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Polyisoprene—multi-wall carbon nanotube composites for sensing strain
Materials Science and Engineering C 27 (2007) 1125 – 1128 www.elsevier.com/locate/msec
Polyisoprene—multi-wall carbon nanotube composites for sensing strain M. Knite a,⁎, V. Tupureina a , A. Fuith b , J. Zavickis a , V. Teteris a a
Institute of Technical Physics, Riga Technical University, 14/24 Azenes str., LV-1048, Riga, Latvia b Institute of Experimental Physics, University of Vienna, Austria Received 6 May 2006; accepted 24 August 2006 Available online 2 October 2006
Abstract Carbon nanotubes offer attractive possibilities for developing new sensors because of superior mechanical and electrical properties. So far most studies relate the mechanical deformation to the change of nano-scale electrical properties. We present an attempt to use the multi-wall carbon nanotubes (MWCNT) to develop a new material for sensing macro-scale strain. Polymer composites containing dispersed nano-size particles, for example, polyisoprene - multi-wall carbon nanotube composites (PMCNTC) were prepared by treatment of the composite matrix with chloroform providing an increase of mobility and better dispersion of the nano-particles within the matrix. MWCNT with a small amount of solvent was carefully ground in a china pestle before adding to the polyisoprene matrix. Both the polyisoprene matrix solution and concentrated MWCNT product were mixed in a mixer with small glass beads at room temperature for 15 min. The product was dried at 40 °C for over 12 h and vulcanized under high pressure at 160 °C for 20 min. PMCNTC shows attractive tensile and compressive strain sensing properties. A mechanism of sensing effects is being investigated. © 2006 Elsevier B.V. All rights reserved. Keywords: Polyisoprene and carbon nanotube composite; Percolation transition; Electrical properties; Strain sensor materials
1. Introduction Pressure and strain sensors as well as tactile sensors for robots are important in many fields of science and engineering. One of the main limitations of existing conventional sensors is that they are discrete point, fixed directional and not flexible sensors, and are separate from the material or structure that is being monitored. There is a need to develop new flexible largearea sensors that can be embedded into the material and can be used for multiple location sensing. Conductive polymer-composites for strain sensing can be obtained when particles of good conductors (carbon black, graphite powder, carbon fibres, particle of metals) are implanted into an insulating polymer matrix [1–5]. A continuous insulator–conductor transition is observed in two-component systems at gradual increase of the number of randomly dispersed conductor particles in an insulator matrix. Most often such transitions, called percolation transitions, are described by the model of statistical percolation [6,7]. The volume con⁎ Corresponding author. Tel.: +371 7089380; fax: +371 7615765. E-mail address: [email protected] (M. Knite). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2006.08.016
centration of conductor particles VC at which the transition proceeds is called the percolation threshold or critical point. In the vicinity of percolation threshold, electric conductivity of the composite changes as: rfjV −VC jt ; (t—critical indices) [7]. If mechanical deformation of composites occurs, the electric conductivity σ will change. This change is the origin of strain sensing effects. The change of electrical resistance with strain and pressure principally can be explained as a result of destruction of the percolation structure of the conductive particle network. New interesting properties are expected in case the composite contains dispersed nano-size conducting particles [8–13]. Polymer - electro-conductive nanostructure composites (PENC) offer attractive alternatives for developing new generation of flexible large-size sensors because of their superior mechanical and electrical properties. Carbon nanotubes in itself change their electrical properties when subjected to strain. Farijan et al. [14] calculated theoretically the I–V characteristics which showed that the current of metallic tube decreases with increased bending while that of
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Fig. 1. TEM image of multi wall carbon nanotube Aldrich 636835 (a) and high structured carbon black nanoparticle Printex XE2 (b). Scale mark 200 nm.
