Synthesis and electrorheology of aniline/pyrrole copolymer

Abstract

An electrorheological (ER) fluid is an electroresponsive system whose rheological and viscoelastic characteristics are drastically changed by an applied electric field. The ER fluid in general consists of polarizable particles and insulating media. Dielectric constant mismatch between polarizable particles and insulating media has been considered to enable ER fluids to form a fibrillar structure [1–3]. To obtain the dielectric constant mismatch, the dielectric constant of the polarizable particles should be greater than that of the media [4]. In the absence of an electric field, the ER fluid exhibits Newtonian fluid behavior. However, in the presence of an applied electric field, polarization of the particles causes the particles to form a column-like structure, producing Bingham plastic characteristics [5]. Dry-based ER fluids were selected to complement the shortcomings of previously developed wet-based ER fluids [6]. Many studies on dry-based ER material have been reported, which include discussion of inorganic and semi-conducting polymeric particles such as zeolite [7], carbonaceous particles [8], semiconducting polyaniline (PANI) [9] and its copolymers [10], and polypyrrole [11]. To systematically control the conductivity of polyaniline and polypyrrole, several methods have been introduced including preparing nanocomposites with insulating clay [12, 13] as well as encapsulation by insulating polymeric materials [14]. In this letter, we examine the ER properties of aniline/pyrrole copolymer particles, synthesized via a chemical oxidation polymerization. Equal amounts (0.075 mol) of aniline and pyrrole were mixed under 200 × 10−6 m3 of 1 M HCl with stirring for 30 min in a reactor. We used ammonium persulfate (APS) as a chemical oxidant initiator, so that 0.15 mol of APS can be mixed with 100 × 10−6 m3 of 1 M HCl. The APS solution was then dropped (by stirring for 30 min) into a reactor which contained aniline and pyrrole monomers in HCl solution at 0 ◦C. Further agitation was applied for 24 h after the dropping process was completed. The product was then washed and filtered three times with de-ionized water to eliminate unreacted oxidants and the oligomers. The electrical conductivity of the resulting polymeric samples, without any after treatment, was too high to be directly used as ER fluids. Note that the conductivity of the PANI particles was 10−9 S cm−1 [9], and it was found that the temperature dependence of dc electrical conductivity of the PAPP (aniline:pyrrole = 8 : 2) follows the quasi 2-dimensional (2D) variable range hopping (VRH) model, while that of the PANI and polypyrrole systems follow the 1D and the 3D VRH models, respectively [15]. Therefore, we performed a de-doping process, which is a treatment of the particles in water by adding 1 M NaOH solution to fix the pH of the particles at approximately 9. The pH-controlled particles were again washed and filtered. The sample was then dried in a convection oven at 60 ◦C for 24 h and further dried in a vacuum oven at 25 ◦C for 48 h. The completely dried samples were ground and filtered through a 100 μm pore sized sieve [15]. The chemical structure