Effects of Multiwalled Carbon Nanotubes on Phase Transformation and Dielectric Properties in Poly(Vinylidene Fluoride-Hexafluoropropylene) Nanocomposites
2019 Volume 60 Issue 8 Pages 1716-1721
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2019 Volume 60 Issue 8 Pages 1716-1721
Poly(Vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] nanocomposites filled with multiwalled carbon nanotubes (MWCNTs) were successfully fabricated by solution casting. The influence of MWCNTs on the phase structure, dielectric and mechanical properties of MWCNTs/P(VDF-HFP) nanocomposites were evaluated. FTIR spectra and X-ray diffraction show that the incorporation of MWNTs promoted the formation of γ phase crystal. The DSC curves demonstrated that the nanocomposites possess higher Tm and Tc than neat P(VDF-HFP), which can be explained by the increasing of the γ phase crystal and the heterogeneous nucleation effect respectively. In addition, the dielectric constant of the nanocomposites is enhanced remarkably with the increasing content of the MWNTs. The highest dielectric constant of the MWCNTs/P(VDF-HFP) nanocomposites was about 106 when the content of fillers was 1.3 vol% at 1 kHz. The percolation threshold of the nanocomposites was predicted at about 1.2 vol%. Moreover, The tensile strength and Young’s modulus of 1.2 vol% MWCNTs/P(VDF-HFP) nanocomposites reach to maximum.
Fig. 5 Dependence of dielectric constant and dielectric loss (tan δ) of composites on contents of MWCNTs measured at 1 kHz and room temperature.
Poly(vinylidene fluoride) (PVDF) and its copolymers, such as those with hexafluoropropylene [P(VDF-HFP)] and trifluoroethylene [P(VDF-TrFE)], are partially crystalline polymers exhibiting extraordinary electrical properties. These ferroelectric polymers have showed their potential to use as high energy density dielectrics for the high dielectric constant capacitor film. PVDF and its copolymers possess the abundance of polymorphic phases, which have at least four crystalline phases, i.e. α, β, γ and δ phases.1–4) The nonpolar α phase is more easily obtained, which was always obtained from melt crystallization below 433.15 K. The oriented β phase can be obtained by mechanical drawing of films originally in the α phase at temperatures between 343.15 K and 363.15 K. The unoriented β phase may be obtained by crystallization from an appropriate solution, if evaporation occurs at below 343.15 K. A mixture of the α and β phases is formed by higher temperatures, and the fraction of the α phase increase with increasing temperature. The polar γ phase may be obtained from both solution and melt crystallization at temperature near the melt temperature (Tm) of the α phase. The annealing temperature close to Tm also result in the γ phase, on account of α → γ solid state phase transformation. For δ phase, a polar version of the α phase, may be obtained by polarizing the α phase at high electric fields.5–8)
Because of its excellent electric properties, Poly(vinylidene fluoride) (PVDF) and its copolymers have attracted more attentions in electromechanical systems. However, for applications such as high charge-storage capacitors, electrostriction system for artificial muscles, pulsed lasers and so on, doping conductive nanofillers is usually necessary to meet the property requirements of higher permittivity. Lots of studies have revealed that the dielectric properties of the nanocomposites were dramatically increased by the introduction of the conductive nanofillers, such as semiconductor,9) metal,10,11) carbon nanofibers (CNFs)12,13) and carbon nanotubes (CNTs).14–16) In addition to the dielectric properties, the crystalline phase of the nanocomposites can also be influenced by the introduction of nanofillers. For example, the addition of CNFs into a PVDF matrix increased the crystallinity and induced the formation of β phase.12,13) These conductive nanofillers can induced phase transformations, and the changes of crystal structure could further impact the dielectric properties of the nanocomposites.
In this paper, we investigated the effects of MWNTs on the phase transformation, degree of crystallinity and dielectric properties of MWCNTs/P(VDF-HFP) composites. The results revealed that the addition of MWCNTs could facilitate the formation of γ crystal. The change of γ-phase behavior of the MWCNTs/P(VDF-HFP) nanocomposites impacts various properties including melting temperature, dielectric properties, etc.
