Effect of MWCNT alignment on mechanical and self-monitoring properties of extruded PET–MWCNT nanocomposites

Composites Science and Technology 72 (2012) 1140–1146

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Effect of MWCNT alignment on mechanical and self-monitoring properties of extruded PET–MWCNT nanocomposites Francesca Nanni a,b,⇑, Beatriz L. Mayoral c, Francesco Madau a,b, Gianpiero Montesperelli a,b, Tony McNally c a

University of Rome ‘‘Tor Vergata’’, Dep. of Industrial Engineering, via del Politecnico 1, 00133 Rome, Italy INSTM – Italian Internuniversity Consortium on Material Science and Technology – UR Roma ‘‘Tor Vergata’’, Italy c School of Mechanical & Aerospace Engineering, Queen’s University Belfast, Belfast, BT9 5AH, UK b

a r t i c l e

i n f o

Article history: Received 25 October 2011 Received in revised form 12 March 2012 Accepted 18 March 2012 Available online 4 April 2012 Keywords: A. Carbon nanotubes B. Electrical properties B. Mechanical properties E. Extrusion

a b s t r a c t Self-monitoring aligned MWCNT loaded PET composites, with different CNT content, were prepared via twin-screw extrusion starting from a PET/MWCNT masterbatch, and fully characterized. All electrically conductive samples showed self-monitoring ability, i.e. a variation in electrical resistance as a function of stress. Moreover, the insertion of MWCNTs resulted in mechanical reinforcement with respect to neat PET. It was found that both self-monitoring behavior and mechanical performance are directly related to MWCNT content and to the direction of applied stress with respect to CNT orientation. In particular, too high MWCNT content decreased sensitivity at low strain, whereas a minimum MWCNT content was required to insure ohmic conductivity. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Since the beginning of the 1990’s a new class of multifunctional composite materials, namely self-monitoring polymer composites, have been proposed [1–3]. The main feature of these materials is their ability to provide structural and sensing properties, becoming sensors in their own right. Both structural and self-monitoring performance depend on composite composition and microstructure, including filler type and level of reinforcement (intrinsic properties, shape, aspect ratio); type of matrix; reinforcement distribution within the matrix (presence of aggregates, degree of alignment, etc.). Since the self-sensing task is achieved by means of the variation of electrical resistance under strain of an electrically conductive phase incorporated within the matrix, the reinforcing component is also required to show a sufficient degree of electrical conductivity. The natural candidate for both conductive and structural reinforcement in composites is carbon, available in different forms: long or short fibers, flakes, nanoparticles and, more recently, graphene and carbon nanotubes (CNTs). Initially, the more traditional long carbon fibers were proposed as the reinforcement and sensing phase [4,5], nevertheless such systems suffer low self-monitoring sensitivity, particularly at low strain [6,7]. This is due to carbon fibers behaving as stiff conductive wires, which offer limited ⇑ Corresponding author at: University of Rome ‘‘Tor Vergata’’, Dep. of Industrial Engineering, via del Politecnico 1, 00133 Rome, Italy. Tel.: +39 06 7259 4496; fax: +39 06 7259 4328. E-mail address: [email protected] (F. Nanni). 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2012.03.015

deformation, and hence little variation in electrical resistance under strain up to failure, where the resistivity suddenly goes to infinity. Subsequently, to increase sensitivity at low strain, the use of carbon particles (CP), either in form of spheres or flakes, was successfully proposed [8,9]. In this case the mechanism of conductivity is related to the formation of a continuous network of conductive particles (i.e. percolation) [10], where the increase in electrical resistance under strain is due to the physical separation of the particles, thus making current flow more difficult. More recently, the introduction of carbon nanotubes (CNTs) opened new opportunities in this field of research, particularly because CNTs are particularly efficient at reaching percolation at very low loadings [11], due to their high intrinsic electrical conductivity and aspect ratio, which facilitates contact among adjacent tubes [12,13]. For CNT or CP loaded systems, both percolation threshold and self-monitoring performance are directly dependent on many features including the intrinsic characteristics of the nanofiller (including morphology, dimensionality, electrical conductivity, etc.) as well as the efficiency of CNT dispersion and distribution within the polymeric matrix. Recently, some authors proposed the use of CNTs to realize strain sensing films: Park and co-authors demonstrated MWCNT–PEO films that were used as strain sensors externally attached to bulk PC dog-bone specimens that were tested in tension [14]. Quijuano et al. [15] proposed MWCNT – polysulfone films that were directly tested by means of monotonic and cyclic tensile tests showing good sensing results under monotonic conditions, while not fully satisfactory results were found under cyclic loading, where traditional metallic strain gauges showed superior performance. Other researchers

