Composites Science and Technology 89 (2013) 69–76

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Composites Science and Technology

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Improvement of the electrical conductivity in multiphase epoxy-basedMWCNT nanocomposites by means of an optimized clay content

0266-3538/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compscitech.2013.09.021

⇑ Corresponding author. Fax: +39 089964218.E-mail address: plamberti@unisa.it (P. Lamberti).

Biagio De Vivo a, Patrizia Lamberti a,⇑, Giovanni Spinelli a, Vincenzo Tucci a, Liberata Guadagno b,Marialuigia Raimondo b, Luigi Vertuccio b, Vittoria Vittoria b

a Department of Information Engineering, Electrical Engineering and Applied Mathematics, University of Salerno - via Giovanni Paolo II, 132 84084 Fisciano, SA, Italyb Department of Industrial Engineering, University of Salerno - via Giovanni Paolo II, 132 84084 Fisciano, SA, Italy

a r t i c l e i n f o

Article history:Received 20 February 2013Received in revised form 2 September 2013Accepted 29 September 2013Available online 9 October 2013

Keywords:A. NanocompositesA. NanoclaysB. Non-linear behaviorB. Electrical propertiesD. Thermogravimetric analysis (TGA)

a b s t r a c t

The improvement of the DC conductivity of nanocomposite based on an epoxy matrix filled with multiwall carbon nanotubes (MWs) by using an optimized Hydrotalcite clay (HT) content is discussed. Multi-phase nanocomposites with fillers content less than 1 wt% are considered in order to exploit the disper-sion mechanism introduced by the clay. A Design of Experiment (DoE) procedure with a coarse parameterspace sampling is adopted in order to explore and optimize the electrical properties of the nanocompositewithout altering sensibly the mechanical ones. The implemented technique puts in evidence a non-lineardependence of the composite electrical conductivity from the clay content. Furthermore an optimized0.6 wt% of HT with 1 wt% MW multiphase composite is found leading to the higher electrical conductivityin the considered parameters space. The optimized design and the non-linear effect are experimentallyvalidated. Some possible mechanisms responsible for the obtained results are discussed.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The development of new added-value materials with specificfunctionalities and improved performances are increasingly criticalfor industrial competitiveness. The availability of new fillers withnanometric dimensions, together with a growing ability to charac-terize and manipulate systems at the nanoscale, will lead to theproduction of materials with higher knowledge content for severalindustrial applications [1,2]. A class of nanocomposites of notableimportance is obtained by considering polymers reinforced withdifferent forms of nanostructured carbon, such as Carbon Nano-Tubes (CNTs), in particular of type Multi Wall (MWs). Due to theirhigh aspect ratio and surface of contact with the matrix, MWs havedefinite advantages for the realization of composites with remark-able electrical conductivity and mechanical properties such ashardness, impact resistance and vibration damping [3–5]. As anexample, these properties are of special interest in aeronauticsfor the realization of the structural parts where multifunctionalmaterials able to dissipate lightning currents and satisfy, at thesame time, electromagnetic (EM) and mechanical constrains arerequired [6,7]. In such applications the electrical conductivity ofthe nanocomposites should be in order of 10 S/m and likely valuescould be theoretically achievable for epoxy-based MW nanocom-

posites with small loading of MWs. In fact, by considering the con-ductivity of MWs (103–106 S/m) and on the basis of a simplemixing rule, one expects that the electrical conductivity of thenanocomposite, i.e. rDC, may reach the target value with quitesmall (a few percent in weight) filler loading [8,9]. However, theconductivity values obtained so far are still lower than those ex-pected due to MWs tendency to agglomerate which determine apoor three dimensional structure of the conductive phase. For thisreason the use of an additional filler, which may improve the dis-persion of the CNTs inside the matrix and favor the conductionmechanisms has been proposed [10–12]. In particular, in [10] theincorporation of supplementary large quantities (i.e. 20–30%) ofdifferent clays is adopted in order to observe the effects on rDC

of the multi-phase composites. In [11] instead the inclusion of asmall concentration of clay in an epoxy based composite obtainedby using nanotube of single wall type (SW) is considered. Theadopted amount of clay content leads to improve the EM proper-ties of the composite without significant modification of themechanical characteristics. In [12] the mechanism leading to thedecrease of the level of aggregation of the CNTs was proposed interms of formation of micelle-like microstructures involving thegeometric-shape difference and mutual exclusion of non-covalentbonding interaction among the pristine nanoparticles in the com-posite systems based on MW and different clays. In these worksthe analysis has been mainly focused on the exploration of themorphological and structural characteristics and their correlationto the physical properties, whereas a systematic study aimed at

70 B. De Vivo et al. / Composites Science and Technology 89 (2013) 69–76

evaluating the combination of CNT and clay leading to most advan-tageous composite performances is still missing.

