elastomer nanocomposites

Accepted Manuscript Graphene/Elastomer Nanocomposites Dimitrios G. Papageorgiou, Ian A. Kinloch, Robert J. Young PII:

S0008-6223(15)30172-X

DOI:

10.1016/j.carbon.2015.08.055

Reference:

CARBON 10220

To appear in:

Carbon

Received Date: 6 May 2015 Revised Date:

7 August 2015

Accepted Date: 17 August 2015

Please cite this article as: D.G. Papageorgiou, I.A. Kinloch, R.J. Young, Graphene/Elastomer Nanocomposites, Carbon (2015), doi: 10.1016/j.carbon.2015.08.055. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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Graphene/Elastomer Nanocomposites

Dimitrios G. Papageorgiou*, Ian A. Kinloch and Robert J. Young*

University of Manchester Oxford Road

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Manchester M13 9PL, UK

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School of Materials and National Graphene Institute

Abstract

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In the decade since the first isolation and identification of graphene, the scientific community is still finding ways to utilize its unique properties. The present review deals with the preparation

and

physicochemical

characterization

of

graphene-based

elastomeric

nanocomposites. The processing and characterization of graphene and graphene oxide are described in detail, since the presence of such fillers in an elastomeric matrix affects dramatically the properties of the nanocomposite samples. Several preparation routes for the

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efficient dispersion of graphene in elastomers are then discussed, while aspects such as the interfacial bonding between the filler and the matrix or interactions between the fillers have been thoroughly analysed. Different types of graphene/elastomer nanocomposites are described in terms of their manufacture and properties and it has been shown that depending

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on the type of graphene employed and the preparation methods, the mechanical, thermal, electrical and barrier properties of the elastomeric matrix can be enhanced due to the presence of graphene, even at relatively-low filler loadings. In most cases, the formation of a filler

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network can play a major role in the improvement of the overall performance of the material.

Keywords: Graphene; Elastomers; Rubber; Polymer Nanocomposites

*Corresponding Authors: e-mail: [email protected] [email protected]

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Contents 1. Introduction ...................................................................................................................... 4 2. Graphene .......................................................................................................................... 6

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2.1 Preparation Methods .................................................................................................... 7 2.1.1 Mechanical exfoliation .......................................................................................... 7 2.1.2 Liquid-phase exfoliation ........................................................................................ 7 2.1.3 Thermal exfoliation ............................................................................................... 9

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2.1.4 Chemical vapour deposition (CVD)....................................................................... 9 2.2 Characterization......................................................................................................... 10

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2.2.1 Microscopy ......................................................................................................... 10 2.2.2 X-ray diffraction ................................................................................................. 11 2.2.3 Raman spectroscopy ............................................................................................ 12 2.2.4 Ultraviolet-visible (UV-vis) spectroscopy ........................................................... 12 2.3 Properties .................................................................................................................. 14

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2.3.1 Mechanical properties ......................................................................................... 14 2.3.2 Thermal conductivity .......................................................................................... 15 2.3.3 Electrical conductivity ......................................................................................... 15

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3. Graphene Oxide .............................................................................................................. 16 3.1 Preparation Methods .................................................................................................. 16

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3.2 Characterization......................................................................................................... 17 3.3 Properties .................................................................................................................. 19

4. Nomeclature for graphene-based materials ...................................................................... 19 5. Graphene/Elastomer Nanocomposites ............................................................................. 21 5.1 Preparation ................................................................................................................ 21 5.1.1 Melt mixing ........................................................................................................ 22 5.1.2 Solution/latex blending........................................................................................ 23 5.1.3 In situ polymerization.......................................................................................... 25 2

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5.2 Characterization......................................................................................................... 26 5.2.1 Fourier Transform InfaRed spectroscopy (FTIR) – X-ray Photoelectron Spectroscopy (XPS) ..................................................................................................... 26

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5.2.3 X-Ray Diffraction (XRD) .................................................................................... 29 5.2.4 Contact angle ...................................................................................................... 30 5.3 Mechanical Properties ............................................................................................... 30 5.3.1 Tensile Properties ................................................................................................ 31

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5.3.2 Dynamic Mechanical Properties .......................................................................... 38 5.4 Thermal properties..................................................................................................... 40

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5.4.1 Thermal Stability................................................................................................. 40 5.4.2 Thermal Conductivity.......................................................................................... 41 5.5 Electrical Properties................................................................................................... 43 5.5.1 Electrical Conductivity ........................................................................................ 43 5.5.2 Dielectric Properties ............................................................................................ 46

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5.6 Barrier properties ....................................................................................................... 47 5.7 Swelling and Vulcanization ....................................................................................... 49 6. Conclusions and Perspectives .......................................................................................... 51 Acknowledgements ............................................................................................................. 52

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References .......................................................................................................................... 53

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1. Introduction Elastomers, according to the general IUPAC definition, are polymers that exhibit rubber-like elasticity [1]. Their general characteristics involve viscoelastic behaviour, a low modulus of

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elasticity, a high failure strain along with very weak inter-molecular forces. Among other properties, elastomers provide good heat resistance, ease of deformation at ambient temperatures and exceptional elongation and flexibility before breaking. These properties have established elastomers as excellent and relatively cheap materials for various applications in many sectors including automotive, industrial, packaging, healthcare and

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many others. The history of elastomers goes at least 3000 years ago, when the ancient Mesoamerican people started collecting and using natural rubber [2] but its use grew rapidly after the development of vulcanization technology in 1839 by Charles Goodyear and Thomas

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Hancock [3]. Nowadays the elastomer industry is huge because of the wide commercial penetration of specific materials accompanied by an increased industrial and academic interest, resulting in a steady rise in global annual revenues, predicted to be US56$ billion by 2020 [4].

This interesting and extremely popular class of materials has inevitably been involved in the

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development of polymer-based nanocomposites which have emerged over recent decades [511]. Elastomers can be used effectively as polymeric matrixes and the polymer nanocomposites that are formed exhibit significantly improved properties. Elastomeric nanocomposites, however, demand preparation procedures that are different than most

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polymer-based nanocomposites; the vulcanization process to which they are subjected demands the use of a cross-linking activator, while the composite samples must be cured at a specific temperature and time prior to use (Figure 1). High-performance elastomeric

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nanocomposites have been produced by several research groups in the past with the incorporation of different types of inorganic fillers such as silica nanoparticles, layered silicates, carbon black, multi-walled carbon nanotubes and other nanomaterials [12-22]. Each type of filler affects the final properties of the nanocomposite in a different way due to their structural and geometrical characteristics, but in order to achieve significant enhancement of the initial physicochemical attributes of the elastomers, a homogeneous and uniform dispersion of the fillers must be achieved. For this reason, nano-fillers are commonly modified chemically with functional groups, in order to increase the filler-matrix interactions

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and bonding whilst simultaneously decreasing the filler-filler interactions and/or increasing the interlayer distance [23].

0000000.0 Vulcanizing 00000000 Additives 00000000 Fillers 0….Mixing

Curing

0000000.0 00000000 Mooney Viscometer Rheometer 00000000 0….

0000000.0 00000000 Compression Moulding 00000000 0….Composite

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Mixing

00000000 00000000 Internal Mixer Two-Roll Mill 00000000 0

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0000000.0 00000000 Elastomer 00000000 0…. NR, SBR etc.

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Figure 1. Flow chart illustrating the formation of an elastomer composite. (After [23])

Apart from the continuous developments in the field of elastomeric materials, significant progress has also been made on the use of inorganic fillers as reinforcements, since the first incorporation of clays in a nylon-6 matrix by the Toyota group [24-26]. Graphene, a carbon allotrope with a planar monolayer of carbon atoms arranged in a 2D honeycomb lattice, is the most exciting materials discovery of the 21st century so far and has attracted the interest of

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many scientific groups as it possesses intriguing properties that can be utilized in various applications such as electronic devices, energy storage materials, shielding and anti-static coatings and obviously, nanocomposites. Its properties include an exceptional modulus of elasticity (≈1 TPa), high thermal conductivity (up to 5000 W/mK), very high electron

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mobility, almost 98% optical transmittance and a large specific surface area [27-32]. Graphene, was successfully isolated and identified in Manchester by Geim and coworkers in 2004 [33], while the previous attempts to prepare free-standing single-layer graphene

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remained unsuccessful, due to the common perception that, as a 2D crystal, it would be thermodynamically unstable [34]. Over recent years, several polymer nanocomposites reinforced by single- and few-layer graphene have been studied in detail and significant enhancements in various physicochemical aspects of those materials have been reported. Another encouraging factor in the use of graphene in polymer nanocomposites, is that it has been realized that even a small quantity of the nano-filler can lead to significant improvements in properties; therefore graphene which (at the moment) is not easy to prepare in bulk quantities, is an ideal candidate for the use in this class of material.

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Several reviews have already dealt with the use of graphene in nanocomposites including thermosets and thermoplastics as polymeric matrices [35-41]. There is, however, a gap in the literature which this review aims to fill, dealing with specifically advances regarding graphene-based elastomer matrix nanocomposites. Up to now, only Sadasivuni et al. have

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dealt with the specific subject, but they included a number of graphitic forms, ranging from graphite to graphene in their review [42]. Emphasis will be given here to the techniques used in the preparation of graphene, elastomers and nanocomposites, along with the

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physicochemical properties obtained and the applications of the final materials.

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2. Graphene

Geim and Novoselov point out in their review [27] that graphene is the mother of all graphitic forms of carbon, since it can be used as a 2D building material for the assembly of carbon nanomaterials in other dimensions; it can produce 0D buckyballs by wrapping, 1D nanotubes by rolling or 3D graphite by stacking (Figure 2). The honeycomb lattice of graphene consists of two equivalent sub-lattices, where the carbon atoms are bonded together with σ bonds. Ideally, graphene is a perfectly flat, single-layer material. In reality, however, ripples are

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formed due to thermal fluctuations, which along with associated waviness may affect its ability to reinforce composite materials. On the other hand, few- and many-layer graphenes

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may be as relevant as single-layer material in applications such as nanocomposites [43].

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Figure 2. Carbon allotropes based upon 2D graphene (Reproduced with permission from [27] Copyright 2007, Nature Publishing Group)

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2.1 Preparation Methods

The increased interest in graphene, especially for nanocomposites, has led to the development of methods for the production of a defect-free, low cost material in large quantities. The thickness, structure and eventual properties of the graphene produced, are strongly dependent

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upon the production method employed. Some of the most popular approaches that are

2.1.1 Mechanical exfoliation

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nowadays used for the production of graphene will be discussed next.

This relatively simple method was employed by Geim and coworkers in 2004 to first prepare monolayer graphene and has subsequently become famous in the scientific community [33]. It involves the repeated peeling of graphene layers from natural graphite or highly-oriented

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pyrolytic graphite using ‘Scotch’ tape. With this method, the number of graphene layers can be controlled, while the dimensions of the graphene flakes are of the order of tens of microns, generally limited by the grain size of the graphite. The specific method can produce highquality graphene with good properties. It cannot, however, be scaled up to produce graphene

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in large enough quantities to be used in nanocomposites, except for research purposes with

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model specimens [44, 45].

2.1.2 Liquid-phase exfoliation The liquid-phase exfoliation method includes the exposure of graphite in organic solvents such as N-methyl-pyrrolidone [46], dimethyl-formamide [47], dimethylsulfoxide [48], tetrahydrofuran [49] or other types of solvents (e.g. perfluorinated aromatic [50]) and proceeds in the following stages; dispersion in a solvent, exfoliation and purification [51] (Figure 3). This process is very effective since the energy that is provided to the solventgraphene mixture through sonication facilitates the exfoliation, while the energy from the graphene-solvent interactions may also aid the process. Therefore it follows that a 7

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prerequisite for this method is to find a suitable solvent with surface tension equal to or higher than the graphene-graphene interaction energy. Typically organic solvents are most effective for use with the specific method, but aqueous-surfactant suspensions have also proven efficient as demonstrated in the work of Coleman et al. [52]. There have also been

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recent reports of the exfoliation being undertaken in aqueous mixtures of graphite and inorganic salts such as NaCl and CuCl2 [53]. The advantage of these methods is that there are a significant number of solvents that can be used for the production of high-quality graphene and the number of commercial applications that can utilize the materials produced is high. The yield efficiency of graphene can, however, be quite small. Also, the electrical properties

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of the graphene prepared using this method are similar to graphene oxide, thought to be the result of poor transport at contacts between the graphene sheets [54]. In terms of composite

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materials, this method is convenient for the production of graphene that can be used in a solution blending procedure with the matrix. However, this process is not eco-friendly, it is expensive and cannot be scaled up easily, which is why it is not currently being used on an

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industrial scale.

Figure 3. Liquid exfoliation process of graphite in the absence (top-right) and presence (bottom-right) of surfactant molecules (Reproduced with permission from [51] Copyright 2014, Royal Society of Chemistry)

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2.1.3 Thermal exfoliation Thermal exfoliation is a method that uses a thermal shock procedure in order to produce exfoliated graphene. Graphite nanoplatelets of the order of 10 nm thick can be produced by the rapid microwave heating of acid-intercalated graphite [55]. This vaporizes the acid within

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the graphite layers and causes rapid expansion of the graphite galleries. The prerequisite for this method when starting with graphite oxide is to induce faster decomposition of the epoxy and hydroxyl groups of graphite oxide in comparison with the diffusion rate of the evolving gases, thus yielding pressures which can exfoliate the graphene sheets by overcoming the van

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der Waals forces between the stacks of graphene [56]. More specifically, high temperatures (1050°C), high pressure (700 m2 g-1) and fast heating rates (>2000°C/min) are required in order to achieve complete exfoliation of graphite [57, 58]. This method generates few- and

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many-layer graphene, which still possess some of the desired properties of single-layer graphene and has the additional advantage that it can produce graphene in bulk quantities. For these reasons, graphene produced by thermal exfoliation is most commonly used for the production of nanocomposite materials, which require large quantities of the filler.

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2.1.4 Chemical vapour deposition (CVD)

Chemical vapour deposition on Ni substrate, first emerged as a method for the production of graphene back in 2008 [59]. During the CVD process, gaseous reactants are deposited onto a substrate. More specifically, the gas species are fed into the reactor and through a hot zone,

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where the hydrocarbon precursors decompose to carbon radicals at the metal substrate surface [60]. When the gases come into contact with the substrate surface within the reaction

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chamber, the reaction that occurs, leads to the formation of a material film on the substrate surface. It is very important to adhere to the guidelines during the method, such as the temperature of the substrate or the disposal of the waste gases, for the production of highquality graphene. Several substrates have been investigated recently for the deposition of graphene [61, 62] and copper is now widely used. A range of different carbon-containing feedstock can also be employed [63]. Even though this method can produce high-quality graphene, there are several problems associated with it. The major issue is the exfoliation of graphene from the substrate, since it can cause significant damage to the structure or alteration of the properties of graphene, while 9

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there is little control of the number of layers, the doping level and the grain size. Additionally, this process is expensive due to high energy consumption and the use of the metal substrates which are often renewed during each preparation procedure. The scale-up of this process has recently taken place by a roll-to-roll production process whereby graphene is

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grown on Cu-coated rolls and then transferred to a polymer film [64]. Graphene produced using CVD is a promising material that can used in a number of applications such as coatings, transparent electrodes, sensors or electronic devices, although it cannot be produced at the moment in large enough quantities for use in nanocomposites. Attempts are currently being made for the production of high-quality, few-layer graphene in bulk quantities by CVD

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[65-67]. Such a process could revolutionize the production of graphene-based nanocomposites and give a massive boost to the whole area of graphene-reinforced

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nanocomposite materials.

