Comparison of DSC and ADSC Measurements

Purpose

The DSC analysis of elastomers often gives rise to a number of weak effects that partially overlap one another. In such cases, the interpretation of a measurement is not always so clear. The purpose of the experiment is to show how ADSC can be used to reliably interpret the measured effects, using SBR as an example.

The diagram shows a conventional DSC curve of unvulcanized SBR with four different interpretation/evaluation possibilities. These are summarized in the table below:

The four interpretation possibilities represent just a few of those available. They lead to completely different results and conclusions. Some of the interpretation possibilities can be excluded with the aid of additional measurements such as heating and cooling experiments. Cooling measurements do not however always lead to the desired result, especially if the sample undergoes irreversible changes (such as chemical reactions). In such cases, ADSC is often very useful for interpretive purposes. 

Sample

Unvulcanized SBR (Krylene 1721) 

Conditions

Measuring cell: DSC822^e with liquid nitrogen cooling option.

Pan: Aluminum 40 µl, pierced lid.

Sample preparation: Cube of approx. 10 mg cut from the starting material. Before the measurement, the sample was cooled from 80 °C to 70 °C at 10 K/min.

DSC measurement: ADSC measurement from 70 °C to 80 °C with an underlying heating rate of 2 K/min, a temperature amplitude of 0.2 K and a period of 48 s. 

Atmosphere: Nitrogen, 50 ml/min

For the calculation, a baseline measurement was subtracted and an ADSC phase correction performed.

To determine the specific heat capacity, a calibration with polystyrene (PS) was performed. This was done by measuring PS under the same conditions with the same period. The calibration factor was determined by dividing the literature value by the measured heat capacity. The heat capacity curve of the sample under investigation was then multiplied by this calibration factor. To compare these values with the total heat flow, the calibrated heat flow curve was multiplied by the underlying heating rate to obtain the reversing heat flow curve. All the calculations were performed using the Mathematics software option. The total heat flow was normalized to the sample weight. 

Interpretation

The figure shows the results obtained from the ADSC evaluation, i.e. the total heat flow (which corresponds to the conventional DSC curve and is equal to the moving average of the ADSC curve) and the reversing heat flow (which corresponds to the heat flow component that is able to follow the heating rate change directly).

While the total heat flow shows all the thermal effects just as in a conventional DSC curve, the reversing heat flow reacts selectively: 

• Glass transitions are measured as a step in the heat capacity curve with the step height, 'cp. The glass transition temperature is shifted to somewhat higher values because of the frequency dependence of the glass transition.
  
• Crystallization phenomena and chemical reactions only show an effect if the process is accompanied by a change in heat capacity. 
  
• Melting processes are measured as a peak whose area depends on the period - with shorter periods, the peak is smaller
  

Taking these points into consideration, one can extrapolate the heat flow above the glass transition from the reversing heat flow curve to lower temperature (blue dashed curve). This curve can also be used as the baseline for the determination of the peak area and the evaluation of the glass transition. A comparison of the two heat flow curves allows the following interpretation of the curve

    a: Glass transition

    b: Exothermic process. No change is detected in the reversing heat flow. It has to do with a     crystallization process that somewhat overlaps the glass transition.

    c: Two endothermic processes that are somewhat better separated at the lower heating rate than in the conventional DSC measurement at 10 K/min. A smaller peak can be observed in the reversing heat flow curve. This has to do with melting processes.

This interpretation corresponds to evaluation A in the conventional DSC curve.  

Evaluation

The following values were obtained for the various thermal events in the total heat flow curve:

The glass transition is a property of the polymer. The crystallization and melting processes relate to transitions of low molecular weight constituents. Compared with the results of conventional DSC, the processes are shifted to lower temperatures. This is due to the low underlying heating rate.

Conclusion

The two curves calculated from the ADSC measurements, i.e. the total and the reversing heat flow, show different behavior for different thermal processes. This can be used to separate processes and to obtain information that allows a reliable interpretation of DSC curves to be made. A comparison of the total heat flow with the reversing heat flow or the complex heat capacity is often a valuable aid for interpretation even without the need to perform the calibration procedure described above. 

The ADSC technique can not only identify the glass transition, but also separate crystallization and melting processes. 

_{Comparison of DSC and ADSC Measurements | Thermal Analysis Handbook No. HB411 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 1}