Macromolecular Nanotechnology Processing and assessment of high-performance poly(butylene terephthalate) nanocomposites reinforced with microwave exfoliated graphite oxide nanosheets Jun Bian a,b,⇑ , Hai Lan Lin a , Fei Xiong He a , Ling Wang a , Xiao Wei Wei a , I-Ta Chang b , Erol Sancaktar b a Key Laboratory of Special Materials and Preparation Technologies, College of Materials Science and Engineering, Xi-hua University, Chengdu, Sichuan 610039, PR China b Polymer Engineering Academic Center, Department of Polymer Engineering, University of Akron, Akron, OH 44325-0301, United States article info Article history: Received 30 November 2012 Received in revised form 21 February 2013 Accepted 23 February 2013 Available online 14 March 2013 Keywords: Poly(butylene terephthalate) (PBT) Microwave exfoliated graphite oxide nanosheets (MEGONSs) Nanocomposites Mechanical property Electrical property abstract To improve the physical properties of poly(butylenes terephthalate) (PBT), a series of nano- composites based on PBT and microwave exfoliated graphite oxide nanosheets (MEGONSs) are prepared via melt compounding technique, and their structures, thermal stabilities, mechanical, rheological and electrical properties are reported. Scanning electron micro- scope and X-ray diffraction exhibit that graphene platelets of MEGONS are well dispersed and exfoliated in the PBT matrix even at high MEGONS content of 4.0 wt.%. DSC cooling and following heating thermograms of the nanocomposites demonstrate that graphene plate- lets of MEGONS play a role as effective nucleating agents for PBT a-phase crystals and thus lead to accelerating the overall crystallization of the nanocomposites. Thermal stability of PBT/MEGONS nanocomposites improved substantially due to the gas barrier effect of graphene platelets of MEGONS dispersed in the PBT matrix. The rheological analysis shows the low frequency plateau of shear modulus and the shear thinning behavior of the nano- composites. The mechanical modulus of the nanocomposites enhanced significantly with increasing MEGONS content. The electrical conductivity test shows a pronounced increase in electrical conductivity from an insulator to almost a semiconductor with increasing MEGONS content. The electrical percolation threshold of the nanocomposites is found to be formed at the MEGONS concentration between 1.0 and 2.0 wt.%. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Among the various nanofillers used in the field of nano- structured materials, graphene represents one of the most promising as demonstrated by the number of recent publi- cations [1–4]. As far the preparation of graphene-based nanocomposite is concerned, the challenge is clearly the attainment of a fine dispersion of the above nanofiller in the polymer matrix [2–4]. Indeed, while it is difficult to obtain homogeneous dispersions of graphene in polymer matrices, it is much easier to do it with single sheets of graphite oxide (GO), which contains hydroxyl and epoxy groups on the basal planes and carboxy groups on the hedges. As GO, due to its hydrophilic nature, can be readily exfoliated and reduced via solvent exfoliation [1] or ther- mal exfoliation (TEGO) [5] to create ‘‘wormlike’’ structures with a high specific surface area. These fully exfoliated graphene oxide platelets have been widely investigated as nanofillers for wide range polymer nanocomposites [1–4]. Still, the main obstacle linked to GO incompatibility with most polymer systems remains. Thus, it is necessary MACROMOLECULAR NANOTECHNOLOGY 0014-3057/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.02.027 ⇑ Corresponding author. Present address: Polymer Engineering Academic Center, Department of Polymer Engineering, University of Akron, Akron, OH 44325-0301, United States. Tel.: +1 330245 9750; fax: +1 330258 2339. E-mail address: [email protected](J. Bian). European Polymer Journal 49 (2013) 1406–1423 Contents lists available at SciVerse ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Transcript
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Contents lists available at SciVerse ScienceDirect
Processing and assessment of high-performance poly(butyleneterephthalate) nanocomposites reinforced with microwaveexfoliated graphite oxide nanosheets
0014-3057/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.eurpolymj.2013.02.027
⇑ Corresponding author. Present address: Polymer EngineeringAcademic Center, Department of Polymer Engineering, University ofAkron, Akron, OH 44325-0301, United States. Tel.: +1 330245 9750;fax: +1 330258 2339.
Jun Bian a,b,⇑, Hai Lan Lin a, Fei Xiong He a, Ling Wang a, Xiao Wei Wei a, I-Ta Chang b,Erol Sancaktar b
a Key Laboratory of Special Materials and Preparation Technologies, College of Materials Science and Engineering, Xi-hua University,Chengdu, Sichuan 610039, PR Chinab Polymer Engineering Academic Center, Department of Polymer Engineering, University of Akron, Akron, OH 44325-0301, United States
a r t i c l e i n f o a b s t r a c t
Article history:Received 30 November 2012Received in revised form 21 February 2013Accepted 23 February 2013Available online 14 March 2013
To improve the physical properties of poly(butylenes terephthalate) (PBT), a series of nano-composites based on PBT and microwave exfoliated graphite oxide nanosheets (MEGONSs)are prepared via melt compounding technique, and their structures, thermal stabilities,mechanical, rheological and electrical properties are reported. Scanning electron micro-scope and X-ray diffraction exhibit that graphene platelets of MEGONS are well dispersedand exfoliated in the PBT matrix even at high MEGONS content of 4.0 wt.%. DSC cooling andfollowing heating thermograms of the nanocomposites demonstrate that graphene plate-lets of MEGONS play a role as effective nucleating agents for PBT a-phase crystals and thuslead to accelerating the overall crystallization of the nanocomposites. Thermal stability ofPBT/MEGONS nanocomposites improved substantially due to the gas barrier effect ofgraphene platelets of MEGONS dispersed in the PBT matrix. The rheological analysis showsthe low frequency plateau of shear modulus and the shear thinning behavior of the nano-composites. The mechanical modulus of the nanocomposites enhanced significantly withincreasing MEGONS content. The electrical conductivity test shows a pronounced increasein electrical conductivity from an insulator to almost a semiconductor with increasingMEGONS content. The electrical percolation threshold of the nanocomposites is found tobe formed at the MEGONS concentration between 1.0 and 2.0 wt.%.
