Progression from Graphene and Graphene Oxide to ...

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Progression from Graphene and Graphene Oxide to High Performance Polymer-Based Nanocomposite: A Review ab

a

b

c

Rahim Shah , Ayesha Kausar , Bakhtiar Muhammad & Sayed Shah a

Nanosciences and Catalysis Division, National Center for Physics, Quaid-i-Azam University Campus, Islamabad, Pakistan b

Department of Chemistry, Hazara University, Mansehra, Pakistan

c

Department of Physics, Swat University, Swat, Pakistan Accepted author version posted online: 25 Sep 2014.Published online: 06 Jan 2015.

Click for updates To cite this article: Rahim Shah, Ayesha Kausar, Bakhtiar Muhammad & Sayed Shah (2015) Progression from Graphene and Graphene Oxide to High Performance Polymer-Based Nanocomposite: A Review, Polymer-Plastics Technology and Engineering, 54:2, 173-183, DOI: 10.1080/03602559.2014.955202 To link to this article: http://dx.doi.org/10.1080/03602559.2014.955202

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Polymer-Plastics Technology and Engineering, 54: 173–183, 2015 Copyright © Taylor & Francis Group, LLC ISSN: 0360-2559 print/1525-6111 online DOI: 10.1080/03602559.2014.955202

Progression from Graphene and Graphene Oxide to High Performance Polymer-Based Nanocomposite: A Review Rahim Shah1,2, Ayesha Kausar1, Bakhtiar Muhammad2, and Sayed Shah3 1

Nanosciences and Catalysis Division, National Center for Physics, Quaid-i-Azam University Campus, Islamabad, Pakistan 2 Department of Chemistry, Hazara University, Mansehra, Pakistan 3 Department of Physics, Swat University, Swat, Pakistan

Downloaded by [39.33.30.11] at 23:29 31 January 2015

GRAPHICAL ABSTRACT

In this article, various types of carbon nanofiller and modification of graphene oxide and graphene for the preparation of polymer-based nanocomposites are reviewed. Recently, polymer/graphene and graphene oxide-based materials have attracted tremendous interest due to high performance even at low filler content. The property enhancement is due to the high aspect ratio, high surface area and excellent electrical, thermal and mechanical properties of nanofiller. Different techniques have been employed to fabricate polymer/ graphene and graphene oxide nanocomposite with uniform dispersion due to fine matrix/ nanofiller interaction. Here we discuss the structure, properties and preparation of these nanocomposites. Keywords

Aspect ratio; Graphene; Graphene oxide; Mechanical properties; Nanocomposite; Thermal properties

Address correspondence to Ayesha Kausar, Nanosciences and Catalysis Division, National Center For Physics, Quaid-i-Azam University Campus, 44000 Islamabad, Pakistan. E-mail: [email protected] Color versions of one or more of the figures in this article can be found online at www.tandfonline.com/lpte.

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INTRODUCTION The field of nanoscience has grown substantially in the past decade[1]. Nanotechnology is known for the study of nanostructure materials with a size order and dimension of 109 that of standard materials. Development in these fields mostly depends upon the preparation of nanoparticles of different shapes and sizes as well as accumulating them capably into complex architecture. Currently nanomaterials have wide range of applications due to their structural features. However the exploration of materials with enhanced physiochemical properties have been the focus of material scientists[2]. Nanostructured materials are usually classified into 0-D (quantum dots, hollow spheres, nanolenses and onions)[3], 1-D (nanowires (NWs), nanotubes (NTs), and nanobelts (NBs))[4], 2-D (nanoplates, nanosheets, nanowalls, and nanodisks, etc.)[3], and 3-D (nanocoils, nanoballs, nanopillers, nanocones and nanoflowers)[3]. The nanocomposite is a material in which at least one dimension of the constituents is in nanometers. Nanoconstituents of nanocomposites generate highly functional materials by combining the properties of component materials. Polymer nanocomposites are one of the most significant applications of carbon nanofillers including graphene and graphene oxdie. A polymer nanocomposite is as a multiphase solid material in which one of the phases has one, two or three dimensions less than 100 nanometers (nm) in polymer matrices. Nanocomposites are suitable for applications as high performance composites, where good dispersion of the filler can be attained and the properties of the nanofiller are substantially different or better than those of the matrix. Graphene with exceptionally high electrical, thermal and mechanical properties is quite suitable as nanofiller in polymer matrices for the development of high performance materials. So, in this concise literature review, we have explored different methods (such as melt blending, solution

