Macromolecular NanotechnologyFabrication and electric stimuli-response of semiconducting poly(3,4-ethylenedioxythiophene)/silica nanocomposite particles
Graphical abstract
Introduction
Electrorheological (ER) fluids composed of electric-stimuli semi-conducting and/or dielectric particles dispersed in a medium with a low dielectric constant are attractive materials that are also referred to as smart and intelligent soft structured materials because their particle structure can be changed dramatically and reversibly by the application of an external electric field [1], [2], [3], [4], [5]. When an electric field is applied, the randomly dispersed particles are polarized due to dielectric constant mismatch between the particles and medium [6]. The dipole-dipole interaction among the polarized particles induces them to be arranged in the form of a chain, altering their rheological properties dramatically [7], [8]. This tunable rheological property of ER fluids controlled by an electric field enables them to be smart [9], and applicable to various engineering fields, such as ER polishing, engine mounts, shock absorbers, ER tactile displays, clutches, and ER valves [10], [11].
Several conducting polymers possessing a π-conjugated structure exhibit ER behavior when they are de-doped in a semi-conducting regime from 10−6 S/cm to 10−9 S/cm [12], [13], [14], [15]. While conducting polyaniline (PANI) and polypyrrole (PPy) have been studied widely owing to their controllable conductivity, low cost, and environmental stability [16], [17], [18], poly(3,4-ethylenedioythiophene) (PEDOT) has recently attracted interest because of its low band gap, great thermal stability, and excellent electrical conductivity [19], [20]. On the other hand, compared to PANI and PPy, PEDOT has higher conductivity that is difficult to control for use in ER materials. This could be resolved by synthesizing its composites with other materials, such as inorganic particles [21].
In general, a wide range of polymer/inorganic hybrids have been studied with hollow or core-shell structured nanofibers, nanotubes, and nanoparticles or ultrathin films [22], [23], [24], [25], [26], including conducting polymers and inorganics, such as SiO2, TiO2, Fe3O4, and clay [27], [28], [29], from different methods of emulsion polymerization, blending, solvent casting, Pickering polymerization, and solvent casting [6], [30], [31]. An alternative approach to the synthesis of colloidal dispersions of conducting polymers also involves the use of an ultrafine aqueous silica sol to prepare nanocomposite particles [21], [32], [33], in which silica has attracted attention for use in polymer/inorganic hybrids because of its good abrasion resistance, high thermal stability, electrical insulation, and comparatively low cost [34], [35], [36], [37]. Concurrently, polymer/inorganic hybrids for ER materials have stimulated considerable interest because of their synergic effects from the advantages of both polymers and inorganic materials [22], [38], [39]. Among them, the PEDOT/silica nanocomposite has been widely researched. In this work, we synthesized it in a simpler way and studied its rheological behavior when dispersed in a silicone oil under an electric field. It is well known that when the PEDOT is directly used as an ER material, its high electrical conductivity easily leads to electrical breakdown through the ER test, so that we synthesized the PEDOT/silica with a relatively lower electrical conductivity. In addition, considered their density and particle size, we finally designed a suitable fabrication scheme to synthesize PEDOT/silica composite particles for ER materials.
In this study, PEDOT/silica nanocomposite particles were synthesized using an ultrafine methanolic silica sol in place of an aqueous silica sol as a novel way. Because of the presence of conducting PEDOT, which is mixed with insulating silica particles, the PEDOT/silica nanocomposite is suitable for use as an ER material because of its reduced conductivity. Both the static and dynamic properties of a de-doped PEDOT/silica nanocomposite-based ER fluid were examined using a rotational rheometer.
Section snippets
Materials and sample preparation
Colloidal silica (LUDOX® TM-40, 40% suspension in water, 22 nm, Sigma-Aldrich), ammonium persulfate (APS, 98%, Daejung), and ethylenedioxy-thiophene (EDOT, 99%, Sigma-Aldrich) were used as received, and ammonium persulfate (APS) was used as an initiator.
For the synthesis of PEDOT/silica nanocomposites, colloidal silica (2.50 g) was dispersed in a mixture solution of DI-water (50 ml) and methanol (50 ml). The APS (1.67 g) was added to the suspension and stirred for 1 h. The EDOT (1.0 g) was then
Results and discussion
Fig. 1 presents TEM images of (a) pure silica and (b) PEDOT/silica nanocomposite. The nano-sized silica particles were spherical. After the polymerization of PEDOT in the dispersed silica system, silica particles aggregated more because of the PEDOT matrix. The size of the PEDOT/silica nanocomposite was approximately 300 nm. Fig. 2 shows SEM images of PEDOT/silica nanocomposite at two different magnifications. The surface of PEDOT/silica nanocomposite was very rough because of the embedded
Conclusion
The electro-response PEDOT/silica nanoparticles were synthesized from colloidal dispersions of conducting polymers that involves the use of an ultrafine methanolic silica sol. SEM and TEM showed that the surface of the PEDOT/silica nanocomposite was quite rough due to attached silica particles. The diameter of the synthesized particles was approximately 300 nm and the amount of PEDOT was calculated quantitatively to be 77.3 wt%. The conductivity of the PEDOT/silica nanoparticles was controlled
Acknowledgements
This research was supported by NRF, Korea (2016R1A2B4008438).
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