Investigation of Carbon Nanotube Grafted Graphene Oxide Hybrid Aerogel for Polystyrene Composites with Reinforced Mechanical Performance

Article Investigation of Carbon Nanotube Grafted Graphene Oxide Hybrid Aerogel for Polystyrene Composites with Reinforced Mechanical Performance

Yanzeng Sun †, Hui Xu †, Zetian Zhao, Lina Zhang, Lichun Ma, Guozheng Zhao, Guojun Song * and Xiaoru Li *

Institute of Polymer Materials, School of Material Science and Engineering, Qingdao University, No. 308 Ningxia Road, Qingdao 266071, China; [email protected] (Y.S.); [email protected] (H.X.); [email protected] (Z.Z.); [email protected] (L.Z.); [email protected] (L.M.); [email protected] (G.Z.) * Correspondence: [email protected] (G.S.); [email protected] (X.L.) † Yanzeng Sun and Hui Xu are co-ﬁrst authors.

Abstract: The rational design of carbon nanomaterials-reinforced polymer matrix composites based on the excellent properties of three-dimensional porous materials still remains a significant challenge. Herein, a novel approach is developed for preparing large-scale 3D carbon nanotubes (CNTs) and graphene oxide (GO) aerogel (GO-CNTA) by direct grafting of CNTs onto GO. Following this, styrene was backfilled into the prepared aerogel and polymerized in situ to form GO–CNTA/polystyrene (PS) nanocomposites. The results of X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy indicate the successful establishment of CNTs and GO-CNT and the excellent mechanical properties of the 3D frameworks using GO-CNT aerogel. The nanocomposite fabricated with around 1.0 wt% GO- CNT aerogel displayed excellent thermal conductivity of 0.127 W/m·K and its mechanical properties Citation: Sun, Y.; Xu, H.; Zhao, Z.; Zhang, L.; Ma, L.; Zhao, G.; Song, G.; were significantly enhanced compared with pristine PS, with its tensile, flexural, and compressive Li, X. Investigation of Carbon strengths increased by 9.01%, 46.8%, and 59.8%, respectively. This facile preparation method provides Nanotube Grafted Graphene Oxide a new route for facilitating their large-scale production. Hybrid Aerogel for Polystyrene Composites with Reinforced Keywords: CNT-GO aerogel; in-situ polymerization; grafting reaction; mechanical properties; ther- Mechanical Performance. Polymers mal conductivity 2021, 13, 735. https://doi.org/ 10.3390/polym13050735

Academic Editor: Iolanda De Marco 1. Introduction Due to their light weight and high performance, graphene oxide (GO)–carbon nan- Received: 27 January 2021 otube (CNT)-polymer nanocomposites have been widely studied and applied in various Accepted: 23 February 2021 Published: 27 February 2021 industries, including aerospace [1], military, and national defense, and some engineering materials [2–4]. Three-dimensional (3D) GO aerogels can act as carriers for CNTs by en-

Publisher’s Note: MDPI stays neutral abling them to disperse evenly in the wall of GO aerogel, followed by filling the matrix with regard to jurisdictional claims in with a suitable polymer material, thereby overcoming poor dispersion of the nanomaterials published maps and institutional affil- within the polymer [5–8]. In previous studies, CNTs were dispersed in GO through physi- iations. cal mixing to form GO-CNT aerogels [9,10]. However, there have been a few systematic studies on the grafting of CNTs onto the surface of GO. Therefore, it is important to explore how the structure and state of the CNTs in the GO aerogel affect the properties of the nanocomposite [11]. In recent years, the use of 3D aerogel-reinforced filler has greatly improved the uneven Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. dispersion of fillers and effectively enhanced the performance of the nanocomposites [12–14]. This article is an open access article For instance, Fan et al. prepared graphene aerogels and filled them with poly(methyl distributed under the terms and methacrylate) (PMMA) to form nanocomposites; the mechanical properties of the nanocom- conditions of the Creative Commons posites were significantly enhanced compared to those prepared using the traditional Attribution (CC BY) license (https:// blending and dispersion method. When the content of the graphene aerogels was increased creativecommons.org/licenses/by/ from 0.67 to 2.50 vol.%, the microhardness of the nanocomposites increased from 303.6 to 4.0/). 462.5 MPa [15]. When the content of GO was 2 wt.%, the mechanical strength and elastic

Polymers 2021, 13, 735. https://doi.org/10.3390/polym13050735 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 735 2 of 11

modulus of the GO/epoxy resin nanocomposites were increased by 50% and 19.6%, respectively, the glass transition temperature increased by 15.59 ◦C, and the thermal conductivity was 1.4 times higher than that of the neat epoxy resin [6]. In addition, the compressive strength and elastic modulus of 3D graphene aerogel-reinforced silicone rubber was higher than that of silicone rubber, with the hardness increasing from 9.2 HA for pure GO to 20.5 HA for the nanocomposite [7]. Li et al. grafted CNTs on GO to form a hybrid filler to be dispersed into an epoxy matrix. The results showed that the tensile modulus and the tensile strength were enhanced by ~36% and ~40%, respectively [16]. However, systematic studies on the influence of the GO and CNT morphologies and structures on the properties of nanocomposites produced by physical mixing and chemical grafting have not yet been conducted [17–19]. In this study, CNTs were grafted onto the surface of GO with a silane coupling agent KH550 (3-Aminopropyltriethoxysilane), and 3D large-scale aerogels were prepared by freeze-drying. A mixture of styrene and an initiator were backfilled into the prepared 3D aerogel and polymerized in situ to form GO-CNT aerogel polystyrene (GO-CNTA/PS) nanocomposites. Subsequently, the mechanical properties of GO-CNT aerogels and GO- CNTA/PS nanocomposites were investigated.

