Evaluation of the Dielectric and Insulating Properties of Newly Synthesized Ethylene/1-Hexene/4-Vinylcyclohexene Terpolymers.

Terpolymerizations of newly synthesized ethylene (E), vinylcyclohexene (VCH), and 1-hexene were carried out with symmetrical metallocene catalysts rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2 (catalyst A) and rac-Et(Ind)2ZrCl2 (catalyst B). X-ray diffractometry (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), high-temperature gel permeation chromatography (GPC), and nuclear magnetic resonance (NMR) spectroscopy were used to evaluate the behavior and microstructure of the polymers. The activity of catalyst B was 1.49 × 106 gm/mmolMt·h), with a T m of 73.45 (°C) and ΔH m of 43.19 (J/g), while catalyst A produced first higher 1-hexene, 19.6 mol %, and VCH contents with a narrow molecular weight distribution (MWD). In previous reports, ethylene propylene monomer dienes (EPDM) had a low content and were used for dielectric and insulating properties with nanomaterials. Second, this paper presents a kind of elastomeric polymers based on E/1-hexene and VCH with a high dielectric constant (k = 6-4) and mechanical properties. In addition, low dielectric loss suggests the suitable application potential of these polymeric materials for the fabrications of capacitors. Also, this work reveals that these polymers can be a better candidate for high-voltage electrical insulation due to their enhanced dielectric, mechanical, and thermal characteristics. To examine the insulating property, the interface characteristics of the polymer were evaluated using electrochemical impedance spectroscopy (EIS) with a frequency range of 1 × 105-0.01 Hz and an amplitude of 5.0 mV. EIS is an effective method to investigate the polymers' interfacial electron transfer characteristics. The EIS Nyquist plot showed high Warburg impedance features in the low-frequency domain with straight lines without a semicircle, suggesting that the property of the polymer owing to the high electrical resistance and poor conductivity for ionic kinetics in the electrolyte may have surpassed that of the semicircle. Although the slope of low frequencies in polymers holding potent exoelectrogenic bacteria (Shewanella oneidensis MR-1) as a charge carrier in the electrolyte could significantly reduce the Warburg resistance, it still could not improve the conductivity, which demonstrated that the external charge supply could not alter the insulating property in the used polymers.

Introduction

Ethylene propylene diene monomer (EPDM) polymeric materials play an important role in daily life and significantly contribute to mechanical and electrical manufacturing. In contrast to inorganic materials such as mica, ceramics, and glasses, polymeric materials have significant advantages, including easy processing, better fabrication, low dielectric losses, and excellent insulating properties. Furthermore, EPDM-based polymeric materials are also used as the reinforcing structural element for electric products.[1]

Ethylene propylene with dienes and EPDM elastomers are manufactured on a large scale, with annual production exceeding 100 million tons.[2−5] These elastomers were produced with heterogeneous Ziegler–Natta (Z-N) catalysts through ion coordination-based polymer reactions. The Ziegler–Natta catalyst system was developed in the 1960s and has been broadly used up to now in polyolefin industries. From the discovery of the Ziegler–Natta catalyst to 1990, the manufacturing of E.P. and EPDM was largely based on vanadium-based catalysts such as VOCl3 and VCl4. The disadvantages of these catalysts were low stability and catalytic activity. In addition, with a large amount of vanadium (V) in the polymeric product, very expensive equipment have to be used to remove the residual V. To overcome these problems, academic and industrial researchers used single-site homogeneous metallocene catalysts for the synthesis of E and P homopolymers, E.P., and EPDM since the 1990s. Numerous metallocene catalysts with different substituents have been investigated to manufacture these polymers.

