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 st orage and the loss compliances of a rheodictic material may yield an a cceptable estimate of the zero-shear-rate stead-state fluidity (the re ciprocal of the zero-shear-rate steady-state viscosity), even when the loss compliance data do not extend sufficiently far into the flow reg ion to determine it from the loss asymptote. The same calculations als o furnish an estimate of the difference between the steady-state and t he glassy compliance. The zero-shear-rate steady-state viscosity can b e obtained from the storage and loss moduli by first converting them t o the compliances. This route is preferable to the traditional ways of estimating it via summation or integration, because those routes yiel d an acceptable value for the viscosity only when the experimental win dow is ''pseudoinfinite,'' i.e., it spans a sufficiently large portion of the frequency scale to approach effectively the ''infinite'' windo w extending on the logarithmic frequency scale from minus to plus infi nity.