2. Experimental 2.1. Materials Graphite powder (about 20 µm) was purchased from the Huayuan Chemical Co., Ltd. (Shanghai, China), plain CNTs (length: 5~15 µm, diameter: ~20 nm, purity: ~97%) were purchased from Shenzhen Nano Company (Shenzen, China), styrene was purchased from Chemical Reagent Co, Ltd. (Tianjin, China), and azobisisobutyronitrile (AIBN) was purchased from the Tianjin Damao Chemical Reagent Factory (Tianjin, China). γ- Aminopropyl triethoxysilane (KH-550), N-hydroxysuccinimide (NHS), and 1-ethyl-3-(3- dimethylaminopropyl) carbodiimide (EDC) were purchased from the Shanghai Machlin Biochemical Co, Ltd. (Shanghai, China). Carboxylated CNTs (oxygen content: ~6.4%) were fabricated by the Institute of Polymer Materials, Qingdao University, China.

2.2. Preparation of the GO-CNT Aerogel GO was prepared via an improved Hummers’ method from graphite flakes. CNTs were oxidized with a mixture of sulfuric acid and nitric acid at 100 ◦C for 8 h, and then the oxidized CNTs were obtained by centrifuging, filtering, and freeze-drying [20]. Next, the obtained CNTs (0.059 g) were mixed with ethanol (2 ml) in deionized (DI) water (21 mL) by sonication for 1 h. The mixture was then transferred to the GO suspension liquid (8.7 mL) and dispersed by sonication for 3 h (the mass ratio of CNTs to GO was 3:7). Following this, the suspension was transferred into a mold and held at −25◦C for 12 h and then freeze-dried for 48 h. The preparation process of GO-CNT aerogel was divided into two steps. First, to prepare the modified CNT-KH550, a mixture of 90 mL ethanol, 10 mL DI water, and 2 mL KH550 in a three-necked flask was ultrasonically dispersed for 1 h. Following this, CNTs were added and the mixture was ultrasonically dispersed for 2 h, followed by stirring at 78 ◦C for 6 h [21,22]. Finally, the prepared mixture was washed with acetone and ethanol, and then freeze-dried. Second, for the preparation of GO-CNT, GO, DI water, and ethanol were combined in a three-necked flask, after which ultrasonic dispersion was conducted for 3 h. Next, EDC/NHS (1:3) was added and the mixture stirred for 2 h at room temperature. Following this, the dispersed CNT-KH550 solution was added dropwise at 35 ◦C for 24 h. A schematic of the reaction mechanism is shown in Figure1[23–25]. Polymers 2021Polymers, 13,2021 735, 13, 735 3 of 11 3 of 12

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FigureFigure 1. 1.TheThe reaction reaction mechanism diagram. diagram.

2.3. Preparation2.3. Preparation of ofGO-CNTA/PS GO-CNTA/PS NanocompositesNanocomposites TheThe prepared prepared GO-CNT GO-CNT aerogel aerogel was was immersed immersed in ain mixture a mixture of styrene of styrene and AIBN and via AIBN via vacuum-assisted impregnation to ensure that the mixture replaced the air in the aerogel. vacuum-assistedThe complexes impregnation were then maintained to ensure at 70that◦C the for mixture 12 h, 80 ◦replacedC for an additionalthe air in 12the h, aerogel. The andcomplexes 90 ◦C for were another then 12 h.maintained The ﬁnal GO-CNTA/PS at 70 °C for nanocomposites 12 h, 80 °C for were an additional obtained after 12 h, and 90 °Ccooling for another and were 12 molded h. The byfinal hot-pressing GO-CNTA/PS at 5 MPa nanocomposites and 170 ◦C for 5 min were to dischargeobtained anyafter cool- ing andtiny pores;were molded the nanocomposites by hot-pressing were only at 5 compressedMPa and 170 by approximately°C for 5 min to 5% discharge during the any tiny pores;hot-pressing the nanocompositesFigure process. 1. The Forreaction comparison, were mechanism only acomp mixturediagram.ressedof GO by and approximately CNTs (a total mass 5% contentduring of the hot- pressing1.0 wt.%, process. and a For mass comparison, ratio of 7:3) was a mixture added to of a GO pristine andPS CNTs matrix (a tototal prepare mass blended content of 1.0 2.3.nanocomposites, Preparation of GO-CNTA/PS deﬁned as GOC/PS. Nanocomposites Abbreviation of the samples was shown in Table1. wt.%, and a mass ratio of 7:3) was added to a pristine PS matrix to prepare blended nano- TheThe hot-pressing prepared conditions GO-CNT aerogel shown inwas Figure immersed2 were in the a samemixture for of all styrene samples. and AIBN via composites, defined as GOC/PS. Abbreviation of the samples was shown in Table 1. The vacuum-assisted impregnation to ensure that the mixture replaced the air in the aerogel. hot-pressingTheTable complexes 1. Abbreviation conditions were ofthen sample.shown maintained in Figu atre 70 2 °C were for 12 the h, same80 °C forfor an all additional samples. 12 h, and 90 °C for another 12Full h. Name The final GO-CNTA/PS nanocomposites Abbreviation were obtained after cool- ing and were molded by hot-pressing at 5 MPa and 170 °C for 5 min to discharge any tiny GO, CNT and polystyrene blending composite GOC/PS pores; the nanocomposites were only compressed by approximately 5% during the hot- polystyren PS pressingCNT process. covalently For comparison, bonded to KH550 a mixture of GO and CNTsCNT-KH550 (a total mass content of 1.0 wt.%, and a CNTmass grafted ratio of GO 7:3) aerogel was added to a pristine PS matrix GO-CNTA to prepare blended nanocomposites,GO-CNT defined aerogel/polystyrene as GOC/PS. composite Abbreviation of the samples GO-CNTA/PS was shown in Table 1. The hot-pressing conditions shown in Figure 2 were the same for all samples.