semiconducting tube increases. Possible application to nanoelectro-mechanical sensors and switches is discussed [14]. An attempt to use the strain sensing capability of single-wall carbon nanotubes (SWCNT) on the nanoscale to develop a strain sensor on the macroscale was made by Dharap et al. [15]. The carbon nanotube film is produced by mixing unpurified SWCTNs with 0.25 mg/ml N,N-dimethylformamide (DMF). The film (10 μm thick) was composed of mechanically entangled randomly oriented nanotube bundles, to which it has isotropic electronic properties [15]. The carbon nanotube film was attached to a brass plate with PVC film between them. A four-point probe was used to measure voltage changes in the carbon nanotube film. The brass specimen was subjected to tension as well as compression cycles. It can be concluded that there is nearly linear relationship between the measured change in voltage and the strains in the carbon nanotube films [15]. The purpose of our paper was the design, elaboration and investigation of the polyisoprene and multiwall carbon nanotube (MWCNT) composites for application in strain sensors as well as to compare them with the polyisoprene and high structure carbon black composites elaborated and prepared by the same technology.
(MWCNT) Aldrich 636835 as well as highly structured carbon black (HSCB) Printex XE2, were prepared by “solution method” as follows. The matrix composition was treated with chloroform providing (1) an increase of the nano-particles mobility and (2) better dispersion of the nano-particles within the matrix. Prepared matrix composition was allowed to stand for swelling ~ 24 h. Nano-size carbon black is carefully grinded with a small amount of solvent in a china pestle before addition to the polyisoprene matrix. Polyisoprene matrix solution and nano-size carbon structure concentrated product was mixed in the mixer with small glass beads at room temperature for 15 min. Product is poured out into little aluminium foil box and stand ∼ 24 h, dried at 40 °C for more than 12 h and vulcanized under high pressure at 160 °C for 20 min. The size of MWCNT: OD = 60–100 nm, ID = 5–10 nm, length = 0.5–500 μm, BET surface area: 40–300 m2/g. A TEM image (Fig. 1) shows the entangled structure of MWCNT. The average particle size of HSCB is 30 nm and its dibutyl phthalate (DBP) absorption is 380 ml/100 g. Its surface area is 950 m2/g. ATEM image (Fig. 2) shows the high structure of Printex XE. A Philips TEM-301 transmission electron microscope equipped with Keen View II CCD camera was used for nanoscopic investigation of fillers. To study deformation at stretch and dependence of electrical resistance of composites on the stretching force, samples of the size of 90 mm × 15 mm × 1.5 mm were cut from the vulcanized sheets with HSCB and samples of the size of 60 mm × 11.5 mm × 2.4 mm from the vulcanized sheets with MWCNT, respectively. Copper foil electrodes were glued on both sides at the ends and each pair of electrodes was short-circuited by copper wiring. On a relaxed sample the distance between electrodes l0 was 50 mm for HSCB and 25 mm for MWCNT, respectively. Sandpaper was glued to the electrodes to fasten the samples in the stretching machine (extensometer). Schematic illustration of the experimental set-up for measurements of the electrical resistance R of the nano-composite as a function of the tensile force is available in [12]. The KEITHLEY Model 6487
2. Samples and experimental procedure Polyisoprene composites containing dispersed nano-size particles, in our case two types multiwall carbon nanotubes
Fig. 2. The electric resistivity ρ of the polyisoprene-MWCNT composite and polyisoprene - HSCB composite vs. concentration (mass parts) of corresponding filler (percolation curves).
M. Knite et al. / Materials Science and Engineering C 27 (2007) 1125–1128
Fig. 3. The electric resistivity ρ of the polyisoprene - MWCNT composite and polyisoprene - HSCB composite vs. small tensile strain ε.
