Epoxy/graphene nanocomposites – processing and properties: a review

Graphene has recently attracted signi ﬁ cant academic and industrial interest because of its excellent performance in mechanical, electrical and thermal applications. Graphene can signi ﬁ cantly improve physical properties of epoxy at extremely small loading when incorporated appropriately. Herein, the structure, preparation and properties of epoxy/graphene nanocomposites are reviewed in general, along with detailed examples drawn from the key scienti ﬁ c literature. The modi ﬁ cation of graphene and the utilization of these materials in the fabrication of nanocomposites with di ﬀ erent processing methods have been explored. This review has been focused on the processing methods and mechanical, electrical, thermal, and ﬁ re retardant properties of the nanocomposites. The synergic e ﬀ ects of graphene and other ﬁ llers in epoxy matrix have been summarised as well.


Introduction
Materials play key roles in every eld of technology such as aeronautics, electronics, energy, health, sensors, etc. 1 It is important to continuously update existing materials and develop new materials with improved properties and multifunctionalities, so they can be exploited for advanced applications. 2Compared to traditional composite materials, nanocomposites exhibit extraordinary properties because of the exceptionally high surface to volume ratio of the nanoller and/ or its exceptionally high aspect ratio. 3Polymer nanocomposites combine the functionalities of polymer matrices, such as low cost, easy processability, 4 with the unique features of the inorganic nanoparticles such as high aspect ratio, excellent toughness and strength and other properties like electrical and thermal conductivities. 5[8][9][10] However, nanollers used in these materials have strong tendency to agglomerate which would cause inhomogeneous dispersion of nanollers in matrices, 11 and reduction in mechanical and thermal properties of these nanocomposites.The optimum enhancement in the properties of polymer matrices can only be achieved if the nanollers are uniformly Besides, he has published several papers dealing with nite element analysis to understand the behaviour of thin-walled composite structures.He has also implemented a user material subroutine in ABAQUS/Explicit to model blast response of Fibre-Metal Laminates and novel micro, nano and hybrid composites.
dispersed. 12Attaining the optimum dispersion is one of the main challenges for processing of nanocomposites and therefore it is essential to review the current processing techniques used for preparing nanocomposites.
However, there has been no review article dealing specically with epoxy/graphene nanocomposites, which is the subject of this paper.Mechanical, electrical, thermal and re retardant properties of epoxy/graphene nanocomposites have been reviewed; besides that, processing methods and properties of nanomaterials have been correlated.Furthermore, some of the listed points have been highlighted in this paper are: (1) Summarised a new method of epoxy/graphene nanocomposites preparationresin impregnation, which impregnating epoxy into a graphene lter cake without pre-mixing.
(2) Summarised the synergic effects of graphene and other llers in epoxy matrix.
(3) Summarised the reason why thermal stability decreased with the incorporation of graphene.
To the best of our knowledge, those points haven't been seen in other reviews and we believe this review covers most of the important publications relating to the processing and properties of epoxy/graphene nanocomposites to date.

Epoxy
Discovered in 1936 by Dr Castan of Switzerland and Dr Greenlee of USA, epoxy based materials are used widely because of their superlative mechanical properties, thermal stability, solvent resistance and ease of processing. 13Epoxies are one of the most adaptable and widely sold high performance material. 14Some of the applications of epoxy and its nanocomposites include aerospace, automotive, marine, sports materials, construction, structures, electrical and electronic systems, biomedical devices, thermal management systems, adhesives, paints and coatings, industrial tooling and other general consumer products. 15Because of its versatile nature, epoxy is replacing many conventional materials, e.g.epoxy based materials have already replaced wood in majority of the boats and various sports goods.
Epoxy resins are thermosetting polymers and dened as a molecule containing more than one epoxide groups, as shown in Fig. 1.
The curing process is a chemical reaction in which the epoxide groups in epoxy resin reacts with a hardener (curing agent) to form a highly crosslinked, three-dimensional network. 16here are wide varieties of curing agents available for epoxy based materials.Depending on the chemical formulation of the hardeners, epoxy resins can be cured at temperatures range from 5 to 150 C. 17 However, epoxy materials with varying engineering applications are oen limited by their brittle nature and poor electrical, thermal properties. 18A simple solution to overcome this problem is to modify the matrix molecular structure or add compatible llers.For example, incorporation of inorganic nanollers has been shown to be a very efficient strategy to increase the performance of the material. 19

