Composites: Part B 60 (2014) 268–273

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Composites: Part B

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Mechanical properties of graphene platelets reinforced syntactic foams

1359-8368/$ - see front matter Published by Elsevier Ltd.http://dx.doi.org/10.1016/j.compositesb.2013.12.040

⇑ Corresponding author. Tel.: +1 225 771 3151.E-mail address: ezegeye@gmail.com (E. Zegeye).

Ephraim Zegeye a,⇑, Ali K. Ghamsari a, Eyassu Woldesenbet a,b

a Next Generation Composite CREST Center (NextGenC3), Mechanical Engineering Dept., Southern University and A & M College, Baton Rouge, LA 70813, USAb Mechanical Engineering Dept., Louisiana State University, Baton Rouge, LA 70803, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 21 September 2013Received in revised form 15 November 2013Accepted 22 December 2013Available online 2 January 2014

Keywords:Graphene platelets (GPs)A. FoamsB. Mechanical properties

Graphene platelets (GPs) are two-dimensional thin plates containing few layers of graphene sheets.Compressive and tensile behaviors of epoxy-based syntactic foams with pristine GPs as additives are dis-cussed in this article. Four sets of syntactic foams containing 0, 0.1, 0.3, and 0.5 vol.% of GPs were fabri-cated and tested. The volume fraction of microballoons in all syntactic foam samples was kept constant at30%. Results indicated that the compressive and tensile moduli of the syntactic foams were significantlyimproved as compared to samples that did not contain GPs. The addition of GPs also enhanced the tensilestrength while the compressive strength was only slightly increased. Optimal property improvementswere obtained for very low filling fraction of approximately 0.3 vol.%. Samples with higher volume frac-tion of GPs (0.5%) showed deterioration in mechanical properties when compared to other GP containingsamples. Transmission microscopy study indicated formation of voids enclosed by undispersed GPs in thesamples which could explain the decline of the properties. High matrix porosity could also play an impor-tant role in this observation. Utilizing surface modified GPs could allow incorporation of higher volumefraction of GPs homogeneously, thus improving the mechanical properties of the syntactic foams.

Published by Elsevier Ltd.

1. Introduction

Syntactic foams are fabricated by dispersing hollow microbal-loons in a polymeric matrix. The presence of the microballoonsprovides a closed cell porosity thereby reducing the weight, andpreventing thermal transport and moisture absorption in thematerial. In addition, the microballoons provide high compressivestrength, high damage tolerance, and good dimensional stability tothe material. Due to these advantages, syntactic foams can findapplications in aerospace industries as a core materials andablative barrier coatings [1]. However, syntactic foams are ductilein compression and extremely brittle in tension. Their tensilestrength decreases further with an increase in the volume fractionof microballoons [2]. In addition, results from microstructural anal-ysis of tensile and shear fracture surfaces revealed that failure ofsyntactic foams is matrix-dominated [3]. In foam-core sandwichstructural members, compressive, tensile, and shear propertiesare critical and need improvement. Specifically, the strength andreliability of these structural members, when used for aerospaceapplications, is very important since their failure could lead toserious life and property loss. Hence, interest in utilizing theadvantage of lightweight syntactic foams for aerospace sandwichstructures drove researchers to explore different reinforcementmethods to improve their properties. Most of the studies on matrix

reinforcement have been focused on the use of long and short mi-cro-sized fibers [4–9]. Results indicated improvements in shearand tensile properties by adding low volume fraction (<5%) offibers. However, the fiber addition did not improve the compres-sive properties of the syntactic foams. It was also observed that,addition of higher volume fractions of fibers did not improve anyof the properties, rather decreased the compressive properties ofthe foams. Ultrasonic imaging techniques showed the presence ofa greater number of voids in foams with higher volume fractionsof fibers, so that degrading the mechanical properties of the foams.In addition, micro-sized fibers in general soften the matrix andhave poor interfacial interaction with the matrix.

