Properties enhancement in multiwalled carbon nanotube-magnetite hybrid-filled polypropylene natural rubber nanocomposites through functionalization and processing methods

2.1 Materials

Multiwalled CNTs (MWCNTs) (Nanocyl NC7000 series; average diameter, 9.5 nm; average length, 1.5 μm; purity, 90%; and surface area, 250–300 m² g^-1) were provided by Nanocyl SA (Sambreville, Belgium), as shown in Figure 1A. Nanocyl NC7000 is an as-grown material produced in an industrial large-scale chemical vapor deposition process. Fe₃O₄ nanoparticles, with the particle size ranging from 20 to 30 nm, were obtained from commercial suppliers in powder form (Nanostructured & Amorphous Materials Inc., Houston, TX, USA). Natural rubber (NR) and PP were supplied by Rubber Research Institute of Malaysia (RRIM, Kuala Lumpur, Malaysia) and Polypropylene Malaysia Sdn. Bhd. (Kuantan, Malaysia), respectively. Liquid natural rubber (LNR) was prepared by the photosynthesized degradation of NR in visible light. Nitric acid (HNO₃, 60%) was obtained from Merck Sdn. Bhd. (Petaling Jaya, Malaysia).

Figure 1

TEM images of fragmented (A) pristine MWCNTs and (B) acid-treated MWCNTs, and complete (C) pristine MWCNTs and (D) acid-treated MWCNTs.

2.2 Preparation of Fe₃O₄-MWCNTs hybrid filler

MWCNTs were dispersed in a round-bottomed flask containing HNO₃ solution with the aid of an ultrasonic water bath for 1 h. The solution was refluxed at 80°C with vigorous mixing for 24 h. The acid-treated MWCNTs were collected by centrifugation, and then they were dried at 80°C for 24 h. From the treatment in boiling nitric acid, the MWCNTs were broken into untangled and straighter forms with open ends, as presented in Figure 1B. Figure 1C and D show the TEM images of complete pristine and acid-treated MWCNTs, respectively, for better comparison. To untie the entanglement of MWCNTs and facilitate the mixing of MWCNTs and Fe₃O₄ nanoparticles, the MWCNTs were mixed with Fe₃O₄ nanoparticles at a weight ratio of 1:1 for 1 h in a ball-milling machine. The porcelain balls in the mill were 15 mm in diameter and 11 g in mass. The ball-to-powder weight ratio was 10:1.

2.3 Preparation of Fe₃O₄-MWCNTs hybrid-filled TPNR nanocomposites

To further disperse the MWCNTs-Fe₃O₄ hybrid in the polymer matrix, MWCNTs-Fe₃O₄ hybrid and PP pellets were pretreated before a melt-blending process. The mixture was ground by a ball-milling process for 2 h to allow the MWCNTs-Fe₃O₄ hybrid to be attached onto the surface of PP pellets. Thereafter, the mixture was melt blended by using a laboratory mixer (Model Thermo Haake 600p). The weight ratio of PP, NR, and LNR was 70:20:10, with LNR being the compatibilizer for the mixture. Blending was carried out at a mixing speed of 100 rpm at 180°C for 13 min. The final nanocomposites contained 2 wt% filler in the TPNR matrix. As a reference, the neat TPNR was also similarly melt blended for thermal and mechanical studies.

2.4 Morphological characterization

The morphology of the MWCNTs before and after acid treatment was observed using a high-resolution transmission electron microscope (TEM; JEOL-1010), with an accelerating voltage of 100 kV. The samples used for TEM observations were prepared by dispersing a small amount of MWCNTs in absolute ethanol followed by ultrasonic vibration for 10 min, then placing a drop of dispersion onto a 200-mesh carbon-coated copper grid, which was then allowed to dry in air. Optical microscopy examinations using Leica DM2500M with a capture camera were carried out on the surface of the prepared nanocomposite samples. A 20× objective was used to examine the samples. The fracture morphology of nanocomposites was examined using a Supra 55VP field emission-scanning electron microscope (FE-SEM). To prepare the specimen for FE-SEM, first, a notch was made in the specimen. Then, the notched specimen was immersed in liquid nitrogen for 10 min and subsequently fractured along the notch. A thin gold layer was then sputter deposited on the fractured surface.

