Investigation of vibration damping properties of electroactive polyanthracene/silicone oil dispersions

Abstract

Electrorheological (ER) fluids generate mechanical responses to applied electric field strength via changing their rheological properties from liquid to solid and vice-versa reversibly. As a result of this, ER fluids can be used in the industrial vibration damping systems. In order to increase applicability of ER fluids, it is necessary to understand electric field induced polarization and ER mechanism of different materials. Therefore, the aim of this study is to illuminate ER and vibration damping properties of polyanthracene (PAT), which is a new material for ER studies. PAT was synthesized from anthracene and characterized by several techniques namely: ATR-FTIR spectroscopy, particle size, SEM image, four-point probe conductivity, and magnetic susceptibility measurements. A series of PAT/silicone oil (SO) dispersions having various concentrations were prepared and subjected to dielectric and ER tests. Then, the colloidal stabilities of 20% PAT/SO and 20% PAT/SO/TritonX systems were determined. Dynamic viscoelastic data obtained by the oscillation tests showed that viscous behavior was dominant under zero electric field, whereas elastic behavior was prevailing under external electric field strength and highlighting the vibration damping characteristics of PAT/SO dispersion. In the creep-recovery measurements, the highest %recovery was recorded to be 62% indicating potential industrial use of PAT/SO dispersion.

Keywords

Vibrating damping dielectric electrorheology polyanthracene

1. Introduction

Electrorheological (ER) fluids are colloidal dispersions of particles, which can polarize under the external electric field, in a non-conducting base liquid. After the electric field is applied, the polarized particles attract to each other to form a column-like structure along the electric field direction within the milliseconds. The electric field induced process leads to reversible and rapid change from liquid-like to a solid-like behavior. Thus, there is a drastic change in flow behavior of ER fluid such as viscosity ( $η$ ) and yield stress ( $τ_{y}$ ) under an applied electric field. Thanks to this phase transformation, ER fluids are useful materials to create mechanical responses to electrical stimuli for active devices such as vibration dampers, shock absorbers, or valves (Dong et al., 2019). Vibration damping system is based on placing a soft isolator between the host structure and the equipment. The isolation system reduces the effect of vibrational force on the structure during the normal operation. Using ER fluids as a soft isolator provides a controllable vibration damping ability to the device due to the capability of being activated by the external electric field (Brennan et al., 1995; Kolekar et al., 2019).

It is important to prepare dry-based ER fluids for industrial applications to prevent evaporation, electrical breakdown, or corrosion problems arising in wet-based ones (Dong et al., 2019). The hydrophobic, conjugated, and semiconductive polymers can polarize under electric field stimulus via hoping of $π$ -electrons and so they can show dry-based ER activity without any polar dopants. Using semiconducting polymers as ER materials have also other advantages such as thermal stability, low density, low current density, low cost, and easy processing. Therefore, semiconducting polymers such as polyaniline, polythiophene, polypyrrole, and their derivatives have recently attracted considerable attention for the development of high performance ER fluids (Liu and Choi, 2012). However, these polymers are not fully water free because the polar pendant group in their structure absorbs the trace amount of water. The fully conjugated and nonpolar polymers such as polyphenylene is truly water free with no affinity to the polar compounds (Krztoń-Maziopa et al., 2005).

To the best our knowledge, the previous ER studies on the fully conjugated and nonpolar polymers were only based on polyphenylene. It is a simple linear polymer meaning the aromatic rings are bonded through a single bond. The fully conjugated, nonpolar, and stepladder polymer, which is defined as a group of aromatic rings connected by a single bond, may be more advantageous for the ER studies. The step ladder structure leads to high degree of conjugation, possess a planar π-electron system ensuring the optimum electron delocalization and allows the structure to do rotational chain motion (Schlüter, 1991). This can provide the formation of strong electric dipoles during the application of electric field stimulus. The structure also shows remarkable properties such as high thermal, mechanical, and chemical stability. Therefore, the objective of this study is to understand the electroactive flow behavior, vibration damping property, and creep-recovery characteristic of fully conjugated, nonpolar and stepladder polymer, polyanthracene (PAT), based ER fluid. PAT is very attractive polymer for semiconductive and optoelectronic devices due to its highly polyaromatic structure (Yoshizawa and Klosterman, 2014). To the best of our knowledge, the ER activity and electro active vibration damping properties of PAT not hitherto been reported in the literature. The previously reported ER active polymers similar to PAT were the radical quinone derivatives of polyacene, which is named as poly(acene quinone) radical (PAQR). PAQR is a type of ladder polymer with polar groups such as carbonyl and hydroxyl. The reported studies have shown that PAQRs have high dielectric constant ( $ε^{'}$ = 3–5) and ER activity with high yield stress of $τ_{y} = ~$ 200 Pa at a shear rate ( $\overset{\cdot}{γ}$ ) of 1 s⁻¹ and electric field strength (E) of 3 kV/mm conditions (Choi et al., 1997; Wang et al., 2016). The reported results on PAQR can support the hypothesis of this research that PAT can be a good candidate for a dry-based ER active smart material having vibration damping capability and long term industrial applicability.