The development of novel catalysts based on group 4 metals has re-examined polyolefin chemistry over the past 30 years due to the fast expansion of technology and materials. The higher activity, better selectivity, control of molecular weight, and a wide range of α-olefins ranging from plastomers to elastomers are all advantages of metallocene catalysts. In addition, many variables influence the elastomeric characteristics of a polymer, including the amount of co- or termonomers present in the polymer, its molecular weight (Mw), and molecular weight distribution (MWD) metallocene catalysts can control.[2,6−9] Due to these advantages of metallocene, it was estimated in the year 2006 that about more than 60 million tons of polyolefin were manufactured, which makes up about 50% of regular-use plastics.[2−5] It means that other daily-use plastics can be replaced with EPDM and other types of polyolefin, such as E with 1-hexene and E/I-hexene/dienes in the future. What is the main reason for this? Its impact on daily life has considerable applications, ranging from simple plastic bags to highly sophisticated capacitors and module fibers.[10−12] Additionally, they are employed as a structural element for electrical devices, strengthening the structure.[1,13−16] Furthermore, olefins are an elastomeric material with outstanding electrical characteristics when adequately formulated, including heat and ozone resistance, an extensive range of hardness and stress resistance, and better chemical resistance, particularly to polar media.[17−19] Their electrical solid insulation qualities make them suitable for use in low- to moderate-voltage wires and cables (up to 35 kV). Measuring and analyzing the dielectric characteristics are a quantitative approach used to understand the charge transfer mechanisms in the polymer.[20−22] These properties provide a different approach for discovering innovative uses for these new polymeric materials. Charge transport capabilities in polymeric materials have been the subject of several studies and theoretical models. Polymeric materials have charge carriers that are localized, which hence move in the material between the localized states either by hopping or tunneling through the potential barriers.[23−25]

In this study, we use a single-site bridged metallocene catalyst for the new kind of E/1H/VCH terpolymers with a higher content of VCH, further chosen as the insulating matrix materials because of their outstanding electrical and mechanical resistance properties and suitable for potential use in electrical wires and cables. They are excellent isolators because they are nonpolar polymers resistant to alkalies and acids.[26] Furthermore, frequency-dependent conducting mechanisms in new materials are an essential aspect to explore. The synthesis and dielectric properties of E/1-H/VCH terpolymers are thoroughly reported in this article. However, the synthesis of these new elastomers to prepare dielectric elastomers has been unreported to date. Therefore, this paper delineates a synthesis of E/1-H/VCH terpolymers catalyzed with catalyst A and catalyst B to prepare dielectric materials with enhanced dielectric properties. These metallocene catalysts allow the possibility to tailor the polyolefin structure in a new way that has not been reported before.

Methods

Materials

The asymmetric metallocene polymerization catalysts rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2 catalyst A (Shanghai Research Institute of Chemical Industry, Shanghai, China), rac-Et(Ind)2ZrCl2 catalyst B (Sigma-Aldrich), and the borate [Ph3C][B(C6F5)4] cocatalyst were donated by Sinochem Lantain, Zhejiang Research Institute of Chemical Industry Co., Ltd. Hangzhou, PR. China. Borate and both catalysts were diluted in toluene, while triisobutylaluminium (TIBA) (Albemarle Co) was diluted in n-heptane and held under nitrogen. Ethylene with 99.9% purity (Zhejiang Gas mixing Co. Hangzhou, China.) was used. 1-Hexane and VCH were bought from Arcos Organics Chemicals China and purified through dehydration using 4 Ă molecular sieves and collected using the vacuum distillation process.

Polymerization

The E/1-H/VCH terpolymerization reactions were carried out in a 100 mL Schlenk round-bottom glass reactor. A pure form of E was added to the polymerization reactor, followed by injections of 1-hexene and VCH. TIBA was then added and allowed to run for 5 min before being followed by a metallocene and borate activator to initiate polymerization. After the planned time, the reaction was quenched with dehydrated ethanol containing 2 percent HCl, which decomposed the TIBA, borate, and metallocene and allowed the polymerization to stop. Before drying under vacuum, the terpolymers were precipitated in ethanol and subjected to an intensive purification technique that included many steps.[27,28]