2.5 Wide-angle X-ray scattering

Wide-angle X-ray scattering (WAXS) studies were performed on a Bruker AXS NANOSTAR system operated at 45 kV and 35 mA. The detector was positioned at the smallest possible distance, 4.8 cm, to provide a 2θ range of 30°.

2.6 Differential scanning calorimetry

Thermal analysis of the neat TPNR and its nanocomposites was performed using differential scanning calorimetry (DSC) with Perkin-Elmer Pyris 1, calibrated with indium standard. For each measurement, a sample of about 5 mg was used, placed in a hermetically sealed aluminum pan. Before a cooling scan, the samples were first heated to 250°C and held in the molten state for 2 min to remove the thermal history. The second heating scan measurements were obtained as the melting data. The measurement was carried out at a scanning rate of 10 K·min^-1 using N₂ as the purging gas.

2.7 Thermogravimetric analysis

Thermal degradation measurements were performed in a Perkin-Elmer TGA-7 thermogravimetric analyzer with a TAC-7/DX thermal analysis controller. Samples of ∼25 mg were heated from 30°C to 850°C at 20 K·min^-1. The purging gas (flow rate: 20 ml·min^-1) was nitrogen from 30°C to 700°C; then, it was changed to air at 700°C.

2.8 Dynamic mechanical properties

Dynamic mechanical analysis (DMA) was carried out with Perkin-Elmer Pyris Diamond DMA. DMA tests were carried out in tensile mode at a fixed frequency of 1 Hz. A rectangular sample with a dimension of 10 mm×10 mm× 0.7 mm was cut and submitted to a temperature sweep from -120°C to 100°C at 2 K·min^-1.

3.1 Morphological characterization

Morphological analysis is very important for the evaluation of the dispersion state of the filler in the polymer matrix. Polarized optical light microscopy allows taking clear images of the fillers embedded in the TPNR matrix. Figure 2 presents the morphologies of TPNR containing 2 wt% filler with different formulations, showing the overall distribution of filler in the TPNR matrix. It can be seen from Figure 2A that the dispersion of pristine MWCNTs was poor in the TPNR matrix. Large MWCNT agglomerates were observed in the TPNR matrix owing to their large length, large surface areas, and their entanglement, which cannot be avoided. Furthermore, the strong intrinsic van der Waals attractions among MWCNTs and the poor interfacial interaction between MWCNTs and the TPNR matrix made the uniform dispersion of MWCNTs in polymer matrix very difficult. The acid-treated MWCNTs formed smaller agglomerates in the TPNR matrix, which can be clearly seen in Figure 2B. During acid treatment, the length of acid-treated MWCNTs was reduced and the entanglement of MWCNTs became loose, and this allowed a finer dispersion in the TPNR matrix compared with the pristine MWCNTs. The existence of functional groups on the surface of MWCNTs after acid treatment might have contributed to the improvement in the interfacial interaction between MWCNTs and the TPNR matrix, which assisted in increasing the dispersion and homogenization of the MWCNTs in the TPNR matrix. Therefore, it is anticipated that the mechanical properties of TPNR with acid-treated MWCNTs as filler would be relatively higher, as strong interfacial interactions between MWCNTs and the TPNR matrix lead to a sufficient load transfer from the polymer to the MWCNTs. Pristine MWCNTs-Fe₃O₄ and acid-treated MWCNTs-Fe₃O₄ showed a better dispersion in the TPNR matrix than pristine MWCNTs and acid-treated MWCNTs, as shown in Figure 2C and D. As the filler content in the nanocomposites was fixed at 2 wt%, the nanocomposites reinforced with MWCNTs-Fe₃O₄ hybrid fillers contained less MWCNTs than those reinforced with a single type of filler, either pristine or acid-treated MWCNTs. This could be the reason for the better dispersion in nanocomposites with hybrid fillers, as higher contents of MWCNTs resulted in a tendency for crowding and entangling. Figure 2E shows the optical microscopic image of the TPNR matrix containing 2 wt% Fe₃O₄. Fe₃O₄ nanoparticles formed agglomerates in the TPNR matrix.