Picoammeter/Voltage Source was used for electrical resistance measurements. The tensoresistance effect was tested at least three times on each of the three samples of each separately prepared composition. The relative standard deviation of resistance measurements was about 20% at strain velocity of 6.67 × 10− 5 m/s. Dispersion of nano-particles in the polymer matrix gives the main contribution in this error. Error of the instrument at strain measurements was 0.1 mm. 3. Results and discussion To estimate roughly the percolation threshold, a series of polyisoprene-carbon nanotube composites (PCNC) were prepared with 6.1, 8.3, 10.9, 12.1 and 14.5 mass parts (m.p.) of MWCNT added to 100 m.p. of polyisoprene. The polyisoprenecarbon black composites (PCBC) with 6.6, 7.7, 8.8, 9.35, 9.9 and 11 mass parts (m.p.) of HSCB added to 100 m.p. of polyisoprene were prepared for confrontation with PCNC using the same preparing procedure. The 8.3 m.p. PCNC composite and 8.8 m.p. PCBC composite appeared to be within the region of percolation threshold (Fig. 2). Samples with 6.6, 7.7 and 11 mass parts of HSCB as well as samples with 6.05, 12.1 and 14.5 mass parts of MWCNT were obviously outside the region of percolation threshold. The samples with 6.6 and 7.7 m.p. of CB were practically insulators, while the samples with 9.9 and 11 m.p. of CB - good conductors. The percolation threshold in case of MWCNT composites was not so sharp as in the case of HSCB composites. For HSCB composites prepared by “solution” method the percolation threshold is slightly shifted to lower values of HSCB concentration than for HSCB composites made by “rolling in” method used in our previous investigations (the composite was made by rolling highly structured nano-size carbon black (Printex XE2, DEGUSSA AG) and the necessary additional ingredients (S, ZnO) into a polyisoprene matrix and vulcanizing under high pressure at 140 °C for 15 min) [12]. This difference can be explained due to better dispersion of HSCB in polyisoprene matrix by “solution” method.
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Furthermore, resistance R of the composites was examined with regard to the force of stretch F and the absolute mechanical deformation Δl in the direction of the force. Results were ranged in two groups — resistivity changes at small deformation and at large deformation respectively. No variation in electric resistance was found for the samples with 7.7 mass parts of HSCB and 3.63 mass of MWCNT parts even at 100% deformation of the composite (Fig. 3). Similarly, there was a relatively weak dependence of R on the deformation at 9.35 mass parts of HSCB (Fig. 3). These results are consistent with our predictions that with too high or too low concentrations of the filler one must expect weak tensoresistance effect or no such an effect at all. The behaviour was essentially different near the percolation threshold. Of all the composites examined, the best results were obtained for the samples with 8.8 mass parts of HSCB and 8.3 mass parts of MWCNT, respectively, which apparently belonged to the region of percolation threshold. The resistivity of these samples changed more than 6 orders upon a 40% stretch for 8.8 mass parts HSCB composite and more than 4 orders for 8.3 mass parts MWCNT composite. After the samples were released, the resistivity of HSCB composite practically returned to its previous value, while in case of MWCNT composite a relatively large hysteresis of ρ(ε) was observed that was explained because of the entangled structure of MWCNT (Fig. 1a) as well as due to high stiffness of nanotubes. The reversible change in electric resistivity of high structure carbon nanoparticle - polyisoprene composite even at large stretch can be explained by the high porosity of carbon agglomerates providing better adhesion to polymeric globules and mobility of nanoparticles, which is not observed in the case of a poorly structured carbon black. Electrical resistivity response to small strain for different polyisoprene nanocomposites is shown in Fig. 4. In this case the resistivity changes in all composites are practically reversible. It was also proved that the model of tunneling currents developed in [12] quite well describes the experimental data at small deformations for both HSCB and MWCNT polyisoprene composites.
Fig. 4. The electric resistivity ρ of the polyisoprene - MWCNT composite and polyisoprene - HSCB composite vs. large tensile strain ε.
