Synthesis and Mechanical Properties of Interconnected ...cmortega/materials_lab/Publications_files/,.pdfand leisure equipments.1 2 Fillers with different composition, size and morphol-ogy

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Copyright copy 2011 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 11 1092ndash1097 2011

Synthesis and Mechanical Properties of InterconnectedCarbon Nanofiber Network ReinforcedPolydimethylsiloxane Composites

Z Y Zhao N D Khatri K Nguyen S Q Song and L Sunlowast

Department of Mechanical Engineering University of Houston Houston TX 77204 USA

Carbon nanofiber (CNF) reinforced elastomer composites with light weight sustainability of largedeformation chemical stability corrosion and fatigue resistance and vibration and noise reductioncapability can have positive impact on a wide range of applications However this type of compositeis still a under studied research area due to the difficulties in material handling and processingTo improve processing control and reproducibility for large scale engineering applications costeffective carbon nanofibers (CNFs) in form of interconnected porous network structure were used asnanofillers Processing microstructure and mechanical properties of carbon nanofibers reinforcedpolydimethylsiloxane (PDMS) have been studied Mechanical measurements on the compositesshow that the CNF-PDMS interfacial bonding can be until failure interfacial debonding happensin the CNF-PDMS composites and the resulted permanent deformation stabilizes with increasingload-unload cycles with significant energy dissipation

Keywords Elastomer Nanocomposite Carbon Nanofiber Mechanical Properties

1 INTRODUCTION

Composite materials and structures have been a fieldattracting both basic science research and practical appli-cation development interests for a very long time Bycombining two or more materials with significantly differ-ent properties and keep their distinct constituent phasescomposites can not only combine the desirable proper-ties of constituents but more often develop novel prop-erties that are not possessed by individual constituentsWith their light weight superior mechanical propertiesfatigue and corrosion resistance and low manufacturingand maintenance cost polymer matrix based compositematerials have found extensive engineering applications inaerospace automobile marine civil electronic sportingand leisure equipments12

Fillers with different composition size and morphol-ogy have been introduced to form polymer compositesSpherical particles such as carbon black have been usedfor a long time to improve the mechanical and elec-trical properties of polymers Various continuous glassfibers organic fibers and carbon fibers have also beendeveloped to reinforce the polymer matrices for structuralcomponent applications In addition to these commercial-ized fillers a wide range of nanomaterials with various

lowastAuthor to whom correspondence should be addressed

dimensionality have been synthesized and introduced topolymers with the rapid development of nanoscience andnanotechnology since the 1990s3ndash9 The polymer nanocom-posites (PNCs) are expected to utilize the size effectslarge interfacial area and anisotropy of the nanofillers todevelop improved performances The two most commonapproaches of polymer nanocomposite (PNCs) fabricationare the direct mixing1011 and in situ polymerization1213

Studies have shown that significant challenges exist in con-trolling the dispersion and alignment of the nanofillerswhich greatly hinder PNC synthesis and performancereproducibility In addition most of current studies oncarbon nanomaterials polymer composite focus on usingnanotubes and often times for mechanical strengtheningeffects in engineering epoxy Very limit studies exist on theinvestigation of elastomer based composites This studywill explore the use of assembled interconnected networkstructure of one dimensional carbon nanomaterials as thebackbone of elastomer composite This approach will notonly improve materials handling and reproducibility butwill also introduce functionalities other than mechanicalstrengtheningIn this study commercially available carbon nanofibers

(CNFs) have been chosen as the filler material due totheir relative low cost mass availability and comparableproperties to the muliwalled carbon nanotubes (MWNTs)

1092 J Nanosci Nanotechnol 2011 Vol 11 No 2 1533-48802011111092006 doi101166jnn20113074


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Zhao et al Synthesis and Mechanical Properties of Interconnected CNF Network Reinforced Polydimethylsiloxane Composites

Through a dispersion and filtration process14ndash16 looseCNFs were first assembled into a self-supportive intercon-nected network structure and the composites were thensynthesized by vacuum assisted polymer infiltration andpolymerization This approach can greatly improve mate-rials handling and processing reproducibility and a higherfiller volume percentage can be achieved More impor-tantly the CNF network structure can help to introducemultiple mechanical electrical and thermal functionalitiesto expand the composite applications in vibrationacousticdamping lightening strike prevention electromagneticimmunity (EMI) shielding and resistive heating applica-tions Combined with elasticity low modulus and highductility of elastomers these CNFs reinforced compositescan have additional application potential for the devel-opment of novel bio stressstrain sensors impact energyabsorbing structures and thermal interface materials

