Article Exploring Impacts of Hyper-Branched Polyester Surface Modification of Graphene Oxide on the Mechanical Performances of Acrylonitrile-Butadiene-Styrene

Xuebing Chen 1, Shulai Lu 2,3,*, Chunfu Sun 2,3, Zhenbiao Song 2,3, Jian Kang 1,* and Ya Cao 1

1 State Key Laboratory of Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China; [email protected] (X.C.); [email protected] (Y.C.) 2 Synthetic Factory of Jilin Petrochemical Company, PetroChina, Jilin 132021, China; [email protected] (C.S.); [email protected] (Z.S.) 3 PetroChina ABS Resin Technology Center, Jilin 132021, China * Correspondence: [email protected] (S.L.); [email protected] (J.K.)

Abstract: In this manuscript, the graphene oxide (GO) was modified by hyper-branched polyester (HBP). The effects of GO or modified GO (HBP-m-GO) on the mechanical performance and wearing properties were investigated. The results of X-ray photoelectron spectra (XPS), Fourier transform infrared spectroscopy (FT-IR), and transmission electron microscopy (TEM) revealed the successful grafting of HBP onto GO. The thermogravimetric analysis (TGA) indicated that the graft amount  of HBP is calculated to be 9.6 wt%. The GO or HBP-m-GO was added into acrylonitrile-butadiene-  styrene (ABS) to prepare the ABS/GO composites. The mechanical properties and wear Citation: Chen, X.; Lu, S.; Sun, C.; performance of the composites were studied to comparatively study the impact of GO modification Song, Z.; Kang, J.; Cao, Y. Exploring on the properties of the composites. The results revealed that the addition of GO has a significant Impacts of Hyper-Branched Polyester effect on the mechanical properties of ABS, and when HBP-m-GO was added, the elastic modulus Surface Modification of Graphene and tensile strength of ABS/HBP-m-GO increased evidently compared with ABS/GO. The tensile Oxide on the Mechanical strength increased from 42.1 ± 0.6 MPa of pure ABS to 55.9 ± 0.9 MPa, up to 30%. Meanwhile, the Performances of Acrylonitrile- elongation at break was significantly higher than ABS/GO to 20.1 ± 1.3%, slightly lower than that of Butadiene-Styrene. Polymers 2021, 13, pure ABS. For wear performance, the addition of raw GO decreased the friction coefficient, and when 2614. https://doi.org/10.3390/ the HBP-m-GO was added, the friction coefficient of the ABS/HBP-m-GO dropped more evidently. polym13162614 Meanwhile, the weight loss during the wear test decreased evidently. The related mechanism

Academic Editors: Fernão was discussed. D. Magalhães and Artur Pinto Keywords: graphene oxide; hyper-branched polyester; acrylonitrile-butadiene-styrene; mechanical Received: 8 July 2021 performance Accepted: 3 August 2021 Published: 6 August 2021

Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in The acrylonitrile-butadiene-styrene (ABS) copolymer is a kind of plastic with excellent published maps and institutional affil- comprehensive properties [1,2]. The presence of high polar polyacrylonitrile component iations. and rubber phase, as well as its unique multiphase composite structure, micro morphology and molecular structure endow it with good surface gloss, excellent mechanical properties, easy surface spraying, and electroplating characteristics [3]. Therefore, ABS resin is widely used in electronic and electrical appliances, automobile and other daily necessities, and Copyright: © 2021 by the authors. industrial products manufacturing. However, the friction and wear resistance of ABS resin Licensee MDPI, Basel, Switzerland. is limited and the friction coefficient is large [4]. On the other hand, it is urgent to have This article is an open access article better mechanical properties [5]. Therefore, adding nano fillers into the ABS resin so as to distributed under the terms and further improve its mechanical and wear properties is considered to be an effective method, conditions of the Creative Commons and also a research hotspot in related fields [6–9]. Attribution (CC BY) license (https:// Graphene is a two-dimensional sheet material, with extremely thin thickness and creativecommons.org/licenses/by/ high specific surface area [10–12], excellent electrical properties, thermal conductivity, 4.0/).

Polymers 2021, 13, 2614. https://doi.org/10.3390/polym13162614 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 2614 2 of 12

