties—however, these forms of damage are inevitable. Self-healing, in which the original properties of the material are recovered, is applied to increase the lifetime and durability of these materials. Self-healing materials can be classified into two categories: extrinsic and intrinsic self-healing. One category is extrinsic self-healing using microcapsules, including healing agents. Under an external stimulus, the healing agents in microcapsules are applied to crack locations, and help prevent crack propagation [12]. The other category is intrinsic self-healing, which inherently restores its original properties using stimuli such as heat, light and microwaves [13–19]. Among intrinsic self-healing, there is much research about the Diels-Alder reaction, which is applicable under mild conditions [20,21]. The Diels-Alder reaction can be applied to various polymers. Most of all, there are many studies that apply the Diels-Alder reaction to acrylate polymer, which is widely

Polymers 2020, 12, 603; doi:10.3390/polym12030603 www.mdpi.com/journal/polymers Polymers 2020, 12, 603 2 of 14 used in coating materials [22–29]. In particular, the application of graphene or modified graphene oxide to self-healing polymers not only improves their mechanical properties, but also their healing efficiency [30–37]. In this study, a polymer containing furan groups was designed to apply a self-healing system based on the Diels-Alder reaction. Furthermore, graphene oxide (GO) with excellent mechanical properties was applied as a filler. To improve the dispersity of the filler and the degree of the Diels-Alder reaction, graphene oxide modified with a maleimide moiety (mGO) was prepared, and nanocomposites with different mGO contents were fabricated. The mechanical properties and self-healing efficiency were investigated by tensile testing. Additionally, to confirm the effects of the mGO, a nanocomposite incorporating GO as a control sample was prepared, and compared with the nanocomposite containing mGO.

2. Materials and Methods

2.1. Materials Furan, maleic anhydride, 2-furoyl chloride (FC) and 2-aminoethanol were purchased by Tokyo Chemical Industry Co., Ltd, Tokyo, Japan. Dibutyltin dilaurate (DBTDL), hexamethylene diisocyanate (HDI), graphite, 2-hydroxyethyl methacrylate (HEMA), poly (ethylene glycol) methyl ether methacrylate Mn300 (PEGMMA) and 1,10-(methylenedi-4,1-phenylene)bismaleimide (BM) were purchased from Sigma-Aldrich Korea Ltd, Yong-In, Korea. For the solvent, tetrahydrofuran (THF), methylene chloride (MC), n-hexane, chloroform, N,N0-dimethylformamide (DMF), methanol and toluene were purchased from Daejung Chemical & Materials Co., Ltd, Si-Heung, Korea. All of these materials were used as received. Azobisisobutyronitrile (AIBN) from Junsei Chemical Co., Ltd, Chuo-ku, Tokyo was used after the recrystallization process.

2.2. Preparation of Maleimide Modified GO (mGO) mGO was prepared as follows. First, to prepare protected maleic anhydride (PM), maleic anhydride and furan were dissolved in toluene, stirred at 80 ◦C for 24 h and cooled to room temperature. As the solution cooled, a white solid was obtained. The crystallized white solid was filtered under reduced pressure, washed with a sufficient amount of methanol and dried overnight in a vacuum. Secondly, protected 2-hydroxyethyl maleimide (PHEM) was prepared. PM dissolved in methanol was placed in a two-necked flask, and 2-aminoethanol dissolved in methanol was dropped into this solution. In this process, a round bottom flask was placed in an ice water bath. After all the reactants were completely dissolved in the solvent, the reaction proceeded under reflux conditions (approximately 105 ◦C) for 24 h. After the reaction, the solution was cooled, and a yellow solid was obtained. Next, the PHEM was deprotected by refluxing in toluene overnight. The solution was then cooled in a freezer to recrystallize the deprotected 2-hydroxyethyl maleimide (HEM) in the form of a white solid. This solid was filtered by vacuum filtration to remove unwanted reactants. Finally, NCO-functionalized maleimide (NCO-M) was synthesized. The HEM obtained by the previous synthesis was dissolved in chloroform. This solution was dropped into a solution containing excessive HDI, and then a few drops of DBTDL were added very slowly. After the drops were added, the reaction proceeded overnight. The resulting solution was poured into excessive n-hexane and the precipitate was filtered and dried under reduced pressure. To obtain mGO, the NCO-M and GO prepared by the modified Hummer’s method [38] were diluted in DMF. To improve the dispersion of the GO, this mixture was sonicated for 30 min at room temperature. After sonication, a few drops of DBTDL were added to the dispersed solution. The reaction proceeded for 24 h at 55 ◦C, and the dispersed solids were filtered using vacuum filtration. The final products were washed with chloroform several times to remove unreacted NCO-functionalized maleimide, and then dried in a vacuum oven. Polymers 2020, 12, 603 3 of 14

2.3. Preparation of Furan Functionalized Polymethacrylate The furan-functionalized methacrylate (FEEMA) monomer was synthesized using our previously reported methods [39]. Subsequently, a series of copolymers with different furan side group contents were prepared by free radical polymerization using FEEMA and PEGMMA. A mixture of FEEMA, PEGMMA and AIBN was stirred under reflux conditions in THF. Next, the mixture was purified twice by precipitation using n-hexane. The copolymers are labeled FEEMA#, where # indicates the molar feed ratio of FEEMA and PEGMMA. For example, the copolymer FEEMA64 has a molar ratio of FEEMA:PEGMMA = 6:4.

2.4. Preparation of Self-Healable Furan-Functionalized Polymethacrylate Nanocomposite Films Nanocomposite films containing nanofillers, FEEMA64 and BM were fabricated as follows. A selected amount of the nanofillers (mGO or GO) was added to a DMF solution containing FEEMA64 and BM. After sonication at room temperature for 1 h, most of the DMF was removed using a rotary evaporator. The concentrated solution was poured onto silicone substrates, and the remaining DMF was evaporated for 36 h at 90 ◦C. Subsequently, the retro Diels-Alder reaction proceeded for 6 h at 120 ◦C, while the Diels-Alder reaction proceeded for 24 h at 50 ◦C, yielding nanocomposite films. A polymer film without nanofillers was also prepared using the above procedure. Nanocomposite films containing 0.015, 0.030 and 0.050 wt% mGO are labeled FEEMA64_mGO0.015 wt%, FEEMA64_mGO0.030 wt% and FEEMA64_mGO0.050 wt%, respectively. The control samples containing GO were labeled FEEMA64_GO0.015 wt%, FEEMA64_GO0.030 wt% and FEEMA64_GO0.050 wt%, respectively. The polymer film without a filler is referred to as the FEEMA64 polymer film.

