Dramatically improved dielectric properties of polymer composites by controlling the alignment of carbon nanotubes in matrix

Abstract

Aligned multi-walled carbon nanotubes (MWCNTs)/polyvinyl alcohol composite films were prepared by using an easy and controllable electrospinning-in situ film-forming (EF) technique. A high dielectric constant (k), a low dielectric loss, a consistently high breakdown strength, and a high energy density were obtained by using this technique. The dramatically improved dielectric properties are ascribed to the good dispersion and alignment of MWCNTs in the matrix, facilitating the formation of a large number of separated nano-capacitors (high k and low direct current (DC) conductance). For comparison purposes, the same composite films were prepared by solution casting (SC). At the same MWCNT content, the SC method yielded a higher k, but a significantly higher dielectric loss and much lower breakdown strength and energy density because of the random dispersion of MWCNTs in the matrix and the formation of a MWCNT network, which result in a large increase in DC conductance. The formation mechanism of the different microstructures and the relationships between the microstructures and dielectric properties are clarified. Our results indicate that high-performance MWCNTs/polymer dielectric composites can be obtained by controlling the microstructure of the composites by using the EF technique, which widens the applications of dielectric materials.

---

1. Introduction

In recent years, polymer-based dielectric materials with high dielectric constant (k), low dielectric loss, high breakdown strength, and high energy density have attracted considerable interest owing to their potential applications such as artificial muscles, energy storage, flexible electronics, and sensors.^1–4 As we know, the dielectric constant of polymers is usually quite low. To improve the dielectric constant, one common strategy is to add high k ceramic fillers such as BaTiO₃ or PbTiO₃ into a polymer matrix. Generally, the k of the composites could be obviously improved by adding up to 50 vol% of these fillers.^5,6 Unfortunately, the mechanical and processing properties were greatly deteriorated as a result of the high loading of these fillers. Another widely used strategy is to prepare conductive polymer composites, among which, multi-walled carbon nanotubes (MWCNTs) is the widely used conductive filler due to their large aspect ratio, unique mechanical properties and high electrical conductivity. Comparing with ceramic/polymer composites, a small loading of MWCNTs is sufficient to obtain a high dielectric constant (k), and thus the mechanical properties can be maintained or even improved over that of the polymer matrix.^7–9 However, the large increase in k of the MWCNTs/polymer composites is usually accompanied by a dramatic increase in dielectric loss, and a sharp decrease in dielectric breakdown strength, which largely limits their practical applications.^10,11

Therefore, innovative approaches are needed to obtain MWCNTs/polymer composites with a high k, a low dielectric loss, a high breakdown strength, and a high energy density.^12–14 Recently, a novel technique was utilized to increase the k and reduce the dielectric loss of MWCNTs/polymer composites by using a unique core–shell structured MWCNTs as fillers, in which the outer wall of MWCNTs was covalently functionalized to become nonconductive, whereas the inner wall of MWCNTs were unfunctionalized and remained electrically conductive. The nonconductive outer wall of MWCNTs resulted in the decrease in dielectric loss and the electrically conductive inner wall of MWCNTs led to the increase in k.^14,15 In addition, MWCNTs/polymer composites with a high k and a low dielectric loss can be obtained by introducing insulating modification layers on the surface of MWCNTs.¹⁵ For example, Yang et al.¹⁵ reported that flexible dielectric MWCNTs/polystyrene composites with a constant high k (≈44), rather low dielectric loss (<0.07), and high energy density (4.95 J cm⁻³) was successfully prepared by coating an organic polypyrrole (PPy) shell on the MWCNTs surface. On the one hand, the insulating shell can ensure the good dispersion of MWCNTs in the polymer matrix, thus improving the interfacial adhesion between the MWCNTs and the matrix. On the other hand, the coating of PPy shell on the MWCNTs surface can effectively prevent the MWCNTs from connecting with one another directly, and screen charge movement to shut off the leakage current, thus can effectively decrease the dielectric loss. However, in these cases, the MWCNTs are randomly dispersed in the polymer matrix, thus easily overlapping with one another, which can result in the increase in dielectric loss and the decrease in breakdown strength. Therefore, in addition to a good dispersion of MWCNTs in the polymer matrix, we need to align the MWCNTs in one direction to reduce the possibility for MWCNTs connecting with one another directly. With both good dispersion and alignment of MWCNTs, the MWCNTs/polymer composites are expected to have excellent dielectric properties.