and mechanical properties [13–15]. As a chemical precursor of graphene, graphene oxide (GO) can have some extent of attractive properties of graphene after chemical or thermal reductions [14,16,17]. GO also has a large number of reactive groups such as hydroxyl group, carboxyl group, and group distributed on its surface, which is easy for further organic modification, and is an ideal reinforcing material for ABS [18–20]. In order to improve the compatibility between GO and ABS resins and improve the dispersion uniformity of GO in ABS resins, it is a common method to use reactive groups on the GO surface to graft modification of macromolecules [21–24]. In recent years, the addition of derivatives of graphene into ABS have been extensively investigated. Wilkie C et al. [1] prepared composites by melt blending various graphites (virgin graphite, expandable graphites, and expanded graphite) with polystyrene (PS) and its (ABS and high-impact polystyrene (HIPS)), and comparatively studied the roles of graphites in enhancing the properties of the composites. Kharrat M et al. [2] systematically studied the impact of 0–7.5 wt% graphite particles in the mechanical and tribological properties in ABS composites, and observed a significant decrease of elastic modulus and failure strain, improvement of friction behavior and anti-wear abilities after the addition of graphite. They suggested that graphite strengthens the wear resistance of ABS composites and effectively reduces its adhesive and ploughing wear, and enhances the formation of a third body with better quality on the sliding stripe. Raza M et al. [8] prepared thermally reduced graphene oxide (TRGO)/ABS composites, and comparatively studied the effects of pristine graphite and TRGO on the mechanical and thermal properties of composites. They reported that the combined solution and melt mixing technique can significantly improve dispersion of TRGO in the ABS matrix. Kar K et al. [3] studied the effects of 0–40 vol% graphite flakes (GFs) in tensile, flexural, impact, hardness, and thermal conductivity performances of ABS/GFs composites, and reported that the GF-reinforced ABS composite with improved thermal conductivity, heat stability, viscoelastic behavior, and flexural modulus can be a promising as well as a suitable for making various electronic and electrical accessories including bipolar plates for fuel cell ap- plications. Hassan A et al. [5] prepared graphene nanoplates reinforced polycarbonate/ABS composites, and studied structural, morphological, mechanical, and thermal properties of the nanocomposites, claiming that the addition of GNP improved the flexural and tensile properties without the loss of extensibility and good thermal properties. Pal K et al. [7] enlightened the effect of three different derivatives of graphene on mechanical, thermal, and rheological properties of ABS composites prepared via the melt-mixing technique, i.e., GO, chemically reduced graphene oxide (rGO-C), and thermally reduced graphene oxide. They noticed that rGO-C acts as a better reinforcement in ABS than other derivatives. Khan N et al. [6] comparatively studied the reinforcement effects of two-dimensional few layer graphene (FLG) and one-dimensional multi-walled carbon nanotubes (MWCNT) in ABS using the tensile testing machine. To obtain the desired improvement of properties of the composites, one key factor is to improve the dispersion of the filler in the ABS matrix. However, current works mainly focus on the sample preparation method as well as the type and concentration of the filler. The impact of functional modification of the filler on the properties of the ABS composites is still urgent to be studied. The hyper-branched polyester has a high degree of branching and a large number of hydroxyl terminated [25,26], low viscosity, high solubility, and high functionality, as well as has a very good compatibility with the ABS resin [27–29]. At the same time, it has a high reactivity with GO. It can be used as the surface graft of GO to further improve the compatibility between GO and ABS resins, which has not been reported before. In this study, GO was modified by hyper-branched polyester (HBP) and then added into ABS resins to prepare the ABS/GO composites. The effects of hyper-branched polyester modification on the mechanical performance and wear properties of ABS/GO composites were studied. Polymers 2021, 13, x FOR PEER REVIEW 3 of 13

Polymers 2021, 13, 2614 3 of 12

In this study, GO was modified by hyper-branched polyester (HBP) and then added 95 into ABS resins to prepare the ABS/GO composites. The effects of hyper-branched poly- 96 ester2. Experimental modification on Section the mechanical performance and wear properties of ABS/GO com- 97 posites2.1. Materials were studied. and Sample Preparation 98 2.1.1. Materials 2. Experimental Section 99 2.1. MaterialsABS granules and Sample (tradename Preparation 0215H, MFI2.16 = 3.1 g/10 min) were kindly provided 100 by Jilin Petrochemical Company, PetroChina Co. Ltd., Jilin, China. The hyper-branched polymer 2.1.1. Materials 101 with the tradename of BOLTORN H202 (molecular weight = 600 g/mol, hydroxyl value ABS granules (tradename 0215H, MFI2.16 = 3.1 g/10 min) were kindly provided by Jilin 102 Petrochemical= 550 mg KOH/g Company, polymer) PetroChina was Co. purchased Ltd., Jilin, China. from ShanghaiThe hyper-branched Bio Biotechnology polymer 103 Co., Ltd., withShanghai, the tradename China. ofN BOLTORN,N-dimethylformamide H202 (molecular (DMF), weight = analytical 600 g/mol, grade,hydroxyl were value purchased = 104 from 550Sinopharm mg KOH/g Chemical polymer) Reagent was purchased Beijing from Co., Shanghai Ltd., Beijing, Bio Biotechnology China. The grapheneCo., Ltd., oxide105 (GO), Shanghai,tradename China. N002-PDE, N, N-dimethylformamide was purchased from(DMF), Guangzhou analytical grade, Angstron were purchased graphene Technology106 fromCo. Ltd.,Sinopharm Guangzhou, Chemical China. Reagent The Beijing average Co., Ltd. thickness, Beijing, ofChina. GO The is lower graphene than oxide 1 nm, 107 the lateral (GO),sizeis trade 0.5name and 1.5N002 micrometers,-PDE, was purchased and the from content Guangzhou of O atomAngstron is higher graphene than Tech- 20%. 108 All of the nology Co. Ltd., Guangzhou, China. The average thickness of GO is lower than 1 nm, the 109 materials were used as received. lateral size is 0.5 and 1.5 micrometers, and the content of O atom is higher than 20%. All 110 of the materials were used as received. 111 2.1.2. Preparation of Surface Modified GO 2.1.2. PreparationThe illustration of Surface of theModified reaction GO between GO hand HBP is shown in Scheme 112 1. The hyper-branchedThe illustration polyester of the reaction HBP between and GO GO were hand dissolved HBP is shown in DMF in Scheme at room 1. temperature,The 113 and hyperthen- irradiatedbranched polyester using an HBP ultrasound and GO were for dissolved 1 h. After in DMF that, at they room were temperature, heated upand to 120114 ◦C and thenreacted irradiated under using magnetic an ultrasound stirring for for 1 24h. After h. After that, that,they were they heated were cooledup to 120 to °C room and temperature 115 reacted under magnetic stirring for 24 h. After that, they were cooled to room temperature 116 slowly and filtered using the PVDF filtration membrane with the average pore diameter of slowly and filtered using the PVDF filtration membrane with the average pore diameter 117 µ of0.22 0.22 m.μm. The The product was was firstly firstly washed washed by acetone by acetone and filtered and filtered several severaltimes to elim- times to118 eliminate inatethe unreacted the unreacted HBP HBP and and extra extra solvent. solvent. Each Each time, time, the the washed washed acetone acetone was taken was takenfor 119 for the FT- theIR analysis,FT-IR analysis, and theand washingthe washing is finishedis finished until until the the washedwashed acetone acetone has has the the same same FT- FT-IR 120 spectra IRcompared spectra compared with pure with acetone. pure acetone. The collected The collected material material was was then then heated heated under under aa vacuum 121 drier vacuumat 80 ◦C drier for 24at 80 h °C to for obtain 24 h theto obtain final the product, final product, which which was was denoted denoted as as HBP-m-GO. HBP-m- 122 GO. 123

