Extensional viscosities and extensional strain rates were calculated
using Eqs. (7) and (9), respectively and the relationship
between these two parameters is shown in Fig. 9. Extensional viscosity
was found to decrease with increase strain rate (i.e. tension
thinning) at the rates examined, and increased with increasing
molecularweight and decreasing melt temperature, in similarmanner
to that shown by shear viscosity. Extensional viscosities were
observed to be a factor of between 260 and 2900 higher than shear
viscosity, which is typical for polymer melts. Ratio of extensional
to shear viscosity increased with increasing molecular weight and
decreasing temperature, i.e. with increasing melt viscosity. The
ratio between extensional viscosity and Newtonian shear viscosity
is termed the Trouton ratio [28] and is 3:1 for Newtonian fluids.
For non-Newtonian polymers at high strain rates, the ratio is several
orders of magnitude greater, and provides an indication of the
relative importance of extensional flow, which is highly dependent
upon molecular structure.