Controlled nanostructure and high loading of single-walled carbon ...

IOP PUBLISHINGNanotechnology 18 (2007) 095708 (7pp)NANOTECHNOLOGYdoi:10.1088/0957-4484/18/9/095708Controlled nanostructure and high loadingof single-walled carbon nanotubesreinforced polycarbonate compositeShiren Wang 1 , Zhiyong Liang 1,3 ,GiangPham 1 , Young-Bin Park 1 ,Ben Wang 1 , Chuck Zhang 1 , Leslie Kramer 2 and Percy Funchess 21 High-Performance Materials Institute (HPMI), Department of Industrial and ManufacturingEngineering, FAMU-FSU College of Engineering, Florida State University, 2525 PottsdamerStreet, Tallahassee, FL 32310-6046, USA2 Lockheed Martin Missiles and Fire Control-Orlando, Orlando, FL 32819-8907, USAE-mail: Liang@eng.fsu.eduReceived 29 September 2006, in final form 19 December 2006Published 24 January 2007Online at stacks.iop.org/Nano/18/095708AbstractThis paper presents an effective technique to fabricate thermoplasticnanocomposites with high loading of well-dispersed single-walled carbonnanotubes (SWNTs). SWNT membranes were made from a multi-stepdispersion and filtration method, and then impregnated with polycarbonatesolution to make thermoplastic nanocomposites. High loading of nanotubeswas achieved by controlling the viscosity of polycarbonate solution. SEMand AFM characterization results revealed the controlled nanostructure in theresultant nanocomposites. Dynamic mechanical property tests indicated thatthe storage modulus of the resulting nanocomposites at 20 wt% nanotubesloading was improved by a factor of 3.4 compared with neat polycarbonatematerial. These results suggest the developed approach is an effective way tofabricate thermoplastic nanocomposites with good dispersion and highSWNT loading.(Some figures in this article are in colour only in the electronic version)1. IntroductionCarbon nanotubes (CNTs) are one of the strongest and stiffestmaterials, having exceptional tensile strength and modulus buta low density [1, 2]. CNTs also possess superior thermaland electrical properties [2]. Lightweight multifunctionalnanocomposites can be produced with outstanding strength,stiffness and electric conductivity by effectively incorporatingCNTs into matrix materials. However, effective utilizationof nanotubes in composites applications is dependent on theability to disperse nanotubes uniformly throughout the matrix.Nanocomposites produced by conventional methods, such asdirect mixing, melt blending or solution casting, usually failto yield significant improvements in composite performancebecause these methods cannot solve several manufacturingproblems, including non-uniform CNTs dispersion, inadequatetube loading and lack of nanotube alignment.3 Author to whom any correspondence should be addressed.Polycarbonate (PC) is a high molecular-weight, amorphousengineering thermoplastic that demonstrates exceptionallyhigh impact strength over a wide temperature range.PC is characterized by an excellent combination of toughness,heat resistance, flame resistance and dimensional stability,making PC advantageous for producing high performanceCNTs/PC nanocomposites. Researchers have fabricatedCNTs/PC nanocomposites via melt processing and solutioncasting [3–6]. However, PC’s high melting temperatureand viscosity makes fabricating high-quality CNTs/PC compositesthrough injection or extrusion molding difficult. Theconcomitant viscosity problem with increasing tube loadingalso presents a challenge in both melting processing and solutioncasting, especially for fabricating high-loading nanotubescomposites. We have demonstrated a resin-infiltration techniquefor fabricating high-loading epoxy composites [7]. Nowwe extend this technique to the nanotubes reinforced thermoplasticcomposites. This paper reports a method of fabricating0957-4484/07/095708+07$30.00 1 © 2007 IOP Publishing Ltd Printed in the UK

