Highly conductive PBFDO-based multifunctional composite for electromagnetic interference shielding, thermal management, and sensing†

Ting Lin^a, Feng Zeng^a, Zhiming Zhong^a, Xiaoling He*^b, Zhenzhong Sun^ac and Jin Xu*^ac
aSchool of Mechanical Engineering, Dongguan University of Technology, Dongguan 523808, China. E-mail: xujin@dgut.edu.cn
^bKey Laboratory of 3D Printing Technology in Stomatology, The First Dongguan Affiliated Hospital, Guangdong Medical University, Dongguan, 523710, China. E-mail: xiaolinghe@gdmu.edu.cn
^cGuangdong-Hong Kong-Macao Joint Laboratory for Neutron Scattering Science and Technology, Dongguan University of Technology, Dongguan 523808, China

First published on 29th May 2025

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1. Introduction

The proliferation of intelligent electronic devices, such as smartphones, wearable gadgets, and autonomous vehicles, has significantly enhanced the quality of life, yet the resultant electromagnetic interference (EMI) has posed risks of not only malfunctioning in these electronic devices but also potential adverse effects on human health.^1–3 The trend towards lighter, thinner, and more flexible wearable electronics presents novel challenges and demands for EMI shielding films. These films must possess not only superior shielding performance but also multifunctional attributes like lightness, elasticity, and efficient thermal management.^4–6

EMI shielding materials are divided into two types by mechanism: absorptive materials dissipate electromagnetic energy via dielectric/magnetic losses, while reflective materials use high conductivity to create impedance mismatches, reflecting waves at interfaces and blocking penetration into protected spaces.^7,8 Compared to absorptive materials, reflective EMI shielding materials offer superior material selection flexibility, with high electrical conductivity as their key feature. The core attribute of EMI shielding materials is their high electrical conductivity. Consequently, while metallic films, such as copper and aluminum foils, have been extensively applied in this domain, their high density and lack of flexibility pose significant impediments to their suitability as next-generation EMI shielding materials.⁹ Stretchable conductive composites, which offer a unique blend of high conductivity and flexibility, present a significant challenge in selecting appropriate conductive fillers and ensuring their uniform dispersion throughout the polymer matrix. Conductive fillers primarily consist of metal-based materials, carbon-based conductive materials, and conductive polymers.^10–13

Metal-based materials, including metal nanowires, metal nanoparticles, and metal carbides, demonstrate remarkable electrical conductivity.^14–17 However, their high surface energy and powerful intermolecular forces make them susceptible to agglomeration and difficult to disperse evenly within a polymer matrix. Furthermore, the elevated density of the metal component makes conductive composites utilizing metallic fillers less optimal for certain applications. Carbon-based conductive fillers, such as carbon nanotubes, graphene, and carbon black, possess high conductivity and low density, yet they also face the challenge of ready agglomeration.¹⁸ To address these challenges, techniques such as ultrasonic dispersion, covalent modification, and surfactant are typically employed.^19–22 However, these approaches may compromise conductive performance and involve relatively intricate procedures. Conductive polymers, characterized by their organic conjugated polymer with a heterocyclic structure, derive their conductive properties from the extensive distribution of delocalized electrons or holes along the conjugated main chain, generated during the doping process. Conductive polymers, including poly(3,4-ethylenedioxythiophene-2,5-diyl):poly(styrenesulfonate) (PEDOT:PSS), polyaniline, and polypyrrole, are well-known for their exceptional conductivity and low density.^23–25 Their aqueous solutions remain stable and exhibit resistance to agglomeration, which enables seamless blending with the polymer matrix, positioning them as an optimal material system for the preparation of conductive composites. The PEDOT:PSS, a prominent conductive polymer that uses water as its solvent, can be uniformly mixed with aqueous elastomer solutions to produce polymer conductive composites with enhanced electromagnetic shielding performance.²⁶ However, the majority of polymer matrices are oil-soluble, posing challenges in forming stable conductive composites with water-soluble PEDOT through straightforward blending techniques.

