- Email: [email protected]
epoxy composites
Composites Part B 143 (2018) 105–112
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Investigation of the mechanical and thermal properties of L-glutathione modified graphene/epoxy composites
T
Suman Chhetria,b, Nitai Chandra Adaka,b, Pranab Samantaa,b, Naresh Chandra Murmua,b, David Huic, Tapas Kuilaa,b,∗, Joong Hee Leed,e,∗∗ a
Surface Engineering and Tribology Division, Council of Scientific and Industrial Research-Central Mechanical Engineering Research Institute, Durgapur 713209, India Academy of Scientific and Innovative Research (AcSIR), CSIR-CMERI, Campus, Durgapur 713209, India c Department of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, USA d Advanced Materials Institute of BIN Convergence Technology (BK Plus Global) & Dept. of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk 54896, South Korea e Carbon Composite Research Centre, Department of Polymer & Nano Science and Technology, Chonbuk National University, Jeonju, Jeonbuk 54896, South Korea b
A R T I C L E I N F O
A B S T R A C T
Keywords: Layered structures Thermosetting resin Fracture toughness Thermomechanical Thermal analysis
Nacre-like graphene nanosheets (GNS) obtained from the L-glutathione mediated reduction of graphene oxide (GO) were used to develop epoxy composites. Field emission scanning electron microscopy (FE-SEM) revealed the layer-by-layer nacre-like structure of GNS. Fourier transform infrared spectra (FT-IR), X-ray diffraction (XRD), Raman spectroscopy, and thermogravimetric analysis (TGA) measurements confirmed the successful reduction of GO. The oxidized product of L-glutathione is expected to perform as capping agent to stabilize the GNS, and also stitches the graphene sheets through hydrogen bonding. Transmission electron microscopy was used to confirm the dispersion of GNS in the epoxy matrix. The GNS/epoxy composites showed significant improvement of ∼91% in fracture toughness (KIC), 46% in flexural strength, and 71% in flexural modulus at 0.25 wt% GNS loadings. The probable toughening mechanism was elucidated from fracture FE-SEM images. The improved compatibility and strong interfacial interaction were reflected in the enhanced storage modulus value. The thermal stability of the composites as investigated by TGA showed appreciable improvement in the degradation temperature.
1. Introduction Discovery of graphene brought about a tectonic shift in many fields of research, on account of its unique blend of properties, which include high specific surface area, mechanical properties, and excellent electrical and thermal conductivity [1–11]. These unique properties make graphene a potential filler, especially in the area of polymer composites, which demands scalable production of high quality single-to-a-fewlayer graphene. Among the widely studied polymers, epoxy resin has been explored more as a model system, due to the suitability of its excellent mechanical and thermal properties for structural applications [12–14]. Despite of its superior mechanical, thermal, and chemical resistance properties, epoxy resin is susceptible to crack, and shows low fracture toughness. One of the approaches to augment the fracture toughness is to improve the energy absorption mechanism by
incorporating a second phase, and significant improvement in fracture toughness and other properties have been achieved by the incorporation of filler, such as rubber particles, organoclays, and silica nanoparticles [15–20]. Of late, graphene has attracted enormous interest as reinforcing components to improve the fracture properties of epoxy composites [21,22]. The majority of current studies have followed the liquid phase exfoliation method to prepare scalable graphene, which includes oxidation of graphite to graphene oxide (GO), followed by reduction [13,14,23,24]. However, the prepared graphene is inevitably prone to agglomeration, and is poorly dispersed in organic solvent [25]. Further, in order to partially reinstate the graphitic structure, toxic reagents are being used to reduce GO. Graphene can also be prepared from the direct exfoliation of graphite through sonication, but graphene prepared from such a direct method cannot withstand agglomeration, either [25]. The
∗ Corresponding author. Surface Engineering and Tribology Division, Council of Scientific and Industrial Research-Central Mechanical Engineering Research Institute, Durgapur 713209, India. ∗∗ Corresponding author. Advanced Materials Institute of BIN Convergence Technology (BK Plus Global) & Dept. of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk 54896, South Korea. E-mail addresses: [email protected] (T. Kuila), [email protected] (J.H. Lee).
https://doi.org/10.1016/j.compositesb.2018.02.004 Received 13 January 2018; Accepted 1 February 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
Composites Part B 143 (2018) 105–112
S. Chhetri et al.
triethylenetetramine (Trade name Lapox AH-713) were purchased from Atul Ltd., Gujarat, India. The viscosity of the epoxy resin is 5 Pa s at 30 °C, and its epoxy equivalent weight is (185–192) gm eq−1. The number average molecular weight (Mn) of epoxy resin is 374 g. Natural graphite flakes were purchased from Sigma-Aldrich. N, N-dimethylformamide (DMF), hydrochloric acid, sulphuric acid, potassium permanganate, and hydrogen peroxide were purchased from Merck, Mumbai, India. L-glutathione was procured from Sigma Aldrich. All the reagents were of analytical grade, and were used as received, without further purification.
only solution to impede agglomeration is alteration of the surface energies of graphene, either by functionalization, or by using surfactants for stabilization. Generally, two methods of functionalization are reported: Covalent, and non-covalent. While covalent functionalization of graphene may improve the interfacial interactions and compatibility with the polymer matrix, it also leads to the generation of defects on the graphene surface, which ultimately may deteriorate its reinforcing competency. But while in the case of non-covalent functionalization the structural integrity and hence extraordinary properties of graphene are preserved, the huge amount of modifier used to stabilize graphene negatively affects the mechanical and thermal properties. Even though surfactant-assisted stabilization of graphene sheets impedes the restacking of layers, the large amount of surfactants required is detrimental to the mechanical and electrical properties [26]. Moreover, the use of surfactants is considered adverse in the field of polymer composites. All these disadvantages reinforce the need to search for a novel organic moiety that can concurrently reduce GO, and stabilize the reduced GO (rGO). Most prior published reports have either used surfactant or stabilizer to stabilize the graphene exfoliated from graphite, or to modify thermally exfoliated graphene. For example, Parviz et al. used pyrene derivatives as stabilizers for pristine graphene, and using these highly stabilized graphene, the authors prepared conducting epoxy composites [27]. Further, the authors revealed that the SDBS treated graphene were not compatible with epoxy matrix, likely due to the phase separation during the composites preparation. Wajid et al. reported two different processing techniques, viz: solution mixing and freeze-drying for the incorporation of polyvinyl pyrrolidon (PVP) stabilized pristine graphene into the epoxy matrix [28]. The composite showed ∼38% improvement in tensile strength, and ∼37% in Young's modulus at the loading of 0.46 wt%. However, no comprehensive study is available on how surfactant stabilized graphene affects the fracture property and thermal stability of the composites. Wan et al. prepared epoxy composites incorporating highly dispersed graphene via a facile surfactant-assisted process using the amphiphilic non-ionic surfactant, Triton X-100 [26]. The highly stabilized graphene induces changes in the mechanical and thermal properties of the composites. However, the impact of a robust interface on the fracture properties of the composites has not been addressed. So far, very few studies are available on how the reductant stabilized graphene affects the mechanical and thermal properties of composites. In the present work, epoxy composites have been prepared that incorporate graphene nano sheets (GNS) obtained from the reduction of GO using L-glutathione. The as-prepared GNS showed good dispersibility in dimethylformamide (DMF) and other organic solvent. Lglutathione is expected to play the dual role of reducing GO, and as a capping agent to stabilize rGO. The oxidized form of L-glutathione contains terminal carboxylic groups bearing negative charge, which is likely to generate electrostatic repulsion sufficient for stable graphene dispersion, which ultimately inhibits the nanosheets from being restacked [29]. The negative charged carboxylate could form hydrogen bond with the residual -OH polarity of carboxylic acid groups present at the edges of rGO. The layer-by-layer nacre-like structure obtained from FE-SEM endorses the probable interlayer stitching of graphene through hydrogen bonding. In reduced form, each L-glutathione is likely to release proton, which may attached to another glutathione to form glutathione disulfide. Fig. 1 shows the probable interaction mechanism of L-glutathione with GO. The flexural, fracture properties and the thermal stability were evaluated to understand the effects of the L-glutathione reduced and stabilized GNS on epoxy composites.
