Mechanical and electrical properties of graphene-coated polyimide yarns improved by nitrogen plasma pre-treatment

Abstract

One-dimensional high-performance yarns with excellent conductivity and flexibility are of considerable interest in the energy and aerospace industries. However, how to achieve highly conductivity, excellent flexibility, extreme condition durability and high mechanical performance in one fiber material is still a great challenge using economically viable materials and synthesis technologies. Herein, we report electrically conductive yarns (modified polyimide (M-PI)/reduced graphene oxide (RGO) yarns) consisting of RGO coated on the surface of nitrogen plasma M-PI yarns, which are fabricated by combining the N₂ plasma pre-treatment and repeated dip-coating and reducing technique. N₂ plasma treatment is used to roughen the surface of the PI yarn and introduce functional groups, contributing to improve wettability, which can provide a stronger adhesion of the graphene coating. The dip-coating and reducing process was repeated 10 times to enhance the loading mass of RGO on the PI yarns, then M-PI/RGO yarns with better conductivity property can be obtained. The effects of N₂ plasma treatment power and time on the M-PI yarns and M-PI/RGO yarns are investigated and discussed. The results demonstrate that the graphene layer is uniformly and densely coated on the PI yarn when being treated at 200 W for 8 min, and the conductivity of the M-PI/RGO yarn reaches 1.51 × 10² S/m. The M-PI/RGO yarn combine the advantages of RGO and PI yarns, retaining the mechanical properties and thermal stability of PI yarn while exploiting the conductive property of RGO. In addition, the enhanced adhesion between the PI yarn and graphene coating endows the composite yarns with excellent fastness and superior flexibility. This work describes an environmentally friendly, controllable and facile method to develop flexible and conductive functional graphene-coated PI yarns with high-performance properties.

Keywords

polyimide yarn reduced graphene oxide N2 plasma pre-treatment conductivity flexibility

Flexible conductive fibers have extensive applications in electrical conduction, electromagnetic interference shielding, wearable electronics, e-textile technologies, energy conversion and storage.^1–4 In particular, since lightweight and wearable electronic devices are becoming smaller and lighter in modern electronics, there is an urgent need to produce flexible conductive materials.^5–8 Polyimide (PI) is a high-performance fiber, which not only possesses the advantages of flexibility and light weight of conventional fiber, but also has excellent mechanical property, good dimensional stability and good chemical and thermal stability properties, making it an ideal supporting scaffold material for preparing fiber-based conductive composite materials.⁹,¹⁰ However, the lower adhesion properties of PI prevent it from being used in such applications.¹¹

Graphene has emerged in recent years as a highly attractive material because of its outstanding properties.¹² Graphene is one of the most widely applied conductive coating materials because of its high electrical conductivity, good chemical stability and outstanding flexibility, which excel in the metal fibers and prevail in the academic and industrial communities.^13–15 Graphene exhibits excellent properties via synthesis by chemical vapor deposition (CVD) and mechanical exfoliation,¹⁶ whereas these methods are unsuitable for fabricating conductive composite materials due to the high manufacturing cost and low production efficiency.¹⁷ In contrast, graphene oxide (GO) and reduced graphene oxide (RGO), which are fabricated via wet chemical methods, are cheap and convenient.¹⁶,¹⁸ Currently, many studies on yarns (cotton,¹⁹ silk,¹⁴ polyamide,²⁰,²¹ polyester,²² aramid²³) coated with a graphene layer have reported very high electrical conductivity, and have proved that there is a great potential for using it in the flexible supercapacitors. The conductivity of reported fibers/yarns with graphene coating is shown in the Table S1. Nevertheless, to the best of our knowledge, there are few studies concerning the interfacial modification of graphene-coated PI yarns by the dip-coating and reducing method.

Nevertheless, it is found that PI shows poor wettability and weak interfacial interaction due to its smooth surface and low surface energy.²⁴,²⁵ Because of this, the conductive and flexible properties of the composite materials are limited. Therefore, in order to improve the surface affinity and the interfacial interaction between PI yarns and the graphene coating, study of the surface modifications of PI yarns is necessary. As is well-known, PI has a strong chemical resistance due to its rigid imide ring structure, which can only be degraded in alkali solution of high concentration.²⁶ As a result, many studies have improved the surface activity of PI fibers or membranes through alkali solution treatment.²⁷ The resulting large amount of alkali liquid waste not only pollutes the water environment, but also wastes resources. Among other surface modification methods, such as plasma, radiation and coupling agents, plasma modification is one of the most widely used and mature technologies for physical modification.^28–31 N₂ plasma treatment not only works on the uppermost layer of the materials, but also roughens the surface and introduces functional groups, without damaging properties such as strength and thermal stability.^32–34 So, it has been widely used in the surface modification of other fibers. In addition, plasma treatment has a desirable practicability with regard to its time-saving, eco-friendly, effective and economical features, which makes it especially suitable for continuous and large-scale processes of materials.^35–37 Herein, N₂ plasma pre-treatment of PI yarns may be feasible for graphene and PI yarns, but this needs demonstration.

