Evaluating the effect of addition of nanodiamond on the synergistic effect of graphene-carbon nanotube hybrid on the mechanical properties of epoxy based composites

Journal Pre-proof Evaluating the effect of addition of nanodiamond on the synergistic effect of graphene-carbon nanotube hybrid on the mechanical properties of epoxy based composites Ankita Bisht, Kinshuk Dasgupta, Debrupa Lahiri PII:

S0142-9418(19)31348-0

DOI:

https://doi.org/10.1016/j.polymertesting.2019.106274

Reference:

POTE 106274

To appear in:

Polymer Testing

Received Date: 5 August 2019 Revised Date:

26 November 2019

Accepted Date: 1 December 2019

Please cite this article as: A. Bisht, K. Dasgupta, D. Lahiri, Evaluating the effect of addition of nanodiamond on the synergistic effect of graphene-carbon nanotube hybrid on the mechanical properties of epoxy based composites, Polymer Testing (2020), doi: https://doi.org/10.1016/ j.polymertesting.2019.106274. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Ankita Bisht: Investigation, Writing - Original Draft, Writing - Review & Editing Kinshuk Dasgupta: Validation, Funding acquisition Debrupa Lahiri: Conceptualization, Supervision, Writing - Review & Editing

Evaluating the effect of addition of nanodiamond on the synergistic effect of graphene-carbon nanotube hybrid on the mechanical properties of epoxy based composites Ankita Bishta, Kinshuk Dasguptab, Debrupa Lahiria* a

Biomaterials and Multiscale Mechanics Lab,

Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Uttarakhand 247667 (INDIA) b

Materials Group,

Bhabha Atomic Research Centre Mumbai 400085 (INDIA) *Corresponding author. Tel: +91-1332-28-5137. E-mail address: [email protected] (D. Lahiri).

Abstract Addition of carbon nanotubes (CNT) to Graphene (Gr) is seen to have synergistic effect as reinforcement to polymer matrix. This is possible as CNTs inhibit stacking of Gr sheets, thus providing larger surface area nanophase to get bonded with polymer matrix and providing mechanical support through load sharing and crack growth inhibition. However, tube like morphology and high aspect ratio of CNT often lead to entanglement, which restricts their effect in exfoliating Gr. The aim of the present study is to investigate the potential of ND in improving the synergistic effect of Gr-CNT hybrid as a reinforcement to epoxy matrix. This study utilizes the power of ultrasonication technique, which is very simple and scalable, for dispersing and incorporating nanofillers into epoxy matrix. Addition of ND to Gr-CNT epoxy composite improved the tensile strength from ~ 46% with 0.5wt% (75Gr:25ND) to ~51% with 0.8wt% (25Gr:25CNT:50ND) as compared to neat epoxy. While the fracture toughness improved from

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~140% with 0.5wt% (25Gr:75CNT) to 165% with 0.8wt% (25Gr:50CNT:25ND). Fractured surfaces of composites revealed improved dispersion and strong interfacial interaction with addition of ND to Gr-CNT hybrid. NDs attaches to the surface of Gr inhibit the stacking of Gr sheets by restricting π-π stabilization. NDs also help in bridging the ends of CNTs together into long chains, thereby increasing the aspect ratio of the fiber like reinforcement. This increases the total available surface area of CNTs and Gr, to interact with epoxy matrix, improves the overall efficiency of Gr-CNT hybrid as a reinforcement, resulting into improvement in mechanical properties of the composite structure. Keywords: Epoxy, Hybrid fillers, Synergistic effect, Mechanical properties 1. Introduction Polymer matrix composites (PMCs) are the most emerging materials for wide variety of engineering applications, including aerospace, automobile, marine, defense, sports, electronics, packaging etc., due to their high specific strength and stiffness, corrosion resistance and ease of fabrication. A wide variety of reinforcements have been used and reported in literature to cater for these applications, like, nanocellulose, boron nitride nanofillers, carbon nanotubes, graphene, carbon fibers, glass fibers, Kevlar, Aramids and many more [1-10]. However, the demand for development of PMCs for these applications can only be met by using reinforcement having high strength and stiffness. Incorporation of carbon nanofillers such as carbon nanofibers (CNFs), carbon nanotubes (CNTs), nanodiamonds (NDs) and graphene (Gr) can enhance the mechanical, thermal, tribological and electrical properties of polymer matrix composites [11-13] and especially epoxy [14-17] at very low content due to their exceptional properties. However, the major challenge encountered with carbon nanofillers is the poor dispersion, which restricts their potential as a reinforcement in polymer matrix.