mixing and in situ technique) for the preparation of different polymer/graphene and graphene oxide nanocomposites and also mechanical, electrical, thermal and other physical properties. CARBON ALLOTROPES The carbon allotrope family consists of 3-dimensional graphite, 2-D graphene, 1-D nanotubes and 0-D fullerenes[5]. Different other forms of carbon have also been explored by the researchers such as synthetic graphite, synthetic diamonds, cokes, carbon black, adsorbent carbon, etc. (Table 1)[6,7]. In each form carbon has unique hybridization properties. The ground state orbital configuration of carbon is 1s2 2s2 and 2p2. The small energy difference between the 2s and 2p orbital can be accomplished by promoting one of the two 2 s electrons to 2p orbital. The promoted electron of carbon allows hybridization into sp, sp2 and sp3 configuration leading to various molecular structures such as chain, planer and tetrahedral structures. Diamond, graphite, fullerene, carbon onion, carbon nanofibers and carbon nanotubes are the allotropic form of carbon. All these are carbon-based nanomaterials[8]. The hybridized states of some typical carbon nanomaterials are summarized in Fig. 1. Diamond Diamond is the hardest allotropic form of carbon and most stable thermodynamically above 60 kbar. It is usually in crystalline form with cubic structure, which contains 8 atoms per unit cell[9-10]. The nanosize diamond was first synthesized in 1960s by a detonation technique in Union of Soviet Socialist Republics (USSR)[11]. Nanodiamond (ND) is also called an ultradispersed diamond (UDD)[12]. Other methods have also been reported for the synthesis of nanodiamond such as laser ablation[13], high energy ball milling of high pressure, high temperature (HPHT) diamond nanocrystal[14], detonation technique[15],

TABLE 1 Carbon allotropes, availability, selected properties and applications[6] Substance Diamond Nanotubes

Carbon fibers

Availability Naturally, artificial Production, Commercially available. Laboratory-scale synthesis, commercially available Industrial production, commercially available Industrial production, Laboratoryscale synthesis, commercially available

Selected properties Thermal conductivity, wide bandgap, Hardness High strength, Electronic properties;

High strength, thermal and electrical conductivity Fullerenes Potential high hardness in composites, optical and electronic properties, superconducting upon doping Carbon onions Laboratory-scale synthesis Under investigation Graphite Naturally occurring, artificial Lubricity, anisotropic electronic production, Commercially available conductivity Graphene Laboratory-scale synthesis Unique electronic properties

Selected applications Jewelry, Abrasives Proposed: nanoelectronics, structural Reinforcement, Pharmaceutics. Structural reinforcer in several applications Nanoelectronics, Hybrid solar cells Pharmaceutics

Under investigation Electrode component, lubricants Under investigation

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FIG. 1. Hybridization state of some carbon nanomaterials.


plasma-assisted chemical vapor deposition (CVD)[16], autoclave synthesis ion irradiation of graphite[17], carbide chlorination[18], ultra-sound cavitations[19], and electron irradiation of carbon onions[20]. Nanodiamond has excellent properties such as good optical properties, high mechanical strength, tunable structure, excellent electrical and thermal conductivity, and high surface area. Due to these outstanding properties the nano-diamond has many potential applications and can be used as filler material for nanocomposities[15,21]. Carbon Nanotubes and Carbon Nanofibers Carbon nanotubes (CNTs) was first discovered by Iijima in 1991[22]. They are the one-dimensional carbon allotrope, composed of rolled-up graphite form cylindrical tubes with diameter on a nanoscale[23]. They show outstanding thermal and electrical conductivity, high surface area, high mechanical and electrochemical stability[24]. The most significant methods such as laser ablation, catalytic chemical vapor deposition (CCVD), and electric arc discharge are used for the production of single and multiwalled carbon nanotubes[25]. CNTs are classified into: multiwalled carbon nanotubes (MWCNTs), double-walled carbon nanotubes (DWCNTs) and single-walled carbon nanotubes (SWCNTs)[22]. Chemical bonding between the carbon atoms in CNTs is sp2 and much stronger than sp3 bonds found in diamonds; therefore, the mechanical properties of CNTs are higher than other carbon materials[26,27]. The tensile strength and Young’s modulus of 50–200 GPa and 1.2 TPa, respectively, has been reported theoretically and experimentally[27]. CNTs are also efficient conducting fillers, but the drawback is their high production cost[2]. Vapor grown carbon nanofibers (VGCNFs) were prepared before CNTs[28]. The mass production of VGCNFs occurred through catalytic chemical vapor deposition. In this process, the deposition of carbon monoxide or hydrocarbon such as ethylene, propane, natural gas, acetylene, benzene, etc. are deposited over the surface of metal or metal alloy (such as Co, Ni, Au, Fe) catalyst. The catalysts are then deposited on substrate. The reaction is generally carried out in a reactor operated at 500–1500°C[29]. The exceptional properties of carbon nanotube and carbon nanofiber include electrical, mechanical, high surface area, strong resistance to acid and base, selective absorption/adsorption agents, composite materials and field emission device[30]. Fullerene Fullerene is the third allotropic form of carbon[31], and was first discovered in 1985[32]. It consists of a C60 spherical