Figure 2

Optical microscopic images of the TPNR matrix containing 2 wt% (A) pristine MWCNTs, (B) acid-treated MWCNTs, (C) pristine MWCNTs-Fe₃O₄, (D) acid-treated MWCNTs-Fe₃O₄, and (E) Fe₃O₄ nanoparticles.

To observe the morphology at the micron and submicron levels, the nanocomposites were further examined using FE-SEM. The FE-SEM images of the cross-sectional fracture of the TPNR matrix with addition of 2 wt% pristine MWCNTs, acid-treated MWCNTs, pristine MWCNTs-Fe₃O₄, acid-treated MWCNTs-Fe₃O₄, and Fe₃O₄ are shown in Figure 3. The dispersed MWCNTs and the Fe₃O₄ nanoparticles are illustrated as bright dots, and some lines are the ends of the broken MWCNTs. It can be seen from Figure 3A that the dispersion of the pristine MWCNTs was poor in the TPNR matrix. The image revealed the agglomerates of MWCNT bundles, where long MWCNTs are entangled together. After acid treatment, a better dispersion was observed as expected, as shown in Figure 3B. It is very interesting to note that most of the pristine MWCNTs were pulled out during the fracture of the nanocomposites, whereas most of the acid-treated MWCNTs were broken rather than pulled out owing to the strong interfacial bonding between the MWCNTs and the polymer matrix. The acid treatment acted as a “cutting” process for the MWCNTs, shortening their length and disentangling them. The untangled and straighter MWCNTs were much better dispersed in the polymer matrix owing to this disentanglement, in spite of the polar groups introduced onto their surface, considering the non-polar nature of the matrix. These observations were consistent for the TPNR matrix containing pristine MWCNTs-Fe₃O₄ and acid-treated MWCNTs-Fe₃O₄, as shown in Figure 3C and D.

Figure 3

FE-SEM images of the cross section of TPNR containing 2 wt% (A) pristine MWCNTs, (B) acid-treated MWCNTs, (C) pristine MWCNTs-Fe₃O₄, (D) acid-treated MWCNTs-Fe₃O₄, and (E) Fe₃O₄ nanoparticles.

3.2 Structure

The introduction of nanoparticles into the TPNR matrix can cause changes in the crystallization rate of the polymer and its crystal conformation. To examine this possibility, the neat TPNR matrix and its nanocomposites were studied by using WAXS in the range of 2θ=10–30°. Figure 4 shows the WAXS patterns of the neat TPNR and its nanocomposites. Comparison of the WAXS patterns revealed no significant difference between the neat TPNR and its nanocomposites. Both the neat TPNR and its nanocomposites with 2 wt% filler displayed characteristic diffraction peaks at 2θ=14.1°, 16.7°, 18.5°, and 21.8°, which can be assigned to the respective (110), (040), (130), and (111) planes of the α-phase crystals of PP. This implies that the addition of filler in the TPNR matrix does not change the crystal structure of the neat TPNR. Similar results have been reported by other researchers [5].

Figure 4

WAXS patterns of the neat TPNR and its nanocomposites.

3.3 Crystallization

The melting temperature (T_m) and heat of fusion (ΔH_m) for the neat TPNR and its nanocomposites were determined from the DSC second heating runs of the samples. The crystallization temperature (T_c) and heat of crystallization (ΔH_c) were determined from the DSC cooling scans of the samples. Many researchers have reported the effect of addition of MWCNTs on the crystallization behavior of polymer matrices. It has been known that MWCNTs either act as nucleating agents [5, 18, 27–29] or as obstacles to crystallization [30] depending on the types of matrices. The DSC first cooling and second heating curves of the neat TPNR matrix and its nanocomposites at a rate of 10°C·min^-1 are presented in Figure 5A and B. The crystallization temperature, crystallization enthalpy, melting temperature, melting enthalpy, and degree of crystallinity of the neat TPNR and its nanocomposites are presented in Table 1. The degree of crystallinity (X_c) was determined from DSC measurements. X_c can be calculated from the heat evolved during crystallization (ΔH_c), using the following equation:

