Impact of MW-CNT/polymer composites matrix type on the electrical and gas-sensitive properties

Abstract

In this paper, polyurethane/MW-CNT, silicone/MW-CNT, and epoxy/MW-CNT nanocomposites were prepared, and their electrical and gas-sensitive properties were investigated. The aerosol-assisted chemical vapor deposition method was used to synthesize MW-CNTs from acetonitrile as a carbon source. These nanocomposites were prepared by an irreversible dispersion method that was developed by our group. SEM analysis results proved that smooth-surfaced, less defective MW-CNTs with 30–60 nm diameter and 60-50 μm length were synthesized successfully. The electrical conductivity of the prepared nanocomposites reveals the correspondence between the degree of uniformity of distribution of MW-CNTs inside polymers and the electrical conductivity of nanocomposites. The electrical conductivity graphs for Epoxy/x MW-CNTs and PU/x MW-CNTs nanocomposites have the same shape, and there is a sharp increase in electrical conductivity from 4% of MW-CNTs, but the values are different: Epoxy/x MW-CNTs (x = 8%) (1000 S/m) and PU/x MW-CNTs (x = 8%) (24.39 S/m). The electrical conductivity graph of silicone/x MW-CNT has a different shape, and percolation began at 2% of MW-CNT and increased sharply till 4%, then there was observed saturation. These results proved that the nanocomposite’s electrical conductivity properties depend on the polymer matrix nature. Simultaneously, the gas-sensitive properties of these nanocomposites were discovered. Thus, it was determined that the highest resistance change was observed for PVAc/x MW-CNT (x = 4%) nanocomposite under CO gas. Except for PVAl/x-MW-CNT (x = 4%), other nanocomposites show a gas-sensitive effect depending on gas types. Moreover, the resistance of all nanocomposites decreases with increasing temperature (from 20 to 120°C) and their behavior as semiconductors. However, the shapes of the graphs of the resistance depending on temperature are different depending on the nanocomposites, and their values are also different, some of them in the Om, KOm, and MOm ranges.

Keywords

Multi-walled carbon nanotubes aerosol-chemical vapor deposition electrically conductive polymer composites irreversible dispersion method gas-sensitive elements

Introduction

The scientific community has been captivated by the development of novel conductive polymer composite materials (CPCs) with specified and fundamentally new properties as a result of the addition of nanometer-scale components.^1–3 The inherently insulating polymers are cheaper in comparison to electrically conductive polymers. Conductive polymers have wide application areas such as electronics and energy storage devices, corrosion inhibitors, pollutant degradation, electromagnetic interference shielding materials, and sensors.^4–9 However, naturally electrically conductive polymers are not stable and their synthesis process is highly costly, which limits their application to large–scale industrial applications. Therefore, obtaining conductive polymers using cheaper insulating polymers with the addition of conductive nano-fillers is the most convenient method.¹⁰ In addition, inherently conductive polymers do not possess sensor effects. However, in our research work, the main goal for the creation of electrically conductive polymer composites was their utilization as gas-sensitive elements.

As an additive material in polymer composites, carbon nanotubes (CNTs) play an important role, owing to their size, shape, surface area, ultralight weight, and unique properties (thermal and electrical conductivity, mechanical, chemical, and thermal stability).^11,12 CNTs provide a highly conductive phase (long-range conductive pathways throughout the matrix) to be integrated into the polymeric matrix of the composite, as well as it is possible to get high electrical conductivity at lower loadings of CNTs due to their emission of electrons at low voltages because of sharp edges in comparison with other reinforcements with spherical or irregularly shaped forms. CNT-filled polymer-based composites have immense potential for technological high-end applications, for instance, gas sensing, antistatic shielding, electronic and food packaging, electromagnetic radiation shielding, energy storage devices, as well as in the electronics and aerospace industries.^13–16