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proposed piezo-resistive bulk composites made of MWCNT in an epoxy resin: Whichmann and co-authors gave particular attention to study the difference of self-monitoring behavior in the elastic and plastic region [16], while Hu et al. focused on the formulation of a 3D mathematical model predicting the piezo-resistivity behavior of randomly dispersed MWCNTs in an epoxy, and found good agreement between numerical and experimental results [17]. Du et al. tried to introduce in a mathematical model the effect of nanotube orientation on the overall piezoresistivity [18], while Zhi-Min et al. analyzed the effect of CNT aspect ratio on the piezoresistivity performance of MWCNT in silicon rubber [19]. Among thermoplastic conductive polymers, recently poly(ethylene terephthalate) (PET) based systems (i.e. PET loaded with conductive particles) gained great attention, due to its very interesting characteristics, such as ease of fabrication, good mechanical properties together with good toughness and high recyclability. Nevertheless, the relatively high melt viscosity of the polymer does not favor filler dispersion, resulting in the presence of aggregates with detrimental effects on both mechanical and electrical performance (increased percolation threshold). Therefore, the attention of many researchers has been focused to assess more efficient methodologies to disperse CNTs in PET. In this regard in situ polymerization [11,20], direct melting [11,20], mechanical mixing of PET powder and SWCNTs, coagulation [21] and melt compounding [22], were described. In this paper, the mechanical and self-monitoring performance of MWCNTs aligned in PET are presented and discussed. Samples with different MWCNT loadings were prepared by melt blending (using a twin-screw extruder) by diluting a PET/10 wt.% MWCNT masterbatch in an attempt to optimize MWCNT dispersion within the polymer. First mechanical tests were carried out to verify the effect of MWCNT addition on tensile mechanical properties. An increase in strength and stiffness with respect to neat PET was obtained, which is dependent on MWCNT loading and orientation. Furthermore, all samples demonstrated a distinct selfmonitoring ability, although with markedly different sensitivity and performance, again dependent on MWCNT content and alignment. The results clearly show that highly dispersed MWCNTs in PET shows the potential of CNTs as both a reinforcing and sensing filler.

2. Materials and methods The poly(ethylene terephthalate) (PET) Polyclear F019 (intrinsic viscosity 0.895 dl/g) used in this study was supplied by Invista Resins & Fibers Gmbh. The multi-wall carbon nanotubes (MWCNTs) used, (NanocylÒ 7000 purity 90%, typical diameter 9.5 nm and an average length 1.5 lm) produced via catalytic carbon vapor deposition (CCVD) and a masterbatch of PET/10 wt.% MWCNTs in pellet form, were both kindly provided by Nanocyl S.A. Belgium. The neat PET and PET/10 wt% MWCNT masterbatch were dried at 120 °C for 12 h and premixed in order to produce samples with MWCNT concentrations of 1 wt.%, 2 wt.% and 4 wt.% in PET (Table 1). The premixed pellet samples were fed into the extruder hopper and melt blended using a co-rotating intermeshing twin-screw extruder Collin GmbH (screw diameter of 25 mm and a barrel length of 750 mm, L/D = 30) with a screw speed of 160 rpm and a residence time of 1 min. The temperature settings over six zones from the feed section to the die head increased from 230 °C to 260 °C resulting in a melt temperature of 280 °C. Extruded sheet of 1 mm thick samples of neat PET, PET/1 wt.% MWCNT, PET/2 wt.% MWCNT and PET/4 wt.% MWCNT were produced and cut to standard into dog-bone specimens for mechanical testing. Two types of specimens for each composite were obtained by cutting parallel to the extrusion direction and perpendicular to extrusion direction (Fig. 1a). Electrical contacts were provided at sample ends by silver

Table 1 Nomenclature of samples.

a b

Direction of cut versus extrusion direction

MWCNT loading (wt.%)

Nomenclature

Parallel Perpendicular Parallel

0% 0% 1–2–4

Perpendicular

1–2–4

MDa_Neat PET TDb_Neat PET MD_wt.% CNT loading TD_wt.% CNT loading

MD: Machine direction. TD: Transverse direction.