2 EXPERIMENTAL DETAILS

Carbon nanofibers used in this study were purchased fromPyrograph Products Inc They have an average diameter of100 nm and length between 50ndash100 m These CNFs wereproduced by decomposition of hydrocarbon gases using Fecatalytic nanoparticles at high temperatures Comparativestudies on the PDMS composites using pristine and acidtreated CNF fillers have been performed A typical acidtreatment procedure includes ultrasonication of the CNFsin a 13 volumetric ratio mixture of nitric acid (680ndash700) and sulfuric acid (950ndash980) for 4 hours10 fol-lowed by multiple times of deionized water rinsing (DIwater) and filtration and finally the CNFs are collectedafter being dried in an oven for overnight at 110 CIn addition to use scanning electron microscopy (SEM)

for CNFs and composite morphology characterizationUV-Visible absorption spectroscopy has been used to char-acterize the CNFs dispersion stability in various liquidsTo fabricate assembled network structures CNFs were

dispersed in acetone at a concentration of 1 gliter using anultrasonic probe homogenizer for 15 minutes at a powerlevel of 60 watts Suspension was then vacuum-filteredthrough a porous polyester membrane (Poretics 200 nmpore size) The precipitated CNFs structure left on filtermembrane was then heated at 110 C to evaporate theremaining acetone During this drying process externalstress was applied to adjust the CNFs volume percent-age Measurement results presented in this paper are fromthe samples fabricated under same atmospheric conditionswithout applying extra stressPolydimethylsiloxane (PDMS) used in this study is the

two components Sylgard 184 from Dow Corning The basepolymer precursor and curing agent were mixed at a 101volume ratio and degassed in vacuum before infiltratinginto the CNFs network structure under vacuum Sampleswere then placed on a spin coater to remove excessive

Fig 1 Dimensions of dog-bone shaped tensile sample

PDMS Finally the samples were put on a hot plate tocure at desired temperatures Results reported here are forthe samples cured at 100 C for 24 hrs

Tensile testing for the pure PDMS and composite sam-ples was performed on an Instron 4467 tensile testerAll test samples had shape and lateral dimensions inaccordance with ASTM standard D 412 (Fig 1) Ten-sile tests were performed at a cross-head moving speedof 1 mmmin Computer recorded simultaneous load andtotal sample deformation data for engineering stress andstrain calculation The strain data used in the presentedresults are presented by the measured deformation dividedby the length of the middle portion of the dog bone shapedsamples

3 RESULTS AND DISCUSSION

31 CNFs Morphology and Dispersion Stability

Pristine and acid treated CNFs were dispersed in acetoneunder the same conditions and a drop from each suspen-sion were placed on a flat substrate for SEM examinationafter acetone evaporation From the microscopic pictureswith same magnifications as shown in Figure 2 it can beseen that acid-treatment process used in this experimentdid improve CNF distribution uniformity and significantlyreduces the size and number of agglomerates At the sametime individual fiber surface morphology does not exhibitsappreciable changes in terms of CNF average diameterand length under higher magnification study It has beenwidely observed that acid treatment can help to removeamorphous carbon and can cause increase in the numberof defects and even breakage in carbon nanotubes17ndash19 Theobservation here maybe due to the large size of the carbonnanofibers and the high temperature graphitizing processused in CNF synthesisStability of CNF dispersed in various liquids was

analyzed from the CNFs characteristic adsorption peakstrength using UV-Visible spectroscopy CNFs were firstmixed with liquid and sonicated for 15 minutes beforethe first adsorption spectrum was taken Figure 3 showsthe summarized adsorption peak intensity data located

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Synthesis and Mechanical Properties of Interconnected CNF Network Reinforced Polydimethylsiloxane Composites Zhao et al

(a) (b)

(c) (d)

(e) (f)

Fig 2 SEM micrographs of carbon nanfibers morphology and dispersion before (a) (c) (e) and after acid treatment (b) (d) (f)

between 265ndash275 nm as a function of time for differentsuspensions CNFs cannot form stable suspension in DIwater (no CNF absorbance was detected) In both alcohol(Ethanol) and acetone the initial dispersion concentrationof CNFs was low and fast precipitation was be observedBy adding surfactant such as sodium laurilsulfate (SDS)the CNFs dispersion concentration in water can be greatincreased with improved stability After acid treatmentCNFs showed improved dispersion concentrations and sus-pension stability in both DI water and acetone than usingthe SDS