Characterization

The NMR spectra were measured using a Varian Mercury plus 300 spectrometers (Varian Corporation) (75 MHz). Dichlorobenzene-d4 was used as the test solvent, and it had a concentration of 10% by weight in the sample. A temperature of 120 °C was employed to obtain NMR spectra, with hexamethyldisiloxane serving as an internal chemical shift standard.[15]

Gel Permeation Chromatography (GPC)

To measure the Mw and MWD, we use PL-gel MIXED-B columns and 1,2,4-trichlorobenzene as the eluent at a flow rate of 1.0 mL/min at a temperature of 150 °C, universal calibration against thin polystyrene standards was performed, and the results were successful for both Mw and MWD.[13]

Differential Scanning Calorimetry (DSC)

A TA Q200 instrument was used to examine the thermal behavior of terpolymers, and it was calibrated with indium and water before being utilized. Initially, the sample was heated to 150 °C for 5 min and then cooled to 20 °C at a rate of 10 °C/min. To eliminate thermal history, the melting curve was finally recorded by heating the sample to 180 °C at a rate of 10 °C/min.[29,30]

Scanning Electron Microscopy (SEM)

Using a Hitachi S-4800 system coupled to an energy-dispersive X-ray spectroscopy (EDS) system, the morphology of the samples was investigated using scanning electron microscopy (SEM).[27]

X-ray Diffractometery (XRD)

The crystal structure of the E/1H/VCH terpolymers and insertion of VCH were also analyzed with an X-ray diffractometer (XRD) from the Zhejiang university chemical engineering department.

Flexural Properties of the E/1H/VCH Terpolymer

E/1-Hexene/VCH Terpolymer Tensile Specimen

The tensile behavior of the newly synthesized E/1-H/VCH terpolymer samples was investigated using an Instron universal tensile machine. Table shows the information about the specimen.

Table 1

Specimen Details

s.no.	description	value (mm)
1	width of graph	25
2	width of graph	5
3	inner radius	25
4	radius for center shape	11
5	length for grip	15
6	gauge length	20
7	total length	50
8	thickness	0.2

The polymer samples were prepared at 80 °C for 5 min using the compression–molding technique; the design of the sample and machine is shown in Figure .

Figure 1

Compression–molding technique.

A dielectric material test fixture measured the capacitance on an LCR meter (TH2826A). The electrode diameter of the sample was 10mm, and the thickness was about 2 mm. The scanning frequency was from 100 Hz to 1 MHz at room temperature. The relative permittivity of the materials was calculated by eq There are four variables in this equation: C (capacitance measurement), z (elastomer thickness), 0 (vacuum permeability), and A (effective area)

Electrochemical Impedance Spectroscopy (EIS)

To examine the insulating property of E/1H/VCH terpolymers, the interface characteristics of the polymer material were evaluated using electrochemical impedance spectroscopy (EIS) with a frequency range of 1 × 105–0.01 Hz and an amplitude of 5.0 mV. The ethylene and diene-based polymer material analyses were performed using the CHI electrochemical workstation (CHI660E, Cheng Hua Instrument Co. Ltd., Shanghai) with a three-electrode system. To operate the system, the polymers served as the working electrode, with platinum (Pt) wire as the counter electrode and an external electrode, saturated calomel, as a reference electrode (SCE, 0.241 V vs SHE).[31]

Furthermore, to test the EIS property of the polymers under mild conditions, the experimental state was characterized at room temperature with two sets of operating conditions using phosphate-buffered saline (PBS, pH 7.2) as an electrolyte without charge carriers and with a negative charge carrier using a potent exoelectrogenic bacterial cell of Shewanella oneidensis MR-1. S. oneidensis MR-1, a model strain of exoelectrogenic bacteria (EEB), can utilize a wide range of extracellular electron acceptors (e.g., iron, manganese oxides, oxygen, and sulfur species) in its cellular metabolism and growth for anaerobic electrode respiration and thus play important roles in environmental bioremediation, energy production from wastes, and biogeochemical cycling of metals in environments.[32] Similarly, to measure the ohmic resistance (R) values of polymers (catalyst A and catalyst B), the potential differences between two points in voltage (V) proportional to the current (I) were calculated through Ohm’s law (R = V/I).[33]