History and properties
Since the historical observation of single layer graphene by Andre Geim and Kostya Novoselov in 2004, 20 this atomically thin carbon sheet has received ever-increasing attention and become a rapidly rising star on the horizon of materials science. 21For example, recently the European Commission has nanced a 10 year research initiative, the European Graphene Flagship, which provides 1 billion Euro in funding and involves more than 140 academic and commercial institutions in 23 countries. 22raphene exhibits many specic and useful properties such as large surface area (2630 m 2 g À1 ), 23 excellent thermal conductivity (5000 W m À1 s À1 ), 24 very high Young's modulus (1 TPa), 25 high value of white light transmittance as to 97.7%, 26 exceptionally high room-temperature electron mobility of 2.5 Â 10 5 cm 2 V À1 s À1 . 27These fascinating properties have attracted extensive research interest in recent years with ever-increasing scientic and technological impetus.
For example, as a conductive nanomaterial, graphene can be used for printed electronics beyond conventional silicon based technologies. 28For energy storage, Yang et al. 29 prepared a supercapacitor with a capacitance of 200-300 F g À1 .Kim et al. 30 used graphene as a transparent electrode and fabricated an organic photovoltaic devices; Prasai et al. 31 incorporated graphene into organic coatings signicantly enhanced its corrosion resistance.Therefore, it can be concluded that signicant achievements have been made at the cross-section of nanotechnology and various applications by employing the specic properties of graphene. 32His expertise includes materials physics and chemistry, processing, characterisation, performance evaluation, applications, materials sustainability and technology commercial exploitation.Dr Inam has produced several high impact publications and authored more than 75 papers in peer-reviewed scientic journals and proceedings to date.He was the founding editor of journal "Epoxy" and on the editorial boards of various journals.

Fabrication
Efforts to exfoliate graphite down to its ultimate constituent can be dated back to 1960s.Fernandez et al. 33 extracted millimetresized graphene sheets (as thin as 5 nm, about 15 layers) from graphite crystals by micromechanical exfoliation for the very rst time.However, it was not until 2004, Andre Geim and Kostya Novoselov 20 isolated individual graphene layers by repeatedly cleaving a graphite crystal with a scotch tape to its limit.This led to the creation of a wonder two dimensional (2D) material, marking the onset of successful fabrication of graphene.
Micromechanical exfoliation, the top-down method, is a simple peeling process as shown in Fig. 2. Similarly, ultrasonication also produce thin graphene sheets. 34Currently, exfoliation of bulk graphite is the most commonly used method for the mass production of small graphene sheets. 35This can be through direct exfoliation in a liquid, with or without the use of surfactant, 36 or in the solid state by edge functionalization, 37 or by rst inserting a chemical species between the graphene layers in graphite to weaken their interaction and then followed by thorough exfoliation. 38ottom-up approaches have also been developed such as Chemical Vapour Deposition (CVD). 40In a typical CVD process, a substrate is exposed to volatile precursors in a reaction chamber and the precursors react and/or decompose on the substrate surface to produce the desired deposit. 41For graphene production, silicon or a transition metal oen serves as the substrate.The CVD chamber is vacuumed and heated in the presence of catalyst and hydrocarbon gases are induced and decomposed.This process deposits a spread of carbon atoms onto the surface of the substrate, thus forming the graphene layers. 42nother advanced method is the chemical reduction of exfoliated graphene oxide, which is an economical and very practical approach to synthesise graphene. 43This process takes the advantage of p-p interactions of graphene oxide and other molecules such as hydrazine, one of the most effective reductive agents, which can effectively return graphene oxide to its original state. 44This method maintains graphene's electrical conductivity, atness and optical properties, but it's not as same as pristine graphene and still contains some signicant oxygen groups and a few irreversible lattice defects. 45here are a number of other growth methods, some of these methods have certain advantages and should be investigated further, such as arc discharge method, 46 template route method, 47 electrochemical synthesis of graphene 48 and total organic synthesis of graphene. 49Many studies have been directed towards developing techniques to create single layer graphene, however, to date, scalable production of single layer graphene is still at exploration stage and there is no mature method to produce good quality graphene in mass quantity. 50In general, mechanical exfoliation, CVD, chemical reduction, epitaxial growth of graphene are among the most notable techniques in graphene production. 513 Graphene oxide (GO) and functionalization of graphene 3.3.1 GO.GO is obtained from the exhaustive oxidation and exfoliation of graphite, and contains a range of oxygen functional groups with specic chemistry.52 It is generally produced by the treatment of graphite using strong mineral acids and oxidizing agents, typically via treatment with KMnO 4 and H 2 SO 4 as in the Hummers 53 method, or KClO 3 (or NaClO 3 ) and HNO 3 as in the Staudenmaier 54 or Brodie 55 methods, or some variation of these methods.There is no unambiguous model to describe the exact structure of GO because there is no single denitive analytical technique available to characterize this material.However, it is generally accepted that the carboxylic groups are mainly located at the edge, while the rest of functional groups (hydroxyl, epoxide, etc.) are present in highest concentration in the basal planes of the graphene layers.56 Fig. 3 shows a proposed structure of graphene oxide that is supported by solid-state nuclear magnetic resonance (SSNMR) experiments on 13 C-labeled GO.
The oxygen functional groups on GO surface are polar and renders GO hydrophilic.GO can be dispersed in many solvents, and particularly well in water. 58In addition, currently the most promising methods for large scale production of graphene are based on exfoliation and reduction of graphene oxide. 59.3.2Functionalization of graphene.Pristine graphene is unsuitable for intercalation with large species, such as polymer chains, because graphene has a pronounced tendency to agglomerate in polymer matrices.60 As observed for other Fig. 2 Scotch tape method of graphene synthesis from graphite block.39 Reproduced with permission from ref. 39.Fig. 3 A proposed schematic (Lerf-Klinowski model) of graphene oxide structure.57 Reproduced with permission from ref. 57.  nollers, the maximum improvements in nal properties can be achieved when the ller is homogeneously dispersed in the matrices because the external load is efficiently transferred through strong polymer/ller interfacial interactions.61 Thus, dispersion and strong interaction between graphene and matrices play important role for the performance of graphene nanocomposites.][64][65][66] The functional groups attached to graphene can be small molecules 67 or long polymer chains, 68 for which various functionalization approaches have been researched such as covalent and non-covalent functionalization of graphene.69 Covalent functionalization is based on the covalent linkage between graphene and other functional groups.70 The structural alteration can take place at the end of the sheets and/or on the surface.71 Covalent functionalization is associated with rehybridization of one or more sp 2 carbon atoms of the carbon network into the sp 3 conguration accompanied by simultaneous loss of electronic conjugation.72 The covalent modication of graphene can be achieved in four different ways: nucleophilic substitution, electrophilic addition, condensation, and addition. 73By conducting an epoxide ring-opening reaction, Yang et al. 74 covalently graed 1-(3-aminopropyl)-3methylimidazolium bromide onto the surface of graphene sheets.The modied graphene showed enhanced solubility in water, N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) at various concentrations and formed long-term stable and homogeneous dispersions.
Non-covalent functionalization helps in networking or connecting the molecules without actually forming chemical bonds.However, this process requires the physical adsorption of suitable molecules on the graphene surface. 757][78][79] For example, Song et al. 80 prepared epoxy/graphene nanocomposites with improved mechanical properties and thermal conductivities by non-covalent functionalization of graphene.The modied graphene showed good dispersibility in acetone, DMF, ethanol, pyridine, methanol, tetrahydrofuran (THF) and water, but only short-term stability in iso-propyl alcohol (IPA), dichlorobenzene (DCB), chloroform, dichloromethane and chlorobenzene.This short-term stability is attributed to the surface functional group 1-pyrenebutyric acid, which is not compatible with these solvents.