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With the successful segregation of free single layer Gr from graphite in 2004, by micromechanical cleavage [18], the focus of researchers has been attracted towards Gr. Gr is an atomically thick, two-dimensional (2-D) sheet composed of sp2 carbon atoms arranged in a honeycomb structure having extraordinary mechanical properties [19]. Compared to other carbon nanofillers, Gr is the most preferred reinforcement for polymer matrix [12,20] specifically for structural applications, as it improves properties along with maintaining the light weight of the structure. The large surface area offered by the 2D structure of Gr allows larger area to come in contact with polymer. This allows better stress transfer between Gr and polymer. However, stacking of Gr sheets due to large van der Waals forces and π-π interaction between the sheets restricts its potential as reinforcement [21,22]. So in order to overcome this problem and achieve high yield of exfoliation, lot of different techniques have been used in the past [23]. The most common of which include functionalization [24], wet chemistry [23] and decorating the surface of Gr with nanoparticles [25-29]. Each one of these techniques have their own setbacks which include severe damage to the structure of Gr due to harsh chemicals and conditions of functionalization [24]; incomplete vaporization of solvents due to relatively high boiling point, high surface tension and strong molecular interactions [30] also sonication when done for long time reduces the size of Gr sheet due to oxidation [23] in wet chemistry; poor interfacial bonding or the compatibility due to mismatch in property and chemistry of foreign nanoparticle decorated on Gr surface with matrix [25-29] hence preferred mostly for applications, like, coatings, energy storage devices. In recent past, few studies have come up with a new approach to combine 2D Gr sheets with 1D CNTs [22, 31-35]. CNTs are suitable for this purpose due to their unique attributes [36-40]. These studies shows that the addition of CNTs to Gr inhibits their stacking by bridging the close

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by Gr sheets. CNTs are 1D tube like structure that have extremely good mechanical, electrical and thermal properties. In last two decades, the potential of CNTs as nano fillers has been evaluated for wide variety of matrices in various applications [39]. CNTs show a remarkable improvement in mechanical and physical properties of polymer, because of their high aspect ratio, high specific surface area, outstanding strength and elasticity. The similar graphitic structure, remarkable properties and compatibility to polymer matrix as Gr makes this approach superior to the rest. Apart from this a few studies are available in literature that investigated the synergy of graphene oxide (GO)-CNT hybrid epoxy composite on mechanical, thermal and tribological properties [41-43]. Various techniques ranging from three roll mill [32], ultrasonication [31], mechanical stirring [33,35] to growing CNTs on Gr surface through CVD [34] were applied for achieving highly exfoliated entanglement free 3D network of Gr-CNT hybrid. All the above mentioned studies have used the Gr-CNT hybrid reinforcement content ranging from 0.5 wt% - 1 wt%. According to literature, as compared to Gr, the reinforcement of Gr-CNT hybrid to epoxy has been reported to higher improvement in tensile properties [31, 34], flexural properties [32] and fracture toughness [32]. The CNTs along with Gr form 3D structure, which inhibits face to face stacking of Gr, thus increasing the surface area between reinforcement and matrix. However, the high aspect ratio, torturous tube like morphology and the strong van der Waals forces between CNTs tend to entangle them, which restricts their effect in exfoliating Gr and as a reinforcement even after using different exfoliating techniques. While on the other hand in CVD technique, once the CNTs are grown on Gr surface, Gr can’t be exfoliated further, also upscaling this process for mass production is not feasible.

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Hence, none of the above mentioned methods could provide a noteworthy improvement in properties. The study that reported highest improvement in tensile properties did not do the fracture toughness studies and vice versa [31-35]. With epoxy being a brittle material investigating fracture toughness becomes very important. Even, the improvement achieved in mechanical properties are at two very different ratios. Thus, a ratio that gives the best combination of tensile properties and fracture toughness is yet to be explored. The improvement in tensile properties and fracture toughness achieved so far is not much significant. As tensile and fracture toughness are two very important properties in structural applications. So a way or a technique needs to be explored to further improve these properties and realize the potential of Gr-CNT hybrid while maintaining its 3D structure and integrity. As per the author’s knowledge, no study is available in open literature that utilizes the potential of ND in improving the synergistic effect of Gr-CNT hybrid as reinforcement in composite structures. So, its potential as a reinforcement to a polymer matrix or any other bulk composites for structural application is yet to be explored. The capacity of ND in improving synergistic effect of Gr-CNT hybrid and there after its potential as a reinforcement to epoxy matrix utilizing the power of ultrasonication technique, which is very simple and scalable, is explored in this study. In depth TEM and SEM analysis of fillers at different stages of exfoliation, with different combinations of nanofillers are investigated. Once the desired exfoliation was achieved, the composite was fabricated and tested for tensile properties and fracture toughness. The fractured surfaces of the tensile samples were then investigated through SEM to get insight of dispersion, interfacial interaction of fillers with matrix and thus validating the differential mechanical properties across compositions. 2. Materials and methods

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2.1 Materials Epoxy monomer Epofine LY5052 (1,4-butanediol diglycidyl ether) and hardener HY5052 (cycloaliphatic amine) were procured from Fine Finish Organics Pvt. Ltd., Mumbai, India. Graphene nanoplatelets (Gr), having purity more than 95%, surface area of 120-150 m2/g and average lateral size of 15 µm, were procured from XG Sciences, USA. Nanodiamonds (NDs), having purity more than 97% and average size of < 10 nm, were procured from Sigma Aldrich, India. Multiwalled Carbon Nanotube (CNT), having more than 95% purity, outer diameter of 4070 nm, length of 1-3 µm and density of 2.1 gm/cm3, was procured from Inframat Corporation (Willington, CT). Acetone (A1060) used for dispersion of nanofillers was purchased from Rankem, India.

