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Materials characterization (SEM, TEM, XRD, XPS)

When a polymer specimen is measured while being heated, which technique is suited to detecting the glass transition temperature Tg, and how does the transition appear?

  1. AIn TGA, as a stepwise decrease in mass at the glass transition
  2. BIn DSC, as a stepwise shift in the heat-flow baseline
  3. CIn DSC, as a sharp exothermic peak like that of cold crystallization
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Answer: B. In DSC, as a stepwise shift in the heat-flow baseline

DSC (differential scanning calorimetry) measures the difference in heat flow into or out of the specimen and a reference. At the glass transition the heat capacity increases discontinuously, so the transition appears as a stepwise shift of the baseline. Melting appears as a sharp endothermic peak and crystallization as an exothermic peak; the glass transition does not appear as a peak. TGA (thermogravimetric analysis) measures changes in mass and is used to study phenomena involving mass gain or loss, such as dehydration, decomposition and oxidation, but the glass transition involves no change in mass and cannot be detected by it. Instruments that measure both simultaneously are also available and are convenient for correlating reactions with mass changes.

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