George Norris Foster

Dynamic Melt Rheometry Used to Study Degradation of Metallocene Polyethylene

At melt processing temperatures (>150°C), the presence of even trace amounts of vinyl end group unsaturation promotes chain enlargement or long chain branching (LCB) versus chain breakage in either unstabilized or under stabilized polymer compositions. Chain breakage leads to decrease in molecular weight (M w ) with breakage of the long chains being favored statistically. As the high M w fraction decreases, melt index (MI) increases, and molecular weight distribution (MWD) and melt flow ratio (MFR) decrease. With chain branching predominating over chain breakage, intrinsic viscosity will decrease. In contrast, low shear viscosity will increase (MI decrease) because of the introduction of LCB. Polydispersity index (PDI) corrected for LCB will increase, as will MFR. Melt rheology measurement provides a sensitive means for following subtle changes in molecular structure resulting from thermal degradation of the polymer as during melt processing. Dynamic oscillatory shear measurements are able to distinguish molecular structure change at low and moderate frequencies for ethylene polymers undergoing thermo-oxidative degradation. The method is simpler, more straightforward, and more sensitive than coupled SEC-viscometry or light scattering methods.