Impacts of polyimide enhanced by amino-modified fluorinated graphene: Thermal,mechanical and tribological behaviors

Abstract

The fluorinated graphene (FG) is modified to get the amino-FG (AFG) which is applied as nanofiller of the polyimide (PI). The repulsion between the C-F bonds endows the AFG with well dispersion in polyimide (PI) matrix, and the amino groups of the AFG react with pyromellitic dianhydride to form strong physical and chemical interactions between the AFG and the PI chains. The results exhibit that the AFG obviously enhances the thermal stability, mechanical performances and wear resistant properties of the PI. When the AFG content reaches 0.5 wt%, the PI/AFG-0.5 composite shows the 35.5% higher tensile stress and 50% higher wear resistance than the PI, which are also higher than the PI/FG-0.5. The strong physical and chemical interactions between the AFG and the PI matrix enhance the interfacial compatibility of the PI/AFG composite, which is better to transfer the stress and heat, so as to enhance the mechanical and tribological properties of the PI. The worn surface changes of the PI/AFG composites indicate that the tribological mechanisms are the synergistic reaction of the abrasive wear and the fatigue wear.

Keywords

polyimide fluorinated graphene mechanical performance thermal stability tribological properties

Introduction

As a super engineering plastic, polyimide (PI) has been used in the fields of electronics, aerospace, cable coatings, and packing materials for its good mechanical, thermal, dielectric and chemical properties.^1-3 However, the high friction and wear of the PI are restricting its safety and reliability under the most demanding environmental conditions.^4-6 Enhancement to the mechanical and tribological properties and other properties of the PI has been the urgent requirements of the industry. Although micromaterials as fillers could improve the mechanical and tribological properties of the PI,^7-10 nanomaterials have been found to be better fillers to enhance the PI.^11,12 Up to now, kinds of nanomaterials, such as silicon dioxide, graphene, carbon nanotube, molybdenum disulfide, boron nitride and so on, have been added to the PI matrix to enhance its mechanical and tribological properties.^13-23

In many available technical solutions, the fluorinated graphene (FG) two-dimension nanomaterials enhanced PI composites have attracted more and more attentions.^24-27 As a graphene derivative, FG can be prepared by exfoliating fluorinated graphite and fluorinating graphene.^28,29 Correspondingly, the FG not only inherits the excellent properties of the graphene but also has the outstanding self-lubricating character and small friction coefficient endowed by the fluorine element.^30,31 When the FG works as fillers of the PI, Zhao et al. found that the fluorination degree of the FG can change the tribological properties of the PI.³² He et al.³³ prepared fluorinated graphene oxide and used it as the filler of PI to enhance the tensile strength and decomposition temperature. Feng et al. obtained the highly transparent, strong, and flexible PI nanocomposite via adding the 2,2^′-bis(trifluoromethyl)-[1,1^′-biphenyl]-4,4^′-diamine modified FG.³⁴ Zhou et al.²⁷ greatly improved the mechanical properties, thermal stability, and tribological properties of PI nanocomposite by using chloroform-treated FG. Our group also prepared a series of PI/FG nanocomposites and found that two-dimensional FG nanosheet, without any chemical modification, can enhance the mechanical properties, thermal stability as well as the antiwear performance of the PI.^26,35

However, the strong chemical forces between the fillers and the polymer matrix always play significant roles in nanomaterials-reinforced polymer composites.^36-40 Therefore, based on the previous research,⁴¹ the two-dimensional FG nanosheets are modified to get the amino-modified FG (AFG) nanosheets. Then, a series of PI, PI/FG, and PI/AFG composites are prepared and tested by kinds of instruments to find the differences in mechanical and tribological properties. Lastly, the strengthening mechanisms of the AFG-enhanced PI composite will be revealed via comparing the different mechanical and tribological properties of the PI, PI/FG, and PI/AFG composites in this work.