Up to now, several techniques have been reported to obtain a good dispersion and alignment of MWCNTs in polymer matrix.^16–20 Among these techniques, electrospinning is a simple, effective, and widely used technique for preparing polymer composite fibers/films. It is easy to realize the good dispersion of MWCNTs in a polymer matrix and the good alignment of both polymer chains and MWCNTs along a direction by using this technique, as has been previously reported.^20,21 In recent years, Nan et al. has conducted a very interesting work, in which modified MWCNTs/polysulfone (PSF) composites nanofibers with a good dispersion and alignment of MWCNTs in PSF matrix have been successfully prepared by using electrospinning technique.²⁰ These nanofibers were then stacked along the same fiber direction into sheets, and then molded by hot-pressing near the softening temperature of PSF. As a result, MWCNTs/PSF composites with high dielectric constant, low dielectric loss and a large energy density were successfully prepared as the content of MWCNTs in the range of 10–25 vol%. Based on Nan's work, in this work, we firstly prepared oriented nanofibers sheet of MWCNTs/polymer composite by using the electrospinning technique. Then, instead of hot pressing, we use a special in situ film-forming technique to prepare the composites films with good dispersion and alignment of MWCNTs in matrix at a low content of MWCNTs (0.5–4 wt%) because hot pressing could lead to the movement of MWCNTs in matrix, and thus could lead to the decrease in the orientation degree of MWCNTs in matrix. In addition, we conduct a comparison work by using solution casting (SC) technique to prepare the composite film with MWCNTs randomly dispersed in the matrix. Then, we carefully investigate the dielectric performance of the composite films with different microstructures prepared by using the two different techniques, and their microstructure-dielectric properties relationship. Poly(vinyl alcohol) (PVA) was used as the matrix because it is nontoxic, water-soluble, and it has high dielectric strength, good charge storage capacity and dopant-dependent electrical and optical properties, and thus could be used in many applications, as reported in some works.^22,23 In addition, PVA has a good spinnability. A commercial available carboxyl-modified carbon nanotube was used to increase the interaction between the MWCNTs and the PVA matrix. Our goal is twofold: (1) to prepare MWCNTs/PVA composite films with the MWCNTs well dispersed in the matrix and well aligned in one direction to obtain high performance MWCNTs/PVA dielectric materials by using a simple and effective method (electrospinning-in situ film-forming (EF) technique) at a low content of MWCNTs, (2) to deeply understand the formation mechanism of the microstructures by using EF technique and the microstructure-dielectric properties relationships.

2. Experimental

2.1. Materials

Carboxyl-modified carbon nanotubes (MWCNTs-COOH) with an average diameter (D) of 8–15 nm, a length (L) of 0.5–2 μm, and a –COOH content of 3.86 wt% were supplied by Chengdu Organic Chemical Co., Ltd. (China). Powdered polyvinyl alcohol (PVA F-17) with a degree of polymerization of 1700 and a degree of alcoholysis of 99% was purchased from OCI Company, LTD (Korea). N,N-dimethyl formamide (DMF) was supplied by Beijing Modern Eastern Fine Chemical (China).

2.2. Preparation of composite films

2.2.1. Preparation of MWCNTs/PVA suspension. The stable suspension of MWCNTs/PVA was prepared by using ultrasonic dispersing technology as follows. First, 10 wt% of PVA powder was dissolved in deionized water and stirred for 2 h at 90 °C. Meanwhile, MWCNTs were suspended in DMF by ultrasonication (100 W) for 2 h. Subsequently, the well-dispersed suspension of MWCNTs/DMF was mixed with the aqueous solution of PVA under stirring for 2 h to obtain a uniform MWCNTs/PVA/H₂O/DMF suspension, in which the mass ratio of DMF to water was 1/4, the content of PVA was 8 wt%, and the ratio of MWCNTs to PVA ranged from 0.0 to 4.0 wt%. As an example, the mass ratio of MWCNTs/PVA//H₂O/DMF suspension with 2.8 wt% MWCNTs is 2.8/100/1000/250. MWCNTs contents higher than 4.0 wt% were not considered because of the poor dispersion and the blocking of the needle during electrospinning.

2.2.2. Electrospinning. The electrospinning process was schematically represented in Fig. 1. The above suspension of MWCNTs/PVA was filled to a 20 mL plastic syringe and injected through a needle (with inner diameter of 0.30 mm) at an injection rate of 0.8 mL h⁻¹ by using a syringe pump. The applied voltage (ES30P, Gamma High Voltage Research Inc., USA) was kept at 14 kV. The electrospun composite nanofibers were then collected in the form of a sheet on a 10 cm diameter rotating drum at a rotation speed of 700 r min⁻¹.

Fig. 1 Schematic of electrospinning.

2.2.3. In situ film forming. In situ film forming technique is conducted as follows. The sheet was cut into small pieces (15 mm × 15 mm). 20 plies of which were stacked along the same fiber direction. These sheets were then molded by dipping a few drops of deionized water of 90 °C onto the surface of them (re-dissolving the PVA to adhere each other). Finally, the compacted MWCNTs/PVA composite film with a thickness of about 0.5 mm was successfully prepared without decreasing the alignment of MWCNTs in PVA matrix. In this case, the orientation degree of MWCNTs is almost the same as that of the as prepared electrospun composites fibers.

2.2.4. Solution casting. A composite film was also prepared by solution casting a well-dispersed suspension of MWCNTs, PVA, H₂O, and DMF on a polytetrafluoroethylene (PTFE) tray, followed by evaporation of H₂O and DMF solvent in air for 24 h, and dried in a vacuum oven at 80 °C for 4 h. The as-obtained MWCNTs/PVA composite film was also 0.5 mm thick.

