Core-Shell-Structured Copolyaniline-Coated Polymeric Nanoparticle Suspension and Its Viscoelastic Response under Various Electric Fields

Semi-conducting poly(n-methylaniline) (PNMA)-coated poly(methyl methacrylate) (PMMA) composite nanoparticles were synthesized using cross-linked and grafted PMMA particles as a core, and then, the PNMA shell was coated via chemical oxidative polymerization on the surface of modified PMMA nanoparticles. Their electroresponsive electrorheological characteristics when dispersed in silicone were confirmed under applied electric fields using a rotational rheometer, focusing on their viscoelastic response. Using a frequency sweep test, the frequency dependence of both the storage and loss moduli was confirmed to increase upon increasing the electric field, with a stable plateau regime over the entire angular frequency range.


Introduction
Electrorheological (ER) fluids are typically prepared by dispersing polarizable or semiconducting particles in an insulating liquid media. When various electric fields are applied, the state of the ER fluid is changed reversibly from liquid-like to solid-like because of the formation of fibril-chain structures. This change occurs because the chains oriented along the electric field direction form under an applied oxidants, concentrations of the reactants such as oxidant and aniline, solvent components, and the presence of additives including stabilizers and surfactants.
In this study, core/shell-structured PMMA-PNMA particles were synthesized using a novel grafting polymerization process. The PMMA seeds were swollen using glycidyl methacrylate (GMA), and ethylene glycol dimethacrylate (EGDMA) was added as a cross-linking agent. Then, the core/shell interaction was enhanced by an epoxy-amine reaction between GMA and oxydianiline (ODA). In our previous experiments, the typical ER performance of a PMMA-PNMA particle-based ER fluid was analyzed using a rotational rheometer in rotation mode [32]. However, in this experiment, the ER properties of the obtained PMMA-PNMA particles were confirmed using a rotational rheometer in oscillatory mode under various applied electric field strengths.
Initially, the PMMA nanoparticles were synthesized using dispersion polymerization. A MMA monomer and PVP as a stabilizer were dispersed in methanol with a radical initiator (AIBN) at room temperature. The polymerization was allowed to proceed at 65 °C for 24 h under continuous stirring. The obtained product was collected by centrifugation, washed with methanol and distilled water, and then dried using a freeze-drier.
The obtained PMMA particles were then dispersed in deionized water containing SDS and swollen using GMA with a radical initiator for 12 h at room temperature. In the course of mild stirring, the SDS was adsorbed on the PMMA surface. The mixture of BPO and EGDMA was placed into the reactor. In this case, EGDMA was used as a crosslinking agent. Then, GMA dissolved in the SDS aqueous solution was added to swell the above mixture for 6 h. The obtained swollen and crosslinked particles (PMMGMA) were then dispersed in acetone containing ODA. The epoxy-amine reaction between glycidyl groups in the GMA and amine groups in the ODA was carried out at 55 °C for 12 h, followed by centrifugation and drying of the ODA-PMMGMA particles.
The synthesized ODA-PMMGMA particles were dissolved in an acidic medium containing PVA as the stabilizer and APS as the initiator. The chemically oxidative polymerization was progressed by adding NMA as the monomer and HCl as the oxidant at 0 °C for 24 h. The final product was collected via centrifuge and then dried for 24 h at 55 °C. Figure 1 shows the detailed experimental mechanism from PMMA seeds to PMMA/PNMA microspheres, and more details on the synthesis can be found in the previous report [32]. The fabricated PMMA-PNMA particles were then subjected to a dedoping process to reduce their conductivity from 9.319 × 10 −4 to 5.35 × 10 −10 S/cm, which is within the semiconducting regime. The PMMA-PNMA particles were then dispersed in silicone oil with a volume fraction of 10% via ultrasonication to achieve good dispersion.

Characterization
The morphology of the synthesized particles was examined using field-emission scanning electron microscopy (FE-SEM) (S-4300, Hitachi, Tokyo, Japan) at a voltage of 15 kV and a working distance of 15 mm. The viscoelastic properties of the PMMA-PNMA particle-based ER fluid were examined using a rotational rheometer (Physica MCR 300, Stuttgart, Germany) equipped with a high-voltage generator (HCP 7E -12500, fug, Schechen, Germany) using a Couette-type sample loading geometry with a bob and cup (CC17, gap size is 0.71 mm) under a dynamic oscillation test.

