Preparation of in situ polymerized polypyrrole-modified braided cord and its electrical conductivity investigation under varied mechanical conditions

2.1 Experimental materials

The polyester braided cords used in this study exhibited a diameter of 0.8 mm with a 16-strand braided structure. The polymerization monomer, pyrrole (CAS: 109-97-7, purity ≥99%), was procured from Shanghai Aladdin Biochemical Technology Co., Ltd. The oxidant and dopant, iron(iii) chloride hexahydrate (FeCl₃·6H₂O, CAS: 10025-77-1, purity ≥98%), was supplied by Sinopharm Chemical Reagent Co., Ltd. Absolute ethanol (CAS: 64-17-5, purity 99.7%) was obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. Deionized water was prepared in-house using laboratory purification systems.

2.2 Preparation of PPy-modified braided cords

The PPy-modified braided cord was prepared using an in situ polymerization method, and its preparation process is illustrated in Figure 1. First, polyester braided cord was immersed in 75% ethanol solution and cleaned for 2 h in an ultrasonic cleaner (80°C, 120 kHz frequency, 500 W power) to remove surface oils, followed by rinsing with deionized water and drying in an oven at 60°C. Second, 0.2–0.8 mol·L⁻¹ pyrrole monomer was dispersed in a 100 mL aqueous solution in a beaker, and a 30 cm-long braided cord was immersed in the pyrrole solution for 1–4 h. Third, 0.05–0.2 mol·L⁻¹ FeCl₃·6H₂O was dissolved in 10 mL aqueous solution and slowly added to the pyrrole solution. Oxidative polymerization was conducted for 6–9 h at 0–6°C and 140 rpm using a refrigerated water bath constant temperature shaker (Changzhou Runhua Electric Co., Ltd., China) to prepare a PPy-modified braided cord. Finally, the modified cord was sequentially washed with anhydrous ethanol and deionized water several times and then dried in an oven at 80°C.

Figure 1

Schematic diagram of the process flow for the preparation of PPy-modified braided cord.

2.3 Experimental program

A four-factor, four-level orthogonal experimental design (as shown in Table 1) was developed, with Factor A as pyrrole monomer concentration (mol·L⁻¹), Factor B as FeCl₃·6H₂O concentration (mol·L⁻¹), Factor C as pre-immersion time (h), and Factor D as polymerization time (h). Without considering interaction effects and under unreplicated experimental conditions, the L₁₆(4⁵) orthogonal array (as shown in Table 2) was selected, with the unassigned column designated as Factor E.

2.4 Property characterization

2.4.7 Flexural properties tests

The flexural performance of the PPy-modified braided cord was tested using a homemade device (shown in Figure 2). The specimen was positioned on the operation table for testing. The tail end of the specimen was secured by Fixture 1, which was routed around an 8 mm diameter cylinder to prevent slippage of the specimen. The first end was then clamped by Fixture 2 at the initial position. Fixture 2 was mounted on a linkage mechanism, which was controlled by a stepper motor to perform flexural tests at angles of 30° and 60°. The specimen underwent 50 flexural cycles. During flexural testing, a multimeter was employed to measure real-time resistance by clamping the electrodes to the ends of a 5 cm segment of the specimen.

Figure 2

Schematic diagram of PPy-modified braided cord flexural performance test.

3.1 Range analysis of orthogonal experiment

The conductivity of PPy-modified braided cord was used as an index to measure the optimal preparation process, and the data of orthogonal experiments were processed statistically and analytically using the range analysis method, and the results are shown in Table 3. Where R is the range, which indicates the magnitude of the conductivity of 1 cm long PPy-modified braided cord under different factors and levels, and K indicates the sum of experimental results at each level. The range analysis of conductivity under the conditions of each reaction factor is shown in Figure 3.

Figure 3

Plot of range analysis of conductivity under each factor condition.

As evidenced by the range analysis in Table 3, the factors influencing the electrical conductivity of PPy-modified braided cords follow a descending order of significance: B (FeCl₃·6H₂O dopant concentration) > A (pyrrole monomer concentration) > D (polymerization time) > C (pre-immersion time). Figure 3 further reveals distinct trends: the conductivity exhibited an inverse correlation with pyrrole monomer concentration, whereas it demonstrated a positive dependence on FeCl₃·6H₂O dopant concentration, pre-immersion time and polymerization time.

The comprehensive analysis shows that the PPy-modified braided cord has the best electrical conductivity with a resistance of 316 Ω and a conductivity of 63 S·m⁻¹ when the concentration of pyrrole monomer is 0.2 mol·L⁻¹, the concentration of FeCl₃ ·6H₂O is 0.2 mol·L⁻¹, the presoaking time is 4 h, and the polymerization reaction time is 9 h.