Nanoclay has also been used to reinforce syntactic foams. Re-ports indicated that nanoclay significantly delayed crack initiationand growth and resulted in enhancement in tensile strength andtoughness [10,11]. The increase in the strength was attributed tothe reinforcing effect of the nanoclay particles. The incorporationof nanoclay resulted in larger amount of interface between nano-clay particles/platelets and the matrix thereby increasing theamount of energy required to de-bond the interfaces betweenthe glass microballoons, nanoclay particles, and matrix resin. How-ever, the inclusion of nanoclay decreased the tensile modulus as aresult of a large increase in fracture strain and toughness of syntac-tic foams. In addition, increase in strength was accompanied withincrease in the stiffness and brittleness, resulting in poor damagetolerance properties [12,13]. It was also observed that nanoclaysyntactic foam structures showed lower load bearing capacity than

E. Zegeye et al. / Composites: Part B 60 (2014) 268–273 269

that of the plain syntactic foam structures. This was due to thepronounced effects of shear stresses that resulted deformationand fracture of the syntactic foam in shear mode. In other report,addition of 2 vol.% of nanoclay decreased the compressive strengthof syntactic foams [14]. When the volume fraction was increased to5 vol.%, compressive strength was improved for lower densityfoams, whereas a decrease in strength was observed for the higherdensity foam. However, the compressive modulus was observed todecrease upon addition of 2 and 5 vol.% of nanoclay. In order to ob-tain better exfoliation of clay particles in the matrix, researchershave limited the nanoclay loading below 5 vol.%, with the mini-mum loading being 1 vol.%. The addition of such volume fractionsof nanoclay affects the density of syntactic foams significantly. Forexample, addition of 2 and 4 vol.% of nanoclay increased the foamsdensity by 17% and 26%, respectively [15]. From the report of an-other work, 3–11% increase in density with addition of 2 vol.%nanoclay was observed [10]. The inclusion of 5 vol.% nanoclayhas also resulted in the increase of the density by 11–20%. Such in-crease in density of the syntactic foams could limit their use inlight-weight applications.

Carbon nanostructures, carbon nanotubes (CNTs) and carbonnanofibers (CNFs) could be other potential reinforcing filler, be-cause of their small size, high aspect ratio, and superior mechanicalproperties. In order for these fillers to influence the properties ofsyntactic foams, they need to be homogeneously dispersed in thematrix. Although several methods of dispersing the nanostructureshave been explored, obtaining homogeneous dispersion has re-mained a fundamental challenge. In addition, these nanostructuresare still very expensive to produce. Consequently, only few reportshave been published on the use of these nanostructures to rein-force syntactic foams [16–18].

Graphene platelets (GPs) are novel potential fillers that can beused to reinforce syntactic foams. GPs are two-dimensional graph-ene thin plates containing few layers of graphene sheets. They haveexcellent performance characteristics compared to other carbonnanostructures. Their surface area is higher than other carbonnanostructures which can provide stronger filler–matrix adhesion,thus maximizing the stress transfer [19]. GPs are also currentlybeing produced in bulk quantities with low cost [20]. Thesenano-materials have been used as fillers in polymeric compositesto tailor the mechanical properties and produce high performancecomposites [19]. Comparative study on the mechanical propertiesof epoxy nanocomposites with GP, single-walled CNT (SWCNT),and multi-walled CNT (MWCNT) additives has been done at a fillerweight fractions of 0.1 ± 0.002% [21]. Results indicated that GPssignificantly out-perform CNT additives [21]. It was observed thataddition of very low volume fraction of GPs improved the tensilemodulus, tensile strength, fracture toughness, and fracture energyproperties of the polymer composite compared to the plain(no-fillers) composite. The effect of these novel fillers on themechanical properties of syntactic foams has not been studiedyet. This paper discusses the mechanical characterization of syn-tactic foams reinforced with graphene platelets (GP-SFs). Compres-sive and tensile properties of GP-SFs at different volume fractionsof GPs are evaluated and presented in this work.

2. Experimental

2.1. Materials and fabrication

The GPs used for this work were pristine and supplied by CheapTube Inc., USA. They had diameters of 1–2 lm, surface area>700 m2/g, and purity of >99 wt%. GPs were first added in Tolueneand sonicated for 5 min. The toluene/GPs suspension was addedinto epoxy (DER 332, DOW chemicals) and sonicated for additional

10 min. In order to remove the solvent, the mixture was kept in adegassing chamber at 60 �C for 15 h. After adding S38 glass micro-balloons (3 M Company, US) and the curing agent (DEH 24, DOWchemicals), the slurry was poured in silicone rubber molds. Thesamples were then allowed to cure for 24 h at room temperatureand post cured for 3 h at 100 �C. Four sets of tensile and compres-sion test samples containing different volume fractions of GPs werefabricated. All fabricated samples comprised of 30% by volume ofglass microballoons. The dimensions of the samples for tensileand compression tests were respectively 147.5 � 28.5 � 7.5 and24.5 � 24.5 � 13 mm. For the compression tests, specimen shapeand size were determined according to ASTM C 365/C 365M-05;whereas, the guideline used for the tensile specimen geometryand dimension was ASTM D 3039/D 3039. Table 1 shows the vol-ume fractions, weigh fractions, density, and porosity of thesamples.