In recent years, hazardous gas sensors and actuators have been attracting more attention in terms of human safety in both domestic and industrial settings.^17,18 Particularly, gases such as methane CH₄ and carbon monoxide CO are harmful to the environment and humanity. The development of gauges for detecting parts per million (ppm) levels of these gases far below their permissible exposure limits is very necessary. As a result, developing compact, portable, affordable, high-effective, and high-selective gas sensors capable of rapidly detecting harmful gas analytes and monitoring air quality in both ambient and indoor locations in real-time is an urgent and sophisticated task.¹⁹

To date, the manufacture and study of active sensitive elements in sensor devices for the rapid and selective detection of various gases is the most urgent task. In the present article, we report on the investigation of highly effective gas-sensitive and highly electrically conductive MW-CNTs/polymer composites. In the literature, there are a number of works devoted to the production and study of sensitive elements for gas sensors based on different carbon nanomaterials, mostly modified nanomaterials, and various polymer matrices for determining these gases.^20–24

Moreover, there are still challenges in the field of gas sensors (sensitivity, selectivity, etc.) that should be solved. Therefore, this research work is devoted to the solvation of these problems as well as the fabrication and investigation of flexible active-sensitive elements based on MWCNT/silicone and MWCNT/polyurethane nanocomposites for these applications. The sensory effects (change of resistance) of the prepared composite thin films of Epoxy/xMW-CNTs, Silicone/xMW-CNTs, and PU/xMW-CNTs (d = 200 m, a = 60 mm, b = 12 mm) were studied for three-combustible (methane, propane), and toxic (carbon monoxide) gases. The used multi-walled carbon nanotubes were grown by aerosol-assisted chemical vapor deposition (A-CVD) method using acetonitrile as a carbon source. MW-CNTs/polymer nanocomposites were prepared by the irreversible dispersion method which was developed by our group in our previous research work.^18,19 SEM analysis was performed in order to reveal the morphology of the synthesized MW-CNTs and fabricated nanocomposites. The electrical conductivity of the prepared nanocomposites was significantly increased using the new optimized irreversible dispersion method in comparison to other methods. The gas sensitivity properties of these nanocomposites were also measured for investigation of their application abilities in the fabrication of gas sensors. Under CO gases, all nanocomposite thin films showed highly sensitive effects depending on the concentration of MW-CNTs. All the prepared nanocomposites show gas-sensitivity effects in the Ohm range.

Experimental

MW-CNTs synthesis process

All reagents used in this work were of analytical grade and utilized deprived of any pre-treatment. Acetonitrile – ACN (99%) and ferrocene, Fc (98%) were procured from Sigma - Aldrich, Chemie GmbH, Germany. Ar (99.999%) and H₂ (99.999%) were purchased from Farafan Gas Co, Tehran, Iran. Acetonitrile was used as a carbon source for the synthesis of MW-CNTs. Ferrocene was used as a precursor for the growth of MW-CNTs. Ferrocene was mixed with ACN in a ratio of 20 mg/mL (optimal concentration). Based upon a series of our experiments, the optimal synthesis temperature was determined to be 950°C. The MW-CNT synthesis was carried out at ambient pressure with the duration of the ACVD process of 85 min. The growth of MW-CNTs was carried out by the aerosol-assisted chemical vapor deposition (ACVD) method on the experimental laboratory setup at Scientific Instruments Dresden GMBH, SCIDRE, Germany in a horizontal laminar reactor (a cylindrical tube of quartz glass with the following sizes: length = 1 m, internal diameter = 33 mm) of an aerosol CVD set up Figure 1.

The reactor was covered by an electric furnace (length = 0.35 m) of the apparatus. For uniform deposition of carbon nanotubes during the synthesis, the electric furnace was automatically moved along the length of the reactor at a fixed rate of 10 mm/min. The detailed synthesis procedure was described in.²⁵

Purification of synthesized multi-walled carbon nanotubes from polyaromatic and tarry impurity compounds was pursued by calcination in an argon flow (flow rate of 500 mL/min) at a temperature of 900°C for 40 min. After the completion of the reaction, the reactor was allowed to cool overnight in an argon stream. The collection of synthesis products from the inner walls of the cooled quartz tube was implemented mechanically.

