Introduction

Carbon nanotubes (CNTs) are important form of fullerenes, which are also known as tubular fullerenes. CNTs are cylindrical graphene sheets of sp2 bonded carbon atoms. It was synthesized by Iijima in 1991 by using arc-evaporation method [1]. CNTs have two types, single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). SWNTs consist of a single graphene sheet rolled upon itself with diameter of 1–2 nm while MWNTs consist of multi-layers of graphene sheets rolled upon itself with diameter ranging from 2 to 50 nm, which depend on the number of graphene tubes. The interlayer distance between these tubes is approximately 0.34 nm [2]. CNTs can be prepared by several ways, but widely used methods are electric arc discharge, laser evaporation and chemical vapor deposition [3–5]. Electrically CNTs may be metallic or semiconductor that depends on graphene sheet rolling pattern and geometry [6]. CNTs exhibit variety of unique mechanical, optical, electrical properties and are also very stable chemically, which asserts its importance in nanotechnology [7, 8]. Also these characteristics make CNTs an important contender in many applications, i.e., as reinforcing material in composites, hydrogen storage, actuators, sensors etc. [9–11].

PBT is an important engineering polymer having high melting point (223 °C). It is a semi-crystalline polymer and has high resistance to solvents like water, methanol, chloroform, etc. PBT is relatively a new comer into polymer industry, but due to their characteristic properties it shows a wide range of applications. It is mechanically very strong and can resist heat up to 150–200 °C, thus it can be used as flame retardant. PBT is also used as an insulator in the electrical and electronics industries due to its unique insulating nature. It is mainly used for housings in electrical engineering, automotive construction as plug connectors and in households, for example in showerheads or irons. In recent years, it is also used in the key caps of some mechanical keyboards, because of its resistance to wear [12].

In the present study, F-SWNTs/PBT nanocomposite films were prepared by solution casting technique. The SWNTs were functionalized by acid treatment, which introduced carboxylic group (COOH) onto the nanotubes. The F-SWNTs showed good compatibility with PBT. Both the neat PBT and F-SWNTs/PBT nanocomposite systems were analyzed and the properties such as thermal, mechanical and solvent uptake were compared with those of neat PBT.

Experimental

Materials

PBT (average molecular weight of 38,000) was purchased from Aldrich. The trifluoroacetic acid was purchased from Across Organics (New Jersey, USA) and was used as received. The SWNTs were provided by Chengdu Organic Chemicals Co., China. The purity was higher than 90 wt %.

Functionalization of SWNTs

The SWNTs were functionalized by acid treatment (HNO3) by the same method as reported in literature [13, 14]. During this process, SWNTs were sonicated in 10 M HNO3 solution, and then refluxed for 10 h. The F-SWNTs were separated from HNO3 by filtration and then washed with distilled water until free from acid and finally dried in oven and stored for further use.

Preparation of F-SWNTs/PBT nanocomposites

The films/sheets of neat PBT and F-SWNTs/PBT nanocomposites were prepared by solution casting techniques. A known amount of F-SWNTs was first dispersed in 5 mL trifluoroacetic acid via sonication (10 min). Then, a known quantity of PBT pellet was also dissolved in 10 mL trifluoroacetic acid. The dispersed F-SWNTs and dissolved PBT were mixed and sonicated for 30 min to acquire a homogeneous solution. The smooth dry films were obtained after the removal of solvent with distilled water. Similar procedure was adopted for 0.5, 1.5 and 2 wt % F-SWNTs/PBT nanocomposite films.

Characterization

The gold-coated fractured surfaces (broken in liquid nitrogen) of the F-SWNTs/PBT nanocomposites were analyzed using a Jeol, JSM_5910, scanning electron microscopy (SEM). The polarized optical microscope (POM) photographs of pure PBT and SWNTs/PBT nanocomposite were obtained using an Optika B-600 POL POM, polarized optical microscope. The sample was melted on a heater and squeezed between two glass slides at 260 °C for 10 min under nitrogen atmosphere. The TG thermograms of neat PBT and F-SWNTs/PBT nanocomposites were obtained in nitrogen atmosphere at a heating rate of 20 °C/min from room temperature to 800 °C using TGA (TG/DTA, Perkin Elmer). The mechanical properties of neat PBT and F-SWNTs/PBT nanocomposites were analyzed using a universal testing machine (Model 100-500 KN, Iestomeric, UK).