2.5. Characterization Methods

1 Fourier-transform infrared (FT-IR) analysis was conducted at 400–4000 cm− using the Alpha-Platinum attenuated total reflectance model from Bruker, Seongnam-si, Korea. The thermal properties were measured using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). For TGA, the TGA 4000 from Perkin Elmer, Seoul, Korea was used under a N2 atmosphere. The sample was heated from 40 ◦C to 700 ◦C at a rate of 10 ◦C/min. For DSC, the Discovery DSC 25 from TA Instruments, Seoul, Korea) was used under a nitrogen atmosphere from -40 ◦C to 180 ◦C, at a heating rate of 10 ◦C/min. Optical microscopy (OM) DIMIS-M Siwon Optical Technology Anyang-si, Korea) with a 100 lens was used for surface analysis. The mechanical properties were characterized × using a Qmesys universal testing machine (UTM, QM100S, Uiwang-si, Korea) at a crosshead speed of 10mm/minute. The sample dimensions were 15 mm 50 mm 0.9 mm. The morphology was × × analyzed by field emission scanning electron microscopy (FE-SEM, JSM-760F PLUS, JEOL, Seoul, Korea). Transmission electron microscopy (TEM, Tecnai G2 F30 from FEI Company, Suwon-si, Korea) was performed at 300 kV. X-ray diffraction (XRD, Empyrean, Malvern PANalytical, Seongnam-si, Korea) patterns were obtained using Cu Kα radiation at room temperature. Elemental analysis was performed using Flash EA 1112 from Thermo Fisher Scientific, Waltham, MA US.

3. Results

3.1. Synthesis and Characterization of mGO GO obtained via the modified Hummer’s method was modified by maleimide functional groups to act as an analogous healing agent. In addition, mGO is expected to improve the mechanical properties of the polymer matrix. For mGO synthesis, NCO-M was prepared as follows. First, the protecting reaction of maleic anhydride was proceeded to prevent the side reaction during subsequent synthetic processes, such as the maleic anhydride ring-opening reaction. The protecting process was performed via the Diels-Alder reaction of maleic anhydride and furan without any catalyst. After this protecting process, the reaction between PM and 2-ethanolamine was conducted. Next, deprotection was performed via the retro Diels-Alder reaction under reflux conditions overnight, yielding HEM. Finally, Polymers 2020, 12, x FOR PEER REVIEW 4 of 14

excessive amount of HDI was used to prevent the production of bismaleimide. The overall process Polymersof NCO2020-M, preparation12, 603 is shown in Scheme 1a. 4 of 14 The mGO was obtained in the presence of DBTDL, which acted as a catalyst to enhance the reaction between the isocyanate group of NCO-M and the hydroxyl group of the GO obtained using the NCO-functionalized maleimide, NCO-M, was obtained via the urethane reaction between the the modified Hummer’s method. Before the reaction, both GO and NCO-M diluted with DMF were hydroxyl group of HEM and the isocyanate group of HDI, where an excessive amount of HDI was sonicated for 30 min to obtain a well-dispersed solution. After the DBTDL catalyst was added, the used to prevent the production of bismaleimide. The overall process of NCO-M preparation is shown reaction proceeded for 24 h at 55 °C. The procedure for mGO preparation is shown in Scheme 1b. in Scheme1a.

Scheme 1. Synthesis of (a) NCO-M and (b) mGO. Scheme 1. Synthesis of (a) NCO-M and (b) mGO. The mGO was obtained in the presence of DBTDL, which acted as a catalyst to enhance the reactionThe betweenmorphologies the isocyanate of graphite, group GO of and NCO-M mGO andwere the observed hydroxyl using group TEM, of the as GOshown obtained in Figure using 1. theThe modifiedgraphite had Hummer’s multiple method. layers, but Before both the the GO reaction, obtained both via GO the modified and NCO-M Hummer diluted’s method with DMF and werethe mGO sonicated had a for single 30 min layer, to obtainwhich awas well-dispersed in agreement solution. with many After previous the DBTDL observations. catalyst was Although added, some rough and curled edges appeared in the mGO but not in the GO, the modification of the GO the reaction proceeded for 24 h at 55 ◦C. The procedure for mGO preparation is shown in Scheme1b. causedThe almost morphologies no significant of graphite, morphological GO and changes mGO were. observed using TEM, as shown in Figure1. The graphite had multiple layers, but both the GO obtained via the modified Hummer’s method and the mGO had a single layer, which was in agreement with many previous observations. Although some rough and curled edges appeared in the mGO but not in the GO, the modification of the GO caused almost no significant morphological changes. The chemical modification of GO to mGO is confirmed via FT-IR and XRD analyses. Figure2a shows the FT-IR spectra of NCO-M, GO and mGO. The presence of characteristic peaks of GO, such as 1 1 1 1 the hydroxyl (3000–3700 cm− ), carbonyl (1730 cm− ), epoxy (1217 cm− ) and alkoxy (1056 cm− ) groups, confirmed that the oxidation reaction occurred [40,41]. In the mGO spectrum, all the signature 1 peaks of NCO-M are also observed, except the characteristic peak at 2260 cm− , which originates from the isocyanate group in NCO-M [42], indicating that GO was successfully modified to mGO via the urethane reaction between GO and NCO-M. The modification of GO by NCO-M was further confirmed by the XRD patterns of graphite, GO and mGO, as shown in Figure2b. The interlayer spacing increases in the order of graphite, GO and mGO. The interlayer spacing of graphite calculated by Bragg’s law at the sharp characteristic diffraction peak 26.530◦ was 0.34 nm; however, GO is found to have an interlayer spacing of 0.83 nm from the 10.675◦ peak [43]. The fact that GO has a larger Polymers 2020, 12, x FOR PEER REVIEW 4 of 14 excessive amount of HDI was used to prevent the production of bismaleimide. The overall process of NCO-M preparation is shown in Scheme 1a. The mGO was obtained in the presence of DBTDL, which acted as a catalyst to enhance the reaction between the isocyanate group of NCO-M and the hydroxyl group of the GO obtained using the modified Hummer’s method. Before the reaction, both GO and NCO-M diluted with DMF were sonicated for 30 min to obtain a well-dispersed solution. After the DBTDL catalyst was added, the reaction proceeded for 24 h at 55 °C. The procedure for mGO preparation is shown in Scheme 1b.