Epoxy/graphene nanocomposites
Epoxy and its composites are versatile materials for plenty of industrial elds like electrical and thermal applications, highperformance nanocomposites in automobiles and aerospace applications but these composites have some limitations as well.As a young rising carbon allotrope, graphene showed a new path to overcome these limitations.The exploration of property enhancement of epoxy/graphene nanocomposites is rapidly advancing as evident in Fig. 4, which shows the dramatic increase in epoxy/graphene nanocomposites research in recent years.

Processing methods
Obtaining a good distribution of the graphene-reinforcement is one of the greatest challenges in the preparation of epoxy/ graphene nanocomposites.A well dispersed state ensures availability of maximum surface area of ller, which will affect the neighbouring polymer chains and, consequently, the properties of the whole nanocomposite. 81For epoxy or any other matrices, dispersion signicantly depends on the processing techniques.3][84][85][86][87][88] The commonly used methods for epoxy/graphene nanocomposites are solution mixing, and recently, a newly emerged method as epoxy impregnation and will be discussed here.
4.1.1Solvent processing.The simplest and most widely used method for processing epoxy/graphene nanocomposites is to take advantage of the presence of functional groups attached on the graphene surface which enables the direct dispersion of graphene in water and many organic solvents.This contributes to strong physical or chemical interaction between the functionalized graphene and polymeric matrices. 89A number of studies explain how the surface modication of graphene has been done by adding various functional groups such as amine, 90 organic phosphate, 91 silane, 92 plasma 93 etc.
Functionalized graphene is normally dispersed in a suitable solvent by, for example, bath sonication, then mixed with epoxy resin, and then solvent is evaporated in a controlled condition. 94he guiding principle is to select solvents compatible with the functional groups on the surface of graphene, and the functional group should be compatible with the epoxy resin as well. 95To achieve better dispersion of functionalized graphene, many solvents have been investigated.Raee et al. 96 prepared epoxy/graphene nanocomposites by dispersing graphene platelets in acetone by tip sonication, mixed graphene/acetone solution with epoxy resin and nally acetone was then   removed by heating the mixture to 70 C.The prepared nanocomposites showed enhanced mechanical properties and resistance to fatigue crack growth at low graphene concentration (0.1 wt%).Fang et al. 97 dispersed graphene in DMF under bath sonication and modied graphene with amine, which provided a mechanical adhesion at the graphene-epoxy interface.The nanocomposites showed improved load transfer efficiency between graphene nanosheets and the matrix, accompanied by the enhanced dissipation capacity of nanocomposites for strain energy during fracture.Tang et al. 98 investigated the inuence of reduced graphene oxide (rGO) dispersion on the mechanical properties of epoxy resin.They found that with the assistance of ball milling in ethanol solution, the blends showed higher dispersibility, which resulted in higher strength and fracture toughness of epoxy resin as well as improved glass transition temperature (T g ) and electrical conductivity.In addition, they also found that the highly dispersed rGO resulted in much more tortuous and ne riverlike structures on the fractured surface.This consumes more fracture energy in comparison with the poorly dispersed rGO, or in other words improves the fracture toughness of the material.Chatterjee et al. 90 investigated the reinforcements of mechanical and thermal properties of a functionalized graphene lled epoxy nanocomposites.The amine functionalized expanded graphene nanoplatelets (GNP) were dispersed within epoxy resins using high-pressure processor followed by three roll milling in the solvent of acetone.The resulting nanocomposite exhibited signicant improvements in mechanical properties and thermal conductivity indicating a favourable interaction at graphene/epoxy interface.Table 1 shows summary of representative investigations on the solvent processing.
As can be seen from Table 1, a wide range of solvents have been used for the dispersion of graphene, such as THF, DMF, acetone, ethanol, water, dichloromethane (DCM), methyl ethyl ketone (MEK), etc. Dispersion techniques like tip sonication, bath sonication, mechanical mix, shear mix, and three roll calendaring have been widely adopted for homogenous dispersion and most of these methods showed good results.
4.1.2Resin impregnation.This method refers to impregnation of epoxy resin into the as-prepared graphene lter cake.It has not been widely reported in the literature until recently as a method for preparing polymer nanocomposites.Im et al. 125 prepared a 60 wt% nanocomposite material by using this method for the very rst time in 2012.They suspended GO particles in H 2 O under ultrasonication and then the prepared mixture was poured into a glass mould which was placed on a silicon oxide membrane.The mixture poured into the glass mould was ltered via vacuum ltration.Aer ltration, the lter cake, which was peeled off from the SiO 2 membrane, was annealed under heating to remove the residual water.Finally, the epoxy containing the curing agent was dropped onto the lter cake and cured under heating.This method infuses epoxy resin into the graphene sheet by capillary driven wetting force and appropriate for fabricating highly concentrated nanocomposites with reasonably high mechanical properties.
Similar approach has been used by Li et al. 126 to fabricate a 11.84 wt% epoxy/graphene nanocomposite.They rst dispersed graphene platelets in the mixture of ethanol and water by ultrasonication and then removed the solvent by vacuum ltration.During the ltration process, self-assembly of the aligned graphene occurred (Fig. 5), aer that they immersed this aligned graphene into epoxy monomer and curing agent.By this method, they prepared a nanocomposite with aligned multilayer graphene in epoxy matrix.The nanocomposite showed a thermal conductivity highly as 33.54 W m À1 K À1 at 90 C.This remarkable improvement in thermal conductivity was due to the unique alignment structure formed during processing.
Low ller percentage nanocomposites could also be prepared by this method.Jia et al. 127 reported the preparation of a 0.1 wt% epoxy/graphene nanocomposite by impregnation of epoxy resin into a three dimensional (3D) graphene-nickel (Ni) foam via chemical vapour deposition, followed by curing of the polymer and etching of the Ni template.This nanocomposite with 0.1 wt% graphene delivered excellent fracture toughness, and the glass transition temperature increased 31 C compared to solid epoxy.More than that, they reported this 3D interconnected graphene network serves as fast channels for charge carriers, giving rise to a remarkable electrical conductivity of the nanocomposite.
4.1.3Other methods.The most widely used method to prepare epoxy/graphene nanocomposites is through solvent processing.However, some derivative methods have also been adopted.Martin et al. 128 dispersed graphene in epoxy monomer by mechanical mixing.The mixture was then mixed with photo initiator and cured by UV irradiation.They reported enhancement in thermal and mechanical properties of the nanocomposite as a result of UV curing.Similarly, Sangermano et al. 129 prepared UV cured epoxy/graphene nanocomposites as well and showed enhanced properties.Yu et al. 130 used hot press in the curing procedure to fabricate the epoxy/graphene nanocomposite which showed several folds of increments in thermal conductivity.However, dispersing graphene in epoxy matrix without using solvent is likely to be less efficient.Hsu et al. 131 mixed graphene, epoxy monomer and curing agent all together using three roll milling at room temperature.Uniform dispersion of graphene was hindered by the high viscosity of epoxy resin, therefore, mixing without solvent might be considered as a less effective dispersion strategy.

