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

A saturated star-shaped polystyrene-b-poly(ethylene-co-propylene block copo lymer, (SEP)(star), was synthesized for use as a viscosity index improver i n lubricants. Polystyrene-b-polyisoprene arms were first made anionically, followed by a linking reaction at the optimum temperature of 60 degreesC wi th divinylbenzene. The resulting star-shaped (SI)(star) was hydrogenated to eliminate the double bonds on the polyisoprene segment, thus forming the s tar-shaped (SEP)(star). The number of arms on each molecule increased with an increase in the mol ratio of divinylbenzene to n-butyl-lithium. Increasi ng the arm length adversely affected the linking efficiency but caused a sl ight increase in the degree of branching. The T-g of the poly(ethylene-co-p ropylene) block was 13 degreesC higher than that of the original polyisopre ne block. Compared with (SI)(star) (SEP)(star) has a thermal decomposition temperature 50 degreesC higher but independent of the arm length or the deg ree of branching. Viscosity measurements for (SEP)(star) revealed that intr insic viscosity depends only on the arm length, but not on the degree of br anching. Adding 1 wt % of (SEP)(star) markedly increased the viscosity inde x 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 l ower 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 le ngth and degree of branching. (C) 2001 John Wiley & Sons, Inc.