When to Measure Glass Transition Temperature (Tg) on DSC, TMA, or DMA

The glass transition temperature (Tg) is a fundamental material property for defining material integrity, manufacturing conditions, and operating guidelines across applications such as polymers, electronics, and advanced materials. Glass transition can be measured by three thermal analysis techniques: DSC, TMA, and DMA, each suited for specific materials and applications. This blog will explain how these techniques measure Tg and when you should choose each one.

The glass transition is the temperature region where an amorphous material changes from a glassy phase to a rubbery phase upon heating, or vice versa if cooling. Glass transition actually occurs over a range of temperatures but is usually reported as a single temperature referred to as Tg.

Understanding glass transition temperature helps answer important questions such as:

• At what temperature is a printed circuit board (PCB) at risk of delamination?
• What is the ideal temperature for injection molding or polymer blending without degradation?
• Will a material warp or crack during end-use?

Glass transition can be measured by DSC, TMA, and DMA. Each of these thermal techniques detects the Tg based on changes in a different material property during the glass transition. Choosing the right method is essential for obtaining reliable data and making quantitative comparisons between materials.

Differential Scanning Calorimetry (DSC) is the most commonly used method to measure glass transition. DSC measures heat flow to and from a specimen relative to an inert reference, revealing heat capacity changes that are used to define the glass transition. These heat capacity changes yield three temperature values: the onset of glass transition, midpoint, and endset of the transition.

Experimental design is very important for DSC results. A higher heating rate can provide higher sensitivity but shifts the Tg due to instrument effects, sample thermal conductivity, and transition kinetics. Still, modern DSC instrumentation offers superior baseline performance, and when combined with a high heating rate, offers a reliable Tg measurement for amorphous, semi-crystalline, and plasticized materials, as well as thermosetting resins. DSC can simultaneously measure melting, crystallization, and cure reactions, providing additional thermal data that other methods lack.

Modulated DSC (MDSC) offers additional information about reversing and non-reversing transitions by applying an oscillating heat flow. Whereas a volume relaxation endotherm can distort Tg on a conventional DSC, MDSC separates the volume relaxation endotherm from the reversing heat capacity change at Tg for a more reliable measurement. MDSC also separates non-reversing transitions (such as cold crystallization and curing) and provides sensitivity to detect heating rate dependent transitions. MDSC is especially valuable for semi-crystalline polymers, multi-layer films, or partially cured thermosets.

Thermomechanical Analysis (TMA) measures the glass transition based on dimensional changes as the sample is heated. Tg can either be measured by a change in thermal expansion or from increased probe penetration as the material softens through the glass transition. Expansion mode is ideal for rigid thermoplastics and materials with a well-defined expansion transition, while penetration mode is better for materials that soften significantly above Tg such as softer polymers, coatings, adhesives, and some elastomers.

TMA is better than DSC for filled, highly crystalline, or crosslinked materials because dimensional changes observed at Tg are usually significant. However, the Tg profiles from TMA are often broad, can be affected by probe loading conditions, and may be complicated by volume relaxation effects.

Dynamic Mechanical Analysis (DMA) measures changes in mechanical stiffness and energy absorption by applying an oscillating mechanical stress or strain. As molecular motion increases through the glass transition, the storage modulus (E’) decreases dramatically, making DMA one of the most sensitive techniques for Tg determinations. This sensitivity is ideal for detecting Tg in highly filled, crystalline, or crosslinked materials with very low amorphous content.

Three DMA events can be used to define the range of the glass transition. Storage modulus onset (E’) marks the beginning of glass transition, loss modulus peak (E”) is the midpoint of glass transition, and tan delta peak represents the upper end of the transition.

DSC, TMA, and DMA each have their own strengths and weaknesses in determining Tg on different materials. The table below summarizes the relative utility of the different thermal techniques for evaluating Tg on different materials based on our research:

For more information and analysis, read the full application note Exploring the Sensitivity of Thermal Analysis Techniques to the Glass Transition. A more detailed breakdown of which thermal instruments suit specific applications is offered in our free Thermal Analysis Selection Guide.