Study on Modified Graphene/Butyl Rubber Nanocomposites.I.Preparation and CharacterizationHuiqin Lian,1,2,3Shuxin Li,1Kelong Liu,1Liangrui Xu,1Kuisheng Wang,2Wenli Guo11Department of Materials Science and Engineering,Beijing Institute of Petrochemical Technology,Beijing,China2Department of Materials Science and Engineering,Beijing University of Chemical Technology,Beijing,China 3Department of Chemical Engineering,Yanbian University,Beijing,ChinaButyl rubber,IIR nanocomposites based on modified graphene sheets,were fabricated by solution process-ing followed by compression molding.MG was pre-pared from natural graphite,NG through graphite oxide route.X-ray diffraction showed that the exfoliated MG was homogeneously dispersed in the IIR matrix with doping levels of1-10wt%as evidenced by the lack of the characteristic graphite reflection in the compo-sites.In contrast,the graphite retained its stacking order and showed the sharp characteristic peak in the NG-IIR composites.Scanning electron microscope images of the fracture surfaces of the IIR matrix showed that MG nanofillers exhibited better compati-bility than NG did.The mechanical properties of the MG-IIR nanocomposites were significantly improved due to the efficient distribution of the large surface area MG sheet.The tensile modulus of nanocomposite with doping level of MG10wt%was16times that of the pure IIR.POLYM.ENG.SCI.,00:000–000,2011.ª2011 Society of Plastics EngineersINTRODUCTIONCommercial butyl rubber(IIR)is a copolymer of isobu-tylene and a small amount of isoprene.It is employed in the inner linings of automobile tires and in other specialty application due to its characteristics of chemical inertness, impermeability to gases and weatherability.However, unsaturated bonds in IIR,due to the presence of isoprene monomer units in the backbone,can be attacked by atmos-pheric ozone leading to oxidative degradation and chain cleavage.Also in somefields,such as aerospace,aircraft and high-vacuum systems,IIR does not meet the extremely high-gas barrier as well as the high mechanical properties requirements.Therefore,there exists a continuous interest in lowering gas permeability and improving the mechanical properties of IIR by various techniques[1,2].It is well known that polymeric nanocomposites are of great interest for both scientific challenges and industrial applications due to their enhanced mechanical properties and unique material properties[3–5].Compared with con-ventional systems,nanomaterials are more effective rein-forcements because the stress transfer from the matrix to the reinforcement is more efficient in nanocomposites due to the increased surface area,assuming good adhesion at the interface.Also,the crack propagation length at the interface becomes longer,improving the strength and toughness.In addition to improving the mechanical prop-erty,nanofillers,such as layered silicate or carbon nanotubes,can provide dramatic improvement in thermal stability,dimensional stability,heat-distortion tempera-ture,and barrier property[6–11].Recently,graphite oxide (GO)has attracted increasing interest as afiller for poly-mer nanocomposites due to its high dispersive capacity, long coherence length and the barrier property[12,13]. GO with a typical pseudo-two-dimensional structure gen-erally contains hydroxyl,carboxyl and ether groups, which causes GO to absorb polar molecules easily and thus GO/polymer composites can be formed.Such struc-tural nanocomposites can provide reinforcement to the base polymer matrix.Also,GO may also have other desirable properties,such as mass diffusion coefficients, coefficients of thermal expansion,dielectric constants, thermal/chemical stability,solvent resistance,selectivity, conductivity,and resistivity to membrane fouling and poisoning.However,as a reinforcement of IIR,the hydro-philic surface of GO makes it difficult to disperse in the hydrophobic IIR matrix.Therefore,it is an importantCorrespondence to:Wenli Guo;e-mail:wlguo2008@Contract grant sponsor:Natural Science Foundation of China(NSFC);contract grant number:51063009;contract grant sponsor:BeijingNatural Science Foundation of