2.2 Methods 2.2.1 Synthesis of nanocomposite Gr, CNT and ND were individually dispersed in acetone using probe sonicator (PKS-750F, PCI Analytics, Mumbai, India), at an amplitude of 70% for 60 min, with 5 seconds ON and 3 seconds OFF cycle and temperature ≤ 10⁰C (maintained in ice bath). To achieve uniform dispersion, 0.01gm of filler/20 ml of acetone was taken for sonication. For Gr-CNT hybrid, one half of the homogeneously sonicated CNT solution was added to Gr solution, followed with sonication for 10 min after the other half of the solution is poured. While, for Gr-CNT-ND hybrid, the homogeneously sonicated ND solution was equally divided into two halves. The one half of the ND solution was mixed into Gr solution in parts, while the other half was mixed into CNT solution in parts, followed with sonication for 10 min after each time the solution is poured. Finally, both the binary solutions were then mixed together and sonicated for another 20 min till

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the homogeneous solution is prepared. After the fillers were homogeneously dispersed in each case, epoxy was poured into the solution in three equal parts with intermittent mixing. Care was taken to confirm proper dispersion of viscous epoxy solution by sonicating the solution for 10 min in between each part of epoxy is added. Sonication was continued for around 20 min after all the epoxy is poured. The solution was then kept over hot plate magnetic stirrer at 70⁰C, for 6-7 hours, rotating at 400 rpm, to remove all the acetone completely. After the acetone is removed completely, the hardener is added to epoxy in the ratio of epoxy: hardener as 100:38 parts by weight. The mixture (resin) is then subjected to vacuum to remove the trapped air and reduce porosity. It is finally poured into silicon rubber molds and left for room temperature curing.

2.3 Characterization methods 2.3.1 Transmission electron microscopy (TEM) After the nanofillers are homogeneously dispersed and exfoliated in acetone, prior to epoxy addition, a drop of the solution is suspended on Cu coated Transmission Electron Microscopy (TEM) grid placed on Teflon sheet and left to dry. The TEM grid is then exposed to Transmission Electron Microscope (TEM, Jeol, JEM 3200FS, USA) operating at 300 kV to verify morphology, dispersability and arrangement nanofillers in unary, binary and ternary combinations. 2.3.2 Field emission microscopy (FESEM) Once the nanofillers are homogeneously dispersed and exfoliated in acetone post each step, prior to epoxy addition, a drop of the solution is suspended on carbon tape and left to air dry. The carbon tapes were then exposed to Field Emission Scanning Microscope (FE-SEM, Carl-Zeiss

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Ultra Plus) operated at 20 kV operating voltage to confirm the morphology, dispersability and arrangement of nanofillers in unary, binary and ternary combinations. The fractured surfaces of composites were also investigated for defects (porosity, cracks etc.), interfacial interaction, active strengthening and toughening mechanism in composites. All samples were sputter coated with gold before observing in FE-SEM.

2.3.3 FT-IR spectroscopic analysis The changes in chemical structures of epoxy with the addition of nanofillers were investigated using the FT-IR (Cary 630, Agilent Technologies, USA). Epoxy and nanofiller reinforced epoxy composite powder was mixed with the KBr powder, pressed into pellets that were subjected to FT-IR analysis. The FT-IR spectra captured for each sample is shown in figure S1. 2.3.4 Tensile test Dog bone samples were made for tensile test according to ASTM D638 (V) standard for nanofiller reinforced epoxy composite. The samples were tested at a strain rate of 1 mm/min using H25K-S UTM from Tinius Olsen Testing Machine Company, USA. The stress-strain curves of the tensile test specimens were used for determination of tensile strength and % strain (at break). Five samples for each composition were tested at a temperature of 27 ⁰C for the repeatability of the results and the average of these quantities, with standard deviation, were reported. Statistical analysis was performed for ultimate tensile strength (UTS) using GraphPad Prism 5.04 (GraphPad Software, San Diego, CA, USA) for the data obtained from tensile tests. For statistical analysis, one-way ANOVA followed by Bonferroni: compare all pair of columns was used. The level of statistical significance was set at p < 0.05. 2.3.5 Fracture toughness

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Single etched notched beam (SENB) samples were tested using 3-point bend test set-up at a strain rate of 1 mm/min using H25K-S UTM from Tinius Olsen Testing Machine Company, USA. Force- displacement curve was used to determine fracture toughness of pure epoxy and composites. ASTM D5045-99 was used for calculating fracture toughness. Five samples for each composition were tested at a temperature of 27 ⁰C for the repeatability of the results and the average of these quantities, with standard deviation, were reported.