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4. Conclusions The use of high structure carbon nanoparticle as filler of a polyisoprene matrix provided about 6 orders of magnitude of reversible change in the electric resistivity at large (40%) stretch. The use of multiwalled carbon nanotube as filler of a polyisoprene matrix provided about 4 orders of magnitude of nonreversible change in the electric resistivity at large (40%) stretch. The maximum change at stretch for both type of composites were observed near the percolation threshold. The reversible change in electric resistivity of high structure carbon nanoparticle - polyisoprene composite even at large stretch can be explained by the high porosity of carbon agglomerates providing better adhesion to polymeric globules and mobility of nanoparticles, which is not observed in the case of a poorly structured carbon black. The non-reversible change in electric resistivity of multiwalled carbon nanotube - polyisoprene composite at large stretch we explain with entangled structure of nanotube. Multiwalled carbon nanotube-polyisoprene composite can be used for small tensile strain sensing but high structure carbon black - polyisoprene composite are preferable for large tensile strain sensing. Acknowledgements The research was supported by the Council of Science of Latvia and by Latvian National Research Program in Materials
Science. The authors are thankful to Dr. Donats Erts and Ms. Jana Andzhane from the Institute of Chemical Physics of the University of Latvia for the TEM investigations. References [1] J.N. Aneli, G.E. Zaikov, I.M. Khananashvili, J. Appl. Polym. Sci. 74 (1999) 601. [2] X.W. Zhang, Y. Pan, Q. Zheng, X.S. Yi, J. Polym. Sci., B 38 (2000) 2739. [3] N.C. Das, T.K. Chaki, D. Khastgir, Polymer Int. 51 (2002) 156. [4] T.M. Wu, J.C. Cheng, J. Appl. Polym. Sci. 88 (2003) 1022. [5] T.M. Wu, M.C. Yan, J.C. Cheng, Polymer 44 (8) (2003) 2553. [6] D. Staufer, A. Aharony, Introduction in to Percolation Theory, Taylor and Francis, Washington, DC, 1992, p. 198. [7] V.I. Roldughin, V.V. Vysotskii, Prog. Org. Coat. N39 (2000) 81. [8] L. Flandin, A. Chang, S. Nazarenko, A. Hiltner, E. Baer, J. Appl. Polym. Sci. 76 (2000) 894. [9] L. Flandin, Y. Brechet, J.Y. Cavaille, Comput. Sci. Technol. 61 (2001) 895. [10] M. Knite, V. Teteris, B. Polyakov, D. Erts, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 19 (2002) 15. [11] M. Knite, V. Teteris, A. Kiploka, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 23 (2003) 787. [12] M. Knite, Valdis Teteris, Aleksandra Kiploka, Jevgenijs Kaupuzs, Sens. Actuators, A, Phys. 110 (2004) 142. [13] M. Knite, V. Teteris, I. Aulika, H. Kabelka, A. Fuith, Adv. Eng. Mater. 6 (2004) 746. [14] A.A. Farajian, B.I. Yakobson, H. Mizuseki, Y. Kawazoe, Phys. Rev., B 67 (2003) 205423-1. [15] P. Dharap, Z. Li, S. Nagarjaiah, E.V. Barrera, Nanotechnology 15 (2004) 379.
Polyisoprene—multi-wall carbon nanotube composites for sensing strain M. Knite a,⁎, V. Tupureina a , A. Fuith b , J. Zavickis a , V. Teteris a a
Institute of Technical Physics, Riga Technical University, 14/24 Azenes str., LV-1048, Riga, Latvia b Institute of Experimental Physics, University of Vienna, Austria Received 6 May 2006; accepted 24 August 2006 Available online 2 October 2006
Abstract Carbon nanotubes offer attractive possibilities for developing new sensors because of superior mechanical and electrical properties. So far most studies relate the mechanical deformation to the change of nano-scale electrical properties. We present an attempt to use the multi-wall carbon nanotubes (MWCNT) to develop a new material for sensing macro-scale strain. Polymer composites containing dispersed nano-size particles, for example, polyisoprene - multi-wall carbon nanotube composites (PMCNTC) were prepared by treatment of the composite matrix with chloroform providing an increase of mobility and better dispersion of the nano-particles within the matrix. MWCNT with a small amount of solvent was carefully ground in a china pestle before adding to the polyisoprene matrix. Both the polyisoprene matrix solution and concentrated MWCNT product were mixed in a mixer with small glass beads at room temperature for 15 min. The product was dried at 40 °C for over 12 h and vulcanized under high pressure at 160 °C for 20 min. PMCNTC shows attractive tensile and compressive strain sensing properties. A mechanism of sensing effects is being investigated. © 2006 Elsevier B.V. All rights reserved. Keywords: Polyisoprene and carbon nanotube composite; Percolation transition; Electrical properties; Strain sensor materials
1. Introduction Pressure and strain sensors as well as tactile sensors for robots are important in many fields of science and engineering. One of the main limitations of existing conventional sensors is that they are discrete point, fixed directional and not flexible sensors, and are separate from the material or structure that is being monitored. There is a need to develop new flexible largearea sensors that can be embedded into the material and can be used for multiple location sensing. Conductive polymer-composites for strain sensing can be obtained when particles of good conductors (carbon black, graphite powder, carbon fibres, particle of metals) are implanted into an insulating polymer matrix [1–5]. A continuous insulator–conductor transition is observed in two-component systems at gradual increase of the number of randomly dispersed conductor particles in an insulator matrix. Most often such transitions, called percolation transitions, are described by the model of statistical percolation [6,7]. The volume con⁎ Corresponding author. Tel.: +371 7089380; fax: +371 7615765. E-mail address: [email protected] (M. Knite). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2006.08.016
