Optimization of Electrical Conductivity, Dielectric Properties, and

Article pubs.acs.org/IECR

Optimization of Electrical Conductivity, Dielectric Properties, and Stress Relaxation Behavior of Conductive Thermoplastic Vulcanizates Based on ENR/COPA Blends by Adjusting Mixing Method and Ionic Liquid Loading Suradet Matchawet,† Azizon Kaesaman,† Norbert Vennemann,‡ Claudia Kummerlöwe,‡ and Charoen Nakason*,§ †

Department of Rubber Technology and Polymer Science, Faculty of Science and Technology, Prince of Songkla University, Pattani 94000, Thailand ‡ Faculty of Engineering and Computer Science, University of Applied Sciences, D-49076 Osnabrück, Germany § Faculty of Science and Industrial Technology, Prince of Songkla University, Surat Thani 84000, Thailand ABSTRACT: Conductive thermoplastic vulcanizates based on epoxidized natural rubber (ENR) and copolyamide (COPA) blends were prepared using two alternative mixing stages to incorporate MWCNTs: after or before dynamic vulcanization (i.e., ADV or BDV, respectively). Effects of ionic liquid (IL) loading on properties of the blends were also studied. The results indicated that both ADV and BDV mixing preferentially localized MWCNTs in the COPA phase. However, with BDV mixing some of the MWCNTs also resided in ENR domains. The TPVs made with BDV showed higher electrical conductivity, dielectric properties, and superior stress relaxation behavior relative to those prepared by ADV. This might be due to better dispersion of MWCNTs in both phases of the blends. In addition, the electrical and dielectric properties were improved by the IL in a dose dependent manner with the IL loading. However, the IL reduced stress relaxation of the TPVs due to its plasticizing effect.

1. INTRODUCTION Conventional vulcanized conductive rubber composites contain conductive fillers such as carbon black, carbon nanotubes (CNTs), or graphene dispersed in the rubber matrix. They have been subjects of academic and industrial research because high electrical conductivity, good flexibility, and considerable elasticity hold great potential for applications to strain sensors, stretchable conductors, and actuators.1−3 However, the preparation of conventional thermoset rubber is a complicated process, also more difficult to recycle than with thermoplastic materials. Therefore, thermoplastic elastomers (TPEs), materials based on blends of elastomers and thermoplastics, have gained considerable importance. The TPEs are materials that exhibit functional properties of conventional thermoset rubber, but they can also be reprocessed and recycled (i.e., they are reprocessable), display good processability, and have economic advantages.4−7 Thermoplastic vulcanizates (TPVs) are an important category of the TPEs based on rubber and thermoplastics. They have been extensively investigated in the past few years because of some specific characteristics, such as special morphological structure, excellent physical and mechanical properties, and unique production process. Therefore, the preparation of conductive polymer composites via dynamic vulcanization is of current interest for the potential to © 2017 American Chemical Society

combine the ease of melt processability of thermoplastics with elastic properties of cross-linked rubbers. Using multiwalled carbon nanotubes (MWCNTs) to create electrically conductive thermoplastic elastomers has received much attention, due to the lower percolation threshold than those of conventional conductive fillers, such as carbon black.8−10 However, the MWCNTs tend to aggregate due to strong van der Waals forces between the nanotubes. Also, the conductivity depends on good dispersion and distribution together with the localization of the conductive filler in the polymer matrix. To improve the dispersion and to increase the electrical conductivity of the filled polymer, it is important to control the phase morphology and achieve a continuous conductive network of MWCNTs in the insulating polymer matrix. The localization of MWCNTs in a polymer blend also affects the microstructure and other related properties. Normally, there are three possible localizations of the filler in the blend: uniform localization across both phases of the blend, selective localization in one phase only, and selective Received: Revised: Accepted: Published: 3629

January 18, 2017 February 22, 2017 March 14, 2017 March 14, 2017 DOI: 10.1021/acs.iecr.7b00252 Ind. Eng. Chem. Res. 2017, 56, 3629−3639

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Industrial & Engineering Chemistry Research

rotor speed of 60 rpm at the initial temperature of 135 °C. Prior to blending, COPA pellets were dried in a hot air oven at 80 °C for 8 h to eliminate their moisture content. To investigate influences of the stage at which MWCNTs are incorporated on the properties of conductive ENR/COPA TPVs, two alternatives were tested, as shown in Table 1. The mixing procedures were as follows.

