1 Introduction

While electronic and electrical equipment have brought great convenience to our lives, they have also caused increasingly serious electromagnetic pollution, such as electronic noise, electromagnetic interference and radio frequency interference [1,2,3]. Electromagnetic waves not only couple and interfere with the normal use of other electronic components, making it impossible for electronic equipment to function properly and posing serious threat to information security, but also affect human health. Studies have shown that when people are exposed to electromagnetic radiation for a long time, the risk of diseases such as cancer, heart disease, skin problems, headaches and other mild or acute diseases will increase. Therefore, the design and development of lightweight, economical and efficient EMI shielding materials are imperative to address the problems of electromagnetic pollution [4,5,6].

Compared with traditional metal-based EMI shielding composites, polymer-based EMI shielding composites have attracted much attention from the scientific and industrial communities due to their lightweight, high specific strength, easy molding and processing, excellent chemical stability, low cost and good sealing properties [7,8,9] Commonly used polymer matrixes are epoxy resin, phenolic resin, polyvinylidene fluoride (PVDF) and polydimethylsiloxane (PDMS). Among them, PDMS has good mechanical properties, high and low temperature resistance, excellent weather resistance, chemical stability and easy processing and molding characteristics, widely used in many fields such as aerospace, automotive industries and microelectronics [10,11,12]. In addition, PDMS has excellent flexibility compared to rigid matrixes such as epoxy resins and can meet the requirements of wearable electronic devices for flexibility in materials. In recent years, PDMS-based EMI shielding composites have made certain research progress, but to achieve the desired EMI shielding effectiveness (EMI SE) usually requires high filler loading, which seriously affects the cost, processability and mechanical properties, largely limiting the application of PDMS-based EMI shielding composites in the field of microelectronics, aircraft and spacecraft [3b), which is mainly attributed to the removal of oxygen-containing functional groups and the carbonization of the cellulose. When the mass ratio of cotton to pre-cooled solution is 5:95, the tangling of fibers within the CCA is more severe (Fig. 3b’). This is due to the limited solubility of the NaOH/urea solution on the cotton, resulting in the tangling of fibers within the CCA. As shown in Fig. 3c, when the GO solution concentration is 7.5 mg mL−1, the CCA@rGO is the homogeneous network structure, with the CCA forming the main framework of the CCA@rGO and the rGO lamellae completely wrap** the fibers, forming the skin-core structure similar to that of a cable. The rGO is similar to the skin and is densely wrapped around the CCA fibers to provide sufficient structural stability for CCA@rGO. The CCA is similar to the core and is wrapped by rGO sheets to provide attachment points and support for the rGO sheets. When the GO solution concentration is 10 mg mL-1, rGO is agglomerated in CCA@rGO, CCA is not uniformly wrapped and CCA@rGO has an uneven network structure (Fig. 3c’). This is due to the high viscosity of the GO solution, which limits its diffusion inside the CA and eventually leads to the agglomeration of rGO in the CCA. After backfilling with PDMS, the skin-core structure of CCA@rGO is well preserved, and the 3D double-layer conductive network structure of CCA@rGO is not significantly damaged (Fig. 3d), and PDMS is more uniformly dispersed in the gaps of the 3D conductive network of CCA@rGO. At the same time, PDMS is uniformly dispersed in the gaps of the 3D conducting network of CCA@rGO.

Fig. 3
figure 3

SEM images of CA (a), CCA (b-b’), CCA@rGO (c-c’) and the CCA@rGO/PDMS EMI shielding composites (d)

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3.3 Electrical Conductivities and EMI Shielding Performances