Properties of nanocomposites
Due to the recent developments in graphene and epoxy/graphene nanocomposites, the literature on this subject is still in its early stage but growing rapidly.However, some interesting studies have already been reported and will be discussed here to illustrate the potential of these new nanocomposites.
4.2.1 Morphology.As property enhancements strongly correlate with nanocomposite microstructure, 132 effective characterization of morphology is important to establish structureproperty relationships for these materials.
Transmission electron microscope (TEM) images of these nanocomposites can provide direct observation of dispersed multilayer graphene platelets.Thicker platelets typically shows adequate contrast against the epoxy matrix, whereas single layer platelets may be difficult to observe directly by TEM. 133Studies on layered nanollers based nanocomposites have suggested the existence of three general states of dispersion on short length scales: stacked, intercalated, or exfoliated, as shown in Fig. 6.
TEM is the most common method for assessing the state of dispersion.Immiscibility of the phases and/or insufficient exfoliation of the graphite or graphene platelet prior to mixing with epoxy can result in large agglomerates consisting of stacked graphene sheets when observed by TEM.Fig. 7 shows an accurate measurement of the number of graphene layers in epoxy matrix.
The drawback of TEM is only a small area of the material could be observed, so cross sectional analysis with scanning electron microscopy (SEM) has also been used to evaluate dispersion of graphene as well as to examine the surface for ller pull-out, which could give insight into the strength of interfacial adhesion. 136However, SEM image cannot resolve the degree of exfoliation of the platelets and is therefore best utilized combined with TEM.Furthermore, atomic force microscope (AFM) and the corresponding height prole graph is an important technique to characterize the pristine or functionalized graphene.The AFM study could give the length and thickness of graphene sheets along with morphology (Fig. 8).For AFM study, the sample is prepared by dispersing graphene in water or solvents and drop casting on a freshly cleaved mica surface.The dried sample is then observed through the instrument.
4.2.2Mechanical properties.As previously mentioned, graphene has excellent mechanical properties namely high Young's modulus, high tensile strength, fracture toughness, etc. 138 These exceptional properties make graphene an ideal candidate as ller for nanocomposite materials.Most of the work on epoxy/graphene nanocomposites is aimed at exploiting the remarkable mechanical enhancement effect of the graphene coupled with the possibility to introduce further functionalities, such as electrical conductivity 139 or thermal stability.Fig. 7 TEM image of layered graphene in epoxy matrix. 135Reproduced with permission from ref. 135.Recently, Bortz et al. 119 conducted the investigation on the mechanical properties of epoxy/graphene oxide nanocomposites.The study showed the inuence of graphene oxide concentration (0.1, 0.25, 0.5 and 1 wt%) on the fracture toughness and exural strength of nanocomposites which are presented in Fig. 9.The graphs showed that with the increase in graphene oxide concentration, the mechanical properties of nanocomposites increased as well.For example, at the concentration of 1 wt%, the nanocomposite showed more than one hundred percent increase in G IC .Qi et al. 141 used thermotropic liquid crystalline epoxy to functionalize the graphene surface.The fabricated nanocomposites showed enhancement in tensile strength from 55.43 MPa to 78.96 MPa at 1 wt% accompanied by nearly one hundred percent increase in impact strength.Similarly, Liu et al. 142 investigated the interphase of epoxy/graphene oxide and reported increase in the modulus and toughness.Fracture toughness and exural modulus were increased with the increasing ller concentration which indicated the signicant enhancement effect of graphene in epoxy matrix.
As discussed in the previous sections, dispersion of graphene plays a very crucial role in the preparation of epoxy/ graphene nanocomposites, 143 for which many different techniques have been investigated.A homogenous dispersion could give better load transfer to ller material which results into better mechanical properties for the graphene nanocomposites. 144For example, Li et al. 115 incorporated 0.5 wt% silane functionalized graphene into epoxy matrix by mechanical mixing and bath sonication and reported 20% increase in elastic modulus and 16% increase in tensile strength as compared to neat epoxy.Similarly, Raee et al. 96 reported a signicant enhancement of Young's modulus at 0.1 wt% of epoxy/graphene nanocomposites processed by shear mixing and tip sonication.For well dispersed nanocomposites, improved mechanical interlocking with polymer chains and graphene can be observed, and slipping of entrapped polymer molecules was suppressed, along with improved dispersion, tensile strength and fracture toughness.Izzuddin et al. 118 reported that the presence of good adhesion between graphene and matrix were the main attributes for these increments.In order to form a strong interface, polyoxyalkyleneamine functionalized graphene were dispersed in epoxy matrix by bath sonication and mechanical mixing, and it was evident that the functionalization treatment increased modulus and fracture properties of the nanocomposites.In their report, the samples with 0.489 vol% functionalized graphene, showed 224% improvement in the fracture toughness as compared to the pure resin.Therefore, functionalization of graphene has signicant positive effect on the mechanical properties of the epoxy nanocomposites.