2. Experimental section

2.1. Materials

Anthracene and FeCl₃ were provided from Merck and used as monomer and catalyst, respectively. Methanol (>99.9%), ethanol (96%–97%), and HCl (36%) were obtained from Merck. Silicon oil (SO) with $ρ$ = 0.97g/mL, $η$ = 1.0 Pa·s and $ε'$ = 2.76 and TritonX-100 (abbreviated as TritonX for ease) were supplied from Aldrich and Fluka, respectively.

2.2. Synthesis of PAT

PAT was synthesized via a modified procedure of oxidative polymerization (Desai et al., 2014; Tanemura et al., 2013). The mass ratio of monomer:oxidative catalyst was arranged to be 1:2, the reaction was performed in nitrobenzene under N₂(g) atmosphere at 150°C and run for 24 h. The polymerization was terminated by adding methanol to the reactor and the crude product of PAT was recovered. The unreacted monomer, Fe²⁺ and Fe³⁺ ions were removed from the crude product by washing with ethanol/HCl(aq) solution and approved with negative potassium thiocyanide (KSCN) test result, which was disappearance of red color of Fe(SCN)₃ (Unal et al., 2014). Eventually, purified PAT homopolymer was obtained as the final product, dried in a vacuum oven at 120°C for 24 h, cryogenically grinded by using a mill (Retsch, MM400, Germany) to obtain fine-grade particles and then stored in a desiccator for further use.

2.3. Characterizations of PAT

The synthesis of PAT from anthracene monomer was detected by ATR-FTIR (Thermo Scientific Nicolet IS5, USA) spectrometer to reveal the chemical structure. The hydrodynamic diameter of PAT was determined in aqueous dispersion containing 10⁻³ M NaCl(aq) solution at pH 7 and 25°C via dynamic light scattering (DLS) method (Malvern Zetasizer, Nano ZS, England). The instrument is equipped with a 4-mW He–Ne laser operating at λ = 633 nm and a noninvasive backscatter system (NIBSTM) which measures the light scattered at a backscatter angle of 173°. The morphology and dimensions of synthesized PAT particles were investigated by scanning electron microscope (SEM, FEI Quanta 400F Field Emission, Netherland) operating with an accelerating voltage of 30 kV. The powdered PAT was compressed, and the pellets were prepared with certain dimensions (height = 0.2 mm, diameter = 13 mm). Then, the conductivity of PAT was measured by four-probe technique at room temperature (Entek Electronic, FPP 470-A, Turkey). Additionally, the density of PAT was calculated from the basic mass/volume equation by using experimental results obtained from compressed pellets with known dimensions and mass. The magnetic susceptibility measurement of PAT was carried out by using Sherwood Scientific, MKI model Gouy scale at room temperature. The powdered PAT was placed in a pyrex tube and the magnetic susceptibility was determined. The experimental readings were turned to the magnetic susceptibility per gram by using equation (1) and the conductivity mechanism of PAT was decided to be whether polaron or bipolaron.

2.4. Dielectric properties of PAT/SO dispersions

SO was vacuum dried in an oven for 24 h at 110°C to remove any moisture present. A series of PAT/SO dispersions having various mass (g)/volume (mL) percentages (5, 10, 15, 20, and 25 m/V%) were prepared. The dispersions were homogenized by using an ultrasonicator (Sonics, Vibracell, USA) before all the measurements carried out. Dielectric analyses of dispersions were carried out by using Agilent E4980A Precision Impedance analyzer/LCR meter (USA) equipped with a liquid test fixture (16452A) at room temperature. The analysis was performed at 1.0 V constant AC voltage in the frequency range of 20 Hz–2 MHz. The dielectric properties were determined from the Cole-Cole equation fitted curves.

2.5. ER activity of PAT/SO dispersions

ER activities of PAT/SO dispersions were investigated with parallel plate torque electrorheometer (Thermo-Haake RS600, Germany) at 25°C under various electric field strengths (E = 0–4 kV/mm with 0.5 kV/mm increments) generated by a DC electric field generator (Fug Electronics, HCL 14, Germany). The gap between the parallel plates was 1.0 mm and the diameters of the upper and lower plates were 35 mm. Pre-mixing ( $\overset{\cdot}{γ}$ = 30 s⁻¹ for 30 s) was applied to all the samples before each measurement. Firstly, ER responses of dispersions at various electric field strengths with time were examined under constant shear rate ( $\overset{\cdot}{γ}$ = 1 s⁻¹). Secondly, the flow measurements were performed with the rate controlled (CR) mode in $\overset{\cdot}{γ}$ = 0.1–200 s⁻¹ range under varying electric field stimuli. To determine the yield stress values, the flow curves were fitted to Cho-Choi-John (CCJ) model.