Figure 5

Heat capacity of (A) first cooling and (B) second heating of the neat TPNR and its nanocomposites obtained at a scanning rate of 10°C·min^-1.

where (ΔHm∗) is the heat of fusion for 100% crystalline isotropic of PP (ΔHm∗=209⋅J×g-1) and θ is the weight fraction of the filler in the composites. It was observed that the melting temperature, T_m, remained practically unchanged with the addition of fillers.

From Figure 5A, it can be seen that the neat TPNR showed a crystallization temperature of about 114°C. Apparently, the introduction of MWCNTs into the polymeric matrix led to an increase of the crystallization temperature. Interestingly, the shift of T_c to high temperatures was more prominent for nanocomposites containing 2 wt% MWCNTs compared with those of hybrid fillers. It is well known that MWCNTs have a nucleating effect on the crystallization of semicrystalline polymer matrices in the composites system [5, 18, 28, 29]. The incorporation of MWCNTs into the TPNR matrix helped induce more polymer chains to crystallize and promoted a faster crystal growth, thus resulting in an enhancement of the crystallization process of the TPNR matrix. The nucleation effect was more evident for the TPNR matrix with pristine MWCNTs as compared with other nanocomposites. The crystallization temperature passed from 114°C to 116°C with the addition of acid-treated MWCNTs and to 117°C with the addition of pristine MWCNTs. A similar result was observed by Sahoo et al. [18]. They suggested that the less evident nucleation effect on a polymer matrix with acid-treated MWCNTs as filler is because the functionalized MWCNTs had stronger interactions with the polymer matrix resulting in a better dispersion of MWCNTs, but had a higher barrier for the mobility of polymer chains. It is also very interesting to note that the addition of 2 wt% Fe₃O₄ nanoparticles did not seem to influence the crystallization temperature of the TPNR matrix. In other words, the nucleation effect promoted by Fe₃O₄ nanoparticles was far less than that of MWCNTs.

3.4 Thermal stability

The thermal stability of nanocomposites was studied using thermogravimetric analysis (TGA) by determining their mass losses during heating. Thermogravimetry (TG), expanded region of TG, and derivative TG (DTG) curves for the neat TPNR and its nanocomposites are represented in Figure 6A, B, and C, respectively, whereas the characteristic thermal decomposition temperatures of the neat TPNR and its nanocomposites are shown in Table 2. Two weight loss transitions can be observed from the TG and DTG curves of all the samples, which correspond to the NR and PP components. The NR component underwent thermal degradation at 350°C, which is extremely close to the decomposition of the PP component. Therefore, both of the decomposition events overlapped in the DTG curve. However, both of the components can be identified by their composition in the samples, which was about 30 wt% for NR and 70 wt% for PP.

Figure 6

(A) TG, (B) expanded region of TG, and (C) DTG curves of the neat TPNR and its nanocomposites at a heating rate of 20°C·min^-1.

From the TG curves, it can be seen that the neat TPNR and its nanocomposites showed relatively good thermal stability, as no remarkable weight loss occurred until 250°C. The 5% and 50% decomposition of the neat TPNR occurred at 367°C and 456°C, respectively. Compared with the neat TPNR, the addition of filler led to a remarkable increase of thermal stability. In the case of addition of 2 wt% pristine MWCNTs to the TPNR matrix, the decomposition temperature increased by 15°C and 11°C at 5% and 50% decomposition. The thermal stabilization effect of MWCNTs could be attributed to the enhancement of crystallinity, as presented in Table 1, as well as the restriction of molecular mobility around the MWCNTs. The polymer chains near the MWCNTs may degrade slower, which helps extend the decomposition temperature to the higher side. The thermal stabilization effect of MWCNTs could be explained by the improved thermal conductivity of the composites that facilitates heat dissipation within the composites [18, 31]. The acid-treated MWCNT-filled TPNR showed the highest decomposition temperature. The decomposition temperatures of acid-treated MWCNT-filled TPNR increased by 16°C at both 5% and 50% weight losses. This enhancement is probably due to the finer dispersion of the acid-treated MWCNTs in the TPNR matrix. The polymer chains were interconnected with the acid-treated MWCNTs by the presence of carboxylic groups on the surface of MWCNTs, which gave a strong interfacial interaction between the TPNR matrix and the MWCNTs [18]. As the polymer thermal degradations begin with chain cleavage and radical formation, the acid-treated MWCNTs in the composites acted as radical scavengers and chain traps in both chain cleavage and radical formation, delayed the onset thermal degradation, and hence improved the thermal stability [18, 32, 33].