Polymers 2020, 12, 603 5 of 14 Polymers 2020, 12, x FOR PEER REVIEW 5 of 14 Scheme 1. Synthesis of (a) NCO-M and (b) mGO. interlayer spacing thanFigure graphite 1. TEM is attributed images of to (a the) graphite presence, (b) ofGO various and (c) oxygen-containingmGO. groups on The morphologies of graphite, GO and mGO were observed using TEM, as shown in Figure 1. the surface of GO [44]. The interlayer spacing of mGO increased further to 1.29 nm, indicating that the The graphite had multiple layers, but both the GO obtained via the modified Hummer’s method and relativelyThe chemical large molecular modification sized NCO-M of GO to was mGO successfully is confirmed introduced via FT- intoIR and the XRD surface analyses of GO.. TheFigure broad 2a the mGO had a single layer, which was in agreement with many previous observations. Although showspeak at the approximately FT-IR spectra 23.668 of NCOin-M, the GO XRD and pattern mGO. The of mGO presence might of becharacteristic due to the formationpeaks of GO of, moresuch some rough and curled edges ◦appeared in the mGO but not in the GO, the modification of the GO asclosely the hydroxyl stacked nanosheets,(3000–3700 cm resulting−1), carbonyl from (1730 a reduction cm−1), epoxy in the (1217 number cm− of1) and oxygen-containing alkoxy (1056 cm groups−1) groups, on caused almost no significant morphological changes. confirmedGO during that modiﬁcation the oxidation [45 ,reaction46]. occurred [40,41]. In the mGO spectrum, all the signature peaks of NCO-M are also observed, except the characteristic peak at 2260 cm−1, which originates from the isocyanate group in NCO-M [42], indicating that GO was successfully modified to mGO via the urethane reaction between GO and NCO-M. The modification of GO by NCO-M was further confirmed by the XRD patterns of graphite, GO and mGO, as shown in Figure 2b. The interlayer spacing increases in the order of graphite, GO and mGO. The interlayer spacing of graphite calculated by Bragg’s law at the sharp characteristic diffraction peak 26.530° was 0.34 nm; however, GO is found to have an interlayer spacing of 0.83 nm from the 10.675° peak [43]. The fact that GO has a larger interlayer spacing than graphite is attributed to the presence of various oxygen-containing groups on the surface of GO [44]. The interlayer spacing of mGO increased further to 1.29 nm, indicating that the relatively large molecular sized NCO-M was successfully introduced into the surface of GO. The broad peak at approximately 23.668° in the XRD pattern of mGO might be due to the formation of more closely stacked nanosheets, resulting from a reduction in the number of oxygen-containing groups on GO during modification [45,46]. Figure 1. TEM images of (a) graphite, (b) GO and (c) mGO.

Figure 2. (a) FT-IR spectra of GO, NCO-M and mGO, (b) XRD patterns of graphite, GO and mGO and Figure 2. (a) FT-IR spectra of GO, NCO-M and mGO, (b) XRD patterns of graphite, GO and mGO and (c) TGA curves of GO, NCO-M and mGO. (c) TGA curves of GO, NCO-M and mGO. TGA was performed to determine the grafting ratio of the maleimide functional group in mGO, TGA was performed to determine the grafting ratio of the maleimide functional group in mGO, and investigate the thermal stability (Figure2c). The initial thermal decomposition of GO occurs and investigate the thermal stability (Figure 2c). The initial thermal decomposition of GO occurs below 160 ◦C by the generation of vapors such as CO, CO2 and H2O[47,48]. The major thermal below 160 °C by the generation of vapors such as CO, CO2 and H2O [47,48]. The major thermal decomposition, from 160 ◦C to 270 ◦C, is attributed to the decomposition of labile oxygen-containing decomposition, from 160 °C to 270 °C , is attributed to the decomposition of labile oxygen-containing groups on the surface of GO, such as the hydroxyl and carbonyl groups [49]. In the three-step thermal groups on the surface of GO, such as the hydroxyl and carbonyl groups [49]. In the three-step thermal decomposition of mGO, the ﬁrst and second thermal decomposition steps are attributed to the vapor decomposition of mGO, the first and second thermal decomposition steps are attributed to the vapor generations and the decomposition of unstable functional groups containing oxygen, respectively, generations and the decomposition of unstable functional groups containing oxygen, respectively, which is similar to the behavior of GO. Considering the thermal decomposition behavior of NCO-M, which is similar to the behavior of GO. Considering the thermal decomposition behavior of NCO-M, the third thermal decomposition step of mGO, which occurred below 490 ◦C, should be attributed to the third thermal decomposition step of mGO, which occurred below 490 °C , should be attributed to the decomposition of NCO-M, which is chemically bonded to the surface of the mGO. The amount the decomposition of NCO-M, which is chemically bonded to the surface of the mGO. The amount of residual mGO at 700 ◦C is found to be 22%, and the amount of residual of GO is 41% at the same of residual mGO at 700 °C is found to be 22%, and the amount of residual of GO is 41% at the same temperature, indicating that 19% of the mGO consists of maleimide functional groups [50]. temperature, indicating that 19% of the mGO consists of maleimide functional groups [50]. 3.2. Preparation of FEEMA# Copolymers 3.2. Preparation of FEEMA# Copolymers The synthetic methods of the furan-functionalized methacrylate (FEEMA) and FEEMA# copolymersThe synthetic were described methods in ourof previous the furan research,-functionalized and are illustrated methacrylate in Scheme (FEEMA)2a,b, respectively and FEEMA# [ 39]. copolymersAdditionally, were the elementaldescribed in analysis our previous was performed research,to an conﬁrmd are illustrated the chemical in Scheme composition 2a,b, respectively of FEEMA [39]. Additionally, the elemental analysis was performed to confirm the chemical composition of FEEMA monomer. Based on the chemical structure of FEEMA monomer, the theoretical value of the

Polymers 2020, 12, x FOR PEER REVIEW 6 of 14

C, H and O elements of FEEMA monomer could be expected to reach 39.286 atomic %, 42.857 atomic % and 17.857 atomic %, respectively. It is confirmed that the experimental value of each element obtained from the elemental analysis is almost the same with one of a theoretical value (Table 1). Thus, it is proved that FEEMA monomer has the expected chemical structure.

