N-substituted copolyaniline for electrorheological material

Abstract

An electrorheological (ER) fluid is a colloidal dispersion consisting of semiconducting particles and insulating oil. When an electric field is imposed, its rheological properties vary by showing a characteristic fibrillation, with the strings of the particles oriented along the electric field direction. This particle structuring is caused by the dielectric constant mismatch of the particles and the suspending oil [1]. Due to this mechanism, many typical properties of ER fluids, such as yield phenomenon, viscoelasticity, and drastic increase of viscosity, have been observed. In an electric field, ER fluids solidify in the order of milliseconds, and fluidize under applied deformation, which can destroy the chain structure. Therefore, ER fluids are often regarded as Bingham fluids. There are two ER fluid systems: wet-base (hydrous) and dry-base (anhydrous). The difference between these systems is the type of carrier species of the particle polarization. The absorbed moisture and the ions move through the electric field in wet-base ER fluids, whereas the electrons move inside the molecules of the particles in dry-base ER fluids. Dry-base ER fluid systems, including polyaniline, have the advantages of a relatively low density, a controllable conductivity, and thermal stability [2]. Polyaniline is very stable in the atmosphere, and also has good thermal properties. Furthermore, it has a relatively higher solubility in general organic solvents than other conducting polymers. In order to develop the processibility of polyaniline further, many workers have investigated polyaniline dispersion [3], the synthesis of copolyaniline, the introduction of a side group into the main chain [4], and types of dopants [5]. Among various copolymers, SO3y and alkyl sulfonate substituents are often put into the polymer main chain. In this type of polymer, the sulfonate ion acts as a self-dopant. Thus, the conductivity of the polymers are little affected by the pH. The value of the conductivity, however, is lower than that of homopolymers, due to the disturbance of the conjugation of the electrons through the main chain. In this study, N-substituted copolyaniline was synthesized and used as the particles in the ER fluid. Its molecular chain structure is supposed as following: To synthesize N-substituted copolyaniline [6], monomer mixtures of aniline and diphenyl-4-sulfonic acid sodium salt (molar feed ratio ˆ 3:1) were put into a l-neck flask of lM HCl. FeSO4 was also added as a catalyst. The flask was kept in a controlled thermostatic bath (0 0.1 8C). A prechilled (NH4)2S2O8 solution in lM HCl was then added dropwise, to the mixture of aniline and lM HCl for one hour in a nitrogen environment. The system was stirred for approximately 2 h to complete the reaction. The solution was then allowed to precipitate for 30 h, and the product was separated using a glass filter. The pH of the product was adjusted to 8.26 by adding either NH4OH or HCl solution. The synthesized polymer was washed three times using distilled water to remove initiator (oxidant), unreacted monomer, and oligomer. It was then further washed with ethanol and cyclohexane, sequentially, in order to make the surface of the synthesized particles hydrophobic [2]. This process is advantageous for wetting between the particles and oil during ER fluid preparation. The product was finally put into a vaccum oven for approximately two days to dry. The conductivity of the particles was measured to be 2.33 3 10y6 S=cm by using the 2-probe method with a pressed disk of polymer. The particle size was controlled by a 38 im sieve, and the size of the particles was measured by a particle analyser (Malvern), using ethanol as the dispersant (average diameter : 15.4 im, median : 7.3 im). The laminate structure of the polymer particles was further identified by a scanning electron microscope. The structural analysis of copolyaniline was performed using FT-IR (BIO-RAD, FRS-40). Fig. 1 shows the FT-IR spectrum determined using KBr pellets. The peaks at 1586 and 1490 cmy1 are from aromatic C–C stretching vibrations, those at 1309 and 1144 cmy1 are from aromatic amine stretching, and the peak at 824 cmy1 is from the out of plane H deformation for aromatic rings, which is from the