Electrical conductivity and humidity sensing properties of PVA/ nanocomposites Mohammadmehdi Aghelinejad and Siu N. Leung, Lassonde School of Engineering, Department of Mechanical Engineering, York University, Toronto, ON, Canada Abstract In this study, multi-walled carbon nanotubes (MW)/polyvinyl alcohol (PVA) nanocomposite films were fabricated and characterized. The effect of MW loadings in the insulating PVA matrix and the influence of relative humidity on the electrical conductivity of the nanocomposite were investigated. The results of different measuring techniques were also characterized and compared together. Experimental results of this research revealed that the electrical conductivity of 1 wt.% MW/PVA nanocomposites increases up to six orders of magnitude by increasing the relative humidity from 0 to 80 percent. This characteristic demonstrates the potential of using MW/PVA nanocomposite thin films in moisture sensing applications. Introduction Polymer nanocomposites have attracted much attention during the last decades in microelectronic industry and sensor applications [1]. In addition to typical advantages of polymers (Such as light-weight, low cost, and good processability), the promotion of electrical properties (e.g., electrical conductivity) by the inclusion of a small amount of conductive fillers into polymer matrices have made polymer nanocomposites versatile multifunctional materials. Among different electrically conductive fillers, carbon nanotubes () represent one of the best candidates for manufacturing conductive polymeric nanocomposites because of their excellent electric properties. With an extremely high aspect ratio, uniform dispersion of a small amount of can produce an efficient conductive network throughout the insulating polymer matrix. Many studies have been devoted to investigate the electrical properties of polymeric composites and nanocomposites with multiwalled carbon nanotubes (MW) or single-walled carbon nanotubes (SW). Aguilar et al. [2] studied the effect of dispersion state (agglomerated vs. uniformly dispersed) on the electrical conductivity of /polymer composite films. They revealed that clustering of, especially for contents close to the percolation threshold, would promote the electrical conductivity of /polymer nanocomposite films. Li et al. [3] conducted experimental studies on electrical properties of MW/epoxy nanocomposites. They successfully prepared electrically conductive polymer nanocomposites with a percolation threshold of as low as 0.05 vol.% chemically modified. Recently, water soluble polyvinyl alcohol (PVA) has brought extensive attention for manufacturing polymeric nanocomposites because of their good processability. Nasr et al. [4] showed that the DC electrical conductivity of MW/PVA nanocomposites increased by several orders of magnitude with the introduction of the conductive nanofiller into the PVA matrix. Some research also indicated humidity dependency of polymer nanocomposites electrical properties. Tang et al. [5] demonstrated the humidity sensitivity of polyimide/mw nanocomposite films. Their results indicated that electrical resistance of this type of composite films increased with relative humidity. They reported a fast and linear response of nanocomposite samples containing 3 wt.% MWNT with relative humidity. Another research by Fei et al. [6] demonstrated the potential of acid treated s/pva nanocomposite films for switching sensor applications. According to their results, resistance of the s-cooh/pva nanocomposite sharply increased when relative humidity went higher than 80%. Ogura et al. [7] synthetized composite films consisted of soluble PAn and PVA with a low percolation threshold of 0.1%. They reported the change in the humidity dependence of the electrical conductivity for nanocomposite prepared with different PAn to PVA ratios. According to their result, the electrical conductivity of Pan-PVA composite films increased about four orders of magnitude by increasing the relative humidity from 0 to 100 percent. In the present paper, we study the effect of humidity on the electrical conductivity of MW/PVA nanocomposites. Nanocomposite films were prepared with different filler content (1, 5, 10, and 20 wt.%) by solution casting method. The electrical conductivity of specimens was measured after moisturizing them in different levels of relative humidity (0, 60, and 80 %RH). Materials Experimental PVA polymer matrix (Average M w = 146000-186000 g mol -1 and 87-89% hydrolyzed) was purchased from Sigma-Aldrich. MW (MW, 1.0 wt.% aqueous dispersed, AQ0101) was provided by Nanocyl. Deionized water was used to prepare the PVA solutions for the fabrication of MW/PVA nanocomposites. Physical properties of MW and PVA are summarized in Tables 1 and 2. SPE ANTEC Indianapolis 2016 / 596