Table 1. Elemental analysis of FEEMA monomer.

C (Atomic %) H (Atomic %) N (Atomic %) O (Atomic %) Polymers 2020Theoretical, 12, 603 value 39.286 42.857 - 17.857 6 of 14 Experimental value 35.484 48.387 - 16.129 monomer.The FEEMA Based on/PEGMMA the chemical ratio structure of the copolymer of FEEMA was monomer, controlled the by theoretical changing value the feed of the ratio C, of H each and Omonomer elements for of polymerization. FEEMA monomer However, could be a expectedpolymer film to reach using 39.286 FEEMA100 atomic was %, 42.857 not obtained atomic, %owing and 17.857to extreme atomic brittleness. %, respectively. Since FEEMA55 It is conﬁrmed had relatively that the few experimental furan-functional value ofgroups, each element it was considered obtained fromto be the unsuitable elemental for analysis showing is almost the effects the same of the with Diels one-Alder of a theoretical reaction valuein this (Table system.1). Thus, Therefore, it is provedFEEMA64 that was FEEMA chosen monomer from among has the the expected various chemicalFEEMA polymers structure. as a model for further analysis.

Scheme 2. (a) Synthesis of FEEMA monomer, (b) synthesis of FEEMA64 polymer and (c) overall Scheme 2. (a) Synthesis of FEEMA monomer, (b) synthesis of FEEMA64 polymer and (c) overall procedure for obtaining nanocomposites. procedure for obtaining nanocomposites. Table 1. Elemental analysis of FEEMA monomer. 3.3. Characterization of FEEMA64 Nanocomposites C (Atomic %) H (Atomic %) N (Atomic %) O (Atomic %) SchemeTheoretical 2c illustrates value the formation 39.286 of self-healable 42.857 nanocomposite -s using FEEMA64, 17.857 BM and a filler. AExperimentals described in value Scheme 2c35.484, the furan moiety 48.387 of FEEMA64 and - maleimide moiety 16.129 of mGO and bismaleimide form self-healable nanocomposites via the Diels-Alder reaction. When damage occurs to the formed nanocomposite and it is heated to a temperature where the retro Diels-Alder reaction The FEEMA/PEGMMA ratio of the copolymer was controlled by changing the feed ratio of each can proceed, the Diels-Alder bond (DA bond) dissociates. When the Diels-Alder reaction temperature monomer for polymerization. However, a polymer ﬁlm using FEEMA100 was not obtained, owing to is applied to the sample again, the healing process take place as the DA bond is recombined. Before extreme brittleness. Since FEEMA55 had relatively few furan-functional groups, it was considered to be unsuitable for showing the eﬀects of the Diels-Alder reaction in this system. Therefore, FEEMA64 was chosen from among the various FEEMA polymers as a model for further analysis.

3.3. Characterization of FEEMA64 Nanocomposites Scheme2c illustrates the formation of self-healable nanocomposites using FEEMA64, BM and a filler. As described in Scheme2c, the furan moiety of FEEMA64 and maleimide moiety of mGO and bismaleimide form self-healable nanocomposites via the Diels-Alder reaction. When damage occurs to the formed nanocomposite and it is heated to a temperature where the retro Diels-Alder reaction Polymers 2020, 12, 603 7 of 14 canPolymers proceed, 2020, 1 the2, x FOR Diels-Alder PEER REVIEW bond (DA bond) dissociates. When the Diels-Alder reaction temperature7 of 14 is applied to the sample again, the healing process take place as the DA bond is recombined. Before the self-healable properties of the nanocomposites are described, the Diels-Alder reaction between the self-healable properties of the nanocomposites are described, the Diels-Alder reaction between the furan-functional group of FEEMA64 and the maleimide group of BM is characterized using FT- the furan-functional group of FEEMA64 and the maleimide group of BM is characterized using FT-IR IR analyses of the FEEMA64 polymer film. analyses of the FEEMA64 polymer film. −1 As shown in Figure 3a, the characteristic peaks at 1581 and 1569 cm1 , which originated from the As shown in Figure3a, the characteristic peaks at 1581 and 1569 cm − , which originated from the furan double bond, decreased after mixing FEEMA64 polymer and BM, showing that the Diels-Alder furan double bond, decreased after mixing FEEMA64 polymer and BM, showing that the Diels-Alder reaction occurred in the FEEMA64 polymer film [51]. Furthermore, in the case of reaction occurred in the FEEMA64 polymer film [51]. Furthermore, in the case of FEEMA64_mGO0.030 FEEMA64_mGO0.030 wt%, the characteristic peaks attributed to the furan double bond are reduced wt%, the characteristic peaks attributed to the furan double bond are reduced compared to the polymer compared to the polymer film; this was additionally confirmed via the peak at 1014 cm−1, attributed film; this was additionally confirmed via the peak at 1014 cm 1, attributed to furan ring breathing [52]. − −1 to furan ring breathing [52]. As shown in the inset graph of 1Figure 3a, the peak at 1014 cm of As shown in the inset graph of Figure3a, the peak at 1014 cm − of FEEMA64_mGO0.030 wt% was FEEMA64_mGO0.030 wt% was lower compared to that of the polymer film. Therefore, it was lower compared to that of the polymer film. Therefore, it was confirmed that the maleimide-functional confirmed that the maleimide-functional groups on the surface of mGO could react with the furan groups on the surface of mGO could react with the furan moiety of FEEMA64 polymer via two moiety of FEEMA64 polymer via two characteristic peak intensity changes. characteristic peak intensity changes.

