Electric field-induced alignment of MWCNTs during the processing of PP/MWCNT composites: effects on electrical, dielectric, and rheological properties

2.1 Materials

PP (T30S) with a melt flow index of 3.0 g/10 min was supplied by Sinopec Group (China). MWCNTs (XFM16) were purchased from Nanjing XFNANO Materials Tech Co., Ltd. (China). These MWCNTs presented an inner diameter of 5–10 nm, an outer diameter of 10–20 nm, a length of 0.5–2 μm, and a specific surface area of about 200 m²/g.

2.2 Sample preparation

The materials (PP and MWCNTs) were dried at 80°C for 4 h in an oven before use. PP/MWCNT composite materials were prepared by mixing PP pellets with MWCNT powder using a Brabender mixer at 180°C for 20 min. The MWCNT weight fractions were 0.1%, 0.5%, 1%, 2%, 3%, 4% and 5%. The mixtures were moved in a self-developed HEF processing device for hot-compression molding. The device is schematically shown in Figure 1. The inducing HEF was established between two plates connected to a high-frequency generator (GJ5-6B-I-JY). The high-frequency generator could provide an output voltage up to 7 kV. The mixtures were made into 1-mm-thick plates of PP/MWCNT composites by hot-compression molding at 180°C with a pressure of 12 MPa for 12 min. HEF was applied in the last 4 min during the hot-pressing process when the composites were in molten stage. After processing, composite plates were quenched immediately in cold water to prevent MWCNTs from returning to the random state again.

Figure 1:

Schematic drawing of the HEF processing device.

2.3 Characterization

3.1 Morphology of PP/MWCNTs

Figure 2 shows the SEM images of PP/MWCNT composites with MWCNT content of 0.5, 1, 2, and 3 wt.%. The dispersion of MWCNTs is relatively homogeneous in the PP matrix and few aggregates of MWCNTs can be observed. Figure 3 shows the TEM images of the ultrathin section of PP/MWCNT composites with MWCNT concentration of 0.5, 1, 2 and 3 wt.% prepared with and without HEF. In the TEM images, MWCNTs are distributed uniformly in the PP matrix. However, the orientation of MWCNTs is irregular in the composites without treatment by HEF (Figure 3A, C, E and G), whereas they show a distinct alignment in the composites with applied HEF, as marked by white arrows in Figure 3B, D, F and H. For samples with 0.5 wt.% MWCNTs, because the content is low, the aligning MWCNTs are almost isolated from one another and have no obvious contacts to generate the conductive pathways. Once the concentration of MWCNTs reaches 1 and 2 wt.%, there can be seen the presence of agglomerates (Figure 3B–F). After the application of the electric field, however, MWCNTs align distinctly and overlap with each other (Figure 3D and F). When MWCNTs increase to 3 wt.%, more aggregations of MWCNTs can be observed in two types of composite, which obviously form a conductive network in the matrix.

Figure 2:

SEM images of untreated PP/MWCNT composites: (A) 0.5% MWCNTs, (B) 1% MWCNTs, (C) 2% MWCNTs and (D) 3% MWCNTs.

Figure 3:

TEM images of PP/MWCNT composites with and without the applied HEF: (A) 0.5% MWCNTs, (B) 0.5% MWCNTs+HEF, (C) 1% MWCNTs, (D) 1% MWCNTs+HEF, (E) 2% MWCNTs, (F) 2% MWCNTs+HEF, (G) 3% MWCNTs and (H) 3% MWCNTs+HEF.

3.2 Electrical conductivity

Figure 4A shows the frequency dependence of AC electrical conductivity measured within the frequency range of 10³–10⁵ Hz for untreated PP/MWCNT composites with MWCNT content of 0.1–5 wt.% at room temperature. The AC electrical conductivity of all composites with 0.1–2 wt.% MWCNTs increase linearly with increasing frequency in the AC electrical conductivity-frequency plotting, indicating typical insulator behavior and the capacitance-dominant mechanism [18]. For the composites with MWCNT content of more than 2 wt.%, its AC electrical conductivity curve appears to be a plateau and remains nearly constant with increasing frequency, which means the formation of a 3D interconnecting network by dispersed MWCNTs within PP matrices. If the MWCNT content is higher than the percolation value, the conductivity is mainly determined by the numerous paths formed by the percolating clusters rather than the capacitors. Therefore, the conductivity of these composites will not change with increasing frequency, and a plateau appears in AC conductivity-frequency plots [19].

