The glass transition in a polymer is a physical change between a metastable glassy state and a rubbery state upon temperature change. It is of utmost interest for polymer processing and use: polymers are often processed above the glass transition where they are less resistant to deformation; and for many applications they need to be kept below the glass transition to avoid softening. Traditionally the glass transition is detected by measuring the change in heat capacity upon heating using calorimetry; however this change is small and often disturbed by other effects: polymer aging, heat transfer, crystallinity, etc. A more direct approach consists in measuring the change in flexibility during the glass transition on the polymer powder encapsulated in a metal frame, using Dynamic Mechanical Analysis (DMA).
During DMA testing, an oscillatory stress is applied on the test specimen held in bending mode and the resulting strain, i.e. deformation, is measured. The ratio between stress and strain corresponds to the complex modulus. It is conceptually identical to Young’s modulus, although the latter is measured in a non-oscillatory experiment. Upon heating in the glass transition region, the two components of the complex modulus (storage and loss moduli) show some change: the storage modulus shows a step decrease, indicating a decrease in stiffness, while the loss modulus shows a peak, indicating an increase in molecular motion; this double feature is typical of a glass transition.
This alternative way to measure the glass transition has the following advantages:
- Glass transition in polymers is directly detected by DMA, as it is accompanied by a change of flexibility.
- A small amount of polymer powder is needed, as for differential scanning calorimetry, typically 5 mg.
- The sensitivity of DMA is higher than that of calorimetry, which avoids erroneous curve interpretations that often come with that technique. The glass transition is even detectable for small amorphous fractions.
- The DMA can operate within a large temperature range: from −170 °C to 500 °C.
Read this application note to learn how this technique works in practice with the analysis of polystyrene powder as an example.