Microencapsulated polyaniline particles for electrorheological materials

Abstract

Electrorheological (ER) fluids are a class of materials whose rheological characteristics are controllable through the application of an electric field. ER fluids are usually made of particle suspensions with a large dielectric constant mismatch between the particles and the fluid [1]. Upon application of an electric field, the suspended dielectric particles in an ER fluid align themselves into chains and columns parallel to the field, thereby inducing yield phenomenon, viscoelasticity, and a drastic increase in viscosity. Because of their controllable viscosity and fast response, ER fluids are regarded as a smart material for active devices, which can transform electric energy to mechanical energy [2]. Recently, much attention has been put on dry-base, nearly anhydrous ER fluids. Examples include carboneous particle [3], zeolite [4] and various polymer semi-conductors, such as acene quinone radical polymers [5, 6], poly (p-phenylene) [7], polyaniline [8, 9], and copolyaniline [10, 11]. Among these ER fluids, polyaniline in its emeraldine base form has advantages with respect to density, conductivity control and thermal stability. Polyaniline is also easy to polymerize by oxidation polymerization at relatively low temperatures [12]. Despite these merits, only the semi-conducting emeraldine base form of polyaniline can be used due to the high conductivity of the hydrochloride form and the resulting high current density causing breakdown in the structure of the fluid. In this note, we microencapsulated homopolyaniline particles with melamine-formaldehyde resin and then investigated their ER characteristics. Because microencapsulation entraps core liquid materials by circulating them with certain polymeric materials, we used this method to modify the polyaniline particles. Based on the finding that melamine-formaldehyde resin is better than urea-formaldehyde resin in hardness and resistance to heat and moisture [13], we adopted the melamine-formaldehyde resin for our microencapsulation process of homopolyaniline particles. At first, the polyaniline particles were polymerized by oxidation polymerization [14]. 1.2 mol of aniline monomer was added to 800 ml of 1 M HCl, and was chilled and stirred for 2 h. A pre-chilled solution of ammonium peroxysulfate (0.72 mol in 400 ml of