FigureFigure 3.3. ((aa)) FT-IRFT-IR spectraspectra ofof FEEMA64FEEMA64 polymer,polymer, FEEMA64 FEEMA64 polymer polymer film film and and FEEMA64_mGO0.030 FEEMA64_mGO0.030 wt%,wt%, (inset) (inset) furan furan ring ring breathing breathing absorption absorption andand ( (bb)) DSC DSC curves curves of of FEEMA64 FEEMA64 polymer polymer film film and and variousvarious nanocomposites,nanocomposites, (inset)(inset) comparisoncomparison ofof glassglass transitiontransition temperaturetemperature andand enthalpyenthalpy atat didifferentfferent mGOmGO contents.contents. On the basis of the FT-IR observations of the FEEMA64 polymer film, the Diels-Alder reaction On the basis of the FT-IR observations of the FEEMA64 polymer film, the Diels-Alder reaction in the nanocomposite is verified by DSC analyses, as shown in Figure3b. Regardless of the filler in the nanocomposite is verified by DSC analyses, as shown in Figure 3b. Regardless of the filler contents, an endothermic peak is observed at 110◦–130 ◦C, owing to the retro Diels-Alder reaction contents, an endothermic peak is observed at 110°–130 °C, owing to the retro Diels-Alder reaction in in the nanocomposite. This behavior indicates that the nanocomposite may also be applicable as athe thermally nanocomposite. self-healable Thismaterial behavior [53 indicates]. While that the glassthe nanocomposite transition temperature may also of be the applicable polymer filmas a representsthermally self0.650-healableC, that material of nanocomposites [53]. While containing the glass transition mGO increased temperature up to 1.720 of theC, polymer as the mGO film − ◦ ◦ contentrepresents of the-0.650 nanocomposite °C, that of nanocomposite increased tos 0.030 containing wt%. ThismGO is increased because anup mGOto 1.720 existing °C , as the between mGO polymercontent chainsof the nanoacts ascomposite a reinforcement increased agent, to 0.030 hindering wt%.the This segmental is because motion an mGO of the existing polymer between chains aspolymer it is already chains known acts as [a54 reinforcement]. Furthermore, agent as shown, hindering in the the inset segmental graph ofmotion Figure of3 b,the the polymer variation chains of theas it enthalpy is already depending known [54 on]. the Furthermore mGO content, as showsshown ain similar the inset tendency graph toof thatFigure of the3b, glassthe variation transition of temperature.the enthalpy depending It indicates on that the the mGO degree content of the shows Diels-Alder a similar reaction tendency increased to that of alongside the glass increasing transition thetemperature. maleimide It groups indicate ons thethat surface the degree of mGO. of the [ 55Diels]. Thus,-Alder it couldreaction be increased concluded alongside that mGO increasing has two functionsthe maleimide in the groups nanocomposites. on the surface First, of mGOmGO. acts [55]. as Thus, a reinforcement it could be agentconcluded to enhance that mGO mechanical has two properties,functions in and the second, nanocomposites the maleimide-functional. First, mGO acts groupsas a reinforcement on the surface agent of mGO to enhance participate mechanical in the Diels-Alderproperties, reactionand second, to improve the maleimide healing e-functionalfficiency. However, groups o inn the casesurface of FEEMA64_mGO0.050 of mGO participate in wt%, the Diels-Alder reaction to improve healing efficiency. However, in the case of FEEMA64_mGO0.050 wt%, the effect of mGO is reduced due to aggregation. This was further confirmed by the analyses of the mechanical properties of the nanocomposites, as described in the next section.

3.4. Mechanical Properties of FEEMA64 Nanocomposite

Polymers 2020, 12, 603 8 of 14 the effect of mGO is reduced due to aggregation. This was further confirmed by the analyses of the mechanical properties of the nanocomposites, as described in the next section. Polymers 2020, 12, x FOR PEER REVIEW 8 of 14 3.4. Mechanical Properties of FEEMA64 Nanocomposite AccordingAccording to to the the DSC DSC analyses, analyses, the mechanicalthe mechanical properties properties of the nanocomposites of the nanocomposites can be expected can be toexpected vary, depending to vary, ondepending the mGO on content. the mGO As shown content. in Figure As shown4a, the in tensile Figure strength 4a, the increased tensile with strength an increasingincreased with mGO an content increasing of up mGO to 0.030 content wt%. Specifically,of up to 0.030 the wt% tensile. Specifically strength of, the FEEMA64_mGO0.030 tensile strength of wt%FEEMA64_mGO0.030 improved significantly, wt% improved to reach significantly, 4.566 MPa, with to reach animprovement 4.566 MPa, with of an 172% improvement compared of to 172 that% ofcompared the FEEMA64 to that of polymer the FEEMA64 film, which polymer contains film, which no nanofiller. contains no However, nanofiller. the However, tensile strength the tensile of FEEMA64_mGO0.050strength of FEEMA64_mGO0.050 wt% decreased wt% todecreased 4.022 MPa, to 4.022 exhibiting MPa, exhibiting behavior similar behavior to similar that of theto that glass of transitionthe glass transition temperature temperature in the DSC in the experiments. DSC experiments. The effect The of effect the maleimide of the maleimide groups groups on the on mGO the surfacemGO surface was confirmed was confirmed by a comparison by a comparison of the tensile of the strength tensile of strength the nanocomposites of the nanocomposite containings GO—thatcontaining is, GO the— FEEMA64_GOthat is, the FEEMA64_GO series (Figure 4 seriesb). Although (Figure the4b). variation Although in tensile the variation strength in change tensile forstrength the FEEMA64_GO change for the series FEEMA64_GO is similar to series that is of similar the FEEMA64_mGO to that of the FEEMA64_mGO series, the absolute series, tensile the strengthsabsolute valuestensile of strengths the entire values FEEMA64_GO of the entire series FEEMA64_GO are lower than series those of are the lower FEEMA64_mGO than those ofseries. the ThisFEEMA64_mGO difference is series. especially This noticeable difference foris especially nanocomposites noticeable with for a nanocomposites 0.030 wt% filler with content. a 0.030 That wt% is, thefiller tensile content strength. That ofis, FEEMA64_mGO0.030 the tensile strength of wt% FEEMA64_mGO0.030 is 4.566 MPa, while wt% that ofis FEEMA64_GO0.0304.566 MPa, while that wt% of isFEEMA64_GO0.030 3.111 MPa, at an increase wt% is of 3.111 117% MPa compared, at an toincrease the FEEMA64 of 117% polymer compared film. to Therefore,the FEEMA64 an additional polymer covalentfilm. Therefore, bonding an based additional on the Diels-Alder covalent bonding reaction based between on the the maleimide Diels-Alder groups reaction on the between surface the of mGOmaleimide and the groups furan on group the surface of FEEMA64 of mGO polymer and the shouldfuran group induce of theFEEMA64 significantly polymer improved should tensileinduce strengththe significant of thely FEEMA64_mGO improved tensile series. strength of the FEEMA64_mGO series.