2.2. Characterization methods

Scanning electron microscopy (SEM) (FEI Quanta 3D FEG DualBeam FIB/SEM) was used for failure analysis. Fracture sampleswere sputter coated with thin gold layer before imaging to maketheir surface conducting. SEM and transmission electron micro-scope (TEM) (JEOL JEM-1011 TEM) were used to study the GPs dis-persion in syntactic foams.

Tensile and compression tests were conducted using QTEST 150universal testing equipment. The compression test was performedat a crosshead speed of 0.5 mm/min. The samples were com-pressed to about 50% of their initial height. On the other hand, ten-sile tests were carried out at a constant deformation of 1 mm/min.The samples were gripped at opposite ends with a gage length of50 mm. Load and displacement data obtained from the testingsystem was used to produce the stress strain plots. Tests were per-formed on at least five samples from each set of compression andtensile samples.

3. Results and discussions

One of the advantages of syntactic foams is their low densityachieved due to the inclusion of hollow microballoons in a polymermatrix. Several reinforcements (glass fibers, carbon fibers, CNTs,etc.) have been used to improve the properties of syntactic foams.These reinforcements usually have higher densities as compared tothe constituents used for fabricating traditional syntactic foams.Addition of high volume percent of these reinforcements maysignificantly affect the overall density of the syntactic foams andcould limit their applications. Nevertheless, several reports indi-cated that only very low volume percent of reinforcements isrequired to bring improvements in the properties of syntacticfoams. Accordingly, the maximum volume fraction of the GPs usedin this study was limited to 0.5%. The densities of the fabricatedsyntactic foams were obtained experimentally by measuring theweight and volume of at least five specimens. The densities ofthe syntactic foams with increasing vol.% are listed in Table 1. Itcan be seen that no significant variation in the density is observeddue to the addition of GPs.

Attempts to study the dispersion of the GPs in syntactic foamsusing SEM were not successful. Due to their planar geometry, onlythe edges of the GPs embedded in the matrix were exposed, anddifferentiating the platelet edges from the edges of fracturedmicroballoons was extremely difficult. Although it was possibleto distinguish the GPs from microballoon fragments using TEM(see Fig. 1a), it was challenging to accurately determine the GPsdispersion as the areal coverage of the TEM grid was too smalland reliable statistics could not be derived. The TEM study,

Table 1Nomenclature, measured density, and matrix porosity of various syntactic foam compositions.

Sample type Volume fraction of GPs (%) Weight fraction of GPs (%) Average measured density (g/cm3) Porosity (%)

Plain 0 0 0.92 ± 0.01 1.34 ± 0.65GP-SF-0.1 0.1 0.23 0.93 + 0.02 1.95 ± 0.28GP-SF-0.3 0.3 0.69 0.92 ± 0.01 3.04 ± 1.37GP-SF-0.5 0.5 1.15 0.92 ± 0.01 5.54 ± 0.51

(a)

1 µm

Microballoon fragments

GPs

100 nm

500 nm

200 nm

Voids

(b)

Fig. 1. TEM micrographs of GP-SFs, (a) GPs and microballoon fragments in a GP-SF-0.3 sample, (b) voids at two different locations in a GP-SF-0.5 sample. The insert at thelower right corner shows the magnifying view of the region indicated by the rectangle.

270 E. Zegeye et al. / Composites: Part B 60 (2014) 268–273

however, revealed aggregated GPs in the GP-SF-0.5 samples. Whena microtome blade passes through aggregated GPs, voids were alsoexposed in the aggregated GPs (Fig. 1b). In addition to the aggrega-tion of the GPs, the presence of such voids might have played a sig-nificant role in decreasing the mechanical properties of the sampleas it will be discussed later.

