THE USE OF LINE SPECTRA IN THE ESTIMATION OF THE ZERO-SHEAR-RATE STEADY-STATE FLUIDITY AND OF THE STEADY-STATE COMPLIANCE

Determination of the retardation line spectra from data on both the storage and the loss compliances of a rheodictic material may yield an acceptable estimate of the zero-shear-rate stead-state fluidity (the reciprocal of the zero-shear-rate steady-state viscosity), even when the loss compliance data do not extend sufficiently far into the flow region to determine it from the loss asymptote. The same calculations also furnish an estimate of the difference between the steady-state and the glassy compliance. The zero-shear-rate steady-state viscosity can be obtained from the storage and loss moduli by first converting them to the compliances. This route is preferable to the traditional ways of estimating it via summation or integration, because those routes yield an acceptable value for the viscosity only when the experimental window is ''pseudoinfinite,'' i.e., it spans a sufficiently large portion of the frequency scale to approach effectively the ''infinite'' window extending on the logarithmic frequency scale from minus to plus infinity.