124 Scheme 1. Illustration of the reaction between GO and hyper-branched polyester. Scheme 1. Illustration of the reaction between GO and hyper-branched polyester. 125

2.1.3. Preparation Preparation of ABS/GO of ABS/GO Composites Composites 126 TheThe composites composites were were prepared prepared by melt byblending melt ABS blending with GO ABS or HBP with-m- GOGO at or 200 HBP-m-GO 127 at °C200 for◦ C10for min 10 using min a usingBrabender a Brabender Plasticorder Plasticorder,, Brabender Technologie, Brabender GMBH Technologie, & CO. KG, GMBH 128 & CO. KG, Duisburg, Germany. The weight percentage of GO or HBP-m-GO was selected as 1.5% of the composites.

2.2. Characterization 2.2.1. Fourier Transform Infrared Spectroscopy (FT-IR) The sample was scanned by Nicolet IS50 FT-IR (Thermo Fisher Scientific Corp., Waltham, MA, USA) with a spectral resolution of 4 cm−1 and test range of 400–4000 cm−1. The sample was prepared by the KBr tablet pressing method [30,31]. Polymers 2021, 13, 2614 4 of 12

2.2.2. Transmission Electron Microscopy (TEM) The transmission electron microscopy (TEM, Tecnai G2 F20, FEI Corp., Hillsboro, OR, USA) was used.

2.2.3. Thermogravimetric Analysis (TGA) ◦ ◦ The samples were heated to 800 C at 20 C/min under the protection of N2 and after equilibrium at 30 ◦C. The values were observed by the TG209F1 thermal analyzer (Netzsch Corp., Selb, Germany).

2.2.4. X-ray Photoelectron Spectra (XPS) The X-ray photoelectron spectra was carried out by the ESCALAB 250 photoelectron spectrometer (Thermo Fisher Scientific Corp., Waltham, MA, USA) with Al Kα (1486.6 eV) as the X-ray source set at 150 W and a pass energy of 30 eV for high resolution scan [32–34].

2.2.5. Mechanical Properties Tensile tests were performed on an INSTRON universal tensile machine equipped with 5 KN load cell and 25 mm displacement extensometer (Instron Corporation, Norwood, MA, USA) [35–37]. The crosshead speed for the tensile test was set at 50 mm/min. All of the tests were carried out at room temperature (25 ± 2 ◦C). Five to eight samples from each sample were tested and the averaged values were obtained [38–40]. The geometry of the samples was according to an international tensile testing norm AFNOR NF T 51-034.

2.2.6. Wear Properties Friction and wear tests were carried out using a reciprocating sliding tribometer connected to a computer monitoring the friction coefficient [41,42]. Tests were performed using a rectangular 30 × 10 × 20 mm3 composite sample sliding against a 35 mm diameter high chromium steel ball under a constant normal load Fn = 17.16 N. The steel ball was kept in stationary and the tangential cyclic motion was applied to the composite specimen using the crank system driven by an electric motor with an electronic speed regulator [9,43–45]. The ball-on-plate machine was set to run with a tangential motion amplitude of 5 mm at 1 Hz. The tests were performed at a temperature of around 25 ◦C and a relative humidity (RH) between 50–60%. Durations of sliding tests were fixed to 1000, 2500, 4000, 5500, 7000, 8500, and 10,000 cycles. At least four tests were performed for each set of conditions.

3. Results and Discussion 3.1. Characterization of the Modified GO The microstructures of GO, modified GO, and HBP were investigated by XPS, FT-IR, TGA, and TEM.

