Trends andFrontiers in Graphene-BasedPolymer …...graphene-basedepoxynanocompos-itescomparedtoSWNT...

Polymers have been combinedwith other plastics to formblends, mixed with talc and

clay to give filled systems, andextruded and molded with fibers andother anisotropic reinforcements toyield composite and hybrid materials.This simple “mix and match”approach has allowed the plasticsengineer to utilize a small library ofpolymers to produce a bewilderingarray of useful products capable ofpossessing extremes of property val-ues. The latest addition to this paletteis graphene, a single atomic layer ofcarbon whose existence had beenknown for a long time but which wasproduced and identified only asrecently at 2004. Andre K. Geim andKonstantin S. Novoselov of theUniversity of Manchester, UK, wereawarded the 2010 Nobel Prize inPhysics for their ability to isolate thissingle sheet of carbon atoms. As aresult of their accomplishment, thelandscape of polymer nanocompositesis changing. It is true that carbon-based materials such as diamond,lonsdaleite, and graphite have beenknown to mankind for ages.However, renewed enthusiasm in thepolymer nanocomposites researchcommunity is primarily due to thespecial properties of graphene thatcan be transferred to plastics and the

fact that graphene is derived frominexpensive precursors. The price andperformance advantages of graphenechallenge carbon nanotubes (CNTs)in nanocomposites, coatings, sensors,and energy-storage-device applica-tions. And then there are applicationsthat can only be dreamed about.Indeed, in the words of Andre Geim,“Graphene is a wonder material withmany superlatives to its name.”1 Thisis evident from the huge surge ofstudies in the current literature. Whysuch an interest in graphene? Is thissimply graphene “hype,” or are thereapplication opportunities forgraphene-based composite structures?This review attempts to address boththese questions based on emergingtrends in graphene-based polymernanocomposites (GPNC). The scopeof this work is further broadened bysuggesting many possibilities thatawait GPNC researchers.

Graphene, the WonderMaterialA frequently cited property ofgraphene is its electron transportcapacity. This means that an electronmoves through it without much scat-tering or resistance. Electron mobili-ties that can be attained are ~ 20,000cm2/V.s, an order of magnitude high-er than that of an Si transistor.2 A

recent review3 suggests that withimproved sample preparation, mobili-ties could even exceed 25,000cm2/V.s. Whether the lack of a bandgap in graphene poses a challengeand whether synthesizing puregraphene (without contaminants) inlarge quantities (be it in ribbon formor any other) is viable or not, onlyfuture research can tell. For themoment, extraordinary electronicproperties put graphene at the top ofthe materials chart. As a result, thepossibility of replacing silicon as thebasic building block of the electronicindustry appears to be tantalizinglyclose. Note that graphene’s electricalconductivity is much higher than thatof copper, but its density is almostfour times lower. Details on electron-ic properties of graphene and theirimplications are abundant in the lit-erature.4,5

Interest in graphene for use innanocomposites is due to its intrinsicproperties. It has been predicted thata single, defect-free graphene plateletcould have an intrinsic tensilestrength higher than that of any othermaterial.6 Indeed, James Hone’sgroup studied the intrinsic breakingstrength of a free-standing monolayergraphene membrane (applyingnanoindentation) in an atomic forcemicroscope. They measured the mean

32 | PLASTICS ENGINEERING | JANUARY 2011 | www.4spe.org

Trends and Frontiers inGraphene-Based PolymerNanocomposites

_______________________________________Prithu Mukhopadhyay* and Rakesh K. Gupta**

*IPEX Technologies Inc., Verdun, Quebec, Canada**West Virginia University, Morgantown, West Virginia, USA

breaking force to be 1770 nN. Theyalso found that the material couldwithstand ultra-high strains (~25%).These measurements allowed theteam to compute the intrinsicstrength of a defect-free graphenesheet to be ~42 Nm–1. Here theintrinsic strength is defined as themaximum stress that can be support-ed by the material prior to failure in apristine defect-free material.7 Themechanical strength of graphene isremarkable in that it corresponds to aYoung’s modulus of ~1.0 TPa. Inother properties, Paul McEuen andco-workers demonstrated that agrapheme membrane only one atomthick is impermeable to standardgases, including helium, providingmany practical application opportu-nities for graphene sealed microcham-bers.8 Graphene exhibits thermalconductivity several times higher thanthat of copper. This means grapheneis able to dissipate heat readily.Recent work on a large graphenemembrane has shown the coefficientof thermal conductivity to be ~ 600W/(m.K).9 Another attribute ofgraphene is its very high specific sur-face area (calculated value, 2630mg–1) compared with that of CNTs(1315 m2g–1), making graphene anattractive candidate for energy-storageapplications. Rod Ruoff ’s group hasused chemically modified graphene(CMG) to demonstrate its superiorultracapacitor cell performance.10

