Flexible and High-Strength Porous Graphene/Polyurea Composite Film for Multifunctional Applications

Abstract

Porous composites possess distinctive structural features and performance advantages, making them promising for applications in various domains such as sensing, energy storage, and acoustics. A simple, efficient, and environmentally friendly method was employed to prepare porous polyurea materials, which were then modified with graphene nanosheets. The resulting graphene/polyurea porous composites demonstrated enhanced mechanical properties, with a 35.04% increase in tensile strength at a graphene content of 5 wt%. These composites exhibited exceptional multifunctionality, achieving a specific capacitance of 35.74 F/g when used as capacitor electrodes. Additionally, they displayed high sensitivity to resistance and capacitance changes under various mechanical loads, such as tensile, torsional, and bending stresses, with a resistance change rate of 57.72% under 180-degree torsion, highlighting their potential as resistive and capacitive sensors. Compared to traditional materials, the multifunctional composites maintained a resistance change rate below 40% and a capacitance retention rate above 95.07% after 10,000 cycles, underscoring their durability and reliability. Moreover, the developed graphene/polyurea porous composites exhibited good corrosion resistance and an impressive sound absorption rate of 30.68% for high-decibel noise, reducing environmental limitations for their applications. These properties position the composite as a durable, high-sensitivity, multifunctional material with significant potential in sensing, energy storage, and noise reduction applications.

1. Introduction

Porous composites are a class of multidimensional porous network materials [1,2,3] that have the advantages of low mass, large specific surface area, and high porosity. Porous composites have enormous application potential in areas such as sensing [4,5,6], energy storage [7,8,9], and noise reduction [10], attracting widespread attention and research interest. However, traditional porous materials typically exhibit poor strength and toughness and often possess only a single functionality, limiting their applications in various fields [11,12]. To address these deficiencies, an increasing number of researchers are dedicated to combining nanomaterials with porous polymers, fully leveraging the advantages of both to develop novel porous composites with superior performance [3,13,14]. Consequently, the incorporation of nano-reinforcements to prepare porous composites has become a research hotspot in the field of porous polymers [15,16,17,18].

To improve the mechanical performance of porous materials, Yao et al. [19] reinforced polyurethane-imide (PUI) with expandable graphite (EG). Due to the high modulus and high-temperature expansion properties of EG, PUI foams with an appropriate EG content exhibited good mechanical performance and thermal stability. Furthermore, it was discovered that the application of silane coupling agents for EG surface modification could effectively enhance the mechanical characteristics of polyurethane-imide. Haiwen Tang et al. [20] reported the fabrication of high-performance epoxy foam utilizing a pre-curing process. The epoxy foam was prepared with E-51/DDS as the polymer matrix and expandable microspheres (EMP) as the physical blowing agent. Although the aforementioned studies have demonstrated relative improvement in the mechanical properties of porous materials, when these porous composites are applied in sensing and energy storage fields, they are often under dynamic load or solution immersion conditions, which brings more severe challenges to the mechanical properties of porous composites [21,22].

