Achieving Large Dielectric Property Improvement in Poly(ethylene

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Achieving Large Dielectric Property Improvement in Poly(ethylene vinyl acetate)/Thermoplastic Polyurethane/Multiwall Carbon Nanotube Nanocomposites by Tailoring Phase Morphology Tingting Zhang,† Jinghui Yang,*,†,‡ Nan Zhang,† Ting Huang,† and Yong Wang*,† †

Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, Erhuan Road, North I, No 111, Chengdu, Sichuan 610031, China ‡ State Key Laboratory of Polymer Materials Engineering, Sichuan University, Yihuan Road, South I, No 24, Chengdu, Sichuan 610065, China S Supporting Information *

ABSTRACT: In this work, multiwall carbon nanotubes (MWCNTs) were melt compounded with a poly(ethylene vinyl acetate)/thermoplastic polyurethane (EVA/TPU) blend to prepare EVA/TPU/MWCNT blend nanocomposites. The effects of the blend morphology and selective localization of MWCNTs on the dielectric properties including dielectric constant and loss were systematically investigated. The results show that when the blend exhibits sea−island morphology accompanying MWCNT selective localization in island droplets, the dielectric constant was increased to 1200@100 Hz with addition of 5 wt % MWCNTs; in addition, the growth of dielectric loss is suppressed and the values of dielectric loss always remain around 1 with varying the loading of MWCNTs. Furthermore, based on the investigations of rheological percolation testing, morphological observation, and selective localization, the microcapacitor model was evoked to explain the underlying mechanism for the improvement of the dielectric constant for the blend with sea−island morphology.

1. INTRODUCTION Polymer nanocomposites with high dielectric constant are appreciated for their unique combination of tunable dielectric properties and the advantage of polymers, such as mechanical flexibility, light weight, and good processing characteristics. They are widely applied in microelectrics,1 aeronautics,2 actuators,3,4 capacitors,5 etc. Currently, introducing conductive fillers such as carbon black (CB),6 carbon fiber (CF),7 carbon nanotubes (CNTs),8 etc. into the polymer matrix is an effective method to prepare nanocomposites with a high dielectric constant. Among these conductive fillers, multiwalled carbon nanotubes (MWCNTs) deserve special attention because of their good combination of electrical property and large aspect ratio, as well as their availability on a large scale.9 On one hand, the nanocomposites with high dielectric constant can be prepared when the loading of MWCNTs increases to the vicinity of the percolation threshold. For example, Xue and coworkers investigated10 the dielectric properties of polyvinylidene fluoride(PVDF)/MWCNT nanocomposites prepared by evaporating suspensions of MWCNTs in PVDF solution, and the dielectric constant sharply increased when the MWCNT loading reached a percolation threshold of 3.8 vol % (∼7.6 wt %). In this case, a proper distance between MWCNTs can be achieved, facilitating the formation of microcapacitors gen© 2017 American Chemical Society

erated by the neighboring MWCNTs. Thus, the charge accumulation in the local electric field would be enhanced, and the high dielectric constant can be achieved. On the other hand, the high dielectric constant of nanocomposites containing MWCNTs is compensated by the large dielectric loss due to its high leakage currents. Because the current leakage results from the conductivity of the MWCNT contacting network, the key challenge in achieving high dielectric constant while maintaining low loss is to form a great amount of microcapacitors without any direct connection of MWCNTs. Thus, modification of MWCNTs to form an insulating layer is regarded as one effective way to disrupt the conductivity of the MWCNT network, thus guaranteeing the low dielectric loss of the nanocomposites. Chen and coworkers11 compounded the polyhedral oligomeric silsesquioxane (POSS)-coated MWCNTs with PVDF, and the nanocomposites exhibited a high dielectric constant (∼120 at 100 Hz) and low dielectric loss (∼1.2 at 100 Hz) when 7 wt % POSS-coated MWCNTs were incorporated. As for PVDF/ Received: Revised: Accepted: Published: 3607

December March 14, March 17, March 17,

13, 2016 2017 2017 2017 DOI: 10.1021/acs.iecr.6b04763 Ind. Eng. Chem. Res. 2017, 56, 3607−3617

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Industrial & Engineering Chemistry Research

received much attention in recent years because of its recyclability, high force output, high mechanical properties, and high dielectric constant.19,20 In this study, it is expected that a dielectric elastomer with tunable constant can be fabricated via tailoring the morphology of EVA/TPU blends, which are immiscible and flexible for both EVA and TPU. Dielectric elastomers are attractive because of their wide applications in sensors,21 actuators,22 capacitors,23 artificial muscle,24 etc. In this work, through the morphology tailoring and selective localization of MWCNTs, EVA/TPU-based nanocomposites with high dielectric constant and low dielectric loss are prepared. For a comprehensive understanding of the relationship between morphology and dielectric properties of EVA/TPU/MWCNT, different component ratios between TPU and EVA are applied to construct various morphologies. Meanwhile, different loadings of MWCNTs are incorporated into polymer blends for discussing the effect of MWCNTs on dielectric properties. Some interesting results are found, and the underlying mechanism is intensively studied based on the morphology control, MWCNT selective localization, and the formation of microcapacitors in polymer blends.