After dielectric and ER studies, the optimum dispersion concentration was determined to be 20 m/V% and this dispersion is designated as 20% PAT/SO for the rest of the study.

2.6. Colloidal stability of PAT/SO dispersion

The colloidal stability of the optimized dispersion, 20% PAT/SO, was determined in a graduated glass tube at room temperature. During the measurements, the height of phase separation between the particle-rich phase and oil-rich phase was recorded as a function of time. The heights were measured by using digital caliper for 30 days. The colloidal stability ratio was calculated by the height percentage of particle rich phase relative to total height of the dispersion. The further colloidal stability test was performed for 20% PAT/SO dispersion by adding 1 m/V% TritonX non-ionic surfactant. The dispersion was defined as 20% PAT/SO/TritonX system.

2.7. Electroactive vibration damping properties of PAT/SO dispersion

Electroactive vibration damping properties of 20% PAT/SO dispersion were investigated via dynamic oscillation tests and creep-recovery measurements under various electric field strengths (E = 0–4 kV/mm with 1.0 kV/mm increments). Before oscillation measurements, linear viscoelastic region (LVR) was determined at $f$ = 1 Hz. In LVR range, frequency (1–100 Hz) and time (0–200 s) dependent dynamic viscoelastic properties that is viscous ( $G ″$ ) and elastic ( $G'$ ) moduli were illuminated by applying a sinusoidal shear deformation to the sample.

The creep-recovery behavior was also analyzed by applying different shear stresses depending on the applied electric field strengths to 20% PAT/SO dispersion for 100 s. The applied shear stresses were selected as $τ$ = 1, 2, 10, 15, and 20 Pa considering the LVR under E = 0, 1, 2, 3, and 4 kV/mm, respectively. Then, the applied stresses were removed and the changes in the strains were measured for 200 s. According to the change in the strains, the percentage recoveries were determined.

3. Results and discussion

3.1. Characterization of PAT

PAT was synthesized from anthracene via oxidative polymerization (Figure 1). The polymerization mechanism proceeds from carbon atoms at 9th and 10th positions of the monomer. This is proven by comparing ATR-FTIR spectra of anthracene and PAT (Figure 2). The peak at 885 cm⁻¹ belongs to the stretching vibration of C−H groups at 9th and 10th positions and the intensity of the peak decreases for PAT because of the formation of new bonds (Desai et al., 2014). It is also found that the peaks indicating neighboring C−H groups shift from 720 to 692 cm⁻¹ and from 957 to 931 cm⁻¹ due to the polymerization reaction. For both PAT and anthracene, aromatic C−H and C−C stretching peaks are observed in 3050, 1620 and 1535 cm⁻¹ regions, respectively.

Figure 1.

Polymerization of anthracene to PAT.

Figure 2.

ATR-FTIR spectra of anthracene and PAT.

The average hydrodynamic diameter of PAT particles was determined in 10⁻³ M NaCl solution at pH 7 by DLS. As reflected from Figure 3, the ground-milled PAT particles have two different particle size distributions. The main distribution was having average hydrodynamic diameter of 600 ± 100 nm corresponding to the 94% of the dispersed particles and the rest was having 110 ± 20 nm corresponding to the remaining 6%. Desai and coworkers reported similar size distribution for the PAT particles synthesized with an identical method (∼500 nm) (Desai et al., 2014, 2017). The nano scale size can improve the colloidal stability of PAT particles when dispersed in SO. Additionally, the presence of smaller particles can be advantageous because they can penetrate between the larger particles, provide maximum packing efficiency, and help to form stronger chain-like structures under applied electric field.

Figure 3.

The hydrodynamic size distribution graph of PAT in 10⁻³ M NaCl at pH 7 and 25°C.

The morphology of synthesized PAT was observed by SEM technique (Figure 4). It was observed that PAT particles show a varying shape and also differing distribution of sizes ranging from submicron to microns by showing no agglomeration. Although PAT particles were cryogenically ground milled it was not possible to obtain a monodisperse distribution but the agglomeration was prevented in contrast to the previously reported study on PAT particles (Desai et al., 2014). On the other hand, this kind of closest packing, filling of gaps between the big particles by small ones, is preferred for ER dispersions.

Figure 4.

The SEM image of PAT particles.