2.2. Preparation of GNS Graphite oxide was prepared from graphite by a modified Hummers method, as reported by our group [30]. The reduction of GO to generate GNS was carried out following the method reported elsewhere [29]. For that, ∼400 mg of graphite oxide was dispersed in distilled water, and sonicated for about 1 h. The un-exfoliated graphite oxide was removed via centrifugation at 3000 rpm for about (15–20) min. In the next step, aqueous solution containing ∼1.2 gm of L-glutathione was added to the GO dispersion under constant stirring. The solution was kept at ∼50 °C for 5 h. The conversion of brown GO dispersion to black colour aggregates confirmed the reduction. The precipitate formed was filtered and vacuum dried at 50 °C for 24 h. 2.3. Preparation of GNS/epoxy composites For epoxy composite preparation, GNS was dispersed in DMF using water bath sonication for 1 h. The GNS dispersion was added into the epoxy resin, and the suspension was further sonicated for 1 h, followed by heating on a hot plate at ∼80 °C for 1 h under reduced pressure, to remove excess solvent. The suspension was kept inside a vacuum oven at ∼80 °C for ∼16 h to remove the residual solvent. The GNS/epoxy composite was cooled to room temperature, and the desired amount of hardener was mixed by using high speed laboratory mechanical mixture at ∼1800 rpm for 4 min. The composite was then placed inside a degassing chamber for ∼30 min, to remove air-bubbles and residual solvent. The mixture was then poured into a silicon mould, and cured at room temperature for 24 h, followed by post curing at (80 and 100) °C for 2 h in each case. 3. Characterization Fourier transform infrared (FT-IR) spectra were recorded with Perkin Elmer RXI FT-IR in the frequency range of (4000–400) cm−1. Xray diffraction (XRD) of GO and GNS was carried out with PANalytical (Model X pert PRO) at a scan rate of 0.106° s−1 in the 2θ range of (10–60)°. Field emission scanning electron microscopy (FE-SEM) was carried out with ∑igma HD, Carl Zeiss, Germany. Transmission electron microscopy (TEM) was carried out using a TEM 2100 (JEOL, Japan) at 200 kV. For TEM observation of GNS/epoxy composites, the ultra-thin sample of thickness ∼70 nm was cut using Leica Ultracut UCT (Leica EMFCS, USA) ultramicrotome at room temperature. Microtomed GNS/ epoxy samples were collected over copper grids for TEM observation. The Raman spectra of the samples were obtained on a Witec alpha300 R using a laser wavelength of 532 nm. Thermogravimetric analysis (TGA) was carried out with Perkin Elmer, Diamond TG/DTA to study the thermal stability of the composites. The samples (∼5.56 mg) were heated from (40–730) °C at a heating rate of 5 °C min−1 under air atmosphere. The flexural measurements were carried out in three-point bending test according to ASTM D790 with specimen dimensions of 128 mm × 13 mm × 4 mm using a Tinius Olsen h50KS universal testing machine at 25 °C with a crosshead speed of 2 mm min−1. Dynamic mechanical analysis (DMA) was carried out with DMA 8000 Perkin Elmer in the temperature range of (30–200) °C with a heating rate of 3 °C min−1 at a constant frequency of 1 Hz at a load strain of 0.10 mm.
2. Experimental 2.1. Materials Diglycidyl ether of bisphenol-A based epoxy (Trade name Lapox-B11) and polyamide hardener composed of tall-oil fatty acids and 106
Composites Part B 143 (2018) 105–112
S. Chhetri et al.
Fig. 1. Schematic of the reduction and stabilization of GO.
increased interlayer spacing. The (002) peak of GO disappeared after reduction with L-glutathione; rather, a hump at 2θ = ∼22° with diminished intensity and considerable peak broadening was observed. The appearance of the peak at 2θ = ∼22° suggested complete exfoliation by the oxidized form of L-glutathione. Raman spectroscopy is a widely used technique for the structural analysis of carbon materials, such as diamond, graphite, graphene and GO [31,32]. Fig. 2 (c) shows the Raman spectra of GO and GNS. The presence of D band at 1349 and 1345 cm−1 for GO and GNS confirmed the lattice disorder. After reduction of GO with L-glutathione, the G peak of GNS shifted from 1589 to 1588 cm−1, thereby indicating the reinstatement of graphitic sp2conjugation. The shifting of G peak towards lower region also indicates the exfoliation of GNS [33]. The ID/IG ratio for GO was around ∼0.88, and in GNS it was ∼1.0. The ID/IG ratio indicates the amount of the in-plane sp2 domain. The increase in ID/IG ratio is an indication of the presence of structural defects due to the emergence of oxygen functionalities on the π network of graphite upon oxidation. The increased ID/IG ratio in this current study was likely due to the incomplete reduction of edge oxygen functionalities. In reduced form, L-glutathione releases proton, and generally proton has more affinity towards epoxide and hydroxyl functionalities. As the GO basal plane holds epoxide and hydroxyl groups, those oxygen functionalities are more prone to reduction. Thus, it is anticipated that in particular, edge –COOH functionalities were not completely removed, due to which defects and disorder in the graphitic structure were retained, and hence the increased sp3/sp2 ratio. The thermal stability of GO and GNS were evaluated using TGA. Fig. 3 shows the TGA results of GO and GNS under air atmosphere. Pristine GO showed weight loss in three stages. The initial mass loss started below 120 °C, which could be due to the presence of moisture and evaporation of interstitial H2O [34]. The next mass loss was observed around 150–240 °C, which is likely due to the decomposition of hydroxyl groups, and carboxyl groups leading to the liberation of CO, CO2, steam, etc. [35,36]. The third stage at around 510–600 °C can be ascribed to the decomposition of the carbon framework of GO structure. On the other hand, GNS showed higher thermal stability than GO. The initial weight loss at 165 °C may be due to the absorbed moisture owing to the hydrogen bonding interaction. As the oxygen functionalities of GO surfaces were eliminated upon reduction and functionlization, GNS showed ∼20% weight loss at 250 °C against ∼36% of GO. The TGA analysis results showed that most of the oxygen functionalities of GO were removed during the chemical reduction of GO.
The fracture toughness test were conducted on a universal tensile machine (Tinius Olsen h50KS) following ASTM D5045 in three-point end notch bending, which has the capacity of 50kN. The single-edge notched bend (SENB) sample of 50 mm length × 10 mm width × 5 mm thickness was placed in three-point bending fixture with a loading span four times the width at a cross-head speed of 1 mm min−1. For testing the specimen, a notch was first introduced using a rotating blade of thickness ∼0.60 mm. The length of the notch was 5 mm, and it was carried out so that the ratio of sample height to crack length should be maintained in the range (0.45–0.55). The mechanized notch was sharpened by sliding a sharp razor blade across the root of the notch. The average notch length (initial notch plus razor slide crack) of the SENB specimen was in the range (5.1–5.2) mm. For each composite, at least four specimens were tested. The fracture toughness, KIC (criticalstress-intensity factor) values of the materials were calculated from the equations [21]:
KIc =
p f (x ) BW 1/2
f (x ) = 6x 1/2
[1.99 − x (1 − x )(2.15 − 3.93x + 2.7x 2)] (1 − 2x )(1 − x )3/2
(1)
(2)
where, x = α/W, is an expression that explains the sample geometry, p is the maximum load in the load-displacement curve (maximum load at failure), B is the sample thickness, W is the material width (overall length), a is the average pre-crack length, and f (a/W) is associated with the sample geometry. 4. Result and discussion 4.1. Morphology and structures In order to confirm the reduction of GO, FT-IR spectroscopy was carried out, and the results are shown in Fig. 2 (a). The characteristic absorption band of GO at 1741 and 1039 cm−1 corresponds to the stretching vibration of carbonyl in -COOH moiety, and the C–O stretching in epoxide group, respectively. The band at 1635 cm−1 can be attributed to the C–C stretching of the graphitic structure. After reduction, all of the peaks associated with the oxygen functionalities of GO disappeared. Only the sharp peak at 1635 cm−1 was observed, which indicates the restoration of graphitic structure. XRD was performed to confirm the exfoliation of graphene (Fig. 2 (b)). The characteristic diffraction peak of GO appeared at 2θ = ∼10°, signifying an 107
Composites Part B 143 (2018) 105–112
S. Chhetri et al.
Fig. 2. (a) FT-IR spectra, (b) XRD pattern, and (c) Raman spectra, of GO and GNS.
Fig. 4 (a) and (b) show the cross-sectional images of GO and GNS. GO shows a loose, wrinkled layered structure (Fig. 4 (a)), whereas for GNS, a layered stacking structure like that of nacre was observed (Fig. 4 (b)). The formation of exceptionally fluffy multilayer graphene may be due to the presence of oxidized L-glutathione, which stitches the interlayer graphene through hydrogen bonding. It can be said that the interaction of oxidized L-glutathione and graphene layers leads to unique structural features, which might have served as competing reinforcing for the epoxy composites. 4.2. Dispersion of GNS in the composites In order to confirm the exfoliation and dispersion of GNS into the epoxy matrix, TEM of microtome composites sample were performed. Fig. 5(a) and (b) show the low and high resolution images of 0.25 wt% GNS/epoxy composites. The micrograph showed that GNS were firmly embedded in the epoxy matrix, which would have resulted from the improved compatibility between GNS and the epoxy matrix. Apparently, the dark regions in the TEM image are evident of aggregated or overlapped GNS. The TEM images well corroborated the good dispersion and embedment of GNS in the epoxy matrix.
Fig. 3. TGA curves of GO and GNS.
Fig. 4. FE-SEM micrographs of (a) GO, and (b) GNS.