MWCNT nanocomposites, a higher dielectric constant (∼400 at 100 Hz) but much higher dielectric loss (∼90 at 100 Hz) were obtained at the same loading of 7 wt % MWCNTs. In addition, it is also noted that the MWCNTs naturally exist as aggregates or bundles because of the super high van der Waals force; thus, the loading of MWCNTs to achieve the high dielectric constant in the nanocomposites is unfavorable for the flexibility of the polymer. Recently, a new research direction for enhancing the dielectric constant of polymer nanocomposites without sacrificing the flexibility of polymer nanocomposites has attracted more attention. Its feature is that the MWCNTs are selectively located in one phase of binary polymer blends. Pötschke and co-workers12,13 reported that the conductivity of blend nanocomposites was dependent on the localization of MWCNTs in blends, especially when the MWCNTs were selectively located in one phase of blends with a cocontinuous morphology; the percolation threshold was greatly reduced because of the double percolation. In our previous work,14 it was found that when the MWCNTs were located at the interface of polycarbonate (PC) and acrylonitrile-butadienestyrene copolymers (ABS) with aid of interfacial compatibilizer, the percolation threshold of PC/ABS/MWCNT nanocomposites was as low as 0.05 wt %. Generally, confining the selective localization of MWCNTs in any one of the continuous phases of immiscible blends could reduce the distance between MWCNTs and lead to formation of more microcapacitors; thus, it is regarded as an efficient approach to enhance the dielectric constant. Dang’s group had found that localization of MWCNTs in continuous PVDF phase in the polystyrene (PS)/ PVDF blend produced a dielectric constant (485 at 100 Hz) that was higher than that in the PS droplets (16 at 100 Hz).15 Yuan et al.16 investigated the effects of morphology of blends on the dielectric property of polymer blends. The results showed that when a cocontinuous structure in the PVDF/low density polyethylene (LDPE) was formed, accompanying the MWCNTs selectively localization in LDPE, an enhanced dielectric constant as high as 470 at 100 Hz was found, while PVDF/LDPE blends with sea−island morphology exhibited a lower constant at the same loading of MWCNTs. It also should be noted that the dielectric loss at 100 Hz of PVDF/LDPE/ MWCNT nanocomposites with a cocontinuous morphology can reach ∼100 at 100 Hz because of the double percolative structure, which is still an unsolved key for obtaining polymer nanocomposites with high dielectric constant and low dielectric loss. In most cases, the cocontinuous phase morphology is regarded as the ideal state for achieving the high dielectric constant because of the double percolation theory, whereas in Gu’s study, the dielectric constant was greatly improved when the MWCNTs were enriched in island droplets. It was found in poly(ether imide) (PEI)/bismaleimide (BD) blends that MWCNTs preferred to be located around the PEI dense zone in the sea−island morphology and arranged normally to the radius of the PEI sphere zone; therefore, the morphology of MWCNTs and the dielectric properties of nanocomposites can be facilely controlled.17 Obviously, the effects of morphology structures for the biphasic polymer blends on the dielectric properties needed to be clearly illustrated. In this paper, thermoplastic polyurethane (TPU) and ethylene-vinyl acetate copolymer (EVA) are selected as the polymer matrix. EVA is a type of widely used thermoplastic elastomer because of its low cost, good processability, and excellent mechanical properties whose VA content can be adjusted easily; thus, its viscosity can be tuned.18 TPU has