distribution at the interfaces between the phases.11 The migration of filler in a multiphase polymer blend tends to occur in the phase that has more affinity to the filler, affected by surface chemistries of polymer and filler: a thermodynamic effect. However, the preferential localization is also affected by kinetic effects, such as blending sequence, blending time, and shear strength.12 Moreover, the dispersion and distribution of MWCNTs and the electrical conductivity can be enhanced by adding IL.13−15 Many attempts to improve the filler dispersion in rubber composites by adding IL have been done with solution styrene butadiene rubber (S-SBR),16 butadiene rubber (BR),17 chloroprene rubber (CR),15,18,19 acrylonitrile−butadiene rubber (NBR),20,21 and carboxylated acrylonitrilebutadiene rubber (XNBR).22,23 Also, thermoplastic matrixes tested include poly(methyl methacrylate) (PMMA)24 and polypropylene (PP)25 on preparing composites with IL. However, no information is available on the application of IL in TPVs based on rubber and thermoplastic. Therefore, incorporating IL in ENR/COPA TPVs with conductive filler might, as a hypothesis, enhance these conductive elastic materials. In the current work, development of new conductive thermoplastic vulcanizates focused on optimizing the alternative blending stages of MWCNTs (i.e., ADV or BDV) and the loading level of ionic liquid, for the best properties of the ENR/COPA TPVs. Furthermore, the localization, dispersion, and distribution of MWCNTs in ENR/COPA TPVs are elucidated. The properties investigated are morphology, electrical conductivity, dielectric properties, and stress relaxation behavior.

Table 1. Mixing Sequences for Preparing Conductive ENR/ COPA TPVs in Two Alternative Ways

2. EXPERIMENTAL SECTION 2.1. Materials. The epoxidized natural rubber with 50 mol % epoxide (ENR-50) was manufactured by Muang Mai Guthrie Public Co., Ltd. (Phuket, Thailand). Copolyamide, COPA (Pebax 3533 SP 01), was produced by Arkema Co., Ltd. (Balan, France). It contains 36.3 wt % polyamide-12 (PA-12) hard segments and 60.1 wt % poly(tetramethylene ether) glycol (PTMO) soft segments, with 3.6 wt % adipic acid linkages. Both ENR-50 and COPA were used as blend components. The conductive filler, MWCNTs (NANOCYL NC7000), was manufactured by Nanocyl S.A. (Sambreville, Belgium). Its nanotubes have average length 1.5 μm, average diameter 9.5 nm, and specific surface area 250−300 m2/g. The ionic liquid (IL), 1-ethyl-3-methylimidazolium chloride, was manufactured by Sigma-Aldrich (Steinheim, Germany). It was used to enhance the electrical and other related properties of ENR/ COPA TPVs. Sulfur was used as a curing agent. It was supplied by Siam Chemical Co., Ltd. (Bangkok, Thailand). The cure accelerator, n-tert-butyl-2-benzothaiazole sulfonamide (TBBS), was manufactured by Flexys (Brussels, Belgium). The other compounding ingredients were cure activators, with stearic acid manufactured by Imperial Chemical Co., Ltd. (Pathumthani, Thailand) and zinc oxide (ZnO) produced by Metoxide Thailand Co., Ltd. (Pathumthani, Thailand). 2.2. Preparation of Conductive Thermoplastic Vulcanizates. Conductive thermoplastic vulcanizates based on dynamically cured ENR/COPA blends were prepared using the fixed blend ratio ENR/COPA = 50/50 wt %. This blend ratio was selected because it gives a simple blend with cocontinuous phases and fine-grained morphology.26,27 The blends were prepared in an internal mixer, Brabender Plasticorder (GmbH & Co. KG, Duisburg, Germany), with a

Method I. Addition of MWCNTs after dynamic vulcanization (ADV). In this method, the COPA was first incorporated in the mixing chamber and the rotors were engaged for 2 min. Then, ENR was incorporated into the mixer and mixed for 1 min. Various compounding ingredients were then incorporated according to the details in Table 2. That is, the activators (i.e., ZnO and steric acid), accelerator, and curing agent (i.e., sulfur and TBBS, respectively) were then sequentially added into the internal mixer until reaching a plateau of the mixing torque, at about 7 min of mixing. Then, half of the MWCNTs were added and the mixing was continued for 5 min. Finally, the other half of MWCNTs was Table 2. Compounding Formulations of Rubber Compounds for the Preparation of Conductive ENR/COPA TPVs ingredients

quantities (phr)a

ENR stearic acid ZnO TBBS sulfur MWCNTs ILb

100 2 5 1 2.5 5 0, 3, 5, 7, 10

a

phr = part per hundred rubber. bIL = 1-ethyl-3-methylimidazolium chloride (only for optimization of ionic liquid loading). 3630