� 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Among the various nanofillers used in the field of nano-structured materials, graphene represents one of the mostpromising as demonstrated by the number of recent publi-cations [1–4]. As far the preparation of graphene-basednanocomposite is concerned, the challenge is clearly theattainment of a fine dispersion of the above nanofiller in
the polymer matrix [2–4]. Indeed, while it is difficult toobtain homogeneous dispersions of graphene in polymermatrices, it is much easier to do it with single sheets ofgraphite oxide (GO), which contains hydroxyl and epoxygroups on the basal planes and carboxy groups on thehedges. As GO, due to its hydrophilic nature, can be readilyexfoliated and reduced via solvent exfoliation [1] or ther-mal exfoliation (TEGO) [5] to create ‘‘wormlike’’ structureswith a high specific surface area. These fully exfoliatedgraphene oxide platelets have been widely investigatedas nanofillers for wide range polymer nanocomposites[1–4]. Still, the main obstacle linked to GO incompatibilitywith most polymer systems remains. Thus, it is necessary
J. Bian et al. / European Polymer Journal 49 (2013) 1406–1423 1407
to proceed to surface property modification of GO by func-tionalization in order to make easier its dispersion in poly-mer matrices. Using GO after chemical modification withisocyanate or amine, composites have also been producedin aprotic solvents with hydrophobic polymers such aspolystyrene (PS) [1] and polyurethane (PU) [6–8]. Anotherroute generally used to disperse graphene into polymers issubjecting GO to microwave treatment to form microwaveexfoliated graphite oxide nanosheets (MEGONSs) beforeblending with polymers [9,10]. Recent literatures surveyshows that MEGONS has a similar structure to TEGO[9,10], which suggests MEGONS may also be dispersedusing melt blending and might afford property enhance-ments comparable to TEGO. Interestingly, the procedurefor making MEGONS is less time and energy intensive thanthe typical TEGO synthesis. Moreover, there are some dif-ferences in the reported physical properties of TEGO andMEGONS, such as different O:C ratios (i.e., a generallyhigher O:C ratio for MEGONS compared to TEGO) and dif-ferent values of electrical conductivity, which could possi-bly affect dispersion and the final composite properties.
Poly(butylene terephthalate) (PBT) is a thermoplasticand semicrystalline polymer and, as a member of thepoly(alkylene terephthalate) family, is derived from a poly-condensate of terephthalic acid with 1,4-butanediol. PBThas relatively rapid crystallization rate and high elasticitycompared with poly(ethylene terephthalate) (PET), but ithas somewhat lower strength and stiffness than PET.Nonetheless, because of its combining excellent mechani-cal properties with robust chemical resistance and dimen-sional stability, PBT has been widely used for anengineering thermoplastic [11,12] or for a component inblends [13–15], copolymers [16–18] and composites [19–21]. However, for high-performance applications, thermaland mechanical properties of PBT need to be enhanced. Be-sides, PBT exhibits some other disadvantages like lownotch impact strength and low heat deflection tempera-ture. In response to demand for high performance materi-als it is often modified by blending with other polymersand use of reinforcements. Among the various reinforce-ments for PBT, calcium carbonate [22], carbon black [23],glass fibers [24], carbon fibers [25], montmorrilonite [26]and carbon nanotubes [21,27] are most often used. How-ever, literature survey reveals that very little researchworks have been published about processing of graphenenanopletes reinforced PBT nanocomposites through themelt blending technique. Recently, Li and Jeong [28] pre-pared PBT/exfoliated graphite nanocomposites by meltblending, which showed the addition of small amounts ofthe above filler could result in a marked improvement inthermal and electrical conductivity (percolation thresholdwas between 3 and 5 wt.%.) of the composites. In-situ poly-merization approach has been applied by Fabbri et al. [29]to prepare PBT/graphene oxide nanocomposites, and theresults indicate that increasing amounts of graphene oxidedid not strongly influence the degree of crystallinity andthe crystallization temperature of PBT, while its thermalstability is significantly increased by the presence ofgraphene oxide. All the PBT/graphene oxide compositesdemonstrated to be electrically conductive and the electricfield assisted thermal annealing of the composites induces
an increase in conductivity. However, in general the use ofgraphene oxide involves a previous oxidation of graphiteand subsequent reduction of GO, in order to restore thematerial electrical conductivity. In this light, the develop-ment of methods able to disperse graphene in one step intoa polymer matrix is a significant current research issue.
In this work, a series of PBT-based nanocompositesreinforced with microwave exfoliated graphene plateletshas been attempted by applying a simple procedure, whichconsists of a preliminary dispersion/exfoliation of graphiteby using microwave exfoliation, and a subsequent meltblending with PBT. The effects of graphene platelets onmelting/crystallization behavior, thermal stability,mechanical modulus, rheological property and electricalconductivity of PBT have been investigated by adoptingvarious measurements. This work differs substantiallyfrom the previous studies, because MEGONS presumablyenhance the interfacial interaction with the PBT matrixand shows a high affinity for PBT, and it could thereforebe used effectively in the fabrication of PBT nanocompos-ites. Herein, we present, to our knowledge, the first reporton the morphology and properties of a MEGONS-filled PBTnanocomposites.
2. Experimental
2.1. Materials
A commercially available PBT particle with intrinsic vis-cosity of 1.25 dL/g was used as polymer matrix. In order toimprove the uniformity of mixing, PBT particles werefirstly frozen in liquid nitrogen then grounded into finepowder. The natural graphite powder (NGP, SP-2, C > 99%,D = 5 lm) was purchased from Qingdao Tianhe GraphiteCo., Ltd. (Qing-Dao, China). KMnO4 (C.P.), NaNO3 (C.P.),H2SO4 (>96%), H2O2 (30%), isopropyl alcohol (IPA) andN,N-dimethylformamide (DMF) were purchased fromKe-Long Reagent, Inc. (Cheng-Du, China). All the chemicalswere used as received without further purification.
2.2. Synthesis of graphite oxide (GO) and MEGONS
GO used in this research was synthesized from NGP bygraphite oxidation with KMnO4 in concentrated H2SO4
according to the procedures depicted in our previous work[30]. The GO was loaded into a glass beaker, put into adomestic microwave oven (General Electric), and heatedfor �20 s to cause rapid exfoliation and reduction of thematerial (the yield of MEGONS relative to the startingamount of GO was �35%). The black, fluffy MEGONS pow-der was collected and kept free to maintain its originalmorphology before use.