Experimental section

Materials

Pyromellitic dianhydride (PMDA) and 4, 4^′-oxydianiline (ODA) were purchased from Sinopharm Chemical Reagent Co., Ltd. and Shanghai Kefeng Chemical Reagent Co., Ltd., respectively. Fluorinated graphite (FGi) was provided by Shanghai CarFluor Chemicals Co., Ltd., (Shanghai, China). Urea was provided by Tianjin Kemiou Chemical Reagent Co., Ltd., (Tianjin, China). The analytical grade N, N^′-dimethylacetamide (DMAc) and N-methyl pyrrolidone (NMP) were used without further purification. Deionized water was used in this work.

Preparation of the FG and AFG

The preparation of exfoliated FG is as follows: The mixed solution formed by 100 mg FGi and 20 mL NMP was heated at 60°C for 2h with magnetic stirring. Then, the mixed solution was placed in ultrasonic instrument to further exfoliate FGi. Lastly, the suspension solution was filtered and dried to obtain the FG. The AFG can be obtained as follows: certain mass ratio of the FG and urea were grounded in a mortar to get the mixture. Then, the mixture was heated at 150°C for 4 h under nitrogen gas flow. Lastly, the mixture was extensively washed with deionized water and dried to get the AFG.

Preparation of the PI, PI/FG, and PI/AFG composites

PI was prepared by two steps: (1) poly (amide acid) (PAA) was synthesized by the ODA and PMDA and (2) PI was achieved by the thermal imidization of the PAA. Based on the preparation of PI, the FG and AFG were applied as the fillers to prepare the PI/FG and PI/AFG composites, respectively. According to the additive amount of the AFG (0 wt.%, 0.25 wt.%, 0.5 wt.%, 0.75 wt.% and 1 wt.%), the corresponding PI/AFG composites were named as PI, PI/AFG-0.25, PI/AFG-0.5, PI/AFG-0.75 and PI/AFG-1. Additionally, the PI/FG-0.5 composite which contained 0.5 wt.% of FG was prepared by the same experimental processes.

Characterizations

The micro-morphologies of the FG and AFG were characterized by using transmission electron microscope (TEM, Tecnai G2 F20 S-Twin). X-ray photoelectron spectroscopy (XPS, Thermo Fisher NEXSA) and X-ray diffractometer (XRD, D8 Advance) were used to identify the chemical differences of the FG and AFG; XRD is tested from 5^o to 80^o. The thickness of FG and AFG was measured by using atomic force microscope (AFM, Dimension icon). Thermogravimetric analysis (TGA, STA449F3) and differential scanning calorimetry (DSC, 204HP) were applied to characterize the thermodynamic performances of the samples, TGA is tested from 25°C to 850°C with the heating rate of 10°C/min, and DSC is tested from 25°C to 500°C with the heating rate of 10°C/min. The mechanical properties were performed by using the universal material testing machine (SUNS, UTM-6000). The tribological properties were performed by using the multifunctional wear testing machine (RTEC, MFT-5000). The morphologies of the worn surface and fracture surface of the composites were detected by using the scanning electron microscope (SEM, FEI, Quanta 250 EFG).

Results and discussion

Micro-properties of the FG and AFG

Both FG and AFG are typical two-dimensional nanomaterials which have large thickness and diameter ratio. As shown in Figures 1(a) and (b), the chemical modification of the urea gives rougher surface of the AFG compared with the flat surface of the FG. Although the urea molecule can react with the C-F bond to modify the FG, the thickness between the FG and AFG has no obvious change. As shown in Figures 1(c) and (d), the thickness of the AFG is 4.8 nm which is close to the thickness of FG (4.9 nm). These changes suggest that the reactions between the urea and the C-F bonds only occur on the surface of the FG.

To further confirm the fact that the urea molecules have survived on the surface of the FG nanosheets via forming strong chemical force, the FG and AFG are tested by the XPS. As shown in Figure 2(a), comparing with the XPS spectrum of the FG, four typical peaks can be found on the XPS spectrum of the AFG. Excepting the peaks of the C, O and F elements, the obvious peak near to the 400 eV belongs to the N element which comes from the urea molecules, and the content of the N of AFG is 3.13%. The towering peak of the AFG contrasts starkly with the smooth curve of the FG among 396 eV–402 eV in the N 1s fine spectra as shown in Figure 2(b). Further, the fitting N 1s spectra of the AFG (Figure 2(c)) give the two peaks located at 398.5 eV and 399.7 eV, which should be corresponded to the O=C-O bond of the urea and the newly generated C-C-N of the AFG, respectively.

