Time Temperature Superposition

Master Curve

Viscoelastic properties, such as storage modulus and loss modulus, are difficult to measure across very high or very low frequencies due to instrument limitations. This superposition technique allows high-frequency behavior to be simulated by conducting tests at lower temperatures, and low-frequency behavior by testing at higher temperatures. Shifting these curves along the frequency axis using the Williams-Landel-Ferry equation creates a comprehensive master curve that describes the material behavior over many decades of frequency.

This data is critical for understanding how polymer parts will perform under high-speed impacts or long-term creep conditions.

Process Simulation

Injection molding simulation software uses these master curves to predict how the melt viscosity will change under the extreme shear rates and cooling speeds encountered during mold filling. This prediction helps engineers design molds that avoid high shear stress zones, which can degrade the polymer and weaken the finished part. It also ensures accurate prediction of the flow behavior in thin-walled sections.

Application Limit

This superposition assumes that the molecular relaxation mechanisms are affected equally by temperature changes across the entire test range. For block copolymers or polymer blends with multiple glass transitions, the different phases respond differently to temperature, which invalidates the single shift factor. Applying this method to unsuitable materials leads to incorrect predictions of flow and mechanical properties.