The conductivity ( $σ$ ) is an important parameter for understanding whether ER fluids can respond to electric field. The conductivity of dispersed particles for ER dispersions are preferred to be in the range of 10⁻¹²–10⁻⁴ S/cm (Dong et al., 2021; Hao, 2002). The conductivity of PAT was measured as (1.2 ± 0.2) × 10⁻⁴ S/cm indicating that it is a suitable semi-conducting material for ER applications. The conductivity of pure polyphenylene was reported to be very low (<1.0 × 10⁻¹³ S/cm) and it was reported to increase to 1.2 × 10⁻⁶ S/cm by doping with 1.1 weight% FeCl₃ (Płocharski et al., 1997). Additionally, the conductivity was reported to be 1.0 × 10⁻⁵ S/cm for PAQR (Desai et al., 2017). The conductivity of PAT in this study is higher than these two reported polymers and this indicates that PAT can respond to the electric field stimulus much better.

The conductivity mechanism of PAT can be determined whether polaron or bipolaron by the help of magnetic susceptibility measurements. The polaron-bipolaron model is frequently used to explain the conductivity and electrochemical properties of conjugated semiconductor polymers (Santos et al., 2007). Polarons (radical cations) and bipolarons (dications) are charged quasiparticles present in the polymer backbone structure. Polarons are formed by the coupling of excess charge carriers, which are holes and electrons, in polarizable structures owing to the ionic vibration and the dielectric polarization (Franchini et al., 2021). Thus, polaron can be identified as a charge carrier with self-induced deformation. On the other hand, bipolarons are formed when the energy for the combination of two polarons are large enough to overcome the Coulombic forces (Fobasso et al., 2020; Volkov et al., 2017). The important difference between polaron and bipolaron is that polarons carry a magnetic moment while bipolarons are spinless, and they can be distinguished by means of the magnetic susceptibility. The positive and negative values of magnetic susceptibility indicate paramagnetic (polaron) and diamagnetic (bipolaron) character of the conduction mechanism. The magnetic susceptibility per gram was calculated via following equation:

X_{g} = \frac{C_{s t d} l (R - R_{0})}{m 10^{9}}

(1)

where $X_{g}$ is gram magnetic susceptibility, $C_{std}$ is scale calibration constant, l is the sample length in the tube, m is mass of the sample, R is the experimental reading for sample in the tube, and $R_{0}$ is the experimental reading for the empty tube.

According to this equation, the magnetic susceptibility of PAT was calculated to be 6.6 × 10⁻⁶ cm³/g. The positive value indicates that PAT is paramagnetic and the conduction occurs through polarons. The estimated conduction mechanism of PAT is represented in Figure 5. This shows that when the voltage or another electrical stimulus is applied to PAT particles, it causes the distortion in the structure and the formation of polarons. The movement of these polarons are responsible for the conduction. Compared to an electron in undistorted structure, the mobility of polarons are lower and the mobile mass are higher due to the interaction between the charge carrier and the self-induced deformation (Bombile et al., 2018). Thus, the conductivity of polarons is low. For PAT based ER fluids, the moderate conductivity is an advantage.

Figure 5.

The polaron structure of PAT.

3.2. Dielectric properties of PAT/SO dispersions

The polarizability and the polarization time of dispersed particles are important properties for ER performance. These properties are related to the dielectric parameters. Firstly, the polarizability is related to the dielectric constant ( $ε^{'}$ ). The large difference between $ε^{'}$ at low and high frequencies ( $Δ ε^{'} = ε_{0}^{'} - ε_{\infty}^{'}$ ) indicates the strong polarization. Secondly, the polarization time is related to the relaxation time ( $λ$ ) which is found from the peak of dielectric loss factor ( $ε^{″}$ ) spectrum. The short polarization time is necessary for fast response of the particles to the applied electric field.

The dielectric analysis was carried out for PAT/SO dispersions for 5%, 10%, 15%, 20%, and 25% concentrations. The Cole-Cole equation, which is a dielectric relaxation model to describe the dielectric properties of polymers, is used to establish a relationship between ER effect and dielectric properties (Cole and Cole, 1941). Therefore, the curves are subjected to the Cole-Cole fitting by using equations (2) and (3):

ε^{*} = ε^{'} - i ε^{″} = ε_{\infty} + \frac{ε_{0} - ε_{\infty}}{(1 + {(i λ)}^{1 - α})}

(2)

ω = 2 π f

(3)

where $ε_{0}^{'}$ and $ε_{\infty}^{'}$ are dielectric constants at low and high frequencies, respectively. $λ$ is relaxation time, $α$ is a parameter indicating broadness of the relaxation time, ω is angular frequency, and f is frequency.