Table 1. Physical properties of aqueous MW solution Grade AQ0101 Description 1.0 wt.% loading MW in deionized water with surfactant ph 7-11 Boiling point 100 C Melting point 0 C Table 2. Physical properties of PVA powders Product No. 363103 Hydrolysis 87.0-89.0% Viscosity 45.0-55.0 cps ph 4.5-6.5 Preparation of MW/PVA nanocomposites PVA powders were gradually dissolved in deionized water at 80 C and mechanically stirred for 2 hours at 450 rpm to obtain a uniform solution. The MW solution was then added to the PVA solution and mechanically stirred for another hour at 1000 rpm at room temperature (23 C). After that, the mixed solution was sonicated in an ice bath for 2 hours to uniformly disperse MW in the PVA solution and to prepare a stable solution for the subsequent solution casting process. The final homogeneous solution was casted into petri dish and dried in fume hood at room temperature for 48 hours. Nanocomposite samples were prepared with four different loadings of MW (i.e., 1, 5, 10, and 20 wt.%) to study the effects of filler loading and relative humidity on the nanocomposite s electrical conductivity. Prior to measuring the electrical conductivity of a moisturized sample, the nanocomposite film was dried in an oven at 60 C for 15 hours (This condition is considered as 0% relative humidity). To moisturize the nanocomposite samples, the samples were put into a humidity controlled chamber preset at different percentage of relative humidity (%RH) at 23 C. In order to study the humidity dependence of MW/PVA nanocomposites electrical properties, their electrical conductivities were measured in three levels of relative humidity (i.e., 0, 60, and 80 %RH). All samples were moisturized at each level of %RH for 15 hours before conducting the measurements. Characterization of MW/PVA nanocomposites Dispersion of MW in the PVA matrix and the nanocomposite morphology were observed by scanning electron microscopy (SEM: FEI Company, Quanta 3D FEG). Cross sections of all nanocomposite samples were exposed by cryo-fracturing the films under liquid nitrogen. Transmission electron microscopy (TEM: Philips EM201 Electron Microscope - Used at 80KV) was also used to investigate the MW networks in the PVA matrix. To prepare for the TEM analysis, samples were impregnated in epoxy resin to properly cut the sections by microtoming. The electrical conductivities of all nanocomposite samples were measured by two methods (i.e., two-wire and four-point probe methods [8]) using a multifunctional source meter (Keithley, Model 2450 SourceMeter). For the four-point probe method, a four-point resistivity probing fixture (Signatone probe S-302-4 with a SP4 probe head) was used. The relative humidity in the moisturizing chamber was controlled by a humidity controller with an accuracy of ±5%RH. The two-wire technique was performed in accordance to the ASTM D257-07 standard. By printing conductive silver-epoxy paste as an electrode on the sample s surface, the voltage difference was introduced through the sample and the flowing current was recorded to calculate the conductivity of the nanocomposite film. The four-point probe measuring method was conducted with the aid of four-point collinear probe technique in accordance to the ASTM F84-02 guideline. By sourcing the current through the outer probes and reading the voltage across the inner probes, the surface conductivities of samples were calculated. The sample s size and thickness correction factors were also taken into consideration when calculating the film s conductivity. In order to increase the measurement accuracy and compare the results, the silver paste electrode was painted on sample in two different configurations. In the first test specimen, electrodes were painted at the two ends of a rectangular specimen, and the current flowed through the surface to measure surface conductivity. In the second one, electrodes were painted on both sides of the sample and the current was introduced across