2.3. Fabrication of PBT/MEGONS nanocomposites
PBT/MEGONS nanocomposites containing variousMEGONS contents were prepared by melt-compoundingmethod. Before processing, all the components were driedin vacuum at 120 �C for 24 h. Since MEGONS has low bulkdensity (0.01–0.02 g/mL), some MEGONS powder was
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easily lost during handling and particularly during themixing step. In order to minimize this problem and to im-prove the dispersion of MEGONS in the nanocomposites,we applied a coating method reported in our previouswork [31]. MEGONS was firstly dispersed in IPA by sonica-tion for 3 h at room temperature, PBT powder was thenadded to this MEGONS solution and sonication was contin-ued for 1 h. Finally, the solvent was evaporated at 80 �Cresulting in complete coverage of PBT powders withMEGONS. The MEGONS contents in the mixtures were setat 0.0, 0.2, 0.4, 0.8, 1.0, 2.0, 4.0, and 8.0 wt.%. The solid mix-tures were melt-extruded through a co-rotating twin-screw extruder (TSE-30A/500-11-40, RuiYa Polymer Pro-cessing Equipment Com., NanJing, China) with a screwdiameter of 30 mm and the L/D (length/diameter) ratio of36. The temperature profile of the extruder from feed zoneto die zone was set at 220, 225, 230, 235, 235, 240, 240,245 and 245 �C. The extruded strands were cooled in waterbath, chopped into pellets, and dried under vacuum at120 �C for 24 h. For structural and property characteriza-tion, melt-quenched film samples of 1 mm thickness wereprepared by compression-molding the pellets in a hot-plate at 245 �C and 18 MPa for 4 min, quenching into aniced-water bath, and then drying in a vacuum oven at30 �C overnight.
2.4. Characterization of PBT/MEGONS nanocomposites
2.4.1. Microscopy and structure (SEM)Morphologies of MEGONS and the dispersion state of
MEGONS in the nanocomposites were characterized byusing a JEOL JSM-820 scanning electron microscope. TheNGP, GO and MEGONS samples were in fine powder form,while PBT and PBT/MEGONS nanocomposites sampleswere from fracture surface specimens. All samples weregold coated prior to SEM investigation to avoid chargingand were examined at 15 kV accelerating voltage.
2.4.2. Atomic Force Microscopy (AFM)The DI MultiMode Scanning Probe Microscopy (SMP)
with the tapping mode Atomic Force Microscopy (AFM)technique was used to examine the topography ofMEGONS. The MEGONS samples for AFM observationswere prepared by depositing dispersion of MEGONS (inDMF) on a metallic sample holder surface and allowingthem to dry in air. Both the height and phase images ofthe sample surfaces were obtained. The scanning sizewas 5–10 lm.
2.4.3. Surface elemental analysisElement analysis was performed on Euro EA 3000 (Euro
Vector S.P.A.). X-ray photoelectron spectra (XPS) of sam-ples were obtained by using an ESCALAB 250 (Thermo-VG Scientific) analyzer.
2.4.4. X-ray diffraction (XRD)These tests were performed on a Rigaku D/max-1200X
Diffractometer (40 kV, 200 mA, CuKa, k = 0.154 nm) atambient temperature. The NGP, GO and MEGONS sampleswere in fine powder form, while PBT and PBT/MEGONS
nanocomposites samples were from hot-compressedspecimens.
2.4.5. Fourier transform infrared spectroscopy (FTIR)The interfacial interaction between PBT and MEGONS in
the nanocomposites was characterized on a Nicolet 380spectrometer (Thermo Electron Corporation, USA) with aresolution of 4 cm�1. The powder specimens were dis-persed into the KBr powder by mortar, and compressedto form disks. The nanocomposite samples were fromhot-compressed specimens. They were dried at 80 �Cunder vacuum for 6 h before analysis.
2.4.6. Differential scanning calorimetric analysis (DSC)Melting and crystallization behaviors of the nanocom-
posites were analyzed by a NETZSCH DSC 200 F3 differen-tial scanning calorimeter thermal analyzer in a N2
environment. The samples were heated from 30 to 280 �Cat a heating rate of 10 �C/min, then cooled to 30 �C and fi-nally heated again to 280 �C at a rate of 10 �C /min. The Tm
was obtained from the second heating run, while the Tc
was obtained from the cooling run.
2.4.7. Thermogravimetric analysis (TGA)Thermal stability of the nanocomposites was investi-
gated by using a Perkin-Elmer TGA7 ThermogravimetricAnalyzer under the nitrogen or oxygen gas condition at aheating rate of 10 �C/min. The degradation temperature(T5% and T30%), maximum rate of degradation temperature(Tmax) and other data were determined from the weightloss curves.
2.4.8. Rheological propertiesRheological characterizations of the pure PBT and PBT/
MEGONS nanocomposites were performed using anAdvanced Rheometric Expansion System (ARES) rheometer(Rheometric, now TA Instruments, New Castle, DE) usinginjection molded disc specimens (approximately0.9–1 mm thickness). All samples were tested at 220 �C un-der flowing nitrogen. The storage modulus (G0), loss modu-lus (G00) and complex viscosity (|g⁄|) were plotted as afunction of oscillation frequency (x). For each sampleloading tested, a dynamic strain sweep at 1 rad/s was per-formed to find the limit of linear viscoelasticity. The max-imum strain before the drop in storage modulus (G0) withincreasing strain was recorded and used as the constantstrain for the frequency sweep test. A frequency sweep atconstant strain was then performed from 0.1 to 500 rad/s.
2.4.9. Mechanical propertiesTensile properties were obtained using an
INSTRON3365 electronic tensile tester with computer con-trol. The rate of cross-head motion was 20 mm/min atroom temperature. Five specimens of each compositionwere tested, and the average values were reported.
2.4.10. Electrical conductivityThe conductivity (r) of nanocomposites was measured
using electrochemical workstation (Solartron 1255B) atroom temperature when r was less than 10�3 S/cm. Whenr of the specimen was greater than 10�3 S/cm, it was mea-
J. Bian et al. / European Polymer Journal 49 (2013) 1406–1423 1409
sured using an SDY-4 four-probe instrument (Guangzhou,China). The detailed procedures and calculation methodshave been reported in our previous works [31,32].