DOI: 10.1021/acs.iecr.7b00252 Ind. Eng. Chem. Res. 2017, 56, 3629−3639

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the SEM specimens. In the first one, the specimens were cryogenically fractured in liquid nitrogen and then examined by SEM. This was used to visualize the MWCNT dispersion in the samples. The second technique involved solvent-etched cryogenically fractured surfaces. That is, the COPA phase was first cracked in liquid nitrogen to create new surfaces. Then the sample was extracted by hot dimethyl sulfoxide (DMSO) to remove the COPA phase so only vulcanized rubber particles remained at the surfaces. This was used to visualize the phase morphology of the dynamically cured ENR/COPA blends. 2.3.3. Stress Relaxation Behavior. Stress relaxation behavior of the ENR/COPA TPVs was investigated using a temperature scanning stress relaxation (TSSR) instrument (Brabender, Duisburg, Germany). The TSSR test method and the details of the instrument are described elsewhere.28,29 In this work, dumbbell-shaped specimens were first cut from the TPV samples according to ISO527 using die type 5A. Then, the samples were placed in the electrically heated test chamber, and consequently subjected to testing in two steps. In the first step, isothermal conditions were sustained at the initial strain of 50%, holding the temperature at 23 °C for 2 h. Thereafter, nonisothermal testing was done with constant approximately 2 K min−1 heating rate from 23 °C until the stress relaxation was completed or the sample ruptured.

eventually added into the mixing chamber and mixed for another 5 min, for a total mixing time of about 20 min. Method II. Addition of MWCNTs before dynamic vulcanization (BDV). This method was performed by incorporating COPA and ENR into the mixing chamber and mixing for 3 min. Then, half of the MWCNTs were added and the mixing was continued for 5 min. Thereafter, the other half of MWCNTs was added into the mixing chamber and the mixing was continued for another 5 min. The other compounding ingredients were then sequentially added as follows: ZnO and steric acid were added separately mixing for 1 min after each, followed by incorporating the accelerator and curing agents (i.e., sulfur and TBBS, respectively) and mixing for 6 min until reaching a plateau of the mixing torque, at approximately 20 min total mixing time. After mixing, the blends were fabricated into thin sheets by compression molding at 160 °C. Effects of IL loading on the conductive ENR/COPA TPVs with 5 phr MWCNTs were investigated with the BDV mixing sequence, using the chemical ingredients given in Table 2. That is, the loading of IL in the conductive ENR/COPA TPVs was one of 0, 3, 5, 7, or 10 phr. It is noted that the IL and MWCNTs were first mixed in a mortar before incorporating their mixture into the mixing chamber. 2.3. Testing and Characterization of TPVs. 2.3.1. Electrical Conductivity and Dielectric Properties. The dc conductivity (σdc) of ENR/COPA TPVs was measured by a two-probe method in a Faraday cage (Fertronik GmbH & Co. KG, Langenfeld, Germany) at room temperature. The dc conductivity (σdc) was calculated as reciprocal of the volume resistivity (ρ): σdc =

1 d = ρ R×S

3. RESULTS AND DISCUSSION 3.1. Optimization of Mixing Methods. 3.1.1. Morphological Properties. Morphological properties of conductive ENR/COPA TPVs that were prepared by ADV and BDV methods were analyzed by scanning electron microscopy (SEM). Figure 1 shows SEM images of ENR/COPA TPV

(1)

where R is the resistance (Ω), d is the thickness of the specimen between two planar electrodes, and S is the cross-sectional area perpendicular to the current. The ac conductivity (σac) and dielectric properties were also measured at room temperature over the frequency range from 0.1 kHz to 200 kHz using an LCR meter (LCR-800, GW Instek, New Taipei City, Taiwan). The samples were prepared to have approximately 16 mm diameter and 1.0−1.5 mm thickness. Both flat sample surfaces were then painted with a high-purity silver paint to form the electrodes. The σac was estimated from the relation: σac = ωε0ε′ tan δ

(2)

where ω is the angular frequency (equal to 2πf with frequency f), and ε0is the dielectric permittivity of free space (8.854 × 10−12 F/m). The dielectric permittivity (ε′) and dielectric loss (ε′′) were calculated based on the capacitance and dissipation factor (tan δ), using the following equations:

Cd Aε0

(3)

ε″ = ε′ × tan δ

(4)

ε′ =

Figure 1. SEM micrographs of conductive ENR/COPA TPVs, before and after extraction on surfaces. The samples were prepared by ADV (a, c) and by BDV mixing (b, d).