2.3. Characterization

An S-4700 scanning electron microscope (SEM) purchased from Hitachi Co. (Japan) was used to observe the morphologies of the composite nanofibers, and the dispersion and alignment of MWCNTs in the MWCNTs/PVA composite films prepared by EF technology and SC. An H-800 transmission electron microscope (TEM) purchased from Hitachi Co. (Japan) was used to observe the dispersion and orientation of the CNTs in the nanofibers. Polarized Raman Spectroscopy (PRS) was performed to determine the alignment of the CNTs in the nanofibers by using a TY-HR 800 Raman spectrometer with laser excitation at 532 nm.

The electrodes, in the form of conductive silver paste, were painted on the upper and lower surfaces of the composites for dielectric measurements, and the samples had a surface area of 1 cm² and a thickness of 0.5 mm. Before the measurement of dielectric properties and dielectric breakdown strength, the samples are dried in vacuum at 60 °C for 12 hours. The dielectric properties of the MWCNTs/PVA composite films were measured by using a HP4294A impedance analyzer (Agilent, U.S.A) in the frequency range of 10²–10⁶ Hz at room temperature. Dielectric breakdown strength was measured according to Chinese national standard GB/T 1695-2005 using a DC high-voltage power source (DW-P303-1AC, China) by sweeping the applied voltage at approximately 10 V s⁻¹ until the point of catastrophic device failure, as evidenced by spurious current changes and pitting of the top electrode. The composite film was placed in an oil bath. The threshold of leakage current was fixed at 0.5 mA. The average values from at least five samples were reported. The error range is about 10%.

3. Results and discussion

3.1. Dispersion and alignment of MWCNTs in PVA/MWCNTs composite films

Fig. 2 shows photographs of the MWCNTs/PVA/H₂O/DMF suspension with a mass ratio of 2.8/100/1000/250 prepared by ultrasonic dispersing technology. For the as-prepared suspension, MWCNTs are well dispersed in the PVA solution without the addition of any surfactant, as shown in Fig 2(a). After 24 h, the uniform dispersion of MWCNTs in the PVA matrix is still observed, and there is no obvious stratification, suggesting that the suspension is very stable. This could be due to the strong hydrogen bonding interactions between the carboxyl groups of MWCNTs and the hydroxyl groups of PVA.²⁴ MWCNTs/PVA composite fibers are then prepared by electrospinning of the stable suspension, followed by the collection of these fibers on a rotating drum along the fiber direction. As shown in Fig. 3(a), most fibers have smooth surfaces and have almost the same diameter of about 200 nm, and they are quite well aligned along the rotary direction of the drum. We then observe the dispersion and alignment of MWCNTs in quite a few composites fibers by using TEM. The typical morphology of the MWCNTs in most composites fibers is shown in Fig. 3(b–d). It is observed that only single MWCNT is well dispersed in the radial direction of a composite fiber with a diameter of about 200 nm and aligned along the fiber axis for all the three fibers because of the confined space of nanometer scale of the composite fibers. This result implies again the strong interfacial interaction between the carboxyl-modified carbon nanotubes and the PVA matrix, which promotes the fine dispersion of MWCNTs in PVA matrix.

Fig. 2 Photographs of MWCNTs/PVA/H₂O/DMF suspension with a mass ratio of 2.8/100/1000/250, indicating stability of MWCNTs in solution.

Fig. 3 SEM micrograph of electronspun MWCNTs/PVA composite fibers with 2.8 wt% MWCNTs (a); TEM micrographs of electronspun MWCNTs/PVA composite fibers with 2.8 wt% MWCNTs (b–d).

The as-obtained composite fibers are then pressed into a film, as described in 2.2. The morphology of the freeze-fractured surface of the electrospun composite film perpendicular to the direction of the fiber axis is then observed by using SEM. The good dispersion and alignment of MWCNTs in PVA matrix is guaranteed by using EF method for all the composites with different content of MWCNTs (0.5–4 wt%), and the corresponding mechanism is described in detail in Section 3.3.1. As an example, only the morphology of the composite with 2.8 wt% MWCNTs is shown in this work (see Fig. 4). We can observe that the MWCNTs are indeed well dispersed in the PVA matrix and they are aligned along fiber direction, as shown in Fig. 4(a). To see clearly the dispersion of MWCNTs in the matrix, SEM images with higher magnification are obtained, as shown in Fig. 4(a′). As expected, many single MWCNTs fibers with a diameter of about 15 nm are well dispersed in the PVA matrix. Meanwhile, some doublet or triplet MWCNT bundles are also well dispersed in the matrix. These MWCNTs or MWCNTs bundles are indeed separated from one another by the PVA matrix and well aligned along the fiber direction. It indicates that the unique EF technique can effectively prevent the MWCNTs from curling, winding, and forming connecting networks. For comparison purposes, a MWCNTs/PVA composite film was prepared by using the SC method. The corresponding morphology is shown in Fig. 4(b) and (b′). Clearly, these MWCNTs are randomly dispersed in the composite film: some are perpendicular to the freeze-fractured surface of the film, and some lie on the surface. As a result, MWCNTs network is formed, in which the MWCNTs agglomerate, curl, and directly connect with one another.

