In situ polymerization approach to poly(arylene ether nitrile)-functionalized multiwalled carbon nanotube composite films: thermal, mechanical, dielectric, and electrical properties

1 Introduction

Poly(arylene ether nitrile) (PEN) has attracted a great deal of attention as a thermoplastic resin in advanced composite materials because of its high thermal stability, excellent mechanical properties, and strong chemical inertia [1], [2], [3]. In particular, a series of PEN and PEN/filler composites formed by introducing functional groups or fillers have many unique properties, including dielectric, optical, electrical, photoelectric, and magnetic properties, which allow a number of exciting potential applications [4], [5], [6]. However, much research is still needed to resolve problems such as the poor interfacial interaction between polymer and filler, high specific gravity and poor dispersion of fillers, and complex surface functionalization of fillers [7], [8].

Carbon nanotubes (CNTs) as a filler have been regarded as ideal for producing high-performance CNT-polymer composites and exhibit unique and remarkable structural, thermal, electrical, and mechanical properties [9], [10]. Moreover, with CNTs becoming easier to produce and cheaper to buy, CNT-based fillers have become one of the major additives for polymer-composite fabrication [11]. Nevertheless, the inert characteristics of the CNT surface and high aspect ratio often lead to significant agglomeration with only a small percentage [12]. Thus, the development of a preparation method to achieve more a favorable dispersion and high CNT loadings in polymer composites is still a challenge.

In this paper, we report the syntheses and properties of PEN-multiwalled CNT (MWNT) composites obtained by in situ polymerization in the presence of MWNTs-COCl. Then, the effect of the good dispersion of MWNT bundles and strong interfacial interaction between the PEN matrix and MWNTs on the morphology and thermal properties was described in detail. Importantly, the mechanical, dielectric, and electrical properties were also discussed with various MWNT-COCl contents. Herein, the investigation of PEN-MWNT films opens up a facile route to obtain a new class of PEN composite materials and use them in a variety of applications.

2 Materials and methods

As shown in Figure 1, MWNTs-COCl were prepared and purified following a procedure described earlier [13]. As described in our previous reports with minor modifications [6], MWNTs-COCl were dispersed into a solution containing N-methylpyrrolidone (NMP), and then 2,6-dichlorobenzonitrile (DCBN), hydroquinone (HQ), resorcinol (RS), and anhydrous K₂CO₃ were slowly added. The molar ratio of HQ and RS is 4:1. It is important to note that the sum of the molar number of HQ and RS should be equal to that of DCBN. The mixture was poured into ethanol to precipitate the product, and then the precipitate was acidified by diluted hydrochloric acid after crushing. Finally, the collected composites were washed with boiling water and dried in a vacuum oven at 140°C for 12 h. Then, PEN-MWNT composites with various MWNT-COCl contents (0, 4, 4.5, 5, 5.5, and 6 wt%) were obtained. PEN-MWNT films were prepared by dissolving PEN-MWNT composites in NMP solvent with 10% concentration solution. The mixture solutions were cast on a clean glass plate and then dried in an oven at 80°C, 100°C, 120°C, 160°C, or 200°C for 2 h. Thus, black and transparent thin films were obtained.

Figure 1:

Schematic illustration of the preparation of PEN-MWNT composite films.

The samples were characterized using a Fourier transform infrared (FTIR) spectrometer (FTIR8400 S) and a scanning electron microscope (SEM; JSM-6490LV). The thermal properties were determined using a differential scanning calorimeter (DSC; TA DSC-Q100) and a thermogravimetric analyzer (TGA; TA TGA-Q50) at a heating rate of 10°C/min under flowing nitrogen. The mechanical properties were measured using an electronic universal testing machine (SANS CMT6104), and the average values for every five samples were reported. The dielectric properties were measured according to ASTM D150 on a HP4284A precision LCR meter (DEA 2970). The electrical properties were monitored according to ASTM D149 on a megohmmeter (GJW-50KV) at room temperature.

3 Results and discussion

The FTIR spectra of samples are shown in Figure 2A. The peaks at 3430 and 1722 cm^-1 can be attributed to the carbonyl vibration, suggesting the grafting of -COOH group onto MWNTs [14]. The weak absorption bands at 650 and 910 cm^-1 correspond to the C-Cl of acyl chloride groups (-COCl). Moreover, a characteristic absorption appeared at 2230 cm^-1, indicating a symmetrical stretching of the cyano group (-CN) [15]. These results indicated that PEN-MWNT composites were successfully synthesized in the presence of MWNTs-COCl. Figure 2B–D show the SEM images of PEN-MWNT films with various MWNTs-COCl contents. Clearly, MWNTs can be easily embedded in the PEN matrix with a homogeneous dispersion. Notably, MWNTs were encapsulated by the polymerized PEN layer, suggesting strong polymer-MWNT interactions.