network structure in which all the carbon atoms are sp2 hybridized to form a football shape (truncated icosahedrons or soccer ball) composed of 12 pentagon and 20 hexagon rings[33]. The fullerene materials are divided into three main groups such as Langmuir-Blodgett film, self-assembled nanolayers, and solution cast or thermally evaporated films, which are obtained directly from a fullerene molecule or from their derivatives[34]. Recently, fullerenes have become particularly interesting materials due to high superconductivity at high temperature[35], ferromagnetism, high mechanical stability, transformation into diamond under high pressure and also other unique chemical reactions[36]. Carbon Onions Carbon onions were actually discovered before fullerene and carbon nanotubes. It consists of spherical closed carbon shells, and their name is due to the concentric layered structure resembling that of an onion. It is also sometimes called carbon nano-onion (CNO) or onion-like carbon (OLC)[37]. The first investigation was given done Iijima in 1980 through the vacuum deposition of amorphous film[38,39]. However in the last 30 years, CNO has been synthesized using many different methods. McDonough and Gogotsi (in 2013) prepared CNOs on large scale using high-temperature annealing of diamond nanoparticles under vacuum[37]. Graphite The word graphite is derived from the Greek word “graphein,” meaning “to write” and has been used to draw or write[6]. Naturally graphite is found in the form of graphite powder or flake with a range of different particle sizes[40]. Graphite occurs in different structural forms such as flat, fibrous and spherical. On the basis of color, size, cleavage, habit and luster, they are divided into many groups[41]. The flake graphite consists of thin layers having thickness smaller than 100 nm[40]. In graphite the carbon atoms are held together by covalent bonds to form a layered structure and held together by van der Waals forces[42]. In layers, carbon atoms are arranged in such a way to form hexagonal sheets. The distance between two carbon atoms in hexagonal sheets is being 1.42A°[43] and there is distance of 3.35Å between the graphite sheets[44]. Each hexagonal sheet of carbon atoms are called graphene layers or graphene sheets in which the carbon atom are sp2-bonded, and their bond angle is 120° with respect to each other[43]. Due to the small distance between the graphite sheets, the intercalation of compound between sheets is very difficult. Therefore, the graphite flakes

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were processed for modification through physical or chemical methods. The chemical treatment involves intercalating different chemical species known as intercalating agents[40]. Chemical oxidation is shown by many methods, using different chemical species in which the van der Waals forces disrupt by the intercalation of atoms, ions and molecules into the graphite layers. This process results in the formation of the graphite intercalation compounds (GIC), also known as graphite oxide (GO)[42]. Graphite shows lubricants properties that are due to their sheets structure and van der Waals forces between the sheets; and, these sheets can easily slide with respect to each other (Fig. 2). Graphite is a good thermal and electrical conductor in the plane directions due to the presence of σ bonds and delocalized π band. On the other hand, they have poor thermal and electrical conductivity in the stacking direction, due to van der Waals forces between the sheets[10]. Graphite Oxide Graphite oxide (GO) typically consists of layer of pseudo– two-dimensional lamellae, usually prepared by the oxidation of natural graphite flakes[45]. It has mostly been prepared using Brodie, Staudenmaier and Hummer’s methods. In all three methods, the oxidation of graphite occurs by oxidants and strong acid. The oxidation levels of graphite oxide are varied on the basis of method of preparation, graphite precursor and reaction conditions[46]. In oxidation results each layer has been attached with polar groups such as epoxide, hydroxyl and carboxylate groups in the six-member aliphatic regions[45]. The hydroxyl and epoxy groups lie above and below on each graphene layer, and near edges carbonyl groups are located as shown in Fig. 3[47]. Due to the presence of these functional groups graphite oxide shows strong hydrophilic nature and can be easily dispersed in water. The graphite oxide may be exfoliated via ultrasonic vibration or thermal shock into two-dimensional graphene oxide nanosheets[48]. Nowadays the most common methods used for the preparation of graphene oxide (GrO) and/or graphite oxide

FIG. 2. Structure of graphite.

FIG. 3. Structure of graphite oxide (GO).