In this study, the PI yarn surface was modified by N₂ plasma pre-treatment to improve its adhesion with the RGO. Then the repeated dip-coating and reducing technique was proposed to fabricate electrically conductive flexible modified polyimide (M-PI)/RGO yarn. The effects of N₂ plasma treatment time and power on the surface of PI yarn, M-PI yarn, PI/RGO yarn and M-PI/RGO yarn were investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, the contact angle (CA) test, the thermogravimetric analysis (TGA), the tensile test, the conductivity test and the flexibility test.

Materials and method

Materials

PI yarns with a linear density of 1100 dtex/504 F and a cross-sectional area of 8.01 × 10^–4 cm² were provided by Jiangsu Aoshen New Materials Company. Single-layer GO powder (diameter 0.5–5 μm, thickness 0.8–1.2 nm, single-layer ratio about 99% and purity about 99%) was purchased from Nanjing XFNANO Materials Tech Co., Ltd, China. Acetone (CH₃COCH₃, 99.5%), ethanol (C₂H₅OH, 99.7%) and L-ascorbic acid (C₆H₈O₆, 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd, China. All reagents were used without further purification. The water used throughout all experiments was purified through a Millipore system.

Preparation

N₂ plasma-modified PI yarns

PI yarns were first ultrasonically cleaned with acetone, ethanol and deionized water to remove the impurities in turn. Then, in order to remove any remaining acetone, ethanol and water absorbed by the PI yarns, the PI yarns were dried in a vacuum oven at 80°C for 3 h. Subsequently, in order to modify the surface both in terms of functional groups and roughness, the precleaned PI yarns were modified by N₂ plasma pre-treatment at a processing pressure of 30 Pa for the different treatment powers and times, which was operated with an HD-300 low-temperature plasma treatment machine (Changzhou ZhongKe ChangTai Plasma Technology, China). The treatment power was controlled at 100, 200, 300 and 350 W under a fixed time of 8 min, and the treatment time was controlled at 4, 8, 12 and 16 min under a fixed power of 200 W. After the N₂ plasma treatments, the M-PI yarns were immediately sealed in a clean plastic bag for further experiments. All the M-PI yarn characterization measurements were completed in the first 2 days after the N₂ plasma treatment.

Preparation of M-PI/RGO yarns

The well-dispersed GO solution (3 mg/ml) was prepared by mixing single-layer GO powder with deionized water in an ultrasound bath for 40 min below 25°C. Then, the M-PI yarns (modified by different N₂ plasma treatment power and time) were immersed in the GO dispersions (3 mg/ml) for 30 min and dried at 80°C for 1 h to obtain the GO-coated M-PI yarns (M-PI/GO yarns). The liquor ratio of the M-PI yarns and GO dispersions was set to 1:50. Afterwards, the M-PI/GO yarns were immersed in a reducing agent containing 10 mg/ml of L-ascorbic acid at 90°C for 30 min to reduce the GO to RGO. After reduction, the RGO-coated M-PI yarns (M-PI/RGO yarns) were subsequently washed with a large amount of deionized water to remove the residual reducing agent and dried at 80°C for 1 h. The dip-coating and reducing process (Figure 1) was repeated 10 times to enhance the loading mass of RGO on the PI yarns for enduring PI yarns with a better conductivity property. For comparison with the M-PI/RGO yarns, a control sample (PI/RGO yarns) was prepared using the same materials and procedure but without the N₂ plasma pre-treatment. Unless otherwise noted, the M-PI/RGO yarn in this article refers to those made from the M-PI yarn modified by N₂ plasma treatment at 200 W and 8 min.

Figure 1.
Illustration of preparation of the modified polyimide (M-PI)/reduced graphene oxide (RGO) yarn.

Characterizations

Surface morphology observation by SEM

The surface morphologies of the pristine PI yarns, M-PI yarns, PI/RGO yarns and M-PI/RGO yarns were observed using SEM (HITACHI S-4800, Tokyo, Japan) under an acceleration voltage of 5 kV. Specimens were coated with a thin layer of gold prior to the observation.

