Preparation and Properties of Waterborne Polyurethane/Carbon Nanotube/Graphene/Cellulose Nanofiber Composites

Abstract

With the rapid advancement of the flexible electronics industry, there is an urgent need to enhance the mechanical properties and thermal stability of flexible electronic devices to expand their range of applications. To address this need, flexible conductive composites have been developed using waterborne polyurethane (WPU) as the matrix, carbon nanotubes (CNTs) and graphene (GA) as conductive fillers, and incorporating cellulose nanofibers (CNFs). The carbon fillers create a conductive and thermal conductivity network within the matrix, while the presence of CNFs improves the dispersion of CNTs and GA, thereby enhancing the overall network structure. The resulting WGNF composites exhibit a resistivity of up to 1.05 × 10⁴ Ω·cm, a tensile strength of 26.74 MPa, and a thermal conductivity of 0.494 W/(m·K). This demonstrates that incorporating cellulose offers an effective solution for producing high-performance polymeric conductive and thermally conductive composites, showing promising potential for flexible wearable devices.

1. Introduction

With the rapid advancement of electronic information technology, flexible electronic devices have garnered significant attention due to their excellent tensile properties and the ability to control their preparation processes [1,2,3]. Conductive polymer materials are prepared by adding conductive fillers, such as nanometallic materials, inorganic materials, and conductive polymers, into polymers [4,5,6]. These materials are applied in wearable devices, 3D printing, and sensors [7,8,9]. Carbon nanotubes have a good aspect ratio, which leads to good electrical conductivity and mechanical properties, and are a good choice for modifying composites [10,11,12]. Currently, research on PU/CNTs conductive composites has achieved significant advancements, such as high electrical conductivity and low filler percolation thresholds [13]. However, due to the strong van der Waals forces between carbon nanotubes, they tend to agglomerate at high concentrations, making it challenging to achieve uniform dispersion within polyurethane [14]. The challenge of improving the compatibility between the filler and the matrix remains to be addressed further.

Cellulose and its derivatives have high crystallinity, high aspect ratios, and remarkable mechanical strength and toughness [15]. Both are abundant and widely available on Earth. Nanocellulose has good biocompatibility, degradability, and nanostructural effect, as well as a high modulus and high tensile strength [16,17], which makes it an excellent choice for reinforcing the properties of composites [18]. Meanwhile, cellulose is amphiphilic, which can produce strong interfacial bonding with polyurethane molecular chains on the one hand and promote the dispersion of fillers in the matrix on the other [19,20,21]. Fei et al. [22] developed electrically conductive cellulose/CNTs aerogels using a freeze-drying technique, and due to the low density, high porosity (90%), and high electrical conductivity of cellulose/CNTs, the conductive polymer composite (CPC) foams exhibited a significantly low percolation threshold and high piezoresistive sensitivity (GF value of 7.84). After many (1000) compression cycles, the composite foam material can remain stable. Wu et al. [23] used ethylcellulose as the second phase polymer, which was blended with polyurethane to prepare a nanofiber membrane as an antimicrobial strain sensor. The sensor had excellent sensitivity performance after the encapsulation of nanosilver and carbon nanotubes with the assistance of poly(dopamine) (PDA) and provided sensitive and regular resistance feedback with good stability for 100 cycles of various human movements. Yang et al. [24] prepared a polyurethane-based cellulose acetate (CA) composite membrane (CA/TPU) with abundant mesopores via electrostatic spinning. They used reduced graphene oxide (rGO) as a conductive filler and graphene oxide (GO) as an insulating layer via ultrasonic impregnation to securely immobilize them on CA/TPU nanofibrous membranes at 0.5% under very small strains, respectively. The GF of this sensor was 3.006 at very small strains. Zhao et al. [25] prepared bilayer-structured MXene/CNC/WPU (MCW-X) composite films via vacuum-assisted alternating filtration. A conductive network was formed between CNC and MXene nanosheets, enhancing the conductivity of the composite film and stabilizing the composite film against water oxidation. On the other hand, the interaction between WPU and MXene nanosheets increased the interlayer sliding of MXene nanosheets, which improved the tensile strength and toughness of the MXene nanosheets.

