Abstract

General Background: Conducting polymers, such as polypyrrole (PPy), offer a unique blend of mechanical flexibility and electronic functionality, making them attractive for diverse technological applications. Specific Background: Incorporating metal nanoparticles into PPy has shown promise in enhancing its electrical properties, particularly DC conductivity. Knowledge Gap: However, a systematic understanding of how varying copper (Cu) nanoparticle concentrations affect charge transport mechanisms and activation energy in PPy/Cu nanocomposites remains limited. Aims: This study aims to synthesize PPy/Cu nanocomposites via in-situ chemical polymerization and evaluate their DC conductivity and activation energy across a temperature range of 293–433 K. Results: The addition of Cu significantly enhanced conductivity, from 1.7×10⁻⁷ S/cm (pure PPy) to 8.6×10⁻⁴ S/cm (10 wt.% Cu), and reduced activation energy from 0.045 eV to 0.023 eV, indicating improved charge transport. Novelty: The findings reveal a clear correlation between Cu concentration and the formation of efficient conduction pathways, supported by percolation theory and interfacial charge transfer dynamics. Implications: These results position PPy/Cu nanocomposites as promising materials for applications in energy storage, sensing, and electromagnetic shielding, where high conductivity and tunable electrical behavior are critical.
Highlight : 

• High Conductivity Gain: Conductivity increased by 10,000× with 10 wt.% Cu addition.

• Efficient Charge Transport: Activation energy dropped from 0.045 eV (pure) to 0.023 eV (10 wt.% Cu).

• Application Potential: Suitable for energy storage, sensors, and electromagnetic shielding

Keywords : Polypyrrole, Copper Nanoparticles, Conductivity Enhancement, Nanocomposites, Activation Energy

1. Introduction

The conducting polymers have cropped up as a new type material that exhibit the mechanical flexibility and processability of traditional polymers as well as the electronic characteristics of metals and semiconductors [1,2]. Among intrinsically conducting polymers, polypyrrole (PPy) stands out due to its exceptional environmental stability, ease of synthesis, and tunable electrical properties [3,4]. The electrical conductivity of PPy originates from the π-conjugated backbone structure, which enables efficient charge transport through delocalized electrons [5].

The incorporation of metallic nanoparticles into conducting polymer matrices has proven to be an effective strategy for enhancing electrical conductivity while maintaining mechanical flexibility [6,7]. Copper nanoparticles, in particular, offer several advantages including high electrical conductivity (5.96×10⁷ S/m), chemical stability, and cost-effectiveness compared to noble metals [8,9]. The synergistic effect between the conducting polymer and metallic nanofillers can result in significant improvements in electrical, thermal, and mechanical properties [10].

Recent advances in polypyrrole-based nanocomposites have demonstrated their potential for diverse applications including supercapacitors, sensors, electromagnetic interference shielding, and flexible electronics [11,12]. The electrical conductivity of these nanocomposites is influenced by several factors including the concentration and dispersion of nanofillers, synthesis method, and processing conditions [13,14].

The charge transport mechanism in conducting polymers involves the creation and mobility of charge carriers such as polarons and bipolarons [15]. The addition of metallic nanoparticles can modify the charge transport pathway by creating additional conduction channels and reducing the activation energy for charge hopping [16,17]. Understanding these fundamental mechanisms is crucial for optimizing the electrical properties of polymer nanocomposites.

The electrical conductivity, DC conductivity of PPy/Cu nanocomposites synthesized through in-situ chemical polymerization, will be the subject of investigation in this study. The influence of copper concentration on the electrical transport properties and activation energy is systematically examined to elucidate the charge transport mechanisms and optimize the nanocomposite performance.

2. Experimental Work

2.1 Materials

Pyrrole monomer (C₄H₅N, 99% purity) was obtained from Sigma-Aldrich and distilled under reduced pressure before use. Anhydrous ferric chloride (FeCl₃, 99.9%) was used as the oxidizing agent without further purification. Copper nanoparticles (10-30 nm diameter, 99.9% purity) were purchased from Nanjing Nano Technology Co., Ltd. All aqueous solutions were prepared using doubly distilled water with resistivity >18 MΩ·cm.

2.2 Synthesis of Pure Polypyrrole

chemical oxidative polymerization was used to form pure polypyrrole. In a normal process, 0.58 M pyrrole monomer was added to a constant dish of distilled water (48 mL) whilst vigorously stirring at a room temperature. Individually, 0.74 M FeCl 3 was dissolved to 50 mL of distilled water. The oxidant solution was added at drop wise rate to pyrrole solution but keeping the solution aggressively stirred. The mixture became dark indicating the commencement of polymerization. The polymerization was carried out at ambient temperature under continuous stirring within 4hours after which the aging was carried out within 24 hours. The black precipitate that was formed was filtrated, washed severally with distilled water until the filtrate turned colorless, and finally dried in a vacuum oven at 70 C in a span of 4 hours.

2.3 Synthesis of PPy/Cu Nanocomposites

PPy/Cu nanocomposites were prepared using the same chemical oxidation method with modifications. Copper nanoparticles (0.5, 1, 5, 7, and 10 wt.% relative to pyrrole) were first dispersed in the pyrrole-water solution using ultrasonic treatment for 30 minutes to ensure uniform dispersion. The FeCl₃ solution was then added dropwise to the pyrrole/Cu mixture under continuous stirring. The polymerization and purification procedures were identical to those used for pure PPy. The resulting nanocomposites showed progressively darker coloration with increasing Cu content. Figure (1a and b) show the PPy powder and the samples of PPy and PPy-Cu nanocomposites, respectively.

Figure 1.(a) Polypyrrole powder (b)PPy and PPy/Cu nanocomposites.