XPS analysis

The surface chemical compositions of the pristine PI yarns, M-PI yarns, PI/RGO yarns and M-PI/RGO yarns were measured using XPS (ESCALAB 250, Thermo Electron VG Scientific, USA). The X-ray source was Al-Ka (1486.6 eV). The deconvolution analysis of C1s peaks was carried out by XPS-PEAK software.

FTIR analysis

The pristine PI yarns, M-PI yarns and M-PI/RGO yarns were characterized by FTIR performed on a Nicolet Nexus 670 (Thermo Fisher Scientific Corporation, USA) using an attenuated total reflection (ATR) accessory in the scanning wavenumber range from 600 to 2400 cm^–1.

Raman analysis

Raman spectra of the pristine PI yarn, M-PI/RGO yarn and graphene were acquired by a Raman spectrometer (InVia-Reflex) using 532 nm laser excitation.

CA measurement

The CAs of the pristine PI yarns and M-PI yarns were measured using an optical CA meter (OCA15EC type tester, Dataphysics Germany, China). The PI fibers were arranged horizontally and closely in parallel to form a thin fiber layer as a CA test sample. The CA value was recorded after 3 s when the water drop began to still on the PI yarn matrixes. ImageJ analysis software was used to calculate the CA of the fiber droplets, so as to compare the change of PI fiber surface wettability before and after the N₂ plasma treatment. The mean angle value on both sides of the distilled water droplets was adopted as the available CA. In addition, the Young–Dupré equation was used to calculate the work of adhesion. The work of adhesion of the pristine PI yarn and M-PI yarn (modified by N₂ plasma pre-treatment at 200 W and 8 min) can be calculated by the following equation
$W = Y_{L V} (1 + \cos θ)$
(1)
where W is the work of adhesion, Y_LV is surface tension of the water and θ is the value of the CA.

TGA

TGA (TGA 5500, USA) was used for thermal analysis of the pristine PI and M-PI/RGO yarn under nitrogen atmosphere. The test samples were heated from 50°C to 850°C at the rate of 10°C/min.

Tensile test

The tensile properties of the pristine PI yarns, M-PI yarns, PI/RGO yarns and M-PI/RGO yarns were tested on a microcomputer control electron universal testing machine (MTS Systems Co., Ltd, China) at a crosshead speed of 100 mm/min with a gauge length of 200 mm at 20°C and 65% relative humidity. At least 10 specimens were tested for each sample, and the means were calculated.

Conductivity and flexibility measurements

A certain length of PI/RGO yarns and M-PI/RGO yarns were fixed on a glass slide, and the ends of the yarn were fixed with silver paste and connected with a thin copper wire. The resistances of the PI/RGO and M-PI/RGO yarns were measured by a UT151B type digital multimeter (Yulide Technology Co., Ltd, China). The conductivity of the PI/RGO and M-PI/RGO yarns can be directly calculated by the following equation
$σ = \frac{L}{R * S}$
(2)
where σ is the conductivity of yarn, L is the length of yarn, R is the resistances of yarn and S is the cross-sectional area of the yarn.

In order to establish the conductive stability and flexibility, the resistances of the M-PI/RGO yarns with different bending, twisting deformation and bending radius status were tested. The M-PI/RGO yarns were deformed by repeated 100 cycles of bending and twisting deformations, in which the bending angles were 90° and 180° and the twisting angles were 180° and 360°, respectively. The M-PI/RGO yarns were tested by home-made cylinders with a different radius. The resistances of the M-PI/RGO yarns used ultrasonically cleaned with deionized water under 360 W in different times were tested, and the resistances of the M-PI/RGO yarns used a home-made abrasion analyzer for different lengths of time were tested, which is to verify the interfacial adhesion between the graphene coating and the PI substrate. The ratio between the resistance values (R) after treatment and initial resistance values (R₀) of the M-PI/RGO yarns (R/R₀) was used to show the electrical conductivity change. The resistance of the PI/RGO and M-PI/RGO yarns is measured at 5 cm lengths. At least five resistance values of the M-PI/RGO yarns were measured to obtain the average value.