The dielectric data, which are $ε^{'}$ , $Δ ε^{'}$ , and $λ$ obtained from Cole-Cole fitted curves are summarized in Table 1. The highest $ε^{'}$ at 10² Hz is obtained for 20% PAT/SO dispersion as $ε^{'}$ = 3.07 (Figure 6). It can be concluded that $ε^{'}$ increases with increasing amount of PAT up to 20% in the dispersion. A similar result was reported for PAQR/SO system as $ε^{'} = ~$ 3.20 at 10² Hz (Wang et al., 2016). Although the viscosities of SO used in this study ( $η$ = 1.0 Pa·s) and the previously reported PAQR/SO system ( $η$ = 0.48 Pa·s) are different, dielectric constants of the silicone oils are almost the same $ε^{'}$ = 2.76 and $ε^{'}$ = 2.75, respectively. We may conclude that since the dielectric constants are the same, PAT/SO and PAQR/SO systems are comparable.

Table 1.

Dielectric properties of PAT/SO dispersions.

PAT/SO (m/V%)	$ε^{'}$ at 10² Hz	$ε_{0}^{'}$	$ε_{\infty}^{'}$	$Δ ε^{'} = ε_{0}^{'} - ε_{\infty}^{'}$	$f_{\max}$ (Hz)	$λ$ (s)
5	2.13	2.61	2.03	0.58	0.98	0.162
10	2.76	4.07	2.62	1.45	1.17	0.136
15	2.81	4.76	2.66	2.10	1.65	0.096
20	3.07	5.84	2.74	3.10	3.01	0.053
25	3.07	7.25	2.76	4.49	0.80	0.198

Figure 6.

Dielectric spectra of 20% PAT/SO dispersion: dielectric constant ( $ε^{'})$ and dielectric loss factor ( $ε^{″}$ ) versus frequency.

On the other hand, conjugated, nonpolar, and linear polymer polyphenylene doped with FeCl₃ had higher $ε^{'} = ~$ 40 at 10 Hz (Plocharski and Wyciślik, 2000).

The polarizabilities of PAT and PAQR can also be compared by considering their $Δ ε^{'}$ values which are calculated to be $Δ ε^{'} =$ 3.10 for 20%PAT/SO system and reported to be $Δ ε^{'} = ~$ 0.8 for PAQR/SO system. This indicates that PAT can exhibit stronger inter-particle interactions by forming stronger chain-like structures and so a much better ER activity may be expected. This expectation may be attributed to the higher mobility of step ladder chains (molecules are connected to each other through a single bond) in the PAT structure compared to the ladder (molecules are connected to each other through multiple points) ones in the PAQR structure.

The relaxation times (λ) of PAT/SO dispersions are calculated from equation (4):

λ = 1 / 2 π f_{\max}

(4)

where $f_{\max}$ is determined from the peak of $ε^{″}$ -f spectrum which stands for the frequency corresponding to the maximum dielectric loss ( $ε^{″}$ ) and f is the frequency.

As seen in Table 1, the relaxation time of PAT/SO dispersions decreases with increasing amount of PAT up to 20% ( $λ =$ 0.053 s). However, the longest relaxation time is calculated to be 0.198 s for 25% PAT/SO system. This indicates that the response of 25% dispersion is very slow compared to the 20% concentration. The reason for the decrease in dielectric properties for 25% dispersion may be attributed to the restriction of the electric field induced diffusion of particles in high viscous medium. Therefore, considering both the high $ε^{'}$ and short relaxation time (λ), the optimum PAT concentration in the dispersion is determined to be 20%. The relaxation time of previously reported PAQR is much shorter (2.75 × 10⁻⁴ s) than PAT (Wang et al., 2016). However, it has been reported that Δ $ε^{'}$ value is more dominant in ER effect than pure λ (Cho et al., 2004) which is the case in our situation.

3.3. ER activity of PAT/SO dispersions

The time depended ER responses of PAT/SO dispersions were investigated by applying increasing electric field strengths (E = 0–4 kV/mm with E = 0.5 kV/mm increments) for 30 s time intervals at a constant shear rate ( $\overset{\cdot}{γ}$ = 1.0 s⁻¹). After applying each electric field strength, it was set back to E = 0 kV/mm for 30 s to bring back the system to its original state to get rid of deposited charge inside the sample. The results depicted in Figure 7 show that there are sudden increases in shear stresses when the external electric field is applied. It is also important to say that PAT/SO dispersions reversibly and suddenly return to their original state when electric field is removed (E = 0 kV/mm). This is consistent with the short relaxation time values obtained from dielectric measurements.

Figure 7.

ER responses of PAT/SO dispersions: 5%, 10%, 15%, 20%, and 25% (m/V%).

In Figure 7, the shear stress of PAT/SO dispersions increase with applied electric field strength and this is due to increasing polarization forces between the dispersed particles. The polarization forces in the direction of applied electric field is defined as (Koyuncu et al., 2012):

F_{p} = \frac{6 ε_{2} r^{6} E^{2}}{h^{4}}

(5)

where $F_{p}$ is polarization force, $ε_{2}$ is dielectric constant, r is radius of the particles, E is electric field strength, and h is interparticle distance.