the thickness direction of the sample to obtain its bulk conductivity. A study conducted by Almuhamed et al. [9] revealed that the electrical conductivities of specimens depended on the pressure being applied during the measurement. Therefore, in order to eliminate measurement errors arising from the pressure of probes on the specimen s surface, conducting wires were attached to the conductive paste on specimen s surface. Results and Discussion Morphology of MW/PVA Nanocomposites SEM and TEM imaging techniques were conducted to investigate the dispersion of MW in the fabricated nanocomposites. Figures 1(a) and (b) show SEM micrographs, at two levels of magnification, that illustrate the dispersion of MW in MW/PVA nanocomposites loaded with 20 wt.% MW. It depicts a uniform dispersion of MW throughout the SPE ANTEC Indianapolis 2016 / 597
nanocomposite. Figures 2(a) and (b) illustrates TEM micrographs that show the dispersion and network of MW in the PVA matrix loaded with 5 wt.% and 20 wt.% of MW, respectively. Figure 2(a) clearly demonstrates scarce and disconnected networks of MWs throughout the insulating PVA matrix. In contrast, Figure 2(b) shows that a high degree of interconnectivity of MW was achieved in the nanocomposite filled with 20 wt.% MW. In conclusion, the SEM and TEM micrographs confirm the efficient processing of nanocomposites and uniform dispersion of MW were achieved. (a) (a) (b) Figure 2. TEM micrographs of MW/PVA nanocomposites loaded with: (a) 5 wt.% MW (b) 20 wt.% MW Electrical conductivity composites (b) Figure 1. SEM micrographs of MW/PVA nanocomposite films loaded with 20 wt.% MW at two different magnifications of MW/PVA nano- The test specimens loaded with different amount of MW possessed different properties that required different characterization techniques to promote the measurement accuracy. In particular, the 4-point probe method is more accurate (especially for high conductive materials) as the lead and contact resistances would not affect the measurement. However, for nanocomposite samples with very low electrical conductivity, this method required higher ranges of voltage and very low values of sourcing current that go beyond the measuring limits of the equipment. Furthermore, when measuring resistance SPE ANTEC Indianapolis 2016 / 598
of very soft or fragile samples such as moisturized thin films and brittle 20 wt.% samples, the probes would easily damage the surface of the samples, and thereby resulted in major measurement errors. The 2-wire method, on the other hand, provides more stable results for highly resistive samples and would cause less damage to the surface of soft films. In measuring surface resistivity of nanocomposite films, there are several other issues that may adversely affect the accuracy and repeatability of results. The electrification time and the voltage range are the most important parameters in measuring conductivity of moderately conductive materials. The electrification time, in this study, for all of measurements was 2 minutes. Tables 3 and 4 summarize the apparent values of electrical conductivity from different MW/PVA nanocomposites obtained by different measurement methods. The source current for the 4-point method and the source voltage for the 2-point method are indicated with results. It is apparent that measurements obtained by different methods had consistent trends. For the bulk electrical conductivity, experimental measurements of the nanocomposites electrical conductivity through the specimen s thickness were different from the results yielded by the other measurement methods. The fourpoint probe and two-point (through surface) methods measured the surface electrical conductivity of the samples while, the two-point (through thickness) method measured the bulk electrical conductivity of samples. As a result the later method is believed to be more accurate in measuring bulk electrical conductivity of specimens. Table 3. Surface electrical conductivity of fully dried nanocomposites (S/square) MW loadings 4-Point method 2-point method 1 wt.% 5 wt.% 10 wt.% 20 wt.% 1.1E-12 1.2E-12 1.8E-10 (10 na) 5.3E-7 (100 na) 1.3E-12 1.2E-12 5.1E-11 1.9E-6 Figure 3. Effect of MW loadings on the surface electrical conductivity of MW/PVA nanocomposites using four-point probe and two-point (through surface) measuring techniques Table 4. Bulk electrical conductivity of fully dried nanocomposites (S/cm) Method 1wt.