(GO) are Hummers and Brodie method. These two methods are different from each other in the acidic medium used (sulfuric acid or nitric acid), and also the type of salts employed (potassium permanganate or potassium chlorate)[49]. Preparation Methods of Graphite Oxide Dreyer et al.[50] in 2010 prepared graphite oxide, at that time the chemistry of graphite was also investigated. He prepared GO through the reaction of graphite powder, fuming nitric acid and potassium chlorate at low temperature[51]. The resulting product was crystalline material but the reflective goniometry failed to measure the interfacial angles because of the limited thickness, imperfect structure and small size. Moreover, GO has oxygen as a constituent, and hydrogen and carbon increased the overall mass of flake graphite. The resulting product was put under further oxidation to increase the oxygen content and composition determined was 61.04: 1.85: 37.11 (C:H:O) with a net molecular formula of C2.1H0.80O1.00[50]. Later, Staudenmair[52] improved Brodie’s experiment by adding concentrated sulfuric acid to increase the acidity of the mixture and by introducing multiple aliquots of potassium chlorate solution. The graphite was highly oxidized in a single reaction vessel (composition was same as the final Brodie product), thus it is mostly simplified graphite oxide synthesis process. However, the Staudenmaier’s method was both hazardous and time consuming. After Staudenmaier, Hummers and Offeman developed the routes for the oxidation. The reaction of graphite with water-free mixture of sodium nitrate, concentrated sulfuric acid, potassium permanganate below 35°C for the final product of oxidation has shown a higher level of oxidation relative to Staudenmaier’s product[53]. Structure of Graphene Oxide The exact structure of graphene oxide is still uncertain; however, there are some main models that proposed the



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FIG. 4. Proposed structure model for graphite oxide (a) Hofmann model; (b) Ruess model; (c) Scholz–Boehm; and (d) Lerf–Klinowski model[54].

structure of graphene oxide. These are Hofmann’s model, Ruess’s model, Sholtz and Boehm’s model, and Lerf and Klinowski’s model. The Hofmann model proposed that only the epoxy (-O-) groups were located on the surface of graphene oxide. However their model did not give any other information about the existence of other chemical groups such as hydrogen. Ruess’s model proposed that the carbon layers in graphene oxide is wrinkled and consists of a cyclohexane chairlike conformation in trans form. He first accounted for the hydrogen content in graphene oxide. Scholz and Boehm improved Ruess’s model. According to the model the ketone groups are present in graphene oxide[54]. In 1998 a new accepted structural model was proposed by Lerf and Kilnowski on the basis of solid state NMR experiments. They proposed that the epoxy (-O-) and hydroxyl (-OH) functional groups are attached above and below each carbon layer, while the carboxyl (-COOH) functional groups are bound to the edges of the carbon layer. Despite the explanation on hydroxyl and epoxy groups, this model has left some unanswered questions. Some questions include such as the hydroxyl groups are located very closely to each other which caused electrical instability due to their electrostatic repulsion and at the edge of each sheet the carboxyl groups are located (Fig. 4). For these reasons this model is need to be revised[54].

GRAPHENE Graphene is a single layer of carbon atoms sp2 hybridized and arranged in two-dimensional honeycomb lattice in which each carbon atom are bonded to three other neighbors carbon atoms[55]. In recent years, graphene has great attention due to its outstanding electrical, mechanical and thermal properties[56]. Graphene has high thermal conductivity (5000 W m1 K1)[57], high electrical properties (6000S/cm)[58], and Young’s modulus of 1 TPa[59], mechanical strength of 130 GPa[60], high optical transmittance (97.7%) and large theoretical surface area (2630 m2 g1)[61]. When GO powder are heated rapidly they produce flake-type graphene because by the thermal decomposition the oxygenated groups are evolved as CO2 and at the same time the sheets are exfoliated into separate graphene sheets. This method is inexpensive and ecofriendly because in reduction the solvent is not utilized[62]. However, different methods have been employed for the preparation of graphene[60] such as top-down approach and bottom-up approach[63]. In the top-down process, the exfoliation or separation of graphite or derivatives of graphite (graphite fluoride and graphite oxide) produced modified graphene or graphene sheets. This method is more suitable for the largescale production of graphene which is required for polymer composites application.