where C is the capacitance of the electrodes sandwiching the sample,d is sample thickness, and A is the area of each electrode. 2.3.2. Morphological Properties. Morphological properties of the ENR/COPA TPVs were characterized by scanning electron microscopy (SEM) (FEI-Quanta 400, FEI, Czech Republic). Two preparation techniques were used to prepare

samples prepared by ADV and BDV methods. Figure 1a,b shows SEM micrographs of the conductive ENR/COPA TPVs from both methods without extraction at the surfaces. The ENR/COPA TPVs did not show clear differences in the phase morphology between the two mixing methods, in terms of the dispersion of MWCNTs. They had smooth fracture surfaces with small white dots representing MWCNTs dispersed in the 3631


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Figure 2. Possible interaction of ENR and COPA phases.

blends. This indicates that the MWCNTs were randomly dispersed and presumably lacked interconnectivity in the conductive ENR/COPA TPVs. However, slightly more homogeneous distribution of MWCNTs was found in the TPV prepared by the BDV method. This might be because the MWCNTs had more time to disperse in the blend. In addition, the SEM micrographs of the extracted fracture surfaces of TPVs, in Figure 1c,d, show similar morphologies in the TPVs from both mixing methods. In the BDV method, the MWCNTs were added before dynamic vulcanization and, similar to ADV case sizes and shapes of the dispersed rubber, domains were observed. This is due to the localization of MWCNTs that were mostly dispersed in the COPA phase, while the small amount of MWCNTs in the ENR phase did not affect much its viscosity. Therefore, the choice between adding MWCNTs before or after dynamic vulcanization did not noticeably impact the morphological properties of conductive ENR/COPA TPVs. However, the addition of MWCNTs before dynamic vulcanization could also influence the diffusion of MWCNTs in the ENR domains, or at domain interfaces, as seen in the left corner of Figure 1d. This might be due to the compatibility of MWCNTs and ENR (i.e., a thermodynamic effect) that caused some MWCNT particles to reside in the ENR domains or in the interfacial regions. Furthermore, micron size domains of dispersed vulcanized ENR phase (from 0.5 to 5.0 μm) were found in the ENR/COPA TPVs with both ADV and BDV methods. The small-sized rubber domains might be due to strong interactions between ENR and COPA

via hydrogen bonding, with a possible reaction shown in Figure 2. These interactions reduced the interfacial tension and enhanced blend compatibility. This increased the shear and extensional viscosities, consequently causing the reduced size of vulcanized rubber domains.30 However, large rubber domains were also found, which might be due to coalescence of newly formed rubber domains during continued dynamic vulcanization. Therefore, it can be concluded that the alternative stages for mixing in MWCNTs did not much differ in effects on the final morphology of thermoplastic vulcanizates based on ENR/COPA blends. Regarding the localization of filler in the blends, surface chemistry of filler and polymer (thermodynamic effects) and viscosity of the polymer (kinetic effects) have been involved as explanatory factors.9,31,32 It is noted that the filler in a blend typically migrates into the phase with lower viscosity. Thus, the MWCNTs prefer to reside in the COPA phase, whose lower viscosity is observed from its lower mixing torque in Figure 3. That is, the mixing torque curves indicate the viscosities of pure ENR and COPA. In Figure 3, it is seen that the torque readily decreased with mixing time reaching its equilibrium after 6 min. However, the torque of pure COPA started to plateau after 3 min due to the melting of hard and soft COPA phases. Therefore, the MWCNTs preferentially chose the COPA phase in TPVs made by ADV or BDV mixing. In addition, the MWCNTs could localize at the interfaces of ENR and COPA. It is noted that the localization of MWCNTs at interfaces can be one of three types.32,33 First, MWCNTs may be parallel to 3632