Figure 4a shows the σ of PCCA/PDMS and CCA/PDMS EMI shielding composites. It can be seen that the σ of PCCA/PDMS EMI shielding composites shows a gradual increase with the increase in the amount of PCCA. When the loading of PCCA is 2.80 wt%, the σ of the PCCA/PDMS EMI shielding composites reaches 0.094 S cm-1. This is mainly due to the fact that the conductive network inside the PCCA/PDMS EMI shielding composites is gradually improved with the increase in PCCA. As the loading of CCA increases, the σ of the CCA/PDMS EMI shielding composites tends to increase and then decrease. When the loading of CCA is 2.24 wt%, the CCA/PDMS EMI shielding composites have the largest σ value (0.47 S cm−1). This is mainly attributed to the gradual improvement of the CCA–CCA conductive network within the CCA/PDMS EMI shielding composites with increasing CCA loading [37]. However, the further increase in the amount of CCA causes the fibers within the CCA to twist into knots, which is detrimental to the formation of the complete conductive pathway and thus has negative impact on the σ. In addition, the σ of the CCA/PDMS EMI shielding composites is consistently much larger than that of the PCCA/PDMS EMI shielding composites for the same amount of CCA or PCCA [38]. At a CCA loading of 2.24 wt%, σ for the CCA/PDMS EMI shielding composites (0.47 S cm−1) is 6.3 times greater than that of the PCCA/PDMS EMI shielding composites (0.075 S cm-1) with the same loading of PCCA. Mainly due to the random distribution of PCCA in the PCCA/PDMS EMI shielding composites, which is difficult to form the effective PCCA–PCCA conductive network. Within the CCA/PDMS EMI shielding composites, CCA has the more complete 3D conductive network structure, giving it an even better σ.

Fig. 4
figure 4

σ of the PCCA/PDMS and CCA/PDMS EMI shielding composites (a) and P(CCA@rGO)/PDMS and CCA@rGO/PDMS EMI shielding composites (b)

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Figure 4b shows the σ comparison diagram of P(CCA@rGO)/PDMS and CCA@rGO/PDMS EMI shielding composites. It can be seen that the σ of the P(CCA@rGO)/PDMS EMI shielding composites tends to increase as the loading of P(CCA@rGO) increases. The σ of the P(CCA@rGO)/PDMS EMI shielding composites reaches 0.117 S cm-1 at a P(CCA@rGO) loading of 3.32 wt%, mainly due to the gradual improvement of the conductive network inside the P(CCA@rGO)/PDMS EMI shielding composites with the increasing P(CCA@rGO) loading. With the increase in the loading of CCA@rGO, the σ of the CCA@rGO/PDMS EMI shielding composites tends to increase and then decrease [39]. When the loaidng of CCA@rGO is 3.05 wt%, the CCA@rGO/PDMS EMI shielding composites have the largest σ value (0.75 S cm-1), which is 59.6% higher than the σ (0.47 S cm−1) of the CCA/PDMS EMI shielding composites (2.24 wt% CCA). This is mainly due to the fact that the rGO wrapped around the CCA gradually forms the second conductive network as the amount of rGO increases based on the first conductive network (2.24 wt% CCA). The synergy of the two conductive networks results in the gradual improvement of the internal conductive network of the CCA@rGO/PDMS EMI shielding composites and the consequent increase in its σ [40]. However, as the amount of rGO increases further, the rGO is prone to agglomeration inside the CCA@rGO and the CCA is not uniformly wrapped, resulting in an imperfect second conductive network, which has negative impact on the σ. It can also be seen that the σ of the CCA@rGO/PDMS EMI shielding composites is consistently much greater than that of the P(CCA@rGO)/PDMS EMI shielding composites for the same loading of CCA@rGO and P(CCA@rGO). When the loading of CCA@rGO is 3.05 wt%, the σ of the CCA@rGO/PDMS EMI shielding composites (0.75 S cm-1) is 7.1 times higher than that of the P(CCA@rGO)/PDMS EMI shielding composites (0.106 S cm-1) with the same loading of P(CCA@rGO). Mainly due to the random distribution of P(CCA@rGO) in the P(CCA@rGO)/PDMS EMI shielding composites, which makes it difficult to form an effective P(CCA@rGO)-P(CCA@rGO) conductive network through point–point laps [41]. In the CCA@rGO/PDMS EMI shielding composites, the CCA@rGO has the more complete 3D conductive network. At the same time, the rGO sheet is wrapped around the CCA fibers to form the double-layer conductive network with the skin-core structure. The CCA–CCA (wire–wire), CCA–rGO (wire–surface) and rGO–rGO (surface–surface) laps form the very complete 3D double-layer conductive network, giving it an even better σ.