centration of conductor particles VC at which the transition proceeds is called the percolation threshold or critical point. In the vicinity of percolation threshold, electric conductivity of the composite changes as: rfjV −VC jt ; (t—critical indices) [7]. If mechanical deformation of composites occurs, the electric conductivity σ will change. This change is the origin of strain sensing effects. The change of electrical resistance with strain and pressure principally can be explained as a result of destruction of the percolation structure of the conductive particle network. New interesting properties are expected in case the composite contains dispersed nano-size conducting particles [8–13]. Polymer - electro-conductive nanostructure composites (PENC) offer attractive alternatives for developing new generation of flexible large-size sensors because of their superior mechanical and electrical properties. Carbon nanotubes in itself change their electrical properties when subjected to strain. Farijan et al. [14] calculated theoretically the I–V characteristics which showed that the current of metallic tube decreases with increased bending while that of
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Fig. 1. TEM image of multi wall carbon nanotube Aldrich 636835 (a) and high structured carbon black nanoparticle Printex XE2 (b). Scale mark 200 nm.
semiconducting tube increases. Possible application to nanoelectro-mechanical sensors and switches is discussed [14]. An attempt to use the strain sensing capability of single-wall carbon nanotubes (SWCNT) on the nanoscale to develop a strain sensor on the macroscale was made by Dharap et al. [15]. The carbon nanotube film is produced by mixing unpurified SWCTNs with 0.25 mg/ml N,N-dimethylformamide (DMF). The film (10 μm thick) was composed of mechanically entangled randomly oriented nanotube bundles, to which it has isotropic electronic properties [15]. The carbon nanotube film was attached to a brass plate with PVC film between them. A four-point probe was used to measure voltage changes in the carbon nanotube film. The brass specimen was subjected to tension as well as compression cycles. It can be concluded that there is nearly linear relationship between the measured change in voltage and the strains in the carbon nanotube films [15]. The purpose of our paper was the design, elaboration and investigation of the polyisoprene and multiwall carbon nanotube (MWCNT) composites for application in strain sensors as well as to compare them with the polyisoprene and high structure carbon black composites elaborated and prepared by the same technology.
(MWCNT) Aldrich 636835 as well as highly structured carbon black (HSCB) Printex XE2, were prepared by “solution method” as follows. The matrix composition was treated with chloroform providing (1) an increase of the nano-particles mobility and (2) better dispersion of the nano-particles within the matrix. Prepared matrix composition was allowed to stand for swelling ~ 24 h. Nano-size carbon black is carefully grinded with a small amount of solvent in a china pestle before addition to the polyisoprene matrix. Polyisoprene matrix solution and nano-size carbon structure concentrated product was mixed in the mixer with small glass beads at room temperature for 15 min. Product is poured out into little aluminium foil box and stand ∼ 24 h, dried at 40 °C for more than 12 h and vulcanized under high pressure at 160 °C for 20 min. The size of MWCNT: OD = 60–100 nm, ID = 5–10 nm, length = 0.5–500 μm, BET surface area: 40–300 m2/g. A TEM image (Fig. 1) shows the entangled structure of MWCNT. The average particle size of HSCB is 30 nm and its dibutyl phthalate (DBP) absorption is 380 ml/100 g. Its surface area is 950 m2/g. ATEM image (Fig. 2) shows the high structure of Printex XE. A Philips TEM-301 transmission electron microscope equipped with Keen View II CCD camera was used for nanoscopic investigation of fillers. To study deformation at stretch and dependence of electrical resistance of composites on the stretching force, samples of the size of 90 mm × 15 mm × 1.5 mm were cut from the vulcanized sheets with HSCB and samples of the size of 60 mm × 11.5 mm × 2.4 mm from the vulcanized sheets with MWCNT, respectively. Copper foil electrodes were glued on both sides at the ends and each pair of electrodes was short-circuited by copper wiring. On a relaxed sample the distance between electrodes l0 was 50 mm for HSCB and 25 mm for MWCNT, respectively. Sandpaper was glued to the electrodes to fasten the samples in the stretching machine (extensometer). Schematic illustration of the experimental set-up for measurements of the electrical resistance R of the nano-composite as a function of the tensile force is available in [12]. The KEITHLEY Model 6487
2. Samples and experimental procedure Polyisoprene composites containing dispersed nano-size particles, in our case two types multiwall carbon nanotubes
Fig. 2. The electric resistivity ρ of the polyisoprene-MWCNT composite and polyisoprene - HSCB composite vs. concentration (mass parts) of corresponding filler (percolation curves).