Figure 2. Preparation process of composite material.

Table 1. Abbreviation of sample.

Full Name Abbreviation GO, CNT and polystyreneFigureFigure blending 2. 2. PreparationPreparation composite process process of of composite composite material. material. GOC/PS polystyren PS 2.4. Characterization CNT covalently bondedTable to KH550 1. Abbreviation of sample. CNT-KH550 The synthesized GO-CNT morphology and chemical structure were analyzed via CNT graftedFourier-transformFull GO Name aerogel infrared spectroscopy (FTIR; Nexus 360 ThermoNicolet, Abbreviation GO-CNTA Singapore). GO-CNTGO, CNT aerogel/polystyren andElemental polystyrene and blending chemicale composite composite compositions were evaluated by using X-ray GOC/PS GO-CNTA/PS photoelectron spec- troscopypolystyren (XPS; Escalab 220i-XL, VG Systems, Ltd., East Grinstead,PS UK). GO and CNT CNT2.4. covalently Characterizationstructures bonded were characterized to KH550 by Raman spectroscopy (InVia 2000, CNT-KH550 Renishaw PLC, Wotton- CNTUnder-Edge, grafted GO UK), aerogel and the structure and composition were analyzed GO-CNTA via X-ray diffraction GO-CNT aerogel/polystyren(XRD;The synthesized D8 Advance,e composite BrukerGO-CNT AXS morphology GmBH, Karlsruhe, and Germany) chemical GO-CNTA/PS with structure Cu-Kα radiationwere analyzed and via Fourier-transform infrared spectroscopy (FTIR; Nexus 360 ThermoNicolet, Singapore). El- emental2.4. Characterization and chemical compositions were evaluated by using X-ray photoelectron spec- troscopyThe (XPS; synthesized Escalab GO-CNT 220i-XL, morphology VG Systems, and Ltd.,chemical East structure Grinstead, were UK).analyzed GO via and CNT structuresFourier-transform were characterized infrared spectroscopy by Raman (FTIR; spec Nexustroscopy 360 ThermoNicolet, (InVia 2000, RenishawSingapore). PLC,El- Wot- ton-Under-Edge,emental and chemical UK), and compositions the structure were and evaluated composition by using were X-ray analyzed photoelectron via X-ray spec- diffrac- tiontroscopy (XRD; D8(XPS; Advance, Escalab 220i-XL,Bruker AXSVG Systems, GmBH, Ltd., Karlsruhe, East Grinstead, Germany) UK). with GO Cu-Kand CNTα radiation andstructures λ = 1.5418 were Å. GO-CNT characterized and by nanocomposite Raman spectroscopy morphologies (InVia 2000, were Renishaw characterized PLC, Wot- by using ton-Under-Edge, UK), and the structure and composition were analyzed via X-ray diffrac- field-emission scanning electron microscopy (FESEM; Hitachi S-5700, Hitachi High-Tech tion (XRD; D8 Advance, Bruker AXS GmBH, Karlsruhe, Germany) with Cu-Kα radiation and λ = 1.5418 Å. GO-CNT and nanocomposite morphologies were characterized by using field-emission scanning electron microscopy (FESEM; Hitachi S-5700, Hitachi High-Tech