2.2 Characterization 2.2.1 Microscopy

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Although it is a nanomaterial, graphene can be observed directly in an optical microscope since a single atomic layer absorbs ~2.3 % of visible light [68]. This absorption is also virtually independent of wavelength. It is also possible to distinguish flakes of graphene with different numbers of atomic layers relatively easily in a transmission optical microscope [69,

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70] (Figure 4). Transmission electron microscopy has also been employed to determine the size of different grains or the atomic structure of grain boundaries, since these features are associated with the electronic [71], magnetic [72] and mechanical [73] properties of the

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material [74, 75]. Novoselov et al., in their initial studies of graphene employed atomic force microscopy (AFM) in order to observe the thickness of the graphene layers and found out that monolayer graphene possesses a thickness of 0.4 nm [33]. Since then, many publications dealing with graphene have used AFM in order to characterize the thickness of the flakes (Figure 4). Scanning electron microscopy and scanning tunnelling microscopy have also been employed in order to observe the ripples, wrinkles and structure of graphene sheets, which ultimately can alter the properties of the initial or “ideal” material [76-80].

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Figure 4. (a) Optical image of graphene with one, two, three and four layers (Reprinted with permission from [70] Copyright 2007, 2007 American Chemical Society.), (b) SEM image of

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graphene (Reproduced with permission from [80] Copyright 2011, Royal Society of Chemistry.), (c) bright-field field TEM image of monolayer graphene on a holey carbon film (Reproduced with permission from [46],, Copyright 2008, Nature Publishing group) , (d) graphene visualized by AFM (Reprod Reproduced with permission from [27] Copyright 2008, Nature

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Publishing group)

2.2.2 X-ray diffraction

X-ray diffraction (XRD) is a technique that can be utilized in order to follow the intercalation and exfoliation of graphite and the formation of graphene. The sharp Bragg reflection of graphite under normal measurement conditions (CuKα (CuK radiation and λ= 0.154 nm)) found at 2θ ≈ 26o, becomes broader with the decreasing number of layers and ultimately disappears sappears in measurements upon monolayer graphene [81]. This can be undertaken upon bulk material; therefore XRD can provide only a relative estimation regarding the average number of layers in graphene (with the aid of Scherrer’s formula).

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2.2.3 Raman spectroscopy One of the most powerful techniques, widely used in extensive studies upon graphene, has proven to be Raman spectroscopy, due to the fact that there is strong resonance Raman scattering from graphene. It is found that even measurements upon a graphene monolayer

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(Figure 5) can provide a strong signal, very useful for the characterization of the material [82, 83]. There are three main characteristic bands of graphene and graphite; the D band at ~1330 cm-1, the G band at ~1580 cm-1 and the G’ (or 2D) at ~2650 cm-1. One of the most important functions of Raman spectroscopy on the study of graphene is the accurate information it can

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provide regarding the number of layers of graphene [84]. Dresselhaus et al. [85] have shown that the characteristic 2D band evolves and displays differences for different numbers of graphene layers (Figure 5). In particular, it broadens and upshifts as the number of graphene

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layers is increased. The D band (not present in the monolayer in Figure 5) is related to the presence of edges and defects [86]. The intensity ratio between the D and G bands indicates the level of defects and for graphene produced by bulk preparation methods, is normally higher than that of the original graphite as the result of damage during exfoliation and the

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formation of edges.

2.2.4 Ultraviolet-visible (UV-vis) spectroscopy UV-vis spectroscopy can be very helpful for the identification of the characteristics of graphene produced by different methods. The UV-vis spectrum of graphene displays a

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pronounced and asymmetric peak at around 4.62 eV, while at lower photon energies (0.6 eV < E < 2 eV) the spectrum is flat. The number of layers does not have a pronounced effect on

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the UV-vis spectrum of graphene, since bi-layer graphene shows similar excitonic effects to single layer graphene, but with a less asymmetric optical absorption at 4.6 eV [87].

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Figure 5. Raman spectra of monolayer graphene showing the complete spectrum with the G and 2D/G' bands (top). Details of the 2D/G' band for monolayer, bilayer, trilayer and manylayer materials (bottom) (Reproduced with permission from [84] Copyright 2013, Royal Society of Chemistry.)

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2.3 Properties 2.3.1 Mechanical properties A detailed analysis of the mechanical properties of graphene and graphene nanocomposites

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has been presented in an earlier review [43]. The general characteristics of monolayer graphene include modulus of elasticity equal to 1000 ± 100 GPa, determined by indentation experiments upon a monolayer flake lying across a hole and an intrinsic strength of σint = 130 GPa [31]. These values of stiffness and strength are similar to those determined earlier, using

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ab initio calculations [88].

Raman spectroscopy has also been used in the study of the molecular deformation of graphene through the observation of the stress-induced band shifts as shown in Figure 6. It

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has been found that the rate of band shift per unit strain can be associated with the Young modulus of carbon fibres [89, 90] and other high-performance fibres [89, 91]. In a similar way it has been shown that the deformation of graphene leads to lattice distortion and bond stretching, which ultimately affect its properties [45, 90, 92-95]. From the slope of the values of the Raman wavenumbers versus strain, the modulus of elasticity for monolayer graphene is calculated, using carbon fibres as a calibration, to be equal to 1000 ± 100 GPa [90], similar to

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values measured directly [31]. During the deformation of the material, the study of G and 2D bands reveals splitting of the bands which gives information regarding the orientation of the crystal lattice relative to the direction of strain [90]. As the number of graphene layers increases, the rate of band shift per unit strain of the 2D

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band to lower wavenumber also tends to decrease [96]. When the number of layers is more than two, a band narrowing occurs, while the normal band broadening is observed for the

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mono- and two-layer material. This has been interpreted as being due to the reversible loss of Bernal stacking in the few-layer graphene [96].

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Figure 6. Shift of the Raman 2D band with strain for a graphene monolayer (Reproduced with permission from [43] Copyright 2012, Elsevier )

2.3.2 Thermal conductivity

The thermal properties of graphene are also important, since it possesses very high in plane

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thermal conductivity which is strongly affected by atomic defects, interfacial interactions, and edges. The thermal conductivity of graphene can then be tuned according to the needs of each application. Balladin et al. found that the thermal conductivity of graphene exceeds 3,000 W mK-1 near room temperature, therefore being higher than the bulk graphite limit [30,

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97, 98]. Additionally, an optothermal Raman study revealed that the thermal conductivity of suspended uncapped few-layer graphene decreases with an increasing number of layers, approaching the value of bulk graphite above 3 or 4 layers [99]. This occurs due to the

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differences in the phonon scattering, resulting from the increasing number of layers whereby more phase-space states become available for phonon scattering, leading to the decrease of thermal conductivity [98].

2.3.3 Electrical conductivity The electrical conductivity of graphene is a property that is being utilized in various applications in nanoelectronics. The fact that graphene is a zero-overlap semimetal, leads to it

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possessing a very high level of electrical conductivity. Very high values of electron mobility have been reported, but these values can increase dramatically if the impurities are limited [27, 29, 100]. In addition, interesting findings have been published on the electrical properties of graphene/polymer nanocomposites, since the addition of graphene can lead to a

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percolation threshold for loadings of as low as ~0.1 vol.% for room temperature conductivity. This could lead to nanocomposites with levels of conductivity being useful for a number of electrical applications [35, 101].

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3. Graphene Oxide

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3.1 Preparation Methods

The preparation of defect-free and single-layer graphene in bulk quantities is a difficult process that is still under development, whereas the preparation of graphite oxide, which consists of stacks of individual graphene oxide (GO) sheets, was first reported by Brodie [102], over 150 years ago. In his attempt to determine the ‘atomic weight of graphite’, Brodie oxidized graphite using potassium chlorate and fuming nitric acid and a ratio of C/O/H of

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2.2/1/0.8 was determined. The method of Brodie was later modified by Staudenmeier [103] and Hummers [104] who obtained graphite oxide by using a mixture of concentrated sulphuric acid, sodium nitrate and potassium permanganate. These reactions achieved similar levels of oxidation (C:O ratio 2:1) and introduced oxygen-related functionalities to the

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material obtained [32]. The interlayer spacing of graphite oxide is higher than that of graphite (ranging from 0.6 to 1 nm, depending on the preparation method) due to the intercalation caused by the water molecules [105]. The major advantage of the graphite oxide route for the

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preparation of GO and the reason why it has been used extensively in polymer nanocomposites, is its capability to be completely exfoliated to individual GO sheets in different solvents, in order to form a stable solution, thereby achieving a homogeneous dispersion of GO in a soluble matrix. Additionally, this method can be used for the scale-up of the production of GO, which is another reason for the increasing interest in this material. Despite its advantages, GO presents some important disadvantages such as its low conductivity due to the presence of the numerous oxygen functional groups and its poor thermal stability.

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The properties of GO can be restored to those of graphene, at least partially, by using methods such as chemical or thermal reduction. Hydrazine hydrate vapour [106], L-ascorbic acid [107] or sodium borohydride [108] can be employed for the chemical restoration of the properties of GO, by the removal of oxygen and the recovery of some of the aromatic double-

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bonded carbons [109]. The thermal reduction method involves rapid heating of dry GO under inert gas and high temperature (~1000 °C). It is worth pointing a stable complex of oxidative debris, which is strongly bounded to the as-produced GO sheets, made by the popular Hummer’s method. This debris causes several problems and can be eliminated if the graphene oxide produced is treated with an aqueous solution of NaOH as shown in Figure 7

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[110]. This base washing reduces the oxygen content of the GO [111], removes its brown

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coloration [112] and increases its conductivity by many orders of magnitude [110].

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Figure 7. Removal of the oxidative debris attached to GO by base washing with NaOH

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(Reproduced with permission from Ref. [110], Copyright 2011, Wiley)

3.2 Characterization

Since graphene oxide has been used in a number of processes and applications, its structure has been examined by a range of characterization techniques. The structure of GO is still an issue of debate, as Dreyer et al. pointed out in their review [32], and the original model of its structure was based upon the solid-state

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C NMR experiments by Lerf et al. [113, 114].

According to their analysis, GO sheets consist of aromatic “islands” of variable size that have not been oxidized and they are separated by 6-membered rings containing C-OH groups, epoxide groups and double bonds (Figure 7) [43]. XPS can also provide information on the 17

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chemical functionalities of graphene oxide, since the narrow, symmetric C1s band which is characteristic for pristine graphite, shows an evolution to a complex band consisting of two maxima for graphite oxide [49]. Electron microscopy has been employed for the characterization of graphene oxide and it was

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shown by TEM that its structure and electron diffraction pattern does not have significant differences compared to neat graphene [115]. In the case of reduced graphene oxide, it was found that it consists of graphitic regions of up to 8 nm2 and holes below 5 nm2. Additionally, the oxidized regions exhibit no order when they form a continuous network across the GO

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sheets [116]. AFM has also been utilized for the observation of the exfoliation process and the thickness of the layers of GO (even though the process faces difficulties due to the wrinkled nature of GO) [57]. SEM images of GO show a material consisting of randomly

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aggregated, thin and crumpled sheets [106]. X-ray diffraction can give significant information regarding the transformation from graphite to GO along with the determination of the distance between the layers of GO. The peak owing to the Bragg reflection from graphite at 2θ≈26o, disappears after the full conversion to GO and a new peak arises at significantly lower angle.

Ultraviolet-visible (UV-vis) spectroscopy can provide important information on the

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identification of GO; the shoulder in the spectra at ~310 nm corresponds to a n-π* plasmon peak, while the feature at 230 nm corresponds to a characteristic π-π* plasmon peak. The intensity and shift of the peak at 230 nm provide information on the electronic conjugation between the GO sheets and by adjusting the position of the peak, the optical and electrical

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properties of GO can be tailored [117]. Thermogravimetric analysis is also useful for the identification of the purity and the extent of the surface functional groups of GO. The mass

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loss curve of GO shows three main degradation steps; the first one is up to 100 °C and corresponds to the moisture that has been absorbed by the sample, while the second one is in the range from 100 to 250 °C and corresponds to the main pyrolysis of the oxygen-containing groups, generating CO, CO2 and steam [118]. The third step appearing at temperatures higher than 350 °C is attributed to the removal of more stable oxygen functionalities such as phenols, carbonyls, etc. In contrast, graphite and graphene are more thermally stable than GO, since they do not contain the different oxygen-containing functional groups. This is why they are preferred in applications where thermal stability is a desirable characteristic.

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The main differences between graphene and graphene oxide in terms of their Raman spectra lie on the broadening of the G band and the presence of a strong D band which is not present in exfoliated graphene. The 2D band is also absent in GO. The D band indicates sp3 bonding in GO and the evolution of the relative intensities of the different bands can provide

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information regarding the structural changes that occur during the reduction processes [119].

3.3 Properties

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The oxygen-containing groups which disrupt the structure of graphene oxide along with the presence of sp3 rather than sp2 bonding, reduce significantly the stiffness and strength of GO,

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compared with those of defect-free monolayer graphene. Moreover the thickening of the sheets caused by oxidation leads to a further reduction in the Young’s modulus. According to the early results presented by Dikin et al. [120] the Young’s modulus of GO paper is only around 30 GPa, depending on the water content and thickness of the samples. Since then, various groups have reported a range of values of GO Young’s modulus, depending on the preparation, measurement methods and number of layers. The generally accepted value of Young’s modulus of GO is around 250 GPa [121] and a detailed presentation of the

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mechanical properties of graphene oxide is given by Young et al. [43].

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4. Nomeclature for graphene-based materials The existence of graphene and the production of several derivatives from the mother form, has caused confusion and misunderstandings between the scientific community and the non-

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expert readers regarding the nomenclature and the standardization of the graphene. An important step towards the elimination of these misunderstandings has been the paper published by Bianco et al. [122], where every member of the graphene family tree is described in detail. More recently, a similar publication by Wick et al. [123] also described clearly graphene and the important vocabulary for graphene-based materials. These two works should be generally promoted by the scientific community in order to avoid generalizations and unwanted complexity on future published work. The vast majority of the researchers whose work is reported in the current review use their own terminology and

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abbreviations; for this reason we will also report briefly the nomenclature of graphene and the related two-dimensional carbon forms, for reference in the following sections. The three fundamental properties which classify the different forms of graphene-based materials are the number of graphene layers, the average lateral dimension and the atomic

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carbon/oxygen ratio [123]. On this basis, each graphene-based material presents different characteristics. As was thoroughly discussed earlier, graphene is isolated, single-atom-thick sheets of hexagonally-arranged, sp2-bonded carbon atoms. Few-layer graphene is considered the 2-5 sheet material, while the 2D material consisting of 5-10 countable and stacked

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graphene layers (sheets) of extended lateral dimension, can be termed as multi-layer graphene. On the other hand, graphite nanoplatelets or nanosheets are the 2D graphite-based materials that have a lateral dimension/thickness less than 100 nm and exfoliated graphite is a

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multi-layered material fabricated by partial exfoliation of graphite and retains the 3D crystal stacking of graphite.

Chemically-modified graphene is widely used in the literature as reinforcing agent in polymer nanocomposites, and especially the monolayer graphene oxide (GO) that originates from the oxidation and exfoliation, accompanied by extensive oxidative modification of the basal plane. The C/O atomic ratio of GO is less than 3and close to 2. More details on graphene

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oxide and its characteristics can be found in the previous section. Reduced graphene oxide (RGO) is the form of GO that is processed by chemical, thermal and other methods in order to reduce the oxygen content, while graphite oxide is a material produced by oxidation of graphite which leads to increased interlayer spacing and functionalization of the basal planes

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of graphite. The same categories which apply for the layers of graphene can be also related to graphene oxide. A useful diagram on the classification of the graphene-based forms can be

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found in the work of Wick et al. [123], where the materials are distinguished by the three fundamental characteristics that differentiate this family of materials (Figure 8).