Therefore, in the present paper a composite system based on anepoxy resin loaded with MWs and a Hydrotalcite (HT) clay is con-sidered in order to evaluate the optimal combination of the twophases leading to improve the DC conductivity without deteriorat-ing the mechanical properties. This problem could be tackled bymeans of a ‘‘trial and error’’ approach or an uniform sampling ofthe CNT and clay contents. In both cases the realization of an a pri-ori unknown large number of samples may be required which, inturn, causes the increase of the costs and time for the experimentalresearch. In order to avoid this problem, in this work an ‘‘ad hoc de-sign’’ is considered by using a Design of Experiment (DoE) ap-proach [13]. In particular, by adopting rDC as a performancefunction (PF) of both the MW and HT loadings, the parameter spaceis ‘‘coarsely’’ sampled in order to experimentally set the most influ-encing parameter and the type of dependence with respect to bothconsidered factors. A Response Surface Methodology (RSM) is thenadopted to obtain a polynomial approximation of the dependenceof the electrical conductivity of the multiphase nanocomposite[13]. The measured rDC of new samples suitably selected in theparameter space confirms the theoretical predictions.

2. Experimental

2.1. Materials

The composites are manufactured by using the epoxy precur-sors DiGlycidil-Ether Bisphenol-A (DGEBA) and as hardener agent4,4-diaminodiphenyl sulfone (DDS), with a weight ratio DGEBA/DDS = 3,5. DGEBA, DDS and the HT were obtained from Sigma–Al-drich. MWs (3100 Grade) were obtained from Nanocyl S.A. Thespecific surface area of MWs determined with the BET method isaround 250–300 m2/g, the carbon purity is >95% with a metaloxide impurity <5% as it results by thermogravimetric analysis(TGA). The fillers were incorporated into Epoxy Precursors by usinga ultrasonication (Hielscher model UP200S-24 KHz high powerultrasonic probe) for 20 min at 100 �C [14]. The filled epoxy andDDS were mixed at 120 �C. All the mixtures were cured at 150 �Cfor 1 h and 220 �C for 3 h.

2.2. Measurement

High resolution scanning electron microscope (HRSEM) micro-graphs were obtained with a Scanning Electron Microscopy(SEM) apparatus (JSM-6700F, JEOL) instrument operating at 3 kV.Atomic Force Microscopy (AFM) images were collected in tappingmode (TMAFM) which reaches lateral resolution <1 nm and heightresolution of less than 1 Å. The data were acquired in an ambientatmosphere (30–40% humidity) with a NanoScope V multimodeAFM (Digital Instruments), Santa Barbara (CA), using microfabri-cated silicon tips/cantilevers. All the images have been recordedsimultaneously in height, in amplitude and in phase. Some of thecomposite sections were cut from the solid samples by a sledgemicrotome. These slices were etched before the observation bySEM microscopy. The etching reagent was prepared by stirring1.0 g potassium permanganate in a solution mixture of 95 ml sul-furic acid (95–97%) and 48 ml orthophosphoric acid (85%). Thesamples were immersed into the fresh etching reagent at roomtemperature and held under agitation for 120 h. Subsequent wash-ings were done using a cold mixture of two parts by volume of con-centrated sulphuric acid and seven parts of water. Afterwards thesamples were washed again with 30% aqueous hydrogen peroxideto remove any manganese dioxide. The samples were finallywashed with distilled water and kept under vacuum for 2 days

using a well-established procedure [15,16]. The transmission elec-tron microscopy (TEM) characterization was performed on a JEOL2010 LaB6 microscope operating at 200 kV. TEM samples werecut from nanocomposite blocks using an ultramicrotome equippedwith a diamond knife at ambient temperature. X-ray diffraction(XRD) measurements were performed with a Brucker D8 Advancediffractometer with Ni-filtered Cu Ka radiation (k = 1.54184 Å).