Novel attributes of graphene havebeen elaborated elsewhere.11

Graphene is typically free of impuri-ties (no metals) as opposed to CNTs,a significant advantage for the con-struction of reliable sensors as well asenergy-storage devices.12 Further,owing to its shape and structure,graphene may pose fewer toxicityconcerns; this is a topic of currentresearch.All these qualities in a single nano-

material have made physicists,chemists, and materials scientists, bethey theorists or experimentalists,excited about graphene’s potential.Nevertheless, the important issue is

to distinguish the hype from the real-ity. The question is not what causedthe graphene hype, but which areascan really benefit from its discoveryand how to exploit its unique proper-ties.Polymer chemists and materials sci-

entists are already ahead of the curveby incorporating graphene and/or itsderivatives in polymer matrices todevelop applications that work.Increasing strength and stiffness byemploying nanoscale reinforcement isa well-known fact. Since the isolationof graphene, its production outputhas increased to 15 tons per annum.Based on a recent report,13 the com-mercial production of graphenenanoplatelets is expected to exceed200 tons per year within the next twoyears.However, graphene sheets that

make up graphite suffer from strongsurface attraction. Unless these sheetsare separated and homogeneously dis-persed within the polymer matrix, thefull potential of grapheme-basednanocomposites cannot be realized.This is quite evident given theresearch that has already been doneon exfoliated graphite and with car-bon nanotubes.To better appreciate the challenges

and the driving force behindgraphene-based polymer nanocom-posites, a short review of the relevantchemistry and production ofgraphene from its precursor graphitewarrants attention.

Production of GraphenesFrom GraphiteThe current literature describes sever-al techniques for graphene produc-tion, including the use of supercriti-cal carbon dioxide for the exfoliationof graphite.14 Each method has itsbenefits and related drawbacks.14-17

Current challenges to scalable nan-otube opening processes for makinggraphene have been documented.17

Kaner et al. reviewed the chemistry,history, production, and possibleapplications of graphene as honey-comb carbon,18 while Ruoff et al.

reviewed the preparation methods,properties, and applications ofgraphene-based materials.19 Anotherreview focused on stable grapheneoxide, highly reduced graphene oxide,and graphene dispersions in aqueousand organic media, highlighting theirmechanical and electrical proper-ties.20 Note that high-quality pris-tine-graphene is crucial to fundamen-tal studies. Clearly, the micromechan-ical cleavage method of Nobelists

Geim and Novoselov is not suitablefor producing large quantities ofgraphene from graphite. Large-scalegraphene-based applications such asgas sensors, ultracapacitors, or trans-parent conductive electrodes use asuspension or colloidal dispersion ofchemically modified graphenes(CMG) or reduced graphene oxide.The latter categories of graphenes arenot pure graphene and are thustermed as “defective graphene.” Thereare several categories of defects withingraphene-like structures.17 Essentiallythe distinction among the productionmethods is whether one wishes to

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Polymer chemists andmaterials scientistsare already aheadof the curve byincorporatinggraphene and/or itsderivatives in polymermatrices to developapplications thatwork.

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synthesize defect-free graphenes(purity) or graphenes with defects(structural and topological containingoxygen and other species onto thesurface). The virtue of graphene withdefects is its lower production costand scalability. However, the primarydrawback of graphene with defects isthe loss of its electronic properties.On the other hand, defects couldprovide reaction sites with the wrin-kled surface in exfoliated graphenesheets, which assist in anchoringpolymers. In consequence, this opensup other application avenues.Therefore, not only the quality ofgraphene but also the applications forwhich it is intended dictate the pro-duction method utilized.Graphene is the basal plane of

graphite, a one-atom-thick two-dimensional honeycomb layer of sp2

bonded carbon. When manygraphene layers are stacked regularlyin three dimensions, graphite is creat-ed. Graphite is unique in its chemicalbehavior. It can act as an oxidizingagent and also as a reducing agent. Alarge number of studies focus ongraphite because of its ability toaccommodate chemical species inter-calated between the basal planes. Theprocess of introducing intercalants isknown as intercalation. In graphiteintercalation compounds (GICs), thegraphene layers either accept elec-trons from or donate electrons tointercalated species without a loss ofplanarity of the carbon atoms. GICsusing potassium as an intercalanthave been known from as early as1841.21 When intercalation occurs,the interlayer spacing between adja-cent graphene layers increases. As aresult, the strength of existing van derWaals forces decreases. This observa-tion led researchers to expand GICfurther. One could take advantage ofthe exfoliation process to make indi-

vidual nanoscale graphene sheets orgraphene nanoplatelets (GNPs orNGPs).The trend to produce graphene and