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3. Results and discussion
3.1. Structural and morphological characterization of fillers
The detailed morphological changes of graphite duringthe oxidation and rapid microwave assistant exfoliationwere characterized by SEM, as can be seen in Fig. 1. Inthese studies, a kind of highly crystalline NGP was usedas starting material in which the parallel graphite sheetsare physically bonded to each other by weak van der Waalsforces and are stacked regularly. It is confirmed that theaverage diameter and thickness of NGP is �500 and�200 mm, respectively (Fig. 1A). On the other hand, GOshows a rougher surface topology as well as increased dis-tance between graphite sheets due to the oxidation whichgenerates oxygen containing functional groups on the sur-faces of graphite sheets (Fig. 1B). After rapid expansion andreduction in microwave, a significant exfoliation of stackedgraphite sheets into thin graphene sheets can be observed(Fig. 1C). Elemental analysis indicates that GO has a C:Oratio of 1.44:1, while MEGONS is found to have a C:O ratioof 3.09:1 (�2.23:1 according to XPS), indicating the pres-ence of oxygen-based functional groups, although in muchlower concentration than GO. The C:O ratio of MEGONS isroughly consistent with other earlier reports [9,10]. Uponpre-coating MEGONS on PBT surface under sonication, itclearly shows that MEGONS has been further exfoliatedand coated homogeneously on the PBT surface (Fig. 1D).
Fig. 1. SEM images of (A) NGP, (B) GO, (C) M
In order to show the thickness of MEGONS sheet andestimate its aspect ratio, AFM images of MEGONS havebeen obtained as shown in Fig. 2. It can be seen thatMEGONS has been exfoliated with cross-section measure-ments indicating a sheet thickness of �50 nm, which indi-cates �80 graphene sheets in a particle. This value isobviously lower than that of NGP or GO. In order to im-prove the exfoliation of the stacked graphene sheets, thesesheets have been subjected to ultrasonication and pre-coating before melt blending (see Experimental part).Microwave treatment reduces the hydrophilic characterof GO sheets due to the decomposition of functional groupsof GO. However, there are still some functional groupswhich remain on the surface of MEGO sheets (as confirmedby FTIR analysis later). Therefore, such MEGO sheets canstill readily form stable dispersions in polar solvents (suchas DMF), consisting of highly exfoliated, functionalizedgraphene oxide sheets, as determined by AFM (Fig. 2Aand B). The large surface area and the observed big pores,in addition to any functional groups, facilitate processingwith polymers.
The structures of NGP, GO, and MEGONS were furtherinvestigated by means of XRD and the results are shownin Fig. 3. The original NGP shows a (002) diffraction peakat 2h = 26.6�, which corresponds to d-spacing of0.335 nm, the typical interlayer spacing of stacked graph-ene sheets in graphite and it supports that the originalgraphite flakes have well-ordered structures. Upon oxida-tion, GO has a broad peak at 2h = 12.2�. Again, this peakcorresponds to the X-ray diffraction from the (002)planes and indicates that the interlayer spacing have ex-panded to 0.73 nm by the accommodation of various
EGONS and (D) MEGONS coated PBT.
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Fig. 2. Height profile (A) and Phase image and (B) of a typical AFM tapping mode images of MEGONS sheets deposited onto a substrate from a DMFdispersion with superimposed cross-section measurements taken along the white line indicating a sheet thickness of �50 nm (bottom image).
Fig. 3. X-ray diffractograms of NGP, GO and MEGONS.
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functional groups on the graphene. Besides, a weak peakat 2h = 25–26� represents diffraction at (002) planes ofgraphite. In the case of MEGONS, the (002) peaks are ob-served in the 2h range of 25–26� when GO is exfoliated toform MEGONS. The shifting in the (002) peaks to lower2h indicates separation of graphite layers. In addition,the (001) peak is absent in either diffractograms, sug-gesting that graphite platelets have been completely
exfoliated and expanded along their thickness directionduring the microwave treatment, as confirmed by theSEM image of MEGONS. Since MEGONS is largely exfoli-ated (relative to GO) before mixing with PBT matrix, itis expected that the nanocomposites will exhibit an exfo-liated morphology. This expectation will be confirmed bysubsequent XRD and SEM characterizations ofnanocomposites.
Fig. 4. SEM images of the fracture surfaces of PBT/MEGONS nanocomposites with various MEGONS contents: (A) pure PBT, and nanocomposites with (B)0.2, (C) 0.4, (D) 0.8, (E) 1.0, (F) 2.0, (G) 4.0, and (H) 8.0 wt.% MEGONS.
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3.2. Dispersion of MEGONS in the nanocomposites
To characterize the dispersion state of MEGONS in thePBT matrix, the fractured surfaces of neat PBT andPBT/MEGONS nanocomposites were examined by usingSEM, as can be seen in Fig. 4. The neat PBT exhibits quite
even and smooth surface, indicating a typical brittle failure(Fig. 4A). In cases of PBT/MEGONS nanocomposites, muchrougher fractured surfaces are seen on adding MEGONSin the PBT matrix, as shown in Fig. 4B–H. Although the de-gree of fractured surface roughness increases with increas-ing MEGONS content in the nanocomposites, no obvious
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agglomerated domains of graphene platelets of MEGONSare observed even for the nanocomposite with highMEGONS content of 1.0 wt.%. It suggests that grapheneplatelets of MEGONS are well delaminated and dispersedin the PBT matrix due to the preliminary dispersion/exfoli-ation of graphite flakes, pre-coating treatment under soni-cation and the high shear force generated during the meltcompounding. In addition, the SEM study reveals thatthere are two distinguished types of morphology on thesample fracture with increasing MEGONS loadings: (i)when MEGONS loading is lower than 1.0 wt.%, surfaceswhere no MEGONS or only very small graphite sheetscan be seen, as indicated by images in Fig. 4A–D; and (ii)at higher MEGONS loading (higher than 1.0 wt.%), surfaceswith big MEGONS agglomerates, and large flat graphiteplatelets buckled and deformed that are shown inFig. 4E–H.