Results and discussion

SEM analysis

The surface morphologies of the pristine PI yarns and M-PI yarns were studied by SEM at the same magnifications, as shown in Figure 2. The purpose of N₂ plasma treatment for the PI yarns is to roughen the yarn surface and generate functional groups. Figure 2 shows that when the PI yarns were modified by N₂ plasma treatment at the different treatment powers and times, obviously, the surface morphologies of the PI yarns are etched and roughened differently. The surface of the pristine PI yarns is relatively clean and smooth (Figure 2(a)). After being treated by N₂ plasma for 200 W and 4 min, a layer of unevenly distributed etching spots is formed on the PI yarn surface (Figure 2(c)). As the treatment time increases to 8 min without changing the power, there is overall uniformly distributed etching spots on the PI yarn surface (Figure 2(b)). When the treatment power increases to 300 W for 8 min, the etching spots on the PI yarns surface become large, deep, uniform and sparse (Figure 2(d)). Thus, it can be concluded that in terms of surface roughness, the appropriate N₂ plasma treatment power and time are 200 W and 8 min, respectively. The increase of fiber surface roughness contributes to better surface wettability and mechanical interlocking effect, which can enhance the interfacial adhesion between the PI yarn and GO.³⁸

Figure 2.
Surface morphologies of the polyimide (PI) yarns: (a) the pristine PI yarns and the modified PI yarns with N₂ plasma treatment of (b) 200 W and 8 min, (c) 200 W and 4 min and (d) 300 W and 8 min.

After the pristine PI and M-PI yarns had undergone the repeated dip-coating and reducing process 10 times, PI/RGO and M-PI/RGO yarns were obtained and are shown in Figure 3. The pristine PI (Figure 3(a)) and M-PI/RGO yarns (Figure 3(b)) are evenly wound on a glass rod. Obviously, the color of the yarns has been changed from yellow to black. A comparison was made of the morphology between the PI/RGO and M-PI/RGO yarns (Figures 3(d) and (e)). The PI/RGO and M-PI/RGO yarns are both coated by RGO. It can be observed that the RGO layer of the PI/RGO yarns is uneven and aggregates in some areas, which would lead to poor conductivity (Figure 3(d)). As shown in Figure 3(e), it can be found that the M-PI/RGO yarns are completely covered with a continuous, uniform, compact and integrated RGO layer, which provides excellent electrically conductive paths, leading to high conductivity.

Figure 3.
Photographs of (a) specimens of the pristine polyimide (PI) yarns and (b) the modified polyimide (M-PI)/reduced graphene oxide (RGO) yarns. Scanning electron microscopy images of (c) the pristine PI yarns, (d) the PI/RGO yarns and (e) the M-PI/RGO yarns.

The single fiber diameters of the pristine PI, PI/RGO and M-PI/RGO yarns were 11.09, 13.50 ± 0.5 and 15.85 ± 0.5 µm, respectively (Figures 3(c)–(e)). Therefore, it can be calculated that the average thicknesses of the RGO layers of the PI/RGO and M-PI/RGO yarns are 1.205 and 2.380 µm, respectively, indicating that the N₂ plasma pre-treated PI yarn can load more RGO mass than the untreated yarn. The rougher morphology of M-PI yarns can lead to more contact area and stronger mechanical interlocking with GO. Thus, M-PI yarns would be beneficial for interfacial adhesion with GO.

XPS analysis

The surface elemental composition of the pristine PI, M-PI, PI/RGO and M-PI/RGO yarns was studied by XPS analysis, as summarized in Table 1. For the pristine PI yarn surface, C, N and O concentrations are 75.54%, 5.50% and 18.96%, respectively, resulting in N to C atom (N1s/C1s) and O to C atom (O1s/C1s) ratios of 0.073 and 0.251, respectively. After the N₂ plasma treatment, the surface C concentration decreases to 72.33%, while the surface N and O concentrations increase to 7.41% and 20.26%, respectively. The increase in the ratio of N1s/C1s and O1s/C1s indicates that the N₂ plasma treatment can introduce nitrogen-containing and oxygen-containing functional groups to the PI yarn surface, which would be beneficial for the wettability and uniform graphene coating on the PI yarn surface. Compared with the pristine PI yarn, the N and O concentrations of the PI/RGO and M-PI/RGO yarns both decrease, which is due to the graphene coating on the PI yarn surface. The O/C ratio of the PI/RGO yarn is higher than that of pristine PI yarn; however, the O/C ratio of the M-PI/RGO yarn is lower than that of the M-PI yarn. This indicates that the GO on the PI yarn without N₂ plasma treatment is not fully reduced to graphene, compared with the M-PI/RGO yarn.