Incorporation of Fe₃O₄ nanoparticles increased the decomposition temperature of TPNR. When comparing the decomposition temperatures at which 5% and 50% weight losses occur, Fe₃O₄-filled TPNR exhibited higher temperatures than the neat TPNR, whereas those temperatures were among the lowest compared with other nanocomposites. The improved thermal stability of TPNR by Fe₃O₄ nanoparticles is related to the barrier of nanoparticles to heat permeability into the TPNR and the reduction of diffusivity of the volatile decomposition products [34]. The hybrid of pristine MWCNTs-Fe₃O₄ and acid-treated MWCNTs-Fe₃O₄ nanocomposites showed slightly delayed decomposition compared with Fe₃O₄ nanocomposites, indicating that MWCNTs have a more pronounced effect in improving the thermal stability of TPNR.

To determine the content of Fe₃O₄ nanoparticles and MWCNTs, the purge gas was switched from nitrogen gas to air at 700°C. It can be seen from the Figure 6B that the remaining residue at 520°C for the neat TPNR was due to the formation of carbonaceous products. At 700°C, the carbonaceous products reacted with oxygen and released carbon dioxide. This explains the negligible remaining residue at the maximum temperature. The remaining residue at 520°C for nanocomposites with the addition of 2 wt% pristine MWCNTs, acid-treated MWCNTs, Fe₃O₄, pristine MWCNTs-Fe₃O₄, and acid-treated MWCNTs-Fe₃O₄ was 2.45%, 2.03%, 2.14%, 1.88%, and 2.04%, respectively, which is close to the added filler amount. Analysis of the filler content, carried out after oxidation, showed that the residues of both pristine MWCNT and acid-treated MWCNT nanocomposites continued burning until almost 100% of samples were degraded at maximum temperature, whereas the residues of pristine MWCNTs-Fe₃O₄ and acid-treated MWCNTs-Fe₃O₄ nanocomposites, after exposing the sample to oxygen, was maintained at about 1%, which corresponds to the amount of inert Fe₃O₄ nanoparticles in the TPNR matrix. The remaining residue of TPNR containing Fe₃O₄ nanoparticles was not affected by the switching of purge gas.

3.5 Dynamic mechanical properties

Nanocomposites with 2 wt% fillers were tested by DMA in a tensile mode at a temperature range from -120°C to 100°C. The same testing was carried out on the neat TPNR samples for comparison purposes. The storage modulus (E′) vs. temperature is shown in Figure 7A. The storage modulus at -90°C obtained from DMA is summarized in Table 3. The value of storage modulus signifies the elastic stiffness of the material. Moving from very low temperature to a higher temperature, all the curves show three distinct regions: a glassy high modulus region where the segmental mobility is restricted, a transition zone where a substantial decrease in the E′ values and the physical properties change drastically with increase of temperature, and a rubbery region where there is a drastic decay in the modulus with temperature. It can be noted that, at low temperature, the modulus of the neat TPNR and its nanocomposites was high. All samples showed transition and a plateau region corresponding to NR and PP. The storage modulus then decreased with increasing temperature, as expected, and finally leveled off at high temperature. The two-step curves are due to the two-phase morphology indicating that the blend between the phases is incompatible. The results are in agreement with those of systems based on blends of nylon copolymer-EPDM rubber by Komalan et al. [35] and blends of PP-EPDM by Karger-Kocsis and Kiss [36].