% 5wt.% 10wt.% 20wt.% 4-point 2-point surface 2-point thickness 5.5E-11 3.9E-11 1.3E-13 4.3E-11 4.1E-11 9.6E-13 7.1E-9 (10 na) 2.0E-9 (10 V) 7.7E-12 1.3E-5 (0.1 µa) 7.6E-5 (10 V) 6.3E-7 Figures 3 and 4 plot the surface and bulk electrical conductivity, measured by different methods, as a function of MW loadings. It can be observed that both the surface and bulk electrical conductivity increased when the amount of MW increased. This phenomenon can be described by the presence of more MW conductive networks, and thereby a higher number of conductive paths throughout the insulating PVA matrix. Figure 4. Effect of MW loadings on the bulk electrical conductivity of MW/PVA nanocomposites using four-point probe, two-point (through surface), and two-point (through thickness) measuring techniques The effect of relative humidity level on MW/PVA nanocomposite s electrical conductivity is plotted in Figures 5 and 6. It can be seen that the humidity SPE ANTEC Indianapolis 2016 / 599
has a significant influence on both the surface and bulk conductivity of MW/PVA nanocomposites loaded with different amount of MW. The electrical conductivity of 1 wt.% MW/PVA nanocomposite increased about six orders of magnitude with the relative humidity change from 0 to 80 percent. This nanocomposite offers higher humidity sensitivity than reported by Fei et al. [6]. By increasing the relative humidity, moisture absorption into the nanocomposite surface increases. Ionic nature of water molecules facilitates electron s mobility through nanocomposite resulting in more conductive materials. Figure 5. Relationship between surface electrical conductivity of nanocomposites with different MW loadings and relative humidity (2-points through surface measurement method) Figure 6. Relationship between bulk electrical conductivity of nanocomposites with different loadings and relative humidity (2-points through thickness measurement method) Experimental results also illustrate that the enhancement in the electric conductivity of MW/ PVA nanocomposite films was more pronounced for samples with lower MW contents. By increasing the MW loadings, highly interconnected and tight MW networks presented in the nanocomposites. Therefore, there would be smaller free volume in the bulk of the materials for penetration of water molecules into amorphous PVA matrix [10]. This resulted in a lower degree of enhancement in the nanocomposite s electrical conductivity. Furthermore, the electrical conductivity of specimens with lower filler content exhibited a more linear relationship with the relative humidity. Conclusion The effects of MW contents and humidity on the electrical conductivity of MW/PVA were investigated. By increasing the MW loading from 1wt.% to 20 wt.%, the electrical conductivity of the nanocomposite significantly improved. The results also revealed that the electrical conductivity of nanocomposite dramatically increased up to six orders of magnitude with increasing the relative humidity of the environment from 0 to 80 percent. As a result, the MW/PVA nanocomposites changes from insulative to moderately conductive material. The sensitivity of PVA/MW nanocomposites with lower filler content is more linear and significant which indicates a promising potential for humidity sensor applications. Acknowledgement The authors are grateful about the financial support by the Natural Sciences and Engineering Research Council of Canada. References 1. C. Min, X. Shen, Z. Shi, L. Chen, and Z. Xu, Polymer-Plastics Technology and Engineering, 12, 49 (2010). 2. J. Aguilar, J. Bautista, and F. Avilés, express Polymer Letters, 5, 4 (2010). 3. J. Li, Z.H. Zhu, and S. Gong, The 19th International Conference on Composite Materials 4. G. Nasr, A. El, A. Klingner, A. Alnozahy, and M. Mourad, J. of Multidisciplinary Eng. Sci. Technol., 5, 2 (2015). 5. Q. Tang, Y. Chan, and K. Zhang, Sensors and Actuators B: Chemical, 1, 152 (2011). 6. T. Fei, K. Jiang, F, Jiang, R. Mu, and T. Zhang, Journal of Applied Polymer Science, 1, 131 (2014). 7. K. Ogura, T. Saino, M. Nakayama, and H. Shiigi, Journal of Materials Chemistry, 12, 7 (1997). 8. ASTM Standard F 43-99 (1999). 9. S. Almuhamed, N. Khenoussi, L. Schacher, D. Adolphe, and H. Balard, Journal of Nanomaterials, (2012). 10. Y. Shirazi and T. Mohammadi, Separation Sci. and Tech., 5, 48 (2013). SPE ANTEC Indianapolis 2016 / 600