Recently, Huang et al. has been successfully synthesized a highly conductive n-type self-doped conducting polymer, poly(benzodifurandione) (PBFDO).²⁷ This polymer, which uses dimethyl sulfoxide (DMSO) as the solvent, demonstrates superior conductivity compared to PEDOT:PSS and has exhibited exceptional performance in applications such as polymer electrodes, photoelectric generators, and electrochemical transistors.^28–32 In view of its excellent electrical conductivity, the conductive composites based on PBFDO exhibit significant potential for applications in various fields, including electromagnetic shielding, thermal management, and sensing.

Inspired by this, we decided to develop a conductive composite material based on PBFDO. Considering that PBFDO is soluble in DMSO, and DMSO can efficiently dissolve the widely used thermoplastic polyurethane (TPU), this study employed a straightforward solution blending technique to prepare the PBFDO-PU conductive solution, followed by fabricating the PBFDO–TPU conductive composite film using vacuum drying technology. The obtained PBFDO–TPU conductive film exhibits remarkable performance in various domains, including electromagnetic shielding, thermal management, and sensing.

2. Experiment section

2.1. Materials

The PBFDO conductive solution was prepared using DMSO as the solvent at a concentration of 10–12 mg mL⁻¹, which was procured from Dongguan Volt-Amp Optoelectronics Technology Co., Ltd. The TPU was obtained from Lubrizol Corporation.

2.2. Preparation of PBFDO–TPU conductive composites

Taking the preparation of PBFDO conductive composite with a quality score of 4% as an example, 528 mg of TPU particles were added to 2 mL of PBFDO conductive ink. To ensure complete dissolution of TPU, approximately 2.2 mL of pure DMSO solvent was added to the solution. The mixture was then placed on a heating plate at 60 °C and stirred for 5 hours to ensure full dissolution of TPU in DMSO and uniform mixing with PBFDO. Subsequently, air in the mixed solution was removed through centrifugation (5000 rpm for 3 minutes). The processed PBFDO–TPU mixed solution was poured into molds and placed in a vacuum oven for drying. Based on the mass ratio of PBFDO at 2%, 4%, 6%, 8%, and 12%, corresponding conductive composite materials were named as PBFDO2-TPU, PBFDO4-TPU, PBFDO6-TPU, PBFDO8-TPU, and PBFDO12-TPU respectively. Please note that we use a PBFDO concentration of 11 mg ml⁻¹ for the above calculations.

2.3. Preparation of PBFDO–TPU flexible sensor and flexible Joule heater

The TPU-PBFDO conductive composites were precisely cut into the desired rectangular size, followed by secure attachment of wires to both ends using conductive silver glue, resulting in successful fabrication of a TPU-PBFDO flexible sensor and flexible Joule heater.

2.4. Characterization

In this study, Fourier transform infrared spectroscopy (FTIR, FRONTIER, PERKIMELMER) was employed to characterize the infrared absorption peaks of various characteristic functional groups in PBFDO–TPU conductive composites. Additionally, X-ray diffraction (XRD, D8 ADVANCE, Bruker) and scanning electron microscopy (SEM, Sigma 500, Zeiss) were utilized to elucidate the crystalline information and surface morphology of the PBFDO–TPU conductive composites respectively. Furthermore, X-ray photoelectron spectroscopy (XPS, ESCALAB XI, Thermo Scientific) was applied to analyze the elemental composition of the PBFDO–TPU conductive composites while their thermal performance was evaluated using differential scanning calorimetry (DSC, DSC 3, Mettler). Finally, a tensile testing machine (KJ-1065, Kejian) was employed to determine the stress–strain curve of the PBFDO–TPU conductive composites.

2.5. Electrical performance test

In this experiment, the electrical conductivity of the conductive composites were assessed using a high-precision four-probe resistivity tester (HPS58006, Helpass). A square film of the composite material, measuring 10 mm × 10 mm, was positioned on the testing platform, and its thickness was meticulously documented. The surface resistance was measured by the four-probe resistivity tester, and the result was based on the average value of three different positions. According to the following equation, the conductivity can be calculated.

	(1)

Among these parameters, σ represents the electrical conductivity, while R_s and d denote the surface resistance and the sample thickness, respectively.