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The second routes is bottom-up approach, in this process graphene is synthesize by a number of methods such as epitaxial growth[64], chemical vapor deposition (CVD)[65], micromechanical exfoliation[66], reduction of GO[67], unzipping carbon nanotubes[68-69], and arc discharge[70-71]. Epitaxial growth and the CVD method often produced graphene sheets, which is free of defects and, if present, only in tiny amounts over large area. For the electronic application and fundamental study this method is more attractive than the mechanical cleavage method. But this method is not applicable for polymer nanocomposities, because it requires large amounts of modified surface graphene sheets[63]. Each method has some advantages and disadvantages. Among all these methods, the chemical method has been found most suitable for the production of graphene[72]. POLYMER/GRAPHENE AND GRAPHENE OXIDE NANOCOMPOSITES Nanocomposites are a new class of materials in which the reinforcement of nanometer size particles (less than 100 nm) into the matrix have been known to enhance the mechanical, electrical and barrier properties of the matrix. Filler such as alumina, silica, clay, silver, graphite nanosheets and carbon nanotubes are used[40]. Presently, polymer nanocomposites have become one of the largest classes of material science, as they efficiently integrate different types of materials in one system and thus increased their performance[60]. Polymer nanocomposites have a large number of applications in industrial sectors and thus organic and inorganic nanocomposites have been extensively studied in recent decades. The nanomaterials are classified into nanolayers, nanoparticles and nanotubes depending on their dimensions[73]. To obtain the high increase in properties of nanocomposites the layers materials (such as exfoliated GO) must be dispersed in the polymer matrix. The highly exfoliated GO can also be produced by ultrasonication as well as by rapid heating. Notably, for electrically conductive composites, mechanical exfoliated GO can be reduced in a separate step, because it retains an insulating structure while thermally expended GO is reduced and can be used for conductive materials[61]. Graphene is suitable filler for the enhancement of physical and mechanical properties of polymer. The polymer/graphene nanocomposites are generally prepared by using different methods. However, it is very difficult to attain their homogeneity because during the process, the graphene sheets are aggregates[74]. And also in polymer matrix the dispersion of pristine graphene is very poor. So the modified graphene is very easily dispersed to form nanocomposites. Modified graphene with organic functionality make its more compatible with organic polymer. The dispersion of graphene depends upon the factors such as composite preparation technique, type of modification, the presence of polar groups in polymer and in filler. Ionic liquid modified graphene, dodecyl amine-modified graphene and organic isocyanate-modified graphene have been used to prepare polymer/graphene nanocomposites by solution mixing.

Various techniques have been used to achieve high dispersion of nanoscale fillers in polymer matrix[75]. The preparation methods mostly depend on the molecular weight, reactive groups, polarity, hydrophobicity, etc. present in the graphene, polymer and solvent. There are a range of methods for the preparation of polymer nanocomposites[76], such as solution intercalation method in which the polymer is first dissolved in suitable solvent and modified graphene or graphene sheets are allowed to swell[77]. The modified graphene or graphene layers is dispersed in appropriate solvent like acetone, water, tetrahydrofuran (THF), toluene, dimethyl formamide (DMF) or chloroform. Moreover, in the modified graphene or graphene sheets the polymers were adsorbed and then solvent was evaporated[78]. Several polymer nanocomposities have been prepared by this method, i.e., epoxy/layered double hydroxide (LDH)[79], polypropylene (PP)/graphene[80], polystyrene (PS)/graphene[81], polyvinylalcohol(PVA)/graphene nanocomposites[82], etc. In situ intercalative polymerization involves the swelling of modified graphene or graphene sheets in liquid monomer, and polymerization is carried out by the addition of suitable initiator and radiation or by heat[48,83]. Several polymer nanocomposites such as polymethylmethacrylate (PMMA)/expanded graphite (EG)[84], polystyrene (PS)/graphene[48,83,85], polyethylene terephthalate (PET)/LDH[86], polyimide (PI)/LDH[87], etc. were synthesized by this method. Melt intercalation is another method for the preparation of polymer nanocomposites. By using this method the modified graphite or graphite in molten state are mixed with polymer matrix at high temperature by using conventional methods such as injunction or extrusion. The polymer chain is then exfoliates or intercalate to form nanocomposites[76,80]. A large number of polymer nanocomposites such as polyamide (PA6)/ expanded graphite (EG)[88], polyethylene sulfide/EG[89], polypropylene/(EG)[89], etc. have been prepared. RECENT ADVANCES IN POLYMER/GRAPHENE AND GRAPHENE OXIDE NANOCOMPOSITES Polymer nanocomposites based on a range of nanofillers such as carbon nanotubes (CNTs), expended graphite (EG), carbon nanofiber (CNF) have been prepared by different research groups. This section focuses on the use of graphene and graphene oxide as fillers in different polymeric matrices such as polystyrene (PS), polyurethane (PU), polyaniline (PANI), polycarbonate (PC), poly(3,4-ethyldioxythiophene) (PEDOT), etc. The following discussion on polymer/graphene and graphene oxide-based nanocomposites may be very helpful for the readers interested in the fabrication of novel polymer/graphene and graphene oxide-based nanocomposites for various applications. Han et al. synthesized polystyrene (PS)/graphene and PS/ graphene oxide (GO) nanocomposites by using the melt blending technique[90]. The nanocomposites flammability was evaluated by cone calorimetry and thermogravimetric analysis (TGA). Both the cone calorimetry and TGA results indicated