Enhancing the mechanical properties of porous composites, such as strength, toughness, and wear resistance, has always been a core objective in this field, and researchers have increasingly recognized the equal importance of functional attributes for practical applications [23,24,25]. Consequently, in recent years, the study of porous composites has also begun to focus on harnessing their unique porous structures to impart sensing, energy storage, sound absorption, and other functional capabilities. He et al. [26] designed and fabricated a flexible piezoresistive sensor using a porous carbon nanotube (CNT)/polydimethylsiloxane (PDMS) (denoted as CP) composite. This sensor could effectively detect compressive strains as low as 0.1% and exhibited stable responses up to 90% compressive strain. Furthermore, the CP sensor exhibited rapid response and recovery times of 54 ms and 65 ms, respectively, along with durability and stability over 2000 cycles. Patole et al. [27] developed 3D GFs-PDMS composites using a straightforward two-step procedure. These composites demonstrated impressive conductivity (2.85 S/m) and highly sensitive piezoresistive properties (with a gauge factor of 178 over a 10% strain range) due to the presence of an interconnected 3D graphene network. Zhang et al. [28] introduced phosphorus-containing graphene oxide (D-GO) to enhance the performance of PUF considerably. The D-GO composited PUF (D-GO/PUF) was synthesized by employing vacuum impregnation technology. The average sound absorption coefficient of D-GO/PUF-2 was 245.45% greater than that of PUF. Hang Ye et al. [29] identified several issues with traditional foam sound-absorbing materials, such as polymer foams, including heavyweight, poor corrosion resistance, and inadequate sound absorption performance in the mid-to-low frequency range. Given the lightweight nature, excellent thermal stability, and low thermal expansion coefficient of carbon materials, efforts have been made to incorporate carbon materials into acoustic porous structures.

Based on findings from previous research, it is evident that current porous composite materials still face several challenges: complex preparation processes, low mechanical properties, and the limited functionality of previously developed functional porous composites, which are typically confined to singular application areas such as sensing or energy storage without achieving multifunctionality. Therefore, enhancing the mechanical properties and achieving multifunctionality in porous composites remain significant challenges.

To address these knowledge gaps, this study aims to develop multifunctional porous composites using a simple, efficient, and environmentally friendly phase transformation method. These composites will be based on a polyurea matrix with graphene as a functional filler, suitable for applications in sensing, energy storage, and sound absorption simultaneously. The resulting graphene/polyurea porous composites are expected to exhibit good stretchability, corrosion resistance, and multifunctionality. Furthermore, these composites should demonstrate reliability and long-term stability under dynamic loading conditions, thereby offering substantial potential for applications in the fields of sensing and energy storage.

2. Materials and Methods

2.1. Materials

Graphite intercalation compound (GIC 1721) was provided by Asbury Carbons (Asbury, NJ, USA). N,N-dimethylformamide (DMF) was reagent grade and obtained from Mingcheng New Material Co., Ltd., Mingcheng, China. A long-chain diamine with a molecular weight of 2000 (D2000) was obtained from Huntsman Co., Ltd., The Woodlands, TX, USA. Diethyl toluene diamine (E100) was purchased from Jinan Nuoshi New Materials Co., Ltd., Jinan, China. Isophorone diisocyanate (IPDI) was purchased from Shanghai Aladdin Biochemical Technology Co., Shanghai, China. All reagents were used as received.

2.2. Preparation of Aliphatic Polyester

In this experiment, the aliphatic polyurea (APU) matrix was synthesized by two-step solution polymerization using D2000 and IPDI as prepolymer monomers, E100 as the chain extender, and N, N-dimethylformamide as the solvent (see Figure 1a). The specific steps are as follows: Weigh 10 g of D2000 and 3.42 g of IPDI, and add the D2000 slowly to the IPDI at room temperature, stirring thoroughly. Transfer the mixture to an oil bath at 85 °C, stirring at a low speed. After two hours, add 1.6 g of chain extender E100 to the prepolymer. Stir for 30 min, pour into a mold, and transfer to a vacuum freeze-drying oven. After removal, place in an oven at 80 °C for drying and curing. Figure 1b illustrates the chemical synthesis of aliphatic polyurea. Aspartic polyurea and aromatic polyurea are typically synthesized from materials containing a higher number of benzene rings, such as Toluene Diisocyanate (TDI) and Methylene Diphenyl Diisocyanate (MDI). In contrast, aliphatic polyureas have fewer benzene rings, which results in weaker absorption of ultraviolet light, making them less susceptible to photodegradation. This characteristic allows aliphatic polyurea to maintain better stability in ultraviolet environments.

