Polystyrene, transparent sheet 0.87 mm thick
Measuring cell: TMA40 in the mode DLTMA (dynamic load TMA) with 3-mm ball-point probe
Pan: No pan; the specimen was placed directly on the fused silica sample support
Sample preparation: A piece was broken off the brittle sheet using pliers; mass approx. 12 mg
Measurement: First heating run from 25 to 145ºC at 10 K/min, uncontrolled cooling to room temperature under dynamic load, second heating run from 25 to 135ºC at 5 K/min
Load: Square wave with the load changing from 0.05 N to every 0.25 N every 6 s
The onset of penetration (indentation) in the first heating curve is 98.8ºC.
Often, the second run is preferred for evaluations. Its upper envelope gives:
The difference of the envelopes is proportional to the compliance (reciprocal Young's modulus). In the diagram, for sake of clarity, the envelopes are slightly shifted with respect to the curve.
Although the physics behind these events is relatively complex, the experiments are easy to perform. If the elastic response is not required, a normal TMA experiment at constant load is sufficient.
Sometimes, the glass transition is not clearly visible in a DSC curve. In such cases TMA is advantageous. Even with highly filled polymers, where DSC may give transitions that are questionable or not visible, DLTMA furnishes clear results, at least in the three-point bending mode.
PS, Glass Transition by DLTMA | Thermal Analysis Handbook No.HB219 | Application published in METTLER TOLEDO TA Application Handbook Thermoplastics