Figure 4. Mechanical properties of nanocomposites containing (a) mGO and (b) GO. Figure 4. Mechanical properties of nanocomposites containing (a) mGO and (b) GO. The dispersion of ﬁllers in the nanocomposites is expected to aﬀect their mechanical properties, as hasThe already dispersion been of discussed fillers in the [56 nanocomposites]. Therefore, the is cryogenicallyexpected to affect fractured their mechanical surface morphology properties, ofas FEEMA64_mGO0.030has already been discussed wt% and[56]. FEEMA64_mGO0.050 Therefore, the cryogenically wt% was fracture comparedd surface using morphology SEM images of (FigureFEEMA64_mGO0.0305a,b). While a wt% smooth and FEEMA64_mGO0.050 surface which originates wt% from was compared relatively using well-dispersed SEM images mGO (Figure is exhibited5a,b). Wh inile FEEMA64_mGO0.030 a smooth surface which wt%, originate non-homogeneouss from relatively and rough well-dispersed surface are mGO observed is exhibited due to the in restackingFEEMA64_mGO0.030 of mGO in thewt% case, non of-homogeneous FEEMA64_mGO0.050 and rough wt%. surface This are restacking observed and due agglomeration to the restacking of mGOof mGO is expected in the case to decrease of FEEMA64_mGO0.050 the chances of the wt% Diels-Alder. This restacking reaction between and agglomeration the maleimide of mGO groups is onexpected the surface to decrease of the the mGO chance ands theof the furan Diels groups-Alder in reaction the FEEMA64 between polymer, the maleimide meaning groups the tensileon the strengthsurface of of the FEEMA64_mGO0.050 mGO and the furan wt%groups should in the decrease FEEMA64 compared polymer, to meaning that of FEEMA64_mGO0.030 the tensile strength of wt%.FEEMA64_mGO0.050 A similar tendency wt% with should a relatively decrease higher compared dispersity to ofthat GO of in FEEMA64_mGO0.030 FEEMA64_GO0.030 wt% wt% than. A onesimilar in FEEMA64_GO0.050 tendency with a relatively wt% is alsohigher observed dispersity in the of nanocompositeGO in FEEMA64_GO0.030 containing GO,wt%as than shown one inin FigureFEEMA64_GO0.0505c,d. This implies wt% thatis also the observed agglomerated in the GO,nanocomposite which could containing induce even GO, theas shown formation in Figure of a 5c,d. This implies that the agglomerated GO, which could induce even the formation of a void, disrupts the orientation of the polymer chain in the nanocomposite [57], which in turn relatively degrades mechanical properties of FEEMA64_GO0.050 wt% [58].

Polymers 2020, 12, 603 9 of 14

Polymersvoid, disrupts 2020, 12, xthe FOR orientation PEER REVIEW of the polymer chain in the nanocomposite [57], which in turn relatively9 of 14 Polymersdegrades 2020 mechanical, 12, x FOR PEER properties REVIEW of FEEMA64_GO0.050 wt% [58]. 9 of 14

Figure 5. SEM images of (a) FEEMA64_mGO0.030 wt%, (b) FEEMA64_mGO0.050 wt%, (c) FEEMA64_GO0.030FigureFigure 5. 5. SEMSEM images wt% images and of of(da)) FEEMA64_GO0.050 ( FEEMA64_mGO0.030a) FEEMA64_mGO0.030 wt%. wt% wt%,, (b) (FEEMA64_mGO0.050b) FEEMA64_mGO0.050 wt%, wt%, (c) FEEMA64_GO0.030(c) FEEMA64_GO0.030 wt% wt% and and (d) FEEMA64_GO0.050 (d) FEEMA64_GO0.050 wt%. wt%. 3.5. Self-Healing Property 3.5. Self-Healing Property 3.5. SelfFirst,-Healing the self Property-healing properties of the nanocomposites were investigated by observing the First, the self-healing properties of the nanocomposites were investigated by observing the degree degreeFirst, of disappearance the self-healing of surface properties cracks of formedthe nanocomposites using a razor blade. were investigatedFigure 6 shows by that observing the optical the of disappearance of surface cracks formed using a razor blade. Figure6 shows that the optical microscopydegree of disappearance images of a FEEMA64_mGO0.030of surface cracks formed wt% using before a razor and blade. after theFigure self 6-healing shows thatprocedure the optical for microscopy images of a FEEMA64_mGO0.030 wt% before and after the self-healing procedure for the themicroscopy retro Diels image-Alders of reactiona FEEMA64_mGO0.030 at 150 °C for 2 wt% h and before the Dandiels -afteAlderr the reaction self-healing at 50 procedure°C for 24 for h, retro Diels-Alder reaction at 150 C for 2 h and the Diels-Alder reaction at 50 C for 24 h, respectively. respectively.the retro Diels Although-Alder reactioncracks still at◦ appear 150 °C in for some 2 h locations and the, Dmostiels -ofAlder the cracks reaction◦ fade atd 50or disappeared°C for 24 h, Although cracks still appear in some locations, most of the cracks faded or disappeared after the afterrespectively. the healing Although procedure. cracks This still implies appear that in some the polymer locations chains, most’ ofmobility the cracks increased faded orsufficiently disappeared so healing procedure. This implies that the polymer chains’ mobility increased suﬃciently so that the thatafter thethe healing polymer procedure. could penetrate This implies the cracksthat the, andpolymer then chains fill them’ mobility by theincreased formation sufficiently of chain so polymer could penetrate the cracks, and then ﬁll them by the formation of chain entanglements during entanglementsthat the poly merduring could the retropenetrate Diels -Alder the cracks reaction., and then fill them by the formation of chain the retro Diels-Alder reaction. entanglements during the retro Diels-Alder reaction.