Table 1.
X-ray photoelectron spectroscopy atomic concentration of the pristine polyimide (PI), modified polyimide (M-PI), PI/reduced graphene oxide (RGO) and M-PI/RGO yarns

Samples Atomic concentration (%)
Proportion

C1s N1s O1s O1s/C1s N1s/C1s

Pristine PI yarn 75.54 5.50 18.96 0.251 0.073

M-PI yarn 72.33 7.41 20.26 0.280 0.102

PI/RGO yarn 75.19 1.67 23.14 0.308 0.022

M-PI/RGO yarn 83.23 2.19 14.59 0.175 0.026

Figure 4 shows the wide-scan survey XPS spectra and C1s XPS spectrum of the pristine PI, M-PI, PI/RGO and M-PI/RGO yarns. Four typical carbon-containing groups with binding energies of 284.6 eV (C-C), 285.1 eV (C-N), 286.1 eV (C-O) and 288.3 eV (C=O) are presented and compared³⁹ in Table 2. Compared with the pristine PI yarn, Table 2 shows that the number of C=O bonds drops significantly and the numbers of C-O and C-N clearly increase after N₂ plasma treatment. This result illustrates that the N₂ plasma treatment of the PI yarn causes chemical structural re-arrangements in the PI yarn surface.⁴⁰ The increase of hydrophilic groups can improve the performance of adhesion between PI yarn and graphene. For the PI/RGO yarn, the concentrations of the C-C, C-N, C-O, C=O groups are 16.18%, 31.48%, 37.73% and 14.61%, respectively. For the M-PI/RGO yarn, the concentrations of the C-C, C-N, C-O, C=O groups are 24.58%, 32.78%, 28.26% and 14.38%, respectively. Obviously, the concentrations of oxygen-containing functional groups for the M-PI/RGO yarn are lower than those of the PI/RGO yarn, which is due to N₂ plasma treatment making the GO coating more uniformly coated on the PI yarn and, therefore, the oxygen-containing functional groups of GO are more completely removed after reduction of the M-PI/GO yarn.

Figure 4.
The wide-scan survey X-ray photoelectron spectroscopy (XPS) spectra and C1s XPS spectrum: (a) the pristine polyimide (PI) yarn; (b) modified polyimide (M-PI) yarn; (c) PI/reduced graphene oxide (RGO) yarn; (d) M-PI/RGO yarn.

Table 2.
The concentration of the different carbon groups on the pristine polyimide (PI), modified polyimide (M-PI), PI/reduced graphene oxide (RGO) and M-PI/RGO yarns

Samples Content of different C species on the yarn surface (%)

C-C C-N C-O C=O

Pristine PI yarn 40.98 27.69 13.88 17.45

M-PI yarn 36.24 28.32 25.20 10.24

PI/RGO yarn 16.18 31.48 37.73 14.61

M-PI/RGO yarn 24.58 32.78 28.26 14.38

FTIR analysis

The chemical compositions of the pristine PI, M-PI and M-PI/RGO yarns were confirmed by FTIR-ATR, as shown in Figure 5. Obviously, for three yarns, there are the bands at 1776 (asymmetrical C=O stretching), 1716 (symmetrical C=O stretching), 1379 (C-N stretching) and 724 cm^–1 (C=O ring deformation), which are the most typical characteristic absorption peaks of imide structures.⁴¹,⁴² The peaks at 1168, 1116 and 1092 cm^–1 are ascribed to deformation vibrations of (CO)₂NC groups. The C-C stretching of the benzene backbone corresponds to the band at 1501 cm^–1. In addition, the absorption bands at 1251 cm^–1 of PI yarn are accounted for by C-O-C stretching. For the M-PI yarn, the intensities of absorption peaks at 1716 (symmetrical C=O stretching) and 724 cm^–1 (C=O ring deformation) are decreased, due to the modification of N₂ plasma treatment. In comparison to the pristine PI and M-PI yarns, the FTIR-ATR spectrum of the M-PI/RGO yarn has no substantial variation, but the intensities of the absorption bands of C-C stretching of the benzene backbone, (CO)₂NC groups and the related imide group are all significantly weakened, from which it can be concluded that RGO was coated on the PI yarn surface.

Figure 5.
Fourier transform infrared spectra of the pristine polyimide (PI), modified polyimide (M-PI) and M-PI/reduced graphene oxide (RGO) yarns.