Result and Discussion
The SEM images were used to determine the surface morphology and particle size. As observed in Figure 2a, the surface of the fabricated PMMA nanoparticles appears smooth with a uniform diameter of 700 nm. However, for the PMMA-PNMA particles in Figure 2b, the particle surface was considerably rough and coated irregularly, with the average particle size being increased to 1.63 μm. This very distinctive difference in their morphology indicates successful coating of the PNMA.
In our previous experiments, the chemical structure of the fabricated PMMA-PNMA microspheres was examined using FT-IR spectroscopy [32]. All the characteristic peaks not only for PMMA and PMMA-PNMA but also for the intermediate steps of PMMGMA and ODA-PMMGMA were carefully identified. The successful fabrication of the core-shell-structured PMMA-PNMA microspheres was then confirmed.
Dynamic-oscillatory testing using a rotational rheometer is an important tool for studying the viscoelastic properties of ER fluids. Initially, the amplitude sweep test was performed to verify the limit of the linear viscoelastic range (γLVE) with a fixed angular frequency of 6.28 rad·s −1 before the main dynamic oscillation test. Figure 3 shows the change in both G', which represents the elastic storage response during the shear process, and G'', which represents the energy dissipation response during the shear process (viscous property) as a function of the strain from 1 × 10 −5 to 1. G' is much larger than G'' at the relevant electric field strength. In addition, the G' and G'' curves exhibited a constant plateau value in the low-amplitude region, which is the so-called γLVE. In the γLVE region, deformation of the structure is considered reversible, and the elasticity term is dominant compared to the viscosity term. The mean γLVE value was approximately 7 × 10 −5 , and this value was selected for the subsequent frequency sweep test. When the applied strain amplitude exceeds the value of γLVE, both G' and G'' decrease sharply because of the irreversible change in the structure, and the value of G'' even exceeds that of G'.  Examining the magnitude of the in-phase (elastic) component of stress (τ' = G'γ) as a function of strain γ is a useful way to illustrate the progressive structural breakdown, as illustrated in Figure 4. The shoulders or maximum values of the elastic stress provide a good estimation of the localization of τy. This approach has been used to explain the existence of two yielding points and progressive structural destruction. At small strain amplitudes (γ < 0.001), the in-phase stress increases linearly with the strain within the linear viscoelastic region. The structure of the PMMA-PNMA particle-based ER fluid began to break down at the limiting strain (γc). γc represents the breaking of intermolecular bonds in the network of the ER fluid [33][34][35][36].

Figure 4.
In-phase stress component (G'γ) as a function of strain replotted using the data presented in Figure 3.
The dependence of τy on the applied electric field strength is analyzed in Figure 5. Typically, under an applied electric field, the correlation between τy and the electric field strength is investigated using the following power-law equation, similar to either the static yield stress or dynamic yield stress: where m = 2.0 is suggested by the polarization model and m = 1.5 is indicated for the conduction model [37]. In Figure 5, all the experimental points for τy fall along a straight line with a slope of 1.5. Thus, τy of this ER suspension is regarded to follow the conduction model. The behavior of G' and G'' as a function of the angular frequency were further examined at various electric field strengths based on the selected critical strain (7 × 10 −5 ), as shown in Figure 6. By increasing the electric field, both G' and G'' increased with a stable plateau regime for of the entire angular frequency range, indicating the frequency independence. Furthermore, G' was always higher than G'', indicating the dominancy of elastic versus viscous behavior in the structure of the ER fluid. Namely, the ER fluid exhibits good solid-like behavior in an electric field. In addition, the complex viscosity (η*) of the PMMA-PNMA particle-based ER suspension was further analyzed as a function of frequency at various electric field strengths, as shown in Figure 7. Without an applied electric field, η* is hardly dependent on the frequency. However, under an electric field, the complex viscosity decreases as the frequency increases over the entire frequency region. In addition, when the electric field is increased, the increase of particle-particle interaction in the ER fluid leads to a higher complex viscosity over the entire frequency range [38]. The ER efficiency is an important factor to assess the changes in the behavior of the ER system with and without the external electric field and is calculated using the following equation: where η* is the complex viscosity in the presence of an electric field and η0 is the field-off complex viscosity. Figure 8 shows the dependence of the ER efficiency on the angular frequency. The ER efficiency increased with increasing electric field strength [39]. The solid-like characteristic of the ER fluids could also be interpreted by examining the stress relaxation behavior. Figure 9 shows the stress relaxation modulus G(t) results, which were calculated from the values of G′ and G″ using the frequency data given in Figure 6 and the numerical formula given in Equation (3): This equation is called the Schwarzl equation [40,41], and G(t) in Figure 9 shows plateau behavior upon increasing the electric field on a logarithmic scale as a function of time. In addition, the Schwarzl equation proves the extremely short-term relaxation behavior of the PMMA-PNMA, which is very difficult to determine experimentally because of the weakness of the mechanical measurement caused by the device itself as well as the intrinsic properties of the ER materials. Figure 9 indicates that under an external electric field, G(t) had a plateau region unlike G(t) without an electric field. Hence, the ER fluid of PMMA-PNMA exhibits a distinctive solid-like behavior under an external electric field because of the strong attractive interactions between the particles.

Conclusions
Core-shell-structured PMMA-PNMA microspheres were successfully prepared by adopting a monodispersed PMMA core and PNMA shell, and the PMMA-PNMA particle-based ER fluid exhibited a viscoelastic ER response in the dynamic oscillation tests under various applied electric field strengths. The slope of 1.5 for the yield stress obtained from the elastic stress indicated that the ER system follows a conduction model. Furthermore, along with drastic increases in the ER efficiency, distinctive changes from a liquid-like to a solid-like phase were observed based on the viscoelastic characteristics of the dynamic moduli and complex viscosity. Using the dynamic moduli data, we also obtained shear relaxation modulus estimated from the Schwarzl equation. Not only the phase change from a liquid-like state to a solid-like state, but also plateau shear modulus increase with increased electric fields was further observed.