Fig. 4 SEM micrographs of fractured surfaces (perpendicular to the direction of fiber axis) of composite film with 2.8 wt% MWCNTs prepared by EF method (a, a′), and SC method (b, b′).

It is generally believed that Polarized Raman Spectroscopy (PRS) is a useful tool to characterize the orientation of MWCNTs in the composite, as has been used in many works.^16–20 Therefore, PRS is used in this work to characterize the orientation of MWCNTs in the composite films prepared by using EF and SC methods, as shown in Fig. 5. For each sample, Raman spectra were recorded parallel to (0°) and perpendicular to (90°) the fiber axis. For all the samples, the two peaks at about 1356 cm⁻¹ and 1601 cm⁻¹ are clearly observed, which correspond to the D mode and G mode, respectively. The former represents a disordered structure, which arises from the multiple phonon scattering of defects or amorphous carbon, and the latter represents the high-frequency E_2g Raman scattering mode of sp²-hybridized carbon.^25–27 The degree of MWCNT alignment is evaluated by the depolarization factor, R, which is defined as the ratio of the peak intensity for the G band in the parallel direction (I₀) to that in the perpendicular direction (I₉₀); i.e., (R = I₀/I₉₀). For the SC composite film, the value of R is approximately equal to 1.15, indicating the low orientation level of MWCNTs in the PVA matrix. However, for the composite film prepared by the EF technique, the value of R increases remarkably to 3.48, suggesting the good alignment of MWCNTs in the PVA matrix. It is noted that the volume fraction of MWCNTs is even higher than 15 vol% to obtain the orientation degree (R= 3.48) for the samples prepared via electrospinning and hot-pressing technique in Nan's work.²⁰ In our work, the volume fraction of MWCNTs is only about 1.4 vol% (mass fraction is 2.8 wt%) to obtain the same orientation degree by using EF technique. In a word, both the SEM images and the PRS spectra indicate that the MWCNTs are well aligned along the fiber direction in the composite film prepared by the EF technique.

Fig. 5 Polarized Raman spectra of MWCNTs/PVA composite films with 2.8 wt% MWCNTs prepared by different methods. 0° refers to spectrum with polarization parallel to fiber axis, and 90° refers to spectrum perpendicular to fiber axis.

3.2. Dielectric properties

The dielectric properties at 100 Hz of the MWCNTs/PVA composite films prepared by the two methods are shown in Fig. 6. For the SC composite film, as shown in Fig. 6(a), k increases sharply with increasing MWCNT content at MWCNT contents higher than the percolation threshold (1.0 wt%). For example, k increases more than 200-fold from pure PVA to the composite film with 2.8 wt% MWCNTs. However, the dielectric loss of the composite film significantly increases with the increase of MWCNTs. There is a 100-fold increase in dielectric loss from pure PVA (0.13) to the composite film with MWCNT content of 2.8 wt% (14). As reported in previous studies, although MWCNTs can greatly increase the k, they also increase the dielectric loss.^28–31 With such high dielectric losses, the composites become semi-conductive, and cannot be used as dielectrics due to the very large leakage. The dielectric properties of the composite film prepared by EF technology are shown in Fig. 6(b). As seen, k is almost linearly increased from 2.0 at 100 Hz for pure PVA to 47 for the composite film with 2.8 wt% MWCNTs and it further increases to ∼105 with the further increase in the content of MWCNTs to 4 wt%. Comparing with pure PVA, the increase in k of the composites with 2.8 wt% and 4 wt% MWCNTs is 23 times and 52 times, respectively, by using EF method. More interestingly, the dielectric loss is only slightly increased with increasing content of MWCNTs. Even for the composite with 4.0 wt% MWCNTs, the dielectric loss remains low at 0.80. This indicates that MWCNTs/PVA composite films with high k and low dielectric loss have indeed been successfully obtained by using EF technology. On the other hand, for SC films, both the k and dielectric loss show a typical percolation behavior: as the content of MWCNTs reaches the percolation threshold (1.0 wt%), the dielectric properties sharply increase, whereas they change only slightly with further increases in the MWCNTs content. However, for EF films, k increases almost linearly with increasing content of MWCNTs in the whole range of 0–4 wt%. In addition, the dielectric loss increases only slightly with increasing content of MWCNTs, indicating that a low dielectric loss could still be obtained even at much higher contents of MWCNTs. Nevertheless, the composites fibers with MWCNT contents higher than 4 wt% are difficult to prepare because the spinnability deteriorates at high MWCNT contents. The different dielectric behaviors of the composites prepared by the two different techniques are ascribed to the different microstructures, as will be discussed in detail in Section 3.