China;contract grant number:KZ200910017001.DOI10.1002/pen.21997Published online in Wiley Online Library().V C2011Society of Plastics EngineersPOLYMER ENGINEERING AND SCIENCE—-2011Administratorissue to improve the compatibility of the GO sheet with the IIR matrix.Moreover,methods used to prepare polymeric nano-composites include in-situ polymerization [14],solution mixing [15],melting compound [16],and cocoagulating of polymeric composite solution [17].Considering the manufacture process of the IIR,slurry and solution pro-cess,the solution mixing is promising to fabricate IIR hybrids in the IIR industry.In this study,we report for the first time the fabrication of well-dispersed modified graphene in IIR composites through solution process.The MG nanosheets are homo-geneously dispersed in the IIR matrix with doping level of 1–10wt%.Compared with the pure IIR,the resulting nanocomposite membranes exhibit dramatic enhancement of mechanical properties.To the best of our knowledge,this is the first report of totally exfoliated graphite to reinforce IIR with outstanding mechanical property.The properties of vulcanization plateau,gas barrier,cure capa-bility,and rubber damping are under study and the results will be published in the near future.EXPERIMENTAL PROCEDURES MaterialsNatural graphite flakes with a partical size of 30l m were purchased from Aladdin Reagent Company (China).Cetyltrimethylammonium bromide (CTAB)was bought from Fuchen Chemical Reagents(Tianjin,China).Butyl rubber (IIR 1751)was obtained from YanShan Petrochem-ical Company of China.The other reagents (NaOH,NaNO 3,and KMnO 4)of analytical grade and 98%H 2SO 4,30%H 2O 2were purchased from Sinopharm Chemical Reagent Co.Ltd.(China)and were used as received with-out further purification.Ultrapure water with resistivity of 18M O was produced by a Milli-Q(Millipore,USA)and was used for solution preparation.Preparation of MG/NG-IIR Composite SheetsThe procedure used to prepare the MG-IIR nano-composite sheets was shown in Fig. 1.First,based on Hummers’method [18],the graphite was oxidized by con-centrated sulfuric acid to create polar hydrophilic groups (ÀÀCOOH,C ¼¼O,ÀÀOH)on the surface.The GO was dispersed in cetyltrimethylammonium bromide solution (20wt%)and ultrasonicated for 0.5h,followed by me-chanical stirring at 258C for 24h.During this process,the tertiary amine reacted with the carboxylic groups on the oxidized surface via an acid-base reaction or via hydrogen bonding between the surface ÀÀOH or C ¼¼O group and the amine groups.The suspension was filtrated and washed three times with water,dried at 408C in a vacuum for 24h.The resulting MG was added into the 15wt%solution of IIR in hexane by sonication for 0.5h to form a colloidal suspension.Then the mixture was stirred for 6h at 258C.The amounts of MG/NG added were 0,1,3,5,10wt%ofthe mass of rubber.The composite solution was then coa-gulated by adding methanol and the precipitated nanocom-posite was dried in a vacuum.Finally,sheet samples were prepared by vacuum compression molding using a 2mm thick spacer at 1008C under 10MPa for ing this procedure,the NG-IIR composite sheets were prepared.CHARACTERIZATION AND MEASUREMENTS The as-made membrane was characterized by X-ray diffraction (XRD,Scintag PAD X diffractometer,Cu K a source,operated at 45kV and 40mA).The samples were scanned with 58/min between 2y of 28–308.SEM observation was performed using Tecnai T12,at an acceleration voltage of 15kV with gold -posite samples were imaged by first fracturing in liquid nitrogen.TGA was performed using a TA Instrument Q500attached to an automatic programmer from ambient tem-perature to 5008C at a heating rate of 108C/min in a nitro-gen atmosphere.A TA instrument Q1000was used to record the DSC traces at a heating rate of 108C/min.Measurements of mechanical properties were con-ducted at 25628C according to relevant ISO standard (ISO 37).Tensile tests were measured on an Autograph AGS-J SHIMADZU universal testing machine at a cross-head speed of 500mm/min.The reported values were the average of five measurements.RESULTS AND DISCUSSIONThe FTIR spectra of NG,GO,and MG were shown in