Multi-arm Star Polyisobutylenes. V. Characterization of Multi-arm Polyisobutylene Stars by Viscometry, Pour Points, Electron Microscopy, and Ultrasonic Shear Degradation

Structure/property relationship of multi-arm star polyisobutylenes [∗-(PIB) n s] were characterized by a variety of techniques, including vis-cometry, pour points, electron microscopy, and ultrasonic degradation. The intrinsic viscosity of ∗-(PIB) n s changes very little with temperature in the 30 to 100°C range, whereas that of linear PIBs of the same molecular weight increases strongly with temperature. Kinematic viscosity measurements of select ∗-(PIB) n s gave viscosity indices in excess of 130. The viscosity of ∗-(PIB) n s is mainly determined by the molecular weight of the arms and much less by the number of arms or overall molecular weights. Electron microscopy of ∗-(PIB) n s indicates a compact spherical morphology, a conclusion that was substantiated by radius of gyration measurements. Pour points of ∗-(PIB) n s are −27°C. Ultrasonic studies gave insight into the mechanism of shear degradation of ∗-(PIB) n s. These characteristics render ∗-(PIB) n s of interest as rheology control additives for motor oils.