3.2 Variance analysis of orthogonal experiment

To assess the statistical significance of pyrrole monomer concentration, FeCl₃·6H₂O concentration, pre-immersion time, and polymerization time on the electrical conductivity of PPy-modified braided cords, and to validate the rationality of the range analysis, an analysis of variance (ANOVA) was performed on the orthogonal experimental results, as summarized in Table 4. The ANOVA results demonstrated that: Pyrrole monomer concentration (Factor A) and FeCl₃·6H₂O dopant concentration (Factor B) exhibited statistically significant effects on conductivity (P ≤ 0.05), with their primary-secondary order aligned with the range analysis results (B > A). Pre-immersion time (Factor C) and polymerization time (Factor D) showed no significant impact (P > 0.1). Therefore, considering practical production efficiency and process requirements, the pre-immersion time and polymerization time can be appropriately reduced to optimize the preparation process of PPy-modified braided cords without compromising their conductivity performance.

3.3 Structural analysis

The apparent morphology and microstructure of polyester braided cord before and after PPy modification were analyzed using an SU3500 SEM, as illustrated in Figure 4a. The unmodified polyester braided cord exhibited a compact structure with tightly interlaced strands, smooth and uniform fiber surfaces, and orderly fiber alignment. Following PPy modification, the cord surface was uniformly coated with granular deposits. Further examination of individual fibers revealed that the fiber surfaces were also covered with particulate matter. Additionally, these granular deposits formed interconnected networks, demonstrating intimate bonding at the fiber surfaces, inter-fiber interfaces, and interstitial gaps between strands.

Figure 4

(a) Structural morphology of polyester braided cord and PPy-modified braided cord. (b) FTIR spectra of PPy, polyester braided cord, and PPy-modified braided cord. (c) XRD curves of PPy, polyester braided cord, and PPy-modified braided cord.

Figure 4b displays the FTIR spectra of PPy, unmodified polyester braided cord and PPy-modified cord. As shown in Figure 4b, the FTIR spectrum of the unmodified polyester braided cord exhibits characteristic peaks: A peak at 1,719 cm⁻¹ is assigned to the stretching vibration of the carbonyl group (C═O). A peak at 1,250 cm⁻¹ corresponds to the asymmetric stretching vibration of the C–O bond adjacent to the carbonyl group in the ester moiety. A peak at 1,102 cm⁻¹ arises from the stretching vibration of C–OH. A peak at 1,020 cm⁻¹ is attributed to the symmetric stretching vibration of the C–O bond linked to the alkyl group in the ester moiety. A peak at 726 cm⁻¹ originates from the in-plane rocking vibration of CH₂ groups in long-chain aliphatic segments (n > 4). In the FTIR spectrum of PPy: A broad absorption band at 3,420 cm⁻¹ is observed, attributed to the stretching vibration of N–H bonds in pyrrole rings. A strong peak at 1,635 cm⁻¹ is assigned to the stretching vibration of conjugated C═C bonds in the PPy backbone. A peak at 1,384 cm⁻¹ corresponds to the stretching vibration of C–N bonds in pyrrole rings, confirming the PPy structure. Peaks near 1,084 cm⁻¹ and 964 cm⁻¹ are associated with the breathing vibrations of pyrrole rings, further supporting the presence of PPy. For the PPy-modified cord: a peak at 3,074 cm⁻¹ is attributed to the stretching vibration of C–H bonds on pyrrole rings. Compared to pure PPy, the position and intensity of this peak shifted due to hydrogen bonding interactions between PPy and the polyester cord. A prominent peak at 1,538 cm⁻¹, characteristic of the stretching vibration of C═C double bonds within pyrrole rings, aligns with the PPy spectrum, confirming the retention of PPy structure. A peak at 1,252 cm⁻¹ corresponds to the stretching vibration of C–N single bonds in pyrrole rings, consistent with PPy’s spectral features. A peak at 1,037 cm⁻¹ is related to the bending or deformation vibrations of C–H bonds on the pyrrole ring. A peak at 878 cm⁻¹ represents the out-of-plane bending vibration of C–H bonds on pyrrole rings. Notably, the FTIR spectrum of the PPy-modified cord exhibits new absorption peaks and significant shifts in peak positions and intensities compared to both unmodified polyester cord and pure PPy. These changes confirm the successful polymerization of PPy on the polyester cord surface, driven by interactions between the modifier and the PPy structure.

Figure 4c presents the XRD curves of PPy, unmodified polyester braided cord and PPy-modified cord. The XRD curve of the unmodified polyester braided cord (Figure 4c) displays characteristic diffraction peaks at 17, 22.5, and 25.5°, corresponding to the diffraction from specific crystallographic planes with defined interplanar spacings in the crystalline structure of polyester. The intensity of these peaks reflects the diffraction efficiency of distinct crystallographic planes (30). In the XRD curve of PPy (Figure 4c), two broad diffraction peaks are observed at 9.5° (low intensity) and 25.5° (higher intensity). For the PPy-modified cord, the XRD curve retains similarities to both components: No new diffraction peaks emerge, indicating that the crystalline structures of polyester and PPy remain largely intact without significant structural disruption or peak shifts (31). The curve combines characteristic peaks of the polyester cord (e.g., at 17° and 22.5°) with those of PPy (e.g., at 25.5°), confirming the successful modification of the cord with PPy. Notably, the intensity of the 25.5° peak differs from both the unmodified cord and pure PPy, likely due to altered intercrystalline interactions and spatial distribution of PPy within the modified cord. Collectively, the XRD curves of the three samples demonstrate that a minimal amount of PPy is uniformly dispersed on the surface of the modified cord, without inducing substantial crystallographic distortion.