Figure 4:

Frequency dependence of AC electrical conductivity of PP/MWCNT composites with different MWCNT contents at room temperature: (A) untreated and (B) HEF treated.

Figure 4B shows the frequency dependence of AC electrical conductivity measured within the frequency range of 10³–10⁵ Hz for HEF-treated PP/MWCNT composites with MWCNT content of 0.1–5 wt.% at room temperature. The AC electrical conductivity of all composites with 0.1–1 wt.% MWCNTs increase linearly with increasing frequency. However, when the MWCNT content is 2 wt.%, its curve is different with the untreated sample, and the AC electrical conductivity remains constant with increasing frequency, which means the formation of conductive networks is brought forward to 2 wt.% when the composite samples are processed under the effect of HEF.

The AC conductivity result at 10³ Hz of PP/MWCNT composites with and without the applied HEF is plotted in Figure 5 as a function of the MWCNT concentration. One can divide both plots into three regions. With increasing MWCNT content up to 0.5 wt.%, the exertion of HEF has little lifting effect on σ. This is because when the MWCNT content is relatively low, it is too far to form a conductive network, although MWCNTs have aligned in the PP matrix under HEF, which corresponds to Figure 3A and B. However, the difference increases from 1 to 2 wt.%, indicating the effectiveness of HEF in increasing the conductivity of composites in this range of the MWCNT content. The σ of 1 wt.% MWCNTs is close to that of untreated PP/MWCNT composites with 2 wt.% MWCNTs. The σ of 2 wt.% MWCNTs has a sharp rise by two orders of magnitude. At even higher concentrations, the σ of composites still increases to some extent after the effect of HEF, but the range of increase is basically kept at one order of magnitude.

Figure 5:

AC conductivity versus MWCNT content of PP/MWCNT composites with and without the applied HEF at 1000 Hz.

The insets show the plot of logσ_m vs. log(m–m_c) [(1) untreated and (2) HF treated] and the straight line represents the best-fitted line of the data in Eq. (1).

According to the classical percolation theory [20], the relationship between the conductivity (σ_m) of a polymer filled with conductive particles and the mass fraction (m) of the particles in the mixture above the percolation threshold mass fraction (m_c) can be expressed according to Eq. (1):

where σ₀ is the conductivity of the conductive filler and t is the critical exponent related to the dimensionality of the system. As shown in Figure 5 (inset), the best linear-fitted logσ_m vs. log(m–m_c) plot was created using Eq. (1) by considering the value for φ_c≈2.42% (untreated) and 1.68% (HEF), and the critical exponent (t) was calculated and found to be ≈1.79±0.3 (untreated) and 2.56±0.24 (HEF) from the slope of logσ_m vs. log(m–m_c) plot. That is, the percolation threshold was reduced from 2.42 to 1.68 wt.% under the effect of HEF on the melt processing of PP/MWCNT composites.

3.3 Dielectric properties

From the percolation power law [21], when the volume fraction of the conductor is slightly lower than the percolation threshold, the dielectric constant of the composite will have a remarkable growth:

where ε and ε₀ are the dielectric permittivity of the composite and polymer matrix, respectively, f_CNT is the volume fraction of MWCNTs, f_c is the percolation threshold, and q is a critical exponent equal to 1.0. When the content of the filling conductor is equal to or higher than the percolation threshold, the composites will change from insulator to conductor and the dielectric properties of materials will disappear.

The dielectric performance of the PP or PP/MWCNTs with and without the applied HEF is shown in Figure 6. As we can see, dielectric permittivity increases with the loading of MWCNT increase. The increase of ε in the polymer composites with the introduction of conductive particles was based on the enhancement of interfacial polarization. MWCNTs effectively increased ε, especially near the percolation threshold of the MWCNT concentration. Tanδ, however, also increased simultaneously.

Figure 6:

Dielectric performance of PP or PP/MWCNTs with and without the applied HEF.