Nanotechnology 18 (2007) 095708SWanget al500 nm(a)(b)Figure 1. (a) AFM image of dispersed nanotubes in aqueous suspension. (b) Diameter measurement through section analysis.CNTs/PC nanocomposites with controlled nanostructure andhigh loading by using preformed nanotube membrane materials.2. ExperimentsSingle-walled carbon nanotubes (SWNTs) were used inthis study, which were P-grade SWNT products with aresidual metal content of 3–12 wt%, produced by CarbonNanotechnologies Inc. The SWNTs were first made intobuckypapers, which are thin films of preformed nanotubenetworks [8]. Aqueous suspensions were prepared by mixingthe SWNTs with suitable surfactant [9, 10] and distilled waterunder ultrasonic power 85 W m −2 for about 60 min. Thesenanotube suspensions, which can remain stable for more thantwo months, were pumped through a nylon filter membrane.After filtration, the SWNT membrane or buckypaper waspeeled from the filter membrane. Finally, the buckypaper wasthoroughly washed with isopropanol to remove the adsorbedsurfactant, resulting in a thin membrane with 10–25 μmthickness.PC solution was prepared by dissolving PC granulesin chloroform (Fisher Scientific Inc. A.C.S. grade) at roomtemperature through intense stirring. After the solution wasstirred for several hours (the stirring time is dependent on thePC concentration), the PC particles were completely dissolvedin the chloroform, resulting in a clear solution. Beforebuckypaper impregnation, the permeability of PC solutionthrough the buckypaper in the thickness direction (z direction)was measured to predict the PC solution infiltration time.Distilled water was used as a testing fluid. Distilled waterflowed through the buckypaper in the thickness directionunder 14.7 psi vacuum pressure. Four parameters—flowrate, vacuum pressure, buckypaper thickness and surfacearea—were recorded to calculate the buckypaper’s z-directionpermeability. The PC infiltration was performed under thesame conditions of the permeability tests, except substitutingPC solution for distilled water. PC used in the researchwas LEXAN 103–112, purchased from GE Plastics Inc.After infiltration, the impregnated buckypaper was air driedfor 12 h and subsequently annealed in a vacuum oven for10 h to remove any residual solvent. A stack of layeredPC-impregnated buckypaper (6 layers) was hot-pressed intonanocomposite films at 580 ◦ F for 10 min and then cooleddown to room temperature (Hot press: Carver LaboratoryEquipment, Hydraulic Unit Model # 3925, Carver Inc.). Hotpressingwas conducted in air, and the thickness of the resultantcomposite was about 0.7 mm, while both length and widthwere 50.8 mm.The nanostructures of buckypaper and SWNTs/PCnanocomposites were characterized with the scanning electronmicroscope (SEM, JSM-7401F) and the atomic forcemicroscope (AFM, Multimode SPM, Nanoscope IIIa, VeecoInc.). The mechanical properties of the nanocomposites weremeasured with a dynamic mechanical analyser (DMA2980, TAInstrument Inc.) using the tensile (film) mode with 5 ◦ Cmin −1ramp rate and 1 Hz single frequency. The temperature rangewas from 25 to 300 ◦ C.3. Result and discussions3.1. SWNT dispersionThe dispersion quality of SWNTs in the suspension wasexamined by the AFM. A small drop of suspension wasspread on a silicon wafer for AFM examination. Figure 1(a)shows the AFM image of the dispersed nanotubes. Diametermeasurements of SWNTs from AFM were based on heightprofiles through section analysis, as shown in figure 1(b).SWNTs length was quantified by the commercial imagingprocessing software, SIMAGIS. This software package has thecapability of automatically analysing multiple AFM images tomeasure length information of SWNTs and their ropes [11].Figure 2(a) reveals the results of the measured diameters.Diameters of nanotube ropes were mostly less than 6 nm, whilea small fraction fell in the range of 6–14 nm. The averagediameter of SWNTs and ropes was about 5.6 nm. Figure 2(b)shows the length measurement results, indicating that smallropes were an average of 547 nm long.Asshowninfigure3(a), the buckypaper has a certainstrength, and it is flexible enough to facilitate handling inthe experiments [12]. Figure 3(b) shows an SEM image ofa buckypaper sample, indicating that the nanotube networkconsists of continuous individual SWNTs or ropes selfassembledby van der Waals force. Figure 4 shows theSIMAGIS analysis results of the rope size measurements,which were based on the horizontal profiles. In the buckypaper,the average diameter of all the SWNT ropes was 13.5 nm. The2

