Synthesis and characterization of phenylethynyl-terminated polyimide/carboxylated multiwalled carbon nanotube composite film

Abstract

Phenylethynyl-terminated polyimide (PI)/carboxylated multiwalled carbon nanotube (MWCNT) composite films were synthesized from 4,4′-oxydianiline, carboxylated MWCNTs, and 3,3′,4,4′-biphenyl tetracarboxylic dianhydride and capped by 4-phenylethynylphthalic anhydride through in situ polymerization under sonication. The results of scanning electron microscopy indicated that the carboxylated MWCNTs were uniformly dispersed in the phenylethynyl-terminated PI matrix. Thermal gravimetric analysis indicated that the initial thermal decomposition temperatures of the films were up to 500°C, and the addition of carboxylic MWCNT have advantage of thermal properties of the phenylethynyl-terminated PI. The dynamic mechanical thermal analysis results clearly indicated that the glass transition temperature increases with increasing carboxylated MWCNT content. In addition, the mechanical properties of the composite films were improved by the carboxylated MWCNTs. The electrical properties were greatly enhanced due to the good conductivity of the carboxylated MWCNTs.

Keywords

Phenylethynyl-terminated polyimide carboxylated multiwalled carbon nanotubes thermal properties dynamic mechanical properties electrical properties mechanical properties

Introduction

Carbon nanotubes (CNTs) were initially discovered by Iijima in 1991¹ and regarded as a potential reinforcement of multifunctional composites,^2
–4 due to their low density, high surface area, and high chemical stability.^5,6 Polyimide (PI) is a kind of polymer with imide ring, which is a high-performance polymer owing to its high thermal stability and chemical resistance. Accordingly, it found applications in the composite and microelectronics industries. Recently, the development of PI/CNT composites has drawn increasing public attention. Their excellent thermal stability and mechanical properties make them prominent in every field.⁷ However, there are also some disadvantages. For instance, the dispersion of the CNTs in the polymer matrix and the weak interfacial interaction between the CNTs and polymer is the most serious problem for composites.^8,9 Chemical modification of the nanotubes is among the most important approaches to overcome these problems. Generally, such modification involves attaching functional groups such as amine, carboxyl, and hydroxy to the CNTs.^10
–12 In this study, aminated CNTs have good compatibility with polymers; however, the quality of PI has been shown to decrease with the addition of amino groups.^13

–16 Hyang Hwa So et al.¹⁷ have discussed the effect of CNTs on the electrical and mechanical properties of PI/CNT nanocomposites. Their results showed that the presence of hydrogen bonds between the MWCNT-COOH and the PI chain affected the properties of the PI matrix. This interaction between the functionalized MWCNTs and the PI matrix greatly enhanced the dispersion as well as the interfacial adhesion, thus strengthening the overall mechanical properties of the composites. Furthermore, in situ polymerization under sonication was beneficial to the dispersion of the CNTs.¹⁸

Phenylethynyl-terminated PI (PETI) is a kind of PI containing reactive end groups and have lower melt or solution viscosity, which attributed to the lower molecular weight. This performance provides a broad melt-processing window, which effectively improves the processing properties of the materials due to its favorable stability at high temperature and cross-linking reaction.^19,20 Furthermore, the triple bond of ethynyl was primarily opened to form double bonds by addition reactions at high temperature, producing materials with a high glass transition temperature (T _g) and excellent mechanical properties and processability.^21,22 PETI/CNT composites also have the performance of PETI. So, PETI/CNT composites have been widely considered for their processing performance and high-temperature thermal stability. In our research, in order to improve the comprehensive performance of composite films and improve the dispersion level of multiwalled carbon nanotube (MWCNT)-COOH in composite films, a new method was used for the ultrasonic dispersion in situ synthesis of PETI/MWCNT-COOH composite films and characterized for their structural and dielectric properties.

Experimental

Materials

3,3′,4,4′-Biphenyl tetracarboxylic dianhydride (BPDA) and 4,4′-oxydianiline (ODA) were supplied by Hebei Academy of Science (China). 4-Phenylethynylphthalic anhydride (PEPA) was obtained from Changzhou Sunlight Pharmaceutical Company (China), and carboxylic MWCNTs (MWCNT-COOHs were supplied by Beijing Boyu High-Tech New Material Technology Company (China). The diameter of the MWCNT-COOH was more than 50 nm, the lengths were 10–20 μm, and the content of −COOH was 0.94%. ODA was placed in a vacuum oven for 2 h at 130°C. BPDA was dehydrated in a vacuum oven at 250°C for 8 h. N,N′-Dimethylacetamide (DMAc) was dehydrated by molecular sieve before use.

Synthesis of PI and PETI

First, the precursor of PI was prepared by reacting ODA (2.1 g, 10.5 mmol) with BPDA (3 g, 10.2 mmol) at room temperature for 4 h under nitrogen. Then, the suspension solution (30 ml) was cast onto a glass plate, and after step curing (at 100, 200, 300, and 371°C for 1 h at each temperature), PI was obtained.