The two ends of the sensor were securely fastened to the fixtures of the tensile testing machine and subjected to stretching at a rate of 30 mm min⁻¹. Concurrently, a digital multimeter (DMM6500, Keithley) was employed to continuously monitor the sensor's resistance in real-time throughout the stretching process, thereby obtaining the resistance–strain relationship curve. The sensor's sensitivity (GF) can be quantified using eqn (2).

	(2)

Among these, ΔR represents the resistance change induced by strain; R₀ denotes the initial resistance value; and ε signifies the strain of the material.

2.6. Electromagnetic shielding performance test

We employed a microwave network analyzer (PNA-N5222A, Agilent) to characterize the electromagnetic shielding performance of PBFDO–TPU conductive composite materials. The X-band frequency range (8.2–12.4 GHz) was utilized for testing using the wave-guide method. The experimental protocol entailed the precise connection of the waveguide test fixture and associated transmission lines, after which a rectangular sample with dimensions of 22.86 mm × 10.16 mm was positioned within the fixture for electromagnetic shielding assessment. The thickness of the samples varied from 30 to 300 μm.

The total electromagnetic shielding effectiveness (SE_T) is typically comprised of three components: the reflection effectiveness (SE_R), absorption effectiveness (SE_A), and multiple reflection effectiveness (SE_M). Among these, SE_R and SE_A are pivotal parameters for quantifying the electromagnetic shielding performance of materials. When the SE_T surpasses 15 dB, the impact of multiple reflections (SE_M) can generally be disregarded, allowing SE_T to be expressed using the following formula.

SET(dB) = SER + SEA	(3)

The parameters of S11r, S11i, S21r and S21i can be obtained through the network analyzer, and the reflectance (R), transmittance (T), absorptance (A), reflection shielding effectiveness (SE_R), absorption shielding effectiveness (SE_A) and electromagnetic shielding efficiency (ESE) can be calculated by using the eqn (4)–(9).

R = (S11r)2 + (S11i)2 = |S11|2	(4)

T = (S21r)2 + (S21i)2 = |S21|2	(5)

A = 1 − T − R	(6)

	(7)

	(8)

	(9)

2.7. Thermal management performance testing

The TPU and PBFDO8-TPU samples were adhered to aluminum foil paper and subsequently placed flat on a heating platform set at 100 °C. An infrared thermal imager (321Q, Fotric) was employed to monitor and record the surface temperature changes over time.

In the Joule heating performance test, a high-precision linear power supply (eTM-L605SPL6, eTOMMENS) was employed to provide a stable voltage to the PBFDO8-TPU composite material. The surface temperature was monitored in real time using an infrared thermal imager, and simultaneous infrared thermal images were captured.

3. Results and discussion

3.1. Chemical composition and morphology of PBFDO–TPU

In contrast to conventional polymer–metal nanowire/carbon nanotube conductive composites, the dual-polymer system (PBFDO–TPU) demonstrates a markedly reduced tendency towards agglomeration when blended. Analysis of their molecular structures uncovers the presence of distinctive functional groups, such as carbonyl and amide groups (Fig. 1(a)), which are capable of forming hydrogen bonds. The establishment of hydrogen bonding between TPU and PBFDO molecules aids in the uniform dispersion of PBFDO within the TPU matrix, ultimately contributing to the improved conductivity observed in the PBFDO–TPU conductive composite (Fig. 1(b)).

As depicted in Fig. 2(a), the infrared characteristic absorption peak of TPU, located at 3334 cm⁻¹ and attributed to the stretching vibrations of the –NH– bond,³³ undergoes a shift to a lower wavenumber of 3316 cm⁻¹ with an increase in the doping ratio of PBFDO, thereby signifying an enhancement in intermolecular hydrogen bonding.³⁴ The infrared spectra exhibit peaks at 1728 cm⁻¹ and 1701 cm⁻¹, attributable to the stretching vibrations of non-hydrogen-bonded and hydrogen-bonded carbonyl groups, respectively.³⁵ Notably, with an increase in the PBFDO content, a gradual decrement in the intensity of the non-hydrogen-bonded carbonyl peak at 1728 cm⁻¹ is observed, whereas the intensity of the hydrogen-bonded carbonyl peak at 1701 cm⁻¹ progressively intensifies. Through the fitting analysis of the characteristic peaks associated with the carbonyl stretching vibrations (Fig. 2(b) and Table S1, ESI†), the quantitative calculation of the carbonyl hydrogen bonding index (R_H) can be achieved according to the following equation.³⁶

