Measurement of the Glass Transition Temperature with DSC - METTLER TOLEDO

Measurement of the Glass Transition Temperature with DSC

Purpose

To evaluate the glass transition using SBR as an example. 

 

Sample

Uncross-linked SBR

 

Conditions 

Measuring Cell: DSC822e with liquid nitrogen cooling option

Pan: Aluminum 40 µl

Sample preparation: Flat disk of approx. 10 mg cut from the starting material. The sample was first cooled from room temperature to -100°C at 5 K/min

DSC measurement: Heating from -100°C to 50°C at 10 K/min

Atmosphere: Nitrogen, 50 ml/min

 

Interpretation

Amorphous and semicrystalline materials exhibit a glass transition after cooling to a sufficiently low temperature. Above the glass transition, the material is a more or less viscous liquid. Crosslinking through vulcanization does not change this situation either. Vulcanization does however restrict the flow to such an extent that the material retains its shape. The molecules can perform liquid-specific cooperative rearrangements in this temperature range. Below the glass transition temperature, cooperative molecular rearrangements are “frozen”. The material is in a glassy state. The change in molecular mobility causes a step in the heat capacity curve at the glass transition. The actual shape of the curve at the glass transition varies depending on the thermal and mechanical history of the sample. If the material is stored below the glass transition temperature, undergoes mechanical stress or is cooled slowly to the glassy state, a peak can often be observed next to the glass transition step on heating. The peak is due to enthalpy relaxation. 

 

Evaluation

The most important quantities in the evaluation of the glass transition are the temperature of the glass transition, Tg, and the change in the heat capacity at the glass transition, Δcp.

These quantities are determined by drawing tangents on the heat flow curve at temperatures above and below the glass transition. The tangents should be selected carefully because they have a significant effect on the quality and reproducibility of the evaluation.

The glass transition temperature is a characteristic temperature for the step in the measurement curve. It can be determined according to various standard methods, e.g. as the temperature at half the step height or as the point of intersection of the bisector of the angle between the tangents with the measurement curve.

In the above diagram, the glass transition temperature of uncross-linked SBR was determined using the ASTM standard method. The value of –19.8 °C obtained is identical to the Tg determined from the point of intersection of the angle bisector (midpoint). 

 

Measurement of the Glass Transition Temperature with DSC | Thermal Analysis Application No. HB400 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 1