Preparation of electrically conductive polymer/MW-CNTs composites

For the preparation of polymer/MW-CNT composites, different types of dispersion techniques were applied. Thus far, sonication, use of surfactants, high shear mixing, and physical and chemical functionalization of the outside wall of the tubes were used. Researchers face the following main challenges while CNTs are incorporated into a polymer matrix: agglomeration (strong attractive Van der Waal forces between individual nanotubes) and restricted dispersion of CNTs; and weak interfacial adhesion between nanotubes and the polymer matrix.^7,8 Therefore, choosing the most effective technique to achieve uniform dispersion of carbon nanotubes within the host matrix is an important prerequisite. In this research work, Polyurethane (PUR 566, Kleiberit (Klebchemie) Germany), silicone (FAB Company, Azerbaijan), and epoxy resin (Spolrk Pro Chemiskou AHutni Vyroby, Сzech Republic) were picked up as polymer matrix for the fabrication of nanocomposites.

The required amount of polymer resin and dispersible samples of MW-CNTs were weighed into a beaker. For the preparation of nanocomposites for each nanocarbon polymer composite, the percentage of added MW-CNTs was calculated based on the weight of the dry polymer. In the case of nanocomposites based on epoxy, the ratio of hardener to epoxy was 1:10. For the preparation of PU-based nanocomposites, the viscosity of PU was reduced by adding the required amount of toluene. MW-CNTs were introduced into polymers at 2, 4, and 8 wt % concentrations. For the preparation of nanocomposites, the irreversible dispersion method of MW-CNTs was used, which was successfully developed by our research group in our previous research work.²⁶

Based on a pasty (semi-liquid) composite, two types of samples were prepared: a bulk nanocomposite for measuring bulk electrical conductivity; and a thin film to study the gas sensor effect. In the first case, a semi-liquid composite was applied with a spatula into a rubber mold with dimensions of 10 mm × 10 mm and a thickness of 1 mm and dried at 70°C for 8 hours. In the second case, for thin-film samples, the prepared semi-liquid composite was applied with a spatula onto a paper substrate measuring 60 mm × 12 mm × 200 m, which was glued to a glass plate of the same size. Photos of the prepared samples are shown in Figure 2.

Gas sensor effect (alteration in the resistance) -measurements of polymer/x MW-CNTs (x = 2; 4; 8%) composites were implemented upon exposure to CO and CH₄ gases (99.9% pure) and propane (98.5%). Carbon monoxide gas (99.9% purity) was produced in a laboratory by virtue of the reaction of sulfuric acid with formic acid and subsequent drying with silica gel. About 99% pure methane gas was extracted from the gas network (Baku City, Azerbaijan). Propane gas (98.5% purity, 1.4% butane admixture) for home use, gas in a cylinder from a gas station (Baku City, Azerbaijan).

Characterization techniques

Analysis of morphological features of the surfaces of synthesized multi-walled carbon nanotube samples as well as prepared nanocomposite specimens was carried out using scanning electron microscopes, SEM (ZEISS). The SEM images were acquired at room temperature with an operating voltage of 10 kV and at various incremental magnifications. The current–voltage (I–V) characteristics of the nanocomposites were measured by a classical two-contact method. For testing, the samples were connected to a linear direct current power supply (DC Power Supply, Mastech Company, HY3005 series, China). This precision laboratory power supply has a smooth adjustment of the output voltage (U = 0–30 V) and an output current limitation (I = 0–5 A), the values of which are monitored on 3-digit indicators. For measuring small currents, microampere meters and milliampere meters (ChiSFJ) have been put on the circuits. To measure the conductivity of prepared composites, copper contacts were glued to the samples with the silver-containing paste “Contactol” (Keller, Germany) and measured in the range of 1–10 V (1, 2, 4, 8, and 16 V). As a consequence of the measurements, I–V characteristics of composite materials were obtained. The gas-sensor effect was measured using electrical conductivity measurement equipment. Prototypes of an active sensing element based on prepared nanocomposites were placed in a glass tube for investigation of gas-sensitive effects. Sensing performance was first tested in air, whereafter measurements of electrical resistance were carried out in the methane, propane, and carbon monoxide gas environments.