that the reduction in peak heat release rate (PHRR) and thermal stability decreased with increasing the oxygen groups on graphene or GOs and the reduction in PHRR of 50% occurred as compared to neat PS (with 5 wt. % of graphene content). Basu et al. synthesized (PS)/ graphene nanocomposites by in situ polymerization method of styrene[91]. The thermal stability, rheological and electrical properties of the nanocomposites were found to be higher than the neat polymer. The loss and storage modulus of the nanocomposites were also enhanced and the enhancements were significant even with 0.25 wt.% of graphene content. It was also noted that the increase in graphene loading decreased the dielectric constant at room temperature and reached to minimum value at a graphene loading of 0.25 wt.%. Stankovich et al. reported PS/isocyanate modified graphene nanocomposites using dimethyl formamide (DMF) is by solution blending method[77]. Dimethyl hydrazine was used for reduction of nanocomposites at 80°C for 24 h. The composites were coagulated by the drop wise addition of DMF into a large volume of methanol. The conductivity of nanocomposite was increased with increasing filler content. Polystyrene (PS)/functionalize and reduce graphene oxide (GO) with p-phenylene diamine (PPD) were prepared by solution blending[92]. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis observed that GO and GO-PPD was highly dispersed in PS. TGA result indicated that the maximum decomposition temperature increased 1°C and 8°C for PS/GO and PS/GO-PPD nanocomposite relative to pristine PS. The electrical conductivity of GO-PPD was improved to 2.1 × 102 S/m nearly 9 orders of magnitude higher than that of GO. Zhao et al. prepared polyaniline/graphene nanosheet/carbon nanotube (PANI/GNS/CNT) composite by in situ polymerization[93]. The working electrode was fabricated by mixing electroactive materials, carbon black and polytetrafluoroethylene (PTFE) with suitable ratio and dispersed in ethanol. The resulting mixture was coated on surface of nickel substrate with spatula followed by drying at 80°C. PANI/GNS/CNT showed higher specific capacitance relative to neat PANI and pure PANI/CNT composites, which is due synergistic effect between the PANI and GNS[94]. The long-term cycle stability of the PANI/GNS/CNT composites was superior to that of the PANI/CNT and PANI/GNS composites. After 1000 cycles, the capacitance of the PANI/GNS/CNT composites was decreased by 6% compared to PANI/GNS (52%) and PANI/CNT (67%) composites. Chemically modified graphene and polyaniline (PANI) nanofiber composites were prepared by in-situ polymerization of aniline monomer in presence of graphene oxide in acidic medium[95]. The resulting PANI/graphene oxide composites were reduced to graphene composites by hydrazine monohydrate followed by reoxidation and re-protonation of the reduced PANI to produce PANI/graphene composites. Pure graphene shows electrical conductivity 277.2 Sm1, which was found to be close to pristine graphite. The difference in electrical conductivity of GO,

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PANI, PANI/graphene and PANI/GO were reported as 0.2, 10.6, 168.7 and 231.2 Sm1, respectively. The lower conductivity of PANI/graphene composites compared with PANI/GO composites was due to the decrease in the degree of doping of PANI and composites morphology was changed during the process of reduction, re-oxidation and re-protonation. Lee et al. reported the waterborne polyurethane (WPU)/functionalized graphene sheet (FGS) nanocomposites by using an in situ method[96]. The FGSs highly dispersed in the WPU matrix was confirmed by TEM analysis. Due to the homogenous dispersion of FGS in WFU matrix increased the electrical conductivity of nanocomposites as compared to pure WPU. Differential scanning calorimetry (DSC) analysis showed that the melting temperature and heat of fusion of soft segment WPU in the nanocomposites was increased by the presence of FGS. Liang et al. prepared three types of nanocomposites by using a solution mixing method[97]. They used different fillers such as sulfonated graphene, isocyanate modified graphene and reduced graphene in thermoplastic polyurethane (TPU) matrix. TGA analysis showed that the rate of thermal decomposition of TPU/reduce graphene and TPU/sulfonated graphene was much lower than TPU/isocyanat-modified graphene nanocomposites. The results indicated that there was less functional group attached to sulfonated graphene relative to compared to isocyanate-modified graphene. Mohamadi et al. fabricated polymethyl methacrylate (PMMA)/graphene nanocomposite by using in situ polymerization method[98]. TGA and Fourier transform infrared spectroscopy (FTIR) analysis showed that PMMA was grafted on the surface of graphene. Grafting of PMMA chain on the surface of graphene occurred via radical reaction of graphene. However the grafting efficiency was very low in coupling reaction. DSC analysis indicated that PMMA was chemically grafted and physically absorbed on graphite surface which enhanced the glass transition temperature. The TGA analysis depicted for physical absorbed polymer only 19.49% weight was lost at the temperature range 291.2–423.14°C, while in chemical grafting of polymer only 40% weight loss at temperature range 501.99–600.44°C. Zeng et al. fabricated poly(methyl methacrylate) (PMMA)/graphene nanocomposites by using solution blending method with different amount of filler (0.1, 0.5, 1, 2 wt.%)[99]. The conductivity of nanocomposites was calculated by using the equation: ρ ¼ R.S/L. It was clear from results that 0.5 and 2 wt.% RGO in nanocomposites increased the electrical conductivity of PMMA about 106 and 1012, respectively, as compared to neat PMMA. At 1 wt.% content of filler, the glass transition temperature of nanocomposite was improved by 37° C. The nanocomposite also exhibited decrease in thermal expansion coefficient by 68% at filler content of 0.1 wt.%. Xu et al.[100] prepared poly(3,4-ethyldioxythiophene) [PEDOT]/sulphonated graphene composite prepared by in situ polymerization (Fig. 5). The novel composites material showed excellent transparency, high electrical conductivity, thermal stability and good flexibility and were easily processed