Macro and micro-properties of the PI/AFG composites

As shown in Figure 3(a), both the PI/AFG films and coatings become more and more black and opaque with the increase of the AFG. In order to explain the fact that the opacity of the AFG causes the color change of the PI/AFG composites rather than the agglomeration of the AFG in the PI matrix, the PI/AFG composites are performed by using XRD. There are no typical peaks of the AFG occurring on the spectra of the PI/AFG composites as shown in Figure 3(b), which confirms that the AFG nanosheets are dispersed uniformly in the PI/AFG-0.25 and PI/AFG-0.5 composites.⁴² At the same time, the repulsion between the C-F bonds of the AFG also suggests that the agglomeration of the AFG in PI matrix can be hindered when the content of the AFG is low.

According to the test results of the TGA, the thermal stability of the PI can be enhanced by the AFG. At 95% weight residue, all PI/AFG composites have higher decomposition temperature than the PI, and the same phenomena can also be found in the high-temperature range from 650°C to 850°C in Figure 4(a). The thermal stability improvement of the PI composite should be attributed to the good dispersion and interfacial collaboration of the AFG with PI matrix. When the disruption of the main molecular chains occurs at the thermal decomposition process of PI/AFG composites, the AFG nanosheets can inhibit the small molecules move into the PI, which can increase the demand for more energy of PI chains to overcome the decomposition process. Figure 4(b) depicts the DSC changes of the PI composites. It can be shown that the addition of AFG into PI matrix has obvious contribution to glass transition temperature (Tg) improvement. During the heated process of PI/AFG composites, the movement of molecules is prevented by the AFG nanosheets, and the Tg is improved correspondingly. Therefore, it can be confirmed that the AFG could enhance the thermal stability and Tg of the PI composites.

Mechanical properties of the PI/AFG composites

The mechanical properties of the PI are important properties for its application in engineering plastic. Figures 5(a) and (b) show the tensile stress and Young’s modulus as a function of the AFG content, respectively. The introduction of the AFG nanosheets obviously increases the tensile stress and the Young’s modulus of the PI. The PI/AFG-0.5 composite exhibits the highest tensile stress (133.5 MPa), which is a 35.5% increase compared with the PI (98.5 MPa). The elongation at break of the PI/AFG composites is reduced with the increase of the AFG, while the Young’s modulus is increased with adding the content of the AFG. Meanwhile, it can be convinced that the PI/AFG-0.5 composite retains good toughness of the PI from the changes of the tensile stress, elongation at break and Young’s modulus. In order to compare the differences between the FG and AFG as the fillers of the PI matrix, respectively, the tensile stress and elongation at break are given in Figure S1. When the addition of the FG is same to the AFG, the tensile stress of the PI/FG-0.5 composite is 122.8 MPa which is higher than the PI and lower than the PI/AFG-0.5 composite. Meanwhile, it can also be found that the FG increases the elongation at break of the PI while the AFG decreases the elongation at break. The reason why the AFG as filler of the PI presents higher tensile stress and lower elongation at break than the FG can be attributed to the strong chemical forces between the AFG and the PI molecules. The well-dispersed AFG in the PI matrix not only has the Van der Waals forces but also has strong chemical forces, which can promote the efficient transfer of stress from the PI matrix to the AFG fillers. The strong interfacial interactions between the AFG and the PI limit the movement of the PI chains, and finally reduce the elongation at break of the PI.

In order to demonstrate the strong interfacial interactions between the AFG and the PI matrix, the cross-sections of the PI/AFG composites are performed by using SEM. As shown in Figure 6(a), the fractured surface of the PI presents plastic deformation, which is a typical tough fracture behavior. The introduction of well-dispersed AFG increases a large number of bulges and hollows on the fractured surface as shown in Figures 6(b)–(e). The bulges and hollows are created by the AFG in the PI matrix, because the PI chains attracted on the surface of the AFG will move as the elongation of the PI/AFG composites until fracture. Therefore, it can be convinced that the strong interfacial interactions exist between the AFG and the PI matrix from the obvious change of the SEM images.