In this paper, we investigate the effects of CNTs, GA, and CNFs on the electrical and mechanical properties of WPU. Through the combination of GA and CNFs, the conductive network was regulated, and the percolation value of the conductive polymer composite was reduced. In addition, the introduction of CNFs improved the dispersion of conductive fillers and improved the properties of the composites. In this study, waterborne polyurethane (WPU) was used as the matrix, with carbon nanotubes (CNTs) and graphene (GA) as conductive fillers to create a conductive network for the preparation of conductive composites. Cellulose nanofibers (CNFs) were introduced to enhance the dispersion of CNTs within the matrix and to promote the delamination of GA nanosheets, thereby improving the performance of the composites. Additionally, CNFs establish strong interfacial bonds with WPU, effectively enhancing the mechanical properties of the composites.

2. Experimental Section

2.1. Materials

Waterborne polyurethane with 42% solids was obtained from Anhui Femtosecond Chemical Co., Ltd., Hefei, China. Carboxylated nanofibrillated cellulose fibers (CNFs) and analytically pure N,N-dimethylformamide (DMF) was obtained from Shanghai McLean Biochemical Technology Co., Ltd., Shanghai, China. Concentrated hydrochloric acid and sodium hydroxide were obtained from China National Pharmaceutical Group Co., Ltd., Beijing, China. Carbon nanotubes (CNTs) were obtained from Shenzhen Turing new materials Co., Ltd., Shenzhen, China. Graphene (GA) was obtained from Shanghai Maclin Biochemical Technology Co., Ltd., Shanghai, China, and the deionized water used in the experiments was produced in the laboratory.

2.2. Experimental Steps

2.2.1. Graphene Pretreatment

Place the graphene (GA) in a beaker and immerse it in a 0.1 mol/L sodium hydroxide solution for 1 h. After soaking, filter the solution until neutral. Then, immerse the filtered GA in a 0.1 mol/L hydrochloric acid solution for 1 h, filter until neutral, and finally dry the GA in an oven at 80 °C for 12 h. This process removes metal ions and other impurities from the GA.

2.2.2. Preparation of Waterborne Polyurethane Composites with Carbon Nanotubes

Weigh a specific amount of waterborne polyurethane and dissolve it in a DMF solution. Add a certain proportion of CNTs (1–5%) to the WPU, then stir the mixture with a magnetic stirrer for 2 h (Power 100 W), and follow this with ultrasonic dispersion for 2 h. Next, coat the mixture onto the surface of a polytetrafluoroethylene (PTFE) mold. Dry it at room temperature for 12 h, then place it in an oven at 50 °C for 12 h to obtain the WPU/CNTs composite, referred to as WN-x.

2.2.3. Preparation of Graphene/Carbon Nanotube Waterborne Polyurethane Composites

Select the WN5 composite material with the best electrical conductivity as the substrate for further optimization. Weigh a specific amount of graphene (1–5%) and add it to the WN mixture (with 5% carbon nanotubes). Stir the mixture with a magnetic stirrer for 1 h, and follow this with ultrasonic dispersion for 3 h (Power 100 W). Then, coat the mixture onto the surface of a polytetrafluoroethylene (PTFE) mold. Dry it at room temperature for 12 h, and then place it in an oven at 50 °C for 12 h to obtain the graphene/carbon nanotube waterborne polyurethane composite, referred to as WGN-x.

2.2.4. Preparation of Cellulose/Graphene/Carbon Nanotube Waterborne Polyurethane Composites

Select the WGN3 composite material with the best electrical conductivity as the substrate for further optimization. Weigh a certain amount of graphene (3%) and cellulose (1~5%), add deionized water and subject the mixture to magnetic stirring for 30 min and then ultrasonic dispersion for 30 min to obtain the graphene/cellulose dispersion after centrifugal separation and drying to obtain a mixed powder. Add the mixed graphene/cellulose powder to the carbon nanotubes aqueous polyurethane composite solution after magnetic stirring for 1 h, and then subject it to ultrasonic dispersion for 3 h (power: 100 W). Then, overcoat it on the polytetrafluoroethylene mold surface, leave it to dry at room temperature for 12 h, and then place in an oven at 50 °C for 12 h to obtain the graphene/carbon nanotube waterborne polyurethane composite material, noted as WGNF-x.