108
Composites Part B 143 (2018) 105–112
S. Chhetri et al.
Fig. 5. (a) and (b) TEM images of GNS/epoxy composite with 0.5 wt% GNS loading.
components, and the aggregation of GNS particles. The wrinkled structure of GNS might also have played an efficient role in mechanical interlocking with the epoxy matrix, subsequently improving the mechanical properties through efficient load transfer. Fracture toughness is a vital design factor, especially for composite materials intended to be used in structural applications. The three-point bending test on pre-crack samples was performed to assess the mode I fracture toughness (KIC) of the GNS/epoxy composites. Fig. 7 (a) and (b) show the typical three-point bending load-displacement curves of pure epoxy and its composites, and the corresponding results of KIC against the weight fraction of GNS, respectively. The critical stress intensity factor, KIC, was used to assess the fracture toughness of the composites. The KIC value of pure epoxy was found to be ∼1.2 MPa m1/2, which is similar to the prior published reports for epoxy composites [13,37,38]. The KIC value of the composites at 0.25 wt% was found to be improved by ∼91% against pure epoxy. The significant improvement in KIC values could be attributed to the high specific surface area of GNS, and the
4.3. Mechanical properties The effects of GNS on the mechanical properties of GNS/epoxy composites were investigated in terms of flexural and single-edge notched bending fracture test. Fig. 6(a)–(c) show the typical flexural stressstrain, corresponding load verse displacement curve, and flexural modulus of the neat epoxy and GNS/epoxy composites with varied amount of GNS. The flexural modulus was derived from the slope of the flexural stress-strain curve. It was observed that with the addition of 0.25 wt% of GNS, the flexural strength and modulus were increased by ∼46 and 71%, respectively. The flexural strength and modulus slightly decreased at 0.5 wt% loading of GNS. The highly dispersed GNS layers might have generated strong interaction with the epoxy matrix, which contributed to efficient stress transfer at the interface, leading to high strength and modulus. However, with the higher loading of GNS, the flexural properties of the composites were found to be diminished, which could be due to the enormous contact interface between the two
Fig. 6. (a) Flexural stress vs. strain response of pure epoxy and GNS/epoxy composites, (b) Load vs. displacement curve of epoxy and GNS incorporated epoxy composites, and (c) Effect of GNS content on the flexural modulus of GNS/epoxy composites.
109
Composites Part B 143 (2018) 105–112
S. Chhetri et al.
Fig. 7. (a) Fracture toughness (KIC), and (b) Representative load-displacement curves of pure epoxy and GNS/epoxy composites at various weight fraction.
linear cracking path, and restraining the rapid proliferation of cracks. The firmly embedded GNS layers forced the propagating cracks to disseminate in a disorderly way, generating many new surfaces, thereby increasing the strain energy required. Crack branching and ditching signify the improved crack resistance. The coarse fracture texture can be linked with the deflection mechanism, where the linear fracture path is being deflected by the imbedded GNS layers. The rougher surface signifies that the usual crack path encounters rigid particles, which force the crack to follow a longer and more tortuous path, thereby inducing plastic deformation, which leads to the absorption of a large amount of energy. Thus, due to crack branching and deflection, a higher amount of energy is absorbed, which is reflected in the higher value of KIC.
robust interfacial interaction with the epoxy matrix, due to the presence of residual oxygen functionalities. 4.4. Microstructure of the GNS/epoxy composites FE-SEM was employed to study the impact of GNS on the failure morphologies of the flexural sample. The fracture surface of pure epoxy exhibited smooth river-like features, evident of low impact resistance (Fig. 8 (a)). The area between the cleavages of flat plane appeared featureless, which is evidence of the rapid proliferation of cracks in a linear manner. Incorporation of GNS produced rough and disordered fracture texture, as shown in Fig. 8 (b). The appearance of rough surface can be attributed to the presence of GNS, which acts as an obstacle to rapidly propagating cracks. The embedded GNS restrained the crack path, and forced the path to change direction, resulting in dissemination of the cracks in a disorderly way, which is an indication of improved load transfer across the interface. Further, the flaky fracture surface is also evidence of strong interfacial interaction between the filler particles and polymer matrix. Thus, the rough and flaky fracture texture corresponded well with the improved flexural strength and modulus.
4.6. Dynamic mechanical properties Fig. 10 (a) and (b) show the temperature dependence of the storage modulus (E′) and tan δ (E''/E′) of the pure epoxy, as well as the GNS/ epoxy composites. The profiles show that the GNS induced reinforcement affected the glassy phase more efficiently than the rubbery phase. The improvement in E′ may be attributed to the restriction imposed by finely dispersed GNS on molecular movement of the matrix. The glass transition temperature (Tg) of composites, derived from the peak position of the tan δ curve, was found to be lower than that of pure epoxy. Many prior studies on graphene/epoxy composites have reported the shifting of Tg towards lower temperature [38,42,43]. Introduction of nanofiller affects the molecular dynamics of the polymer matrix, which leads to change in the relaxation mode of the bulk polymer matrix. The altercation in the chain dynamics leads to the shifting and broadening of the tan δ peak (Tg), either towards lower or higher temperature. The glass transition temperature (Tg) of GNS/ epoxy composites, derived from the peak position of tan δ vs temperature, was found to be lower than that of the pure epoxy. The shifting of Tg towards lower temperature can be explained in terms of the chemical reorganization of the epoxy matrix in the immediate proximity of GNS, or in the inter-phase region. The polymer layers that
4.5. Probable fracture mechanism Fractography is a valuable tool for comprehending the fracture behaviour and toughening mechanisms of composites [39,40]. Fig. 9 (a)–(d) show FE-SEM images of the three-point bending fractured sample of pure epoxy and GNS/epoxy composites. Pure epoxy showed relatively smooth river-like features with linear proliferated cracks that are evenly aligned in the direction of crack propagation (Fig. 9 (a) and (b)). The un-branched smooth and flat fracture indicates typical brittle failure characteristics. The area between the cleavage planes appeared smooth, as shown in the high-magnification image, which signifies high propagation velocity [41]. In contrast to the pure epoxy, the GNS/ epoxy composites showed rough and disordered fracture surface (Fig. 9 (c) and (d)). This can be attributed to the presence of GNS impeding the
Fig. 8. FE-SEM images of the flexural fractured surface of (a) neat epoxy, and (b) GNS (0.25 wt %)/epoxy composite.
110
Composites Part B 143 (2018) 105–112
S. Chhetri et al.
Fig. 9. FE-SEM images of the fractured surface of mode I fracture toughness specimens for (a) and (b) pure epoxy, and (c) and (d) GNS (0.25 wt %)/epoxy composite.
form inter-phase with GNS would show different properties than those of the corresponding polymer chain present in the bulk. The polymer layers, which are firmly attached to the GNS due to the interfacial interactions, would be strongly affected by the filler layers, and it is likely that GNS would perturb the cross-linking reaction during the curing process. The interruption in the cross-link formation leads to the reduction in cross-linking density, and so the Tg of the composites. The altercation in curing kinetics due to the presence of GNS, use of DMF for GNS dispersion, and agglomeration at higher loading can be the probable reasons for the shift of Tg towards lower temperature.
4.7. Thermogravimetric analysis TGA was performed to investigate the thermal degradation behaviour of the GNS/epoxy composites. Fig. 11 shows the TGA curve of the GNS/epoxy composites. The TGA curve exhibited one-step degradation mechanism for the neat epoxy and its composites with GNS, demonstrating that the existence of GNS did not significantly alter the degradation mechanism of the epoxy matrix. First, GNS composites showed a small decrease in degradation temperature with respect to the pure epoxy. The initial decrease in degradation temperature is likely due to the residual solvent and moisture trap in the composites, residual oxygen functionalities on the surface of GNS, and defects in the polymeric network. The temperature for 10 and 50% (T-10% and T-50%)
Fig. 11. TGA curves of GNS/epoxy composites.
weight loss were selected to characterize the thermal stability of pure epoxy and its composites with GNS. The pure epoxy showed T-10% and T-50% at around 336 and 386 °C, respectively, while 0.25 wt% GNS/ epoxy composites showed at around 338 and 399 °C. The improvement
Fig. 10. (a) Variation of storage modulus (E′) with temperature for GNS epoxy composites,and(b) Variation of tan δ with temperature for GNS epoxy composites.
111
Composites Part B 143 (2018) 105–112
S. Chhetri et al.
in degradation temperature (T-10% and T-50%) may be due to the barrier property provided by GNS to oxygen molecules from inflowing into the composites, which impeded the further oxidation of epoxy. The char yield of the cured epoxy composites, as revealed by the residue weight at 650 °C, was found higher for the entire weight fraction studied against the neat epoxy. This is likely due to the optimum dispersion of GNS in the epoxy matrix generating robust interfacial interaction, which acts as an obstruction, and impedes the volatilization of polymer decompositions from being eluded outside.