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λ = 1.5418 Å. GO-CNT and nanocomposite morphologies were characterized by using Analyticalﬁeld-emission Science, scanning Abingdon, electron UK) microscopy with gold (FESEM;for 30 s. Microhardness Hitachi S-5700, was Hitachi measured High-Tech with anAnalytical HXD-1000TMC Science, (Cany Abingdon, Precision UK) Instruments with gold Co., for 30Ltd., s. Shanghai, Microhardness China) was digital measured micro- hardnesswith an HXD-1000TMC tester using a 245 (Cany mN Precisionindentation Instruments force for 15 Co., s. Ltd., Shanghai, China) digital microhardnessComposite tester tensile, using bending, a 245 mNimpact, indentation and compression force for 15tests s. were performed according toComposite National Standards tensile, bending, GB/T 1040.1-2006/ISO impact, and compression 527-1:1993, tests GB/T were 9341-2008/ISO performed according178:2001, GB/Tto National 1041-2008/ISO Standards 604:2002, GB/T 1040.1-2006/ISO and GB/T 1043.1-2008/ISO 527-1:1993, 179-1:2000 GB/T 9341-2008/ISO using an a1-7000 178:2001, ten- sileGB/T tester 1041-2008/ISO (GOTECH, Taiwan, 604:2002, China) and GB/T over 1043.1-2008/ISO0–30,000 N with 2 179-1:2000 mm/min tensile using antest a1-7000 speed andtensile a standard tester (GOTECH, 25 mm sample-spacing. Taiwan, China) overThe 0–30,000test speed N for with the 2 mm/minplastic bending tensile test test was speed 1 mm/min,and a standard with 32 25 mm mm sample-spacing. sample-spacing. Compre The testssion speed tests for were the performed plastic bending using testthe uni- was versal1 mm/min, testing with machine 32 mm with sample-spacing. a speed of 5 mm/min, Compression and impact tests tests were were performed performed using using the theuniversal universal testing testing machine machine with with a speed a simple-supported of 5 mm/min, andbeam impact impact tests tester were with performed 7.5 J im- pactusing energy the universal and a 32 testing mm sample machine span. with a simple-supported beam impact tester with 7.5 J impact energy and a 32 mm sample span. 3. Results and Discussion 3. Results and Discussion 3.1. Chemical Composition and Morphology of GO, CNT, CNT-KH550, and GO-CNT 3.1. Chemical Composition and Morphology of GO, CNT, CNT-KH550, and GO-CNT Figure 3 shows the structures and compositions of GO, CNTs, CNT-KH550, and GO- Figure3 shows the structures and compositions of GO, CNTs, CNT-KH550, and GO- CNT determined via Fourier-transform infrared spectroscopy (FTIR). Absorption peaks CNT determined via Fourier-transform infrared spectroscopy (FTIR). Absorption peaks for GO and CNTs were located at 3208 cm−1 (O-H tensile vibration), as well as 1721 and for GO and CNTs were located at 3208 cm−1 (O-H tensile vibration), as well as 1721 and −1 1045 cm− 1(C=O(C=O and and C-O C-O tensile tensile vibration, vibration, respectively), respectively), indicating indicating that that they they contained a largelarge number number of of hydroxyl hydroxyl and and carboxyl carboxyl grou groups.ps. However, However, CNT-KH550 had an absorption −1 peak at 1031 cm −,1 ,which which was was attributed attributed to to Si-O-C Si-O-C tensile tensile vibration, vibration, indicating indicating that that KH550 KH550 had been successfullysuccessfully grafted grafted onto onto the the surface surface of theof the CNTs. CNTs. The The peak peak intensity intensity at 3208 at cm3208−1 −1 cmfor GO-CNT for GO-CNT decreased, decreased, indicating indicating that thethat O-H the O-H groups groups had decreasedhad decreased signiﬁcantly significantly and andCNT-KH550 CNT-KH550 had beenhad been successfully successfully grafted grafted onto onto the GO the surface.GO surface.

Figure 3. FTIRFTIR spectrum of GO, CNT,CNT, CNT-KH550CNT-KH550 and and GO-CNT. GO-CNT. (color (color ﬁgure figure can can be be viewed viewed online). online). Figure4 shows Raman spectroscopy results for GO, CNTs, CNT-KH550, and GO-CNT. TwoFigure strong bands4 shows at Raman 1351 (the spectroscopy D band) and results 1594 cm for− 1GO,(the CNTs, G band) CNT-KH550, correspond and with GO- the CNT.surface Two disorder strong andbands active at 1351 groups (the ofD band) GO or and CNT, 1594 respectively. cm−1 (the G Itband) can becorrespond seen from with the theID/ surfaceIG ratio disorder that the degreeand active ofdisorder groups of and GO reactivity or CNT, respectively. of unoxidized It can CNTs be (1.076),seen from CNTs the I(1.270),D/IG ratio and that CNT-KH550 the degree (1.466)of disorder gradually and reacti increased.vity of Thus, unoxidized the degree CNTs of (1.076), disorder CNTs and (1.270),surface and activity CNT-KH550 for CNT-KH550 (1.466) gradually were high. in Comparedcreased. Thus, with the GO degree (1.705), of thedisorder degree and of

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surface activity for CNT-KH550 were high. Compared with GO (1.705), the degree of dis- orderdisorder and and reactivity reactivity of GO-CNT of GO-CNT (1.533) (1.533) was was reduced reduced to a to certain a certain extent, extent, which which was was due due to theto the reaction reaction of ofCNT-KH550 CNT-KH550 with with the the active active gr groupsoups on on the the GO GO surface, surface, signifying signifying that CNTsCNTs had been grafted onto the surface of the GO aerogel.