XRD patterns of the neat PBT and nanocomposites withvarious MEGONS contents are shown in Fig. 5. It is foundthat, in pure PBT the diffraction peaks are observed at 2hBraggs angle of 8.9�, 15.5�, 16.8�, 20.2�, 22.9� and 24.2�,corresponding to diffraction planes (001), (011), (010),(110), (100) and (111) which are characteristic of thea-form [33] of PBT with triclinic crystal structure (curvea). In contrast, the PBT/MEGONS nanocomposites showmore intense crystalline diffraction peaks with increasingMEGONS content in the nanocomposites. The sharp dif-fraction peaks for the nanocomposites are identified to becaused by the a-phase crystals of PBT. It has been knownthat PBT has two crystal structures of a- and b-phasesand that the reversible transition between two crystalphases takes place depending on the mechanical deforma-tion, that is, the a-phase transforms to b-phase by appliedstress for elongation and vice versa by relaxation [34,35].Therefore, it is expected that graphene platelets of ME-GONS contribute dominantly to the formation of PBT a-
Fig. 5. X-ray diffractograms of PBT/MEGONS nanocomposites with various MEGO(h) 8.0 wt.% MEGONS.
phase crystals. Interestingly, we have compared XRD dif-fraction patterns of PBT/MEGONS nanocomposites beforeand after mechanical tensile (not shown here), and noany structural transition (a- and b-phases phase transfor-mation) has been observed during/after mechanical defor-mation because no any new diffraction peaks appearexcept some slight changes in peak intensities. On theother hand, it should be mentioned that there is no anycharacteristic diffraction peak of ordered graphene plate-lets or natural graphite when MEGONS loading is lowerthan 4.0 wt.% (curves from b to f). It supports the fact thatgraphene platelets of MEGONS exist in an exfoliated anddisordered state in the nanocomposites. However, thecharacteristic diffraction peak observed at 2h Braggs angleof 26� of ordered graphene platelets when MEGONS load-ing is higher than 4.0 wt.% can be seen (curves g and h),probably due to slightly agglomerates of MEGONS.
3.3. Surface properties of fillers and possible interactions innanocomposites
The possibility of chemical interactions in polymercomposites can be identified by FTIR spectra. It is knownthat, if two polymers are compatible, a distinct interaction(hydrogen-bonding or dipolar interaction) can existbetween the chains of the two polymers, causing the infra-red spectra of the composite to change (e.g. band shifts,broadening) [36]. Consequently, FTIR can identify segmen-tal interactions and provide information about the phasebehavior of polymer composites. To understand the inter-facial interaction between MEGONS and PBT matrix, theFTIR spectra of the fillers and nanocomposite films wereobtained as shown in Fig. 6. Fig. 6A shows the FTIR spectraof NGP and MEGONS, while FTIR spectra of PBT and PBT/MEGONS nanocomposites are shown in Fig. 6B and C. Asshown in Fig. 6A, no obvious absorbance peaks can be de-
NS contents: (a) pure PBT, (b) 0.2, (c) 0.4, (d) 0.8, (e) 1.0, (f) 2.0, (g) 4.0, and
Fig. 6. FTIR spectra of (A) NGP and MEGONS, and (B) PBT/MEGONS nanocomposites in the range from 4000 cm�1 to 400 cm�1 and (C) in the range from2000 cm�1 to 1000 cm�1, with various MEGONS contents: (a) pure PBT, (b) 0.2, (c) 0.4, (d) 0.8, (e) 1.0, (f) 2.0, (g) 4.0, and (h) 8.0 wt.% MEGONS.
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tected on the NGP surface, indicating the chemical inertproperty of NGP. In contrast, the surface property of ME-GONS is clearly different from that of NGF. As for MEGONS,the absorbance peak at 1720 cm�1 is assigned to thestretching vibrations of acid carbonyl (>C@O) group ofthe carboxylic acid groups present on the MEGONS [37].The broad band centered at around 3430 cm�1 is attributedto the OH stretching mode in carboxylic acid group andalso inferred to the presence of hydroxyl (AOH) group onthe MEGONS surface [38]. The absorption peaks featuredat 1580 cm�1 is attributed to the CAC bond stretchingvibration of carbon skeleton of the MEGONS bulk [39].The peak at 1040 cm�1 is referred to the vibration ofCAO bond in primary alcohol present on the MEGONS sur-
face. The strong absorption peak at around 1130 cm�1 isidentified as the CAO stretching mode of the characteristicof ether linkage (CAOAC) [40]. The peaks at 2920 and2850 cm�1 correspond to the stretching vibrations ofCAH (ACH3, ACH2A and >CHA groups) of the MEGONSskeleton. The small peak at 1622 cm�1 is assigned to thestretching vibration of C@C bond in aromatic ring of ME-GONS strand [41]. We believe that these surface character-istics are helpful in improving the interfacial interactionsbetween MEGONS and polymeric matrices, thus resultingin improvement of mechanical properties.
The FTIR spectra of neat PBT and PBT/MEGONSnanocomposites are shown in Fig. 6B, which helps to inves-tigate the interface interactions between the functional
Fig. 6. (continued)
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groups of MEGONS and the PBT matrix. The typical FTIRbands of PBT appear at 2925, 2853 (CH2 stretching), 1717(C@O stretch), 1415 (CAH bending in CH2 group), 1340(arom. ring), 1246 (COAO stretch in esters), 1115 (OACH2),1013 (arom. ring) and 720 cm�1 (aromatic CAH bending).In cases of nanocomposites, overall FTIR spectra are quitesimilar to that of the neat PBT (Fig. 6B). Interestingly, it isfound that the stretching vibration of C@O of PBT chainsshifts slightly to the lower wavenumber from 1717 cm�1
to 1707 cm�1, when we compare pure PBT with the nano-composite with 8.0 wt.% MEGONS (Fig. 6C, curve h), whichis presumably related to the hydrogen bond formation be-tween the ACOOH functional groups which reside on thesurface of MEGONS and the PBT matrix. Furthermore, themagnitude of hydrogen bond increases with increasingMEGONS content, which can be confirmed by the shiftsof wavenumbers near 1717 cm�1 (from curve b to h).Besides, it is found that the bands at 1340 cm�1 and1048 cm�1 relating to the stretching vibration of aromaticrings of PBT chains disappear when MEGONS is incorpo-rated. These bands changes with respect to the MEGONScontent of nanocomposites demonstrates that the phenylrings of PBT chains are specifically interacting with graph-ene platelets of MEGONS via aromatic–aromatic (p–p)interaction at the interface of nanocomposites. In sum-mary, it can be argued that interfacial interactions exist be-tween PBT and MEGONS, which stems from the fact thatthe completely exfoliated MEGONS is composed of func-tionalized graphene sheets that still contain the polar func-tional groups which remain after microwave treatment.
