Glass Transition of Vulcanized and Unvulcanized Silicone Elastomers

Purpose

The glass transition temperature of elastomers depends on the degree of crosslinking. In the example of SBR described in Section 4.2.4 it was shown that the glass transition temperature increases with an increasing degree of cross-linking, without the step height changing to any great extent. The increase in the glass transition temperature is also observed in the isothermal vulcanization of NBR (Section 4.2.1. Change of the glass transition on vulcanization). With other elastomers, and especially with semicrystalline elastomers, the influence of crystallinity on the glass transition dominates. A silicone elastomer was used as an example to show that here cross-linking has a secondary influence on the glass transition. 

Sample

Silicone elastomers, vulcanized and unvulcanized.

Conditions

Measuring cell: DSC822^ewith liquid nitrogen cooling option 

Pan: Aluminum 40 µl, pierced lid Sample preparation: Cubes of approx. 20 mg cut from the starting material

DSC measurement: Heating at 10 K/min from –140 °C to 100 °C. The sample was placed in the measuring cell at room temperature and then cooled at the maximum rate to the starting temperature.

Atmosphere: Nitrogen, 50 ml/min

Interpretation

The glass transition temperatures of the unvulcanized and vulcanized samples are the same, namely –123.2 °C. There are small differences in the step Δcp - the step height of the unvulcanized material is less than that of the vulcanized material (0.18 J/g and 0.23 J/g respectively). The difference has to do with differences in crystallinity and not direct with the degree of cross-linking. Using the method described in Section 4.1.3. Influence of crystallization on the glass transition of silicone rubber, total peak areas, Δh, of 9.17 J/g for the unvulcanized material and 5.76 J/g for the vulcanized material were determined from the crystallization and melting peaks. This means that, on cooling to the starting temperature, the vulcanized material crystallizes more slowly than the uncross-linked material. The factor decisive for the differences at the glass transition is therefore the crystallinity and not the degree of cross-linking. This is why the values of Δcp and Δh correlate very well with the values for different crystallinity from Section 4.1.3. Influence of crystallization on the glass transition of silicone rubber. The values obtained from the measurements presented here are marked in red in the following diagram. 

The three black points on the right of the diagram are the results of measurements performed on samples with larger degrees of crystallinity taken from Section 4.1.3. Influence of crystallization on the glass transition of silicone rubber. In these experiments, cooling rates of 20, 10 and 5 K/min were used. The black measurement point on the left was obtained from a sample that was almost amorphous. In this case, the sample was placed in the measuring cell that had been cooled beforehand. This allowed a very rapid cooling rate of more than 100 K/min to be achieved. The red points originate from measurements in which the samples were cooled in the measuring cell at approx. 50 K/min. 

Conclusions

Cross-linking through vulcanization does not directly influence the glass transition with the material investigated. More important is that cross-linking lowers the crystallization rate. The resulting differences in crystallinity cause differences in the glass transition. 

_{Glass Transition of Vulcanized and Unvulcanized Silicone Elastomers | Thermal Analysis Application No. HB450 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2}