FigureFigure 4. 4. RamanRaman spectroscopy spectroscopy (color (color figure ﬁgure can be viewed online).

FigureFigure 5 shows XRD patterns of graphite, graphite, GO, GO, unoxidized unoxidized CNTs, CNTs, CNTs, CNTs, CNT-KH550, and GO-CNT. The diffraction peak at 26.51◦ corresponds to 0.34 nm interlayer spacing. and GO-CNT. The diffraction peak at 26.51° corresponds to 0.34 nm interlayer spacing. GO had a large interlayer distance (~0.86 nm) due to hydroxyl and carboxyl group for- GO had a large interlayer distance (~0.86 nm) due to hydroxyl and carboxyl group for- mation, indicating that graphite was successfully exfoliated during the chemical reaction. mation, indicating that graphite was successfully exfoliated during the chemical reaction. The diffraction peak at 25.95◦ for unoxidized CNTs, CNTs, and CNT-KH550 conﬁrmed The diffraction peak at 25.95° for unoxidized CNTs, CNTs, and CNT-KH550 confirmed that the interlayer distances for these did not change because the hydroxyl and carboxyl Polymers 2021, 13, 735 that the interlayer distances for these did not change because the hydroxyl and carboxyl6 of 12 groups do not affect the interlayer distance. However, the interlayer spacing of GO-CNT groups do not affect the interlayer distance. However, the interlayer spacing of GO-CNT (~0.842 nm) was smaller than that of GO (~0.86 nm), suggesting that GO had reacted. (~0.842 nm) was smaller than that of GO (~0.86 nm), suggesting that GO had reacted. Figure 6 shows the surface chemical composition of GO, CNTs, CNT-KH550, and GO-CNT analyzed via XPS. In Figure 6a, the peaks at 285 and 530 EV correspond to C and O, respectively, in GO, CNTs, CNT-KH550, and GO-CNT. Figure 6b is an enlarged view of Figure 6a for the range of 0–250 eV. The peaks at 50 and 101 eV correspond to Si 2p and Si 2s, respectively, which helps to further indicate that KH550 and CNT-KH550 had been grafted onto the CNTs and GO, respectively. Figure 6c,d shows the C 1s regions of CNT- KH550 and GO-CNT, respectively, deconvoluted into six peaks, which were attributed to the binding energies of C-C (284.4 EV), C-O-C (286.5 eV), C=O (287.2 eV), C-OH (284.8 eV), and C (C=O)-OH (288.5eV), respectively. Figure 6d,f shows the Si 2p regions of CNT- KH550 and GO-CNT, respectively, especially the binding energy of Si-O-C at 1031 eV. Therefore, there were a lot of Si-O-C bonds present on the GO surface after the CNTs had been grafted.

FigureFigure 5. 5. XRDXRD patterns patterns of of Graphite, Graphite, GO, GO, unoxidiz unoxidizeded CNT, CNT, CNT-KH550 andand GO-CNT.GO-CNT. Figure6 shows the surface chemical composition of GO, CNTs, CNT-KH550, and GO- CNT analyzed via XPS. In Figure6a, the peaks at 285 and 530 EV correspond to C and O,

Figure 6. XPS spectra of GO, CNT, CNT-KH550 and CNT-GO, (a) and (b) XPS wide spectrum, (c) C1s spectrum of CNT-KH550, (d) Si2p spectrum of CNT-KH550, (e) C1s spectrum of GO-CNT, (f) Si2p spectrum of GO-CNT. (color figure can be viewed on line).

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respectively, in GO, CNTs, CNT-KH550, and GO-CNT. Figure6b is an enlarged view of Figure6a for the range of 0–250 eV. The peaks at 50 and 101 eV correspond to Si 2p and Si 2s, respectively, which helps to further indicate that KH550 and CNT-KH550 had been grafted onto the CNTs and GO, respectively. Figure6c,d shows the C 1s regions of CNT-KH550 and GO-CNT, respectively, deconvoluted into six peaks, which were attributed to the binding energies of C-C (284.4 EV), C-O-C (286.5 eV), C=O (287.2 eV), C-OH (284.8 eV), and C (C=O)-OH (288.5eV), respectively. Figure6d,f shows the Si 2p regions of CNT-KH550

and GO-CNT, respectively, especially the binding energy of Si-O-C at 1031 eV. Therefore, Figurethere were5. XRD a lotpatterns of Si-O-C of Graphite, bonds presentGO, unoxidiz on theed GO CNT, surface CNT, afterCNT-KH550 the CNTs and had GO-CNT. been grafted.

FigureFigure 6. 6. XPSXPS spectra spectra of GO, CNT, CNT-KH550CNT-KH550 andand CNT-GO, CNT-GO, ( a()a and) and (b ()b XPS) XPS wide wide spectrum, spectrum, (c )(c C1s) C1sspectrum spectrum of CNT-KH550, of CNT-KH550, (d) Si2p(d) Si2p spectrum spectrum of CNT-KH550, of CNT-KH550, (e) C1s(e) C1s spectrum spectrum of GO-CNT, of GO-CNT, (f) Si2p(f) Si2p spectrum of GO-CNT. (color figure can be viewed on line). spectrum of GO-CNT. (color ﬁgure can be viewed on line).