Fig.2.The FT-IR spectrum of NG showed no significant features.While that of GO showed quite differentFIG.1.Schematic representation for the fabrication of MG-IIR nano-composite membrane.2POLYMER ENGINEERING AND SCIENCE—-2011DOI 10.1002/pencharacter by the presence of new bands.The broad bandat3405cm21could be assigned to stretching of the ÀÀOH groups on the GO surface.The bands at1720and 1070cm21were associated with stretching of the C¼¼Oand CÀÀO stretching vibrations of carboxylic groupsrespectively.The FTIR spectrum of MG confirmed theeffective functionalization of graphene.The double bandsat2849and2919cm21were antisymmetric and symmet-ric CÀÀH stretching vibrations of theÀÀCH2ÀÀgroups from surfactant molecules[19]respectively.The bands at 1463and1127cm21were corresponding to CÀÀH bend-ing and the C-N stretching vibration respectively.The spectrum also showed a C¼¼C peak at1574cm21corre-sponding to the skeletal vibration of graphene sheets[20]. These spectral features showed that MG was successfully synthesized.The XRD patterns of the NG,GO,MG were shown inFig.3.The sharp diffraction peak around26.5o for pris-tine graphite(Fig.3a)showed that the basal spacing was0.34nm.Because of the strong Van der Waals force andstatic electric force between the sheets of graphite,thesheet was difficult to disperse.Thus a relatively strongoxidative acid was used to oxidize the graphite creatingpolar groups on the surface of graphite sheet.The surfac-tant of cetyltrimethyl ammonium bromide was used to functionalize the oxidized graphite through acid-base reaction to obtain stable exfoliated graphene sheets.As shown in Fig.3a,the GO showed two diffraction peaks at 2y of9.7o and25.3o,corresponding to a d-spacing of0.91 and0.35nm,respectively,and indicated that the GO was not fully oxidized and the additional peak at25.3o was that of unoxidized graphite.From Fig.3b,MG showed no characteristic peak indicating that the modified graphene sheet had been exfoliated completely.The XRD patterns of MG/NG-IIR nanocomposites membranes with different doping levels were presented in Fig.4.As shown in Fig.4a,the broad peak of2y around 15o appeared in IIR and NG-IIR membranes,due to the amorphous phase of IIR.Toki et al.[21]reported that the amorphous peak of IIR changed during uniaxial deforma-tion.In this case,with NG loading increase,the broad peak shifted slightly,from2y of14.7o in IIR to14.4o in 10wt%NG-IIR composite.It was deduced that NG did not influence the crystallization behavior of IIR very much.From Fig.4a,the diffraction peak at2y around 26.5o appeared in NG and NG-IIR composites because the NG retained its stacking order in the composite.XRD diffraction curves of MG-IR nanocomposites were shown in Fig.4b.The diffraction ascribed to graphite or oxidized graphite did not appear in all of the XRD patterns of composites,indicating the complete exfoliation of the MG in the IIR matrix.SEM images of the fractured surfaces of the as-made MG-IIR and NG-IIR composites were shown in Fig.5. SEM images of NG-IIR composites showed a smooth to-pography with the NG remained its stacking order in the IIR matrix(Fig.5a and b).In contrast,the MG-IIR com-posites appeared a rough surface and the MG dispersed in IIR matrix homogenously(Fig.5c and d).This is likely due to the organic modifier on the surface of MG produc-ing good compatibility with IIR matrix.These results were coincident with the XRD test.TGA under a nitrogen atmosphere was performed on NG,MG,IIR and MG-IIR composites to obtain the struc-ture of MG and composites,as well as to determine the effects of the MG on the thermal stability of the compo-sites.The resulting curves were shown in Fig.6.From FIG.2.FT-IR spectra of NG,GO andMG.FIG.3.The XRD patterns of(a)NG,GO and(b)MG.The curves are shifted vertically for clarity.DOI10.1002/pen POLYMER ENGINEERING AND SCIENCE—-20113FIG.4.The XRD patterns of composites(a)NG-IIR and(b)MG-IIR.The curves are shifted vertically forclarity.FIG.5.SEM images of fracture surface of(a,b)NG-IIR nanocomposites and(c,d)MG-IIR nanocompo-sites.4POLYMER ENGINEERING AND SCIENCE—-2011DOI10.1002/penNG curve of Fig.6a,graphite maintained