graphene nanoplatelets in bulk quan-tities centers on chemical exfoliationof graphite. In this process, graphiteis oxidized (similar to intercalation)with a strong oxidizer to formgraphite oxide (GO), which results inan increase in the interlayer spacingbetween the oxygen-containinggraphene layers. Remarkably, oxidiz-ing methods (or modified therefrom)that are used today were developedover 50,22 100,23 and 15024 yearsago. GO can contain a variety of oxy-gen functionalities, including hydrox-yl and epoxide groups, on the basalplanes with carbonyl and carboxylgroups formed on the edges.25 Thesefunctional groups make GO sheetshighly hydrophilic and render themprone to swelling quickly and there-fore easy to disperse in water. Evenwith increasing humidity, the inter-layer distance between the graphenelayer sheets has been found toincrease reversibly from 6 to 12angstroms.26 Taking advantage ofthis, Ruoff ’s group first demonstrateda solution-based process to producealmost 1-nm-thick single-layergraphene sheets.27 To reducehydrophilicity, GO sheets have beenreacted with organic isocyanates, andit has been demonstrated28 that theisocyanate derivatized GO forms astable and completely exfoliated dis-persion in DMF (polar aprotic sol-vent). As for the reaction, the forma-tion of carbamate and amide func-tionalities has been proposed. Thispaved the way to synthesizegraphene-polymer nanocompositeswhere organic polymers could be dis-persed in polar aprotic solvents. Onenagging problem with GO, though,is its reduced electrical conductivity.

This could be alleviated by thermalannealing or by chemical reduction ofGO. Solvothermal reduction couldprovide another option.29 It isimportant to note that completereduction of GO (removal of defectsdue to oxygen-containing species) isimpossible to achieve.30 This is whyelectrical conductivity is perhaps themost reliable indicator of the extentto which graphite oxide has beenreduced to grapheme.31 This, howev-er, raises the issue of dispersibility inaqueous and/or organic matrices.Aggregation can be a substantialproblem if reduced GO sheets are notdispersed well in a polymer matrix.Both chemical functionalization32

and electrostatic stabilization33 couldalleviate aggregation of exfoliated GOsheets. Yet the possibility of the exfo-liated graphene’s polydispersityremains. Hersam provided a lucidperspective of polydispersity ofgrapheme, including challenges toproduce a dispersion of monodispersegraphene and how density gradientultracentrifugation could be effectivein isolating graphene sheets withvarying thicknesses.34 Utilizing thesame strategy as carbon nanotubes toavoid aggregation due to liquidswhose surface energy matched that ofnanotubes, Coleman’s group pro-duced a stable dispersion of grapheneby liquid-phase exfoliation ofgraphite.35-36 Extending these worksfurther, the same group reported onsurfactant-exfoliated graphene usingN-methylpyrrolidone as solvent.37

Recently, they proposed a low-cost,scalable process using water-sodiumcholate solution for a stable graphenedispersion.38

Another route to producinggraphene in bulk quantities isthrough thermal expansion ofgraphite oxide. Studies have shownthat for a successful GO exfoliation

GRAPHENE-BASED NANOCOMPOSITES CONTINUED

process, an enhancement of the c-axisspacing to 0.7 nm, brought about bycompletely eliminating the 0.34-nmgraphite interlayer (center to center)

spacing, is necessary. Exfoliatedgraphene sheets produced by thismethod are termed functionalizedgraphene sheets (FGSs).39 Figure 1ais an SEM image of a typicalgraphene flake, while Figure 1b is ahigh-resolution transmission electronmicroscopy (HRTEM) image of theedge of a typical FGS flake; thisFigure indicates that each flake iscomposed of ~ 2–3 individualgraphene sheets.Although size reduction and distor-

tion in the flat graphene structureoccurs because of thermal expansion,FGS still has high electrical conduc-tivity. However, it is important to dis-tinguish between different degrees ofexfoliation. Expanded graphite (EG)and/or worm-like exfoliated graphite(WEG) are not fully exfoliatedgraphene products. This is evidentfrom the diffraction peak at 2 inthe XRD pattern that provides the d-spacing. This is shown in Figure 2; itdifferentiates among structures ofgraphite, FGS, EG, and GO.55