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FIG. 5. Poly(3,4-ethyldioxythiophene)[PEDOT]/sulphonated nanocomposite[100].

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graphene

in both organic and aqueous media. The conductivity of PEDOT/graphene films was deposited on quartz and PMMA substrate was 7 and 10.8 Sm1. The results indicated that the deposition of composites on PMMA has greater conductivity then on quartz substrate. The conductivity of composites film was independent on film thickness. PEDOT/graphene film with thickness of 103, 76, 58 and 33 have optical transmittance 36%, 51%, 76% and 96% at a wavelength of 550 nm. The PEDOT/graphene composites had high thermal stability and very little mass loss below 297°C and 19% loss below 325°C. Zhao et al. has synthesized graphene nanosheet (GN)/polythiophene (PTh) composite by in situ polymerization technique[101]. SEM of the nanocomposites showed that the GN was finely dispersed in the PTh matrix. The electrical and magnetic properties of composites were reached to 12.17 × 103 and 15.33 emu/g, respectively, at 0.4% mass content of GN. Liang et al. synthesized poly(vinyl alcohol) (PVA)/graphene nanocomposites by using water as a processing solvent to mix the graphene oxide GO into PVA matrix[82]. X-ray diffraction (XRD) pattern exhibited that the GO (2θ ¼ 10.9°) sheets were fully exfoliated into individual graphene sheets in matrix. At 0.7 wt % loading of graphene oxide into PVA matrix, Young’s modulus and tensile strength increased by 62% (from 2.13 to 3.45 GPa) and 76% (from 49.9 to 87.6 MPa) respectively which was due to the molecular level dispersion of graphene sheets in PVA matrix due to H-bonging between PVA and graphene[102,103]. Zhao et al. prepared PVA/fully exfoliated graphene sheets nanocomposites in aqueous solution[104]. XRD pattern indicated that the peak of graphene was disappeared in nanocomposite due to dispersion of graphene sheets matrix. The tensile strength of nanocomposites was improved 150% upon the 1.8 vol. % of graphene loading into the matrix. Ansari and Giannelis synthesized poly(vinylidene fluride) (PVDF)/functionalized graphene sheets (FGS) from graphene oxide and expended graphite by solution processing and compression molding[105]. DSC and XRD result indicated that there was no considerable change in glass transition temperature (Tg) and crystallinity. The thermal stability of PVDF/FGS composites was higher than PVDF/EG composites. The small