2.3. Characterization

A high-resolution field emission scanning electron microscope (SEM) model SU8010 was used to observe the micro-morphology of the composite film sections, which were gold-sprayed and accelerated with an accelerating voltage of 10 KV. XRD analysis of the composites was carried out using a polycrystalline X-ray diffraction analyzer (Sharp shadow Empyrean) with a scanning angle of 5~90° and a scanning speed of 10°/min. An intelligent electronic tensile machine(Shenzhen Mester/CMT4204) was used to test the mechanical properties of the composites at room temperature. An ST2643 ultra-high resistance micro-current tester was used to test film resistivity. The sample was dried and processed, and the test was conducted at room temperature. The average value was taken (the sample is a 50 mm diameter disc). A thermal analyzer was used to test the thermal conductivity of the composites, and the average value was taken for several tests. A Novocontrol, Germany, CONCEPT40 dielectric constant meter was used to test the dielectric constant of the composites; the sample was 2.5 mm in diameter and 1 mm in thickness and was tested under dry conditions at room temperature (frequency 10~10⁷ Hz).

3. Results and Discussion

3.1. SEM

As shown in Figure 1a, the scanning electron microscope image of the WPU cross-section reveals a relatively flat surface. Figure 1b–d display the micro-morphology of the WN5 and WGN3 sections, respectively. These images illustrate that carbon nanotubes and graphene are poorly dispersed within the matrix. Due to the high surface energy of the carbon fillers, carbon nanotubes tend to entangle and form large agglomerates due to van der Waals forces, while graphene also shows signs of aggregation and partial detachment when the material fractures. This indicates poor compatibility between the carbon fillers and the WPU matrix, making it challenging to achieve a uniform dispersion at higher filler concentrations. In contrast, Figure 1e,f show the cross-sectional SEM images of WGNF2, where the addition of cellulose nanofibers (CNFs) significantly reduces the aggregation of carbon nanotubes. Cellulose is observed to adhere to the graphene flakes, promoting their delamination. This improvement in dispersion suggests that the incorporation of cellulose enhances the interfacial compatibility between the filler and the matrix, thereby improving the overall performance of the composite.

3.2. XRD

The XRD spectrum of GA is shown in Figure 2a, from which it can be seen that there exists a strong diffraction peak of GA around 2θ = 26.8°, corresponding to the 001 crystal plane of GA. As shown in Figure 2b for the XRD spectrum of the composite, it can be seen that there exists a broader characteristic peak for WPU around 2θ = 20.07°, indicating that the composite is an amorphous aggregated state structure. After the addition of CNTs, the intensity of the characteristic peak of the composite at this place decreases, while after the addition of GA and CNFs, the characteristic peak of the composite at this place shows a tendency to decrease and then increase with the increase in the GA and CNF content; however, it is always lower than that of pure WPU, which suggests that the addition of fillers destroys the orientation of the composite’s hard segment chain and decreases the aggregation of the composite in the region of hard segments. The 001 crystal surface diffraction peak of GA appeared in the composite at 2θ = 26.8°.

3.3. FT-IR

Figure 3a shows the infrared spectra of CNFs, GA, and CNTs. It can be seen that the -OH stretching vibration peak, the –CH₂ stretching vibration peak, and the OH bending vibration peak appear in CNFs at 3430 cm⁻¹, 2830 cm⁻¹, and 1630 cm⁻¹, respectively. As shown in Figure 3b, the FT-IR map of the composite material shows the N-H stretching vibration peak and bending vibration peak of WPU at 3350 cm⁻¹ and 1533 cm⁻¹, respectively, and the C=O absorption peak at 1727 cm⁻¹. Compared with pure WPU, the N-H and C=O peaks of WGNF2 shifted to lower waves, which is due to the interaction of N-H and C=O with CNFs in WPU, resulting in load shifting of their tensile vibrations [26], and there is no significant change in the IR pattern because of the relatively small content of CNFs.