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Figure 8. Classification grid for the distribution of different graphene types, based on their fundamental properties. The materials drawn at the edges of the grid represent the ideal cases

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on the basis of the number of layers, the C/O ratio and the average lateral dimension (Reproduced with permission from [123] Copyright 2014, Wiley)

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5. Graphene/Elastomer Nanocomposites

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5.1 Preparation

Several methods have been reported in the literature for the preparation of graphene/elastomer nanocomposites. The properties of the nanocomposites are strongly dependent upon the dispersion of the filler and the matrix-filler and filler-filler interactions. Therefore various methods such as melt mixing and solution blending tend to be employed, while taking into account the cost of each procedure and the time needed for the production of the material. It should be pointed out that each method can attribute different characteristics to the resultant nanocomposite, due to the different states of dispersion of the inorganic filler, as has been shown by Kim et al. [124] and others [125]. Sometimes, a combination of processing techniques is applied, since the preparation of elastomeric 21

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nanocomposites is not a single-step process (Figure 1). A number of different ingredients, such as curing or crosslinking agents, processing aids, catalysts etc., need to be incorporated in the elastomer/filler mixture, in order to produce the final material with the appropriate properties. Hence a number of stages are often employed, with elastomer being

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solution/latex-blended with the graphene or graphene oxide and the curing and crosslinking procedures carried out in a two-roll mill [125-136]. The most important preparation techniques described in literature will be discussed next.

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5.1.1 Melt mixing

Melt mixing is a preparation technique favoured by industry, since it combines low cost and

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speed [134, 137-151]. The general principle of this method involves the dispersion of the fillers in the elastomer matrix in the molten state, by applying a shear force. This technique can involve the chemical modification of the filler, the use of compatibilizers to enhance filler-matrix interactions, while it also can be used for both polar and non-polar elastomers [42]. Despite the advantages of this method, some problems can be encountered since the elevated temperatures that are needed in order to incorporate the inorganic fillers properly

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into the elastomeric matrix, make the materials prone to degradation. The high viscosity of the polymeric matrix, along with the use of higher filler fractions, can also obstruct the effective dispersion of the filler [141], while the high shear forces that are usually applied in order to overcome the viscosity of the matrix, can lead to the breakage of the graphene or

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graphene oxide sheets. In comparison with the other popular methods for the preparation of graphene/elastomer nanocomposites, it is the one that leads to the poorest of dispersion of the filler, as Kim et al. [124] have demonstrated for their thermoplastic polyurethane (TPU) filled

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with graphite and thermally-reduced graphene (TRG) (Figure 9). This report paved the way for several other investigations, since it described in detail the advantages and disadvantages of each preparation method upon the properties of graphene/elastomer nanocomposites. It is important to note that even though melt mixing does not result in the highest homogeneity of filler, several property enhancements have been reported in the literature, as will be described in the following section.

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Figure 9. Comparison of the different preparation methods on the dispersion of graphene in a

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polyurethane matrix: (a) 5 wt % (2.7 vol %) graphite, (b, c) melt-blended, (d) solvent-mixed, (e, f) in situ polymerized ∼3 wt % (1.6 vol %) TRG, (g) solvent-mixed 3 wt % (1.6 vol %)

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Ph-iGO, (h) AcPh-iGO, and (i) in situ polymerized 2.8 wt % (1.5 vol %) GO. (Reprinted (adapted) with permission from [124] Copyright 2010, American Chemical Society.)

5.1.2 Solution/latex blending Solution/latex blending is a technique that is most commonly employed in academic studies for the production of graphene/elastomer nanocomposites [112, 124-135, 152-187]. The reasons for this are that graphene suspensions can be used and incorporated into the matrix without significant further processing. Additionally, according to reports on elastomers

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reinforced with sheet-like inorganic fillers [188], solution blending ensures good dispersion and exfoliation of the filler in the elastomeric matrix, even without a chemical modification of the filler. During the solution blending process, the elastomeric matrix is dissolved either with the graphene in the same solvent or it is already in solution and is mixed with the

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graphene by high-speed shear mixing, ultra-sonication or stirring. Latex blending is similar to the solution-blending process, the only difference being that the elastomer is in the form of latex, which presents advantages from an environmental viewpoint. Several procedures have been introduced into this process in order to ensure the

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strong interactions between the matrix and the fillers and disperse better the reinforcing materials (Figure 10). They include co-coagulation or latex mixing/co-coagulation, which can improve the quality of dispersion, since the aggregation of the fillers can be prevented

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kinetically by the faster coagulation process of the emulsion [128-134, 141, 153, 156, 172]. The co-coagulation process can only be applied during solution blending and, according to Wu et al. [189], it leads to a partly-exfoliated structure in which the layered filler can be dispersed simultaneously in the matrix as individual layers, or as a layered-filler without the presence of the elastomer between its sheets. Non-exfoliated layers are formed by the reaggregation of the initially-exfoliated layers during the co-coagulating process.

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Despite the number of advantages of solution mixing, the effective removal of the solvents used during the procedure remains a significant problem for the extensive use of this particular method. Furthermore, the high cost of solvents and their disposal act negatively upon the scale-up and therefore the eventual adaptation of this method by industry. Finally,

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this is a very sensitive procedure due to the fact that the details of each component or the processing (ie. quantity and quality of solvents, mixing time and speed, sonication etc.) can

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affect dramatically the outcome of the process.

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Figure 10. Schematic description of a multi-step process for the preparation of graphenefilled NR nanocomposites (Reproduced with permission from [156] Copyright 2014, Elsevier)

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5.1.3 In situ polymerization

In situ polymerization is a method which has been used successfully in the past, in order to produce polymer/layered-silicate nanocomposites with an exfoliated structure, since the macromolecular chains of the polymer can be effectively incorporated between the sheets of

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the filler and exfoliate the filler with this method [5, 190-193]. The general principle of in situ polymerization in polymer nanocomposites involves the mixing of the inorganic filler with the monomers in a solvent, followed by in situ polymerization. It has therefore been

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employed successfully for the preparation of graphene/elastomer nanocomposites due to the similar layered structure of graphene [124, 194, 195]. The in situ polymerization method is still not a popular method for the preparation of the specific set of materials, since it demands a low elastomer viscosity. Another limiting factor is that during this procedure the macromolecular chains of the polymer may become attached to the graphene, therefore not allowing the filler to from an interconnecting network, which ultimately leads to lower electrical conductivity values. On the other hand, this process may be advantageous for mechanical reinforcement, resulting in better stress transfer from the matrix to the filler.

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5.2 Characterization The interactions between the various elements of a complex multicomponent system, such as the ones found in graphene/elastomer nanocomposites, play a major role on the final physicochemical properties of the material. For this reason, in much of the literature, surface

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chemistry is applied to the different components of the system, in order to ensure chemical compatibility between them. In addition, it can lead to the formation of chemical bonds focused towards the improvement of the properties of the initial material and a satisfactory dispersion of the filler. Some of these approaches include surface functionalization, in situ

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mini-emulsion polymerization, the use of surfactants, etc. The specific processes, however, have not yet been perfected, since the degree of functionalization is most of the times uncertain, while the use of expensive solvents and chemicals and the need for time-

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consuming experiments makes these processes unattractive. For the evaluation of the interactions and confirmation of the functionalization of the filler, different spectroscopic techniques can be employed such as FTIR, Raman, XPS and others. In addition, the dispersion of the fillers is best studied with the use of electron microscopy methods such as

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TEM and SEM, or even AFM.

5.2.1 Fourier Transform InfaRed spectroscopy (FTIR) – X-ray Photoelectron Spectroscopy (XPS)

Wei and Qiu [138] functionalized graphene oxide and thermally-reduced graphene oxide with

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allyl groups in order to enhance the interactions between the filler and a fluoroelastomer (FKM) matrix. The nanocomposites were prepared by mixing in an open twin-roll miller and

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cured by hot pressing at 177 °C. The presence of the allyl groups was confirmed by FTIR and XPS and the final results showed that the introduction of the allyl groups not only improved the interactions between FKM and graphene, the tensile properties and the crosslinking density but also accelerated the vulcanization rates. Nawaz et al. [170] also functionalized graphene oxide with octadecylamine and incorporated the functionalized filler into a thermoplastic polyurethane (TPU). The functionalization was evidenced from FTIR, while mechanical percolation was initiated at a volume fraction of 2.5 vol% with the samples exhibiting an increase of stiffness and low-strain stress. Tang et al. [130] proposed an interesting technique for the preparation of butadiene-styrene-vinyl pyridine rubber (VPR)

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composites containing graphene oxide (GO), by combining the popular co-coagulation process and in situ interface tailoring. According to their work, a hydrogen bonding interaction was formed between the hydroxyl groups present in the basal plane of the GO and the nitrogen of VPR by using CaCl2 or HCl as flocculants. In order to evaluate the

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interactions between GO and VPR, XPS and FTIR were successfully employed and the results for the composites containing 1.5 vol% GO and coagulated by CaCl2 (CaVPR-1.5) or

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HCl (HVPR-1.5) are shown in Figure 11.

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Figure 11. (left graph) XPS N 1s core-level spectra and (right graph) FTIR spectra of (a) neat VPR, (b) HVPR-1.5 and (c) CaVPR-1.5 (Both figures reproduced with permission [130] Copyright 2012, Royal Society of Chemistry.)

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The changes in XPS binding energies can give information regarding the interactions between components in systems such as those in polymer nanocomposites. The downshift of

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the two species in the composite samples (Figure 11-left) suggests the hydrogen bonding between GO and VPR. Furthermore, the presence of the third peak for the HVPR-1.5 composite indicates the presence of N+ in the sample, providing evidence of an electrostatic interaction between the negatively-charged GO and N+ of the VPR. The above results show that hydrogen bonding occurs in the CaVPR/GO composites while both hydrogen and ionic bonding exists in HVPR/GO. FTIR was also utilized in the study (Figure 11-right) for the confirmation of interactions between the filler and the matrix, and the results were consistent with those from XPS. In detail, the shift to higher wavenumber of the peak due to the -C=N bond, indicated the presence of hydrogen bonding. Furthermore, the new peak at 1608 cm-1

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which is assigned to the protonated pyridine group (-C=NH+) confirms once again the ionic bonding for HVPR-1.5 sample. The results from this study [130] showed that the formation of strong ionic bonds between the matrix and the filler induced a fine dispersion of GO and enhanced significantly the mechanical properties and gas permeability of the composite

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materials. The research described in this section demonstrates clearly the benefits of both spectroscopic techniques. In particular, the evaluation of the bonds formed between the filler and the matrix from the preparation procedure can be attributed to the distinct characteristics of the nanocomposite materials, which generally lead to an improvement in their

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physicochemical properties.

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5.2.2 Electron Microscopy

The utilization of electron microscopy is particularly useful in the study of the morphology and dispersion of the filler in graphene/elastomer nanocomposites [149]. Liu et al. [158] produced composites consisting of graphene oxide (GO) nanosheets and a thermoplastic polyurethane (TPU) and used both TEM and SEM for the observation of the microstructural characteristics of their materials (Figure 12). Their results showed that the GO was dispersed

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uniformly in the matrix even at higher loadings due to the strong interfacial interactions with the matrix and the spin-flash drying technique which was employed during their solution blending procedure. The alignment of the filler which can be observed, was ascribed to the hydrogen bonds between the oxygen groups from GO and the N-H groups from TPU [158].

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Finally, the SEM micrographs in Figure 12 show that the GO nanosheets are all coated by TPU, confirming once again the strong interfacial adhesion in this specific system. The use of microscopy techniques is greatly encouraged for most polymer composites in order to make

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firm conclusions regarding the dispersion of the filler, which is always related directly to the preparation procedure which has been employed. TEM is preferable for this task since it provides information on the internal composition of the nanocomposite at the highest magnification. SEM can also prove useful for surface characterisation and, in particular, the dispersion of filler from fracture surfaces.

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Figure 12. TEM micrographs of TPU filled with (a) 0.5 vol.%, (b) 2 vol.% GO nanosheets and SEM micrographs of fractured surfaces of the TPU/GO nanocomposites with (c) 0.5

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vol.% and (d) 2 vol.% (Reproduced with permission from [158], Copyright 2014, Elsevier )

5.2.3 X-Ray Diffraction (XRD)

Another technique that can be employed in order to characterize the levels of exfoliation/intercalation of the graphene layers in the elastomeric matrix is X-ray diffraction

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(XRD). Bai et al. [165] prepared hydrogenated carboxylated nitrile-butadiene rubber (HXNBR) reinforced with exfoliated GO and used XRD in order to study both the fillers and

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the composites. The Bragg peak of graphite shifted to very low angles due to the significantly higher interlayer spacing, indicating the successful exfoliation of the GO. In the nanocomposite filled with 1.3 vol.%, a broad diffraction peak indicated that the compounding process had disrupted the periodic structure of the GO and also that the GO had been exfoliated into monolayers or few-layer material. Similar results were also reported by Xiong et al. [167] for their bromobutyl rubber (BIIR) nanocomposites which they reinforced with ionic-liquid-modified GO. Both studies are good examples of the simple, but important information that XRD can provide on graphene/elastomer nanocomposites; the exfoliation or intercalation of the graphene sheets in the composite materials. Additionally, the degree of

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crystallinity can be also obtained from the diffractograms and this can also play a major role in controlling the physicochemical properties of the material.

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5.2.4 Contact angle Interfacial energy is another factor which can be used for the evaluation of interactions between the components of polymer nanocomposites, by measuring the contact angles of liquids on the materials. Chen et al. [127] used contact angle measurements upon their

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graphene oxide reinforced ethylene-propylene-diene rubber (EPDM)/petroleum resin (PR) blends. The introduction of GO was found to change slightly both the dispersive and polar components of the surface energy of the EPDM/PR blends and this was found to contribute to

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the homogeneous dispersion of the filler in the blend. Therefore, contact angle can be also utilized in order to give indications regarding the effect of the filler on the hydrophobicity or hydrophilicity of the composite samples along with its dispersion.

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5.3 Mechanical Properties

The introduction of inorganic nanoscale fillers has been proved a successful way of improving the properties of a polymeric matrix, since the fillers can “incorporate” their own high stiffness and strength to the bulk material and inhibit the propagation of cracks which

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can ultimately delay the breakdown of the material. The size and the geometrical characteristics of the filler are important for the improvement of different physicochemical properties but the most significant feature controlling the properties in these nanocomposites

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is the dispersion of the filler in the matrix. The formation of aggregates, bundles and stacks of fillers can lead to early failure, since these features will act as defects in the material. Several different types of fillers have been incorporated to polymeric matrixes for the improvement of their properties, such as silica [8], carbon nanotubes [16, 21, 196], clays [197] and others [198] but none of these fillers exhibit the unique properties that single layer and defect-free graphene exhibits; an in-plane Young’s modulus of almost 1 TPa, an ultimate tensile strength of almost 130 GPa and the ability to elongate by a strain of 25% up to failure [31, 125]. Following the incorporation of graphene into a matrix, the filler is invariably wrinkled which on one hand may reduce its intrinsic properties, but on the other may enable it to act as a 30

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crack propagation barrier and also increase the mechanical interlocking between the matrix and the filler [199]. The low value of Young’s modulus that most elastomers exhibit can be increased significantly upon the homogeneous incorporation of graphene thereby making elastomer/graphene nanocomposites attractive for a range of applications. The mechanics of

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graphene nanocomposites with a rigid polymer matrix have been recently reviewed by our group [43] and in the following section, the mechanical properties of graphene/elastomer nanocomposites will be presented.

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5.3.1 Tensile Properties

There are a number of reports in the literature upon investigations into the mechanical

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properties of graphene-based/elastomer nanocomposites [112, 124-127, 129-131, 133, 134, 138, 139, 141, 143, 144, 146, 147, 150-153, 155-159, 162, 163, 165, 166, 170-172, 175, 176, 179, 181-183, 185, 187, 194, 195, 200-204]. In most cases some type of reduced graphene oxide was employed as the filler rather than graphene itself. The vast majority of these reports showed significant improvements in the Young’s modulus and the tensile strength of the nanocomposite samples compared with the pure elastomer. The results from the

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elongation at break of the nanocomposites, however, were found to depend upon the processing procedure and other important factors; sometimes they reported improvements at low filler contents, with the presence of the graphene accompanied by a decrease in elongation at higher contents, due to the inevitable formation of aggregates. Other

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investigations just showed a decrease in elongation to failure with the incorporation of the filler, which is common for composite materials.