Tribological properties of the PI/AFG composites

The tribological properties of the PI and PI/AFG composites are given in this work. As shown in Figure 7(a), the friction coefficient of the PI is about 0.43; the introduction of the AFG can reduce the friction coefficient. The corresponding wear volumes are also changed obviously as shown in Figure 7(b). The PI/AFG-0.5 composite has the lowest wear volume (0.006 mm³) among the PI/AFG composites, which is a 50% reduction compared with the wear volume (0.012 mm³) of the PI. At the same time, PI/FG-0.5 shows a higher wear volume (0.008 mm³) than PI/AFG-0.5 as shown in Figure S2.

The three-dimensional and two-dimensional wear scars of the PI/AFG composites are described in Figure 8 and it is basically the physical representation of the wear volume. This three-dimensional wear scar images show the same trend with the wear volume. The PI/AFG-0.5 composite shows the smallest wear scar compared with the other samples. The corresponding three-dimensional wear scar of the PI/FG-0.5 is larger than the PI/AFG-0.5 in Figure S3. The two-dimensional wear scar images show that the PI/AFG-0.5 composite also has the lowest depth among the samples. Therefore, the AFG really enhances the tribological properties of the PI.

Figure 9 shows the SEM images of the worn surfaces of the PI and PI/AFG composites. The parallel furrows observed on the worn surface of both the PI and PI/AFG composites are caused by the initial surface roughness of the samples as shown in Figure S4, which indicates the corresponding tribological mechanism should be the abrasive wear. Meanwhile, a few plastic deformations associated with furrows appear on the worn surface suggesting the fatigue wear mechanism. It can also be found that some cracks are generated by the load or stress applied to the PI during the sliding process, while no or few cracks can be found on the worn surface of the PI/AFG composites. According to the worn surface in Figure S5, the tribological mechanism of the PI/FG-0.5 is similar to that of the PI/AFG-0.5.

On the basis of comprehensive analysis, the AFG is really a better nanofiller than the FG, because there is only physical interaction between the FG and the PI while physical and chemical interactions can be formed between the AFG and the PI. Due to the strong physical and chemical interactions, the interfacial compatibility between the AFG and the PI chains will be better than the FG and the PI as fillers, which leads to more effective transfer of the frictional heat and friction shear force from the PI to the AFG, and finally reduces the friction coefficient and wear volume. Meanwhile, the AFG plays an important role in the friction reduction because the load applied on the PI can be partly supported by the AFG. Moreover, the PI/AFG-0.5 composite shows the best tensile stress which can support larger loads without cracking and causing severe wear, and finally enhances the wear resistance obviously.

Conclusion

The thermal, mechanical, and tribological properties of the PI can be enhanced by adding the AFG. The physical and chemical properties of the AFG make it well dispersed in the PI matrix and form strong physical and chemical interactions with the PI chains. The PI/AFG-0.5 shows 35.5% higher tensile stress and 50% higher wear resistance than the PI, which are also higher than the PI/FG-0.5. The corresponding mechanisms can be listed as follows: (1) the strong physical and chemical interactions between the AFG and the PI matrix enhance the interfacial compatibility of the PI/AFG composite, which is better to transfer the stress and heat; (2) the well dispersed AFG can partly support the load applied on the PI to reduce the wear; (3) the suitable addition of the AFG endows the PI with optimal mechanical performances to limit the crack of the PI matrix and finally enhances the wear resistance.

Supplemental Material

sj-pdf-1-hip-10.1177_09540083221079507 – Supplemental Material for Impacts of polyimide enhanced by amino-modified fluorinated graphene: Thermal, mechanical and tribological behaviors

Supplemental Material, sj-pdf-1-hip-10.1177_09540083221079507 for Impacts of polyimide enhanced by amino-modified fluorinated graphene: Thermal, mechanical and tribological behaviors by Xiangyuan Ye and Meigui Wang in High Performance Polymers

Footnotes

Declaration of conflicting interests

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

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (Grant No. 51805007) and the Youth Innovation Team Foundation of Shaanxi Provincial Department of Education (Grant No. 21JP004).

ORCID iD

Xiangyuan Ye

Supplemental Material

Supplemental material for this article is available online.

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