Dynamic-mechanical properties of the samples were performedwith a dynamic mechanical thermo-analyzer (TA instrument-DMA2980). Solid samples with dimensions 4 � 10 � 35 mm were testedby applying a variable flexural deformation in dual cantilevermode. The displacement amplitude was set to 0.1%, whereas themeasurements were performed at the frequency of 1 Hz. The rangeof temperature was from 30 �C to 300 �C at the scanning rate of3 �C/min.

The measurements of the DC volume conductivity of the com-posites were performed by using disk-shaped specimens of about2 mm thickness and 50 mm diameter. In accordance with CEI15–23 standard, the samples metallization is of circular form,about 22 mm of diameter and it is obtained by using a silver paint(Alpha Silver Coated Copper Compound Screening, with a thicknessof about 50 lm and a resistivity of 0.7 X-square). In order to avoidsurface leakage currents, a guard ring was adopted. The measure-ment system, remotely controlled by the software LABVIEW�, iscomposed by a multimeter Keithley 6517A with function of voltagegenerator (max ± 1000 V) and voltmeter (max ± 200 V), the amme-ter HP34401A (min current 0.1 lA) and a suitable shielded cellwith temperature control. The applied electric field ranges from0 to 1.5 kV/m as a function of the conductivity of the sample to re-duce the thermal effect. The conductivity was independent in therange selected by the applied field.

3. Results and discussion

3.1. Electrical conductivity

In order to verify the impact of the clay incorporation in a MW-epoxy composite (named EpoxyCNT) the DC electrical conductivity(rDC) has been measured and used as PF for the double-phase(named EpoxyCNT-0HT) and the multiphase nanocomposite(named EpoxyCNT-wtHTHT with wtHT being the weight percentof the clay content). By fixing wtHT to a given value, the conductiv-ity is plotted vs. the MW concentration leading to typical percola-tion curves like those reported in Fig. 1, where the average valuesand the error bars, associated to the unavoidable variability of theprocess parameters, are shown (to improve the readability only theconductivity values obtained above the percolation threshold arepresented). From the plots of Fig. 1 several interesting observationscan be obtained. Firstly, it has to be noticed that the percolationthreshold is very low for all studied systems and, differently fromthe results obtained in [11] with the SWs, not sensibly affected bythe incorporation of the clay. An increase of several orders of mag-nitudes with respect to the value of the neat epoxy resin (equal to8.7 � 10�14 S/m) is, in fact, obtained at 0.05% CNT concentration.Then the conductivity of all composites increases when the CNTloading is varied between 0.1% and 1%. However, between 0.1%and 0.5%, the variability is more marked for the two multiphasesystems than for the double-phase composite. In particular, atwtCNT = 0.1% the average values of the conductivity for Epox-yCNT-01HT and EpoxyCNT-1HT samples are 10.29 and 15.76 mS/m respectively, whereas that concerning the EpoxyCNT-0HT sam-ple is 28.15 mS/m. At wtCNT = 0.3% the values are 146.0 and125.0 mS/m for EpoxyCNT-01HT and EpoxyCNT-1HT, whereas thatconcerning the EpoxyCNT-0HT system is 94.9 mS/m. AtwtCNT = 0.5% the conductivity of EpoxyCNT-01HT and EpoxyCNT-

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 110

-5

10-4

10-3

10-2

10-1

100

wtCNT

[%]

DC

[S/

m]

EpoxyCNT-0HTEpoxyCNT-01HTEpoxyCNT-1HT

σ

Fig. 1. Conductivity of nanocomposite systems as a function of the CNTs concentrations (wtCNT). The electrical measurements were performed on a set of five samples foreach composite. The scale bar, related to the dispersion of the characteristics of the composites due to the manufacturing process, is shown.