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The reinforcement of natural rubber with reduced graphene oxide (RGO) has been studied extensively by Potts et al. [131, 136]. They found that there was exceptional enhancement in multifunctional properties in nanocomposites produced by the coagulation of RGO colloidal suspensions and natural rubber latex [131]. In a subsequent study [136] they investigated the effect of using a conventional rubber melt compounding procedure for dispersing thermallyexfoliated graphene oxide (TEGO) into natural rubber by employing a two-roll mill and without the use of solvents. Their results showed that there was a significant reinforcement of the natural rubber upon the addition of the TEGO as shown from the stress-strain curves in Figure 13, which exhibit clearly the increase of the modulus and the stress at break in the nanocomposite materials. Although the addition of the TEGO above 4 phr (parts by weight 31

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per hundred parts of rubber) increased the stiffness of the material, when they subjected the TEGO to a latex premixing process (termed L-TEGO) significantly better stiffening was obtained for all loadings, with a reduction, however, in the strain to break (Figure 13a). It was found using transmission electron microscopy that that the premixing step led to better

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exfoliation, improved the dispersion of the TEGO in the matrix and the platelets appears less

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wrinkled, all of which appeared to lead to the better overall mechanical properties [136].

Figure 13. Representative stress–strain curves of (a) L-TEGO/NR nanocomposites and (b) TEGO/NR nanocomposites, each at various loadings (Reproduced with permission from

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[136] Copyright 2013, Elsevier)

The stress-strain curves in Figure 13 show the improvement in the modulus and the stress at break and the overall mechanical properties of the material are summarised in Table 1 for 5 phr of these and other fillers in the natural rubber (NR). It can be seen that the increase in

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stiffness is somewhat less than that obtained by Potts et al. [131] in their earlier study on RGO, but an increased failure strain was achieved. The findings do show however, that

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reasonable mechanical properties can be obtained using material processed using a two-roll mill. It is also demonstrated in Table 1 that significantly higher levels of stiffness and strength can be obtained by reinforcing natural rubber with GO than with carbon black. As well as increases in stiffness and strength there have also been recent reports of greatly improved fatigue resistance for the incorporation of only 1 phr of GO in natural rubber [187]. These studies, along with the significant work of Kim et al. [124] which was discussed earlier, exhibit clearly how strongly the preparation procedure affects the final properties of the nanocomposite material. Solution blending gave the best results. However, environmental concerns from the use of solvents and the difficulties in the scale-up combine to make the process difficult for industrial use. On the other hand, the fact that researchers have obtained 32

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encouraging results upon the properties of the nanocomposites made by employing a two-roll mill, commonly used in industry, can lead to the conclusion that the production of

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graphene/elastomer nanocomposites on an industrial scale is only a matter of time.

Table 1. Comparison of the mechanical properties of natural rubber (NR) reinforced by 5 phr of different carbon-based fillers. E100 and E300 are the modulus at 100% and 300% strain respectively and σf is the failure strength. The mechanical properties of the natural rubber permission from [136], Copyright 2013, Elsevier)

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reinforced with carbon black (CB) are also given for comparison. (Reproduced with

E100 (MPa)

E300 (MPa)

σf (MPa)

Strain at break

Neat NR

0.41

0.84

5.15

8.44

TEGO

0.43

1.28

6.05

8.96

L-TEGO

1.07

5.19

10.90

5.05

RGO [131]

1.59

9.01

10.18

3.19

CB

0.51

1.19

6.28

8.34

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Mülhaupt and coworkers investigated the reinforcement of styrene-butadiene rubber (SBR) with reduced GO [201] and functionalized graphene [159]. They obtained significant increases in stiffness and strength upon the addition of both chemically-reduced GO (CRGO)

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and thermally-reduced GO (TRGO) to SBR and their data are summarised in Table 2. Data for the same elastomer reinforced with both carbon black (CB) and carbon nanotubes (CNT) is also given and it can be seen that the best overall properties are again obtained with the

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GO-based fillers. This is another example of the advantages of graphene-based fillers compared with the predecessors; the stiffness of graphene-based composites is higher than for ones filled with carbon black or CNTs, since the tensile strength at 100% (or 50%) and 300% strain is always higher than in such nanocomposites. However, it should be taken into account that the process followed for the preparation of the composites was tailored for the exfoliation of sheet-like fillers, therefore it was reasonable for the graphene-based materials to have a higher degree of dispersion and better mechanical properties.

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Table 2. Mechanical properties of SBR elastomers reinforced with 25 phr different carbonbased fillers (Reproduced with permission from [159] Copyright 2014, Wiley). The parameters are the same as those in Table 1 except for E50 which is the modulus at 50% strain. E300 (MPa)

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Neat SBR

0.8

4.7

5.1

CRGO

2.8

12.4

12.6

TRGO

10.9

-

17.5

CB

1.2

6.7

8.2

CNT

2.1

-

9.2

Strain at break

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E50 (MPa)

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Filler

3.24

2.95

0.88

3.65

2.60

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NB. Failure took place for the TRGO- and CNT-modified SBR below 300% strain.

Khan et al. [175] investigated the reinforcement of an elastomeric polyurethane by pristine graphene produced by solvent exfoliation and characterized by Raman spectroscopy. Figure 14 shows a series of stress-strain curves for different loadings of the graphene by weight. It can be seen that there is a large increase in the slope of the stress-strain curve (the Young’s

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modulus increases by a factor of the order of 100 for their highest loadings) and the strain to failure decreases with graphene loading. Khan et al. [175] also found that for a given loading of graphene, the reinforcement effect decreased as the graphene flake size decreased, which they pointed out was consistent with the findings of Gong et al. [44] upon the need to have

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large lateral graphene flake dimensions for good reinforcement in nanocomposites. This was recently confirmed by Seyedin et al. [205] who demonstrated that better reinforcement was

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obtained with larger-sized GO sheets in elastomeric composite fibres. The flake size is a critical aspect of the reinforcement for flake-reinforced nanocomposite materials [44] since the larger flakes can increase the interactions with the matrix due to their high aspect ratio.

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Figure 14. Stress–strain curves for nanocomposites consisting of solvent-exfoliated graphene in a polyurethane matrix reinforced with different loadings of graphene (by weight %) (Reproduced with permission from [175] Copyright 2014, Elsevier)

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An interesting report by Chen et al. [127] showed that the addition of GO in ethylenepropylene-diene rubber (EPDM) and EPDM/petroleum resin (PR) blends enhanced slightly both the tensile modulus and the stress and the elongation at break of the nanocomposites. The reason for this behaviour, according to the authors, was the dilution effect [206] caused

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by the presence of GO in the matrix and the blend; the volume fraction of the matrix or the blend decreased with increasing GO loading, which led to the decrease in the crosslinking in

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the composite material and a simultaneous increase of the elongation at break. The particular phenomenon observed in this work, may have been additionally facilitated by the fact that the GO contents in the blend were very small (0.2 and 0.5 wt.%) and the dispersion was relatively homogeneous.

Chen and Lu [171] constructed sacrificial bonds and hidden lengths at the interface of the graphene nanosheet (GN)/polyurethane (PU) nanocomposites, by employing covalently and non-covalently functionalized GNs. Their original target was to enhance the mechanical properties of the GN/PU nanocomposites, but not at the expense of the ductility and toughness of the final material. The functionalization of GNs was undertaken by reduction 35

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with hydrazine and the hydroxyl and epoxide groups in GNs were bonded covalently with the PU oligomer chains by reacting with diisocyanete and polyethylene glycol oligomer. In addition, non-covalently covalently bonded PU oligomer chains were we formed by the π-π interactions between GNs and pyrene derivatives (Figure ( 15a). The tensile testing analysis showed

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enhancement of the fracture strength and the tensile modulus along with no deterioration of the elongation at break, contrary to some of the results on GN/PU nanocomposites reported previously [174-176]. The most efficient improvement came from the nanocomposite containing 1 wt% HO-GN sample ple while the specific behavio behaviour was attributed ted to the interfacial bonding formed in the composites, which consisted of covalently and nonnon

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covalently (by π-π interaction) linked PU oligomer chains ((Figure 15b). These two studies [127, 171] can be considered significant since the elongation and strength at break of the

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elastomers are factors that make them useful for a number of different applications; ations; therefore the use of these techniques enables the preparation of nanocomposite materials with increased strength, but not at the expense of elongation or strength at break. Itt should be stated, stated however, that these methods can be employed only for the incorporation of small amounts of graphene, since the use of higher amounts (> 3-4 wt.%) can easily lead to an increase in the

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cross-link density of the elastomer, which will diminish the effect of the functionalization. functionalization

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Figure 15. (a) Synthesis route for covalently and non-covalently functionalized graphene nanosheets (HO-GNs). (b) Toughening mechanism involving sacrificial bond rupture and

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hidden length release in mechanical tests of GN/PU composites (Both figures reproduced with permission from [171] Copyright 2012, Royal Society of Chemistry)

Zhan et al. [129] exhibited the significance of the crosslinking procedure in the graphene/elastomer nanocomposites by showing that uncrosslinked graphene (GE)/natural rubber (NR) nanocomposites, exhibited poor mechanical properties, since their flexibility is

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lower than that of the matrix due to the segregated network of fillers. However, after the in situ vulcanization by adding sulphur and other additives followed by hot pressing at 150 °C, both the tensile strength and elongation at break of the nanocomposites increased significantly. This work highlights nicely the adjustment of the preparation method on the

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basis of the application that is being targeted. If good mechanical properties (e.g. an increase in the Young’s modulus or stress at break) are desirable, then the route of vulcanization

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should be followed. On the other hand, if high electrical conductivity values are the main objective, uncrosslinked materials can exhibit a very low percolation threshold and high conductivity values.

Tang et al. [207] have also shown recently the importance of the surface chemistry of the filler in controlling both the dispersion and interfacial adhesion between graphene produced by the in situ chemical reduction of GO and styrene/butadiene rubber, both of which affect the resultant properties of the nanocomposite. The determinant factor on the dispersion of graphene, according to their work, is the COx fraction. The dispersion was homogeneous at COx fractions lower than 0.2, which resulted in significantly enhanced mechanical properties; 37

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the moduli was improved by 530% and the strength by about 490%, for the sample containing 3 phr graphene and a high level of ammonia obtained through a doping method. The authors attributed the improvements to the improved interfacial interactions, the good dispersion and the high N-doping level. This is a good example of how the surface chemistry

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of the filler can affect the properties of the nanocomposite, even without a functionalization procedure.

The overall picture that emerges is that graphene-based fillers can increase the stiffness and strength of elastomers significantly, with the best improvements being obtained with well-

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exfoliated, uniformly-dispersed fillers. The combination of the selected preparation method and functionalization of the filler, along with the content of the filler can control the desired characteristics each time. In the case of the nanocomposites with the greatest improvements

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in stiffness and strength, this phenomenon is normally accompanied by a reduction in the strain to break as a result of the high stresses to which the nanocomposite material becomes subjected. Some of the reported increases in stiffness of elastomers (typically more than an order of magnitude) can appear to be very large relative to the properties of the matrix, compared with rigid-matrix polymer nanocomposites [43, 208]. This can be understood if we take into account that the Young’s modulus of rigid polymers is of the order of 109 Pa

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compared with only 106 Pa for elastomers. Hence since the modulus of the filler (up to 1012 Pa) is much higher than the modulus of the elastomer matrix, for a given volume fraction of filler, the effect upon the nanocomposite modulus after the addition of rigid particles will be

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much more pronounced for elastomers than for rigid polymers.

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5.3.2 Dynamic Mechanical Properties Dynamic mechanical analysis can also provide important information regarding both the molecular structure and viscoelastic characteristics of graphene/elastomer nanocomposites, by probing the mechanical relaxations of the matrix [125, 127, 130, 131, 135, 138, 141, 143, 145-147, 151, 155, 156, 161, 163, 165, 171, 181, 182, 195, 200]. The molecular mobility is highly dependent on the structure of the nanocomposite and it reveals information about the different modes of chain motion. The increase of glass transition temperatures of nanocomposite samples can give indication on the mobility of the macromolecular chains in the matrix. Additionally, the storage modulus can provide information on the reinforcement of the matrix due to the presence of inorganic fillers, such as graphene. 38

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Araby et al. [125] observed an enhancement of the storage modulus of styrene-butadiene rubber (SBR) reinforced with 1-5 layer graphene nanoplatelets, seven times higher than that of neat SBR in the rubbery region, where the elastomer is soft (Figure 16), while Potts et al. [131] observed an impressive near twenty-fold increase for a natural rubber reinforced with

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GO by solution treatment, in the same region. These are indications of increased interactions between the fillers and the matrix, which were intensified by the solution blending procedure followed in both cases. When the samples in these studies were prepared with a melt mixing procedure, using a three-roll mill, the mechanical properties were inferior due to the weak interactions and the formation of aggregates which act as failure points during mechanical

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testing. Araby et al. [125] also found an increase of the glass transition temperature for their set of samples which was a representation of the reduced chain mobility of the matrix.

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However, a decrease of the glass transition can sometimes also be observed in graphene/elastomer nanocomposites and a possible reason may be a plasticization or a

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dilution effect, which reduces the physical entanglements in the matrix [127].

Figure 16. Storage modulus and tanδ of graphene-reinforced SBR, as a function of temperature (Reproduced with permission from [125] Copyright 2014, Elsevier)

In general, most DMA experiments on graphene-based elastomeric nanocomposites report improvements in the storage modulus of the matrix as a result of the high aspect ratio of

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graphene, which provides high interfacial area in the nanocomposite sample, disrupts the mobility of the macromolecular chains at high temperatures and causes a nanoconfinement effect on the macromolecular chains. Furthermore, the homogeneous dispersion of the filler and the level of interaction between the macromolecular chains of the matrix and the

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graphene can be also reflected on the viscoelastic properties of the sample. As with the tensile properties, the large differences between the modulus of the matrix and the filler accentuate the improvement of the storage modulus of the nanocomposite and large increases have been

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recorded in general in a number of literature reports.

5.4 Thermal properties

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5.4.1 Thermal Stability

The thermal stability of polymer nanocomposites is important since it determines the working temperature at which the materials can be used. Thermogravimetric analysis (TGA) is a technique that measures the mass loss percentage with increasing temperature and has been extensively applied in nanocomposites, in order to observe the effect of the inorganic fillers on the thermal stability of the matrixes. There are a number of reports in literature on the

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effect of the addition of graphene-based materials upon the thermal stability of the host elastomeric matrixes [127, 137, 141, 143, 145, 152, 155, 161, 163, 167, 171, 177-179, 181, 183, 195, 209]. Clearly, the behaviour each time is dependent upon several factors, such as the elastomeric matrix being reinforced, the type of graphene that has been used, the

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preparation method, the dispersion of graphene, and the chemical functionalization of the filler, along with the presence of other additives such as surfactants.

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The enhancement of the thermal stability in a graphene/elastomer nanocomposite can be attributed to the physical barrier effect of the filler which inhibits the emission of the gaseous molecules occurring from the pyrolysis of the material and nano-confinement to which the macromolecular chains are subjected, whose movement can be restricted by the homogeneous dispersion of graphene, delaying the degradation of the matrix. On the other hand, the use of surfactants such as CTAB or others during the preparation by solution blending procedures [161], or the functionalization of the filler with organic groups [138] which are less stable than the matrix, may initiate premature decomposition since they decompose at lower temperatures than the host elastomeric matrixes (Figure 17). In general, 40

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the presence of graphene does not appear to change the degradation pathway of the elastomer significantly, while a general statement regarding the thermal stability of the nanocomposites cannot be made, since the results in literature are somewhat ambiguous and depend upon a

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number of different factors in each case.