Results and Discussion

Before analyzing the dielectric properties, we first synthesized E/1-H/VCH terpolymers with different mole ratios of 1-hexene and VCH using two symmetrical metallocene-catalyzed polymerization techniques in a nonpolar solvent; the structure of the asymmetric metallocenes is shown in Figure . Both catalysts showed C2 symmetry but different ligand structures, which means that they produce E/1-hexene/VCH terpolymers with various activities and different monomer compositions, depending on the electronic impact and size of ligands attached to the Zr metal. According to the literature, catalyst B is one of the simplest C2-symmetric polymeric catalysts systematically studied and well known to produce polymers with a high polymerization and diene content.

Figure 2

Structure of the metallocene catalysts and active species for polymerization.

One of the most popular industrial methods is the slow coordination polymerization method to produce polymers with <5% dienes. To increase the insertion of termonomers (diene) in E-1-hexene, we chose VCH with a less steric effect than other cyclic dienes (NB, VNB, ENB, etc.), which permitted us to insert it in the polymer chain through the exocyclic bond; see Scheme .

Scheme 1

Synthesis of E/1-H/VCH Terpolymers with Symmetrical Metallocene

To increase the insertion of the exocyclic π bond of VCH and the 1-hexene π bond silly bridge and ethylene bridge catalyst system was the best choice.

Our previous study on E/P/dienes (80/20/0.06 and 80/20/0.12 mole ratios) catalyzed with metallocene/TIBA/borate and that of Jongsomjit et al. on E/P/dienes (50/50/0.06 mole ratios) found that enhancing VCH concentrations over 0.06 mol/L lowered the catalytic activity considerably.[13,34] Similarly, a higher amount of cyclic nonconjugated diene addition could result in a declined activity of metallocene polymerization as investigated by Malmberg and Löfgren et al., although it should be noted that large amounts of (∼2–5 mol %) of dienes (VCH) were technically required to produce the dielectric properties of the polymers.[35−38] Following on that study, at first, E/1-H/VCH terpolymerization was run at a constant E/1-hexene/VCH feed ratio of 0.12/0.06. The amount of 0.06 mol/ L VCH was enough to maintain the higher activities of both catalysts because of the low steric flexibility of the double bond in the cyclohexene ring.[13] As shown in Table , catalyst B showed higher activity than catalyst A and so did the formerly produced terpolymer with a lower VCH content than that with catalyst A. According to Fan et al., the copolymerization of E/P (50/50) with the addition of VCH caused evident activity enhancement.[34] In contrast with the catalyst A-catalyzed E/P/diene terpolymers, more negative effects of diene addition were reported in the literature.[27,39] In addition, compared to propylene, a higher content of 1-hexene in the feed caused a more substantial activation effect. According to Mortazavi, among the E/P/diene terpolymers, reducing the ethylene content of the monomer feed caused a more significant deactivation effect by the diene.[27,40] It is interesting to compare the activities; for E/P/VCH with catalyst B/methylaluminoxane (MAO) and catalyst B /TIBA/borate, the activities were about 2.8 × 106 and 3.14 × 106 gmpolym/molMt*h, which are relatively lower than the activity of E/1-H/VCH terpolymers catalyzed with catalyst B/borate/TIBA under similar conditions.[13,15] It means that 1-hexene is an effective comonomer for synthesizing elastomers with higher activity. However, the increased activities were more pronounced with VCH in both cases.[13,14,34] The comonomer effect can explain increases in the polymerization activity. Guo et al. claimed that the copolymerization activity of E/P was greater than their homopolymerization activity.[13] They observed that the faster insertion of ethylene molecules could not explain the increased rate but was likely the result of activating dormant catalyst sites by ethylene.[14] Some other characteristic explanations have also been reported in the literature to describe this phenomenon, including the improved diffusion rates and preventing the solubilization of active sites of a catalyst by incorporating co- or termonomers and trigger mechanisms.[8,27] However, based on this explanation, addition of 1-hexene would improve the attachment of ethylene monomers to the catalyst’s active sites, resulting in increased activity. In contrast, propylene inhibits the attachment of ethylene to the catalyst’s active sites through the solubilization of active sites.[14,41]