3.4 Conductivity analysis under different mechanical working conditions

Figure 5a illustrates the apparent morphology of the PPy-modified braided cord after friction testing. As shown in Figure 5a, the unabraded sample exhibits uniform polymerization of PPy on individual fibers within the cord. However, after five cycles of friction with 3000-grit sandpaper, partial detachment of PPy from the fiber surfaces was observed under mechanical friction. Following thirty cycles of friction, a more pronounced removal of PPy and significant alterations in apparent morphology were evident. This phenomenon is attributed to the weak interfacial adhesion between PPy and polyester fibers, as the pyrrole monomer was solely deposited on the cord via in situ polymerization without additional bonding agents. Even under mild frictional forces, PPy tends to delaminate from the cord surface. Progressive accumulation of friction cycles exacerbates the detachment, leading to an incremental loss of PPy coating.

Figure 5

(a) Apparent morphology of PPy-modified braided cord after friction property test. (b) Electrical conductivity of PPy-modified braided cord under friction test condition (3000 grit sandpaper, 0.1 N load). (c) Electrical conductivity of PPy-modified braided cord under constant load tensile test condition. (d) Electrical conductivity of PPy-modified braided cord under constant load cyclic tensile test condition of 20 N. (e) Electrical conductivity of PPy-modified braided cord under flexural test condition.

Figure 5b illustrates the relationship between the electrical conductivity of the PPy-modified braided cord (prepared under optimal conditions) and the number of friction cycles. As shown, the conductivity gradually decreases with increasing friction cycles. After 5 cycles of friction with 3000-grit sandpaper, the conductivity decreased from 63 to 50 S·m⁻¹ (a 20.63% reduction), indicating relatively good friction resistance and electrical stability under low friction and limited cycles. After 30 cycles, the conductivity further dropped to 32.82 S·m⁻¹ (a 30.21% reduction). This decline is attributed to the progressive and uneven detachment of PPy from the cord surface under repeated friction, which disrupts the conductive pathways formed by PPy, increases resistance, and leads to cumulative degradation of electrical performance.

Figure 5c depicts the correlation between the electrical conductivity of the PPy-modified cord and applied constant tensile loads. The conductivity decreases gradually with increasing load. At 2 N, the conductivity declined from 63 to 56 S·m⁻¹ (11.11% reduction), while at 10 N, it further decreased to 54 S·m⁻¹ (14.29% reduction). This behavior arises from the structural elongation of the cord under tensile stress. Since PPy is deposited on the cord surface via in situ polymerization and cannot elongate synchronously with the substrate, the increased spacing between PPy particles reduces conductive pathways, elevating resistance and degrading conductivity. Notably, the conductivity reduction between 2 and 10 N was only an additional 2.76%, demonstrating excellent tensile resistance and electrical stability under low loads.

Figure 5d presents the real-time resistivity variation rate of the PPy-modified cord during cyclic tensile testing under a 20 N constant load. The resistivity fluctuates reciprocally with increasing cycles, showing a larger initial amplitude and an overall trend of rising followed by stabilization. During cyclic loading, the cord undergoes elongation, reducing conductive pathways and increasing resistance. Upon load release, partial recovery of the cord’s structure restores some pathways, lowering resistance. However, due to the cord’s limited elasticity, incomplete recovery after repeated cycles causes irreversible conductive pathway damage, leading to an upward trend in resistivity variation during initial cycles. After 220 s, the resistivity stabilizes as the cord adapts to the test conditions. After 800 s, the resistivity variation rate increased by 474.46%, indicating fairly good electrical stability under prolonged cyclic loading.

Figure 5(e) shows the resistivity variation of the PPy-modified cord during cyclic flexural tests at 30° and 60°. Under both conditions, the resistivity fluctuates with increasing cycles, exhibiting larger initial amplitudes and stabilizing over time. After 50 cycles, the resistivity variation rates increased by 1% (30°) and 4% (60°), respectively. These results confirm the cord’s robust electrical stability under flexural cyclic. During flexural test, the outer side of the cord experiences tensile stress, while the inner side undergoes compression. The tensile-induced resistance increase follows the same mechanism as cyclic tensile testing, with higher flexural angles amplifying tensile stress and resistivity variation. Conversely, the compressive side maintains stable conductivity due to minimal structural deformation, ensuring sustained contact between PPy particles and intact conductive pathways.