2. EXPERIMENTAL SECTION 2.1. Materials. All the materials used in this study are commercially available. MWCNTs (TNM5) were purchased from Chengdu Institute of Organic Chemistry of the Chinese Academic of Science (China). The outer and inner diameters of MWCNTs are 20−30 nm and 5−10 nm, respectively. The length is about 10−20 μm. The purity is above 95% in weight base. EVA [Elvax 40 L-03; density = 0.97 g/cm3; content of VA = 40 wt %; MFR = 3 g/10 min (190 °C/2.16 kg)], was supplied by DuPont, United States. TPU (WHT-1570; density = 1.1 g/cm3) was supplied by Wanhua Chemical Group Co., Ltd. China. 2.2. Preparation of EVA/TPU/MWCNT Nanocomposites. The EVA/TPU/MWCNT nanocomposites were prepared by a two-step method through melt mixing. First, MWCNTs were melt blending with EVA by a torque rheometer (ZJL-300, Changchun Intelligent Instrument and Equipment Co., Ltd.) at a mixing speed of 80 rpm at 100 °C for 6 min to prepare the masterbatch with MWCNT loading of 10 wt %. Then the appropriate amounts of masterbatch, EVA, and TPU were mixed at 160 °C for 6 min at a mixing speed of 80 rpm, and the mixture cooled to room temperature. Finally, the resulting blend was compressed by hot-pressing at 160 °C and 5 MPa for ∼10 min into specific specimens with a thickness of 1 mm and a diameter of 30 mm for rheological measurements; a thickness of 4 mm and a diameter of 20 mm for dielectric properties measurements; and dumbbell-shaped samples with a length of 30 mm, a width of 5 mm, and a thickness of 1 mm for mechanical properties. The resultant products were coded as x/ y/z, where x, y, and z represent the total mass fraction of EVA, TPU, and MWCNTs in the composites, respectively. For a comparative investigation, the binary EVA/MWCNT and TPU/MWCNT nanocomposites with varying MWCNT loading were also melt blended. 2.3. Characterization. Dielectric studies in the frequency range of 102−107 Hz were tested using a Broadband Dielectric/ Impedance Spectrometer (Novocontrol Tecnologies, Germany) at room temperature. Morphologies of the nanocomposites were observed using a scanning electron microscopy instrument (SEM, FEI inspect, United States) with an accelerating voltage of 5.0 kV. Prior to observation, samples 3608

DOI: 10.1021/acs.iecr.6b04763 Ind. Eng. Chem. Res. 2017, 56, 3607−3617

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Industrial & Engineering Chemistry Research were cryogenically fractured in liquid nitrogen, then the cryogenically fractured surfaces were etched in N,N-dimethylformamide (DMF) at 40 °C for 24 h to remove TPU phase. Then the etched surfaces were carefully washed with fresh DMF and ethanol with the aid of sonication. For untreated samples, the fractured surfaces were prepared in liquid nitrogen, and all of the samples were sputtered with gold in vacuum. The dispersion state of MWCNTs was investigated using a transmission electron microscopy instrument (TEM) Tecnai G2 F20 S-TWIN (FEI, United States) with an operating voltage of 200 kV. An ultrathin section with a thickness of about 90 nm, which was cut from the compression molded samples using a cryo-diamond knife on a microtome EM UC6/ FC6 (LEICA, Germany), was used for TEM characterization. Rheological measurements were performed on a DHR-1 Rheometer (TA Instruments, United States). All rheological tests were carried out at 160 °C in a nitrogen atmosphere in the frequency range of 10−2−102 Hz. In the dynamic measurements, the peak strain was kept at 5%, which is within the linear limit. A Fourier transform infrared spectroscopy instrument (FTIR, Nicolet 5700 Thermo Nicolet, United States), equipped with an attenuated total reflectance (ATR) probe, was used to study hydrogen bonds of TPU and the responding nanocomposites. All the spectra were obtained at a resolution of 2 cm−1 in a wavenumber range of 400−4000 cm−1. The mechanical properties measurements were accomplished on dumbbell-shaped specimens using a Universal Testing Machine (20 kN) (RGM-4020, Shenzhen Reger Instrument Co., LTD, China) series with a grip separation of 30 mm and a speed of 500 mm/min at 23 °C.

3. RESULTS AND DISCUSSION The frequency dependence of the dielectric properties of EVA/ TPU/MWCNT nanocomposites with different loading of MWCNTs and different component ratio were investigated, as shown in Figure 1. It is found that with increasing loading of MWCNTs, the dielectric constant increases as well as the dielectric loss. It should be emphasized that among all the samples, the extent of the increase for the EVA/TPU blends with weight ratio of 90/10, 80/20 and 70/30 is significantly higher than that of blends with weight ratio of 60/40 and 50/ 50. Meanwhile, the dielectric constant of blends with weight ratio of 80/20 rises sharply; the maximum dielectric constant can reach 1200 at 100 Hz, which is far higher than that of the pure EVA/TPU blend. On the other hand, as shown in Figure 1, a moderate increase of dielectric constant for blends with weight ratio of 90/10 and 70/30 can also be found. The conductivities of all nanocomposites exhibit a similar increase effect with increasing loading of MWCNTs (shown in the Supporting Information, Figure S1). As for all the samples, it is seen that in the range of the lower (102−104 Hz) and higher (106−108 Hz) frequencies, the values of dielectric loss gradually increase with increasing loading of MWCNTs. On one hand, broad peaks of tan δ at 105−106 Hz can be associated with the dipole polarization arising from the polymer matrix; on the other hand, with increasing loading of MWCNTs, the increases of tan δ at 102−104 Hz are dominant, which indicates the existence of the interfacial polarization relaxation in the MWCNT filled polymer nanocomposites. It should be mentioned that when the loading of MWCNTs is 5 wt %, the dielectric constant of EVA/MWCNT is close to 20 @100 Hz, and that of TPU/MWCNT is only 15 @100 Hz, which are much lower than that of ternary composites.

