Synergistic Integration of Polypyrrole, Graphene Oxide, and Silver Nanowires into Flexible Polymeric Films for EMI Shielding Applications

Academic Editor: Grzegorz D. Sulka Received: 10 September 2025 Revised: 27 October 2025 Accepted: 27 October 2025 Published: 29 October 2025 Citation: Gaji´c, B.; Radoiˇci´c, M.; Yasir, M.; Saeed, W.; Bolka, S.; Nardin, B.; Potoˇcnik, J.; Bajuk-Bogdanovi´c, D.; ´ Ciri´c-Marjanovi´c, G.; Šaponji´c, Z.; et al. Synergistic Integration of Polypyrrole, Graphene Oxide, and Silver Nanowires into Flexible Polymeric Films for EMI Shielding Applications. Molecules 2025,30, 4221. https:// doi.org/10.3390/molecules30214221 Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/). Article Synergistic Integration of Polypyrrole, Graphene Oxide, and Silver Nanowires into Flexible Polymeric Films for EMI Shielding Applications Brankica Gaji´c 1 , Marija Radoiˇci´c 1, * , Muhammad Yasir 2 , Warda Saeed 2 , Silvester Bolka 3 , Blaž Nardin 3 , Jelena Potoˇcnik 1, Danica Bajuk-Bogdanovi´c 4, Gordana ´ Ciri´c-Marjanovi´c 4, Zoran Šaponji´c 5 and Svetlana Jovanovi´c 1 1“Vinˇca” Institute of Nuclear Sciences, National Institute of Republic of Serbia, University of Belgrade, Mike Petovi´ca Alasa 12-14, 11000 Belgrade, Serbia; [email protected] (B.G.); [email protected] (J.P.); [email protected] (S.J.) 2Division of Microrobotics and Control Engineering, Department of Computing Science, Carl von Ossietzky Universität Oldenburg, 26129 Oldenburg, Germany; [email protected] (M.Y.); [email protected] (W.S.) 3Faculty of Polymer Technology, Ozare 19, 2380 Slovenj Gradec, Slovenia; silvester[email protected] (S.B.); [email protected] (B.N.) 4Faculty of Physical Chemistry, University of Belgrade, Studentski Trg 12-16, 11158 Belgrade, Serbia; [email protected] (D.B.-B.); gor[email protected] (G. ´ C.-M.) 5Institute of General and Physical Chemistry, Studentski Trg 12-16, 11158 Belgrade, Serbia; [email protected] *Correspondence: [email protected] Abstract The remarkable growth of high-frequency electronic systems has raised concerns about electromagnetic interference (EMI), emphasizing the need for lightweight and efficient shielding materials. In this study, ternary composites based on polypyrrole (PPy), graphene oxide (GO), and silver nanowires (AgNWs) were synthesized through chemical oxidative polymerization of pyrrole monomer and embedded into polycaprolactone (PCL) matrices to create flexible films. Structural and morphological analyses confirmed the successful incorporation of all components, with scanning electron microscopy showing granular PPy, sheet-like GO, and fibrous AgNWs, while spectroscopic studies indicated strong interfacial interactions without damaging the PPy backbone. Thermomechanical analysis revealed that GO increased stiffness and defined the glass transition, whereas AgNWs improved toughness and energy dissipation; their combined use resulted in balanced properties. EMI shielding effectiveness (SE) was tested in the X-band (8–12 GHz). Pure PPy exhibited poor shielding ability, while the addition of GO and AgNWs significantly enhanced performance. The highest EMI SE values were observed in PPy/GO–AgNWs composites, with an average SE of 16.05 dB at 20 wt% of the composite in the PCL matrix, equivalent to about 84.4% attenuation of incident waves. These results demonstrate that the synergistic integration of GO and AgNWs into PPy matrices enables the creation of lightweight, flexible films with advanced EMI shielding properties, showing great potential for next-generation electronic and aerospace applications. Keywords: polypyrrole; silver nanowires; graphene oxide; electromagnetic interference shielding; composites Molecules 2025,30, 4221 https://doi.org/10.3390/molecules30214221 Molecules 2025,30, 4221 2 of 18 1. Introduction The rapid growth in the use of high-frequency electronic systems—ranging from wireless communication devices and smart robotics to aerospace instruments and wearable electronics—has made electromagnetic interference (EMI) an increasingly important issue. EMI, caused by the unintentional emission of electromagnetic radiation by electronic components, not only disrupts sensitive nearby devices but also potentially harms human health. With the development of compact, multifunctional, and densely integrated electronics, creating efficient, lightweight, and environmentally durable shielding materials has become crucial in ensuring electromagnetic compatibility and protecting