Fig. 6 Dielectric constant and dielectric loss of MWCNTs/PVA composite films prepared by SC method (a) and EF method (b) at 100 Hz.

The frequency dependence of dielectric properties of a dielectric material is also very important for its wide application. Therefore, in this work, the frequency dependence of dielectric properties of pure PVA and MWCNTs/PVA composite films obtained by using the two different techniques were carefully investigated at room temperature, and the results are shown in Fig. 7. For both films prepared by two different methods, the k is obviously decreased with increasing frequency at MWCNT contents above a certain value, whereas k remains constant at MWCNT contents below a certain value. However, the dielectric properties of the composite films prepared by SC are much more sensitive to frequency than those of the composite films prepared by EF technology. For example, as the frequency increases from 10² to 10⁴ Hz, the k of the SC film with 2.0 wt% MWCNTs is sharply decreased from ∼400 to ∼30, while the k of the EF composite film with the same amount of MWCNTs is only slightly decreased from ∼40 to ∼20, as shown in Fig. 7(A-a) and (C-c). More interestingly, for the SC film with the content of MWCNTs beyond 1.0 wt% (percolation threshold), the dielectric loss shows an obvious decrease in the whole frequency range of 10²–10⁶ Hz, whereas for the electrospun composite film with MWCNTs content beyond 1.0 wt%, the dielectric loss shows a relatively low frequency dependence in the frequency range of 10²–10⁴ Hz, and even keeps almost constant in the high frequency range of 10⁴–10⁶ Hz, as shown in Fig. 7(B-b) and (D-d). The underlying mechanism for the different dielectric behaviors will be discussed in detail in Section 3.3.

Fig. 7 Frequency dependence of dielectric constant (A, a, C, c) and dielectric loss (B, b, D, d) of MWCNTs/PVA composite films prepared by EF method (A, a, B, b) and SC method (C, c, D, d).

Another two important properties of a dielectric material are the dielectric breakdown strength and the energy density, which are also carefully studied in this work, as shown in Fig. 8. Clearly, the breakdown strengths of the electrospun films with 1.0 wt% and 2.8 wt% MWCNTs are only slightly decreased from 50 MV m⁻¹ for pure PVA to 45 MV m⁻¹ and 40 MV m⁻¹, respectively, as shown in Fig. 8(a). However, the breakdown strengths of the SC films with 1.0 wt% MWCNTs and 2.8 wt% MWCNTs are sharply decreased to 10 MV m⁻¹ and 7 MV m⁻¹, respectively. These results are ascribed to the different microstructures of the composites films prepared by the two different techniques, as will be discussed in detail in the following section. On the other hand, the energy density of the films calculated by using the equation J = k₀kE²/2 are shown in Fig. 8(b).^32,33 As seen, the energy density of pure PVA film at the frequency of 1000 Hz is only 0.01 kJ L⁻¹ for both the SC film and the electrospun film due to the very low k of PVA. For the electrospun composite film, the energy density is obviously improved to 0.1 kJ L⁻¹ with the addition of 1.0 wt% MWCNTs, and it shows a further enhancement to 0.24 kJ L⁻¹ for the film with 2.8 wt% MWCNTs. Comparing with pure PVA, the improvements in energy density are 10 times and 24 times over that of pure PVA, respectively. These improvements are mainly ascribed to the greatly enhanced k and the consistently high breakdown strength of the electrospun composite film, as shown in Fig. 7 and 8(a), respectively. For the SC film, the energy density is firstly increased to 0.08 kJ L⁻¹ with the addition of 1.0 wt% of MWCNTs, and then sharply decreased to 0.02 kJ L⁻¹ as the MWCNT content increases to 2.8 wt%. The reason is that although k is greatly improved by the addition of 2.8 wt% of MWCNTs, the breakdown strength is sharply reduced to 7 MV m⁻¹, which leads to a significantly decreased energy density. In a word, these results suggest that a consistently high dielectric breakdown strength and a dramatically enhanced energy density have been successfully obtained by using the EF technique.

Fig. 8 The dielectric breakdown strength (a) and the energy density (b) of MWCNTs/PVA composite films prepared by EF and SC methods.

In a word, comparing with conventional SC method, the advantages of EF method are as following. First, the good dispersion and alignment of MWCNTs in matrix by using EF method lead to the formation of a large number of separated nano-capacitors, resulting in the large increase in dielectric constant. Second, the good dispersion and alignment of MWCNTs in matrix also results in a quite low DC conductance, and thus a low dielectric loss, a consistently high breakdown strength, a high energy density, and a relatively low frequency dependence of dielectric properties.