Figure 1. (a) 3D plots of the dielectric constant−content of TPU phase−frequency for composites. (b) 3D plots of the dielectric loss− content of TPU phase−frequency for composites.

For a more clear observation, when the loading of MWCNTs is fixed at 5 wt %, the dielectric constant, the dielectric loss, as well as the conductivity at 100 Hz of blends with different weight ratio of TPU and EVA are summarized in Figure 2a,b. One can find that the dielectric constant is sharply increased to 1200 for the blends with weight ratio of 80/20, while no remarkable change can be found for other blends. As for the dielectric loss, the changing trend with increasing the content of EVA in Figure 2a is steady, and all the values of dielectric loss are below 1. Compared with others’ work,25,26 the dielectric loss of the dielectric elastomer in this work is not high with similar contents of conductive fillers. On the other hand, the maximum conductivity reaches 10−7 S·cm−1, which is regarded as nonconductive through the entire samples. Additionally, the conductivity of nanocomposites with the same loading of 5 wt % of MWCNTs varies from the weight ratio of blend nanocomposites shown in Figure 2b. With decreasing content of TPU, the AC conductivity increases. The underlying mechanism may be attributed to the reduced distance between MWCNTs and the increased number of microcapacitors in the nanocomposites. To deeply investigate the effect of phase morphology on the dielectric properties, the morphologies of all samples were 3609

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Figure 2. Summary of the (a) dielectric properties and (b) AC conductivity of the EVA/TPU/MWCNT nanocomposites with different weight ratio of TPU when the loading of MWCNTs is 5 wt % at 100 Hz.

observed by SEM, as shown in Figure 3. The SEM images of the EVA/TPU/MWCNT nanocomposites in panels a1, b1, c1, d1, and e1 of Figure 3 represent 90/10/5, 80/20/5, 70/30/5, 60/40/5, and 50/50/5, respectively, at lower magnification, and the images of a2, b2, c2, d2, and e2 of Figure 3 represent the corresponding blends at higher magnifications. It should be noted that the TPU has been removed by chemical etching; thus, the black zones in images represent the TPU. One can observe that as for the blends with weight ratio of 90/10, 80/ 20, and 70/30, typical sea−islands morphology can be found, which means the TPU-rich phase forms spherical domains in the EVA matrix phase. As the content of TPU increases, the TPU domain size gradually increases while the distance between TPU domains decreases, as shown in the Supporting Information (Figure S2). When the content of TPU reaches 40%−50%, the blend nanocomposites exhibit a cocontinuous phase structure. Besides blend morphology, the localization of MWCNTs in the blend matrix is also worthy of investigation. However, MWCNTs can seldom be found in Figure 3. It is possible that the MWCNTs are removed with the etching of TPU, inferring that the MWCNTs are favorably located in the TPU phase. Based on this phenomenon, the fracture surfaces of EVA/TPU/MWCNT nanocomposites (80/20/5 and 50/50/ 5) are observed without any treatment, as shown in Figure 4. It is not easy to differentiate the TPU from EVA; however, one can find that clusters of MWCNTs are homogeneously

Figure 3. SEM images of fracture surfaces of (a) 90/10/5, (b) 80/20/ 5, (c) 70/30/5, (d) 60/40/5, and (e) 50/50/5 at different magnifications. The TPU phase was etched by DMF.

dispersed in the blend matrix. In the image of the nanocomposite (80/20/5) with higher magnification (Figure 4a2), the MWCNTs are confined in a spherical TPU domain which is well matched with the morphology shown in Figure 3. For a comparative study, the morphology of binary EVA/MWCNT and TPU/MWCNT composites were observed, as shown in panels c and d of Figure 4, respectively. It can be observed that MWCNTs were homogeneously dispersed in the EVA or TPU matrix, and aggregates of MWCNTs can be found occasionally. For a clear observation of MWCNT dispersion in the blend composites, TEM was applied to explore the localization of MWCNTs. As shown in Figure 5a, the blend composites with weight ratio of 80/20 and 5 wt % MWCNTs exhibit typical sea−island morphology, and the TPU droplets are filled with MWCNTs. The cocontinuous morphology can be found in the blend composites with weight ratio of 50/50 and 5 wt % MWCNTs, as shown in Figure 5b. The MWCNTs are selectively localized in the TPU domain. As we know, the 3610

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Figure 4. SEM images of fracture surfaces of (a) 80/20/5, (b) 50/50/ 5, (c) EVA5, and (d) TPU5 at different magnifications.

Figure 6. Rheological properties of samples with different content of TPU phase when the loading of MWCNTs is 5 wt %: (a) storage modulus vs frequency, (b) loss modulus vs frequency, and (c) complex viscosity vs frequency for nanocomposites.

Figure 5. TEM images of (a) 80/20/5 and (b) 50/50/5 at different magnifications.