B. De Vivo et al. / Composites Science and Technology 89 (2013) 69–76 71

1HT samples is 461.0 and 341.0 mS/m respectively, while the dou-ble-phase sample exhibits a value of 142.8 mS/m. At the highestconsidered CNTs concentration (wtCNT = 1%) the rDC values forthe two multiphase composites and the double-phase one are al-most the same. In fact, the measured values are 528.5, 689.0 and818.5 mS/m for EpoxyCNT-0HT, EpoxyCNT-01HT and EpoxyCNT-1HT samples, respectively. Among the different features of the per-colation curves shown in Fig. 1, which deserve further analysis tobe presented in a forthcoming paper, the non-linear dependenceof rDC with respect to the clay content is evident. In the presentstudy, indeed, the attention has been focused on the possibilityto obtain, through a suitable procedure, an optimized filler combi-nation of the multiphase composites leading to a maximum in thevalue of the electrical conductivity.

3.2. Optimization of the multiphase composite content

The objective is to improve the PF, i.e. the rDC, by finding theoptimal combination of the controllable input factors, i.e. the con-tent of the MW (x1 = wtCNT) and clay fillers (x2 = wtHT). In order toavoid either an inefficient and time consuming ‘‘trial and error’’plan, a DoE approach is adopted. In particular the parameter space,i.e. {x=(wtCNT, wtHT)=(x1, x2)e[x1min, x1max]�[x2min, x2max]} �R2 issampled by considering a full factorial array with different levelsfor each factors. The adopted ranges for the input factors are{0.05, 1} and {0, 1} for x1 and x2 respectively. In particular, thewtCNT minimum (0.05%) is chosen close to the percolation thresh-old found for epoxy-CNT systems, whereas the maximum (1%) isdetermined on the basis of the limitations in the preparation pro-cedure imposed by the very high viscosity of the compositeachieved at such concentrations. As it concerns the HT content,the maximum value (1%) is selected by fixing a max decrease of10% in the storage modulus at room temperature (T = 25 �C), ashighlighted by the mechanical characterization reported in Section3.5. A non-uniform five levels segmentation of the correspondingrange is considered for the CNT content, i.e. x1 e {0.05, 0.1, 0.3,0.5, 1}, whereas a non-uniform 3-levels segmentation is assumedfor the HT loading, i.e. x2 e {0,0.1,1} [13]. Therefore, the full facto-rial array requires the characterization of a set of 5 � 3 = 15 possi-ble type of specimens. However, all the experimental results ofelectrical characterization reported are obtained by measuringten samples for each identified composition. The Dex scatter Plot(DsP) and the corresponding Main factor Plot (MfP) of the conduc-tivity vs. wtCNT and wtHT allow to establish the dependency orderand trend on the two input factors, as shown in Fig. 2. From the

analysis of these graphs it is possible to observe that, as expected,the final conductivity of the composite is strongly influenced bythe concentration of the conductive phase. In fact, the MfP forthe MW content is characterized by a straight-line with a highslope (a = 0.84). As it concerns the effect of HT, Fig. 2d) puts in evi-dence a small (a = 0.09), but positive dependence of the compositeconductivity from such factor. This means that effectively theincorporation of clay can improve the conducting performance ofthe multiphase composites. On the same data set a second orderRSM interpolation of the PF is considered in the Matlab� environ-mental, obtaining the following equation for a confidence level of90% [17]:

rDCðwtCNT;wtHTÞ¼ f ðx1;x2Þ¼b0þb1x1þb2x2þb11x21þb22x2

2þb12x1x2

¼�0:1þ0:5x1þ1:38x2þ0:28x21�1:32x2

2þ0:19x1x2 ð1Þ

The 3D RSM representation of the rDC as a function of eachindependent parameters (wtCNT, wtHT) is reported in Fig. 3. Thismodel is adopted to find the set of input factorsx� ¼ wt�CNT;wt�HT

� �leading to the best performance of the multi-

phase composite in terms of rDC. The analysis of Fig. 3 allows inter-esting remarks. As it concerns the increase of the conductivity dueto the variation of MW content, it is possible to observe thatapproximately a first order dependence occurs. Also the b1 valuehigher than both b11 and b12 confirms such a quasi-linear depen-dence. Unlikely, the dependence on nanoclay concentration ap-pears of a parabolic type. This means that, for a given MWloading, there is a HT content in the considered range for whichthe composite conductivity assumes a maximum. In fact, in the re-sponse surface shown in Fig. 3, the red marker highlights the exis-tence of a theoretical maximum of rDC = 1.14 ± 0.60 S/m obtainedfor an HT content wt�HT ¼ 0:55%. Therefore, a composite whose fill-ers concentrations are selected by considering the described ad-hoc design can exhibit a conductivity higher than the largest oneexperimentally measured (i.e. 0.9 S/m for the sample EpoxyCNT-01HT with 1% wtCNT defined in the pre-planning phase).