	(10)

Among these, A₁₇₀₁ and A₁₇₂₈ respectively denote the areas of the peaks with peak positions at 1701 cm⁻¹ and 1728 cm⁻¹. The relationship between R_H and PBFDO content, as depicted in Fig. 2(c), reveals a distinct positive correlation, with the R_H value of pure TPU being 0.75, increasing to 1.61 for the composite containing 2 wt% PBFDO, and further to 3.26 for the composite with 12 wt% PBFDO. This observation further reinforces the idea that the carbonyl groups found in PBFDO are capable of forming intermolecular hydrogen bonds with the hydrogen atoms on the imino group of TPU, thereby enhancing their compatibility and blending process.

To further investigate the intermolecular interactions, rheological frequency sweep testing (0.1–100 rad s⁻¹) was conducted to analyze the storage modulus (G′) and loss modulus (G′′) of both TPU and PBFDO8-TPU. As shown in Fig. S1 (ESI†), the consistent dominance of G′ over G′′ across the entire frequency spectrum confirms the characteristic elastomeric behavior of both materials. Notably, PBFDO8-TPU exhibited a 63% reduction in G′ (60 kPa) compared to pristine TPU (160 kPa). This marked decrease in mechanical rigidity can be attributed to the competitive hydrogen-bonding interactions between PBFDO molecules and TPU chains. Specifically, the newly formed hydrogen bonds between PBFDO and TPU components partially disrupt the original hydrogen-bonding network within the TPU matrix, thereby modifying the polymer's viscoelastic response.

Additionally, X-ray photoelectron spectroscopy (XPS) was utilized to conduct full-spectrum and detailed scans of carbon (C), nitrogen (N), and oxygen (O) on both pure TPU and PBFDO8-TPU composite, as shown in Fig. 2(d)–(f). The XPS full spectrum depicted in Fig. 2(d) exhibits characteristic peaks corresponding to C1s (284.80 eV), O1s (532.08 eV), and N1s (399.55 eV) in both TPU and PBFDO8-TPU composite. The high-resolution C1s XPS spectrum presented in Fig. 2(e) reveals the presence of four distinct carbon species within TPU and PBFDO8-TPU: C–C (284.80 eV), C–O (285.90 eV), C–N (286.05 eV), and O–CO (288.90 eV).³⁷ Notably, in comparison to TPU, a higher proportion of the O–CO component is observed in PBFDO8-TPU, attributed to its presence as a functional group of PBFDO. Furthermore, the high-resolution O1s XPS spectrum illustrated in Fig. 2(f) demonstrates two different forms of oxygen present within TPU: O–C (532.08 eV) and –CO (533.62 eV).³⁸ In contrast, PBFDO8-TPU showcases an additional type of oxygen: C–OH (533.06 eV), which likely originates from a small quantity of unreacted enol groups within PBFDO.²⁷

The scanning electron microscope (SEM) images of the PBFDO–TPU conductive composites are presented in Fig. 3. The pure TPU film exhibits a smooth interface with numerous micrometer-sized pores on its surface, which may be attributed to the evaporation of the processing solvent DMSO during the curing process (Fig. 3(a) and (b)). Upon incorporation of a small amount of PBFDO (2–4 wt%), the pore size in the composite material significantly increases, displaying a more dense distribution and clearer interface contour (Fig. 3(c)–(f)), indicating that hydrogen bonding between PBFDO and TPU molecules influences the aggregation morphology of TPU. With further increase in PBFDO doping content (6–12 wt%), the distinct porous interface within the composite material gradually becomes blurred, resembling cotton-like connections (Fig. 3(g)–(l)).