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MWCNTs cannot easily diffuse into the solid shells of the vulcanized rubber domains in the ENR/COPA TPVs with ADV mixing. Also, the MWCNTs could not be observed in Figure 1c due to the extraction of the COPA phase. On the other hand, with BDV mixing, the MWCNTs were incorporated in unvulcanized cocontinuous materials. In Figure 1d, clear observation was found of the MWCNTs at the interfacial areas and might be some parts located inside the ENR domains. Despite the high viscosity of the ENR phase, the polar functional groups in ENR and MWCNTs might interact so nanotubes end up in the ENR phase; the chemical interactions of filler and polymer can be considered thermodynamic effects. Therefore, after the cocontinuous phase structure with some MWCNTs residing in the micron sized vulcanized rubber domains which dispersed in the COPA matrix, MWCNTs still remained in the ENR domains. The ENR/COPA TPVs prepared by BDV mixing contain the majority of MWCNTs in the COPA phase, while some filler can reside in the ENR domains. 3.1.2. Electrical Conductivity and Dielectric Properties. The variation of ac conductivity at room temperature as a function of frequency for conductive ENR/COPA TPVs is shown in Figure 5. It is clearly seen that at low frequency, the TPV prepared with BDV mixing exhibited significantly higher ac conductivity than the one prepared with the ADV method. This is because in BDV mixing, the MWCNTs were first mixed with the unvulcanized cocontinuous ENR/COPA blend, and only thereafter, dynamic vulcanization took place. This removed MWCNTs from the vulcanizing rubber phase, so the majority of MWCNTs were located in the continuous COPA phase while some still resided in the ENR domains. Also the prolonged mixing time gave MWCNTs improved dispersion or distribution in the blend. Thus, the electrical conductivity in the ENR/COPA TPV with BDV mixing was improved, due to good contact between MWCNTs in the continuous COPA phase or at the interfaces of ENR domains and COPA matrix, together with the MWCNTs in the ENR domains. Nevertheless, the conductivity of both types of TPVs (i.e., with BDV or ADV mixing) was low at about 10−9 to 10−10 S/cm at 1 kHz. This might be due to the dispersed ENR domains in the COPA

Figure 3. Mixing torque of pure ENR and pure COPA at 60 rpm rotor speed and 135 °C.

the interface. Second, MWCNTs may bridge the two phases across the interface. Finally, agglomerates may form at interfaces due to the high aspect ratio this filler. Thus, agglomeration at the interfaces is likely to take place in a high viscosity system, due to the nature of MWCNTs with high aspect ratio and tendency to agglomerate. A schematic illustration of MWCNTs localization in the conductive ENR/ COPA TPVs, prepared by ADV or BDV mixing, is shown in Figure 4. It is seen that the morphology of ENR/COPA = 50/ 50 wt % in both cases had a cocontinuous structure before addition of the curing agent.26 Furthermore, the preparation of ENR/COPA thermoplastic vulcanizates by adding the rubber curatives, the transformation from cocontinuous phase structure to the dispersed phase morphology was found.27 In the ADV, the MWCNTs were added after dynamic vulcanization, the cocontinuous phase morphology was transformed so that micron-size ENR domains were dispersed in the COPA matrix before incorporation of the MWCNTs. Therefore, the MWCNTs might reside in the lower viscosity continuous COPA phase because of kinetic effect. Furthermore, the

Figure 4. Schematic illustration of MWCNT filler localization in the conductive TPVs with incorporation of MWCNTs at two alternative stages. 3633


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Figure 6. Dielectric permittivity (a) and dielectric loss (b) as functions of frequency for the conductive TPVs based on dynamically cured ENR/COPA blends.

available for dielectric relaxation or dipole orientation during each cycle.39 3.1.3. Stress Relaxation Behavior. Stress relaxation behavior of conductive ENR/COPA TPVs prepared with two alternative mixing methods, ADV and BDV, was characterized by TSSR in isothermal and nonisothermal conditions. The isothermal relaxation at 23 °C of normalized stress with time is shown in Figure 7. It can be seen that both types of TPVs displayed

Figure 5. The ac conductivity as a function of frequency for conductive TPVs based on dynamically cured ENR/COPA blends.

matrix being electrical insulators. Furthermore, both types of ENR/COPA TPVs showed frequency dependent conductivity. It has been well established that the conductivity of composites typically exhibits frequency dependent behavior (i.e., the conductivity increases with increasing frequency) at low filler loadings below the percolation threshold. 34−36 This is consistent with the insulating behavior, but the conductivity is independent of the frequency around the percolation threshold, when conductive filler paths are forming a percolating network. Therefore, it can be concluded that MWCNTs at 5 phr, when added via ADV or BDV mixing, did not approach the percolation threshold. The increase of ac conductivity (real part, σ′) with frequency in isothermal conditions can be expressed in terms of contributions from dc and ac conductivity, as follows:37,38 σ ′(ω) = σ(0) + σac(ω) = σdc + Aωs