Representative compressive stress–strain curves of all syntacticfoams fabricated in this study are presented in Fig. 2. A similarstress–strain profile consisting of a linear elastic region followedby a strain relaxing region, characterized by a sharp drop in stresswas observed. When the samples are subjected to a compressivestress, initial cracks generate in the specimens due to the shearstresses or the secondary tensile stresses [8]. Weaker microbal-

Strain (%)

0 10 20 30 40 50 60

Stre

ss (

MPa

)

0

20

40

60

80

100

120

140

160

Plain

GP-SF-0.1

GP-SF-0.3

GP-SF-0.5

Fig. 2. Typical compressive stress–strain curves of various syntactic foam samples.

loons could easily be fractured at low stress and leave a cavity thatcould act as a region for the evolution of these cracks. The crackscould also be initiated from defective regions such as pores. Suchcracks normally propagate in the matrix material with fracture offew microballoons. In addition, the cracks could de-bond smallnumber of microballoons from the matrix. The combined effectof these phenomena explains the sharp drop in the stress at theend of the linear region. It can be observed in Fig. 2 that the de-crease in stress in all samples is about the same. However, whenthe samples were further compressed, distinct stress–strain behav-ior was noticed between the plain and GP-SF samples. A clearly de-fined plateau region was observed before the rising of the stress inthe case of plain samples. For the GP-SF samples, however, thestress started to rise with no recognizable plateau region. In addi-tion, the increase in stress is faster as compared to plain syntacticfoam samples. This confirmed the attribution of the GPs to thetoughening of the matrix.

From the compressive stress–strain curves it can be observedthat the yield strain of the plain syntactic foam sample was higherthan the GP-SFs. However, the compressive modulus, which is theslope of the initial linear region of the stress–strain curve, wassignificantly lower than all GP-SFs. A comparison of the averagecompressive modulus obtained from the tests is presented inFig. 3. As the GPs content increased from 0.1 to 0.3 vol.%, an impor-tant improvement in the compressive modulus was observed.However, addition of more than 0.3 vol.% GPs tends to lower themodulus slightly. When compared to the plain syntactic foam,samples containing 0.3 vol.% GPs provided the maximum improve-ment (26.6%) in compressive modulus.

The average compressive strength of the samples is presentedin Fig. 3. In order to obtain strength values, the first peak stressin the stress–strain curves of five samples from each set was takenand averaged. Addition of GPs has only marginally affected the

Sample type

Plain GP-SF-0.1 GP-SF-0.3 GP-SF-0.5

Mod

ulus

(M

Pa)

0

200

400

600

800

1000

1200

Stre

ngth

(M

Pa)

0

20

40

60

80

100

120

140ModulusStrength

Fig. 3. Comparison of compressive modulus and strength of syntactic foams. Strain (%)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Stre

ss (

MPa

)

0

5

10

15

20

25

30

35

PlainGP-SF-0.1GP-SF-0.3GP-SF-0.5

Fig. 5. Typical tensile stress–strain curves of syntactic foam samples.

Sample type

Plain GP-SF-0.1 GP-SF-0.3 GP-SF-0.5

Stre

ngth

(M

Pa)

0

10

20

30

40

Mod

ulus

(M

Pa)

0

200

400

600

800

1000

1200

1400

1600

StrengthModulus

Fig. 6. Comparison of tensile modulus and strength of various syntactic foamsamples.

E. Zegeye et al. / Composites: Part B 60 (2014) 268–273 271

compressive strength of syntactic foams. Similar to the trendobserved for the modulus, the compressive strength increases withaddition of low vol.% and decreases with addition of high vol.% ofGPs. A maximum strength enhancement of 3.6% and 2.4% wasobtained with addition of 0.1 vol.% and 0.3 vol.% GPs, respectively.

Under the compressive loading conditions, most microballoonsfractured as shown in Fig. 4, which was obtained from the com-pressive fracture surface of a GP-SF-0.5 specimen. The compositefracture mode was dominated by the deformation and fracture ofthe microballoons. No significant difference was observed amongthe fracture morphologies of plain and GP-SF specimens, as allsamples contained the same volume fraction of microballoons.