3.2. Morphology and Dispersion of GO-CNT Aerogel and GO-CNTA/PS Nanocomposites The microstructure of the GO-CNT aerogel and GO-CNTA/PS nanocomposite were

investigated via FE-SEM imaging, as shown in Figure7. Figure7a shows the GO-CNT aerogel images with different magniﬁcation, and Figure7b exhibits nanocomposite fractured surface microstructures with different magniﬁcation. In Figure7a, it can be seen that GO-CNT had a uniform 3D network, and the pores were interconnected with each other. However, the aerogel wall can be seen in the composite fractured surface as shown in Figure7b1. The bright streaking is the wall of the aerogel, which was made of CNTs Polymers 2021, 13, 735 7 of 12

3.2. Morphology and Dispersion of GO-CNT Aerogel and GO-CNTA/PS Nanocomposites The microstructure of the GO-CNT aerogel and GO-CNTA/PS nanocomposite were investigated via FE-SEM imaging, as shown in Figure 7. Figure 7a shows the GO-CNT aerogel images with different magnification, and Figure 7b exhibits nanocomposite frac- Polymers 2021, 13, 735 tured surface microstructures with different magnification. In Figure 7a, it can be seen7 that of 11 GO-CNT had a uniform 3D network, and the pores were interconnected with each other. However, the aerogel wall can be seen in the composite fractured surface as shown in Figure 7b1. The bright streaking is the wall of the aerogel, which was made of CNTs bonded to GO sheets. Seen Seen from from Figure Figure 77b2,b2, thethe redred dotteddotted lineslines showshow thethe wallswalls ofof thethe aerogel, and and the the uniform uniform parts parts between between the the walls walls are are PS. PS. Figure Figure 7b37b3 shows shows the the GO GO aerogel aero- wallgel wall uniformly uniformly coated coated with with CNTs; CNTs; it can it canbe clearly be clearly seen seen that thatthe CNTs the CNTs were were only only on the on surfacethe surface of GO of GOand anddid didnot penetrate not penetrate the GO the sheet. GO sheet. A cross-section A cross-section of GO-CNTA/PS of GO-CNTA/PS (Fig- ure(Figure 7b1)7 b1)indicatesindicates that thatthe matrix the matrix and reinfo and reinforcementrcement in the in nanocomposites the nanocomposites appear appear alternately,alternately, indicating indicating that thatPS completely PS completely filled ﬁlled the pores the pores in the in aerogel. the aerogel.

Figure 7. SEM images of (a1–a3) CNT-GO aerogelsaerogels ((b1b1––b3b3)) thethe fracturedfractured surfacessurfaces ofof thethe CNT-GOA/PS.CNT-GOA/PS.

Figure8 8 shows shows compression compression curves curves after after two two successive successive compressions compressions of theof the columnar colum- naraerogel aerogel by 40%by 40% of itsof its height. height. It canIt can be be seen seen that that it it bounced bounced back back naturally naturally afterafter thethe ﬁrstfirst compression with with a a compression compression strength strength of of0.26 0.26 MPa, MPa, and and maintained maintained a strength a strength of 0.19 of Polymers 2021, 13, 735 8 of 12 MPa0.19 MPaafter after two twosuccessive successive compressions. compressions. This This is isdue due toto the the enhancement enhancement of of the the aerogel through the GO and CNTs beingbeing linkedlinked together.together.

3.3. Mechanical Properties of the Nanocomposites We explored the mechanical properties of pristine PS, GOC/PS, and GO-CNTA/PS using tensile, flexural, compressive, and impact measurements. Table 2 summarizes the various parameters for tensile strength, tensile strain, flexural strength, flexural strain, compressive strength, compressive strain, and impact strength. As shown in the stress– strain curves of the nanocomposites in Figure 9, we determined that the tensile, impact, and compressive strength values of the GO-CNTA/PS nanocomposites were improved by varying degrees compared with pristine PS: the tensile strength was improved by 9.01%, and that the bending strength was improved by 46.88%. Hence, the distribution of the CNTs in the GO aerogel and PS filling the aerogel pores had a direct effect on the mechanical properties of the nanocomposites.

FigureFigure 8. 8. Stress-strainStress-strain curves curves of of compression compression for for GO-CNT GO-CNT aerogel. aerogel.

3.3. MechanicalAs shown Propertiesin Figure of10, the it Nanocompositescan be seen that the electrical and thermal conductivity valuesWe of exploredGO-CNTA/PS the mechanical were 0.156 properties ms/m and of 0.128 pristine W/mK, PS, GOC/PS,respectively. and Compared GO-CNTA/PS with pureusing PS tensile, with no ﬂexural, electrical compressive, conductivity, and th impacte thermal measurements. conductivity Tableincreased2 summarizes by 60%. This the is because the excellent electrical and thermal conductivity were enhanced by the CNTs being connected and evenly dispersed. However, the electrical and thermal conductivity values of GO-CNTA/PS were lower than those of GOC/PS. The possible reasons are as follows: (1) In the process of hot pressing, the structure of 3D aerogel may be damaged to a certain extent, resulting in lower electrical and thermal conductivity properties of the GO-CNTA/PS composite than that of the GOC/PS composite. (2) The grafting of carbon nanotubes onto the GO surface resulted in a decrease in the amount of CNTs between the GO sheets on the aerogel wall, and therefore the electrical and thermal conductivity of the GO-CNTA/PS composites was lower than that of the GOC/PS composites. The electrical and thermal conductivity of the composites is expected to be better if the CNT were doped in GO sheets. It also provides a reference and idea for the design of new composite materials.