Raman analysis

To verify the deposition of RGO on the surface of PI yarn, Raman spectra of the pristine PI yarn, graphene and M-PI/RGO yarn were measured, as shown in Figure 6. From Figure 6, it can be seen that the PI yarn shows three clearly characteristic absorption peaks at 1395 (C-N-C axial vibration stretch), 1610 (C=C bonding in the aromatic phenylene ring stretch) and 1784 cm^–1 (C=O stretch). Typical D (1336 cm^–1) and G (1589 cm^–1) bands for crystalline graphitic carbon are observed for graphene.⁴³ The Raman spectra of M-PI/RGO yarn shows two new absorption peaks at 1345 (D band) and 1596 cm^–1 (G band) compared to the PI yarn. For M-PI/RGO yarn, there are both characteristic peaks of PI yarn and graphene, and the peak intensity of the characteristic peaks of the PI yarn is weakened, indicating that the graphene was successfully deposited on the surface of the PI yarn. In addition, the intensity ratio of the D band to the G band (I_D/I_G) for M-PI/RGO yarn is 1.28, indicating that most of the GO coated on the surface of PI yarns reduced to RGO.

Figure 6.
Raman spectra of the pristine polyimide (PI) yarn, graphene and modified polyimide (M-PI)/reduced graphene oxide (RGO) yarn.

Contact angle analysis

The CA analysis is valuable in characterizing the surface wettability. Figure 7 shows the average water CAs of the pristine PI and M-PI yarns. The pristine PI yarn is hydrophobic with a large static CA of 113.86°. Conversely, after N₂ plasma treatment, the M-PI yarn is hydrophilic with a smaller CA of 66.42°, which is 41.67% smaller than that of the pristine PI yarn, contributing to the wettability of the PI yarn. Wenzel’s theory shows that the factors of wettability are composed of the chemical composition of the solid surface and the micro-geometric structure. The increase of the surface polar group can reduce the CA. However, when the surface is rougher, the CA of the hydrophilic surface is lower and the CA of the hydrophobic surface is larger. Thus, the CA of the M-PI yarn surface decreases mainly due to the introduction of polar groups induced by N₂ plasma treatment. In addition, based on Equation (1), the works of adhesion of the pristine PI and M-PI yarns are calculated as 42.86 and 100.76 mJ/m², respectively. It can be concluded that the increase of work of adhesion of PI yarn provides stronger adhesion with graphene, therefore contributing to more graphene evenly coated on the PI yarn surface.⁴⁴

Figure 7.
Photographs of water droplets on (a) the pristine polyimide (PI) yarn and (b) modified polyimide (M-PI) yarn.

TGA

Thermostability is an important property for electrically conductive yarns. An excellent heat resistance performance can avoid the yarn melting. The TGA of the pristine PI and M-PI/RGO yarns is shown in Figure 8. It can be seen that the pyrolysis of the M-PI/RGO yarn is similar to that of the pristine PI yarn. The pristine PI and M-PI/RGO yarns start to degrade from 560°C, indicating that they both have excellent thermal stability. The temperatures of 39.5% weight loss (T_39.5) for the PI and M-PI/RGO yarns are both 780°C. The residual masses of the PI and M-PI/RGO yarns are 57.49% and 58.07%, respectively. The results suggest that the RGO coating has almost no effect on the thermal stability of the PI yarn and the M-PI/RGO yarn also has excellent thermal stability.

Figure 8.
Thermogravimetric analysis curves of the pristine polyimide (PI) and modified polyimide (M-PI)/reduced graphene oxide (RGO) yarns.

Tensile properties analysis

The mechanical properties of the pristine PI and M-PI yarns are shown in Figure 9. Figure 9(b) shows that the tensile strengths of the M-PI yarns being treated by N₂ plasma for different treatment powers under a fixed treatment time of 8 min are all higher than those of the pristine PI yarn, suggesting that the N₂ plasma treatment can enhance the mechanical properties of the PI yarns. It can be observed that when the treatment power is 200 W, the tensile strength of the M-PI yarn reaches a maximum from 625.57 to 659.27 MPa (approximately 5.39% increase over the pristine PI yarns). Figure 9(d) shows the tensile strength of the M-PI yarn under the influence of treatment time with a fixed treatment power of 200 W. When the treatment time is 12 min, the strength of M-PI yarn reaches a maximum value of 665.13 MPa (approximately 6.32% increase over the pristine PI yarn). However, when the treatment time is 16 min, the M-PI yarn strength reaches a minimum value of 621.43 MPa, which is slightly smaller than that of the pristine PI yarn (only 0.66% decrease by the pristine PI yarn). This demonstrates that the M-PI yarn could improve mechanical properties when the treatment time is within 12 min. The improvement mechanical property is due to the increased surface roughness, which provides a larger contact area and enhances the cohesion between the fibers of PI yarn, thus provided stronger mechanical interlocking on the interfaces between the PI fibers. However, when the N₂ plasma treatment time is 16 min, the etching and modification of the PI yarn surface will increase the damage on the yarn surface, which will cause the weight of the yarn to be damaged. In addition, because the N₂ plasma treatment only affects the yarn surface within 100 nm, the mechanical performance is barely changed. The results indicate that when PI yarn is used as a matrix material for RGO, the N₂ plasma pre-treated PI yarn is not only beneficial to improve the interfacial bond strength with graphene, but also to improve the overall performance of yarns.