3.3. Experimental validation

In order to verify the predicted ‘‘optimized’’ choice of the inputparameters, additional composites with specific compositions havebeen produced. Such composites are manufactured by consideringa feasible step for the HT concentrations (0.1 wtHT). By assuming aMW loading of 1%, i.e. wt�CNT, supplementary composite systemshave been prepared with a clay concentration of 0.3%, 0.5%, 0.6%and 0.7% respectively, in order to explore in greater detail the

Fig. 2. DoE representation of the measured rDC for the wtCNT and for the wtHT factors: (a) and (b) DsP; (c) and (d) MfP. The DsP shows the values of the performance functionfor selected levels of each independent variable for different combinations of levels of the other factor. The MfP is obtained from the DsP by graphing the segment between themean values of the PF achieved at the lowest and highest level of each parameter. The slope of the straight-line, as reported in (c) and (d), represents the influence of theparticular design parameter on the PF. A null value of the slope indicates that the PF is independent on the factor, whereas a positive/negative figure provides informationabout the (increasing/decreasing) trend of the dependence. A steeper slope corresponds to a higher dependency order.

00.2

0.40.6

0.81

0

0.5

1-0.5

0

0.5

1

1.5

wtHT [%]

(wt*CNT , wt*HT)

*

wtCNT [%]

σDC

[S/m

]

Fig. 3. 3D response surface representation of the conductivity dependence on thetwo fillers concentrations obtained by means of (1). The marker highlighted by theasterisk shows the existence of the theoretical absolute maximum forwt�HT ¼ 0:55%;wt�CNT ¼ 1%.

0 0.2 0.4 0.6 0.8 1

0.8

1

1.2

1.4

1.6

1.8

2

2.2

wtHT [%]

σD

C [S

/m]

Fig. 4. Conductivity of the composites vs. clay content (wtHT) withwtCNT ¼ wt�CNT ¼ 1%.

72 B. De Vivo et al. / Composites Science and Technology 89 (2013) 69–76

region where the maximum of the electrical conductivity is pre-dicted (wtHT around 0.55%). The measured rDC on the selectednew specimens is shown in Fig. 4. A quasi-parabolic behavior ofthe dependence of rDC on wtHT is experimentally found accordingto the theoretical predicted one. In fact, rDC in Fig. 4 range from aminimum (i.e. 0.7 S/m), corresponding to the double-phase com-posite system with 1% of MW, to a maximum value (i.e. 2.02 S/m) for the composites in which a 0.6% of clay content is used.Moreover the experimental detected maximum is near the theoret-ical one given for wt�HT (i.e. 1.74 = 1.14 + 0.60 S/m). Also the sam-ples with wtHT = 0.5% and wtHT = 0.7% exhibit an highconductivity value around 1.6 S/m, that is in the computed rangeof the theoretical optimum. These values are the best obtainedfor the assumed matrix and also 2 order of magnitude higher thanthat obtained in [11] for SW-based composites. There may be dif-ferent reasons to justify the non-linear (parabolic) behavior of theconductivity of the multiphase composite due to the introductionof the clay in suitable concentrations. In fact, according to the ex-cluded volume theory, the formation of tactoids of clay at micro/

meso-scale may affect the network of nanotubes and take part tothe conductive mechanisms in different way, with dependenceon the interaction with the CNTs [18]. In Fig. 5 a schematic illustra-tion of the representative volume for the two phase nanocompos-ite is reported, together with the possible combination of the twofiller in the case of low or high concentration of CNTs. In particular,when an high content of CNTs is considered (i.e. the samples Epox-yCNT-xxHT with 1 wt% MW), percolative paths are expected: theclay takes part to the conduction mechanism like a planar conduc-tive particle because it is covered by the CNTs. Vice-versa, when alow content of CNTs is considered (i.e. the samples Epoxy-01CNT-xxHT), the few percolation paths are hindered by the reduction ofthe available CNTs: the clay captures the CNTs and subtracts themfrom the conduction mechanism. Therefore, by comparing the con-ductivity of double and multi-phase composites, it can be deducedthat HT positively contributes to the conduction mechanism in thecase of composites with high CNTs content, whereas it lowers thevalue in composites filled with small nanotubes concentrations.Moreover, this effect is not monotonic and it is influenced by the

Fig. 5. Schematic representation of the morphological structure of clay/matrix/CNT.

