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Thermal transitions of polymers

04/08/2025

We have all heard of glass transition temperature when dealing with adhesives or other polymeric materials, but do we know what it means?

When we take an amorphous material, melt it and let it cool, it solidifies from a viscous liquid to a solid. And finally, when all the molecules have lost their thermal agitation, it becomes a glassy solid. This last change is called the glass transition temperature, Tg, and is characteristic of polymers or amorphous regions of semi-crystalline materials when they change from a rubbery state to a hard, brittle state.

The crystalline melting transition, Tm, is the temperature at which a polymer changes from a microcrystalline solid to a viscous liquid. It would correspond to the melting point of a non-polymeric compound.

In general, we can say that semi-crystalline polymers when heated will present 2 transitions, a Tg of their amorphous region and a melting temperature Tm of their crystals, while amorphous materials will present only one transition, the Tg.

The value of the Tm will depend on the chain structure, intermolecular forces and cross-linking of the chains. Tm is always higher than the glass transition temperature.

There are different techniques and experimental methods to determine the Tg, but due to the thermodynamic character of this transition each method provides a different Tg value and it is always convenient to specify the method by which its value was determined.

To study these thermal transitions and see what happens when the polymer is heated, one of the most commonly used techniques is DSC or differential scanning calorimetry.

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