2.3. Preparation of Graphene/Polyurea Porous Composite

In this study, aliphatic polyurea was used as the matrix for the porous composites. Figure 1 illustrates the preparation method using a 5 wt% graphene/polyurea porous composite as an example. For this specific mass fraction, 1 g of the aliphatic polyurea matrix was added to 15 mL of DMF solution and then transferred to an oil bath maintained at 110 °C. After the polyurea matrix was completely dissolved, 0.53 g of graphene powder was added to the solution and thoroughly stirred to ensure even dispersion. The mixed graphene/polyurea solution was inverted onto a glass mold and spread into a thin, homogeneous layer using a glass rod, which was then gently placed in a distilled water coagulation bath. After the mixed solution was transformed into a solid phase in the coagulation bath, the glass mold containing the graphene/polyurea composite was removed and dried at room temperature for 24 h. The completely dried graphene/polyurea film was then removed and stored in an airtight bag for later use. Following this procedure, graphene/polyurea porous composites with 0 wt%–8 wt% graphene content were prepared.

2.4. Characterization

The morphology of polyurea porous composites with different graphene contents was observed using a scanning electron microscope (SEM) (SU 8010, Hitachi, Japan) at 5 kV.

Fourier transform infrared spectrometer (FT-IR) (Nicolet iS20, Thermo Scientific, America) was employed to characterize polyurea. The specific steps involved placing the ATR accessory in the optical path of the spectrometer, scanning the air background, ensuring the sample surface was in close contact with the crystal surface of the ATR accessory, and then collecting the infrared spectrum of the sample. The measurements were performed with a resolution of 4 cm⁻¹, and the testing wavenumber range was set from 400 to 4000 cm⁻¹.

A universal tensile testing machine (GX-SF001, Shenzhen Shared Instrument Equipment Co., Ltd., Shenzhen, China) was utilized to conduct quasi-static uniaxial tensile tests, assessing both the mechanical and piezoresistive properties of the porous composite film under tensile load. The machine, equipped with a 2 kN load cell, operated at a crosshead speed of 0.5 mm/min. From the recorded data, stress–strain curves were plotted to calculate the tensile strength, elongation at break, and Young’s modulus of the composite films. Each sample underwent five tests to obtain average values. The porous composite film exhibited diverse resistance characteristics under various mechanical deformations, including tension, compression, bending, and twisting. The FLUKE 2638A data acquisition system was employed to measure the relative resistance changes under these mechanical loads, using a current source of 100 μA and a voltage of 12 V. During tensile tests, conductive silver paste connected the composite film to one end of a wire, while the other end was linked to the FLUKE system, facilitating the recording of resistance changes under tensile strain.

The electrochemical performance of the graphene/polyurea porous composite film was evaluated using an electrochemical workstation (CHI660E B19038, Chenhua Instruments, Shanghai, China). Specific capacitance values were determined from cyclic voltammetry (CV) experiments using Formula (1) and from galvanostatic charge–discharge (GCD) experiments using Formula (2). These calculations provide insights into the capacitive behavior of the composite.

$$\mathrm{C}={\displaystyle \frac{{\int }_{V_1}^{V_n}IdV}{mv∆V}},$$

(1)

$${\mathrm{C}}_{\mathrm{sp}}={\displaystyle \frac{I∆t}{m∆V}},$$

(2)

The fatigue test was conducted using a universal tensile machine (GX-SF001, Shenzhen Shared Instrument Equipment Co., Ltd., China), with a testing range set at 10% strain. A total of 10,000 cycles was tested at a frequency of 3.33 Hz. The resistance of the composite was continuously monitored during the test using a FLUKE 2638A data acquisition system.

For the corrosion resistance test, the graphene/polyurea porous composite was placed in solutions of H₂SO₄, NaOH, and NaCl, each with a concentration of 2%. An electrochemical workstation was connected to apply a specific current, and the corrosion resistance was evaluated by comparing and analyzing the breakdown voltage of the composite.