[12] He Y, Li Q, Kuila T, Kim NH, Jiang T, Lau K, Lee JH. Micro-crack behavior of carbon fiber reinforced thermoplastic modified epoxy composites for cryogenic applications. Composites Part B 2013;44(1):533–9. [13] Jiang T, Kuila T, Kim NH, Lee JH. Effects of surface-modified silica nanoparticles attached graphene oxide using isocyanate terminated flexible polymer chains on themechanical properties of epoxy composites. J Mater Chem 2014;2(27):10557–67. [14] Li Q, Siddaramaiah, Kim NH, Hui D, Lee JH. Effects of dual component microcapsules of resin and curing agent on the self-healing efficiency of epoxy. Composites Part B 2013;55:79–85. [15] Balakrishnan S, Start PR, Raghavan D, Hudson SD. The influence of clay and elastomer concentration on the morphology and fracture energy of preformed acrylic rubber dispersed clay filled epoxy nanocomposites. Polymer 2005;46(25):11255–62. [16] Hsu YG, Liang CW. Properties and behavior of CTBN-modified epoxy with IPN structure. J Appl Polym Sci 2007;106(3):1576–84. [17] Qian JY, Pearson RA, Dimonie VL, El-Aasser MS. Synthesis and application of core–shell particles as toughening agents for epoxies. J Appl Polym Sci 1995;58(2):439–48. [18] Wu S, Guo Q, Peng S, Hameed N, Kraska M, Stühn B, Mai YW. Toughening epoxy thermosets with block ionomer complexes: a nanostructure mechanical property correlation. Macromolecules 2012;45(9):3829–40. [19] Rosso P, Ye L, Friedrich K, Sprenger S. A toughened epoxy resin by silica nanoparticle reinforcement. J Appl Polym Sci 2006;100(2):1849–55. [20] Raju T, Ding YM, He YL, Yang L, Paula M, Yang WM, Tibor C, Sabu T. Miscibility, morphology, thermal, and mechanical properties of a DGEBA based epoxy resin toughened with a liquid rubber. Polymer 2008;49(1):278–94. [21] Chandrasekaran S, Sato N, To¨lle F, Mu¨lhaupt R, Fiedler B, Schulte K. Fracture toughness and failure mechanism of graphene based epoxy composites. Compos Sci Technol 2014;97:90–9. [22] Zaman I, Kuan HC, Meng QS,Michelmore A, Kawashima N, Pitt T, Zhang L, Gouda S, Luong L. A facile approach to chemically modified graphene and its polymer nanocomposites. Adv Funct Mater 2012;22(13):2735–43. [23] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, Wu Y, Nguyen ST, Ruoff RS. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007;45:1558–65. [24] Kim H, Macosko CW. Morphology and properties of polyester/exfoliated graphite nanocomposites. Macromolecules 2008;41(9):3317–27. [25] Israelachvili JN. Intermolecular and surface forces. second ed. New York: Academic Press; 1991. [26] Wan YJ, Tang LC, Yan D, Zhao L, Li YB, Wu LB, Jiang JX, Lai GQ. Improved dispersion and interface in the graphene/epoxy composites via a facile surfactant-assisted process. Compos Sci Technol 2013;82:60–8. [27] Parviz D, Das S, Tanvir AHS, Irin F, Bhattacharia S, Green MJ. Dispersions of noncovalently functionalized graphene with minimal stabilizer. ACS Nano 2012;6(10):8857–67. [28] Wajid AS, Tanvir AHS, Das S, Irin F, Jankowski AF, Green M. High-performance pristine graphene/epoxy composites with enhanced mechanical and electrical properties. Macromol Mater Eng 2013;298(3):339–47. [29] Pham TA, Kim JS, Kim JS, Jeong YT. One-step reduction of graphene oxide with lglutathione. J Colloids Surf A 2011;384:543–8. [30] Kuila T, Mishra AK, Khanra P, Kim NH, Uddi Md E, Lee JH. Facile method for the preparation of water dispersible graphene using sulfonated poly(ether–ether–ketone) and its application as energy storage materials. Langmuir 2012;28(25):9825–33. [31] Livneh T, Haslett TL, Moskovits M. Distinguishing disorder induced bands from allowed Raman bands in graphite. Phys Rev B 2002;66(195110):1–11. [32] Krishnamoorthy K, Veerapandian M, Yun K, Kim SJ. The chemical and structural analysis of graphene oxide with different degrees of oxidation. Carbon 2013;53:38–49. [33] Duan BR, Cao Q. Hierarchically porous Co3O4 film prepared by hydrothermal synthesis method based on colloidal crystal template for supercapacitor application. Electrochim Acta 2012;64:154–61. [34] Zangmeister CD. Preparation and evaluation of graphite oxide reduced at 220 °C. Chem Mater 2010;22(19):5625–9. [35] Jeong HK, Jin MH, So KP, Lim SC, Lee YH. Tailoring the characteristics of graphite oxides by different oxidation times. J Phys D Appl Phys 2009;42(6):65418. [36] Stankovich, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, Wu Y, Nguyen ST, Ruoff RS. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007;45(7):1558–65. [37] Wang X, Jin J, Song M. An investigation of the mechanism of graphene toughening epoxy. Carbon 2013;6:324–33. [38] Chhetri S, Adak NC, Samanta P, Murmu NC, Kuila T. Functionalized reduced graphene oxide/epoxy composites with enhanced mechanical properties and thermal stability. Polym Test 2017;63:1–11. [39] Bortz DR, Heras EG, Martin GI. Impressive fatigue life and fracture toughness improvements in graphene oxide/epoxy composites. Macromolecules 2012;45(1):238–45. [40] Zaman I, Phan TT, Kuan HC, Meng Q, La LTB, Luong L, Youssf O, Ma J. Epoxy/ graphene platelets nanocomposites with two levels of interface strength. Polymer 2011;52(7):1603–11. [41] Wang J, Song Jin M. An investigation of the mechanism of graphene toughening epoxy. Carbon 2013;65:324–33. [42] Liu W, Koh KL, Lu J, Yang L, Phua S, Kong J, Xuehong Lu ZC. Simultaneous catalyzing and reinforcing effects of imidazole-functionalized graphene in anhydridecured epoxies. J Mater Chem 2012;22(35):18395–402. [43] Fang M, Zhen Z, Li J, Zhang H, Lu H, Yang Y. Constructing hierarchically structured interphases for strong and tough epoxy nanocomposites by amine-rich graphene surfaces. J Mater Chem 2010;20(43):9635–43.
5. Conclusions In summary, one-step reduction and stabilization of GO was carried out, using environment friendly L-glutathione. FE-SEM image revealed the compact nacre-like structure of the GNS. The flexural, fracture, and dynamic mechanical properties of the epoxy composites were evaluated to understand the effects of the L-glutathione reduced GO. It was anticipated that the oxidized product of L-glutathione would behave as a capping agent to stabilize the rGO and promote the dispersion and exfoliation of GNS in the epoxy matrix. The significant improvement in the mechanical properties like fracture toughness (KIC), flexural strength and flexural modulus were recorded as 91, 46 and 71% higher with 0.25 wt% of GNS loadings as compared to the pure epoxy corraborating the excellent reinforcing competency of GNS. The probable toughening mechanism was proposed based on the fracture structure of the composites obtained from FE-SEM. The firm attachment of GNS within the epoxy matrix through chemical interaction was reflected in the enhanced storage modulus value. The thermal stability of the composites was found to be enhanced against the neat epoxy. Acknowledgement The authors are thankful to the Director of CSIR-CMERI. The authors are also thankful to the Department of Science and Technology, New Delhi, India for the financial support (GAP-215312&GAP-211412), and the Council of Scientific and Industrial Research, New Delhi, India, for funding the Fast Track Translational Project MLP-210812. It is also supported by the X-mind Corps Program (2017H1D8A2030449) through National Research Foundation (NRF) funded by the Ministry of Science and ICT of Korea. References [1] Lee C, Wei XD, Kysar JW, Hone J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008;321(5887):385–8. [2] Stoller MD, Park SJ, Zhu YW, An JH, Ruoff RS. Graphene based ultracapacitors. Nano Lett 2008;8(10):3498–502. [3] Nair RR, Blake P, Grigorenko AN, Novoselov KS, Booth TJ, Stauber T, Peres NMR, Geim AK. Fine structure constant defines visual transparency of graphene. Science 2008;320(5881):1308. [4] Balandin AA, Ghosh S, Bao W, Calizo I, Teweldebrhan D, Miao F, Lau CN. Superior thermal conductivity of single-layer graphene. Nano Lett 2008;8(3):902–7. [5] Hung PY, Lau KT, Fox B, Hameed N, Lee JH, Hui D. Surface modification of carbon fibre using graphene related materials for multifunctional composites. Composites Part B 2018;133:240–57. [6] Layek RK, Uddin ME, Kim NH, Lau AKT, Lee JH. Noncovalent functionalization of reduced graphene oxide with pluronic F127 and its nanocomposites with gum Arabic. Composites Part B 2017;128:155–63. [7] Sharmaa K, Maiti K, Kim NH, Hui D, Lee JH. Green synthesis of glucose-reduced graphene oxide supported Ag-Cu2O nanocomposites for the enhanced visible-light photocatalytic activity. Composites Part B 2018;138:35–44. [8] Bandyopadhyay P, Nguyen TT, Li X, Kim NH, Lee JH. Enhanced hydrogen gas barrier performance of diaminoalkane functionalized stitched graphene oxide/ polyurethane composites. Composites Part B 2017;117:101–10. [9] Gopalsamy K, Balamurugan J, Thanh TD, Kim NH, Hui D, Lee JH. Surfactant-free synthesis of NiPd nanoalloy/graphene bifunctional nanocomposite for fuel cell. Composites Part B 2017;114:319–27. [10] Park WB, Bandyopadhyay P, Nguyen TT, Kuila T, Kim NH, Lee JH. Effect of high molecular weight polyethyleneimine functionalized graphene oxide coated polyethylene terephthalate film on the hydrogen gas barrier properties. Composites Part B 2016;106:316–23. [11] Liu H, Bandyopadhyay P, Kshetri T, Kim NH, Ku BC, Moon B, Lee JH. Layer-by-layer assembled polyelectrolyte-decorated graphene multilayer film for hydrogen gas barrier application. Composites Part B 2017;114:339–47.