M. Knite et al. / Materials Science and Engineering C 27 (2007) 1125–1128
Fig. 3. The electric resistivity ρ of the polyisoprene - MWCNT composite and polyisoprene - HSCB composite vs. small tensile strain ε.
Picoammeter/Voltage Source was used for electrical resistance measurements. The tensoresistance effect was tested at least three times on each of the three samples of each separately prepared composition. The relative standard deviation of resistance measurements was about 20% at strain velocity of 6.67 × 10− 5 m/s. Dispersion of nano-particles in the polymer matrix gives the main contribution in this error. Error of the instrument at strain measurements was 0.1 mm. 3. Results and discussion To estimate roughly the percolation threshold, a series of polyisoprene-carbon nanotube composites (PCNC) were prepared with 6.1, 8.3, 10.9, 12.1 and 14.5 mass parts (m.p.) of MWCNT added to 100 m.p. of polyisoprene. The polyisoprenecarbon black composites (PCBC) with 6.6, 7.7, 8.8, 9.35, 9.9 and 11 mass parts (m.p.) of HSCB added to 100 m.p. of polyisoprene were prepared for confrontation with PCNC using the same preparing procedure. The 8.3 m.p. PCNC composite and 8.8 m.p. PCBC composite appeared to be within the region of percolation threshold (Fig. 2). Samples with 6.6, 7.7 and 11 mass parts of HSCB as well as samples with 6.05, 12.1 and 14.5 mass parts of MWCNT were obviously outside the region of percolation threshold. The samples with 6.6 and 7.7 m.p. of CB were practically insulators, while the samples with 9.9 and 11 m.p. of CB - good conductors. The percolation threshold in case of MWCNT composites was not so sharp as in the case of HSCB composites. For HSCB composites prepared by “solution” method the percolation threshold is slightly shifted to lower values of HSCB concentration than for HSCB composites made by “rolling in” method used in our previous investigations (the composite was made by rolling highly structured nano-size carbon black (Printex XE2, DEGUSSA AG) and the necessary additional ingredients (S, ZnO) into a polyisoprene matrix and vulcanizing under high pressure at 140 °C for 15 min) [12]. This difference can be explained due to better dispersion of HSCB in polyisoprene matrix by “solution” method.
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Furthermore, resistance R of the composites was examined with regard to the force of stretch F and the absolute mechanical deformation Δl in the direction of the force. Results were ranged in two groups — resistivity changes at small deformation and at large deformation respectively. No variation in electric resistance was found for the samples with 7.7 mass parts of HSCB and 3.63 mass of MWCNT parts even at 100% deformation of the composite (Fig. 3). Similarly, there was a relatively weak dependence of R on the deformation at 9.35 mass parts of HSCB (Fig. 3). These results are consistent with our predictions that with too high or too low concentrations of the filler one must expect weak tensoresistance effect or no such an effect at all. The behaviour was essentially different near the percolation threshold. Of all the composites examined, the best results were obtained for the samples with 8.8 mass parts of HSCB and 8.3 mass parts of MWCNT, respectively, which apparently belonged to the region of percolation threshold. The resistivity of these samples changed more than 6 orders upon a 40% stretch for 8.8 mass parts HSCB composite and more than 4 orders for 8.3 mass parts MWCNT composite. After the samples were released, the resistivity of HSCB composite practically returned to its previous value, while in case of MWCNT composite a relatively large hysteresis of ρ(ε) was observed that was explained because of the entangled structure of MWCNT (Fig. 1a) as well as due to high stiffness of nanotubes. The reversible change in electric resistivity of high structure carbon nanoparticle - polyisoprene composite even at large stretch can be explained by the high porosity of carbon agglomerates providing better adhesion to polymeric globules and mobility of nanoparticles, which is not observed in the case of a poorly structured carbon black. Electrical resistivity response to small strain for different polyisoprene nanocomposites is shown in Fig. 4. In this case the resistivity changes in all composites are practically reversible. It was also proved that the model of tunneling currents developed in [12] quite well describes the experimental data at small deformations for both HSCB and MWCNT polyisoprene composites.
