What is Time Temperature Superposition, and Why is it Useful?

Developing high-performance materials for demanding applications requires predicting their behavior over a wide range of time scales. Rheology and DMA are powerful tools to understand and predict viscoelastic behavior, including how materials will respond to sudden forces or years of continuous use.

Examples abound from everyday products to high-performance materials:

• Adhesive bandages must stick to skin and stay in place but need to detach quickly when pulled off.
• A rubber gasket forms a seal when it is initially compressed, but how do we know how long it will maintain a seal?
• The plastic cover on your car’s headlights must have the impact resistance to withstand being hit by a piece of gravel at 80 mph without cracking.
• New construction composites need to be characterized beyond instantaneous deformation; it’s critical to understand how it will deform over years to avoid structural failure.
• A material used in noise-canceling headphones needs to comfortably stay in place for your long flight and provide the damping properties to insulate from high frequency noise.

High frequency / short timescale behavior (like ripping off the bandage, headlight impact resistance, sound vibration) cannot be directly measured. Low frequency / long time scale behavior (like long-term adhesion, rubber seal longevity, and construction beam stability) are wildly impractical to measure in real-time; as single test would take a year or more! Luckily, material science has a principle to predict these behaviors.

Time-Temperature Superposition (TTS) measurements allow materials scientists to access a material’s time-dependent behavior, which is otherwise inaccessible, accelerating insights into real-world material performance.

Time Temperature Superposition is based on the principle that changing the temperature has the same effect as changing the deformation time for many viscoelastic materials. Increasing the temperature will show how the material will behave over longer times, and decreasing the temperature will show short time scale deformation behavior. In practice, by changing temperature we are able to rescale time.

TTS measurements are executed by performing oscillatory frequency sweeps at multiple temperatures. This is easily set up in TA’s TRIOS Software using an Oscillation Temperature Sweep, which will step through temperatures at defined intervals (typically 5 or 10 °C), holding at each temperature to complete a frequency sweep.

Factors to consider when designing your TTS experiment:

• To predict behavior at low frequencies / longer times, include temperatures above your reference temperature. For higher frequencies / short time scales, you’ll want to measure at lower temperatures.
• Measurements must be within the linear viscoelastic region to produce accurate TTS Master Curves. See the application note Determining the Linear Viscoelastic Region in Oscillatory Measurements for further explanation.
• Over wide ranges of temperature, your material’s modulus may change significantly. The Auto Strain feature automatically adjusts the deformation to ensure data quality across the temperature range.
• Your sample will shrink when it is cooled and expand when it is heated. Use the Axial Force Control feature in your procedure to ensure the instrument maintains contact with the sample.

Creep or Stress Relaxation measurements can also be used for TTS. Alternatively, frequency sweep TTS Master Curves can be mathematically transformed into the relaxation or retardation spectrum to predict material behavior over time under constant deformation.

TRIOS software makes it simple to generate a TTS Master Curve. After collecting a series of frequency sweeps at different temperatures, use the TTS toolbar to select a reference temperature – this curve will be held in place. Next, click “Shift.” All the other curves will be automatically shifted along the x axis to overlay with the reference curve. TRIOS software will provide a table and graph of shift factors and allow you to make any adjustments. Finally, click “Master” to generate a continuous data set of your master curve.

In the above example, experimental data was obtained from 10^-1 to 10² rad/s. TRIOS software then generated a master curve using the average moduli data, extending the analysis to a much wider frequency range.

Time Temperature Superposition is a powerful technique for materials scientists to expand their insights beyond what can be directly measured, to predict performance and optimize properties to meet demanding applications. TTS data from the Discovery Hybrid Rheometer, DMA 850, ARES-G2, RSA-G2 and ElectroForce DMA 3200 are quickly and easily converted to Master Curves in TA Instruments’ TRIOS software. Contact TA Instruments’ experts for personalized guidance on finding the right instrument to facilitate TTS and your material innovation.