Table 1. Surface Energies of EVA, TPU, and MWCNT component

γdi

γpi

γi

EVA TPU MWCNT

39.5 29.2 17.6

0.1 4.0 10.2

39.6 33.2 27.8

location of MWCNTs in a polymer blend is generally dictated by the state of the minimum interfacial energy. According to Young’s equation, one can find the thermodynamic equilibrium filler position by evaluating the wetting coefficient, ωa. γ − γCB ωa = CA γAB (1)

Table 2. Interfacial Tension between MWCNT and EVA, MWCNT and TPU, and EVA and TPU and the Wetting Coefficient Calculated According to Young’s Equation interfacial energies (mJ·m−2)

γCA

γCB

γAB

ωa

harmonic equation geometric equation

18.4 13.1

5.6 2.9

5.4 3.9

2.4 2.6

Here, γCA, γCB, and γAB are the interfacial energies between MWCNTs and polymer A, MWCNTs and polymer B, and polymers A and B, respectively. The values ωa > 1, −1 < ωa < 1, and ωa < −1 imply MWCNTs localization at polymer B, at the interface between two polymers, and at polymer A, respectively. Two approaches can be used depending on the type of phase 3611

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Figure 8. FTIR spectra of 80/20, 80/20/5, and 50/50/5 in the ranges of (a) 400−4000 cm−1 and (b) 3600−3200 cm−1 showing disruption of hydrogen bonding of TPU in composites. Figure 7. (a) Dielectric properties and (b) AC conductivity of TPU/ MWCNT nanocomposites as functions of frequency. TPU10 and TPU20 represent TPU-based nanocomposites containing 10 and 20 wt %, respectively.

selective localization of MWCNTs in TPU phase. As for the blends with weight ratio of 50/50, the cocontinuous morphology is confirmed, and also the MWCNTs are dispersed only in TPU phase. It is further confirmed that the MWCNTs preferably locate in the TPU phase either in sea−island morphology or in cocontinuous morphology. Based on the selective localization of MWCNTs in the TPU phase, a three-dimensional (3D) double percolated structure can possibly form in the EVA/TPU (50/50) blends with cocontinuous morphology. In previous work, it was found16,29 that a percolative structure, especially a double percolated structure, nanocomposite allowed for enhancing the electrical conductivity of polymer nanocomposites because the conductive pathway of MWCNTs was formed in only one polymer phase, not through the entire matrix. The results also revealed that such a percolated structure is also beneficial for enhancing the dielectric constant of the blend which is caused by the polarization effect and the electron motion owing to the absence of the conductive path. However, the results in our study are different from the previous work; no improvement can be found for the blends with cocontinuous morphology. In addition, it is surprising to find that the dielectric constants of blends with sea−island morphology exhibit significant enhancement with incorporation of MWCNTs. Thus, a question arises: how does the morphology influence the dielectric properties of EVA/TPU/MWCNT nanocomposites?

surfaces: harmonic mean equation and geometric mean equation, as shown in eqs 2 and 3, respectively.27,28 ⎛ γ dγ d γ pγ p ⎞ γ12 = γ1 + γ2 − 4⎜⎜ d 1 2 d + p 1 2 p ⎟⎟ γ1 + γ2 ⎠ ⎝ γ1 + γ2

(2)

γ12 = γ1 + γ2 − 2( γ1dγ2d +

(3)

γ1pγ2p )

where γ1 and γ2 are the surface tensions of components 1 and 2, respectively; γd1 and γd2 are the dispersive parts of the surface energies of components 1 and 2, respectively; and γp1 and γp2 are the polar parts of the surface tension of components 1 and 2, respectively. The interfacial energy γ12 derives from the surface free energies of phase 1 (γ1) and phase 2 (γ2), corresponding to the γCA, γCB, and γAB. In our study, EVA is chosen as phase A, and TPU is chosen as phase B. Surface energies of TPU, EVA, and MWCNTs and the calculated interfacial energies between the polymers and the MWCNTs are reported in Tables 1 and 2, respectively. The wetting coefficients (ωa) are calculated to be 2.4 and 2.6 according to the two different approaches. Both values are greater than 1, which indicates that the MWCNTs should explicitly disperse into the TPU phase. Such a prediction is well matched with our SEM observations on the 3612