Chemical exfoliation from bulkgraphite has revealed that both thelateral size and the crystallinity of thestarting graphite determine if thefinal graphene products are composedof a single layer, a single and a doublelayer, or more layers. Startinggraphite materials, such as artificialgraphite, flake graphite powder, kishgraphite, natural flake graphite, andhighly oriented pyrolytic graphite,also influence the grapheneproducts.40 It appears that artificialgraphite is also suitable for the pro-duction of single-layer graphene.The direct thermal treatment tem-

perature and time of the treatmentcan be varied to design and produceranges of graphene nanoplatelets(NGPs). Jang et al. claim to have pro-duced NGPs having mostly one-layerto five-layer structures by the thermalshock (at 1050°C) exfoliation ofGIC, followed by mechanical shear-ing.41 Different thickness values arisefrom the nonuniform expansion ofGIC. Note that graphenenanoplatelets are formed by stacking

of two to ten or even morenanographene sheets. It has beenshown that the electronic structureevolves rapidly, with a concomitantevolution in properties, with thenumber of graphene layers, approach-ing the 3D limit of graphite at as fewas ten layers.42 Moreover, owing tovarying thicknesses and sizes, NGPstend to agglomerate, and this couldpose dispersion problems in a poly-mer. Therefore, for the purpose ofdeveloping graphene-based polymernanocomposites, the importance ofthe synthesis method of the startinggraphene material is crucial.Manipulating graphene chemistry aswell as its layer architecture can pro-vide control over the final products.

Graphene-Based PolymerNanocompositesGraphite is cheap and abundant innature. Gene and MildredDresselhaus started working withgraphite (multilayered graphene) sev-eral decades ago, and their group’swork on graphite intercalated com-pounds until 1980 has been docu-mented.43 In terms of properties(stiffness, thermal, and electrical),graphite is superior to clay, and there-fore it provides a unique opportunityfor polymer reinforcement. The key,however, is to exfoliate graphite’s lay-ered structure and utilize it as ananoreinforcement. For the past 20years, researchers have incorporatedeither intercalated, exfoliated, orexpanded graphite platelets into poly-mers to produce nanocomposites.Mechanical exfoliation has, of course,been known for a while.44 Graphitenanosheets have been prepared bysonicating expanded graphite inaqueous alcohol solution. Thengraphite nanosheet-based polymethylmethacrylate (PMMA) was preparedvia in-situ polymerization of MMAand nanographite sheets by sonica-tion.45 Drzal’s group46 exfoliatedgraphite using graphite intercalatedcompounds, and Jang’s team usednatural flake graphite to intercalate toform expanded graphite and then to


Figure 1. a) SEM image of a typicalfunctionalized graphene flakedeposited on a silicon wafer forimaging; b) HRTEM image of theedges of a typical graphene flakeshowing ~2–3 graphene layers. Theinset shows measured electron dif-fraction pattern, typical for a few lay-ered graphene. From: M.A. Rafiee, J.Rafiee, I. Srivastava, Z.Wang, H.Song, Z-Z.Yu and N. Koratkar; Small,6, pp. 179–83 (2010) (Reference 61).Published byWiley.

Figure 2. XRD patterns of graphite,FGS, GO and EG. From: S. Ansariand E.P. Giannellis, J. Polym. Sci.Part B: Polym. Phys., 47, pp. 888–97(2009) (Reference 55). Published byWiley.

a

b


nanographene platelets.47 In a similarapproach, using graphite that hadalready been intercalated and exfoliat-ed, Kaner et al.48 re-intercalated thematerial with an alkali metal to makea first-stage compound, only to fur-ther exfoliate with ethanol.Subsequently they used microwaveradiation heating to produce NGP ofhigh aspect ratio with thicknessesdown to 2–10 nm. In a twist,graphite nanoplatelets were incorpo-rated into a polyacrylonitrile polymerfiber matrix by an electrospinningprocess to create polymer-basednanocomposite fibrils.49

Undoubtedly, the experimental dis-covery of graphene as a nanomaterialhas opened up new challenges forpolymer nanocomposites research. Infact, different approaches to makinggraphene with one or a few layershave led scientists to a better under-standing of graphene’s chemistry andits intrinsic properties, thus providingthe knowledge necessary to usegraphene as a nanofiller for polymer-based nanocomposites design toenhance electrical, thermal, barrier,and/or mechanical properties.Not only does the number of

graphene layers matter, but also theirthickness, area, and shape. The criti-cal factor in the success of polymerreinforcement is the length-to-thick-ness ratio, known as the aspect ratio.When the size of reinforcing particlesis greatly reduced, they tend toagglomerate and become difficult todisperse in a polymer matrix.Separation of graphene layers is cru-cial to avoid restacking due to vander Waals surface forces. The otherissue with pristine graphene is its lowwettability, i.e., low surface energy.Studies50 have shown that at roomtemperature, the surface energy ofgraphene is only 46.7 mJ/m2. Thisresults in poor dispersion in polymer