3.4. Thermal analyses of PBT/MEGONS nanocomposites
The effects of MEGONS on the non-isothermal crystalli-zation behavior of PBT nanocomposites were analyzedwith DSC. DSC cooling and heating thermograms of neat
PBT and PBT/MEGONS nanocomposites are shown inFig. 7. The thermal properties such as the melting onsettemperature (Tm,o), peak melting temperature (Tm,p), heatof melting (DHm), crystallization onset temperature (Tc,o),crystallization peak temperature (Tc,p), heat of crystalliza-tion (DHc), and degree of crystallinity (Xc) of the purePBT and PBT nanocomposites are summarized in Table 1.The crystallinity percentage of PBT constituent in nano-composite is determined according to the following equa-tion proposed by Liu et al. [42].
Xc ð%;CrystallinityÞ ¼ DHm
DH�mð1�wf Þ� 100%
where DH�m is the heat of fusion (140 J/g) for 100% crystal-line PBT [21] and wf is the mass fraction of filler in thenanocomposites. In the cooling thermograms (Fig. 7A),the melt crystallization peaks shift to higher temperaturesfrom 194.8 �C for the neat PBT (curve a) to 207.4 �C for thenanocomposite with 8.0 wt.% MEGONS (curve h), and theyalso become broader with the increment of MEGONS con-tent in the nanocomposites. This result indicates that thedispersed graphene platelets of MEGONS act not only asseeds for faster formation of PBT nuclei but also as barriersto the formation of large PBT crystallites. Nonetheless,graphene platelets of MEGONS accelerate the overallmelt-crystallization of PBT in the nanocomposites. To char-acterize the melting behavior of PBT crystals formed dur-ing the cooling runs, the following heating thermogramswere obtained as shown in Fig. 6B. All the samples showtypical multiple melting endotherms, which is interpretedin terms of the melting-recrystallization-remelting pro-cesses occurring during the 2nd heating and has been re-ported earlier also [26]. In other words, the lowermelting endotherm is associated with the melting of theoriginal crystals developed during the melt-crystallizationand the higher melting endotherm is related with the
Fig. 7. DSC cooling (A) and following heating and (B) thermograms of the neat PBT and PBT/MEGONS nanocomposites with various MEGONS contents: (a)pure PBT, (b) 0.2, (c) 0.4, (d) 0.8, (e) 1.0, (f) 2.0, (g) 4.0, and (h) 8.0 wt.% MEGONS.
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remelting of crystals formed by recrystallization during theheating process. It is also observed that the lower meltingpeaks are slightly shifted to higher temperatures from215.2 to 223.2 �C with increasing MEGONS content, whilethe higher melting peaks remain unchanged, regardlessof the MEGONS content in the nanocomposites. The shiftsof the lower melting peaks to higher temperatures demon-strate that the original crystals developed during the cool-ing runs are thicker and more stable for thenanocomposites with higher MEGONS content. Overall, itis believed that the graphene platelets of MEGONS dis-persed in the PBT matrix contributes to the formation ofstable and thick PBT crystals.
Further analysis from Table 1 and Fig. 7, the Tc,p and theTm,p of pure PBT is 194.8 and 222.1 �C, respectively. Incor-porating MEGONS provides approximately 12.6 �C increasein Tc,p and 1.1 �C increase in Tm,p, respectively, for 8.0 wt.%of MEGONS. The improvements in Tc,p and Tm,p can be cor-related to the processing of nanocomposites into two steps.First, break up of NGP into graphite nanosheets by micro-wave exfoliation, and second, uniform dispersion and coat-ing of graphite nanosheets on the surface of PBT particlesduring pre-mixing. It has been confirmed by fractographythat these steps enable the fine dispersibility of graphitenanosheets. Further analysis from Table 1 indicates thatthe crystallization process of PBT is accelerated as
Table 1DSC results of neat PBT and PBT/MEGONS nanocomposites with different MEGONS contents.
Tc,o = Onset crystallization temperature.Tc,e = End crystallization temperature.Tc,p = The peak temperature of crystallization.DHc = The heat of crystallization.Tm,o = Onset melting temperature.Tm,e = End melting temperature.Tm,p = The peak temperature of melting.DHm = The heat of melting.Xc = The relative degree of crystallinity.DT = Degree of supercooling (Tm,p � Tc,p).
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evidenced by the narrowing of the crystallization peakwidth (Fig. 6A), also can be indicated by DT value listedin Table 1). The results show that MEGONS dispersed inPBT matrix play the role of hetero-nucleating agent, whichcan facilitate the crystallization of PBT. The supercoolingdegree (DT = Tm,p � Tc,p) as a thermodynamic driver ofcrystallization required for PBT crystallization in the nano-composites is lower compared with that required for purePBT. The drop in the DT indicates an accelerated nucleatingprocess for PBT. Overall, the DSC analysis clearly showsthat incorporating MEOGNS in PBT results in an increasein Tc,p and Tm,p, and is attributed to the mixing method uti-lized. PBT/MEGNOS obtained from coating mixing disruptsthe graphite layers, separating them and thus providingmore surfaces for nucleation. Similar behaviors have beenobserved and reported in PP-clay [43], PPS-graphite [44]and PES-graphite [31] composites.
Thermal stability of PBT/MEGONS nanocomposites un-der nitrogen and oxygen gas conditions was investigatedusing a thermogravimetric analyzer. Representative TGAcurves of the neat PBT and nanocomposites with differentMEGONS content under the nitrogen gas condition areshown in Fig. 8A. To evaluate quantitatively the enhancedthermal stability of nanocomposites, the characteristicthermal degradation temperatures for 5%, 10%, 30% andthe maximum weight loss (T5, T10, T30 and Tmax) are sum-marized in Table 2. It is found that thermal stability ofthe pure PBT is enhanced by the incorporation of MEGONSnot only under nitrogen gas condition but also under oxy-gen gas condition, which is evident from the increasedthermal degradation temperatures for T5, T10, T30 and Tmax.For instance, T5, T10, T30 and Tmax values of the nanocom-posite with 8.0 wt.% MEGONS under the nitrogen gas con-dition are characterized to be 385.5, 393.8, 408.1 and417.5 �C, respectively, which are 22.4, 21.7, 22.7 and20.8 �C higher than those of the neat PBT. In considerationof higher heat conductivity and thermal diffusivity ofMEGONS in comparison to PBT, the distinct improvementin thermal stability may be associated with the two dimen-sional planar structure of well-dispersed MEGONS. As
such, the MEGONS may serve as a barrier preventing spee-dy removal of degradation products from the underlyingPBT matrix, thus delaying the process of degradation. Wealso note that the excellent thermal stability of MEGONSand the interfacial interactions present between the PBTmatrix and MEGONS which reduce significantly the PBT-MEGONS thermal boundary resistance, and results insmooth transfer of heat from the PBT matrix to theMEGONS. Consequently, it facilitates uniform heat distri-bution throughout the nanocomposites without the crea-tion of any ‘‘hot spots’’, a characteristic which may leadto lower decomposition temperatures.