both equipment and users [ 1 ]. Traditionally, metals such as copper, aluminum, and silver have been used as standard EMI shielding materials because of their high electrical conductivity and reflectivity. However, these conventional materials are often heavy, rigid, prone to corrosion, and unsuitable for flexible or miniaturized devices. Additionally, metallic shielding mainly relies on reflection, which can lead to secondary electromagnetic pollution, making absorption-based shielding more desirable in modern technology [ 2 , 3 ]. Recently, conducting polymers have emerged as promising materials for EMI shielding due to their low density, tunable electrical conductivity, chemical stability, and ease of processing. Among these, PPy has attracted significant interest because of its excellent intrinsic conductivity, environmental durability, and ability to form various microand nanostructures that help attenuate waves through dipole polarization and conduction losses [ 4 ]. However, pure PPy’s EMI shielding effectiveness is often limited by its brittleness, high conductivity, and poor interfacial compatibility within composite matrices [ 5 ]. To overcome these issues, hybrid composite strategies incorporating carbonbased nanomaterials and metal nanoparticles have proven highly effective [ 6 , 7 ]. Graphene oxide (GO), for example, offers a large specific surface area, functional surface chemistry, and high electrical conductivity (upon partial or full reduction), making it ideal for enhancing interfacial polarization and electrical pathways in the polymer matrix [ 8 , 9 ]. In parallel, incorporating silver nanostructures—such as silver nanowires (AgNWs) or nanoparticles (AgNPs)—into conducting polymer matrices has been shown to significantly improve electrical conductivity and EMI SE by creating additional conductive networks and promoting surface plasmon resonance effects [ 9 , 10 ]. These metallic inclusions contribute not only through reflection and absorption but also facilitate Joule heating and eddy current losses, further enhancing overall shielding performance [ 11 ]. Yu et al. studied conductive composites containing silver nanowires (AgNWs) and silver nanoparticles (AgNPs), demonstrating that Ag nanostructures offer superior electrical conductivity and form more efficient conductive pathways within the polymer matrix, resulting in higher EMI shielding effectiveness compared to bulk metallic fillers [ 12 ]. Previous studies have extensively demonstrated the potential of conducting polymers and hybrid nanostructures for EMI shielding. For instance, PPy-based nanostructures embedded in a silicone matrix show notable EMI shielding ability, emphasizing the importance of morphology and electrical conductivity in the shielding mechanism [ 13 ]. Composites made with GO and AgNWs reveal that combining 2D GO and 1D AgNWs improves charge transport and overall shielding efficiency [ 14 ]. GO-functionalized carbon-fiber/cement composites demonstrate better dispersion and interfacial contact, leading to a significant increase in shielding effectiveness compared to pure systems [ 15 ]. AgNW-coated textiles maintain high flexibility and light weight while providing effective EMI shielding [ 16 ]. Additionally, AgNWs/PPy hybrid systems are effective in shielding due to the synergistic interaction between metallic and conjugated polymer phases [ 17 ]. The combined integration of PPy, GO, and Ag nanostructures within a single composite can create multifunctional materials that are lightweight, flexible, and offer high EMI attenuation, wide absorption bandwidths, and environmental stability. These ternary nanocomposites utilize Molecules 2025,30, 4221 3 of 18 multiple shielding mechanisms—including dielectric polarization, interfacial and internal reflections, and better impedance matching—making them suitable for next-generation electromagnetic shielding, especially in the Xand Ku-bands. [18,19]. Although many studies focus on conducting polymer-based composites for EMI shielding, most reported systems still face issues like poor flexibility, limited processability, and discontinuous conductive networks, especially when using free-standing films or rigid matrices. Furthermore, while binary PPy/GO and PPy/AgNWs composites have demonstrated promising electrical properties, their combination in a flexible polymer matrix like polycaprolactone (PCL) has not been thoroughly investigated. The synergistic integration of PPy, GO, and