Tensile properties of the GP-SFs were also investigated. Fig. 5presents representative tensile stress–strain curves for plain andGP-SF samples. All curves were linear at lower strains and showedslight nonlinearity at higher strains. This nonlinearity may beattributed to the viscoelastic behavior of the polymer that domi-nates in the samples. The tensile modulus of each of the sampleswas obtained from the slope of the linear portion of stress–straincurve. Fig. 6 depicts the calculated tensile modulus for the plainand GP-SF samples. The tensile modulus of all GP-SF sampleswas higher than that of the plain samples. Incorporation of0.1 vol.% GPs enhanced the tensile modulus by 4.57% as comparedto the plain samples. The highest enhancement (14.70%) was ob-tained with addition of 0.3 vol.% GPs. As it can be observed fromFig. 6, the tensile modulus tend to decrease when the vol.% ofGPs exceed 0.3. It was also noted that the tensile modulus was22–38% higher than the compressive modulus.

The addition of GPs has also increased the tensile strength. Upto 15.9% and 14.7% enhancement in strength was obtained ascompared to plain samples with addition of 0.1 and 0.3 vol.% of

(a)

200 µm

Fig. 4. Compressive failure features of GP-SF-0.5 sp

GPs, respectively. Similar to the results obtained in the compres-sion tests, the strength decreased when the content of GPsexceeded 0.3 vol.%. However, large standard deviations were ob-served in the tensile strength measurements as it can be observedin Fig. 6. This was attributed to the porosity and microstructuralinhomogeneity of the matrix, which has a profound effect on themechanical strength measurements, especially in tension [22].The fracture strain of the GP-SFs also showed significant improve-ment as compared to the plain ones. With addition of 0.1 vol.% GPs,23.9% increase in the fracture strain was obtained. Nevertheless, itshould be noted that matrix porosity and microstructural inhomo-geneity could highly affect the fracture strain.

(b)

50 µm

ecimen at two different magnification levels.

272 E. Zegeye et al. / Composites: Part B 60 (2014) 268–273

The tensile fracture morphologies of several compositions ofsyntactic foam samples fabricated in this study are shown inFig. 7. Only few microballoon fragments were observed as com-pared to the morphology obtained for compressive fracture inFig. 4. Most microballoons were seen intact in the matrix after frac-ture in tension. The tensile fracture mechanism seemed to bemainly related to particle–matrix de-bonding and matrix fracture.The morphological features of GP-SF matrix were having several fi-ner sliding marks as compared to that of the plain samples. It canalso be observed that as the GPs content increases, the number ofsliding marks also increases. From Table 1, addition of high volumefraction of GPs increased the porosity of the matrix. In sampleswith high matrix porosity, several micro cracks can be initiatedfrom the pores. Only few of these micro cracks can potentiallypropagate to become large-cracks and reach to the critical cracklength for fracture to occur, with other cracks imprint a large num-ber of fine sliding marks. In the GP-SF-0.5 samples, the propagationand coalescence of many of the micro cracks lead to the formationof fine matrix fragments during the failure of the samples.

The mechanical properties enhancement observed in GP-SFs isgenerally attributed to the GP’s high specific surface area, excellentmechanical properties, and their capacity to deflect crack growthin a more effective way than zero-dimensional (e.g. nanoparticle)and one-dimensional (e.g. nanotube) fillers [23]. Stronger interac-tion between GP layers and the polymer matrix effectively re-strains the motion of polymer chains. Thus, GPs restrict thepolymer chains mobility during the loading and plays a significantrole in delaying crack initiation and growth. According to the re-sults of this work, when the volume fraction of GPs is less than�0.3 vol.%, GPs are more effective in enhancing the compressiveand tensile properties of syntactic foams. Samples containing0.5 vol.% of GPs showed lowest properties as compared to all GP-SFs. The improvement in the mechanical properties of the syntacticfoams depends on the distribution of GPs in the polymer matrix aswell as interfacial bonding between the GP layers and polymer ma-trix. Dispersion of high volume percent of two-dimensional sheets

(a)

(c)

50 µm

50 µm

Fig. 7. Typical micrographs of fractured GP-SFs (a) p

in a polymer matrix could be significantly more challenging ascompared to one-dimensional fibers. Voids such as the one shownin Fig. 2b could be formed when the two-dimensional undispersedGPs wrap each other. Due to the voids, strain transfer betweenaggregated GPs was highly hindered. In addition to the entrapmentof air bubbles during fabrication of the samples, the voids alsocontribute to the increase in the porosity of the sample (Table 1).Consequently, the combined effect of the aggregated GPs and thevoids they form, as well as the higher porosity could be the reasonfor the decline of the mechanical properties in the GP-SF-0.5samples.