Results and discussion

SEM analysis of the synthesized MW-CNTs and the prepared nanocomposites

An SEM image of the synthesized MW-CNTs is shown in Figure 3. From the picture, it is clear that synthesized MW-CNTs from acetonitrile are in a threadlike shape and arranged in a parallel manner. The diameter of the synthesized MW-CNTs changes in the range of (30-60) nm, and the average length are in 50–60 microns (Figure 3). Their surfaces are very smooth, with fewer defects.

SEM micrographs of the dispersed MW-CNTs inside epoxy, polyurethane, and silicone polymers are shown in Figure 4(a–c). Figure 4 shows that using the irreversible dispersion method of MW-CNTs inside polymers, it is possible to obtain homogenously dispersed of MW-CNTs inside polymers without agglomeration, and the best dispersion was observed inside silicone matrices. Thus, the type of matrices directly impacts the dispersion ability of MW-CNTs even though the same method was used for the preparation of nanocomposites.

Because of the homogeneous dispersion of MW-CNTs, continuous electrically conductive chains were formed inside polymers. The high dispersion observed for Silicone/x MW-CNT and Epoxy/xMW-CNT nanocomposites, thus, the electrical conductivity of silicon/x MW-CNT and epoxy/x MW-CNT nanocomposites is expected to be higher than that of the PU/x MW-CNT nanocomposite (Figure 4(a–c)).

Electrical conductivity of the nanocomposites

For all the prepared nanocomposites, the current-voltage characteristics were measured, and the resulting plots are displayed in Figures 5–7. For the epoxy/x MW-CNT (x = 2%; x = 4%) polymer nanocomposites, with an increase in the voltage value in the range of 2–16 V, a slight increase in electrical current is observed. For the epoxy/x MW-CNT (x = 8%) sample, this current value increases more than 10 times in this range (Figure 5).

The I-V characteristics of the PU/x-MW-CNT (x = 2; 4; 8%) show that only the electrical current of the PU/x-MW-CNT (x = 8%) increases with increasing voltage and even this value is not so high.

Table 1 represents the electrical conductivity values of all the prepared nanocomposites prepared by the irreversible dispersion method.

Table 1.

Electrical conductivity values of the Polymers/xMW-CNTs (x = 2 wt %, 4 wt %, and 8 wt %).

Name of samples	R, Ohm	ρ, Ohm·m	σ, S·m⁻¹
Epoxy	∼ ∞	∼ ∞	0
Epoxy/x MW-CNTs (x = 2%)	8.3	0.0083	120.48
Epoxy/x MW-CNTs (x = 4%)	6.1	0.0061	163.93
Epoxy/x MW-CNTs (x = 8%)	1.03	0.001	1000
Silicon	∼ ∞	∼ ∞	0
Silicon/x MW-CNTs (x = 2%)	95.23	0.095	10.526
Silicon/x MW-CNTs (x = 4%)	1.7	0.0017	588.235
Silicon/x MW-CNTs (x = 8%)	1.48	0.0015	666.667
PU	∼ ∞	∼ ∞	0
PU/x MW-CNTs (x = 2%)	533.33	0.533	1.876
PU/x MW-CNTs (x = 4%)	266.67	0.267	3.745
PU/x MW-CNTs (x = 8%)	41.03	0.0041	24.39

With the exception of PU/x MW-CNTs (x = 4%), the conductivity of Epoxy/x MW-CNTs (x = 4%) and Silicone/x MW-CNTs (x = 4%) samples is several orders of magnitude higher than that of nanocomposites based on PVAc, PVAl, and PS which was used as matrices in our previous research work.¹⁸ Thus, we can conclude that the electrical conductivity of MW-CNT-filled PU, Epoxy, and Silicone polymer-based composites can vary quite a lot, which is explained by the strong difference in the nature of the polymer matrices.