3.2. Physical and electrical properties of PBFDO–TPU

The thermal performance of PBFDO–TPU conductive composites was characterized using differential scanning calorimetry (DSC). Fig. 4(a) depicts the variation of heat flow with temperature during the cooling phase. TPU exhibits a prominent exothermic peak at 90 °C, which corresponds to its glass transition temperature. During this cooling stage, as the polymer molecular chains transition from a mobile to an immobile state at the glass transition temperature, heat is released as a consequence of decreasing their mobility. A distinct glass transition temperature is observable in PBFDO–TPU conductive composites with low doping ratios (2 wt% to 6 wt%). Conversely, at higher doping ratios (8 wt% to 12 wt%), no discernible glass transition temperature is evident, likely due to the substantial restriction of TPU molecular chain mobility by the rigid PBFDO and the intermolecular hydrogen bonding between PBFDO and TPU.

The XRD pattern of pure TPU film (Fig. 4(b)) exhibits a significant broad diffraction peak at approximately 20.4°, which corresponds to the crystallization behavior of polyurethane soft segments.³⁹ As the PBFDO doping ratio increases, this diffraction peak gradually shifts towards higher angles, indicating a reduction in the lattice spacing of polyurethane soft segments due to hydrogen bonding between TPU and PBFDO. Moreover, an additional diffraction peak emerges near 27.3° as a result of π–π stacking (010) of PBFDO molecules.²⁷ The intensity of this peak increases with higher PBFDO content, indicating a more ordered arrangement of the stacked PBFDO molecules. These findings suggest that PBFDO molecules are not randomly distributed within the TPU matrix but instead exhibit an organized stacked structure, which contributes to the enhanced conductivity attributed to PBFDO.

The stress–strain relationship of PBFDO–TPU conductive composites is illustrated in Fig. 4(c). As the concentration of PBFDO increases, there is a notable decline in both the fracture strength and fracture elongation of the composite material. Specifically, when the PBFDO content reaches 2 wt%, the fracture elongation decreases from 975% in pure TPU to 745%. Moreover, as the PBFDO content rises to 4 wt%, the fracture elongation further diminishes to 480%. Notably, when the PBFDO content exceeds 6 wt%, there is a substantial reduction in fracture elongation, approaching as low as 40%. On one hand, at elevated levels of PBFDO content, intermolecular hydrogen bonding between PBFDO and TPU substantially restricts the chain mobility of TPU, leading to a significant reduction in the toughness of the composites. This is due to the fact that the high toughness of polymers predominantly depends on unimpeded segmental movement. On the other hand, PBFDO, being a rigid polymer without side chains, exhibits no segmental motion, as indicated by the absence of a discernible glass transition temperature. Consequently, an increase in the doping concentration of PBFDO further enhances the overall rigidity of the composite.

Electrical conductivity serve as a critical performance indicator for conductive composites. Fig. 4(d) illustrates the electrical conductivity of the PBFDO–TPU. Due to equipment limitations, the reference value for pure TPU's electrical conductivity is reported as 6.76 × 10⁻¹⁴ S cm⁻¹.⁴⁰ Adding 0.5 wt% PBFDO increases conductivity by about nine orders of magnitude to 5.8 × 10⁻⁵ S cm⁻¹, thanks to PBFDO's high conductivity and ordered structure in the TPU matrix. Further increasing the PBFDO content to 2 wt% results in an enhanced electrical conductivity close to 0.71 S cm⁻¹, while contents of 4% and 12% lead to respective electrical conductivity of up to 81 S cm⁻¹ and 312 S cm⁻¹. This exceptional blend uniformity enables a low percolation threshold at only 1%, facilitating formation of a conductive network even with minimal amounts of PBFDO. Additionally, Table S2 provides the electrical conductivity data of other highly conductive composites (e.g., TPU/PEDOT:PSS, TPU/MWCNT, TPU/AgNWs), specifically demonstrating that PBFDO–TPU achieves comparable electrical conductivity to these systems.