matrices with decreased mechanicalproperties of the resulting nanocom-posites. By contrast, oxidizedgraphene (GO) has a surface energyof 62.1 mJ/m2. In other words, oxi-dation and/or functionalization(introducing defects) of graphenehelp improve its dispersion in a poly-mer matrix but risk losing conductiv-ity of the resulting nanocomposites.The main challenge in designinggraphene-based polymer nanocom-posites (GPNCs) with improvedproperties is, therefore, to dispersethe individual graphene sheets in thepolymer matrix. Although the prepa-ration of PP/GO nanocomposites viain-situ Ziegler-Natta polymeriza-tion51 has been reported recently,solution phase mixing and meltblending appear to be dominant inthe current GPNC literature. Currentwork directions in these areas arehighlighted here.Stankovich and Dikin’s work

showed that it was, indeed, possibleto prepare a well-dispersed, homoge-neous mixture of graphenenanosheets52 in polystyrene. Torestore electrical properties of thegraphene-based PS-nanocomposites,the group carried out reduction ofthe dispersed materials (phenyl iso-cyanate-treated GO with PS) withdimethylhydrazine, which also pre-vented agglomeration of thegraphene. Furthermore, theresearchers found that the percolationthreshold of graphene in polystyrene-graphene composites was close to 0.1vol%, which was one-third than thatof any other two-dimensional filler.Subsequent work confirmed that ahomogeneous dispersion of function-alized grapheme in PMMA evencompeted against single-walled car-bon nanotube-PMMA composites.53

It has been found that surface func-tionality and the wrinkled morpholo-

gy of FGS endow superior mechani-cal and thermal properties to FGS-PMMA nanocomposites. At 0.01wt% graphene loading, the Young’s

modulus of PMMA increases by over30%. A further increase in grapheneloading to 1 wt% enhances theYoung’s modulus to 80% above thatof PMMA. However, at 1 wt% load-ing, the ultimate tensile strengthincreases only modestly, by 20%.When these results are compared toPMMA composites containinggraphite and expanded graphite (notfully exfoliated), mechanical propertyimprovements are not as significantas those for fully exfoliated compos-ites.54 Giannellis55 incorporated FGSand exfoliated graphite (EG) in aPVDF matrix by solution mixing andby melt blending. FGS-based PVDFnanocomposites showed percolationaround 2 wt% compared to 5 wt%for EG-filled PVDF composites. Thelower percolation threshold wasascribed to the greater aspect ratio ofFGS compared to EG. This differ-ence reveals itself in improved electri-cal conductivity of FGS-based versusEG-based composites, as shown inFigure 3.


Figure 3. Electrical conductivity ofFGS-PVDF and EG-PVDF nanocom-posites. From: S. Ansari and E.P.Giannellis, J. Polym. Sci. Part B:Polym. Phys., 47, pp. 888–97 (2009)(Reference 55). Published byWiley.

It was found that the resistance ofFGS-PVDF nanocompositesdecreased with increasing tempera-ture, indicating a negative tempera-ture coefficient, while EG-PVDFcomposites showed an increasedresistance with increasing temperature(positive temperature coefficient).Yongsheng Chen’s group used similarsolution-processable functionalizedgraphene to make graphene/poly(3-hexylthiophene) composites.56 Thesame group prepared solution-processable functionalizedgraphene/epoxy composites57 andfound a low percolation threshold of0.52 vol%. Previously, expanded-graphite-reinforced epoxy-nanocom-posites were prepared by differentprocessing methods (direct, sonica-tion, shear, and combined shear andsonication). When these methodswere compared, shear mixing provid-ed the best exfoliation and dispersionof graphite nanosheets and conse-quently the highest modulus of theresulting nanocomposites.58 To miti-gate the trade-off between mechanicaland electrical conductivity, a covalentbonding approach between thegraphene and epoxy matrix was uti-lized, and this showed a five- order-of-magnitude increase in electricalconductivity, 30% improvement instrength, and a 50% increase in stiff-ness in functionalized graphite-epoxynanocomposites.59 In another study,Koratkar60 compared mechanicalproperties of grapheme-based epoxynanocomposites to single-walled andmultiwalled based epoxy nanocom-posites at a nanofiller weight fractionof 0.1 ± 0.002%. Not only weremechanical properties better forgraphene-based epoxy nanocompos-ites compared to SWNT- andMWNT-based epoxy nanocompos-ites, but the fatigue resistance behav-ior was also decidedly superior.61

Improved properties with graphene-based composites were ascribed togreater specific surface area(graphene-epoxy interactions) andmechanical interlocking (graphene’swrinkled surface as shown in Figure