AgNWs in a PCL matrix aims to solve these problems by combining the high electrical conductivity of AgNWs, the large surface area and interfacial polarization of GO, and the environmental stability of PPy within a flexible, processable host polymer. This method creates a new pathway toward lightweight, mechanically robust, and highly effective EMI shielding materials suitable for flexible electronics and environmental protection. To the best of our knowledge, this study reports for the first time the synthesis of flexible and processable films based on polypyrrole (PPy), graphene oxide (GO), and silver nanowires (AgNWs), embedded in a polycaprolactone (PCL) matrix, demonstrating promising electromagnetic interference (EMI) shielding capabilities. 2. Results and Discussion 2.1. Morphological Properties of Synthesized Materials SEM micrographs of all synthesized samples are shown in Figure 1. To verify the chemical composition of the examined samples, elemental mapping was performed using EDS. The results are presented in Figure 2, along with the corresponding spectra, shown in the 0 to 5 keV energy range for better clarity. In the elemental maps, different colors were assigned to each element to emphasize their spatial distribution: carbon (C) in red, oxygen (O) in green, nitrogen (N) in yellow, and silver (Ag) in cyan. Figure 1. SEM micrographs of neat PPy (a), PPy/GO (b), PPy–AgNWs (c), and PPy/GO–AgNWs (d). Molecules 2025,30, 4221 4 of 18 Figure 2. EDS elemental mapping and spectra for (a) PPy, (b) PPy/GO, (c) PPy/AgNWs, and (d) PPy/GO–AgNWs. Elemental distribution is shown using different colors: red for carbon, green for oxygen, yellow for nitrogen, and cyan for silver. Regarding polymerization under acidic conditions, the pure PPy sample (Figure 1a) exhibited typical granular morphology, with an average particle size of about 150 nm [ 20 ]. The granular particle size and distribution are shown in SI, Figure S1. The original sizes of the pure GO and AgNWs samples before in situ polymerization of pyrrole are available from our previous study by Milenkovi´c et al. [ 19 ]. Adding GO dispersion to the polymerization mixture resulted in a mixed morphology PPy/GO composite that displayed features of both components, such as a two-dimensional sheet-like graphene structure and granular-shaped PPy. SEM images of the PPy/GO composite reveal that the GO sheet’s lateral size ranges from a few hundred nanometers to several micrometers (Figure 1b). After polymerization, some differences from the original dimensions are noticeable, caused by the formation of a polypyrrole layer on the GO surface, confirmed by the presence of N atoms in elemental mapping (Figure 2b). In the pure GO sample, only C and O atoms were detected, with no N atoms present, as expected (SI, Figure S2). When pyrrole polymerization was carried out in the presence of AgNWs, the resulting composite materials displayed silver nanowires coated with a granular PPy layer (Figure 1c). The PPy-coated nanowires are over 4 µ m long, with an average diameter of about 600 nm, depending on the thickness of the PPy layer on the surface of the AgNWs. The detection Molecules 2025,30, 4221 5 of 18 of nitrogen in the elemental mapping of GO and AgNWs clearly confirms the successful deposition of the polypyrrole layer on their surfaces. Furthermore, after synthesizing the hybrid PPy/GO–AgNWs system, the distinct morphological features of all three components are clearly visible, demonstrating that each component maintained its original structure while being evenly coated with a PPy layer (Figure 1d). Elemental mapping also confirmed the presence of nitrogen on both GO and AgNWs surfaces, providing direct evidence that these components are covered with a polypyrrole layer (Figure 2d). Besides microscopy detection, AgNWs are clearly visible in elemental mapping (Figure 2c,d), confirming their presence in the PPy/AgNWs and PPy/GO–AgNWs samples. The EDS spectra from the examined areas display characteristic peaks at 0.27 keV, 0.38 keV, and 0.52 keV, corresponding to the K lines of carbon, nitrogen, and oxygen, respectively. The presence of silver is clearly identified by its L lines at 2.62 keV, 2.98 keV, and 3.15 keV (Figure 2c,d). Additionally, peaks near 2.1 keV and 2.3 keV originate from the M lines of gold, which was used during sample preparation for SEM imaging. EDS elemental mapping clearly revealed a uniform distribution of nitrogen across all composite samples, confirming the successful deposition of a PPy layer onto their surfaces (PPy/GO, PPy/AgNWs, PPy/GO–AgNWs). The minor variations in elemental composition likely stem from local morphological differences. Since polypyrrole is the only nitrogen-containing component, these results strongly support the formation of a continuous PPy coating on the surfaces of GO and AgNWs, explaining the dimensional differences observed compared to the initial precursors. 