For the data processing of the sound-absorbing/attenuation experiment, Magic Music Editor Software (version: v4.1.6) was used to collect decibel values. The voice frequencies in MP3 format of the collected controlled trials were turned on in the software, and the absorption rates were calculated according to the displayed decibel value.

3. Results and Discussion

3.1. FT-IR

Fourier transform infrared (FT-IR) spectroscopy was employed to study the functional group variations [30,31,32]. Figure 2 shows FT-IR spectra of polyurea. At 1629 cm⁻¹, a strong absorption peak was identified, attributed to the stretching vibration of the -C=O- urea group. The polyurea spectrum also exhibited peaks at 1552 cm⁻¹, 1238 cm⁻¹, and 959 cm⁻¹, corresponding to amide II, amide III, and N-H bending vibration, respectively.

Additionally, a single peak was observed around 3328 cm⁻¹, indicating the presence of hydrogen-bonded amino groups with no evidence of free amino groups in the region. The peak at 1629 cm⁻¹ was attributed to the ordered hydrogen-bonded urea carbonyl group, and notably, no absorption peak corresponding to free urea carbonyl groups was observed. This suggests the formation of hydrogen-bonded urea carbonyl groups in the material. These infrared analysis results collectively indicate the successful synthesis of the polyurea material.

3.2. Morphology and Mechanical Property

Scanning electron microscopy (SEM, SU 8010, Hitachi, Japan) has been widely used to observe the microstructure of composites. In this study, polyurea materials with varying graphene contents were chosen for characterization and further analysis. Figure 3 shows the microscopic morphology of porous films with graphene contents of 1 wt%, 3 wt%, 5 wt%, and 8 wt%, respectively. The porous morphology of these films is further observed at different magnifications, both at the micron and nanometer scales.

The results show that in the range of 1 wt% to 5 wt%, with an increase in graphene content, the number of nanopores in the composite film increased significantly; on the contrary, in the range of 5 wt% to 8 wt%, with an increase in graphene content, the increment of nanopores did not increase greatly. This phenomenon has been similarly confirmed in other studies: Liang et al. [33] showed that even with the addition of a small amount of graphene oxide, the pore structure of polyurethane became finer. This may be attributed to the reaction between the oxygen-containing functional groups of graphene oxide and isocyanate, with graphene oxide possibly serving as nucleation points for pore growth, resulting in a denser pore structure that increases the pore surface area. Li et al. [34] described the micropores generated during the pre-oxidation and carbonization processes, which offer good charge accommodation. Moreover, in Figure 3(d3), it can be clearly seen that the shape of the nanopore is chaotic rather than strictly circular. The possible reason is that as the content of graphene increases, the solution is squeezed by graphene during phase transformation.

The pore size and porosity of porous composites with different components were further analyzed, and the results are shown in Figure 4a–e. It is evident that the pore quantity of the porous composite increases with the addition of graphene, and the porosity rises from 12.6% to 49.89%. Interestingly, with the increase in graphene content, the average pore size of the porous composite film initially increases and then decreases. The average pore sizes of polyurethane composites containing 3 wt% GnPs and 5 wt% GnPs increased by 76.79% and 19.01%, respectively, while the average pore size of polyurethane composites containing 8 wt% GnPs decreased by 62.61%. The reason for this trend might be that a small amount of graphene is conducive to the formation of pores during the phase separation of polyurea and solvent. However, as the internal graphene content of the composite gradually increases, it somewhat hinders the diffusion of the solvent during phase separation, leading to a reduction in pore size. In summary, a polyurethane composite containing 5 wt% GnPs maintains a reasonable pore size while ensuring porosity. Therefore, composites with this composition were selected for subsequent experimental studies.