Table 2

Ethylene/1-Hexene/VCH Terpolymerization Catalyzed by Symmetrical Metalloceneabe

run	cat	yield (g)	activity (106 gm/mmolMt·h)	Mw (kg/mol)	Đ	Tm (°C)	ΔHm (J/g)
1	catalyst A	0.61	1.48	59.00	2.22	123.1	0.3
2	catalyst B	1.49	3.6	56.30	2.66	92.6	58.8

Reaction conditions: Catalyst 1.25 μmol, borate 2.5 μmol, TIBA 1000 μmol, 1-hexene 0.12 mol/L, VCH 0.06 mol/L, ethylene pressure = 0.1 MPa, temperature = 50 °C, and solvent: toluene (50 mL).

Determined by GPC.

Determined by DSC.

Thermal Properties

The thermal properties (Tm and ΔHm) of E/1-hexene/VCH were obtained from DSC and are listed in Table . According to the 1H NMR results, the content of 1-hexene and VCH is more in the catalyst A-catalyzed polymer, leading to reduced crystallinity properties. The Tm of catalyst A-catalyzed E/1-hexene/VCH elastomers is 123.1 (°C), which is higher than that of catalyst B-catalyzed polymers. In contrast, catalyst B-catalyzed E/1-hexene/VCH terpolymers with a higher content of E become crystalline materials; see Figure . Similar results were reported by Phoowakeereewiwat et al. work where the content of E increased in the terpolymerization of E/P/diene catalyzed with catalyst B/MMAO.[13,34] In conclusion, the thermal properties are firmly dependent on the mole percent of VCH and 1-hexene. It is required to determine the incorporation of VCH in the polymer chain, which is explained in the next part of this paper.

Figure 3

Thermal properties of E/1H/VCH terpolymerization with catalyst A and catalyst B under similar conditions.

The morphological investigations of the synthesized polymers were done through scanning electron microscopy analysis (SEM), as displayed in Figure a–f. The catalyst A-catalyzed polymer SEM images exhibited highly dense and interlinked thick polymeric structures with little porosity. The more elastomeric-like nature of the polymer is reflected in the corresponding SEM micrographs. (Figure a–c). Multiple small islands like bumpy surfaces revealed the compactness and homogeneity of the surface. However, the catalyst B-catalyzed polymer revealed a typical flatter surface morphology that comprises thin polymeric strings interconnected throughout the network, as seen in Figure d–f. The high-resolution SEM micrographs further clarify polymeric strings’ networking and homogenous surface covering. From the SEM analysis, it was found that both the polymers offered homogeneous and clear surface morphology.

Figure 4

SEM images of E/1H/VCH terpolymers with catalyzed A (a–c) and E/1H/VCH terpolymers with catalyze B (d–f).

NMR

It is well known that NMR spectroscopy is one of the sanative and powerful techniques to identify the polymer microstructure and co- or termonomer mole %. It is required to determine the incorporation of VCH in the polymer chain through exocyclic π bonds or endocyclic π-bonds. The characteristic 1H NMR spectra for E/1-hexene/VCH with both metallocene catalysts are illustrated in Figure . It can be observed that the 1H NMR spectra for E/1-hexene/VCH have peaks in the region of 5.4–5.6 ppm for VCH and 1.5–0.5 ppm for E and 1-hexene, which clearly show the insertion of VCH exocyclic π bonds. In addition, these peaks indicate that more than enough amount of 1-hexene and VCH is incorporated into the polymer chain under the specified conditions; see Table . In our previous study, we observed more than 0.06 mol/L VCH during the insertion of a propylene comonomer. Jongsomjit and his group described comparable results in the copolymerization of ethylene with higher α-olefins such as 1-octene and 1-decene.[34]

Figure 5

1H NMR spectra of the E/1-H/VCH terpolymer catalyzed by the metallocene/borate/TIBA catalyst system.