Table 2. Mechanical properties of different composites.

Tensile Flexural Impact Compressive Tensile Strain Flexural Compressive Sample Strength Strength Strength Strength (%) Strain (%) Strain (%) (MPa) (MPa) (KJ/m2) (MPa) PS 8.29 0.858 18.74 0.95 2.09 33.45 7.56 GOC/PS 6.8 0.785 22.18 0.79 1.96 38.72 6.08 GO-CNTA/PS 9.037 1.074 27.525 1.405 2.168 53.441 10.853

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various parameters for tensile strength, tensile strain, flexural strength, flexural strain, compressive strength, compressive strain, and impact strength. As shown in the stress– strain curves of the nanocomposites in Figure9, we determined that the tensile, impact, and compressive strength values of the GO-CNTA/PS nanocomposites were improved by varying degrees compared with pristine PS: the tensile strength was improved by 9.01%, and that the bending strength was improved by 46.88%. Hence, the distribution of the CNTs in the GO aerogel and PS filling the aerogel pores had a direct effect on the mechanical properties of the nanocomposites.

Table 2. Mechanical properties of different composites.

Flexural Impact Compressive Tensile Strength Tensile Strain Flexural Strength Strength Compressive Sample (MPa) (%) Strength Strain (%) Strain (%) (MPa) (KJ/m2) (MPa) PS 8.29 0.858 18.74 0.95 2.09 33.45 7.56 GOC/PS 6.8 0.785 22.18 0.79 1.96 38.72 6.08 Polymers 2021, 13, GO-CNTA/PS735 9.037 1.074 27.525 1.405 2.168 53.4419 of 12 10.853

Figure 9. MechanicalFigure 9. properties Mechanical of PS,properties GOC/PS, of PS, and GOC/PS, GO-CNTA/PS: and GO-CNTA/PS: (a) typical stress-strain (a) typical curves,stress-strain (b) stress-strain curves, curves of ﬂexural, (c)(b impact) stress-strain strength, curves (d) stress-strain of flexural, curves(c) impact of compression. strength, (d) (colorstress-strain ﬁgure cancurves be viewedof compression. online). (color figure can be viewed online). As shown in Figure 10, it can be seen that the electrical and thermal conductivity values of GO-CNTA/PS were 0.156 ms/m and 0.128 W/mK, respectively. Compared with pure PS with no electrical conductivity, the thermal conductivity increased by 60%. This is because the excellent electrical and thermal conductivity were enhanced by the CNTs being connected and evenly dispersed. However, the electrical and thermal conductivity values of GO-CNTA/PS were lower than those of GOC/PS. The possible reasons are as follows: (1) In the process of hot pressing, the structure of 3D aerogel may be damaged to a certain extent, resulting in lower electrical and thermal conductivity properties of the GO-CNTA/PS composite than that of the GOC/PS composite. (2) The grafting of carbon nanotubes onto the GO surface resulted in a decrease in the amount of CNTs between the GO sheets on the

Figure 10. Electrical and thermal conductivity of neat PS, GOCA/PS and GO-CNTA/PS.

The thermal properties of the nanocomposites were also analyzed. Figure 11 shows the differential scanning calorimetry curves for the different samples. It can be seen that the glass transition temperature of GO-CNTA/PS was the highest, which was due to the physical crosslinking effect in the complete GO-CNT network in the nanocomposite. However, the 3D network structure for GO-CNT was incomplete and thus not able to improve the glass transition temperature of the nanocomposites. In addition, the crystal- lization tendency of polystyrene was enhanced by the induction of GO, and the crystalli- zation temperature of GOCA/PS and GO-CNTA/PS composites was lower than that of pure polystyrene.

Polymers 2021, 13, 735 9 of 12

Polymers 2021, 13, 735 9 of 11

aerogel wall, and therefore the electrical and thermal conductivity of the GO-CNTA/PS compositesFigure 9. Mechanical was lower properties than that of PS, of GOC/PS, the GOC/PS and GO-CNTA/PS: composites. (a The) typical electrical stress-strain and thermal curves, conductivity(b) stress-strain of curves the composites of flexural, is (c expected) impact strength, to be better (d) stress-strain if the CNT curves were doped of compression. in GO sheets. It(color also figure provides can be a referenceviewed online). and idea for the design of new composite materials.

Figure 10. Electrical and thermal conductivity of neat PS, GOCA/PS and and GO-CNTA/PS. GO-CNTA/PS.