First, the precursor of PI was prepared by reacting ODA (2.1 g, 10.5 mmol) with BPDA (3 g, 10.2 mmol) at room temperature for 4 h under nitrogen. Then, PEPA (0.152 g, 0.61 mmol) was added to the system at room temperature for 3 h. Finally, the suspension solution was cast onto a glass plate, and after step curing (at 100, 200, 300, and 371°C for 1 h at each temperature), PETI was obtained.

Synthesis of PETI/MWCNT-COOH

MWCNT-COOH (0.1–4%, related to total amount of ODA and BPDA) and ODA (2.1 g, 10.5 mmol) were dissolved in DMAc (35 ml), and the system was dispersed by sonication for 2 h. BPDA (3 g, 10.2 mmol) was added to the system under nitrogen and magnetic stirring for 4 h, and then PEPA (0.152 g, 0.61 mmol) was added. The suspension solution was stirred for 3 h. Finally, the suspension solution was cast onto a glass plate, and after step curing (at 100, 200, 300, and 371°C for 1 h at each temperature), the PETI/MWCNT-COOH composite films were obtained, and the thickness is about 0.07–0.11 mm. The reaction formula of MWCNT-COOH and ODA and BPDA and PEPA is shown in Figure 1.

Characterization

The thermal properties of the samples were determined using a Sta449C Netzsch thermal gravimetric analyzer (Germany) under nitrogen atmosphere, and the temperature was raised to 900°C at 10°C min⁻¹. Fourier transform infrared (FTIR, ThermoFisher, USA) spectra of the products were collected on an FTS-135 Fourier transform infrared spectrometer. The dynamic mechanical thermal analysis was performed on a Q800 dynamic mechanical thermal analyzer (TA Instruments, New Castle, Delaware, USA) with a heating rate of 10°C min⁻¹ and a test frequency of 1 Hz. Scanning electron microscopy (SEM) was used to measure the surface morphology of the composites. Mechanical properties were tested by a microcomputer-controlled electronic universal testing machine; gauge length of the samples is 25 mm, width is 6 mm, and drafting rate is 50 mm min⁻¹. The electrical properties were measured by a ZC-36, Shanghai Jingke high resistance tester; the length and width of samples is 60 × 60 mm², and the main testing process is short circuit, discharge; test, open circuit.

Results and discussion

FTIR analysis

Figure 2 presents the FTIR spectra of PETI cured at 100°C (60 min), 200°C (60 min), 300°C (60 min), 371°C (30 min), and 371°C (60 min). It is clear that the amide C–N stretching vibration of polyamide acid (PAA) at 1539 cm⁻¹ disappeared gradually with increasing temperature, and the characteristic absorption peaks of C=O in the imide group near 1776, 1710, and 736 cm⁻¹, and C–N stretching at 1370 cm⁻¹ were observed in the spectra of the thermal imidization samples.^23,24 These results suggest that polyamide acid was successfully dehydrated to form PI. It can also be observed that when the composite was cured at 371°C for 60 min, the characteristic absorption peaks of the phenylethynyl group at 2215 cm⁻¹ decreased gradually and almost disappeared, indicating that the cross-linking reaction between phenylethynyl groups occurred. The studies of Fang^25,26 have shown that the main addition reaction to the triple bond generates a double bond during the cross-linking reaction.

SEM observations

SEM was performed to observe the microstructure of the MWCNT-COOH and PETI/MWCNT-COOH composites. Figure 3 shows the morphology of MWCNT-COOHs with a nearly uniform length to diameter ratio. Figure 4(a), (b), (c), and (d) show the SEM images of PETI/MWCNT-COOH composite films containing 0.1, 1, 1.5, and 3 wt% MWCNT-COOH, respectively. Some bright and uniform lines were observed in the images, which were attributed to the good dispersion of the MWCNT-COOH. These observations indicate that MWCNT-COOH exhibited high dispersibility in the PETI matrix. The –O– groups of PETI and MWCNT-COOH may have interacted through hydrogen bonding.^17,27

Thermal gravimetric analysis

The thermal gravimetric analysis results for PI, PETI, and PETI/MWCNT-COOH films are shown in Figure 5, revealing that all three have good thermal stability, with initial thermal decomposition temperatures of the films were up to 500°C. The temperature of weight loss of 5% was observed at approximately 570°C and 10% at approximately 600°C. The detailed data of loss temperature are presented in Table 1. The table shows that PETI have relatively high thermal stability compared with PI, which may be attributed to cross-linking reaction of phenylethynyl groups and bond energy in molecular backbone. In addition, because of excellent thermal properties of MWCNT-COOH, PETI/MWCNT-COOH composite films that contained 1 and 3 wt% MWCNT-COOH have higher thermal properties than PETI. Thus it can be seen that the addition of MWCNT-COOH has advantage of thermal properties of the PETI.