With applied HEF, the dielectric performance of PP/MWCNTs has a considerable and interesting change, especially in 1 and 2 wt.% MWCNTs (Figure 7). In samples with 1 wt.% MWCNTs, the ε of the sample treated by HEF has a great improvement compared to the untreated sample, whereas the tanδ of the sample still remains stable at about 0.5. This is important for dielectric applications. When the MWCNT content increases to 2 wt.%, the effect of HEF also leads a dielectric permittivity lifting, but the tanδ of composites reaches a fairly high level and it cannot be used as a dielectric material. With regard to MWCNT/polymer composites, the increase in dielectric permittivity could be attributed to the interfacial polarization effect, which is ascribed to the accumulation of many charge carriers at the internal interfaces between MWCNTs and PP. In this work, it is much easier for MWCNTs to directly connect with one another and form an MWCNT network in the composites when treated by HEF than in untreated composites, as already confirmed in TEM graph and the result of the electrical conductivity measurement. Thus, at the same content of MWCNTs, the dielectric constant of HEF-treated composites is obviously higher than that of untreated composites because of the formation of the MWCNT network in HEF composites. On the contrary, the direct connection of MWCNTs in HEF composites and the smaller distance between adjacent MWCNTs leads to a high DC conductance, which contributes to a higher dielectric loss, especially in 2% MWCNTs.

Figure 7:

Effect of HEF to the dielectric performance of PP/MWCNT composites: (A) 1 wt.% and (B) 2 wt.%.

3.4 Rheological properties

Owing to their high aspect ratio and specific surface area, the orientation behavior of MWCNTs in the matrix due to HEF is bound to affect the microstructure of composite materials and subsequently affect the rheological behavior of the composites. Therefore, through the analysis of the rheological properties of composite materials, we can further study the relationship between the alignment of MWCNTs in the matrix and the rheological properties of the composites.

The storage modulus represents the ability of viscoelastic materials to store elastic energy under loading. The storage modulus of PP/MWCNT composites with and without applied HEF is shown in Figure 8. At low frequency (0.25 rad/s), it can be seen that the more MWCNTs are added into composites, the more the storage modulus is improved, which confirms the reinforcement effect of MWCNTs on the PP composites. What’s more, the storage modulus of the composites processed under the effect of HEF is larger than the composites produced without exposure to HEF. Besides, this kind of lifting effect is more obvious with the increase of the MWCNT content. However, at high frequency (100 rad/s), this is no longer the enhancement of MWCNTs to the PP matrix as well as the improvement along the direction of HEF.

Figure 8:

Storage modulus versus MWCNT content of PP/MWCNT composites with and without the applied HEF at 0.25 and 100 rad/s.

The improvement on the storage modulus at relatively low frequency (0.25 rad/s) is based on the interconnection between MWCNTs and the matrix as well as the network structure formed by MWCNTs in the matrix. The shear force at low frequency is not large enough to overcome this obstacle in the flow process and the storage modulus of the composites increases macroscopically. Under the effect of HEF, the alignment of MWCNTs accelerates the formation of the MWCNT network in the matrix and thereby forms more entanglements between the network structure and the molecular chain of PP as the obstacle in the flow process. Therefore, in the same content, the storage modulus of PP/MWCNT composites with exposure to HEF is higher. However, for high frequency (100 rad/s), the network structure of MWCNTs is destroyed seriously by high loading and the reinforcement effect is no longer obvious [22]. Similar results [15] were reported previously.

Figure 9 shows a plot of the storage modulus G′ versus the loss modulus G″ with frequency. Such plots were used by Han and Jinhwan [23] to investigate temperature-induced changes in the microstructure of homopolymers, block copolymers, and blends. It was proposed that if the microstructure does not change with temperature, the curves of logG′ vs. logG″ at different temperatures should coincide. Harrell and Nakajima [24] used logG′-logG″ curves to investigate the influence of the branching and molecular weight distribution width of polyethylene on its microstructure. In addition, logG′-logG″ curves were used to research multiphase systems and analyze the structure transformation between the matrix and the filler in systems at a given temperature. Pötschke et al. [25] used such plots to explore the rheological behavior of PP/MWCNT composites and found that the slope of G′ vs. G″ decreases with increasing MWCNT content, which means the microstructure of composite changes.

Figure 9:

Storage modulus G′ as a function of loss modulus G″ of PP/MWCNT composites with and without the applied HEF.

For the logG′-logG″ curves of PP/MWCNT composites with and without the effect of HEF in Figure 9, the storage modulus G′ (for a given loss modulus G″) increases significantly with the increasing content of MWCNTs. The slope of G′ vs. G″ can be obtained by linear fitting and the slope of each curve is listed in Table 1. It can be clearly seen that the slope of the G′ vs. G″ curve of the untreated sample is larger than the HEF sample in every MWCNT content. This interesting result indicated that the microstructure of composites is changed under the effect of HEF. This could be attributed to the alignment of MWCNTs in HEF forming a more perfect network structure in the PP matrix.