Figure 2:

Characterization of MWNTs, MWNTs-COOH, MWNTs-COCl, and PEN-MWNT films with 4 wt% MWNTs-COCl content: (A) FTIR spectra. SEM images of PEN-MWNT films depending on the MWNTs-COCl content: (B) 4 wt%, (C) 5 wt%, and (D) 6 wt%.

The thermal stability of the PEN-MWNT films was evaluated by the glass transition temperatures (T_g) and 5% weight loss temperatures (T_5%), which are summarized in Table 1. The T_g of the PEN-MWNTs is not affected by the addition of MWNTs-COCl compared to pure PEN, although the T_5% of all the composites are stable up to 480°C. These results are suggestive of the MWNTs-COCl loading, which has only a minor influence on the cross-link density, rigidity, and flexibility of the polymer molecules. In addition, this result suggests that there is a good interaction between PEN and MWNTs and the nanotube surface is uniformly coated by PEN [16]. The mechanical properties of PEN-MWNT films are listed in Table 1. The tensile strength of all films with various loadings of MWNTs-COCl increased compared to that of neat PEN, and the maximum was up to 98 MPa under 4.5 wt% MWNTs-COCl loading. This was primarily attributable to the strong interfacial adhesion resulting from in situ polymerization and good-quality dispersion of MWNTs in the PEN matrix. In contrast, the breaking elongation showed a decrease with increasing MWNTs-COCl content probably because of the addition of MWNTs. Furthermore, this results in the fracture of the PEN-MWNT interface (Figure 1).

The dielectric properties of PEN-MWNT films were measured as a function of MWNTs-COCl. Figure 3A and B describes the frequency dependence of the dielectric constant and dielectric loss of the films, respectively. The dielectric constant of the films decreased slightly with respect to frequency and increased abruptly with increasing MWNTs-COCl content. This occurred mostly because the mechanisms contributing to the dielectric constant are associated with Maxwell-Wagner polarization of polymer-conductive MWNT interfaces [17], [18]. Then, the dielectric loss of all the films demonstrates similar characteristics with a minor fluctuation in the 0.02–60.00 kHz range, with the exception of the film (6 wt% MWNTs-COCl loading), showing a dramatic decrease with increasing frequency in the range of 60–200 kHz, revealing that it is difficult for PEN to process the long-range intermolecular hopping of electrons, and they show a high dielectric loss with the addition of MWNTs. More importantly, the formation of the excellent planar conductivity of MWNTs with a highly conjugated structure in the PEN matrix could play a dominant role in determining electrical performance.

Figure 3:

Dielectric properties of PEN-MWNT films depending on the MWNTs-COCl content: (A) dielectric constant and (B) dielectric loss. DC conductivity of PEN-MWNT films: (C) frequency dependence and (D) at 1 kHz.

The DC conductivities of PEN-MWNT films are exhibited as a function of frequency from 0.02 to 200 kHz in Figure 3C. The DC conductivities of all the films demonstrate similar characteristics with respect to frequency, showing a relatively steady increase with increasing frequency in the range of 1–200 kHz. Furthermore, the DC conductivities gradually increased with increasing MWNTs-COCl loading. Figure 3D describes the MWNTs-COCl content dependence of the DC conductivities of the PEN-MWNT films at 1 kHz. As shown in Figure 3D, the DC conductivities increased slightly with increasing MWNTs-COCl loading, exhibiting a rapid increase above the percolation threshold [19], [20]. It may be suggested that the formation of polymer-nanotube interconnected networks significantly improved the moderate ionic conductivity induced by PEN with electrical conductivity supported by the highly conductive MWNTs, which is also attributed to this tunneling conduction mechanism [21].

4 Conclusions

We successfully prepared PEN-MWNT composite films by in situ polymerization in the presence of MWNTs-COCl. By employing a nucleophilic substitution polymerization, the macromolecular chains of PEN were effectively coated onto MWNTs. MWNTs showed good-quality dispersion in the PEN matrix. Although the thermal stability of PEN-MWNT films remained stable, the tensile strength and dielectric constant and loss were greatly improved with enhanced interfacial interaction. Meanwhile, PEN-MWNT films displayed significantly improved conductivity even at high MWNT loadings. Importantly, the in situ polymerization of PEN-MWNT films opens up a facile route to effectively obtain a new class of PEN composite materials for more extensive applications.