Nanotechnology 18 (2007) 095708SWanget al(a)Cumulative Percentage(%)Counts0 2 4 6 8 10 12 14 1699.9999580604020510.10.01 28262422201816141210864200 2 4 6 8 10 12 14 16Diameter of SWNT and ropes(nm)(b)Cumulative Percentage(%)Counts0 200 400 600 800 1000 1200 1400 160099.9999580604020510.10.011210864200 200 400 600 800 1000 1200 1400 1600Length of SWNT and ropes(nm)Figure 2. (a) Diameter distribution of nanotube ropes. (b) Length distribution of nanotube ropes.(a)(b)25mmFigure 3. SWNT buckypaper: (a) buckypaper sample; (b) SEM image of random buckypaper.diameter distribution of SWNT ropes seems to shift slightlyto the right as compared to that of the SWNT ropes in theaqueous suspension measured by AFM sectional analysis.This difference seems to be from the measurement methodby SIMAGIS, since this software measures the diametersaccording to the horizontal profiles. In spite of this change,almost 80% of SWNT ropes showed small diameters less than16 nm, indicating that the buckypaper provides an effectiveway to produce well-dispersed SWNTs in the bulk samples.3.2. PC solution infiltrationCurrently it is difficult to fabricate well-dispersed, highloadingthermoplastic nanocomposites through a conventionalmelting process due to high viscosity. This paper haspresented a method involving buckypaper impregnation(solution process) to overcome the manufacturing difficultiesof the melting process. The buckypaper impregnation method,or solution process, provides the capability to realize thehigh-loading composite with controlled nanostructure. Thebuckypaper pore size was measured through the absorptionof nitrogen gas by porosimetry (TriStar 3000, MicromeriticsInstrument Corporation). The test results indicated the porediameters of buckypaper range from 2 to 311 nm, and about80% are above 50 nm. Since the porous structures ofthe buckypaper are at nanoscale, the comparability betweenbuckypaper pore size and the dimension of the polycarbonatemolecules must be checked. The polycarbonate resin used inthis study had a weight-average molar mass (M W ) of about36 000 g mol −1 or about 140 repeat units in each PC molecule.Figure 5 shows the molecular structure of PC repeat unit usingmolecular dynamic (MD) simulation. The MD simulation wascarried out with Materials Studio using the COMPASS forcefield by considering one molecule in a near-infinite dilutionlimit [13]. The size of PC molecule was acquired at thesimulation of 200 ps, indicating that a single polycarbonatechain is close to the dimension of 9.7 nm× 8.6 nm× 0.9 nm,which is small enough to penetrate the buckypaper pores.For an in-plane wetting of buckypapers, the driving forceprimarily comes from surface tension. Due to nanoscale pore3

Nanotechnology 18 (2007) 095708SWanget alCumulative Percentage (%)99.9999580604020510.10.014 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34120100Count8060100nm4020(a)04 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34Diameter of SWNT ropes (nm)(b)Figure 4. (a) Smart identifying SWNT ropes of buckypaper SEM image in SIMAGIS software. (b) Rope diameter distribution of thebuckypaper.Figure 5. Molecular structure of polycarbonate repeat unit.size, the in-plane permeability of the buckypapers is verysmall, and therefore impregnating buckypaper through in-planewetting is extremely difficult [14]. Therefore, a throughthicknessinfiltration system was designed to impregnatepolycarbonate solution into the buckypapers. Throughthickness (z direction) flow behaviour of the buckypaper can bemodelled with one-dimensional Darcy’s law. The z-directionpermeability K z (saturated permeability) was calculated usingthe following equation derived from Darcy’s law [12]:K z = QηL(1)APwhere Q is the flow rate; η is the viscosity of fluid; L isthe thickness of buckypaper; P is the vacuum pressure acrossbuckypaper; A is the