Introduction

Carbon nanotube (CNT) and graphene (GN), due to their exceptional properties, have been extensively used to develop multifunctional nanocomposites in various fields such as aerospace and automotive, batteries and biosensors. However, their poor dispersion states in the polymer matrices and high costs hindered further applications [1–5]. In comparison with quasi-1-D CNT, GN with 2-D lattice of carbon possesses similar physical properties but larger surface areas, which can be considered as an unrolled CNT [6, 7]. Therefore, simultaneous incorporation of CNT and GN into polymers can remedy partial shortness of surface area between fillers and polymers. That will bring better stress transfer and thus enhanced mechanical properties. Moreover, GN is very cost-effective [8]. Consequently, much CNT is not required and cost-effective CNT/GN/polymer composites can be fabricated. However, former studies on the preparations and properties of the CNT/GN/polymer systems are quite limited [9] and no CNT/GN/thermoplastic composites have been ever reported. As engineering thermoplastics, PEN exhibits excellent mechanical strength, radiation resistance and chemical inertia [10]. The polar nitrile groups on the aromatic ring in PEN promote adhesion of the polymer to many substrates. To date, several attempts have been made in our lab to further multifunctional PEN-based composites for special areas such as aerospace and military [11–14], for example, PEN/CNT nanocomposites.

In this paper, we report our efforts to show the joint effect of CNT and GN on the mechanical and thermal properties of PEN. Herein, CNT and GN were first covalently functionalized by strong polar cyanogen groups to achieve better dispersion states as we reported before [11, 14]. Then, based on our previous researches [11, 13], the mass of functionalized carbon nanofillers was confined as 2 wt% and CNT-CN/GN-CN/PEN nanocomposites were prepared via a facile solution casting method. The mechanical, morphological and thermal properties of as-prepared nanocomposites were investigated to show joint effects of CNT and GN. Consequently, a synergistic effect of one-dimensional (1-D) CNT and two-dimensional (2-D) GN on the mechanical and thermal properties have been demonstrated in these systems.

Experimental

PEN was kindly provided by Union Laboratory of Special Polymers of UESTC-FEIYA, Chengdu, China. Its inherent viscosity is 1.22 dL/g (0.005 g/mL in N-methyl-2-pyrrolidinone (NMP)). The multi-walled carbon nanotube (CNT) (diameter: 10–30 nm, length: 3–10 μm, purity: >95%), synthesized via chemical vapor deposition, was supplied by Chengdu Organic Chemicals CO. LTD. Chinese Academy of Science. Graphene (GN) (thickness: 10–30 nm, diameter: 5–20 μm) was in our lab [14]. Cyanogen functionalized GN and CNT (CNT-CN and CNT-CN) were synthesized and purified as we reported before [11, 14]. Previous researches revealed that the properties of 2 wt% CNT/PEN composite were the best [11, 13], therefore; in this paper, the mass of CNT-CN and GN-CN was fixed as 2 wt% for a comparison. Then CNT-CN/GN-CN/PEN nanocomposites were prepared via a facile solution casting method, as presented in Fig. 1. A weight-measured of CNT-CN and GN-CN were sonicated in NMP for 1 h. Meanwhile; a certain weight of PEN was refluxed for 30 min after PEN was totally dissolved. Then, sonicated carbon nanofiller suspensions were added to PEN with a mechanical stirrer for 1 h; the mixture was then put on a clean preheated glass plate and cast solvents at 60 °C, 80 °C, 100 °C, 120 °C, 140 °C, 160 °C and 200 °C for 1 h, respectively. Then it was cooled to room temperature gradually, the CNT-CN/GN-CN/PEN nanocomposite films were obtained.

Fig. 1
figure 1

Preparation process of CNT-CN/GN-CN/PEN nanocomposites: SEM images of a pure GN and b GN-CN; TEM images of pure c CNT and d CNT-CN; e GN and f CNT dispersion in NMP (0.5 mg/mL); g photograph of CNT-CN/GN-CN/PEN 4-4 composites

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The morphology of GN, GN-CN and fracture surfaces of the CNT-CN/GN-CN/PEN nanocomposites was observed with scanning electron microscope (JEOL JSM-5900LV, Tokyo, Japan) with an acceleration voltage of 20 kV. The SEM samples were coated with a thin layer of gold prior to examinations. The morphology of CNT and CNT-CN was observed with transmission electron microscopy (TEM) (Hitach H600, Tokyo, Japan). The tensile test of the PEN nanocomposites, with the sample size as 10 × 100 mm, was performed with a SANS CMT6104 Series Desktop Electromechanical Universal Testing Machine (CMT6104, Shenzhen, China) at room temperature, with a crosshead rate of 5 mm/min for tensile tests. The final result was the average values of five replicate measurements. Glass transition temperature (T g ) was measured on TA instrument DSC Q100 (USA), at a heating rate of 10 °C/min. Thermogravimeric analysis (TGA, USA) of as-synthesized CNT-CN/GN-CN/PEN composites for the initial decomposition temperatures at weight loss 5%, 10%, 30% (T d5% , T d10% , T d30% ) and maximal decomposition temperature (T max ) was carried out under N2 atmosphere at a heating rate of 10 °C /min using TA Q50 series analyzer system combination with data processing station.