P(VDF-HFP) was purchased from Arkema corporation, France. MWCNTs (OD: 10–30 nm, ID: 5–10 nm, length: 10–30 µm, purity: >98%) was provided by Chengdu Organic Chemicals Co. Ltd. AR grade N, N-dimethylformamide (DMF) was purchased from Tianjin Reagents Co. Ltd.
2.2 Sample preparationThe MWCNTs/P(VDF-HFP) nanocomposites studied here were prepared as films via a solution casting method. An appropriate amount of P(VDF-HFP) powders were dissolved in DMF. MWCNTs with different amount were ultrasonically dispersed in another DMF for 1 h. Then the two solutions were mixed by mechanical stirring for 2 h, followed by sonication in a bath-type sonicator for additional 3 h at 338.15 K. The mixture was cast onto clean glass plates and kept in a drying oven at 373.15 K for 1 h, followed at 433.15 K under vacuum for 8 h, and then slowly cooled down to room temperature. The thickness of the achieved nanocomposite film was about 50 µm.
2.3 CharacterizaionFTIR spectrum were carried on an IR spectrometer (E55+FRA106) at 0.5 cm−1 resolution within the 600–4000 cm−1 wave number range. X-ray diffraction carried out via a M03XHF22 X-ray diffractometer with Cu Ka radiation (λ = 0.154 nm) and a scanning range of 2θ = 10°–55°. DSC analysis was performed on a Netzsch STA449C under Ar protection at a heating rate of 278.15 K/min. The morphology of the samples was characterized by emission scanning electron microscopy (S-4800, hitachi, Japan). The dielectric constant and loss were tested on precision impedance analyzer (Wayne Kerr Electronics 6500B, England) at different frequencies ranging from 100 Hz to 1 MHz. The tensile properties of all samples were measured by an electronic universal testing machine (Inspekt 100, Germany) according to standard ASTM D 638 at a testing speed of 1 mm/min.
Microstructure of fracture surfaces of the P(VDF-HFP) composites with different MWCNTs with a volume fraction of 1.2 vol% and 1.3 vol% are shown in Fig. 1. Figure 1(a) and (c) show the dispersion of MWCNTs in 1.2 vol% MWCNTs/P(VDF-HFP) and 1.3 vol% MWCNTs/P(VDF-HFP) at relative low magnification, respectively. It can be clearly recognized that the surfaces of both samples are relatively smooth. Figure 1(b) and (d) show the detailed morphological characterization of the both samples at relative high magnification, indicating a good homogeneous dispersion for 1.2 vol% MWCNTs composite film, but a little MWCNTs form agglomeration due to the high interface energy and some MWCNTs stretch out of the failure surface for 1.3 vol% MWCNTs composite film.
The cross-section SEM images of MWCNTs/P(VDF-HFP) composite films: (a), (b) 1.2 vol% MWCNTs; (c), (d) 1.3 vol% MWCNTs.
FTIR spectra used to examine the crystalline phases of the pure P(VDF-HFP) and its composites with different MWCNTs concentrations are plotted in Fig. 2. The peak at 876 cm−1 was attributed to the amorphous phase of P(VDF-HFP) and could not be used to identify any of the crystalline phases.17) The P(VDF-HFP) film presented a mixture of α and γ phases, which can be seen by the characteristic absorption bands of α phase at 613, 763, 1072 and 1042 cm−1 and the characteristic absorption bands of γ phase at 812, 834, and 1232 cm−1.5–6,18–20) This was as expected because P(VDF-HFP) was crystallized from the melt at temperatures above 423.15 K for a long time, which resulted in these two phases.6,8,21) With increasing content of MWCNTs, the characteristic α phase absorption bands at 763 cm−1 disappeared, while the intensity of γ phase at 834 and 1232 cm−1 increases. This means the introduction of MWCNTs induces the occurrence of the α → γ solid-state phase transition and promote the increase of γ phase ratio with increasing filler content.
FTIR spectra of pure P(VDF-HFP) and MWCNTs/P(VDF-HFP) composite films.