Figure 5 shows the comparison graph of the EMI shielding effectiveness (EMI SE) results of PCCA/PDMS, CCA/PDMS, P(CCA@rGO)/PDMS and CCA@rGO/PDMS EMI shielding composites. As shown in Fig. 5a, the EMI SET of the PCCA/PDMS EMI shielding composites tends to increase as the loading of PCCA increases. When the loading of PCCA is 2.80 wt%, the EMI SET of the PCCA/PDMS EMI shielding composites is 12 dB. This is mainly due to the PCCA–PCCA conductive network within the PCCA/PDMS EMI shielding composites gradually improving with increasing PCCA loading, resulting in an increased ability to reflect and absorb incident electromagnetic waves, which is reflected in the increase in EMI SET value [42]. Figure 5b shows that as the loading of CCA increases, the EMI SET of the CCA/PDMS EMI shielding composite material first increases and then decreases. When the loading of CCA is 2.24 wt%, the CCA/PDMS EMI shielding composites have the best EMI SET (40 dB), which is 20 times than that of pure PDMS (2 dB). This is because the density of the CCA–CCA conductive network within the CCA/PDMS EMI shielding composites increases as the amount of CCA increases [43]. At the same time, the two-phase interface with the PDMS matrix increases, resulting in enhanced conductive losses, impedance mismatch and interfacial polarization losses between the incident electromagnetic waves and the CCA–CCA conductive network, thus significantly improving the EMI SET of the CCA/PDMS EMI shielding composite [44]. However, when the amount of CCA is too high, the fibers in CCA tend to twist into knots, which reduces the conductive network density of CCA and reduces the two-phase interface between CCA and PDMS substrate, thus reducing its EMI SET. As shown in Fig. 5c, the EMI SET of the P(CCA@rGO)/PDMS EMI shielding composites tends to increase gradually as the loading of P(CCA@rGO) increase. The EMI SET of P(CCA@rGO)/PDMS EMI shielding composites is 14 dB when the loading of P(CCA@rGO) is 3.32 wt%. This is mainly due to the gradual improvement of the conductive network inside the P(CCA@rGO)/PDMS EMI shielding composites with the increase in the loading of P(CCA@rGO), which leads to the enhancement of its ability to reflect and absorb the incident electromagnetic waves, manifested in the increase in the EMI SET [45, 46]. Figure 5d shows that the EMI SET of CCA@rGO/PDMS EMI shielding composites tend to increase and then decrease as the loading of CCA@rGO increase. When the loading of CCA@rGO is 3.05 wt%, the CCA@rGO/PDMS EMI shielding composites have the best EMI SET (51 dB), which is 27.5% higher than the EMI SET (40 dB) of the CCA/PDMS EMI shielding composites (2.24 wt% CCA) and 25.5 times higher than that of the pure PDMS (2 dB). This is because as the amount of CCA@rGO increases, the CCA (first conductive network) wrapped with rGO gradually forms the perfect second conductive network, and the skin-core structure of CCA@rGO makes the two conductive networks work together to form the perfect 3D double-layer conductive network [47]. At the same time, the interfaces between rGO and CCA, rGO and rGO, and CCA@rGO and PDMS matrix are increased, so that the conductive loss, impedance mismatch and interface polarization loss between CCA@rGO/PDMS EMI shielding composite and incident electromagnetic waves are enhanced, which significantly improves the EMI SET of CCA@rGO/PDMS EMI shielding composites. However, when the loading of rGO is too high, rGO tends to agglomerate in CCA@rGO, resulting in an imperfect second conductive network and reducing the conductive network density. At the same time, it reduces the two-phase interface between CCA@rGO and PDMS matrix, which adversely affects the EMI SET of CCA@rGO/PDMS EMI shielding composites [48].

Fig. 5
figure 5

EMI SET of the PCCA/PDMS EMI shielding composites (a), EMI SET of the CCA/PDMS EMI shielding composites (b), EMI SET of the P(CCA@rGO)/PDMS EMI shielding composites (c), EMI SET (d), EMI SEA and SER (d’) of the CCA@rGO/PDMS EMI shielding composites, schematic illustration of EMI shielding mechanism (e)

