Rheological measurements on polycarbonate

Rheological properties of polycarbonate were measured at constant shear stress and at constant shear rate using a capillary rheometer. The melt viscosity of polycarbonate was determined as a function of: (1) weight‐average molecular weight and molecular weight distribution in the range of M̄w = 13,000–80,000 and M̄w/M̄n = 1.1–3, respectively; (2) temperature at 260–316°C.; (3) shear rates of 1–300 sec.–1; (4) “Processability” as experienced in the injection‐molding range. A straight‐line relationship between long ηm and log M̄w was found with a slope of 3.2. A plot of log ηm against log M̄n resulted in a family of straight lines of the same slope but whose intercepts depended on the polydispersity ratios. The activation energy of viscous flow was calculated from a plot of log ηm versus 1/T at various shear rates. It varied from 26–30 kcal./mole at low shear rates to about 16 kcal./mole at higher shear rates. A shear stress versus shear rate plot showed that polycarbonate exhibited almost Newtonian behavior at temperatures above 290°C. Finally, a good correlation was established between the data obtained on the capillary rheometer and that obtained in an injection‐molding test using a spiral mold.