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Figure 17. Mass loss (%) curves of natural rubber (NR) filled with reduced graphene oxide (rGO) under a synthetic air atmosphere (Reproduced with permission from [161] Copyright

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2014, Elsevier)

5.4.2 Thermal Conductivity

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Another aspect of the utilization of the unique properties of graphene, is the enhancement of the thermal conductivity of elastomers. The thermal conductivity of graphene at room temperature has been found to be almost 5000 W/m K when freely suspended [30] and its sheet like morphology provides lower interfacial resistance and higher conductivity as well as a greater heat capacity than the host elastomer. Phonons are considered responsible for the thermal conduction in amorphous polymers such as elastomers; therefore the phonon scattering or acoustic impedance mismatch must be reduced in a thermally conducting composite. The presence of exfoliated graphene can ensure this reduction by forming strong bonds with the elastomeric matrix. Other factors which are associated with the improvement of thermal conductivity in a graphene-based nanocomposite are obviously the homogeneous 41

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dispersion of the filler and the formation of interconnecting networks. Limiting factors can include the formation of aggregates and the poor thermal coupling between the elastomer and graphene or between the graphene flakes, which leads to high interfacial resistance (Kapitza resistance) [36].

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The vast majority of the literature upon the thermal conductivity of graphene/elastomer composites reports enhancement due to the presence of the graphene [125, 131, 137, 141, 148, 151, 154, 155, 167, 209] and this has to do mainly with the intrinsic thermal conductivity of the filler itself. In the interesting work of Potts et al. [131], the thermal

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conductivities of natural rubber (NR) reinforced with reduced graphene (RG) were measured in terms of the production method employed. It was found that the interconnecting network in solution-treated nanocomposites, acted more beneficially for the improvement of the thermal

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conductivity of the nanocomposites, than materials produced by the milling method (Figure 18). Milling may not always be a problem; the fact that the composites produced with a threeroll mill are more thermally insulating can lead to them finding use in applications that demand temperature control. Moreover, the ability to control thermal conductivity of elastomer nanocomposites is a highly desirable property which can be exploited in applications which may require a material to be thermally conductive but electrically

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insulating such as in power electronics, electric motors etc.

Figure 18. Thermal conductivities of milled and solution-treated RG-O/NR nanocomposites at various loadings (Reprinted (adapted) with permission from [131] Copyright 2012, American Chemical Society.)

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5.5 Electrical Properties 5.5.1 Electrical Conductivity The introduction of electrically-conductive fillers into non-conductive polymeric matrixes is

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a simple and efficient way to produce electrically-conductive multifunctional polymer nanocomposites, while maintaining the low cost and processability of the initial material [210, 211]. Graphene, once again, is an “ideal” filler for the preparation of electrically conductive nanocomposites, due to its high inherent electrical conductivity and its extremely high surface area. As pointed out earlier, the fewer impurities in the filler, the higher the

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electrical conductivity would be for a nanocomposite material with a given loading. Therefore, the quality of graphene is once again a determinant factor for this property.

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As with all other physicochemical properties of graphene nanocomposites, the dispersion of the filler is the key to obtaining significant property enhancement. The ideal dispersion for the preparation of a graphene-based conductive polymer nanocomposite, involves the formation of a network in which the fillers are close to each other and conduction takes place by a tunnelling phenomenon through the polymer layers which surround the filler. Contrary to some other properties such as mechanical properties, which demand homogeneous

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dispersions, electrical conductivity may be enhanced by the partial segregation, since the formation of a sheath of matrix between and isolated filler flakes, acts negatively on electrical conductivity. Percolation theory can be applied in these composites and, at the critical filler concentration (or percolation threshold), the electrical conductivity of the materials presents a

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rapid increase [211]. The percolation threshold can be also used as a rough estimate of the characteristics of the dispersion. Apart from the dispersion characteristics and the surface area, the electrical conductivity of graphene-based nanocomposites can be improved by

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functionalization of the filler, which can induce improved interactions with the matrix. The orientation of the fillers is also critical since the parallel alignment of the particles increases the percolation threshold, while a random orientation is more beneficial. A number of attempts have been reported for the improvement of the conductivity of elastomers by incorporating graphene-based nanoparticles [124, 125, 129, 131, 143, 144, 151, 152, 154, 156-159, 161-163, 177, 180, 181, 183, 212] with the majority of them being successful. An important conclusion from some of these studies is that graphene is capable of enhancing the conductivity of the composites at lower loadings than other fillers such as carbon black, carbon fibres or even graphite. 43

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Zhan et al. [129] obtained the electrical conductivity in vulcanized natural rubber composites reinforced with graphene by constructing an interconnected network of self-assembled graphene via latex mixing. This work highlights nicely the effect of the preparation procedure on the conductivity of the composite samples. The composite samples exhibited a low

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percolation threshold (~0.62 vol%) and maximum conductivity 0.03 S m-1. The comparison of their method (where the crucial part was the formation of a segregated network) with others at the same loading fraction, revealed that their method led to nanocomposites with electrical conductivity enhanced by 5 orders of magnitude compared with material produced by other methods (Figure 19). It is therefore clear that a segregated network of fillers within

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the matrix enhances the tunnelling effect, which in turn increases the conductivity values.

Figure 19. Electrical conductivity of natural rubber (NR) composites as a function of

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graphene (GE) content, prepared by different methods. NRLGES: crosslinked GE/NR composites with the segregated network prepared by self-assembly in latex and direct hot pressing. NRLGES-TR: cross-linked GE/NR composites without the segregated network prepared by latex mixing and twin-roll mixing. NRLGE: uncrosslinked GE/NR composites with a segregated network prepared by self-assembly in latex and direct hot pressing. NRLGE-TR: uncrosslinked GE/NR composites without a segregated network prepared by latex mixing and twin-roll mixing. NRGE-TR: composites prepared by direct twin-roll mixing of GE powders and rubber. NRGE-HM: composites prepared by direct Haake mixing

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of GE powders and rubber (Reproduced with permission from [129] Copyright 2012, Royal Society of Chemistry)

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In an interesting report by Yan et al. [143], polyamide 12 (PA12)/TRGO nanocomposites were prepared by melt compounding and maleated polyethylene-octene (POE-g-MA) rubber was added as a compatibilizer. The authors studied the effect of the compounding sequence on the electrical conductivity of the nanocomposites. It was demonstrated that the best results were obtained by initially compounding the PA12 with the TRGO, but the presence of

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maleated rubber on the final composite sample induced a volume-exclusion effect where the TRGO was selectively located in the PA12 rather than in the POE-g-MA phase as

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demonstrated by TEM. This work is a good example of how the melt compounding, which is a common industrial procedure, can be utilized to prepare electrically-conductive, rubberbased nanocomposites with values even higher than some systems prepared by solution mixing or in situ polymerization.

Although the loading and the dispersion of the graphene-based materials in an elastomer are the main factors that control the resistivity, it is also found to change with deformation of the

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nanocomposite. In a recent study, Coleman and coworkers [213] reported the development of graphene/elastomer based strain sensors and they demonstrated their utility as bodily motion sensors. These sensors were made by infusing liquid-phase exfoliated graphene into a swollen elastomer by soaking the elastomer in a graphene/NMP dispersion for 15-48 h

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(Figure 20). It was found that the resistance of such material was less than 100 kΩ and also sensitive to strain. Moreover, the electrical conductivity was as high as 0.1 S/m. The example shown in Figure 20 is of the dynamic resistance change for a sensor applied to a human throat

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to monitor breathing from the associated bodily movement. The specific report by Coleman’s group is a notable illustration of how elastomer/graphene nanocomposites can be utilized for advanced applications, while their preparation methods do not have to be over-complicated or particularly expensive.

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Figure 20. Graphene-elastomer strain sensors showing the preparation by liquid-phase

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infusion into a swollen elastomer and the change in resistance through straining by breathing (Reprinted (adapted) with permission from [213] Copyright 2014, American Chemical

5.5.2 Dielectric Properties

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Society).

The dynamics of the macromolecular chains of polymer composites can be also studied by dielectric analysis. The relaxation spectra as a function of either temperature or frequency, can provide information on the intermolecular cooperative motion and hindered dipolar

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rotation, hence this technique has also been applied to several graphene-based/elastomer nanocomposites [112, 128, 134, 140, 142, 144, 152, 158, 160, 180, 212]. The aspect ratio of the graphene and its adhesion with the matrix can affect the dielectric properties of the nanocomposite samples significantly, which then can be exploited in flexible electronics, and

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other applications [214]. The polymer-filler interfacial bonding can be considered as the analogous to the donor-acceptor complexes, described by the Maxwell-Wagner-Sillars

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(MWS) process for dielectric properties. When a current flows through a nanocomposite, the charges can accumulate at the interfaces between two dielectrics with different relaxation times [112].

Wu et al. [128] studied the chain dynamics of butadiene-styrene-vinyl pyridine (VPR)/GO nanocomposites thoroughly by dielectric analysis. They prepared nanocomposites with different types of interfacial bonding (ionic and hydrogen), by using two flocculants (HCl and CaCl2) for co-coagulation and found out that the nature of the interfacial interactions, due to the different bonding characteristics, slowed down the segmental relaxations of the composites with the ionic interfaces (HVPR-GO) (Figure 21). In contrast, the segmental 46

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dynamics of (CaVPR-GO) were not affected by the presence of GO at low filler content, while for contents higher than 2.5 vol.% the dielectric loss peaks began to be eroded in the low frequency region unlike in the HVPR-GO in which they were completely shielded [128]. As a conclusion, the parameters which ultimately affect the dielectric properties of graphene-

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based elastomers are as with most properties, the homogeneity and the reinforcing ability of

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the filler, along with the thickness of the specimen and the loading of the filler.

Figure 21. Alteration of chain dynamics due to the different bonding in VPR/GO nanocomposites (Reproduced with permission from [128] Copyright 2013, Royal Society of

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Chemistry).

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5.6 Barrier properties

The control and optimization of gas and liquid barrier properties in polymer nanocomposites could be one of the most significant steps in the commercialization of these materials. Elastomers generally have poor barrier properties, so the physical barrier that the inorganic fillers impose on the gases and liquids that are attempting to flow through the material is an obvious reason for their use in elastomer-based nanocomposites. The application of graphene in this class of materials is very important for this specific goal, since its defect-free monolayers are impermeable to all gases including helium [215]. Recently Su et al. found that the exceptional barrier properties of multilayer graphitic films can be attributed to the 47

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high degree of graphitization of the laminates and little structural damage during the reduction process [216]. Several attempts have been made to enhance the barrier properties of elastomeric materials, that possess large free volume and therefore normally exhibit poor gas barrier properties

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[112, 124, 126, 129, 132-134, 162, 172, 181, 183, 186, 217]. The use of graphene as reinforcement in this type of material, can form a percolating network, that provides a “tortuous path” and so also inhibits molecular diffusion through the matrix [10]. Once again, the permeability as a function of the filler content, can give indication about the state of

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dispersion of the filler. In addition to the dispersion of the fillers and the formation of the percolating network, the orientation of the filler also plays a significant role in platelet-

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reinforced nanocomposites.

Wu et al. [134] compared the air permeability of a surface-functionalized graphene oxide (SGO)/natural rubber (NR) composite with that of a thermally-reduced graphene oxide (TRGO)/NR composite. It was found out that the presence of wrinkles and waves in the surface of TRGO due to the superheating process along with the reduction of the diameter of the TRGO sheets, led to the TRGO/NR composite having inferior gas barrier properties. From this work it can be understood that the unavoidable wrinkling of graphene that occurs

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during the mixing with the matrix always acts negatively on the gas barrier properties of graphene/elastomer composites. Therefore, since graphene without wrinkles is difficult to produce in practice, the preparation of an impermeable nanocomposite material is

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challenging.

Scherillo et al. [132] demonstrated the importance of a segregated morphology upon the oxygen barrier properties of segregated and non-segregated graphene oxide (GO)/natural

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rubber (NR) nanocomposites. According to their findings, the formation of a segregated network reduces significantly the gas permeability since the latex spheres become completely coated by impervious GO platelets. The formation of this network can lead to a significantly longer diffusion path for low molecular weight molecules (Figure 22). This is another good example of the advantages of the formation of a segregated network of fillers; it increases simultaneously both the electrical conductivity and the gas barrier properties of the nanocomposite materials. During the assessment of the liquid barrier properties of graphene/elastomer nanocomposites, it has been found that additional factors need be taken into consideration, such as the 48

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interaction of the solvent with the graphene, the nature of the polymer, the interface region, the clustering of solvent molecules and the crazing or partial dissolution of the composites in

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the presence solvent [112].

Figure 22. (a) Representation of the two morphologies for the GO/NR nanocomposites, (b) oxygen permeability for the samples with the different morphologies, as a function of

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volumetric fraction of graphene (Reprinted (adapted) with permission from [132] Copyright 2014, American Chemical Society).

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5.7 Swelling and Vulcanization

Swelling measurements are commonly reported in literature for the observation of the

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transport of solvent into the nanocomposite and the determination of the crosslink density between the filler and the elastomer or between the macromolecular chains of the elastomer [134, 139, 141, 148, 151, 152, 159, 161, 200]. The lamellar nanostructure of layered graphene fillers once again acts as a barrier that hinders the permeation of the solvent with increasing filler content. Moreover, the formation of networks, between the filler particles/layers that hinder the movement of the macromolecular chains or the strong interaction between the electron acceptor/electron donating groups of the components of the system can act favourably towards the apparent increase of crosslinking density. For this reason, the graphene or graphene oxide which is used as reinforcing agent is functionalized, 49

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in order to form strong bonds with the matrix and increase the crosslinking density (Figure 23). The increased crosslinking network can contribute to the enhanced mechanical and thermal performance of the final nanocomposite. This can also be verified from measurements of the torque/time curves during the curing or processing of the

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graphene/elastomer nanocomposites [130, 134, 138, 141, 146, 148, 152, 172]. The curing curves are normally shifted towards a shorter time with increased filler loading, which indicates an acceleration of the vulcanization phenomenon, while the maximum torque values

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of the curing curves may be increased by the presence of graphene.

Figure 23. (a) Schematic representation of the effect of functionalization of graphene oxide (GO) to the formation of crosslinks and linkages with the natural rubber (NR) matrix (Reproduced with permission from [134] Copyright 2013, Elsevier) (b) curing curves of

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graphene (GE)/natural rubber (NR) composites (Reproduced with permission from [155] Copyright 2013, Elsevier)

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6. Conclusions and Perspectives The synthesis and properties of graphene, graphene oxide and graphene-based elastomeric nanocomposites have been reviewed. It has been shown that both graphene and graphene oxide with their unique assets can be utilized for a range of applications and show excellent

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promise as reinforcing agents in high-end elastomeric nanocomposites. The combination of the desirable characteristics that conventional fillers such as layered silicates and carbon nanotubes offer individually, makes graphene/elastomer nanocomposite materials attractive,

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since their properties can be exploited in a number of different ways.