All polymer matrices before the addition of MW-CNTs were insulators since the current-voltage characteristics of these polymers (without the addition of CNTs) in the range of 1–16 V did not show electrical conductivity even in microamperes. The electrical conductivity is provided by the addition of MW-CNTs to polymers and increases with an increase in MW-CNTs concentration from 2 to 8%.

Measurement of the electrical conductivity of composites reveals the correspondence between the degree of distribution uniformity of MW-CNTs inside polymers and the electrical conductivity of nanocomposites. At 4% MW-CNTs, a sharp increase in the electrical conductivity of Epoxy/x MW-CNTs (x = 4%) and PU/xMW-CNTs (x = 4%) composites is observed. This is due to the formation of a continuous, distributed, three-dimensional network of MW-CNTs, which allows charge carriers to move along the entire polymer matrix according to percolation theory.^19,20 This critical concentration can be regarded as the percolation threshold.

The plot of the dependence of the electrical conductivity of the nanocomposites in the case of epoxy is similar to the plot of polyurethane (Figure 8). The electrical conductivity graph of silicone/x MW-CNT has a different shape. As can be seen from Figure 8, for the silicone/x MW-CNT nanocomposite, percolation began at 2% of MW-CNT and increased sharply to 4%, then there was observed saturation Table 2.

Table 2.

The standard error estimation results of the electrical conductivity of the prepared nanocomposites.

Epoxy/MW-CNT	σ1	σ2	σ3
Epoxy/MW-CNT	±5.7	±1	±5.7
Silicone/MW-CNT	±0.5	±0	±0.5
PU/MW-CNT	±0.5	±1.5	±1.2

In spite of the fact that the shapes of the electrical conductivity graphs of PU/MW-CNT and Epoxy/MW-CNT are similar, their electrical conductivity values are different. Thus, at a percolation threshold of 4%, the electrical conductivity of Epoxy/x MW-CNTs (163.93 S/m) is almost 44 times higher than that of PU/x MW-CNTs (3.745 S/m). Above this value, with an increase in the concentration of nanotubes, the number of conducting paths in the composite increases accordingly, which contributes to an increase in conductivity. This can be explained by the difference in the nature of the epoxy matrix. At the same time, at an MW-CNT content of 8%, the electrical conductivity of PU/x MW-CNTs (24.39 S/m) is lower than that of epoxy/x MW-CNTs (1000 S/m) and silicon/x MW-CNTs (666.667 S/m) by almost 41 and 27 times, respectively. To evaluate the benefits of the irreversible dispersion method of MW-CNTs inside polymers, the calculated electrical conductivity values are compared with the results of various methods described in the literature. For example, using different preparation methods like solution mixing, a high-shear mixing approach, and vacuum stirred method the electrical conductivity of the Epoxy/x MW-CNTs, PU/MWCNTS, and silicone nanocomposites mostly changes in the range of (10⁻¹–10⁻³) S/m range.²⁷ However, by using the irreversible dispersion method the electrical conductivity of these nanocomposites’ minimum value is 3.745 S/m.

Gas sensing of composites

The sensory effects of the prepared nanocomposites were determined by the change in their resistance after exposure to CH_4, CO, and propane gases.^28–34 The sensitivity was first tested in air, followed by resistance measurements in methane, propane, and carbon monoxide. In this research work, three nanocomposites -Epoxy/x MW-CNT (x = 4%), Silicone/x MWCNTs (x = 4%), PU/x MWCNTs (x = 4%) were fabricated, and the gas-sensitive effect investigated. However, at the same time, for comparison and analysis of the results, three other nanocomposites that had been prepared using the same irreversible dispersion method and investigated in the same conditions before by our research group were added to the table.²⁴

The results of these measurements are summarized in Table 3.