3.1.1. XPS The XPS spectrum of neat GO, HBP-m-GO, and HBP are shown in Figure1. From Figure1, the detailed analysis on the C 1s and O1s were carried out, the peak fitting results have been shown in Figures1b and2 and Table1. As can be seen from the XPS results, the O and C element contents of HBP, GO, and HBP-m-GO are greatly different. The C atom content of GO was 83.7%, which was higher than that of HBP (39.5%) and HBP-m-GO (72.3%). On the other hand, the content of O element in GO is 16.7%, which is lower than that in HBP-m-GO (27.7%) and HBP (60.5%). The ratio n(O)/n(C) of O element and C element was calculated, and it can be found that the n(O)/n(C) of GO is 0.19, and the n(O)/n(C) of HBP is 1.53. After the grafting reaction, the n(O)/n(C) of HBP-m-GO increased from 0.19 to 0.38, indicating that the content of oxygen element was significantly increased. As can be seen from the detailed analysis in Figure2 and Table1, after grafting HBP, the percentage of C=O of HBP-m-GO decreases compared with neat GO, while the percentage of C–O of HBP-m-GO increases. In summary, all of the XPS results above confirmed the success of the grafting reaction. Polymers 2021, 13, x FOR PEER REVIEW 5 of 13

Polymers 2021, 13, x FOR PEER REVIEW 5 of 13 2.2.6. Wear Properties Friction and wear tests were carried out using a reciprocating sliding tribometer con- nected2.2.6. toWear a computer Properties monitoring the friction coefficient [41,42]. Tests were performed us- 3 ing a rectangularFriction and 30wear × 10tests × 20were mm carried composite out using sample a reciprocating sliding againstsliding tribometera 35 mm con-diameter highnected chromium to a computer steel ballmonitoring under thea constant friction coefficientnormal load [41,42]. Fn = Tests 17.16 were N. Theperformed steel ball us- was kepting in a stationaryrectangular and 30 × the 10 tangential× 20 mm3 composite cyclic motion sample was sliding applied against to the a 35composite mm diameter specimen usinghigh thechromium crank systemsteel ball driven under bya constant an electric normal motor load with Fn = 17.16an electronic N. The steel speed ball regulatorwas [9,43–45].kept in stationary The ball-on-plate and the tangential machine cyclic was setmotion to run was with applied a tangential to the composite motion specimen amplitude of 5 mmusing at the 1 Hz. crank The system tests weredriven performed by an electric at a motortemperature with an of electronic around 25speed °C andregulator a relative humidity[9,43–45]. (RH) The ball-on-platebetween 50–60%. machine Durations was set to of run sliding with atests tangential were fixed motion to amplitude1000, 2500, of 4000, 5500,5 mm 7000, at 1 8500,Hz. The and tests 10,000 were cycles. performed At least at a fotemperatureur tests were of aroundperformed 25 °C for and each a relative set of con- ditions.humidity (RH) between 50–60%. Durations of sliding tests were fixed to 1000, 2500, 4000, 5500, 7000, 8500, and 10,000 cycles. At least four tests were performed for each set of con- ditions. 3. Results and Discussion 3.1.3. CharacterizationResults and Discussion of the Modified GO 3.1.The Characterization microstructures of the Modifiedof GO, modified GO GO, and HBP were investigated by XPS, FT-IR, TGA, andThe TEM.microstructures of GO, modified GO, and HBP were investigated by XPS, FT-IR, TGA, and TEM. 3.1.1. XPS 3.1.1.The XPS XPS spectrum of neat GO, HBP-m-GO, and HBP are shown in Figure 1. From Polymers 2021, 13, 2614 FigureThe 1, the XPS detailed spectrum analysis of neat on GO, the HBP-m-GO, C1s and O1s and were HBP carried are shown out, the in Figurepeak5 of 12fitting 1. From results haveFigure been 1, shownthe detailed in Figure analysis 1b, on Figure the C 1s2 andand O Table1s were 1. carried out, the peak fitting results have been shown in Figure 1b, Figure 2 and Table 1.

Figure 1. (a) XPS profiles of HBP, GO, and HBP-m-GO and (b) atomic percentages of O1s and C1s. Figure 1. (a) XPS profiles of HBP, GO, and HBP-m-GO and (b) atomic percentages of O1s and C1s. Figure 1. (a) XPS profiles of HBP, GO, and HBP-m-GO and (b) atomic percentages of O1s and C1s.

Figure 2. Detailed analysis of the O1s profile of XPS results of (a) GO and (b) HBP-m-GO.

Figure 2.Figure Detailed 2. Detailed analysis analysis of the of the O1s O 1sprofileprofile of XPSXPS results results of ( aof) GO (a) and GO (b and) HBP-m-GO. (b) HBP-m-GO.

Table 1. Detailed analysis of the O1s profile of XPS results of GO, HBP-m-GO, and HBP.

Type Position (eV) GO (%) HBP-m-GO (%) HBP (%) C−O 531.7 37.0 47.3 63.1 C=O 533.4 64.0 52.7 36.9

3.1.2. FT-IR Figure3 is the infrared spectrum of the sample. For HBP, the peak at 3420 cm −1 corresponds to the vibration of –OH group, while signals at the wavenumbers of 2920 and −1 2888 cm represent the asymmetric and symmetric vibrations of –CH3 and –CH2, respec- tively [46–48]. The peaks at the wavenumber of 1722 cm−1 are the stretching vibration peak of carbonyl [49,50]. On the other hand, by comparing the infrared spectra of GO and HBP-m-GO, it can be found that, compared with pure GO, new characteristic peaks appear on the infrared spectra of HBP-m-GO, which are located at 2921, 2888, and 1734 cm−1 [51–53]. These three absorption peaks correspond to the asymmetric and symmetric vibrations of –CH3 and –CH2, and stretching vibration peak of carbonyl, respectively, suggesting that the HBP is Polymers 2021, 13, x FOR PEER REVIEW 6 of 13

Table 1. Detailed analysis of the O1s profile of XPS results of GO, HBP-m-GO, and HBP.