The graphene structure, the number of layers, the specific surface and the chemical functionalization of these fillers all play a major role in the final performance of the reinforced elastomers. Moreover, the variety of preparation methods that have been presented in this review offer alternative pathways, depending upon the perceived application of the

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material. For example, solution blending and in-situ polymerization produce composites with a high degree of homogeneity for research purposes, but these processes are very difficult to scale up, while the environmental concerns need also to be taken into account. On the other hand, the conventional melt-mixing process with the use of multi-roll mills, commonly

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employed in industry, may produce nanocomposites with properties somewhat inferior than for other preparation methods, but which may still be adequate for some applications. The scaling-up of the production of high-quality graphene is a determinant factor for the

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production on commercial scale of elastomeric nanocomposites with useful properties. The different processes being investigated for the production of few-layer, high-quality graphene in bulk have been reviewed. The properties of the graphene/elastomer nanocomposite materials depend intimately upon the preparation method which in turn, affects the dispersion of the filler and ultimately its final physicochemical properties. The dispersion can be evaluated in several ways; microscopy is always the default option for the direct observation of the fillers in the matrix, while the study of the percolation threshold for properties such as the electrical conductivity, the gas permeability, the thermal stability or even the mechanical properties, offer indirect information on the state of dispersion. The 51

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tuning of the structure/surface of the filler to be compatible with the elastomer can ensure an interaction between the components of the system is also commonly employed for the formation of a strong interface which will have a positive effect upon the material. The modification of the structure of the filler or the matrix, however, may improve a number of

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properties but worsen others. It has been shown that random orientation can improve the electrical conductivity, while the formation of an oriented network of fillers can improve the gas permeability or mechanical properties in certain directions. In this context, since it is impossible to improve all properties in a nanocomposite simultaneously, the preparation

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method needs to be tailored each time for the targeted application.

So far, the incorporation of graphene and graphene oxide in elastomeric matrixes has been shown to lead to a significant enhancement in the mechanical, thermal and barrier properties.

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Several preparation strategies were evaluated in terms of the dispersion of the fillers and the filler-filler and filler-matrix interactions, while a number of characterization techniques were discussed. As a general conclusion we can affirm that the use of graphene in this specific class of materials can overcome the some of the disadvantages of elastomers and produce high-quality elastomeric nanocomposites. On several occasions, it has been shown that the combination of the choice of the type of graphene and carefully-followed preparation

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methods can avoid the traditional problems observed in such nanocomposites, such as low deformation extensibility. It is clear that in the near future, this family of materials will be able to find use in both conventional and advanced applications ranging from tyres, automotive components, seals of all kinds, hydraulic hoses, footwear and water-proof

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clothing to tribological, biomedical, aerospace, sensing devices and nanoelectronics.

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Acknowledgements

This research has been supported by funding from the European Union Seventh Framework Programme under grant agreement n°604391 Graphene Flagship and the EPSRC (award no. EP/I023879/1). The authors wish to acknowledge part-funding for this research by BP through the BP International Centre for Advanced Materials.

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References

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[1] McNaught AD, McNaught AD. Compendium of chemical terminology: Blackwell Science Oxford 1997. [2] Hosler D, Burkett SL, Tarkanian MJ. Prehistoric polymers: Rubber processing in ancient mesoamerica. Science. 1999;284(5422):1988-91. [3] Heinrich G, Basak GC. Advanced rubber composites: Springer 2011. [4] Ceresana. Market study: Synthetic rubber (uc-4905). 2012 [cited; Available from: http://www.ceresana.com/en/market-studies/plastics/synthetic-rubber/market-study-syntheticrubber.html [5] Alexandre M, Dubois P. Polymer-layered silicate nanocomposites: Preparation, properties and uses of a new class of materials. Materials Science and Engineering R: Reports. 2000;28(1):1-63. [6] Moniruzzaman M, Winey KI. Polymer nanocomposites containing carbon nanotubes. Macromolecules. 2006;39(16):5194-205. [7] Pavlidou S, Papaspyrides CD. A review on polymer-layered silicate nanocomposites. Progress in Polymer Science (Oxford). 2008;33(12):1119-98. [8] Zou H, Wu S, Shen J. Polymer/silica nanocomposites: Preparation, characterization, properties, and applications. Chemical Reviews. 2008;108(9):3893-957. [9] Sinha Ray S, Okamoto M. Polymer/layered silicate nanocomposites: A review from preparation to processing. Progress in Polymer Science (Oxford). 2003;28(11):1539-641. [10] Paul DR, Robeson LM. Polymer nanotechnology: Nanocomposites. Polymer. 2008;49(15):3187-204. [11] Heinrich G, Klüppel M, Vilgis TA. Reinforcement of elastomers. Current Opinion in Solid State and Materials Science. 2002;6(3):195-203. [12] Koerner H, Price G, Pearce NA, Alexander M, Vaia RA. Remotely actuated polymer nanocomposites--stress-recovery of carbon-nanotube-filled thermoplastic elastomers. Nature Materials. 2004;3(2):115-20. [13] Bokobza L. Multiwall carbon nanotube elastomeric composites: A review. Polymer. 2007;48(17):4907-20. [14] Dannenberg EM. Carbon black, physics, chemistry, and elastomer reinforcement. New York: John Wiley & Sons, Inc. 1977. [15] Leblanc JL. Rubber–filler interactions and rheological properties in filled compounds. Progress in Polymer Science. 2002;27(4):627-87. [16] Frogley MD, Ravich D, Wagner HD. Mechanical properties of carbon nanoparticlereinforced elastomers. Composites Science and Technology. 2003;63(11):1647-54. [17] Donnet JB. Nano and microcomposites of polymers elastomers and their reinforcement. Composites Science and Technology. 2003;63(8):1085-8. [18] Boonstra BB. Role of particulate fillers in elastomer reinforcement: A review. Polymer. 1979;20(6):691-704. [19] Edwards DC. Polymer-filler interactions in rubber reinforcement. Journal of Materials Science. 1990;25(10):4175-85. [20] Joly S, Garnaud G, Ollitrault R, Bokobza L, Mark JE. Organically modified layered silicates as reinforcing fillers for natural rubber. Chemistry of Materials. 2002;14(10):4202-8. [21] Ponnamma D, Sadasivuni KK, Grohens Y, Guo Q, Thomas S. Carbon nanotube based elastomer composites - an approach towards multifunctional materials. Journal of Materials Chemistry C. 2014;2(40):8446-85. [22] Sherif A, Qingshi M, Liqun Z, Izzuddin Z, Peter M, Jun M. Elastomeric composites based on carbon nanomaterials. Nanotechnology. 2015;26(11):112001. 53

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[23] Basu D, Das A, Stöckelhuber KW, Wagenknecht U, Heinrich G. Advances in layered double hydroxide (ldh)-based elastomer composites. Progress in Polymer Science. 2014;39(3):594-626. [24] Usuki A, Kojima Y, Kawasumi M, Okada A, Fukushima Y, Kurauchi T, et al. Synthesis of nylon 6-clay hybrid. Journal of Materials Research. 1993;8(05):1179-84. [25] Kojima Y, Usuki A, Kawasumi M, Okada A, Kurauchi T, Kamigaito O. Synthesis of nylon 6–clay hybrid by montmorillonite intercalated with ϵ‐caprolactam. Journal of Polymer Science Part A: Polymer Chemistry. 1993;31(4):983-6. [26] Kojima Y, Usuki A, Kawasumi M, Okada A, Fukushima Y, Kurauchi T, et al. Mechanical properties of nylon 6-clay hybrid. Journal of Materials Research. 1993;8(05):1185-9. [27] Geim AK, Novoselov KS. The rise of graphene. Nature Materials. 2007;6(3):183-91. [28] Singh V, Joung D, Zhai L, Das S, Khondaker SI, Seal S. Graphene based materials: Past, present and future. Progress in Materials Science. 2011;56(8):1178-271. [29] Novoselov KS, Geim AK, Morozov SV, Jiang D, Katsnelson MI, Grigorieva IV, et al. Two-dimensional gas of massless dirac fermions in graphene. Nature. 2005;438(7065):197200. [30] Balandin AA, Ghosh S, Bao W, Calizo I, Teweldebrhan D, Miao F, et al. Superior thermal conductivity of single-layer graphene. Nano Letters. 2008;8(3):902-7. [31] Lee C, Wei X, Kysar JW, Hone J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science. 2008;321(5887):385-8. [32] Dreyer DR, Park S, Bielawski CW, Ruoff RS. The chemistry of graphene oxide. Chemical Society Reviews. 2010;39(1):228-40. [33] Novoselov KS, Geim AK, Morozov S, Jiang D, Zhang Y, Dubonos S, et al. Electric field effect in atomically thin carbon films. Science. 2004;306(5696):666-9. [34] Peierls R. Quelques propriétés typiques des corps solides. Annales de l'institut Henri Poincaré; 1935: Presses universitaires de France; 1935. p. 177-222. [35] Potts JR, Dreyer DR, Bielawski CW, Ruoff RS. Graphene-based polymer nanocomposites. Polymer. 2011;52(1):5-25. [36] Kim H, Abdala AA, Macosko CW. Graphene/polymer nanocomposites. Macromolecules. 2010;43(16):6515-30. [37] Sengupta R, Bhattacharya M, Bandyopadhyay S, Bhowmick AK. A review on the mechanical and electrical properties of graphite and modified graphite reinforced polymer composites. Progress in Polymer Science. 2011;36(5):638-70. [38] Terrones M, Martín O, González M, Pozuelo J, Serrano B, Cabanelas JC, et al. Interphases in graphene polymer‐based nanocomposites: Achievements and challenges. Advanced Materials. 2011;23(44):5302-10. [39] Cai D, Song M. Recent advance in functionalized graphene/polymer nanocomposites. Journal of Materials Chemistry. 2010;20(37):7906-15. [40] Kuilla T, Bhadra S, Yao D, Kim NH, Bose S, Lee JH. Recent advances in graphene based polymer composites. Progress in Polymer Science. 2010;35(11):1350-75. [41] Young RJ, Kinloch IA, Nicolais L. Graphene composites. Wiley encyclopedia of composites: John Wiley & Sons, Inc. 2011. [42] Sadasivuni KK, Ponnamma D, Thomas S, Grohens Y. Evolution from graphite to graphene elastomer composites. Progress in Polymer Science. 2014;39(4):749-80. [43] Young RJ, Kinloch IA, Gong L, Novoselov KS. The mechanics of graphene nanocomposites: A review. Composites Science and Technology. 2012;72(12):1459-76. [44] Gong L, Kinloch IA, Young RJ, Riaz I, Jalil R, Novoselov KS. Interfacial stress transfer in a graphene monolayer nanocomposite. Advanced Materials. 2010;22(24):2694-7.

54

ACCEPTED MANUSCRIPT

AC C

EP

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SC

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[45] Young RJ, Gong L, Kinloch IA, Riaz I, Jalil R, Novoselov KS. Strain mapping in a graphene monolayer nanocomposite. ACS Nano. 2011;5(4):3079-84. [46] Hernandez Y, Nicolosi V, Lotya M, Blighe FM, Sun Z, De S, et al. High-yield production of graphene by liquid-phase exfoliation of graphite. Nature Nanotechnology. 2008;3(9):563-8. [47] Blake P, Brimicombe PD, Nair RR, Booth TJ, Jiang D, Schedin F, et al. Graphenebased liquid crystal device. Nano Letters. 2008;8(6):1704-8. [48] Park S, An J, Jung I, Piner RD, An SJ, Li X, et al. Colloidal suspensions of highly reduced graphene oxide in a wide variety of organic solvents. Nano Letters. 2009;9(4):15937. [49] Paredes JI, Villar-Rodil S, Martínez-Alonso A, Tascón JMD. Graphene oxide dispersions in organic solvents. Langmuir. 2008;24(19):10560-4. [50] Bourlinos AB, Georgakilas V, Zboril R, Steriotis TA, Stubos AK. Liquid-phase exfoliation of graphite towards solubilized graphenes. Small. 2009;5(16):1841-5. [51] Ciesielski A, Samori P. Graphene via sonication assisted liquid-phase exfoliation. Chemical Society Reviews. 2014;43(1):381-98. [52] Lotya M, Hernandez Y, King PJ, Smith RJ, Nicolosi V, Karlsson LS, et al. Liquid phase production of graphene by exfoliation of graphite in surfactant/water solutions. Journal of the American Chemical Society. 2009;131(10):3611-20. [53] Niu L, Li M, Tao X, Xie Z, Zhou X, Raju APA, et al. Salt-assisted direct exfoliation of graphite into high-quality, large-size, few-layer graphene sheets. Nanoscale. 2013;5(16):7202-8. [54] Emtsev KV, Bostwick A, Horn K, Jobst J, Kellogg GL, Ley L, et al. Towards wafersize graphene layers by atmospheric pressure graphitization of silicon carbide. Nature Materials. 2009;8(3):203-7. [55] Xiang JL, Drzal LT. Thermal conductivity of exfoliated graphite nanoplatelet paper. Carbon. 2011;49(3):773-8. [56] McAllister MJ, Li J-L, Adamson DH, Schniepp HC, Abdala AA, Liu J, et al. Single sheet functionalized graphene by oxidation and thermal expansion of graphite. Chemistry of Materials. 2007;19(18):4396-404. [57] McAllister MJ, Li J-L, Adamson DH, Schniepp HC, Abdala AA, Liu J, et al. Single sheet functionalized graphene by oxidation and thermal expansion of graphite. Chemistry of Materials. 2007;19(18):4396-404. [58] Zhang H-B, Wang J-W, Yan Q, Zheng W-G, Chen C, Yu Z-Z. Vacuum-assisted synthesis of graphene from thermal exfoliation and reduction of graphite oxide. Journal of Materials Chemistry. 2011;21(14):5392-7. [59] Yu Q, Lian J, Siriponglert S, Li H, Chen YP, Pei S-S. Graphene segregated on ni surfaces and transferred to insulators. Applied Physics Letters. 2008;93(113101):1-3. [60] Zhang Y, Zhang L, Zhou C. Review of chemical vapor deposition of graphene and related applications. Accounts of Chemical Research. 2013;46(10):2329-39. [61] Suk JW, Kitt A, Magnuson CW, Hao Y, Ahmed S, An J, et al. Transfer of cvd-grown monolayer graphene onto arbitrary substrates. ACS Nano. 2011;5(9):6916-24. [62] Li X, Magnuson CW, Venugopal A, Tromp RM, Hannon JB, Vogel EM, et al. Largearea graphene single crystals grown by low-pressure chemical vapor deposition of methane on copper. Journal of the American Chemical Society. 2011;133(9):2816-9. [63] Ruan G, Sun Z, Peng Z, Tour JM. Growth of graphene from food, insects, and waste. ACS Nano. 2011;5(9):7601-7. [64] Li X, Cai W, An J, Kim S, Nah J, Yang D, et al. Large-area synthesis of high-quality and uniform graphene films on copper foils. Science. 2009;324(5932):1312-4.