2.2. Molecular Structure The molecular structure of the synthesized materials was analyzed using Raman and FTIR spectroscopies (Figure 3). Figure 3. Raman (a) and FTIR (b) spectra of synthesized neat PPy and PPy-based composite materials. Molecules 2025,30, 4221 6 of 18 The Raman spectra of all composites based on neat PPy and all PPy-based nanocomposites (PPy/GO, PPy/AgNWs, and PPy/GO–AgNWs) are shown in Figure 3a. The most intense band in all spectra, observed at 1588 cm −1 , corresponds to the stretching vibrations of C=C bonds within the polymer backbone, indicating the presence of a π -conjugated system in PPy [ 21 ]. The band at 1370 cm −1 is attributed to stretching vibrations within the pyrrole ring, while the band at 1248 cm −1 is associated with the stretching of bonds connecting individual pyrrole units along the polymer chain, confirming the successful formation of the polymer structure [ 22 – 24 ]. The band at 1048 cm −1 may relate to C–H bending or in-ring deformation modes [ 23 ]. Particularly prominent bands at 968 and 930 cm −1 are assigned to ring deformations caused by charged species, specifically polarons and bipolarons, further confirming the conductive nature of the obtained PPy. Although GO is present in the composite structure, the Raman spectra of the PPy/GO composite closely resemble that of pure PPy, without clearly visible D and G bands typically associated with GO (D band at ~1350 cm −1 and G band between 1580 and 1600 cm −1 ) [ 25 ]. This suggests that GO is likely not just physically mixed but is instead incorporated and well dispersed within the polymer matrix, so that PPy chains largely cover GO sheets [ 26 ]. Strong interactions between the PPy chains and the functional groups on the GO surface, as well as the lower GO content relative to PPy, may cause the GO bands to be masked or less distinguishable in the composite spectrum. AgNWs do not show their own characteristic Raman bands and are mainly used as substrates to enhance signals in SERS (Surface-Enhanced Raman Scattering) systems [ 27 ]. If the polymer matrix completely covers the AgNWs or if they are not exposed on the surface, their contribution to the Raman spectrum is minimal. This may explain why there are no significant differences between the spectrum of pure PPy and that of the PPy/AgNWs composite [ 28 ]. Due to the reasons discussed for binary composites, the Raman spectrum of the ternary composite PPy/GO–AgNWs is also dominated by PPy bands and is very similar to the spectra of PPy, PPy/GO, and PPy/AgNWs. FTIR spectra of all synthesized materials are presented in Figure 3b. The spectra of pure PPy, along with PPy/GO, PPy/AgNWs, and PPy/GO–AgNWs composites, show characteristic vibrational bands of PPy, confirming that the polymer’s core chemical structure remains intact [ 29 ]. This indicates that the synthesis of the composites did not cause significant chemical changes in the PPy component. In all spectra, the following bands typical of PPy were observed. The broad band at 3425 cm −1 is attributed to N–H stretching vibrations, while the bands at 1540 cm −1 and 1450 cm −1 correspond to C–C and C–N stretching vibrations in the pyrrole ring, respectively [ 21 , 22 , 30 , 31 ]. The band at approximately 1300 cm −1 is associated with inplane deformation vibrations of the C–N and C–H groups, whereas the band at 1167 cm −1 corresponds to the pyrrole ring “breathing” vibrations [ 21 , 32 ]. The band at 1038 cm −1 can be assigned to C–H and N–H deformation vibrations, while a weaker band at 888 cm −1 relates to out-of-plane C–H vibrations characteristic of the pyrrole ring [22,29]. In the spectra of the PPy/GO and PPy/GO–AgNWs composites, the bands originating from GO (C=O stretching at ~1735 cm −1 and epoxy C–O–C stretching at ~1087 cm −1 ) are not clearly visible. The strong broad band at 3425 cm −1 , due to N-H stretching in PPy [ 25 , 33 ], most likely overlaps with the GO band from O-H stretching, which appears at a nearby position of ~3440 cm −1 [ 34 ]. This can be explained by the