The relationship between graphene content and tensile strength, as well as fracture elongation, is illustrated in Figure 4f. The tensile strength of porous composites exhibits an initial increase followed by a subsequent decrease as the graphene content is increased. The incorporation of graphene as a rigid nanofiller enhances the mechanical properties of porous composites. However, it is worth noting that excessively high porosity in porous materials may compromise the mechanical properties of the composites. When the graphene content reaches 5 wt%, the tensile strength increases from 2.34 MPa to 3.16 MPa, an increase of 35.04%. It decreases slightly beyond 5 wt% graphene. Additionally, the elongation at break decreases from 170% to 109%, representing a reduction of 35.88% compared to the initial value. This decrease is attributed to the addition of graphene, a rigid material with a single-layer, two-dimensional structure. Therefore, the tensile strength of the 5 wt% graphene porous composite is increased by 35.04%, and the elongation at break is reduced by 35.88%, which can better balance the tensile strength and toughness of the material.

3.3. Strain Sensing Performance

The rate of relative resistance change serves as an indicator of the sensing sensitivity of porous composites [35,36,37]. Figure 5a illustrates the resistance change rate under tensile loading for composites with varying graphene content. As anticipated, the resistance change rate of all three composites increases with the application of tensile stress. This phenomenon occurs because, under tensile stress, the graphene flakes within the porous composite separate and lose contact, gradually reducing the number of conductive paths and thereby increasing the relative resistance (Figure 5b). Notably, the composite with a graphene content of 5 wt% exhibits a more pronounced resistance change. This observation can be attributed to the following reasons: with low graphene content, the conductivity is poor, resulting in a weaker relative change under tensile load. On the other hand, with high graphene content, even under the same tensile strain load, a greater number of overlapping conductive fillers remain, forming conductive paths within the composite and leading to a slower increase in resistance.

A sensitivity analysis was conducted on the graphene/polyurea porous composite by applying both torsional and bending loads while monitoring the relative resistance changes. Figure 5c,d depict the variation in relative resistance for the 5 wt% graphene/polyurea porous composite under different loading stress angles. It is evident that as the bending and torsional angles increase, the resistance decreases, and the rate of resistance change increases. This behavior can be attributed to the fact that bending and torsional stresses alter the orientation of graphene flakes within the composite, enabling previously separated graphene nanosheets to come into contact or increase their overlapping area. This results in an increase in the number of conductive pathways, thereby reducing the resistance. When the torsional and bending angles reach their maximum (180°), the resistance change rates reach 57.72% and 40.31%, respectively. These results indicate that the sensor demonstrates good sensitivity and holds the potential for sensing and detecting angular changes in future applications.

The durability of aerospace composites is a decisive factor in determining their stability [38]; in this section, the fatigue resistance of graphene/polyurea porous composites is tested. The graphene/polyurea porous composite was subjected to 10% strain and 10,000 tensile loads at a frequency of 3.33 Hz to test its resistance change during use and verify its stability.

The fatigue resistance performance of the graphene/polyurethane porous composite is illustrated in Figure 5e, which presents the tensile loading-unloading cycles. During the initial 1000 cycles, the corresponding electrical response signals appear chaotic. This can be attributed to microcracks or damage within the composite during the initial tensile loading. These damages may sever some conductive paths or alter the routes of electron movement, resulting in changes in resistance [39,40]. As the number of tensile cycles increases, the number of severed conductive paths also increases until a certain quantity is reached, at which point the rate of resistance change in the composite stabilizes.

Subsequently, a portion of the intermediate cycling was selected for analysis, revealing that the electrical response signals exhibited a stable state, indicating that the composite possesses a certain degree of fatigue resistance to the applied strain. Within this range, the loading and unloading curves show small gaps during the first hundred cycles and the last hundred cycles, suggesting that the composite is sensitive to resistance changes with low hysteresis. This sensitivity can be attributed to the high elasticity of the polyurea matrix, the conductivity of the graphene fillers, and the thorough integration of both.

Furthermore, after approximately ten thousand cycles, an upward trend in the resistance change rate is observed, and the resistance does not return to its original value upon unloading. This behavior is likely due to the accumulation of fatigue damage in the composite with increasing fatigue cycles [41]. Consequently, a portion of the conductive pathways are irreparably damaged and unable to undergo self-repair.