Results and discussions

The procedure for preparing CNT-CN/GN-CN/PEN composites is presented in Fig. 1. Figure 1(a) and (b) show that the diameter of GN-CN (around 10 μm) decreased compared with that of GN (20–30 μm). It can be observed that pure GN has a strong tendency to stack or stamp while GN-CN is quite separated, indicating that the agglomerate tendency of GN after functionalization is lowered. From TEM images of pure CNT and CNT-CN, pure CNT aggregates heavily with black dots and its surface is quite smooth (Fig. 1(c)). However, after functionalization, most of the impurities such as amorphous carbon were removed and several obvious branches on the surface of CNT can be observed, indicating the existence of -CN groups (Fig. 1d). Dispersion of carbon nanofillers is a key element to realize the potential of these materials. Compared with pure CNT and GN, CNT-CN and GN-CN could be well dispersed into NMP (Fig. 1(e) and (f)). Their changed dispersibility certified the successful functionalization of -CN groups on the CNT and GN, but also offered convenience for preparing uniform nanocomposite films. Figure 1(g) also gives a photo of CNT-CN/GN-CN/PEN 4/4 composite film with about 85 μm thickness.

Among mechanical properties, the tensile properties are one of the most frequently used for comparison [15, 16]. Figure 2. shows the tensile properties of PEN nanocomposites. The tensile strength and modulus of both CNT-CN/PEN (69 and 2164 MPa) and GN-CN/PEN (71 and 2255 MPa) composites are increased in comparison with those of pure PEN (64 and 2064 MPa). However, tensile increments of GN-CN/PEN composites are more obvious due to two important reasons: (1) the contact area is larger among GN-CN as they contact by means of plane-to-plane [6, 7]; (2) stress transfer can be taken in 2-D rather than in 1-D. Nevertheless, tensile increments of all CNT-CN/GN-CN/PEN composites are higher than those of GN-CN only composites. As CNT-CN/GN-CN weight ratio increases, the tensile strength and modulus increase firstly and decrease afterwards; they reach their maximums when the CNT-CN/GN-CN weight ratio is 5/3. At that point, the tensile strength increases from 71 MPa in GN-CN/PEN composite to 83 MPa (16.9% increase) and the tensile modulus increases from 2255 MPa in GN-CN/PEN composite to 2593 MPa (15.0% increase). These results indicated that simultaneous incorporation of CNT and GN into PEN had synergetic effect on the mechanical properties.

Fig. 2
figure 2

Effects of GN-CN content on the mechanical properties of CNT-CN/GN-CN/PEN composites: a tensile strength; b tensile modulus; c elongation at break and d photograph of CNT-CN/GN-CN/PEN 4/4 composites

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Generally, incorporation of CNT or GN into polymers will decrease the elongation of composites with increasing filler content [7, 16]. However, as can be seen in Fig. 2(c), the elongation of CNT-CN/GN-CN/PEN composite increases and then decreases with increasing CNT-CN/GN-CN ratio, which once again confirmed the synergetic effects of CNT-CN and GN-CN. And the elongation reaches its peak of 5.37% when the CNT-CN/GN-CN weight ratio is 4/4, very close to that of neat PEN (5.36%). Figure 2(d) also gives a photograph of the CNT-CN/GN-CN/PEN 4/4, showing that the film is rather smooth and flexible.