The improvements in mechanical properties achieved by addingGPs are in the same order of magnitude with enhancements re-ported by incorporating other nanofillers such as CNTs/CNFs andnanoclay [10,14–17]. However, the type of matrix, microballoons,and amount of constituents could influence the interactions be-tween the nanofiller, matrix, and microballoons and significantlyaffect the properties of the composite. Therefore, it would be diffi-cult for cross-comparison to be made among the results. Neverthe-less, reports indicated that mechanical properties improvementsobtained using CNTs/CNFs and nanoclay as fillers were associatedwith adverse effects in some of the mechanical and physical prop-erties of syntactic foams as it was discussed in the introductorysection. Based on the current study, GP-SFs have better compres-sive and tensile properties as compared to plain ones. Suchimprovement was obtained without significantly affecting the den-sity of the foams. Nevertheless, more work need to be done inorder to determine the benefits of GPs over other nanofillers. Asstated in the experimental section, the GPs used in this researchwere pristine, which could have a tendency to agglomerate. Thepossibility of formation of GP aggregates in a polymer matrix canbe minimized by modifying the surface properties of the GPs. Oxy-genating and functionalizing the GPs (bearing hydroxyl, carboxyl,and amine functional groups) can alter the van der Waals interac-tions significantly and improve the GPs dispersion in the polymer.In addition, such modifications create defects on the surface of the

(d)

(b)

50 µm

50 µm

lain, (b) GP-SF-0.1, (c) GP-SF-0.3, (d) GP-SF-0.5.

E. Zegeye et al. / Composites: Part B 60 (2014) 268–273 273

GPs and lead to a wrinkled topology at the nanoscale [24,25]. Thisnanoscale surface roughness expected to enhance mechanicalinterlocking of the GPs with the polymer chains and potentiallyresult in better adhesion as it has been suggested by moleculardynamics studies [26]. Therefore, the use of oxygenated and/orfunctionalized GPs could bring significant improvements in themechanical properties of syntactic foams as compared to othernanofillers.

4. Conclusions

Graphene platelets (GPs) are two-dimensional carbon nanofil-lers having rough and wrinkled surface texture caused by a veryhigh density surface defects and exhibit excellent mechanicalproperties. In this work, pristine GPs were used to reinforce syntac-tic foams that contains 30 vol.% of glass microballoons. The volumefraction of the GPs in the syntactic foam matrix was varied from 0to 0.5. GP reinforced syntactic foams showed improvements ascompared to samples that did not contain GPs (plain syntacticfoams). When compared to plain samples, a maximum enhance-ment of 26.0% in compressive modulus and 14.7% in tensile modu-lus was obtained with addition of 0.3 vol.% GPs. The tensilestrength of the syntactic foams was also improved by 15.9% withaddition of 0.1 vol.% GPs, whereas, GPs only marginally increasedthe compressive strength. GPs have also increased significantlythe tensile failure strain resulting in increased toughness of thefoams. The enhancement in the mechanical properties was cred-ited to the strong interaction between GP layers and the polymermatrix. This interaction effectively restrains the motion of polymerchains and plays a significant role in delaying the initiation andgrowth of cracks during the loading.

Mechanical properties can be further increased by improvingthe dispersion of the GPs in the matrix. It was observed that, inho-mogeneous dispersion of the GPs in the matrix could lead toformation of voids as the platelets wraps and entangled oneanother. This resulted in less modulus and strength in syntacticfoams with 0.5 vol.% GPs as compared to samples containing lowerGPs. Oxygenating and functionalizing the GPs could change the vander Waals interactions and improve their dispersion in the matrixthereby providing better mechanical properties.

References

[1] Ash M, Ash I. Handbook of fillers, extenders, and diluents. USA: SynapseInformation Resources; 2007.

[2] Gupta N, Nagorny R. Tensile properties of glass microballoon–epoxy resinsyntactic foams. J Appl Polym Sci 2006;102(2):1254–61.

[3] Ashida K. Foamed composites. In: Landrock AH, editor. Handbook of plasticfoams. Park Ridge, New Jersy: Noyes Publications; 1995.

[4] Huang Y-J. Fiber-reinforced syntactic foams. Los Angeles, CA: University ofSouthern California; 2009.