Direction of extrusion

(a)

(b)

Fig. 1. (a) Sample manufacturing from extruded lamina: MD samples with major axis parallel to extrusion direction and TD samples with major axis perpendicular to it (b) dog-bone samples with electrical contacts prior to testing.

glue (Fig. 1b). Percolation was assessed (Keithley 2700 DMM digital multimeter) prior to mechanical testing. I/V tests (Solarton SI 1287 potenziostat) were carried out to evaluate the predominant mode of conductivity (i.e. tunneling or ohmic). Mechanical testing consisted of simple tensile tests carried out at 2 mm/min (Instron 5569 J, 3 replicates), while self-monitoring was assessed by measuring the variation in electrical resistance of samples under stress, by means of a digital multi-meter in a two probe configuration. To verify the self-monitoring accuracy at low strain, strain gauges were mounted on samples (HBM Spider 8) for comparison. The microstructure of the composites was investigated by FEG–SEM (Leo Supra 35). The degree of MWCNT dispersion in the PET matrix was examined using transmission electron microscopy (TEM). Specimens for TEM examination of approximately 75 nm thickness were cryogenically microtomed and cut with a diamond knife. Images were obtained using a FEI Tecnai F20 field-emission HRTEM using an accelerating voltage of 200 kV. 3. Results and discussion 3.1. Percolation and I–V characterization The electrical resistance of both MD and TD specimens for each composite was measured and percolation assessed (Fig. 2). From the electric measurements it is clear that percolation occurred around 1 wt.% A numerical fitting of the experimental results based on the power law relationship (1) of classical percolation theory [23]:

q / ðU  Ut Þt

ð1Þ

(where U = filler weight fraction, Ut = weight fraction at the percolation threshold and t is the critical exponent which depends on system dimensions) resulted in calculated values Ut of UtMD = 0.9 wt.% and UtTD = 0.91 wt.%, respectively, with tMD = 2.26 and tTD = 2.18. Such values of percolation are in good agreement with the published literature [24,25]. Microstructural analysis (TEM micrographs of Fig. 3a aand b) clearly show the formation of a more uniform network of nanotubes passing from the 1 wt.% to 2 wt.% MWCNT samples. Moreover the increase of nanofiller

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Fig. 2. Percolation curves of MD and TD samples (measured on 0–0.5–1–2–4 wt.% CNT samples). Insert: magnification of the percolation cures above percolation.

loading results in a marked CNT alignment in the extrusion direction (SEM micrographs Fig. 3c and d referred to as 4 wt.% samples). Above percolation, a slightly higher conductivity was found in MD samples (Fig. 2), and this can be ascribed to the fact that in systems with CNT oriented preferentially, assuming the CNTs remain interconnected, conductivity is favoured in the direction parallel to that of CNT alignment [26,27], since in this case CNT aspect ratio is fully exploited and mainly electron flow occurs inside the carbon tubes, while only short distances external to nanotube ends have to be covered (see schematic diagram in Fig. 5). Since the PET/1 wt.% MWCNT composite lies just above percolation, it was expected that in such samples a significant contribution

to conductivity (much more than in other samples) can be related to tunnelling. Since it is important to assess the predominant type of conductivity (i.e. tunnelling or ohmic) with regard self-monitoring testing [17,25], I–V tests were carried out on all samples. Tunnelling is characterized by non-linear I–V behavior, while when conductivity is due to direct contact among conductive particles, Ohm’s law is obeyed and a linear I–V curve is expected. The PET/2 wt.% MWCNT and PET/4 wt.% MWCNT composites in both the MD and TD showed a linear I–V behavior (graphs not shown), despite in such systems a number of tunnelling junctions remains always active. PET/1 wt.% MWCNT composite in the MD and TD showed instead a non-ohmic behavior (Fig. 4a and b). Moreover, from Fig. 4b it can be seen that in this samples there is a decrease in electrical resistance with increasing voltage, which is typical of conductors where electron tunnelling predominates. It has been reported previously that in such systems an increase in temperature improves conductivity, as a consequence of the addition of kinetic energy to electrons that can more easily pass from one conductor to another [28]. Sheng [29] described the dependence of electrical resistance with temperature for tunnelling with the following Eq. (2):