1a). This study raises another crucialissue: whether or not effective loadtransfer across the graphene-hostpolymer interface for high perform-ance composites occurs! The answercomes from a Raman spectroscopic

study62 in which a single pristinegraphene sheet was sandwichedbetween two coated layers of poly-mers (epoxy and poly-methylmethacrylate) to measure interfacialstress transfer in model nanocompos-ites (Figures 4a, 4b).Using continuum mechanics, this

study showed that the interface

between the graphene and the poly-mer breaks down at a shear stress ofthe order of 2.3 MPa. The studyobserved stress-induced shift of theG’ band (Figure 5). A linear shift ofthe G’ band up to 0.4% strain wasnoted when the stepwise deformationwas halted. The G’ band positionafter loading and unloading exhibitedslippage behavior of untreated andpristine graphene in the composites.This study demonstrated that Ramanspectroscopy could be used to evalu-ate the effectiveness of graphene as areinforcement for polymers.In the absence of chemical bonding

between graphene and the matrixpolymer, an effective approach couldbe to use hydrogen bonding tostrengthen the interface and to avoidde-bonding. For example, work ongraphene/polyvinyl alcohol nanocom-posites has been reported where a76% increase in tensile strength and a62% improvement in Young’s modu-lus were documented by introducingonly 0.7 wt% of GO.63 That hydro-gen bonding is critical to improvingmechanical properties of GO-poly-mer nanocomposites has been shownby the vacuum-assisted self assembly(VASA) technique.64 Chitosan-basedbiopolymer GO nanocomposites haveshown a substantial improvement intensile strength (122%) and Young’smodulus (64%) compared to chi-tosan with 1 wt% addition of GO.65

The anti-scratch property was alsomarkedly improved when isocyanateterminated PU was combined withGO nanoplatelets.66 Another studyused functionalized graphene sheet(FGS) and thermoplasticpolyurethane (TPU) in DMF solventto produce a cast nanocomposite filmthat enhanced the electrical conduc-tivity of TPU.67 Exfoliated graphenelayers were dispersed into TPU usingthree dispersion methods: melt com-pounding, solvent mixing, and in-situpolymerization. Comparative resultsindicated that the solvent-basedprocess was more effective in dispers-ing graphenes into TPU than meltprocessing.68 GNP-filled TPU


Figure 4. a) Optical micrographshowing the monolayer grapheneflake; b) schematic diagram (not toscale) of a section through the com-posite. From: L. Gong, I.A. Kinloch,R.J.Young, I. Riaz, R. Jalil and K.S.Novoselov, Advanced Materials, 22,pp. 2694–97 (2010) (Reference 62).Published byWiley Interscience.

Figure 5. Shift of the G’ band peakposition as a function of strain. (Theblue circles indicate where the load-ing was halted to map the strainacross the flake.) From: L. Gong, I.A.Kinloch, R.J.Young, I. Riaz, R. Jalil,and K.S. Novoselov, AdvancedMaterials, 22, pp. 2694–97 (2010)(Reference 62). Published byWileyInterscience.


nanocomposites made by solutionblending showed improved flameretardancy of the TPU matrix byreducing its heat release rate.69

Drzal et al.70 prepared GNP-PPnanocomposites by melt mixing usinga twin-screw extruder and then injec-tion-molded the GPNC to study itsthermal, viscoelastic, and barrierproperties. A loading of 3 vol% wasfound to reduce the coefficient ofthermal expansion of PP by ~ 25% inboth transverse and longitudinaldirections. In addition, the thermalconductivity was significantlyenhanced. Also, the oxygen perme-ability was reduced. Electrically con-

ductive PET-based graphenenanocomposites have been preparedby melt compounding.71 Macoskoreported on melt-compounded poly-carbonate-graphene nanocompositesand found both dispersion and orien-tation to be important to enhancingcomposite properties.72 Using solventblending and melt-mixing tech-niques, J.M. Tour introduced GO asa flame retardant nanoadditive intothermoplastics such as HIPS, ABS,and PC. This study73 showed thatGO could be used to fabricate poly-mer nanocomposites where decreasedflammability is desired. Melt-extrud-ed thermally reduced graphite oxide

(TrGO) or exfoliated GO based poly-mer nanocomposites using severalpolymers as matrices were studied.74

Owing to the low bulk density ofTrGO, solution blending was donewith the base polymers to premixTrGO at various loadings and then tomelt-compound the additive with thebase polymer in a mini-twin-screwextruder. TEM images of SAN andPC based TrGO nanocomposites(Figure 6) show effective exfoliationand uniform distribution of TrGO.Mechanical and electrical properties

of TrGO-based polymer nanocom-posites were then compared to multi-walled carbon nanotube (MWCNT)and conducting carbon black(CB)–based polymer nanocompos-ites.74 Kim made a detailed solutionand melt blended study by incorpo-rating FGS and isocyanate-treatedGO (iGO) in different model poly-meric systems.75 Modifying graphitenanosheets (GN) with surfactant, andincorporating it into HDPE, a melt-blended nanocomposite was preparedthat showed pronounced improve-ments in mechanical properties.76

