Cold Crystallization Using Silicone Rubber as an Example - METTLER TOLEDO

Cold Crystallization Using Silicone Rubber as an Example

Purpose

To demonstrate the effect of cold crystallization 

 

Sample

Vulcanized unfilled silicone rubber

 

Conditions

Measuring Cells: DSC822e with liquid nitrogen cooling option

Pan: Aluminum 40 µl, with pierced lid

Sample preparation: Elastomer sample of 34.146 mg. The sample was placed in the cold measuring cell (-140°C). The shock-cooled sample (cooling rate approx. 10 K/s) did not crystallize. 

DSC measurement: Heating from -140°C to -20°C at 10 K/min

Atmosphere: Nitrogen, 50 ml/min

 

Interpretation

The glass transition is observed as a step at 125 °C after the initial startup deflection. Crystallization takes place in the range 100 °C and 80 °C. The melting peak follows immediately afterward with a peak maximum at approx. 40 °C. 

 

Evaluation

An exothermic peak due to cold crystallization occurs before the endothermic melting peak. If the degree of crystallinity of the sample before the measurement is to be determined, both peak areas have to be added, paying attention of course to their mathematical signs: The actual peak areas are in fact subtracted from one another.

To obtain reproducible results, it is often best to integrate both peaks together. In the case considered, a total peak area von 0.61 J/g is obtained. The areas of the crystallization and melting peaks are almost the same. This means that nearly all the crystallites that melt were produced during cold crystallization during the heating measurement. The shock-cooled sample was therefore almost completely amorphous before the measurement.

 

Conclusion

Materials with a relatively low rate of crystallization can undergo cold crystallization on heating to temperatures above their glass transition temperature. The corresponding crystallization peak must be taken into account when determining the degree of crystallinity

 

Comments

The size of the cold crystallization peak and its temperature range depend not only on the cooling rate used before the measurement but also on the molecular structure of the material and the number of crystal nuclei in the material. Impurities and additives such as fillers can act as athermal crystal nuclei and thereby accelerate cold crystallization. 

 

Cold Crystallization Using Silicone Rubber as an Example | Thermal Analysis Handbook No.HB405 | Application published in METTLER TOLEDO TA Application Handbook Elastomers Volume 1