R ¼ R0 exp



T1 T þ T0

 ð2Þ

where R is the electrical resistance at the temperature of test environment T, R0 is the initial electrical resistance and T0, T1 are constants that depend on a variety of parameters including: area available for conductivity, system barrier potential, film thickness and others (including, Boltzman and Plank constants, mass and charge of the electron). In this instance the former parameter is probably the predominant factor, since for samples taken in the TD, area available for conductivity is less than that for the samples

Fig. 3. (a) and (b) TEM micrographs of PET/1 wt.% and PET/2 wt.% CNT samples showing conductive network formation. (c) and (d) SEM micrographs of PET/4 wt.% MWCNT composite, taken in MD, showing nanotubes preferential orientation in extrusion direction.

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Fig. 4. Electric assessment of PET/1 wt.% MWCNT MD and TD samples (a) I/V curves (b) DR/R0%/V curves.

Fig. 5. Schematic representation of electron flow in (a) MD and (b) TD where CNT aspect ratio is not fully exploited resulting in more pronounced tunnelling.

taken in the MD, due to CNT orientation. Consequently, for TD samples, the decrease of electrical resistance is more marked, in agreement with Eq. (2). Considering a possible schematic representation of current flow within both types of materials (Fig. 5), it can be hypothesized that for the samples taken in the MD, where the direction of current is parallel to that of preferential CNT alignment, electronic flow primary occurs within the CNTs, with only short paths among CNT ends, while for the TD samples the distance covered by the electrons inside and outside the CNTs is more comparable, increasing the contribution from tunnelling. 3.2. Assessment of mechanical properties In this section the results of mechanical testing are discussed (Fig. 6, Table 2) with a view to highlighting the effect of MWCNT loading on the mechanical properties of the nanocomposites with respect to those of neat PET. It is evident that there is a clear effect of MWCNT loading on the resulting mechanical properties: an increase of tensile yield strength along with a decrease of strain at rupture were recorded in both MD and TD samples with increasing MWCNT content, while only a slight improvement of elastic modulus was observed. The primary effect of increasing MWCNT loading can be recognized by a drastic reduction of strain at rupture passing from neat PET to the composites with addition of 1 wt.% MWCNT (Fig. 6). All mechanical properties are influenced by the direction of the applied stress with respect to that of the extrusion direction, and for MWCNT loaded samples, this coincides with preferential orientation of the MWCNTs. Some degree of anisotropy was found in neat PET samples too (Table 2), which can be directly ascribed to alignment of the polymer chains during the extrusion process, as a consequence of the design of the die used. For MWCNT loaded samples, the tensile yield strength was greater in the MD compared to the TD, due to the favorable orientation of CNT along the stress direction. The difference in mechanical performance of samples taken in the MD or TD became more evident

Fig. 6. Stress versus strain for neat PET and PET/MWCNT composites in the MD.

with increasing MWCNT content above 1 wt.%. For the PET/1 wt.% MWCNT composite there was just a slight increase in elastic modulus in the MD with respect to the TD, while tensile strength for both was practically equal. 3.3. Self-monitoring assessment As previously reported, assessment of self-monitoring consists of simultaneous mechanical and electrical testing. All samples containing MWCNTs showed clear self-monitoring behavior in the sense that a variation of electrical resistance can be distinctly associated with an increase of strain (Fig. 7, Table 3). Moreover, both the sensitivity at low strain (elastic zone at the beginning of the test) and at high strain (plastic plateau above yield strength) depends on MWCNT loading. In particular, for the samples containing 1 wt.% MWCNT in both the MD and TD (Fig. 7a) show similar selfmonitoring ability, with slighter higher sensitivity associated with the TD samples. Such a result can be explained considering the model proposed in Fig. 5: in the MD a large part of current flow occurs inside the MWCNTs and it is therefore less affected by MWCNT separation, while in the TD an increase in electrical resistance is more susceptible to MWCNT separation induced as a consequence of an applied strain. The variation in electrical resistance for both types of samples (MD and TD) actually ceases at the yield point. In fact, since 1 wt.% MWCNT content is relatively small (it that it is just above percolation), bundles of MWCNTs easily separate under strain and when yielding occurs the interconnected network of MWCNTs disintegrates, hindering any possibility of electron conduction, including tunnelling. As MWCNT content increased to 2 wt.% and 4 wt.% in both MD and TD, see Fig. 7b and c, a decrease in sensitivity at low strain was