Type Position (eV) GO (%) HBP-m-GO (%) HBP (%) C−O 531.7 37.0 47.3 63.1 C=O 533.4 64.0 52.7 36.9

As can be seen from the XPS results, the O and C element contents of HBP, GO, and HBP-m-GO are greatly different. The C atom content of GO was 83.7%, which was higher than that of HBP (39.5%) and HBP-m-GO (72.3%). On the other hand, the content of O element in GO is 16.7%, which is lower than that in HBP-m-GO (27.7%) and HBP (60.5%). The ratio n(O)/n(C) of O element and C element was calculated, and it can be found that the n(O)/n(C) of GO is 0.19, and the n(O)/n(C) of HBP is 1.53. After the grafting reaction, the n(O)/n(C) of HBP-m-GO increased from 0.19 to 0.38, indicating that the content of ox- ygen element was significantly increased. As can be seen from the detailed analysis in Figures 2 and Table 1, after grafting HBP, the percentage of C=O of HBP-m-GO decreases compared with neat GO, while the percentage of C–O of HBP-m-GO increases. In sum- mary, all of the XPS results above confirmed the success of the grafting reaction.

3.1.2. FT-IR Figure 3 is the infrared spectrum of the sample. For HBP, the peak at 3420 cm−1 cor- responds to the vibration of –OH group, while signals at the wavenumbers of 2920 and 2888 cm−1 represent the asymmetric and symmetric vibrations of –CH3 and –CH2, respec- tively [46–48]. The peaks at the wavenumber of 1722 cm−1 are the stretching vibration peak of carbonyl [49,50]. On the other hand, by comparing the infrared spectra of GO and HBP-m-GO, it can be found that, compared with pure GO, new characteristic peaks appear on the infrared Polymers 2021, 13, 2614 6 of 12 spectra of HBP-m-GO, which are located at 2921, 2888, and 1734 cm−1 [51–53]. These three absorption peaks correspond to the asymmetric and symmetric vibrations of –CH3 and – CH2, and stretching vibration peak of carbonyl, respectively, suggesting that the HBP is successfullysuccessfully grafted grafted onto onto GO. GO. Obviously, Obviously, these these three peaksthree allpeaks come all from come HBP, from indicating HBP, indicating thatthat HBP HBP was was successfully successfully grafted grafted onto onto GO. GO.

FigureFigure 3. 3.FT-IR FT-IR spectrum spectrum of HBP, of HBP, GO, andGO, HBP-m-GO. and HBP-m-GO.

3.1.3. TGA 3.1.3. TGA Figure4 is the TGA curve of the sample. It can be seen that the heat resistance of HBP isFigure poor, and 4 is its the weight TGA losscurve takes of the place sample. in the range It can of be about seen 300–450that the◦ C,heat and resistance almost of HBP Polymers 2021, 13, x FOR PEER REVIEWis poor, and its weight loss takes place in the range of about 300–450◦ °C, and almost 7com- of 13 completely decomposes when the temperature is higher than 450 C. GO has the least thermalpletely weight decomposes loss and when the best the thermal temperature stability. is However, higher than HBP-m-GO 450 °C. exhibitsGO has athe certain least thermal degreeweight of loss weight and loss the in best the rangethermal of 300–457stability.◦C, However, which is causedHBP-m-GO by the exhibits grafting ofa certain HBP degree ◦ ontoItof can weight it. Itbe can seen loss be seen fromin the from Figure range Figure 4of that,4 300–457that, at at the the °C, temperature temperature which is caused of of 457 457 by C°C (indicatedthe (indicated grafting by abyof short aHBP short onto dash it. dash line in the figure), the residue weights of GO and HBP-m-GO are 85.1 and 75.5 wt% line in the figure), the residue weights of GO and HBP-m-GO are 85.1 and 75.5 wt% re- respectively, and the graft amount of HBP is calculated to be about 9.6 wt%. spectively, and the graft amount of HBP is calculated to be about 9.6 wt%.

FigureFigure 4. 4.TGA TGA results results of HBP, of HBP, GO, andGO, HBP-m-GO.and HBP-m-GO.

3.1.4.3.1.4. TEM TEM To directly observe the morphology of the fillers, TEM was performed as shown in To directly observe the morphology of the fillers, TEM was performed as shown in Figure5. Clearly, compared with neat GO, many dark dots can be observed from the TEMFigure image 5. Clearly, of HBP-m-GO, compared which with might neat be GO, the evidence many dark for the dots successful can be observed grating of from HBP the TEM ontoimage GO. of HBP-m-GO, which might be the evidence for the successful grating of HBP onto GO.

Figure 5. TEM results of (a) neat GO and (b) HBP-m-GO.