Table 3

Monomers Composition in Ethylene/1-Hexene/VCH Terpolymerization Calculated by H NMR Spectroscopya

run	cat	ethylene (mol %)	1-hexene (mol %)	VCH (mol %)
1	catalyst A	79.3	19.9	0.77
2	catalyst B	93.8	6.0	0.16

Conditions: Solvent = dichlorobenzene-d4, sample concentration 10% by weight, temperature of 120 °C, number of scans 400, and hexamethyldisiloxane serving as an internal chemical shift standard.

In addition, the XRD pattern of E/1H/VCH is shown in Figure , and the characteristic peaks of E/1H/VCH terpolymers produced with catalyst A and catalyst B can be observed, where the red peak shows catalyst A and blue peak shows catalyst B, compared with E/P/diene terpolymers,[42] which give evidence of VCH insertion same as in EPDM.

Figure 6

XRD pattern of E/1H/VCH terpolymers produced with catalyst A and catalyst B.

Molecular Weight (MW)

Based on our investigation data, the effect of catalyst ligands and bridging atoms inserted between the metal and ligands of the catalysts could be drawn from the resulting E/1-hexene/VCH terpolymer activities, VCH incorporation, thermal properties, molecular weight, and molecular weight distribution. The addition of VCH/1-hexene (0.06/0.12 mol/L) could modify the behaviors of E/1-hexene/VCH terpolymers using an asymmetrical zirconocene/borate/TIBA catalyst system. The Mw of E/1-hexene/VCH is higher than that of E/P/VCH terpolymers. Generally, sufficient higher Mw is required for outstanding tensile properties of the EPDM that work as an elastomer. As seen in Table , the E/1-hexene/VCH terpolymers produced with catalyst A have higher Mw than with catalyst B. However, in our previous study, catalyst B/TIBA/Borate showed a higher (51.0 kg/mol) Mw when VCH was inserted as a termonomer in the E/P copolymer.[8,27] The polydispersity index (Đ) of the E/1-hexene/VCH terpolymers produced with both catalysts is narrow (see Figure ). Its value was still obvious and larger than 2, which is closer to the true single-site metallocene catalyst, suggesting the existence of several active sites in the catalytic system involved in inserting E, 1-hexene, and VCH in the polymer backbone.

Figure 7

Molecular weight distribution of E/1H/VCH polymers under the same polymerization conditions.

Dielectric Properties of the E/1-Hexene/VCH Terpolymer

The silicone rubber and EPDM polymeric material are generally used for reinforced insulation due to their excellent insulation performance. Following this study, we chose E/1-H/VCH terpolymers as dielectric materials because they exhibit a higher amount of 1-hexene and VCH. First, we calculate the dielectric properties of catalyst A-catalyzed E/1-H/VCH terpolymers at room temperature and then for catalyst B-catalyzed polymer. The dielectric constant and dielectric loss variation were investigated by varying the applied field’s frequency. As shown in Figure , the sample shows high dielectric constant values of around 6 and 4 at a lower frequency. However, with a further increase in the frequency from 1 Hz to 1 MHz in the applied field, the dielectric constant values were slightly increased. For example, the dielectric constant was reduced from 6.00 to 4.65 for catalyst A-catalyzed E/1-H/VCH terpolymers within the frequency range from 100 to 1 kHz. In contrast, catalyst B-catalyzed E/1H/VCH terpolymers demonstrated comparatively lower dielectric constants, varying from 4.00 to 4.50 under the same frequency level. However, as the frequency was increased from 100 Hz to 1 kHz, the dielectric constant of catalyst B-catalyzed polymers increased. In addition, as the applied field frequency was increased, both catalysts exhibited comparable dielectric constants. According to Kan Zhang, the dielectric constant of the polybenzoxazines was decreased when the frequency of the applied field was increased to 100 Hz; catalyst A-catalyzed E/1-H/VCH terpolymers showed similar behavior at a low-frequency level.[43] In general, at 1 MHz, the dielectric constants of conventional EPDM and benzoxazine are typically in the range of 3–4.[44] As expected, newly synthesized E/1-H/VCH terpolymers with moderate mol % of VCH catalyzed by the symmetrical metallocene catalyst system exhibited a high dielectric constant, which is higher than that previously reported in the literature with norbornene-functionalized benzoxazine. Notably, the higher dielectric constants of the E/1-H/VCH terpolymers are comparable to those of ethylene/propylene/diene (EPDM)/NB nanocomposites.[20] Therefore, the introduction of VCH and 1-hexene with ethylene instead of propylene increased the dielectric constant significantly, and 1-hexene importantly increased the incorporation rate of VCH as compared to P. In the terpolymer chain, the network formed by connection of the exocyclic π-bonds of VCH and π-bonds of 1-hexene might have a higher dielectric constant.