Figure 9.
(a) Stress–strain curves and (b) tensile strength of the pristine polyimide (PI) and modified polyimide (M-PI) yarns with different N2 plasma treatment power. (c) Stress–strain curves and (d) tensile strength of the pristine PI and M-PI yarns with different N2 plasma treatment time. (e) Stress–strain curves and (f) tensile strength of the pristine PI, PI/reduced graphene oxide (RGO) and M-PI/RGO yarns.

Figures 9(e) and (f) show the tensile properties of the pristine PI, PI/RGO and M-PI/RGO yarns. The tensile strength of the PI/RGO and M-PI/RGO yarns improves compared with that of the pristine PI yarn. The tensile strength of the PI/RGO (632.84 MPa) and M-PI/RGO yarns (672.45 MPa) increases by 1.16% and 7.49% more than that of the pristine PI yarn (625.57 MPa), respectively. A comparison was made of morphology between the pristine PI, PI/RGO and M-PI/RGO yarns (Figure 10). The fibers in the pristine PI yarn are arranged in parallel without adhesion (Figure 10(a)). Figures 10(b) and (c) show that the M-PI/RGO yarn is better wrapped by graphene than the PI/RGO yarn. Therefore, the increase in strength can be attributed to the formation of the continuous RGO coating on the PI yarn (Figure 10), which probably enhances the interfacial bonding between the adjacent fibers. In addition, this is an indication that the adhesion between the PI yarn and the RGO coating is quite stable.

Figure 10.
(a) Scanning electron microcopy images of (a) the pristine polyimide (PI), (b) PI/reduced graphene oxide (RGO) and (c) the modified PI/RGO yarns.

Conductivity and flexibility analysis

The amount of graphene coating on the PI yarns directly affects the formation of conductive paths, and ultimately affects the electrical conductivity. The weight content of graphene of the PI/RGO yarn is 6.25% (Figure 11). However, the graphene weight gain of the M-PI/RGO yarn by being treated at 200 W for 8 min is 10% (Figure 11), which is the highest value in all samples. This is due to the fact that the overall uniformly distributed etching spots and functional groups on the surface of the PI yarn increase the specific surface area and surface energy of the yarn, thereby loading more GO. As the N₂ plasma treatment power and time increase, the etching spots on the PI yarn surface become large and sparse, which leads to a reduction of the amount of graphene coating on the PI yarns. The conductivity of the PI/RGO and M-PI/RGO yarns was tested and is shown in Figure 11. The N₂ plasma treatment power and time have an important influence on the conductivity. Figure 11 clearly show that the conductivity of M-PI/RGO yarn by being treated at 200 W for 8 min reaches a maximum of 1.51 × 10² S/m, which is 54.35% higher than that of the PI/RGO yarn (97.69 S/m). This is ascribed to the stronger interfacial contact and the evenly distributed graphene coating, which is consistent with the SEM (Figure 3(e)) and weight content of the graphene results (Figure 11). Therefore, the optimum N₂ plasma treatment power and time are 200 W and 8 min, respectively.

Figure 11.
The weight content of graphene and conductivity of the modified polyimide/reduced graphene oxide yarn with different N2 plasma treatment (a) power and (b) time.