112
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Investigation of the mechanical and thermal properties of L-glutathione modified graphene/epoxy composites
T
Suman Chhetria,b, Nitai Chandra Adaka,b, Pranab Samantaa,b, Naresh Chandra Murmua,b, David Huic, Tapas Kuilaa,b,∗, Joong Hee Leed,e,∗∗ a
Surface Engineering and Tribology Division, Council of Scientific and Industrial Research-Central Mechanical Engineering Research Institute, Durgapur 713209, India Academy of Scientific and Innovative Research (AcSIR), CSIR-CMERI, Campus, Durgapur 713209, India c Department of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, USA d Advanced Materials Institute of BIN Convergence Technology (BK Plus Global) & Dept. of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk 54896, South Korea e Carbon Composite Research Centre, Department of Polymer & Nano Science and Technology, Chonbuk National University, Jeonju, Jeonbuk 54896, South Korea b
A R T I C L E I N F O
A B S T R A C T
Keywords: Layered structures Thermosetting resin Fracture toughness Thermomechanical Thermal analysis
Nacre-like graphene nanosheets (GNS) obtained from the L-glutathione mediated reduction of graphene oxide (GO) were used to develop epoxy composites. Field emission scanning electron microscopy (FE-SEM) revealed the layer-by-layer nacre-like structure of GNS. Fourier transform infrared spectra (FT-IR), X-ray diffraction (XRD), Raman spectroscopy, and thermogravimetric analysis (TGA) measurements confirmed the successful reduction of GO. The oxidized product of L-glutathione is expected to perform as capping agent to stabilize the GNS, and also stitches the graphene sheets through hydrogen bonding. Transmission electron microscopy was used to confirm the dispersion of GNS in the epoxy matrix. The GNS/epoxy composites showed significant improvement of ∼91% in fracture toughness (KIC), 46% in flexural strength, and 71% in flexural modulus at 0.25 wt% GNS loadings. The probable toughening mechanism was elucidated from fracture FE-SEM images. The improved compatibility and strong interfacial interaction were reflected in the enhanced storage modulus value. The thermal stability of the composites as investigated by TGA showed appreciable improvement in the degradation temperature.
1. Introduction Discovery of graphene brought about a tectonic shift in many fields of research, on account of its unique blend of properties, which include high specific surface area, mechanical properties, and excellent electrical and thermal conductivity [1–11]. These unique properties make graphene a potential filler, especially in the area of polymer composites, which demands scalable production of high quality single-to-a-fewlayer graphene. Among the widely studied polymers, epoxy resin has been explored more as a model system, due to the suitability of its excellent mechanical and thermal properties for structural applications [12–14]. Despite of its superior mechanical, thermal, and chemical resistance properties, epoxy resin is susceptible to crack, and shows low fracture toughness. One of the approaches to augment the fracture toughness is to improve the energy absorption mechanism by
incorporating a second phase, and significant improvement in fracture toughness and other properties have been achieved by the incorporation of filler, such as rubber particles, organoclays, and silica nanoparticles [15–20]. Of late, graphene has attracted enormous interest as reinforcing components to improve the fracture properties of epoxy composites [21,22]. The majority of current studies have followed the liquid phase exfoliation method to prepare scalable graphene, which includes oxidation of graphite to graphene oxide (GO), followed by reduction [13,14,23,24]. However, the prepared graphene is inevitably prone to agglomeration, and is poorly dispersed in organic solvent [25]. Further, in order to partially reinstate the graphitic structure, toxic reagents are being used to reduce GO. Graphene can also be prepared from the direct exfoliation of graphite through sonication, but graphene prepared from such a direct method cannot withstand agglomeration, either [25]. The
∗ Corresponding author. Surface Engineering and Tribology Division, Council of Scientific and Industrial Research-Central Mechanical Engineering Research Institute, Durgapur 713209, India. ∗∗ Corresponding author. Advanced Materials Institute of BIN Convergence Technology (BK Plus Global) & Dept. of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk 54896, South Korea. E-mail addresses: [email protected] (T. Kuila), [email protected] (J.H. Lee).
https://doi.org/10.1016/j.compositesb.2018.02.004 Received 13 January 2018; Accepted 1 February 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
Composites Part B 143 (2018) 105–112
S. Chhetri et al.
triethylenetetramine (Trade name Lapox AH-713) were purchased from Atul Ltd., Gujarat, India. The viscosity of the epoxy resin is 5 Pa s at 30 °C, and its epoxy equivalent weight is (185–192) gm eq−1. The number average molecular weight (Mn) of epoxy resin is 374 g. Natural graphite flakes were purchased from Sigma-Aldrich. N, N-dimethylformamide (DMF), hydrochloric acid, sulphuric acid, potassium permanganate, and hydrogen peroxide were purchased from Merck, Mumbai, India. L-glutathione was procured from Sigma Aldrich. All the reagents were of analytical grade, and were used as received, without further purification.
only solution to impede agglomeration is alteration of the surface energies of graphene, either by functionalization, or by using surfactants for stabilization. Generally, two methods of functionalization are reported: Covalent, and non-covalent. While covalent functionalization of graphene may improve the interfacial interactions and compatibility with the polymer matrix, it also leads to the generation of defects on the graphene surface, which ultimately may deteriorate its reinforcing competency. But while in the case of non-covalent functionalization the structural integrity and hence extraordinary properties of graphene are preserved, the huge amount of modifier used to stabilize graphene negatively affects the mechanical and thermal properties. Even though surfactant-assisted stabilization of graphene sheets impedes the restacking of layers, the large amount of surfactants required is detrimental to the mechanical and electrical properties [26]. Moreover, the use of surfactants is considered adverse in the field of polymer composites. All these disadvantages reinforce the need to search for a novel organic moiety that can concurrently reduce GO, and stabilize the reduced GO (rGO). Most prior published reports have either used surfactant or stabilizer to stabilize the graphene exfoliated from graphite, or to modify thermally exfoliated graphene. For example, Parviz et al. used pyrene derivatives as stabilizers for pristine graphene, and using these highly stabilized graphene, the authors prepared conducting epoxy composites [27]. Further, the authors revealed that the SDBS treated graphene were not compatible with epoxy matrix, likely due to the phase separation during the composites preparation. Wajid et al. reported two different processing techniques, viz: solution mixing and freeze-drying for the incorporation of polyvinyl pyrrolidon (PVP) stabilized pristine graphene into the epoxy matrix [28]. The composite showed ∼38% improvement in tensile strength, and ∼37% in Young's modulus at the loading of 0.46 wt%. However, no comprehensive study is available on how surfactant stabilized graphene affects the fracture property and thermal stability of the composites. Wan et al. prepared epoxy composites incorporating highly dispersed graphene via a facile surfactant-assisted process using the amphiphilic non-ionic surfactant, Triton X-100 [26]. The highly stabilized graphene induces changes in the mechanical and thermal properties of the composites. However, the impact of a robust interface on the fracture properties of the composites has not been addressed. So far, very few studies are available on how the reductant stabilized graphene affects the mechanical and thermal properties of composites. In the present work, epoxy composites have been prepared that incorporate graphene nano sheets (GNS) obtained from the reduction of GO using L-glutathione. The as-prepared GNS showed good dispersibility in dimethylformamide (DMF) and other organic solvent. Lglutathione is expected to play the dual role of reducing GO, and as a capping agent to stabilize rGO. The oxidized form of L-glutathione contains terminal carboxylic groups bearing negative charge, which is likely to generate electrostatic repulsion sufficient for stable graphene dispersion, which ultimately inhibits the nanosheets from being restacked [29]. The negative charged carboxylate could form hydrogen bond with the residual -OH polarity of carboxylic acid groups present at the edges of rGO. The layer-by-layer nacre-like structure obtained from FE-SEM endorses the probable interlayer stitching of graphene through hydrogen bonding. In reduced form, each L-glutathione is likely to release proton, which may attached to another glutathione to form glutathione disulfide. Fig. 1 shows the probable interaction mechanism of L-glutathione with GO. The flexural, fracture properties and the thermal stability were evaluated to understand the effects of the L-glutathione reduced and stabilized GNS on epoxy composites.