3.2. Mechanical Performance Figure 6 shows the typical tensile stress-strain curves for the neat ABS and its com- posites. It is shown that the typical yield behavior occurs in stress strain curves for the composites with GO or HBP-m-GO. The addition of GO or HBP-m-GO reduces the tensile tendency of the material, meaning that the greater tensile stress is needed to stretch and yield. It is clear from the results in Figure 6 and Table 2 that when the unmodified GO is added, the tensile strength of ABS/GO increased from 42.1 ± 0.6 MPa of pure ABS to 42.8 ± 1.3 MPa, while the elastic modulus increased from 1.15 ± 0.03 GPa to 1.21 ± 0.05 GPa. Meanwhile, the elongation at break decreased drastically from 23.2 ± 1.2% of pure ABS to 11.3 ± 2.0%. These trends indicate that the addition of GO has a significant effect on the mechanical properties of ABS, which is mainly attributed to the weak interface interaction between ABS and GO and the lack of chemical bonds. Interestingly, when the modified GO was added, the elastic modulus and tensile strength of ABS/HBP-m-GO increased evidently. The tensile strength increased from 42.1 ± 0.6 MPa of pure ABS to 55.9 ± 0.9 MPa, up to 30%. Meanwhile, the elongation at break

Polymers 2021, 13, x FOR PEER REVIEW 7 of 13

It can be seen from Figure 4 that, at the temperature of 457 °C (indicated by a short dash line in the figure), the residue weights of GO and HBP-m-GO are 85.1 and 75.5 wt% re- spectively, and the graft amount of HBP is calculated to be about 9.6 wt%.

Figure 4. TGA results of HBP, GO, and HBP-m-GO.

3.1.4. TEM To directly observe the morphology of the fillers, TEM was performed as shown in Polymers 2021, 13, 2614 Figure 5. Clearly, compared with neat GO, many dark dots can be observed from the7 of TEM 12 image of HBP-m-GO, which might be the evidence for the successful grating of HBP onto GO.

FigureFigure 5. 5. TEMTEM results results of (a)) neatneat GOGO and and ( b(b)) HBP-m-GO. HBP-m-GO.

3.2.3.2. Mechanical Mechanical Performance Performance

Polymers 2021, 13, x FOR PEER REVIEWFigure Figure 6 showsshows the the typical typical tensile tensile stress-strain stress-strain curves curves for the for neat the ABSneat andABS its and compos- its com- 8 of 13 posites.ites. It isIt shown is shown that that the typical the typical yield behavioryield beha occursvior inoccurs stress strainin stress curves strain for thecurves compos- for the compositesites with GO with or HBP-m-GO. GO or HBP-m-GO. The addition The ofadditi GO oron HBP-m-GO of GO or HBP-m-GO reduces the reduces tensile tendency the tensile tendencyof the material, of the meaningmaterial, thatmeaning the greater that the tensile grea stresster tensile is needed stress to is stretch needed and to yield. stretch It is and yield.clearof theIt from is sampleclear the results from was the in significantly Figure results6 andin Figure Table higher2 6 that and than when Table ABS/GO, the 2 that unmodified when to 20.1 the GO ±unmodified is1.3%, added, slightly the GO is lower than ± ± added,tensilethat theof strength pure tensile ofABS strength ABS/GO resin. of increasedThis ABS/GO might from increased be 42.1 attrib 0.6fromuted MPa 42.1 to of the± pure 0.6 significantly ABSMPa toof42.8 pure enhancedABS1.3 MPa, to 42.8 interaction ± ± ± while1.3 MPa, the elasticwhile modulusthe elastic increased modulus from increased 1.15 0.03 from GPa 1.15 to 1.21± 0.03 0.05GPa GPato 1.21. Meanwhile, ± 0.05 GPa. thebetween elongation the at ABS break matrix decreased with drastically GO after from surfac 23.2 ±e grafting1.2% of pure modification. ABS to 11.3 ± During2.0%. stretching, Meanwhile, the elongation at break decreased drastically from 23.2 ± 1.2% of pure ABS to Thesethe area trends of indicate weak interfaces that the addition between of GO ABS has an ad significant HBP-m-GO effect significantly on the mechanical decreased, result- 11.3 ± 2.0%. These trends indicate that the addition of GO has a significant effect on the propertiesing in the of maintaining ABS, which is of mainly elongation attributed at break to the at weak a higher interface level. interaction Meanwhile, between the elastic mod- mechanicalABSulus and and GO properties tensile and the strength lack of ABS, of chemical whichare significantly is bonds. mainly attributed enhanced. to the weak interface interaction between ABS and GO and the lack of chemical bonds. Interestingly, when the modified GO was added, the elastic modulus and tensile strength of ABS/HBP-m-GO increased evidently. The tensile strength increased from 42.1 ± 0.6 MPa of pure ABS to 55.9 ± 0.9 MPa, up to 30%. Meanwhile, the elongation at break

FigureFigure 6. Stress-strain6. Stress-strain curves curves of ABS, of ABS/GO, ABS, ABS/GO, and ABS/HBP-m-GO. and ABS/HBP-m-GO.

Table 2. Mechanical parameters of ABS, ABS/GO, and ABS/HBP-m-GO. Table 2. Mechanical parameters of ABS, ABS/GO, and ABS/HBP-m-GO. Composition (wt%) Elastic Modulus Tensile Strength Elongation at Break Samples Composition (wt%) Elastic Modulus Tensile Elongation at Samples ABS GO HBP-m-GO (GPa) (MPa) (%) ABS GO HBP-m-GO (GPa) Strength (MPa) Break (%) ABS 100 - - 1.15 ± 0.03 42.1 ± 0.6 23.0 ± 1.2 ABSABS/GO 98.5100 1.5 - - 1.21 ± 0.05 - 1.15 46.8 ± 0.031.3 42.1 11.3 ± 0.6± 2.0 23.0 ± 1.2 ABS/GOABS/HBP-m-GO 98.5 - 1.5 1.5 1.43 ± 0.04 - 1.21 55.9 ± 0.050.9 46.8 20.1 ± 1.3± 1.3 11.3 ± 2.0 ABS/HBP-m-GO 98.5 - 1.5 1.43 ± 0.04 55.9 ± 0.9 20.1 ± 1.3