Fig. 4. The electric resistivity ρ of the polyisoprene - MWCNT composite and polyisoprene - HSCB composite vs. large tensile strain ε.
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4. Conclusions The use of high structure carbon nanoparticle as filler of a polyisoprene matrix provided about 6 orders of magnitude of reversible change in the electric resistivity at large (40%) stretch. The use of multiwalled carbon nanotube as filler of a polyisoprene matrix provided about 4 orders of magnitude of nonreversible change in the electric resistivity at large (40%) stretch. The maximum change at stretch for both type of composites were observed near the percolation threshold. The reversible change in electric resistivity of high structure carbon nanoparticle - polyisoprene composite even at large stretch can be explained by the high porosity of carbon agglomerates providing better adhesion to polymeric globules and mobility of nanoparticles, which is not observed in the case of a poorly structured carbon black. The non-reversible change in electric resistivity of multiwalled carbon nanotube - polyisoprene composite at large stretch we explain with entangled structure of nanotube. Multiwalled carbon nanotube-polyisoprene composite can be used for small tensile strain sensing but high structure carbon black - polyisoprene composite are preferable for large tensile strain sensing. Acknowledgements The research was supported by the Council of Science of Latvia and by Latvian National Research Program in Materials
Science. The authors are thankful to Dr. Donats Erts and Ms. Jana Andzhane from the Institute of Chemical Physics of the University of Latvia for the TEM investigations. References [1] J.N. Aneli, G.E. Zaikov, I.M. Khananashvili, J. Appl. Polym. Sci. 74 (1999) 601. [2] X.W. Zhang, Y. Pan, Q. Zheng, X.S. Yi, J. Polym. Sci., B 38 (2000) 2739. [3] N.C. Das, T.K. Chaki, D. Khastgir, Polymer Int. 51 (2002) 156. [4] T.M. Wu, J.C. Cheng, J. Appl. Polym. Sci. 88 (2003) 1022. [5] T.M. Wu, M.C. Yan, J.C. Cheng, Polymer 44 (8) (2003) 2553. [6] D. Staufer, A. Aharony, Introduction in to Percolation Theory, Taylor and Francis, Washington, DC, 1992, p. 198. [7] V.I. Roldughin, V.V. Vysotskii, Prog. Org. Coat. N39 (2000) 81. [8] L. Flandin, A. Chang, S. Nazarenko, A. Hiltner, E. Baer, J. Appl. Polym. Sci. 76 (2000) 894. [9] L. Flandin, Y. Brechet, J.Y. Cavaille, Comput. Sci. Technol. 61 (2001) 895. [10] M. Knite, V. Teteris, B. Polyakov, D. Erts, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 19 (2002) 15. [11] M. Knite, V. Teteris, A. Kiploka, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 23 (2003) 787. [12] M. Knite, Valdis Teteris, Aleksandra Kiploka, Jevgenijs Kaupuzs, Sens. Actuators, A, Phys. 110 (2004) 142. [13] M. Knite, V. Teteris, I. Aulika, H. Kabelka, A. Fuith, Adv. Eng. Mater. 6 (2004) 746. [14] A.A. Farajian, B.I. Yakobson, H. Mizuseki, Y. Kawazoe, Phys. Rev., B 67 (2003) 205423-1. [15] P. Dharap, Z. Li, S. Nagarjaiah, E.V. Barrera, Nanotechnology 15 (2004) 379.