3.3. EMI shielding performance of PBFDO–TPU

The significantly improved electrical conductivity imparts excellent EMI shielding effectiveness on PBFDO–TPU conductive composites (Fig. 5(a)). Pure TPU (280 um) demonstrates negligible EMI shielding effectiveness (SE_T: 0.57) in X-band (8–12 GHz). However, incorporation of just 2% PBFDO increases SE_T to 18.38, effectively shielding most electromagnetic waves. Additionally, increasing the PBFDO content to 12% further enhances the SE_T to 40.57, demonstrating outstanding EMI shielding performance. The specific EMI absorption shielding effectiveness (SE_A) of the PBFDO–TPU conductive composites exhibits a significant enhancement with the increment of PBFDO content, as illustrated in Fig. 5(b), varying from 11.4 dB (PBFDO2-TPU) to 27.6 dB (PBFDO12-TPU). This improvement can be attributed to the elevated conductivity, which promotes the more efficient generation of induced currents, thus accelerating the transformation of electromagnetic energy into thermal energy and consequently enhancing the SE_A. Concurrently, a parallel trend is noted for the specific EMI reflection shielding effectiveness (SE_R), escalating from 7 dB (PBFDO2-TPU) to 13 dB (PBFDO12-TPU). This phenomenon may be ascribed to the increased conductivity inducing impedance mismatch at the boundary between air and the conductive composites.

To further elucidate the EMI shielding mechanism of PBFDO–TPU conductive composites, the coefficients of reflectivity (R), absorptivity (A), and transmissivity (T) were calculated as shown in Fig. 5(c). In PBFDO–TPU conductive composites, with the exception of the low-conductivity PBFDO2-TPU, where the value of T approximates zero, all other doping ratios demonstrate an R value exceeding 93%. Consequently, reflection plays a predominant role in the electromagnetic shielding mechanism of PBFDO–TPU conductive composites (Fig. S2, ESI†). Fig. 5(d) illustrates the relationship between different doping ratios of PBFDO and Electric Shielding Efficiency (ESE) for the conductive composites. It is evident that pure TPU material only exhibited an ESE value of 12.19%, indicating negligible influence on EMI shielding performance. Conversely, TPU composite material containing 2 wt% PBFDO attained an impressive ESE value of 98.54%, demonstrating outstanding EMI shielding performance. Furthermore, increasing the content of PBFDO to 12 wt% resulted in an elevated ESE value reaching up to 99.99%, which satisfies most requirements.

In light of the reflective EMI shielding mechanism of PBFDO–TPU, it is essential to consider electrical conductivity as the primary determinant of EMI shielding effectiveness, rather than the thickness of composites. Therefore, we evaluated the EMI shielding capability of thinner PBFDO–TPU films (30–100 μm) (Fig. S3, ESI†). Due to its lower electrical conductivity, the reduction in film thickness resulted in a decline in the electromagnetic shielding effectiveness of PBFDO2-TPU (100 μm), reaching 6 dB. Conversely, in PBFDO–TPU with a higher doping ratio of PBFDO, which exhibits improved electrical conductivity, the decrease in thickness had a minimal impact on its EMI shielding effectiveness, maintaining a level above 20 dB. Remarkably, even when the initial thickness of PBFDO12-TPU was reduced from 300 μm to 30 μm, it still achieved an outstanding electromagnetic shielding effect of 28 dB.

To further validate the EMI shielding performance of the PBFDO–TPU composite material, we conducted an experiment to shield a wireless mouse Bluetooth connection signal, as depicted in Fig. 6. Initially, the mouse was completely encased in aluminum foil, which led to the disruption of the Bluetooth connection signal as a result of the EMI shielding properties of aluminum foil (Fig. 6(a)). Subsequently, a small aperture was created in the aluminum foil. The mouse promptly re-established the Bluetooth connection with the computer, thereby enabling normal cursor control (Fig. 6(b)). Finally, the pinhole in the aluminum foil was sealed using the PBFDO8-TPU material. It was observed that the Bluetooth connection between the mouse and the computer was interrupted once more, indicating that PBFDO8-TPU possesses excellent EMI shielding properties (Fig. 6(c)).

