End-chain and Mid-chain Functional Macrophotoinitiators of Poly(ε-caprolactone) and Their Molar Mass Effects on Strong Electrorheological Response

Abstract

ABSTRACT:Electrorheological (ER) fluids are suspensions composed of ionomer solid particles dispersed in insulating liquids, which show abrupt changes of rheological properties under the stimulation of an external electric field. In this study, electrical and ER properties of end-chain and mid-chain functional macrophotoinitiators of poly-("-caprolactone) were investigated. For this purpose 4 different samples of poly("-caprolactone) were synthesized with different molar masses. They were then ground milled for a few hours to obtain micron size. The particle sizes of the poly("-caprolactone)s were determined by dynamic light scattering. Suspensions of poly("-caprolactone)s were prepared in silicone oil (SO), at a series of concentrations (c ¼ 5{20%, m/m). The gravitational stability of suspensions against sedimentation was determined at constant temperature (T ¼ 25 C). Flow times of the suspensions were measured under no electric field (E ¼ 0 kV/mm), and under an external applied electric field (E 6 ¼ 0 kV/mm) strengths and a strong ER activities were observed for all the poly("-caprolactone)s/SO suspensions. Further, the effects of suspension concentration, molar mass of the poly("-caprolactone)s, shear rate, electric field strength, frequency, promoter and temperature onto ER activities of suspensions were investigated. [doi:10.1295/polymj.PJ2006048] KEY WORDS Electrorheological Fluids / Ring-Opening Polymerization (ROP) / Poly("-caprolactone) / Molar Mass / Shear Stress / Electrorheological (ER) fluids display remarkable rheological behavior, being able to convert rapidly and repeatedly from a fluid to a solid-like when an external electric field (E) is applied or removed. The wide potential of ER fluids in industrial applications have been attracted much attention, and a great deal of research activities has been carried out in this area.1,2 However, as have been usually recognized, the luck of ER fluids with strong ER effect has become the bottleneck of the development of ER technology, which includes materials design, preparation, and characterization, application establishment and controlling.3 Electrorheological fluids have attracted much industrial attention because of their possible use in devices such as valves, dampers, clutches, brakes and robotic actuators. 4,5 Industrially desired shear stress of ER fluids is usually reported to be not less than 5 kPa under low shear rates. 6 We have recently reported the preparation of ER active suspensions composed of poly(Li-2-hydroxyethyl methacrylate)/SO system, which shown a good ER response with a ¼ 13:75 kPa shear stress.7 To further enhance the ER properties of ER system we prepared a new series of poly("-caprolactone), and reported the synthesis, characterization and ionomer preparation in an our earlier article.8 For this ER active ionomer suspensions, the high dielectric constant (933) and conductivity ratios (3:88 Â 10 À8 Sm À1 ) between particles and SO have been considered to be responsible for the strong ER effect ( ¼ 7:4 kPa).However, to prepare E...