Fig. 6. AFM image of the composite with wtCNT = 0.5% and wtHT = 1% (scan size: 504 nm).

1 For interpretation of color in Fig. 7b, the reader is referred to the web version ofthis article.

B. De Vivo et al. / Composites Science and Technology 89 (2013) 69–76 73

degree of the clay exfoliation as discussed in the next sub-section.In addition, depending on the filler concentrations, the affinity le-vel between the two different phases and the different viscosity ofthe compound before the cure cycle may determine diverse inter-action with the epoxy matrix. As a consequence, it leads to varia-tions in the height of the potential barrier involved in thetunneling mechanism that sensibly influence the conductivity ofthe composites [19,20]. However, these important issues are stillunder analysis and will be discussed in detail in a future paper.

3.4. Morphological and structural characterization

In order to investigate the influence of clay on the dispersionstate of MWs, AFM, SEM TEM and X-ray investigations were per-formed on fractured cross-sections of the composites. AFM investi-gation was performed, in addition to SEM and TEM, because, whileelectron microscopy approaches (SEM) can provide the needed res-olution only after an etching procedure, AFM images were col-lected on the fractured cross-sections of the composites withoutchemical treatments. AFM image of the composite EpoxyCNT-1HT containing 0.5 wt% MW is shown in Fig. 6. The amplitude im-age clearly displays stratifications signals due to the clay layers. Infact, this particular stratified structure is not shown in unloadedsamples and in samples filled only with MWs (AFM images not re-ported here). From height and phase images, we can deduce that inthe stratification signals a different phase (see Fig. 6 – phase) withsmudges in the form of spidery branches (see Fig. 6 – height) ispresent.

In order to understand why the systems loaded with HT clayshow these morphological features, SEM images performed onetched samples were collected. Fig. 7 shows the SEM images for

two multiphase composite systems loaded with the same HT(wtHT = 1%) and two different CNTs (wtCNT = 0.5% and 1%) concen-trations. In particular, in Fig. 7a and in the zoom of Fig. 7b theSEM images concerning the sample obtained with wtCNT = 0.5%and wtHT = 1% are shown, while in Fig. 7c that achieved for thesample with wtCNT = 1% and wtHT = 1% is presented. These picturesallow to correlate the morphological characteristics with the elec-trical behavior for the nanocomposites whose electrical conductiv-ity variability is illustrated by the blue line in Fig. 1 (i.e. HT contentfixed at 1 wt% and variation of the MWs amount). SEM images ofFig. 7a and c put in evidence a very peculiar feature. For bothMW content, the resin/layered clay with MWs appears as leaf-likemicro/nanostructure homogeneously covered by MWs, as sche-matically represented in Fig. 5. This feature is well distinguishablein the magnifications of Fig. 7b for the different regions (Zones 1, 2and 3) reported for the sample filled with 0.5% of MWs. In partic-ular, the magnification of the Zone 1 highlights that many MWsare aligned along the leaf of lamellar hydrotalcyte with an orienta-tion clearly visible on the lamellar ends (see the zone indicated inFig. 7b by the red1arrow). These observation suggests the existenceof a synergy between clay and CNTs that results in a very peculiarmicro/nanostructure development. This interesting synergic effectwas already found for systems such as clay/carbon black [21–23]and for polymer composite filled with SW/Clay [9]. This effect wasexplained by Liu and Grunlan in [11] for epoxy resin/SW/clay sys-tems in which the clay is sodium based montmorillonite (MMT).Although the composite systems are different (in our case a differentclay and multi walled nanotubes were embedded in the same epoxy

Fig. 7. SEM image of two multiphase nanocomposites with wtHT = 1% and differentwtCNT in different zones and at various magnifications: (a) wtCNT = 0.5% with threehighlighted zones; (b) magnification of the zones highlighted in (a) and (c)wtCNT = 1% with highlighted a zone exhibiting ‘‘uncovered’’ tactoides of clays.