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Scheme 1. Schematic Diagram of Difference in Dielectric Properties between Samples with Different Content of TPU

blends, the curves of storage modulus versus frequency of the blends with 5 wt % MWCNTs are presented in Figure 6. It is well-known that during oscillatory melt rheology measurements the rheological behavior at low frequencies is sensitive to the nano- and microstructure of filled polymer nanocomposites, such that the percolative state of the MWCNTs can be detected by the behavior that the storage modulus of the material is less dependent on low frequency. As shown in Figure 6, it can be found that when the loading of MWCNTs is fixed at 5 wt %, the blends with different weight ratio show similar frequencyindependent behavior in the lower-frequency range, which means MWCNTs percolative structures can be found in the blend nanocomposites with 5 wt % MWCNTs. It needs to be clarified that although the loading of MWCNTs has reached 5 wt % and rheological percolation can be found in Figure 6, no frequency independence of the AC conductivity can be found, as shown in Figure 2. It is well-known that when the MWCNT percolating conductive paths are formed and the electrons can move freely in the entire frequency range, the frequency independence of the AC conductivity can be observed.16,30 Here, the rheological percolative structure is obviously different from the conductive paths. In detail, as for the blends with weight ratio of 80/20, the plateau of rheological percolative network appears at a frequency of 10−1−10−2 Hz during the frequency scanning. With increasing content of TPU, the percolative structures seem be to less effective in the lowfrequency region, reflected by the obscure plateau with smaller frequency range. For example, as for the blends with weight ratio of 50/50, the plateau of percolative structure locates in the lower-frequency range (around 10−1 Hz) during the frequency scanning. Although a double percolation phenomenon was observed in the blends with weight ratio of 50/50, the local concentration of MWCNTs in droplets of TPU phase facilitates an effective percolative network. On the other hand, according to the dielectric constant as shown in Figure 1, the dielectric percolation threshold can be evaluated via the jumping of dielectric constant and AC conductivity, as displayed in ref 31. In addition, it can be found that for the blend composites 80/ 20/z, the dielectric percolation threshold is located between 3

Figure 9. (a) Mechanical properties of the EVA/TPU/MWCNT (80/ 20/z) and (b) the stress−strain curve of EVA/TPU/MWCNT (80/ 20/z). z represents the content of the MWCNTs.

First, rheological testing was applied to confirm whether the 3D percolated structure was produced in our study. As indicated by previous work, with increasing the content of MWCNTs, the storage modulus, loss modulus, as well as complex viscosity increase, as shown in Figure 6. To check the effect of morphology on the MWCNTs percolated structure in 3613

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Industrial & Engineering Chemistry Research Table 3. Summary of Parameters of the EVA/TPU/MWCNT (80/20/z) sample

dielectric constant ε at 100 Hz

EVA TPU 80/20 80/20/1 80/20/2 80/20/3 80/20/5

6.6 10.6 6.8 8.4 11.2 35.1 1215.7

elastic modulus Y (MPa)

electromechanical sensitivity β = ε/Y (MPa−1 × 103)

elongation at break (%)

± ± ± ± ± ± ±

0.66 0.15 0.31 0.37 0.53 1.46 46.76

− − 921.0 ± 46.8 1054.8 ± 101.0 1122.5 ± 51.7 382.7 ± 34.4 557.4 ± 34.9

0.01 0.07 0.022 0.023 0.021 0.024 0.026

0.0003 0.004 0.002 0.002 0.0003 0.004 0.002

and 5 wt % because of the sharp increase of dielectric constant from 85 at 100 Hz for 80/20/3 to 1200 at 100 Hz for 80/20/5. No significant sharp increase can be found in the blend composites 50/50/z, which is well-matched with the rheological results. The same results can be found in PP/ NR/MWCNT nanocomposites,32 in which the MWCNTs in distorted droplets of NR phase benefited the effective percolated network of MWCNTs with bridging of some of MWCNTs from the NR droplet to the PP continuous phase. For the blend nanocomposites with weight ratio of 80/20, the AC conductivity also exhibits partial frequency independence in the low frequencies. This arises from the limitation of conductive paths and the contribution of microcapacitors to the conductivity.16 It should be noted that the loss curves at low frequencies have a distinct dependence on the MWCNTs loading, and a loss peak at 103 Hz can be observed for the blend composite 80/20/5, which is proposed to be related to the heterogeneous distribution of MWCNTs in the TPU droplets. As opposed to traditional polymer/conductive filler dielectric composites, the conductive TPU droplets filled by MWCNTs are separated by an EVA matrix so that the conductive pathways cannot go through the entire sample. In this case, when a frequency-dependent electric field is applied, the charges induced by interfacial polarization would accumulate at the interface between EVA and TPU, resulting in the increase of AC conductivity, as shown in Figure 2b. When the frequency of the dielectric field decreases, there is more time for approaching the equilibrium; therefore, more charges would be accumulated at the interface between EVA and TPU, which is regarded as being responsible for the appearance of the loss peak at low frequency. This similar loss peak can be found in PVDF-based multilayered dielectrics containing alternating layers of confined carbon black fabricated using a layermultiplying extrusion.33 For the blends with weight ratio of 50/ 50 showing the cocontinuous morphology, the rheological percolation is possibly related to the reduced distance between MWCNTs with increasing loading of MWCNTs, because the AC conductivity remains “nonconductive”. It can be concluded that the sea−island morphology with MWCNT localization in droplets enhances the percolative structure which has positive effects on the improvement of the dielectric constant. On the other hand, the microcapacitor principle can be invoked as being responsible for such increment of dielectric constants based on the MWCNT distribution in TPU droplets. A network of microcapacitors with the MWCNT filled TPU phase as electrodes, and a very thin EVA layer in between as insulating layer, can be formed in the blends with sea−island morphololgy, which is beneficial for more charge accumulation on the interfaces between TPU and EVA phases. As we know, for the blend composite 50/50/5, the maximum loading of MWCNTs in TPU domains would be 10 wt %; for the blend composite 80/20/5, the maximum loading of MWCNTs in