32 Mechanical Properties Comparison

Figure 4 shows the representative initial loading curvesfor pure PDMS pristine CNF reinforced PDMS compos-ite and acid treated CNF reinforced PDMS composites

Density of CNF and PDMS are 19 gcm3 0965 gcm3respectively Knowing the CNFPDMS mass and sampledimension the CNF concentration for the two compositesamples presented here are determined to be 320 voland 749 vol respectively Corresponding weigh percent-age of the two samples are 611 wt and 1375 wtFor more than 50 samples of each type studied here it isalways observed that the acid-treated CNFs always packedalmost twice denser than the untreated CNF in the sheetsform This may also be an indication of improvement infiber uniformity and could be the result of better disper-sion For the two composite samples tested here they haveequal amount of total CNFs but with different thickness(145 mm for un-acided treated smaple and 070 mm foracid treated sample) due to the CNFs packing densityTensile test performed on the pure PDMS sample shows

a nonlinear elastic behavior up to the fracture point

1094 J Nanosci Nanotechnol 11 1092ndash1097 2011


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Fig 3 Dispersion stability of CNFs in liquids determined by UV-Visible spectroscopy adsorption strength The starting CNFs concentra-tion is 01 gliter

with completely reversible the load and unload curvesas been discussed in a previous study6 Both compositesshow more significant non-linear mechanical responsesand reduced ductility Enhancement in the initial mechan-ical modulus is evident When a linear fit is used upto a strain value of 005 the calculated moduli for purePDMS pristine CNF composite and acid treated CNF

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Fig 4 (a) Stress-strain measurement results for pure PDMS CNFs-PDMS and acid treated CNFs-PDMS samples and (b) linear fit of stress-strain curves to 005 strain for the comparison of initial sample moduli

composite are 12 MPa 53 MPa and 204 MPa respec-tively Considering the CNF concentration in the acidtreated sample is twice of that of un-acid treated samplethe disproportion of the initial modulus increase confirmsthe significant effect of interfacial bonding strength to theload transfer and mechanical response of materials Previ-ous studies on CNT indicated the introduction of surfacefunctionalization groups and remove of amophrous carbonlayers after acid treatment2021 The other notable observa-tion is that there is only a small difference in the failurestrength for the three samples Combined with later crosssection micrograph analysis it can be concluded that thesenanocomposite failure is due to the interfacial debondingand failure initiated in the polymer matrix Here the frac-ture strength of acid-treated sample is a little lower thanthat of the un-acid treated composite Sample fracture isvery sensitive to defects formation growth and propaga-tion which may not directly link to the interfacial bondingstrengthUnlike pure PDMS composites samples have much

smaller elastic deformation range As shown in Figure 5the second and consequent loading curves are very differ-ent from the initial loading curve For composite samples

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Fig 5 Stress-strain behavior of (a) PDMS with pristine CNFs and(b) PDMS with acid-treated CNFs as a function of number of load-unloadcycles



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with pristine CNFs after initial applied strain of 030the unrecoverable strain after unloading was around 005In comparison the composite sample containing acid-treated CNFs had a larger unrecoverable strain over 01

(a) (b)

(c) (d)

(e) (f)

(g) (h)

Fig 6 SEM micrographs of (a) cut composite surface with pristine CNFs (b) cut composite surface with acid-treated CNFs (c) fractured compositesurface with pristine CNFs and (d) fractured composite surface with acid-treated CNFs (e)ndash(f) are higher magnification micrograph corresponding to(a)ndash(d)

after 30 elongation The elastic deformation regions forboth samples were less than 003 for multiple samplesBeyond the elastic deformation regions both compositesamples exhibit hysteresis between loading and unloading



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As shown in the figures load and unload curves sta-bilize with increasing number of cycles For compositewith untreated CNFs the hysteresis between load andunload curves persisted after 100 cycles indicating signif-icant amount of energy dissipation during the cyclic load-ing On the other hand for composite with acid-treatedCNFs energy dissipation during load and unload decreaseswith the number of cycles and both curves almost over-lapped Since pure PDMS showed no appreciable intrinsicenergy dissipation the load-unload hysteresis in compos-ites should be attributed to the interfacial debonding andresulted interfacial friction effects The measured differ-ences in the composites indicate very different nature ofsurface bonding and further study is needed to quantifythis difference

