Influence of Sample Pretreatment on the Glass Transition - METTLER TOLEDO

Influence of Sample Pretreatment on the Glass Transition

Purpose

To demonstrate the influence of the cooling conditions on the glass transition temperature using an amorphous elastomer as an example. The fictive temperature (Richardson method) and ASTM evaluation methods are compared.

 

Sample

Unvulcanized L-SBR (solvent-polymerized SBR)

 

Conditions

Measuring Cell: DSC822e with liquid nitrogen cooling option 

Pan: Aluminum 40 µl, with pierced lid

Sample preparation: Elastomer sample of 11.970 mg. The sample was cooled from 10°C to below the glass transition (-50°C) at cooling rates between 40 K/min and 0.1 K/min

DSC measurement: Heating rate: 10 K/min

Atmosphere: Nitrogen, 40 ml/min

 

Evaluation

The glass transition temperatures were determined from the curves according to the ASTM and fictive temperature (Richardson) methods. In the evaluation, care was taken to ensure that the tangents above and below the glass transition were in each case parallel to one another. It can be seen that the glass transition temperatures obtained depend on the cooling rate of the sample prior to the measurement. In the following diagram, the glass transition temperatures obtained were plotted as a function of the logarithm of the cooling rate. 

Interpretation

As can be seen in the measurement curve, it is possible that the step in the glass transition and the enthalpy relaxation peak overlap. The nature of this peak depends on the sample history and the measurement conditions. In the experiment described, no peak occurs if the cooling rate is greater than the heating rate (10 K/min). The relaxation peak becomes larger the greater the heating rate is compared with the cooling rate.

As discussed above, one expects the fictive temperature at the glass transition to be lower at lower cooling rates. From theoretical considerations of the glass transition, a linear relationship between the logarithm of the cooling rate and the glass transition temperature is expected. This relationship is confirmed if the glass transition temperature is evaluated according to the Richardson method. With the other methods for determining the glass transition temperature, the result is influenced by the actual shape of the curve. In those cases in which no enthalpy relaxation peak occurs, the glass transition temperatures determined according to the ASTM and Richardson methods show a similar dependence on the heating rate. The glass temperature according to the ASTM method is however about 1 K higher. As the relaxation peak increases, the ASTM glass transition temperature shifts to larger values. All the methods used for the determination of the glass transition temperature that depend on the shape of the curve (e.g. half step height method, point of inflection, etc.) show a similar dependence of the glass transition temperature on the heating rate as the ASTM method.

 

Conclusion

Whenever glass transition temperatures are quoted, the method of determination used should be stated.

When glass transition measurements are compared, sample pretreatment should be identical.

 

Influence of Sample Pretreatment on the Glass Transition | Thermal Analysis Application No. HB402 | Application published in METTLER TOLEDO TA Application Handbook Elastomers Volume 1