3.4. Mechanical Characteristics

The mechanical strength of the composites is shown in Figure 4. The tensile strength of pure WPU is 14.07 MPa, and when the content of CNTs is 5%, the tensile strength of the composite material can reach 18.88 MPa, which is 34.2% higher than that of pure WPU (as shown in Figure 4a). This is due to the fact that the CNTs have a good aspect ratio, allowing for the formation of a cross-linking structure with WPU using the CNTs as a node. The force can be added to load part of the force on the CNTs during stretching, thus enhancing the mechanical properties of the composite. The tensile strength trend in WGN is shown in Figure 4b. As shown in this figure, the mechanical strength of the composites after the addition of GA shows a trend of increasing and then decreasing, and the tensile strength of WGN is the highest when the content of GA is 2%, reaching 22.43 MPa, which is an improvement of 18.8% compared with WN5. This is due to the fact that GA has good mechanical properties and can play a role in enhancing the mechanical properties of composites. On the other hand, due to the strong surface energy of the carbon materials, it is easy to form agglomerates in the WPU matrix when the content is high. The mechanical properties of the composites start to decrease when the GA content exceeds 2%. The reason for this phenomenon is due to the aggregation of GA in the WPU matrix, which results in the appearance of pores in the matrix, destroying the interfacial bonding between the filler and the matrix and the cohesion of the WPU matrix itself, which results in decreased mechanical strength. In order to investigate the effect of CNFs on the mechanical properties of the composites, WGN3 material was selected as the basis for introducing CNFs for modification. As shown in Figure 4c for the trend of the mechanical properties of the composites after the addition of CNFs, it can be seen that the mechanical properties of the composites increase firstly and then decrease with the increased CNF content, which is due to the presence of OH groups on the surface of the CNFs, producing a strong interfacial bonding with the molecular chains of the WPU through hydrogen bonding, which enhances the rigidity of the composites, and at the same time, the C-H bonding of the surface of the CNFs can repel the carbon atoms on the surface of CNTs. This has a certain compatibility with GA, which can be adsorbed on the surface of GA to promote its delamination, enhance the dispersibility of CNTs and reduce the degree of aggregation of GA, enhancing the tensile strength of the composites. The tensile strength reaches the maximum value of 26.74 MPa when the CNF content is 2%, and when the content of CNFs is more than 2%, the CNFs begin to aggregate in the WPU matrix, which destroys the interfacial bonding strength between the CNFs and the WPU, and thus, reduces the mechanical properties of the composite materials.

3.5. Electrical Conductivity

From Figure 5a, it can be seen that when the content of CNTs is 5%, the resistivity of the composite is 5.3 × 10⁵ Ω·cm, which is 5 orders of magnitude lower than that of the pure WPU, which is attributed to the fact that CNTs have a good aspect ratio, which endows CNTs with excellent conductivity, making it possible to construct a complete conductive pathway in the WPU; this trend is similar to that reported by Zhang et al. [20]. It can be seen that the incorporation of GA leads to a certain decrease in the resistivity of the composites (as shown in Figure 5b). When the GA content is 3%, the resistivity of WGN reaches its lowest at 2.7 × 10⁴ Ω·cm, which is an order of magnitude higher compared to WN. This may be due to the fact that by using CNTs and GA in combination, the over-diffusion value of the composites can be significantly reduced through the synergistic effect between the fillers, which further enhances the electrical conductivity of the composites. In addition, GA has high electrical conductivity, and a small amount of GA can improve the construction of the conductive pathway under the more complete conductive pathway constructed by CNTs. When the GA content exceeds 3%, due to the increase in the content of conductive fillers and carbon materials with high surface energy, it is difficult to disperse in the matrix to produce an aggregate, leading to decreased conductivity. Figure 5c shows the resistivity trend of WGNF. The addition of a small amount of CNFs can make a certain improvement to the conductivity of the composite material; the resistivity of the composite material when the content of CNFs is 2% is 1.05 × 10⁴ Ω·cm. This may be due to the fact that the C-H bond on the surface of the CNFs is repulsed by the carbon atoms on the surface of the CNTs, and the CNFs are adsorbed on the surface of GA, which, on the one hand, can promote the CNTs in the substrate, and on the other hand, can promote the CNTs in the substrate, which has a high surface energy and is difficult to disperse. On the one hand, it can promote the dispersion of CNTs in the matrix, and it can promote the delamination of GA, reduce the aggregation of GA, improve the construction of the conductive pathway in the WPU matrix, and enhance the interfacial compatibility between the carbon material and the WPU matrix so as to improve the conductivity of the composite material. On the other hand, although the CNFs themselves have insulating properties, in the case of a low content, the current passing through the CNFs can produce electronic jumps, forming a tunneling effect. When too much is added, it can cause part of the conductive pathway to disconnect, leading to decreased electrical conductivity.