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Combining Fig. 5c, d also shows that the EMI SET of CCA@rGO/PDMS EMI shielding composites is always better than that of P(CCA@rGO)/PDMS EMI shielding composites at the same CCA@rGO and P(CCA@rGO) loading. When the amount of CCA@rGO is 3.05 wt%, the EMI SET of CCA@rGO/PDMS EMI shielding composites is 51 dB, which is 3.9 times higher than that of P(CCA@rGO)/PDMS EMI shielding composites (13 dB) with the same loading of filler. This is because the conductive fillers in P(CCA@rGO)/PDMS EMI shielding composites are randomly distributed, and the efficiency of lap bonding through P(CCA@rGO)-P(CCA@rGO) (point–point) is extremely low [49]. At the same time, P(CCA@rGO) has high surface energy and are prone to agglomeration within the PDMS matrix, which makes it difficult to form an effective conductive network, thus affecting the reflectivity and dissipation ability of the P(CCA@rGO)/PDMS EMI shielding composites for incident electromagnetic waves, and therefore, its EMI SET enhancement effect is poor [50]. For the CCA@rGO/PDMS EMI shielding composites, the skin-core structure allows CCA@rGO to form the 3D double-layer conductive network structure with a high conductive network density, which enhances the conductive loss and impedance mismatch between the incident electromagnetic waves and the CCA@rGO/PDMS EMI shielding composites (Fig. 5e). Meanwhile, the introduction of rGO leads to more two-phase interfaces between rGO and CCA, rGO and rGO, and CCA@rGO and PDMS matrix, which significantly improves the interfacial polarization loss capability of CCA@rGO/PDMS EMI shielding composites to incident electromagnetic waves [51]. The synergistic effect of the two aspects makes the CCA@rGO/PDMS EMI shielding composites have relatively stronger reflection, scattering and absorption of incident electromagnetic waves, so that their EMI SET is consistently better than that of P(CCA@rGO)/PDMS and CCA@rGO/PDMS EMI shielding composites [52].

Figure 5d’ shows that the EMI SEA and EMI SER of CCA@rGO/PDMS EMI shielding composites also tend to increase and then decrease as the loading of CCA@rGO increases. When the loading of CCA@rGO is 3.05 wt%, the EMI SER and EMI SEA of CCA@rGO/PDMS EMI shielding composites reach the maximum values of 7 dB and 44 dB, respectively. This is because the continuous increase in CCA@rGO provides more mobile charge, which enhances the impedance mismatch between the CCA@rGO/PDMS EMI shielding composites and the incident electromagnetic wave, hence the EMI SER increase [53]. Meanwhile, the CCA@rGO-CCA@rGO double-layer conductive network is gradually improved with the increase in CCA@rGO loading, which can provide more carriers for dissipating electromagnetic waves, so its EMI SEA is improved [54]. However, as the loading of CCA@rGO increases further, rGO tends to agglomerate inside CCA@rGO and CCA is not uniformly wrapped, reducing the internal conductive network density of CCA@rGO/PDMS EMI shielding composites and decreasing the two-phase interface between CCA@rGO and PDMS matrix [55]. This weakens the ability of the CCA@rGO/PDMS EMI shielding composite to reflect, scatter and absorb incident electromagnetic waves, resulting in lower EMI SER and EMI SEA.

3.4 Thermal Conductivities

Figure 6 shows the λ (a), thermal diffusivity (α, b), 3D infrared thermal images (c) and surface temperature curves vs heating time (d) of the CCA@rGO/PDMS EMI shielding composites. Figure 6a, b shows that λ and α of CCA@rGO/PDMS EMI shielding composites both tend to increase and then decrease as the amount of CCA@rGO increases. When the loading of CCA@rGO is 3.05 wt%, the CCA@rGO/PDMS EMI shielding composites have the largest λ (0.65 W mK-1) and α (1.082 mm2 s-1), which are 3.3 and 3.4 times higher than λ (0.20 W mK-1) and α (0.3185 mm2 s-1) of pure PDMS. This is because, as the loading of CCA@rGO increases, rGO gradually wraps the CCA fibers to form the 3D double-layer thermal conductivity network with the skin-core structure, which improves the thermal conductivities of CCA@rGO/PDMS EMI shielding composites. However, as the amount of CCA@rGO increases further, the internal rGO of CCA@rGO tends to agglomerate, which reduces the density of the thermal conductivity network inside the CCA@rGO/PDMS electromagnetic shielding composites [56, 57]. However, with the further increase in the loading of CCA@rGO, the rGO inside CCA@rGO tends to agglomerate, which decreases the density of the thermal conductivity network inside the CCA@rGO/PDMS EMI shielding composites, thus adversely affecting the λ and α of the CCA/PDMS EMI shielding composites [58].