2.2. Preparation of GNS Graphite oxide was prepared from graphite by a modified Hummers method, as reported by our group [30]. The reduction of GO to generate GNS was carried out following the method reported elsewhere [29]. For that, ∼400 mg of graphite oxide was dispersed in distilled water, and sonicated for about 1 h. The un-exfoliated graphite oxide was removed via centrifugation at 3000 rpm for about (15–20) min. In the next step, aqueous solution containing ∼1.2 gm of L-glutathione was added to the GO dispersion under constant stirring. The solution was kept at ∼50 °C for 5 h. The conversion of brown GO dispersion to black colour aggregates confirmed the reduction. The precipitate formed was filtered and vacuum dried at 50 °C for 24 h. 2.3. Preparation of GNS/epoxy composites For epoxy composite preparation, GNS was dispersed in DMF using water bath sonication for 1 h. The GNS dispersion was added into the epoxy resin, and the suspension was further sonicated for 1 h, followed by heating on a hot plate at ∼80 °C for 1 h under reduced pressure, to remove excess solvent. The suspension was kept inside a vacuum oven at ∼80 °C for ∼16 h to remove the residual solvent. The GNS/epoxy composite was cooled to room temperature, and the desired amount of hardener was mixed by using high speed laboratory mechanical mixture at ∼1800 rpm for 4 min. The composite was then placed inside a degassing chamber for ∼30 min, to remove air-bubbles and residual solvent. The mixture was then poured into a silicon mould, and cured at room temperature for 24 h, followed by post curing at (80 and 100) °C for 2 h in each case. 3. Characterization Fourier transform infrared (FT-IR) spectra were recorded with Perkin Elmer RXI FT-IR in the frequency range of (4000–400) cm−1. Xray diffraction (XRD) of GO and GNS was carried out with PANalytical (Model X pert PRO) at a scan rate of 0.106° s−1 in the 2θ range of (10–60)°. Field emission scanning electron microscopy (FE-SEM) was carried out with ∑igma HD, Carl Zeiss, Germany. Transmission electron microscopy (TEM) was carried out using a TEM 2100 (JEOL, Japan) at 200 kV. For TEM observation of GNS/epoxy composites, the ultra-thin sample of thickness ∼70 nm was cut using Leica Ultracut UCT (Leica EMFCS, USA) ultramicrotome at room temperature. Microtomed GNS/ epoxy samples were collected over copper grids for TEM observation. The Raman spectra of the samples were obtained on a Witec alpha300 R using a laser wavelength of 532 nm. Thermogravimetric analysis (TGA) was carried out with Perkin Elmer, Diamond TG/DTA to study the thermal stability of the composites. The samples (∼5.56 mg) were heated from (40–730) °C at a heating rate of 5 °C min−1 under air atmosphere. The flexural measurements were carried out in three-point bending test according to ASTM D790 with specimen dimensions of 128 mm × 13 mm × 4 mm using a Tinius Olsen h50KS universal testing machine at 25 °C with a crosshead speed of 2 mm min−1. Dynamic mechanical analysis (DMA) was carried out with DMA 8000 Perkin Elmer in the temperature range of (30–200) °C with a heating rate of 3 °C min−1 at a constant frequency of 1 Hz at a load strain of 0.10 mm.
2. Experimental 2.1. Materials Diglycidyl ether of bisphenol-A based epoxy (Trade name Lapox-B11) and polyamide hardener composed of tall-oil fatty acids and 106
Composites Part B 143 (2018) 105–112
S. Chhetri et al.
Fig. 1. Schematic of the reduction and stabilization of GO.
increased interlayer spacing. The (002) peak of GO disappeared after reduction with L-glutathione; rather, a hump at 2θ = ∼22° with diminished intensity and considerable peak broadening was observed. The appearance of the peak at 2θ = ∼22° suggested complete exfoliation by the oxidized form of L-glutathione. Raman spectroscopy is a widely used technique for the structural analysis of carbon materials, such as diamond, graphite, graphene and GO [31,32]. Fig. 2 (c) shows the Raman spectra of GO and GNS. The presence of D band at 1349 and 1345 cm−1 for GO and GNS confirmed the lattice disorder. After reduction of GO with L-glutathione, the G peak of GNS shifted from 1589 to 1588 cm−1, thereby indicating the reinstatement of graphitic sp2conjugation. The shifting of G peak towards lower region also indicates the exfoliation of GNS [33]. The ID/IG ratio for GO was around ∼0.88, and in GNS it was ∼1.0. The ID/IG ratio indicates the amount of the in-plane sp2 domain. The increase in ID/IG ratio is an indication of the presence of structural defects due to the emergence of oxygen functionalities on the π network of graphite upon oxidation. The increased ID/IG ratio in this current study was likely due to the incomplete reduction of edge oxygen functionalities. In reduced form, L-glutathione releases proton, and generally proton has more affinity towards epoxide and hydroxyl functionalities. As the GO basal plane holds epoxide and hydroxyl groups, those oxygen functionalities are more prone to reduction. Thus, it is anticipated that in particular, edge –COOH functionalities were not completely removed, due to which defects and disorder in the graphitic structure were retained, and hence the increased sp3/sp2 ratio. The thermal stability of GO and GNS were evaluated using TGA. Fig. 3 shows the TGA results of GO and GNS under air atmosphere. Pristine GO showed weight loss in three stages. The initial mass loss started below 120 °C, which could be due to the presence of moisture and evaporation of interstitial H2O [34]. The next mass loss was observed around 150–240 °C, which is likely due to the decomposition of hydroxyl groups, and carboxyl groups leading to the liberation of CO, CO2, steam, etc. [35,36]. The third stage at around 510–600 °C can be ascribed to the decomposition of the carbon framework of GO structure. On the other hand, GNS showed higher thermal stability than GO. The initial weight loss at 165 °C may be due to the absorbed moisture owing to the hydrogen bonding interaction. As the oxygen functionalities of GO surfaces were eliminated upon reduction and functionlization, GNS showed ∼20% weight loss at 250 °C against ∼36% of GO. The TGA analysis results showed that most of the oxygen functionalities of GO were removed during the chemical reduction of GO.
The fracture toughness test were conducted on a universal tensile machine (Tinius Olsen h50KS) following ASTM D5045 in three-point end notch bending, which has the capacity of 50kN. The single-edge notched bend (SENB) sample of 50 mm length × 10 mm width × 5 mm thickness was placed in three-point bending fixture with a loading span four times the width at a cross-head speed of 1 mm min−1. For testing the specimen, a notch was first introduced using a rotating blade of thickness ∼0.60 mm. The length of the notch was 5 mm, and it was carried out so that the ratio of sample height to crack length should be maintained in the range (0.45–0.55). The mechanized notch was sharpened by sliding a sharp razor blade across the root of the notch. The average notch length (initial notch plus razor slide crack) of the SENB specimen was in the range (5.1–5.2) mm. For each composite, at least four specimens were tested. The fracture toughness, KIC (criticalstress-intensity factor) values of the materials were calculated from the equations [21]:
KIc =
p f (x ) BW 1/2
f (x ) = 6x 1/2
[1.99 − x (1 − x )(2.15 − 3.93x + 2.7x 2)] (1 − 2x )(1 − x )3/2
(1)
(2)
where, x = α/W, is an expression that explains the sample geometry, p is the maximum load in the load-displacement curve (maximum load at failure), B is the sample thickness, W is the material width (overall length), a is the average pre-crack length, and f (a/W) is associated with the sample geometry. 4. Result and discussion 4.1. Morphology and structures In order to confirm the reduction of GO, FT-IR spectroscopy was carried out, and the results are shown in Fig. 2 (a). The characteristic absorption band of GO at 1741 and 1039 cm−1 corresponds to the stretching vibration of carbonyl in -COOH moiety, and the C–O stretching in epoxide group, respectively. The band at 1635 cm−1 can be attributed to the C–C stretching of the graphitic structure. After reduction, all of the peaks associated with the oxygen functionalities of GO disappeared. Only the sharp peak at 1635 cm−1 was observed, which indicates the restoration of graphitic structure. XRD was performed to confirm the exfoliation of graphene (Fig. 2 (b)). The characteristic diffraction peak of GO appeared at 2θ = ∼10°, signifying an 107
Composites Part B 143 (2018) 105–112
S. Chhetri et al.
Fig. 2. (a) FT-IR spectra, (b) XRD pattern, and (c) Raman spectra, of GO and GNS.
Fig. 4 (a) and (b) show the cross-sectional images of GO and GNS. GO shows a loose, wrinkled layered structure (Fig. 4 (a)), whereas for GNS, a layered stacking structure like that of nacre was observed (Fig. 4 (b)). The formation of exceptionally fluffy multilayer graphene may be due to the presence of oxidized L-glutathione, which stitches the interlayer graphene through hydrogen bonding. It can be said that the interaction of oxidized L-glutathione and graphene layers leads to unique structural features, which might have served as competing reinforcing for the epoxy composites. 4.2. Dispersion of GNS in the composites In order to confirm the exfoliation and dispersion of GNS into the epoxy matrix, TEM of microtome composites sample were performed. Fig. 5(a) and (b) show the low and high resolution images of 0.25 wt% GNS/epoxy composites. The micrograph showed that GNS were firmly embedded in the epoxy matrix, which would have resulted from the improved compatibility between GNS and the epoxy matrix. Apparently, the dark regions in the TEM image are evident of aggregated or overlapped GNS. The TEM images well corroborated the good dispersion and embedment of GNS in the epoxy matrix.
Fig. 3. TGA curves of GO and GNS.
Fig. 4. FE-SEM micrographs of (a) GO, and (b) GNS.