TPU domains would be 20 wt %. To evaluate the conductivity of electrodes, the dielectric and electrical properties of TPU/ MWCNT composites with 10 and 20 wt % were measured, as shown in Figure 7. Correspondingly, AC conductivity of binary TPU/MWCNT composites containing 10 wt % MWCNTs exhibits the frequency dependence, and the value of AC conductivity at low frequency is as low as 10−11 S·cm−1, which indicates the nonconductivity of TPU containing 10 wt % MWCNTs. It should be noted that no frequency dependence of the AC conductivity for TPU containing 20 wt % MWCNTs can be found, and the value of AC conductivity has reached 10−4 S·cm−1, which indicates that the TPU/MWCNT is conductive when 20 wt % MWCNTs were incorporated into the TPU. In conclusion, the TPU droplets filled by MWCNTs are conductive and can serve as electrodes. Neighbored TPU droplets with the EVA layer can form a microcapacitor. Each microcapacitor contributes an abnormally large capacitance, which can then be correlated with the significant increase in the dielectric constants. A similar model regarding matrix-free dielectrics based on core@double shell can be found in recent work.34 The inner shell polymer had either high dielectric constant or high electrical conductivity to provide large polarization, similar to the domains consisting of MWCNTs in our work, while the encapsulating outer shell polymer was more insulating to maintain a large resistivity and low loss, which behaved like the interlayer polymer matrix between two domains in this work. This result demonstrated that confinement of the more conductive phase in this nanostructure is the key to achieving a high dielectric constant and a low loss, which is mainly attributed to large space charge polarization of the conductive polymer domain induced by the large charge carrier density. Similarly, in our work, the movement of electrons is confined in the TPU island phases; thus, the large charge carrier density can be enhanced in the conductive MWCNT filled TPU droplets, and the free movement of electrons is confined by interface and limited through the entire nanocomposites, leading to high constant and low loss. Obviously, the dielectric property depends on the domain size and distance between two droplets, because only the blends with weight ratio of 80/20 exhibits great increase, whereas the blends with weight ratio of 90/10 and 70/30 show moderate increase although they also present the sea−island morphology. A qualitative description about the effects of domain size and distance between two droplets will be further discussed in future work. Another question arises: why are the cocontinuous blends with double percolation unable to improve the dielectric properties? First, compared with the blends with sea−island morphology, the increase of TPU content dilutes the dispersion of MWCNTs in the TPU phase. Therefore, compared with the MWCNTs filled in the droplets, the distances between MWCNTs are reduced, which can be evidenced by the 3614

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the dielectric constant show great increase with increasing loading of MWCNTs. With widespread application of polymer-based nanocomposites, multifunctional nanocomposites are in urgent need. A great amount of MWCNTs can lead to aggregates, which would induce stress concentration and finally result in the loss of mechanical properties. Therefore, the mechanical properties, including tensile stress, Young’s modulus, and elongation at break, were measured, and the results are shown in Figure 9. It is obvious that with increasing content of MWCNTs, the tensile strength as well as the elongation at break increases when the loading of MWCNTs reaches 2 wt %. Even though more MWCNTs (5 wt %) are incorporated into the blend, the elongation at break is still up to 557%, showing that the nanocomposite still retains good flexibility. With further increases in the loading of MWCNTs, the elongation decreases, possibly because of the aggregates of MWCNTs, while the tensile strength continues to increase. The parameters of mechanical properties and dielectric properties of blend nanocomposites with weight ratio of 80/20 and pure EVA/ TPU blend are summarized in Table 3. Electromechanical sensitivity (β) is defined as the ratio of the dielectric constant (ε) to the elastic modulus (Y) (β = ε/Y), which is an important parameter for determining whether the dielectric elastomeric has the potential to have high actuated strain at a low electric field.36 Generally, a substantial increase in the β value (a large relative dielectric constant, ε, and a low elastic modulus, Y37,38) is helpful for achieving high actuation performance. In this work, with the incorporation of 5 wt % MWCNTs into the blend, the electromechanical sensitivity increases by almost 150 times compared to that of pure EVA/TPU with weight ratio of 80/20, showing that the nanocomposite has the potential of a high actuated strain at a low electric field without any loss of modulus. Therefore, tailoring the dielectric properties via controlling the morphology of blends is an effective way to fabricate the polymer nanocomposites without sacrificing the mechanical properties.