3.4. Thermal management performance of PBFDO–TPU

Modern electronic devices are evolving towards miniaturization and thinness, which has led to an ongoing challenge in heat dissipation. Excellent EMI shielding materials exhibit superior performance owing to their high electrical conductivity, which concurrently bestows these materials with enhanced thermal conductivity. For the purpose of facilitating the testing process, both PBFDO8-TPU and TPU were affixed to the surface of aluminum foil and positioned on a heating plate set at 100 °C. As shown in Fig. 7(a), the digital photographs and corresponding infrared thermal images at various time points are displayed. Initially, both materials had similar surface temperatures. When placed on the heating plate, their surface temperatures quickly rose to 77 °C after 9.2 seconds. However, the heating rate of PBFDO8-TPU then slowed significantly. After 36.3 seconds, its surface temperature reached 90.8 °C, close to the temperature of the surrounding aluminum foil. In contrast, TPU continued to heat rapidly, reaching 98.6 °C after 36.3 seconds, which surpassed the temperature of the surrounding aluminum foil. Fig. 7(b) illustrates the temporal variations in the surface temperatures of TPU, PBFDO8-TPU, and aluminum foil. The temperature–time curves of PBFDO8-TPU and aluminum foil are almost identical, with both having significantly lower steady-state temperatures than that of TPU. In materials with high thermal conductivity (PBFDO8-TPU and aluminum), heat is effectively transferred from the heated side to the upper surface of the film and then dissipated into the surrounding environment, thereby preventing excessive surface temperature buildup. Conversely, for materials characterized by low thermal conductivity, their limited capacity to transfer heat to the surrounding environment results in heat accumulation on the surface, consequently leading to a relatively elevated surface temperature.

Joule heating refers to the thermal energy produced when an electric current flows through a resistive medium. Under conditions of constant voltage, the magnitude of Joule heating is inversely proportional to the resistance of the material. Consequently, the PBFDO8-TPU conductive composite, characterized by its superior conductivity, is anticipated to demonstrate exceptional electrothermal conversion performance. Fig. 8 illustrates the Joule heating data for the PBFDO8-TPU conductive composite under various applied voltages. As illustrated in Fig. 8(a), at each constant driving voltage, the surface temperature of the Joule heater device rises rapidly and subsequently stabilizes as a result of the equilibrium between heat generation and dissipation. Within a range of driving voltages from 0.5 V to 2.5 V, PBFDO8-TPU enables continuous temperature control between 28 °C and 101 °C. Moreover, thanks to its exceptional thermal conductivity performance, once power supply is discontinued, the surface temperature quickly returns to room temperature level. The characteristic growth time constant (τ_g = 9.5 s) and decay time constant (τ_d = 8 s) were determined through fitting the time-temperature data. These parameters demonstrate the system's rapid thermal response, with τ_g representing the heating phase to steady-state and τ_d characterizing the cooling phase to ambient temperature. The efficiency of heat transfer (h_r+c) was determined to be 47.1 W m⁻² K⁻¹, demonstrating the system's high thermal energy transfer efficiency.

The relationship curve between steady-state temperature obtained from thermal imaging images (Fig. 8(b)) and its corresponding driving voltage was plotted (Fig. 8(c)). By fitting experimental data using a quadratic equation model, we derived an equation describing this relationship as T = 12.10V² + 27.44 (Fig. 8(c)). The experimental data aligns well with this fitted curve and confirms our expectation that steady-state temperature should be proportional to square of driving voltage in terms of thermal characteristics analysis. According to this equation, precise temperature control can be achieved by adjusting the voltage to attain the desired temperatures. The magnitude of the coefficient associated with V², which exhibits a clear positive correlation with the material's conductivity, significantly influences the electrothermal conversion efficiency of the Joule heater. Consequently, benefiting from the superior conductivity of PBFDO8-TPU, this heater exhibits significantly enhanced electrothermal conversion efficiency in comparison with previous studies.^41–43 After conducting multiple cycles of power-on and power-off experiments, Fig. 8(d) illustrates the temperature–time variation curve of the heater under a driving voltage of 2 V. During the repeated testing process, the Joule heater consistently demonstrates stable heating and cooling rates, as well as a reliable steady-state temperature, making it exceptionally well-suited for potential future commercialization scenarios.