The equation shows that the polarization forces are proportional to the electric field strength and inversely proportional to the interparticle distance. Therefore, the higher amount of PAT leads to increase in the shear stress (Figure 7) due to decreasing interparticle distance and thus increasing interparticle interactions between PAT particles in PAT/SO dispersion. The shear stress is reached to 292 Pa at E = 4 kV/mm for 25% PAT/SO dispersion system. However, the increment is not significant between 20% ( $τ$ = 283 Pa at E = 4 kV/mm) and 25% ( $τ$ = 292 Pa at E = 4 kV/mm) dispersions. Both ER and dielectric results supports that the optimum dispersion of PAT/SO should be 20%. Therefore, this concentration is used for the further measurements.

The flow behavior of optimum 20% PAT/SO dispersion under applied electric field (E = 0–4 kV/mm with E = 0.5 kV/mm increments) is revealed via investigating the changes of shear stress and viscosity with increasing shear rate. Non-Newtonian shear thinning flow behavior is observed for the dispersion. The shear stress values observed to increase and show larger plateau region in a low shear rate range with increasing applied electric field strength (Figure 8). When shear deformation applied to a material under external electric field, two types of interactions, namely; electrostatic and hydrodynamic, between dispersed particles compete with each other to dominate the flow behavior. The electrostatic and hydrodynamic interactions are induced by the applied electric field and the shear deformation, respectively. In low shear rate range, the electrostatic interaction is dominant, and the shear deformation can be recovered in time. Thus, the shear stress remains constant for certain shear rate range in the plateau region. As shear rate increases, the hydrodynamic interactions become prevailing, and the dispersion starts to flow as the interactions are not enough to recover the electric field induced chain-like structure in expected time. The dynamic yield stress is a minimum shear stress at which the dispersion starts to flow. The extrapolation of the data of shear stress to zero shear rate gives the dynamic yield stress and it is a good measure of magnitude of the ER effect. In the previous studies, the Bingham model was used to explain the flow behavior and to determine the yield stress of ER dispersions, but Cho and coworkers proposed a new model (Cho-Choi-John, CCJ) (equation (6)) including six parameters for better fitting and explanation of flow behaviors of conducting polymer based ER fluids (Cho et al., 2005). The first and second parts of CCJ model can explain the flow behavior phenomena at low shear rate and high shear rate regions, respectively.

τ = \frac{τ_{y}}{(1 + {(t_{1} \overset{\cdot}{γ})}^{α})} + η_{\infty} (1 + \frac{1}{{(t_{2} \overset{\cdot}{γ})}^{β}} \overset{\cdot}{γ})

(6)

where, t₁ and t₂ are time constants, $\overset{\cdot}{γ}$ is shear rate, $τ_{y}$ is yield stress, $η_{\infty}$ is viscosity at high shear rate, $α$ is related to decrease in the shear stress, and $β$ is a constant ( $0 < β ⩽ 1$ ).

Figure 8.

The effect of shear rate on shear stress (solid lines for CCJ fitting) (a) and viscosity (b) of 20% PAT/SO dispersion under various applied electric field strengths.

The electric field induced flow curves of 20% PAT/SO dispersion are fitted to CCJ model. The dependence of dynamic yield stress obtained from CCJ model to applied electric field strengths is illustrated in Figure 9. The yield stress increases with applied electric field strength due to the enhanced polarization of dispersed particles, so the highest yield stress is determined to be 283 Pa for E = 4 kV/mm for PAT/SO system. For PAQR/SO and polyphenylene/SO, the yield stress values were reported to be ∼100 and ∼240 Pa at E = 2 kV/mm (Plocharski et al., 2005; Wang et al., 2016). There is a significant difference in electric field induced yield stress value of PAT when compared to the previously reported PAQR and polyphenylene. It is well known that, dispersing medium viscosity affects the yield strength of an ER fluid and makes the comparisons difficult. Therefore, it is more reliable to compare the ER efficiencies of various systems in which the effect of viscosity of dispersing medium is discarded by subtracting off field (E = 0 kV) from the on field (E ≠ 0 kV) yield stresses.

Figure 9.

The change in dynamic yield stress values of 20% PAT/SO dispersion obtained from CCJ model with increasing applied electric field strengths.

The ER efficiency was calculated by using the following equation:

ER Efficiency = \frac{τ_{E} - τ_{0}}{τ_{0}}

(7)

where $τ_{E}$ and $τ_{0}$ are the shear stresses with and without electric field, respectively.