3.6. Dielectric Properties

The results of the dielectric constant test of the composites are presented in Figure 6. It can be observed from this figure that the dielectric constant of the composite material exhibits a decreasing trend with the increased operating frequency. This reflects the process of the dielectric relaxation of the WPU molecules. In the low-frequency interval, the polarization mode of the composite material is predominantly interfacial polarization. Furthermore, as the operating frequency increases, the polarization frequency of the composite molecules cannot keep pace with the change in the electric field frequency, preventing the dipole from being fully polarized. This results in a decrease in the dielectric constant of the composite material. The dielectric constant of the composites is enhanced with an increased concentration of conductive fillers within the WPU matrix. This phenomenon occurs due to the fact that, in the absence of an applied electric field, the WPU molecules contain numerous disordered molecular dipole moments. Although the polarity between CNTs, GA, and the WPU matrix is distinct, CNTs possess a considerable aspect ratio. At a specific content, CNTs contact one another to form an electron transport channel, which can facilitate the formation of a conductive network within the matrix. The application of an electric field results in the ordered arrangement of disordered dipole moments in WPU, thereby enhancing the molecular polarization of the composite. The addition of GA/CNF hybrid powder results in the formation of a conductive network within the WPU matrix, which is further enhanced by the high conductivity of GA and the amphiphilicity of CNFs. This leads to the dispersion of CNTs within the matrix, thereby improving the interfacial and molecular polarization of the composites. Consequently, the dielectric constant of the composite is enhanced. The incorporation of GA with high electrical conductivity results in an increase in the number of charged loads in the composites, accompanied by a corresponding intensification of polarization. GA and CNTs typically form a more compact and electrically conductive network structure in the composites. This structure can provide a more effective charge transport path at high frequencies, thereby increasing the dielectric constant. Additionally, at high frequencies, it can effectively reduce dielectric loss and maintain a high dielectric constant. The addition of CNFs may lead to a sparser network structure, which can reduce the enhancement effect of conductivity and polarization at high frequencies. As a result, the dielectric constant of WGNF2 is lower than that of WGN3 at frequencies above 10³ Hz. Therefore, the dielectric constant of the composites increases with the increase in the content of CNTs and GA and improves with the introduction of CNFs.

3.7. Thermal Conductivity

Figure 7 illustrates the thermal conductivity of the composites. As illustrated in Figure 7, the thermal conductivity of pure WPU is 0.261 W/(m·K), whereas the thermal conductivity of the composite material is enhanced to 0.367 W/(m·K) following the addition of 5% CNTs, representing a 40.6% increase in thermal conductivity compared to pure WPU. This is due to the excellent thermal conductivity of CNTs and their large aspect ratio, which facilitates the construction of a robust and effective thermal conductivity network within the matrix, thereby enhancing the thermal conductivity of the composite. The addition of 3% GA to WN5 resulted in a 79.7% improvement in the thermal conductivity of the composite material, reaching 0.469 W/(m·K). This value is 3.5 times higher than that of pure WPU. This is due to the good thermal conductivity of GA, which is mainly attributed to the sp2 hybridized orbitals between the carbon atoms in GA, which allows the electrons to move freely in GA to achieve efficient thermal conductivity. Furthermore, the lamellar structure facilitates the formation of a thermally conductive network within the matrix, which is conducive to heat transfer. The incorporation of GA into the thermal conductive network of CNTs further enhances the conductive network of the composites, improving the thermal conductivity of the composites. The addition of CNFs increased the thermal conductivity of the composites to 0.494 W/(m·K). This was attributed to the amphiphilic nature of CNFs, which facilitated the dispersion of CNTs in the matrix and promoted the delamination of GA nanosheets. This enabled the carbon fillers to contact each other in the matrix, further refining the thermal conductivity of the network.

4. Conclusions

In this paper, conductive composites were prepared using WPU as the matrix and CNTs and GA as the conductive fillers, and CNFs were used as the dispersant to promote the dispersion of the conductive fillers. The amphiphilicity of CNFs was utilized to produce strong interfacial bonding with the WPU matrix through hydrogen bonding while promoting the dispersion of CNTs and the delamination of GA. This is helpful in preventing the excessive aggregation of fillers in the matrix that leads to the deterioration of material properties. This improved the construction of the conductive and thermally conductive networks in the matrix. The resistivity of the composites can be reduced to as low as 1.05 × 10⁴ Ω·cm. The tensile strength can be as high as 26.74 MPa. The dielectric constant improves with the increase in filler, and the thermal conductivity can be as high as 0.494 W/(m·K). In this study, the mechanical properties and electrical conductivity of the conductive composites were enhanced via the incorporation of cellulose; in addition, cellulose and conductive fillers create a brittle conductive network within the matrix, enhancing the material’s sensitivity and providing valuable insights for their application in flexible sensors.