3.3. Wear Performance Figure 7a shows the relationship between the coefficient of friction and the sliding cycles of pure ABS and ABS/GO composites. It is shown that for all the samples, the coef- ficient of friction increases monotonically with the number of sliding times. Pure ABS ex- hibits a high coefficient of friction (0.41) after 1000 sliding cycles. As the number of cycles increases, the friction coefficient gradually increases, after 10,000 cycles, the pure ABS specimen wears out severely and the friction coefficient increases significantly to 0.62. Compared with pure ABS, the addition of raw GO decreases the friction coefficient com- pared to pure ABS, a rule consistent in 10,000 cyclic tests. On the other hand, when the modified GO is added, the friction coefficient of the ABS/HBP-m-GO drops further, at 0.27 after 1000 sliding cycles, and after 10,000 cycles, at 0.395. This shows that the addition of the modified GO significantly reduces the coefficient of friction of the ABS.

Polymers 2021, 13, 2614 8 of 12

Interestingly, when the modified GO was added, the elastic modulus and tensile strength of ABS/HBP-m-GO increased evidently. The tensile strength increased from 42.1 ± 0.6 MPa of pure ABS to 55.9 ± 0.9 MPa, up to 30%. Meanwhile, the elongation at break of the sample was significantly higher than ABS/GO, to 20.1 ± 1.3%, slightly lower than that of pure ABS resin. This might be attributed to the significantly enhanced interaction between the ABS matrix with GO after surface grafting modification. During stretching, the area of weak interfaces between ABS and HBP-m-GO significantly decreased, resulting in the maintaining of elongation at break at a higher level. Meanwhile, the elastic modulus and tensile strength are significantly enhanced.

3.3. Wear Performance Figure7a shows the relationship between the coefficient of friction and the sliding cycles of pure ABS and ABS/GO composites. It is shown that for all the samples, the coefficient of friction increases monotonically with the number of sliding times. Pure ABS exhibits a high coefficient of friction (0.41) after 1000 sliding cycles. As the number of cycles increases, the friction coefficient gradually increases, after 10,000 cycles, the pure ABS specimen wears out severely and the friction coefficient increases significantly to 0.62. Compared with pure ABS, the addition of raw GO decreases the friction coefficient compared to pure ABS, a rule consistent in 10,000 cyclic tests. On the other hand, when the modified GO is added, the friction coefficient of the ABS/HBP-m-GO drops further, at Polymers 2021, 13, x FOR PEER REVIEW 0.27 after 1000 sliding cycles, and after 10,000 cycles, at 0.395. This shows that the addition 9 of 13

of the modified GO significantly reduces the coefficient of friction of the ABS.

Figure 7.Figure (a) Friction 7. (a) Friction coefficient coefficient and and (b) (weightb) weight losses losses (%) (%) versus versus number number of of sliding sliding cycles cycles for ABS for andABS ABS/GO and ABS/GO composites. composites.

On Onthe the other other hand, hand, itit isis well well known known that that the worsethe worse the wear the resistance, wear resistance, the greater the the greater wear losses. From the curve of the sample mass loss-cyclic friction number in Figure7b, the wear losses. From the curve of the sample mass loss-cyclic friction number in Figure the pure ABS resin has the lowest wear resistance and the largest mass loss in the friction 7b, thecycle pure test, andABS after resin 10,000 has cyclic the lowest frictions, wear it reaches resistance 0.24%. Afterand addingthe largest the unmodified mass loss in the frictionGO, thecycle wear test, amount and after is greatly 10,000 reduced, cyclic indicating frictions, that it thereaches GO added 0.24%. improves After theadding wear the un- modifiedresistance GO, of the the wear ABS amount resin. When is greatly the modified reduced, GO indicating is added, the that wear the amountGO added of the improves the wearcomposite resistance decreases of the further. ABS After resin. 10,000 When cyclic the frictions, modified the GO wear is onlyadded, reaches the wear 0.07%, amount of thewhich composite is 29% of thedecreases same condition further. as After pure ABS. 10,0 The00 cyclic above resultsfrictions, show the that wear the surface only reaches modification of the GO can significantly improve the friction resistance of the ABS resin, 0.07%,possibly which since is 29% the surface of the modificationsame condition of the as GO pure makes ABS. it a moreThe above uniform results dispersion show in that the surfacethe ABSmodification resin and has of athe stronger GO can interface significantl bindingy forceimprove with the ABSfriction matrix. resistance of the ABS resin, possiblyThe SEM since observation the surface was usedmodification to investigate of the the GO effect makes of the it fillers a more on theuniform worn disper- sionsurface in the morphologicalABS resin and features has a ofstronger ABS after interface 1000, 4000, binding and 10,000 force sliding with cycles the ABS (Figure matrix.8). The SEM observation was used to investigate the effect of the fillers on the worn sur- face morphological features of ABS after 1000, 4000, and 10,000 sliding cycles (Figure 8).

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Figure 8. SEM images of the worn surface morphological features of pure ABS, ABS/GO and ABS/HBP-m-GO after (a–c) 1000, (d–f) 4000 and (g–i) 10,000 sliding cycles, respectively.