In this study, the ER efficiency of PAT/SO system under E = 3 kV/mm is calculated to be 81 which is much higher than the one reported for PAQR (23.75 at $\overset{\cdot}{γ}$ = 1 s⁻¹ and E = 3 kV/mm) (Wang et al., 2016). It is an expected result because $Δ ε^{'}$ of PAT is higher than of PAQR. Furthermore, the ER efficiency of polyphenylene was not calculated in the reported study but when we have deduced from the reported available data it was corresponding to 2.6 at E = 2 kV/mm which is lower than the ER efficiency of PAT/SO system. This indicates that, PAT/SO system is a good candidate for potential industrial vibration damping applications due to the better performance and lower energy consumption.

The dynamic yield stress versus applied electric field strength graph also provides information about the ER activity mechanism of PAT/SO system. The dependence of the yield stress on the electric field strength is correlated in equation (8):

τ_{y} \propto E^{m}

(8)

where $τ_{y}$ is dynamic yields stress, E is applied electric field strength, and m is an exponent varying between 1.0 to 2.0.

There are six ER mechanism models namely: (i) fibrillation, (ii) electrical double layer, (iii) water/surfactant bridge, (iv) polarization, (v) conduction, and (vi) dielectric loss models. According to equation (8), if m equals to 1.5 or 2; the ER mechanism follows the conduction or polarization model, respectively. (Dong et al., 2019). The electric field induced structure forms due to the conductivity and dielectric constant differences between dispersed particles and dispersing medium for conduction and polarization models, respectively (Krztoń-Maziopa et al., 2006). For PAT/SO dispersion system, the slope of the fitting line is 1.5 indicating that the driving force is the conduction model (Figure 9). The ER mechanism of previously reported conjugated, nonpolar, and linear polyphenylene was reported to be obeying the polarization model and the magnitude of polarizability attributed to the amount of doped FeCl₃.

3.4. Colloidal stability of PAT/SO dispersion

The colloidal stability of ER fluids is an important parameter for industrial applications. The sedimentation of particles causes the ER activity of dispersion to decrease in time. According to the Stoke’s law, the density mismatch between dispersed particles and dispersing medium results in the phase separation with time (Cavicchi et al., 2018; Uemura et al., 1995; Whitmer and Luijten, 2011). The colloidal stability of optimized 20% PAT/SO dispersion system is identified according to the colloidal stability ratio versus time graph shown in Figure 10. The colloidal stability ratio is defined in equation (9):

Colloidal stability ratio = \frac{T h e h e i g h t o f p a r t i c l e r e a c h p h a s e}{T h e t o t a l h e i g h t o f t h e f l u i d} x 100 %

(9)

The results indicate that the colloidal stability ratio of PAT/SO system is below 50% after 10 days and only 40% of the dispersion remains unsettled after 30 days. Although the average hydrodynamic size of PAT is relatively small (600 nm by DLS), the colloidal stability measured in non-polar SO was not as high as expected. This may be attributed to the density mismatch between PAT ( $ρ$ =1.03 g/mL) and SO ( $ρ$ =0.97 g/mL). Thus, a nonionic surfactant, TritonX, was added to the PAT/SO system to enhance the colloidal stability in trace amount. As seen in Figure 10, the colloidal stability of PAT/SO/TritonX system measured to increase up to 88% at the end of 30 days, which is a good result in terms of potential industrial vibration damping applications.

Figure 10.

The colloidal stability ratio of 20% PAT/SO and 20% PAT/SO/TritonX dispersions for 30 days.

3.5. Electroactive vibration damping properties of PAT/SO dispersion system

To the best of our knowledge, the electroactive viscoelastic, creep-recovery, or vibration damping properties of PAT not hitherto been reported in the literature. For the industrial applications, it is important to obtain strong and sustainable elastic character for ER fluids; therefore, the dependence of $G'$ and $G ″$ on stress, frequency, and time under various applied electric field strengths were examined for PAT/SO smart fluid system. Firstly, the linear viscoelastic region (LVR) and the critical shear stress values were determined for 20% PAT/SO dispersion system (Figure 11(a) and (b)). In LVR, materials can return back to its original state without destruction in the structure so the viscoelastic measurements and creep-recovery tests can be carried out in this region. The critical stress is the limit of shear stress of material to withstand. The stress values greater than the critical stress causes the deterioration of structure, decrease in $G'$ , and so the loss of elasticity. As seen in Figure 11(b), the critical shear stress values increase with increasing electric field strength. Furthermore, the viscoelastic properties such as elastic modulus ( $G'$ ), viscous modulus ( $G ″$ ), and $\tan δ = G ″$ / $G'$ were examined in LVR to understand the vibration damping capability of PAT/SO dispersion system. The vibration reduction is improved by both increasing the stiffness ( $G'$ ) and the energy dissipation ( $G ″$ ) (Chung, 2001). For ER fluids, the stiffness significantly changes under electric field because of the formation of chain-like structures. As seen in Figure 12(a) and (b), the viscous response ( $\tan δ$ > 1) is dominant for PAT/SO dispersion system under off field. This means that only the viscous friction forces withstand to the applied damping force. However, when the external electric field is applied to PAT/SO dispersion, the elastic character becomes more dominant ( $\tan δ$ =1). The electric field induced interactions between polarized PAT particles lead to both the viscous friction and the Coulombic forces which resist to the applied damping force while increasing the stiffness and vibration damping capability of PAT/SO system (Brennan et al., 1995; Goilkar, 2017; Gumus et al., 2011). According to Figure 12(a), the improvement in elastic behavior with applied electric field is very high ( $G^{'} ≅$ 10⁵ Pa at E = 4 kV/mm) which is consistent with the high value of $Δ ε$ (=3.10) determined from dielectric measurements. For electric field induced stiff PAT/SO systems, the energy dissipation can be compared by considering values of $\tan δ$ = $G ″$ / $G'$ (Figure 12(b)). It was observed that there was no significant difference between the $\tan δ$ values at varying electric fields. It was concluded that PAT/SO system has a potential industrial use as an electro active vibration damping material (Liu et al., 2005).