Mechanical properties of the nanofiller/polymer systems are related to the dispersion, orientation, adhesion, and stabilization of the nanofillers in the polymer matrices [9]. Hence, fracture surface images of obtained composites were given to further evaluate their mechanical properties. Figure 3(a) shows that the fracture surface of neat PEN is quite smooth and homogeneous. For those of GN-CN only composites, with uniform dispersion state and layered structure, most of GN are both horizontally and vertically oriented (Fig. 3(b)). In comparison, the dispersion state of CNT in the PEN matrix is also well, but the phenomenon of “pull-out” is observed in the CNT-CN/PEN nanocomposites (Fig. 3(c)). These results confirmed that GN-CN/PEN composites exhibit higher mechanical performances than CNT-CN/PEN. The fracture surface of CNT-CN/GN-CN/PEN 4/4 composites shows that GN has good adhesion to PEN matrix and GN is horizontally oriented (Fig. 3(d)). From enlarged Fig. 3(d) and (f), both CNT and GN are tightly held to the PEN matrix, which can prevent the sliding of CNT and GN during tension. These results can be attributed to the benefit of functionalized –CN groups, promoting adhesion among CNT-CN, GN-CN and PEN. The fracture surface of the composite clearly shows many broken segments of CNT-CN ropes rather than just pull out. Furthermore, some line-like CNT-CN are embedded in the GN-CN, resulting in 3-D CNT-GN networks. These structures are especially beneficial to their mechanical properties. During the tension, GN-CN acted as the main stress transfer plane while line-like CNT-CN stabilized the horizontally oriented GN. Consequently, they are capable of transferring stress load and thus decreasing interface loss due to synergetic effects of CNT-CN and GN-CN in the PEN matrix. And these synergetic effects maximize when the weight ratio of CNT-CN/GN-CN is around 4/4.

Fig. 3
figure 3

SEM images of a neat PEN, b GN-CN/PEN composites, c CNT-CN/PEN composites, d CNT-CN/GN-CN/PEN 4/4 composites, e enlarged d, and f CNT-CN/GN-CN/PEN 5/3 composites

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To further investigate the synergetic effects of GN and CNT, thermal properties of CNT-CN/GN-CN/PEN composites such as glass transition temperature (T g ), the initial decomposition temperatures at weight loss of 5%, 10%, 30% (T d5% , T d10% , T d30% ) and maximal decomposition temperature (T max ) were displayed in Table 1.

Table 1 Effects of CNT-CN/GN-CN content on the mechanical properties of CNT-CN/GN-CN/PEN composites
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In comparison with neat PEN, All PEN-based composites showed slightly higher T g s in comparison with neat PEN. However, the improvement of T d is quite encouraging. PEN/GN-CN composites exhibit a T d5% of 507 °C, 10 °C higher than PEN/CNT-CN composites. In addition, T d10% , T d30% and T max of PEN/GN-CN composites are all higher than those of PEN/CNT-CN composites. These results indicated that GN is more effective to enhance thermal properties of PEN, which is in accordance with mechanical results. The possible reason is that when GN was rolled to CNT, surface area is decreased, which inevitably affect mechanical and thermal performances. Consequently, incorporation of CNT into the GN/PEN systems to form CNT-CN/GN-CN/PEN composites showed decreased thermal properties at beginning. However, as CNT-CN/GN-CN ratio increased to 4/4, thermal performances reached the maximum values, which are higher than those of PEN/GN-CN composites. Then with increasing CNT-CN/GN-CN mass ratio, the thermal values slightly decrease, but they are still larger than those of PEN/CNT-CN composites. This improvement is mainly attributed to joint barrier effect of CNT and GN and strong PEN-CNT-GN interactions, as well as GN’s layered structures. With the introduction of GN-CN and CNT-CN into the PEN, GN serve as the mass transfer barriers against the volatile pyrolyzed products in the PEN matrix while CNT stabilized GN, eventually retarding thermal degradation of the nanocomposites. All these results agree well with CNT/GN-CNs’ synergetic effects on the mechanical and morphological properties observed above. Therefore, simultaneous incorporation of GN and CNT into polymer matrices not only can have synergetic effects on mechanical and thermal properties, but also can cover high cost of CNT, which is beneficial to large-scale use of polymer/filler composites from scientific and social points of view.

Conclusions

Cost-effective CNT-GN/GN-CN/PEN nanocomposite films were prepared by a facile solution casting method and investigated for their mechanical and morphological properties. The weight ratio of CNT/GN was varied from GN-dominated to CNT-dominated for studying their joint effects on the mechanical and morphological properties of PEN nanocomposites. Consequently, the mechanical performances of the composites increase, then decrease with increasing of CNT-GN/GN-CN ratio, and reach their maximums when CNT-CN/GN-CN is around 4/4. These results are due to good dispersion states, strong adhesion with PEN matrix, especially GN-CN horizontal orientation and CNT deep-thrust into GN. Thermal properties of CNT-GN/GN-CN/PEN nanocomposite once again confirmed the synergistic effect of CNT-CN and GN-CN, leading to improved thermal properties. Therefore, a synergistic effect on mechanical and thermal properties between 1-D CNT and 2-D GN was demonstrated in these systems. In addition, simultaneous incorporation of GN and CNT into polymer matrix is beneficial to large-scale use of polymer/filler composites from scientific and social points of view.