On the other hand, TGA curves under oxygen gas condi-tion are completely different from those under nitrogengas condition, because oxygen gas plays an important rolein thermo-oxidative degradation behavior of nanocompos-ites. In Fig. 8B, the degradation of PBT and PBT/MEGONSnanocomposite is divided into two steps. This unusual phe-nomenon is due to the decomposition of initial materialsand the following consumption of char under the high oxy-gen presence. In addition, like the nitrogen gas condition, itis apparent that the thermal stability of PBT/MEGONSnanocomposites under the oxygen gas condition isincreased because of the presence of graphene plateletsdispersed homogeneously in the PBT matrix. It is foundthat T5, T10, T30 and Tmax values of the nanocomposite with8.0 wt.% MEGONS under oxygen gas condition are 6.4, 7.5,8.0 and 6.5 �C higher than those of the neat PBT, as listed inTable 2. It is noticeable that the increment in T5, T10, T30
and Tmax values of the nanocomposites under oxygen gascondition in comparison with the neat PBT is much smallerthan that under the inert nitrogen gas condition, and thesevalues are far lower than the corresponding temperaturesexamined under the nitrogen gas condition. Recently, ithas been reported that the gas permeation in graphene-based polymer nanocomposites is remarkably reduced,which is caused by the fact that the graphene plateletscan serve as diffusion barriers in polymeric membranes[7]. Thus, it is reasonable to contend that the significantenhancement in thermal stability of PBT/MEGONS
Fig. 8. TGA thermograms of the neat PBT and PBT/MEGONS nanocomposites with various MEGONS contents examined under (A) nitrogen and (B) oxygengas conditions.
Table 2TG results of neat PBT and PBT/MEGONS nanocomposites with different MEGONS contents.
Maximum improvement 22.4 21.7 22.7 20.8 6.4 7.5 8.0 6.5
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nanocomposites is attributed to the barrier effect of graph-ene platelets to the gas permeating through the PBTmatrix.
3.5. Rheological properties of PBT/MEGONS nanocomposites
Dynamic frequency sweep tests were used to explorenetwork formation and microstructure of the nanocom-posites. The storage modulus (G0), loss modulus (G00) andcomplex viscosity (|g⁄|) of neat PBT and PBT/MEGONSnanocomposites measured at 220 �C are logarithmicallyplotted as a function of angular frequency in Fig. 9. It can
Fig. 9. Rheographs depict the variation of (A) storage modulus, G0 , (B) loss moduwith different MEGONS loadings with angular frequency at temperature and sh
be found that both moduli of PBT/MEGONS nanocompos-ites are higher than that of PBT at all frequency regions.As shown in Fig. 9A, the G0 value significantly increaseswith increase in wt.% of MEGONS loading. Moreover, theincrease in G0 is more significant particularly at low fre-quency region as compared to the high frequency region.At low frequency region, the relaxation exponent (n) ofthe power law (G0 �xn) drastically drops with increasein MEGONS content resulting a transition of viscoelasticresponse from ‘liquid-like’ to ‘solid-like’ of PBT nanocom-posites that beginning at 0.2 wt.% MEGONS content [45].PBT nanocomposite with 4.0 wt.% MEGONS shows higher
lus, G00 and (C) complex viscosity, |g⁄|, of neat PBT and its nanocompositesear strain of 220 �C and 0.98%, respectively.
Fig. 9. (continued)
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storage modulus and lower terminal region slope that indi-cates the higher solid-like behavior. At higher frequencyregion, the viscoelastic behavior of all PBT/MEGONSnanocomposites is almost same. The increment in valueof G0 for 4.0 wt.% filled MEGONS based PBT nanocompos-ites is significant as compare to the pristine counterpart.However, the rate of increase in value of G0 decreases withincrease in wt.% of filler loading due to aggregation of theMEGONS. The loss modulus (G00) of the PBT/MEGONSshows the similar trend as that of G0. The G00 increasesand the frequency dependence decreases with MEGONSloading compared with that of PBT matrix, as shown inFig. 9B. Besides, notable rheological plateaus, as have de-picted in some earlier reports [46,47], are not observedfrom Fig. 9 even MEGONS loading reaches 4.0 wt.%. How-ever, the slopes of the terminal zone of G0 and G00 for thePBT nanocomposites decrease gradually (Fig. 9A and B).The slopes of the terminal zone of G0 and G00 are relativelyhigher than other reports [48,49], which indicates thenon-terminal behavior with the power-law dependencefor G0 and G00 of the PBT nanocomposites. Similar non-ter-minal low-frequency rheological behaviors have been ob-served and reported by Rosedalev and Bates [50] andLarson et al. [51].
Fig. 9C represents the frequency dependency of com-plex viscosity (|g⁄|) for pristine PBT and PBT/MEGONSnanocomposites. The magnitude of g⁄ of PBT/MEGONSnanocomposites substantially increases with increase inMEGONS loading because of the establishment of PBT-ME-GONS and MEGONS–MEGONS interactions, which notice-ably oppose the segmental chain molecular mobility ofthe PBT matrix. Furthermore, the enhancement of g⁄ withMEGONS loading is especially significant at lower fre-quency region due to the dramatic increase in the G0,whereas the change in g⁄ diminishes at higher frequencyregion. The g⁄ value remarkably reduces with increase in
frequency due to decrease in shear thinning exponent (n)of the power law (|g⁄| �x�n). The rate of decrease in g⁄
with rise in frequency that prominent at higher frequencyregion may be due to the strong shear thinning effect [52].At higher frequency region, the dispersed MEGONS tendsto align along the direction of strong shear stress, whichis responsible for the demolition of the PBT chain cross-linked networks formation as a result strong shear thin-ning behavior is shown in MEGONS filled systems. The g⁄
almost decreases linearly with increase in frequency,whereas the slope of curve or reduction gradient becomeshigh at higher wt.% MEGONS loading as a consequence theg⁄ difference is significantly low at the high frequencyregion for PBT/MEGONS nanocomposites [53].