3.3. Discussion for the formation mechanism of different microstructures and microstructure-dielectric properties relationships

3.3.1. Formation mechanism of different microstructures by using EF and SC techniques. A schematic representation of the microstructure formation in MWCNTs/PVA composites by using the EF and SC methods is shown in Fig. 9(a). Initially, the MWCNTs are uniformly dispersed in PVA solution due to the hydrogen bonding between the carboxyl groups of MWCNTs and the hydroxyl groups of PVA. This can also be evidenced by the good spinnability of all the samples with 0.5–4 wt% MWCNTs. Here, it should be noted that if MWCNTs are aggregated in PVA suspension, the spinnability deteriorates. Actually, we find that MWCNTs contents higher than 4.0 wt% lead to the blocking of the needle during electrospinning because of the poor dispersion of MWCNTs in PVA suspension. In the EF method, MWCNTs are highly aligned along the fiber direction under electrostatic force because of the extremely high elongation rate (up to 1 000 000 s⁻¹) during electrospinning (especially during the bending instability).^34,35 Then, the solvent evaporates very quickly owing to the special large surface area of the nanofibers, promoting the transformation of PVA into the glassy state, in which the molecular chains no longer move freely. As a result, the deformation and relaxation of MWCNTs in the matrix are stopped, and the good dispersion of MWCNTs in the composite fibers and the good alignment of MWCNTs along the fiber direction are fixed. In addition, by using the in situ film-forming technology, the PVA matrix in the electrospun composite fiber can act as a sheath to prevent the MWCNTs in different fibers from direct connection. Thus, a uniform dispersion of MWCNTs in the nanofibers is achieved. In addition, the orientation of both MWCNTs along the fiber direction can be well maintained, as confirmed by Fig. 3–5.^36–38 However, in the SC method, although the same uniform MWCNTs/PVA suspension is used, a severe re-aggregation and random dispersion of MWCNTs in the PVA matrix are observed because of the slow evaporation of solvent, resulting in the formation of MWCNTs network.

Fig. 9 Schematic representation of (a) microstructures and (b) dielectric mechanism of MWCNTs/PVA composite films prepared by using EF and SC methods.

3.3.2. Microstructure-dielectric properties relationship. As we all know, the dielectric properties depend strongly on the microstructure of the polymer composites. Thus, the microstructure-dielectric properties relationship is carefully studied in this work, as schematically represented in Fig. 9(b). With regard to MWCNTs/polymer composite, the increase in dielectric constant could be attributed to two mechanisms.³⁹ The first one is the nano-capacitance-structure model, which postulates that many parallel or serial nano-capacitors connected with one another are formed. The second one is the interfacial polarization effect (also named Maxwell–Wagner–Sillars (MWS) effect), which is ascribed to the accumulation of many charge carriers at the internal interfaces between MWCNTs and PVA. For both mechanisms, the dispersion and spatial distribution of MWCNTs in the matrix are the key factors to affect the dielectric performance. In this work, it is much easier for MWCNTs to directly connect with one another and form a MWCNTs network in SC composites than in EF composites due to the random dispersion of MWCNTs in the PVA matrix in SC composites, as already confirmed in Section 3.1. As a result, at the same content of MWCNTs, the dielectric constant of SC composites is obviously higher than that of EF composites because of the formation of MWNTs network in SC composites. On the other hand, in SC composites, the plot of dielectric constant versus mass fraction of MWCNTs shows a typical percolation behavior (see Fig. 6(a)), as described in Section 3.2. In this case, the MWS effect should be the main mechanism, since the nano-capacitors are not easy to form because of the random dispersion of MWCNTs in the matrix. In EF composites, the MWCNTs are separated by the PVA matrix and cannot directly connect with one another owing to the good dispersion and alignment of MWCNTs in the PVA matrix. However, more nano-capacitors (MWCNTs–PVA–MWCNTs) are formed in EF composites than in SC composites because of the high orientation of MWCNTs along the fiber direction in EF composites. The higher the content of MWCNTs, the larger the number of nano-capacitors. As a result, k is linearly increased with increasing content of MWCNTs (see Fig. 6(b)), and a relatively high k can still be obtained at contents of MWCNTs above 2.8 wt% because of the formation of a large number of nano-capacitors.

On the other hand, for MWCNTs/PVA composites, the frequency dependence of dielectric constant in the low-frequency range is dominated by the MWS effect because the nano-capacitor structure model is frequency-independent, as reported in previous studies.³⁹ Thus, it is easy to understand that a much stronger frequency dependence of the dielectric properties is obtained in SC composites than that in EF composites, since the MWS effect dominates in SC composites, whereas the nano-capacitor mechanism dominates in EF composites, because of their different microstructures.