Fig. 6
figure 6

λ (a), α (b), 3D infrared thermal images (c) and surface temperature curves vs heating time (d) of the CCA@rGO/PDMS EMI shielding composites

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As shown in Fig. 6c, the heat flow conduction rate is significantly higher inside the CCA@rGO/PDMS EMI shielding composites compared to the CCA/PDMS EMI shielding composites for the same temperature thermal stage and heating time, indicating their excellent thermal conductivities [59]. Meanwhile, with the increase in the amount of CCA@rGO, the heat flow conduction rate inside the CCA@rGO/PDMS EMI shielding composites becomes faster and then slower, indicating that the appropriate amount of CCA@rGO (3.05 wt%) is beneficial to further improving the thermal conductivities of the CCA@rGO/PDMS EMI shielding composites, which is consistent with the experimental results of Fig. 6a, b. In addition, the heat flow is uniformly conducted inside the CCA@rGO/PDMS EMI shielding composites, indicating the relatively uniform dispersion of CCA@rGO in the CCA@rGO/PDMS EMI shielding composites (consistent with Fig. 3d).

The surface temperature change of the CCA@rGO/PDMS EMI shielding composites is divided into two stages as the heating time increases (Fig. 6d). The first stage is the 0 to 40 s heating time period, where the surface temperature of the CCA@rGO/PDMS EMI shielding composite increases rapidly. This is mainly attributed to the low initial temperature of the CCA@rGO/PDMS EMI shielding composites, which causes the large temperature difference between them and the heat table, and therefore, the heat propagation rate is fast [60]. The second stage is the 40 to 80 s heating time period, where the surface temperature of the CCA@rGO/PDMS EMI shielding composites increase slowly. This is mainly attributed to the fact that after 40 s of heating, the temperature of the CCA@rGO/PDMS EMI shielding composites start to increase and the temperature difference between them and the hot table are smaller, so the heat propagation rate become slower [61]. It is also observed that the heating rate of the surface temperature of the CCA@rGO/PDMS EMI shielding composites tend to increase and then decrease in the first heating stage with the increase in the loading of CCA@rGO. In the case of both heating times is 40 s and the loading of CCA@rGO is 3.05 wt%, the surface temperature of CCA@rGO/PDMS EMI shielding composites reach the maximum value of 89.2 °C indicates that the appropriate CCA@rGO (3.05 wt%) is beneficial to efficiently enhance the thermal conductivities of CCA@rGO/PDMS EMI shielding composites [62].

3.5 Mechanical Performances

Figure 7 shows the stress–strain curves (a), tensile strength (b), elongation at break (c) and hardness (d) of the CCA@rGO/PDMS EMI shielding composites. The tensile strength and elongation at break of the CCA@rGO/PDMS EMI shielding composites show a decreasing trend with the increase in the loading of CCA@rGO (Fig. 7a-c). When the loading of CCA@rGO is 3.05 wt%, the tensile strength and elongation at break of CCA@rGO/PDMS EMI shielding composites are 4.1 MPa and 77.3%, respectively, which are 36.9% and 35.6% lower than the tensile strength (6.5 MPa) and elongation at break (120%) of pure PDMS. It is mainly attributed to more two-phase interfaces (weak interfacial connections) between rGO and CCA, rGO and rGO, and CCA@rGO and PDMS matrix with the increase in CCA@rGO loading. It is easy to develop microcracks and voids inside the CCA@rGO/PDMS EMI shielding composites, resulting in the reduction of bond strength. When subjected to external forces, its internal defects will become stress concentration points and rapidly trigger the expansion and fracture of internal microcracks, thus reducing the tensile strength and elongation at break of CCA@rGO/PDMS EMI shielding composites.

Fig. 7
figure 7

Stress–strain curves (a), tensile strength (b), elongation at break (c) and hardness (d) of the CCA@rGO/PDMS EMI shielding composites

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As shown in Fig. 7d, the hardness of the CCA@rGO/PDMS EMI shielding composites illustrates a gradual increase with the increase in the loading of CCA@rGO. When the loading of CCA@rGO is 3.05 wt%, the hardness of CCA@rGO/PDMS EMI shielding composites reach 42 HA, which is 50% higher than that of pure PDMS (28 HA). This is mainly attributed to the fact that the network density of the rigid skeleton CCA@rGO gradually increases with the loading of CCA@rGO, forming more hard two-phase interfacial layers with the PDMS matrix, which effectively hinders the deformation of the CCA@rGO/PDMS EMI shielding composites under pressure, resulting in the increase in the hardness.