When volume resistivity propertiesbetween melt-blended HDPE/GNand HDPE/carbon black (CB)nanocomposites were compared,HDPE/GN nanocomposites exhibit-ed a lower percolation threshold thanthat of HDPE/CB. Even HDPE/GNnanocomposites had better melt-flowproperties than HDPE/CB.77 A newdesign twist was imparted whenGNPs were coated with paraffin toreinforce LLDPE. Results indicatedthat the percolation threshold wasdramatically decreased as comparedto the uncoated GNP-LLDPEnanocomposites.78 A recent study hascompared melt-blended followed byinjection-molded GNP-HDPEnanocomposites to composites filledwith commercial carbon fibers (CF),


Figure 6. TEM images of SAN (top) and PC (bottom) nanocomposites contain-ing 7.5 wt% TrGO with different magnifications. From: P. Steurer, R.Wissert, R.Thomann, and R. Mulhaupt, Macromolecular Rapid Comm., 30, pp. 316–27(2009) (Reference 74). Published byWiley Interscience.

carbon black (CB), and glass fibers(GF). GNP-HDPE nanocompositesshowed equivalent flexural stiffnessand strength to HDPE compositesreinforced with CB and GF butslightly less than that of CF compos-ites at the same volume fraction(Figure 7a). The impact strength ofthe GNP-HDPE nanocomposites wassignificantly higher79 than that withthe use of all other reinforcements(Figure 7b).Recently, Cai and Song discussed

the behavior of various graphene-polymer nanocomposite systems andthe difficulties they present.80 Theconclusion is that graphene acts as areinforcing phase in polymer matri-ces. The challenge in dispersinggraphene platelets in commodity andengineering thermoplastics duringprocessing is the clue to the commer-cial success of GPNC development.Current research trends provide sever-al strategies to mitigate the dispersionproblem while exploiting the multi-functional properties of graphene as ananomaterial.

Possibilities AboundThe superiority of graphenes overcarbon nanotubes as reinforcementsstems from easy access to thegraphitic precursor material, the cost,the scalable method, and its orienta-tion flexibility (morphology). GPNCscan be formulated via different strate-

gies and with different types of poly-mers such as thermoplastics, ther-mosets, elastomers, and TPEs.Potential application areas includelaser mode locker to thermal to bipo-lar plates to energy storage to sensorsto structural adhesives to barrier togases. The diversity of applications isobvious from an examination of thefollowing studies.Owing to wavelength-independent

ultrafast saturable absorption, Tang etal. used a grapheme-based PVDFnanocomposite membrane as a sat-urable absorber for a high-power fiberlaser mode locker.81 The opticallytriggered actuation property isdependent on the integrity of the aro-matic network of graphene.Exploiting this characteristic,graphene-based thermoplasticpolyurethane nanocomposites wereshown to have an excellent infraredlight–triggered actuation.82 Thegroup also showed how functional-ized graphene-epoxy compositescould have commercial potential forlightweight shielding materials forelectromagnetic radiation.83 An earli-er study showed the potential forgraphite nanosheets to act as conduc-tive pigments for resin shielding coat-ings.84 Graphite nanoplatelets-basedpaste showed promise for use in ther-mal-interface materials.85 NGP-poly-mer composites have been reported86

to show impressive bulk conductivity

over 200 S/cm, which could findapplications in fuel-cell bipolar plates.Graphene-based nanocomposites canbe a promising catalyst support mate-rial for polymer electrolyte membranefuel cell (PEMFC).87-88 A biopoly-mer (chitin)-based hybrid nanocom-posite was used as a glucose biosen-sor.89 Recently, Pumera90 reviewed indetail progress in constructing high-performance electrochemical sensorsand biosensors. The possibility of uti-lizing NGPs in brominated and non-brominated vinyl ester nanocompos-ites for flammability performanceimprovements has been explored.91-92 Functional graphene-based poly-mer nanocomposites have been stud-ied for gas-barrier applications.Recently, the trustees of PrincetonUniversity received a patent forgraphene-elastomer nanocompositeswhere FGS had been dispersed invulcanized natural rubber, styrenebutadiene rubber, PS-isoprene-PS,and PDMS.93 The patented workcould find a wide range of industrialapplications, including food packag-ing, gasketing, and automotive. FGS-silicone foam nanocompositesshowed enhanced thermal stabilityand heat dissipation efficiency.94

Many commercial applicationopportunities for GPNC have recent-ly begun to surface.Conductive inks based on graphene

can be used as a cheaper alternative to


Figure 7. a) Flexural strength of various HDPE composites; b) impact strength of various HDPE composites. From: X.Jiang and L.T. Drzal, Polym. Compos., 31, pp. 1091–98 (2010) (Reference 79). Published byWiley Interscience and theSociety of Plastics Engineers.


silver-based ink to print RFID anten-nas and electrical contacts for flexibledisplays. Similarly, NGP-based mate-rials could protect an aircraft againstlightning strikes. Sheet molding com-pounds (SMC) are being developedfor automotive industries. Automotiveapplications could include fuel sys-tems (charge dissipation), tires (heatdissipation while enhancing stiffness),bumpers/fenders, and body compo-nents that require electrostatic spraypainting.