In addition, for conductor/polymer composites, the dielectric loss is mainly regarded as the contribution of two distinct effects: direct current (DC) conductance and interfacial polarization, as reported in many works.^28,40,41 The loss of DC conductance dominates at low frequencies, whereas the loss of interfacial polarization dominates at high frequencies.⁴² In this work, the direct connection of MWCNTs (MWCNTs network) in SC composites or the much smaller distance between MWCNTs leads to a high DC conductance,^42,43 which contributes to a high dielectric loss and a strong frequency dependence of dielectric loss at low frequencies range. For SC composites, interfacial polarization also plays an important role in the dielectric properties and contributes greatly to the dielectric loss at high frequencies. As a result, a strong frequency dependence of dielectric loss is obtained even in the high frequency range of 10⁴–10⁶ Hz. For EF composites, the uniform dispersion and high orientation of MWCNTs in the matrix leads to a larger distance between nanotubes, and thus a low DC conductance because tunneling conduction is suppressed.⁴⁴ As a result, a rather low dielectric loss and a relatively weak frequency dependence of dielectric loss in the relatively low frequency range of 10²–10⁴ Hz are obtained. In addition, for EF composites, interfacial polarization plays a less important role in dielectric performance than the nano-capacitor effect, probably resulting in a low dielectric loss at high frequencies.

Furthermore, the dielectric breakdown of conductor/polymer dielectric materials could be due to the greatly increased electrical conductance under a given electrical field and/or the greatly enhanced thermal energy originated from the dielectric loss, and/or the air voids. In this work, the effect of air voids on dielectric breakdown strength should be neglected for both SC composites and EF composites because of the quite low content of MWNTs (less than 4 wt%) and the strong hydrogen bonding interactions between the carboxyl groups of MWCNTs and the hydroxyl groups of PVA. This is different from polymer/high k ceramic fillers composites, for which the decrease in dielectric breakdown strength could also be due to the local distribution of air voids, as reported in some works.⁴⁴ In this work, the greatly decreased dielectric breakdown strength of SC composites at contents of MWCNTs above the percolation threshold (1.0 wt%) should be mainly due to the high DC conductance and high dielectric loss caused by the connection of MWCNTs. The low breakdown strength in turn leads to a low energy density of the SC composites. On the other hand, the consistently high dielectric breakdown strength of EF composites should be mainly due to the low DC conductance and low dielectric loss because of the good dispersion and alignment of MWCNTs in the matrix. In addition, such high breakdown strength and dielectric constant lead to the high energy density of EF composites at contents of MWCNTs reaching 2.8 wt%.

4. Conclusions and perspective

MWCNTs/PVA dielectric composites with the MWCNTs well dispersed in the PVA matrix and well aligned along the fiber direction are successfully prepared by using an easy and controllable EF technique. A large number of separated nano-capacitors are formed, which lead to a high k. Meanwhile, the DC conductance is quite low, resulting in a low dielectric loss, a consistently high breakdown strength, a high energy density, and a relatively low frequency dependence of dielectric properties. On the other hand, in the films prepared by the SC method, the MWCNTs are randomly dispersed in the PVA matrix, and the MWCNTs networks with the MWCNTs directly connecting with one another were formed even at very low MWCNT loadings. These films have a higher k because of the direct connection of MWCNTs. However, the very high DC conductance leads to a particularly high dielectric loss, a very low breakdown strength and energy density, and a high frequency dependence of dielectric properties. To sum up, high-performance MWCNTs/polymer dielectric composites have been successfully prepared by using the EF technique, which could facilitate the wider applications of these dielectric materials. Furthermore, a good understanding of the relationships between microstructure and dielectric properties help us design high performance dielectric materials by controlling the microstructure of the composites.

Acknowledgements

We would like to express our sincere thanks to the National Natural Science Foundation of China (Grant no. 51173007 and no. 51221002) and the National Basic Research Program of China (973 Program) (Grant no. 2011CB932603) for financial supports. In addition, we would like to thank Dr Tung W. Chan from Department of Materials Science and Engineering in Virginia Polytechnic Institute and State University for improving written English.