Table 1.

Thermal performance analysis of PI, PETI, and its composite films.

Composite films	5% Loss temperature (°C)	10% Loss temperature (°C)
PI	574.4	595.2
PETI	580.3	598.04
PETI/MWCNT-COOH (1%)	579.7	596.1
PETI/MWCNT-COOH (3%)	589	606

PI: polyimide; PETI: phenylethynyl-terminated polyimide; MWCNT-COOH: carboxylic multiwalled carbon nanotube.

Dynamic mechanical thermal analysis

The storage modulus (E′) and loss factor (tan δ) of the PETI and PETI/MWCNT-COOH composite films determined by dynamic mechanical thermal analysis are shown in Figure 6. The values of E′ correlated with the elastic modulus of the composites and the glass transitions of all the composite films occurred at 300–320°C according to the E′ curves. The improvement of the T _g was attributed to a strong interfacial adhesion and possible interaction between PETI and MWCNT-COOH. The maximum of the tan δ curves, which represented the T _g of the PETI and composites, shifted to higher temperatures with increasing MWCNT-COOH content. The results suggest that the segment mobility of the PETI chains was restricted by the entangled structure, which formed through the interaction of MWCNT-COOH with the PETI matrix.^9,28,29

MWCNT-COOH influence on the electric properties of PETI

To study the electrical properties of the composite films, high-resistance measurements were conducted. The sample was placed between two electrodes, and the ratio of the direct voltage and the surface current on the sample was defined as surface resistance (R _s). If the sample length is 1 cm, and the specimen width between electrodes is 1 cm, the values of R _s equivalent to specific surface resistance (ρ _s) can be defined as:

ρ_{s} = R_{s} \frac{L}{b} = R_{s} \frac{2 π}{ln \frac{D_{2}}{D_{1}}} .

In the same way, specific volume resistance (ρ _v) can be defined as:

ρ_{v} = R_{v} \frac{S}{d} = R_{v} \frac{π R^{2}}{d},

where L represents the width of the specimen surface through the current, b is the distance between the electrodes, D ₁ is the diameter of measuring electrode, D ₂ is the diameter of ring electrode, S is electrode surface area, and d is the thickness of the composites.

Figures 7 and 8 give the curves of the electrical properties of the PETI/MWCNT-COOH composites. The ρ _s and ρ _v values of the pure PETI films are all larger than the composites. In addition, with the incorporation of MWCNT-COOH, ρ _s and ρ _v decreased gradually. This result provided a clear indication of the conductive properties of the composites, which showed significant increase with increasing MWCNT-COOH content. However, when the MWCNT-COOH content is higher than 0.8 wt%, the values of ρ _s and ρ _v show little change.

MWCNT-COOH influence on the mechanical properties of PETI

Figure 9 shows the influence of MWCNT-COOH content on the mechanical properties of PETI/MWCNT-COOH and PI/MWCNT-COOH composites. The tensile strength of PETI/MWCNT-COOH was higher than the tensile strength of PI/MWCNT-COOH within the range of 0–0.3 wt% MWCNT-COOH. The results further show that the end-capping agent influences the structure of the films, which indicates that the tensile strength of PETI/MWCNT-COOH is increased by the cross-linking reaction of the phenylethynyl groups. When the MWCNT-COOH content is higher than 0.3 wt%, the tensile strength of the PI/MWCNT-COOH composites and the PETI/MWCNT-COOH composites show little difference. This result may be attributed to the greater influence of the MWCNT-COOH compared to the capping agent on the mechanical properties of the films. As shown in Figure 9, by the influence of the cross-linking reaction, elongation at break of PETI is smaller than PI. It is an indicator of the material flexibility, which shows that the inclusion of net structure makes the PI stronger but more brittle.

Conclusions

In this work, the properties of PETI and PETI/MWCNT-COOH were investigated. PETI and PETI/MWCNT-COOH were found to exhibit excellent thermal stability, and the addition of MWCNT-COOH has advantage of thermal properties of the PETI. The dynamic mechanical thermal analysis results clearly showed that T _g increases with increasing MWCNT-COOH content. The strong interfacial interaction between the PETI and MWCNT-COOH improved the mechanical properties of the composite films. The electrical properties were greatly enhanced due to the good conductivity of the MWCNT-COOH. Clearly, the electrical properties were strongly dependent on the MWCNT-COOH. The tensile strength of the PETI/MWCNT-COOH composite films was higher than the PI/MWCNT-COOH films within the range of 0–0.3 wt% MWCNT-COOH. With higher MWCNT-COOH content, the tensile strength of the PI/MWCNT-COOH composites and the PETI/MWCNT-COOH composites showed little difference.

Footnotes

Acknowledgment

The authors would like to thank Hebei University of Science and Technology for financial support of this research under Contract No. F215 and Hebei Academy of Science for providing the instruments and materials necessary for this research.

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) received no financial support for the research, authorship, and/or publication of this article.

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