The creation of several start-upsthat provide graphene materials—including XG Sciences, GrapheneEnergy, Angstrom Materials, VorbeckMaterials, Graphene Solutions, andGraphene Industries—confirms thecommercial potential for graphene-based nanocomposites (GPNC).Indeed, the different applications list-ed above are rapidly becoming a reali-ty. Nanomaterials suppliers such asCheaptubes Inc., Ovation Polymers,

Graphene Supermarket, Avanzare,and Xiamen Knano GraphiteTechnology Corp. Ltd. have addedgraphene to their product portfolio.Depending on graphene type (e.g.,dimensions, conductivity, layer num-bers, oxide layer thickness, disper-sions) and its intended application,graphene could cost from $0.25 to$2000 per gram. Key findings fromLux Research suggest graphene’sprice/performance profile threatensMWNTs in composites, coatings,and energy-storage applications. Asproduction increases, the prices ofgraphene would decrease even more.According to this study,95 graphenesales are poised to grow from a valueof $196,000 in 2008 to $59 millionin 2015.

Conclusions and OutlookGraphene research has exploded,catching the world’s attention,because of the materials’ electron-transport capability. The chemistrypart has just started to unfold.Chemically-modified graphene hasprovided the platform for exploringstructure-property relations ofnanocomposites.In addition to conventional poly-

merization methods, other techniquessuch as atom transfer radical poly-merization (ATRP),96-98 reversibleaddition fragmentation chain transferpolymerization (RAFT)99-101 ornitroxide-mediated radical polymer-ization (NMRP), or layer-by-layerassembly are also available today topolymer chemists. Microwave irradia-tion is gaining attention with a viewto exfoliating chemically modifiedgraphene. These techniques will beused increasingly in the future to tai-lor application-specific GNPCs.Work will continue to focus on the

surface chemistry of graphene and itsderivatives. The synthesis of GO and

control of type and quantity of oxy-gen-containing species in GO havebeen the primary research focus ofthe graphene-based nanocompositescommunity. More studies are expect-ed using molecular-dynamics simula-tions to reveal the structural evolu-tion and chemistry of reducedgraphene oxide.102 How efficientlythe knowledge gained is transferredto the real world of processing willdetermine the true commercial suc-cess of GNPC. For instance, anobstacle to melt-mixing grapheneinto polymers is its low bulk density.Strategies will surely evolve to mixgraphene with polymers.Electron microscopic techniques

have played a vital role in elucidatingthe morphology of GNPCs includingintercalation and exfoliation in assess-ing the interfacial structure betweenthe graphene and the polymer matrixwith nanometer resolution.Morphological details allowresearchers to understand theresponse of all the structural details ofthe composite towards applied load,enabling the design of tailored mate-rials. On the other hand, Ramanspectroscopy has not only been usedto characterize graphene103 and dis-tinguish it from graphene withdefects, it has also provided the keyto understanding graphene edgechemistry104 and to monitoring stresstransfer efficiency in a GPNC. Thesestudies could further open up newvistas for grapheme-based nanocom-posites. We believe Raman spec-troscopy and electron microscopictechniques such as TEM will playmajor roles in designing application-specific GPNCs.Can incorporating graphene in an

elastomeric matrix provide flexiblenanocomposite materials exhibitingmultifunctional properties? GPNCdesign could involve biodegradable


■

Conductive inksbased on graphenecan be used as acheaper alternative tosilver-based ink toprint RFID antennasand electricalcontacts for flexibledisplays.

■

polymeric systems105 or stimuli-responsive polymeric systems.Inserting a self-healing component ina GPNC to auto-heal or by externalstimulus could address the fatigueissue that one encounters during serv-ice conditions. These novel graphene-based composites could be used inwell-established application areassuch as cars, aircrafts, fuel cells, mem-branes, and medical devices and sys-tems.Studies are piling up to develop

engineering data and design guide-lines for graphene-based polymernanocomposites. Graphene hasproven to be a multifunctional nano-material and is entering a crucial seg-ment in its product lifecycle frominnovation to applications.Opportunities for the future willdepend on the effective use ofgraphene defects to design GPNCs.The main challenge that remains:how to produce a large enough vol-ume of graphene safely and in a cost-efficient manner. That’s where therace begins!

AcknowledgmentAuthor PM wishes to thank LouisDaigneault of IPEX Technologies Inc.for his valuable suggestions.

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