Figure 7

DMA of the neat TPNR and its nanocomposites: (A) storage modulus vs. temperature and (B) tan(δ) vs. temperature.

It was observed that at -90°C, the TPNR matrix containing acid-treated MWCNTs had the highest storage modulus, whereas the neat TPNR sample had the lowest. The addition of 2 wt% pristine MWCNTs has no significant effect on the storage modulus of TPNR. This is probably attributed to the poor dispersion and weak interfacial adhesion of MWCNTs in the TPNR matrix. However, the presence of acid-treated MWCNTs in the TPNR matrix increased the storage modulus of the TPNR matrix. The same trends have been observed by Špitalský et al. [6], Pan et al. [8], and Sahoo et al. [18] on the mechanical properties of acid-treated filled PP, epoxy, and nylon-6 composites. They reported that the enhanced storage modulus of the nanocomposites are due to the effect of the fine dispersion of the acid-treated MWCNTs into the polymer matrix, and the enhanced interaction between functional groups of MWCNTs and the polymer chains. This result is consistent with the TGA result as reported above. TPNR filled with acid-treated MWCNTs-Fe₃O₄ showed a significantly high storage modulus, which is slightly lower than that of acid-treated MWCNT-filled TPNR. The addition of 2 wt% acid-treated MWCNTs-Fe₃O₄ to TPNR increased its storage modulus from 16.1 to 17.7 GPa, i.e., about 10% improvement over the neat TPNR. However, the storage modulus for TPNR containing Fe₃O₄ and pristine MWCNTs-Fe₃O₄ are relatively low. For TPNR containing Fe₃O₄, the low storage modulus is probably caused by the preferential agglomeration of Fe₃O₄ nanoparticles due to the strong magneto-dipole interactions between particles [37], which weakens the adhesion of polymer molecules on the nanofiller surface, as shown in Figure 2E and 3E.

Figure 7B shows the variation of loss tangent (tan(δ)) with temperature for the neat TPNR and its nanocomposites. The tan(δ) of each nanocomposite has a wider distribution than that of the neat TPNR because of the incorporation of filler into the TPNR matrix. Examining the tan(δ) variation curves of the TPNR and its nanocomposites, two distinct peaks can be observed where each peak corresponds to the glass transition of NR and PP indicating immiscibility between the phases. The temperature corresponding to the maxima of a tan(δ) peak is normally associated with the glass transition temperature T_g. Table 2 indicates the effect of the addition of filler on the T_g of the TPNR matrix. It was noted that there was no significant change in the T_g values of the NR phase with the addition of fillers into the neat TPNR in the tan(δ) curve. Komalan et al. [35] reported that this phenomenon could be attributed to two types of constraints imposed on the motions of the molecular segments in the vicinity of the interface at the lower T_g. One is due to the presence of another phase, and the other is due to the presence of chemical bonds between the two phases. As the blends of TPNR goes through the lower T_g, one phase, which has a higher T_g, retains its glassiness. The constraints imposed by the glassy phase apparently dominate the presence of chemical bonds between phases. Consequently, addition of fillers makes no difference to the T_g of the NR phase.

From the variation of tan(δ) with temperature curve, the T_g of the neat TPNR appeared at approximately -73.6°C for the NR phase and at 2.73°C for the PP phase. The T_g of the neat TPNR for the PP phase was slightly shifted to the higher temperature by the addition of acid-treated MWCNTs. This phenomenon could be explained in two ways. First, this is due to the interaction between TPNR and acid-treated MWCNTs; the adsorption of the polymer chains on the surface of MWCNTs restricted the mobility of the molecular segment in the zone surrounding the MWCNTs [2, 8, 18]. Second, this effect can be explained in terms of decreasing the free volume of polymer. When MWCNTs are added into the TPNR matrix, absorption of polymer molecules will cause the matrix phase to become more compacted and the free volume will be decreased. Consequently, the T_g will be increased.