Glass Transition Temperature

Molecular Mobility

Physical properties change when a material crosses its characteristic transition point. Above the glass transition temperature, the free volume within the molecular structure increases, allowing for greater flexibility and impact resistance. Below this point, the chains are frozen in a disordered state, leading to high stiffness but increased brittleness.

This shift is not a first-order phase transition like melting but a second-order kinetic event.

Measurement Technique

Differential scanning calorimetry detects this event as a step-change in the heat capacity of the sample. The glass transition temperature is typically reported as the midpoint of this step, though the onset and end temperatures are also relevant for design. Dynamic mechanical analysis provides a sensitive measurement by monitoring changes in the storage and loss moduli under cyclic loading.

Both methods show that the transition occurs over a temperature range rather than at a single discrete point.

Cure Correlation

Thermosetting resins see an increase in this thermal boundary as the density of cross-links grows. Monitoring the glass transition temperature during a production cycle allows engineers to determine the degree of cure achieved by the part. A fully cured resin reaches a maximum value that serves as a benchmark for material quality.

If the measured value is lower than the specification, the part may require additional post-curing to reach its full mechanical potential.