(5)

where ω is the angular frequency (2πf), σdc is limiting conductivity as ω → 0, A is a temperature-dependent constant, and s is an exponent dependent on both frequency and temperature. The variation of the dielectric properties dielectric permittivity (ε′) and dielectric loss (ε′′) as functions of frequency, for the dynamically cured ENR/COPA blends at room temperature, is shown in parts a and b of Figure 6, respectively. It is seen that at low frequencies the dielectric permittivity and the dielectric loss of TPVs with BDV mixing was higher than with ADV mixing, but there was no significant difference at higher frequencies. The higher dielectric permittivity and dielectric loss in the ENR/COPA TPV prepared by BDV can be explained by higher polarization due to interfacial polarization or Maxwell−Wagner−Sillars (MWS) polarization, and charge-dipoles due to good dispersion of MWCNTs in the COPA matrix, in interfacial areas, and in the vulcanized ENR domains. Furthermore, it is also seen that the dielectric permittivity and dielectric loss were frequency dependent in both cases and slightly decreased with frequency tending to a constant at high frequencies. The decrease of ε′ with frequency might be due to lesser time

Figure 7. Time dependence of normalized stress in the conductive ENR/COPA TPVs prepared with two alternative mixing sequences (ADV and BDV). The inset displays the negatives of slopes of the normalized stress curves.

relaxation of stress with time. The relaxation modulus (E) is normally a function of time and temperature, E = E (t, T). At isothermal conditions, the stress σ(t) is given by28,40 σ(t ) = E iso(t ) × e0

(6)

where Eiso(t) is the modulus that depends on time under isothermal conditions, and e0 is the applied strain. The time dependent modulus can expressed as a composition of first-order relaxations by reference to the Laplace transform. 3634


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E iso(t ) =

∫−∞

H(τ ) × e−t / τ d ln τ + E∞

Furthermore, the increased force corresponds to the amount of cross-links or, in this case, relates to the polymer−filler interactions of MWCNTs and COPA and also the interactions of ENR domains with COPA. It is also seen that the ENR/ COPA TPVs prepared with ADV or BDV mixing had decreasing modulus in the temperature range 50−70 °C. This is attributed to the breakdown of filler−filler interactions as well as to the physical desorption of polymer−filler, which is typical at comparatively low temperatures while the chemical bonding is affected at higher temperatures.28 Thus, the ENR/COPA TPV prepared with BDV method exhibited slightly higher relaxation moduli than when prepared with the ADV method because of stronger polymer−filler interactions and better dispersion of MWCNTs. Nevertheless, the stress in both ENR/ COPA TPV materials approached zero at temperatures above 100 °C. This is due to the desorption of COPA and MWCNTs as well as the melting of COPA together with the softening of ENR domains. Figure 8b shows the relaxation spectra of the ENR/COPA TPVs prepared by ADV and BDV mixing methods. Two main peaks are observed for both samples at similar temperatures of about 70 and 120 °C. Generally, the positions of relaxation peaks correspond to the strength of relevant interactions. That is, the physical interactions of polymer and filler could be broken down at a low temperature, but the stronger chemical interactions require higher temperatures.28 Therefore, the lower peak corresponds to the amount of polymer chains desorbed from the MWCNT surfaces. As expected, the higher peak is found for the ENR/COPA TPV prepared with the BDV method. This is attributed to the better dispersion and distribution giving larger available surface area of MWCNTs for rubber−filler interactions. Also, the second peak at the higher temperature might be assigned to chemically induced relaxation by the melting of COPA phase together with the degradation of rubber phase during the TSSR test. In Figure 8b, it is clear that the ENR/COPA TPV prepared with the BDV method had stronger relaxation. This is possibly due to more homogeneous dispersion of MWCNTs with greater available surface area in the COPA matrix and in the ENR domains. Therefore, the ENR/COPA TPV prepared with the BDV method exhibited more chemical interactions than the one prepared with the ADV method. 3.2. Optimization of Ionic Liquid Loading. 3.2.1. Electrical Conductivity and Dielectric Properties. The dependence of dc conductivity at room temperature of conductive ENR/ COPA TPVs on the IL loading is seen in Figure 9. The incorporation of IL enhanced the conductivity of the blends, as expected. That is, the conductivity increased from about 3 × 10−6 S m−1 (without IL) to about 3 × 10−5 with IL. This increase in electrical conductivity might be due to the high conductivity of the ionic liquid. However, the limited increase in conductivity (about 1 order of magnitude) on adding IL into the blend could be due to the insulating rubber domains finely dispersed in the COPA matrix interrupting the conductive pathways. Even the TPV filled MWCNTs mostly dispersed in the COPA matrix and at the interfaces with some amount in the ENR domains (i.e., the TPV prepared with BDV mixing) did not reach the percolation threshold. Moreover, the phase separation of IL also influenced the electrical conductivity of ENR/COPA TPVs. Therefore, the conductivity only slightly improved with IL in the blends. Figure 10 shows the dielectric permittivity of conductive ENR/COPA TPVs versus IL loading and ac frequency. It can be seen that the IL slightly increased permittivity consistently.