Figure 8

Frequency dependence of the dielectric constant of the catalyst A- and catalyst B-catalyzed E/1-H/VCH terpolymer samples in the frequency range of 100 Hz–1 MHz.

The change in dielectric loss with increasing frequency is because of the displacement of electric charges inside the polymer. The variation of dielectric loss was calculated at different frequencies[20] for both metallocene-catalyzed terpolymers, as shown in Figure .

Figure 9

Frequency dependence of dielectric loss of the catalyst A- and catalyst B-catalyzed E/1-H/VCH terpolymer samples in the frequency range of 100 Hz–1 MHz.

The graph shows that the samples show a low dielectric loss, values of around 0.2 and 0.16, at a lower frequency of 100–1 kHz. We observed that the dielectric loss shows good consistency with increased frequency. A high dielectric constant and low loss have always been contradictory points, and the loss of high-dielectric materials is generally higher. Dielectric loss for both samples shows a low-value plateau-like polymer material at higher frequencies. At low frequencies, the dielectric loss values were found to decrease. The dielectric loss is characterized by relaxation frequency for both samples. The change in dielectric loss with increasing frequency is because of the displacement of electric charges inside the polymer[24] and due to high resistivity caused by grain boundaries. Moreover, the catalyst A-catalyzed sample shows less dielectric losses than the catalyst B-catalyzed sample, suggesting the suitable application of the polymeric material in the fabrications of capacitors.[45] The results again demonstrate that the electromechanical properties and performance of dipole elastomers are affected by the combined influence of the structure and the content of the dipoles.

EIS is a powerful technique, which was used to examine the properties of interfacial electron transfer processes in polymers. The obtained data by EIS are fitted with a simple electrical equivalent circuit model using ZView software integrated with SAI measurement software (RS (C(RCT.WS))) (Figure a). The fitting data could estimate the values of individual resistances (i.e., ohmic resistance, Warburg resistance, etc.) in this study. The E/1-hexene/VCH terpolymers were used as the working electrode, with platinum (Pt) wire as the counter electrode and an external electrode, saturated calomel, as a reference electrode (SCE, 0.241 V vs SHE).[31] The three-electrode system is depicted schematically in Scheme .

Figure 10

(a) Nyquist plots of EIS spectra from catalyst A- and catalyst B-catalyzed polymers in the electrolytes without a charge carrier using phosphate-buffered saline (PBS) and with a negative charge carrier using exoelectrogenic S. oneidensis MR-1 (The blue arrow shows the low-frequency domain on EIS Nyquist plots). (b) Obtained linear curves from polymers as a function of voltage directly proportional to current in terms of ohmic resistance values.