3.4. Capacitor Performance

The porous network structure and electrical conductivity of the graphene/polyurea composite enhance its internal specific surface area and electrically conductive paths, thereby improving its electric charge transport capacity [42,43,44]. The electrochemical performance of the graphene/polyurea composite was investigated using cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) techniques.

Table 1. Mass of active material and area under the CV curve for graphene/polyurea porous composites with different compositions.

	1 wt%	3 wt%	5 wt%	8 wt%
m (g)	0.24	0.10	0.07	0.046
${\int }_{v_1}^{v_n}Idv$	3.65	5.36	9.34	7.16

provides detailed data on the materials used in the experiment. Figure 6a presents the CV curves of the composite within a potential range of 0–0.8 V, showcasing the characteristics of electrochemical double-layer capacitance behavior and ideal capacitance performance attributable to its high porosity, conductivity, and mechanical strength. The specific capacitances were calculated to be 19.05, 23.38, 35.74, and 24.43 F/g for graphene contents of 1, 3, 5, and 8 wt%, respectively, indicating that 5 wt% GnPs is the optimal content for achieving the highest electrochemical performance.

Table 2. Discharge duration of polyurea porous composites containing 5 wt% graphene at varying scan rates.

	0.5 A/g	1 A/g	3 A/g	5 A/g
$∆t$ (s)	35.90	12.60	2.85	1.45

provides detailed data on the materials used in the Charge-Discharge Test. In Figure 6c, the charge–discharge time decreases as the current density increases. For current densities of 5, 3, 1, and 0.5 A/g, the corresponding specific capacitance values of the 5 wt% graphene porous composite are 9.06, 10.69, 15.75, and 22.44 F/g, respectively. The symmetry between the charging and discharging times in the charge–discharge curve typically indicates that the electrochemical system has good reversibility and minimal polarization. This implies that the electrode reactions during charging and discharging have similar kinetics, with the transport rates of electrons and ions being relatively consistent and no significant transport barriers or side reactions present.

The cycling capacity of the graphene/polyurea film electrode in the electrolyte was measured within a potential range of 0–0.8 V and at a current density of 0.5 A/g. Figure 6d demonstrates that the specific capacitance of the synthesized film remains stable without significant deterioration over 20,000 cycles. The overall capacitance retention stays above 95.07%, which can be attributed to the electrical conductivity stability of the graphene flakes. The slight decrease in capacitance may be due to damage to the graphene flakes during the cycling process [45].

Figure 6e illustrates the capacitance variation in graphene/polyurea porous composite within the stress range of 0–5 kPa. It is evident that the specific capacitance of the composites increases with the growing compressive load. This is attributed to the reduction in interlayer distance between graphene layers when the composite film bears the compressive load, resulting in an increased number of conductive pathways and an enlarged capacitance (Figure 6h). When the pressure load reaches 5 kPa, the relative capacitance change rate reaches 58.93%. Meanwhile, Figure 6f presents the capacitance variation rates of the porous composite under bending and torsional stresses, reaching the maximum capacitance change rates of 48.54% and 45.92%, respectively. The increase in capacitance under bending and torsional loads may be attributed to the stress-induced arrangement of the graphene sheets, which promotes partial contact between the sheets, thereby increasing the number of internal conductive pathways and resulting in reduced resistance (Figure 6i).

Alternating current impedance testing was conducted on the 5 wt% graphene/polyurea porous composites. Figure 6g shows the corresponding fitted Nyquist plot. The plot represents the relationship between the real (Z′) and imaginary (Z″) components of the impedance over a range of frequencies, providing insight into the electrochemical behavior of the material. In a typical Nyquist plot, the semicircle observed at higher frequencies corresponds to the charge transfer resistance, while the linear portion at lower frequencies is associated with diffusion processes or Warburg impedance. The measured Nyquist plot for this composite reveals a well-defined semicircle, indicating that the composite exhibits capacitive behavior alongside its inherent resistance. The fitting analysis of the impedance data shows that the resistance of the composite is 128.30 ohms, reflecting its conductive properties and effectiveness as a functional material in potential applications.