rheological testing as shown in Figure 5, giving rise to the reduced number of microcapacitors generated by the neighboring MWCNTs. Second, the potential aggregates or bundles of MWCNTs in the continuous phase could weaken the generation of efficient microcapacitors. Third, it is worth noting that a large number of hydrogen bonds exist in TPU, in which the N−H group acts as the donor and either the carbonyl group from the hard segment or the ester oxygen group from the soft segment acts as the acceptor.18 These hydrogen bonds limit the mobility of the polarized groups of TPU chains, thus limiting the dipole orientation polarization ability of TPU chains, leading to low dielectric constant of TPU. Figure 8 shows the FTIR spectra of EVA/TPU blends with 5 wt % MWCNTs (80/20/5 and 50/50/5) and EVA/ TPU blend without MWCNTs (80/20) to analyze the effect of MWCNTs on hydrogen bonding. The broad peak in the range of 3320−3330 cm−1 is assigned to the hydrogen bond of TPU.35 In detail, seen from Figure 8b, the enlarged pictures in the hydrogen bond region show that when the MWCNTs are incorporated into EVA/TPU blends, the peak of hydrogen bond was weakened and blue-shifted from 3334 cm−1 for pure EVA/TPU blend to 3441 cm−1 for EVA/TPU/MWCNT (80/ 20/5) blend nanocomposite, indicating the disruption of the hydrogen bond by MWCNTs, thus increasing the dipole orientation polarization ability of TPU chains. This phenomenon can be found in TPU/GO nanocomposites,35 in which the containing oxygen groups destroy the hydrogen bonds in TPU phase and further induce the increment of dielectric constant. On the other hand, the MWCNTs are incorporated into the blends with a weight ratio of 50/50; the MWCNT network is comparatively weak so that it cannot effectively destroy the hydrogen bonding of TPU. Finally, the hydrogen bonds in blend composite 50/50/5 exhibit no change compared with neat EVA/TPU blend, leading to low polarization ability of TPU chains and low constant of blend composite 50/50/5. Based on the microcapacitor model and the effect of MWCNTs in TPU domains, a more visual schematic representation for the underlying mechanism for the improvement of dielectric properties is shown in Scheme 1. The MWCNTs are selectively localized in TPU phase, and the morphology of blends varies from sea−island to cocontinuous morphology with increasing content of TPU. MWCNTs localization in TPU droplets facilitates the formation of microcapacitors and especially benefits the accumulation of charge in the TPU domain, which may further enhance the charge polarization. On the other hand, although a percolative structure of MWCNTs can be found in the continuous TPU phase, the density of MWCNT network in TPU is comparatively weakened and the number of microcapacitors arising from the neighboring MWCNTs is reduced with increasing content of TPU. Meanwhile, because of the confinement of electrons in the TPU island morphology, the movement of electrons is limited and the growth of the dielectric loss in the blend nanocomposites with sea−island morphology is suppressed. In addition, the hydrogen bonds of TPU were also separated by the addition of MWCNTs, and it is obvious that the effect of hydrogen bonds on dielectric constants is more efficient in blends with cocontinuous morphology than that in sea−island morphology in which the denser MWCNT network may totally destroy the hydrogen bonds in TPU droplets. Therefore, only when the blends with weight ratio of 80/20 exhibit the sea−island morphology does

4. CONCLUSIONS This work has provided a comprehensive study concerning the effects of phase morphology and MWCNT selective localization on the dielectric properties of EVA/TPU/MWCNT nanocomposites. The dielectric constant attains great improvement only when the blend morphology exhibits sea−island morphology. In detail, only when the weight ratio of the blend is 80/20 do the dielectric constant and dielectric loss increase with increasing MWCNT loading. In order to determine the underlying mechanism for obtaining high dielectric constant and low dielectric loss, based on the investigation of rheological percolation structure, SEM morphological observation, and selective localization of MWCNTs, the microcapacitor model has been evoked, in which the TPU droplets containing MWCNTs act as electrodes and benefit the charge accumulation, which is responsible for the improvement of dielectric constants. The electron movements are limited because of the confinement of MWCNTs in the TPU island; therefore, the growth of dielectric loss is suppressed. In addition, the blend with sea−island morphology retains remarkable mechanical properties. Therefore, tailoring the blend morphology is regarded as an effective way to fabricate polymer nanocomposites with high dielectric constant while maintaining the mechanical properties. 3615

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b04763. 3D plots of the AC conductivity, content of TPU phase, and frequency for nanocomposites; particle sizes of the TPU domains and the distances between domains in the blend nanocomposites (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86 28 87602714. E-mail: [email protected]. *Tel: +86 28 87603042. E-mail: [email protected]. ORCID

Yong Wang: 0000-0003-0655-7507 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We express our sincere thanks to the National Natural Science Foundation of China (50973090) and Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (SKLPME 2016-4-27) for financial support.

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REFERENCES

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