mount of loading (2 wt.%) of EG and FGS improved the storage modulus of pure PVDF from 1275 MPa to 1739 and 1859 MPa, respectively. The mechanical properties of both PVDF/FGS and PVDF/EG composites were superior then pure PVDF. Hu et al. has used sol-gel method for the synthesis of PVDF doped graphene oxide (GO) to improved the pyro-electric properties of PVDF in DMF solvent with different concentration of GO[106]. The film was deposited on ITO/PET substrate and annealed with different temperature to modified β-phase crystallization of PVDF. The XRD analysis indicated that the best annealing temperature for β-phase crystallization was 80°C at 1 wt.% loading of GO. The pyroelectric properties of PVDF increased with the increasing content of GO up to 1 wt.%. Wang et al. prepared a series of poly(l-lactic acid) (PLLA)/ graphene oxide (GO) nanocomposites with different ratio of GO using DMF as a mutual solvent by solution mixing method[107]. Wide-angle X-ray diffraction (WAXD) pattern showed that the peak of GO was disappeared in nanocomposites, suggesting that GO layers was highly exfoliate in nanocomposites. TEM results indicated that the exfoliated GO was highly dispersed in PLLA matrix. However, by increasing amount of GO in nanocomposites some aggregation occurred with 2 wt.% GO. The isothermal melt crystallization temperature was considerable higher in nanocomposites than pure PLLA. As the crystallization temperature increased for both PLLA/GO nanocomposites and pure PLLA the isothermal melt crystallization temperature decreased at different loading of GO. El Achaby and Qaiss prepared high density polyethylene (HDPE)/graphene nanosheets (GNs) and HDPE/multi-walled carbon nanotubes (MWCNTs) nanocomposites by a melt mixing process[108]. Both the nanocomposites were compared with the same amount of filler. At 3 wt.% filler content the Young’s modulus and tensile strength of graphene nanocomposites increased by 87% and 77%, while the MWCNTs nanocomposites was enhanced by 57 and 58% than the pristine polymer. The higher performance of the graphene nanocomposites was due the high aspect ratio, high surface area and nanoscale 2-D flat surface as compared to carbon nanotubes, which had only 1-D linear contact with polymer. Kim et al. prepared polycarbonate (PC) nanocomposites with graphite and functionalized graphene sheets (FGS) using melt compounding[109]. XRD and TEM showed that FGS were highly exfoliated and homogeneously delaminated in PC matrix. Rigidity percolation was achieved in PC/FGS composite at FGS content of 1–1.5 wt.%, where as this percolation was achieved at a graphite loading between 3 and 5 wt.%. The electrical conductivity percolation was also accomplished by lower FGS loading as compared to graphite. The mechanical performance of PC/FGS nanocomposites was higher than PC/G nanocomposites. Shen et al. synthesized polycarbonate (PC)/thermally reduced graphene (TRG) nanocomposites by using melt mixing method with different duration times and studied interfacial interaction between PC and TRG[110].



Marginal improvement in strength and tensile modulus was observed when mixing time was 10 min. However, for 0.5 h mixing time, a decrease in mechanical properties was observed. The FTIR result suggested that the chemical bond between the carboxylic group of TRG and carbonate group of PC leading to trans-esterification. Zhang et al. have fabricated polyethylene terephthalate (PET)/ graphene nanocomposites using the melt compounding process[111]. TEM analysis of nanocomposites disclosed that the graphene network was composed of large number of thin stack of a few sheets of monolayer graphene. A small increase of graphene content from 0.47 to 1.2 vol.% enhanced the electrical conductivity of PET/graphene nanocomposites to 7.4 × 102 Sm1 from 2.0 × 1013 Sm1. CONCLUSION Graphene is a two-dimensional carbon nanofiller with a single planer sheet which is one atom thick with the atom arranged in a honeycomb-shaped lattice. Graphene has received extensive attention in recent years due to mechanical, thermal and electrical properties suitable for electrically and thermally conducting reinforcement nanomaterials, sensors, solar cells, circuits, and flexible electrodes for displayed and transparent. Graphene is considered the best nanofiller among all nanofillers due to their high aspect ratio, high surface area high electrical and thermal conductivity. Different approaches such as top-down and bottom-up have been reported in scientific literature to produced graphene. The top-down approach is very efficient and more commonly applied for the production of graphene. Covalent and noncovalent surface modification has been developed to enhance their distribution and properties. Polymer/graphene and graphene oxide-based nanocomposites obtained through different routes (solution mixing, melt blending and in situ polymerization). The significant enhancement in mechanical, electrical, thermal, gas barrier and rheological properties have been reported at lower filler loading. The polymer/graphene-based nanocomposites have been found to show significant enhancement in mechanical properties as compared to pure matrix. Polymer/graphene and graphene oxide-based nanocomposites exhibited remarkably superior thermal stability and electrical conductivity as compared to the pristine polymer. The enhancement in thermal, electrical and mechanical properties of polymer/graphene and graphene oxide nanocomposites was not only dependent upon the characteristics of graphene and graphene oxide but was also dependent on the type of polymer matrix. REFERENCES 1. Porter, A.L.; Youtie, J.; Shapira, P.; Schoeneck, D.J. Refining search terms for nanotechnology. J. Nanopart. Res. 2008, 10, 715–728. 2. Ashori, A.; Bahrami, R. Modification of physico-mechanical properties of chitosan-tapioca starch blend films using nano graphene. Polym.Plast. Technol. Eng. 2014, 53, 312–318.

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