Shear Degradation of Polyisobutene

Abstract

Abstract A polyisobutene of Mw 1.98×106, Mw/Mn 1.8, was extruded in an Instron capillary rheometer. Shear degradation occurred at high shear stresses, approaching melt fracture, and was more prominent at lower extrusion temperatures for tests at 60–140°C. The capillary was 51 mm long with a length/diameter ratio of 66.7 and a 90° entrance angle. Repetitive extrusions at constant shear rate caused a decrease in a molecular weight and a simultaneous narrowing of the molecular weight distribution. Extrudate expansion was measured after each successive capillary pass for tests at 80°C. Extrudate swelling correlated well with (Mz+1)Mz/Mw, except for the two first passes, where melt fracture was pronounced. The correlation with equilibrium extrudate expansion was almost as good for (Mz/Mw)3.7 (Mill's correlation) and for Mz+1 alone. The efficiency of bond rupture is low, with the energy required to rupture 1 mol of bonds being about 840 MJ at 80°C.