its weight under the test condition.The curve of MG indicated that MG was composed of 17wt%graphene and 83wt%organicmodifiers.The functional surface contributed to the dis-persion of MG in the IIR solution as well as to the com-patibility with the IIR matrix in the composite membrane.Figure 6b indicated that the IIR began to decompose at 2698C and degraded completely at 4158C.In the case of MG/IIR composites,the weight loss below 3008C was attributed to the decomposition of the small organic molecules on the surface of graphene.Table 1listed the temperature at 5wt%loss of the TGA curve of IIR and MG-IIR composites.The pure IIR lost 5wt%at 3248C and the value was the highest of all the curvesobtained.FIG.6.(a)TG curves of NG and MG;(b)TG curves of MG-IIR nano-composites and (c)DTG curves of MG-IIR nanocomposites.TABLE 1.Thermal propertiesofpureIIRandMG-IIRnanocomposites.IIR 1%MG-IIR 3%MG-IIR 5%MG-IIR 10%MG-IIRT 5wt%(8C)a 324297321256236T mrv (8C)b 372368375378383a Temperature at 5wt%loss.bTemperature at the maximum reactivevelocity.FIG.7.DSC curves of MG-IIRnanocomposites.FIG.8.Tensile stress-strain curve of (a)MG-IIR nanocomposites and (b)NG-IIR nanocomposites.DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-20115The temperature at5wt%loss decreased with the increas-ing MG content when the MG contents were3,5,and10wt%in composites respectively.This might be due to theamount of small molecule of organic modifier increasedwith the increasing MG content.Thefirst derivative of the TGA curve(DTA curvesshown in Fig.6c showed the variation in weight withtime(dW/dT)as a function of temperature.The DTApeaks indicated the temperature of the maximum reactivevelocity.Table1listed the temperature at the maximumreactive velocity of the DTA curve of IIR and MG-IIRcomposites.Pure rubber reached the maximum reactivevelocity at3728C.The value of1wt%MG-IIR nanocom-posite was3688C and it was lower than that of IIR.Thismight be that the amount of graphene was too low toinfluence the thermal stability of the composite,whilesmall organic molecules on the surface of graphene withlow decomposition temperature induced the thermaldecomposition.For all other composites,the temperatureat the maximum reactive velocity increased with increas-ing graphene content,and for the10wt%composite itwas increased by118C compared with pure IIR.Thisindicated that the thermal stability of IIR was improvedby the addition of graphene.A significant number ofpapers had reported increased thermal stability of variouspolymers using graphene asfiller[15,22,23].T.Kuilaet al.reported that the better thermal stability of gra-phene/polyethylene composite system was due to the highaspect ratio of the monodispersed graphene layers whichacted as a barrier and inhibited the emission of small gas-eous molecules[23].DSC for the MG/IIR nanocomposites was shown inFig.7.The glass transition(T g)temperature of IIR wasobserved at270.58C,and the MG loading increased,theT g values of composites which were269.28C,268.68C, 268.98C,and269.18C with the loading levels of1,3,5, 10wt%,respectively.All the composites showed aslightly higher T g than that of pure IIR;among them thehighest one was268.68C of3wt%MG-IIR nanocompo-site.It indicated that the effective exfoliation of the MGincreased the T g due to the interaction between MG andpared with the results of graphene/polystyrenenanocomposite[24],the incorporation of PS nanoparticleson graphene sheets resulted in an increase in T g by178C,while the exfoliated MG did not influence the T g of MG-IIR nanocomposite very much.Therefore,the properties of nano-filler influenced the phase behavior of polymer matrix.The interaction between MG and IIR is under further study.The typical stress–strain curves for the MG-IIR and NG-IIR compositesfilms were presented in Fig.8.The mechanical performance of MG/IIR and NG/IIR compos-itefilms was significantly increased compared with pure IIRfilm(Fig.8a and b).The effect of MG content on mechanical properties of the nanocomposites was shown in Table 2.Stresses at 100%,200,and300%elongation of composites increased along with the MG loading increase.Therefore,in this