Figure 8 shows the σ (a) and EMI SET (b) results of CCA@rGO/PDMS EMI shielding composites after bending fatigue. The σ and EMI SET of CCA@rGO/PDMS EMI shielding composites show a slight decrease with the increase in the number of bending fatigues. When the bending fatigue reach 2000 times, the σ and EMI SET of CCA@rGO/PDMS EMI shielding composites are 0.745 S cm−1 and 50 dB (3.05 wt% CCA@rGO), respectively, which were only 0.7% and 2.0% lower than the σ (0.75 S cm−1) and EMI SET (51 dB) of the CCA@rGO/PDMS EMI shielding composites without bending fatigue, which indicates that the CCA@rGO/PDMS EMI shielding composites have good bending fatigue resistance.

Fig. 8
figure 8

σ (a) and EMI SET (b) values of CCA@rGO/PDMS EMI shielding composites after bending fatigue

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3.6 Thermal Stabilities

Figures 9a, b shows the DSC and TGA curves of the CCA@rGO/PDMS EMI shielding composites, respectively, and Table 1 shows the corresponding thermal characteristic data. Figure 9a and Table 1 show that the Tg of the CCA@rGO/PDMS EMI shielding composites gradually increases with the increase in the loading of CCA@rGO. When the loading of CCA@rGO is 3.05 wt%, the Tg of CCA@rGO/PDMS EMI shielding composites is −43.4 °C, which is 5.7 °C higher than that of pure PDMS. This is mainly attributed to the fact that the hard two-phase interfacial layer between CCA@rGO and PDMS matrix increases with the increase in CCA@rGO loading, which restricts the movement of PDMS molecular chains and makes Tg increase [63]. As shown in Fig. 9b and Table 1, the T5, T30 and the corresponding THRI of the CCA@rGO/PDMS EMI shielding composites show a gradual increase with the increase in the CCA@rGO loading. When the loading of CCA@rGO is 3.05 wt%, the T5, T30 and the corresponding THRI of the CCA@rGO/PDMS EMI shielding composites are 318.1, 394.4 and 178.3 °C, respectively. which are 25.8, 362.5 and 163.9 °C higher than The T5, T30 and THRI of the pure PDMS (292.3, 362.5 and 163.9 °C). It is mainly attributed to the fact that the introduction of rGO with excellent heat resistance can help improve the heat resistance of CCA@rGO/PDMS EMI shielding composites [64]. Meanwhile, the good interfacial compatibility between CCA@rGO and PDMS matrix can effectively prevent the oxygen penetration and thermal degradation behavior of CCA@rGO/PDMS EMI shielding composites [65, 66]. The synergy of the two aspects leads to the significant improvement in the heat resistance of CCA@rGO/PDMS EMI shielding composites compared to pure PDMS.

Fig. 9
figure 9

DSC curves (a) and TGA curves (b) of the CCA@rGO/PDMS EMI shielding composites

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Table 1 Thermal characteristic data of the CCA@rGO/PDMS EMI shielding composites
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4 Conclusions

rGO was successfully wrapped on the surface of CCA to form CCA@rGO with the 3D double-layer conductive network skin-core structure, and its 3D conductive network structure was not significantly damaged during backfilling with PDMS. When the loading of CCA@rGO is 3.05 wt%, CCA@rGO/PDMS EMI shielding composites have the best EMI SET (51.0 dB). At this time, the CCA@rGO/PDMS EMI shielding composites have outstanding thermal conductivities (λ is 0.65 W mK−1), excellent mechanical properties (tensile strength and hardness are 4.1 MPa and 42 HA, respectively) and excellent thermal stabilities (THRI of 178.3 °C). Excellent EMI shielding performances and thermal stabilities, as well as good thermal conductivities, make CCA@rGO/PDMS EMI shielding composites have great application prospects in lightweight, flexible electromagnetic shielding composites and portable and wearable electronic devices.