Electrical conductivity.
A number of studies, related to the electrical properties of polymer/graphene nanocomposites, have been conducted. 145The combination of graphene and polymer matrices offers new attractive electrical properties and novel conducting polymers.These polymers can be used for various engineering applications like, electrical conducting adhesives, antistatic coating and lms, electromagnetic interference shielding materials for electronic devices, thermal interface materials, etc. 146 These conducting nanocomposites follow the principle of percolation theory which basically explains the transition from insulator to conductor in materials.The percolation threshold is the concentration at which the electrical conductivity of an insulating polymer matrices increases dramatically.A conductive continuous network of ller is created and electrons can be transported by direct contact among nanoller particles, beyond this concentration, the conductivity of the nanocomposite increases marginally. 147he electrical conduction in a nanocomposite is due to the formation of a continuous conductive network formed by the llers.Therefore, aligned nanollers have higher probabilities to percolate at lower volumetric concentrations than spherical nanollers. 148Graphene becomes an ideal candidate to achieve this percolated network at low loading fractions due to its intrinsically high conductivity and the 2D structure.Wajid et al. 110 reported the ultra-low electrical percolation threshold at 0.088 vol% in epoxy/graphene nanocomposites, by dispersing graphene with the assistance of tip sonication, mechanical mixing and shear mixing.Similarly, Liang et al. 124 also reported signicant increase in the electrical conductivity by incorporating graphene in epoxy nanocomposites matrix by bath sonication and mechanical mixing.The conductivity was improved from 0.8 Â 10 À10 to 0.8 Â 10 À2 by incorporating 8 vol% reduced graphene oxide into epoxy.Such improvements are only possible when graphene are thoroughly de-bundled and homogenously dispersed in epoxy matrix.Monti et al. 149 Fig. 9 Quasi-static mechanical properties of epoxy nanocomposites. 119Reproduced with permission from ref. 119.dispersed graphene into epoxy to study the electrical conductivities.In order to improve the dispersion, they processed the mixture by tip sonication and mechanical mixing and used different solvents such as chloroform or THF.The highest electrical conductivity was observed for sample with 3 wt% graphene.It was also shown that the thermal conductivity increased with the increase in graphene concentration.
4.2.4Thermal conductivity.As opposed to the electrical conductivity, the thermal conductivity of epoxy/graphene nanocomposites has received less attention to date.As compared to electrical conductivity enhancements of several orders of magnitude, thermal conductivity enhancement by the carbon nanollers is not as signicant. 150However, several folds increment in thermal conductivity can easily be obtained, as it has been reported that the 2D shape platelets like graphene nanosheets can improve thermal conductivity more effectively than 1D rod like carbon nanotube (CNT). 92,151As given by Kapitza resistance, the transfer of thermal energy are carried out by the free electrons interaction and lattice vibration between the two contacted interface.Therefore, poor coupling at the ller/polymer interfaces will signicantly impact on thermal resistance. 152Hence, a strong ller/polymer interface is required to achieve good thermal conductivity. 153eca et al. 151 applied alcohol and oxidative acid treatment with the assistance of extended and vigorous sonication to thermally expanded graphite.Carbon nanosheets were found well dispersed in epoxy matrix with a thickness of less than 10 nm.The incorporation of 33 vol% carbon nanosheets could improve the in plane thermal conductivity of epoxy nanocomposites to 80 W m À1 K À1 .However, the across-plane thermal conductivity was found only one-tenth to one-h of the average in-plane value.This highly anisotropic nature resulted from the 2D structure of the graphene sheets.Wang et al. 154 reported that 5% graphite oxide (prepared via thermal expansion) increased the thermal conductivity of epoxy to over 0.8 W m À1 K À1 and decreased the coefficient of thermal expansion by 31.7% below T g .Ganguli et al. 92 found that 20 wt% silane functionalized thermally expanded graphite enhanced the thermal conductivity of epoxy from 0.2 to 5.8 W m À1 K À1 .It was interesting to nd that silane functionalization could form covalent bonding with epoxy and improved the interfacial heat transfer between two components by reducing acoustic impedance mismatch in the interfacial area.However, excessive functionalization also tends to reduce the intrinsic thermal conductivity of carbon materials.Fig. 10 shows a larger contribution of graphene for the thermal conductivity for epoxy matrix as compared to CNTs and carbon black.