33 Fractured Surface Morphology

The cross section morphological micrographs of compos-ite samples reinforced by pristine and acid-treated CNFsare shown Figure 6 The tensile fractured surfaces areshown together with the cut surfaces created by a razorblade without additional stress For the purpose of illus-tration only mid-range magnification images are shownThe micrographs show uniform and random distribution ofCNFs for both samples The sample with pristine CNFshas a much smoother surface compared to that of the sam-ple with acid treated CNFs Also more polymer materialattaches to the acid treated CNFs These observations con-firm that the acid-treatment can significantly improve fiber-PDMS interfacial bonding When compare the cut andfractured surfaces it is observed that the exposed CNFs arehave more uniform however shorter length for the cut sur-face than that of acid-treat composite and the compositesfailure is dominated by fiber pullout and polymer matrixfailure

4 SUMMARY

Assemble loose carbon nonofibers into interconnectedand self-supportive network structure can improve thehandling and reproducibility of polymer nanocompositesynthesis These composites show significantly enhancedelastic modulus with improved energy dissipation capabil-ity for cyclic loading Also the composites have highertoughness when compared to pure PDMS Acid treat-ment can help to introduce hydrophilicity and improveCNF dispersion and stability in water Acid treated CNFsalso forms better bonding with PDMS which is evidentfrom the increase in initial composite elastic modulus and

more attachment of polymer on the fiber surface Interfacedebonding and polymer facture is the main failure mech-anism in these composite materials With their improvedmodulus strength toughness and dynamic damping prop-erties these CNFs reinforced elastomer composites canhave a wide range application as structural components

Acknowledgments The authors acknowledge financialsupport from NSF through Grant No CMMI-0800866and a grant from NASA through the Institute for SpaceSystems Operations

References and Notes

1 P K Mallick High-Performance Structural Fibers for AdvancedPolymer Matrix Composites Marcel Dekker Inc New York NYUnited States (1993)

2 High-Performance Structural Fibers for Advanced Polymer MatrixComposites National Research Council The National AcademicPress (2005)

3 L Li and D D L Chung Composites 25 215 (1994)4 L S Schadler S C Giannaris and P M Ajayan Appl Phys Lett

73 3842 (1998)5 S R C Vivekchand K C Kam G Gundiah A Govindraj A K

Cheetham and C N R Rao J Mat Chem 15 4922 (2005)6 K Keshoju and L Sun J Appl Phys 105 023515 (2009)7 S D Burnside and E P Giannelis J Poly Sci B 38 1595 (2000)8 R Haggenmueller W Zhou J E Fischer and K I Winey JNN

3 105 (2003)9 M C Weisenberger E A Grulke D Jacques T Rantell and

R Andrews JNN 3 535 (2003)10 S Porro S Musso M Vinante L Vanzetti M Anderle F Trotta

and A Tagliaferro Physica E 37 58 (2007)11 M Knite K Ozols J Zavickis V Tupureina I Klemenoks and

R Orlovs J Nanosci Nanotechnol 9 3587 (2009)12 S J Park M S Cho S T Lim H J Choi and M S Jhon Macro-

mol Rapid Commun 24 1070 (2003)13 C C Zeng and L J Lee Macromolecules 34 4098 (2001)14 H Muramatsu T Hayashi Y A Kim D Shimamoto Y J Kim

K Tantrakarn M Endo M Terrones and M S Dresselhaus ChemPhy Let 414 444 (2005)

15 R H Baughman A A Zakhidov and W A Heer Science 297 787(2002)

16 R H Baughman C X Cui A A Zakhidov Z Iqbal J N BarisciG M Spinks G G Wallace A Mazzoldi D D Rossi A GRinzler O Jaschinski S Roth and M Kertesz Science 284 1340(1999)

17 S Nagasawa M Yudasaka K Hirahara T Ichihashi and S IijimaChem Phys Lett 328 374 (2000)

18 S R C Vivekchand and A Govindaraj Proc Indian Acad Sci115 509 (2003)

19 T Saito K Matsushige and K Tanaka Physica B 323 280 (2002)20 M Yoonessi H Toghiani R Wheeler L Porcar S Kline and C U

Pittman Jr Carbon 46 577 (2008)21 S Kumar T Rath R N Mahaling C S Reddy C K Das K N

Pandey R B Srivastava and S B Yadaw Mater Sci Eng B141 61 (2007)