55

ACCEPTED MANUSCRIPT

AC C

EP

TE D

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[65] Herron CR, Coleman KS, Edwards RS, Mendis BG. Simple and scalable route for the 'bottom-up' synthesis of few-layer graphene platelets and thin films. Journal of Materials Chemistry. 2011;21(10):3378-83. [66] Edwards RS, Coleman KS. Graphene synthesis: Relationship to applications. Nanoscale. 2013;5(1):38-51. [67] Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Kim KS, et al. Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature. 2009;457(7230):70610. [68] Nair RR, Blake P, Grigorenko AN, Novoselov KS, Booth TJ, Stauber T, et al. Fine structure constant defines visual transparency of graphene. Science. 2008;320(5881):1308. [69] Blake P, Hill E, Neto AC, Novoselov K, Jiang D, Yang R, et al. Making graphene visible. Applied Physics Letters. 2007;91(6):063124. [70] Ni ZH, Wang HM, Kasim J, Fan HM, Yu T, Wu YH, et al. Graphene thickness determination using reflection and contrast spectroscopy. Nano Letters. 2007;7(9):2758-63. [71] Yazyev OV, Louie SG. Electronic transport in polycrystalline graphene. Nature Materials. 2010;9(10):806-9. [72] Červenka J, Katsnelson M, Flipse C. Room-temperature ferromagnetism in graphite driven by two-dimensional networks of point defects. Nature Physics. 2009;5(11):840-4. [73] Yazyev OV, Louie SG. Topological defects in graphene: Dislocations and grain boundaries. Physical Review B. 2010;81(19):195420. [74] Huang PY, Ruiz-Vargas CS, van der Zande AM, Whitney WS, Levendorf MP, Kevek JW, et al. Grains and grain boundaries in single-layer graphene atomic patchwork quilts. Nature. 2011;469(7330):389-92. [75] Meyer JC, Geim AK, Katsnelson MI, Novoselov KS, Booth TJ, Roth S. The structure of suspended graphene sheets. Nature. 2007;446(7131):60-3. [76] Tung VC, Allen MJ, Yang Y, Kaner RB. High-throughput solution processing of large-scale graphene. Nature Nanotechnology. 2009;4(1):25-9. [77] Xu K, Cao P, Heath JR. Scanning tunneling microscopy characterization of the electrical properties of wrinkles in exfoliated graphene monolayers. Nano Letters. 2009;9(12):4446-51. [78] Decker Rg, Wang Y, Brar VW, Regan W, Tsai H-Z, Wu Q, et al. Local electronic properties of graphene on a bn substrate via scanning tunneling microscopy. Nano Letters. 2011;11(6):2291-5. [79] Bao W, Miao F, Chen Z, Zhang H, Jang W, Dames C, et al. Controlled ripple texturing of suspended graphene and ultrathin graphite membranes. Nature Nanotechnology. 2009;4(9):562-6. [80] Xu B, Yue S, Sui Z, Zhang X, Hou S, Cao G, et al. What is the choice for supercapacitors: Graphene or graphene oxide? Energy & Environmental Science. 2011;4(8):2826-30. [81] Khanra P, Kuila T, Bae SH, Kim NH, Lee JH. Electrochemically exfoliated graphene using 9-anthracene carboxylic acid for supercapacitor application. Journal of Materials Chemistry. 2012;22(46):24403-10. [82] Graf D, Molitor F, Ensslin K, Stampfer C, Jungen A, Hierold C, et al. Spatially resolved raman spectroscopy of single- and few-layer graphene. Nano Letters. 2007;7(2):23842. [83] Ferrari A, Meyer J, Scardaci V, Casiraghi C, Lazzeri M, Mauri F, et al. Raman spectrum of graphene and graphene layers. Physical Review Letters. 2006;97(18):187401. [84] Young RJ, Kinloch IA. Graphene and graphene-based nanocomposites. Nanoscience: Volume 1: Nanostructures through chemistry: The Royal Society of Chemistry 2013:145-79.

56

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

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[85] Malard LM, Pimenta MA, Dresselhaus G, Dresselhaus MS. Raman spectroscopy in graphene. Physics Reports. 2009;473(5–6):51-87. [86] Eckmann A, Felten A, Verzhbitskiy I, Davey R, Casiraghi C. Raman study on defective graphene: Effect of the excitation energy, type, and amount of defects. Physical Review B. 2013;88(3):035426. [87] Luo Z, Yu T, Shang J, Wang Y, Lim S, Liu L, et al. Large-scale synthesis of bi-layer graphene in strongly coupled stacking order. Advanced Functional Materials. 2011;21(5):911-7. [88] Liu F, Ming PM, Li J. Ab initio calculation of ideal strength and phonon instability of graphene under tension. Physical Review B. 2007;76(6):064120. [89] Young RJ. Monitoring deformation processes in high-performance fibres using raman spectroscopy. The Journal of The Textile Institute. 1995;86(2):360-81. [90] Frank O, Tsoukleri G, Riaz I, Papagelis K, Parthenios J, Ferrari AC, et al. Development of a universal stress sensor for graphene and carbon fibres. Nature Communications. 2011;2:255. [91] Andrews M, Young R. Analysis of the deformation of aramid fibres and composites using raman spectroscopy. Journal of Raman spectroscopy. 1993;24(8):539-44. [92] Raju APA, Lewis A, Derby B, Young RJ, Kinloch IA, Zan R, et al. Wide-area strain sensors based upon graphene-polymer composite coatings probed by Raman spectroscopy. Advanced Functional Materials. 2014;24(19):2865-74. [93] Li Z, Young RJ, Kinloch IA. Interfacial stress transfer in graphene oxide nanocomposites. ACS Applied Materials & Interfaces. 2012;5(2):456-63. [94] Low T, Perebeinos V, Tersoff J, Avouris P. Deformation and scattering in graphene over substrate steps. Physical Review Letters. 2012;108(9):096601. [95] Abdelouahed S, Ernst A, Henk J, Maznichenko I, Mertig I. Spin-split electronic states in graphene: Effects due to lattice deformation, rashba effect, and adatoms by first principles. Physical Review B. 2010;82(12):125424. [96] Gong L, Young RJ, Kinloch IA, Haigh SJ, Warner JH, Hinks JA, et al. Reversible loss of bernal stacking during the deformation of few-layer graphene in nanocomposites. ACS Nano. 2013;7(8):7287-94. [97] Ghosh S, Nika DL, Pokatilov EP, Balandin AA. Heat conduction in graphene: Experimental study and theoretical interpretation. New Journal of Physics. 2009;11(9):095012. [98] Balandin AA. Thermal properties of graphene and nanostructured carbon materials. Nature Materials. 2011;10(8):569-81. [99] Ghosh S, Bao W, Nika DL, Subrina S, Pokatilov EP, Lau CN, et al. Dimensional crossover of thermal transport in few-layer graphene. Nature Materials. 2010;9(7):555-8. [100] Zhang Y, Tan Y-W, Stormer HL, Kim P. Experimental observation of the quantum hall effect and berry's phase in graphene. Nature. 2005;438(7065):201-4. [101] Stauffer D, Aharony A. Introduction to percolation theory: Taylor and Francis 1991. [102] Brodie B. Sur le poids atomique du graphite. Annales de Chimie et de Physique. 1860;59(1860):466-72. [103] Staudenmaier L. Verfahren zur darstellung der graphitsäure. Berichte der deutschen chemischen Gesellschaft. 1898;31(2):1481-7. [104] Hummers Jr WS, Offeman RE. Preparation of graphitic oxide. Journal of the American Chemical Society. 1958;80(6):1339. [105] Park S, Ruoff RS. Chemical methods for the production of graphenes. Nature Nanotechnology. 2009;4(4):217-24.

57

ACCEPTED MANUSCRIPT

AC C

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[106] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon. 2007;45(7):1558-65. [107] Zhang J, Yang H, Shen G, Cheng P, Zhang J, Guo S. Reduction of graphene oxide vial-ascorbic acid. Chemical Communications. 2010;46(7):1112-4. [108] Shin HJ, Kim KK, Benayad A, Yoon SM, Park HK, Jung IS, et al. Efficient reduction of graphite oxide by sodium borohydride and its effect on electrical conductance. Advanced Functional Materials. 2009;19(12):1987-92. [109] Robinson JT, Perkins FK, Snow ES, Wei Z, Sheehan PE. Reduced graphene oxide molecular sensors. Nano Letters. 2008;8(10):3137-40. [110] Rourke JP, Pandey PA, Moore JJ, Bates M, Kinloch IA, Young RJ, et al. The real graphene oxide revealed: Stripping the oxidative debris from the graphene-like sheets. Angewandte Chemie International Edition. 2011;50(14):3173-7. [111] Thomas HR, Day SP, Woodruff WE, Valles C, Young RJ, Kinloch IA, et al. Deoxygenation of graphene oxide: Reduction or cleaning? Chemistry of Materials. 2013;25(18):3580-8. [112] Kumar SK, Castro M, Saiter A, Delbreilh L, Feller JF, Thomas S, et al. Development of poly (isobutylene-co-isoprene)/reduced graphene oxide nanocomposites for barrier, dielectric and sensingapplications. Materials Letters. 2013;96:109-12. [113] He H, Klinowski J, Forster M, Lerf A. A new structural model for graphite oxide. Chemical Physics Letters. 1998;287(1-2):53-6. [114] Lerf A, He H, Forster M, Klinowski J. Structure of graphite oxide revisited. Journal of Physical Chemistry B. 1998;102(23):4477-82. [115] Wilson NR, Pandey PA, Beanland R, Young RJ, Kinloch IA, Gong L, et al. Graphene oxide: Structural analysis and application as a highly transparent support for electron microscopy. ACS Nano. 2009;3(9):2547-56. [116] Erickson K, Erni R, Lee Z, Alem N, Gannett W, Zettl A. Determination of the local chemical structure of graphene oxide and reduced graphene oxide. Advanced Materials. 2010;22(40):4467-72. [117] Li D, Mueller MB, Gilje S, Kaner RB, Wallace GG. Processable aqueous dispersions of graphene nanosheets. Nature Nanotechnology. 2008;3(2):101-5. [118] Lerf A, He H, Forster M, Klinowski J. Structure of graphite oxide revisited. The Journal of Physical Chemistry B. 1998;102(23):4477-82. [119] Kudin KN, Ozbas B, Schniepp HC, Prud'Homme RK, Aksay IA, Car R. Raman spectra of graphite oxide and functionalized graphene sheets. Nano Letters. 2008;8(1):36-41. [120] Dikin DA, Stankovich S, Zimney EJ, Piner RD, Dommett GHB, Evmenenko G, et al. Preparation and characterization of graphene oxide paper. Nature. 2007;448(7152):457-60. [121] Gomez-Navarro C, Burghard M, Kern K. Elastic properties of chemically derived single graphene sheets. Nano Letters. 2008;8(7):2045-9. [122] Bianco A, Cheng H-M, Enoki T, Gogotsi Y, Hurt RH, Koratkar N, et al. All in the graphene family – a recommended nomenclature for two-dimensional carbon materials. Carbon. 2013;65(0):1-6. [123] Wick P, Louw-Gaume AE, Kucki M, Krug HF, Kostarelos K, Fadeel B, et al. Classification framework for graphene-based materials. Angewandte Chemie International Edition. 2014;53(30):7714-8. [124] Kim H, Miura Y, Macosko CW. Graphene/polyurethane nanocomposites for improved gas barrier and electrical conductivity. Chemistry of Materials. 2010;22(11):344150.

58

ACCEPTED MANUSCRIPT

AC C

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[125] Araby S, Meng Q, Zhang L, Kang H, Majewski P, Tang Y, et al. Electrically and thermally conductive elastomer/graphene nanocomposites by solution mixing. Polymer. 2014;55(1):201-10. [126] Sadasivuni K, Saiter A, Gautier N, Thomas S, Grohens Y. Effect of molecular interactions on the performance of poly(isobutylene-co-isoprene)/graphene and clay nanocomposites. Colloid Polym Sci. 2013;291(7):1729-40. [127] Chen B, Ma N, Bai X, Zhang H, Zhang Y. Effects of graphene oxide on surface energy, mechanical, damping and thermal properties of ethylene-propylene-diene rubber/petroleum resin blends. RSC Advances. 2012;2(11):4683-9. [128] Wu S, Tang Z, Guo B, Zhang L, Jia D. Effects of interfacial interaction on chain dynamics of rubber/graphene oxide hybrids: A dielectric relaxation spectroscopy study. RSC Advances. 2013;3(34):14549-59. [129] Zhan Y, Lavorgna M, Buonocore G, Xia H. Enhancing electrical conductivity of rubber composites by constructing interconnected network of self-assembled graphene with latex mixing. Journal of Materials Chemistry. 2012;22(21):10464-8. [130] Tang Z, Wu X, Guo B, Zhang L, Jia D. Preparation of butadiene–styrene–vinyl pyridine rubber–graphene oxide hybrids through co-coagulation process and in situ interface tailoring. Journal of Materials Chemistry. 2012;22(15):7492-501. [131] Potts JR, Shankar O, Du L, Ruoff RS. Processing–morphology–property relationships and composite theory analysis of reduced graphene oxide/natural rubber nanocomposites. Macromolecules. 2012;45(15):6045-55. [132] Scherillo G, Lavorgna M, Buonocore GG, Zhan YH, Xia HS, Mensitieri G, et al. Tailoring assembly of reduced graphene oxide nanosheets to control gas barrier properties of natural rubber nanocomposites. ACS Applied Materials & Interfaces. 2014;6(4):2230-4. [133] Kang H, Zuo K, Wang Z, Zhang L, Liu L, Guo B. Using a green method to develop graphene oxide/elastomers nanocomposites with combination of high barrier and mechanical performance. Composites Science and Technology. 2014;92(0):1-8. [134] Wu J, Huang G, Li H, Wu S, Liu Y, Zheng J. Enhanced mechanical and gas barrier properties of rubber nanocomposites with surface functionalized graphene oxide at low content. Polymer. 2013;54(7):1930-7. [135] Khajehpour M, Sadeghi S, Zehtab Yazdi A, Sundararaj U. Tuning the curing behavior of fluoroelastomer (fkm) by incorporation of nitrogen doped graphene nanoribbons (cnxgnrs). Polymer. 2014;55(24):6293-302. [136] Potts JR, Shankar O, Murali S, Du L, Ruoff RS. Latex and two-roll mill processing of thermally-exfoliated graphite oxide/natural rubber nanocomposites. Composites Science and Technology. 2013;74(0):166-72. [137] Dao TD, Lee H-i, Jeong HM. Alumina-coated graphene nanosheet and its composite of acrylic rubber. Journal of Colloid and Interface Science. 2014;416:38-43. [138] Wei J, Qiu J. Allyl-functionalization enhanced thermally stable graphene/fluoroelastomer nanocomposites. Polymer. 2014;55(16):3818-24. [139] Allahbakhsh A, Mazinani S, Kalaee MR, Sharif F. Cure kinetics and chemorheology of epdm/graphene oxide nanocomposites. Thermochimica Acta. 2013;563:22-32. [140] Al-Hartomy OA, Al-Ghamdi A, Dishovsky N, Shtarkova R, Iliev V, Mutlay I, et al. Dielectric and microwave properties of natural rubber based nanocomposites containing graphene. Materials Sciences and Applications. 2012;3:453. [141] Zhan Y, Wu J, Xia H, Yan N, Fei G, Yuan G. Dispersion and exfoliation of graphene in rubber by an ultrasonically‐assisted latex mixing and in situ reduction process. Macromolecular Materials and Engineering. 2011;296(7):590-602.