4.2.5 Thermal stability.Several studies have evaluated the effect of graphene on the thermal properties in many polymer matrices, such as thermal degradation temperature, 156 glass transition temperature, 157 melting temperature 158 and polymer crystallinity of the nanocomposites. 159However, there is no melting temperature for epoxy because of its thermosetting nature, so glass transition and thermal degradation behaviours are among the most important characters to characterize the thermal stability of epoxy nanocomposites which have been discussed in this section.
It is generally observed that graphene would enhance the T g of epoxy matrix. 160This is due to the adhesion force between epoxy and graphene which reduces the mobility of epoxy chains on graphene surface.Contrarily, decrease in T g is expected for weakly adhering llers and unstable interfaces facilitating the chain polymer mobility, thus lowering the T g . 161Li et al. 162 reported the increase in T g of epoxy by hindering segmental motion of polymer chains via mechanical interlocking and hydrogen bonding with surface oxygen functionalities.Similarly, a T g increase of 14 C in epoxy/graphene nanocomposites has been measured by Park et al. 163 at 1 phr (parts per hundred resin) of graphene in epoxy matrix.This is an expected outcome of the strong ller-matrix adhesion and because of the conformational changes of the epoxy matrix at the epoxy/graphene interface.
In contrary, a signicant volume of research reported the opposite trend and will be discussed here.It has been vastly reported that graphene reduces the glass transition or thermal degradation temperature of epoxy matrix and there is no unanimous agreement for this negative trend.Galpaya et al. 164 proposed the theory that the T g of nanocomposites depends on the balance of two effects, i.e., inuence on reaction conversion and molecular connement.Graphene sheets are stiffer than epoxy matrix which could lead to signicant connement on the polymer chains.On the other hand, graphene sheets may impede the epoxy curing reaction.The reason could be the functional groups on graphene surface reacting with the curing agent and/or epoxy resin, or graphene sheets covering the reactive sites in the resin due to its high surface area.If the latter one plays the dominant role, it would be expected to reduce the polymer cross link density and would also increase polymer chain mobility.Liao et al. 70 and Kim et al. 123 reported similar conclusions as well.According to them, the incorporation of graphene reduces the cross link density of the epoxy matrix, which results in the decrease of T g .Some research groups like Saurín et al., 165 Liu et al., 166 and Guo et al. 91 reported that graphene acts as reactive plasticizer and has a plasticizing effect on epoxy resin, thus increasing the exibility of chain segments of the epoxy matrix.Liu et al. 120 prepared an epoxy/ imidazole functionalized graphene nanocomposite and reported that the short molecular chains of functional group on graphene surface are exible and would result in an overall T g decrease.There are also some other claims, such as Liu et al. 167 incorporated edge functionalized graphene into epoxy resin and found that the T g decreased because of the existence of graphene sheets that could result in increased exibility of the network.Zhang et al. 168 prepared magnetic graphene reinforced epoxy nanocomposites and reported that the rigid structure of graphene nanoplatelets would cause extra enlarged free volume, which is detrimental to the thermal stability of the matrix.
For thermal decomposition temperatures (T d ), which are characterized by the maximum weight loss rate in thermogravimetry, shis up to 30 C for epoxy nanocomposites by incorporating 0.5 wt% functionalized graphene were reported. 105Decomposition of graphene nanocomposites is substantially slower than neat epoxy, which is attributed to restricted chain mobility of polymers near the graphene surface.
Prolongo et al. 169 reported that 0.5 wt% graphene nanoplatelet can push the thermal degradation temperature of epoxy from 377 C to 397 C. Youse et al. 114 reported that both graphene oxide and reduced graphene oxide improved the thermal decomposition temperature of epoxy matrix.Fig. 11 shows the shi in thermal gravimetric analysis (TGA) curves to higher temperature, which means a higher thermal stability due to the incorporation of graphene.
Wang et al. 170 and Xin et al. 171 incorporated functionalized graphene into epoxy matrix and reported decrease in T d .This was due to the presence of thermally unstable chemicals, which on decomposition, lowered T d as compared to monolithic epoxy.Feng et al. 111 used epoxy resin to modify graphene rst, and then mixed with the epoxy matrix.They found that in some cases, the T d of the nanocomposite decreased because the ller might causes defects in the polymeric networks during the curing process.
In general, it is widely acknowledged that graphene could enhance the thermal stability of epoxy.However, there are still many controversies where many researchers reported a decrease in thermal properties of epoxy with the incorporation of graphene.The reason of such has not been fully explained and requires deeper understanding via extensive further research.