Therefore, we endeavored to employ the Joule heater in the realm of thermal therapy heating patches, renowned for their flexibility, thinness, and low operational voltage. The thermal therapy heating patch was affixed to the dorsal side of the hand using a constant driving voltage of 1.5 V. Fig. S4 (ESI†) illustrates real-time temperature measurements along with corresponding thermal imaging data. Following a power-on time of 12 seconds, the temperature at the contact area with the heater swiftly escalated from ambient temperature to 37.5 °C and gradually ascended to approximately 56.5 °C within one minute. Flexible Joule heaters can effectively alleviate and mitigate the discomfort associated with cold sensitivity caused by various conditions, while also significantly reducing menstrual cramps in women.

3.5. Sensing performance of PBFDO–TPU

As a stretchable conductive composite, PBFDO–TPU shows significant potential in sensing. We chose PBFDO4-TPU for further sensor testing due to its conductivity and stretchability. Fig. S5 (ESI†) illustrates the resistance change rate of the sensor under 0–100% strain, which was analyzed through linear fitting to obtain sensitivities ranging from 1.11 to 12.44. The sensor has a remarkably thin profile (thickness: 200 um) and can be affixed to various joints and body parts for precise motion detection on the human body.

As depicted in Fig. 9(a), when the flexible sensor is closely attached to the subject's finger joint, it exhibits exceptional capability in real-time discrimination of sensor signal variations across three distinct bending states of the finger. Notably, an augmented bending angle corresponds to an increased resistance change in the sensor. Furthermore, by positioning the flexible sensor on the throat region, as illustrated in Fig. 9(b), it becomes feasible to achieve continuous monitoring of resistance alterations caused by swallowing activity within the throat area. In order to capture dynamic information pertaining to knee movement during flexion testing, sensor was securely adhered onto the anterior aspect of knees as demonstrated in Fig. 9(c). This arrangement enables recording changes in resistance throughout knee flexion and facilitates visualization of knee movement dynamics. Similarly, through tight attachment of sensor on heels during running exercises, step frequency can be effectively monitored. When heel contacts ground surface during each stride cycle, pressure exerted on sensors leads to increased resistance; conversely, when heel lifts off from ground surface at toe-off phase, pressure diminishes resulting in decreased resistance values. By analyzing signals derived from Fig. 9(d), the running frequency of the tester was inferred as 180 steps min⁻¹. Real-time monitoring of running frequency provides valuable insights into runners’ rhythmical patterns, enabling comprehensive performance analysis and optimization of training plans.

4. Conclusions

This study developed high-performance PBFDO–TPU conductive composites by incorporating highly conductive polymer PBFDO, and successfully implemented these composites in applications such as EMI shielding, thermal management, and sensing. As the PBFDO content increased, the hydrogen bonding interactions within the PBFDO–TPU composites progressively intensified, leading to a more uniform and ordered dispersion of PBFDO within the TPU matrix. Consequently, the maximum electrical conductivity of PBFDO–TPU reached 312 S cm⁻¹. The PBFDO12-TPU achieves exceptional EMI shielding, with SE_T values of 40 dB (300 μm) and 28 dB (30 μm). Moreover, the PBFDO8-TPU-based conductive composites exhibited remarkable thermal conductivity and efficient electrothermal conversion capabilities. Within the voltage range of 0.5 V to 2.5 V, the Joule heater fabricated from this material could maintain a stable temperature between 28 °C and 100 °C. Lastly, the PBFDO4-TPU-based flexible strain sensor demonstrated high sensitivity (1.11–12.44), enabling precise detection of human joint movement.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Data availability

The data supporting this article have been included in the manuscript and ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the support of characterization from Dongguan University of Technology Analytical and Testing Center. The research described in this paper was financially supported by the National Natural Science Foundation of China (grant no. 62005043) and the PhD Research Initiation Project of Guangdong Medical University (GDMUB2021023).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc01612f

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