Figure 11.

(a) The change of elastic modulus ( $G'$ ) with shear stress at various electric filed strengths and (b) change of critical stress with electric field strength for 20% PAT/SO dispersion.

Figure 12.

The change in (a) the elastic ( $G'$ ) and viscous ( $G ″$ ) moduli and (b) tanδ with frequency for 20% PAT/SO dispersion.

The time dependent change in $G'$ at f = 1 Hz (Figure 13) demonstrates that the electric field induced elastic properties are continues and constant. This is a desired behavior for industrial vibration damping applications.

Figure 13.

The change in the elastic ( $G'$ ) modulus with time for 20% PAT/SO dispersion.

Viscoelastic properties are also investigated by the creep-recovery tests under various electric field strengths and the recoverable elastic portion of deformation are determined (Figure 14). The shear stresses were applied to the 20% PAT/SO dispersion system for 100 s and then the strain values are detected. The recovered strain and constant deformation are related to elastic and viscous parts of the material, respectively. The percentage of recoveries are calculated using equation (10):

% Recovery = \frac{γ_{i} - γ_{f}}{γ_{i}} x 100

(10)

where, $γ_{i}$ is the strain at the end of the creep phase, and $γ_{f}$ is the average steady state strain at the end of the recovery phase. As seen in Figure 14, the PAT/SO dispersion system shows almost pure viscous deformation under E = 0 kV/mm with small recovery by indicating Newtonian flow behavior. The recovery is improved by application of electric field because of increase in elastic character by showing non-Newtonian and non-linear flow behavior. According to Table 2, the percentage of recovery increases up to 62% with rising applied electric field strength because of the better energy storing capacity arising from the strong electric field induced polarization of PAT particles.

Figure 14.

The creep-recovery behavior of 20% PAT/SO dispersion.

Table 2.

Percentage of recoveries of 20% PAT/SO dispersion.

E (kV/mm)	0	1	2	3	4
% Recovery	15	43	46	54	62

4. Conclusion

The dielectric and time dependent ER response results indicate that the optimum PAT content is 20 m/V%. The dielectric analyses indicate that the polarizability of PAT/SO dispersion system is higher than the previously reported PAQR/SO dispersion. The electric field induced flow tests have shown that 20% PAT/SO dispersion system have ER activity following the conduction model with 283 Pa yield stress at E = 4 kV/mm. Additionally, its ER efficiency is better than the previously reported PAQR/SO and polyphenylene/SO dispersions, which have comparable structures. To the best of our knowledge, the vibration damping properties of PAT/SO system have not hitherto been reported and the experimental results in this study show that PAT/SO dispersion system can be promising material for the industrial vibration damping applications. The investigations on viscoelastic properties depicted that PAT/SO dispersion system has viscous behavior under E = 0 kV/mm; on the other hand, the elastic behavior is dominant under externally electric field induced conditions. The $G'$ , indicating energy storing capability, increases with increasing external electric field strength and the time depended $G'$ results show that the elastic behavior is continues. Dominant elastic behavior is necessary for vibration damping applications, in this way the material can store applied force without structural destruction which was confirmed by $\tan δ$ < 1 value. The creep-recovery results indicate that in LVR, PAT/SO dispersion system can recover up to 62% of strain at E = 4 kV/mm and concluding that it may be useful for long term industrial applications which is also supported by good sedimentation resistance of 88% against gravitational forces.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors thank to Gazi University Research Fund (Grant No.: 05/2020-12) for financial support of this work.

ORCID iDs

Gokce Calis Ismetoglu

Halil Ibrahim Unal

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