108
Composites Part B 143 (2018) 105–112
S. Chhetri et al.
Fig. 5. (a) and (b) TEM images of GNS/epoxy composite with 0.5 wt% GNS loading.
components, and the aggregation of GNS particles. The wrinkled structure of GNS might also have played an efficient role in mechanical interlocking with the epoxy matrix, subsequently improving the mechanical properties through efficient load transfer. Fracture toughness is a vital design factor, especially for composite materials intended to be used in structural applications. The three-point bending test on pre-crack samples was performed to assess the mode I fracture toughness (KIC) of the GNS/epoxy composites. Fig. 7 (a) and (b) show the typical three-point bending load-displacement curves of pure epoxy and its composites, and the corresponding results of KIC against the weight fraction of GNS, respectively. The critical stress intensity factor, KIC, was used to assess the fracture toughness of the composites. The KIC value of pure epoxy was found to be ∼1.2 MPa m1/2, which is similar to the prior published reports for epoxy composites [13,37,38]. The KIC value of the composites at 0.25 wt% was found to be improved by ∼91% against pure epoxy. The significant improvement in KIC values could be attributed to the high specific surface area of GNS, and the
4.3. Mechanical properties The effects of GNS on the mechanical properties of GNS/epoxy composites were investigated in terms of flexural and single-edge notched bending fracture test. Fig. 6(a)–(c) show the typical flexural stressstrain, corresponding load verse displacement curve, and flexural modulus of the neat epoxy and GNS/epoxy composites with varied amount of GNS. The flexural modulus was derived from the slope of the flexural stress-strain curve. It was observed that with the addition of 0.25 wt% of GNS, the flexural strength and modulus were increased by ∼46 and 71%, respectively. The flexural strength and modulus slightly decreased at 0.5 wt% loading of GNS. The highly dispersed GNS layers might have generated strong interaction with the epoxy matrix, which contributed to efficient stress transfer at the interface, leading to high strength and modulus. However, with the higher loading of GNS, the flexural properties of the composites were found to be diminished, which could be due to the enormous contact interface between the two
Fig. 6. (a) Flexural stress vs. strain response of pure epoxy and GNS/epoxy composites, (b) Load vs. displacement curve of epoxy and GNS incorporated epoxy composites, and (c) Effect of GNS content on the flexural modulus of GNS/epoxy composites.
109
Composites Part B 143 (2018) 105–112
S. Chhetri et al.
Fig. 7. (a) Fracture toughness (KIC), and (b) Representative load-displacement curves of pure epoxy and GNS/epoxy composites at various weight fraction.
linear cracking path, and restraining the rapid proliferation of cracks. The firmly embedded GNS layers forced the propagating cracks to disseminate in a disorderly way, generating many new surfaces, thereby increasing the strain energy required. Crack branching and ditching signify the improved crack resistance. The coarse fracture texture can be linked with the deflection mechanism, where the linear fracture path is being deflected by the imbedded GNS layers. The rougher surface signifies that the usual crack path encounters rigid particles, which force the crack to follow a longer and more tortuous path, thereby inducing plastic deformation, which leads to the absorption of a large amount of energy. Thus, due to crack branching and deflection, a higher amount of energy is absorbed, which is reflected in the higher value of KIC.
robust interfacial interaction with the epoxy matrix, due to the presence of residual oxygen functionalities. 4.4. Microstructure of the GNS/epoxy composites FE-SEM was employed to study the impact of GNS on the failure morphologies of the flexural sample. The fracture surface of pure epoxy exhibited smooth river-like features, evident of low impact resistance (Fig. 8 (a)). The area between the cleavages of flat plane appeared featureless, which is evidence of the rapid proliferation of cracks in a linear manner. Incorporation of GNS produced rough and disordered fracture texture, as shown in Fig. 8 (b). The appearance of rough surface can be attributed to the presence of GNS, which acts as an obstacle to rapidly propagating cracks. The embedded GNS restrained the crack path, and forced the path to change direction, resulting in dissemination of the cracks in a disorderly way, which is an indication of improved load transfer across the interface. Further, the flaky fracture surface is also evidence of strong interfacial interaction between the filler particles and polymer matrix. Thus, the rough and flaky fracture texture corresponded well with the improved flexural strength and modulus.
4.6. Dynamic mechanical properties Fig. 10 (a) and (b) show the temperature dependence of the storage modulus (E′) and tan δ (E''/E′) of the pure epoxy, as well as the GNS/ epoxy composites. The profiles show that the GNS induced reinforcement affected the glassy phase more efficiently than the rubbery phase. The improvement in E′ may be attributed to the restriction imposed by finely dispersed GNS on molecular movement of the matrix. The glass transition temperature (Tg) of composites, derived from the peak position of the tan δ curve, was found to be lower than that of pure epoxy. Many prior studies on graphene/epoxy composites have reported the shifting of Tg towards lower temperature [38,42,43]. Introduction of nanofiller affects the molecular dynamics of the polymer matrix, which leads to change in the relaxation mode of the bulk polymer matrix. The altercation in the chain dynamics leads to the shifting and broadening of the tan δ peak (Tg), either towards lower or higher temperature. The glass transition temperature (Tg) of GNS/ epoxy composites, derived from the peak position of tan δ vs temperature, was found to be lower than that of the pure epoxy. The shifting of Tg towards lower temperature can be explained in terms of the chemical reorganization of the epoxy matrix in the immediate proximity of GNS, or in the inter-phase region. The polymer layers that
4.5. Probable fracture mechanism Fractography is a valuable tool for comprehending the fracture behaviour and toughening mechanisms of composites [39,40]. Fig. 9 (a)–(d) show FE-SEM images of the three-point bending fractured sample of pure epoxy and GNS/epoxy composites. Pure epoxy showed relatively smooth river-like features with linear proliferated cracks that are evenly aligned in the direction of crack propagation (Fig. 9 (a) and (b)). The un-branched smooth and flat fracture indicates typical brittle failure characteristics. The area between the cleavage planes appeared smooth, as shown in the high-magnification image, which signifies high propagation velocity [41]. In contrast to the pure epoxy, the GNS/ epoxy composites showed rough and disordered fracture surface (Fig. 9 (c) and (d)). This can be attributed to the presence of GNS impeding the
Fig. 8. FE-SEM images of the flexural fractured surface of (a) neat epoxy, and (b) GNS (0.25 wt %)/epoxy composite.
110
Composites Part B 143 (2018) 105–112
S. Chhetri et al.
Fig. 9. FE-SEM images of the fractured surface of mode I fracture toughness specimens for (a) and (b) pure epoxy, and (c) and (d) GNS (0.25 wt %)/epoxy composite.
form inter-phase with GNS would show different properties than those of the corresponding polymer chain present in the bulk. The polymer layers, which are firmly attached to the GNS due to the interfacial interactions, would be strongly affected by the filler layers, and it is likely that GNS would perturb the cross-linking reaction during the curing process. The interruption in the cross-link formation leads to the reduction in cross-linking density, and so the Tg of the composites. The altercation in curing kinetics due to the presence of GNS, use of DMF for GNS dispersion, and agglomeration at higher loading can be the probable reasons for the shift of Tg towards lower temperature.
4.7. Thermogravimetric analysis TGA was performed to investigate the thermal degradation behaviour of the GNS/epoxy composites. Fig. 11 shows the TGA curve of the GNS/epoxy composites. The TGA curve exhibited one-step degradation mechanism for the neat epoxy and its composites with GNS, demonstrating that the existence of GNS did not significantly alter the degradation mechanism of the epoxy matrix. First, GNS composites showed a small decrease in degradation temperature with respect to the pure epoxy. The initial decrease in degradation temperature is likely due to the residual solvent and moisture trap in the composites, residual oxygen functionalities on the surface of GNS, and defects in the polymeric network. The temperature for 10 and 50% (T-10% and T-50%)
Fig. 11. TGA curves of GNS/epoxy composites.
weight loss were selected to characterize the thermal stability of pure epoxy and its composites with GNS. The pure epoxy showed T-10% and T-50% at around 336 and 386 °C, respectively, while 0.25 wt% GNS/ epoxy composites showed at around 338 and 399 °C. The improvement
Fig. 10. (a) Variation of storage modulus (E′) with temperature for GNS epoxy composites,and(b) Variation of tan δ with temperature for GNS epoxy composites.
111
Composites Part B 143 (2018) 105–112
S. Chhetri et al.
in degradation temperature (T-10% and T-50%) may be due to the barrier property provided by GNS to oxygen molecules from inflowing into the composites, which impeded the further oxidation of epoxy. The char yield of the cured epoxy composites, as revealed by the residue weight at 650 °C, was found higher for the entire weight fraction studied against the neat epoxy. This is likely due to the optimum dispersion of GNS in the epoxy matrix generating robust interfacial interaction, which acts as an obstruction, and impedes the volatilization of polymer decompositions from being eluded outside.