study,MG-IIR nanocomposites had higher mechanical properties than that of pure IIR.On the basis of these stress–strain curves of tensile tests(Fig.8a and b),Young’s moduli were taken as the linear regression of the initial linear part of stress–strain curves.Figure9a showed the representative calculated Young’s moduli of IIR,NG-IIR,and MG-IIR.The corre-sponding Young’s modulus values were shown in Fig.9b. The addition of MG grapheneflakes significantly increased the Young’s modulus.Remarkably,the Young’s modulus of10wt%MG-IIR was16times that of pure IIR.In comparison,the10wt%NG-IIR compositesTABLE 2.Mechanical properties of pure IIR and MG-IIR nanocomposites.Stress at 100%(MPa)Stress at200%(MPa)Stress at300%(MPa)IIR0.170.190.181%MG-IIR0.250.230.183%MG-IIR0.440.380.295%MG-IIR0.540.430.3110%MG-IIR0.770.590.40FIG.9.(a)Representative calculated Young’s moduli of IIR,NG-IIR,and MG-IIR based on the slope of the elastic region.(b)Dependence ofYoung’s modulus on loading offillers in MG-IIR and NG-IIR nanocom-posites.6POLYMER ENGINEERING AND SCIENCE—-2011DOI10.1002/penimproved3.5times only,as shown in Fig.9b.It might be that the NG stacks did not exfoliate or intercalate in the IIR matrix.This result coincided with the results of the XRD and SEM analysis.Therefore,from the result of mechanical properties,it was deduced that the modifier on the surface of the gra-phene sheet not only caused the sheet to disperse in the IIR matrix,but also bonded to the large rubber chain in two dimensions,which gave the nanocomposite better tensile performance.According to the reference[25],gra-phene layers in poly(vinyl chloride)matrix enhanced the mechanical properties of PVC,because of the strong interfacial adhesion.Therefore,in our case,MG-IIR nano-composites had higher mechanical properties than the pure IIR,which may be due to the homogenous distribu-tion of exfoliated MG and the good compatibility with polymer matrix.CONCLUSIONMG-IIR nanocomposites were prepared by solution processing.SEM and XRD analysis indicated that the exfoliated MG was homogeneously disperse in the IIR matrix.The addition of MG greatly improved the mechan-ical property of the nanocomposites.The nanocomposites with10wt%MG showed the highest Young’s modulus, 3.4MPa,which was about16times higher than that of pure IIR,0.21MPa.REFERENCES1.S.Takahashi,H.A.Goldberg,C.A.Feeney,D.P.Karim,M.Farrell,K.O’Leary,and R.Paul,Polymer,47,3083 (2006).2.Y.Liang,W.Cao,Z.Li,Y.Wang,Y.Wu,and L.Zhang,Polym.Test.,27,270(2008).3.N.A.Kotov,Nature,442,254(2006).4.M.A.Pulickel and M.T.James,Nature,447,1066(2007).5.E.P.Giannelis,Adv.Mater.,8,29(1996).6.Camenzinda,W.R.Caserib,and S.E.Pratsinis,Nano Today,5,48(2010).7.S.C.Tjong,Mater.Sci.Eng.,R53,73(2006).8.Suresha,B.N.Ravi Kumar,M.Venkataramareddy,and T.Jayaraju,Mater.Des.,31,1993(2010).9.H.Kim and C.W.Macosko,Polymer,50,3797(2009).10.Y.Liang,Y.Wang,Y.Wu,Y.Lu,H.Zhang,and L.Zhang,Polym.Test.,24,12(2005).11.E.Burgaz,H.Lian,R.H.Alonso,L.Estevez,and E.P.Gian-nelis,Polymer,50,2348(2009).12.Y.Lian,Y.Liu,T.Jiang,J.Shu,H.Lian,and M.Cao,J.Phys.Chem.C,114,21(2010).13.K.Kalaitzidou,H.Fukushima,and L.T.Drzal,Compos.Sci.Technol.,67,2045(2007).14.J.M.Herrera-Alonso,Z.Sedlakova,and E.Marand,J.Membr.Sci.,349,251(2010).15.S.Ansari and E.P.Giannelis,J.Polym.Sci.Part B:Polym.Phys.,47,888(2009).uki, A.Tukigase,and M.Kato,Polymer,43,2185(2002).17.L.Q.Zhang,Y.Z.Wang,Y.Q.Wang,Y.Sui,and D.S.Yu,J.Appl.Polym.Sci.,78,1873(2000).18.W.S.Hummers and R.E.Offeman,J.Am.Chem.Soc.,80,1339(1958).19.P.J.Thistlethwaite and M.S.Hook,Langmuir,16,4993(2000).20.T.Szabo,O.Berkesi,and I.Dekany,Carbon,43,3186(2005).21.S.Toki,I.Sics,B.S.Hsiao,S.Murakami,M.Tosaka,S.Poompradub,S.Kohjiya,and Y.Ikeda,J.Polym.Sci.Part B:Polym.Phys.,42,956(2004).22.Q.L.Bao,H.Zhang,J.X.Yang,S.Wang,D.Y.Tong,R.Jose,S.Ramakrishna,C.T.Lim,and K.P.Loh,Adv.Funct.Mater.,20,1(2010).23.T.Kuila,S.Bose,C.E.Hong,M.E.Uddin,P.Khanra,N.H.Kim,and J.H.Lee,Carbon,49,1033(2011).24.A.S.Patole,S.P.Patole,H.Kang,J.Yoo,T.Kim,and J.Ahn,J.Colloid Interface Sci.,350,530(2010).25.S.Vadukumpully,J.Paul,N.Mahanta,and S.Valiyaveettil,Carbon,49,198(2011).DOI10.1002/pen POLYMER ENGINEERING AND SCIENCE—-20117。