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Table 2 Tensile test results as a function of MWCNT loading. MWCNT (wt.%)

E_MD (GPa)

E_TD (GPa)

Yield strength MD (MPa)

Yield strength TD (MPa)

eat rupture MD (%)

Neat PET 1% 2% 4%

2.0 ± 0.1 2.25 ± 0.07 2.57 ± 0.08 2.80 ± 0.32

1.93 ± 0.01 2.13 ± 0.08 2.34 ± 0.20 2.75 ± 0.08

62.6 ± 0.7 66.9 ± 0.8 72.5 ± 0.2 89.1 ± 4.2

59.0 ± 1.2 65.4 ± 0.7 68.2 ± 1.0 76.5 ± 0.8

>300 18 10 8

Fig. 7. Variation in electrical resistance as a function of deformation and the corresponding stress (MPa) versus strain curves for PET/MWCNT composites in both MD and TD.

Table 3 Variation in electrical resistance at 0.5% and 2.5% strain for MD and TD samples with different CNT loading. Nome

DR/R at e = 0.5%

DR/R at e = 2.5%

MD_1% TD_1% MD_2% TD_2% MD_4% TD_4%

0.71 ± 0.16 0934 ± 0.5 0.88 ± 0.14 1041 ± 0.21 0.78 ± 0.05 0968 ± 0.07

9.72 ± 0.56 11.53 ± 0.91 7.11 ± 0.32 8.96 ± 0.49 4.99 ± 0.44 6.75 ± 0.05

recorded. This is associated with the larger number of available conductive elements that, in the same volume of material, translated into more numerous contacts and conductive pathways. Under these conditions more strain is required to produce a noticeable increase in electrical resistance. On the contrary, the composites with higher MWCNT content, both in TD and MD configurations, can more precisely record material yielding, depicting a maximum in the (DR/R0)/e curve. Nevertheless, a noticeable difference in self-monitoring performance between samples taken in the TD and MD was observed, which became more evident with

increasing MWCNT content, from 2 wt.% to 4 wt.%, with the samples taken in TD being more sensitive due to the aforementioned mechanism of current flow. For the PET/2 wt.% MWCNT composite in both the MD and TD a steep increase in electrical resistance was recorded just above the yield point, at the beginning of the plastic plateau, and this again is directly associated with the overall amount of conductive elements not sufficient to preserve a conductive network above yielding. For the 4 wt.% MWCNT samples in the MD and TD, a more marked and different behavior above yielding was obtained. However, for the same composite in the TD, good self-monitoring behavior was preserved in the plastic region, despite a clear recovery of electrical resistance after yielding. In contrast, in the MD a very interesting peculiarity was seen above yielding, a distinct and noticeable recovery of electrical resistance was recorded (its value becomes lower than that at the beginning of the test in the unstressed state), revealing an improvement in electrical conductivity. Such a surprising result can only be explained by a rearrangement of CNTs at high strain, so that formerly separated CNTs come close together, giving rise to a newly formed and efficient network of conductors. This takes place in the plastic plateau, where necking propagates throughout the specimen gauge length. It can be assumed that under these conditions the

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Stressed state

Un- stressed state

(a)

(b)

Un- stressed state

Stressed state

(c) Fig. 8. SEM micrographs after fracture of the necking region of PET/4 wt.% MWCNT composites in (a) and (b) MD showing MWCNTs concentrated in highly conductive specific areas and (c) in the TD showing a more isotropic distribution of MWCNTs. Schematic representations of CNT rearrangement during necking in both MD and TD are shown.

