Preparation and characterization of a star-shaped polystyrene-b-poly(ethylene-co-propylene) block copolymer as a viscosity index improver of lubricant

Abstract

A saturated star-shaped polystyrene-b-poly(ethylene-co-propylene) block copolymer, (SEP)star, was synthesized for use as a viscosity index improver in lubricants. Polystyrene-b-polyisoprene arms were first made anionically, followed by a linking reaction at the optimum temperature of 60°C with divinylbenzene. The resulting star-shaped (SI)star was hydrogenated to eliminate the double bonds on the polyisoprene segment, thus forming the star-shaped (SEP)star. The number of arms on each molecule increased with an increase in the mol ratio of divinylbenzene to n-butyllithium. Increasing the arm length adversely affected the linking efficiency but caused a slight increase in the degree of branching. The Tg of the poly(ethylene-co-propylene) block was 13°C higher than that of the original polyisoprene block. Compared with (SI)star, (SEP)star has a thermal decomposition temperature 50°C higher but independent of the arm length or the degree of branching. Viscosity measurements for (SEP)star revealed that intrinsic viscosity depends only on the arm length, but not on the degree of branching. Adding 1 wt % of (SEP)star markedly increased the viscosity index of a HN base oil. With a fixed arm length, a (SEP)star having a higher degree of branching increased the viscosity index more than that having a lower degree of branching. On the other hand, the viscosity index increased with an increase in the arm length when the degree of branching was fixed. The addition of 1wt % of (SEP)star increased the vioscosity index up to a number between 111 and 166, with the exact number depending upon its arm length and degree of branching. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1838–1846, 2001