Figure 6. OM images (a) before and (b) after healing of FEEMA64_mGO0.030 wt%. Figure 6. OM images (a) before and (b) after healing of FEEMA64_mGO0.030 wt%. To measureFigure 6. the OM self-healing images (a) before efficiency and (b) quantitatively,after healing of FEEMA64_ the mechanicalmGO0.030 propertieswt%. of the nanocompositesTo measure are the analyzed self-healing using efficiency a UTM, quantitatively, as shown in Figure the 7mechanical. For the healing properties procedure, of the nanocompositean as-preparedTo measures neatare the analyzed nanocomposite self-healing using a film efficiency UTM is, cut as usingshown quantitatively, scissors, in Figure and 7.the theFor mechanical damaged the healing film propertiesprocedure, is then placed ofan asthe on- preparednanocompositethe same neat silicone nanocomposites are mold analyzed that was filmusing used is acut toUTM fabricateusing, as scissorsshown the neat ,in and Figure sample. the damaged7. TheFor the damaged filhealingm is nanocompositethen procedure, placed on an film,the as- sameprepared silicone neat mold nanocomposite that was used film tois fabricatecut using thescissors neat , sample.and the Thedamaged damaged film nanocompositeis then placed on film the, undersame siliconepressure mold from that the siliconewas used mold to fabricate, is placed the on neat a hot sample. plate at The 150 damaged°C for 2 h. nanocomposite In this process, film DA, adductsunder pressure in the polymer from the matrix silicone were mold dissociated., is placed A onfter a hotthat plate, the sampleat 150 °C was for heated 2 h. In at this 50 process,°C for 24 DA h, adducts in the polymer matrix were dissociated. After that, the sample was heated at 50 °C for 24 h,

Polymers 2020, 12, 603 10 of 14

Polymers 2020, 12, x FOR PEER REVIEW 10 of 14 under pressure from the silicone mold, is placed on a hot plate at 150 ◦C for 2 h. In this process, DA adductswhich promotes in the polymer the recombination matrix were dissociated.of DA adduct Afters. The that, sample the sample obtained was by heated this process at 50 ◦ isC called for 24 the h, whichhealed promotes sample. the recombination of DA adducts. The sample obtained by this process is called the healed sample.

FigureFigure 7. 7. Stress-strainStress-strain curves curves of of neat neat andand healed healed samples samples for for FEEMA64 FEEMA64 polymer polymer ﬁlm film and and FEEMA64_mGO0.030FEEMA64_mGO0.030 wt%. wt%.

TheThe self-healing self-healing eefficiencyfficiency is obtained obtained as as the the ratio ratio of ofthe the elongation elongation at break at break values values of the of healed the healedsamples samples to those to of those the of neat the ones neat from ones the from UTM the experiments UTM experiments [53]. The [53 self].- Thehealing self-healing efficiency effiincreasedciency increased gradually gradually with the withincreasing the increasing mGO content mGO contentof the nanocomposites. of the nanocomposites. In particular In particular,, the self- thehealing self-healing efficiency effi incciencyreased increased dramatically dramatically from 23.560 from% for 23.560% FEEMA64_mGO0.015 for FEEMA64_mGO0.015 wt% to 81.228% wt% tofor 81.228%FEEMA64_mGO0.030 for FEEMA64_mGO0.030 wt%. The self wt%.-healing The efficiency self-healing of FEEMA64_mGO0.050 efficiency of FEEMA64_mGO0.050 wt% is similar to wt% that isof similar FEEMA64_mGO0.030 to that of FEEMA64_mGO0.030 wt%. The self-healing wt%. property The self-healing of the FEEMA64_mGO property of the series FEEMA64_mGO is summarized seriesin Table is summarized 2. Considering in Table the2 . tensile Considering strength the variation tensile strength of the variationFEEMA64_mGO of the FEEMA64_mGO series, both the series,dispersity both theof mGO dispersity in the of nanocompo mGO in thesite nanocomposite and the degree and of the the degree Diels- ofAlder the Diels-Alder reaction between reaction the betweenmaleimide the groups maleimide of the groups mGO ofsurface the mGO and surfacethe furan and groups the furan in the groups FEEMA64 in the polymer FEEMA64 should polymer be key shouldfactors be affecting key factors the self aff-ectinghealing the efficiency self-healing of the e ffiFEEMA64_mGOciency of the FEEMA64_mGO series; this was further series; confirmed this was furtherby the confirmed self-healing by the efficiency self-healing of ethefficiency FEEMA64_GO of the FEEMA64_GO series. The series. self- Thehealing self-healing efficiency effi ciency of the ofFEEMA64_mGO the FEEMA64_mGO series seriesis much is muchhigher higher than that than of that the ofFEEM theA64 FEEMA64_GO_GO series. series.For example, For example, the self- thehealing self-healing did not did occur not occur in inFEEMA64_GO0.050 FEEMA64_GO0.050 wt% wt%.. However However,, the the selfself-healing-healing e efficiencyfficiency of of FEEMA64_mGO0.050FEEMA64_mGO0.050 wt% wt% is is 82.033%. 82.033%. The The reason reason for for this this is is thought thought to to be be that that the the maleimide maleimide groups groups onon the the mGO mGO surface, surface, which which participate participate in in the the Diels-Alder Diels-Alder reaction, reaction improve, improve the the self-healing self-healing ability ability of of thethe FEEMA64_mGO FEEMA64_mGO series series [59 [59].]. A heart-shaped specimen was prepared to confirm the effect of the reversibility of the Diels-AlderTable 2. reaction Self-healing on self-healingefficiencies and property. mechanical The properties healing of procedure FEEMA64_ describedmGO series above before was and appliedafter to thethe heart-shaped healing test. specimen, which was cut into two pieces using scissors. The two pieces of FEEMA64_GO0.030 wt%B didefore not Healing bond withTest each other, indicatingAfter Healing that FEEMA64_GO0.030Test wt% is not capable of reversibleTensile healing. By contrast, as shownTensile in Figure 8, FEEMA64_mGO0.030Self-Healing wt% Sample Elongation at Elongation at was restored to its originalStrength shape, even after a secondStrength self-healing procedure. ThisEfficiency result proves 1 (%) Break (%) Break (%) that reversible self-healing,(MPa) which is a characteristic of the(MPa) Diels-Alder reaction, is possible for the FEEMA64_mGOFEEMA64 specimens. 2.648 ± 0.257 214.667 ± 14.687 1.663 ± 0.261 12.600 ± 0.898 5.724 polymer film FEEMA64_mG 4.021 ± 0.229 198.235 ± 10.232 3.611 ± 0.155 36.465 ± 12.070 23.560 O0.015 wt% FEEMA64_mG 4.566 ± 0.564 69.035 ± 9.383 2.916 ± 0.171 61.465 ± 1.322 81.228 O0.030 wt%