Table 3.

Electrical resistance values of the prepared samples in air, methane, propane, and carbon monoxide gases.

Specimens	R, Ohm (Air)	R, Ohm (CH₄)	R, Ohm (Propane)	R, Ohm (CO)
Epoxy/xMWCNTs (x = 4%)	0.633*10³	0.620*10³	0.582*10³	0.630*10³
Silicon/xMWCNTs (x = 4%)	0.375*10³	0.372*10³	0.438*10³	0.375*10³
PU/xMWCNTs (x = 4%)	340.6*10³	328.2*10³	340.2*10³	332.0*10³
PS/xMWCNTs (x = 4%) *	0.921*10³	0.921*10³	0.93910³	0.947*10³
PVAc/xMWCNTs (x = 4%)*	1.371*10⁶	1.138*10⁶	1.277*10⁶	9.92*10⁶
PVAl/xMWCNTs (x = 4%)*	6.011*10³	6.011*10³	6.004*10³	6.011*10³

* – Changes in electrical resistance values of the nanocomposites under gases²⁴

As can be seen from Table 3, no gas sensory effect was observed for the PVAl/x MWCNTs (x = 4%) sample. However, the highest gas sensory effect was observed for PVAc/x MWCNTs (x = 4%) nanocomposite under CO gas (from 1.371*10⁶ to 9.92*10⁶). At the same time, for the silicone/x MW-CNTs (x = 4%) sample, the resistance change was observed only under propane gas (0.375*10³Ω to 0.438*10³ Ω). Depending on gas types different nanocomposites react in a different ways.

For example, epoxy/xMWCNTs (x = 4%) nanocomposite react only to methane and propane gases, silicone/xMWCNTs (x = 4%) responds only to propane gas, PU/xMWCNTs (x = 4%) nanocomposite response to methane and CO gases.

Thereby, all these nanocomposites except PVAc/x MW-CNT are useful for the preparation of active-sensitive elements for the fabrication of gas sensors depending on gas types.

In Table 4, we can see the results of the quantitative analysis of the electrical resistance values of the prepared nanocomposites for 4% MW-CNT concentrations.

Table 4.

Quantitative analysis results of electrical resistance values of the prepared samples in methane, propane, and carbon monoxide gases.

Names	CH₄ (%)	C₄H₈ (%)	CO (%)
MW-CNT/epoxy	−1.6	−8.06	−0.47
MW-CNT/silicon	−0.8	16.8	0
MW-CNT/PU	3.7	−0.12	−2.52
MW-CNT/PS	0	1.95	2.82
MW-CNT/PVAc	−16.99	−6.86	623
MW-CNT/PVAl	0	−0.12	0

A minus sign shows the decrease in resistance, and a positive sign shows the increase in resistance of the nanocomposites. This table makes it very easy to determine useful and unuseful nanocomposites for gas-sensor applications. For example, PVAl/xMW-CNT (x = 4%) is not useful for the preparation of active gas-sensitive elements, as their qualitative analysis of the change of electrical resistance showed zero percentage (0%) nearly for all three gases. The highest resistance change was observed for the PVAc/xMW-CNT (x = 4%) composite, whose qualitative percentage value is 623%.

Temperature dependence of electrical resistance in the x MW-CNT/polymer composites (x = 4%)

As these nanocomposites show a gas-sensor effect, it is very interesting to determine their nature of them. Therefore, their thermo-resistive properties were investigated.^34–36 The temperature dependence of electrical resistance was measured using the two-contact method. The thermo-resistive response of the prepared nanocomposites was characterized in an air atmosphere under heating conditions (20°C–120°C). The results are shown in Figures 9 and 10(a)–(f).