Figure 3 shows the XRD spectra of pure P(VDF-HFP) and MWCNTs/P(VDF-HFP) composite films. The diffractogram of P(VDF-HFP) presents peaks at 2θ = 17.8°, 19.8°, referent to the diffractions in planes (100) and (110) respectively, characteristic of the α phase. The peaks at 2θ = 26.8° can be seen as the characteristic of both the (021) of α phase and the (022) of γ phase.7,22) Increasing the concentration of MWCNTs causes formation of γ crystals and diminishing of α crystals, and the intensification of peaks at 26.8° and the decrease of (100) reflection at 17.8°. In addition, the diffraction peaks at 18.4° is also strengthened with the filler concentration increasing, which is identified as the (020) of α phase overlapped with the (020) of γ phase.22–24) The peaks at 19.8° shifted to 20.1° which can be considered as (110) plane10) of γ phase with the content of MWCNTs increased. These results are consistent with the FTIR results.
XRD spectra of pure P(VDF-HFP) and MWCNTs/P(VDF-HFP) composite films.
The DSC curves of pure P(VDF-HFP) and its composites with different filler concentrations are plotted in Fig. 4. The corresponding DSC thermal dynamic data values are listed in Table 1. It can be seen that pure P(VDF-HFP) shows the melting temperature (Tm) and the crystallization temperature (Tc) at 427.85 K and 377.45 K, respectively. The Tm of MWCNTs/P(VDF-HFP) composites from 430.85 K (0.5 vol% MWCNTs) increases to 432.45 K (1.3 vol% MWCNTs). This might be considered resulting from the formation of γ phase crystal in the P(VDF-HFP), because the Tm of the γ phase is higher than that of the α phase.6,17) In addition, the Tc also rises with the filler concentration increasing. This should be owing to the strong interaction between MWCNTs and P(VDF-HFP). MWCNTs act as nucleation agents in the polymer matrix during crystallizing of the nanocomposites. The heterogeneous nucleation effect of MWCNTs accelerate the crystallization rate of P(VDF-HFP). While the crystallinity (Xc) of the composites decreases as the filler concentration increases, which is opposite to some literature.11,15) This could be explained that a lot of nucleus generate and simultaneously grow in a limited space with the increasing of MWCNTs, which dramatically confine the movements of polymer chains. Furthermore, the quick evaporation of DMF (373.15 K) cannot provide sufficient time for the P(VDF-HFP)-MWCNTs interaction and rearrangement of polymer chains.25,26)
DSC curves of P(VDF-HFP) and its composites with various amounts of MWCNTs: (a) heating, (b) cooling.
Figure 5 shows the variation in dielectric constant of MWCNTs/P(VDF-HFP) with increasing MWCNTs content. As shown in Fig. 5, the dielectric constant increases rapidly after 1.2 vol% as well as the dielectric loss. There are two plausible causes for this. First, such improvement may be related to the γ crystals with the increase of MWCNTs content which can enhance the polarity of polymer. Second, the major effect for improving the dielectric constant can be understood by a minicapacitor principle.16) In the MWCNTs/P(VDF-HFP) composites, the MWCNTs were well-distributed in the insulated P(VDF-HFP), forming a lot of micro-capacitors. The number of the micro-capacitors increases with more and more MWCNTs added into the polymer, which finally leads to the increasing of the dielectric constant of the composites.
Dependence of dielectric constant and dielectric loss (tan δ) of composites on contents of MWCNTs measured at 1 kHz and room temperature.
