Newtonian shear viscosity of polymer melts

This section will be devoted to the Newtonian viscosity i]0, that is to situations where the shear rate is proportional to the shear stress. This is the case under steady-state conditions at low shear rates. Although rj0 may be directly measured at low shear rates in a cone and plate rheometer, it is in general not measured directly but found by extrapolation of viscosity values, as measured in a capillary rheometer, as a function of shear rate  

This may introduce a certain inaccuracy, however, into the values of rj0. 

The best method to determine the zero shear viscosity is to calculate rj0 from the loss modulus G measured in dynamic shear experiments at a series of frequencies such low that by plotting log G vs. log co a straight line is obtained with slope equal to 1  

The parameters which r)a is dependent on for a given polymer are molar mass, temperature and hydrostatic pressure. Table 15.3 gives some typical values of //Q for different polymers. 

As is to be expected, the viscosity of polymer melts increases with increasing molar mass. The difference in behaviour of polymers from low-molar mass substances becomes striking, however, for molecular mass higher than a certain critical value, Mcr. In this instance 

---

Van Krevelen DW and Hoftyzer PJ (1976) Newtonian shear viscosity of polymer melts. Angew Makromol Chem 52 101-9. 

---