Fig. 8. TEM images of two resin/layered nanocomposites: (a) with wtCNT = 1% andwtHT = 1% and (b) with wtCNT = 1% and wtHT = 0.7%.

74 B. De Vivo et al. / Composites Science and Technology 89 (2013) 69–76

precursor hardened with a different curing agent), the synergic effectcan be explained with the same considerations reported in [11]. Inparticular, among the proposed mechanisms, the most influencingones can be considered the strong affinity between the two nanopar-ticles and the increase in the viscosity of the liquid initial epoxy pre-cursor (Epon 828) due to clay inclusion, that makes very difficult fornanotubes to migrate and re-aggregate after sonication procedure.According to Liu and Grunlan [11], another convincing explanationfor this effect is associated to the ‘‘excluded volume’’ formed bythe micro-scale clay clusters that may effectively create segregatednetworks of nanotubes [24]. Studies are in progress to better inves-tigate this effect from a chemical point of view. These interactionscould be responsible for the highest recorded value of the storagemodulus.

Moreover, as highlighted in the SEM image of Fig. 7c, the exis-tence of un-exfoliated tactoids clays, not covered by MWs, is alsodetected (see the red-framed zone in Fig. 7c). From the electricalpoint of view these neat clay particles cross cut the electrical con-duction paths formed by the clusters of ‘‘pure’’ MWs and the leafsof HT covered by MWs. Therefore the presence of an increasingamount of these tactoids may adversely affect the conductivity ofthe multi-phase composites.

This morphological arrangement is also detected in many re-gions by means of TEM investigations. In fact, from the TEM imageconcerning the composite loaded with wtHT = 1% and wtCNT = 1%reported in Fig. 8a, it is possible to see the uncovered clay tactoidsbetween the leafs of HT covered by MWs (see the red arrow inFig. 8a). A similar morphological arrangement among the differentphases is also evident for sample filled with a lower concentrationof HT, as shown in Fig. 8b where the TEM image for the compositesystems loaded with wtHT = 1% and wtCNT = 0.7% is reported. In thislast case neat leaf of HT is interposed between two clay tactoidscovered by MWs (see the red arrow in Fig. 8b).

Fig. 9 shows, for comparison, the SEM image of a cross-sectionof the composite filled only with carbon nanotubes (EpoxyCNT-0HT with 0.5 wt% MWs). It is clearly observable a very differentdistribution of CNTs inside the epoxy resin. In fact the absence ofparticular synergies between the inorganic phases (MW and HT)causes a 3-dimensional network of CNTs without any preferentialorientation. For all the compositions, in the resin containing onlyCNTs, the tubes are randomly dispersed into the epoxy matrixand, for amount equal or higher than the electrical percolationthreshold, they are well connected inside the matrix.

Fig. 9. SEM image of the composite containing with wtCNT = 0.5%.

B. De Vivo et al. / Composites Science and Technology 89 (2013) 69–76 75

To better investigate the presence of un-exfoliated clay (tac-toids) in the analysed nanocomposites, in Fig. 10a the X-ray pow-der diffraction patterns of the pure Epoxy and the Epoxy filled withwtCNT = 1% at different percentage of HT contents are shown. In thesame Figure the pattern of the pure hydrotalcite is also reported forcomparison (see Fig. 10b). X-ray data highlight that the resin filledwith a percentage of HT equal or higher than 0.5% by weight showsthe reflection characteristic of clay tactoids (see the appearance ofthe weak peak between 10� and 12� of 2h). These results indicatethat in the adopted experimental conditions the complete exfolia-tion of HT can occur up to a concentration lower than 0.5%,whereas at higher HT concentrations the samples cannot be con-sidered free of big clay tactoids evidenced by SEM images ofFig. 7c. The presence of clay tactoids is more relevant at high con-centration of HT, in fact we can see that the signal of clay tactoidsslightly increases by increasing HT percentage (see X-ray profilesin Fig. 10b). Such an increasing content of uncovered clay layersor tactoids can be deemed responsible of the electrical conductivitydecrease for HT percentage higher than the optimized value of0.6%. A more detailed study would be necessary to confirm sucha conjecture, but this would require a very onerous statistical studybased on combined SEM and TEM micrographic analysis. However,

Fig. 10. X-ray diffractogram of the pure hydrotalcite (a) and of EpoxyCNTnanocomposites filled at different HT content (b) and fixed wtCNT = 1%.

this remarkable issue is beyond the scope of the present study andwill be dealt with in a future paper.