[12] He Y, Li Q, Kuila T, Kim NH, Jiang T, Lau K, Lee JH. Micro-crack behavior of carbon fiber reinforced thermoplastic modified epoxy composites for cryogenic applications. Composites Part B 2013;44(1):533–9. [13] Jiang T, Kuila T, Kim NH, Lee JH. Effects of surface-modified silica nanoparticles attached graphene oxide using isocyanate terminated flexible polymer chains on themechanical properties of epoxy composites. J Mater Chem 2014;2(27):10557–67. [14] Li Q, Siddaramaiah, Kim NH, Hui D, Lee JH. Effects of dual component microcapsules of resin and curing agent on the self-healing efficiency of epoxy. Composites Part B 2013;55:79–85. [15] Balakrishnan S, Start PR, Raghavan D, Hudson SD. The influence of clay and elastomer concentration on the morphology and fracture energy of preformed acrylic rubber dispersed clay filled epoxy nanocomposites. Polymer 2005;46(25):11255–62. [16] Hsu YG, Liang CW. Properties and behavior of CTBN-modified epoxy with IPN structure. J Appl Polym Sci 2007;106(3):1576–84. [17] Qian JY, Pearson RA, Dimonie VL, El-Aasser MS. Synthesis and application of core–shell particles as toughening agents for epoxies. J Appl Polym Sci 1995;58(2):439–48. [18] Wu S, Guo Q, Peng S, Hameed N, Kraska M, Stühn B, Mai YW. Toughening epoxy thermosets with block ionomer complexes: a nanostructure mechanical property correlation. Macromolecules 2012;45(9):3829–40. [19] Rosso P, Ye L, Friedrich K, Sprenger S. A toughened epoxy resin by silica nanoparticle reinforcement. J Appl Polym Sci 2006;100(2):1849–55. [20] Raju T, Ding YM, He YL, Yang L, Paula M, Yang WM, Tibor C, Sabu T. Miscibility, morphology, thermal, and mechanical properties of a DGEBA based epoxy resin toughened with a liquid rubber. Polymer 2008;49(1):278–94. [21] Chandrasekaran S, Sato N, To¨lle F, Mu¨lhaupt R, Fiedler B, Schulte K. Fracture toughness and failure mechanism of graphene based epoxy composites. Compos Sci Technol 2014;97:90–9. [22] Zaman I, Kuan HC, Meng QS,Michelmore A, Kawashima N, Pitt T, Zhang L, Gouda S, Luong L. A facile approach to chemically modified graphene and its polymer nanocomposites. Adv Funct Mater 2012;22(13):2735–43. [23] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, Wu Y, Nguyen ST, Ruoff RS. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007;45:1558–65. [24] Kim H, Macosko CW. Morphology and properties of polyester/exfoliated graphite nanocomposites. Macromolecules 2008;41(9):3317–27. [25] Israelachvili JN. Intermolecular and surface forces. second ed. New York: Academic Press; 1991. [26] Wan YJ, Tang LC, Yan D, Zhao L, Li YB, Wu LB, Jiang JX, Lai GQ. Improved dispersion and interface in the graphene/epoxy composites via a facile surfactant-assisted process. Compos Sci Technol 2013;82:60–8. [27] Parviz D, Das S, Tanvir AHS, Irin F, Bhattacharia S, Green MJ. Dispersions of noncovalently functionalized graphene with minimal stabilizer. ACS Nano 2012;6(10):8857–67. [28] Wajid AS, Tanvir AHS, Das S, Irin F, Jankowski AF, Green M. High-performance pristine graphene/epoxy composites with enhanced mechanical and electrical properties. Macromol Mater Eng 2013;298(3):339–47. [29] Pham TA, Kim JS, Kim JS, Jeong YT. One-step reduction of graphene oxide with lglutathione. J Colloids Surf A 2011;384:543–8. [30] Kuila T, Mishra AK, Khanra P, Kim NH, Uddi Md E, Lee JH. Facile method for the preparation of water dispersible graphene using sulfonated poly(ether–ether–ketone) and its application as energy storage materials. Langmuir 2012;28(25):9825–33. [31] Livneh T, Haslett TL, Moskovits M. Distinguishing disorder induced bands from allowed Raman bands in graphite. Phys Rev B 2002;66(195110):1–11. [32] Krishnamoorthy K, Veerapandian M, Yun K, Kim SJ. The chemical and structural analysis of graphene oxide with different degrees of oxidation. Carbon 2013;53:38–49. [33] Duan BR, Cao Q. Hierarchically porous Co3O4 film prepared by hydrothermal synthesis method based on colloidal crystal template for supercapacitor application. Electrochim Acta 2012;64:154–61. [34] Zangmeister CD. Preparation and evaluation of graphite oxide reduced at 220 °C. Chem Mater 2010;22(19):5625–9. [35] Jeong HK, Jin MH, So KP, Lim SC, Lee YH. Tailoring the characteristics of graphite oxides by different oxidation times. J Phys D Appl Phys 2009;42(6):65418. [36] Stankovich, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, Wu Y, Nguyen ST, Ruoff RS. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007;45(7):1558–65. [37] Wang X, Jin J, Song M. An investigation of the mechanism of graphene toughening epoxy. Carbon 2013;6:324–33. [38] Chhetri S, Adak NC, Samanta P, Murmu NC, Kuila T. Functionalized reduced graphene oxide/epoxy composites with enhanced mechanical properties and thermal stability. Polym Test 2017;63:1–11. [39] Bortz DR, Heras EG, Martin GI. Impressive fatigue life and fracture toughness improvements in graphene oxide/epoxy composites. Macromolecules 2012;45(1):238–45. [40] Zaman I, Phan TT, Kuan HC, Meng Q, La LTB, Luong L, Youssf O, Ma J. Epoxy/ graphene platelets nanocomposites with two levels of interface strength. Polymer 2011;52(7):1603–11. [41] Wang J, Song Jin M. An investigation of the mechanism of graphene toughening epoxy. Carbon 2013;65:324–33. [42] Liu W, Koh KL, Lu J, Yang L, Phua S, Kong J, Xuehong Lu ZC. Simultaneous catalyzing and reinforcing effects of imidazole-functionalized graphene in anhydridecured epoxies. J Mater Chem 2012;22(35):18395–402. [43] Fang M, Zhen Z, Li J, Zhang H, Lu H, Yang Y. Constructing hierarchically structured interphases for strong and tough epoxy nanocomposites by amine-rich graphene surfaces. J Mater Chem 2010;20(43):9635–43.
5. Conclusions In summary, one-step reduction and stabilization of GO was carried out, using environment friendly L-glutathione. FE-SEM image revealed the compact nacre-like structure of the GNS. The flexural, fracture, and dynamic mechanical properties of the epoxy composites were evaluated to understand the effects of the L-glutathione reduced GO. It was anticipated that the oxidized product of L-glutathione would behave as a capping agent to stabilize the rGO and promote the dispersion and exfoliation of GNS in the epoxy matrix. The significant improvement in the mechanical properties like fracture toughness (KIC), flexural strength and flexural modulus were recorded as 91, 46 and 71% higher with 0.25 wt% of GNS loadings as compared to the pure epoxy corraborating the excellent reinforcing competency of GNS. The probable toughening mechanism was proposed based on the fracture structure of the composites obtained from FE-SEM. The firm attachment of GNS within the epoxy matrix through chemical interaction was reflected in the enhanced storage modulus value. The thermal stability of the composites was found to be enhanced against the neat epoxy. Acknowledgement The authors are thankful to the Director of CSIR-CMERI. The authors are also thankful to the Department of Science and Technology, New Delhi, India for the financial support (GAP-215312&GAP-211412), and the Council of Scientific and Industrial Research, New Delhi, India, for funding the Fast Track Translational Project MLP-210812. It is also supported by the X-mind Corps Program (2017H1D8A2030449) through National Research Foundation (NRF) funded by the Ministry of Science and ICT of Korea. References [1] Lee C, Wei XD, Kysar JW, Hone J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008;321(5887):385–8. [2] Stoller MD, Park SJ, Zhu YW, An JH, Ruoff RS. Graphene based ultracapacitors. Nano Lett 2008;8(10):3498–502. [3] Nair RR, Blake P, Grigorenko AN, Novoselov KS, Booth TJ, Stauber T, Peres NMR, Geim AK. Fine structure constant defines visual transparency of graphene. Science 2008;320(5881):1308. [4] Balandin AA, Ghosh S, Bao W, Calizo I, Teweldebrhan D, Miao F, Lau CN. Superior thermal conductivity of single-layer graphene. Nano Lett 2008;8(3):902–7. [5] Hung PY, Lau KT, Fox B, Hameed N, Lee JH, Hui D. Surface modification of carbon fibre using graphene related materials for multifunctional composites. Composites Part B 2018;133:240–57. [6] Layek RK, Uddin ME, Kim NH, Lau AKT, Lee JH. Noncovalent functionalization of reduced graphene oxide with pluronic F127 and its nanocomposites with gum Arabic. Composites Part B 2017;128:155–63. [7] Sharmaa K, Maiti K, Kim NH, Hui D, Lee JH. Green synthesis of glucose-reduced graphene oxide supported Ag-Cu2O nanocomposites for the enhanced visible-light photocatalytic activity. Composites Part B 2018;138:35–44. [8] Bandyopadhyay P, Nguyen TT, Li X, Kim NH, Lee JH. Enhanced hydrogen gas barrier performance of diaminoalkane functionalized stitched graphene oxide/ polyurethane composites. Composites Part B 2017;117:101–10. [9] Gopalsamy K, Balamurugan J, Thanh TD, Kim NH, Hui D, Lee JH. Surfactant-free synthesis of NiPd nanoalloy/graphene bifunctional nanocomposite for fuel cell. Composites Part B 2017;114:319–27. [10] Park WB, Bandyopadhyay P, Nguyen TT, Kuila T, Kim NH, Lee JH. Effect of high molecular weight polyethyleneimine functionalized graphene oxide coated polyethylene terephthalate film on the hydrogen gas barrier properties. Composites Part B 2016;106:316–23. [11] Liu H, Bandyopadhyay P, Kshetri T, Kim NH, Ku BC, Moon B, Lee JH. Layer-by-layer assembled polyelectrolyte-decorated graphene multilayer film for hydrogen gas barrier application. Composites Part B 2017;114:339–47.
112