3 Result and discussion The unary composites, with only Gr/CNT/ND in epoxy matrix, were evaluated for 0, 0.1, 0.2, 0.3, 0.5 & 1 wt% reinforcement content. The composites were tested for mechanical properties and optimized for the composition, which gave best combination of properties. For binary composite of Gr-CNT, two compositions were selected, i.e. the highest value out of the individual best (optimized unary composition) and the sum of individual best of Gr/CNT-epoxy composite. Both compositions were tested for 5 different ratios of Gr:CNT i.e. 100:0, 75:25, 50:50, 25:75, 0:100. Similarly, for ternary combination two compositions were selected i.e. the sum of individual best (optimized composition) of Gr/CNT/ND-epoxy composite and the other composition is the one that offered best combination of properties for binary composite. Both the composition were tested for 4 different ratio of Gr:CNT:ND, namely, 25:25:50, 25:50:25, 50:25:25 and 33:33:33. 3.1 Morphology of Gr, CNT, ND and their hybrids The TEM morphology of Gr, CNT, ND and their hybrids are shown in figure 1. The TEM image in figure 1(a) shows that the morphology of Gr is in the form of stacked wrinkled nanoplatelets, as evidenced from the opaque dark black colour. The nanosheet has dimensions of ~ 15 µm along the length and ~ 10 µm along the width. Figure 1(b) reveals tube like morphology of CNT 9

with outer diameter of ~ 40 nm. They are found entangled together, due to their high aspect ratio. The stacking of Gr nanosheets and the entanglement of CNTs reduces the overall exposed surface area [32, 44]. Thus, the total interface area with the polymer matrix is reduced in the composite structure. The entanglements also lead to improper impregnation of graphene in these regions. The forces responsible for the stacking, as well as, entanglement are weak. Altogether, this make them less effective as reinforcement for strengthening and toughening, due to slipping and untangling during application of stress. Hence, to improve exfoliation and prevent the restacking of Gr nanosheets during exfoliation the sonicated solution of CNTs were added to Gr solution during sonication. The improved exfoliation of Gr with the addition of CNT can be evidenced in figure 1c. Similar observations were also reported in other studies [31-35]. CNTs attach to the surface of Gr sheets preventing them from restacking. However, as mentioned the CNTs tend to entangle due to the tube like morphology [44], reducing the total number of CNTs interacting with the Gr sheets. In order to overcome this drawback and to further improve the exfoliation, NDs were added to Gr-CNT hybrid. The TEM analysis (figure 1d) of the ND revealed the spherical morphology and the size of individual ND, which is ~10 nm. However due to the small size and high surface energy, NDs tends to cluster in a group of 10-15 in number. The addition of ND to the presynthesized Gr-CNT mixture did not lead to good exfoliation as shown in figure S2. Hence, all three nanofillers i.e. Gr, CNT and ND were exfoliated separately. Then, half of ND solution was added to Gr and the other half to CNT and sonicated. The TEM image of these binary combinations are presented in figure 1(e & f). Addition of ND improved exfoliation of Gr sheets. The Gr sheets, which appear opaque and black due to stacking of multiple sheets (Figure 1(a)) before addition of ND, are found to be quite transparent after the addition of ND figure 1(e). The transparency indicates less number of graphene sheets stacked in the structure. The surface of Gr sheets is also seen decorated with ND, which restricted the π–π stabilization and hence inhibited stacking of Gr sheets. In case of ND-CNT mixture, the NDs cluster and attach 10

themselves to the ends of CNTs bridging them together into long chains (figure 1(f)). This prevents the entanglement of CNTs to some extent, thus increasing the overall aspect ratio in the hybrid structure and thus, efficiency as reinforcement. Gr-ND and CNT-ND solutions are then added together and sonicated. The exfoliation achieved for nanofillers (figure 1g) was better compared to what was achieved in figure S2(a). The morphology, exfoliation and placement of nanofillers for unary, binary and ternary combinations were further verified through SEM images (figure 2(a-g)). For unary nanofillers, the SEM images (figure 2(a, b & c)) reaffirms the size and stacking of Gr sheets, diameter and entanglement of CNTs as well as shape and clustering of ND. SEM images for binary combinations of nanofillers confirms the improved exfoliation of Gr sheets, with the addition of CNTs and NDs (figure 2(d & e)), which were similar to that reported in literature [32-34,45,46]. A comparison of Gr, Gr-CNT and Gr-ND images (figure 2 (a, d & e)), clearly shows that the Gr sheets shows better exfoliation with addition of ND. Graphene sheets appear thick, torn, folded due to multiple sheets of different sizes stacked randomly one over other in figure 1a. They transform to, thin, less crumpled sheets with CNTs uniformly distributed over the entire surface (figure 1c) and extremely thin ones with no traces of broken or folded sheets on the surface with ND covering the entire surface uniformly (figure 1e) [32, 46]. The exfoliation achieved with ND is much better than that with CNTs, due to entanglement in the latter reducing their effective number to restricting the stacking of Gr. This is evidenced by the presence of only few CNTs on Gr surface and more clustered on the surrounding area (figure 2(d)). Figure 2(f) confirms the bridging of CNTs by ND clusters, preventing the entanglement of CNTs. This was further confirmed by TGA studies (figure S3). The SEM image for ternary combination of Gr-CNT-ND (by method adopted for this study) reveals improved exfoliation (figure 2g), as compared to all binary combinations, as well as, the ternary combination achieved by adding ND to the premixed Gr-CNT combination (figure S4). Therefore, both TEM and SEM analysis confirms that