The known weight of each sample (dried at 80 °C) was immersed in a vial containing 10 mL of the solvent and kept at room temperature. Each sample was withdrawn from the vial at regular intervals, wiped between filter papers to remove excess surface water and then weighed. The solvent uptake was calculated using the following equation:

$${\text{Solvent}}\;{\text{uptake}}\;\left( \% \right) = \frac{{W_{0} - W}}{W} \times 100$$

Where, W o was the weight of the swollen sample and W was the weight of the dried sample.

Results and discussion

Morphological study

The dispersion of carbon nanotubes in polymer matrix plays an important role on the mechanical, electrical and thermal properties of that polymer. The CNTs are usually present in the form of bundles due to van der Vaal’s forces and it is very difficult to disperse them within the polymer matrix or solvents. To solve this problem chemical functionalization of CNTs is the best method for the dispersion of CNTs within the polymer matrix or solvents. In our study, we used the functionalized SWNTs as filler in PBT matrix. Figure 1 shows the SEM images of cryofractured surface of neat PBT and F-SWNTs/PBT nanocomposite films. The SEM micrograph (Fig. 1b) of nanocomposites showed that the F-SWNTs were dispersed and embedded well within the PBT matrix, which might be due to more favorable compatibility of F-SWNTs with polymer matrix.

Fig. 1
figure 1

SEM images of a neat PBT and b 1.5 wt % SWNTs/PBT

Full size image

POM study

Figure 2 shows the POM microphotographs for neat PBT and SWNTs (2 wt %)/PBT nanocomposite, which were prepared by cooling from the melt state. The pure PBT upon crystallization showed considerable size of crystalline spherulites (Fig. 1a). The spherulites were of normal Maltese type. The POM microphotograph (Fig. 3b) shows smaller size of spherulites for F-SWNTs (0.5 and 2 wt %)/PBT nanocomposite. The decrease in size of spherulites in the case of SWNTs (2 wt %)/PBT nanocomposite might be due to the nucleation role of F-SWNTs in F-SWNTs/PBT nanocomposite. Ratna et al. [15] and Chen et al. [16] also studied the crystallization behavior of clay/PEO nanocomposites and they found that the sizes of PEO spherulites were reduced when clay was incorporated into the polymer. They reported that initially the crystallinity increased with incorporation of clay (acting as nucleation agent) and it dropped by further addition of clay due to hindrance offered by randomly oriented clay.

Fig. 2
figure 2

POM micrographs of a pure PBT, b 0.5 wt % SWNTs/PBT and c 2 wt % SWNTs/PBT

Full size image
Fig. 3
figure 3

DSC melting temperature of a pure BT, b 1 wt % F-SWNTs/PBT and c 2 wt % F-SWNTs/PBT

Full size image

Thermal properties of F-SWNTs/PBT nanocomposite

DSC is an important tool for the analysis of crystallization and melting temperature of polymer as well as filler-filled polymer composites. The F-SWNTs/PBT nanocomposite samples with different amounts of nanotubes were subjected to DSC analysis to study the effect of F-SWNTs on the melting temperature (T m) (Fig. 3). The T m of pure PBT was found to be 224 °C, which was slightly increased by the addition of F-SWNTs into PBT polymer matrix. The T m of 1 wt % F-SWNTs/PBT nanocomposites was 226.8 °C while the T m of 2 wt % F-SWNTs/PBT was about 226.12 °C. Similarly, Deka et al. [17] reported the increased trend in T m in the case of polyurethane/clay nanocomposites. They reported that this increase might be due to the formation of compact structure, which might have formed through different types of molecular interactions with loading of the nanoclay.

The thermal study of pure PBT and its nanocomposites with F-SWNTs was carried out to determine the effect of F-SWNTs on the degradation temperature and variation in their thermal stability. The TGA thermograms of pure PBT and F-SWNTs filled PBT nanocomposites are collectively shown in Fig. 4. The TGA curves of all samples show that the weights of all the samples remain constant till 360 °C, but after that the weight loss occurs rapidly and the degradation is completed between 360 and 450 °C.