When the filler content is near the percolation threshold (1.2 vol%), the fillers connect with each other and form a continuous path, the increasing of the dielectric constant of the composites can be explained by the power law:16,27)
| \begin{equation*} \varepsilon = \varepsilon_{m}(f_{c} - f)^{-s},\ \text{for $f_{c}>f$} \end{equation*} |
Figure 6 depicts the frequency dependence of the dielectric constant of the composites measured at room temperature. In the range of 100 Hz to 1 MHz, the dielectric constant is nearly independent of frequency except for sample containing 1.2 vol% and 1.3 vol% MWCNTs. However, The dielectric constant decreases with increasing frequency. This effect is more pronounced in case of P(VDF-HFP) composites above the percolation threshold (1.2 vol%). Compared to the pure P(VDF-HFP) dielectric constant of 9.58, the dielectric constant of P(VDF-HFP) in the composites is large enough reaching 13.6, 25.0, 28.7, 54.99, 148.9 at 100 Hz for the P(VDF-HFP) composites with 0.5 vol%, 1.0 vol%, 1.1 vol%, 1.2 vol%, 1.3 vol%, respectively. Furthermore, the dielectric constant at 100 Hz–105 Hz increases with increasing MWCNTs contents in the P(VDF-HFP) composites. On one hand, this enhancement in dielectric constant with increasing vol% of filler may be attributed to the increased γ-phase fraction of P(VDF-HFP) in prepared composites, and on the other hand, the enhancement may be due to the enhanced interfacial interaction and the formation of the microcapacitor networks between MWCNTs and P(VDF-HFP), which are based on the homogenous dispersion of MWCNTs in the P(VDF-HFP) polymer matrix.27,28)
Effect of frequency on the dielectric constant of MWCNTs/P(VDF-HFP) composites at different amounts of MWCNTs.
Figure 7 shows the dielectric loss behavior of P(VDF-HFP) composites with different content of MWCNTs versus frequency at room temperature. The dielectric loss (tan δ) of all samples is decreased continually as frequency increased to a certain high frequency, and subsequently increased with frequency further increase. This is mainly due to the conductance loss and the polarization loss. At a low frequency, the main reason caused the dielectric loss is the conductance loss, which reduces with increasing frequency. At the higher frequency, the dominated factor is the polarization loss increasing with the frequency.17) In addition, when the filler content is above 1.2 vol%, the dielectric loss suddenly jumped above 1, this is mainly because the continuous conducting path is formed near the percolation threshold.
Effect of frequency on the dielectric loss (tan δ) of composites at different amounts of MWCNTs.
The mechanical properties of P(VDF-HFP) and MWCNTs/P(VDF-HFP) composites are shown in Fig. 8. All samples present a linear elastic behavior at the beginning of the tensile tests and a region of plastic deformation before the fractures. Comparison of the tensile strength, Young’s modulus and elongation at breakage of all samples are shown in Table 2. It can be clearly seen that the elongation at breakage sustainably decrease with increasing the content of MWCNTs. For instance, the tensile strength and Young’s modulus of 1.2 vol% MWCNTs/P(VDF-HFP) composites are maximum of 38.00 MPa and 0.48 GPa, respectively. This enhancement could be attributed to homogeneous dispersion and reinforcing effect of MWCNTs,29) which is beneficial for the optimization of the dielectric properties.
The typical stress-strain curves of P(VDF-HFP) and MWCNTs/P(VDF-HFP) composites.
In this article, we have fabricated MWCNTs/P(VDF-HFP) nanocomposites by adding a small fraction of MWCNTs for high permittivity purpose. The percolation threshold of the MWCNTs/P(VDF-HFP) nanocomposites is lower (about 1.2 vol%) than other polymer-based nanocomposites. The 1.2 vol% MWCNTs/P(VDF-HFP) composites exhibit an excellent dielectric constant of 54.99, approximately 574% greater that of the pure PVDF (9.58) and 100%–200% higher than other polymer-based nanocomposites loaded with similar contents of the conductive fillers. The large dielectric constant can be attributed to the micro-capacitors structure of MWCNTs/P(VDF-HFP) nanocomposites. Moreover, the crystallinity of the composites decreases as well as the crystallization temperature increases with the MWCNTs content increasing, which can be attributed to the heterogeneous nucleation effect. γ-phase is intensified after the addition of MWCNTs causing the melting temperature shifting to a higher temperature. The tensile strength and Young’s modulus of MWCNTs/P(VDF-HFP) composites (≥1.2 vol% MWCNTs) improved mainly because of the dispersion of MWCNTs.
This work was funded by Science and Technology Program of Handan (1721211052), Key Project of Science and Technology Research of Higher Education in Hebei Province (ZD2018302).