shrinkage associated with necking and continuous alignment of polymer chains in the stress direction forces the CNTs to draw up, giving rise to the formation of new electric pathways. SEM micrographs of fractured surfaces of the PET/4 wt.% MWCNT composite (MD) in the necking region (Fig. 8a and b) confirm this hypothesis, showing that, in fractured MD samples, CNTs are mostly concentrated in well-defined areas, forming bundles of conductive MWCNTs, where current flow is much favoured (a schematic diagram of this is proposed in Fig. 8 also). The authors believe that such CNT concentration most probably originates in correspondence to pre-existing CNT aggregates. The fact that such morphology was found only in highly loaded samples, where the achievement of good dispersion is more difficult, seems to confirm this theory. The same phenomenon of necking and MWCNT rearrangement leads to a different self-monitoring behavior in the TD, since the MWCNTs are initially oriented perpendicular to the stress direction are forced together to draw up (as an effect of shrinkage) and rotate to align in the stress direction (Fig. 8b and schematic representation on the right). The balancing of these two effects, that continues up to fracture, results in a more uniform and isotropic distribution of CNTs (SEM micrograph, Fig. 8c) and thus in more efficient self-monitoring performance.

higher MWCNT content, i.e. 2 wt.% and 4 wt.%, showed true selfmonitoring behavior. The electrical resistance variation as a function of strain, follows the mechanical stress/strain curve, depicting yielding of the polymeric matrix. Moreover, it was found that MWCNT loading and orientation, strongly affect self-monitoring performance. In the former case, the presence of a large amount of conductive elements translates to the formation of numerous conductive pathways, which allows increased sensitivity performance up to high strain, where large matrix deformation is needed to separate the conductive CNTs. Less sensitivity was instead found in samples with CNTs oriented in the stress direction (i.e. MD), as in such samples current flow mainly occurs inside the CNTs, and it is therefore less influenced by their physical separation. Finally, it was found that the mechanism of necking and alignment of polymer chains during plastic deformation affects self-monitoring performance of highly loaded samples, since this results in a rearrangement of the CNTs within the matrix, which in turn leads to, for samples taken in the MD, the formation of newly electrically conductive interconnected CNT pathways. In contrast in the TD due to necking (reduction of sample thickness) and rotation of CNTs in the stress direction, a more uniform filler distribution and efficient self-monitoring performance result.

4. Conclusions

References

In this paper self-monitoring aligned MWCNT loaded PET composites, with different MWCNT content (1 wt.%, 2 wt.% and 4 wt.%) were prepared and fully characterized. SEM observations revealed the MWCNTs are homogenously and uniformly dispersed in the matrix, with the consequence that the filler provides an effective reinforcement, as testified by an increase in both tensile strength and Young’s modulus in both MD and TD samples. Electrical conductivity of all composites was measured in an attempt to assess the self-monitoring performance of these materials. Samples with 1 wt.% MWCNTs showed predominantly tunnelling conductivity, characterized by non-linear I–V behavior, which is not suitable for self-monitoring. Instead, samples with

[1] Schulte K, Baron C. Load and failure analyses of CFRP laminates by means of electrical resistivity measurements. Comp Sci Technol 1989;36:63–76. [2] Prabhakaran P. Damage assessment through electrical resistance measurements in graphite fiber reinforced composites. Experiment Tech 1990;14:16–20. [3] Chung DDL. Structural health monitoring by electrical resistance measurement. Smart Mater Struct 2001;10:624–36. [4] Muto N, Arai Y, Shin SG, Matsubara H, Yanagida H, Sugita M, et al. Hybrid composites with self-diagnosis function preventing fatal fracture. Compos Sci Technol 2001;61:8750–883. [5] Wang S, Chung DDL. Interlaminar interface in carbon fiber polymer matrix composites studied by contact electrical resistivity measurement. Compos Interfaces 1999;6:497–506. [6] Nanni F, Ruscito G, Forte G, Gusmano G. Design, manufacture and testing of self-sensing carbon fiber-glass-fiber self-sensing polymer rods. Smart Mater Struct 2007;16:2368–74.