Fig. 4
figure 4

TGA thermograms of PBT and SWNT/PBT nanocomposites

Full size image

In pure PBT, the degradation starts at 360 °C and it is completed at 430 °C. While in the case of F-SWNT/PBT nanocomposites, the degradation temperature is shifted toward higher temperature (about 20–30 °C higher than pure PBT). The residual quantity, contributed by F-SWNTs, remains at higher temperature.

Mechanical properties of PBT and F-SWNT/PBT nanocomposites

The mechanical properties of PBT and F-SWNT/PBT nanocomposites are shown in Table 1. The stress yield and modulus of pure PBT are 1.79 and 166.6 MPa, respectively. It is evident in Table 1 that the stress yield and moduli of nanocomposites are enhanced as compared to neat PBT. The stress yields of 0.5, 1.5 and 2 wt % F-SWNTs/PBT nanocomposites are 2.56, 5.3 and 4.33 MPa (about 0.77–3.51 MPa higher than pure PBT). The stress moduli of 0.5, 1.5 and 2 wt % F-SWNTs/PBT nanocomposites are 635, 1,023 and 646 MPa, respectively. The result also reveals that the mechanical properties of the nanocomposite film are decreased when 1.5 wt % F-SWNTs are incorporated into PBT polymer matrix. This decrease in mechanical properties might be due to the agglomeration of F-SWNTs occurred at higher concentration.

Table 1 Mechanical properties of PBT and A-SWNT/PBT nanocomposites
Full size table

Solvent uptake study of PBT and F-SWNTs/PBT nanocomposites

The equilibrium degree of swelling is an important property of polymeric materials. From swelling study one can understand the value of permeability and mechanical properties of the given material, which is necessary for selection of membrane in research areas including drug delivery and pervaporation [18]. The solvent uptake by polymer depends upon different factors. These include interaction ratio between filler and polymer, cross-linking phenomena and tortuosity.

The solvent uptake of neat PBT and nanocomposites was studied in selected solvents, i.e., distilled water, kerosene and HNO3 (2 molar solution). Tables 2, 3, 4 show the percent solvent uptake of neat PBT and F-SWNTs/PBT nanocomposites. The percent solvent uptake of neat PBT polymer and its nanocomposites shows gradual increase initially, which becomes steady after specific intervals for each sample. By comparing the results, it becomes clear that the incorporation of F-SWNTs slightly decreases the solvent uptake of PBT. This decrease in the solvent uptake with increase in concentration is due to the hydrophobic nature of F-SWNTs and their interactions with the polymers, which are bound with the polymer. This interaction leads to cross-linking and tortuosity in the polymer, which inhibit its solvent uptake ability. Similar results are also reported by Mishra et al. [19] who worked on thermoplastic vulcanizate (TPV)/organoclay nanocomposite. The minimum solvent uptake is observed in the case of kerosene, which is probably due to its higher molecular size. In case of distilled water and HNO3 (2 M solution), the solvent uptake is relatively high, which might be due to the compatibility of these solvents with F-SWNTs/PBT nanocomposites and their low molecular size (Table 3).

Table 2 Uptake of distilled water by neat PBT and F-SWNTs/PBT nanocomposites (%)
Full size table
Table 3 Uptake of kerosene by neat PBT and F-SWNTs/PBT nanocomposites (%)
Full size table
Table 4 Uptake of 2 M HNO3 solution by neat PBT and F-SWNTs/PBT nanocomposites (%)
Full size table

Conclusion

It is concluded that the F-SWNTs were dispersed well within the PBT matrix. The spherulite sizes of neat PBT were about 15–35 μm, which were dropped by the addition of F-SWNTs (nucleation effect of F-SWNTs). The tensile strength, moduli and thermal properties of PBT were improved by the incorporation of SWNTs into the polymer matrix. The degradation temperature of F-SWNTs/PBT nanocomposites (about 20–30 °C) was higher than neat PBT. The DSC analysis showed that the T m of neat PBT was 224 °C, which was slightly increased by incorporation of F-SWNTs into PBT polymer matrix. It was also found that the solvents (distilled water, kerosene, uptake 2 M HNO3) uptake of F-SWNTs/PBT nanocomposite was lower as compared to neat PBT polymer.