[5] Karthikeyan C, Kishore, Sankaran S. Comparison of compressive properties offiber-free and fiber-bearing syntactic foams. J Reinf Plast Compos.2000;19(9):732–42.

[6] Karthikeyan C, Sankaran S, Kishore. Influence of chopped strand fibers on theflexural behavior of a syntactic foam core system. Polym Int2000;49(2):158–62.

[7] Gupta N, Karthikeyan CS, Sankaran S, Kishore. Correlation of processingmethodology to the physical and mechanical properties of syntactic foamswith and without fibers. Mater Charact 1999;43(4):271–7.

[8] Gupta N, Woldesenbet E, Kishore K. Compressive fracture features of syntacticfoams-microscopic examination. J Mater Sci 2002;37(15):3199–209.

[9] Karthikeyan C, Murthy C, Sankaran S, Kishore K. Characterization of reinforcedsyntactic foams using ultrasonic imaging technique. Bull Mater Sci1999;22(4):811–5.

[10] Maharsia RR, Jerro HD. Enhancing tensile strength and toughness in syntacticfoams through nanoclay reinforcement. Mater Sci Eng A – Struct 2007;454–455:416–22.

[11] Gupta N, Maharsia R. Enhancement of energy absorption in syntactic foams bynanoclay incorporation for sandwich core applications. Appl Compos Mater2005;12(3–4):247–61.

[12] Woldensenbet E, Sankella N. Flexural properties of nanoclay syntactic foamsandwich structures. J Sandwich Struct Mater 2009;11(5):425–44.

[13] Maharsia R, Gupta N, Jerro HD. Investigation of flexural strength properties ofrubber and nanoclay reinforced hybrid syntactic foams. Mater Sci Eng A –Struct 2006;417(1–2):249–58.

[14] Maharsia R, Nikhil G, Guoqiang L. Compressive properties of nanoclaysyntactic foams for sandwich core applications. SEM X internationalcongress & exposition on experimental & applied mechanics costa mesa. CA:2004; p. 1–5.

[15] John B, Nair CPR, Ninan KN. Effect of nanoclay on the mechanical, dynamicmechanical and thermal properties of cyanate ester syntactic foams. Mater SciEng, A – Struct 2010;527(21–22):5435–43.

[16] Zegeye E, Woldesenbet E. Processing and mechanical characterization ofcarbon nanotube reinforced syntactic foams. J Reinf Plast Compos2012;31(15):1045–52.

[17] Dimchev M, Caeti R, Gupta N. Effect of carbon nanofibers on tensile andcompressive characteristics of hollow particle filled composites. Mater Des2010;31(3):1332–7.

[18] Poveda RL, Achar S, Gupta N. Viscoelastic properties of carbon nanofiberreinforced multiscale syntactic foam. Compos Part B - Eng 2014;58:208–16.

[19] Rafiee MA, Rafiee J, Yu Z-Z, Koratkar N. Buckling resistant graphenenanocomposites. Appl Phys Lett 2009;95(22):223103.

[20] Potts JR, Dreyer DR, Bielawski CW, Ruoff RS. Graphene-based polymernanocomposites. Polym J 2011;52(1):5–25.

[21] Rafiee MA, Rafiee J, Wang Z, Song H, Yu Z-Z, Koratkar N. Enhanced mechanicalproperties of nanocomposites at low graphene content. ACS Nano2009;3(12):3884–90.

[22] Meyers MA, Mishra A, Benson DJ. Mechanical properties of nanocrystallinematerials. Prog Mater Sci 2006;51:427–556.

[23] Rafiee MA, Rafiee J, Srivastava I, Wang Z, Song H, Yu Z-Z, et al. Fracture andfatigue in graphene nanocomposites. Small 2010;6(2):179–83.

[24] McAllister MJ, Li J-L, Adamson DH, Schniepp HC, Abdala AA, Liu J, et al. Singlesheet functionalized graphene by oxidation and thermal expansion ofgraphite. Chem Mater 2007;19(18):4396–404.

[25] Schniepp HC, Li J-L, McAllister MJ, Sai H, Herrera-Alonso M, Adamson DH, et al.Functionalized single graphene sheets derived from splitting graphite oxide. JPhys Chem B 2006;110(17):8535–9.

[26] Starr FW, Schrøder TB, Glotzer SC. Molecular dynamics simulation of apolymer melt with a nanoscopic particle. ACS Macro Lett2002;35(11):4481–92.