4.2.6 Flame retardant properties.Engineering materials are required to resist degradation during an unlikely event of re in many critical applications like skyscrapers, boats, or airplanes. 172In fact, some studies reported that about 20% of victims of airplane crashes are killed not by the crash itself but by ensuing res. 173Materials used in aviation should be designed to inhibit, suppress, or delay the production of ames to prevent the spread of re.Flame retardant materials are mainly based on halogen, phosphorus, inorganic, and melamine compounds, 174 however, among these ame retardants, only inorganic llers are normally nontoxic. 1757][178] For example, Li et al. 179 used 2-(diphenylphosphino)ethyltriethoxy silane modied graphene oxide, and then incorporated this modied graphene oxide into epoxy matrix.They found that the limiting oxygen index increased from 20 to 36, which means a huge transition of material's nature from ammable to non-ammable.Jiang et al. 180 prepared epoxy/graphene-ZnS nanocomposites and reported that with the incorporation of ZnS decorated graphene, the carbon monoxide production rate for the nanocomposites is much lower than that of pure epoxy along with decreased total smoke release.Wang et al. 181 prepared Ni-Fe Layered Double Hydroxide (LDH) modied graphene/epoxy nanocomposites.They found that with the incorporation of 2 wt% Ni-Fe LDH modied graphene, the ignition time of epoxy matrix increased from 68 s to 89 s, the total heat release decreased from 113.1 MJ m À2 to 44.2 MJ m À2 , and the re growth index decreased from 13.3 kW m À2 s À1 to 4.8 kW m À2 s À1 .Fig. 12 shows the drastic decrease of heat release rate with the incorporation of graphene and Ni-Fe LDH modied graphene.
Zhuo et al. 182 proposed a ame retarding mechanism for polymer matrices when lled with graphene.According to Zhuo et al., 182 the barrier effect of graphene plays a dominant role in ame retardancy.Graphene walls make excellent gas barriers which delay the oxidative degradation of epoxy during a re, moreover, the large surface area of graphene can induce a large amount of char which prevents the resin from suffering heat.
In general, the addition of graphene into epoxy matrix results in improving ame retardancy and thermal stability of epoxy along with improved mechanical properties.Moreover, no environmental or toxicity issues have been reported for graphene.Therefore, it can be concluded that graphene has a great potential to be one of the most promising ame retarding llers for nanocomposites in near future.
4.2.7 Synergic effects with other llers.Synergic effect or hybridisation means incorporation of two or more llers together for enhanced functionality which is not possible to achieve with single ller alone.Recently, a tremendous research effort can be witnessed to generate enhanced properties by synergistically combining different llers as reported by Inam et al. 183 The group fabricated multiscale epoxy composites which showed enhanced mechanical properties with the combination of carbon nanotubes and carbon bers.Chatterjee et al. 184 found that the CNT : GNP ratio is an interesting factor inuencing the properties of the epoxy based nanocomposites.At nanoller concentration of 0.5 wt%, highest CNT content (9 : 1) showed marked improvement in fracture toughness of 76%.Kumar et al. 185 suggested that by bringing together two nanollers like CNT and GNP, they could form a co-supporting network.This net-resemble structure could shield the llers from fracture and damage during processing, while still allowing full dispersion of both ller during high power sonication, thus resulting improved properties.Apart from the mechanical properties, incorporation of carbon llers in polymer matrices attained signicance for the applications where enhanced thermal and electrical conductivity were required together.
Epoxy resins containing a binary mixture of GNP and single wall carbon nanotube (SWCNT) in 3 : 1 weight ratio have higher thermal conductivity than those reinforced with either individual llers.Yu et al. 186 explained this synergistic effect by bridging interactions between GNP and SWCNT which can reduce the interfacial resistance for thermal conduction.Also, a remarkable synergetic effect between graphene platelets and multiwalled carbon nanotubes (MWCNTs) in improving the mechanical properties and thermal conductivity of epoxy nanocomposites was demonstrated by Yang et al. 159 The tensile strength and thermal conductivity were increased by 35.4% and 146.9% respectively by using MWCNT/graphene llers as compared to either ller for epoxy nanocomposites.They found that stacking of individual 2D graphene is effectively inhibited by introducing 1D MWCNTs.Long and tortuous MWCNTs can bridge adjacent graphene platelets and inhibit their agglomeration, resulting in a high contact area between the MWCNT/ graphene structures and the polymer matrices. 159n general, the exact mechanism responsible for this dramatic enhancement is not entirely understood.It is widely believed that molecular level interactions between the nanomaterials and polymer matrices play a major role.The large interface area available for such interactions clearly hold the key for this dramatic enhancement in mechanical properties. 187able 2 lists some representative papers which adopt multi ller or hybridisation approach to modify the properties of epoxy matrix.