3.5. Mechanical characterization

In order to understand the chosen limit for the HT content andto find correlation with the morphological characterization, thebehavior of the storage modulus and tand for the pristine resin,the EpoxyCNT and the EpoxyCNT-wtHTHT with 1 wt% MWs are re-ported in Fig. 11. The storage modulus of the multiphase systemEpoxyCNT-1HT between 75 and 150 �C is higher than both the un-filled resin and the resin EpoxyCNT-0HT (Fig. 11a), whereas themaximum in tand cure of the multiphase is at an intermediatetemperature (180 �C) between the composite filled only with CNTs(170 �C) and the unfilled composite (190 �C) (Fig. 11b). This resultcan be explained in light of a previous work [25] where a lower Tg

of the composite filled only with CNTs was attributed to unreactedmolecular segments that cause inhomogeneities from regions ofvarying crosslink density. Such an inhomogeneity caused by CNTsthat break the regular crosslinking due to the reaction betweenoxirane rings and hardener is expected. According to the resultson the morphological investigation, an intermediate Tg for the mul-tiphase is also expected because the hydrotalcyte (in the form ofblocks or small lamellae aggregates subtracts a percentage of CNTsfrom the epoxy matrix core). The higher value in Tg of the multi-phase system is also due to an increase in the degree of cure (dc)due to hydrotalcyte phase. The value of dc determined by differen-tial scanning calorimetry for the resin EpoxyCNT-0HT (with 1 wt%MWs) is 230, whereas the value for the composite filled also withclay (EpoxyCNT-1HT with 1 wt% CNTS) is 287 with an increase of25% with respect to the sample filled only with CNTs. This resultcan be explained by considering that clay component actives other

Fig. 11. Storage modulus (a) and loss factor (b) for pristine resin, EpoxyCNT without(0HT) and with (1HT) hydrotalcite with wtCNT = 1%.

76 B. De Vivo et al. / Composites Science and Technology 89 (2013) 69–76

crosslinking reactions due to the nucleophilic character of OH andcarbonate groups.

4. Conclusions

The use of a DoE approach for the design of the multiphasenanocomposites in conveniently exploited in order to identify themain effective factor and type of dependency of the electrical con-ductivity with respect to the conductive filler and the dispersiveone. As is attempt, the main factor results the conductive filler(MW) and it shows also a quite linear dependences. Also the claycontent can influence positively the rDC of the nanocompositesand a non-linear dependence is detected with respect its contrib-ute. Furthermore a second order dependence is adopted. It suggestthe existence of an optimized amount of dispersive filler (0.6 wt%HT) in the considered starting range leading to select the bestnanocomposite in terms of rDC with only 15 samples. A maximumrDC of about 2 S/m is experimentally confirmed by using ad-hocmade new specimens. The possibility to make nanocomposite byincorporating dispersive nanofiller, in addition to the conductiveone, is confirmed as an alternative to the polymer blends to im-prove the properties of the polymeric materials. This thanks tothe synergy between clay and carbon nanotubes (for suitable con-centrations) that results in a very peculiar micro/nanostructuredevelopment that influence the electrical performance of the com-posite. A morphological interpretation of the HT’s role in the con-duction mechanisms is given in order to understand suchfeature. The adopted methodology is capable to substitute the typ-ical ‘‘trial and error’’ approach with an ‘‘ad-hoc’’ design in order tohave a tailored material as well as build systems with controlledand reproducible properties.

Acknowledgements

This work has been partly supported by the Italian Ministry forEducation, University and Research (MIUR) within the NationalProject PRIN 2008NMRHJS ‘‘Development and ElectromagneticCharacterization of Nano Structured Carbon Based Polymer Com-posites (DENSE)’’.

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