SAN-Na+-montmorillonite nanocomposite for electrorheological material

Abstract

Recently, the study of molecular composites in which inorganic solids are associated with organic parts has opened the way for the development of new materials. Undoubtedly, this type of nanoscale composite is expected to offer characteristic and potential properties that are drastically different from their bulk counterparts [1]. Various nanocomposite materials were synthesized by different preparation methods, one being polymer intercalation into the layer of clay. Evidence of intercalation by an emulsion polymerization was recently observed by Lee and Jang [2], even though the mechanism of intercalation was not fully explained. Clay minerals are introduced into the field of nanocomposites because of their small particle size and layer expanding properties, especially in the application of reinforcement materials with polymers [3]. A large number of polymer-clay nanocomposites have become accessible in the form of end-functionalized derivatives [4, 5]. Montmorillonite (MMT) clay has been widely used for this purpose. The structural characteristics of MMT exhibit an octahedral aluminate sheet sandwiched between tetrahedral silicate layers. When the clay is exchanged with Na+, it possesses a large swelling rate for water, and its interlayer spacing becomes large enough to be penetrated by a monomer. The emulsion system consisting of an aqueous medium can contribute to maximization of the affinity between the hydrophilic host and hydrophobic guest by the action of the emulsifier. Furthermore, the emulsion polymerization has the advantage of being able to simultaneously attain both high molecular weights and high reaction rates. Using this concept, we performed an emulsion polymerization for styrene-acrylonitrile (SAN)-clay nanocomposite and confirmed intercalation. We then investigated the electrorheological (ER) characteristics of this SAN-clay nanocomposite suspended ER system. In general, ER fluids are heterogeneous colloidal suspensions whose properties strongly depend on the applied electric field, where a characteristic fibrillation with the strings of particles oriented along the direction of an electric field is observed. This reversible fibrillation of particles due to the electric field produces a significant increase in apparent viscosity [6, 7]. ER fluids have been the subject of intense theoretical and experimental research due to their emerging technological applications. Attention has recently been paid to the synthesis and characterization of various anhydrous dry-base ER systems such as semiconducting polyaniline and copolyaniline. These systems could overcome several shortcomings of hydrous wet-base systems, such as the temperature limitation caused by the presence of water, the density mismatch between the particle and the oil, and an insufficient yield stress. Choi et al. [8] recently reported that the ER performance of the polyaniline system can be improved by increasing the conductivity of the polyaniline particles. Furthermore, using N-substituted copolyaniline, Choi et al. [9] also found that the better ER performance of the copolyaniline system can be explained from the electrical relaxation analysis obtained from the dielectric spectrum. The Na+-exchanged MMT sample in this study was emulsion polymerized with SAN in the presence of potassium persulfate (K2S2O8) and sodium lauryl sulfate. At first, a 2 l fourneck flask fitted with a mechanical stirrer, thermometer, and condenser was charged with 600 ml of aqueous dispersions containing 5 wt % Na+-MMT. After being sonicated for 1 h at 60 ◦C, styrene/acrylonitrile (70/30 wt %) monomer, sodium lauryl sulfate, and potassium persulfate were added while applying a constant and gentle agitation. The temperature was raised to 82 ◦C to initiate copolymerization. The polymerization was allowed to continue for 8 h with a stirring rate of 450 rpm and was then terminated by the addition of 700 ml of aluminum sulfate solution (10 wt %). The products were vacuum dried at 60 ◦C for 2 days, and the major part of the product was exposed to hot tetrahydrofuran (THF) extraction for 5 days. Each component of the clay and the SAN was identified by Fourier transform infrared (FTIR) spectroscopy. Thermogravimetric analysis (TGA) was used to study the thermal stability of the nanocomposite particle and X-ray diffraction (XRD) was adopted to confirm the intercalation of the SAN into the MMT interlayer. The molecular weight and polydispersity of SAN were determined by gel permeation chromatography