59

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[142] Al-Hartomy O, Al-Ghamdi A, Al-Salamy F, Dishovsky N, Shtarkova R, Iliev V, et al. Effect of carbon nanotubes and graphene nanoplatelets on the dielectric and microwave properties of natural rubber composites. Advanced Composite Materials. 2013;22(5):361-76. [143] Yan D, Zhang HB, Jia Y, Hu J, Qi XY, Zhang Z, et al. Improved electrical conductivity of polyamide 12/graphene nanocomposites with maleated polyethylene-octene rubber prepared by melt compounding. ACS Applied Materials & Interfaces. 2012;4(9):47405. [144] Hernández M, Bernal MdM, Verdejo R, Ezquerra TA, López-Manchado MA. Overall performance of natural rubber/graphene nanocomposites. Composites Science and Technology. 2012;73:40-6. [145] Valentini L, Bolognini A, Alvino A, Bittolo Bon S, Martin-Gallego M, LopezManchado M. Pyroshock testing on graphene based EPDM nanocomposites. Composites Part B: Engineering. 2014;60:479-84. [146] Varghese TV, Ajith Kumar H, Anitha S, Ratheesh S, Rajeev R, Lakshmana Rao V. Reinforcement of acrylonitrile butadiene rubber using pristine few layer graphene and its hybrid fillers. Carbon. 2013;61:476-86. [147] Das A, Kasaliwal GR, Jurk R, Boldt R, Fischer D, Stöckelhuber KW, et al. Rubber composites based on graphene nanoplatelets, expanded graphite, carbon nanotubes and their combination: A comparative study. Composites Science and Technology. 2012;72(16):19617. [148] Lin Y, Liu K, Chen Y, Liu L. Influence of graphene functionalized with zinc dimethacrylate on the mechanical and thermal properties of natural rubber nanocomposites. Polymer Composites. 2014:Article In Press.( DOI: 10.1002/pc.23021) [149] Das A, Boldt R, Jurk R, Jehnichen D, Fischer D, Stockelhuber KW, et al. Nano-scale morphological analysis of graphene-rubber composites using 3d transmission electron microscopy. RSC Advances. 2014;4(18):9300-7. [150] Moghaddam SZ, Sabury S, Sharif F. Dispersion of rgo in polymeric matrices by thermodynamically favorable self-assembly of go at oil-water interfaces. RSC Advances. 2014;4(17):8711-9. [151] Sherif A, Izzuddin Z, Qingshi M, Nobuyuki K, Andrew M, Hsu-Chiang K, et al. Melt compounding with graphene to develop functional, high-performance elastomers. Nanotechnology. 2013;24(16):165601. [152] Mensah B, Kim S, Arepalli S, Nah C. A study of graphene oxide-reinforced rubber nanocomposite. Journal of Applied Polymer Science. 2014;131(16):40640. [153] Yan N, Xia H, Wu J, Zhan Y, Fei G, Chen C. Compatibilization of natural rubber/high density polyethylene thermoplastic vulcanizate with graphene oxide through ultrasonically assisted latex mixing. Journal of Applied Polymer Science. 2013;127(2):93341. [154] Hu H, Zhao L, Liu J, Liu Y, Cheng J, Luo J, et al. Enhanced dispersion of carbon nanotube in silicone rubber assisted by graphene. Polymer. 2012;53(15):3378-85. [155] Wu J, Xing W, Huang G, Li H, Tang M, Wu S, et al. Vulcanization kinetics of graphene/natural rubber nanocomposites. Polymer. 2013;54(13):3314-23. [156] Luo Y, Zhao P, Yang Q, He D, Kong L, Peng Z. Fabrication of conductive elastic nanocomposites via framing intact interconnected graphene networks. Composites Science and Technology. 2014;100(0):143-51. [157] Yang H, Liu P, Zhang T, Duan Y, Zhang J. Fabrication of natural rubber nanocomposites with high graphene contents via vacuum-assisted self-assembly. RSC Advances. 2014;4(53):27687-90.

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[158] Liu S, Tian M, Yan B, Yao Y, Zhang L, Nishi T, et al. High performance dielectric elastomers by partially reduced graphene oxide and disruption of hydrogen bonding of polyurethanes. Polymer. 2014;56:375-384. [159] Beckert F, Trenkle S, Thomann R, Mülhaupt R. Mechanochemical route to functionalized graphene and carbon nanofillers for graphene/SBR nanocomposites. Macromolecular Materials and Engineering. 2014;299(12):1513-20. [160] Singh VK, Shukla A, Patra MK, Saini L, Jani RK, Vadera SR, et al. Microwave absorbing properties of a thermally reduced graphene oxide/nitrile butadiene rubber composite. Carbon. 2012;50(6):2202-8. [161] Matos CF, Galembeck F, Zarbin AJ. Multifunctional and environmentally friendly nanocomposites between natural rubber and graphene or graphene oxide. Carbon. 2014;78:469-79. [162] Ozbas B, O'Neill CD, Register RA, Aksay IA, Prud'homme RK, Adamson DH. Multifunctional elastomer nanocomposites with functionalized graphene single sheets. Journal of Polymer Science Part B: Polymer Physics. 2012;50(13):910-6. [163] Barrett JS, Abdala AA, Srienc F. Poly (hydroxyalkanoate) elastomers and their graphene nanocomposites. Macromolecules. 2014;47(12):3926-41. [164] Bokobza L, Bruneel J-L, Couzi M. Raman spectroscopy as a tool for the analysis of carbon-based materials (highly oriented pyrolitic graphite, multilayer graphene and multiwall carbon nanotubes) and of some of their elastomeric composites. Vibrational Spectroscopy. 2014;74(0):57-63. [165] Bai X, Wan C, Zhang Y, Zhai Y. Reinforcement of hydrogenated carboxylated nitrile–butadiene rubber with exfoliated graphene oxide. Carbon. 2011;49(5):1608-13. [166] Ozbas B, Toki S, Hsiao BS, Chu B, Register RA, Aksay IA, et al. Strain-induced crystallization and mechanical properties of functionalized graphene sheet-filled natural rubber. Journal of Polymer Science Part B: Polymer Physics. 2012;50(10):718-23. [167] Xiong X, Wang J, Jia H, Fang E, Ding L. Structure, thermal conductivity, and thermal stability of bromobutyl rubber nanocomposites with ionic liquid modified graphene oxide. Polymer Degradation and Stability. 2013;98(11):2208-14. [168] Ponnamma D, Sadasivuni KK, Strankowski M, Guo Q, Thomas S. Synergistic effect of multi walled carbon nanotubes and reduced graphene oxides in natural rubber for sensing application. Soft Matter. 2013;9(43):10343-53. [169] Pradhan B, Srivastava SK. Synergistic effect of three-dimensional multi-walled carbon nanotube–graphene nanofiller in enhancing the mechanical and thermal properties of high-performance silicone rubber. Polymer International. 2014;63(7):1219-28. [170] Nawaz K, Khan U, Ul-Haq N, May P, O’Neill A, Coleman JN. Observation of mechanical percolation in functionalized graphene oxide/elastomer composites. Carbon. 2012;50(12):4489-94. [171] Chen Z, Lu H. Constructing sacrificial bonds and hidden lengths for ductile graphene/polyurethane elastomers with improved strength and toughness. Journal of Materials Chemistry. 2012;22(25):12479-90. [172] Wei J, Jacob S, Qiu J. Graphene oxide-integrated high-temperature durable fluoroelastomer for petroleum oil sealing. Composites Science and Technology. 2014;92(0):126-33. [173] Loomis J, Fan X, Khosravi F, Xu P, Fletcher M, Cohn RW, et al. Graphene/elastomer composite-based photo-thermal nanopositioners. Scientific Reports. 2013;3:1900. [174] Raghu AV, Lee YR, Jeong HM, Shin CM. Preparation and physical properties of waterborne polyurethane/functionalized graphene sheet nanocomposites. Macromolecular Chemistry and Physics. 2008;209(24):2487-93.

61

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

[175] Khan U, May P, O’Neill A, Coleman JN. Development of stiff, strong, yet tough composites by the addition of solvent exfoliated graphene to polyurethane. Carbon. 2010;48(14):4035-41. [176] Choi J, Kim D, Ryu K, Lee H-i, Jeong H, Shin C, et al. Functionalized graphene sheet/polyurethane nanocomposites: Effect of particle size on physical properties. Macromol Res. 2011;19(8):809-14. [177] Kim JS, Yun JH, Kim I, Shim SE. Electrical properties of graphene/sbr nanocomposite prepared by latex heterocoagulation process at room temperature. Journal of Industrial and Engineering Chemistry. 2011;17(2):325-30. [178] Lian H, Li S, Liu K, Xu L, Wang K, Guo W. Study on modified graphene/butyl rubber nanocomposites. I. Preparation and characterization. Polymer Engineering & Science. 2011;51(11):2254-60. [179] Gan L, Shang S, Yuen CWM, Jiang S-x, Luo NM. Facile preparation of graphene nanoribbon filled silicone rubber nanocomposite with improved thermal and mechanical properties. Composites Part B: Engineering. 2015;69(0):237-42. [180] Tian M, Zhang J, Zhang L, Liu S, Zan X, Nishi T, et al. Graphene encapsulated rubber latex composites with high dielectric constant, low dielectric loss and low percolation threshold. Journal of Colloid and Interface Science. 2014;430(0):249-56. [181] Xing W, Wu J, Huang G, Li H, Tang M, Fu X. Enhanced mechanical properties of graphene/natural rubber nanocomposites at low content. Polymer International. 2014;63(9):1674-81. [182] Liu X, Kuang W, Guo B. Preparation of rubber/graphene oxide composites with insitu interfacial design. Polymer. 2015;56:553-62. [183] Stanier DC, Patil AJ, Sriwong C, Rahatekar SS, Ciambella J. The reinforcement effect of exfoliated graphene oxide nanoplatelets on the mechanical and viscoelastic properties of natural rubber. Composites Science and Technology. 2014;95(0):59-66. [184] Li FY, Yan N, Zhan YH, Fei GX, Xia HS. Probing the reinforcing mechanism of graphene and graphene oxide in natural rubber. Journal of Applied Polymer Science. 2013;129(4):2342-51. [185] Xing W, Tang MZ, Wu JR, Huang GS, Li H, Lei ZY, et al. Multifunctional properties of graphene/rubber nanocomposites fabricated by a modified latex compounding method. Composites Science and Technology. 2014;99:67-74. [186] Yan N, Buonocore G, Lavorgna M, Kaciulis S, Balijepalli SK, Zhan YH, et al. The role of reduced graphene oxide on chemical, mechanical and barrier properties of natural rubber composites. Composites Science and Technology. 2014;102:74-81. [187] Dong B, Liu C, Zhang L, Wu Y. Preparation, fracture, and fatigue of exfoliated graphene oxide/natural rubber composites. RSC Advances. 2015;5(22):17140-8. [188] Sadhu S, Bhowmick AK. Morphology study of rubber based nanocomposites by transmission electron microscopy and atomic force microscopy. Journal of Materials Science. 2005;40(7):1633-42. [189] Wu Y-P, Wang Y-Q, Zhang H-F, Wang Y-Z, Yu D-S, Zhang L-Q, et al. Rubber– pristine clay nanocomposites prepared by co-coagulating rubber latex and clay aqueous suspension. Composites Science and Technology. 2005;65(7–8):1195-202. [190] Vassiliou AA, Chrissafis K, Bikiaris DN. In situ prepared pet nanocomposites: Effect of organically modified montmorillonite and fumed silica nanoparticles on pet physical properties and thermal degradation kinetics. Thermochimica Acta. 2010;500(1–2):21-9. [191] Leroux F, Besse J-P. Polymer interleaved layered double hydroxide:  A new emerging class of nanocomposites. Chemistry of Materials. 2001;13(10):3507-15.

62

ACCEPTED MANUSCRIPT

AC C

EP

TE D

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SC

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[192] Paul M-A, Alexandre M, Degée P, Calberg C, Jérôme R, Dubois P. Exfoliated polylactide/clay nanocomposites by in-situ coordination–insertion polymerization. Macromolecular Rapid Communications. 2003;24(9):561-6. [193] Ma J, Xu J, Ren J-H, Yu Z-Z, Mai Y-W. A new approach to polymer/montmorillonite nanocomposites. Polymer. 2003;44(16):4619-24. [194] Paszkiewicz S, Szymczyk A, Špitalský Z, Mosnáček J, Kwiatkowski K, Rosłaniec Z. Structure and properties of nanocomposites based on ptt-block-ptmo copolymer and graphene oxide prepared by in situ polymerization. European Polymer Journal. 2014;50(0):69-77. [195] Lee YR, Raghu AV, Jeong HM, Kim BK. Properties of waterborne polyurethane/functionalized graphene sheet nanocomposites prepared by an in situ method. Macromolecular Chemistry and Physics. 2009;210(15):1247-54. [196] Coleman JN, Khan U, Blau WJ, Gun’ko YK. Small but strong: A review of the mechanical properties of carbon nanotube–polymer composites. Carbon. 2006;44(9):1624-52. [197] Lan T, Pinnavaia TJ. Clay-reinforced epoxy nanocomposites. Chemistry of Materials. 1994;6(12):2216-9. [198] Favier V, Chanzy H, Cavaille J. Polymer nanocomposites reinforced by cellulose whiskers. Macromolecules. 1995;28(18):6365-7. [199] Wakabayashi K, Pierre C, Dikin DA, Ruoff RS, Ramanathan T, Brinson LC, et al. Polymer-graphite nanocomposites: Effective dispersion and major property enhancement via solid-state shear pulverization. Macromolecules. 2008;41(6):1905-8. [200] Li C, Feng C, Peng Z, Gong W, Kong L. Ammonium‐assisted green fabrication of graphene/natural rubber latex composite. Polymer Composites. 2013;34(1):88-95. [201] Schopp S, Thomann R, Ratzsch KF, Kerling S, Altstadt V, Mulhaupt R. Functionalized graphene and carbon materials as components of styrene- butadiene rubber nanocomposites prepared by aqueous dispersion blending. Macromolecular Materials and Engineering. 2014;299(3):319-29. [202] Kumar V, Hanel T, Giannini L, Galimberti M, Giese U. Graphene/styrene butadiene rubber nanocomposite. KGK-Kautschuk Gummi Kunststoffe. 2014;67(9):29-36. [203] Kumar V, Hanel T, Giannini L, Galimberti M, Giese U. Graphene reinforced synthetic isoprene rubber nanocomposites. KGK-Kautschuk Gummi Kunststoffe. 2014;67(10):38-46. [204] She XD, He CZ, Peng Z, Kong LX. Molecular-level dispersion of graphene into epoxidized natural rubber: Morphology, interfacial interaction and mechanical reinforcement. Polymer. 2014;55(26):6803-10. [205] Seyedin MZ, Razal JM, Innis PC, Jalili R, Wallace GG. Achieving outstanding mechanical performance in reinforced elastomeric composite fibers using large sheets of graphene oxide. Advanced Functional Materials. 2015;25(1):94-104. [206] Sae-oui P, Sirisinha C, Thepsuwan U, Hatthapanit K. Dependence of mechanical and aging properties of chloroprene rubber on silica and ethylene thiourea loadings. European Polymer Journal. 2007;43(1):185-93. [207] Tang ZH, Zhang LQ, Feng WJ, Guo BC, Liu F, Jia DM. Rational design of graphene surface chemistry for high-performance rubber/graphene composites. Macromolecules. 2014;47(24):8663-73. [208] Bar-On B, Wagner HD. Effective moduli of multi-scale composites. Composites Science and Technology. 2012;72(5):566-73. [209] Verdejo R, Barroso-Bujans F, Rodriguez-Perez MA, de Saja JA, Lopez-Manchado MA. Functionalized graphene sheet filled silicone foam nanocomposites. Journal of Materials Chemistry. 2008;18(19):2221-6. [210] Gangopadhyay R, De A. Conducting polymer nanocomposites: A brief overview. Chemistry of Materials. 2000;12(3):608-22. 63

ACCEPTED MANUSCRIPT

AC C

EP

TE D

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[211] Bauhofer W, Kovacs JZ. A review and analysis of electrical percolation in carbon nanotube polymer composites. Composites Science and Technology. 2009;69(10):1486-98. [212] Wang Z, Nelson J, Hillborg H, Zhao S, Schadler L. Nonlinear conductivity and dielectric response of graphene oxide filled silicone rubber nanocomposites. Electrical Insulation and Dielectric Phenomena (CEIDP), 2012 Annual Report Conference on; 2012: IEEE; 2012. p. 40-3. [213] Boland CS, Khan U, Backes C, O’Neill A, McCauley J, Duane S, et al. Sensitive, high-strain, high-rate bodily motion sensors based on graphene–rubber composites. ACS Nano. 2014;8(9):8819-30. [214] Nelson JK. Dielectric polymer nanocomposites: Springer 2010. [215] Bunch JS, Verbridge SS, Alden JS, van der Zande AM, Parpia JM, Craighead HG, et al. Impermeable atomic membranes from graphene sheets. Nano Letters. 2008;8(8):2458-62. [216] Su Y, Kravets VG, Wong SL, Waters J, Geim AK, Nair RR. Impermeable barrier films and protective coatings based on reduced graphene oxide. Nature Communications. 2014;5:4843. [217] Bhattacharya M, Biswas S, Bhowmick AK. Permeation characteristics and modeling of barrier properties of multifunctional rubber nanocomposites. Polymer. 2011;52(7):156276.

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