surface area of buckypaper.The results of z-direction permeability test on the resultingbuckypaper show the average value of buckypapers’ z-direction permeability was about 2 × 10 −19 m 2 [12]. On theother hand, infiltration flow rate is the ratio of flow volume toinfiltration time, as shown in equation (2):Q = Vt= ALtwhere Q is the flow rate; L is the thickness of buckypaper; Ais the surface area of buckypaper; L is the flow distance in(2)the infiltration time of t; t is the infiltration time; V isthe flow volume in the infiltration time of t.Substituting equation (2) into equation (1) results inequation (3) to determine PC infiltration time:t = ηLL(3)K z Pwhere η is the viscosity of PC solution; L is the thickness ofbuckypaper film; P is the vacuum pressure across buckypaper.L is the flow distance in the infiltration time of t; K z is thez-direction permeability.Integrating equation (3) yields the formulation of theinfiltration time:∫ LηLt =0 K z P dl = 1 ηL 2(4)2 K z Pwhere η is the viscosity of PC solution; L is the thickness ofbuckypaper film; P is the vacuum pressure across buckypaper;K z is the z-direction permeability.Basedonequation(4), infiltration time can be estimated,asshowninfigure6. In the experiments, the infiltration timewas measured by a stopwatch from the switching-on of thepressure till observation of the solution coming out of thebuckypaper bottom. Infiltration time can be controlled viaPC solution viscosity, as shown in figure 6(a). Furthermore,it was observed that higher solution viscosity yielded higherPC concentration, thus resulting in lower SWNT loading in thefinal nanocomposites, as shown in figure 6(b). In other words,desired infiltration time and final nanotubes loading of thenanocomposites can be achieved with tailored viscosity of PCsolution. For instance, using a viscosity of about 10 cp wouldallow a short infiltration time and the final nanotubes loadingof around 50% in the materials. Hence, this simple techniqueis effective in achieving high, controlled SWNT concentrationand good tube dispersion.4

Nanotechnology 18 (2007) 095708SWanget al(a)Infiltration Time (second)140012001000800600400200Theoretical valueExperimental value(b)Tube loading (%)4540353025201510500 10 20 30 40 50 60 70Viscosity (centipoise)00 10 20 30 40 50 60 70Viscosity (centipoise)Figure 6. Effect of viscosity on the buckypaper impregnation: (a) infiltration time versus viscosity; (b) tube loading versus viscosity.(a)(b)Figure 7. Impregnated buckypaper: (a) macroscopic image of impregnated buckypaper; (b) SEM image of a cross section for the impregnatedbuckypaper.Figure 7(a) shows a 6.5 ′′ × 6.5 ′′ dried PC-impregnatedbuckypaper. The buckypaper surface was thoroughly wettedby polycarbonate. Figure 7(b) shows an SEM image ofthe cross section of the impregnated buckypaper, whichindicates even impregnation throughout and intercalation withnanotubes. Nanotube ropes were homogeneously spreadover the polycarbonate, and continuous intercalation networksformed in the resultant composite. Aggregations of nanotubeswere not observed. The formative tube networks in thenanocomposite were almost the same as those in the previousimpregnated buckypaper and original buckypaper. Thisindicated that the impregnation of polycarbonate solutionthrough buckypaper yielded controlled nanostructure in thenanocomposite. Since nanocomposite with various nanotubeloading can be achieved by adjusting the viscosity ofpolycarbonate solution, this impregnation technique appears tobe an effective way to achieve thermoplastic composites withhigh-loading of nanotubes.3.3. Nanostructure and properties of SWNT-buckypaper/PCnanocompositesThe SWNT-buckypaper/PC nanocomposite used for mechanicalcharacterization was fabricated by hot-pressing six layersof PC-impregnated buckypaper in a thickness-controlledStorage Modulus (MPa)Temperature (°C)Figure 8. Dynamic mechanical analysis of SWNT reinforcedpolycarbonate composites: (A) pristine PC, (B) 2 wt% SWNTs/PCcomposite, (C) 20 wt% SWNT-buckypaper/PC composite.flat mold. DMA tests were performed in single-frequency,temperature-ramping mode. Storage modulus indicates theamount of energy stored in the composite as elastic energy,which is highly influenced by the reinforcement mechanicalproperties, geometric characteristics, reinforcement loadingand interfacial bonding between reinforcement and the matrix.5