Based on graphs of the temperature dependence of electrical resistance, it is clear that the properties of the nanocomposites are similar to those of semiconductors. As a result, the resistance of all nanocomposites decreases as the temperature rises. However, the shape of graphs differs between nanocomposites. Moreover, their values are different, some of them in the Om, KOm, and MOm ranges. Details about their values are shown in Tables 5 and 6. All active sensitive elements in gas sensors, according to the literature, are semiconductors. It means that, precisely, semiconductor materials show a response to gases. The gas-sensitive effect of our nanocomposites can also be explained by their semiconductor nature.

Table 5.

The resistance values of the nanocomposites depend on temperature.

T (t = ⁰C)	Resistance CNT/Silicone	Resistance CNT/Epoxy	Resistance CNT/PU	Resistance CNT/PS	Resistance CNT/PVAc	Resistance CNT/PVAl
20	375 Om	633 Om	340.6 kOm	921 Om	1.371 Mom	6.01 KOm
40	369 Om	611 Om	277.7 kOM	920 Om	1.264 MOm	5.94 KOm
60	358 Om	521 Om	240.2 kOm	918 Om	1.195 MOm	5.85 KOm
80	349 Om	315 Om	210.0 kOm	917 Om	1.133 MOm	5.73 KOm
100	346 Om	271 Om	189.1 kOm	916 Om	1.082 MOm	5.64 KOm
120	340 Om	239 Om	175.5 kOm	916 Om	1.007 MoM	5.51 KOm

Table 6.

The standard error estimation results of the gas-sensitive properties of the prepared nanocomposites.

Epoxy/MW-CNT	σ1	σ2	σ3	σ4	σ5	σ6
Epoxy/MW-CNT	±2.6	±2.3	±1.1	±1.5	±2	±2.5
Silicone/MW-CNT	±7	±5.2	±2.8	±2.8	±4	±6.4
PS/MW-CNT	±1.5	±0.6	±0.2	±0.2	±0.6	±0.6
PVAc/MW-CNT	±0.07	±0.03	±0.08	±0.02	±0.04	±0.07
PVAl/MW-CNT	±0.08	±0.03	±0.05	±0.08	±0.05	±0.07
PU/MW-CNT	±1	±2.3	±0.2	±2	±1	±0.5

Conclusion

This study analyzes the impact of polymer nature on the electrical conductivity and gas-sensor effects of the epoxy/xMW-CNT (x = 2, 4, 8%), silicone/x MW-CNTs (x = 2, 4, 8%) and PU/x MW-CNTs (x = 8%) composites. The following conclusions from the research work can be deduced.

SEM microscopy determined that high-quality MW-CNTs without non-tubular forms of carbon (soot or coke) were obtained. Morphology analysis results proved that using the irreversible dispersion method developed by our group, it was possible to increase the homogeneous dispersion of MW-CNTs inside polymers, which directly increased their electrical conductivity properties in comparison to literature results. The electrical conductivity of epoxy/x MW-CNTs (x = 8%) is 1000 S/m; that of silicone/x MW-CNTs (x = 8%) is 666.667 S/m; and that of PU/x MW-CNTs (x = 8%) is 24.39 S/m. Thus, high electrical conductivity is observed for epoxy and silicone-based nanocomposites, which show highly dispersed MW-CNTs inside these polymers. These nanocomposites exhibit gas sensitivity in addition to electrical conductivity. It was revealed that the highest gas-sensitive effect for PVAc/xMW-CNT (x = 4%) nanocomposite was observed under CO gas. Except for PVAl-based nanocomposite, other nanocomposites showed gas sensitivity effects depending on gas types. In addition, the temperature dependence of the electrical resistance of the prepared nanocomposites showed that these nanocomposites behave like semiconductors since their resistance decreases with increasing temperature.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research work was supported by the Science Development Foundation under the President of the Republic of Azerbaijan - Grant № (EIF-BGM-4-RFTF-1/2017).

ORCID iD

Samira Mammadova

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