Transformation Equation
Empirical relations describe the temperature dependence of the shift factors used to build master curves for viscoelastic materials. The williams-landel-ferry shift calculates the horizontal distance required to align experimental data points across a logarithmic time scale. It applies specifically to temperatures above the glass transition point of a polymer.
Free Volume
Molecular motion is governed by the amount of empty space available for chain segments to move. The williams-landel-ferry shift assumes that this free volume increases linearly with temperature once the material enters its rubbery state. This model provides the mathematical basis for shifting data sets to a common reference temperature.
Polymer Mobility
Calculating the shift factor allows for the consolidation of various stiffness measurements into a single predictive model. In the context of PCB lamination, the williams-landel-ferry shift helps manufacturers understand the flow characteristics of prepreg during the press cycle. The equation uses two material specific constants that are determined through curve fitting of experimental results.
While the model is effective for temperatures within one hundred degrees of the glass transition, it becomes less accurate at very high or very low extremes. Engineers use these shifts to optimize the dwell times and pressure stages of the vacuum press to ensure complete wet out of the copper features. The resulting master curve provides a standard for evaluating the rheological consistency of different resin batches.
Quality depends on this model.