Received 4 December 2009 Accepted 27 January 2010



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Through a dispersion and filtration process14ndash16 looseCNFs were first assembled into a self-supportive intercon-nected network structure and the composites were thensynthesized by vacuum assisted polymer infiltration andpolymerization This approach can greatly improve mate-rials handling and processing reproducibility and a higherfiller volume percentage can be achieved More impor-tantly the CNF network structure can help to introducemultiple mechanical electrical and thermal functionalitiesto expand the composite applications in vibrationacousticdamping lightening strike prevention electromagneticimmunity (EMI) shielding and resistive heating applica-tions Combined with elasticity low modulus and highductility of elastomers these CNFs reinforced compositescan have additional application potential for the devel-opment of novel bio stressstrain sensors impact energyabsorbing structures and thermal interface materials

2 EXPERIMENTAL DETAILS

Carbon nanofibers used in this study were purchased fromPyrograph Products Inc They have an average diameter of100 nm and length between 50ndash100 m These CNFs wereproduced by decomposition of hydrocarbon gases using Fecatalytic nanoparticles at high temperatures Comparativestudies on the PDMS composites using pristine and acidtreated CNF fillers have been performed A typical acidtreatment procedure includes ultrasonication of the CNFsin a 13 volumetric ratio mixture of nitric acid (680ndash700) and sulfuric acid (950ndash980) for 4 hours10 fol-lowed by multiple times of deionized water rinsing (DIwater) and filtration and finally the CNFs are collectedafter being dried in an oven for overnight at 110 CIn addition to use scanning electron microscopy (SEM)

for CNFs and composite morphology characterizationUV-Visible absorption spectroscopy has been used to char-acterize the CNFs dispersion stability in various liquidsTo fabricate assembled network structures CNFs were

dispersed in acetone at a concentration of 1 gliter using anultrasonic probe homogenizer for 15 minutes at a powerlevel of 60 watts Suspension was then vacuum-filteredthrough a porous polyester membrane (Poretics 200 nmpore size) The precipitated CNFs structure left on filtermembrane was then heated at 110 C to evaporate theremaining acetone During this drying process externalstress was applied to adjust the CNFs volume percent-age Measurement results presented in this paper are fromthe samples fabricated under same atmospheric conditionswithout applying extra stressPolydimethylsiloxane (PDMS) used in this study is the

two components Sylgard 184 from Dow Corning The basepolymer precursor and curing agent were mixed at a 101volume ratio and degassed in vacuum before infiltratinginto the CNFs network structure under vacuum Sampleswere then placed on a spin coater to remove excessive

Fig 1 Dimensions of dog-bone shaped tensile sample

PDMS Finally the samples were put on a hot plate tocure at desired temperatures Results reported here are forthe samples cured at 100 C for 24 hrs

Tensile testing for the pure PDMS and composite sam-ples was performed on an Instron 4467 tensile testerAll test samples had shape and lateral dimensions inaccordance with ASTM standard D 412 (Fig 1) Ten-sile tests were performed at a cross-head moving speedof 1 mmmin Computer recorded simultaneous load andtotal sample deformation data for engineering stress andstrain calculation The strain data used in the presentedresults are presented by the measured deformation dividedby the length of the middle portion of the dog bone shapedsamples

3 RESULTS AND DISCUSSION

31 CNFs Morphology and Dispersion Stability

Pristine and acid treated CNFs were dispersed in acetoneunder the same conditions and a drop from each suspen-sion were placed on a flat substrate for SEM examinationafter acetone evaporation From the microscopic pictureswith same magnifications as shown in Figure 2 it can beseen that acid-treatment process used in this experimentdid improve CNF distribution uniformity and significantlyreduces the size and number of agglomerates At the sametime individual fiber surface morphology does not exhibitsappreciable changes in terms of CNF average diameterand length under higher magnification study It has beenwidely observed that acid treatment can help to removeamorphous carbon and can cause increase in the numberof defects and even breakage in carbon nanotubes17ndash19 Theobservation here maybe due to the large size of the carbonnanofibers and the high temperature graphitizing processused in CNF synthesisStability of CNF dispersed in various liquids was

analyzed from the CNFs characteristic adsorption peakstrength using UV-Visible spectroscopy CNFs were firstmixed with liquid and sonicated for 15 minutes beforethe first adsorption spectrum was taken Figure 3 showsthe summarized adsorption peak intensity data located



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Documents

Synthesis and Mechanical Properties of Interconnected ...cmortega/materials_lab/Publications_files/,.pdfand leisure equipments.1 2 Fillers with different composition, size and morphol-ogy