Polymers 2021, 13, x FOR PEER REVIEW 9 of 13

Figure 7. (a) Friction coefficient and (b) weight losses (%) versus number of sliding cycles for ABS and ABS/GO composites.

On the other hand, it is well known that the worse the wear resistance, the greater the wear losses. From the curve of the sample mass loss-cyclic friction number in Figure 7b, the pure ABS resin has the lowest wear resistance and the largest mass loss in the friction cycle test, and after 10,000 cyclic frictions, it reaches 0.24%. After adding the un- modified GO, the wear amount is greatly reduced, indicating that the GO added improves the wear resistance of the ABS resin. When the modified GO is added, the wear amount of the composite decreases further. After 10,000 cyclic frictions, the wear only reaches 0.07%, which is 29% of the same condition as pure ABS. The above results show that the surface modification of the GO can significantly improve the friction resistance of the ABS resin, possibly since the surface modification of the GO makes it a more uniform disper- Polymers 2021, 13, 2614sion in the ABS resin and has a stronger interface binding force with the ABS matrix. 9 of 12 The SEM observation was used to investigate the effect of the fillers on the worn sur- face morphological features of ABS after 1000, 4000, and 10,000 sliding cycles (Figure 8).

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Figure 8. SEM images of the worn surface morphological features of pure ABS, ABS/GO and Figure 8. SEM images of the worn surface morphological features of pure ABS, ABS/GO and ABS/HBP-m-GO after (a–c) ABS/HBP-m-GO after (a–c) 1000, (d–f) 4000 and (g–i) 10,000 sliding cycles, respectively. 1000, (d–f) 4000 and (g–i) 10,000 sliding cycles, respectively.

Morphologies of the worn surfaces formed after 1000 sliding cycles are shown in

Figure8a–c. For the neat ABS (Figure8a), a large area of surface deformation can be seen, characterized by plastic deformation and plough constitute predominant surface modification, indicating that the adhesive wear mechanism is activated. Meanwhile, the deformed area of ABS/GO is much smaller than the neat ABS, and almost no deformation can be seen from ABS/HBP-m-GO, which is in accord with the result in Figure7. At 4000 sliding cycles (Figure8d–f), a similar trend can be observed. The surface of neat ABS is seriously worn, while the severity of the wear of ABS/HBP-m-GO is lowest. At 10,000 sliding cycles (Figure8h–j), the seriously worn surface of neat ABS can be seen, characterized by severe peeling off and plastic deformation (Figure8h), corre- sponding to its worse tribological properties. When raw GO was added, it can be seen that the worn surface is relatively smooth compared with the neat ABS, reflecting that the addition of GO benefits for wear reduction. However, it should be emphasized that some debris can be noticed for ABS/GO, which might be attributed to the limited compatibility between raw GO and ABS. For ABS/HBP-m-GO, its worn surface is obviously smoother compared with the neat ABS and ABS/GO and almost no debris can be seen, indicating the evidently enhancement of tribological properties, which might be attributed to the surface modification of GO and the strengthened compatibility between HBP-m-GO and the polymer matrix.

4. Conclusions In this manuscript, the GO surface was modified by hyper-branched polyester and the ABS/GO and ABS/HBP-m-GO composites were prepared. The effects of GO or modified GO on the mechanical performance and wearing properties were investigated, as follows: Polymers 2021, 13, 2614 10 of 12

(1) The results of XPS, FT-IR, and TEM revealed the successful grafting of HBP onto GO. In addition, TGA indicated that the graft amount of HBP is calculated to be about 9.6 wt%. (2) ABS/GO composites have been prepared, and the mechanical properties and wear performance were studied in detail. The results herein reviewed that the addition of GO has a significant effect on the mechanical properties of ABS, and when the modified GO was added, the elastic modulus and tensile strength of ABS/HBP-m-GO increased evidently. The tensile strength increased from 42.1 ± 0.6 MPa of pure ABS to 55.9 ± 0.9 MPa, up to 30%. Meanwhile, the elongation at break of the sample was significantly higher than ABS/GO, to 20.1 ± 1.3%, slightly lower than that of pure ABS resin. For wear performance, the addition of raw GO decreases the friction coefficient compared with pure ABS, and when the modified GO is added, the friction coefficient of the ABS/HBP-m-GO drops further, at 0.27 after 1000 sliding cycles, and after 10,000 cycles, at 0.395, revealing that the addition of the modified GO significantly reduces the coefficient of friction of the ABS. Meanwhile, the weight loss during the wear test decreases evidently. Finally, the morphological analysis was conducted and the related mechanism was analyzed.

Author Contributions: The manuscript was written through the contributions of all authors. Con- ceptualization: J.K., S.L. and X.C.; methodology, C.S.; software, Z.S. and Y.C.; validation, X.C. and S.L.; formal analysis, X.C. and S.L.; investigation, X.C. and J.K.; resources, X.C. and J.K.; data curation, X.C., and J.K.; writing—original draft preparation, X.C.; writing—review and editing, S.L. and J.K.; visualization, X.C.; supervision, Y.C.; project administration, Y.C.; funding acquisition, Y.C. All authors have read and agreed to the published version of the manuscript. Funding: We gratefully acknowledge the financial support from the National Natural Science Foundation of China (NSFC, Grant Nos. 51503134, 51702282). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conflicts of Interest: The authors declare no conflict of interest.

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