References

1. Z.-M. Dang, D. Xie and C.-Y. Shi, Appl. Phys. Lett., 2007, 91, 222902.
2. Y. Shen, Y. Lin and C. W. Nan, Adv. Funct. Mater., 2007, 17, 2405.
3. G. Gallone, F. Carpi, D. De Rossi, G. Levita and A. Marchetti, Mater. Sci. Eng., C, 2007, 27, 110.
4. M. Arbatti, X. Shan and Z. Y. Cheng, Adv. Mater., 2007, 19, 1369.
5. D.-H. Kuo, C.-C. Chang, T.-Y. Su, W.-K. Wang and B.-Y. Lin, J. Eur. Ceram. Soc., 2001, 21, 1171.
6. G. M. Joshi, S. Khatake, S. Kaleemulla, N. M. Rao and T. Cuberes, Curr. Appl. Phys., 2011, 11, 1322.
7. P. Pötschke, S. M. Dudkin and I. Alig, Polymer, 2003, 44, 5023.
8. Z.-M. Dang, S.-H. Yao and H.-P. Xu, Appl. Phys. Lett., 2007, 90, 012907.
9. S.-H. Yao, Z.-M. Dang, M.-J. Jiang, H.-P. Xu and J. Bai, Appl. Phys. Lett., 2007, 91, 212901.
10. Q. Li, Q. Xue, L. Hao, X. Gao and Q. Zheng, Compos. Sci. Technol., 2008, 68, 2290.
11. X. Zhang, G. Liang, J. Chang, A. Gu, L. Yuan and W. Zhang, Carbon, 2012, 50, 4995.
12. M.-J. Jiang, Z.-M. Dang and H.-P. Xu, Appl. Phys. Lett., 2007, 90, 042914.
13. X. Zhao, A. A. Koos, B. T. T. Chu, C. Johnston, N. Grobert and P. S. Grant, Carbon, 2009, 47, 561.
14. Q. Li, Q. Xue, Q. Zheng, L. Hao and X. Gao, Mater. Lett., 2008, 62, 4229.
15. C. Yang, Y. Lin and C. Nan, Carbon, 2009, 47, 1096.
16. Q.-F. Cheng, M.-Z. Li, L. Jiang and Z.-Y. Tang, Adv. Mater., 2012, 24, 1838.
17. Q.-F. Cheng, B. Wang, C. Zhang and Z.-Y. Liang, Small, 2010, 6, 763.
18. Q.-F. Cheng, J.-W. Bao, J.-Y. Park, Z.-Y. Liang, C. Zhang and B. Wang, Adv. Funct. Mater., 2009, 19, 3219.
19. K. Koziol, J. Vilatela, A. Moisala, M. Motta, P. Cunniff, M. Sennett and A. Windle, Science, 2007, 318, 1892.
20. H. Liu, Y. Shen, Y. Song, C. W. Nan, Y. Lin and X. Yang, Adv. Mater., 2011, 23, 5104.
21. A. Koski, K. Yim and S. Shivkumar, Mater. Lett., 2004, 58, 493.
22. B. Chandar Shekar, V. Veeravazhuthi, S. Sakthivel, D. Mangalaraj and S. K. Narayandass, Thin Solid Films, 1999, 348, 122.
23. M. Abdelaziz and M. M. Ghannam, Phys. B, 2010, 405, 958.
24. J. S. Jeong, S. Y. Jeon and T. Y. Lee, Diamond Relat. Mater., 2012, 4, 4398.
25. W. Ren and H.-M. Cheng, J. Phys. Chem. B, 2005, 109, 7169.
26. S. Repalle, J. Chen, V. Drozd and W. Choi, J. Phys. Chem. Solids, 2010, 71, 1150.
27. A. R. Bhattacharyya, T. Sreekumar, T. Liu, S. Kumar, L. M. Ericson, R. H. Hauge and R. E. Smalley, Polymer, 2003, 44, 2373.
28. Y. Li, C. Chen, J.-T. Li, S. Zhang, Y. Ni, S. Cai and J. Huang, Nanoscale Res. Lett., 2010, 5, 1170.
29. R. R. Kohlmeyer, A. Javadi, B. Pradhan, S. Pilla, K. Setyowati, J. Chen and S. Gong, J. Phys. Chem. C, 2009, 113, 17626.
30. J. Chang, G. Liang, A. Gu, S. Cai and L. Yuan, Carbon, 2012, 50, 689.
31. S.-L. Shi, L.-Z. Zhang and J.-S. Li, J. Polym. Res., 2009, 16, 395.
32. B. Chu, X. Zhou, K. Ren, B. Neese, M. Lin, Q. Wang, F. Bauer and Q. Zhang, Science, 2006, 313, 334.
33. N. J. Smith, B. Rangarajan, M. T. Lanagan and C. G. Pantano, Mater. Lett., 2009, 63, 1245.
34. D. H. Reneker and I. Chun, Nanotechnology, 1996, 7, 216.
35. D. R. Salem, Structure formation in polymeric fibers, Hanser Verlag, 2001.
36. N. Kimura, H. K. Kim, B. S. Kim, K. H. Lee and I. S. Kim, Macromol. Mater. Eng., 2010, 295, 1090.
37. H. R. Pant, W.-i. Baek, K.-T. Nam, I.-S. Jeong, N. A. Barakat and H. Y. Kim, Polymer, 2011, 52, 4851.
38. Y. Liu, L. Cui, F. Guan, Y. Gao, N. E. Hedin, L. Zhu and H. Fong, Macromolecules, 2007, 40, 6283.
39. F. He, S. Lau, H. L. Chan and J. Fan, Adv. Mater., 2009, 21, 710.
40. Z.-M. Dang, J.-P. Wu, H.-P. Xu, S.-H. Yao, M.-J. Jiang and J. Bai, Appl. Phys. Lett., 2007, 91, 072912.
41. J. Yacubowicz, M. Narkis and L. Benguigui, Polym. Eng. Sci., 1990, 30, 459.
42. J. Zhang, M. Mine, D. Zhu and M. Matsuo, Carbon, 2009, 47, 1311.
43. C. Kerr, Y. Huang, J. Marshall and E. Terentjev, J. Appl. Phys., 2011, 109, 094109.
44. P. Kim, N. M. Doss, J. P. Tillotson, P. J. Hotchkiss, M.-J. Pan, S. R. Marder, J. Li, J. P. Calame and J. W. Perry, ACS Nano, 2009, 3, 2581.

---