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the addition of ND improves the exfoliation of the binary combination of Gr-CNT, increasing its efficiency as a reinforcement. 3.2 Fracture Surface Analysis of epoxy composite reinforced with Gr, CNT, ND and their hybrids The fractured surfaces of composites were investigated through SEM to study the dispersion of nanofillers and the strengthening effect imparted as a result (figure 3). At 0.3 wt% Gr addition, the fractured surface of composite shows agglomeration at few places (figure 3(a)), while CNT and ND reveal good dispersion and compatibility with the epoxy matrix (Figure 3(b & c)). The agglomeration of Gr, even though is not that severe, may still act as a site for failure, when subjected to stress due stress concentration and weak interface [17-47]. With increase in reinforcement content further from 0.3wt% to 0.5wt%, the size and severity of agglomeration becomes detrimental in case of Gr, with only one end embedded in epoxy matrix and rest suspended out (figure 3(d)). Unlike Gr, the agglomerate size is quite small and completely coated with epoxy in case of CNT and ND, making it less prone to failure (figure 3(e & f)). Researchers have reported similar observations while comparing Gr and CNT as reinforcement to epoxy matrix [34, 48]. Figure 3(g-i) shows the fractured surface morphology for various combination of Gr-CNT hybrid composite. As compared to Gr/CNT-epoxy composite, the Gr-CNT hybrid epoxy composite showed better dispersion and interaction with the epoxy matrix even at higher reinforcement content i.e. at 0.5 wt% content. However, the interaction, noted here is mainly physical in nature and not chemical. This has been inferred from the FTIR spectroscopic analysis of the composite structures (figure S1 in supplementary document), which does not show any prominent peak, denoting the presence of extra chemical bond or major modification in the structure of epoxy. The addition of CNT to Gr led to improved dispersion, as, CNTs bridged the Gr sheets, thus preventing their restacking and thereby reducing entanglement of CNTs. The presence of uniformly dispersed Gr-CNT lead to strong interaction with the matrix, which can be witnessed 12

by the sharp edges of the Gr sheets and the ends of CNTs protruding from all the three combinations. However, still the sharp edges of Gr sheets at some places can be seen devoid of epoxy, especially with the combinations having high Gr ratio. To further improve the dispersion, ND was added to Gr-CNT Hybrid. As already mentioned in the previous section, instead of adding complete ND to Gr-CNT hybrid. It was divided in two equal halves, each of which was separately added to Gr and CNT. Both the solutions were sonicated separately, then mixed together and further sonicated. Comparative analysis of SEM images in figure 3(j-m) show that the dispersion and the interfacial interaction with the matrix for ternary reinforcement was much better even at high content of 0.8wt%. In fact, the dispersion in this case was even much better than binary (figure (g-i)) and unary reinforcement (figure (a-f)) at even lower content. No traces of agglomeration can be seen on the fractured surface for all combinations with ternary reinforcement. The fracture surface morphology of composite with ternary reinforcement looks very similar. However, the addition of ND to Gr-CNT has changed the morphology of fractured surface significantly. Sharp edges and wrinkles on the 2D Gr sheets, which are predominant on the fracture surface of Gr-epoxy composite (figure 3 a & d) diminishes in Gr-CNT epoxy composite (figure g-i), subsided considerably in presence of NDs. Attachment of NDs on the surface of Gr not only helps in inhibiting the stacking of Gr sheets by restricting π-π stabilization, but also knits the CNTs into a network, thereby reducing the entanglement. This increases the number of CNTs available for interaction, which further inhibits the stacking of Gr and improves the efficiency of CNTs as a reinforcement. The improved number and interaction of CNTs and ND on the Gr surface and edges prevent the Gr features to be reflected evidently on fractured surface of composite. This proves that the interaction between the Gr-CNT is improved both quantitatively and qualitatively with the addition of ND. 3.3 Mechanical Behavior of Composites 3.3.1 Tensile Properties 13

Tensile properties were studied to understand the effect of improved exfoliation of nanofillers with binary and ternary combinations on epoxy matrix. The representative stress strain plots for Gr-epoxy, CNT-epoxy and ND-epoxy for 5 different compositions of 0.1, 0.2, 0.3, 0.5 and 1wt% are shown in figure S5. Further, the stress strain plots for Gr-CNT-epoxy and Gr-CNT-NDepoxy composite for different compositions are shown in figure 4 (a, b, c & d). The quantitatively values of UTS and % strain for different compositions and combinations, calculated from the stress strain plots and from ANOVA test, are presented in table 1. The highest improvement in UTS was ~25% and ~ 18% with 0.2 wt% Gr and 0.3 wt% CNT, above which it reduces and becomes less than that for pure epoxy with ≥ 0.5 wt % Gr or CNT-epoxy composite. Others have also reported decrease in mechanical properties at higher content of Gr and CNT, due to agglomeration [49]. The SEM micrographs in figures 3(a, b, d & e) confirm the same. The micrographs clearly show that with 0.3wt% Gr the agglomeration is not that evident (figure 3a). However, with 0.5 wt% Gr, the agglomeration becomes severe with multiple Gr sheets settled at one place with their edges dangling out, having no traces of epoxy on the surface (figure 3d). This clearly indicates weak interface between Gr and epoxy matrix at high content, thereby leading to failure of composite at considerably low load. On the other hand, figure (3b & S6) shows no agglomeration with 0.3 wt% CNT. The CNTs can be seen properly dispersed throughout the matrix. The ones which were aligned along the direction of applied load protruding out of the fractured surface as marked by an arrow, while the one encircled is aligned in the transverse direction have their surface exposed, as seen in the inset in figure 3b. Agglomeration starts with increase in CNTs content to 0.5 wt%. But it is not as severe as with 0.5wt% Gr, as the CNT clusters can be seen coated with epoxy. The agglomerated CNTs act as a site for stress concentration and crack generation, which leads to premature failure and low UTS. The highest UTS value was recorded with 0.2 wt% Gr and 0.3 wt% CNT. So, two compositions were selected for Gr-CNT hybrid composite as following: the highest value out of the individual best i.e. 0.3 wt% and; sum of individual best i.e. 0.5 wt%. These two compositions were tested for 5 different combinations of Gr:CNT i.e. 100:0, 75:25, 50:50, 25:75 and 0:100, the representative stress strain plots for which is shown in figure 4(a & b)). The UTS was higher for 14