The thermal properties of the nanocompositesnanocomposites were alsoalso analyzed.analyzed. Figure 1111 showsshows the differential scanning calorimetry curves forfor the different samples. It can bebe seenseen thatthat the glass glass transition transition temperature temperature of of GO-CNTA/ GO-CNTA/PSPS was was the the highest, highest, which which was was due due to the to thephysical physical crosslinking crosslinking effect effect in inthe the complete complete GO-CNT GO-CNT network network in in the the nanocomposite. However, the 3D network structure for GO-CNT was incomplete and thus not able toto improve the glass transition temperature of the nanocomposites. In addition, the crystal- lization tendency of polystyrene was enhanced by the induction of GO, and the crystal- lization tendency of polystyrene was enhanced by the induction of GO, and the crystalli- Polymers 2021, 13, 735 lization temperature of GOCA/PS and GO-CNTA/PS composites was lower than that10 of of12 zation temperature of GOCA/PS and GO-CNTA/PS composites was lower than that of pure polystyrene. pure polystyrene.

Figure 11. DSC curves of PS, GOC/PS and and GO-CNTA/PS. GO-CNTA/PS.

4. Conclusions To thethe bestbest ofof our knowledge, testing of standard splines has been rarely reported inin the literature. In this work, we demonstrated a facile preparation method to fabricate a the literature. In this work, we demonstrated a facile preparation method to fabricate a GO-CNT aerogel and GO-CNTA/PS nanocomposite on a large-scale and implemented standard spline cutting and testing. The surface energy of GO was reduced by surface modification, and the compatibility between inorganic materials and polymers was increased, thus improving the mechanical properties of the nanocomposites. The results show that the GO-CNT aerogel framework has high compression performance: the tensile, flexural, impact, and compressive strength of the obtained GO-CNTA/PS nanocomposites were significantly stronger than pristine PS, with increases of 9.01%, 46.8%, 3.73%, and 59.8%, respectively. Moreover, the thermal conductivity of the GO-CNTA/PS nanocomposites showed an increase of 60% to 0.128 W/m∙K compared to pristine PS. To sum up, the innovative points of this work are as follows: (1) large-scale GO-CNT aerogel was prepared, (2) large-scale GO-CNTA/PS composite sheet was prepared via in situ polymerization, (3) realized the mechanical test of the standard sample.

Author Contributions: Conceptualization, G.S.; Data curation, Z.Z. and L.Z.; Software, L.M.; V., G.Z.; Writing—original draft, Y.S. and H.X.; Writing—review & editing, X.L. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the project of Natural Science Foundation of China (NO. 51903129, 51803102), China postdoctoral Science Foundation (No. 2017M612196), Natural Science Foundation of Shandong Province (No. 2014ZRB01840, 201807070028), Qingdao Postdoctoral Sci- entific Research Foundation, and Industry and Education Cooperation Program of The Ministry of Education (No. 201802230009, NO. 201901078008). 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. Acknowledgments: This work was supported by the project of Natural Science Foundation of China (NO. 51903129, 51803102), China postdoctoral Science Foundation (No. 2017M612196), Nat- ural Science Foundation of Shandong Province (No. 2014ZRB01840, 201807070028), Qingdao Post- doctoral Scientific Research Foundation, and Industry and Education Cooperation Program of The Ministry of Education (No. 201802230009, NO. 201901078008).

Polymers 2021, 13, 735 10 of 11

GO-CNT aerogel and GO-CNTA/PS nanocomposite on a large-scale and implemented standard spline cutting and testing. The surface energy of GO was reduced by surface modification, and the compatibility between inorganic materials and polymers was increased, thus improving the mechanical properties of the nanocomposites. The results show that the GO-CNT aerogel framework has high compression performance: the tensile, flexural, impact, and compressive strength of the obtained GO-CNTA/PS nanocomposites were significantly stronger than pristine PS, with increases of 9.01%, 46.8%, 3.73%, and 59.8%, respectively. Moreover, the thermal conductivity of the GO-CNTA/PS nanocomposites showed an increase of 60% to 0.128 W/m·K compared to pristine PS. To sum up, the innovative points of this work are as follows: (1) large-scale GO-CNT aerogel was prepared, (2) large-scale GO-CNTA/PS composite sheet was prepared via in situ polymerization, (3) realized the mechanical test of the standard sample.

Author Contributions: Conceptualization, G.S.; Data curation, Z.Z. and L.Z.; Software, L.M.; G.Z.; Writing—original draft, Y.S. and H.X.; Writing—review & editing, X.L. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the project of Natural Science Foundation of China (NO. 51903129, 51803102), China postdoctoral Science Foundation (No. 2017M612196), Natural Science Foundation of Shandong Province (No. 2014ZRB01840, 201807070028), Qingdao Postdoctoral Sci- entific Research Foundation, and Industry and Education Cooperation Program of The Ministry of Education (No. 201802230009, NO. 201901078008). 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. Acknowledgments: This work was supported by the project of Natural Science Foundation of China (NO. 51903129, 51803102), China postdoctoral Science Foundation (No. 2017M612196), Natural Science Foundation of Shandong Province (No. 2014ZRB01840, 201807070028), Qingdao Postdoctoral Scientific Research Foundation, and Industry and Education Cooperation Program of The Ministry of Education (No. 201802230009, NO. 201901078008). Conflicts of Interest: The authors declare no conflict of interest.

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