Flexibility and mechanical stability are important characteristics affecting the reliability of e-textiles for smart textile and wearable electronic applications. The stability of the conductivity of the M-PI/RGO yarn is shown in Figure 12. The M-PI/RGO yarn underwent bending and twisting deformations, in which the bending angles are 90° and 180° and the twisting angles are 180°and 360°, respectively. Figure 12(a) shows that the M-PI/RGO yarn is flexible, and the electrical resistance does not change markedly after repeated bending and twisting. The flexibility of the M-PI/RGO yarn was investigated by four home-made cylinders with different radii,⁴⁰ which were made of insulation polyester film. When the M-PI/RGO yarn was attached to the surface of cylinders with different radii and durations, the electrical resistance only slightly changed (Figure 12(b)). In addition, the M-PI/RGO yarn were tested by a water wash (Figure 12(c)). It was ultrasonically washed in water at a frequency of 40 kHz for different times, and then dried at 80°C for 2 h. It can be observed that the resistance of the M-PI/RGO yarn changed a little. Compared with the M-PI/RGO yarn without a water wash, the resistance of M-PI/RGO yarn with washing in water for 30 min is only increased by 18.3%, suggesting that the RGO coating binds strongly to PI yarn substrate. The M-PI/RGO yarn was tested by a home-made abrasion analyzer a different number of times. A diagram of the home-made abrasion analyzer is shown in the embedded picture in Figure 12(d). The resistance of the M-PI/RGO yarn increases as the number of abrasions increases. Before rubbing 1500 times, the resistance of the M-PI/RGO yarn increased slowly. The resistance of the M-PI/RGO yarn is only 17.02% larger than the initial value when the number of abrasions is 1500. However, when the number of abrasions increases to 2000, 2500 and 3000, the resistance of the M-PI/RGO yarn increases 23.40%, 27.66% and 38.3%, respectively. Thus, it can be concluded that the M-PI/RGO yarn has an excellent fastness, thereby contributing to superior flexibility.

Figure 12.
The stability and flexibility test of the conductivity of the modified polyimide/reduced graphene oxide yarn on (a) bending and twisting deformations, (b) home-made cylinders with different radii and (c) a water wash under different ultrasonic cleaning times and (d) abrasion a different number of times.

Conclusions

In summary, the flexible conductive M-PI/RGO yarn was obtained by N₂ plasma pre-treatment combined with the repeated dip-coating and reducing technique. The roughness, wettability and mechanical bonding of the PI yarn surface were increased after N₂ plasma pre-treatment, which enhanced the chemical bonding and mechanical interlocking between the PI yarn and graphene coating, resulting in conductive M-PI/RGO yarn with a continuous and compact RGO coating. The conductivity and adhesion of graphene coating on the PI yarn were different due to the N₂ plasma treatment power and time. It was found that surface morphology and hydrophobicity are critically important substrate conditions that determine the improvement of the electrical properties of M-PI yarn. The results show that the optimal N₂ plasma treatment power and time are 200 W and 8 min, respectively, with which the obtained M-PI/RGO yarn shows better conductivity, flexibility and thermal stability and keeps the inherent tensile strength of the pristine PI yarn. This flexible conductive M-PI/RGO yarn, made in an environmentally friendly, controllable and facile way, has great potential applications, such as in energy-storage devices for high-performance flexible, portable, stretchable and wearable electronics.

Samples	Atomic concentration (%)	Proportion
Pristine PI yarn	75.54	5.50	18.96	0.251	0.073
M-PI yarn	72.33	7.41	20.26	0.280	0.102
PI/RGO yarn	75.19	1.67	23.14	0.308	0.022
M-PI/RGO yarn	83.23	2.19	14.59	0.175	0.026

Samples	Content of different C species on the yarn surface (%)
Pristine PI yarn	40.98	27.69	13.88	17.45
M-PI yarn	36.24	28.32	25.20	10.24
PI/RGO yarn	16.18	31.48	37.73	14.61
M-PI/RGO yarn	24.58	32.78	28.26	14.38

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship and/or publication of this article: This work was supported by the Fundamental Research Funds for the Central Universities (grant no. 2232020A-05, 2232020D-09 and 2232020G-06) and the Innovation of Doctoral Dissertation of Donghua University (Grant No. 17D310116).

ORCID iD

Jinhua Jiang

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Samples	Atomic concentration (%)			Proportion
Samples	C1s	N1s	O1s	O1s/C1s	N1s/C1s
Pristine PI yarn	75.54	5.50	18.96	0.251	0.073
M-PI yarn	72.33	7.41	20.26	0.280	0.102
PI/RGO yarn	75.19	1.67	23.14	0.308	0.022
M-PI/RGO yarn	83.23	2.19	14.59	0.175	0.026

Mechanical and electrical properties of graphene-coated polyimide yarns improved by nitrogen plasma pre-treatment

Abstract

Keywords

Materials and method

Materials

Preparation

N2 plasma-modified PI yarns

Preparation of M-PI/RGO yarns

Characterizations

Surface morphology observation by SEM

XPS analysis

FTIR analysis

Raman analysis

CA measurement

TGA

Tensile test

Conductivity and flexibility measurements

Results and discussion

SEM analysis

XPS analysis

FTIR analysis

Raman analysis

Contact angle analysis

TGA

Tensile properties analysis

Conductivity and flexibility analysis

Conclusions

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References

N₂ plasma-modified PI yarns