all combinations of Gr:CNT epoxy composite as compared to epoxy matrix and Gr/CNT-epoxy composite. The maximum improvement achieved in UTS was ~ 42% and ~ 60% in 0.5 wt% (75 Gr: 25CNT), as compared to pure epoxy and 0.5 wt% Gr, respectively. The improvement in tensile strength obtained, in present study, is better than that reported in literature that used the similar synthesis technique [31]. The fractured surface of Gr-CNT hybrid does not show any traces of severe agglomeration with 0.3 wt % or 0.5 wt % reinforcement content (figure 3(g-i)). This reveals that the CNTs have the potential to improve the exfoliation of Gr. However, the combination with high Gr content (figure 3g) at few places shows small edges of pulled out Gr. For further improving the synergistic effect of Gr-CNT hybrid, ND was added. The tube like morphology of CNT, having high aspect, tend to cause entanglement. This leads to reduction in the overall aspect ratio, surface area and efficiency of CNT in exfoliating Gr, as well as, reinforcement. The Gr-CNT-ND hybrid were tested for two compositions, i.e., 0.5wt% and 0.8 wt% for 4 different combinations of Gr:CNT:ND i.e. 25:25:50, 25:50:25, 50:25:25 and 33:33:33. The highest improvement in UTS was ~34% with 0.5wt% (33Gr:33CNT:33ND) as compared to pure epoxy. However, the improvement is not as much as expected and it is less than that obtained with Gr-CNT hybrid. As seen from the table 1 that combinations of Gr-CNT-ND having higher ND ratio had higher UTS. This means that the amount of ND is not enough to achieve the desired exfoliation leading to the morphology similar to figure S4, where Gr is not properly exfoliated and ND bridged CNTs are seen sitting at very few places. As a result, epoxy matrix has Gr and ND bridged CNTs acting as two different reinforcement phases, which is further confirmed through DMA studies (figure S7). To overcome this drawback, the reinforcement content was raised from 0.5 wt% to 0.8 wt% for the same ratios of nanofillers. This was decided as the total reinforcement content being the sum of individual best (optimized composition) of Gr, CNT and ND-epoxy composites, i.e., 0.2 wt%, 0.3 wt% and 0.3 wt%, respectively. The highest improvement in UTS was obtained with 0.8 wt% (25Gr:25CNT:50ND) to be ~51% and ~20%, respectively, as compared to pure epoxy and 0.5 wt% (25Gr:25CNT:50ND). The higher value of UTS for all combinations with increase in reinforcement content from 0.5 wt% to 0.8 wt% proves the potential of ND in improving 15

exfoliation of Gr and reducing entanglement of CNTs. The fracture surface morphology in figure 3(j-m) shows very uniform dispersion, with no trace of agglomeration or free lying Gr, CNT or ND. These observations suggest strong interfacial interaction and no discontinuity or stress concentration. The fractured surface morphology for ternary reinforcement has more resemblance to ND epoxy composite, rather than wave or ripple like morphology of Gr epoxy composite formed due to the sharp edges of Gr. As, due to the good exfoliation of Gr sheets, its surface as well as the edges are completely covered with ND and ND bridged CNT which is then effectively wrapped in epoxy, making their presence less evident on fractured surface. Similarly, the ends of CNTs, which were earlier seen protruding out of the fractured surface throughout the matrix, are rarely seen due to the ends of CNTs bridged by NDs. This uniformly dispersed well connected network of Gr-CNT-ND, having strong interfacial interaction with epoxy, is the key to efficient load transfer from matrix to reinforcement. Any discontinuity, whether agglomeration (stress concentration or crack generation) or weak interface, will lead to inefficient load transfer or premature failure when loaded externally. Along with the improvement in strength, the simultaneous improvement in % strain of epoxy matrix is recorded with the addition of ND. The spherical morphology of NDs is believed to play the key role. When, a tensile stress is applied to a polymer composite, sliding of the polymer chains starts. The motion/rotation of NDs, further, facilitates this process, thus increasing failure strain or ductility [50]. 3.3.2 Fracture Toughness The composites were also evaluated for fracture toughness, as it is another very important mechanical property that needs to be evaluated for structural applications, especially with brittle epoxy matrix. The presence of reinforcement not only changes the fracture toughness of matrix, but its morphology affects the mode of fracture and the active toughening mechanism [17, 45, 49]. Thus, fracture toughness is the property, which is affected the most by the extent of dispersion as well as the morphology of reinforcement. The fracture toughness values, calculated for unary composition from 3-point bend test, are shown in figure S8, and in figure 5(a & b) for 16