(7)

According to Alfrey’s law, the relaxation spectrum H(τ) at τ = t (where τ is the relaxation time constant) is obtained by differentiating Eiso(t) with respect to ln τ:28 ⎛ d E (t ) ⎞ ⎛ d E (t ) ⎞ H(τ ) = −⎜ iso ⎟ = −t × ⎜ iso ⎟ ⎝ d ln t ⎠t = τ ⎝ dt ⎠ t = τ

(8)

The stress relaxation with time can be explained as the relaxation of polymer chains by sliding and moving past each other. Also, the physical desorption of MWCNT nanotubes and COPA or of vulcanized rubber domains and COPA contributes to relaxation. This reduces the modulus with testing time. In addition, the negative slope of the normalized stress relaxation curve, for each type of TPVs, is displayed in the inset of Figure 7. It is seen that the negative slope of the ENR/COPA TPV based on the ADV method showed larger negative slopes than the one based on the BDV method. With ADV mixing, the MWCNTs were only located in the COPA matrix, with less time to disperse and distribute causing more agglomeration. Thus, the ENR/COPA TPV based on the ADV method had larger stress relaxation with larger negative slopes during stretching, due to the breakdown of filler−filler networks, sub networks, or some MWCNT agglomerates. This matches well the observed electrical conductivity (Figure 5) and dielectric properties (Figure 6), which were low due to MWCNT agglomeration. The significant initial decrease in stress in Figure 7 was further characterized by peaks in the nonisothermal relaxation, as shown in Figure 8a. The modulus in

Figure 8. Relaxation modulus (a) and relaxation spectrum (b) across the tested temperature range for the conductive TPVs based on dynamically cured ENR/COPA blends.

the nonisothermal test (Enoniso) at a constant heating rate (β) can be displayed as a relaxation modulus vs temperature curve. In addition, the relaxation spectrum H(T) was calculated by differentiating Enoniso(T) with respect to temperature T:29 ⎛ dE ⎞ H(T ) = −ΔT × ⎜ noniso ⎟ ⎝ dΔT ⎠ β=ΔT / t = const

(9)

Figure 8a shows the relaxation modulus versus temperature for the ENR/COPA TPVs prepared by the two mixing methods: ADV and BDV. It is seen that the relaxation modulus of both types of TPVs slightly increased with temperatures in 23−50 °C. This might be due to the entropy effect.29,40 3635


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Figure 11 shows the dielectric loss (ε′′) of conductive ENR/ COPA TPVs in relation to IL loading and ac frequency. It is seen that the IL in TPVs did not significantly affect the loss factor within the range of loadings tested. That is, the loss factor had a practically constant value at high frequencies. This might be due to the small effects of IL on conductivity. However, at low frequencies the behavior of ε′′ was similar to that of ε′. That is, ε′′ increased with IL loading. This is related to the total contributions from three factors, namely, conduction loss, interfacial charge polarization, and dipole orientation (Debye loss factor), described by the following equation:43−45 ε″ = εdc″ + εMW ″ + εD″

(10)

where εdc″, εMW″, and εD″ are the loss factors of dc conductance, interfacial charge polarization, and dipole orientation, respectively. 3.2.2. Stress Relaxation Behavior. Figure 12 shows the influences of IL loading on isothermal relaxation at 23 °C observed by TSSR measurements, for the conductive ENR/ COPA TPVs. It is seen that the normalized stress (Figure 12a) decreased with time in all cases, due to relaxation of COPA and ENR molecules by sliding and moving past each other. Moreover, the stress diminished with IL loading owing to the plasticizing effects of IL, which decreased the polymer viscosity. However, the ENR/COPA TPV with 10 phr of IL had higher modulus than the one with 7 phr. This is also seen in the negative slope in Figure 12b, after slight initial increase with IL loading to the maximum at 7 phr loading. This behavior could be explained by the morphology changes in the SEM micrographs of Figure 13. It can be clearly seen that the IL dispersed in the blends (dark blue color in the color contour images) with the size of the IL droplets becoming larger with IL loading. However, at 10 phr loading of IL, larger clusters of it were emerging. This affected the physical desorption between

Figure 9. The dc conductivity of the conductive ENR/COPA TPVs with various IL contents.