Nanotechnology 18 (2007) 095708SWanget al(a)(b)Figure 9. SEM images of SWNTs/PC composite fracture surfaces: (a) 2 wt% SWNTs loading casting sample; (b) 20 wt% SWNT loadingbuckypaper composite sample.Table 1. Modulus of CNTs/PC composites.Tubes loading by weight Storage modulus ReferredApproaches (%) (GPa) literatureBuckypaper SWNT: 20 6.2 This studyimpregnationMelting mixing MWNT: 1.5–15 ∼1.0 [15]Solution casting SWNT: 0.05–0.25 1.6–2.1 [3]Functionalized 2.35–2.52 [16]SWNT: 0.5–2MWNT: 5–20 1.1–2.42 [17, 18]Figure 8 shows the storage modulus of a pristine polycarbonateis 1.8 GPa. Comparatively, the modulus of 2 wt% SWNT reinforcedpolycarbonate (produced by solution casting method)increased to 2.4 GPa, a factor of 1.3 increment, and the modulusof 20 wt% SWNT-buckypaper reinforced polycarbonatejumped to 6.2 GPa, a factor of 3.4 increment. Obviously,the controlled nanostructure and high loading contributed tothe improved mechanical performance in the thermoplasticnanocomposites. Table 1 lists the storage modulus of CNTsreinforced polycarbonate composites prepared from conventionalmethods in the reported literature. It can be seen that thebuckypaper/PC solution infiltration method achieved a highermechanical property.The fracture surfaces of the SWNT-buckypaper/PCcomposites with 20 wt% tube loading were characterizedwith SEM, as shown in figure 9. The dimpled morphologyin the fracture surface indicates that considerable level oflocalized plastic deformation occurred to prevent formationor coalescence of microvoids, which can potentially lead tocrack propagation. This type of dimple fracture suggests thatmechanical properties of SWNT-buckypaper/PC compositeswere enhanced due to the well-dispersed SWNT network.Theoretical modulus of PC/SWNTs composite wasestimated by the Cox–Krench model [11]. Computationresults showed that the elastic modulus of nanocompositeswas 3.7 GPa for 2 wt% tube loading and 64.25 GPa for20 wt% loading. These theoretical values are much higherthan the experimental measurements. Theoretical calculationmay overestimate the modulus by using individual nanotubemodulus rather than bundle modulus, which have a much lowermodulus [19]. It is very difficult to achieve the individual tubedispersion in the experiments. Another possible major reasonof this mismatch may arise from the weak interfacial bonding,a consequence of the smooth and chemically inactive surfaceof SWNTs. The weak interfacial bonding significantly reducedthe efficiency of the load transfer and resulted in low modulusin the experiments. In addition, the low experimental valuescould also stem from the short SWNT lengths, which were547 nm on average [20]. Therefore, even though this paperhas demonstrated an effective method to fabricate high-loadingthermoplastics nanocomposites with controlled nanostructure,much more efforts, such as nanotube functionalization, areneeded in order to realize the full potentials of carbonnanotubes in the composite applications.4. ConclusionsAn effective approach was presented to acquire good dispersionand high loading of nanotubes in the fabrication ofSWNTs/PC composites, which is very difficult using conventionalmanufacturing techniques. A controlled nanostructure ofSWNTs/PC composite can be achieved through impregnatingpreformed SWNT networks or buckypaper with PC solution.This method can also be extended to other suitable thermoplasticresins for high nanotube loading nanocomposites. Thecomparison between experimental value and theoretical predictionindicates that functionalization is necessary to improveload transfer efficiency. In addition, this method also makes itfeasible to fabricate aligned nanocomposites if buckypaper iscomposed of aligned nanotubes.6

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