binary and ternary compositions. The fracture toughness values for all compositions and combinations are higher than that for pure epoxy and the % improvement in fracture toughness w.r.t epoxy is presented in table 1. The increase in fracture toughness is due to the presence of nanofillers, which changes the mode of fracture by impeding the path of crack. In case of epoxy, the crack once initiated propagates through the sample up till fracture, without getting deflected, bridged or pinned down by any impenetrable obstacle [17,45,49]. However, with the addition of nanofillers, the crack front encounters a series of impenetrable obstacle in the form of Gr, CNT or ND [17,45,49]. In case of nanofiller reinforced epoxy composite, the toughening is caused by matrix deformation, interfacial debonding, nanofiller rupture, crack deflection, crack bridging or crack pinning [17]. The crack is deflected, bridged or pinned due to interaction with nanofillers, which creates the requirement of more energy for further propagation of the crack. A crack may even get fully suppressed after encountering multiple of such occasions or it may finally find the path of least resistance and propagate causing failure. The increase the fracture toughness of epoxy matrix is noted as ~ 101%, 154%, 140% and 165% for 0.3 wt% Gr, 0.5 wt% CNT, 0.5wt% (25Gr:75CNT) and 0.8wt% (25Gr:50CNT:25ND), respectively, which was higher than most of the studies reported [20, 32, 49]. Higher filler content for Gr-epoxy (0.5 wt%) and Gr-CNT (1 wt%) composites lead to reduction in fracture toughness, due to agglomeration of reinforcement phases, as seen from figure 3(d & e). Agglomerated nanofillers act as sites for crack generation, due to stress concentration. Further, agglomeration reduces the effective spatial distribution of reinforcement phases, leading to reduction in number of probable sites for crack deflection. In addition, weak interface between matrix and agglomerate allows the crack to easily propagate through the interface. All of these result into deterioration of overall fracture toughness of the composite structure. Addition of CNTs improved the fracture toughness of Gr epoxy composite. The improvement was ~ 31% and ~ 9%, as compared to 0.5 wt% Gr and 0.3 wt% (25Gr:75CNT), respectively, with 0.5 wt% (25Gr:75CNT), which was higher than that reported by Chatterjee et al. [32]. The 17

CNTs gets attached to the surface of Gr, preventing the restacking of Gr during exfoliation. To improve the dispersion further, specifically at higher filler concentration, ND was added to GrCNT epoxy composite. With 0.5 wt% filler content, the Gr-CNT-ND epoxy composite showed higher fracture toughness than epoxy but lower than Gr-CNT epoxy composite. The quantity of ND was not sufficient to achieve the desired exfoliation, as confirmed from tensile test as well as through DMA studies (Figure S7). Hence, the total reinforcement content was increased from 0.5 wt% to 0.8 wt%. The improvement in fracture toughness was found quite significant with increase in content to 0.8 wt% (25Gr:50CNT:25ND). The resulting improvement was ~ 40% and ~ 44%, as compared to 0.5 wt% (25Gr:75CNT) and 0.5 wt% Gr. The improvement achieved was the highest, out of all the compositions so far. This proves the potential of ND in achieving better synergy of Gr-CNT hybrid at higher reinforcement content and the key is better exfoliation and dispersion. Conclusion In this study, a well exfoliated, close knitted 3D network of Gr-CNT-ND reinforcement was achieved by addition of ND to Gr-CNT hybrid. TEM, as well as SEM analysis confirmed the role played by ND in improving exfoliation of Gr by inhibiting the stacking of Gr sheets and preventing the entanglement of CNTs by acting as a bridge, which connects CNTs end to end. Addition of ND to Gr-CNT epoxy composite improved the tensile strength from ~46% with 0.5 wt% (75Gr:25ND) and ~51% with 0.8 wt% (25Gr:25CNT:50ND), as compared to neat epoxy. While, fracture toughness is improved by ~140% with 0.5 wt% (25Gr:75CNT) and 165% with 0.8 wt% (25Gr:50CNT:25ND). The fractured surfaces of composites, fabricated with Gr-CNTND reinforcement, revealed better dispersion and strong interfacial interaction as compared to Gr-CNT reinforcement. Thus, the present study establishes the potential of ND in improving the synergistic effect of Gr-CNT hybrid and thereafter discovers its prospect as a reinforcement in bulk composite for structural application. 18

Notes The authors declare no competing financial interest

ACKNOWLEDGMENTS DL acknowledges the financial support from Board of Research in Nuclear Sciences (BRNS), India (36(2)/14/17/2016-BRNS) for carrying out this research. The authors also wish to thank the laboratory staffs from the Department of Metallurgical and Materials Engineering, IIT Roorkee, for maintaining the experimental facilities.