3.6. Mechanical properties of PBT/MEGONS nanocomposites
To investigate the effects of MEGONS content onmechanical properties of the nanocomposites, mechanicalmeasurements of the nanocomposites with variousMEGONS contents were carried out as shown in Fig. 10.Fig. 10A presents the results of tensile strength and modu-lus for the nanocomposite systems. It is found that the ten-sile strength increases slightly with the incorporation ofMEGONS and shows an enhancement of 20.4% at 4.0 wt.%MEGONS addition. Also the tensile modulus is found toincrease linearly and dramatically with the addition ofMEGONS. At 8.0 wt.% MEGONS addition it is enhanced by201%. The elongations at break results are shown inFig. 10B. A drastic drop in the elongation at break is ob-served in nanocomposites as compared to neat PBT. How-ever, there is a gradual decrease in elongation with anincrease in MEGONS content. This result is in accordancewith those reported earlier and can be explained in termsof the stress concentration effect and the restriction im-posed by MEGONS on deformation of PBT matrix during
Fig. 10. Tensile strength and tensile modulus (A) and elongation at break and izod impact strength and (B) of PBT/MEGONS nanocomposites with differentMEGONS loadings.
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tensile loading. Also izod impact strength is found todecrease in the composite systems with the addition ofMEGONS (Fig. 10B). In nanofiller reinforced composites,under tensile loading the applied load is transferred frommatrix to fillers through shear stresses. The extent of stresstransfer depends on the interfacial bond between thematrix and filler surface. This improvement in strengthand modulus can be attributed to the efficient transferfrom PBT matrix to graphene platelets of MEGONS, whichis caused by the uniform distribution and good interfacialadhesion of graphene platelets in the PBT matrix. As con-firmed by FTIR and SEM observations. Further analysisfrom Fig. 10 and it indicates that the mechanical reinforc-ing effect of graphene platelets is much dominant for thenanocomposites with lower MEGONS contents(<2.0 wt.%). The decrease of tensile strength of the
nanocomposites with higher MEGONS contents is believedto stem from the structural distortion and partial cluster-ing of graphene platelets of MEGONS in the matrix.
3.7. Electrical properties of PBT/MEGONS nanocomposites
Fig. 11 displays the electrical conductivity of the PBT/MEGONS nanocomposites as a function of the MEGONScontent. The electrical conductivity of neat PBT is evalu-ated to be in the order of �10�18 S/cm, indicating thatPBT is a typical electrically insulating material. In cases ofthe nanocomposites, the electrical conductivity increasesslightly with the increment of the MEGONS content up to�1.0 wt.% and there is a dramatic increase in electrical con-ductivity at certain MEGONS content between 1 and2.0 wt.%. This substantial increase in the electrical conduc-
Fig. 11. Electrical conductivity of PBT/MEGONS nanocomposites with different MEGONS loadings.
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tivity is associated with the formation of the electrical con-duction path. When the MEGONS content reaches a perco-lation threshold between 1 and 2.0 wt.%, the distanceamong graphene platelets in the matrix is close enoughin a few nanometer ranges for the electrical conductionvia the electron hopping mechanism [54,55]. Thus, theelectrical conductivity of the nanocomposite with2.0 wt.% MEGONS content is achieved to be �10�6 S/cm.It has been reported that the nanocomposites with thehigh electrical conductivity of �10�6 S/cm can be usedfor thermoplastic applications requiring the electrostaticdissipation and partial electromagnetic insulation [56]. Inthe meantime, it might be interesting to compare the elec-trical conductivity of PBT-based composites including ME-GONS or carbon black (CB). In case of PBT/CB composites, itwas reported that the electrical percolation is achieved atcertain CB content between 4 and 6 wt.% [23], which isslightly higher conductive filler content when comparedwithPBT/MEGONS nanocomposites in this study.
4. Conclusions
A series of PBT nanocomposites reinforced with ME-GONS were prepared via the melt-compounding method.SEM images and X-ray diffraction patterns confirmed thatthe graphene platelets were well dispersed and remainedexfoliated in the PBT matrix, even for the nanocompositewith high MEGONS content of 4.0 wt.%. FTIR spectroscopicanalysis confirmed that there exists an interactionbetween PBT chains and MEGONS at the interface of nano-composites. This interfacial interaction helped in improv-ing the mechanical and thermal properties. Also thethermal analysis results proved the nucleation efficiencyof MEGONS with a corresponding increase in % crystallinityand also enhancement in thermal stability. Rheologicalanalysis determined that the G0 and G00 incremented withincrease in MEGONS content and variation of g⁄ with ap-plied frequency showed a shear-thinning characteristic.
Both tensile strength and modulus of the nanocompositeswere also enhanced due to the uniform distribution andgood interfacial adhesion of graphene platelets in the PBTmatrix. In addition, the percolation threshold for electricalconduction in the PBT/MEGONS nanocomposites wasfound to be at certain MEGONS content between 1 and2.0 wt.%. Thus from this study it can be concluded thatthe use of MEGONS in PBT provides good combination ofmechanical, thermal and electrical properties, and it is thusconsidered that PBT/MEGONS nanocomposites can be usedas advanced materials in the area requiring improvedphysical properties. Since it is cheap and easily availableit also would hopefully provide a cost effective solutionto composite manufacturers.
Acknowledgements
The authors thank the Chunhui Cooperation Project ofthe Ministry of Education (Grant No. Z2010097), SichuanProvince Youth Science Grant Program (Grant No.11ZB005), the Open Research Fund of Key Laboratory ofSpecial Materials and Preparation Technologies of XihuaUniversity, and Automobile High Performance Materialsand Molding Technology Key Laboratory of Sichuan Prov-ince College (in Xihua University) for the financial supportsof this work. Special thanks are also due to the PolymerEngineering Academic Center in the University of Akronfor the structural characterizations.
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