1146

F. Nanni et al. / Composites Science and Technology 72 (2012) 1140–1146

[7] Bakis CE, Nanni A, Terosky JA, Koeler SW. Self-monitoring, pseudo-ductile, hybrid FRP reinforcement rods for concrete applications. Compos Sci Technol 2001;61:815–23. [8] Okuhara Y, Matsubara H. Carbon-matrix composites with continuous glass fiber and carbon black for mazimum strain sensing. Carbon 2007;45:1152–9. [9] Indada H, Okuhara Y, Kumagai H. Health monitoring of concrete structure using self-diagnosis materials. In: Asari F, editor. Sens Issues Civil Struct Health Monitor. Berlin: Springer; 2005. p. 239–48. [10] Kirkpatrik S. Percolation and conduction. Rev Mod Phys 1973;45(4):574–88. [11] Logakis E, Pissis P, Pospiech D, Korwitz A, Krause B, Reuter U, Pötschke P. Low electrical percolation threshold in poly(ethylene terephthalate)/multi-walled carbon nanotube nanocomposites. European Polymer Journal 2010;46:928–10. [12] Hu N, Masuda Z, Yan C, Yamamoto G, Fukunaga H, Hashida T. The electrical properties of polymer nanocomposites with carbon nanotube fillers. Nanotechnology 2008;19(21):215701. [13] Al-Saleh MH, Sundararaj U. A review of vapor grown carbon nanofiber/ polymer conductive composites. Carbon 2009;47:2–22. [14] Park M, Kim H, Youngblood YP. Strain-dependent electrical resistance of multi-walled carbon nanotube/polymer composite films. Nanotechnology 2008:19–055705. [15] Quijano JRB, Avilésa F, Aguilara JO, Tapiab A. Strain sensing capabilities of a piezoresistive MWCNT-polysulfone film. Sensor Actuat A 2010;159:135–40. [16] Wichmann MHG, Buschhorn ST, Gehrmann J, Schulte K. Piezoresistive response of epoxy composites with carbon nanoparticles under tensile load. Phys Rev B 2009;80:245437. [17] Hu N, Yoshifumi K, Cheng Y, Zen M, Fukunaga H. Tunneling effect in a polymer/ carbon nanotube nanocomposite strain sensor. Acta Mater 2008;56:2929–36. [18] Du F, Fischer JE, Winey KI. Effect of nanotube alignment on percolation conductivity in carbon nanotube/polymer composites. Phys Rev B 2005;72:121404-1–4-4.

[19] Zhi-Min D, Jiang MJ, Xie D, Yao SH, Zhang LQ, Bai J. Supersensitive linear piezoresistive property in carbon nanotubes/silicone rubber nanocomposites. J Appl Phys 2008;104:024114. [20] Hernández JJ, García-Gutiérrez MC, Nogales A, Rueda DR, Kwiatkowska M, Szymczyk A, et al. Influence of preparation procedure on the conductivity and transparency of SWCNT-polymer nanocomposites. Comp Sci Technol 2009;69:1867–72. [21] Hu G, Zhao C, Zhang S, Yang M, Wang Z. Low percolation thresholds of electrical conductivity and rheology in poly(ethylene terephthalate) through the networks of multi-walled carbon nanotubes. Polymer 2006;47:480–8. [22] Anoop Anand K, Agarwal US, Rani J. Carbon nanotubes-reinforced PET nanocomposite by melt-compounding. J Appl Polym Sci 2007;104:3090–5. [23] Stauffer DS, Aharony A. Introduction to Percolation Theory. Taylor & Francis; 1994. [24] Anoop Anand K, Agarwal US, Nisal A, Rani J. PET–SWNT nanocomposites through ultrasound assisted dissolution evaporation. Eur Polym J 2007;43:2279–85. [25] Hu N, Karube Y, Arai M, Watanabe T, Yan C, Li Y, et al. Investigation on sensitivity of a polymer/carbon nanotube composite strain sensor. Carbon 2010;48:680–7. [26] Wang Q, Dai J, Li W, Wei Z, Jiang J. The effects of CNT alignment on electrical conductivity and mechanical properties of SWNT/epoxy nanocomposites. Compos Sci Technol 2008;68:1644–8. [27] Sennett M, Welsh E, Wright JB, Li WZ, Wen JG, Ren ZF. Dispersion and alignment of carbon nanotubes in polycarbonate. Appl Phys A 2003;46:111–3. [28] Simsek Y, Ozyuzer L, Seyhan AT, Tanoglu M, Schulte K. Temperature dependence of electrical conductivity in double-wall and multi-wall carbon nanotube/polyester nanocomposites. J Mater Sci 2007;42:9689–95. [29] Sheng P. Fluctuation-induced tunneling conduction in disordered materials Phis. Review B 1980;21(6):2180–95.