2.4 Characterization and Measurements

2.4.1 DC Electrical Conductivity Measurements

Electrical conductivity measurements were performed using a four-probe method to eliminate contact resistance effects. Samples were prepared by pressing the dried powder into pellets (10 mm diameter, 2 mm thickness) under 10 MPa pressure. Silver paste was applied to ensure good electrical contact. The measurements were conducted in a temperature-controlled environment from 293 K to 433 K with 10 K intervals using a Keithley 6517B electrometer. The electrical conductivity (σ) was calculated using (σ = L/(R×A)) where L is the sample thickness, R is the measured resistance, and A is the cross-sectional area.

2.4.2 Activation Energy Determination

The electrical conductivity of temperature will be analyzed with the help of Arrhenius Equation ( (σ(T) = σ₀ × exp(-Eₐ/kᵦT)), is equal to the pre-exponential factor, activation energy is Eₐ the Boltzmann constant is equal to kᵦ and the absolute temperature is T ) van der Waals parameters were calculated as activation energy depending on the slope of the ln(σ) versus 1000/T plots

3. Results and Discussion

3.1 DC Electrical Conductivity

Figure (2) shows the variation of DC electrical conductivity with temperature for pure PPy and PPy/Cu nanocomposites. The electrical conductivity of all samples increases with temperature, indicating thermally activated transport behavior typical of semiconducting materials. For pure polypyrrole, the conductivity increases from 1.7×10⁻⁷ S/cm at 293 K to 1.6×10⁻⁵ S/cm at 433 K, representing a two-order magnitude enhancement.

Figure 2.The change of DC electrical conductivity in relation to temperature with pure PPy and PPy/Cu nanocomposites

The incorporation of copper nanoparticles significantly improves the electrical conductivity across the entire temperature range. At room temperature (293 K), the conductivity increases systematically with Cu concentration: 5.16×10⁻⁵ S/cm (0.5 wt.%), 1.2×10⁻⁴ S/cm (1 wt.%), 3.8×10⁻⁴ S/cm (5 wt.%), 6.2×10⁻⁴ S/cm (7 wt.%), and 8.6×10⁻⁴ S/cm (10 wt.%). The highest conductivity achieved represents a four-order magnitude improvement compared to pure PPy.

3.2 Charge Transport Mechanism

The enhanced conductivity in PPy/Cu nanocomposites can be attributed to several factors:

First: (Percolation Theory), The addition of highly conductive Cu nanoparticles creates additional conduction pathways through the polymer matrix. At higher concentrations, these pathways form a percolating network that significantly reduces the overall resistance [18,19].

Second: (Charge Carrier Modification) The presence of Cu nanoparticles introduces additional charge carriers and modifies the electronic structure of the polymer. This leads to an increase in the concentration of polarons and bipolarons, which are the primary charge carriers in conducting polymers [20,21].

Third: (Interfacial Effects) The interface between Cu nanoparticles and the PPy matrix creates regions with enhanced charge transfer efficiency. The high surface area of nanoparticles maximizes these interfacial effects [22].

3.3 Activation Energy Analysis

The temperature dependence of electrical conductivity was analyzed using Arrhenius plots (ln(σ) vs. 1000/T), as shown in Figure (3). All samples exhibit linear behavior, indicating a single activation process governs the charge transport.

Figure 3.Variation of ln (σ) with reciprocal temperature for PPy and PPy/Cu nanocomposites

The activation energy (listed in the table 2) decreases systematically with increasing Cu concentration, from 0.045 eV for pure PPy to 0.023 eV for PPy/Cu (10 wt.%). This reduction indicates that Cu nanoparticles facilitate charge transport by reducing the energy barrier for charge hopping between polymer chains

Figure (4) illustrates the relationship between Cu concentration and activation energy. The nearly linear decrease in activation energy with increasing Cu content confirms the effectiveness of metallic nanoparticles in enhancing charge transport efficiency.

Figure 4.The relationship between Cu concentration and activation energy

The conductivity values obtained in this study are consistent with recent reports on PPy-based nanocomposites. The enhancement factor of 10⁴ achieved with 10 wt.% Cu loading compares favorably with other metallic nanofillers reported in the literature [23,24]. The activation energy values are also within the typical range for conducting polymers, confirming the reliability of the experimental results.

The improvements of the electrical properties of the PPy/Cu nanocomposites usher in potential applications, such as energy storage: These factors designate the material as a good candidate of supercapacitor electrode material or battery material [25]. And sensors: Conductivity variation to stimuli in the environment can be ridden to gas sensors and biosensors [26].

4. Conclusions

This study successfully demonstrates the synthesis and characterization of PPy/Cu nanocomposites with enhanced DC electrical conductivity. The key findings are:

1. Significant Conductivity Enhancement: The incorporation of Cu nanoparticles resulted in a four-order magnitude improvement in electrical conductivity, from 1.7×10⁻⁷ S/cm for pure PPy to 8.6×10⁻⁴ S/cm for PPy/Cu (10 wt.%).
2. Systematic Improvement: The electrical conductivity increased systematically with Cu concentration, indicating effective incorporation and dispersion of nanoparticles.
3. Reduced Activation Energy: The activation energy decreased from 0.045 eV for pure PPy to 0.023 eV for PPy/Cu (10 wt.%), demonstrating improved charge transport efficiency.
4. Thermally Activated Transport: All samples exhibited thermally activated transport behavior, with conductivity increasing exponentially with temperature according to the Arrhenius relationship.
5. Applications: These nanocomposites can be used in many applications such as building energy storage devices, sensors, and uses in electromagnetic shielding applications due to the improved electrical properties.

The results provide valuable insights into the charge transport mechanisms in conducting polymer nanocomposites and demonstrate the potential of PPy/Cu systems for advanced electrical applications.

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