Conclusions
Graphene shows great potential as ller for the next generation advanced nanocomposite materials.Numerous efforts have been made to prepare useful epoxy/graphene nanocomposites.However, the development and applicability of epoxy/graphene nanocomposites will be signicantly related to the dispersion  and the interfacial bonding of graphene in epoxy matrix, which are the two most critical factors to determine the performance of these novel nanocomposites.Thus, the key to prepare advanced epoxy/graphene nanocomposites is to improve the techniques for the dispersion of graphene and engineering the graphene-epoxy interface.This review provided a detailed introduction of epoxy/graphene nanocomposites and critical analyses on recent research investigations.Following conclusions can be drawn from the existing reported research: Graphene has signicant potential for epoxy based composites.Extremely enhanced multi-functional properties can be achieved subject to homogenous dispersion and strong interfacial interactions.Chemical functionalization of graphene can also signicantly improve the graphene-epoxy interfacial interactions.
Solvent processing is the most widely adopted method to prepare epoxy/graphene nanocomposites.The high viscosity of epoxy may hinder the uniform dispersion of graphene and therefore, it is also difficult to adopt solvent-free processing approach.
Mechanical properties, electrical conductivity, thermal conductivity, thermal stability and ame retardant properties are generally increased with the incorporation of graphene.
Graphene could increase the glass transition and thermal degradation temperatures of epoxy nanocomposite.However, this needs to be further explored as some investigations have reported the negative trend.
In general, epoxy/graphene materials have remarkably high thermal and electrical conductivities, as well as improved mechanical strength and thermal stability.Because of these excellent properties, graphene reinforced epoxy nanocomposites possess great potential to be used in automotive, electronics, aerospace and for other sectors.However, a lot is still required to be understood before such applications can be materialised.
Department of Mechanical and Construction Engineering, Faculty of Engineering and Environment, Northumbria University, Newcastle upon Tyne NE1 8ST, UK.E-mail: fawad.inam@northumbria.ac.uk;Tel: +44 (0)1912273741 Jiacheng Wei received his Master degree in 2013 from Southwest University of Science and Technology, China.Currently he studies as a PhD candidate in Northumbria University under the supervision of Dr Fawad Inam and Dr Thuc Vo.His focus of research is preparation and development of polymer based materials, especially epoxy/ graphene nanocomposites.Jiacheng is fully funded for his PhD research studentship by the Northumbria University in Newcastle upon Tyne, UK since 2013.Dr Thuc Vo is a senior lecturer at Northumbria University in Newcastle upon Tyne, UK since 2013.Dr Vo's most recent efforts have been devoted towards the development of rened shear deformation theories to investigate static, vibration and buckling responses of composite structures (laminate, functional graded material and sandwich).

a
Abbreviations in table:s: tensile strength, E: elastic modulus, K IC : fracture toughness, G IC : energy released, a x increase: increase in order of magnitude, k: electrical conductivity, l: thermal conductivity, T g : glass transition temperature, T d : thermal degradation temperature, G: graphene, GO: graphene oxide, m-G: modied graphene, m-GO: modied graphene oxide, rGO: reduced graphene oxide, m-Gi: modied graphite, bath sonic: bath sonication, tip sonic: tip sonication.

Fig. 5
Fig. 5 Schematic diagram of the preparation of aligned epoxy/graphene. 126Reproduced with permission from ref. 126.

Fig. 8
Fig.8AFM image and height profile of graphene.137Reproduced with permission from ref.137.

Fig. 11
Fig.11TGA curves of epoxy nanocomposites containing GO and rGO.114Reproduced with permission from ref.114.

Fig. 12
Fig.12Heat release rate versus time curves of epoxy and its nanocomposites.181Reproduced with permission from ref.181.

Table 1
Different graphene dispersion method and the properties enhancement a

Table 2
Synergic effect of graphene and other fillers in epoxy matrix