The permittivity is a measure of the total polarization in a material, arising from contributions of electronic, atomic, molecular, and ionic mechanisms.41 Incorporation of IL in these heterogeneous blends increased polarization in the materials. The permittivity includes also interfacial polarization or Maxwell−Wagner−Sillars (MWS) polarization19 at the interfaces of MWCNTs, IL, COPA, and ENR. The ionic motions also influenced permittivity. That is, the dielectric permittivity tended to increase with IL loading. Moreover, the dielectric permittivity improved as frequency was reduced. This might be due to dielectric relaxation and displacement polarization effects that involve dipole orientation, which requires sufficient time follow the cycling of the electric field.42

Figure 10. Relationship of dielectric permittivity to IL loading and ac frequency for the conductive TPVs based on dynamically cured ENR/COPA blends. 3636


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Figure 11. Relationship of dielectric loss to IL loading and ac frequency for the conductive TPVs based on dynamically cured ENR/COPA blends.

Figure 14 shows the relaxation modulus and relaxation spectrum across the tested temperature range for the ENR/ COPA TPVs with various IL loadings. In Figure 14a, the initial modulus at the starting temperature had a decreasing trend with IL loading due to the plasticizing nature of IL. However, adding 10 phr of IL gave higher initial modulus than with 7 phr of IL. This could be explained by the poor dispersion of IL forming larger clusters. In addition, at temperatures above 70 °C, the relaxation moduli of all samples decreased and slowly approached zero. This behavior might be caused by the desorption of MWCNTs from COPA and/or ENR domains. Also the melting of COPA in this temperature range abruptly reduced the relaxation modulus of the TPV samples. Figure 14b shows the relaxation spectra across tested temperatures for the ENR/COPA TPVs. It can be seen that there are two relaxation peaks at a lower and a higher temperature. These peaks tended to split more clearly as the IL loading increased. This might be due to phase separation caused by the IL. It is anticipated that the first peak at lower temperature indicates physical interactions and the second peak indicates stronger chemical interactions.28 Therefore, the peak at about 70 °C corresponded to breakdown of filler−filler

Figure 12. Normalized stress (a) and negative of the slope of the relaxation curve (b) for the conductive TPVs based on dynamically cured ENR/COPA blends with various IL loadings.

IL and MWCNTs, COPA, and ENR and caused the eventual negative slope in Figure 12b.

Figure 13. SEM micrographs (above) also with false coloring (below) of the conductive TPVs based on dynamically cured ENR/COPA blends with various IL loadings. 3637


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other hand, with ADV mixing the MWCNTs in the TPV were localized in the COPA phase and at the phase interfaces. The optimal IL loading in the ENR/COPA TPVs was also studied. It was found that increasing the IL loading in the blends enhanced electrical conductivity and dielectric properties (i.e., the dielectric permittivity and dielectric loss). However, the stress relaxation was negatively impacted by IL loading due to its plasticizing effects and microphase separation in the blends. In summary, conductive thermoplastic vulcanizates based on dynamically cured ENR/COPA blends with BDV mixing of carbon nanotube filler, and with addition of IL, gave superior electrical conductivity and dielectric properties. However, the IL reduced stress relaxation of the TPVs, especially at excessive contents beyond 7 phr.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Charoen Nakason: 0000-0003-1631-9369 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge the financial support for an overseas thesis research study, in the form of a scholarship from the graduate school of Prince of Songkla University, Thailand. The Higher Education Research Promotion and National Research University Project of Thailand and Prince of Songkla University, Contract No. SAT540523M is also acknowledged. The authors gratefully acknowledge the facility support from the Faculty of Engineering and Computer Science, University of Applied Sciences Osnabrück, Germany. In addition, we would like to thank Assoc. Prof. Dr. Seppo Karrila for assistance with manuscript preparation.

Figure 14. Relaxation modulus (a), and relaxation spectrum (b) across the tested temperature range for the conductive TPVs based on dynamically cured ENR/COPA blends with various IL loadings.

interactions and desorption of COPA from ENR. However, the intensity of this peak diminished with IL content due to decreased filler−filler interactions of the MWCNTs because the IL acted as a lubricant. On the other hand, the higher temperature peak at about 120 °C corresponds to the relaxation by melting of COPA phase together with softening and eventually degrading of the rubber molecule networks during the TSSR test. Also this second relaxation peak height significantly decreased with IL loading. This could be explained by the plasticizing effects that increased flexibility of the molecular chains. Nevertheless, the addition of IL at 10 phr gave higher relaxation peak height but lower rupture temperature than 7 phr. This might be due to an excess of IL causing phase separation at too high loadings, which induced weak points in the samples and reduced the relaxation.

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REFERENCES

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Optimization of Electrical Conductivity, Dielectric Properties, and

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