3.5. Corrosion Resistance Performance

Due to their unique internal porous structure, graphene/polyurea composites offer favorable conditions for charge transfer and are commonly employed as electrode materials. The tolerance of electrolyte solutions is a crucial factor influencing the performance of electrode materials. This necessitates an evaluation of the acid, alkali, and salt resistance of graphene/polyurea porous composites. The electrochemical polarization curve is an effective means to evaluate the corrosion resistance of polymer materials [46,47].

Figure 7a–c, respectively, show the electrochemical corrosion curves of different components of graphene/polyurea porous composites in acid, base, and salt. The breakdown voltage represents the tolerance to acid, base, and salt solutions, and the greater the breakdown voltage, the greater the tolerance to the test solution. Obviously, the graphene/polyurea porous composite has the highest tolerance to acid, followed by salt, and the lowest tolerance to alkali. In addition, the experimental material with a 5 wt% composition showed superior environmental tolerance compared to other composites.

3.6. Sound Absorption Performance

The conventional aircraft engine is characterized by its substantial size, formidable power, and rapid airflow during flight, resulting in heightened noise levels that necessitate enhanced noise reduction capabilities of aerospace materials [48]. Porous and sponge materials have a certain effect on noise reduction due to their loose and porous structures. Therefore, the noise reduction performance of graphene/polyurea porous composites was studied in this section. Figure 8 shows the research method in this paper: Two experimental boxes were used for testing—one empty box and one with the inner wall covered with the functional composite. As can be seen from Figure 8, the test box with the inner wall covered by a graphene/polyurea functional composite has a better sound absorption effect. At high decibels, the sound absorption rate of the empty box was 11.27%, and that of the experimental box filled with graphene/polyurea porous composite was 30.68%. At low decibels, the sound absorption rate of the empty chamber was 15.56%, and the sound absorption rate of the test chamber filled with graphene/polyurea porous composite was 25.45%.

This decrease in decibels can be attributed to two main factors. Firstly, the unique structure of the porous material obstructs sound waves, causing them to undergo multiple reflections. This results in a decrease in intensity as the waves propagate through the porous composite. Secondly, multiple surface reflections of the sound waves lead to interference, resulting in the cancellation or attenuation of certain frequencies [48,49]. These factors demonstrate that porous composites can effectively reduce environmental noise levels, thereby enhancing acoustic comfort.

4. Conclusions

In conclusion, a simple, efficient, and environmentally friendly method was employed to prepare porous polyurea materials, which were subsequently modified with graphene nanoplatelets. The resulting graphene/polyurea porous composites exhibited good mechanical properties, with a 35.04% increase in tensile strength at a graphene content of 5 wt%. These composites demonstrated exceptional multifunctionality, achieving a specific capacitance of 35.74 F/g when used as capacitor electrodes. Furthermore, their sensitivity to resistance and capacitance changes under various applied loads, such as tensile, torsional, and bending stresses, was notably high. Specifically, under 180-degree torsion, the resistance change rate reached 57.72%, highlighting their potential as both resistive and capacitive sensors.

Compared to conventional materials, the multifunctional composite maintained a resistance change rate below 40% and a capacitance retention above 95.07% after 10,000 cycles, underscoring its durability and reliability. Additionally, the developed graphene/polyurea porous composite exhibited good corrosion resistance and achieved a noise absorption rate of 30.68% for high-decibel noise, reducing environmental constraints on its applications. These characteristics render the composite a durable, highly sensitive, multifunctional material with significant application potential in sensing, energy storage, and noise reduction domains.