fact that PPy coats the composite surface and dominates the IR absorption, while GO and AgNWs are mostly embedded in the matrix and present in smaller amounts, causing their signals to be masked by the polymer’s bands. In the spectra of the composites, the band caused by N-H stretching vibration is at nearly the same position as in pristine PPy (at 3425 cm −1 ). Differences in its shape and relative intensity among the spectra of the synthesized materials could result Molecules 2025,30, 4221 7 of 18 from various factors, such as changes in hydrogen bonding interactions (e.g., replacement of N-H ···· N-H in pure PPy with N-H ···· O-H in composites due to the presence of GO) or different amounts of residual adsorbed water, which depend on their surface properties like hydrophilicity. These spectroscopy results confirm the successful incorporation of GO and AgNWs into the PPy matrix while maintaining the polymer’s core structure. 2.3. Thermogravimetric Analysis Figure 4shows the TGA results of pristine PPy, its composite PPy/GO, as well as the PPy/AgNWs and PPy/GO–AgNWs nanocomposites. All tested materials exhibit distinct mass loss behaviors. The initial weight loss, between 65 and 78 ◦ C, for all samples is caused by the removal of adsorbed water from the surface [ 28 , 35 ]. During this stage, the mass loss of the PPy/GO composite (Figure 4b,c) is lower than that of neat PPy (Figure 4a), confirming the formation of a PPy layer on the GO sheets and AgNWs, along with a slight improvement in thermal stability during the first degradation stage [ 36 , 37 ]. A minor deviation is observed for the PPy/AgNWs sample, where the degradation temperature is lower than that of pure PPy, while the mass loss in the first degradation stage is slightly higher due to the presence of AgNWs. The minimal mass loss of 1.54% observed for the PPy/GO–AgNWs composite (Figure 4d) is attributed to a synergistic effect between the GO sheets and AgNWs [ 18 ], which, to some extent, alters the thermal stability of the PPy matrix. This effect results from improved heat distribution and increased mechanical strength, which slow down the thermal degradation of the polymer [38]. The thermogram of pristine PPy (Figure 4a) shows that the polymer powder loses 41.22% of its mass in N 2 atmosphere during the second degradation step (differentiation maximum at 249.43 ◦ C) and 55.23% during the third degradation step (differentiation maximum at 618.46 ◦ C). A total mass loss of approximately 99.99% occurs above 620 ◦ C in O 2 atmosphere, indicating complete polymer decomposition [ 39 ]. A similar thermal behavior is observed for the PPy/GO composite (Figure 4b), with mass losses in similar temperature ranges: 40.08% (max 243.53 ◦ C) during the second and 57.31% (max 614.35 ◦ C) during the first degradation step. This demonstrates that adding GO does not change the overall degradation pattern but may slightly influence the sample’s thermal stability. Results from the TGA analysis of the PPy/AgNWs and PPy/GO–AgNWs composites (Figure 3) show their enhanced thermal stability during the third degradation stage compared to pure PPy. Specifically, during this phase, the weight loss for the PPy/AgNWs sample is 54.58% at a maximum temperature of 632.56 ◦ C, while the ternary PPy/GO– AgNWs nanocomposites experience a 60.71% loss at a maximum of 619.18 ◦C. The final decomposition showed total mass losses of 94.47% for PPy/AgNWs and 97.77% for PPy/GO–AgNWs. The remaining masses were 5.53% for the PPy/AgNWs composite and 2.23% for the PPy/GO–AgNWs hybrid, which are due to the presence of thermally stable silver nanowires. The slight increase in thermal stability results from a synergistic interaction between the polymer and the AgNWs and GO/AgNWs, which act as thermal stabilizers and only slightly slow down polymer degradation. 2.4. Thermomechanical Analysis Results of the TMA for all synthesized PCL film samples are shown in Figure 5. The filler content in the PCL matrix was maintained at 20 wt% for each sample. The tan δ maxima for neat PPy/PCL (58.15 ◦ C, Figure 5a) and PPy/GO/PCL (58.02 ◦ C, Figure 5b) were nearly identical, indicating that adding GO did not significantly alter the segmental mobility of the PCL matrix. Molecules 2025,30, 4221 8 of 18 Figure 4. Thermogravimetric analysis of synthesized powder samples: PPy (a), PPy/GO (b), PPy/AgNWs (c), and PPy/GO–AgNWs (d). Molecules 2025,30, 4221 9 of 18 Figure 5. 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