Scheme 2

Electrochemical Workstation with the Polymer (E/1H/VCH) Working Electrode

As shown in Figure a, all EIS Nyquist plots exhibited high Warburg impedance features in the low-frequency domain with straight lines without semicircles,[46] suggesting that the property of the polymer owing to the high electrical resistance and poor conductivity for ionic kinetics in the electrolyte may have surpassed the semicircle. Nevertheless, the area of low frequencies in the catalyst B-catalyzed polymer exhibited a smaller domain than the catalyst A-catalyzed polymer. Although the slope of low frequencies in polymers holding S. oneidensis MR-1 as a charge carrier in the electrolyte could significantly reduce the Warburg resistance, it still could not improve the conductivity, which demonstrated that the external charge supply could not alter the insulating property in the used polymers. As a result, the obtained linear curves between voltage and current in two polymers could determine high ohmic resistance values of 4.28 and 4.77 kΩ for catalyst A and catalyst B, respectively (Figure b). The findings indicated that the polymers owing to their distinct insulating properties have poor electrical conductivity.

Mechanical Properties of the E/1H/VCH Terpolymer

The mechanical properties of the E/1-H/VCH terpolymer produced with catalyst A and catalyst B were calculated at room temperature using an Instron universal testing machine (model 5566, Norwood, MA). The ability to resist external forces in the polymer is mainly due to the chemical bonding of the main chain, interchange interaction, and length of the chain. Therefore, it is reasonable to improve polymer polarity, chain nitration, and chain length to increase its mechanical properties.[46] The effect of 1-hexene and VCH contents in E/1-H/VCH on tensile properties has been analyzed. The stress–strain curves of E/1-H/VCH and fracture are shown in Figure .

Figure 11

Mechanical properties of the E/1H/VCH terpolymer produced with catalyst A and catalyst B.

It reveals that the E/1-H/VCH sample produced with catalyst A shows a higher strength than that of catalyst B because catalyst A produced the E/1-H/VCH polymer with a higher content of 1-hexene (19.06 mol %) and VCH (0.77 mol %) with enhancement in the tensile strength of the polymer, which can be described by increasing length of the chain structure, which was confirmed by the characterization of high-temperature GPC and 1H NMR. Higher insertion of 1-hexene and VCH increases the polymer chain length, and when an external force is applied, more energy is required to fracture the catalyst A-catalyzed E/1-H/VCH polymer. On the other hand, compared with EPDM, due to the higher insertion of propylene, it showed higher energy fracture than the E/1-H/VCH polymer.[47] Thermally stable E/1-hexene/VCH with a higher content of 1-hexene (10–20%) has been prepared with good tensile properties that can be used in the communication, construction, automotive, aerospace, and synthetic rubber industries. In conclusion, the results indicated that the newly synthesized E/1H/VCH terpolymers with both catalysts showed moderate flexural strengths.

Conclusions

In this work, a symmetrical metallocene catalyst system was used to produce a new type of E/1-hexene/VCH-based elastomer with excellent insulating properties. Catalyst A produced the polymer with higher 1-hexene and VCH contents and high molecular weight, while catalyst B showed a higher activity of 1.49 × 106 gm/mmolMt·h and crystalline (ΔHm of 43.19 J/g) properties. Both catalysts inserted the VCH through exocyclic π bonds, which was confirmed by 1H NMR. In addition, numerous spectroscopic techniques such as 1H NMR, XRD, DSC, GPC, and SEM were used to investigate the microstructure and behavior of the polymers. Second, EIS was used to investigate the polymer interfacial electron transfer properties. The Warburg impedance features in the low-frequency domain with straight lines without semicircles, suggesting the property of the polymer owing to the high electrical resistance. The slope of low frequencies in polymers holding S. oneidensis MR-1 as a charge carrier in the electrolyte could significantly reduce the Warburg resistance, which demonstrated that the external charge supply could not alter the insulating property in the E/1-hexene/VCH polymers. These polymers also present a high dielectric constant (k = 6–4) and mechanical properties. Low dielectric loss suggests the suitable application potential of these polymeric materials for the fabrications of capacitors.