Polymers 2020, 12, x FOR PEER REVIEW 11 of 14 Polymers 2020, 12, 603 11 of 14 FEEMA64_mG 4.022 ± 0.542 155.510 ± 6.980 2.927 ± 0.151 141.333 ± 7.588 82.033 O0.050 wt% Table 2. Self-healing eﬃciencies and mechanical properties of FEEMA64_mGO series before and after 1 the healing test. Healing efficiency calculated using elongation at break.

A heart-shaped specimen wasBefore prepared Healing Test to confirm the effect After Healing of the Test reversibility of the Diels- Self-Healing Sample Alder reaction on self-healingTensile property. Strength TheElongation healing at procedureTensile Strength describedElongation above at wasE appliedfficiency 1 (%)to the (MPa) Break (%) (MPa) Break (%) heart-shaped specimen, which was cut into two pieces using scissors. The two pieces of 214.667 FEEMA64 polymer film 2.648 0.257 ± 1.663 0.261 12.600 0.898 5.724 FEEMA64_GO0.030 wt% did not± bond with each14.687 other, indicating± that FEEMA64_GO0.030± wt% is not capable of reversible healing. By contrast,198.235 as shown in Figure 8, FEEMA64_mGO0.030 wt% was FEEMA64_mGO0.015 wt% 4.021 0.229 ± 3.611 0.155 36.465 12.070 23.560 restored to its original shape, even± after a second10.232 self-healing± procedure.± This result proves that FEEMA64_mGO0.030 wt% 4.566 0.564 69.035 9.383 2.916 0.171 61.465 1.322 81.228 reversible self-healing, which is± a characteristic± of the Diels± -Alder reaction± , is possible for the FEEMA64_mGO0.050 wt% 4.022 0.542 155.510 6.980 2.927 0.151 141.333 7.588 82.033 FEEMA64_mGO specimens. ± ± ± ± 1 Healing efficiency calculated using elongation at break.

Figure 8. Confirmation of self-healing using FEEMA64_mGO0.030 wt%. Figure 8. Confirmation of self-healing using FEEMA64_mGO0.030 wt%. 4. Conclusions 4. Conclusions In this report, a furan-functionalized polymer (FEEMA64) was prepared for a polymer matrix. FEEMA64In this polymer report, a film furan was-functionalized then fabricated polymer using the(FEEMA64) Diels-Alder was reaction prepared between for a polymer FEEMA64 matrix. and bismaleimide.FEEMA64 polymer To enhance film was the then mechanical fabricated properties using the of the Diels FEEMA64-Alder reaction polymer between film, a graphene-based FEEMA64 and fillerbismaleimide. was introduced To enhance into polymer the mechanical film. The properties graphene-based of the FEEMA64 filler was modified polymer film, using a NCO-Mgraphene to- preventbased filler aggregation. was introduced Additionally, into polymer the maleimide film. The moiety graphene on the-based surface filler of was mGO modified could participate using NCO in- theM to Diels-Alder prevent reaction.aggregation. Thus, Additionally, the specimens th containinge maleimide mGO moiety exhibited on thebetter surface mechanical of mGO properties could andparticipate self-healing in the effi Dielsciency-Alder in this reaction. self-healing Thus, system. the specimens The tensile containing strength of mGO FEEMA64_mGO0.030 exhibited better wt%mechanical improved properties to 4.566 and MPa, self which-healing is anefficiency improvement in this ofself 172%-healing compared system. to The that tensile of polymer strength film. of TheFEEMA64_mGO0.030 self-healing efficiency wt% of improved FEEMA64_mGO0.030 to 4.566 MPa wt%, which is 81.228%, is an improvement which is significantly of 172% compared better than to thatthat ofof polymerpolymer film. film. Therefore, The self-healing the mechanical efficiency and of self-healingFEEMA64_mGO0.030 properties of wt% brittle is 8 polymethacrylate1.228%, which is derivativessignificantly could better bethan increased that of polymer by the addition film. Therefore, of a very the smallmechanical amount and of self mGO-healing in this properties system. Thisof brittle nanocomposite, polymethacrylate based onderivatives the Diels-Alder could be reaction, increased is expected by the addition to extend of thea very lifetime small of amount materials of bymGO applying in this smart system. technology This nanocomposite to coating materials., based on the Diels-Alder reaction, is expected to extend the lifetime of materials by applying smart technology to coating materials. Author Contributions: Conceptualization, S.-H.C.; Methodology, S.-H.C.; Formal analysis, S.-H.C.; Data analysis,Author Contributions: W.-J.L.; Writing, Conceptualization, W.-J.L.; Editing, S.-H.C. S.-H.C.; All authors Methodology, have read S.- andH.C.; agreed Formal to the analysis, published S.-H.C.; version Data of the manuscript. analysis, W.-J.L.; Writing, W.-J.L.; Editing, S.-H.C. All authors have read and agreed to the published version of Funding:the manuscript.This research received no external funding.

Acknowledgments:Funding: This researchThis received work was no supportedexternal fundin by Kyonggig. University’s Graduate Research Assistantship 2020. Conﬂicts of Interest: The authors declare no conﬂict of interest. Acknowledgments: This work was supported by Kyonggi University’s Graduate Research Assistantship 2020.

ReferencesConflicts of Interest: The authors declare no conflict of interest.

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