Introduction

Carbon fibers (CFs) have been widely used as a reinforcement in advanced composites because of their relatively low cost and excellent properties, such as the high specific strength and stiffness, the low expansion coefficient, and the relatively good flexibility. However, when applied without a previous surface modification, the physicochemical interaction between carbon fibers and a matrix is not strong enough because of the inert surface of the fibers, which directly affects the degree of interfacial adhesion in the composite system. Numerous methods concerning their surface treatment [1–3] have been developed to increase the quantity of surface functional groups and thus to enhance the ability to establish a strong interaction between the fibers and a matrix. Moreover, it is very important to design and introduce suitable surface functional groups on carbon fibers for different resin matrices [4–6].

Carbon nanotubes (CNTs) have attracted interest since Iijima identified the structure of single-walled carbon nanotubes (SWCNTs) in 1991 [7]. CNTs possess excellent mechanical properties, low density, a high surface area, high chemical stability, and good electrical and thermal conductivities [8]. The mechanical and electrical properties of polymer matrices can be improved significantly by the addition of carbon nanotubes. Carbon nanofibers are characterized by a high degree of orientation of their graphitic basal planes parallel to the fiber axis, and they possess excellent mechanical strength and electrical conductivity [9].

A poor dispersion of SWCNTs not only significantly lowers their efficiency as a reinforcement, but also causes SWCNTs to slip relative to each other when forces are applied to the composites [10]. Furthermore, the poor adhesion usually observed between SWCNTs and most polymer matrices results from the atomically smooth nonreactive surface of the nanotubes. It is also well known that a polymer composite consisting of an insulating polymer matrix and conductive fillers becomes electrically conductive as the filler content exceeds a certain critical value.

In the present work, the preparation of CNT-toughened PI and their nanocomposites and the investigation of their mechanical and morphological behavior are considered.

Experimental

Materials

The reinforcing elements were polyacrylonitrile-based unmodified and unsized high-strength carbon fibers (supplied by Shanghai Sxcarbon Technology Co. Ltd, China) with the following properties: tensile strength 2500 MPa, elastic modulus 200 GPa, density 1760 kg/m−3, diameter 7 μm, and length 75 μm.

The multiwalled carbon nanotubes were obtained from the Nanotech Port Company, Shenzhen, China. The diameter of the MWCNTs was 40–60 nm and the length 0.5-40 mm.

Preparation process

In this study, polyimide composites were prepared by mixing the polymer with different weight percents of carbon fibers. MWCNTs were added to the blends to produce polyimide composites. The following compositions were prepared: CF/PI 30/70, CNT/PI 30/70, and CF/CNT/PI 30/15/65.

Mechanical tests and measurements

The tests were carried out in a laboratory at a temperature of 25°C and humidity of 50% by using a Zwick Z010 tension testing machine. The tension rate of nonreinforced and reinforced test specimens was 5 and 0.5, respectively. The tests were carried out at least five times for each specimen, and the results were averaged arithmetically.

The flexural strength and the flexural modulus of specimens were measured by using a Testometric Micro 500 machine according to ASTM D790. The notched impact strength was tested according to ASTM D256 on a TMI testing machine. The morphology of the fracture surface of specimens was examined by a scanning electron microscope.

Results and Discussion

Mechanical properties

Figures 1–2 show that the tensile and flexural strengths of CF/PI is higher than those of the neat PI, but the notched Izod impact strength of the neat PI is higher than that of the CF/PI composite. However, the tensile strength, flexural strength, and notched Izod impact strength of CNT/PI are higher than those of the neat PI, but the tensile and flexural strengths of CNT/PI are higher than those of the CF/PI composite.

Fig. 1
figure 1

Tensile (a) and flexural (b) strengths σt and σf and moduli E t and E f of the composites investigated.

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Fig. 2
figure 2

Impact strength σimp of the composites investigated.

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Upon addition of CNTs, the tensile and flexural strengths and modulus of both the composite systems increased with fiber content, while the impact strength decreased.

Figure 1a shows the tensile properties of carbon-fiber-reinforced polyimide, CNT/PI, and CF/CNT/PI. As seen, the tensile properties of CNT/PI nanocomposites are superior to those of PI composites at the same content of carbon fibers. The figure also reveals that the tensile properties of CF/CNT/PI exceed those of the CF/PI composite. Since the aspect ratio of the fibers is larger than that of the CNTs, the fibers contribute more to the tensile properties. Figure 1b discloses that the flexural properties of fiber-reinforced PI composites are better than those of the neat PI.

The energy of crack propagation was determined from the notched Izod impact test. Figure 2 shows that the notched Izod impact strength of the composites decreases with fiber content.

From Figs. 1a and b, the effect of addition of CNTs on the tensile and flexural properties of CF/PI composites can be seen. CNTs probably act as bridges between the polar fibers and the nonpolar matrix, leading to a better interlock of the fibers. Therefore, the addition of CNT yields a composite with higher mechanical characteristics than those of nonmodified ones.

This means that, in the case of carbon fibers, a strong three-dimensional network is created by the introduction of CNTs, which prevents the initiation of shear failure in resulting composites.

The bonding between the fibers and matrix is a crucial factor controlling the load transfer between the composite constituents. The presence of CNTs as a filler of micropores at the fiber-polymer interface could strengthen the bonding by providing a mechanical interlocking between the fibers and matrix by means of nanotubes.

Composite microstructure

To investigate the influence of CNTs on the mechanical properties of the composites, the scanning electron microscopy (SEM) was performed on their fractured cross sections. The fracture behavior of the composites with and without CNTs is shown in Figs. 3a and b. The pullout of carbon fibers seems to be more pronounced in the composite without CNTs. In addition, we do not see any nanotube pullout or breakage as in case of neat CNT-based polymer composites. This fact confirms that the addition of nanotubes favors the creation of interlockings between the fibers and matrix. The shear delamination of the hybrid composite is thus reduced, which is reflected in their improved mechanical properties.

Fig. 3
figure 3

SEM micrographs of the fracture surface of composites with (a) and without CNTs (b).

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The presence of CNT aggregates suggests an interaction between CFs and CNT particles, which not only affects their distribution, but also their state of aggregation. This synergy between CFs and CNTs has a profound influence on the mechanical behavior of the composites.

Conclusion

The tensile and flexural strengths of CNT/PI containing 30 wt.% CNTs is higher than that of PI containing 30 wt.% carbon fibers. The notched Izod impact strength of the composites decreases with addition of carbon fibers, while the addition of CNTs increases the toughness of the CF/PI composites.