Data Availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data forms part of an ongoing study.

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Figure 1 TEM images of (a) Gr, (b) CNT, (c) Gr-CNT, (d) ND, (e) Gr-ND, (f) CNT-ND and (g) Gr-CNT-ND nanofillers Figure 2 SEM images of (a) Gr, (b) CNT, (c) ND, (d) Gr-CNT, (e) Gr-ND, (f) CNT-ND and (g) Gr-CNT-ND nanofillers

24

Figure 3 FE-SEM images for fractured surface of (a) 0.3wt% Gr, (b) 0.3wt% CNT, (c) 0.3wt% ND, (d) 0.5wt% Gr, (e) 0.5wt% CNT, (f) 0.5wt% ND, (g) 0.5wt % (75Gr:25CNT), (h) 0.5wt% (50Gr:50CNT), (i) 0.5wt % (25Gr:75CNT), (j) 0.8wt% (50Gr:25CNT:25ND), (k) 0.8wt% (25Gr:50CNT:25ND), (l) 0.8wt% (25Gr:25CNT:50ND) and (m) 0.8wt% (33Gr:33CNT:33ND) epoxy composite Figure 4 Representative stress-strain plots for various combinations of (a) 0.3 wt% Gr-CNT, (b) 0.5 wt% Gr-CNT, (c) 0.5wt% Gr-CNT-ND and (d) 0.8wt% Gr-CNT-ND epoxy composite Figure 5 Fracture toughness plots for various compositions and combinations of (a) Gr-CNT, (b) Gr-CNT-ND epoxy composite Table 1. Ultimate tensile strength, % strain, % improvement in Fracture Toughness for various composition and combination of Gr-CNT and Gr-CNT-ND epoxy composites

Improvemen t in KIC (%)

42.5 ± 1.2

3.05 ± 0.2

-

-

100Gr:0CNT 75Gr:25CNT 50Gr:50CNT 25Gr:75CNT 0Gr:100CNT 100Gr:0CNT 75Gr:25CNT 50Gr:50CNT 25Gr:75CNT

48.7 ± 1.1 53.8 ± 1.2 43.8 ± 1.6 54.9 ± 1.5 49.9 ± 1.5 38.9 ± 3.0 62.1 ± 0.8 59.2 ± 0.6 60.0 ± 0.8

2.72 ± 0.1 3.02 ± 0.2 2.65 ± 0.1 2.56 ± 0.1 2.61 ± 0.1 2.36 ± 0.1 3.28 ± 0.2 3.06 ± 0.2 3.10 ± 0.1

101 111 117 123 128 86 123 128 140

0Gr:100CNT

39.5 ± 3.0

2.42 ± 0.2

154

∗∗ ∗∗∗∗ ns ∗∗∗∗ ∗∗∗ ns ∗∗∗∗ ∗∗∗∗ ∗∗∗∗ ns

0.5 wt%

0.3 wt%

Epoxy

∗∗ ∗ ∗∗ ∗ ∗∗∗∗ ∗∗∗∗ ∗∗∗∗ ns

25Gr:75CNT

50Gr:50CNT

75Gr:25CNT

Statistical Analysis One-way Anova [Post Test : Bonferroni multiple comparison test] at confidence level P < 0.5 100Gr:0CNT

% strain

Epoxy

Composition

UTS (MPa)

∗∗∗ ∗∗ ns

∗∗∗ ∗∗

∗∗

ns ns ∗∗∗∗

ns ∗∗∗∗

∗∗∗∗

25

2.47 ± 0.1 3.18 ± 0.2 3.06 ± 0.2 3.62 ± 0.2 3.95 ± 0.2 3.52 ± 0.2 4.72 ± 0.2 4.38 ± 0.2

80 91 78 87 155 165 148 158

ns ns ns ∗∗∗∗ ∗∗∗∗ ∗∗

50Gr:25CNT:25ND

∗∗∗ ∗∗∗ ∗∗∗ ∗∗∗∗ ∗∗∗∗ ∗∗∗∗ ∗∗∗∗ ∗∗∗∗

25Gr:50CNT:25ND

25Gr:25CNT:50ND

53.5 ± 1.5 51.0 ± 1.5 52.2 ± 2.0 56.8 ± 2.0 64.2 ± 1.0 54.2 ± 0.8 55.9 ± 0.8 59.8 ± 1.0

Epoxy 0.5 wt% 0.8 wt%

25Gr:25CNT:50ND 25Gr:50CNT:25ND 50Gr:25CNT:25ND 33Gr:33CNT:33ND 25Gr:25CNT:50ND 25Gr:50CNT:25ND 50Gr:25CNT:25ND 33Gr:33CNT:33ND

ns ∗

ns

ns ∗∗∗

∗∗

Figure 1

26

Figure 2

27

Figure 3

28

Figure 4 29

Figure 5

30

ND addition to Gr-CNT hybrid help achieve close knitted 3D network of reinforcement ND improved exfoliation of Gr sheets by acting as barrier and preventing restacking ND prevented entanglement of CNTs by acting as a bridge connecting CNTs end to end Mechanical properties of epoxy improved significantly with Gr-CNT-ND reinforcement